Recovery of Rare Earths from Polishing Waste

This is an essay in the field of metallurgical engineering. Using rare earth polishing powder waste as raw material, an orthogonal experimental design was used to convert rare earth oxides into sulfates by first conducting roasting experiments of the waste with a mixture of ammonium sulfate and ammonium bisulfate. The orthogonal experiment with 3 factors and 3 levels was carried out by using the orthogonal table L₉(3⁴) and statistically analyzed the test results: the roasting temperature was 480 ℃, the roasting time was 3 h, and the mass ratio was 1.8:1. In the second step, 0.2% thiourea was added to the acid leaching solution as reducing agent, and the technological conditions of leaching rare earth from the calcined solid phase with dilute sulfuric acid were studied. The orthogonal experiment with 4 factors and 4 levels was carried out by using the orthogonal table L₁₆(4⁵). The experimental result shows that on the condition of lixiviating temperature being 90 ℃, sulfuric acid thickness being 0.5 mol/L, lixiviating time being 4 h, liquid-to-solid quality ratio being 4∶1, the extraction rate of rare earths will reach 97.8%~98.0% in mass fraction.