Selective removal of chromium from laterite residue with high content of iron by the co-operation of phosphoric acid and oxalic acid

With the rising demand for nickel and cobalt, laterite residue containing chromium is extensively generated through the laterite hydrometallurgical process, posing a significant burden on environmental protection and the sustainable development of the nickel industry. This study presents a novel and environmentally friendly method for selectively extracting chromium from laterite residue and yielding iron phosphate concentrate. In this method, iron, as the major component, is simultaneously transformed into iron phosphate as solid state, while chromium is extracted into solution by a minimal amount of phosphoric acid. The effects of phosphoric acid and oxalic acid usage, temperature, liquid-to-solid ratio, and leaching time on the transformation and removal were investigated. Thermodynamic analysis and experimental results indicated that the addition of oxalic acid promoted chromium extraction with minimal impact on the transformation of hematite and iron phosphate. The results of SEM, XPS, and XRD revealed the conversion of laterite residue to iron phosphate. At an oxalic acid dosage of 10 %, almost 90 % of the iron remained in the residue as iron phosphate, while over 60 % of the chromium was extracted into the solution, resulting in a chromium concentration of 1.1 g/L. The solution underwent a two-step treatment process, primarily to recycle the soluble iron species. The extracted chromium was captured by adding calcium hydroxide at pH similar to 3.0 in the form of phosphate.