# Optimization of Electric Field Assisted Mining Process Applied to Rare Earths in Soils
## 3 Results and Discussion
### Real Soil Electromining  
From the six proposed optimization problems, the P\({}_{\text{Y3}}\) experimental configuration, corresponding to an electric field of 0.5386 V cm\({}^{-1}\) and electrolyte concentration of 0.1 mol L\({}^{-1}\), was chosen to be reproduced in a real soil EM. In this experiment the Y\({}^{3+}\) migration profiles, the pH profiles of the medium, and electromining efficiency were analyzed.  
Figure 7 shows the Y\({}^{3+}\) concentration profile in the reactor CC during the experiment, in which it was observed that the Y\({}^{3+}\) migration in the EM-RS was slower than the EMs performed in synthetic soils (Figure 3). This behavior can be explained by some peculiarities of the real soil, such as a higher composition heterogeneity, the presence of organic matter, and the higher values of soil resistivity and pH (Table 1). This set of characteristics contributes to the inhibition of Y\({}^{3+}\) desorption and the migration of the species through the porous medium. However, the EM-RS can be considered satisfactory, as an advance in the Y\({}^{3+}\) migrational front through time towards the CC was observed, resulting in an electromining efficiency of 41.48% for the species.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Optimization Problem**} & **Electric Current Density Limit (mA cm\({}^{-2}\))** & **Coded Electric Field** & **Electric Field** & **Coded Electrolyte** & **Electrolyte** & **Total Mass of Y\({}^{3+}\) Extracted Concentration (mol L\({}^{-1}\))** & **Y\({}^{3+}\) Extracted (mg)** \\ \hline P\({}_{\text{Y1}}\) & 0.0500 & \(-\)0.3299 & 0.3036 & \(-\)1.0000 & 0.1000 & 11.2135 \\ P\({}_{\text{Y2}}\) & 0.0750 & 0.3164 & 0.4211 & \(-\)1.0000 & 0.1000 & 13.9113 \\ P\({}_{\text{Y3}}\) & 0.1000 & 0.9628 & 0.5386 & \(-\)1.0000 & 0.1000 & 16.6090 \\ P\({}_{\text{Y4}}\) & 0.1300 & 1.0000 & 0.5454 & \(-\)1.0000 & 0.1000 & 16.7644 \\ P\({}_{\text{Y5}}\) & 0.1600 & 1.0000 & 0.5454 & \(-\)1.0000 & 0.1000 & 16.7644 \\ P\({}_{\text{Y6}}\) & Unlimited & 1.0000 & 0.5454 & \(-\)1.0000 & 0.1000 & 16.7644 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Optimal configurations considering different limits for electric current density.  
Figure 6: Extracted mass function considering the coded design variables containing the representations of the limit electric current density isolines.  
As the EM-RS was conducted by the potentiostatic method, the electric current density presented a stable behavior with few oscillations and an average current density of 0.059 \(\pm\) 0.004 mA cm\({}^{-2}\). Although this test was performed using experimental conditions similar to EM-03, the average electric current density was lower than the majority of the EMs (Table 3). This result is considered satisfactory, as it indicates that the parallel reactions on the surface of the electrodes did not develop significantly. Thus, the electric filed is more available to the Y\({}^{3+}\) migration. On the other hand, due to higher complexity matrix, the migration of species was slower than in the other EMs, resulting in a time approximately three times longer. However, in the EM-RS, the electrodes degradation was not observed, nor the adsorption of species in their surfaces.