Thermodynamic study on recovery of lithium using phosphate precipitation method

The thermodynamic equilibrium diagram of Li+/Fe3+ − PO43− − H2O system at 298 K was established and the thermodynamic analysis conducted on the precipitation of lithium using phosphates and the decomposition of Li3PO4 using ferric salts. The results revealed that phosphates preferentially reacted with Fe(III) rather than Li(I) to form FePO4 in the pH range of 0-5.5. The FePO4 could be transformed to Fe(OH)3 in the pH range of 5.5-14.0. The optimum pH window for the precipitation of Li3PO4 was in the range of 11.0-13.0 and the concentration of lithium in the solution can be decreased to 10−2.5 mol/L with a total phosphorus concentration of 0.005 mol/L at equilibrium. The precipitate of Li3PO4 could be effectively decomposed using ferric salts in the pH range of 2.5-3.5 to form FePO4 and corresponding lithium salts, resulting in both the concentrations of Fe(III) and P in filtrates less than 10−3 mol/L. The verification experiment results showed that the precipitation of Li(I) reached 96.5% by adding 110% theoretical amount of Na3PO4·12H2O at equilibrium pH of 11.0. After the addition of 105% theoretical amount of FeCl3·6H2O and the adjustment of pH to 2.5, the decomposition of Li3PO4 reached as high as 99.7%, while the concentrations of Fe(III) and P in LiCl solutions were only 0.21 g/L and 0.038 g/L, respectively, which was consistent with the thermodynamic analysis.