## 4 Conclusions  
Normally, the spent LAB electrolytes can be directly filtered and reused on the condition that the electrolytes were collected and handled properly in the dissembling plant. For dissembling plants without appropriate collecting setups, electrolytes are often polluted with foreign ions, since electrolytes are often discharged onto the ground directly. These polluted electrolytes cannot be purified by simple filtration, and thus require more reasonable recycling techniques. Instead, it is more reasonable and profitable to recycle and stabilize SO\({}_{4}\)2\({}^{-}\) in the form of CaSO\({}_{4}\)2H\({}_{2}\)O, which can be widely used as a fertilizer and for building materials. In this study, the polluted electrolyte sample was recycled with chemical precipitation and carbonation techniques. To understand the whole recycling process, thermodynamic simulations in the precipitation and carbonation processes were conducted. The thermodynamic calculations of saturated Ca(OH)\({}_{2}\), CaCO\({}_{3}\), and CaSO\({}_{4}\) solutions helped us to understand the whole process, since Ca(OH)\({}_{2}\) and CaSO\({}_{4}\) appear in both precipitation and carbonation processes when the pH is higher than 13. Additionally, CaCO\({}_{3}\) appears in the carbonation process with and without filtration. The theoretical limit for the removal ratio of SO\({}_{4}\)2\({}^{-}\) was predicted to be 99.15% with 1.53 M SO\({}_{4}\)2\({}^{-}\) and 1.55 M Ca(OH)\({}_{2}\). Also, by comparing the carbonation process with and without pre-filtration of the precipitates obtained from the precipitation process, the theoretical removal efficiency of SO\({}_{4}\)2\({}^{-}\) was calculated to be 99.15% and 69.61%, respectively. With the increase of CO\({}_{2}\) concentrations in the solution, the removal efficiency of SO\({}_{4}\)2\({}^{-}\) further decreased. Furthermore, the chemical precipitation and carbonation of lead in the spent electrolyte sample were calculated; no precipitation existed in the precipitation process due to the appearance of SO\({}_{4}\)2\({}^{-}\), while 96.35% of lead was precipitated as PbCO\({}_{3}\)(s) in the carbonation process, which stabilized lead with CaCO\({}_{3}\). The thermodynamic calculations helped us to design and understanding the whole removal process quantitatively. Based on the thermodynamic calculation results, the optimized removal process of  
Figure 8: Calculated lead related species distributions of carbonation process with filtration at different pH: (**a**) \(\text{C}_{\text{CO}_{2}}\) = 0.1 M with log concentration vs. pH; and (**b**) \(\text{C}_{\text{CO}_{2}}\) = 0.1 M with fraction of lead vs. pH. SO4\({}^{2-}\) should be precipitation without contact with ambient CO\({}_{2}\), since PbCO\({}_{3}\) might precipitate around pH 9, which would lead to the existence of lead in CaSO4\({}^{\cdot}\)2H\({}_{2}\)O. Additionally, filtration should be undertaken before the carbonation process to avoid the re-dissolution of CaSO4\({}^{\cdot}\)2H\({}_{2}\)O. After filtration, the final product of the recycling process (CaSO4\({}^{\cdot}\)2H\({}_{2}\)O) is obtained. In the following carbonation process, the flow rate of CO\({}_{2}\) and pH should be controlled properly to stabilize lead and extra calcium in the form of PbCO\({}_{3}\) and CaCO\({}_{3}\), respectively. At last, after chemical precipitation and carbonation, there is still around 13 mM SO4\({}^{2-}\) in the solution, which needs to be removed by adsorption or ion-exchange techniques. The overall theoretical chemical precipitation limit for removal of SO4\({}^{2-}\) was 99.15%. The limitation of the simulation was: (1) in the simulation process of carbonation, where the system was assumed to be a closed system with all CO\({}_{2}\) dissolved in the solution; and (2) equilibrium at each pH is achieved by both carbonation reaction and addition of HCl or NaOH when needed.