Increase in gold dissolution in copper ammonia thiosulfate solution via cobalt surface modification

Abstract

When gold was leached using copper ammonia thiosulfate solution, the dissolution rate of gold decreased as the leaching time increased. The dissolution of gold powder with 44 and 77 μm particles was studied，and the results indicated that the dissolution rate was not decreased by surface passivation or the decrease in copper concentration but by the changes of the electronic structure of the gold surface caused by the change in the exposed crystal surface. The transmission electron microscopy images of the gold powder revealed that the high-index (220) and (311) crystal surfaces almost disappeared during leaching. Furthermore, the surface of gold consisted mainly of the low-index (111) crystal surface during the later stages of leaching. Owing to the changes in crystal surface, the work function of the gold surface increased from 6.56 to 6.95 eV, and the increase in work function hindered the loss of electrons from the gold surface. When cobalt was used to modify the gold surface, the work function decreased from 6.56 to 5.99 eV. The leaching results indicated that the cobalt modification increased gold dissolution, and the gold leaching percentage was increased by 40% within 8 h. the tendency of gold to dissolve more easily after it underwent surface modification was confirmed by the apparent activation energy of the surface-modified being lower than that of the fresh gold powder, the formation of intermediate products of Co3O4 with high oxidation ability and the galvanic interaction between cobalt and gold electrodes. Dry-grind was applied for gold concentrate calcine and different contents of cobalt powder to study the possibility of industrial application of this effect. The results showed that only adding 50 mg cobalt to 100 g gold ores was beneficial to gold leaching