Good use of a pressure cooker

It’s the only one in New Zealand, and one of only 10 in the world processing gold, but the autoclave at Oceana Gold Corporation’s Macraes site is a key to the viability of both its Macraes and Reefton goldmines. By HUGH DE LACY.

Without its autoclave, only 70 percent of the gold in the rock at Macraes could be recovered, and only an uneconomic 15 percent at Reefton. With it, the gold extraction rate for both is around 85 percent.

Autoclaves are essentially pressure-cookers and there are thousands of them working in a range of industrial processes throughout the world, but they come into their own in the goldmining industry when dealing with highly refractory ores from which the gold is difficult to extract.

The Macraes autoclave, which handles ore concentrates from the Frasers underground and Macraes opencast mines, as well as concentrate railed down from Reefton, was built by the former owner of the East Otago mine, Perth-base Gold Resource Developments (GRD), which spun the Macraes tenement off as listed company, Oceana Gold, in 2004.

GRD sank $20 million into the development and commissioning of the machine which was based on pressure oxidation technology licensed by its developer, Australian multinational Newmont Mining Corporation. Since then the technology has been further refined at Macraes under the direction of Oceana’s chief operations officer, Mark Cadzow.

“The reason the autoclave is so economic for us is that we produce a concentrate first – a lot of other autoclaves around the world uses whole-of-ore oxidation pressure,” Cadzow told Q&M. They include the Twin Creeks project in Nevada where Newmont developed the technology, making GDR/Oceana’s use of a flotation concentrate a world first.

Aside from reducing the bulk of material to be processed, concentration raises the sulphur grade beyond the minimum of six percent needed to make the reaction exothermic, or heat-producing, thus obviating the need to add steam to the autoclave except at start-up. The Macraes concentrate has about 12 percent of sulphur while Reefton runs to 22-23 percent.

The autoclave has a capacity of 57 cubic metres, is built from 40mm steel plate, has an overall length of 12.6 metres and an inside diameter of 3.5 metres. It’s lined with a proprietary membrane and two layers of acid-resistant brick, with the latter also used to divide it into three compartments. The first of these contains two agitators, and there is one in each of the others.

Apart from start-up steam, oxygen and quench water are all that is added to the autoclave compartments as the concentrate undergoes oxidation. The plant producing the oxygen is the second biggest in New Zealand after that at the Glenbrook steel mill near Hamilton. At the rate of 180 tonnes a day, the plant takes oxygen from the air, cleans it and compresses it to liquid for storage in one 20-tonne high-pressure and two 75-tonne low-pressure tanks. The oxygen plant is manned five days a week but for the rest of the time is run remotely from Auckland, 1500 kilometres away.

The autoclave’s first compartment operates at temperatures ranging from 205ºC to 220ºC and the other two at 225ºC, with concentrate being fed in initially at the rate of up to 29 tonnes per hour at the operating density of 40 percent solids, though subsequent modifications now allow it to handle up to 40 tonnes an hour.

The machine’s life expectancy is about 30 years. Most parts are replaceable, and regular maintenance involves changing the acid-resistant bricks, as well as pipes and valves. Macraes is a round-the-clock operation and, with stops only for maintenance, the autoclave is in operation 95 percent of the time.