Experimental investigation of recovering gold from maghemite-rich magnetic concentrates by roasting

Abstract

Roasting of double refractory ores found in the Carlin Trend is necessary to oxidize the sulfides and remove the organic matter prior to leaching for gold extraction. Oxidation of the iron sulfides often leads to the formation of maghemite, an intermediary product of non-porous calcine, which negatively affects cyanide leaching. Magnetic recovery circuits are often used to capture the highly magnetic maghemite from the leach tails. The magnetic concentrates are either routed to autoclaves for pretreatment or as part of the raw feed to the roasters. The component that goes into the autoclave is ground finer prior to pressure oxidation. This fine grinding presents an added processing cost. The main objective of this project was to explore the possibility of recovering gold from the maghemite-rich magnetic concentrates by roasting. Preliminary roasting tests were conducted on pure maghemite standard samples and the results obtained were used as a basis for further roasting the magnetic concentrate samples. The magnetic concentrates were characterized before and after the experiments to determine any changes in mineralogy, composition, and surface characteristics. Roasting at 650oC or higher resulted in at least 50% maghemite-to-hematite conversion. The roast calcines were cyanide-leached for gold recovery. Results suggest that roasting improves gold recovery from the magnetic concentrates from no extraction to about 85% extraction. Enriching the roast air with oxygen did not yield in any appreciable increase in gold recovery. Additional test results have also shown that finer grinding of the magnetic concentrates and/or prolonging the roasting time barely reduces the arsenic, organic carbon and sulfide sulfur content of the resulting calcine. Therefore, these processes do not result in any appreciable increase in gold recovery. Even though roasting has been proven as a feasible means of recovering appreciable amount of the gold held in the maghemite-rich magnetic concentrates, it is highly recommended that its application on a large scale be preceded by a detailed economic analysis.