## 2 Production of Copper slag
### Copper Slag  
Copper slag is a by-product generated during the pyrometallurgical process used to extract concentrated copper from sulfide mineral ores. It typically contains materials such as FeO, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), CaO, and residual Cu [3]. The pyrometallurgical stage, known as smelting, involves the use of furnaces operating at very-high temperatures. During this process, the intense heat generates two immiscible liquid phases: a Cu-rich matte and a copper slag referred to as the oxide phase [19]. Generally, copper slag is composed of oxides such as FeO and Fe\({}_{3}\)O\({}_{4}\) (30-40%), SiO\({}_{2}\) (35-40%), Al\({}_{2}\)O\({}_{3}\) (0-10%), and CaO (0-10%) [3] which vary their presence in the product slag due to (i) the nature of the minerals, (ii) the nature of the fluxes, (iii) the nature of the concentrates, and (iv) the operating conditions, as well as other factors related to the production process [27]. Copper slag is one of the main solid wastes from the pyrometallurgical process that is applied to concentrates in copper industry plants. Chile produces approximately 4.5 million tons of slag per year, and more than 30 million tons of copper slag are produced annually in the world [28], demonstrating the magnitude of the waste from mining mineral processing. Furthermore, copper slag often contains a large amount of valuable metal and is currently considered a secondary metal resource [6]. These metals can be Fe, Cr, Cu, Al, Zn, Co, Ni, Nb, Ta, Au, and Ag, and they can be recovered by units operated in the mineral processing of slags, such as (i) crushing, (ii) grinding, (iii) magnetic separation, (iv) flotation, (v) leaching, and (vi) roasting [9], respectively. Materials used for cathodes have become increasingly important [29]. Therefore, recycling the metals present in copper slag can not only recover strategic metals but also contribute to carbon neutrality. Regarding their appearance and properties, naturally cooled copper slags have a black color and a glassy surface, and are dense, lumpy, hard, and brittle (see Table 1). Important properties to consider for use in a thermochemical solar process are the unit weight (transport), bulk density  
Figure 1: Scheme of Cu slag production, process, product, and by-product (modified of [21]). (storage), and hardness (abrasive material). However, depending on the cooling pattern, which can be natural or water quenched, they can differ in appearance, density, and shape (see Table 2) [30]. In recent years (1999-2019), approximately 752 Mt [20] of copper smelting slag have been generated from the 13 main producing countries (see Figure 2). The largest generation of this waste occurs in South America (40%), followed by Asia (13%), USA (8%), Oceania (6%), Europe (6%), Africa (6%), and others (15%). \begin{table}
\begin{tabular}{l l} \hline
**Property** & **Value** \\ \hline Unit weight (T/m\({}^{3}\)) & 2.8–3.8 \\ Absorption & 0.13 \\ Bulk density (T/m\({}^{3}\)) & 2.3–2.6 \\ Conductivity (\(\mu\)s/cm) & 500 \\ Specific gravity & 2.8–3.8 \\ Hardness (Moh) & 6–7 \\ Moisture (ppm) & \(<\)5 \\ Abrasion loss (\%) & 2.4–10 \\ Internal friction angle & 40–53 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical and mechanical properties of copper slag [3, 31]. \begin{table}
\begin{tabular}{l l l} \hline
**Property** & **Description** & **Ref.** \\ \hline \multirow{2}{*}{Particle shape} & Angular & [32; 33; 34] \\  & Irregular & [31; 35; 36] \\  & Multifaceted & [37] \\ \hline \multirow{3}{*}{Surface texture} & Glassy & [24; 32; 34; 36] \\  & Smooth & [6; 9; 31; 33; 38; 39] \\  & Granular rough & [34; 40; 41] \\ \hline \multirow{3}{*}{Color} & Black & [32; 42] \\  & Blackish gray & [43; 44] \\ \cline{1-1}  & Brown with green, red, or black tint & [45] \\ \hline \end{tabular}
\end{table} 
Table 2: Visual description of copper smelting slag. Figure 2: Copper slag was generated worldwide between 1999 and 2019 (adapted from [20]). In Chile there are seven copper smelters, five of them are state-owned and the other two are private.