Selective separation of lanthanide group in spent NiMH battery acidic leaching solutions

Abstract

A novel selective separation of lanthanide group (Ln) known as rare earth elements (REEs) from acidic leaching solutions of spent nickel metal hydride (NiMH) batteries was examined using precipitation at low pH with disodium phosphate, Na2HPO4, as precipitation agent. The acidic leaching solutions containing lanthanides (La, Ce, and Nd) and base metals (Ni, Cd, Mn, Fe, Al, and Zn) were from spent NiMH batteries subject to subcritical water extraction (SWE) process using 0.5 mol/L of HCl, HNO3, and H2SO4 solution, respectively. The effects of important parameters such as the molar ratio of lanthanides to phosphate ions, Ln/P, to form precipitates of lanthanide phosphates (Ln(PO4)), pH and temperature were investigated. The recovery efficiency of lanthanides increased when decreasing Ln/P, while increased with increasing pH and temperature. The decrease in the concentration of Ln remaining in HCl and HNO3 leaching solutions was proportional to the decrease in the P concentration in leaching solutions, while different results were found in the leaching solution of H2SO4 due to the formation of another insoluble lanthanide mineral, double sulfate (NaLn(SO4)2·H2O). This separation process demonstrated that the high-purity lanthanide group including La, Ce, and Nd from spent NiMH batteries could be obtained simply by adding phosphate ions at low pH. It is efficient, effective, and selective for critical rare metals recovery from spent NiMH batteries.