Development of process scheme for recovery of rare earths from leachate of coal flyash

Abstract

The present paper expounds development of a two stage solvent extraction scheme for separation of rare earths (REE) from a sulfuric acid leachate obtained from an Indian coal flyash sample containing 2160 ppm REE. The leachate has low concentration of REE (305 mg/L) and high content of impurities (32.6 g/L). The problem of gel formation due to presence of Si in the leachate was prevented by gelatin strike which lowered dissolved Si content. About 94% of HREE and 86% of Light Rare Earths (LREE) values could be recovered in first and second stages of solvent extraction stages using D2EHPA solvent of optimized concentrations 12% (v/v) and 40% (v/v) saponified up to 40%, respectively. The two organic streams, former rich in HREE and the later rich in LREE, were subjected to multi stage cross current stripping at low A/O ratio using 6 mol/L HCl solution to obtain strip solutions concentrated in HREE and LREE respectively. The two strip liquors were neutralized with NaOH from which REE were precipitated using oxalic acid dihydrate to produce mixed REE concentrates, assaying 13% HREE and 17.5% LREE. The present study was a maiden attempt to recover REE from the actual leachate, of complex chemistry, obtained from an India flyash sample. Further, the flowsheet was up scaled to bench scale which includes easily scalable processes like leaching at ambient temperature (25 °C) and high solids concentration (20% w/v) followed by solvent extraction and precipitation stages. This study establishes a high potential for the recovery of REE from an Indian coal flyash at industrial scale. Scope exists for future studies on separation of individual rare earths oxides from the mixed REE oxalates that can be produced according to the present study.