Improvement of a hydrometallurgical process for recovering Mn from electric-arc furnace slag

Abstract

Electric-arc furnace (EAF) slag is a waste generated in large quantities by the steelmaking industry. This waste contains high levels of Ca, Fe, Mn and Si. In the present study, an improved hydrometallurgical process was demonstrated for efficient recovery of Mn from EAF slag. Firstly, the EAF slag was dissolved in a mixture of nitric and hydrochloric acids to remove the insoluble Si-containing mixture. The generated leaching acid contained 6 g/L Mn, 26.9 g/L Ca, 15.9 g/L Fe and small amounts of other impurities. Secondly, Ca precipitated into anhydrite when concentrated sulphuric acid was added, and the residual Ca ion concentration was 213 mg/L. Thirdly, after Ca was precipitated, nearly 100% Fe was removed as hematite nanoparticles with 67.7% of Fe during the hydrometallurgical processes by adding methanol at an Mmethanol/Mnitrate ratio of 0.75, the retention rate of Mn was higher than 98.5%. Without adding methanol, only 54.8% of Fe was precipitated as an irregular hematite block. Fourthly, the residual Mn content is 5.8 g/L, which was recovered as hausmannite with Mn content of 60.3% when solution pH is adjusted to 8. Hydrolysis of Fe was not observed at 80 °C but accelerated when temperature was higher than 140 °C. Ethanol exhibited removal efficiency of Fe similar to that of methanol, but the optimal Methanol/Mnitrate molar ratio was 0.3, which was 2/5 that of methanol. By using the improved method, high-purity hausmannite product and hematite by-product were effectively recovered from the EAF slag without generating secondary waste.