Hidden gold in fire assay of gold telluride ores

Abstract

The recovery of precious metals from gold and silver tellurides is a significant challenge because of the refractory behavior of these species during standard cyanide leaching. Some alternatives have been proposed to treat these ores, among others the use of non-cyanide leaching agents, oxidative pretreatments before cyanidation or the addition of some salts to the standard cyanidation process. In the search of new leaching techniques to recover precious metals from tellurides at CINVESTAV Saltillo, Mexico, some leaching methods have recovered more dissolved gold in the leachate than was originally measured in the feed solids by fire assay. The same results were found in other telluride ores, but not in gold ores that did not contain gold telluride. This fact may suggest that fire assays underestimate the gold levels when gold is also present as gold telluride. This work examines the impact of gold telluride on the determination of gold levels using fire assay.Fire assays were performed using pure species, a gold telluride ore and a non-telluride gold ore. The cupels and the slags generated in these tests were also fire assayed. The results obtained confirm the underestimation of gold. For the investigated gold telluride containing ore, 20% of the gold is absorbed by the cupel, and 36% is found in the slag: the fire assay technique is only determining 44% of the total gold. Roasting the ore at 650 °C for 24 or 72 h in the presence of air before the fire assay permits to increase the reported gold from 44% to 57 and 62%, respectively.Since fire assay is the common method to determine gold in mine operations, it is important to consider the impact that the presence of gold telluride has on resource estimation and operational processes.