Optimization of Pilot-Scale Solvent Extraction Unit for the Recovery of Scandium from "Bioacid"-Leached West KY Coarse Coal Refuse

Loading, scrubbing, and stripping trials using Cyanex® 272 as extractant for the liquid-liquid extraction of scandium were performed using naturally formed acid mine drainage (AMD) and pregnant leachate solution (PLS) generated from mine waste material. Results from the AMD tests showed that the recovery of Sc from AMD was highly effective using 1 vol% extractant in Orfom® diluent. The extraction efficiency of Sc was over 85% while using an aqueous-to-organic (A:O) ratio of 50:1 (v/v) and rejected 94.4% of Fe in the loading stage. Stripping using 2M sulfuric acid solution was able to effectively recover Sc from the loaded organic phase. A continuous solvent extraction system using 2 stages of loading and 2 stage of stripping was able to concentrate Sc from roughly 0.15 ppm to a 2% concentration solution.

Based on the preliminary findings from the AMD test, the process was assessed on Sc extraction from PLS obtained from mine waste material. The PLS was obtained from leaching roasted West Kentucky Coal Seam 13 coarse refuse with sulfuric media generated through oxidation of fine pyrite with Acidithiobacillus ferrooxidans. The acidity of the plant-generated “bioacid” was at a pH of 1.56. After leaching at 200 g/L solid concentration under 75 °C for 5 hours, the pregnant leachate solution contained 0.65 ppm of Sc with a final pH of 2.811. Majority of the Fe(III) was neutralized during the leaching process due to the elevated pH. Loading trials were performed using various A:O ratios, and a 50:1 aqueous-to-organic ratio was found to be optimal for scandium loading and limitation of iron loading. However, accumulation of Al was observed in the organic phase provided due to the high A:O ratio. Scrubbing trials were carried out to selectively remove Al ions using NaOH solution. An organic layer of iron precipitates formed during the scrubbing tests as a result of the presence of Fe in the organic. Further work is suggested to 1) generate the leachate solution at lower pH so that the pH of the PLS is below 2.0, and thus reducing the loading efficiency of Fe in the loading stage and 2) reduce ferric ions to ferrous ions to prevent Fe loading into SX circuitry. Stripping trials were not re-tested at this time, as similar initial pH and Eh constraints of the feed suggest stripping trial data from AMD work will continue to hold.