Separation and recovery of iron impurity from a vanadium-bearing stone coal via an oxalic acid leaching-reduction precipitation process

Abstract

Sulfuric acid leaching solutions of vanadium-bearing stone coal usually contain massive iron impurity, which is detrimental to vanadium extraction because iron is easy to precipitate or co-extracted during the solvent extraction and ion exchange. Meanwhile, sulfuric acid in wastewaters is environmentally hazardous. In this paper, a vanadium-bearing stone coal was leached with oxalic acid, which has the advantages of cheap biological sources and good environmental compatibility. The iron impurity was separated from the oxalic-acid leaching vanadium-bearing solution (OLVS) via reduction precipitation. During the oxalic acid leaching, 80.5% of the vanadium can be recovered, and 50.2% of the iron impurity was leached with vanadium under the conditions: a CaF2 addition of 5 wt%, an oxalic acid dosage of 60 wt%, a leaching time of 4 h, a leaching temperature of 95 °C and a water-mineral ratio of 1.0 mL/g. However, the concentrations of vanadium and iron were 1777.5 ppm and 6552.5 ppm, respectively, in the OLVS. By adopting reduction precipitation, 83.8% of the iron was precipitated as FeC2O4·2H2O with only a 2.5% vanadium loss when an iron powder dosage was 2.5 times that of the stoichiometry for 90 min at 25 °C. The oxalic-acid leaching vanadium-bearing solution after reduction precipitation (OLVS-P) contained 1733.6 ppm of vanadium and only 1059.8 ppm of iron. Analyses indicated that the as-produced FeC2O4·2H2O (P-FeC2O4·2H2O) was in a high crystalline state with a purity of 98.2%. Therefore, the iron impurity can be separated and recovered from the vanadium-bearing stone coal via the oxalic acid leaching-reduction precipitation process.