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To completely recover valuable elements and reduce the amount of waste, the impact of phosphoric acid on the decomposition of rare earth, fluorine and phosphorus during cyclic leaching was studied based on the characteristics of low-temperature sulfuric acid decomposition. When a single monazite was leached using 75 wt% H2SO4 solution with phosphoric acid, the size and number of monazite particles in the washing slag gradually decrease with the increase in phosphoric acid content in the leaching solution. The monazite phase can hardly be found in the slag when the phosphoric acid content reaches 70 g/L, which indicates that phosphoric acid is favorable for monazite decomposition. The mixed rare earth concentrate was leached by 75 wt% H2SO4 containing 70 g/L phosphoric acid, the mineral compositions of the washing slag are only gypsum and unwashed rare earth sulfuric acid. After cyclic leaching of 75 wt% H2SO4, the mineral compositions of the primary leaching washing slag are mainly undecomposed monazite, rare earth sulfate and calcium sulfate. However, monazite is not found in the mineral phase of the second and third leaching washing slag. The leaching rates of rare earth and phosphorus gradually increase with the increase in cyclic leaching times. In addition, the phosphoric acid content in the leaching solution increases with the increase in the number of cyclic leaching time. However, the rising trend decreases when the phosphoric acid content reaches 50 g/L by adsorption and crystallization of phosphoric acid. A small amount of water can be used to clean the leaching residue before washing to recover the more soluble phosphorus acid according to the difference of dissolution between phosphoric acid and rare earth sulfuric acid.
Hou et al. (Mon,) studied this question.