BIOREACTOR PERFORMANCE VERSUS SOLIDS CONCENTRATION IN COAL BIODEPYRITIZATION

Solids concentration is a critical technico-economic parameter in bioh ydrometallurgical processing since it decisively affects bioreactor si ze and therefore investment and operating costs. Researchers have repe atedly provided evidence, since the early times of biohydrometallurgy, that bioreactor leaching rates of metal sulphide concentrates drastic ally decrease whenever percent solids concentration exceeds 16-20. Thi s limitation, should it hold also for coal biodepyritization processes , would place a serious constraint on their economics. The paper prese nts the results of an investigation carried out running in parallel th ree different types of laboratory reactors fed with the same pulp proc essed in the 7.5 m(3) stirred-tank reactors (STRs) installed in the Po rto Torres coal biodepyritization pilot plant. Evidence is provided th at biodepyritization performance of the reactors remains practically u nchanged in the solids concentration range from 5% to 40%, whereas fai lure of pure pyrite bioleaching at 30% solids concentration is observe d in the same reactors. This result seems to support the view that sol ids concentration influence on bioleaching performance is related to t he proportion of oxidizable mineral present in the solid phase. Compar ison of laboratory- and semi-commercial-scale STRs seems to show that scale-up imparts an improvement in machine performance.