Possibilities to use industrial oxidic by-products as neutralising agent in bioleaching and the effect of chloride on biooxidation
Abstract: The cost for neutralisation is the second largest cost in a bioleaching operation for which, possibilities to replace generally used lime/limestone was tested. Industrial oxidic by-products generated form Swedish industries were investigated for neutralising capacities by chemical leaching with sulphuric acid at pH 1.5, which is the optimum pH for bioleaching operations. The by-products used for the study comprised of five different steel slags from ore and scrap based steel making, electric arc furnace (EAF) dust from scrap based steel plant, Mesalime from paper and pulp industry and three different types of ashes from combustion for energy production. All the by-products showed a good neutralising capacity, while some of them had higher capacities than the reference Ca(OH)2. Due to the good neutralising potential of the by-products obtained from the chemical leaching, attempts were made to use them as neutralising agent in batch bioleaching of pyrite in stirred tank reactor to determine their neutralising potential, eventual toxic effects on the microorganisms and pyrite oxidation. Pyrite oxidation in all the batch bioleaching was in the range of 69-80%, except the Waste ash experiment which was 59%. Neutralising capacity was high for all the by-products except Waste ash and Coal & Tyres ash compared to slaked lime. No remarkable toxic effects due to the by-products were observed except in the Waste ash experiment, which was probably due to the high content of chloride. To confirm if the chloride in the Waste ash caused any toxic effect on the bioxidation activity, batch bioleaching studies were conducted with Ca(OH)2 + NaCl as neutralizing agent with a similar chloride concentration profile obtained in the Waste ash experiment. Effect of the chloride on the biooxidation of pyrite by sudden exposure of 2 g/L, 3 g/L, and 4 g/L of chloride in the log phase of the biooxidation of pyrite was investigated. Addition of 2 g/L chloride resulted jarosite precipitation with a lower pyrite recovery than the reference experiment, whereas the addition of 3 g/L chloride temporarily chocked the microorganisms but activity was regained after a short period of adaptation. Population dynamics study conducted on the experiment with 3 g/L chloride showed the variation in the microbial species at different stages of the biooxidation of pyrite. The study with sudden exposure of 4 g/L of chloride was found to be lethal to the microbes. Out of all the by-products used in batch bioleaching studies, Mesalime and Electric Arc Furnace (EAF) dust were used as a neutralising agent in continuous biooxidation of refractory gold concentrate. The neutralising capacity of EAF dust was lower, while the Mesalime was similar to the Ca(OH)2 reference. The arsenopyrite oxidation in the experiments ranged from 85-90%, whereas the pyrite oxidation was 63-74%. In subsequent cyanidation 90% of the gold was achieved in the bioresidues from Mesalime and Ca(OH)2, while 85% of gold was recovered in bioresidue from EAF dust. A probable explanation for the low recovery of gold from the EAF dust experiment could be due to the encapsulation of the part of the gold by high elemental sulphur content present in the EAF dust. Cyanide consumption was relatively high and ranged from 8.1-9.2 kg/tonne feed after 24 hours of cyanidation. Both Mesalime and EAF dust proved to be feasible options as neutralising agents in bioleaching operations. Studies on the modelling of ferrous iron oxidation by a Leptospirillum ferriphilum-dominated culture was conducted with 9 g/L or 18 g/L ferrous iron in a chemostat. Modelling data suggested that the kinetics and yield parameters changed with the overall solution composition. The apparent Fe3+ inhibition on specific Fe2+ utilisation rate was a direct consequence of the declining biomass yield on the Fe2+ oxidation, when dilution rate was decreased. The maintenance activity contributed up to 90% of the maximum specific Fe2+ utilisation rate, which appears close to the critical dilution rate. Determination of the toxic limit of chloride were studied both in batch and chemostat conditions. Batch studies showed a toxic limit at 12 g/L chloride, while chemostat studies showed a toxic limit of 4 g/L. Modelling of the ferrous iron oxidation in chloride environment showed a decrease in maximum specific growth rate and increase in the substrate constant. The biomass concentration decreased with the increase in chloride concentration due to the toxic effect on the microorganisms. The maintenance coefficient decreased by 70% in the chloride environment.
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