## 3 Discussion
The principal minerals and oxides present in the copper slag, including those that represent 1% of the chemical composition, have melting temperatures (_Tm_) very different from each other, which are detailed below: Fe\({}_{2}\)SiO\({}_{4}\) (_Tm_ = 1216.9 \({}^{\circ}\)C) \(<\) Cu\({}_{2}\)O (_Tm_ = 1243.6 \({}^{\circ}\)C) \(<\) FeO (_Tm_ = 1376.9 \({}^{\circ}\)C) \(<\) Fe\({}_{3}\)O\({}_{4}\) (_Tm_ = 1596.9 \({}^{\circ}\)C) \(<\) SiO\({}_{2}\) (_Tm_ = 1722.9 \({}^{\circ}\)C) \(<\) MnO (_Tm_ = 1841.9 \({}^{\circ}\)C) \(<\) ZnO (_Tm_ = 1701.9 \({}^{\circ}\)C) \(<\) Al\({}_{2}\)O\({}_{3}\) (_Tm_ = 2053.9 \({}^{\circ}\)C) \(<\) MgO (_Tm_ = 2831.9 \({}^{\circ}\)C) \(<\) CaO (_Tm_ = 2926.9 \({}^{\circ}\)C) [67]. These temperatures allow us to see that metal oxides could volatilize when exposed to high temperatures and in contact with water steam. [PERSON] et al. [68] indicate that the interaction of high-temperature water steam with oxides to form volatile hydroxides leads to material loss. The following reactions are according to the supposed reaction that could occur during the thermolysis process. The thermal decomposition of Fe\({}_{2}\)SiO\({}_{4}\) or 2 FeO-SiO\({}_{2}\) mainly involves its transformation into Fe\({}_{2}\)O\({}_{3}\), Fe\({}_{3}\)O\({}_{4}\), and amorphous silica (SiO\({}_{2}\)) [3; 64; 69]. The oxidation of the slag includes the decomposition of Fe\({}_{2}\)SiO\({}_{4}\), and the adsorption and diffusion of superoxide ions (O\({}^{2-}\)) and other species, promoting the formation of FeO, Fe\({}_{2}\)O\({}_{3}\), Fe\({}_{3}\)O\({}_{4}\), and SiO\({}_{2}\)[64]. [PERSON] et al. [70] investigated the oxidation behavior of Fe\({}_{2}\)SiO\({}_{4}\) in water steam at 1000 \({}^{\circ}\)C, reporting that Fe\({}_{2}\)SiO\({}_{4}\) was completely decomposed into Fe\({}_{3}\)O\({}_{4}\) (58.8 wt%), Fe\({}_{2}\)O\({}_{3}\) (21.6 wt%), and SiO\({}_{2}\) (19.6 wt%) (see Equation (5)) and obtained a hydrogen production capacity of 24.42 mL/g. Equation (5) is as follows:  
\[3\text{Fe}_{2}\text{SiO}_{4}+2\text{H}_{2}\text{O}\ \to 2\text{Fe}_{3} \text{O}_{4}+3\text{SiO}_{2}+2\text{H}_{2} \tag{5}\]  
The use of magnetite has been widely investigated, but the production of H\({}_{2}\) from the use of Fe\({}_{3}\)O\({}_{4}\) was proposed by [PERSON] et al. in 1977 [69; 71; 72; 73; 74; 75; 76] according to Equation (6), which is as follows:  
\[2\text{Fe}_{3}\text{O}_{4}+\ \text{H}_{2}\text{O}\ \to 3\text{Fe}_{2} \text{O}_{3}+\ \text{H}_{2} \tag{6}\]  
However, other researchers indicate that when Fe\({}_{3}\)O\({}_{4}\) is in contact with water steam it can produce volatile metal oxides (Equation (7)) [77], as can be seen in the following equation:  
\[\text{Fe}_{3}\text{O}_{4}+2\text{H}_{2}\text{O}\ \to\text{Fe}(\text{OH})_{2}+2 \text{FeO}(\text{OH}) \tag{7}\]  
(Mg\({}_{1-x}\)Fe\({}_{x}\))\({}_{2}\)SiO\({}_{4}\)[78], which is also written as 2(Mg\({}_{1-x}\)Fe\({}_{x}\))O-SiO\({}_{2}\), is a mineral formed between two metals (Fe and Mg) and SiO\({}_{2}\) and is found in a solid solution between Fe\({}_{2}\)SiO\({}_{4}\) and Mg\({}_{2}\)SiO\({}_{4}\)[79]. [PERSON] et al. [80] proposed that, if H\({}_{2}\)O is incorporated into the (Mg\({}_{1-x}\)Fe\({}_{x}\))\({}_{2}\)SiO\({}_{4}\) structure, its presence can create extrinsic point defects that are associated with H\({}_{2}\) production due to the formation of OH ions. But the structure of (Mg\({}_{1-x}\)Fe\({}_{x}\))\({}_{2}\)SiO\({}_{4}\) has mainly been investigated for the influence it has on the production of H\({}_{2}\) during a serpentinization reaction which is accompanied by volume expansion (serpentine (Mg\({}_{3}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)) possibly results from the reaction of water with ferrous minerals rich in Fe that are contained in ultramafic rocks). The generation of H\({}_{2}\) during the serpentinization mechanism is a result of the reaction of water with ferrous iron (Fe\({}^{2+}\)) derived from minerals, mainly olivine and pyroxene, to ferric iron (Fe\({}^{3+}\)), which normally precipitates as magnetite.