### A PREVIEW OF ANODE COKE QUALITY IN 2007
Franz Vogt, Robert Tonti, Maia Hunt, Les Edwards CII Carbon LLC, 1615 East Judge Perez Drive Chalmette, LA, 70044, USA

Keywords: Calcined Coke, Green Coke, Anode, Isotropic

Abstract
Demand for petroleum coke for aluminum smelting anodes reached about 10.4 million metric tons in 2003. This created a coke calciner demand of 13.8 million metric tons of green coke suitable for making anodes. The green coke demand will increase to about 16.7 million metric tons by 2007. Although the oil refining industry will produce ample green coke to meet this demand, the quality will be quite different from that used in the past. During the last few years, the calcining industry has begun using green cokes of a quality that would not have been considered suitable for anode use ten years ago. This paper surveys the available quality of green cokes worldwide, and makes projections on the quality of anode coke by 2007. The calcined coke of the future will be higher in sulphur and vanadium, higher in porosity and more isotropic. The change in structure could eventually lead to higher thermal shock sensitivity of anodes. Smelters will need to adapt by making process changes and improvements.

Introduction
For the past five years or so, it has been apparent to the coke calcining industry that high quality raw materials will become harder to source. In a paper published in 2001 [1], it was predicted that the growth of green petroleum coke of the quality traditionally used by the aluminum industry would not keep pace with the demand. By 2003, it was predicted that low sulphur, low vanadium cokes would be fully used by the industry and higher sulphur and vanadium cokes would be drawn into the raw material blend. This has happened. Cokes with vanadium levels as high as 600 ppm are being used as blend cokes, and this trend will likely continue.
The causes of the imbalance between demand and supply of good quality anode grade green cokes (growth of calcined coke demand, crude oil quality changes, refining industry changes) were explained in the previous paper. These factors remain generally the same today.
For this paper, the authors have examined the raw materials available in 2003 through 2007, worldwide, to produce calcined coke for the aluminum industry. The regional demand and supply was considered, and the impact of the changing raw materials on key quality parameters was reviewed.
World Anode Grade Green Coke Demand and Supply
Figure 1 shows the demand and supply for anode grade green coke for the years 1995 through 2007. The demand is based on figures for aluminum production, past history and future projections, supplied by CRU [2], factored by a usage rate of 0.39 tons calcined coke per ton of metal, and a conservative calciner recovery rate of 0.78 tons calcined coke per ton of dry green coke.

Figure 1: Green Coke Demand and Availability

7500 10000 12500 15000 17500

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YearGreen Coke (Tons)

Green Coke Demand Traditional Green Coke Supply

From Figure 1, it can be seen that the green coke requirement of 13.8 million tons in 2003 will rise to 16.7 million tons in 2007. This 21% increase is solely due to the demand from the aluminum smelting industry worldwide. Until 2002, the availability of traditional quality, low sulphur, low vanadium, green cokes exceeded demand, and an excess supply of traditional coke was available. This situation changed in 2003 due to the lack ofgrowth in new anode grade coke sources. As a result, nontraditional cokes with higher vanadium levels and a more isotropic structure have been blended with traditional cokes to meet the shortfall. From this time, an increasing proportion of non-traditional cokes will meet the demand.
Regional Supply and Demand for Green Cokes in 2007
Figure 2 shows the global distribution of the 16.7 million tons of green cokes that will be used for anode grade calcined coke manufacture in 2007. Obviously, a number of estimates were made to produce such a graph. The green coke production distribution will not show much difference from 2003 except in Latin America. Brazil anticipates a doubling of the production of good quality green coke that may enter the anode market by 2007. Light Metals 2004 Edited by Alton T. Tabereaux TMS (The Minerals, Metals & Materials Society), 2004

489Figure 2: Distribution of Green Coke in 2007

N.America

32%

W.Europe

11% E.Europe

1% CIS 14% CHINA 17% Middle East 5% SE Asia

8% Latin America 11% Africa

1%

North America: North America will remain the largest producer of green coke for the industry. This is consistent with the fact that nearly 60% of the world’s green coke is produced there.
Based on CRU projections for the period 2003 – 2007, demand for anode green coke created by smelters in North America is well below the supply. North America has a large calcining industry, which was developed to take advantage of the plentiful raw material supply of the past. This industry has for many years exported much of its production to other regions where coke was less available. These large calciners have the benefit of being able to source and blend a wide range of green cokes to produce calcined coke of suitable quality for smelters around the world. However, because of the large requirement of the North American calciners, there has been a deficit of high quality anode grade coke in North America for some years, and, in the future, raw material must be imported in ever increasing quantities.

China: By 2007, the aluminum smelting industry in China will become the world’s largest, and that anode coke requirement will affect the supply worldwide. China will also be the second largest anode grade green coke producer, assuming that the crude oils refined there continue to support a quality that can be used by the industry. In spite of that, China will become a net importer of about one million tons (about 35%) of anode grade green coke in 2007.

Middle East: The Middle East will see an expansion of its aluminum industry. The calcining industry there will use virtually all of the available green coke in that region.

Latin America : In 2007, Brazil will double their green coke availability, exceeding the regional demand by some 600,000 tons. This will be a usable raw material for anode coke production.
Other Regions: Other regions examined, Europe, CIS, SE Asia, Africa, and Australasia will increase their coke requirements, but the anode coke availability will not materially change, except for SE Asia. There, more of the cokes produced today will be drawn into the raw material supply for anode coke production.

Anode Coke Quality Changes (Excluding CIS and China)
Projections of calcined coke quality between 2003 and 2007 based on worldwide green coke quality, excluding coke produced in the CIS and China, are shown below. The projections assume no net growth in the availability of traditional quality cokes but a steady increase in the use of non-traditional cokes to make up this deficit as depicted in Figure 3.
Figure 3: Western World Green Coke Demand

Compared to Supply of Traditional Cokes

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2001 2002 2003 2004 2005 2006 2007 YearGreen Coke Demand

Shortfall to be Met by Non-traditional Cokes Coke Demand

Traditional Quality Coke

The quality projections are based on knowledge of the next available quality of cokes likely to be used by the calcining industry. There is too much uncertainty in the data for the CIS and China to incorporate these two regions. These projections must be viewed as averages for all the anode coke used in the world except for CIS and China. One should expect that any given smelter would see quality that varies above and below the average quality.
Figure 4 shows that sulphur will increase only slightly on a worldwide basis. The order of magnitude will be about 0.1% (absolute). This may not seem like a big increase, but it comes at a time when pressure to reduce sulphur emissions from smelters is increasing. The tempering effect on sulphur increasing, especially in 2005, would be caused by the introduction of the new Brazilian coke.
Figure 4: World Excluding CIS and China Changing Calcined Coke Quality - Sulphur

1.80 1.84 1.88 1.92 1.96 2.00

2000 2001 2002 2003 2004 2005 2006 2007 2008 YearSulphurContent (wt%)

Figure 5 shows that vanadium will increase about 20% to nearly 270 ppm on average in 2007. This is the most dramatic change in quality expected. The next green cokes available are significantly higher in vanadium; some are above 600 ppm.

Figure 5: World Excluding CIS and China Changing Calcined Coke Quality- Vanadium

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2000 2001 2002 2003 2004 2005 2006 2007 2008 YearVanadium (ppm)

490Changes in impurity levels such as sulphur and vanadium are easy to measure and understand but changes in coke structure are more subtle. Delayed sponge coke used by the aluminum industry today typically has a low micro-porosity with a moderate degree of anisotropy with large domain sizes. Many of the non-traditional cokes used in the future will be more isotropic in character with small domain sizes and a fine-grained texture. They are also typically higher in micro-porosity.
Figure 6 shows a micrograph of a traditional delayed sponge coke that has been used successfully by the aluminum industry for many years. Figure 7 shows a more isotropic sponge coke that is being used today in small quantities as a blend component. Thedifference in structure should be obvious. Many of the nontraditional cokes used in the future will have isotropic structures like those shown in Figure 7.

Figure 6: Micrograph of a Typical Sponge Coke (100x)

Figure 7: Micrograph of an Isotropic Sponge Coke (100x)
Two calcined coke properties related to coke structure that can be expected to change in the future as more non-traditional cokes are used in anode manufacture are micro-porosity (as measured by mercury apparent density) and coefficient of thermal expansion (CTE). Figure 8 shows a projection for the change in mercury apparent density (measured via the Pechiney method). Mercury apparent densities are expected to drop on average by 0.01g/cm 3 from the recent past. Coke bulk densities are also likely to decrease on average due to the higher volatile matter content of non-traditional green cokes. Figure 8: World Excluding CIS and China Changing Coke Quality - Apparent Density

1.70 1.71 1.72 1.73 1.74 1.75 1.76

2000 2001 2002 2003 2004 2005 2006 2007 2008 YearApparent Density(g/cm3)

Figure 9 shows that the CTE of anodes made in 2007 will be higher than at present. The projection is based on a measurement of CTE’s on bench scale anodes produced with traditional cokes and more isotropic, non-traditional cokes. The projection is somewhat speculative since only limited CTE data is available. The trend toward a higher CTE can be expected however, since more isotropic cokes have a higher coefficient of thermal expansion. This is one of the primary reasons why the industry avoids using shot cokes. Although shot coke packs to a high bulk density, its high CTE makes it very sensitive to thermal shock problems when used in anodes. No use of shot coke is assumed in these projections.

Figure 9: World Excluding CIS and China Changing Coke Quality - CTE

4.00 4.05 4.10 4.15 4.20 4.25

2000 2001 2002 2003 2004 2005 2006 2007 2008 YearCTE

One Typical New Coke
Table 1 compares the properties of the coke shown in Figure 7 with the average properties of a range of traditional cokes being used today. The properties were measured on bench s cale anodes produced with 100% of each di fferent coke. The non-traditional coke is typical of the next tier of quality available to the calcining industry for anode grade coke production.
The property differences that stand out with anodes produced from the new, non-traditional coke are as follows:

Baked anode density is slightly lower

Compressive strength is significantly higher

Modulus of elasticity is significantly higher

CTE is significantly higher

Air reactivity residue is lower

Sulphur and vanadium are higher

491Table 1: Comparison of Bench Scale Anode Properties of Traditional Cokes with a Non-Traditional Coke
Property Traditional Cokes (Range) Traditional Cokes (Average) NonTraditional Coke Baked Density, g/cm 3 1.48-1.53 1.51 1.46 Specific electrical resistivity (  m) 67-82 75 66
Compressive Strength, MPa 21-27 23 48
Modulus of Elasticity, GPa 1.8-2.4 2.18 4.50
CTE, 10 -6 /K 3.7-4.5 4.00 5.49 Thermal Conductivity W/mK 2.2-3.3 2.6 3.1
Xylene Density (g/cm 3) 2.07-2.10 2.08 2.05 Air Permeability, nPm 2.0-2.2 2.5 2.3
CO 2 Reactivity Residue, % 87-97 93.7 94.2
Air Reactivity Residue, % 60-98 79.2 54.5
S, % 0.98-3.31 2.35 3.77 V, ppm 44-350 167 578

It would not be feasible for the industry to produce anodes with 100% of a coke like this today, but it has been used successfully as a blend coke. The challenge that the industry faces is that a greater proportion of this type of coke quality will need to be used in the future.
How Can the Aluminum Industry Best Adapt?
The situation facing the aluminum industry relative to coke quality is somewhat analogous to the situation with coal tar pitch. The closing of coke ovens for economic and environmental reasons has left coal tar producers and users searching for strategies to deal with the shrinking of supply of coal tar [3]. The industry is adapting however, and many smelters are now using coal tar pitch/petroleum pitch ble nds with good success.
The coke changes predicted are inevitable, and the aluminum industry will need to adapt to using progressively more isotropic cokes or other carbon based materials. Many people ask about coal, but high ash levels mean that expensive pre-treatments are required to reduce the ash to accep table levels. A study by Goodes et al [4] in the mid-1980’s concluded that good results were possible using a flash pyrolysis process, but the economics were unfavorable. Research is underway again today in this area but it is too early to predict the technical and commercial viability.
In the meantime, as the aluminum industry’s demand for carbon increases, the use of more isotropic cokes will become necessary. What could a smelter using a moderately anisotropic sponge coke with a low metals content expect today if it switched to a high metals, high sulfur, isotropic coke? The following negative consequences might be expected: Anode CTE would increase along with the potential for significant thermal shock cracking problems.

Anode air reactivity would increase resulting in higher excess carbon consumption.

Metal purity would decrease along with possible decreases in cell current efficiency.

The coke may be more difficult to grind and poor pitch penetration of coke would reduce anode strength and increase permeability.

Environmental problems associated with handling higher sulfur carbon materials would increase.
It is unlikely that any smelter today or in the future could performwell if it switched fully to using a highly isotropic coke. Smallscale plant trials have been attempted in the past, but the results were disappointing, if not unexpected. Mannweiler et al. [5] reported on the results of a smelter trial with a highly isotropic Venezuelan coke containing 2280 ppm vanadium, 530 ppm nickel and 3.9% Sulphur. The coke had a low real density and the air reactivity was an order of magnitude higher than traditional materials. A relatively small number of anodes were produced but nearly all cracked when placed in the cell. The trial was aborted in favor of a trial with a 50/50 mixture of isotropic coke and a more traditional anode grade coke. More acceptable results were produced in this second trial and cracking in the cells was eliminated.
Below are some of the adaptations or technologies the industry may need to explore in order to solve the type of problems experienced when utilizing more isotropic cokes.
High CTE and Thermal Shock Cracking : This is a significant negative consequence of using isotropic cokes. Limiting the amount of high CTE coke in the blend is one way to control this along with other steps such as:

Modifying the anode granulometry or particle size distribution to a recipe that offers more thermal shock resistance [6].

Limiting the physical size of anodes used in the cell

Utilizing anode pre-heating before setting new anodes in a cell

Reducing cell operating temperatures and cell process variations

Eliminating the thermal shock problem completely by developing a modern, continuous pre-bake anode technology.
Air Reactivity : The susceptibility of anodes to excess consumption through air oxidation or airburn is dependent on both the coke quality and the cell design and cover practices. Modern well-covered cells are not as sensitive to anode airburn problems as older cells, where maintaining a sufficient level of cover is difficult. Adaptations to limit anode airburn may include:

Changing to deep cavity cell designs which allows good control of anode cover

Adding small quantities of oxidation inhibitors to the carbon (This is common practice in the ceramics industry for materials such as silicon carbide and graphite.)
Metal Purity and Loss of Current Efficiency : This is arguably the most difficult problem to solve when using cokes with high metals contents and will ultimately limit the quantity of these materials

492that can be used in a blend. Contaminants such as nickel, vanadium and silicon will report to the metal and may result in a loss of current efficiency as has been reported in previous studies [3]. Post-metal treatment (such as addition of boron) can be used to remove such impurities but there is a significant cost associated with this.
Grinding and Pitch Penetration Problems : These problems are potentially the simplest to solve by changing process equipment or targeting particular size fractions for the isotropic coke. The aluminum industry has been conservative in its approach to using new technology in the past but this is changing. In the last 10 years, the industry has em braced fabrication tec hnology that has been in use in other industries for some time such as Eirich Intensive Mixer/Coolers, vacuum vibro-formers, and new crushing and grinding equipment. Recently, vacuum hydraulic presses with fully programmable pressing cycles have been suggested. A combination of new technology can easily overcome some of the processing problems that may be experienced when utilizing more isotropic cokes.

Environmental : The use of higher sulphur cokes will generate more SO 2 and it will become increasingly necessary for smelters to remove SO 2 from the cell off gases before they are released to the atmosphere. Scrubbing technology is available today for this and the problem becomes an economic one rather than a technological one.
Finally, it is worth noting that all of the above problems could be eliminated if the industry developed a viable inert anode. Despite a lot of attention in 2001, this technology still appears to a long way from commercial reality, and in the meantime, the industry will need to learn to utilize progressively more isotropic cokes.

Conclusions
The following conclusions can be drawn from the data presented in this paper.
1. Non-traditional cokes with more isotropic structure and higher sulphur and vanadium levels will largely meet a 21% increase in anode gr ade green coke demand between 2003 and 2007. 2. The major change in calcined coke quality predicted between 2003 and 2007 is the 20% increase in vanadium. Other negative changes for the aluminum industry will be higher coke CTE’s, increasing sulphur content and density decreases. 3. Smelters must prepare for the future changes in anode quality by modifying their processes and developing strategies for removing vanadium from their product metal. References
1. “Global Trends in Anode Grade Coke Availability and Quality for the Aluminium Industry,” F. Vogt, R. Tonti, L. Edwards, Proceedings of the 7 th Australasian Smelting Technology Conference, Melbourne, Australia, November 11-15, 2001. 2. Aluminium Quarterly Industry and Market Outlook; CRU International Ltd., London, England, July 2003 3. “Developing Coal Tar/Petroleum Residues,” R.H. Wombles and M.D. Kiser, Light Metals 2000, p537 4. “Production of Anode Grade Carbon from Coal,” C.G. Goodes, J.A. Eady and J.C. Nixon, Light Metals 1987, p459. 5. “High Vanadium Venezuelan Petroleum Coke, a Raw Material for the Aluminum Industry?,” U. Mannweiler, W. Schmidt-Hatting, D. Rodriguez and A. Maitland, Light Metals, 1989, p449. 6. “An Approach for a Complete Evaluation of Resistance to Thermal Shock: Applying to the Case of Anodes and Cathode s,” C. Dreyer and B. S amanos, Light Me tals, 1997, p 585.

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