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Houston-based Viridian Resources LLC believes that its patented technology to recover nickel using plants will enable it to become major 'mining' company. Viridian and its associates have developed a process known as phytomining, which uses naturally-occurring plants that have evolved to thrive on nickel-bearing soils. Viridian has evaluated some 300 species of nickel-accumulator plants from around the world and collected, propagated and cross-bred the most promising varieties to produce the plants that it uses. These perennial plants (which grow for several years) are 'hyperaccumulators' of nickel. They are capable, when fully mature, of accumulating 1.75-2.9% nickel (by weight) in their leaves. The plants can then be burned and the nickel recovered using a variety of conventional process routes. The company says that its technology will enable it to produce nickel at a negative net operating cost, and to develop deposits that are uneconomic using alternative process routes.
According to Viridian, for its process to be viable soils must have a minimum nickel content of 0.05% Ni. Viridian notes that soils with a higher nickel content would not lead to higher nickel yields in the plants, but would extend the life of a phytomining operation. The company says that nickel-rich soils should produce 20 t/ha of nickel- bearing biomass, which when burned should yield an ash at a grade of 30-40% Ni. The final yield is around 350 kg of nickel per hectare. In addition, if the plant matter is used as a fuel for electricity generation, in excess of 20 MWh of energy would be generated from each harvested hectare. Viridian estimates that in Australia, Turkey, Brazil, South Africa and New Caledonia alone, there are in excess of 1.6 Mha of soils suitable for phytomining.
According to Viridian, the phytomining process involves the sowing of seeds of the company's perennial plants on suitable land, and the harvesting and field drying of the plants to produce hay using conventional agricultural equipment.The dried plants are then baled and incinerated.
According to Viridian, the resulting ash can be processed to extract nickel, cobalt and PGM using a variety of conventional techniques. The company notes that in its ongoing joint venture with Inco Ltd of Canada, ash produced from plants grown on contaminated land at the Port Colborne nickel smelter has been successfully added to the nickel converters. Nickel can also be recovered from the ash in the form of intermediate products or as nickel cathode following solvent-extraction and electrowinning. Furthermore, Viridian believes that in some countries, such as the US, operations based on its process could generate additional revenues from carbon- credits/allowances for the growing of the plants, and further credits/allowances for producing energy from a renewable source.
Viridian believes that in the US, using a 10,000 ha operation (approximately 3,500 t/y of nickel), it will be able to produce nickel at a negative net operating cash cost of US$0.28/lb if by-product revenues and credits are taken into account. Without credits, such an operation would have a cash cost of less than US$1.35/lb. The forecast capital costs of the process are US$2/lb/y compared with those of a conventional operation in the region of US$30/lb/y.
In addition to Inco, Viridian says that it is in negotiations with a number of other mining companies and organisations relating to the use of its technique, on greenfield projects, at existing mine sites and on contaminated land.
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