Indirect bio-oxidation: Treatment of refractory gold and silver sulfides

Abstract

In this research, the process of gold and silver extraction from a refractory gold-silver sulfide ore, sourced from the Gran Chimú Province in the La Libertad department of Peru, was evaluated. The extraction was carried out in two stages, the first being called indirect bio-oxidation, and the second involving conventional cyanidation. In the first stage, an acidic solution from the area was used to cultivate Thiobacillus ferrooxidans bacteria, which was promoted by the addition of salts (9K). The objective of this stage was the transformation of ferrous cations to ferric, which was evidenced by the color change of the acidic water to red. In the second stage, a solution of sodium cyanide and calcium oxide (lime) was used, and the total cyanidation time was 51 hours at an average pH of 10.5 and ambient temperature. The quantification of gold and silver was done through a dry method (fire assay). The evaluated process variables were the concentration of silver salt (0, 0.75, and 1.5) mg/g, mineral particle size (80% passing 100 mesh, 80% passing 200 mesh, and 80% passing 325 mesh), and temperature of indirect bio-oxidation (25, 45, and 65)°C. To evaluate the effect of the bio-oxidation pretreatment, conventional cyanidation tests were conducted, resulting inextraction percentages of 44.61% for gold and 48.98% for silver, using 80% passing 325 mesh particle size. This indicated the need for the mentioned pretreatment to release the gold and silver from the ore. Meanwhile, the extraction percentages using the two stages were 61.44% for gold (80% passing 325 mesh, 65°C, 0.0 mg/g) and 94.24% for silver (80% passing 200 mesh, 65°C, 0.75 mg/g) in the best cases, and 35.98% for gold (80% passing 100 mesh, 45°C, 0.0 mg/g) and 41.44% for silver (80% passing 100 mesh, 25°C, 0.0 mg/g) as the lowest recorded values..