Enhancement of Precious Metal Recovery by Geothermal Heat

Abstract

Nevada is the leading producer of gold in the United States. Processing large volumes of low grade ore is accomplished by cyanide heap leaching. The process entails mining, crushing, grading and stacking the ore which is then soaked with a dilute solution of sodium cyanide. The cyanide dissolved both gold and silver, which is eventually recovered by additional processing. Heap leaching takes many months to complete, and the operation depends heavily on the weather. Active mines that use cyanide heap leaching either shut down during the winter, or use expensive oil emersion heaters to prevent freezing in the heap and pipelines. The rate at which cyanide can dissolve gold and silver increased, like most chemical reactions, with the application of heat. Laboratory experiments indicate that the recovery of precious metals was 17 to 40 percent greater when "geothermal" heat was applied to the cyanide leaching process. Increasing cyanide solution reaction rates decreased the time required for batch heap leaching and increases productivity.