The Use of Copper Slag in the Thermolysis Process for Solar Hydrogen Production--A Novel Alternative for the Circular Economy  
[PERSON]  
1 Centro de Desarrollo Energitecnto Antofagasta, Universidad de Antofagasta, Antofagasta 1240000, Chile; [EMAIL_ADDRESS] (S.L.-G.); [EMAIL_ADDRESS] (F.S.); [EMAIL_ADDRESS] (E.F.)  
2 Departamento de Ingenieria Mecanica, Universidad de Tarapaca, Arica 1100000, Chile; asagade  
also been used as a fine aggregate in Portland cement concrete. [PERSON] et al. [2], indicate that the most common management for copper slag is recycling to produce value-added products, such as abrasive tools, and granule cutting tools for roofing, tiles, and road base construction [2; 3; 4; 5]. Research related to copper slag is aimed at finding reuse applications for it as a construction material [3], using high-temperature, direct reduction in slag as a means of waste management [6], as a material for the manufacture of high-strength concrete [7], as a potential catalyst (iron present in the slag) for the depletion of phenol present in water during hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) production [8], in the recovery of valuable metals from slag [9; 10; 11], in the recovery of iron from copper slag by direct reduction based on coal [1], as a bifunctional photocatalyst for the degradation of alcohols and hydrogen production [12; 13], in thermal energy storage systems [14; 15], and in the recovery of copper and magnetite from copper slag using concentrated solar power (CSP) [16].  
Among the research carried out with copper slag, its main use has focused on the recovery of valuable metals such as Co and Cu. Currently, solar thermochemical cycles are based on the use of materials from rare earths such as cerium oxide (CeO\({}_{2}\)), which is insufficient and expensive [17; 18]. However, there is no evidence in the scientific literature of the direct use of copper slag as a thermo-catalytic material for the generation of gaseous hydrogen through a solar thermochemical process. In addition to the above, the abundance of copper slag in the Atacama Desert, mainly that deposited in abandoned landfills, generates one of the largest environmental liabilities related to the processes of obtaining copper [19], which to date has no solution [20].  
The smelting of copper concentrate (20-30% Cu) composed of bornite (Cu\({}_{5}\)FeS\({}_{4}\)), chalcopyrite (CuFeS\({}_{4}\)), pyrite (FeS\({}_{2}\)), covellite (CuS) and chalcocite (Cu\({}_{2}\)S) [21; 22], enters the smelting furnace at temperatures of 1200-1300 \({}^{\circ}\)C, releasing gases such as O\({}_{2}\), SO\({}_{2}\), N\({}_{2}\), CO, CO\({}_{2}\), and H\({}_{2}\) due to the compositions of the fluxes used such as natural gas, thermal coal, and metallurgical coke [23], with the resultant (Matte) containing a Cu content between 35 and 75%. The copper slag generated normally contains FeO, Fe\({}_{3}\)O\({}_{4}\), SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), MgO, CaO, and Cu\({}_{2}\)O [24]. [PERSON] et al. have investigated in-depth the presence of TiO\({}_{2}\), MnO, As, Pb, and other chemical elements [25], which makes it a complex multi-component residual material. However, it is mainly composed of Fe (40 wt%), Si (15-37 wt%), and Cu (0.8-13 wt%) [21].  
This communication highlights the importance of copper slag in the thermolysis process for H\({}_{2}\) production, examining its role in the circular economy and the potential benefits of treating slag waste through thermochemical methods. By integrating copper slag into solar-driven thermolysis, it is possible to add economic value to existing applications--improving energy efficiency, enabling hydrogen generation, and recovering valuable metals such as Fe\({}_{3}\)O\({}_{4}\) y Fe\({}_{2}\)O\({}_{3}\). This approach helps reduce the environmental impact of Cu production by transforming industrial waste into a useful raw material.