Rare earth elements recovery using staged precipitation from a leachate generated from coarse coal refuse

Abstract

Recovery of rare earth elements (REEs), especially heavy and critical REEs, from alternative resources such as coal refuse has recently become increasingly important. A natural leachate containing 6.14 ppm of REEs with a pH value of 2.70 was collected from a coal coarse refuse pile and utilized as a feedstock for REE recovery tests. The pH value of the natural leachate was gradually increased in a step-wise manner using a sodium hydroxide solution which produced several precipitates. A precipitate containing about 1.1% total REEs was obtained in the pH range of 4.85–6.11 while recovering >80% of the REEs contained in the original leachate. The leaching and staged precipitation process enriched both the critical and heavy REE fractions. The ratios of heavy-to-light (H/L) and critical-to-uncritical (C/UC) REEs in the precipitate were much higher than the coarse refuse material (H/L: 1.54 vs 0.21, C/UC: 1.51 vs 0.34). Additionally, the precipitate contained 18.4% Al, 1.7% Zn, 1.4% Cu, 1.1% Mn, 0.5% Ni and 0.2% Co. A solution chemistry study indicated that the hydroxide and hydroxysulphate precipitates of Al, Si and Cu produced in the pH range of 4.85–6.11 contributed to the fractionation of the REEs and other metal ions such as Zn, Mn and Co to the precipitate. The iron hydroxide precipitates formed at lower pH values did not contain a significant amount of REEs. An explanation may be the competitive adsorption between Al, which was present at higher solution concentrations, and REEs on the iron hydroxide and/or hydroxysulphate precipitate surfaces as proven by results of precipitation tests conducted on model solutions. A product containing 94% of a rare earth oxide mix was obtained by re-dissolution of the REE enriched precipitates followed by selective precipitation using oxalic acid. The overall results including an economic assessment indicate that leachate solutions generated naturally from coarse coal refuse may be a valuable source of critical and heavy REEs that can be effectively recovered and concentrated using staged precipitation.