## 3 Discussion  
Copper slag is a massive by-product of the mining industry, generated in large volumes annually worldwide. Traditionally considered a waste material, its composition rich in metal oxides makes it attractive for advanced applications, such as its use as a cathodic electrode to promote hydrogen evolution reactions (HERs) during the thermochemical cathodic subprocess. Its implementation as a cathode material for H\({}_{2}\) production represents a disruptive innovation for several reasons: (i) instead of being discarded or underutilized, copper slag can be treated and transformed into functional electrodes, extending its life cycle and reducing the need to extract new resources--thus closing the material use loop, a core principle of the circular economy; (ii) the presence of iron oxides and iron silicate (Fe\({}_{2}\)SiO\({}_{4}\)) gives the slag a porous structure and sufficient conductivity to act as a redox-supporting conductor in systems for water treatment, due to its tendency to produce H\({}_{2}\)O\({}_{2}\), or in alkaline electrolysis for HERs; (iii) reusing copper slag as an electrode prevents its disposal in legal or illegal landfills, reducing the occupation of large volumes of public land. Moreover, by replacing exotic materials like Ti, Pt, or Ni, it reduces reliance on costly and uncommon metals, and lowers the carbon footprint and energy consumption in the fabrication of electrochemical devices; (iv) manufacturing electrodes from copper slag allows a significant reduction in costs, which is crucial in efforts to democratize technologies such as wastewater treatment or distributed hydrogen generation [2; 3; 4; 5]. Figure 3: Location, capacity, and production of the Chilean smelters. The elemental characterization of copper slag in Chile has been extensively reported by [PERSON] et al. [21; 24], and the researchers indicate that modern copper slags possess high contents of SiO\({}_{2}\) (10-71%) and FeO (0.7-62%) and the predominant chemical elements correspond to Cu, Pb, Ni, and Zn, with its mineralogical phases being mainly Fe\({}_{2}\)SiO\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)[24; 47; 48]. The variation in the elemental and mineralogical chemical composition of copper slag is mainly influenced by the type of (i) concentrate, (ii) type of fluxes, (iii) type of furnace, and (iv) cooling rates during and after the smelting and converting processes [25]. Table 3 shows the different compositions of copper slag provided through the scientific literature, in which the high contents of Fe, Fe\({}_{2}\)O\({}_{3}\), FeO, and SiO\({}_{2}\) stand out. However, the extensive scientific literature indicates that compounds containing Fe, FeO, Fe\({}_{2}\)O\({}_{3}\), and Fe\({}_{3}\)O\({}_{4}\) react with water steam at high temperatures to generate H\({}_{2}\) gas [49]. It is reported that at 900 \({}^{\circ}\)C, 1.43 mol H\({}_{2}\)/kg was produced from high-temperature steam steel slag [50; 51]. Nevertheless, the mineralogical study in Chile of the copper slag indicates that it is composed of 50% Fe\({}_{2}\)SiO\({}_{4}\), 39% Fe\({}_{3}\)O\({}_{4}\), and 10% (Mg\({}_{2}\)Fe)\({}_{2}\)SiO\({}_{4}\)[7]. There is then the possibility that copper slag deposited in Chile reacts with water steam to generate H\({}_{2}\) because its main compounds contain iron oxides. The XRD analysis of the copper slag from the Chilean smelter indicates that it is composed of, Fe\({}_{2}\)SiO\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\), KAl\({}_{2}\)[AlSi\({}_{3}\)O\({}_{10}\)](OH)\({}_{2}\), Cu\({}_{6}\)Si\({}_{2}\)S\({}_{7}\), (Mg\({}_{2}\)Fe)\({}_{2}\)SiO\({}_{4}\), and Ca\({}_{2}\)Mg\({}_{5}\)Si\({}_{8}\)O\({}_{22}\)(OH)\({}_{2}\), show-up in the Figure 4a. Apart from the X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) examination shown in Figure 4, the findings in Figure 5 provide additional support for the chemical and mineralogical description of copper slag from Chile. Fe, Si, and O are the most common components in the sample, as shown by the SEM-EDS image.