Effect of nutrient concentration, dissolved salts, and mineralogy at high temperature column bioleaching of a pyrrhotite-pyrite secondary ore

Over the past several decades, extensive research has focused on improving bioleaching of different ores, and increasing research on applying bioleaching for the processing of tailings produced from mining operations around the world. As part of the research project Near-zero-waste recycling of low-grade sulfide mining waste for critical-metal, mineral and construction raw-material production in a circular economy (NEMO), we have studied the effect of nutrient availability, the effect of high concentration of dissolved salts (Al and Mg sulfates) on metal (Cu, Co, Mn, Ni, Zn) extraction rates by bioleaching in laboratory column tests at high temperature (48 °C and 60 °C), using secondary ore from the Sotkamo mine (Terrafame operation, Finland). Metal dissolution rates showed two stages: a rapid first stage, where acid addition and initial irrigation released acid-soluble metal species, and a second stage, with slower rates of metal dissolution by microbial oxidation of residual metal sulfides with Fe(III) as the oxidant. Experiments showed that adequate Fe(II) oxidation rates depended on the addition of salts but not so much as the addition of nutrients (P as phosphate, N as ammonium and K). High dissolved salt concentrations resulted in low redox potentials (745–760 mV vs standard hydrogen electrode), limiting Co dissolution rates. For example, Co extraction yield at 48 °C when added salts plus nutrients was 57 %, and without salts (with or without nutrients) ∼75 %. Other metal dissolution rates were not affected by the composition of the irrigation solution or temperature, e.g. Ni extraction rates were between 88 % and 92 % in all the experiments. This was mostly due to the high metal content as readily soluble phases formed during the first heap leaching as metal dissolution in mine waste (e.g. tailings) highly depends on the metal speciation. Recycling of the leaching solution for irrigation, a common practice in heap bioleaching, affects negatively metal extraction rates, the precise reason for this is still to be determined in further experiments, possible effects including changes in solution chemistry affecting mineral oxidation, precipitation of insoluble salts, or toxicity or inhibition of microbial growth rates limiting microbial activity and mineral leaching.