Bringing Nickel back via ore sorting technology
03 December 2014
Sensor Ore Sorting (now also available in the new Mobile Sorting Solution) has been applied to pre-concentrate nickel sulphide ores, and the nature of such ores offers the metallurgist a number of properties of the valuable mineral and the host minerals to achieve a separation assuming that some liberation occurs at coarse sizes.
Annual consumption of new nickel metal is around 1 million tonnes with other supply coming from stainless steel scrap recycling. Nickel is an attractive silvery-white metal with a high resistance to corrosion and oxidation, excellent strength and toughness, and alloys readily with many other metals. Nickel has a wide range of applications encompassing domestic consumer, heavy industrial, aerospace, marine, and metal plating applications.
It is mainly used in the form of stainless steels which are produced in a wide range of compositions to meet specific applications requiring say corrosion and heat resistance, in heavy industry or hygienic surfaces for food processing
More than 70% of the world’s nickel occurs in laterites with remainder being mostly in sulphides. Laterites are produced by groundwater action and other weathering mechanisms which concentrate valuable metals.
Nickel laterites usually contain cobalt and vary with the location of formation; generally wet deposits in the tropics e.g. New Caledonia are higher grade compared to the older dry laterites such as those found in Australia which also have a higher clay content. Processing of laterites is quite complex and there are number methods in practice including:-
- High Pressure Acid Leaching (HPAL) which produces Nickel and Cobalt metal in the form of briquettes.
- Pyro-metallurgical processes to produce Nickel-Iron alloys
- Ferro-Nickel containing up to 40% Ni the remainder being iron plus some minor contaminants
- Nickel Pig Iron (NPI) – a cheaper process which produces a much lower Nickel content alloy ~ 12% Ni and used to make what is commonly called “Rusty Stainless”
Current nickel production is predominantly from Sulphide ores as Capital and Operating costs of process plants are much cheaper, however, unless considerably more sulphide ore bodies are found production from laterites will inevitably increase based on forecast demand.
The principal nickel sulphide mineral is Pentlandite and commonly occurs with other sulphides such as chalcopyrite, pyrrhotite and pyrite, and often with a mineral broadly described as Serpentenite. Pentlandite is an iron-nickel sulphide, (Fe, Ni) 9 S8 and is usually described as having a Ni: Fe ratio of 1:1.
The close association of Pentlandite with the mineral Pyrrhotite ~ (Fe7 S8) is due to similar conditions during geologic formation . The two minerals are difficult to differentiate visually with both being a metallic yellow colour, dense and hard. Pyrrhotite is usually weakly magnetic, and this property assists in identification.
Beneficiation of sulphide ores at most nickel mines around the world follow a standard process flowsheet comprising crushing, grinding and flotation to make nickel concentrates for despatch to smelters and refiners.
In any process flowsheet development if ores are amenable to pre-concentration at coarse sizes, an opportunity presents to reject a portion of the crushed ore between the crushing and grinding circuits as a barren fraction, which means the size of all downstream processes will be reduced, or conversely, the overall treatment rate increased, which could be particularly significant in terms of operating cost and product output.
Pre-concentration can add previously uneconomic zones to Ore Reserves help manage ore blending programs more effectively and can be applied to selective upgrading of low grade stockpiles and even waste dumps to recover value from previously uneconomic material, as well as reduce environmental risks and costs, reduce tailings dam or tailings filter size, and reduce milling energy requirements.
Pre-concentration of ore at remote sites can reduce haulage costs. Satellite ore-bodies can be mined more effectively if pre-concentration scalping is carried out in-pit or close to pit.