Effect of organic extractant LIX 84IC, pH and temperature changes on bioleaching microorganisms during SX treatment

Abstract

During the solvent extraction process, the bioleaching solution undergoes changes. Basically these changes consist of an organic phase incorporation as well as changes in temperature and pH profiles. The resulting conditions have an effect on the following stages and more importantly on the behavior of the microbial community.The influence of the increase in temperature and acidity during solvent extraction on a mixed culture of bioleaching microorganisms was investigated. The effect of both factors was tested separately and together, where temperature was increased from 20 to 30°C and pH was decreased from 2.0 to 1.5 and then to 1.0 (according to SX plant data). At the same time, the effect of an organic phase composed of 24% LIX 84IC as the extractant and Oxform SX12 as the solvent on the same microbial community was studied. Quantitative and qualitative changes of the microbial population were investigated using optical microscopy, MPN (Most Probable Number) and qPCR.Decreasing pH had a stronger negative effect on biomass numbers and activity (oxidation of ferrous iron) than temperature that showed almost no effect. Total cell number and activity decreased around one and one and a half orders of magnitude respectively when pH decreased to 1.0.The number of active ferrous iron-oxidizing cells was reduced more by contact with the organic phase than by the decrease in pH, and the detrimental impact of both stress factors together was even greater (decrease of over two orders of magnitude at pH 1.0). The increase of the SX agent concentration further decreased the number of active iron-oxidizing cells. On the other hand, the decrease in the organic extractant concentration did not show a significant improvement in the measured iron-oxidizing activity. Relatively constant cell numbers were recovered from the organic phase after contact, with the predominance of Acidithiobacillus thiooxidans.Sulfobacillus thermosulfidooxidans and Alicyclobacillus spp. showed the smallest decrease in cell numbers of all the species quantified when acidity was increased and during contact with the organic extractant.The results gathered in this study show the clear impact of SX on bioleaching communities, and therefore possible influences on subsequent downstream stages. Preventing the leakage of organic extractant from the SX step to the following stages is essential to minimize the deteriorating effect of this agent on the overall microbial activity.