Silver catalyzed bioleaching of low-grade copper ores.: Part III:: Column reactors

A study of the effect of different variables (inoculation, [Ag], silver addition mode, reactivation of the ore (i.e. delayed silver addition), composition of the pregnant liquid solution, ore particle size, pH and aeration) on the silver-catalyzed bioleaching of two different low-grade copper (>600 mu m) has been investigated in column reactors. The two low-grade copper ores tested, lower K-ore and Pinto Valley Dump ore, behaved similarly during the silver catalyzed process. After more than 300 days of leaching, the addition of both silver and bacteria improved the copper extraction from both low-grade chalcopyritic ores. Copper extraction in the presence of silver displays paralinear kinetics with two stages very well defined, with the first stage being much faster than the second one. Copper extraction from the K-ore surpassed 70% after 350 days of bioleaching using low concentrations of silver (1.4 g Ag/kg Cu). The addition of silver was effective even after the bioleaching process was started. Conversely, silver addition mode and aeration were not significant variables in the bioleaching process. The effect of a pregnant liquid solution collected from a first bioleaching experiment demonstrated that the build up of the solution can negatively affect the copper extraction during the silver-catalyzed chalcopyrite process. Copper extraction from PVD ore surpassed 60% after 300 days of bioleaching using 1.24 g Ag/kg Cu. Copper extraction was substantially affected by both [Ag] and ore particle size. For the ranges between 0.25 and 1.00 g Ag/kg Cu and between <5.08 x 10(4) and < 1.27 x 10(4) mu m, the dissolution of copper was favoured with the highest amount of silver and the smallest particle size. On the other hand, silver concentration, pH and silver addition mode were not influential variables in the process. Chemical analyses, XRD and SEM/EDX studies were carried out on different residues after 100, 200 and 325 days of experimentation. Chemical analysis showed that copper is selectively bioleached in the presence of silver and bacteria, while silver remains in the solid phase. The SEM study showed that pyrite remains mainly unattacked during the bioleaching of chalcopyrite from the low-grade copper ore. Jarosite, gypsum and elemental sulphur precipitates have been identified on the residues. (c) 2007 Elsevier B.V. All rights reserved.