Analysis of reasons for decline of bioleaching efficiency of spent Zn-Mn batteries at high pulp densities and exploration measure for improving performance

► Bioleaching was used for extraction of valuable Zn and Mn from the spent batteries at high pulp densities. ► Bioleaching dynamics of both Zn and Mn under different pulp densities were compared. ► Reasons for decrease of bioleaching efficiency at high pulp densities were unveiled. ► Feasible measures for improving efficiency at high pulp density of 4% were proposed. ► Extraction yields of 100% for Zn and 89% for Mn at 4% of pulp density were attained.