Nickel from bacteria
Pekka Perä Chief Executive Officer, Talvivaara plc, talks to Michael Forrest about his company’s technology for nickel recovery in Finland.
The world production of nickel is around 1.4Mt, of which two thirds is recovered from sulphide deposits with the remainder from oxide ores, usually nickel laterites. Nickel mines often have credits from chromium, cobalt, copper and platinum group metals that can significantly increase the value of the ore. Where these are of low concentration or absent, there is a cut-off grade below which mining, crushing and concentrating nickel-bearing minerals is uneconomic.
Hydrometallurgy offers a technology with reduced costs for metal recovery and works effectively on many oxide ores where weak sulphuric acid acts as the solvent providing a pregnant metal-rich solution that can form a concentrate by precipitation. However, sulphide ores are not amenable to acid leaching under atmospheric pressure, and accordingly most nickel sulphide ores are treated by concentrating, roasting and smelting – an expensive option for low-grade ores.
The action of bacteria on sulphide minerals has been known for over 3,000 years. In 1947 the role of bacteria was identified in leaching and a few years later Thiobacillus ferrooxidans and Thiobacillus thiooxidans were indentified (Thiobacillus has been renamed Acidithiobacillus) as principal bacteria in breaking-down sulphide minerals. In the 1960s industrial-scale bacteria heap-leaching of copper ores was established followed by gold leaching in the 1980s. Today 40 plants around the world use bioleaching technology to produce copper, gold, zinc and cobalt. ‘Talvivaara,’ states Perä, ‘is the first company to apply bioleaching to the production of nickel.’
The project had a long gestation beginning with the work of the mining company Outokumpu, Espoo, Finland, that extensively explored and tested the Talvivaara deposit, 450kms north east of Helsinki, discovered by the Geological Survey of Finland in the 1970s. The mineralisation is hosted in recrystallised carbon and sulphide-rich black shales metamorphosed to schist grade surrounded by quartz-mica schists and quartzites of the Karelia supergroup, that in turn lie on Archean basement. At Talvivaara, two polymetallic ore bodies (Kuusilampi and Kolmisoppi) have been identified over a distance of four kilometres and it is likely that they are linked.
Talvivaara is one of Europe’s largest sulphide nickel deposits. Measured and indicated resources are 642Mt with an additional inferred resource of 362Mt using a cut-off grade of 0.07% nickel, giving a total of over one billion tonnes at an average grade of 0.22%. ‘The deposit is flat-lying, near-surface and ideal for open pit mining with minimal stripping ratio of ore to waste of one to one. In addition to nickel there are commercially significant amounts of copper, cobalt and zinc as well as uranium and manganese,’ states Perä.
The key to this deposit is the ability to recover metals from the complex mineralogy. Heap leaching is ideal, but requires the breakdown of the sulphide minerals. Outokumpu’s work, including process trials lead the way to Talvivaara’s current flow sheet. ‘It enabled our scientists and engineers to develop a bioleaching system that recovers close to 100% of a number of metals, and offers the possibility of more in the future’ says Perä.
‘The concept relies on the effect of bacteria that grow naturally in the ore and convert insoluble metals sulphides into water-soluble sulphates. Current understanding is that mineral oxidation is driven by chemistry rather than biology, for example increasing temperature speeds the reaction, and the role of the microbes is to produce leaching chemicals. Microbes also provide the most efficient reaction space for bioleaching to occur through contact leaching where ferrous-ferric cycling releases thiosulphate and/or sulphur dependent on the mineralogy. These sulphur compounds in solution act as an energy source for unattached bacteria.’
He explains, ‘Our research has shown that a number of factors affect leaching including temperature and the need to keep ferric iron and metals in solution. Diverse microbial cultures have proved more robust than single strains. The grain size of the ore is also important as leaching is proportional to the mineral surface area as is the porosity of the mineral that also increases contact area’.
The bioleaching microbes used are acidophiles, active in the pH range one to three. They get their energy by oxidising iron and/or inorganic sulphur compounds with carbon needed for growth from atmospheric carbon dioxide. They can tolerate high metal concentrations and over 10bln microbes can be found in a teaspoon of solution.
Going with the flow
The flow circuit for the leaching begins with crushing the ore to eight millimetres. The ore is then treated with the addition of sulphuric acid and placed on large-scale leach pads with a residence time of 15-18 months for the primary pad, recovering >80% of metals. The material is then reclaimed and conveyed to the secondary heap pad, recovering metals from ores where the primary solution had poor contact. This secondary leaching increases nickel recovery to >90%. The metals are recovered sequentially from the pH2.3 pregnant leaching solution (PLS). The addition of gaseous hydrogen sulphide precipitates copper and zinc sulphides at pH1.5 to 2.0. At pH3.3 to 4.0, nickel and cobalt are recovered from solution. Increasing pH to 5.5 by adding limestone precipitates iron and aluminium, while adjusting pH to 9.5 by adding slaked lime brings out manganese and magnesium.
‘The mine [covers] over 61km2. The leach pads are 2,400m by 800m with 6,700km of bioleaching irrigation and recycling pipelines transporting 30,000 cubic metres of solution every hour. Significant aeration is achieved by pipes embedded in the heaps. The benefits of the hydrometallurgy are numerous. It is a simple and inexpensive process with substantially lower capex and opex than in traditional smelting and refining processes. There are no sulphur dioxide emissions as in smelters, no need for high pressures or temperatures as in pressure acid leach, and the residues are less active than those from physico-chemical processes. It is ideal for low-grade sulphide ores often making possible recovery of metals from lower grade ores economic’, adds Perä.
Full production will begin in 2012, but the company expects to mine 24Mt of ore and 16Mt of waste this year. At full capacity, metal recovery will be approximately 50,000t of nickel and 90,000t of zinc. At year’s end production costs are estimated to be $3.1/lb ($6,832/t) comparing favourably with prices of around $19,000/t.
Nickel will be the prime income and Talvivaara has secured a 10-year off-take agreement for 100% of its main output of nickel and cobalt with Moscow-based Norilsk Nickel and entered into a long term zinc streaming arrangement with Nyrstar NV for 1.25Mt of zinc concentrate. The company has also lodged an application in accordance with the Finnish Nuclear Energy Act to the Ministry of Employment and Economy for the extraction of uranium as a by-product. The solvent extraction plant is expected to have an annual production of 350t/y of uranium at a capital cost of 30 million euros at an estimated production cost of 2.60euros/lb. The current yellowcake price is c US$50/lb.