Improvement of Silver Extraction by Ultrafine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore

Abstract

Ultrafine grinding (UFG) was proposed as a pretreatment method prior to cyanide leaching of old plant tailings of a refractory silver ore. Direct cyanidation of the as-received tailings (d80: 100 µm) led to a low silver extraction of only ≤43% over 24 h. A size-based diagnostic study has demonstrated that the refractoriness of the tailings is essentially physical in character. Therefore, effects of particle size/UFG (d99: 5–75 µm), concentrations of NaCN (0.75–2.25 g/L) and Pb(NO3)2 (0–500 g/t) on the extraction of silver were investigated using a Box–Behnken design. The statistical analysis of the experimental data (%Ag extraction at 1 h) revealed that particle size/UFG was the most significant parameter for the extraction of silver, which was substantially improved after UFG. A positive effect of increasing the concentration of NaCN was also observed particularly at finer particle sizes while the effect of concentration of Pb(NO3)2 was insignificant. Almost complete extraction for silver (i.e., ≥99%) was demonstrated to be possible at even higher concentrations of cyanide (>2.25 g/L NaCN) when using the finest material (d99: 5 µm) over only 1 h of leaching. Albeit, the consumption of cyanide tended to increase with UFG or increasing the initial concentration of cyanide. The findings showed that UFG can be used as a suitable and environmentally sound pretreatment method to improve the extraction of silver from the refractory silver ore tailings. The diagnostic approach adopted in the current study has proved to be a useful analytical tool to determine the amenability of the ore to ultrafine-grinding as a pretreatment process.