A novel process for the selective precipitation of valuable metals from lepidolite

The treatment of lepidolite by a multistage leaching process was studied for the extraction of alkali metals as alkali alums and brines. The precipitation characteristics of the aluminum and fluorine in the solution were also investigated. The transformation of the mineralogical species in lepidolite into albite and silicon dioxide was tentatively identified by X-ray diffraction measurements. The crystallization was aimed at the separation of rubidium and cesium from an acidic solution containing potassium as the main contaminant. The alum mixture contained on average 7.5% potassium, 1.3% rubidium and 0.30% cesium. The main crystalline components were aluminum sulfate and AlF(SO4)·5H2O after concentration. The aluminum in the solution was removed by adding potassium sulfate with a K/Al mole ratio of 2 at various pHs to yield an overall aluminum precipitation of 62.1–66.6%. Under weakly acidic and neutral conditions, soluble aluminum may transform into insoluble aluminum, such as Al(OH)3. Additionally, soluble AlFx(3−x)+ species tended to precipitate directly with OH−, thereby achieving the formation of Al-OH-F complexes. The AlFx(3−x)+ species at high pH were partially dissociated into F− and AlOH2+, Al(OH)2+ and Al(OH)4−, and the buffering effect of F− inhibited the transformation of the precipitated Al(OH)3 into soluble aluminates due to the formation of Al-OH-F complexes. The residual aluminum and fluorine contents were on average 0.0065 g/L and 0.042 g/L, respectively, in the mother liquor after neutralization at pH 9. The loss of lithium increased from 1.1% to 5.4% as the pH increased from 5 to 9. These findings imply that aluminum and fluorine can play significant roles in the recovery of alkali metals from lepidolite, and the precipitation of aluminum from leaching solutions as alums can cause a loss of lithium.