## 3 Discussion
While H\({}_{2}\) production with copper slag is aligned with the principles of the circular economy (see Figure 6), it is essential to assess its environmental impact. The H\({}_{2}\) product of this process can be \"reused\" as fuel in mining processes, reducing the use of fossil fuels (for example, diesel which is widely used in mining trucks). In addition, by-products such as Fe\({}_{3}\)O\({}_{4}\) y Fe\({}_{2}\)O\({}_{3}\) can be \"recycled\" for the recovery of Fe, an element in high demand in Cu smelting. \[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]4 Scheme of the circular economy model applied to copper slag. The release of heavy metals (Pb, As, and Cd) through volatile metal oxides is unlikely because the thermolysis process must be carried out at temperatures between 200 and 1000 \({}^{\circ}\)C, and the majority compound of Chilean copper slag is Fe\({}_{2}\)SiO\({}_{4}\) (melting temperature is 1217 \({}^{\circ}\)C) and Fe\({}_{3}\)O\({}_{4}\) (melting temperature is 1597 \({}^{\circ}\)C) [67; 99]. To ensure a complete circular economy cycle, the waste generated after thermolysis of copper slag (magnetic and hematite) must be analyzed for possible reuse or Fe recovery, rather than being disposed of, thus maximizing material efficiency. The presence of toxic elements such as As, Cd, and Pb in copper slags depends primarily on the mineralogical composition of the copper concentrate, as well as on the conditions of the smelting process. Complex minerals such as enargite (Cu\({}_{3}\)AsS\({}_{4}\)) [100; 101; 102; 103], tetrahedrite-tennantite ((Cu,Fe)\({}_{12}\)(Sb,As)\({}_{4}\)S\({}_{13}\)) [104; 105; 102], galena (PbS) [104], and sphalerite (ZnS) [106] containing traces of Cd are the main sources of toxic elements in copper slag. This contamination is the result of flotation processes applied to sulfide ores without strict control over the hydrometallurgical stages. Such operational shortcomings can cause metallic impurities to be carried over into the copper concentrate used in pyrometallurgical processing. In addition, smelters often process blended concentrates from different mining origins, which increases the presence of difficult-to-manage impurities, such as Pb in the form of PbS. During smelting, factors such as temperature, redox atmosphere, and the type of fluxes used influence the final composition of copper slag. Moreover, the recycling of metallurgical dusts or secondary residues can enrich the slag with trace amounts of heavy metals. Therefore, process control and proper stabilization of the slag are essential to minimize the risk of copper slag contamination with toxic materials. At high temperatures (thermolysis process), toxic compounds may pass into the gas phase if not adequately controlled. Currently, copper smelters implement a combination of physical, chemical, and environmental strategies to eliminate or control contaminants such as As and Cd. It is worth noting that Pb is not commonly found as a by-product in copper mining. The removal or control of contaminants in copper smelting is achieved through a combination of physical, chemical, and operational approaches, in which the toxic elements are stabilized as oxides, sulfides, or silicates within glassy or crystalline matrices, reducing their mobility. Simultaneously, at high temperatures during smelting, some of these elements volatilize and are captured by gas treatment systems, producing metallic dusts that require specialized handling. In addition, hydrometallurgical processes are employed to recover or stabilize impurities, such as the removal of As in the form of insoluble compounds like scorodite (FeAsO\({}_{4}\cdot\)H\({}_{2}\)O) [105] or calcium arsenate (Ca\({}_{3}\)(AsO\({}_{4}\))\({}_{2}\)) [107]. The final disposal of slags and dusts typically involves their stabilization and confinement in controlled storage facilities, with physicochemical encapsulation to prevent leaching.