## 3 Results and Discussion
### Crystalline Phases  
In Table 4, the crystalline phases found in the reduced and oxidized residues of OCs and possible reactions are reported. The reported crystalline phases are not exclusive, so the existence of others not reported by the equipment is probable, since the software used did not identify some of the peaks of the diffractograms. In addition, when analyzing this type of sample, where different compounds exist, it is possible that the peaks overlap, thus preventing identification of any of them. In the literature, there are no chromite reduction oxide reactions described. However, the crystalline phases identified in the oxidized residue of CRSI003 indicate that the active phase corresponds to iron oxide. On the other hand, no chromium oxides were present in the oxidized state, since Cr(III) is not oxidized to Cr(VI) at temperatures lower than 1200 \({}^{\circ}\)C. However, this reaction can be favored at temperatures between 600 and 900 \({}^{\circ}\)C due to the presence of alkalis (Na, K) and alkaline earth (Ca, Mg) metals [56]. In the oxidized residue of FEMA004, only Fe\({}_{2}\)O\({}_{3}\) was identified, corresponding to the Oxidation Reaction 7. However, the oxidation of ilmenite (FeTiO\({}_{3}\)) to pseudobrookite (Fe\({}_{2}\)TiO\({}_{5}\)) according to Reaction 5 was not identified. While analyzing the diffractogram of this pure compound (Fe\({}_{2}\)TiO\({}_{5}\)), it can be observed that it comes close to the unidentified peaks in the diffractogram of oxidized FEMA004 residues. For FEMA011 residues evaluated with H\({}_{2}\) gas and CH\({}_{4}\), the oxidation reactions 6, 7, and 8 were theoretically expected and, according to XRD, the hematite and magnetite analyses were the only two crystalline phases identified in the residues evaluated with the two reducing gases. The phases identified in the oxidized state of ILME007 indicate that reactions 4 and 5 occurred, however, no wustite (FeO) or magnetite (Fe\({}_{3}\)O\({}_{4}\)) from the reduction of hematite (Fe\({}_{2}\)O\({}_{3}\)) was found as indicated by reactions 2 and 6. The diffractogram of the reduced sample could not be fully identified. Based on the chemical balance calculations provided by the HSC Chemistry software 6.1, it follows that the unidentified phases may be Fe\({}_{3}\)O\({}_{4}\), TiO\({}_{2}\)(A), Fe\({}_{2}\)TiO\({}_{5}\), and/or FeO. The material OXMN009 is constituted by more than 90% Mn\({}_{3}\)O\({}_{4}\), it is therefore expected that the crystalline phases would be present in reactions 9 and 10; nevertheless, Mn\({}_{3}\)O\({}_{4}\) and Mn\({}_{2}\)O\({}_{3}\) can be identified in the oxidized state residues. According to the literature, the latter compound is not stable at temperatures higher than 900 \({}^{\circ}\)C [39]. In spite of the above, according to the chemical balance calculations provided by HSC Chemistry software 6.1, it is possible to find Mn\({}_{2}\)O\({}_{3}\) in the operating conditions used, while taking into account that Mn\({}_{3}\)O\({}_{4}\) is an intermediate phase in the reduction/oxidation reactions between Mn\({}_{2}\)O\({}_{3}\) and MnO. Consequentially, the presence of Mn\({}_{2}\)O\({}_{3}\) suggests that reaction 10 was carried out until the formation of Mn\({}_{2}\)O\({}_{3}\). Figure 2: Crushing strength. Finally, the fresh OXMN010A material is composed mainly of rhodonite (MnSiO\({}_{3}\)) and a small percentage of wustite (FeO). This silicon and manganese oxide (MnSiO\({}_{3}\)) has been studied in CLC technology as an OC with oxygen decoupling (CLOU) [41; 42], and reactions 11 and 12 are expected according to the literature. In the residues of the material OXMN010A evaluated with methane gas, no braunite was present (Mn\(\gamma\)SiO\({}_{12}\)), but the oxidized crystalline phase of rhodonite and tephroite (Mn\({}_{2}\)SiO\({}_{4}\)) was detected.