Recovery of rare earth elements from phosphogypsum waste in resin-in-leach process by eluting with biodegradable complexing agents

Phosphogypsum waste is a potential, secondary rare earth element (REE) source that is available in very large volumes. Extraction and purification of REE from this complex waste was studied using resin-in-leach (RIL) followed by batch elution. Multiple consecutive RIL cycles with Purolite S940 resin in dilute H2SO4 solution were performed for an industrial phosphogypsum waste. REE loading (max. 0.92 equiv./kg) and purity over calcium (70%) increased up to seven cycles. REE and the impurities were separated by treating the loaded resin in a packed bed column with a two-step elution procedure. Calcium was efficiently removed with 0.06 M HCl. Alkaline solutions of biodegradable chelating agents N,N–dicarboxymethyl glutamic acid (GLDA), methylglycinediacetic acid (MGDA), oxalic acid, citric acid and iminodiacetate were studied for the second elution step. MGDA and GLDA were both suitable candidates for effective REE elution from S940 resin. A calcium free, high purity REE fraction (up to 99.01%) was obtained with MGDA. The most likely REE:MGDA complex stoichiometry was found to be 1:1 by using the method of continuous variation.