Biorecovery of metal sulfides from leachates obtained through zinc-carbon battery recycling

One of the most commonly used methods of recycling of spent zinc-carbon batteries is hydrometallurgical leaching with sulfuric acid. The technology is highly efficient and enables the recovery of the majority of deposited metals. Unfortunately it also generates leachates that remain after the treatment. The leachates are characterized by low pH (0.5-1.0) and relatively high concentrations of sulfates (20-30 g/l) and metal ions (even up to 25g/l of Zn and 10g/l of Mn). Herein we present a biotechnology for the management of effluents generated during the hydrometallurgical recycling of spent zinc-carbon batteries and for the recovery of metals (in the form of sulfides), which have typically been considered off-balance material. The main driving force of the biotechnology are sulfate-reducing bacteria (SRB), which generate alkalinty (by organic compound decomposition) and produce sulfides (as a result of sulfate reduction) that co-precipitate with metals. To maximize the efficiency of metal sulfide biorecovery by SRB, the leachates are pretreated with sodium hydroxide and ammonia produced by urea-degrading bacteria. Titration with NaOH leads to pH adjustment to 3.5 and subsequent precipitation of iron compounds. In turn, the addition of biogenic ammonia contributes to a further pH increase up to 5.0 and the reduction of the effluent toxicity.