Solubilisation of monazite in organic acids.

The recovery was investigated of rare earth elements from a monazite concentrate containing 14.36 wt% Ce, 8.65 wt% La, 16.25wt% Fe and 7.24 wt% P. Dissolution experiments were conducted using tartaric, citric, oxalic and maleic acids. The effects of pH, residence time and the use of tailings mine site water were also studied. Oxalic acid exhibited the highest dissolution potential and was selected for further study. The results showed that the effectiveness of each acid correlated fairly well with their ability to bind rare earths, but citric acid exhibited greater complex stability with rare earths than oxalic acid indicating that reprecipitation of the oxalate is important for increased mineral dissolution. The system exhibited surprising selectivity for the dissolution of monazite over the iron phase, as oxalic acid has a greater binding strength with iron than with rare earths. This may be due to other interactions with the phosphate, the different stability of the specific phases, a kinetic limitation on iron dissolution or preferential adsorption to the monazite surface. The conversion of monazite to rare earth oxalate appeared to exhibit a rate limitation with time possibly due to the precipitating material forming a passive layer on the surface of the monazite or consumption of the oxalate by dissolving ions. A rare earth hydroxide could be produced from the leach residue, allowing the process to be applied to conventional solvent extraction. Tailings water used to investigate the effect of dissolved solids on the process showed a marked decrease in phosphorus release, suggesting a significant decrease in monazite conversion.
The recovery was investigated of rare earth elements from a monazite concentrate containing 14.36 wt% Ce, 8.65 wt% La, 16.25wt% Fe and 7.24 wt% P. Dissolution experiments were conducted using tartaric, citric, oxalic and maleic acids. The effects of pH, residence time and the use of tailings mine site water were also studied. Oxalic acid exhibited the highest dissolution potential and was selected for further study. The results showed that the effectiveness of each acid correlated fairly well with their ability to bind rare earths, but citric acid exhibited greater complex stability with rare earths than oxalic acid indicating that reprecipitation of the oxalate is important for increased mineral dissolution. The system exhibited surprising selectivity for the dissolution of monazite over the iron phase, as oxalic acid has a greater binding strength with iron than with rare earths. This may be due to other interactions with the phosphate, the different stability of the specific phases, a kinetic limitation on iron dissolution or preferential adsorption to the monazite surface. The conversion of monazite to rare earth oxalate appeared to exhibit a rate limitation with time possibly due to the precipitating material forming a passive layer on the surface of the monazite or consumption of the oxalate by dissolving ions. A rare earth hydroxide could be produced from the leach residue, allowing the process to be applied to conventional solvent extraction. Tailings water used to investigate the effect of dissolved solids on the process showed a marked decrease in phosphorus release, suggesting a significant decrease in monazite conversion.