Optimisation of ultrasound-assisted alkali-acid leaching of rare earth elements from run-of-mine and discard coal using the response surface methodology

This paper presents the results of an ultrasonic-assisted alkali-acid leaching process for concurrently recovering rare earth elements (REEs) and coal from South African run-of-mine (RMC) and discard (DC) coal. The response surface methodology (RSM) was used during the alkali leaching of DC to evaluate and optimise the process parameters. REE recovery increased while ash content decreased with increasing alkali leaching parameters (temperature, 20 °C-80 °C; concentration, 20%-40%; time, 30–120 min; and ultrasound frequency, 20–40 kHz). The optimal parameters achieved for DC were also used to pre-treat RMC. Pre-treatment of DC with the caustic solution at optimised conditions increased REE concentrations in RMC from 225.3 to 270.4 ppm and in DC from 245.5 to 301.5 ppm, respectively. Leaching of the alkalised samples with 7.5% hydrochloric acid (HCl) at 50 °C for 30 min, led to a TREE recovery of 88.8% and 79.6% from RMC and DC, respectively. The ash content in the samples was reduced in RMC from 50.9% to 36% and in DC from 68.3% to 59.3%. RMC proved to be a suitable source for the simultaneous extraction of the REEs and clean coal. Whereas, DC seemed to be a more viable secondary source of REEs only due to its higher TREE content and comparative REE recovery. This study demonstrated that coal used can be a valuable source of critical REEs that can be recovered effectively.