Selective removal of magnesium from a lithium-concentrated anolyte by magnesium ammonium phosphate precipitation

Magnesium was selectively removed from a lithium-concentrated anolyte (containing Li 3.5 g L−1 and Mg 14.4 g L−1) by MgNH4PO4 precipitation method. Firstly, thermodynamic analysis of the systems Li+-Mg2+-PO43−-H2O and Li+-Mg2+-PO43−-NH4+-H2O were studied, and the optimal operating windows in which magnesium removal is the most probable were identified. Based on the theoretical considerations, Mg3(PO4)2 and MgNH4PO4 precipitation methods were used to remove magnesium from our lithium-concentrated anolyte, and the results showed that MgNH4PO4 precipitation method exhibits better performance than that of Mg3(PO4)2 on the complete removal of magnesium. Under the conditions of 1.0 times of theoretical amount dosage of (NH4)3PO4 and pH value 8.0, the concentration of magnesium was reduced to 11.1 mg L−1, and the lithium concentration maintained 3.46 g L−1, corresponding with the magnesium removal rate 99.92%, and the lithium recovery rate 98.91%. The obtained MgNH4PO4(s) precipitate was a well-crystallized bulky particle, and it is in favour of filtration. Especially, it can be used as NP compound fertilizer, which is great beneficial for reducing the costs of magnesium removal.