Lateritic nickel ore deposits are surficial, weathered rinds formed on

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

rocks. They account for 73% of the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions In geography, regions, otherwise referred to as zones, lands or territories, are areas t ...

al world

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

resources and will be in the future the dominant source for the mining of nickel.

Genesis and types of nickel laterites

Lateritic nickel ores formed by intensive tropical

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

of olivine-rich ultramafic rocks such as dunite, peridotite and komatiite and their serpentinized derivatives,

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''se ...

which consist largely of the

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

silicate serpentine and contains approx. 0.3% nickel. This initial nickel content is strongly enriched in the course of lateritization. Two kinds of lateritic nickel ore have to be distinguished: limonite types and silicate types. Limonite type laterites (or oxide type) are highly enriched in iron due to very strong leaching of magnesium and silica. They consist largely of

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

and contain 1-2% nickel incorporated in goethite. Absence of the limonite zone in the ore deposits is due to erosion. Strong weathering of ultramafic rocks at the earth's surface in humid conditions causes nickel resources to form inside nickel laterites. Laterites are formed by the breakdown of minerals which then leach into groundwater, the leftover minerals join to form the new mineral known as laterites. Nickel is turned into usable quality ore grade by being merged into the newly formed stable minerals. Silicate type (or saprolite type) nickel ore formed beneath the limonite zone. It contains generally 1.5-2.5% nickel and consists largely of Mg-depleted serpentine in which nickel is incorporated. In pockets and fissures of the serpentinite rock green

garnierite Garnierite is a general name for a green nickel ore which is found in pockets and veins within weathered and serpentinized ultramafic rocks. It forms by lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits in the ...

can be present in minor quantities, but with high nickel contents - mostly 20-40%. It is bound in newly formed

phyllosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

minerals. All the nickel in the silicate zone is leached downwards (absolute nickel concentration) from the overlying goethite zone.

Ore deposits

Typical nickel laterite ore deposits are very large

tonnage Tonnage is a measure of the cargo-carrying capacity of a ship, and is commonly used to assess fees on commercial shipping. The term derives from the taxation paid on ''tuns'' or casks of wine. In modern maritime usage, "tonnage" specifically r ...

, 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 A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

. Ore deposits of this type are restricted to the weathering mantle developed above ultramafic rocks.Golightly, J.P. (1981): Nickeliferous Laterite Deposits. Economic Geology 75th Anniversary Volume, 710-735 As such they tend to be tabular, flat and really 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 (currently second to sulfide nickel ore deposits). Nickel laterites are generally mined via

open cut mining Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique of extracting rock or minerals from the earth from an open-air pit, sometimes known as a borrow. This form of mining ...

methods. Nickel is extracted from the ore by a variety of process routes. Hydrometallurgical processes include high-pressure acid leach (HPAL) and

heap leach Heap leaching is an industrial mining process used to extract precious metals, copper, uranium, and other compounds from ore using a series of chemical reactions that absorb specific minerals and re-separate them after their division from other e ...

, both of which are generally followed by solvent extraction - electrowinning ( SX-EW) for recovery of nickel. Another hydrometallurgical routes is the

Caron process A caron (), háček or haček (, or ; plural ''háčeks'' or ''háčky'') also known as a hachek, wedge, check, kvačica, strešica, mäkčeň, varnelė, inverted circumflex, inverted hat, flying bird, inverted chevron, is a diacritic mark (� ...

, which consists of roasting followed by ammonia leaching and precipitation as nickel carbonate. Additionally, ferronickel is produced by the

rotary kiln electric furnace process Rotary may refer to: General * Rotary motion Engineering and technology * Rotary dial, a rotating telephone dial * Rotary engine (disambiguation), multiple types of engines called "rotary" * Rotary latch * Rotary milking shed, a type of milking ...

(RKEF process).

HPAL process

High pressure acid leach processing is employed for two types of nickel laterite ores: # Ores with a limonitic character such as the deposits of the Moa district in Cuba and southeast New Caledonia at Goro where nickel is bound in

and asbolan. # Ores of a predominantly nontronitic character, such as many deposits in Western Australia, where nickel is bound within

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

or secondary silicate substrates in the ores. The nickel (+/-

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...

) metal is liberated from such minerals only at low pH and high temperatures, generally in excess of 250 °C. 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 applicable to clay-poor oxide-rich ore types where clay contents are low enough to allow

percolation Percolation (from Latin ''percolare'', "to filter" or "trickle through"), in physics, chemistry and materials science, refers to the movement and filtering of fluids through porous materials. It is described by Darcy's law. Broader applicatio ...

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 Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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.

FerroNickel process

A recent development in the extraction of nickel laterite ores is a particular grade of tropical deposits, typified by examples at Acoje in the Philippines, developed on

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

sequence ultramafics. This ore is so rich in limonite (generally grading 47% to 59%

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

, 0.8 to 1.5%

and trace

) 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.

Nitric acid hydrometallurgical tank leach

Another new method of extracting nickel from laterite ores is currently being demonstrated at a pilot scale test plant at the CSIRO facility in Perth Australia. The DNi process uses nitric acid, instead of sulphuric acid, to extract the nickel within a few hours and then the nitric acid is recycled. The DNi process has the major advantage of being able to treat both limonite and saprolite lateritic ores and is estimated to have less than half the capital and operating costs of HPAL or FerroNickel processes.

References

{{ores Economic geology Ore deposits Nickel Regolith