Extraction of silver from a refractory silver ore by sono-cyanidation

In this study, effect of ultrasound on silver extraction from a refractory silver ore containing both native silver and various silver sulphide minerals was investigated. Main effects and interaction effects of pulp density, ultrasonic frequency, cyanide concentration, air flow rate and agitation speed on the extraction rate of silver were studied by a two-level fractional factorial experimental design. A few additional cyanidation tests were also conducted to verify the findings of the designed experiments. It was found that the overall extraction yield was varied from 67% to 90% depending on the operating conditions used in the sono-cyanidation tests (48 h). However, it was varied from 63% to 80% by same operating conditions used in the direct cyanidation tests at the same cyanidation time. It was observed that an increase in the ultrasonic frequency has a negligible effect on the silver recovery. More importantly, it was determined that there were insignificant differences between the 24-h sono-cyanidation results and the 48-h direct cyanidation results for each cyanidation conditions. This finding, which is very important from the cyanidation practice standpoint, indicates that the cyanidation time can be reduced up to 50%, or the capacity of an operating silver extraction plant can be increased up to 100% by the sono-cyanidation by the refractory silver ores. In order to describe the rate of silver dissolution in the cyanide solutions, the experimental data were analysed using shrinking core models. It was found that there is a good fit between the experimental data and the models, indicating the rate of silver dissolution in cyanide can be described by a two-stage, porous layer diffusion controlled, shrinking core model.