Lateritic nickel ore deposits

Lateritic nickel ore deposits are surficial, weathered rinds formed on ultramafic rocks which contain 0.2% or more of nickel, resulting in a goethite-limonite or nontronite clay containing 0.5% to 2.5% nickel.

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Ore Genesis

Nickeliferous laterite is formed by weathering and oxidation of ultramafic rocks which preferentially enriches the laterite (a residual regolith material) in nickel at the expense of magnesium and silica which are removed by dissolution in the environment.

Lateritization of ultramafic igneous rocks (serpentinite, dunite, or peridotite containing about 02 - 0.3% nickel) often results in a considerable nickel concentration.

Types of nickel laterites

Two kinds of lateritic nickel ore have to be distinguished: goethite-limonite types and nontronitic clay types.^[1]

Limonite type laterites (or oxide type) contain 1-2% Ni bound in goethite which is highly enriched due to very strong leaching of magnesium and silica. Beneath this zone nickel silicate ore can be formed, frequently containing > 2% Ni that is incorporated in silicate minerals primarily serpentine and chrysoprase. In pockets and fissures of the serpentinite rock green garnierite can be present in minor quantities, but with high nickel contents - mostly 20-40%. It is bound in newly formed phyllosilicate minerals. All the nickel in the silicate zone is leached downwards (absolute nickel concentration) from the overlying goethite zone. Absence of this zone is due to erosion.

Saprolite type laterites are formed beneath the oxide zone, which may sometimes be removed by erosion. Saprolite nickel ores may contain green nontronitic nickel bearing clays, garnierite, chrysoprase and are generally deficient in nickel bound in goethite-limonite.

Ore deposits

Typical nickel laterite ore deposits are very large tonnage, low-grade deposits located close to the surface. They are typically in the range of 20 million tonnes and upwards (this being a contained resource of 200,000 tonnes of nickel at 1%) with some examples approaching a billion tonnes of material. Thus, typically, nickel laterite ore deposits contain many billions of dollars of in-situ value of contained metal.

Ore deposits of this type are restricted to the oxide and saprolite profiles developed above ultramafic rocks.^[2] As such they tend to be tabular, flat and areally large, covering many square kilometres of the Earth's surface. However, at any one time the area of a deposit being worked for the nickel ore is much smaller, usually only a few hectares. The typical nickel laterite mine often operates as either an open cut mine or a strip mine.

Extraction

Nickel laterites are a very important type of nickel ore deposit. They are growing to become the most important source of nickel metal for world demand, and are second for now to sulphide nickel ore deposits.

Nickel laterites are generally mined via open cut mining methods with ore extracted via some form of hydrometallurgy process, with two main process routes; high-pressure acid leach (HPAL) and for some types of limonite type nickel laterites, heap leach-SX-EW process routes are viable.

HPAL Processing

High Pressure Acid Leach processing is required for nickel laterite ores with a predominantly nontronitic character where nickel is bound within clay or secondary silicate substrates in the ores. The nickel (+/- cobalt) metal is liberated from such minerals only at low pH and high temperatures, generally in excess of 250 degrees celsius.

The advantages of HPAL plants are that they are not as selective toward the type of ore minerals, grades and nature of mineralisation. The disadvantage is the energy required to heat the ore material and acid, and the wear and tear hot acid causes upon plant and equipment. Higher energy costs demand higher ore grades.

Heap (Atmospheric) Leach

Heap leach treatment of nickel laterites is primarily possible only for clay-poor oxide-rich ore types where clay contents are low enough to allow percolation of acid through the heap. Generally, this route of production is much cheaper - up to half the cost of production - due to the lack of need to heat and pressurise the ore and acid.

Ore is ground, agglomerated, and perhaps mixed with clay-poor rock, to prevent compaction of the clay-like materials and so maintain permeability. The ore is stacked on impermeable plastic membranes and acid is percolated over the heap, generally for 3 to 4 months, at which stage 60% to 70% of the nickel-cobalt content is liberated into acid solution, which is then neutralised with limestone and a nickel-cobalt hydroxide intermediate product is generated, generally then sent to a smelter for refining.

The advantage of heap leach treatment of nickeliferous laterite ores is that the plant and mine infrastructure are much cheaper - up to 25% of the cost of a HPAL plant - and less risky from a technological point of view. However, they are somewhat limited in the types of ore which can be treated.

Pig iron oxide ores

A recent development in the extraction of nickel laterite ores is a partcular grade of tropical deposits, typified by examples at Acoje in the Philippines, developed on ophiolite sequence ultramafics. This ore is so rich in limonite (generally grading 47% to 59% iron, 0.8 to 1.5% nickel and trace cobalt) that it is essentially similar to low-grade iron ore. As such, certain steel smelters in China have developed a process for blending nickel limonite ore with conventional iron ore to produce stainless steel feed products. This pig iron process short-circuits the typical costly hydrometallurgical route for producing nickel, which is then used in stainless steel anyway.

See also

• Ore genesis
• Laterite
• Open cut mining
• Nickel

References

1. ^ Schellmann, W. (1983): Geochemical principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, Sao Paulo, 119-135
2. ^ Golightly, J.P. (1981): Nickeliferous Laterite Deposits. Economic Geology 75, 710-735