Bacterial oxidation technology — A viable process for refractory gold ores and base metals recovery

Abstract

Bacterial oxidation technology can be applied to refractory sulphide ores and concentrates to break down the mineral matrix and release the precious and base metals. The precious metals and some base metals remain in the oxidized residues and can be recovered in a conventional CIP or other chemical leaching step. Some base metals, such as copper, zinc and nickel, enter the acidic solution and may be recovered directly by conventional solvent extraction and electrowinning. A moderately thermophilic bacterial culture has been developed which can operate at high ambient temperatures without the need to cool the process reactors. The ability to operate at these higher ambient temperatures makes the culture more suitable for use in hot tropical or hot arid regions than the Thiobacillus species, which can only operate at temperatures up to 40 °C. The moderately thermophilic culture has been proven in the gold fields of Western Australia during the summer when ambient temperatures were as high as 49 °C. The maximum reactor temperature reached 55 °C and regularly exceeded 50 °C. This bacterial culture has been applied to a range of refractory gold ores and concentrates from Australia, South-East Asia and North America. Types of samples tested included pyrite ore and concentrate, arsenopyrite concentrate, arsenical bulk concentrate, copper — gold concentrate, carbonaceous pyrite concentrate and antimonial concentrate. The culture has shown resistance to high levels of arsenic present in some concentrates and has been applied at concentrations up to 25 g/L arsenic. Antimony has not affected the bacterial oxidation process as it was relatively insoluble in the sulphate environment and probably precipitated as antimony oxides. One refractory gold sample has been extensively tested at a mine site over a 6 month period to prove the technology and to obtain design data for a commercial plant. The bacterial culture proved extremely robust under typical operating conditions. The technology was simple to apply and the plant was operated by site personnel for the majority of the test period. A feasibility study showed the operating costs were very competitive and that the estimated capital cost could be recovered over a relatively short operating period. Base metals present in refractory gold concentrates were released as the bacterial oxidation progressed, often in the early stages of oxidation. One concentrate containing 0.5 to 1.0% nickel as a nickel arsenic sulphide gave nickel extractions greater than 95%. A second concentrate contained significant copper. More than 88% of this copper was removed during the oxidation which was necessary to improve gold recovery and reduce the cyanide consumption. The copper could have been recovered by solvent extraction and electrowinning. A third concentrate contained 7% zinc and 110 ppm silver. Both metals were released from the mineral matrix as the concentrate was oxidized and could be recovered, the silver with the gold in CIP leaching and the zinc by precipitation from the bacterial oxidation solution. The bacterial culture has been also applied for the extraction of base metals from sulphides not containing precious metals. A nickel - iron sulphide ore was oxidized using bacterial oxidation treatment and gave a nickel extraction of 93% compared with only 33% by conventional iron (III) sulphate leaching. The nickel did not prove toxic to the bacteria nor did it retard the progress of the oxidation. A second application was to use bacterial oxidation to boost the recovery of copper from a chalcocite -chalcopyrite ore. Recovery after oxidation was about 95%. The economics of these base metal extraction processes are being determined.