Application of alkaline pretreatments for gold and silver recovery from refractory ores

In this study, the recovery of gold and silver from Akoluk (Ordu) antimony ore containing gold and silver during cyanidation following alkaline and alkaline sulfide pre-treatment was investigated. The study was initiated by detailed mineralogical analysis (ore microscopy, SEM-EDS and EPMA) and diagnostic leaching tests for characterization of the ore. This was followed by the investigation of the effects of ultrafine grinding, roasting and lead nitrate on gold and silver recovery during cyanidation. After these preliminary studies, in order to increase the efficiency of cyanide leaching, the decomposition of antimony minerals was performed by using Na2S+NaOH alkaline sulfide leaching, NaOH, KOH and lime alkaline leaching. In these experiments, the effects of parameters such as the concentration of reagent, temperature, particle size and leaching time on the efficiency of antimony dissolution were determined. Later, gold and silver recoveries during subsequent cyanidation were investigated.Low gold (?49% Au) and silver (?24% Ag) recoveries were obtained in direct cyanidation from ore. According to the detailed mineralogical investigations, it was showed that gold particles (<3 µm) associated with sulphides minerals consist of 10-15% Ag in ore. The majority of silver was found as andorit (Ag3SbS3) mineral to be insoluble in cyanide solutions. Disseminated gold particles were found within the andorite minerals. Framboidal pyrites contains Sb and Ag-ringed zones. Low gold and silver recoveries during cyanidation are mainly due to the presence of gold and silver inclusions within Pb-Sb-S minerals. The diagnostic leaching tests exhibited that refractoriness of ore derived from associated with sulfide minerals of a large portion of the gold and almost all a part of silver. Although lead nitrate addition and pretreatments such as fine grinding, roasting provide improvements for gold and silver recoveries during cyanidation, the desired recoveries were not obtained. These results suggest that ore have refractory properties and a chemical decomposition pretreatment was required to achieve high metal recoveries.In alkaline and alkaline sulfide experiments, the antimony dissolution was improved with increasing of reagent concentration, temperature, leach time and decreasing particle size. Gold and silver recoveries were improved during cyanidation after decomposition. 82.6% gold and 90.2% silver recoveries were obtained with 95.1% antimony removal by alkaline sulfide leach (4 mol/L Na2S and NaOH, 800C, d80:15 µm). 82.2% gold and 80.6% silver recoveries were obtained with 70.1% antimony removal by NaOH alkaline leach (5 mol/L NaOH, 800C, d80:15 µm). 82.4% gold and 86.1% silver recoveries were obtained with 85.5% antimony removal by KOH alkaline leach (5 mol/L NaOH, 800C, d80:15 µm). Compared to low gold and silver recoveries in directly cyanidation from ore, gold and silver recoveries were improved with a significant amount of 33.2-33.6% Au and 67.2-77.2% Ag proportions during cyanidation after alkaline and alkaline sulfide leachs.As a result, as well as Akoluk ore, it was determined that alkaline or NaOH and KOH alkaline leach were highly effective on decomposition of antimony minerals containing gold and silver. These processes can be effectively applied as pre-treatments on recovery of gold and silver during cyanidation.Key Words: Refractory gold-silver ore, antimony, roasting, ultrafine grinding, alkaline sulfide, alkaline, leach, cyanide, gold and silver recovery