## 3 Discussion
At a preventive level, many smelters also apply source minimization strategies, ensuring the quality control of the concentrate and limiting the input of minerals with high levels of penalized elements. A major challenge for researchers using copper slag for H\({}_{2}\) production is the potential release of impure gases, such as SO\({}_{2}\), SO\({}_{3}\), and volatile metal species (Pb, As, etc. ), during high-temperature reactions. These gases can reduce the purity of the H\({}_{2}\), requiring post-treatment processes before its use in fuel cells or specific industrial applications. In general, the reduction of Fe\({}_{2}\)O\({}_{3}\) does not lead directly to metallic Fe. If the reduction temperature is below 570 \({}^{\circ}\)C, the reduction to Fe occurs stepwise from Fe\({}_{2}\)O\({}_{3}\) to Fe\({}_{3}\)O\({}_{4}\), and continues to Fe. The intermediate oxide, like Fe\({}_{(1-x)}\)O, is not stable at temperatures below 570 \({}^{\circ}\)C.  
At reduction temperatures above 570 \({}^{\circ}\)C, wustite must also be considered in the reduction process. In this case, the reduction proceeds from Fe\({}_{2}\)O\({}_{3}\) through Fe\({}_{3}\)O\({}_{4}\) to Fe\({}_{(1-x)}\)O and then continues to Fe. The following equations show the reduction process at different temperatures [108]:\[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]5 \[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]6 (13) \[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]7 (14) \[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]8  
The presence of sulfur-containing phases such as Cu\({}_{2}\)S and FeS\({}_{2}\) would lead to SO\({}_{2}\) emissions, while carbon residues from smelting processes can contribute to CO\({}_{2}\) generation. To mitigate these effects, pretreatment methods or gas scrubbing technologies can be employed to remove impurities after thermolysis. In addition, post-process gas separation techniques, such as selective membranes or pressure swing adsorption (PSA), need to be explored to ensure the high purity of the hydrogen produced. The efficiency of copper slag in producing H\({}_{2}\) by thermolysis depends on temperature, pressure, and its chemical composition. Studies on iron-based thermochemical cycles have shown that FeO can react with H\({}_{2}\)O at temperatures below 1000 \({}^{\circ}\)C to generate H\({}_{2}\). However, Fe\({}_{2}\)SiO\({}_{4}\) and other silicates in copper slag can reduce the reaction efficiency by stabilizing iron oxides in less reactive forms. To improve the reactivity of copper slag, strategies such as thermal pre-activation (reduction in H\({}_{2}\) or CO atmosphere), the addition of catalytic promoters (Ni, Co), or surface modifications can be explored. To justify the viability of H\({}_{2}\) production with copper slag, a comparative technical-economic analysis is necessary, evaluating energy efficiency, process scalability, and market demand. In the future, hybrid approaches could be explored, where copper slag is processed in conjunction with other industrial waste to improve overall resource recovery efficiency. However, a substantial advantage is that the copper slag thermolysis process for H\({}_{2}\) production can be integrated with renewable energy, taking advantage of the high solar potential in desert regions such as the Atacama Desert, for example using concentrated solar power (CSP) such as the CSP plant called Cerro Dominator in Chile for high-temperature slag oxidation, medium-temperature concentrated solar power (using parabolic troughs) for steam generation, and photovoltaic energy for electricity consumption. This allows for the design of a completely sustainable system where the slag acts as a reactive material for the production of H\({}_{2}\) without the need for fossil fuels. This approach allows for closing the resource loop in the mining and energy industries by reusing waste and avoiding dependence on fossil fuels, thereby reducing the carbon footprint of the H\({}_{2}\) production process.