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 usua ...

rocks. They account for 80% of the

continent A continent is any of several large geographical regions. Continents are generally identified by convention (norm), convention rather than any strict criteria. A continent could be a single large landmass, a part of a very large landmass, as ...

al world

nickel Nickel is a chemical element; it has 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 slo ...

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

Genesis and types of nickel laterites

Lateritic nickel

ores Ore is natural Rock (geology), rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the ...

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, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

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

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

which consist largely of the

magnesium Magnesium is a chemical element; it has Symbol (chemistry), 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 ...

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

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 Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

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 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, the crystalline forms of silica (silicon dio ...

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 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 refers to a cal ...

, 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 () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

. Ore deposits of this type are restricted to the weathering mantle developed above ultramafic rocks. 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 methods. Nickel is extracted from the ore by a variety of process routes. Hydrometallurgical processes include high-pressure acid leach (HPAL) and heap leach, both of which are generally followed by solvent extraction – electrowinning ( SX-EW) for recovery of nickel. Another hydrometallurgical routes is the Caron process, which consists of roasting followed by ammonia leaching and precipitation as nickel carbonate. Additionally, ferronickel is produced by the rotary kiln electric furnace process ( 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 New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...

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. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

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

cobalt Cobalt is a chemical element; it has Symbol (chemistry), 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. ...

) 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 In physics, chemistry, and materials science, percolation () refers to the movement and filtration, filtering of fluids through porous materials. It is described by Darcy's law. Broader applications have since been developed that cover connecti ...

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 is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and a nickel-cobalt hydroxide intermediate product is generated, generally then sent to a

smelter Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron, copper, silver, tin, lead and zin ...

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 (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

sequence ultramafics. This ore is so rich in

(generally grading 47% to 59%

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, 0.8 to 1.5%

and trace

) that it is essentially similar to low-grade

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the f ...

. As such, certain steel smelters in China have developed a process for blending nickel limonite ore with conventional iron ore to produce

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

feed products like nickel pig iron.

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