Nickel mineralization is normally present in one of two forms, as oxide minerals or as sulphide minerals. Historically, nickel sulphides have been preferred for nickel extraction as the composition of the minerals makes the nickel more efficient to extract, first by flotation to concentrate the nickel (e.g. to 20% purity) and second by smelting to produce anickel matte (e.g. to 80% purity). Nickel purity is increased during refining.
Nickel laterite makes up 70% of the world’s nickel reserves, and at the rate that nickel sulphide deposits are being depleted, production is switching to laterites which now account for 60% of the world’s nickel. Nickel laterite is normally processed by pyrometallurgy (i.e. heat), hydrometallurgy (i.e. acid), or in some cases blending with iron ore (i.e.ferronickel). Normally nickel laterite requires concentrating close to the minesite before it can be shipped any significant distance for further processing.
The global demand for nickel has been increasing ata rate of 4% annually. With the recent development of electric vehicles, the batteries of which are heavily dependent on nickel, global nickel demand is expected to increase at an even greater rate. In 2021 the US government putnickel on a critical minerals list citing the possibility of serious shortages during the time period 2025-2035.
Nickel laterite is formed by the weathering of nickel-bearing silicate minerals such as peridotite, dunite and harzburgite.These minerals are magnesium silicates containing iron, nickel, cobalt and minor concentrations of platinum and palladium (PGM). Weathering results from the surface exposure of these easily altered minerals to sunlight and precipitation.The silicates breakdown and wash away leaving higher concentrations of iron, nickel, and cobalt behind. The starting concentration of an unaltered peridotite can be range from 0.15-0.25% nickel whereas the laterite is typically 1% nickel and higher. Nickel grade usually increases with depth to a maximum near or at the unaltered basement rocks.
The main advantage of nickel laterites is that theyare at-surface deposits extending over a large lateral distance. This results in a large tonnage deposit which can be extracted at lower mining costs (e.g.open pit compared to underground mining).
Nickel laterites appear as soil and as altered rocks (corestone). Usually the laterite is more soil-rich closer to surface and becomes more rock abundant at depth. The soil has a higher nickel content compared to the corestone, but the latter is also economic. The saprolite zone,composed of hydrous magnesium silicates (serpentine, chlorite, garnierite), occurs closest to surface. This can be followed by a clay zone known assmectite. The lower limonite zone has a higher abundance of iron and generally contains the highest nickel grades. In some highly altered laterites, there isan iron cap at surface. There is often little to no significant soil material (suitablefor vegetation) overlying a laterite.
While nickel laterites are normally found close to the equator, there is an extensive occurrence of laterites that extend from northern California into southern Oregon. These deposits were formed over the past 3-5 million years. Mining of laterites in Oregon took place from the 1950s to the 1980s at the Riddle Mine located near Riddle, Oregon. The mine closed in 1984. Significant deposits of nickel laterite remain in Oregon, including at Cleopatra and Red Flat, both located in Curry County.
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