## 3 Discussion
Another study focused on the production of syngas from H\({}_{2}\)O and CO\({}_{2}\) in a two-step cycle through Zn/ZnO redox reactions [92]. And recently, [PERSON] [93] has proposed a hybrid solar thermochemical cycle of ZnSO\({}_{4}\)/ZnO for water splitting (WS), with an oxidation step through the reaction ZnO + SO\({}_{2}\) + H\({}_{2}\)O \(\rightarrow\) ZnSO\({}_{4}\) + H\({}_{2}\) at a temperature of 127 \({}^{\circ}\)C. [PERSON] et al. [68] indicate that alumina volatilizes in environments containing water steam at high temperatures through the reaction 1/2 Al\({}_{2}\)O\({}_{3}\) + 3/2 O\({}_{2}\) \(\rightarrow\) Al(OH)\({}_{3}\), but they mention that the volatility of alumina due to the formation of Al(OH)\({}_{3}\) is not a problem at temperatures! 1300 \({}^{\circ}\)C. [PERSON] et al. [94] indicate that Mg is a good candidate for H\({}_{2}\) generation due to its very low cost, high availability, high theoretical H\({}_{2}\) yield, and the formation of environmentally friendly by-products. The reaction between Mg and H\({}_{2}\)O can be described as Mg + 2H\({}_{2}\)O \(\rightarrow\) Mg(OH)\({}_{2}\) + H\({}_{2}\), and this reaction is interrupted due to the formation of a passive layer of Mg(OH)\({}_{2}\) on the unreacted Mg particles. [PERSON] et al. [95] investigated the reaction rate of Ca oxidation with water steam in the temperature range of 25 and 300 \({}^{\circ}\)C, obtaining a reaction rate of linear type (at 25 \({}^{\circ}\)C) and logarithmic (at 300 \({}^{\circ}\)C). In the temperature range of 20-150 \({}^{\circ}\)C, Ca reacts according to Ca + 2H\({}_{2}\)O \(\rightarrow\) Ca(OH)\({}_{2}\) + H\({}_{2}\), and at temperatures above 250 \({}^{\circ}\)C it reacts according to the mechanism Ca + H\({}_{2}\)O \(\rightarrow\) CaO + H\({}_{2}\). [PERSON] [96] thermodynamically analyzed the CaO/Ca-based WS cycle (Ca-WS) usingMetso's chemical reaction and equilibrium software (HSC Chemistry 9.9). Furthermore, they indicate that during the reoxidation of Ca, the result was the formation of H\({}_{2}\). The oxidation reaction of metal oxides (see Equation (9)), present in copper slag (2 FeO-SiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), MgO, CaO, MnO, ZnO, K\({}_{2}\)O, TiO\({}_{2}\), etc.) [24], can be applied, with the equation as follows:  
\[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]1  
There is abundant scientific information explaining the interaction between water vapor (H\({}_{2}\)O(g)) and Fe under high temperature conditions (200 to 1000 \({}^{\circ}\)C), where it is specified that H\({}_{2}\) is produced by the oxidation reaction of Fe and FeO according to Equations (10) and (11) [71; 72; 97], which are as follows:  
\[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]2  
\[\Delta G^{\circ}=-RT\ln(K) \tag{1}\]3  
On the other hand, the dissociation reaction of water in contact with FeO is favorable at temperatures below 800 \({}^{\circ}\)C, when \(\mathrm{\Delta}G^{\circ}<\) 0 (the reaction is spontaneous and the theoretical conversion of FeO decreases with increasing temperature) [74; 98]. Our expectations are that oxides such as Fe\({}_{2}\)SiO\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\), and (Mg, Fe)\({}_{2}\)SiO\({}_{4}\) will react with water steam at high temperatures (in a range of 200-1000 \({}^{\circ}\)C and 1 atm), generating products such as Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), Mg\({}_{2}\)SiO\({}_{4}\), H\({}_{2}\), and possible volatile metal oxides (M\({}_{x}\)O\({}_{y}\)(OH)\({}_{2}\)). The circular economy in mining seeks the recovery, vaporization, and reuse of mining waste, transforming it into inputs for new applications instead of disposing of it. According to the above, the use of copper slag in the production of H\({}_{2}\) through thermolysis would represent an important opportunity to transform a waste product that currently plagues the world, especially in desert areas such as the Atacama Desert where mining activity is prevalent, into a high-value-added material, prioritizing its reuse as an alternative application in solar energy.