Oxygen for gold leaching (CIL and CIP)

Why the leach needs oxygen

An Eriez article republished by Austmine in October 2020 notes that the gold-cyanide reaction summed up by the Elsner equation needs both oxygen and cyanide to progress. A 1992 review by K. E. Haque, published by the Canadian Institute of Mining, Metallurgy and Petroleum, explains why oxygen matters: most gold occurs in the elemental state, its oxidation is vital for dissolution in the cyanide solution, and cyanidation at high pH favours that oxidation with a mild oxidant such as air or oxygen.

Haque also notes that preaerating the slurry with air or oxygen alleviates the harmful effects of soluble sulphides, arsenates and antimonates on cyanidation, and that refractory gold can be liberated by roasting in the presence of oxygen or by leaching under oxygen pressure.

CIP and CIL

In carbon-in-pulp (CIP), as described in US patent 4,816,234 (1989), milled ore is first leached in a series of agitated vessels, where the gold is largely dissolved, and the pulp then passes through vessels where granules of activated carbon adsorb the gold from the solution. Carbon-in-leach (CIL) does both at once: the same patent describes dissolution and adsorption as essentially simultaneous, and a 2019 Cyanide Code audit of a CIL plant describes it as leaching and adsorbing gold onto activated carbon at the same time.

Air, enriched air or pure oxygen

Eriez explains that injected oxygen must first dissolve, and that transfer through the gas-liquid interface, the slower of the two steps, typically controls the rate; injecting (sparging) the gas directly into the leach tank enlarges that interface. Peroxide addition is another way to raise dissolved oxygen, but its use has tailed off, mainly because of its high cost, and gas injection is the most common method.

Air, oxygen-enriched air or high-purity oxygen are typically used. Henry's law sets an upper limit to the dissolved oxygen that air can give; beyond it, the oxygen partial pressure has to be raised by enriching the air or using pure oxygen. Eriez notes that oxygen costs more than air but that the metallurgical gains of oxygen sparging more often than not offset the cost, while site-specific considerations sometimes make air the preferred gas.

Published figures

• In a 1995 study of one antimony-bearing gold ore (M. Wong, Canadian Institute of Mining, Metallurgy and Petroleum), extracting 94% of the gold took 1 g/L of sodium cyanide and 48 hours at 7 ppm dissolved oxygen with 90% of the ore below 200 mesh, but 0.25 g/L and 16 hours at 20 ppm with 80% below 200 mesh. For that ore, oxygen-enriched leaching cut retention time by three times and cyanide consumption by four times.
• A March 2019 Cyanide Code recertification audit of one gold mine records an average dissolved oxygen of about 24 ppm in its first two leach tanks, two PSA oxygen generators producing about 8 tonnes of oxygen a day at 92% purity for the CIL tanks, and additional oxygen dosed as hydrogen peroxide on leach tanks 3 and 4.

These figures describe one ore and one plant.

On-site oxygen from ULTRAOX VSA

Ultra Controlo lists gold leaching in mines among the applications of ULTRAOX VSA, its industrial oxygen plant. ULTRAOX VSA delivers 75% to 95% oxygen at 50 kPa, with an additional oxygen booster for higher pressures, comes on a skid or in a container, and can replace cryogenic systems with liquid oxygen tanks or work as the main source of supply. Its seven models deliver 18 to 240 m³/h at 93% oxygen; see industrial oxygen generators.