Refractory ores - Russian gold mining technology

Materials World magazine
,
10 Feb 2013

Michael Forrest talks to Peter Hambro about the technological changes required to maintain production in Russia’s gold mines.

Spending on mineral exploration in Russia accounts for 6% of the world total – just one sixth of Canada’s outlay, despite having a surface area 1.7 times larger. But this is not a reflection on the lack of mineral deposits in Russia – it is a legacy of political history. Russia is simply not on the radar of junior exploration companies, and only a few majors have explored here.

There are, however, exceptions to this trend. The foremost company is Petropavlovsk plc, whose Chairman, Peter Hambro, has been active in the country for some time. In 1994, the owner of a gold mining licence for a deposit in the Amur region of the Russian far east, Pavel Maslovskiy, was seeking finance to develop it. Hambro assisted in financing the project and the two began the development of a gold mining industry in the remote region. Hambro explains, ‘Pavel and I began what was to become Petropavlovsk, but to this day the partnership has endured without the normal legal contracts that usually bind partners. Our agreement is based on trust.’

Gold mining in a remote region is never easy, and although the company is now the second largest gold producer in Russia, it has plans in place to ensure exploration and production are maintained. The original mining was based on Soviet era resources at the Pokrovskiy mine, which by 1999 was producing gold from a heap-leach/Merrill Crowe operation. Two years later, a greenfield prospect at Pioneer was licensed to the company. Other prospects were explored and by 2005, licenses for the Malomir and Albyn deposits were under development. In 2012, Hambro forecasted that around 700,000 ounces of gold will be recovered.

To be successful in such an inhospitable location, labour, equipment and technology have to be readily available. As a result, the company has developed its own process and exploration laboratory facilities – the largest of their kind in Russia. It also has its own engineering and construction companies, its own process engineering company in St Petersburg and a mining process and engineering institute in Irkutsk. Having these facilities has enabled development of a series of mines whose initial production was based upon oxide ores. However, although these are still in production, the resources have a limited mine life. For example, the Pioneer mine that began operation in 2008 has around four years’ worth of oxide ores left at the current rate of production, while the Malomir mine, commissioned in 2010, has only two years of production from oxide resources.

In Russia, as in many other countries, the easiest gold deposits have been exploited, leaving the more difficult ones as the resource base. It may be those at great depth, as in South Africa, but by far the most common are the refractory resources. These are the difficult-to-treat ores that commonly contain gold within the lattice or coated by other minerals, commonly sulphides. They do not respond to gravity upgrading nor do they allow the cyanide dissolution of the gold grains. Petropavlovsk has been working on this problem for a number of years to maintain gold production from its refractory ores. And this is not just a company issue – around 80% of all Russian gold resources are classified as refractory or part-refractory, including Sukhoi Log, the nation’s largest deposit.

POX in Petropavlosk

According to the company’s technical teams, nearly 95% of the JORC-compliant reserves at the Malomir and 60% at the Pioneer mine are refractory, and a resolution to this problem is required within the next two years if they are to maintain production. Around the world, a number of solutions to the recovery problem have been developed, which broadly fall into three categories. The oldest technology is roasting – literally heating a concentrate of the ore in a furnace to oxidise the sulphide mineral, leaving a gold-containing residue that can be leached using a traditional cyanide plant. However, it is environmentally unfriendly as the exhaust gas contains sulphur dioxide that, when it meets moisture in the atmosphere, produces acid rain.

A more recent technology uses sulphide-consuming bacteria to break down the minerals, so that cyanide solutions can contact the microscale gold particles. However, the ambient winter temperature in the Russian far east is too low to facilitate the growth of bacteria. This leaves the third option of pressure leaching the gold-bearing minerals in an oxygen-charged autoclave.

Pressure oxygen leaching (POX) is a well-established technology. The first base-metal pressure leach was commissioned in 1952 by Sherritt Gordon in Canada to treat nickel sulphide ores, and the first gold POX plant was at the McLaughlin mine in California, USA, in 1985. More recently, Agnico-Eagle uses POX technology at its Kittila mine in Finland, and Barrick is using similar plants at Pueblo Viejo, in the Dominican Republic. Petropavlovsk has determined that this is the best technology for treating its refractory ores and has been undertaking bulk testing and process optimisation at its pilot plant at Blagoveschensk, where it has a 40-litre replica of a full-scale industrial autoclave. This work showed that it would be best to initially install four autoclaves at its Pokrovskiy mine, which will act as a central processing hub for its mines in the Amur region.

POX is not the only change that will be required in the mining and processing of its refractory ores. At present, oxide ores are crushed and milled for treatment in a conventional resin in pulp (RIP) cyanide process, followed by electrowinning and doré production. As it would be economically impractical to build a POX plant at each mine, flotation will be added to the process circuit to enable a concentrate to be trucked to the central hub at Pokrovskiy.

The first capital addition of the move to refractory ores is the need to produce the concentrate. The new processing route will start with crushing and grinding, followed by industry-standard flotation tanks that will concentrate the gold-bearing sulphides. Waste material will be dewatered and stored in a tailings facility and concentrates from both Pioneer and Malomir will be trucked to the Pokrovskiy hub. The second capital element is the autoclave process unit and the attendant oxygen plant. In addition, a new quarry for limestone will be built to neutralise the sulphuric acid by-product. ‘Test work in our pilot plant has indicated a 93–98% recovery, and we have partnered with Outotec to ensure installation is optimised to the highest level,’ says Hambro. The autoclaves are 14x3m diameter and lined with ceramic tiles. Residence time will be 45 minutes, replicating millions of years of atmospheric oxidation. Of the 80m3 capacity, 60m3 will be concentrate and the remainder oxygen. Following pressurisation, the concentrate (now highly acidic from the breakdown of the sulphides) requires neutralisation, as RIP cyanidation demands an alkaline feed. The choice of Pokrovskiy is not only geographic – a large amount of equipment and expertise, including the RIP, was already on-site.

The necessary move to refractory ore processing has an all-up capital cost of around US$500m, including the satellite flotation plants and the POX hub. Operating costs will change, with an increase in processing and transport costs offset by reduced mining costs reflecting the lower stripping ratios of the refractory ores. Says Hambro, ‘Overall, the operating cost per ounce of gold will rise by about US$100 to around US$750. The move to refractory ore will double the amount of gold reserves and ensure production into the future’.