## 2 Materials and Methods  
### Experimental Development--Laboratory Tests  
Table 2 shows the chemical analysis of the synthetic copper PLS used in the assays, which was carried out at room temperature (25 \({}^{\circ}\)C). The chemical composition of the PLS solutions used for all the tests was determined using inductively coupled plasma mass spectrometry (ICP\(-\)MS). The molybdenum adsorbed at the resin's surface was determined using the difference in molybdenum concentrations from the initial and final solutions. Figure 2: (**a**) Molybdenum Pourbaix diagram. (**b**) Molybdenum speciation [40; 41; 42]. Since the chloride concentration in the copper PLS solution changes substantially during the industrial operation, it is necessary to study its influence on the molybdenum adsorption results, and to a lesser degree, the process's performance may be hindered by the presence of ferrous ions and zinc. For this reason, the effect of three different concentrations of chloride ions in synthetic solution was also investigated: 1, 5, and 10 g/L. Since copper is the foundation of Chile's mining industry, it is critical to underline that the fundamental condition for completing this project is that the composition of the copper\(-\)bearing solution cannot be changed. For this reason, the ion exchange method was the best choice. The experimental development of this work comprised two stages: static\(-\) and continuous\(-\)type tests. Figure 3 illustrates the static tests [43]:  
The adsorption properties of the anion\(-\)exchange resins available for use were determined using static ion\(-\)exchange tests. Two ion exchange resins were available for the experiments: DOWEX M\(-\)43 (Delft, The Netherlands), a strongly basic resin, and LEWAITI MP 62 WS (Cologne, Germany), a weakly basic resin. Both resins are quaternary amine based. The goal of these tests was to identify the key adsorption parameters for each resin using the copper leaching solution from Table 2 based on the Langmuir theory. The resins were pre\(-\)conditioned with a 10% hydrochloric acid solution prior to the ion exchange tests to boost the resin affinity for the molybdenum anion of interest. These tests were made using a constant volume of 6000 cm\({}^{3}\) copper leaching solution and 5 different quantities of the resins under study, 0.1, 0.25, 0.5, 0.75, and 1.0 g, for 24 h and at room temperature. The molybdenum recovery process was as follows [44]:  
1. Resin preconditioning: Both resins need a HCl preconditioning to increase their affinity for molybdenum anions: \[R^{+}(OH)^{-}+H^{+}Cl^{-}\to R^{+}Cl^{-}+H^{+}OH^{-}\] (1)
2. Molybdenum recovery from the copper PLS: Molybdenum can be recovered and exchanged during the elution stage. According to the Figure 2b, molybdenum, at pH values 1.0-2.5, was present mostly as Mo\({}_{7}\)O\({}_{21}\)(OH)\({}_{3}\)\({}^{3-}\). Equation (2) illustrates the ion exchange mechanism: \[3R^{+}Cl^{-}+Mo_{7}O_{21}(OH)_{3}^{3-}\to R_{3}^{+}Mo_{7}O_{21}(OH)_{3}^{3-}+3 Cl ^{-}\] (2) \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Cu (g/L)** & **Mo (mg/L)** & **Fe (g/L)** & **Zn (g/L)** & **CI (g/L)** & **SO\({}_{4}\)\({}^{2-}\) (g/L)** & **pH** & **Eh** \\ \hline
2.5 & 50 & 5 & 0.5 & 1.0\(-\)5.0\(-\)10.0 & 91.8 & 1.0\(-\)1.5 & 0.3\(-\)0.5 \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of the copper leaching solution. Figure 3: Static ion exchange tests. 3. Molybdenum elution: According to the Pourbaix diagram from Figure 2a, molybdenum can be effectively recovered from the resin phase using a NaOH solution, because in these conditions, the prevalent species are the anions. Equation (3) shows the ion exchange: \[R_{3}^{+}M\omega_{7}O_{21}(OH)_{3}^{3-}+OH^{-}\to 3R^{+}OH^{-}+M\omega_{7}O_{21}(OH)_{3}^{3-}\] (3) For pH values higher than 7, molybdenum will be present as \(\text{MoO}_{4}^{-}\). 4. Resin regeneration: To carry out the process continuously, the resin conditioning must start again, as described in Equation (1).