Nitric nickel - recovery through nitric acid process
Michael Forrest talks to Andrew Bell, Chairman of UK firm Regency Mines
plc, about a new process for recovery of nickel from laterites.
World annual demand for nickel is around two million tonnes and over the past decades there has been a move to recover nickel from laterite deposits that were formed in tropical weathering environments.
In terms of resources, the United States Geological Survey (USGS) estimates that some 60% of global resources grading more than 1% nickel are found in laterites where the nickel is held in oxides and hydroxides, with the remaining 40% in sulphide deposits. For lower grade deposits the percentage is higher. New sulphide deposits are proving harder to find, while known laterites are extensive, particularly in the Pacific Rim, where nickel-containing basic and ultrabasic rocks have been deeply weathered.
Laterites are best developed on gentle topography where the erosion products of topical wet and dry season chemical erosion remain in situ. This results in the development of limonitic, iron-rich material that overlays saprolitic magnesium-rich horizons, which in turn overlays bedrock. Highly concentrated forms of nickel can be found in unweathered rock as oxides or phyllosilicates such as garnierite. As weathering and groundwater circulation are the principal processes, very large deposits can be found.
Such is the case with the Mambare project, located in the foothills of the north coast of Papua New Guinea. Andrew Bell, Chairman of AIM-listed Regency Mines plc based in London, UK, explains that its Mambare venture has an indicated and inferred JORC-compliant resource of 162.5Mt grading 0.94% nickel and 0.09% cobalt. Here, extensive limonite and saprolite horizons are developed to more than 30 metres from the surface, and recent drilling at depth has revealed high-grade saprolites – for example, 5.8 metres at 2.82% nickel. Exploration drilling has so far only covered 6% of the Mambare licence area and less the 3% of the plateau, which is the primary exploration target. The consistency of results across the drilled area indicates the possible world-class scale of lateritic mineralisation that consistently covers the underlying mafic bedrock, which is apparent in local drainage systems across the entire tenement.
In 2011, Regency entered a joint agreement with Direct Nickel, an Australian company that, along with its technical partners CSIRO and Teck Resources, has developed a new method for recovering nickel from laterite deposits. Lateritic nickel has been exploited in a number of countries with a variety of techniques. Two principal methods have been established, namely pyrometallurgy and acid leaching. Pyrometallurgy produces ferronickel or nickel pig iron (NPI – a low-grade ferronickel) product for steelmaking, or matte (a mixture of copper and iron sulphides) for downstream smelting, while acid leaching recovers nickel from solution. Ferronickel, NPI and matte production require the removal of the large amount of water present in lateritic ores, and a high-temperature electric furnace to reduce the nickel oxides. And with grades greater than 2% desirable, energy requirements for the process are high.
Conventional sulphuric acid leaching requires high temperatures (greater than 250°C) and atmospheric pressures (up to 44 atmospheres) to achieve economic recovery from the ore. First used in Cuba in 1959, high-pressure acid leaching has a large technical input in the construction of the plant and high acid consumption in operation. Modern high-pressure acid leach (HPAL) requires titanium-lined autoclaves and sulphuric acid generation. Even so, many HPAL projects have suffered from technical difficulties and cost overruns.
Direct Nickel has determined that its technology will only be licensed to those companies that will enter a joint venture. ‘Regency’s joint venture at Mambare is the first project to secure a licence to use Direct Nickel’s nitric acid recovery process,’ says Bell. Exploration at Mambare during the 1960s and 1970s only identified the upper limonitic horizon, with lower levels described as weathered bedrock. Regency drilling revealed that the lower levels were in fact hard saprolites that early drills had failed to penetrate. In early test work, analysis of samples from the separate horizons revealed nickel concentration of 1.1% in the limonite and 1.5% in the saprolite. However, says Bell, ‘this would provide a challenge for the conventional HPAL using sulphuric acid, as separate process circuits would be required for the saprolite and limonite layers’. The major problem with the saprolite is its high magnesium oxide content, which consumes large amounts of acid to achieve nickel-rich solutions. Furthermore, because the residual leached waste contains sulphur, neutralisation is required to meet environmental standards.
Although nitric acid is more expensive than sulphuric acid, it offers a number of advantages. Foremost is its ability to treat both the limonite and saprolite lateritic ores in the same process circuit, and recycling of the nitric acid allows for very low acid consumption. HPAL is constrained to recover the nickel in limonite, as the high magnesium content in saprolites leads to uneconomically high acid consumption.
Nitric acid leaching of lateritic ores takes place at atmospheric pressure and in temperatures reaching 100°C. Run-of-mine ore is crushed and delivered to leach tanks, to which the acid is then added. Residence time is short – two to four hours – and results in a pregnant leach solution containing nickel, cobalt, iron, aluminium and magnesium. Insoluble material (mostly silica) is removed by filtration, and iron is also removed this way following hydrolysis. Aluminium is then precipitated by the addition of magnesium oxide and filtered out. Finally, a mixed hydroxide product (typically 40–45% nickel and 2% cobalt) is precipitated and removed via filtration, leaving behind a barren leached solution of magnesium nitrate.
From an economic perspective, the most critical component is the recycling of magnesium nitrate to recover nitric acid. This is achieved through evaporation of the barren solution followed by thermal decomposition to magnesium oxide and nitrous oxide gases. The magnesia is a high-quality, saleable product (though some is retained for the precipitation steps), while the gases are recovered to nitric acid and recycled.
Compared with HPAL, the system has several additional advantages:
- Capital costs are greatly reduced, as welded 304- or 316-grade stainless steel can be used for the acid tanks, replacing the expensive, explosively This is due to nitric acid’s passivating reaction with stainless steel, which obviates high-cost corrosion-resistant linings.
- As process pressure is atmospheric, the operating intensity and operating safety are much more manageable than HPAL.
- The recycling of the acid provides the biggest contrast in acid consumption – around 20–40kg/t ore compared with more than 500kg/t for HPAL.
- Waste is considerably reduced to around half the ore volumes, through the production of iron and magnesium by-products. Leach residues – mainly silicates with residual nitrates – are expected to be able to promote plant growth over tailings dumps. HPAL waste, on the other hand, contains sulphides and ore – usually much greater in volume than that of the original ore.
In addition to the joint venture, Regency has a 7.5% equity stake in Direct Nickel. ‘We have been encouraged by the successful recovery of nickel from a mixed feed of 25% limonite and 75% saprolite of an Indonesian ore source – this indicates a robust technology,’ says Bell. Furthermore, acid recovery tests conducted in 2010 at the Direct Nickel pilot plant in North Carolina, USA, proved positive and have since been verified by Aker Solutions Pty Ltd.
The next phase is to refine the nitric acid process at demonstration scale. To this end, Direct Nickel is focusing its efforts on optimising the continuous process at its test plant in Perth, Australia, with completion expected by the end of 2013. Regency has engaged technical consultants at Australian firm RMDSTEM to review the project, to identify the key issues needing to be addressed ahead of project development and to help to streamline feasibility studies to maximise value. With the piloting process nearly complete and the results due shortly, Bell hopes that the technology will soon not only be generally recognised by the industry, but prove to be a revolution in nickel production.