China Nonferrous Metallurgy

Monazite is a phosphate rare earth ore with abundant light rare earth content. The decomposition rate of monazite by hot alkali leaching method can reach 97%, which has high economic benefits. At present, there are relatively few theories on the alkali decomposition of monazite, and there is a lack of research on the transformation laws of rare earth elements (Ce, Pr, Nd) in monazite. This article uses thermodynamic calculations to draw the lgc-pH equilibrium diagram of the dissolved components of alkali decomposed monazite RE-P-H₂O system (RE is La, Ce, Pr, Nd) under temperature conditions of 25℃ and 160℃. The thermodynamic equilibrium phase diagram is used to analyze alkali decomposed monazite. The results show that at 25℃, within the pH range studied, the region where LaPO₄ solid-phase stability exists is 0.08＜pH＜14.62, The region where REPO4 (RE is Ce, Pr, Nd) solid-phase stability exists is 0＜pH＜14.62; When pH ≥ 14.62, the pH values of Pr(OH)₃, Ce(OH)₃, La(OH)₃, and Nd(OH)3 precipitated sequentially are 14.62, 15.06, 15.24, and 15.46; after the system temperature increased to 160℃, the initial precipitation pH of RE(OH)₃ decreased to 10.70, and the precipitation order was Ce(OH)₃, Nd(OH)₃, La(OH)₃, Pr(OH)₃. Therefore, from theoretical analysis, it can be seen that the alkaline decomposition of solitary stones can be achieved at room temperature and pressure, and the high-temperature and high-pressure conditions have a significant promoting effect on this decomposition process, which is consistent with the existing alkaline decomposition process of solitary stones.