Mn bio-dissolution from low-grade MnO2 ore and simultaneous Fe precipitation in presence of waste electrolytic manganese anolyte as nitrogen source and iron scavenger

Abstract

A bioleachate containing high-content Mn and low-content Fe is required for the production of electrolytic manganese metal (EMM). To produce the qualified bioleachate, bioleaching of Mn from a low-grade MnO2 ore by a mixed autotrophic culture in presence of combined energy matters of pyrite and sulfur and simultaneous removal of Fe using waste electrolytic manganese anolyte (WEMA) containing high concentration of NH4+ as both nitrogen source and iron scavenger were investigated. The optimal conditions for both the maximum Mn release from the MnO2 ore and the maximum Fe precipitation from the bioleachate were determined via Plackett-Burman design, Steepest Ascent design and Box-Behnken design, which were listed as follows: 6% (v/v) of WEMA addition, 140 rpm of shaking speed, 250 mesh of ore particle size, 1.0 g L−1 of KH2PO4, pH value adjustment at 2.4, ratio of pyrite to sulfur at 14:10 (g·L−1), incubation temperature at 31 °C, and bioleaching period of 14 days at a fixed pulp density of 100 g L−1. The predicted values were 79.6% for the Mn extraction efficiency and 0.69 g L−1 for the total Fe residual concentration respectively, being very close to the measured values of 78.5% and 0.73 g L−1 in the confirmation experiments. The addition of 6% WEMA instead of (NH4)2SO4 resulted in an increase of 41% in the cell density from 1.20 × 108 to 1.69 × 108 mL−1 after 14 days of culture, displaying that the WEMA was competent to be an nitrogen source for the bioleaching. On the other hand, the addition of WEMA enhanced the formation of more ammoniojarosite to greatly reduce the Fe residual concentration from 2.19 to 0.73 g L−1, accompanying 7% of loss rate in Mn extraction due to possible Mn adsorption onto the ammoniojarosite. The present study demonstrated that the WEMA as a byproduct of EMM industry is qualified to be both the nitrogen source for efficient bioleaching of low grade MnO2 ore and the Fe scavenger for simultaneous removal of Fe from the bioleachate, which is advantageous for both reduction of bioleaching cost and the closed loop recycling of waste in preparation of EMM.