Mechanism and Kinetics of Scandium Extraction from High-Pressure Leachate of Iron-Aluminum Residue

Abstract

Laterite nickel ore, a crucial resource for nickel production, contains associated rare earth scandium (Sc) at 50–80 g/ton, which is difficult to extract directly from the raw ore. Currently, the high-pressure acid leaching (HPAL) process for laterite nickel ore is becoming mainstream; during this primary HPAL step, Sc remains in the solid residue because the leaching conditions are optimized for Ni and Co recovery. The Sc-bearing iron‑aluminum residue is then subjected to a second HPAL step with higher acidity and temperature, which effectively dissolves Sc. However, this second HPAL step dissolved not only Sc but also Fe and Al, necessitating selective Sc extraction from the leachate. This study systematically investigated the extraction efficiency of Sc using a binary system of bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) and di(2-ethylhexyl) (2-ethylhexyl) phosphonate (Cextrant 230), employing leachate generated from prior experiments. At a Cyanex 272 molar fraction of 0.6 (total extractant concentration 0.1 mol/L, room temperature, aqueous-to-organic phase ratio 1:1), the system exhibited optimal synergistic extraction for Sc, achieving separation coefficients of 2.89 and 2.35 for Fe and Al, respectively. Based on these findings, the chemical formula of the final extracted complex was determined via the slope analysis method. Furthermore, the kinetics and mass transfer processes of the extraction reaction were systematically studied using the constant interfacial cell technique. The extraction of Sc³⁺ by the Cyanex 272-Cextrant 230 system was identified as a first-order reaction. The forward extraction rate was found to be governed by a mixed diffusion-and-chemical-reaction mechanism, with the reaction occurring at the organic-aqueous interface. A rate equation was derived based on the extraction mechanism calculations. This research provides valuable insights for Sc recovery from laterite nickel ore.