Effect of silver and bismuth on bioleaching of copper sulphide concentrates with thermophilic microorganisms

Abstract

Silver ion has been used successfully as catalytic agent in chalcopyrite leaching with mesophilic microorganisms to increase the copper dissolution rate. Although it could appear logical to add Ag⁺ to accelerate the process at high temperatures, thermophilic bacteria are less resistant than mesophilic microorganims to silver and so its application is limited. Therefore other cations must be studied to increase dissolution efficiency at high temperatures. In this work, a comparative study is performed using silver and bismuth in the bioleaching of a copper concentrate with thermophilic (68 °C) and mesophilic (35 °C) bacteria. The experiments were carried out in shake flasks at 1 % pulp density.

The quantity of silver added to both the thermophilic and mesophilic cultures was lg Ag/kg concentrate. Silver was precipitated as Ag₂S, which was then dissolved by microorganism activity. The concentration of silver increased up to a maximum value of 18μg Ag/1 causing bacterial inhibition. Then, there was a new precipitation of silver. It would prove silver dissolution is related to bacterial activity. Consequently, copper extraction was low (40% of mineral copper).

The thermophilic culture with 10g Bi/kg concentrate showed the highest copper dissolution rate, maximum extraction being attained in 48 hours (60% of mineral copper soulbilised). There was no effect when 1g Bi/kg concentrate was used. In both cases, soluble bismuth concentration increased, probably due to the dissolution of bismuth hydrolisis products.