# Making sustainable aluminum by recycling scrap: The science of “dirty” alloys

Dierk Raabe, Dirk Ponge, Peter Uggowitzer, Moritz Roscher, Mario Paolantonio, Chuanlai Liu, Helmut Antrekowitsch, Ernst Kozeschnik, David Seidmann, Baptiste Gault, et al.

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Dierk Raabe, Dirk Ponge, Peter Uggowitzer, Moritz Roscher, Mario Paolantonio, et al. Making sustainable aluminum by recycling scrap: The science of “dirty” alloys. Progress in Materials Science, 2022, 128, 10.1016/j.pmatsci.2022.100947. [https://doi.org/10.1016/j.pmatsci.2022.100947](https://doi.org/10.1016/j.pmatsci.2022.100947)

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# Making sustainable aluminum by recycling scrap: The science of “dirty” alloys

Dierk Raabe<sup>a, *</sup>, Dirk Ponge<sup>a</sup>, Peter J. Uggowitzer<sup>b</sup>, Moritz Roscher<sup>a</sup>, Mario Paolantonio<sup>a</sup>, Chuanlai Liu<sup>a</sup>, Helmut Antrekowitsch<sup>b</sup>, Ernst Kozeschnik<sup>c</sup>, David Seidmann<sup>d</sup>, Baptiste Gault<sup>a, e</sup>, Frédéric De Geuser<sup>f</sup>, Alexis Deschamps<sup>f</sup>, Christopher Hutchinson<sup>g</sup>, Chunhui Liu<sup>h</sup>, Zhiming Li<sup>i</sup>, Philip Prangnell<sup>j</sup>, Joseph Robson<sup>j</sup>, Pratheek Shanthraj<sup>j</sup>, Samad Vakili<sup>a</sup>, Chad Sinclair<sup>k</sup>, Laure Bourgeois<sup>l</sup>, Stefan Pogatscher<sup>m, *</sup>

## ARTICLE INFO

**Keywords:**
* Aluminum
* Alloy design
* Precipitation
* Sustainability
* Recycling
* Properties
* Processing
* Corrosion
* Thermodynamics

## ABSTRACT

There are several facets of aluminum when it comes to sustainability. While it helps to save fuel due to its low density, producing it from ores is very energy-intensive. Recycling it shifts the balance towards higher sustainability, because the energy needed to melt aluminum from scrap is only about 5% of that consumed in ore reduction. The amount of aluminum available for recycling is estimated to double by 2050. This offers an opportunity to bring the metallurgical sector closer to a circular economy. A challenge is that large amounts of scrap are post-consumer scrap, containing high levels of elemental contamination. This has to be taken into account in more sustainable alloy design strategies. A “green aluminum” trend has already triggered a new trading platform for low-carbon aluminum at the London Metal Exchange (2020). The trend may lead to limits on the use of less-sustainable materials in future products. The shift from primary synthesis (ore reduction) to secondary synthesis (scrap melting) requires to gain better understanding of how multiple scrap-related contaminant elements act on aluminum alloys and how future alloys can be designed upfront to become scrap-compatible and composition-tolerant. The paper

<footnote><sup>a</sup> Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237 Düsseldorf, Germany</footnote>

<footnote><sup>b</sup> Chair of Nonferrous Metallurgy, Montanuniversität Leoben, Franz-Josef-Str. 18, 8700 Leoben, Austria</footnote>

<footnote><sup>c</sup> Institute of Materials Science and Technology, Technische Universität Wien, Getreidemarkt 9, 1060 Wien, Austria</footnote>

<footnote><sup>d</sup> Northwestern University, Cook Hall, 2220 Campus Drive, Evanston, IL 60208, USA</footnote>

<footnote><sup>e</sup> Department of Materials, Imperial College, South Kensington, London SW7 2AZ, UK</footnote>

<footnote><sup>f</sup> Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMAP, F-38000 Grenoble, France</footnote>

<footnote><sup>g</sup> Department of Materials Science and Engineering, Monash University, Room 249, Building 82, Clayton Campus, Clayton, VIC 3800, Australia</footnote>

<footnote><sup>h</sup> Light Alloy Research Institute, State Key Laboratory of High-Performance Complex Manufacturing, Central South University, Changsha 410083, China</footnote>

<footnote><sup>i</sup> School of Materials Science and Engineering, Central South University, Changsha 410083, China</footnote>

<footnote><sup>j</sup> Henry Royce Institue, University of Manchester, Royce Hub Building, Manchester M13 9PL, UK</footnote>

<footnote><sup>k</sup> Department of Materials Engineering, The University of British Columbia, 309-6350 Stores Rd., Vancouver V6T 1Z4, Canada</footnote>

<footnote><sup>l</sup> Monash Centre for Electron Microscopy, and Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia</footnote>

<footnote><sup>m</sup> Christian Doppler Laboratory for Advanced Aluminum Alloys, Chair of Nonferrous Metallurgy, Montanuniversität Leoben, Franz-Josef-Str. 18, 8700 Leoben, Austria</footnote>

<footnote>* Corresponding authors.
E-mail addresses: d.raabe@mpie.de (D. Raabe), stefan.pogatscher@unileoben.ac.at (S. Pogatscher).</footnote>
therefore discusses the influence of scrap-related impurities on the thermodynamics and kinetics of precipitation reactions and their mechanical and electrochemical effects; impurity effects on precipitation-free zones around grain boundaries; their effects on casting microstructures; and the possibilities presented by adjusting processing parameters and the associated mechanical, functional and chemical properties. The objective is to foster the design and production of aluminum alloys with the highest possible scrap fractions, using even low-quality scrap and scrap types which match only a few target alloys when recycled.

# 1. Introduction and motivation for scrap-based aluminum alloys

About 100 Mt (million metric tons) of aluminum are currently produced per year, ~35% of which comes from scrap while ~40% has already been scrapped in the manufacturing chain [1,2]. Aluminum is a material with two facets when it comes to sustainability [3,4]. On the one hand, it reduces energy consumption in products and processes, e.g. in lightweight transport, packaging and construction, due to its low mass density (2.7 kg dm⁻³). It is also used for low-resistive, low-weight electrical conduction, as with 37 × 10⁶ A (Vm)⁻¹it reaches 64% of the conductivity of pure Cu with about 3 times less mass. On the other hand, aluminum is also one of the greatest greenhouse gas (GHG) producers and energy-intensive industrial metals produced from ores [5,6]. Its GHG emissions include carbon dioxide, methane, nitrous oxides, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride. Globally, aluminum production contributes ~3% of all GHG emissions (~15% of all emissions in the industrial sector), with a ca. 1.1 Gt carbon dioxide equivalent per year [1,7,8]. This means that aluminum from primary production generates about 12–16.5 t of GHG per t of metal produced [1,3,9–12]. About 65% of these emissions occur because ~67% of the electricity used for electrolysis is produced from fossil fuels [1,9,13,14]. Aluminum production requires ~13 Exa J energy per year, which is ~1% of total global energy consumption [14]. The synthesis of aluminum also creates multiple harmful by-products during mining and electrolysis [3]. This impact is accelerating due to growing population (and the growth of the middle class, now about half of the total population) and per-capita consumption of aluminum, which is driven by several current trends in transportation, urbanization, electrification and manufacturing. For the first time in history the production of aluminum is now facing sustainability limits (see Fig. 1).

Aluminum is an infinitely recyclable material [21–24]: today, about 75% of all the aluminum produced in history – nearly a billion metric tons – is still in use [25,26]. Recycling involves re-melting the metal, which requires only 5% of the energy used to make new aluminum from bauxite ore [6]: the primary synthesis of Al from ores expands 45 kWh/kg because of the large enthalpy of its oxide, and about 12 kg CO₂/kg, while re-melting Al scrap (secondary synthesis) expands only 2.8 kWh/kg because of its low melting point (660 °C), and about 0.6 kg CO₂/kg [27]. This means that recycling has the potential to shift aluminum’s energy and carbon balance substantially towards much greater sustainability [2].

On average, a third of the aluminum used today is produced from recycled scrap [21], although this fraction is predicted to rise to 50% by 2050, according to data (2019) from the International Aluminum Institute (see Fig. 2) [1,9,13]. The huge differences between primary and secondary synthesis in terms of GHG and energy consumption make aluminum alloys, as an important pillar of a circular economy, important subjects for sustainable metallurgy research [4,28,29].

In scrap recycling a differentiation must be made between pre-consumer scrap, produced during manufacturing and often featuring well-defined alloy classification, and mixed, low-quality post-consumer scrap, which must be sorted before remelting (see Fig. 3) [1,2].

A good example of circularity is aluminum beverage cans. In some regions, the recycling rates for such alloy grades reach 95% [30]. However, even though used beverage cans (UBC) are easily collected, have a short lifetime and can theoretically be made of a single

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Aluminum alloys made entirely from scrap can reduce energy consumption and GHG emissions by > 90% compared to alloys made by primary synthesis [1]. Advanced alloy design aims at improving and developing alloys via built-in recyclability, for maximum scrap compatibility and use [15]. On average the values for the embodied energy for primary aluminum production range between 53 and 65 kWh/kg (190 – 235 MJ / kg), depending on slightly different approximations and year [6,16–20].</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Development of primary and recycled aluminum through 2050, based on numbers in 2019 for end-of-life product collection rates (data sourced in 2021 from the International Aluminum Institute (IAI)). MT: million metric tons.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Development of global aluminum scrap intake in metric tons. Since 1990, post-consumer scrap availability from end-of-life (EOL) products has exceeded pre-consumer scrap (data sourced in 2021 from the International Aluminum Institute (IAI)).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Carbon footprint of an aluminum can, analysed by Eric Onstad of Reuters, using data from the Carbon Trust. The minimum and maximum values give the grams of CO₂ embedded in a 300 ml standard package unit.</figcaption></figure>

alloy (today two are typically used) [31], the can-to-can cycle is still not a closed loop [30]. An analysis by Reuters, using data from the Carbon Trust, shows that the overall sustainability of aluminum can packaging depends greatly on the recycled fraction and on the origin of the energy used for melting and for generating the additional primary material (see Fig. 4).

The challenge presented by such issues increases exponentially in multi-material products such as vehicles [32-36]. This issue needs to be addressed, because the use of aluminum in cars is increasing. Figures from 2019 indeed show that most post-consumer scrap, in terms of tonnage, now stems from packaging and vehicles (see Fig. 5).

Fig. 6 shows an example where using recycled Al (instead of material from primary synthesis) in a vehicle generates not only a 28% reduction in mass (and therefore fuel) but also a 35% reduction in energy consumption and GHG emissions during manufacture.

Today aluminum recycling is conducted primarily within established alloy classes, and only small scrap tonnages are exchanged among compositionally dissimilar alloy groups [37-40]. The recycling rates for aluminum are generally rather high, but they differ substantially by product, alloy and region, ranging from as low as 20% to as high as 80% for some packaging products [25,26].

The quality, abundance and flow of scrap in this field vary greatly, as do the target alloys and their composition requirements [1]. They range from contaminated and mixed low-price scraps (also referred to as old scrap or post-consumer scrap) and lower-purity-
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Numbers from 2019 showing the distribution of the global aluminum post-consumer scrap intake as a function of manufacturing methods and sector (data represent the year 2019; source: the International Aluminum Institute (IAI)).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Example revealing the dual effect of aluminum alloys made from scrap: weight reduction and the associated fuel savings (left) plus reduction in energy consumption and GHG output from synthesis when using alloys made from scrap (right). Step 1 (data from Al Association) shows how much mass is saved when increasing Al from 9 to 37 wt%. Step 2 shows that this generates a 28% reduction in mass plus a 35% reduction in energy consumption and GHG emission when using scrap-based Al.</figcaption></figure>

sensitive commodity alloys to well-defined high-quality secondary material (retrieved from closed-loop industry processes where little further improvement can be achieved) and very impurity-sensitive high-performance alloys with little tolerance for tramp elements [34,41–43]. It is this variety in the scrap landscape and the great diversity in the types of aluminum alloys used, which set the stage for our investigation of the role of scrap-usage in aluminum alloys and their potential recycling-friendly design. Here several questions are of interest with respect to the metallurgy of these alloys [44]. One set of questions concerns the number and quantities of scrap-related tramp elements that can be tolerated in existing high-performance aluminum alloys, and what can be done to make them more impurity-tolerant while retaining comparable properties [45–47]. The extensive use of scrap for the synthesis of aluminum alloys turns the underlying phase diagrams into multi-component equilibria scenarios: while conventional phase diagrams for alloys reside in low-dimensional composition corners with two or three prevalent alloying elements, the phase diagrams required to design scrap-tolerant alloys may have to juggle up to 15 composition axes. This is particularly important for aluminum alloys, due to their low solubility for most scrap-related tramp elements.

Another set of challenges lies in whether new alloy design strategies might target a compromise between higher scrap usage on the one hand, and modestly reduced properties that satisfy the requirements of a broader range of commodity products on the other. This trade-off could be quantified by balancing the reduction in energy use and GHG production through higher scrap use with performance loss and the associated sustainability reduction when the material is in service. Other questions concern the design of so-called crossover or uni-alloys [48], with chemical overlap between separate alloy classes [34]. If sufficient performance is achieved, such materials would be more recycling-friendly from the outset. Another avenue would be to develop compositionally lean alloys, which realize properties less through fine-tuning of composition than through processing and microstructure tuning. This allows the decoupling of properties from compositions and permits compositionally simple alloys to be more readily recycled and mixed.

The following article addresses the current status of established and novel aluminum alloys and their development, processing and optimization for high performance if we use recycling as the production route. We provide guidelines for creating materials with more built-in recyclability, launching a new field, which we call “sustainability alloy design”. It targets a shift from primary synthesis (reducing ores) to secondary synthesis (melting scrap). In this context it must be emphasized that the amount of energy needed to melt aluminum from scrap is much lower than that needed to reduce ores. It should also be noted that scrap availability, quality and supply
chains obviously impose limits, although markets are currently nowhere near reaching even these [1,49–52]. In Europe, the amount of aluminum available for recycling is estimated to more than double by 2050 [49,53]. However, in this scenario supply chains and increasing costs due to demand growth will also play important roles. This means that a large-scale business such as secondary aluminum production will only prosper if the supply chains for recycled material are logistically robust, quality-controlled (in both their pre- and post-consumer scrap fractions and their sorting technology), sufficient in magnitude and economically competitive.

Identifying pathways towards sustainability alloy design requires us to understand how multiple scrap-related contaminant elements act on aluminum alloys, and to utilize the knowledge. Here we have coined a further term: the “science of dirty alloys” (SoDA). This term addresses the fundamental question of how alloys can be designed upfront to be scrap-compatible and composition-tolerant, in order to maximize the use of scrap. Most of the aluminum scrap today is post-consumer scrap [1,2,40,54]. This scrap usually has a high contaminant content, meaning the intrusion of mostly unwanted tramp elements. In other words, most scrap in the next few decades will be “dirty”. This article explores how what initially sounds inauspicious can become the basis for a new and fruitful science.

The article presents and discusses the scientific and engineering tasks required to achieve the goal of a circular aluminum economy. These have been broken down into specific metallurgy research topics such as the influence of scrap-related contaminant elements on the thermodynamics and kinetics of precipitation reactions [55] and their mechanical and electrochemical effects; precipitation free zones around grain boundaries; casting microstructures; the adjustments required in processing parameters; and the resulting mechanical, functional and chemical properties. We also review advanced experimental methods to identify the partitioning of tramp elements among lattice defects, precipitates and the matrix and the associated effects on precipitation kinetics, thermodynamics and mechanical response. We follow two main research directions. One involves an asymptotic approach, where we evaluate how to improve existing alloys and make them more scrap-tolerant. The other, where we discuss ideas for developing new generations of compositionally robust and lean alloys that are more scrap-tolerant from the outset, is more disruptive. Ultimately we wish to steer the design and production of aluminum alloys towards using the highest possible scrap fractions, low-quality post-consumer scrap, and scrap types which serve only a few matching target alloys when recycled.

The “green aluminum” trend (encompassing both primary synthesis on the basis of hydropower and scrap in conjunction with recycling) resonates with international markets. The London Metal Exchange announced in June 2020 a new trading platform for sustainable aluminum. The future may see a scenario where alloys that fail to achieve a certain level of sustainability face limitations on use in products. Investor, consumer and legislative pressure is intensifying on industries which pollute, and manufacturers are increasingly being forced to source materials responsibly. These trends are starting to create premium markets in cleaner-footprint metal, and sustainable metallurgy is attracting market awareness in turn.

Exploring SoDA – the science behind sustainable metallurgy and green alloys – in this way can guide the development of a generation of more sustainable metallic materials. This will contribute not only to the metallurgical science, but also to a more sustainable society and a circular economy where sustainable materials can be translated into successful, safe, high-performance products that meet the highest quality standards.

## 2. What defines a sustainable aluminum alloy?

### 2.1. Sustainability and circular economy measures for aluminum alloys

To frame the leverage effects of individual sustainability measures, we first need to describe what defines a sustainable aluminum alloy. Several academic studies, legislative measures and industry-driven initiatives have made suggestions for defining sustainable metallic alloys, processes and elements of a corresponding circular metallurgical economy [3,15,56,57]. This section applies the essence of these suggestions to aluminum alloys.

The sustainability of materials must be broadly defined [10,18,22,58]. It needs to consider the environmentally relevant aspects associated with mining, processing, chemistry, waste and by-product treatment. It has to include energetic, recycling and metallurgical aspects, the amount of material scrapped during manufacturing, and the responsible and lean use of alloying elements [37,40,59,60]. It must also incorporate social, ethical and work standards. These dimensions must be applied to the entire product and to supply chain pathways, and must include implementation of sustainability risk analysis and risk mitigation strategies. These must be transparent, well-documented and quantifiable by internationally acknowledged standardization methods, since the market for aluminum and aluminum products is global. This requires transparent life cycle assessments (LCA) based on data and boundaries that are agreed upon by research and economic associations, customers, legislators and standardization boards [58,61–63]. The tracking of sustainability measures must be extended over the entire value chain. For the specific case of aluminum this involves all stages of mining, production and processing, including bauxite mining and transport; alumina refining; primary and secondary aluminum production including energy sources; red mud treatment and waste management; semi-finished processing such as casting, hot rolling, cold rolling, heat treatment, extrusion and forging; and the minimization, collection, alloy-specific sorting, refining and re-melting of scrap [15,41,52,64].

The colossal production, GHG, scrap and energy figures outlined above speak for themselves and illustrate the urgency of this approach. It is also important to identify the measures that have the biggest effects on the sustainability of aluminum alloys. For different alloy groups, Fig. 7 shows the technology readiness level (TRL) versus scaled potential for the impact of different sustainability measures throughout value chains [3]. In the specific case of aluminum alloys, using electrical energy from renewable resources such as hydropower leverages the highest impact at high TRL, making it one of the most efficient measures. Particularly in the area of primary synthesis, aluminum production can only get as sustainable as the power source used to operate electrolysis [65–67] (see Fig. 4). This reflects the trend in the market, which shows that primary aluminum synthesis from renewable energy sources,
<figure><img src="image_9.png" /><figcaption>Fig. 7. Technology readiness and scaled potential for impact of different sustainability measures throughout the value chains of different alloy groups. Modified version reproduced with permission from [3].</figcaption></figure>

particularly from hydropower, has in some regions advanced market share (see Fig. 8) [13].

High leverage for improved sustainability also comes from electrification (from renewable sources) and avoidance of CO2 related energy and heating sources for the entire downstream manufacturing chain that follows primary smelting. One trend, for instance, is to use hydrogen-based energy carriers instead of fossil fuels to heat furnaces.

Further sustainability measures with high leverage but different degrees of technology readiness are shown in Fig. 7. They are related to the main topic of this paper, i.e. secondary synthesis [68,69]. They include measures associated with the role of scrap-related tramp elements and scrap-compatible alloy design [4,28,70]. The impact analysis suggests that strong leverage is expected from advanced and machine-learning-assisted scrap sorting and separation techniques; the general increase in scrap fractions used in alloys (where scrap collected during or after production, and post-consumer scrap, must be separated from each other); upfront recycling-oriented alloy design; reduced chipping and scrapping during manufacturing; and within-alloy-family recycling.

If we consider the origin and sustainability of energy sources (for both primary and secondary synthesis), the quantity and quality

<figure><img src="image_10.png" /><figcaption>Fig. 8. Power sources for primary aluminum synthesis by electrolysis, plotted after data from the International Aluminum Institute (IAI) [13].</figcaption></figure>
of the scrap and the impact of mining and labour conditions (evaluated on the basis of transparent and quantitative LCA validation methods), it becomes apparent that a sustainable alloy does not necessarily have to be based on secondary synthesis. However, in most cases secondary synthesis provides considerable benefits, as LCAs show [41,52,61]. It is therefore conceivable that one and the same alloy can be produced with very different levels of sustainability. This underlines the fact that commonly accepted standards must be introduced to document the origin and sustainability of the material before it is used in products or processes.

Fig. 7 also suggests that for both primary and secondary synthesis, downstream production benefits from deploying manufacturing processes with low energy consumption. These can involve avoidance of re-heating of melts and slabs; lower thickness reduction in sheet manufacturing; temperature and time optimized heat treatments; and near-net-shape casting processes. Such methods are also advantageous in that the coarse intermetallic precipitates that can form in materials with high scrap-related impurity intrusion remain smaller than in conventional thick slab casting.

Kümmerer et al. [57] have translated these aspects into rules for the integration of chemical and materials products and processes into a circular economy. Although formulated for all material classes, many of these rules also apply to aluminum. One rule suggests keeping the chemical complexity and number of components used in materials to the minimum required for achieving a certain performance. This should include recycling and all end-of-life considerations. Increasing chemical complexity requires more complex synthesis pathways and makes recycling more challenging. Another rule suggests building recycling considerations upfront into material design, manufacturing processes and scrap collection streams, including auxiliaries, unused substrates and – for aluminum – particular by-products such as red mud. Further rules suggest reducing alloy, material and product diversity, dynamics and flows, and making alloys with the lowest possible resource intensity. This can in fact be realized in the case of aluminum alloys made from scrap, which are also suitable to serve as scrap in new alloys. In this article reduction in product diversity is addressed in the section on uni- or crossover alloys [48]. This is a new alloy design concept which aims to unify in one material the properties of several alloy types with originally different compositions.

Reaching higher circularity for aluminum also requires bringing the innovation speed for new alloys and products into better synchrony with the speed of recycling. In particular, the use of rare and expensive alloy ingredients should be reduced; if they are required, they must be suitable for efficient recovery and recycling to prevent their gradual loss and dissipation as landfill. The authors called such scenarios “entropic losses” [46,71,72].

Something else that should be avoided are so-called “rebound effects” [57]. These can arise when sustainable technologies are applied that on the one hand help to reduce GHG emissions, but on the other generate higher demand for the metals needed for the technology. Examples are wind parks, photovoltaics and the electrification of transport and industry: manufacturing all of the involved parts and the associated infrastructures to put these technologies into service over the next few decades will require substantial additional amounts of metal and production processes which can be very carbon-intense. LCAs must take into account such unintended leverage effects. The rule that circular metrics should be developed and adopted by the producer and customer community, encompassing responsibility for toxic waste or the use of by-products, can engender similar effects. Using fewer high-strength aluminum alloys, for example, could in some applications lead to greater use of stainless steels, carbon-fiber-reinforced composites or titanium alloys – i.e. materials which can generate even higher energy costs and GHG emissions.

Producers should also take full responsibility for their alloys and products throughout their respective life cycles. This includes recycling protocols. Digital measures might be involved, such as digital passports which carry traceable, transparent and forgery-proof information on alloy composition and production and manufacturing processes. Finally, processing could be simplified. For aluminum this means that processing has to be streamlined for efficiency, e.g. avoiding homogenization treatments where appropriate, and favoring temperature- and time-optimized annealing treatments. Of considerable importance are also near-net-shape and thin-strip casting, i.e. avoidance of energy losses due to unnecessary cooling and re-heating through just-in-time workflows. Other efficiency measures include the flexible operation of electrolysis cells. This approach uses electrical power when smart grids provide inexpensive surplus energy and/or when ample sustainable energy is available, such as on windy or sunny days. Similar advantages are provided by just-in-time delivery of pre-alloyed liquid material (rather than pre-alloyed slabs) to customers. This turns liquid aluminum alloys into batteries, as their high heat capacity stores huge amounts of energy efficiently, thereby qualifying the technique as an efficient means of buffering power.

The essence of the rules suggested by Kümmerer et al. is summarized in Table 1. In the case of aluminum this means that certain community-agreed limits on GHG emissions, energy consumption, waste and by-products must be adhered to if a material is to qualify as a sustainable alloy. This leverages the use of the most efficient processing methods.

On the metallurgical side, the essence of the rules is that alloys must be designed from the outset to be impurity-tolerant, making them suitable for high levels of recycled content. This would mean that they can be collected and used again as scrap in the most compatible way possible for the secondary synthesis of new alloys, i.e. without poisoning the latter [28]. Thus the title of the current paper, “the science of dirty alloys”. In other words, sustainable alloys should be “scrap-compatible”, and designed not for single use but rather for repeated use and recycling. This means that scrap-compatible alloys must be created which accept maximum scrap content when synthesized, i.e. they must be tolerant of tramp elements and minor composition variations, including rare impurities stemming e.g. from mixed post-consumer, automotive and electronic scrap.

Impurity tolerance is particularly important in the design of aluminum alloys. This is because aluminum, unlike many steels, has small solubility for many elements (except Mg and Zn) (see Fig. 9) [73-75]. This means that intermetallic phases form more readily than in many transition metal alloys when contamination by tramp elements occurs. In conventional alloys, modest changes in the solid solution state are usually associated with relatively modest property changes, while altering the material’s precipitation state can be more disruptive and cause brittleness, corrosion or hydrogen embrittlement.

The low solubility of aluminum for tramp elements suggests that from a sustainability perspective compositionally lean alloys are
<table><caption>**Table 1**
Elements of a circular economy applied to aluminum alloys.</caption><thead><tr><th colspan="3">Circular economy measures related to aluminum alloys</th></tr><tr><th>General</th><th>Specific</th><th>Metallurgical and engineering measures</th></tr></thead><tbody><tr><td rowspan="10">Use of renewable energy sources and responsible use of resources</td><td>Use of renewable energy sources</td><td>Transition to use of hydro-, solar and wind power for electrolysis and manufacturing</td></tr><tr><td>Use of sources with reduced GHG emissions</td><td>Avoidance of carbon carriers as fuels for re-heating, aging heat treatments and related processes</td></tr><tr><td>Energy efficient processes</td><td>Use of high scrap fractions in alloys; near-net-shape manufacturing; thin-slab casting; thin-strip casting; reduction of required thickness; in-line processing, i.e. avoidance of reheating of melts and slabs through just-in-time inter-process logistics; low-temperature heat treatments</td></tr><tr><td>Waste and by-product management</td><td>Responsible deposition or recycling of mining and metallurgical waste by-products and of toxic substances; reduced use of products, additives and by-products that cannot be separated and recycled</td></tr><tr><td>Adherence to social and ethical standards</td><td>Absence of child and slave-like labour; appropriate salary and commitment to labour and union rights; fair working conditions and payment; adherence to inclusion and diversity standards; professional development and career training; adequate equal opportunity and diversity measures</td></tr><tr><td>Process simplification and product portfolio streamlining</td><td>Develop process workflows with minimum number of process steps, auxiliaries and energy requirements; develop low-temperature warm rolling protocols, omit/ optimize homogenization, low temperature annealing treatments; reduce product diversity</td></tr><tr><td>Quantification of measures by life cycle assessment</td><td>Develop and apply suitable metrics that capture and quantify sustainability measures; make results of such calculations open</td></tr><tr><td>Responsible use of alloying elements</td><td>Use alloying elements with low energy and GHG footprint; reduce use of critical alloying elements; ensure efficient recovery and recycling of critical alloying elements; reduce alloy complexity in terms of required processing and chemical composition</td></tr><tr><td>Circular economy business models</td><td>Develop closed-loop business models with customers</td></tr><tr><td rowspan="5">Scrap management</td><td>Advanced scrap sorting</td><td>Automated chipping, spectroscopy and artificial-intelligence-assisted alloy detection, classification and sorting</td></tr><tr><td>Avoidance of scrap during manufacturing</td><td>Product and process design for less scrap</td></tr><tr><td>In-production scrap collection</td><td>Alloy-specific scrap collection during synthesis and manufacturing; develop processes to collect and sort scrap during production</td></tr><tr><td>Post-consumer scrap use</td><td>Develop low-grade and composition-tolerant alloys and identify products that can accommodate higher scrap-related contaminant content in alloys</td></tr><tr><td>Green branding for alloys containing high levels of scrap</td><td>Develop marketing methods to brand high-scrap-content alloys</td></tr><tr><td rowspan="6">Digitalization</td><td>Digital twin of products and processes</td><td>Thermodynamic, kinetic, process and mechanical simulations for knowledge-based sustainable design of alloys, processes and products; identify thermodynamic, kinetic and mechanical limits of tramp-related effects; establish lifetime and corrosion simulations for higher product longevity; higher digitalization degree of manufacturing enables better tracking and management of material, scrap and by-product flows; sustainability considerations and quantitative life-cycle-relevant measures should become an integral part of digitalization, sensoring and tracking strategy</td></tr><tr><td>Improvement of databases</td><td>Maintenance and improvement of thermodynamic and kinetic databases to include scrap-related tramp elements, specifically taking into account metastable and intermetallic phases and contaminant effects on vacancies</td></tr><tr><td>Digital material and product passports</td><td>Alloys and products should be fully traceable, e.g. by using product- and alloy-specific digital passports and history trackers that include the composition of products, components, and processes; this also implies claiming responsibility and an awareness of the history of products throughout their life cycles, including recycling; establish digital tracking and digital workflows with customers; using digital material passports for chemical composition of products, components, and processes can become part of product marketing to demonstrate and "brand" sustainable products and processes</td></tr><tr><td>Novel approaches to sustainable alloy design</td><td>Most compositions in use today were developed many decades ago. With today's understanding of physical metallurgy, thermodynamics and kinetics, new opportunities in sustainable alloy design are emerging</td></tr><tr><td>Lean alloys</td><td>Design of alloys with minimal use of processing steps and chemical ingredients; achieve properties more by microstructure and less by composition adjustment</td></tr><tr><td>Less use of harmful and critical alloying elements</td><td>Selection of alloying elements according to lowest energy consumption and GHG production; avoidance of rare and less-responsible elements as ingredients</td></tr><tr><td rowspan="3">Alloy design</td><td>Fewer alloys</td><td>Design of uni-alloys and crossover alloys that combine usually separated or even contradictory features of different alloys or different alloy classes</td></tr><tr><td>Scrap-tolerant alloys</td><td>Design of alloys for high-scrap-related tramp element content</td></tr><tr><td>High-strength alloys</td><td>Alloys that enable reduced vehicle weights and enhanced fuel efficiency</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 1 (continued)</caption><caption>Circular economy measures related to aluminum alloys</caption><thead><tr><th>General</th><th>Specific</th><th>Metallurgical and engineering measures</th></tr></thead><tbody><tr><td rowspan="4">Product use</td><td>Recycling as part of alloy design</td><td>Ensure that alloys and by-products can be collected and recycled; recyclability should be an integral part of alloy and process design and that of scrap collection loops in-house and with customers; processes and alloys should be designed for optimal material recovery of auxiliaries, scrap and unintended by-products</td></tr><tr><td>Longevity</td><td>Alloys with high resistance to corrosion, hydrogen embrittlement and abrasion</td></tr><tr><td>Damage tolerance and self-repair</td><td>Alloys with high damage tolerance and self-healing properties</td></tr><tr><td>Use optimization</td><td>Re-manufacturing, repair, reuse and sharing of products</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Solubility limits of tramp and alloying elements in aluminum [73].</figcaption></figure>

generally preferable to over-alloyed materials as acceptors and donators of scrap. It is a general feature and an advantage of structural alloys that they obtain many of their mechanical properties primarily through microstructure. This means that what matters is not only global composition, but the way it helps to realize specific microstructures. Of course, certain effects such as precipitate formation do not occur without alloying, but the fine-tuning of properties (which today often takes place through composition adjustment) can in part be replaced by the tweaking of microstructure, for two main reasons. First, composition is a conserved quantity, while micro-structure does not depend on conservation laws. This means that any alteration of an alloy through composition adjustment will reappear in the form of a modified mass balance somewhere in a circular economy, as every single atom is conserved. This does not apply to microstructure, which can be modified over multiple orders of magnitude in terms of dislocation density, grain size, pre-cipitate dispersion, clusters or crystallographic textures without hindrance from any conservation law except mass balance. Second, tweaking microstructure is very efficient for improving mechanical properties. Among the various microstructure-based hardening and ductilization methods, controlling and increasing precipitate dispersion (at given total volume of precipitates) are effective because, due to the high mobility of the dislocations in Al-alloys, they reduce the mean free path of the gliding dislocations (which translates inversely into strength increase) to much lower values than do grain size control, dislocation cell size control or strain hardening alone. Formability, strength and surface appearance can also be substantially influenced by the crystallographic texture.

The effectiveness of microstructure control is seen in Fig. 10, where the yield strength is plotted for a variety of commercial wrought aluminum alloys as a function of both chemical composition and microstructure variation [74]. The compilation shows that a variation in strength of up to 50% is achieved for the same composition. Alloys of the 1xxx group, on the left-hand side, are relatively pure packaging grades. The 3xxx series alloys, blended primarily with manganese, serve in cans and buildings. Next are the medium-strength alloys, the 5xxx and 6xxx series. Alloys of the 5xxx series are essentially blended with magnesium, while those of the 6xxx series use magnesium and silicon. These two alloy classes are deployed in large quantities in vehicles, mobile phones and computers [76–80]. 2xxx alloys use copper as their primary alloying element. They serve mostly in highly mechanically loaded parts in the aerospace sector. Even higher strength levels can be achieved if aluminum is doped with copper, zinc, and magnesium, producing the 7xxx alloys. These are used in electrical vehicles and aerospace applications. This means that from left to right, most increases in strength are obtained by adjusting chemical compositions, together with the corresponding heat treatment required to achieve the desired nano-precipitation state. In essence Fig. 10 thus shows that in many of the high-strength aluminum alloys a broad range of tensile strengths can be obtained by adequate adjustment of microstructures rather than alterations in chemistry.

Opportunities for improved microstructure control lie in the development of more complex thermal treatments in particular. Fifty years ago, when most alloys in use today were originally developed, the thermal profiles deployed for precipitation heat treatments were quite simple as far as ramps, isothermal holding and cooling are concerned [55,81]. An important consideration in the development of well-tailored future thermal treatments is the large scope for designing composition-specific and non-isothermal thermal profiles to control precipitation and increase efficiency in advanced microstructure and property design [82–85]. Such heat treatments have to be robustly tolerant of scrap-related impurities and their effect on solutionising and precipitation.

Crossover alloys [48], also referred to as uni-alloys, occupy a special niche, as they aim to merge existing alloy concepts to make
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Overview of the flow stress values for wrought aluminum alloys. The trend shows the strength spectrum accessible through composition variation, while the vertical variation bars reveal how much the strength can be influenced – for the same chemical composition – by microstructure alone. The trend may be of special interest for the future design of thermal treatments with composition-specific and non-isothermal profiles for advanced properties that can possibly even tolerate minor variations in tramp elements [82–85]. The figure is reproduced with permission, from the original version published in [74].</figcaption></figure>

new materials which combine averaged composition concepts from different alloy groups. This could lead to less chemical sensitivity and to a smaller number of alloys in use and in scrap. The advantage of crossover alloys is also shown in their capability of strength adjustment by microstructure manipulation. Alloy 5182, shown in Fig. 10, with a relatively narrow range, spans a tensile strength of 300 to 550 MPa in the crossover variant, i.e. after modification with Zn, Cu and Ag [69].

The influence of different alloying elements on an alloy's overall sustainability was discussed by Jarfors et al. [88]. They suggested imposing energy- and GHG-related measures for more responsible use of alloying elements. More specifically, they show that certain elements, such as Mn, offer a much higher gain in strength (and also in Young's modulus) than other alloying elements for a given sustainability footprint. The aim of their calculations was to show how to achieve a maximum increase in strength at the lowest possible embodied energy and GHG footprint associated with the specific choice of alloying element.

Although the rules for sustainable alloy design outlined above provide a framework for moving forward, fundamental challenges remain. The compositional tolerance values discussed are alloy-, customer-, and application-specific. This means that not all aluminum products can be provided easily by secondary synthesis alone, as several classes of high-end products, such as those needed for safety-relevant parts in transportation or conductive devices, require alloys which are carefully engineered with respect to toughness, corrosion, hydrogen, fatigue, conductivity or surface finish – which usually tolerate only small variations or uncertainties introduced from scrap. However, it is important to emphasize that even small variations in the tolerance limits, sometimes also coupled with adequate alloying or processing counter-measures, can generate considerable changes in the level of acceptable scrap content. Also, apart from these sensitive parts, there are many products in architecture, design, construction, machines, consumer electronics and packaging where over-engineered alloys are not actually needed. This means that designing scrap-tolerant alloys must proceed in concert with the market, i.e. green-branded alloys would first target market niches where sustainability plays a particularly important role and customers are inclined to pay more for green branded products, or niches where minor property variations can be tolerated.

In areas which are entirely cost-driven, the scrap content must be high enough to beat the price of aluminum products that come from primary synthesis. In such fields, new scrap-tolerant / highly sustainable alloys and novel crossover alloys must both also meet the highest quality standards in their properties in order to compete with existing primary synthesis alloys that are potentially less sustainable [48]. In many cases this might mean that alloys with high scrap fractions will cost more (we do not give price ranges here because trading and prices vary too much from month to month). This could be balanced by green branding strategies. In addition, metallic materials are sometimes not very visible to customers in the products that contain them, and they frequently comprise a small price fraction relative to the added value of the entire product. This means that the higher costs accrued by using a more sustainable alloy often change the price of the entire product only modestly.

The next question is: what concrete, specific metallurgical research challenges do the elements which define a sustainable alloy present? In this context, the main topics addressed in the following sections are:
(i) What effects do contaminants have, individually and collectively, on alloy properties? How (for instance) do they influence interface decohesion, phase formation, precipitation free zones, precipitation kinetics, surface finish, mechanical properties and corrosion?

(ii) Are thermodynamic and kinetic databases sufficiently detailed and reliable as foundations for the development of “dirty alloys”, in particular in the areas of spinodal, metastable and intermetallic phases and contaminant effects on vacancies?

(iii) Can scrap-related contaminants get trapped at lattice defects and inside precipitates? Could such trapping be used to render them harmless? Which types of thermal treatment should be applied to this end? Are all contaminant-related phases that form harmful, or are there beneficial features associated with any of the tramp elements?

(iv) How impurity-tolerant can we make green near-commercial Al-alloys, and which contaminants are most relevant? What upper limits apply to contaminants in near-engineering Al-alloys?

(v) Which crossover alloys are the most promising for combining beneficial mechanisms across established alloy families? Such broad-band, multi-purpose alloys must be designed as universal and not as niche alloys. They should be broad in their composition tolerance and application range.

(vi) What combinatorial high-throughput methods are suitable for revealing scrap-related composition-microstructure-kinetics-property trends and ranges? How can we probe damage-tolerance in the associated experiments?

(vii) For which phases is it necessary to use atomistic simulation methods to understand phase (meta-) stability, impurity trapping, sublattice occupancy and phase stoichiometry ranges? In general, are there sufficiently suitable modeling techniques available by which we can analyse the effects of tramp elements on aluminum alloys?

(viii) How does deploying meso- and near atomic-scale characterization methods address these challenges? How can the main effects associated with tramp elements be observed and identified?

(ix) Can supervised, knowledge-informed machine learning identify suitable nanostructure-composition-processing-property relations for the field of sustainability alloy design?

(x) How must solidification, solutionizing and heat treatment processes be adjusted to deal with the effects of contaminant elements and the associated intermetallic phases?

(xi) What measures and approaches promise to be the most effective in improving the sustainability of secondary synthesis?

## 2.2. Balance between sustainable primary metal and scrap use in green aluminum

Primary aluminum will continue to play an important role in alloy production because of the need for both dilution and alloy composition adjustment even in closed-loop scrap streams. Moreover, it is projected that demand for aluminum products will continue to outstrip scrap supply, requiring significant primary aluminum production well into the future (see Fig. 2). It is clear, though, that solutions are needed which allow increased scrap uptake without downcycling in secondary aluminum production. Here we are also required to consider how improvements in the production of primary aluminum might reduce its environmental impact. As seen in Table 2 [15], it is primary aluminum production, particularly electrolysis, that generates most of the energy consumption and CO₂ emissions during the manufacture of a product from aluminum. For example, if we compare the energy required during aluminum beverage can production stages, nearly the same amount of energy is required to produce the proportion of the liquid metal arising from scrap as is needed to produce the much smaller amount of primary aluminum used as a sweetener [8]. Anything – increased scrap usage, improved processing, or both – that reduces these negative energy and emissions effects would be a great boon.

One specific component in the electrolysis process is the electrode material, which is a particularly strong source of CO₂ emissions, even where renewable electrical energy feeds the overall process. The standard primary aluminum production method deploys the Hall-Héroult electrolysis process, which involves consumable carbon electrodes [89,90]. In this process, alumina is dissolved in a sodium-aluminum-fluoride (cryolite and aluminum fluoride) salt at 960 °C, where the species containing aluminum is reduced to metallic aluminum at the cathode and the species containing oxide is oxidized to carbon dioxide at the (carbon) anode following the reaction.

<caption>**Table 2**</caption>

Estimated energy consumption (on an annual basis) and emissions (per metric ton of product) for aluminum manufacturing processes, adapted from [15]. The data reveal that the main steps required for primary synthesis, i.e. bauxite extraction, alumina production and electrolysis, generate a much greater environmental burden than secondary production through scrap preparation, sorting and refining.

<table><thead><tr><th>Process</th><th>Energy Consumption (EJ/year)</th><th>Emissions (t CO<sub>2</sub>/t)</th></tr></thead><tbody><tr><td>Bauxite extraction + Alumina production</td><td>1.1</td><td>1.02</td></tr><tr><td>Electrolysis</td><td>5</td><td>5.5</td></tr><tr><td>Scrap preparation + re-melting</td><td>0.04</td><td>0.6</td></tr><tr><td>Scrap refining</td><td>0.15</td><td>0.6</td></tr><tr><td>Ingot casting</td><td>0.05</td><td>0.2</td></tr><tr><td>Hot + cold rolling</td><td>0.23</td><td>0.2</td></tr><tr><td>Extrusion</td><td>0.09</td><td>0.3</td></tr><tr><td>Wire drawing</td><td>0.09</td><td>0.3</td></tr><tr><td>Shape casting</td><td>0.17</td><td>0.5</td></tr></tbody></table>
$$ \frac{1}{2} \text{Al}_2\text{O}_3(\text{dissolved}) + \frac{3}{4} \text{C}_{(\text{s})} = \text{Al}_{(\text{l})} + \frac{3}{4} \text{CO}_{2(\text{g})} \quad (1) $$

In addition to CO₂, CO (via the Boudouard reaction) and perfluorocarbons (tetrafluoromethane, CF₄, and hexafluoroethane, C₂F₆) are generated, the latter resulting from insufficient oxide content of the electrolyte. All of these products contribute significantly to GHG and global warming. On top of this, the significant energy consumption required to drive the above reaction causes indirect emissions via electricity generation from non-renewable sources. A worrying trend is emerging from the circumstance that the fraction of renewable energy being used in the production of primary aluminum is in decline. While in 2000, 50% of primary aluminum was produced using low emission, renewable or nuclear power, by 2018 this figure had dropped to only 29% [89].

Recognizing the CO₂ emissions accruing from traditional consumable carbon anodes as a serious issue, research and development over the past 40 years has focused on the development of inert anodes that can achieve the reduction with only oxygen as a product:

$$ \frac{1}{2} \text{Al}_2\text{O}_3(\text{s}) = \text{Al}_{(\text{l})} + \frac{3}{4} \text{O}_{2(\text{g})} \quad (2) $$

This reaction completely eliminates the formation of CO₂, CO and perfluorocarbons. It also (potentially) eliminates the need for an anode plant and rodding shop and reduces the frequency of anode changes. Technologically, however, the development of inert anode technologies has been limited by the availability of suitable materials. Anode materials must demonstrate (among other properties) a combination of high electrical conductivity, fluoridation resistance, stability with respect to oxide formation, low cost, easy fabrication, high thermal shock resistance and mechanical strength [91,92]. Candidate materials have been proposed which range from ceramics to cermets to metals [91,92] and metal anodes are the current preferred choice [92,93]. Some significant hurdles have apparently been overcome in this context, as Rusal [92] and a joint Alcoa / Rio Tinto Aluminum venture (Elysis) [94] will launch commercial products in 2021 and 2024, respectively.

Superficially, the environmental benefits of inert anodes appear clear in that they eliminate CO₂ as a product of electrolysis. This, however, comes at the cost of the increased energy required to perform the reduction via reaction (2) compared to (1). Considering only the energy required to heat the reactants and to supply the heat of reaction, Solheim [93] has estimated the minimum energy costs involved in reduction via carbon and inert anodes, respectively. His calculations predict that carbon anodes require a minimum of ~ 6.2 kWh/kg Al, while inert anodes require a minimum of ~ 9.2 kWh/kg Al. It is notable that the current average energy consumed globally in conventional Hall-Héroult processes is ~ 12.0 kWh/kg Al [93]. This is not very far from the minimum energy required by the inert anode process.

If renewable hydroelectric energy or, alternatively, nuclear energy are used to provide electricity for electrolysis then the benefits in terms of reduced CO₂ emissions from inert anodes are clear. However, as shown above, <1/3 of current aluminum is produced in this fashion. Thus this analysis must be expanded to include the CO₂ emissions arising from non-renewable electricity generation. If we consider the use of inert anodes in connection with energy produced from either coal or natural gas-based electricity, the advantages of using inert anodes evaporate. This is because most CO₂ emissions come from electricity production, not electrolysis. For example, in the conventional Hall-Héroult process ~ 12.9 t CO₂/t Al are generated if coal-fired electricity is used, this accounting for ~ 87% of all the CO₂ generated in the process [93]. In the case of inert anodes ~ 15.9 t CO₂/t Al would be generated, far surpassing the amount saved due to the reduction in CO₂ emissions from the reduction reaction itself [93].

In conclusion, because the capacity for renewable energy to drive primary aluminum production is limited, the prospects for inert anode production as a generator of “clean” primary aluminum are also limited. Indeed, the value of diverting renewable electricity to aluminum production rather than using it for other purposes is questionable [15]. Solheim [93] argued that carbon capture may be a more promising avenue for reducing environmental impact, but this comes at the cost of increased energy demand. Ultimately, all of this provides a further incentive to reduce the use of primary aluminum and increase the use of scrap for synthesis.

# 3. Scrap-related production steps and their sensitivity to tramp elements

## 3.1. Recycling rates: Definitions, industry branches and regional aspects

The estimates of different aluminum associations suggest that 1–1.5 billion metric tons of aluminum have been produced since the Hall-Héroult and Bayer processes were introduced in 1886–1892. Roughly three quarters of this enormous quantity are still in use. Of this about one third each are found in buildings / infrastructure, electrical devices, and transport. Aluminum material used in packaging can return to the recycling stream within a few weeks. Approximately 30% of today’s annual global aluminum production comes from scrap. However, substantial regional deviations apply: for example, the rate in the US is about 30%, versus about 50% in the EU. On average the total worldwide recycling rates for aluminum are estimated to be ~ 40% (see Fig. 2 and Fig. 3). At about 98% the yield in scrap-based aluminum production is very high, as the metal lost from recycling processes once collected is usually below 2%. Today the collection figures for recycled aluminum produced from old scrap are circa 33% from the transportation sector, 26% from packaging, 13% from cables and electrical devices and 16% from building applications (see Fig. 5).

The growth of the aluminum scrap market parallels that of the entire aluminum market. In 2009, 18 million metric tons of aluminum scrap were available globally. In 2019 this increased to 33 million metric tons, translating to an average annual growth rate of 6% (see Fig. 2 and Fig. 3). The amount of aluminum produced from old scrap increased from one million metric tons in 1980 to as many as 20 million metric tons in 2019. Obtaining an overview of the recycled aluminum amount can be challenging, however, because academia, governmental agencies and industry use different indicators. This applies particularly to recycling performance
indicators: the recycling input rate, the total (or overall) recycling rate and the end-of-life recycling rate [40,54].

The recycling input rate describes the amount of aluminum that is produced from new scrap and old scrap (see definitions in section 3.1) as a fraction of the total amount of aluminum, including that obtained from primary and secondary (recycled) sources, that is shipped to manufacturing markets. The overall recycling efficiency rate describes the total fraction of recycled aluminum that is produced from both new scrap and old scrap as a percentage of the aluminum that is available from new and old scrap sources. This means that the recycling efficiency rate describes how much of the available scrap is actually turned into recycled aluminum. The end-of-life recycling efficiency rate measures the amount of recycled aluminum that is made from old scrap as a fraction of the aluminum available from old scrap sources. This means that the end-of-life recycling rate quantifies how much of the post-consumer aluminum scrap is really recycled into new material. The same applies to the end-of-life collection rate, which describes the fraction of aluminum collected from old scrap as a percentage of the aluminum that is in principle available when collecting from all old scrap sources. Along the same lines, the end-of-life processing rate is the aluminum produced from old scrap as a percentage of the material collected from old scrap sources [26].

Industrial operators of re-melting plants often take their total scrap input (counting both new and old scrap) divided by the total amount of aluminum they produce to measure their recycling. Depending on region, company and specialization their values can range between 60% and 95%. It is also important to differentiate between end-of-life recycling performance as a key indicator and the total recycled amount of aluminum. Although it is usually best from a metallurgical perspective to recycle an alloy-specific product which has reached the end of its lifetime directly into its new counterpart, this is not always economically worthwhile. Re-melting an old product into an equivalent new one obviously provides the best composition match, but the sheer longevity of many large-volume aluminum products, for instance in transportation, buildings and construction, often renders it less attractive from an economic and sustainability perspective.

The fact that overall global aluminum demand exceeds the available scrap by about two thirds (with substantial regional variations) makes aluminum scrap a product of limited availability. This means that aluminum should be reused as soon as possible after it returns to the market as scrap, and not just when the exact same alloy is needed. Short-lived products such as cans or related packing products, however, are recycled directly into new equivalent products [31,52,95,96].

The global aluminum recycling fractions for scraped parts from the transportation and building sectors, with their characteristically long lifetimes, range between 80% and 95% depending on region (differences are mainly due to market access and logistics). From the time systematic scrap collection began, the transportation sector has been the most important source of aluminum from end-of-life products. Scrap aluminum from buildings (which have greater longevity) only became available in the 2000s, and originally only in the modest quantities used when the buildings were erected 30–40 years previously. Scrap from aluminum beverage cans, which have an average lifetime of only a few weeks, is collected and recycled at rates of 30%–100%, with very high differences among regions.

Due to its large market share it is of interest to take a closer look at the aluminum which comes from the transportation sector [97–102]. Here alloys that return as scrap include mainly body sheet, frame, bumper and structure parts from the 2xxx alloys class (Al-Cu), the 5xxx alloy class (Al-Mg), the 6xxx alloy class (Al-Mg-Si), and the 7xxx alloys class (Al-Zn-Mg-/Cu). A large fraction of cast Al-Si alloys, such as A356, 360 and A380, is also returned from engine blocks. Among these classes, some are incompatible with each other: the high Si content inherited from cast parts is not admissible in wrought alloys, and the high Zn and Cu content returning from the 2xxx and 7xxx alloys is not compatible with 5xxx and 6xxx sheet materials [27,103].

For buildings, the scrap collection scenario is quite different. The average aluminum content of buildings is below 1%, but the total percentage recovered from this is high (about 95%) because many aluminum parts in buildings, such as claddings and sun protection devices, are peripheral and can be easily dismantled. This aluminum can be sold at a good price relative to other materials retrieved from buildings, which are often an economic burden rather than a trading good. The market is very heterogeneous, and there are substantial regional variations. Factories, commercial and office buildings have a much higher aluminum content than residential houses, and building in warm regions use about 20 times more aluminum than those in cold climates. Trends in architecture point to an increase, and the markets for both new aluminum in buildings and the amount returning as scrap are growing. While 5xxx and 6xxx class alloys have traditionally been used in buildings due to their high corrosion resistance paired with high strength, modest cost and good joinability, 3xxx alloys (Al-Mn) and 5xxx alloys containing large amounts of Mg are gaining momentum. While some of these alloys can be mixed to a minor extent, it is economically and ecologically much more sensible to collect them separately, also in view of large and growing transportation, construction and building markets which demand chemically well-controlled high-end alloys [27,39,61,104,105].

## 3.2. Advanced aluminum scrap sorting and pre-treatment

### 3.2.1. Brief introduction to scrap sorting

The quality of the input materials in a metallurgical process has a strong influence on both the metal yield and the quality of the final product. This is especially true for aluminum, with its low solubility. Because of this it is very important that treatment of aluminum scrap be high-quality, and therefore the technologies in this area are continually improving. Here, to close the loop in aluminum recycling in the future, increased interplay between scrap sorting and the melting process will be necessary [106].

Because of the growing importance of aluminum as a construction and lightweight material, more alloys and material composites are being produced which guarantee aluminum product functionality. Here sorting and melting processes must be optimized. Of particular importance here, as an essential prerequisite for excellent product properties and high metal yield [106,107], is the reproducible adjustment of a product-specific melt quality with regard to non-metallic and intermetallic inclusions. Due to both
increasing scrap rates and the higher impurity content of input material, it is necessary to optimize scrap sorting and the quality of aluminum melts along the process chain in a secondary aluminum smelter [108,109]. However, legal requirements such as those set out in the European End-of-Life Vehicles Directive or the Reach Regulation also have an impact on the recycling of aluminum. Furthermore, the limited refining possibilities of aluminum smelters demand optimized scrap sorting and collection if high alloy quality is to be achieved [54,72]. In addition, increased mixing of scrap and high organic content are also driving the need to develop new alloying systems [106,110].

The scrap sorting technologies used today are well developed, but are reaching their limit due to the large number of alloys and composites on the market. The following sections explain how the main mechanical and thermal processing techniques are deployed to turn aluminum scrap into high-quality input material for recycling. The aim is on the one hand to facilitate production of high-quality aluminum alloys, and on the other to create the conditions for developing recycling-friendly alloys [110,111].

### 3.2.2. Types of scrap and valuation

In the recycling of aluminum, only scrap and other materials containing aluminum in metallic form (dross, salt and slag) are considered as input materials [110–112]. Aluminum in non-metallic form, for example oxides, nitrides, chlorides, carbides etc., cannot in principle be reprocessed into metallic aluminum. The main types of scrap are circulating scrap, new scrap and old scrap [110]. Circulating scrap is not usually recorded in statistics and is obtained directly “in-line” during the production process in a clean and unmixed form. If possible, this scrap is immediately remelted in the same factory [110]. Examples are sort-specific scraps from sheet cutting or surface grinding which do not leave the shop floor for any downstream operation but are directly collected for in-house secondary synthesis.

New scrap (pre-consumer scrap) is created during the production and processing of aluminum products, usually at customer sites, which distinguishes it from circulating scrap. New scrap is statistically recorded and is usually remelted into aluminum alloys in smelters (remelters or refiners). In recent years the amount of this type of scrap has increased significantly. In order to guarantee closed-loop recycling of new scrap, the companies involved are required to guarantee complete, sorted scrap collection. The scrap is remelted in the smelters to wrought and cast alloys. New scrap can occur in a variety of forms and ranges from large pieces to highly contaminated, mixed chips. New scrap also includes the dross produced during aluminum production, which is remelted in refiners using salt [110,111].

Old scrap (post-consumer scrap) is produced in all aluminum material applications (automotive, aerospace, construction, food, engineering, packaging, etc.) in widely varying compositions. Some old scrap must be extensively processed to enable its use in subsequent melting processes. Return quantities also depend very much on service life, which can range from weeks to decades (e.g., packaging versus buildings). In addition, collected scrap varies greatly, ranging from packaged beverage cans to shredded scrap from old cars [110].

Scrap is evaluated according to its condition and purity (chemical analysis), type and quantity of foreign components and how they are mixed with the aluminum material [111]. The main parameter for evaluation is purity, i.e. the content of the main alloying elements (Si, Cu, Mg, Zn, Mn), the secondary components (Fe, Cr, Ni, Pb, Sn, Ti etc.) and minor elements (Li, Sr, Sb, Ca, Bi etc.) [110]. Representative sampling is very important for a suitable evaluation. The sample must have a certain minimum size, which is difficult to achieve with heavily mixed scrap. For this reason, adequate sorting technology is also important [110,111].

How scrap is stored plays a decisive role in the output of the melting process and in the purity of the melt. Particularly if there is an unfavorable surface-to-volume ratio (e.g. chips and foils), alloys may have oxide layers and need pre-treatment. Excess moisture may also affect the melting process if scrap has been stored outdoors. At low temperatures, if there are cavities in the scrap ice or water can even get underneath the melt and cause dangerous explosions [110,113]. In principle, dross must always be stored under a roof.

Suitable scrap preparation which generates high-grade products for the melting process is crucial for maintaining high-quality application standards [111,114]. Aluminum recycling treatment, which includes all the processes required to provide high-quality input material, can be both mechanical and thermal.

### 3.2.3. Mechanical scrap sorting

Mechanical scrap processes mainly include comminution, classification, sorting, agglomeration, and dehydration [106,110]. Comminution plays an important role in the context of aluminum scrap. It enables the separation of foreign substances which can lead to problems in melting. Comminution and/or agglomeration are also beneficial for transport, storage and charging. Non-metallic and metallic components are separated from each other by classification and sorting. There are a wide range of aluminum scrap treatment technologies which are utilised depending on the quality of the raw materials. Fig. 11 shows the various possible processing options and workflows. Comminution is mostly performed by imposing mechanical stresses (tension, pressure, shearing, torsion, bending, cutting, etc.) [111,115].

To sort scrap it is necessary to divide the materials into different size classes. In this sizing process it is possible to remove fine and coarse particles which would be disadvantageous in subsequent processing steps. A sorting process can also be carried out in a sizing procedure if (for example) the metallic aluminum contained in the salt slag remains coarse (deformation process) and the salt and oxidic components break up finely. The materials can then be sorted by subsequent screening [109,111].

Sorting represents an essential step in the processing of scrap for aluminum recycling. The aim is, as far as possible, to separate non-metallic components and other metals (Fe, Mg, Zn, Cu etc.) from scrap which contains aluminum [31,106,111]. Specifically, this involves separating metals from non-metals; light metals from heavy metals; different types of light metal (i.e. aluminum and magnesium); and different aluminum alloys. Various sorting methods are available [106]. The most important are described briefly below.

Density sorting makes use of the difference in mass density between individual materials and a fluid with a certain density (liquid or
<figure><img src="image_13.png" /><figcaption>Fig. 11. Processing methods for aluminum scrap. Reprinted with permission from [110].</figcaption></figure>

air) [111,116–118]. Magnetic separation is used to separate ferromagnetic materials from aluminum. Fe plays an important role in aluminum alloy metallurgy through its formation of intermetallic particles, and therefore needs to be controlled carefully. Magnetic sorting is thus often used in the processing of aluminum scrap [111]. A further method for separating metals and non-metals and metals from each other is eddy current separation. A time-variable magnetic field generates electromotive forces, and in a conductive material the induced electromotive forces generate currents in the material. This magnetic field is counter-directed to the inducing magnetic field [75,111], generating repulsion, which depends on the ratio of conductivity/density (see Table 3). This technology is being used more and more frequently, especially for aluminum, because it is an efficient way to sort weakly conducting materials or non-metals.

Manual picking is still used today where there are low volume flows and expensive technologies cannot be economically justified. This applies, for example, to portable batteries, electric motors, etc. In recent years, however, sensor-based sorting has become more common. Originally this addressed visual identification features, but over time that has been supplemented or replaced by opto-electronic electromagnetic, X-ray, spectroscopic and other identification methods [106,119]. In sensor-based sorting, determining the exact position is also crucial. Two main identification methods are used for aluminum alloys [106,111]. The first is X-ray transmission measurement (XRT) with spectral resolution (RSA) (reverse saturable absorption), where elements are determined according to their atomic density. This technique is currently used to separate AlCu and AlZn alloys from other aluminum alloys [111]. The second method is laser-induced breakdown spectroscopy (LIBS), where material evaporation and spectral analysis are performed [111].

<table><caption>**Table 3**
Ratio of conductivity/density for different metals. Reprinted with permission from [111].</caption><thead><tr><td>Material</td><td>Conductivity / density [m<sup>2</sup> 10<sup>3</sup> / Ω kg]</td></tr></thead><tbody><tr><td>Aluminum</td><td>13</td></tr><tr><td>Magnesium</td><td>12</td></tr><tr><td>Copper</td><td>7</td></tr><tr><td>Al-alloy</td><td>5...23</td></tr><tr><td>Zinc</td><td>2.4</td></tr><tr><td>Gold</td><td>2.3</td></tr><tr><td>Brass</td><td>1.8</td></tr><tr><td>Nickel</td><td>1.3</td></tr><tr><td>Tin</td><td>1.2</td></tr><tr><td>Lead</td><td>1.0</td></tr><tr><td>Iron, steel, alloyed steels</td><td>1.0...1.3</td></tr></tbody></table>
### 3.2.4. Thermal pre-treatment

Aluminum scrap may contain large amounts of hydrocarbons, which can enhance emissions, remelting loss and dross formation, and reduce alloy purity. However, scrap with a high organic content is also an energy source which allows fuel savings [120–122].

Not only is the type of scrap (used beverage cans (UBCs), chips, offcuts, used aluminum packaging etc.) important, but also its nature (e.g. loose, compacted, clean, coated) and components. Scrap impurities include non-metallic components, free iron, moisture, and inert, volatile and contaminating substances. The main categories containing them are coated scrap (covered by paint, varnish, printing ink, plastic, paper or metal); scrap containing residual non-metallic substances (such as water, oil, grease, wood, plastic, glass, etc.); scrap containing moisture (with liquid that adheres to the material); and scrap containing volatile substances (which can be removed by appropriate heat treatment before or during melting or by dedusting).

So-called low-grade materials with a non-metallic organic content of >15% by weight are problematic in new scrap. These materials change in the furnace from endothermic to exothermic scrap with an excess of energy in the metallurgical aggregate [121]. Unfortunately, however, the mechanical or thermal treatment required to deal with them is currently either technically inadequate or uneconomical.

Due to the versatility of aluminum as a material, there are many different types of aluminum scrap. The amount of metal each contains can vary considerably. In dry, single-variety raw material, the valuable metal content is largely determined by the weight of the coating. As seen in Table 4, this can vary between 2 and 90% [123,124].

There are different types of coatings on aluminum, and each behaves differently during pyrolysis. The emissions released during this process are related to the compounds contained in the coating. Coatings are mainly composed of plastics, paper, paints, varnishes, inks and various oils, often in the form of multiple layers such as in packaging. If the structure is complex, the aluminum content will be lower. The risk of toxic emissions during pyrolysis increases with higher plastic content [106,124]. Coatings can contain organic and inorganic components. The former comprise carbon compounds, which crack when the temperature is increased [124] while heating has no effect on inorganic components. After heat treatment, this fraction is composed of ashes and metal oxides (pigments) [126], such as titanium or zinc oxides. These substances do not volatilise during thermal pre-treatment. Some substances can be discharged via exhaust gas in the form of dust. When coated material is heated directly in a furnace, waste gas is released, due to the decomposition of the plastics. Besides gas, carbon particles are formed which react with the metallic aluminum and form carbides (Al₄C₃) [126], which tend to accumulate in the salt slag or dross. During intermediate storage or processing of the dross, these carbides can react with water and form methane. Carbides can also enter the aluminum melt and negatively affect alloy properties. These problems can be avoided by thermal treatment (pyrolysis) of the scrap before subjecting it to smelting [127].

To describe the behavior of the input materials in more detail, it is useful to divide the process into two steps: the splitting or thermal decomposition of the hydrocarbons present in the coating, and the combustion of the decomposition products. The first step depends on the type of coating and the metal substrate. During combustion, the coke which is produced during pyrolysis and remains on the surface reacts with the oxygen to form CO and CO₂ [127]. During pyrolysis, a reducing atmosphere is applied, which means that the material is melted without substantial oxidation losses. The gas generated by the thermal treatment contains tars, oils, dust and pollutants, components which can crack during high-temperature treatment, converting the hydrocarbons into CO and H₂. The gas produced in this way has a calorific value of 4.5 to 5.5 MJ/m³. A typical composition for such a pyrolysis gas is about 22 vol-% H₂, 14 vol-% CO, 12. Vol.-% CO₂, 2 vol-% CH₄ and 50 vol-% N₂ [121,128].

## 3.3. Secondary synthesis: Process steps for aluminum scrap treatment and remelting

### 3.3.1. Brief introduction to the metallurgical fundamentals

Aluminum reacts with different gases, reducing metal yield and negatively influencing the quality of secondary alloys. The most important reactions and their relation to impurity effects [106,110,111] are briefly discussed in the following.

Aluminum is highly reactive with oxygen and such oxides can no longer be reduced in conventional recycling cycles. Therefore reactions with oxygen plays a special role in the field of recycling. Oxygen itself is practically insoluble as a gas in liquid aluminum [110,111]. Oxidation of an aluminum melt takes place above 727 °C. Due to its high affinity for oxygen, an oxide layer forms instantly on fresh surfaces of solid and liquid aluminum in air. This layer is dense, and protects the aluminum from further oxidation. The reaction takes place with atmospheric oxygen and humidity. However, aluminum oxide can also be formed in a reaction with components in the flame or the combustion gases (CO₂, H₂O), or arise aluminothermically with other metal oxides, e.g. iron oxide which

<table><caption>**Table 4**

Properties of different scrap types with organic coating in the field of aluminum recycling. Reprinted with permission from [125].</caption><thead><tr><td>Scrap Type</td><td>Coating</td><td>Mass of coating [%]</td></tr></thead><tbody><tr><td>Foil (blank)</td><td>Oil residues from the rolling process</td><td>&lt; 10</td></tr><tr><td>Printed foil</td><td>Color/Varnish</td><td>7</td></tr><tr><td>Laminated paper</td><td>Paper/Ink</td><td>50–70</td></tr><tr><td>Laminated plastic</td><td>Polymer/Ink/Wax</td><td>40–90</td></tr><tr><td>Laminated tubes</td><td>Polymer/Varnish</td><td>70</td></tr><tr><td>Window frames</td><td>Varnish/sheathing</td><td>21</td></tr><tr><td>U.B.C *</td><td>Varnish/Color</td><td>2–3</td></tr></tbody></table>

<footnote>*...Used beverage cans.</footnote>
<table><caption>**Table 5**
Free reaction enthalpy of Al and different aluminum compounds to aluminum oxide [110].</caption><thead><tr><th>Reaction</th><th>ΔG<sup>0</sup><sub>600° C</sub>kJ/mol Al<sub>2</sub>O<sub>3</sub></th><th>ΔG<sup>0</sup><sub>800° C</sub>kJ/mol Al<sub>2</sub>O<sub>3</sub></th></tr></thead><tbody><tr><td>2Al<sup>v/l</sup> + 3/2O<sup>g</sup> ↔ Al<sub>2</sub>O<sub>3</sub><sup>i</sup></td><td>-1402,7</td><td>-1336,0</td></tr><tr><td>2Al<sup>f</sup> + 3H<sub>2</sub>O<sup>g</sup> ↔ Al<sub>2</sub>O<sub>3</sub><sup>g</sup> + 3H<sub>2</sub>Al</td><td>-804,4</td><td>-770,9</td></tr><tr><td>2AlN<sup>f</sup> + 3H<sub>2</sub>O<sup>g</sup> ↔ Al<sub>2</sub>O<sub>3</sub><sup>i</sup> + 3NH<sub>3</sub><sup>g</sup></td><td>-256,7</td><td>-223,5</td></tr><tr><td>Al<sub>4</sub>C<sub>3</sub><sup>g</sup> + 6H<sub>2</sub>O<sup>g</sup> ↔ 2Al<sub>2</sub>O<sub>3</sub><sup>i</sup> + 3CH<sub>4</sub><sup>g</sup></td><td>-708,5</td><td>-651,6</td></tr></tbody></table>

adheres to tools, or silicon dioxide from (e.g.) the furnace lining. In addition, not only aluminum itself, but also aluminum nitride and carbide formed secondarily in the melt can react to form aluminum oxide (see Table 5) [110,111].

When aluminum oxidizes in the solid state, in dry air the resulting layer consists of amorphous aluminum oxide with a thickness of 2 to 10 nm, which increases only slowly below 300 °C. When exposed to humid air, such layers become much thicker, initially forming a thin non-porous barrier layer on which a porous, hydrous top layer grows, the thickness of which depends on the temperature and can reach a few 0.1 µm. When heated above the aluminum melting point, the amorphous oxide transforms into γ-Al<sub>2</sub>O<sub>3</sub>. At temperatures above 700 °C the transformation of γ-Al<sub>2</sub>O<sub>3</sub> to α-Al<sub>2</sub>O<sub>3</sub> (corundum) starts, with a time delay. This causes a decrease in volume, leading to cracks. Oxygen can penetrate through these cracks, causing further oxidation. The speed of oxidation increases with rising temperature and increases very strongly from about 780 °C. For this reason it is essential to avoid overheating, which can be caused by burning dross, a high organic content or incorrectly adjusted burners [110,111].

During the oxidation of alloyed aluminum melts, oxidation can be either selective or non-selective towards aluminum, depending on the oxygen affinity of the respective alloying elements. The alloying constituents thus have a considerable influence on the amount of oxide formed and on the structure and composition of the oxide layers. Selectively oxidizing elements (e.g. Mg, Sr and Ca) have a higher affinity for oxygen than aluminum. They tend to oxidize out of the melt at a high rate and form separate phases, where the oxidation rate increases with the increasing temperature of the melt and with increasing content. In non-selective oxidation (of e.g. Cu, Fe, Zn), which takes place at a low rate, the oxides that form are incorporated into the Al<sub>2</sub>O<sub>3</sub> lattice depending on the atomic or ionic size of the element, and generate a change in the oxidation behavior by altering the oxide layer's structure and density [110,111].

In wrought alloys, oxide inclusion and other non-metallic or intermetallic inclusions impair not only the materials' mechanical properties but also influence corrosion, which can lead to problems during pickling, polishing and anodizing [129]. In cast alloys problems can arise during surface treatment, e.g. polishing (as mentioned) and anodic oxidation. Pressure tightness, weldability, corrosion resistance and machinability are also reduced by non-metallic particles. Melts which contain oxides are more viscous, and thus have poorer mold filling and feeding properties. Die casting processes also cause increased mold wear [110].

In order to conduct the entire recycling process of aluminum and produce a high metal yield, the reaction with oxygen must be reduced during storage, transport, smelting (where technique is crucial), holding and casting. The extent of oxidation depends strongly not only on the melt composition but also on temperature and time. The important factors influencing it are oxide, moisture and the impurity content of the input material; temperature control during melting; duration of exposure to atmospheres containing oxygen and moisture; flow conditions during melting, transfer and potting; composition of the fluxes used; and the ratio of the bath surface to the volume of the melt. To melt aluminum scrap with minimum loss, the following conditions must therefore be met [110,111]:

- Oxidation and moisture absorption of the raw materials should be avoided as far as possible (dry storage under a roof, rapid processing, etc.).

- Melting must be carried out at the lowest possible temperature.

- Melting time must be short and unnecessary holding times should be avoided.

- Small and fine material must be stirred into the metal or salt melt quickly to protect it from oxidation.

- Turbulent flows during transfer and casting must be avoided to reduce tearing of the protective oxide layer.

- Bath surface must be small to minimise contact with the atmosphere.

- Optimised fluxes should be used.

Using optimised fluxes is especially important. Here oxide layers must be completely removed from the scrap, and aluminum droplets must be enabled to coalesce in the molten salt [110,111,130].

Oxide layers also play an important role when the material is liquid. This produces dross: a mixture of aluminum, the alloying and impurity elements and their oxides. Halogenides, carbides or nitrides are also found in the dross. The greater the amount of dross produced, the more liquid aluminum is bound within it due to capillary effects. The formation of dross and the associated loss of aluminum can be described in terms of the following steps [123,126]:

- Formation of the oxide skin on the melt

- Breaking of the oxide skin by the bath movement

- Sinking and floating of the oxide particles

- Adhesion of the oxide particles

- Filling of the interstices with metallic Al (capillary action)

- Oxidation of aluminum dispersed in the dross, resulting in additional dross
- Removal of the dross from the molten bath

- Partial dripping of liquid aluminum or further oxidation if no appropriate treatment (rapid cooling) takes place

Oxides also enter the melting process via scrap due to the passive layers. They have a thickness of 4–10 nm in dry air and up to a maximum of 10 µm in moist air. A 2–4 nm thin barrier layer is formed immediately upon contact of the free surface with any oxygen-containing atmosphere. This layer consists of pure Al and amorphous $Al_2O_3$ and, depending on the aluminum alloy, may also contain mixed oxides such as magnesium oxide. If the humidity is appropriate, a non-porous top layer will form on the porous barrier layer. Here aluminum hydroxide $Al(OH)_3$ is produced [127]. The structure and growth of this top layer depends on surface impurities or coatings and on air humidity. The growth rate depends on time and temperature. In dry air, the barrier layer has already reached its maximum thickness after about one hour, whereas only increased humidity in the ambient atmosphere will cause the growth of the top layer [79,127,129].

Hydrogen is soluble in aluminum, but little is known about the specific reactions which place except the fact that embrittlement and stress-corrosion cracking can occur in high-strength alloy grades [131–133]. The hydrogen solubility in aluminum increases in the solid and liquid state with temperature and depends on the hydrogen partial pressure [110]. Due to reduced solubility in the solid state compared to that of the liquid state, bubbles and pores form in the material during solidification. However, hydrogen precipitation can be suppressed with the help of a high cooling rate, which is deployed for example in continuous casting or die casting. In subsequent treatment steps (e.g. heat treatment, hot forming, etc.), the forcibly dissolved hydrogen is released and in turn generates annealing bubbles, grain boundary porosity, etc. These greatly impair the mechanical properties of the aluminum alloys, but can be prevented in advance via appropriate melt treatment processes [111].

In castings, any gas porosity that occurs can be used as a measure of hydrogen solubility. However, the tendency towards pore formation depends not only on the hydrogen content, but also, crucially, on the solidification rate, the solidification morphology and the microstructure state.

An equilibrium is established between the aluminum melt and the hydrogen content in the surrounding atmosphere. This defines the proportion of dissolved hydrogen, where environmental water vapour is the main cause of the increased hydrogen content. Water vapour is decomposed by liquid aluminum. The atomic hydrogen formed is quickly dissolved in the molten aluminum, and the oxygen reacts exothermically to form aluminum oxide, which creates a dense oxide layer on the molten aluminum. This reaction increases not only the hydrogen content in the melt, but also reduces the metal yield [110].

Oxide layers on aluminum melts in humid atmospheres provide a certain protection against further reaction with water vapour. However, if this oxide layer is destroyed by turbulent flows (casting and pouring off, gas purging treatment, bath movement, etc.), a reaction with the water vapour of the environment again takes place which is exothermic. This leads to overheating, which favors hydrogen absorption. In the same way, organic substances adhering to the scrap also react with liquid aluminum to form carbon or aluminum carbide, with the resulting hydrogen then being absorbed by the melt. The same reaction takes place in the incomplete combustion of organic compounds. In addition, aluminum carbide can in turn be converted into aluminum oxide in the course of the smelting process [110, 111, 134].

Hydrogen can also be introduced via the input material. The oxide layer adhering to the scrap is, for example, a source of hydrogen. The naturally formed oxide skin consists of porous aluminum oxide and contains moisture. When using small-sized material (chips, foils, etc.) or dross, the risk of water absorption is particularly high due to capillary effects. The same applies to moist melting salt. Other sources of hydrogen are new crucibles, newly lined furnaces and freshly coated melting tools. In practice, melts with a large hydrogen content also often have a greater oxide content, and vice versa [110,111].

Alloying and trace elements can also influence hydrogen solubility [110]. Elements that increase it are Mg, Ti, Na, Sr and Ca; those that only modestly affect it are Cr and Fe; and elements that decrease it in Al-alloys are Cu, Mn, Ni, Si, Zn and Sn. The aim must therefore be to reduce hydrogen absorption and the associated oxide inclusion. Measures for achieving this include using dry scrap with as little oxidation as possible; rapid melting or submergence of fine scrap; avoiding unnecessary holding times; using dried melting salts and dry gases; and avoiding melt overheating and drying of new linings of furnaces, transfer troughs, ladles, melt treatment and casting equipment.

Nitrogen can also react with solid and liquid aluminum [110] to form aluminum nitride. Similar to oxygen, a thin, dense layer of aluminum nitride is formed on the melt and protects it from further nitrogen absorption. Aluminum nitrides occur not only in the melt, but also in the dross or in salts. The cause of aluminum nitride formation is gas purging treatment with nitrogen and a reaction with atmospheric oxygen, e.g. during local overheating. In dross and salt slags, aluminum nitride reacts with atmospheric humidity or with water during salt slag processing and causes the formation of ammonia ($NH_3$). AlN can also be present in solid form, suspended in aluminum melts. If no suitable melt treatment is carried out, it will reduce the quality of the resulting products [110].

The high chemical reactivity of liquid aluminum and the alloying elements present in the melts (e.g. Mg) mean that liquid aluminum reacts not only with gases but also with solids. Particularly important are reactions with the refractories ($SiO_2$, $Fe_2O_3$ etc.) and alloying additions (master alloys and their impurities), as well as reactions with oxides in (for example) pigments of organic coatings from scrap ($TiO_2$, $ZnO$, $Pb_3O_4$, $Fe_2O_3$, $Cr_2O_3$ etc.). The reaction products not only generate impurities in the aluminum melt, but also reduce the yield when aluminum is recycled due to the aluminum oxides that usually form [110,111].

### 3.3.2. *Process and plant technology*

Recycling of secondary raw material is carried out for aluminum depending on the degree of impurity. It is necessary to use different furnace systems for efficient recycling of the very different materials involved [110,111,106]. Direct processing of high-purity input materials, which is conducted during production and further downstream processing, can take place in the foundries of the
primary smelters, in semi-finished product plants, recycling plants and foundries. This frequently involves the company's own scrap, which is usually not recorded statistically but treated as a closed-loop contribution [110,111].

Today, new and old scrap are processed in recycling plants, and a distinction is made between remelting for purer scrap and refining for lower-quality scrap. While remelters mostly produce wrought alloys, the products of refiners are cast alloys [110,111,130]. The basic process steps are the same in the production of both [135,110]. They involve preparing the input materials (if necessary); melting; melt treatment (alloying, gas purging treatment, temperature and microstructure adjustment and, if necessary, filtration); and casting.

The main difference in the recycling of aluminum is whether salt is used (refiner) or not (remelter), and this depends on the corresponding input material. The amount of salt depends on the process technology and the material used. An overview of the steps in recycling of aluminum is shown in Fig. 12 [110].

The smelting salts used in the recycling of low-grade scrap comprise mainly sodium chloride (NaCl) and potassium chloride (KCl) with added fluorides, such as fluorspar (CaF₂) or cryolite (Na₃AlF₆) or other alkali fluorides (NaF, KF). The tasks of the salt are (i) to break up the oxide skins; (ii) to absorb the non-metallic impurities (oxides, carbides, nitrides, etc.); (iii) to protect the liquid metal from the furnace atmosphere by covering the melt; and (iv) to improve the coagulation of the metal droplets [110].

Important criteria in commercial smelting technology are high metal yield; low dross formation and low salt input; low energy requirements; high quality of products with a large scrap component (low primary aluminum); low oxidation of aluminum; and low emissions [110,111].

Important improvements have been made to fuel-heated plants via a reduction in the amount of additional materials such as smelting salt; improved burner technology; combustion with oxygen; and waste heat recovery. Controllable medium-frequency induction furnaces are mostly used in electrically heated plants, with high efficiency and improved energy consumption. All melting furnaces are equipped with off gas collection and off gas purification systems, which guarantee compliance with legally prescribed emission values. Off gas purification processes, especially for organically contaminated off gases, have solved former problems associated with the occurrence of PCDD/F (polychlorinated dibenzodioxins and polychlorinated dibenzofurans, often referred to as "dioxins") [54,110,111].

In the context of secondary materials, a distinction can be made between foundries and semi-finished product plants, remelting plants (remelters) and melting plants (refiners) [115]. The individual areas are explained below, with particular reference to furnace technology, melting technology and the materials used. Foundries and semi-finished product plants are often not considered part of the recycling process because they mainly use primary metal or products from secondary smelters (casting ingots or liquid aluminum). In addition, they use sorted scrap which accumulates during production (scrap from foundries, parts of sheet metal, profiles or chips). This type of scrap does not appear on any scrap balance sheet, and quantitative registration usually only takes place within the plant. Nevertheless, because the amount of these high-quality materials has increased in recent years due to the increasing depth of processing and rising production, these furnace systems represent an important part of the overall recycling cycle. The units they use are mostly from tilting or stationary hearth furnaces with a capacity of up to 130 metric tons, although induction furnaces (crucible or

<figure><img src="image_14.png" /><figcaption>Fig. 12. Process routes (with and without salt) in the recycling of secondary aluminum. Reprinted with permission from [110].</figcaption></figure>
channel furnaces) are also deployed in both recycling plants and foundries.

Remelter plants mainly use low-contaminated, wrought alloy scrap with a low number of alloying elements. Large quantities of sorted scrap with the same composition are usually charged. The treatment of the scrap only consists of making these materials chargeable by cutting or, if necessary, agglomeration. The process sequence in a remelter is shown in Fig. 13 [75,110,111,115,116,135].

Scrap melting usually takes place in fuel-heated single- or multi-chamber furnaces, which increasingly have integrated preheating chambers to smolder adhering organic compounds. These furnaces also have low-temperature carbonization gas and oxygen burners and facilities for heat recovery. In hearth furnaces, the melting process can be accelerated by stirring devices or pumping. Heating is carried out via oil and gas burners to about 950 °C, with the use of oxygen burners on the increase. For more contaminated materials, especially those with higher organic content, multi-chamber furnaces are used. These consist of a reheating and smoldering area, a melting chamber (forehearth) and a heating chamber (main hearth) [115,134,136]. In the smoldering section of the furnace, the off gases from the heating chamber are used to smolder the organic compounds under a reducing atmosphere. These gases heat the scrap and at the same time largely prevent oxidation of the aluminum. In the subsequent charging process, the smoldered scrap is transported by new scrap into the forehearth and melts in the aluminum bath. The main hearth provides the heat necessary for the burning of the smoldered gases from the forehearth.

Other furnace possibilities include rotary drum furnaces without added salt, crucible induction furnaces, and more rarely, channel induction furnaces and hearth shaft furnaces. After the input materials are melted, the liquid aluminum is charged via troughs or by ladle transport in fuel-heated holding furnaces. These are also used as furnaces for the casting process. The gas purging treatment takes place either in the holding furnaces and/or in plants which are arranged "in-line" between the casting furnace and the continuous casting, and where filtration also takes place. In the holding furnaces, which are designed as tilting hearth furnaces, the melt is alloyed, if necessary grain-refined, and cast or delivered in liquid form.

Highly contaminated, oxidized input materials are typically processed by refiners. Mixed scrap from all alloy classes, sometimes with a high organic content, as well as dross, dross and slag grains or scrap with a high surface/volume ratio, are often charged. Melting usually takes place using salt in fuel-fired rotary drum furnaces. Low-salt tilting drum furnaces and hearth or multi-chamber furnaces with a low salt addition are also deployed. After the melting process the liquid aluminum is transferred to holding furnaces, which are also fuel-heated. Here the liquid metal is alloyed, treated for cleaning and microstructure adjustment and finally cast or poured into liquid transport containers. Refiner products are cast alloys in the form of ingots or liquid aluminum. Similarly producted deoxidized aluminum for the steel industry is mainly sold in the form of granules [110,111,130].

Fig. 14 shows the process scheme for producing cast alloys by processing highly contaminated materials in refiners. The criterion which differentiates rotary and tilting drum furnaces is the salt factor [75,110,111,135].

<figure><img src="image_15.png" /><figcaption>Fig. 13. Illustration of the process steps in aluminum recycling for the production of wrought alloys. Reprinted with permission from [111].</figcaption></figure>
The salt factor (mass of salt / mass of non-metallics) is between 0.5 (tilting drum furnace) and 1.5 (rotary drum furnace), depending on the scrap quality and the furnace system. This correlates to about 300 to 500 kg of salt per metric ton of secondary aluminum produced. The metal content in the salt is between 5 and 20%. The quantities of salt depend on the degree of impurity and the specific surface area of the primary materials [110,137].

A distinction is made between rotary drum and tilt drum furnaces. With a salt factor > 1, a liquid salt slag is present (salt bath melting). The rotary drum furnaces used are relatively safe and can be operated without complex controls. If the salt factor is significantly below 1 (approx. 0.5), we speak of low-salt smelting. In this case tilting drum furnaces are used. The amount of added salt must be carefully adjusted to the input material, because there is a risk of increased aluminum oxidation as the amount of salt decreases. Drum, hearth and multi-chamber furnaces are also used for salt-free melting in the production of cast alloys [110,111,137,138].

Regardless of which melting concept is used, measures must be taken that enable (i) high energy efficiency; (ii) low salt consumption resulting in a low amount of salt slag and filter dust; and (iv) low environmental pollution. The standard melting unit for contaminated scrap is the rotary drum furnace [75]. It consists of a cylindrical steel vessel, lined with refractory material, which is mounted on two rollers via races on the furnace shell so that it can rotate around a rigid axis of rotation. It is driven electrically by friction-driven rollers, chains or gears, with a stepless speed control typically set to 1–8 revolutions per minute. The furnace capacity is 10–60 t and heating is performed with oil or gas burners. Great improvements in heating are, however, achieved with the help of oxygen burners and additional oxygen lances. This also makes it possible to completely burn off the smoldering gases of organic fractions that are produced during heating, thus gaining additional heat for melting. Compared to stationary hearth furnaces, rotary drum furnaces have the following advantages [75,110,111,137–139]: (i) The rotating movement enables a mixing and stirring effect between liquid salt, metal and the charged scrap. (ii) The burner heats up the furnace wall, which is then submerged in the molten metal by the rotary movement, thus achieving a very good heat transfer. The scrap is mechanically charged through the furnace door, and the melting process operates at about 800 °C. Tapping openings arranged around the circumference of the furnace shell are used first to recover liquid metal and then to pour off the salt slag. Particularly noteworthy is the improved coagulation of the aluminum droplets due to the rotary motion [110,111,139].

A follow-on of the rotary drum furnace is the tilting drum furnace. Due to the small amount of salt in this furnace operation, a crumbly salt slag (salt cake) is produced which must be emptied by tilting the furnace. The tilting drum furnace operates with a reversing flame, where additional oxygen lances can be used. The principle of the tilting drum furnace is similar to that of a top blow rotary converter, which is often used in the copper industry. Characteristic of these furnaces are their hydraulic tilting mechanism and the use of natural gas-oxygen burners, which are arranged on the front side in the furnace lid. Their capacity is 6–10 t. Their main advantages are less salt slag accumulation and less gas consumption and flue gas volume than those of the rotary drum furnace [12,140,141].

<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Illustration of the process sequence for Al recycling with salt slag [110,111]. Reprinted with permission from [111].</figcaption></figure>
When melting with salt, care must be taken to ensure that the off gases are specially treated. The off gases contain not only the combustion products (CO₂, CO, H₂O, SO₂, residues of organic materials), but also low-temperature carbonization products, dioxins, evaporated salt (NaCl, KCl, NaF, KF), evaporated MgCl₂ and HF. The resulting filter dust usually has to be disposed of in specialised landfill [137,142].

### 3.3.3. Residual materials produced in the secondary metallurgy of aluminum

In aluminum secondary metallurgy, a distinction must be made between smelting with and without salt. In all smelting units without the use of a chloride mixture, a dross is produced that can contain up to 80% metallic aluminum. The high degree of metallization shows that this residue is a high-quality raw material. If a salt is used in the melting of Al scrap, then a salt cake or salt slag is produced depending on the amount of salt added. Additionally, many scrap pre-treatment units and the melting processes themselves produce dusts that contain valuable components to be recovered, such as salts and metallic aluminum. Another recoverable residual material is the refractory lining impregnated with the chloride mixture and the Al (furnace spoil) [110,137,143]. Table 6 gives an overview of the residues that occur in the production process of aluminum from scrap.

Drosses are those residues containing aluminum that are produced during smelting, melt treatment and casting of aluminum and its alloys in primary smelters, foundries and recycling works. They are classified as new scrap. In addition to very different degrees of metallic aluminum content, dross contains aluminum oxide, spinels, silicates and the like as well as chlorides and fluorides originating from gas purging and smelting salt treatment (see Table 7). They are processed directly or after prior mechanical treatment in the smelters [110,137,143,144].

Oxidation in the solid and especially in the liquid state of aluminum is the cause of dross. Newly formed oxides in the smelting process or oxides from the input material generate considerable amounts of this residue. Statistically, about 20–30 kg of dross is produced per metric ton of secondary aluminum produced [143,145].

Due to the high metallic content of dross, processing is essential. The residual material produced in primary or secondary production is first subjected to dross treatment, which involves either the processing of hot dross or the mechanical treatment of already cooled dross. The processing of the extracted hot material is performed by shaking out, stirring out, pressing out or centrifuging, whereby a recovery of up to 50% by mass of the metallic aluminum contained is possible. Here the pressing processes are the most effective, as higher metal losses occur due to the initiated movement during the stirring processes. Another possibility in the context of dross preparation is rapid cooling after drossing, such as inert gas cooling with argon [144]. The cooled dross can then be processed by repeated crushing and screening, where the residual materials produced are critical due to their reactivity (formation of H₂, ammonia, chlorides, etc.) and the low metallic Al content.

Mechanical treatment is a method for material separation. Oxide residues accumulate in the fine material, whereas metal-containing lumps accumulate in the coarse-grained fraction. The metal content differs in the products of the individual processing steps (see Table 8). The further processing of the coarse material (dross, lumps and flakes), which has a higher Al content, is carried out in rotary drum furnaces with the addition of salt. Dross residues can be used as casting powder in the steel industry under limited conditions, but the resulting dusts currently have no subsequent use. Disposing of them in landfill is problematic because gases form if they come into contact with water. Another issue is their high reactivity, as they secrete damaging substances (ammonia, hydrogen, etc.) into the environment when exposed to moisture. This pollutes not only the surrounding air, but also the groundwater. The aim is

<table><caption>**Table 6**
Presentation of the residues arising in the production of secondary aluminum [110].</caption><thead><tr><th></th><th>Salt slag</th><th>Filter dust</th><th>Furnace lining</th><th>Dross</th></tr></thead><tbody><tr><td>Accumulation (in kg/<br>tAl)</td><td>300–500</td><td>10–40</td><td>2–3</td><td>20–30</td></tr><tr><td>Accumulation point</td><td>Smelting in a rotary drum<br>furnace</td><td>Exhaust gas treatment<br>Rotary drum and holding<br>furnace</td><td>Rotary drum furnace and holding<br>furnace</td><td>Melt refining, foundry</td></tr><tr><td>Reduction/<br>avoidance</td><td>Possible</td><td>Hardly possible</td><td>Not possible</td><td>Hardly possible</td></tr><tr><td>Environmental<br>relevance</td><td>Gas formation,<br>Elucidability</td><td>Elucidability</td><td>Elucidability</td><td>Gas formation,<br>Elucidability</td></tr><tr><td>Utilization</td><td>Dissolution-crystallization<br>process</td><td>Dissolution-crystallization<br>process</td><td>–</td><td>Dross processing, melting<br>processes</td></tr><tr><td>Removal</td><td>Landfill ban</td><td>Underground deposition</td><td>Deposition</td><td>Landfill ban</td></tr></tbody></table>

<table><caption>**Table 7**
Classification of dross into grain classes with associated metal content and listing of other constituents [110].</caption><thead><tr><th>Classes</th><th>Metallic aluminum</th><th>Other components</th></tr></thead><tbody><tr><td>Unprocessed dross</td><td>&lt; 80 wt-%</td><td>oxides: Al<sub>2</sub>O<sub>3</sub>, Al<sub>2</sub>O<sub>3</sub>•MgO, Al<sub>2</sub>O<sub>3</sub>•SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>•FeO etc.</td></tr><tr><td>Boulders</td><td>&lt; 90 wt-%</td><td>chlorides: AlCl<sub>3</sub>, NaCl, KCl etc.</td></tr><tr><td>Dross skimmings</td><td>70 – 50 wt-%</td><td>fluorides: CaF<sub>2</sub>, NaF, AlF<sub>3</sub> etc.</td></tr><tr><td>Dross residue</td><td>50 – 20 wt-%</td><td></td></tr><tr><td>Dross dust</td><td>&lt; 20 wt-%</td><td></td></tr></tbody></table>
<table><caption>**Table 8**
Average compositions and exact analyses of accumulating liquid and crumb salt slags [110,140,149,153,155–157].</caption><thead><tr><th rowspan="2">Component</th><th colspan="3">Salt slag</th><th colspan="3">Salt cake</th></tr><tr><th>wt.-%<sup>1)</sup></th><th>wt.-%<sup>2)</sup></th><th>wt.-%<sup>2)</sup></th><th>wt.-%<sup>3)</sup></th><th>wt.-%<sup>4)</sup></th><th>wt.-%<sup>5)</sup></th></tr></thead><tbody><tr><td>NaCl</td><td>40–50</td><td>45–75</td><td>62.9</td><td>49.50</td><td>23.2<sup>6)</sup></td><td>20–80</td></tr><tr><td>KCl</td><td>18–20</td><td></td><td></td><td>10.40</td><td></td><td></td></tr><tr><td>Other chlorides</td><td>&lt; 10</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>:Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, MgO:Al<sub>2</sub>O<sub>3</sub> and other complex oxides and hydroxides</td><td>20–25</td><td>20–40</td><td>28.0</td><td>61.3</td><td>67.8</td><td>20–60</td></tr><tr><td>CaF<sub>2</sub></td><td>1–2</td><td></td><td>1.6</td><td></td><td>0.7<sup>6)</sup></td><td></td></tr><tr><td>Al<sub>met</sub></td><td>4–10</td><td>5–20</td><td>7.5</td><td>n.a.</td><td>8.1</td><td>3–10</td></tr><tr><td>PCDD/F</td><td>20 ng ITE<sup>7)</sup>/kg</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

1) Specification from Krone and Essafi.

2) Specification from Gerke and Boin.

3) Specification Gil.

4) Specification from Prillhofer.

5) Specification from Peterson.

6) Specification in Cl und F.

7) International toxic equivalent.

therefore to develop suitable reprocessing methods that meet all the ecological requirements, or to achieve a chemical composition through targeted treatment which will allow this residual material to be used in alternative industries (e.g. the building materials industry) [75,135,144,146,147].

Dross is mainly processed via the smelting technique. In combination with subsequent salt slag processing, residues destined for landfill can be avoided. The salt slag produced during pyrometallurgical cross recycling is then further processed to close the recycling loop. This generates products such as metallic aluminum and salt, and an alumina-containing residue that is used (e.g.) in the building materials industry. In the context of cross processing, mechanical and pyrometallurgical recycling in combination with salt slag processing should aim for closed-loop recycling [143,148–151].

More advanced processes only introduced into the recycling of aluminum dross in recent years are based on plasma technology. The aims here are to release the metallic aluminum trapped in the dross while producing residues with minimal environmental impact. The advantage of these technologies lies primarily in the elimination of salt additives and the resulting salt slag. In this context, the ALUREC, ECOCENT, DROWRITE and DROSCAR processes are of particular mention [148,149,152].

The processing of low quality scrap or high oxide materials in horizontal or tilting rotary drum furnaces involves the application of smelting salt. The salt requirement depends on the purity of the scrap. The composition of the salt system used in the smelting process is of great importance for the aluminum yield and subsequent slag treatment. In addition to the residues of the salt used, the salt slag after the smelting process contains both metallic aluminum and various products of the chemical reactions between the salt, the metal and impurities. The PCDD/F contents are negligible (see Table 8). Industrial salt slags have an aluminum content of between 5 and 20%, and the recoverable fraction during recycling is 3 to 8% [153,154].

A large part of the aluminum contained in salt slag is in the form of finely dispersed particles and can only be partially recovered. Up to 50% of the metallic Al is contained in particles with a grain size range of 0.5 to 1.6 mm [36]. The coarser components can be separated by mechanical processing. Table 8 shows average compositions of liquid and crumbly salt slags (salt cakes). These essentially contain NaCl and KCl with additions of CaF<sub>2</sub>, Na<sub>3</sub>AlF<sub>6</sub>, NaF, or KF [110,153].

During the solidification of a salt slag, the formation of spinels and silicates depends on the alloy. Metallic silicon and Mg-Al compounds occur in some cases, and magnetite (Fe<sub>3</sub>O<sub>4</sub>) is also found in some slags [153]. Today the salt slag composition cannot be classified according to specific types of alloy because a complex mix of scrap with unknown alloying element content is generally used in the smelting process [158]. Essential properties of the salt slag are (i) thermochemical and thermal stability at the operating temperatures; (ii) low viscosity; (iii) the highest possible density difference from the liquid metal; (iv) optimal interfacial tension to aluminum or alumina; and (v) the highest possible compatibility with the furnace lining [121,153,156].

The tasks of the smelting salt are to break up the oxide layer; absorb non-metallic impurities and broken oxide skins; promote coagulation of metal droplets to form a melt; and cover the melt to protect it from further oxidation.

The composition of the salt mixture varies depending on the application, scrap type and furnace type. >100 different mixtures are used in the aluminum recycling industry [143]. In Europe a mixture of 70 mass % NaCl and 30 mass % KCl is usual. In the USA and Canada, on the other hand, an equimolar eutectic mixture of 50 mol-% NaCl and 50 mol-% KCl is used, which corresponds to 44 mass-% NaCl and 56 mass-% KCl [121,153,156,157]. The binary phase diagram for the NaCl-KCl system is shown in Fig. 15 [159]. Here a comparison is made between data determined by the thermodynamic calculation program FactSage and previously published data. It can be seen that the eutectic point of the equimolar mixture is below the melting temperature of pure aluminum and the salt mixture therefore begins to melt before the metal, which is why good coverage is ensured. The European mixture, on the other hand, has liquidus and solidus temperatures of 690–712 °C and 644–670 °C, respectively [153,158].

Fluorides are also added to the NaCl-KCl mixture, whereby fluorespar in the range of 2–5 mass % is often used for European applications. This can lead to an increase in the melting point. It should also be noted that the NaCl-KCl-CaF<sub>2</sub> mixture tends to evaporate.

In 1956 the process of detaching the compact oxide layer from the surface of the aluminum was investigated and formulated as a step-by-step process with three stages [146,154,160]. These stages are the formation of defects in the adherent oxide layer, which can
<figure><img src="image_17.png" /><figcaption>Fig. 15. Binary phase diagram of the NaCl-KCl system. Reprinted with permission from [158].</figcaption></figure>

occur due to thermal stresses and phase transformations; penetration of the molten salt between the oxide layer and the aluminum; and detachment of the oxide layer. The mechanism of oxide layer detachment draws on the fact that the interfacial tension between molten salt and oxide is lower than that between oxide and aluminum. Adding fluorides results in the local accumulation of these surface-active elements, which leads to a reduction of the interfacial tension between salt and aluminum. The resulting tension gradient increases the force for separation. The chlorine anions in the liquid salt also play an essential role in attacking the oxide layer [161]. The stages of oxide layer detachment are shown in Fig. 16. The sub-steps which occur are the contact of molten salt and the oxide layer; the attack of the oxide layer by chlorides; oxide separation by penetration of the liquid salt and separated phases (liquid aluminum and oxide particles) [75,146,161].

The separation of the oxide particles now suspended in the melt takes place by wetting with salt slag. These then rise to the surface and pass into the salt slag phase [162]. After the oxide layer has been stripped off, the aluminum droplets can coagulate and collect at

<figure><img src="image_18.png" /><figcaption>Fig. 16. Schematic representation of the oxide layer separation during the recycling of aluminum using molten salt. Reprinted with permission from [161].</figcaption></figure>
the bottom of the furnace due to the difference in density. Bath agitation, as occurs in rotary and tilting drum furnaces, improves the coalescence of the metal droplets [146,162,163]. The coalescence efficiency depends on the one hand on the density and viscosity of the liquid salt and on the other on interfacial tensions, which play an essential role [164].

The ecological problem associated with salt slag lies in the water-solubility of the chlorides it contains, which can lead to high salt content in groundwater. Here slag also produces harmful gases such as hydrogen, methane, ammonia, hydrogen sulphide and hydrogen phosphide when it comes into contact with water or moisture. However, the valuable materials slag contains, which include metallic aluminum and salt, still make recycling interesting from an economic standpoint [106,110,111]. Salt slag recycling processes that pursue a zero-waste strategy are thus becoming increasingly important due to their economic and sustainability advantages.

The leaching and crystallization process (e.g. the BUS process) is the most widespread technology for the recycling of salt slag. It essentially comprises five process steps. The coarse-grained metallic aluminum is removed in appropriate crushing and classification processes, and the remaining slag is ground in mills. Due to the ductility of the aluminum, it accumulates in the form of finely rolled flakes. The metal obtained is called “coarse” and is returned directly to the smelting process. By leaching the remaining fines (<0.4 mm) with water at elevated temperature (65–80 °C) for a duration of two hours in a multi-stage process, the salts contained dissolve. The metals bound to the halides also go into solution [110,111]. After dedusting of the resulting off gases, treatment with diluted sulphuric acid takes place. In the process the ammonia present reacts to form ammonium sulphate, which is used in fertiliser production. This process is carried out in washing columns. The residual pollutants (e.g. hydrogen sulphide and phosphine) are then adsorbed in a two-stage treatment in an activated carbon adsorption plant. A solid-liquid separation is carried out in lamella thickeners, where the accumulating underflow comprises the oxide residue, and the overflow is fed into evaporation-crystallization plants to recover the salt. The resulting mixed salt has an average residual moisture content of 2–4%. The saturated mother liquor must then be fed to the last two evaporation stages, followed by filtration of the condensate. The BUS process produces metal (80% Al), recycling salt, oxide residue (partly used in the cement industry) and ammonium sulphate (used in the fertiliser industry) [110,111].

Dust which contains aluminum is mainly produced during smelting. However, although the analysis of these varies greatly, corresponding quantities are also produced during treatment of scrap and residual materials. Due to the fineness of the dusts from the secondary aluminum industry and their composition (organics, heavy metals, etc.), developing a suitable processing method is of particular interest. The main types of dust are briefly addressed below [137,145,165].

Filter dust forms during the melting of scrap in recycling furnaces. Here processes involving salt are of particular relevance. In the salt bath process in the rotary drum furnace, dusts occur when volatile components evaporate in the form of (e.g.) chlorides and fluorides; when fine particles are transported with the off gas stream; from CaO added to the off gas; or from organic material. Due to the pyrohydrolysis that takes place, aluminum oxide, hydrogen fluoride and hydrochloric acid are formed from aluminum chloride or fluoride and water. Free chlorine enters the filter dust through the merging of the off gas systems of refining and smelting furnaces. The off gas is treated with calcium hydroxide. Hydrogen fluoride, hydrochloric acid, chlorine and sulphur dioxide (from the fuel) adsorb at 300–500 °C on the sorbent. Calcium chloride, fluoride and sulphate are produced as reaction products. After cooling to below 120 °C, the dust is separated in bag filters [110]. In rotary and tilting drum furnaces operated with salt, the filter dusts consist largely of this chloride-fluoride mixture and unreacted hydrated lime, while those from the salt-free smelting process have lower CaO and salt content. The metallic aluminum nitrate content can be as high as 17.7%. Due to the carbon and the fluctuating oxygen content with the presence of copper as a catalyst, ideal conditions exist for the formation of polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF), plus polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) [110,166]. As with salt slag, problems arise with landfill due to water-soluble chlorides, the possible formation of gases (hydrogen, methane, ammonia) when water is added, the fineness of the dust, and PCDD/F contamination [110].

Filter dust comprises waste that is subject to monitoring, which is why such materials may only be deposited in dedicated, specialised landfill. This generates considerable extra costs. Potential recycling processes are therefore being sought, but at present nothing is economically viable and disposal in landfill predominates [110].

In addition to the processes already mentioned, such as BUS technology, there are alternative areas of application where dusts can be utilised. For example, the filter dust produced in rotary drum furnaces can be used as an additive in the pre-filter of exhaust air purification. The fine-particle material from this is in turn used in rotary drum furnaces as cover salt [167]. Another possibility is to add dust to the desulphurization agent (CaO-CaF₂-Al₂O₃) in the steel industry. Here, however, the heavy metals dusts contain are problematic, as they lead to a deterioration in steel quality and generate halide emissions [110,167]. During the treatment (e.g. shredding) of scrap before it is used in the smelting process, dust from scrap processing accumulates to 25 kg/t Al which is composed of organic substances, metallic aluminum, aluminum oxide and free carbon. Currently this fine-scale material also goes to landfill [110].

Finally, residues from refractories need to be considered. These include products made from CaO, MgO, Al₂O₃, ZrO₂, Cr₂O₃, SiO₂ and C [166]. Due to increasingly strict legal requirements and the lack of landfill options, recycling of refractory materials is becoming more and more important. For every metric ton of secondary aluminum, approximately 2 kg of furnace waste is produced, generating a considerable amount of residue on a global scale [110,166]. The refractory industry is trying to minimise the amount of waste destined for landfill, firstly by increasing the service life of the linings, and secondly by recycling the material. The motivation for recycling high-temperature-resistant residual materials lies in the added value through savings in landfill costs, and the reduction of CO₂ emissions in the energy-intensive processes of primary extraction. A further advantage is the familiarity and consistency of the lining composition. It is extremely difficult to collect only one type of material, because diverse refractory products are deployed within one aggregate alone. Further impurities are generated by infiltrations and the attack of slag and metal baths. Approximately 50% of the refractory bricks used in the metallurgical sector are consumed by material conversion, enrichment with foreign substances and mixing with other lining zones, which means that reuse in the form of secondary products is not possible [166]. It is therefore advisable to consider using materials destined for landfill in alternative industrial sectors (e.g. the building materials industry). Appropriate
treatment processes can be deployed to separate valuable metals, and the remaining raw material can be used, for example, as an additive in road construction. Application in cement production and addition to abrasives and slag formers are also conceivable [166].

### 3.4. Mixing scrap-based melts with primary aluminum in secondary synthesis

If we follow the material flow in the Sankey diagram shown in Fig. 17, we see that only a relatively small proportion of the total end-of-life aluminum scrap from products is returned to secondary processing and used for the subsequent production of wrought aluminum alloys [8]. This reflects the complexity involved in collecting and separating different scrap streams in order to achieve the tight chemical tolerances required for wrought aluminum alloys. There are three ways to use end-of-life scrap: (1) downcycle to fewer alloy-chemistry-sensitive applications; (2) use closed-loop recycling, as in used beverage containers; and (3) dilute “lower grade” scrap by adding primary aluminum.

Aluminum from automotive scrap, for example, has traditionally been downcycled to casting alloys because of the difficulty of separating the alloys it contains, particularly wrought and casting alloys. Based on data collected in 2012 [8] it has been estimated that 6.1 Mt of wrought scrap globally are downcycled every year to cast alloys. Increasingly, however, scrap dealers are forced to deal with the fact that more automotive scrap is available than higher-quality scrap from closed-loop sources. This lower-quality mixed automotive scrap would traditionally have been downcycled to casting alloys, but supply is currently exceeding demand [35]. This trend is exacerbated by the future decrease in demand for automotive aluminum castings, forecast due to the growth in electric vehicle sales [168]. It has been estimated that the way things are going, by 2030 there may be an excess supply of > 6 Mt of scrap which is not recyclable due to the prevalence of alloy scrap collected from open-loop recycling. For this scrap the only option at present is dilution by the addition of primary aluminum. Dilution, or “sweetening”, of scrap in secondary processing by adding primary aluminum, is traditionally performed on a limited basis due to the added expense incurred. This is amplified if we consider the amount of primary aluminum needed to dilute scrap from different sources to render it usable in wrought alloy production. Fig. 18 illustrates the results of a case study [169] which examined the amount (%) of scrap (the scrap source is shown on the periphery) that can be used to generate four specific alloys, two wrought and two cast. If we focus on the two wrought alloys, it is apparent that the 3104 alloy – the alloy used for beverage can bodies – can be produced from 100% scrap. However, apart from this specific combination of alloy and source, it is also apparent that, using the technology available today, generating the wrought alloys will typically require at least 50% primary aluminum in order to achieve the desired wrought alloy chemistries. It is notable that even producing 6061 from 6xxx scrap requires significant dilution due to the significantly different levels of impurity tolerance for minor elements among various alloys within this class [29].

Fig. 18 highlights the significant advantages that might be achieved if aluminum scrap separation could be improved according to scrap source [17]. Taking end-of-life vehicles as an example, the advantages that would arise simply from separating cast components (engine and transmission components) from wrought components are apparent. Owing to the high alloy content of the castings, mixing these two sources otherwise significantly increases the amount of primary aluminum required to achieve wrought alloy chemistries. This would be a motivation for scrap standards that focus on sorting based on product applications [169].

Even in the best-case scenario of closed-loop recycling, as used for beverage cans, a significant proportion of primary aluminum is

<figure><img src="image_19.png" /><figcaption>**Fig. 17.** Sankey diagram (used with permission from [8]) showing materials flow (in millions of metric tons per year; 2007 numbers) showing the flow of aluminum from liquid metal (left) into products (right), including the flow of end-of-life scrap (left, center) into secondary processing.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. 18. Case study illustrating an idealized view of maximum scrap utilization (%) for two wrought (3104, 6061) and two cast (355.0, 380.0) aluminum alloys starting from the typical scrap mixes from the sources listed on the periphery of the diagram [169]. Image taken with permission from [169].</figcaption></figure>

still partially required because production outstrips the supply of scrap. The most recent report on aluminum beverage can recycling by the Aluminum Association [170] shows that in the US, 66% of an average aluminum can's weight comprises recycled content (43% from recycled beverage cans and 23% from post-industrial scrap arising from manufacturing); 7% comprises non-beverage can, end-of life, scrap; and 27% comprises primary aluminum. The repeated recycling of used beverage cans in a closed loop can then lead to the accumulation of minor impurity elements that will eventually exceed alloy tolerances. Løvik and Muller [96] provided a simplified description of such impurity accumulation in the closed-loop recycling of beverage cans. They specifically considered the accumulation of Ti coming from lacquer on the can body, the amount of Ti introduced in each cycle being conservatively estimated to be $x_i \sim 0.1\%$ of the weight of the can body. After $n$ cycles, the concentration of impurity, $C_i$ will have risen from its initial value, $C_i^0(n=0)$, to.

$$C_i(n) = (\dot{R})^n C_i^0 + \left[ \frac{x_i}{(1 - f_{lid})} \right] \left[ \frac{\dot{R}(1 - \dot{R}^n)}{1 - \dot{R}} \right] \quad (3)$$

where $f_{lid}$ is the fraction of the can mass contained in the lid (~20% [96]) and $\dot{R}$ is the end-of-life recycling rate for beverage cans. In the case of Ti, for which $C_i^0 \approx 0$, the results obtained as a function of the number of cycles is shown in Fig. 19a using different recycling rates to illustrate its effect on Ti accumulation. As $n \to \infty$ the impurity concentration approaches a saturation value, this saturation level being shown as a function of recycling rate and initial impurity fraction in Fig. 19b. This highly simplified model illustrates the important compromise that must be considered; increasing the scrap intake, even in the case of closed-loop recycling, can lead to undesirable accumulation of impurities that are hard, if not impossible, to remove in secondary processing. This can set an upper limit on the level of recycled content that can be deployed in the manufacture of new products according to current alloy design strategies.

In summary, primary aluminum is still an important mainstay in wrought alloy production today. Compared to wrought alloys, the demand for cast aluminum alloys is decreasing [72,171,172] and this may lead to increased demand for primary aluminum for sweetening. To most effectively use wrought alloy scrap, significant efforts will thus be required in the areas of alloy separation and tramp element removal if we are to recycle automotive wrought alloys in particular [36].

<figure><img src="image_21.png" /><figcaption>Fig. 19. Prediction of the accumulation of impurity Ti due to recycling of closed-loop beverage cans, reproduced with permission from [96]. (a) The concentration of impurity as a function of the number of recycling loops for different assumptions of the recycling rate of end-of-life beverage cans. (b) In (a) one can see that impurity levels saturate as the number of recycling loops increases. Here this saturation concentration is plotted as a function of the recovery rate for different assumed levels of contamination (% of can weight). The x-axis is truncated at a recovery rate of 80% because the remaining 20% of the recycled can mass comes from the can lids, which do not contribute to Ti contamination via lacquer.</figcaption></figure>
### 3.4.1. Refining of aluminum melts containing scrap, and casting

#### 3.4.1.1. Brief introduction to the metallurgical fundamentals of the refining process.
The reactivity of aluminum with other elements presents refining challenges. These will intensify significantly in the coming years due to the increasing share of low-quality scrap and partially insufficient scrap-sorting quality. Recycling companies will also be confronted with more customer demands with regard to alloy recycling rates, meaning that standard dilution with primary aluminum will no longer be as easy. In the future, besides the usual refining processes such as purging gas technology, filtration and, in some cases, melt settling time, further technologies will have to be deployed to guarantee the quality of materials required. The essential scientific conditions for treating liquid aluminum are explained briefly in the following.

The solubility of almost all metals in aluminum melts and the reduction capacity of base aluminum in oxides each cause an increase in the foreign metal content. Liquid aluminum also has a high affinity to gases, which means that its selective use for melt purification must be restricted to only a few elements, such as Sr, Na, Ca, Li and Mg. Besides the soluble components, solid compounds in the form of oxides, nitrides, chlorides and carbides also cause contamination of the melt and a significant decrease in the quality of the material properties. Many elements form intermetallic compounds (IMCs) with aluminum, especially during the solidification process, which in turn also significantly impair the material properties. Higher quality standards despite the scenario of increasing scrap content coupled with decreasing scrap quality can only be achieved in the future via (i) improved scrap collection and treatment; (ii) optimization of melting technology; (iii) optimization of refining methods; (iv) development of new smelting and refining technologies; and (v) exploitation of casting methods which enable high solidification rates [173–176].

Table 9 and Table 10 give examples of dissolved and solid impurities and explain their origins, the consequences for quality and possible removal technologies. It should be mentioned, however, that the elements listed only represent a small selection of the potential impurities; main alloying elements such as Cu, Si, and Mn and secondary elements such as Cr, V, Zr, Ti and Cd are not mentioned, but nevertheless enter the melt with the scrap in varying quality. Further elements also enter the melt through low-quality scrap; their effects are not yet known and they are frequently not recorded analytically. Notable examples of such metals are rare earth elements, In, Se, Te, and Bi [110,111,177,178].

The effects of solid impurities can vary greatly according to composition and origin, which is why targeted removal is necessary prior to the solidification process. Otherwise not only the material properties (elongation, toughness, yield strength, corrosion property, surface quality, etc.), but also the subsequent process steps, such as those during forming, can be negatively affected [110,179,180].

Of greater importance are oxides, predominantly aluminum oxide (Al₂O₃). Aluminum oxide can, however, also occur with other oxides: MgO or mixed oxides of aluminum oxide with magnesium oxide (spinel, MgO·Al₂O₃), and oxides of other alloying elements, e. g. silicon dioxide (SiO₂). Oxides from the refractory can enter the melt through abrasion of the furnace and channel lining, and are usually silicates or other complex oxides, which may also contain Na, K and Ca. Less frequently, nitrides are present in the metal itself. Similar to oxides, they can be formed by a reaction of the melt with the atmosphere or during gas purging with nitrogen. They mainly occur in the dross or salt slag region, where they largely remain bound. Al carbides play a role in secondary aluminum, for example, when they are introduced via primary metallurgy or input material (organics) and may convert into oxides or other compounds. Chlorides may originate from the salt slags used in smelting or be formed during refining treatment of the melt with chlorine or chlorine-containing gases. They occur as Na, Ca and possibly also Mg chloride inclusions. Intermetallic phases are also present, which either form in high-alloy aluminum melts or are introduced into the melt by master alloys (e.g. Al₃Ti, Al₃Zr) or during grain refining treatment (e.g. TiB₂) [177,181,182].

The sizes of solid particles range from a few micrometres to a few millimetres. The particles often agglomerate, so that oxides are

<table><caption>**Table 9**
Examples of dissolved elements/impurities, their origins and qualitative effects, plus their refining possibilities in aluminum melts [110].</caption><thead><tr><td>Element</td><td>Origin</td><td>Consequences/influence on</td><td>Possible removal means</td></tr></thead><tbody><tr><td>H</td><td>Reaction with the atmosphere and combustion gases; moist feed materials and refractory lining; alloying additives</td><td>Gas bubbles, gas porosity, annealing bubbles (W); mechanical properties</td><td>Steady-state treatment, purge gas treatment (vacuum treatment)</td></tr><tr><td>Li</td><td>Input materials (Al-Li-alloys)</td><td>Dross formation tendency; “blue corrosion” (W)</td><td>Treatment with reactive gases and salts</td></tr><tr><td>Na</td><td>Reaction with refractory materials</td><td>Dross formation tendency; edge cracking (W); “pick-up”-formation (W); casting properties</td><td>Reaction with reactive gases and salts</td></tr><tr><td>Ca</td><td>Input materials; reaction with refractory materials</td><td>Dross formation tendency; casting properties (C)</td><td>Reaction with reactive gases and salts</td></tr><tr><td>Sb</td><td>Input materials (modified casting alloys)</td><td>Analytical tolerance</td><td>None</td></tr><tr><td>Pb</td><td>Input materials (adherent, free-cutting alloy, bearing metal)</td><td>Analytical tolerance; casting properties (C)</td><td>None (vacuum distillation)</td></tr><tr><td>Fe</td><td>Input materials (foreign component)</td><td>Analytical tolerance</td><td>None (crystallization)</td></tr><tr><td>Mg</td><td>Input materials (alloying element)</td><td>Analytical tolerance</td><td>Treatment with reactive gases (vacuum distillation)</td></tr><tr><td>Bi</td><td>Input materials (free-cutting alloy)</td><td>Analytical tolerance; casting properties (C)</td><td>None</td></tr><tr><td>Zn</td><td>Input materials (alloying element)</td><td>Analytical tolerance</td><td>(Vacuum distillation)</td></tr><tr><td>Sn</td><td>Input materials (adherent, bearing metal)</td><td>Analytical tolerance; casting properties (C)</td><td>None</td></tr></tbody></table>

C = Casting alloys, W = Wrought alloys.
<caption>**Table 10**</caption>

Examples of solid impurities, their origins and the qualitative effects, plus their refining possibilities in aluminum melts [110].

<table><thead><tr><th>Contamination</th><th>Origin</th><th>Consequences/influence on</th><th>Possible removal means</th></tr></thead><tbody><tr><td>Oxides Al<sub>2</sub>O<sub>3</sub>, MgO</td><td>Reaction with the atmosphere;</td><td rowspan="5">Tool damage during processing (W); tool wear during processing (C); mechanical properties; staining, polishing and shining behavior (W); “gray lines” (C)</td><td rowspan="5">Steady-state treatment; treatment with reactive gases; filtration</td></tr><tr><td>Spinels Al<sub>2</sub>O<sub>3</sub>-MgO</td><td>input materials; erosion of refractory lining</td></tr><tr><td>Silicates CaSiO<sub>3</sub></td><td></td></tr><tr><td>Aluminates CaAl<sub>2</sub>O<sub>4</sub></td><td></td></tr><tr><td>Nitrides AlN</td><td>Reaction with the atmosphere; purge gas treatment with N<sub>2</sub></td></tr><tr><td>Silicates CaSiO<sub>3</sub></td><td>Refractory lining</td><td></td><td></td></tr><tr><td>Aluminates CaAl<sub>2</sub>O<sub>4</sub></td><td></td><td></td><td></td></tr><tr><td>Carbides SiC, TiC</td><td>Reaction of the melt with carbon; abrasion of the refractory lining with SiC, master alloys</td><td></td><td></td></tr><tr><td>Chlorides NaCl, KCl</td><td>Melting process with salt; purge gas treatment with Cl<sub>2</sub></td><td></td><td></td></tr><tr><td>Intermetallic compounds Al<sub>3</sub>Zr, Al<sub>3</sub>Ti, AlTiZr, etc.</td><td>Master alloys; reaction in the melt with unfavorable alloy compositions</td><td></td><td>(Crystallization, filtration)</td></tr></tbody></table>

C = Casting alloys, W = Wrought alloys.

mainly present in the melt as so-called oxide flakes. In addition to size, the particle concentration (characterized by the inclusion number, i.e. the number of particles per unit area), and above all shape and distribution play a decisive role. Solid impurities are usually removed by means of gas purging in combination with filtration. However, due to the variety and size of the various compounds, existing refining methods are generally not suitable for complete separation.

In aluminum melts, alloying elements which are more noble than aluminum are usually reduced by dilution with primary aluminum because no economical refining processes are available. Hydrogen is removed by gas purging. Salts (described in detail in section 3.3) are also used in the course of smelter purification, sometimes in the form of slags, but also selectively as powders, granules or tablets added during melting. They can be added via immersion bells, via the gas flow or by charging onto the melt surface; they act in the same way as purging gases [106,110,177].

The scrap treatment described in section 3.1 also has an important impact on the purity of the melt. However, the various influencing variables mean that complete grade purity cannot be achieved, and extensive research and development are still needed in the area of refining liquid aluminum for sustainable production of aluminum alloys with high recycling rates. The two common refining methods used to purify molten aluminum are detailed below [110,183].

### 3.4.2. Gas purging for aluminum melts

In the course of gas purging, a flotation effect brings about the removal of dissolved hydrogen; elements which have a higher affinity to the gas than aluminum; and solid components. In discussing the process a distinction must be made between inert gases (argon and nitrogen) and reactive gases (chlorine and SF<sub>6</sub>). Reactive gases are normally used in mixtures (in combination with argon or nitrogen) [177,184]. With inert gases, dissolved hydrogen (H) diffuses into gas bubbles and H<sub>2</sub> is discharged. Here bubble size is an essential factor which depends on the injection system. In gas purging four main process technologies are used to inject the gases. The process can take place in the furnace or in-line in the launder. As the bubble diameter decreases, the specific bubble surface area and thus the mass transfer area increase sharply, greatly increasing the gas purging efficiency. Here not only the removal of hydrogen, but also removal of solid components via a flotation effect play an important role. In gas refining with reactive mixtures (e.g. chlorine-argon with up to 15% chlorine), additional elements such as Sr, Na, Ca, Li and Mg can be removed.

### 3.4.3. Filtration of impurity-contaminated aluminum melts

Filtration treatments are important for alloys which contain large amounts of scrap. This is mainly due to the intrusion of iron, which is a typical and highly problematic tramp element because of its tendency to form or enrich in ternary or quaternary intermetallic compounds (IMCs) [119]. Advanced filtration strategies are thus gaining momentum for melts with a high fraction of post-consumer (old) scrap, as they typically contain higher iron fractions [185].

Besides binary IMCs Al<sub>3</sub>Fe, typical ternary phases containing iron such as Al<sub>3</sub>FeSi, β-AlFeSi, α-AlFeSi, Al<sub>15</sub>Mn<sub>3</sub>Si<sub>2</sub>, Al<sub>4</sub>FeSi<sub>2</sub> and related compounds have been reported. The literature uses several different notations for some of these phases, which can be confusing. The notations AlFeSi, β-AlFeSi or α-AlFeSi are actually abbreviations for different types of intermetallic with more complex compositions. For instance, the β-AlFeSi phase is actually stoichiometrically a β-Al<sub>5</sub>FeSi phase, and the α-AlFeSi phase is stoichiometrically an α-Al<sub>8</sub>Fe<sub>2</sub>Si phase. Most of these phases are metastable, which means that the specific Fe-rich phase observed depends on cooling rate and heat treatment. For instance, the α-Al<sub>8</sub>Fe<sub>2</sub>Si phase tends to form at higher cooling rates instead of the β-Al<sub>5</sub>FeSi phase [186–188].

The variety of possible intermetallics naturally increases if other tramp elements such as Mn, Cr, Cu, Ni, Zn Mg, Na and Li also enrich in the scrap and in the melt [37,71]. For instance, as discussed below in more detail, doping with Mn can change the acicular α-Al<sub>8</sub>Fe<sub>2</sub>Si phase into a more granular-shaped α-Al<sub>8</sub>(Fe<sub>2</sub>Mn)<sub>3</sub>Si phase or into the α-Al<sub>12</sub>(Fe,Mn)<sub>3</sub>Si phase (see Fig. 20).

Scrap-related phases containing Fe, Cu, C and Ni can have a particularly detrimental influence on the alloys' corrosion resistance, as
<figure><img src="image_22.png" /><figcaption>Fig. 20. Aluminum-rich regions of phase-diagram sections and projections for the most important tramp elements Fe, Si and Mn. (a) Aluminum-rich side of the Al-Fe-Si system. (b) Projection of the liquidus lines of an Al-Fe-Mn phase diagram. (c) Polythermal projection of the solidification surface of the quartenary Al-Fe-Mn-Si system. Figures reproduced with permission from [187].</figcaption></figure>

they form local galvanic elements which nucleate and accelerate corrosion. Some IMCs, however, can be beneficial when they act (for instance) as grain refiners. Examples of this effect are some IMCs containing Cr and Zr. Overviews of the phases, their thermodynamics and measures for removing them by filtration, centrifugal separation or electromagnetic (eddy-current) separation were provided by Gnatko et al. [189] and Gaustad et al. [37].

Packed bed filters and ceramic foam filters are used for the filtration of aluminum melts. In applying filter systems a distinction can be made between (i) cake filtration and (ii) deep-bed filtration [190]. In cake filtration a layer of inclusions, the so-called filter cake, is formed on the filter surface through which melt must flow. Cake filtration occurs primarily at high inclusion numbers, with the separation of large inclusions on the surface of the filter cake occurring by mechanical influence [191]. In deep-bed filtration, inclusions are removed from the melt after accumulation and deposition by means of adhesion to the inner surface of the filter medium. Deep bed filtration is particularly effective for the removal of small, low-content inclusions (<20 µm). Surface effects are predominantly responsible for removal [177].

The two filtration mechanisms occur together and/or sequentially. Therefore a filter cake can form on the surface of a deep-bed filter. In cake filtration, however, the filter cake itself acts as a deep-bed filter. The concentration of inclusions in the melt is important in that filter cakes, which improve filtration, only form at high inclusion concentrations. However, very high concentrations can quickly lead to large filter cakes, which result in enhanced filter impermeability to the molten aluminum and, in turn, the necessity for particle pre-separation measures, e.g. gas purging. It is assumed that due to surface effects, particles > 30 µm are isolated mainly by mechanical separation, and particles < 30 µm mainly by adhesion [110,177]. Surface effects are ultimately responsible for the final deposition of particles on the inner filter surface. Particle adhesion is caused by van der Waals and chemical bonding forces, enhanced by hydrodynamic pressure forces. The predominant mechanism in each case depends on (i) particle shape, size, and type; (ii) melt temperature, composition, and flow rate; and (iii) filter material structure, morphology, and composition [192].

Due to the various influencing factors, various types of filter are deployed in aluminum industry gas purging systems: (i) net filters; (ii) packed bed filters; (iii) ceramic foam filters; and (iv) ceramic tube filters. They are frequently used in combination to meet quality requirements, especially in the context of wrought alloys. Due to the challenges inherent in using old scrap, filtration in combination with gas purging technology is likely to receive increased attention in the future [110,177,191].

### 3.4.4. Alternative refining processes for aluminum melts

Aluminum melts can only be refined to a limited extent, and generally only a few elements (alkali and alkaline earth metals), dissolved hydrogen and solid particles can be removed. To remove higher-content alloying elements and impurity metals, other physical and electrochemical processes can in principle also be considered., these are (i) intermetallic precipitation; (ii) segregation (in combination with centrifuges); (iii) distillation; (iv) ultrasonic treatment for hydrogen removal; and (v) molten salt refining electrolysis (only for very high Al purities). These processes are currently not economically feasible. However, lack of quality scrap, insufficient scrap treatment processes, the limitations of existing refining processes, higher quality standards and increasing recycling rates mean that alternative refining processes for aluminum melts will become ever more pivotal in addressing industry demands and practicalities [193,194].

### 3.4.5. Continuous casting of aluminum and the roles of solidification rate and scrap-related impurity elements

The invention of today's continuous casting process by Walter Roth in 1936 [195] led to significant developments in the solidification technology of metals, including vertical, horizontal, thin-strip and composite casting (Fig. 21). For aluminum these process technologies are mainly used for the semi-finished product formats of rolling ingots and extruded billets, and for cast ingots. The main differences lie in the mold used and the number of molds [196–200].

Continuous casting is a generic term for a variety of continuous and semi-continuous casting (direct chill casting) processes for metallic materials. It produces elongated castings with a constant cross-section. Continuous casting has many advantages, in particular
(i) high yield; (ii) low energy requirements; (iii) low investment costs; and (iv) high surface and microstructure quality [196-198,201,202].

In addition to affecting the quality of the liquid aluminum (hydrogen, inclusion, alkali metal and alkaline earth metal content), mold technology also has a significant influence on continuous casting quality. This is determined by the mold-length-dependent formation of the strand shell by indirect heat dissipation (which causes edge segregations) and the melt supply, which effects the solidification time, the microstructure and the cooling condition after the mold [196-198].

In semi-continuous direct-chill (DC) casting, water flows directly from the mold to the ingot surface (direct chill). During this process there are three different cooling zones: (i) primary mold cooling; (ii) secondary water cooling; and (iii) bottom block cooling [203].

Vertical continuous (direct chill) casting is the central technology in the aluminum industry. In recent decades, numerous molds have been developed for the purpose. Selected processes and mold technologies are discussed below as examples [204,205]. With the exception of some hot-top molds, in the procedure the metal is fed via a system comprising a pouring nozzle with a plug for metal flow control. The further distribution of the liquid metal in the mold is performed by special glass fiber distributor bags, the structure of which is highly dependent on format and alloy [196,206].

Clad sheets or plates are interesting product alternatives to monolithic material production, particularly if alloys contain large amounts of scrap. Cladding can become necessary because high-strength aluminum alloys contaminated with tramp elements but intended for special applications sometimes lack satisfactory corrosion and/or surface finish properties. Here a thin layer of pure aluminum is applied to ensure a better protective effect [196]. Often the cladding process is carried out by rolling the different materials. This production process is costly and requires high surface purity at the interfaces of the cladding and the core material [207-209]. Novelis and Wagstaff have therefore developed a casting process for the production of already-clad rolling ingots (FusionTM). The main challenges here are the various thermal expansion coefficients and liquidus temperatures of the materials, which require a special cooling regime. Two different melts must also be provided for the casting, each with a separate melt supply. Doubled-sided cladding therefore needs three melt distribution systems (nozzle, plug, distributor bag, etc.). The control effort and the required degree of automation are also major challenges compared to other casting processes [196,207-209].

The various solidification and casting strategies described above all act on the resulting as-cast microstructures [173-176]. Important features in the context of this paper are the Scheil-type macrosegration profiles through the cast material; the resulting spatial and size distributions; the particle shapes; the remaining degree of solute supersaturation; and the alignment and morphology of intermetallic phase stringers along cell boundaries. Because the freezing rate changes en route from the surface to the center layer regions, the gradients of features between the center and the near-surface regions also matter.

More specifically, higher cooling rates generate finer dendrite cell sizes and thinner phase morphologies. If cooling rates are too slow, the larger intermetallics in particular become problematic. While these are general trends for all aluminum alloys, a specific challenge in the context of SoDA is how scrap-related tramp elements act and distribute themselves in as-cast slabs. A systematic literature overview and study of the related effects was conducted by Wagstaff et al. [45,210] for a 3104 alloy. They cast ingots with an Fe content higher than that of the commercial standard composition, and investigated the macrosegregation profiles and spatial distribution of the resulting intermetallic particles through the slab thickness. Interestingly, they reported that increased iron content – representative of impurity intrusion from using more recycled material – has a refining effect on the constituent particles. This was particularly true of the center regions of the as-cast ingots. They also suggested that higher particle numbers and reduced particle size improve recrystallization kinetics through particle-stimulated nucleation.

<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Continuous casting technologies for the production of aluminum. Reproduced with permission from [196].</figcaption></figure>
Lazaro-Nebreda et al. [173] also studied the solidification of Al-alloys made from scrap containing high levels of Fe, with a special focus on the formation of Fe-rich intermetallic phases and the effects of different melt processing conditions. Their work revealed that the formation and dispersion of Fe-rich IMCs depends not only on cooling rate and holding time prior to solidification, but also substantially on the preceding melt treatment and its effect on IMC nucleation and growth. They also studied the beneficial impact of Mn on IMC dispersion and shape change, and reported that high melt-shearing increased nucleation and thus Fe-rich IMC dispersion.

Jerina et al. [174] studied the solidification behavior of a recycled 7075 alloy using cooling curve analysis and differential scanning calorimetry. They found that a larger specific surface area generates greater melt loss – up to 11 wt% – and a large number of inclusions. They observed the lowest enthalpy of solidification in materials with the largest specific surfaces, and concluded that high-melting inclusions formed.

A detailed review of enhanced solidification rates and their impact on microstructure and mechanical properties was presented by Lavernia et al. [211], who focused on 7XXX aluminum alloys containing differing amounts of tramp elements, e.g. Fe, Ni, Cr and Zr.

### 3.5. Near-net-shape, thin-strip, belt and chain conveyor casting methods for higher impurity tolerance

In most near-net-shape manufacturing, casting and rolling are closely combined. With the help of a casting nozzle, liquid metal is fed into the roll gap. The rolls subsequently have a double function, i.e. heat extraction for solidification and forming of the solidified strip. This eliminates the need to reheat the semi-finished product for the forming process. Release agents based on water, graphite or emulsions are used to prevent adhesion to the rolls. The main challenges of this casting technology are (i) the melt distribution system; (ii) metal level control; (iii) temperature control; (iv) release agent optimization; (v) roll cooling; and (vi) roll gap control [196,198]. Several reviews have been published of the properties and benefits of aluminum thin slab and sheet production via twin roll and belt casting methods (see Table 11) [212–214].

Belt and twin roll casters produce much thinner slabs than DC casting, in a combined solidification and deformation process with much higher solidification rates and less required in-line and/or downstream deformation, although at smaller overall production rates. Table 11, taken from the overview paper of Sanders [214], gives an overview of the various near-net-shape casting methods.

Twin roll casting involves pouring the melt into the gap between two rotating water-cooled cylindrical rolls, where it solidifies rapidly and produces sheets with a thickness of a few mm. In twin roll casters, the aluminum solidifies almost entirely on the roll surfaces prior to reaching the roll gap, where the hot material is exposed to an in-line deformation step. Compared to conventional DC casting, this method requires less capital, generates lower operating costs and lower energy consumption due to the reduced total thickness reduction, and has a reduced scrap rate – but also has a lower total production rate. Several methods, including varying in-line and downstream deformation and the subsequent heat treatments, have been developed to adjust the crystallographic texture and the microstructure of the sheets.

The attractiveness of the comparably low cost and low deformation work offered by near-net-shape aluminum casting is lessened for some alloy variants by lower surface quality and lower microstructure homogeneity compared to material produced via DC casting and downstream forming. This applies particularly to surface-sensitive can stock and automotive alloy grades. Material of good quality can commonly be produced via the twin roll route for the low-alloyed aluminum grades 1xxx and 8xxx and for 3xxx alloys. Twin-belt machines, mainly of the Hazelett design, are used to produce 3xxx and 5xxx series alloy sheet products with moderate strength, surface requirements and good corrosion resistance. The greater solid solution supersaturation during twin-roll casting leads to finer dispersion of primary phases and finer grains elongated in the casting direction. Therefore – in contrast to the processing routes established for DC cast alloys – processing routes for producing twin-roll cast materials require adjustment.

The solidification rate achieved in twin-roll strip casting depends on the strip thickness and can vary between $10^2$ and $10^4$ K/s [215], this figure tending to be higher at the surface than at the centerline of the strip [216]. Higher casting rates increase productivity but can cause macro-segregation in the central part of the strip. The two bounding surfaces of the twin roll cast sheets solidify first, inducing the solute enrichment of the liquid. When the two solidification fronts reach the strip center, solute-enriched areas with eutectic structures aligned along the casting direction are formed.

The microstructure and texture of twin roll cast sheets can differ between near-surface and center regions, particularly in particle density and grain size. The number of dispersoids precipitated during heat treatment is usually lower in the near-surface region. Grains in the central region are to a greater or lesser extent equiaxed, while those at the surface may be elongated in the casting direction or are very fine due to the recovery and recrystallization caused by the hot in-line deformation step characteristic of the twin-roll casting process [217].

<caption>**Table 11**</caption>

Overview of the various near-net-shape casting methods. Reproduced with permission from [214].

<table><thead><tr><th>DC Casting</th><th></th><th>Twin-Belt/Block</th><th></th><th>Twin-Roll</th></tr></thead><tbody><tr><td>Ingot casting</td><td>450–500 mm</td><td>Slab casting</td><td>15–20 mm</td><td>Roll casting 5–6 mm</td></tr><tr><td>Scalping</td><td>Remove 10–50 mm</td><td></td><td>None</td><td>None</td></tr><tr><td>Homogenization</td><td>500–530 °C</td><td></td><td>None</td><td>None</td></tr><tr><td>breakdown hot rolling</td><td>To 25–40 mm (>430 °C)</td><td></td><td>None</td><td>None</td></tr><tr><td>Tandem rolling</td><td>To 3–8 mm (430 → 300 °C)</td><td>Tandem rolling</td><td>To 3 mm (500 → 300 °C)</td><td>Cold rolling To 1 mm</td></tr><tr><td></td><td>Annealing (optional)</td><td>Annealing (optional)</td><td>>350 °C</td><td>Annealing > 350 °C</td></tr><tr><td>Cold rolling</td><td>To 1 mm</td><td>Cold rolling</td><td>To 1 mm</td><td></td></tr><tr><td>Annealing</td><td>>350 °C</td><td>Annealing</td><td>>350 °C</td><td></td></tr></tbody></table>
The various casting methods produce different slab dimensions which require specific downstream deformation protocols and heat treatment procedures. The main focus of this article, however, is the specific ability of near neat shape casting methods to provide the rapid cooling rates attractive for casting aluminum alloys which contain large amounts of scrap, i.e. those with greater trap element content. We have therefore reviewed some of the recent work in the field, with attention to the suitability of twin rollers for producing alloys with greater scrap content than commonly feasible in DC casting routes.

Suzuki et al. [218] used a vertical-type high speed twin-roll strip casting device equipped with a symmetrically arranged pair of water-cooled pure copper rolls to produce 6063 alloy base aluminum strips with systematically elevated Fe content, ranging from 0.7 to 6 wt%. The cooling rates were estimated to vary between 4500 K/s at the surface and 100 K/s in the mid-thickness region of the strips. The authors observed a refinement of the primary intermetallic Al-Fe-Si particles in the outer regions of the sheets, but also some coarser precipitates, stemming from a centerline segregation in the sheet center regions. After downstream cold rolling and annealing to T4 condition, or artificial aging to T6 condition, mechanical testing was conducted. The authors reported that no deterioration of bendability was found in materials with up to 3 wt% Fe and no cracking occurred upon hemming. However, age-hardenability was altered for sheets with 1 wt% Fe and above. Good results regarding bendability, increased proof stress and high ultimate tensile strength relative to the reference 6063 alloy with 0.35 wt% Fe were found in both T4 and T6 conditions for a material with 0.7 wt% Fe. This means that with a near-net-shape manufacturing method of this type, it is possible to produce sheets with the same or even better mechanical properties but with twice the Fe content tolerated in DC casting. The authors concluded that due to the high cooling rate provided by their roll caster setup, this production pathway is suitable for reducing the detrimental effects of enhanced impurity Fe content in melts containing large amounts of scrap.

In a similar study, Haga et al. [219] investigated the dependence of the microstructures and properties of 6111 base alloys produced by a high-speed twin roll caster featuring elevated Fe impurity content, in mimicry of using melts prepared from recycled aluminum alloys. They varied the Fe content between 0.14 and 1.02 wt%. They referred to their setup as a high-speed twin roller, as the device was claimed to provide higher cooling rates than conventional twin toll casters with casting speeds of up to 60 m/min. They reported equiaxed or globular microstructure in the as-cast strips. For up to 0.4 wt% Fe no changes to the tensile test results after T6 heat treatment and no cracking after hemming were observed.

In another study, Haga et al. [220] cast strips of recycled 6016 aluminum alloys for auto body sheets at 60 m/min on a vertically arranged high-speed twin roll caster equipped with 1500 mm diameter rolls. The thickness of the as-cast strips was 3.4 mm in both cases. The authors studied two material variants: one standard 6016 alloy with 0.16 wt% Fe as reference material, and another variant with 0.45 wt% Fe, representing the recycled material variant. The authors reported that both types of cast strip could be cold rolled, without any homogenization treatment, to 1 mm sheet thickness, and that T4 heat-treated strips of the 6016 alloy with high Fe content could be deformed by hemming and tensile testing like the reference materials, with no property loss.

Several strip casting studies were also conducted on 5xxx series alloys. Kumar et al. [221] studied twin roll casting and downstream processing of a 5754 alloy with a high added impurity content. The as-cast high-impurity strips generated coarse grains and complex secondary phases. When an Al-Ti-B grain refiner was added, the grain size and centerline segregation decreased significantly. The authors found secondary phases, such as Fe-bearing intermetallics and Mg₂Si, which formed a well interconnected network in the interdendritic regions. Homogenization reduced these networks and modified the secondary phases into a more equiaxed morphology. The equiaxed secondary phases aligned along the direction of rolling. X-ray diffraction showed that the predominating secondary phases produced during casting were α-Al(FeMn)Si and Mg₂Si, and that these phases prevailed during downstream processing. The strip cast and processed high-impurity alloy variant had higher strength and formability relative to the same material processed from standard mold ingot casting. The authors suggested that twin roll casting tolerates higher impurity levels than conventional mold casting.

Komeda et al. [222] and Haga et al. [223] conducted similar investigations on 5xxx series alloys. Both studies cast a recycled 5182 alloy strip using a vertical type high speed twin roll caster at 60 m/min using twin-positioned steel rolls. The authors mimicked a recycled alloy by enhancing its Fe content up to 0.6 wt%, and found properties in the processed strips that were comparable to those of the un-doped reference alloy.

Al-Helal et al. [202,224] conducted several studies of DC cast and strip cast aluminum products, using a 5754 reference alloy. The caster variant they deployed was a continuously operating high-shear melt conditioning twin-roll casting process, which was custom-designed for the production of recycled aluminum alloy strip. They observed that the method provided as-cast sheets with fine and equiaxed grains and a homogenized matrix structure, while the centerline segregation reported in other studies was eliminated. They also found that due to the imposed shear, the cast strip material showed clear recrystallization after thermomechanical treatment at 430 °C for 30 min and had >50% higher elongation and about 4% higher ultimate tensile strength compared to conventionally produced twin-roll cast strips.

Several other studies reporting similar trends for microstructure homogeneity, centerline segregation, mechanical properties and generally improved impurity tolerance of sheets produced via the strip casting route were also published about alloys based on 7050 [225], Al-Mn-Zr (with variations of Si and Fe) [226], Al-Mg-Cu [227], and Al-Mn-Fe (with variations in Mn, Si and Fe content) [228].

## 3.6. Advanced processing of alloys containing large amounts of scrap

### 3.6.1. Introduction and overview

The previous sections focused on conventional and thin-strip processing methods and how increased use of mixed scrap aluminum may impact them. However, there are also a few less common processing routes which can have advantages when dealing with material contaminated with scrap.
Two main approaches, namely conventional remelting and solid-state (meltless) recycling [229–231], have been adopted for the recycling of aluminum alloy scrap (Fig. 22). Their respective recycling efficiencies, energy consumption, cost and environmental impact during practical application need to be considered. Solid-state recycling has recently been applied to waste material from machining, chipping and turning [229–234]. This process incorporates annealing and heavy working after initial cold compaction. Large plastic strain was deployed to create metal-to-metal bonding by breaking down the oxide layers, a mechanism referred to as solid welding of scrap [229,232,234]. Such solid-state processes are estimated to reduce environmental impact in terms of energy consumption and carbon emission by 50–70% relative to conventional primary production [230]. The severe deformation and deformation-enhanced diffusion are critical for microstructure control, specifically for grain refinement. The influence of several parameters such as extrusion ratio/rate and temperature on the mechanical properties of solid-state aluminum alloys recycled by hot extrusion has been studied [229,233]. In hot extrusion most of the energy is required to heat the material. Thus, cold solid-state recycling of scrap, specifically cold profile extrusion and cold rolling of swarf material [232], was developed to reduce costs [232,234]. However, if extrusion alone is used, large residual voids remain in the recycled aluminum. A fine dispersion of particles containing Si and a reduction in the number of voids were achieved by combining extrusion and rolling, resulting in a significant increase in the density and strength of recycled Al-Si alloys.

Along similar lines, severe plastic deformation (SPD) can also be utilized for solid-state recycling of aluminum alloys [231]. Several solid-state recovery technologies have been proposed based on SPD, including equal channel angular extrusion (ECAP), cyclic extrusion compression (CEC), friction stir extrusion (FSE) and screw extrusion [231–233]. SPD processes have been successfully applied with the aim of consolidating aluminum scrap by imposing large shear deformation. In addition, SPD processes are capable of producing materials with ultra-fine grains and even nanostructured alloys which in some cases exhibit better mechanical properties than the original ingot.

Subsequent to SPD processing, homogenization and annealing towards precipitation, recovery and recrystallization can be used to control the grain size/morphology and crystallographic texture of the final products [235–237].

When applying such workflows to scrap-based alloys, tramp-element-related brittle intermetallic phases in particular can be harmful. For example the transformation of the needle-like β-Al~5~FeSi phase into a more skeleton-like α-Al~12~(Fe~x~Mn~1~−~x~)~3~Si is of importance for neutralizing harmful effects resulting from accumulated Fe in recycled aluminum alloys [236,238–240]. The fraction and shape of these and other scrap-related particles is influenced by the heat treatment, and can be understood by using computational thermodynamics, also taking into account integrated solidification and homogenization [238]. Deploying a high solutionizing temperature without initializing the pre-melting of Cu-rich phases results in the fragmentation and dissolution of the Fe-rich phases, an aspect beneficial to the mechanical properties in the recycled AlSi~9~Cu~3~ cast alloy [240]. Another example is seen in the 7xxx alloys, which were successfully fabricated using recycled beverage cans [241]. Several advanced processing approaches for aluminum alloys

<figure><img src="image_24.png" /><figcaption>**Fig. 22.** Microstructure modification of alloys containing large amounts of scrap through processing.</figcaption></figure>
containing large amounts of scrap are discussed in more detail in the next sections.

### 3.6.2. Liquid-based processing

The traditional practice for aluminum scrap recycling is to either return sorted scrap to secondary processing for smelting and casting or to directly downcycle mixed alloy scrap to cast alloys, where the process and alloys are more tolerant of composition variations. A certain number of impurity elements are thus inevitably introduced into the recycled aluminum alloys, leading to the formation of coarse intermetallic compounds. These compounds can cause excessive shrinkage porosity defects, which may have an adverse effect on mechanical properties (especially ductility) [239,240,242]. Such defects tend to be the source of cracks due to stress concentration, thus damaging fatigue strength and toughness. To mitigate these negative effects, the intermetallic compounds are traditionally modified by adding transition elements (Co, Cr, Mn, V, etc.) [243,244]. Advanced casting methods are also commonly implemented to improve the tolerance limit of the impurity elements in aluminum alloys containing large amounts of scrap [243,245–249].

High-pressure die-casting (HPDC) is widely used in the production of mixed recycled aluminum alloys due to enhanced castability and relatively low cost [245]. Fe normally has a negative effect on the mechanical properties of the cast aluminum alloys, but using HPDC as a casting and forming method can reduce their tendency to form fragile and coarse second phases [245].

Squeeze casting can refine the grain size and the Fe-rich compounds with the formation of a nano-sized Fe-rich α phase [248,249]. For this reason aluminum alloys produced by squeeze casting exhibit excellent tensile properties, especially in terms of elongation. The squeeze-cast 2A16 alloy with 0.5 wt% Fe still meets the requirements of the safety-critical suspension components used in the automotive industry (UTS > 380 MPa, elongation > 7%) [248], aiding the efficient utilization of a secondary aluminum alloy.

The application of external fields, e.g. ultrasonic, electromagnetic and force fields, during the casting process is also effective in controlling the microstructure of the recycled aluminum alloys [250–253]. Ultrasonic melt processing (USP) can not only effectively refine and enhance the peritectic transformation of the Fe-rich phases, but can also reduce the volume fraction of pores in the condensation process [250], thus prospectively modifying the microstructure and improving the mechanical properties of the recycled aluminum alloys. Zhao et. al. [251] used combined fields, i.e. ultrasonic vibration (USP) and applied pressure (squeeze casting), during the solidification of recycled aluminum alloys. They found that using combined fields can significantly refine the α-Al phase and the Chinese-script Fe-rich phases. This method also promotes dendritic fragmentation and reduces solute segregation on grain boundaries.

An intermediate approach under active development is rheoforming [254,255]. Rheoforming is a semi-solid processing technique that involves the controlled cooling of an alloy melt into a semi-solid state [256,257]. This semi-solid alloy is then actively stirred while being injected into a die. Owing to the low temperature and continuous shearing of the semi-solid mix as it is injected into the die, it is possible to achieve a fine microstructure with little macroscopic solute segregation. Unlike thixoforming, rheoforming is less sensitive to minor changes in the liquid fraction. This is beneficial for alloys considered here where small changes in composition and temperature may lead to large changes in liquid fraction [257]. This makes rheoforming feasible for a wide range of alloy types, ranging from traditional cast alloys to wrought alloys such as heat-treatable 2xxx [258], 6xxx [259] and 7xxx [259–261] alloys and non-heat-treatable 5xxx alloys [262]. Solution treatment followed by aging can bring the strength of these alloys close to their expected T6 condition, albeit with lower ductility [257,259,261]. This lower ductility has been attributed to both porosity and to partial remelting due to microsegregation [259]. While rheoforming appears a promising technique worth further exploration, much remains to be done to optimize the process for a wider range of alloys and to achieve properties comparable to those from more conventional processes. Despite this, efforts are underway to produce rheoformed parts to replace parts made by conventional die casting or forging [263].

A second near-net-shape solidification-based process of recent interest for its potential to handle a wide range of recycled alloy scrap is spray forming, alternatively known as liquid dynamic compaction or the Osprey process [264]. In this process, liquid metal is inert-gas atomized (particle sizes of 5–250 µm) and the atomized droplets are deposited onto a substrate in a mix of liquid, semi-solid and solid particles. Cooling rates for particles in flight can be very high (>>10² K/s) depending on particle size, while cooling of the densifying deposit occurs much more slowly (0.1–10 K/s) [265,266]. Much of the interest in the potential of spray forming as a technique for handling recycled aluminum comes from its ability to refine, modify or even possibly eliminate intermetallic particles deleterious to subsequent formability and produce a fine microstructure [267–271]. A recent study on the spray forming of a 6061 alloy intentionally “contaminated” with 1.4 wt% Fe showed the preferential formation of α-Al₁₅(Fe,Mn)₃Si₂ over the undesirable plate-like β-Al₅FeSi [269]. The as-produced alloy was subsequently hot-extruded and then subjected to homogenization and aging treatments. It was shown that the mechanical properties of the heat-treated material, despite a high Fe content, were very similar to those of conventionally processed 6061. An additional advantage of the spray-forming approach noted by the authors was relatively short homogenization time due to the fine as spray-cast microstructure [269]. Studies on 7xxx series alloys show a similar ability to achieve strengths comparable to those obtained through conventional processing, although with evidence of lower ductility and corrosion resistance [270,271].

### 3.6.3. Solid-state processing

Rheoforming and spray forming require scrap to be re-melted during processing. The conversion of solid scrap back into liquid comes at a significant cost, both in terms of the energy required for melting (and potentially for sweetening the melt) but also in the pre-treatments required to clean the scrap. In the case of fine scrap (e.g. turnings from machining) there are additional challenges that can result in recycling yields of only approximately 50% [272]. It has been estimated that the energy savings that would arise from a fully solid-state process for aluminum scrap recycling could be as high as 90% [273,274]. This is not a new discovery; the first patent for a fully solid-state-based process for recycling aluminum scrap can be traced to the middle of the last century [273].

A wide variety of solid-state processes for recycling the fine aluminum scrap arising from (e.g.) turnings or extrusions have been
studied; a recent review is found in [231]. These processes can be separated into two groups: those based on powder metallurgy, and those based on deformation. Powder-metallurgy-based processing (milling of turnings followed by sintering, e.g. [275] or direct sintering of recycled turnings [276,277]) remains much less explored than deformation-based processing. In this context a variety of techniques have been explored, such as hot and cold extrusion [231,233,274,278], severe plastic deformation (equal channel angular pressing, high pressure torsion, accumulative roll bonding [279-281]) and friction stir processing [282]. The common feature of these processes is that they take advantage of large plastic strains to cause the fracture of oxide layers and "cold" welding of the individual particles, assisted by the heat generated during the process and, potentially, by heat supplied externally [273]. Among these processes, the most well developed are those based on extrusion. Güley et al. [274] showed the successful hot extrusion of 6061 scrap and mixed 1050 and 6061 scrap starting directly from compacted recycled extrusion pins and turnings from machining. More recent work has produced complex-shaped extrusions made directly from recycled scrap. For example, Chiba and Yoshimura [233] compared two sources of scrap aluminum (milling chips and turnings from machining) in the hot extrusion of parts from an Al-Si alloy. While both source materials could be successfully extruded following cold compaction, two key observations were made. First, fully dense materials required such large strains that it was a challenge to achieve them uniformly given the complex flow pattern through the die. Second, there was a difference in consolidation of the two source materials. While the extrusions made from the milling chips appeared fully dense at the largest extrusion ratio, significant porosity remained in the extrusion manufactured from the turnings. This points to the importance of the handling of the source material prior to consolidation [283].

### 3.6.4. Processing to achieve hybrid/architected recycling-friendly alloys

Regardless of the process used to convert mixed scrap into new products, concerns about performance in downstream processing or in service remain. For example, one of the problems that rheoforming, spray forming and strip casting aim to circumvent is the formation of coarse, brittle, intermetallics detrimental to ductility. Rather than tackling the issue solely through microstructure manipulation of the alloy, one approach takes into account the material's architecture [284-286]. Aluminum production has long used this approach for clad products. To improve the corrosion resistance of aerospace alloys (e.g. 2024) it is common to roll clad the surface with a thin sheet of a more corrosion-resistant alloy (e.g. 1200), while in for brazing sheets a low melting point alloy (e.g. 4004) is roll bonded onto a high melting point alloy (e.g. 3003) [287]. To address concerns regarding either the corrosion resistance or mechanical performance of recycling-friendly alloys, one solution might therefore be to take a hybrid design approach which combines two or more alloys with a particular spatial arrangement to achieve improved overall performance.

In the case of corrosion resistance, the advantages afforded by roll cladding are obvious. The benefits of a chemically distinct surface layer in controlling mechanical properties can also be quite significant, particularly in the areas of strain localization [288] or fracture [289,290]. In particular, aluminum sheet product performance is often limited by bendability issues. This is the case in automotive applications where hemming operations require a severe 180° bend. Here the most demanding mechanical need exists at the outer surface of the material, where the strains are greatest. Roll cladding made of high-strength, low-ductility aluminum alloys (e. g. 6xxx) combined with a lower strength, ductile alloy (e.g. 8xxx) has been shown to significantly improve hemming performance [290]. In this case, fracture was suppressed for a full 180° bend with a clad layer of 10% of the sheet thickness. Compare this with a monolithic 6016 sheet that exhibited significant surface cracks at bending angles of <120°.

Roll cladding is only one of the possible processes that can be used to manipulate the properties of a material's surface. Another, commercially successful, process involves the direct chill co-casting of two alloys to generate a continuously cast ingot consisting of a core made of one alloy and a surface made of another [287,291].

Roll bonding and co-casting are only two of the potential processes for generating hybrid recycling-friendly alloys. Chemical and/or mechanical treatments of the surface that change the composition or microstructure of the near-surface layer may provide an ideal opportunity to optimize the combination of environmental, mechanical and chemical performance. Taking inspiration from work on the design of architectured materials from other fields might also prove highly productive in broadening the commercial appeal of recycling-friendly aluminum alloys.

### 3.7. Sheet-forming challenges for scrap-based alloys

Flat rolled product (foil, sheet and plate) accounts for ~30%-55% of the Al used today, varying by company, market and region. Of this, ~95% consists of the non-age-hardenable 1xxx, 3xxx and 5xxx grades [292]. In the types of application where these alloys are used, the formability of the sheet is a critical factor during processing. The formability of sheet from age-hardenable alloys (2xxx, 6xxx and 7xxx) is also important, especially for transportation applications, but the forming is usually performed in the naturally aged (T4) state (i.e. before the age-hardening step) so most of the solute is in solid solution. This solute in solution has important effects on the stability of plastic flow and the surface finishes obtained. The microstructural considerations for forming are similar to those for the non-age-hardenable classes, except that one must also consider the effect of the (de)formed microstructure on the subsequent age-hardening reaction, for example during a paint-bake treatment for automotive applications.

Some forming operations for Al-alloys – warm forming or creep forming [293–295] – are performed at elevated temperatures. Apart from the expected effects of temperature on thermally-activated dislocation processes, in the age-hardenable grades one may also observe significant changes in the precipitate state during forming, e.g. [81]. The coupling of plasticity and dynamic precipitation adds an additional layer of complexity to questions surrounding the formability of scrap-based alloys [81,85,296]. The impurity elements accumulated by including scrap can affect both processes together during the forming operation. Such contaminant couplings have not yet been studied in any detail.

There is no one quantitative measure of formability; nor can formability be simply related to any single material property. Different
sheet forming operations are used industrially and each combines different elements of stretching, bending, drawing and ironing. Stampings, such as those used for automotive body panels, are usually stretching-dominated operations, whereas beverage cans are deep drawn.

In general, formability depends on.

* the strain hardening capacity and strain-rate sensitivity of the material (which can be a strong function of the solutes in solution, and hence also of the impurity elements present);

* the crystallographic texture (which depends on the sheet rolling and subsequent recrystallization annealing steps);

* the R-value (which is the ratio of the width to thickness strain and hence is a measure of the resistance to thinning during deformation);

* the resistance to local fracture (which can be sensitive to the constitutive particles present and hence also to the presence of scrap-related impurity elements).

The relative importance of each of the above factors depends on the specific sheet-forming operation, particularly the balance between stretching and drawing. Failure during sheet forming can be of two types: local fracture may occur as a result of reaching a critical local stress or strain (e.g. at a large intermetallic particle), or excessive local thinning (necking) of the sheet may occur due to a mechanical instability in the flow. Both modes of failure can be affected by compositional modifications generated by using scrap.

Forming operations that are dominated by stretching operations (e.g. sheet stampings) often represent formability in terms of a Forming Limit Curve (FLC). FLCs are experimentally generated by stretching a clamped sheet over a hemispherical punch and monitoring the strains at which failure occurs. Fig. 23a shows a schematic FLC which highlights the strain states. Successful forming requires one to remain below the curve for the major and minor strains.

Texture, strain hardening capacity, strain-rate sensitivity, R- and ΔR-values, and resistance to fracture are all properties that can be significantly affected by compositional modifications due to the use of scrap (see Fig. 23b). Examples of these effects are presented in the ensuing sections.

### 3.7.1. Fracture and the role of constituent particles

The contaminant from scrap that has received the most attention is Fe, and with good reason. Discussed in section 3.7 in the context of twin-roll casting, it is currently an issue for recycling. Al recycling usually leads to Fe accumulation, manifesting as (micron-sized) Fe-containing intermetallic particles in alloys. These large constituent particles can have important effects on fracture, and sheet-forming operations that are fracture-strain-limited can be significantly affected. Bending is particularly sensitive to fracture strain. Fig. 24 shows an example in 6xxx alloys from Lloyd [298]. Here the bendability of the sheet is characterized by the minimum radius to which a sheet of thickness t can be bent without failure. The outside of the sheet surface is in tension and the inside surface is in compression. Failure usually occurs through cracking on the outer surface. Fig. 24 shows data for 6111 with three differing Fe content figures. On average, as the Fe content increases, the minimum bend radius also increases, correlating with decreased bendability. Even though these Fe additions were shown to have little impact on the yield strength, tensile strength, strain hardening or elongation, they do affect the reduction in cross-section during tensile tests. Failure occurs through void formation at the large intermetallic particles, which then coalesce, leading to failure. The correlation between bendability and reduction in thickness (due to the increase in intermetallic particles with increased Fe) is shown in Fig. 24.

<figure><img src="image_25.png" /><figcaption>**Fig. 23.** (a) Schematic Forming Limit Curve (FLC). (b) FLC for two 6022 alloys containing different low and elevated Fe content. Data extracted from [297].</figcaption></figure>