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Nickel laterite deposits – geological overview, resources and exploitation
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Nickel laterite deposits – geological overview, resources and exploitation
M. Elias
Mick Elias Associates, CSA Australia Pty Ltd, PO Box 139, Burswood WA 6100, Australia
In: Giant Ore Deposits: Characteristics, genesis and exploration, eds DR Cooke and J Pongratz. CODES Special Publication 4, Centre for Ore Deposit Research, University of Tasmania, pp 205-220.
This paper reviews the nature and genesis of nickel laterite mineralisation, and describes the relationship between deposit characteristics (both geological and non-geological) and the successful development of lateritic deposits as commercial nickel producers. The importance of nickel laterites lies in their huge resource base, which could potentially provide a much greater share of global nickel production than their current level compared to nickel from sulphides.
Most of the world’s terrestrial nickel resources are hosted in nickel laterites, the products of intense weathering of ultramafic rocks at the surface of the earth in humid climatic conditions. The process of lateritisation involves the breakdown of primary minerals and release of their chemical components into groundwater, the leaching of mobile components, the residual concentration of immobile or insoluble components, and the formation of new minerals which are stable in the weathering environment. The combined effects of these processes is to produce a vertical succession of horizons of differing chemistry and mineralogy (the laterite profile), the overall structure of which is governed by the differential mobility of the elements in the weathering zone. The detailed structure of the profile varies greatly, and in any one place is the result of the dynamic interplay of climatic and geological factors such as topography, drainage, tectonics, structure and parent rock lithology. Nickel can be enriched to ore grade in parts of the profile by being incorporated into the structure of the newly formed stable minerals or into the alteration products of primary minerals.
Exploitation of nickel laterites provides about 40% of the world’s production of nickel.
Three process routes are used commercially, each of which is suited to only part of the laterite profile. Hydrometallurgical processes of sulphuric acid leaching and reduction roast-ammonia leaching are used to extract nickel and cobalt from the upper, low- magnesium part of the profile, and smelting is used for the high-magnesium silicates lower in the profile. The economics of nickel laterite processing are strongly dependent on grade and composition of ore feed, economies of scale, location, availability of low-cost energy and well-developed infrastructure. Historically, nickel laterite projects have proven difficult to develop and reach their nameplate capacity, but the enormous surface resources of lateritic nickel provide compelling incentive to overcome engineering challenges inherent in their successful treatment. The outlook is for a greater proportion of nickel production in the future to come from lateritic sources.
Introduction
Laterites are the residual products of chemical weathering of rocks at the surface of the earth, in which various original or primary minerals unstable in the presence of water, dissolve or break down and new minerals are formed that are more stable to the environment. Laterites are important as hosts to economic ore deposits, as the chemical interactions which together make up the lateritisation process can in certain cases be very efficient in concentrating some elements.
Well-known examples of important lateritic ore deposits are aluminous bauxite and enriched iron ore deposits, but lesser known examples include lateritic gold deposits (e.g., Boddington in Western Australia) (Evans, 1993).
Nickel laterites are the product of lateritisation of Mg-rich or ultramafic rocks which have primary Ni contents of 0.2-0.4% (Golightly, 1981). Such rocks are generally dunites, harzburgites and peridotites occurring in ophiolite complexes, and to a lesser extent komatiites and layered mafic-ultramafic intrusive rocks in cratonic platform settings (Brand et al, 1998).
Lateritisation processes result in the concentration by factors of 3 to 30 times the nickel and cobalt contents of the parent rock. The processes, and the character of the resulting laterite, are controlled on regional and local scales by the dynamic interplay of factors such as climate, topography, tectonics, primary rock type and structure.
Figure 1: Global distribution of sulphide and laterite nickel deposits
CUBA
INDONESIA
AUSTRALIA
LATERITES SULPHIDES
NEW CALEDONIA
PHILIPPINES 22oN
22oS
Most lateritic nickel resources occur within a band about 22 degrees of latitude either side of the equator (Fig. 1) and the giant, and in some cases highest grade, deposits are concentrated in tectonically active plate collision zones (eg Indonesia, the Philippines and New Caledonia) where extensive obducted ophiolite sheets are exposed to aggressive chemical weathering in tropical conditions of high rainfall and warm temperatures, and there is the greatest opportunity for supergene enrichment. Resources in cratonic settings can be large but tend to be lower in grade (e.g. Murrin Murrin in Western Australia). Cratonic shield deposits in West Africa (Nahon et al, 1982) and Brazil (Schobbenhaus, 1986) are within the equatorial zone, but those in the Balkans (Greece, Albania and former Yugoslavia) (Valeton et al, 1987) and the Yilgarn craton in Western Australia occur at higher latitudes. The latter two examples are “fossil” deposits, currently situated in temperate or arid climates quite different from the warm, humid conditions under which they formed.
Nickel laterites play an important part in the global nickel industry and currently account for around 40% of the total nickel production of about 1 million tonnes. About 70% of all continental or terrestrial nickel resources are contained in laterites. Production of nickel from lateritic sources as a proportion of total (sulphide plus laterite) nickel production has remained fairly constant over the last ten years (Fig. 2), but is expected to grow with time as easily-won sulphide resources are depleted. The main barriers to more rapid growth in lateritic nickel production are the high capital cost of processing facilities, high energy requirements in the pyrometallurgical process routes, and the technical challenges of making hydrometallurgical processing more efficient.
This paper is divided into two sections. Part 1 describes the processes by which lateritic deposits enriched in nickel are developed over ultramafic rocks, the environmental factors controlling the processes, and the nature of the lateritic profile formed as a result of these processes. Part 2 discusses the production of nickel from laterites, the extraction processes used on a commercial scale, the structure of the nickel laterite industry and describes the factors that characterise commercially successful operations.
0 100 200 300 400 500 600 700 800
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
tonnes Ni ('000)
30.0%
32.0%
34.0%
36.0%
38.0%
40.0%
42.0%
44.0%
46.0%
48.0%
50.0%
% Lateritic Ni
Sulphide Laterite
% Laterite
Figure 2: World nickel production by ore type, 1988-2001.
Part 1 – Geology of nickel laterite deposits
Geology of laterite profiles on ultramafic rocks
The process referred to as “lateritisation” is essentially chemical weathering taking place in seasonally humid climates over long periods of time in conditions of relative tectonic stability, allowing the formation of a thick regolith with distinctive characteristics (Trescases, in Butt and Zeegers, 1992). Table 1 lists the main effects of chemical weathering of rocks in general, and how these processes are manifested in the weathering of ultramafic rocks. In summary, the process of lateritisation involves the breakdown of primary minerals and release of some of their chemical components into groundwater, the leaching of mobile components, the residual concentration of immobile or insoluble components, and the formation of new minerals which are stable in the weathering environment. The net effect of the mineral transformations and the differential mobility of elements involved produces a stratified or layered mantle of weathered material overlying the parent rock from which it was formed, which is generally referred to as the
“laterite profile”.
Figure 3: Schematic laterite profile developed on ultramafic rock in a tropical climate (Fe oxide-dominant limonite zone), showing indicative chemical compositions in wt%. See Figure 4 for indicative thicknesses of units.
Fe MgO Ni Co
>50% <0.5% <0.6% <0.1%
40-50% 0.5-5% 0.8-1.5% 0.1-0.2%
10-25% 15-35% 1.5-3% 0.02- 0.1%
5% 35-45% 0.3% 0.01%
Bedrock Saprock Saprolite
“Limonite”
Ferricrete
The process is dynamic and gradual, and the gross stratification that one sees in the laterite profile (Fig. 3) is essentially a snapshot of lateritisation in progress. The lowest layer reflects early stages of weathering of the bedrock, and each layer further up represents a transformation of that lying immediately below it, displaying progressively advancing stages of the process. In the lowermost part of the profile (saprock), weathering takes place at contacts between minerals and at fracture boundaries and there is abundant fresh rock and little alteration product. Further up the profile, the proportion of surviving primary minerals decreases, and more strongly fractured zones are completely altered eventually leaving detached boulders of intact bedrock
“floating” in a mixture of primary and alteration minerals in which the primary rock fabric is preserved (saprolite). Higher layers consist totally of alteration minerals, and are marked by the eventual loss of primary fabric. The zone above the saprolite is termed the pedolith in the technical literature (Butt and Zeegers, 1992), but the term is seldom used in practice. The pedolith is more often referred to as the limonite zone, the latter term being derived from the dominant mineralogy (goethite and hematite) in this zone in oxide laterites.
Table 1: Main processes of chemical weathering and their effects in ultramafic rocks (after Butt and Zeegers, 1992, p. 10)
General processes Effects in ultramafic rocks 1. Leaching of mobile constituents:
alkalis, alkaline earths
Breakdown of olivine, pyroxene, serpentine and leaching of Mg, Ni, Mn, Co
2. Formation of stable secondary minerals: Fe and Al oxides, clays
Goethite formation, smectite formation, adsorption of Ni from solution
3. Partial leaching of less mobile components: silica, alumina, Ti
Leaching of silica in rainforest and moist savanna climates
4. Mobilisation and partial
reprecipitation of redox-controlled constituents: Fe, Mn
Precipitation of Mn oxides and
adsorption of Ni and Co from solution 5. Retention and residual
concentration of resistant minerals:
zircon, chromite, quartz
Residual chromite concentration
Factors influencing weathering and profile development
The processes and conditions that govern and control lateritisation of ultramafic rocks are numerous and varied on all scales, and consequently the nature of the profile varies in detail from place to place in thickness, chemical and mineralogical composition, and the relative development of individual profile zones. The main factors influencing the efficiency and extent of chemical weathering, and consequently the nature of the profile, are:
Climate: Rainfall determines the amount of water passing through the soil, which influences the intensity of leaching and removal of soluble components. In addition to the amount, the effectiveness of rainfall (the extent to which water is allowed to pass down through the profile rather than running off) is important. Higher mean soil temperature (which is close to mean
surface air temperature) increases the kinetics of weathering processes (Butt and Zeegers, 1992).
Topography: Relief and slope geometry influence drainage, the extent to which water passes into the soil, and water table level.
Drainage: Drainage affects the net water budget available for leaching from the whole landscape.
Tectonics: Tectonic uplift increases erosion of the top of the profile, increases topographic relief and lowers the water table. Tectonic stability allows planation of the landscape, slowing groundwater movement.
Parent rock type: Mineralogy determines susceptibility of rocks to weathering and the elements available for recombination as new minerals.
Structure: Faults and shears provide discrete zones of bedrock permeability; jointing and cleavage improve pervasive alteration potential.
Clearly many of these climatic and geological factors are closely interrelated, and the characteristics of a profile at any one place can best be described as due to the combined effect of all the individual factors acting over time, rather than being dominated by any single factor.
The thickness of the laterite profile is determined by the balance between the rates of chemical weathering at the base of the profile and physical removal of the top of the profile by erosion.
The rate of chemical weathering varies from 10 to 50 metres per million years, is generally proportional to the quantity of water percolating through the profile, and is 2-3 times faster in ultramafic rocks than sialic rocks (Nahon, 1986). Trescases (1975) has estimated that the rate of downward movement in the base of weathering in the highlands of southern New Caledonia to be from 125 to 140 metres per million years, but one-tenth this rate in the plateaux and terraces.
Rapid erosion rates restrict profile development to thicknesses of 50 to 100 metres, and their age is therefore probably less than 1 million years.
Estimated rates of chemical weathering in stable cratonic settings where relief is subdued and rainfall somewhat lower (e.g. Brazil and West Africa) are also in the range of 10 metres per million years (Golightly, 1981). In these areas, tectonic stability and low erosion rates allow the development of profiles over 100 metres thick, but these are older than those in tectonically active settings.
Laterite profile types
Despite the complexity and interplay of controls, there are a number of broad features of the laterite profile that are common to most examples, and it is possible to describe the range of laterite types formed over ultramafic rocks in terms of three main categories on the basis of the dominant mineralogy developed in the profile:
Oxide laterites: comprise largely Fe hydroxides and oxides in the upper part of the profile, overlying altered or fresh bedrock;
Clay laterites: comprise largely smectitic clays in the upper part of the profile, and Silicate laterites: comprise hydrated Mg-Ni silicates (serpentine, garnierite) occurring deeper in the profile, which may be overlain by oxide laterites.
Oxide laterites: These are the most common final products of lateritisation of ultramafic rocks.
In the presence of water, primary rock-forming minerals (mainly olivine and/or serpentine, orthopyroxene and less commonly clinopyroxene) break down by hydrolysis releasing their constituents as ions in aqueous solution. Olivine is the most unstable mineral and is the first to be weathered; in humid tropical environments its Mg2+ is totally leached and lost to groundwater, and Si is largely leached and removed. Fe2+ is also released but is oxidised and precipitated as ferric hydroxide, initially amorphous or poorly crystalline but progressively recrystallising to goethite which forms pseudomorphs after olivine. Orthopyroxene and serpentine hydrolyse after olivine, also releasing Mg, Si and being replaced by goethitic pseudomorphs. Initially, while co- existing ferro-magnesium minerals remain unweathered and support the rock fabric, the transformation is isovolumetric and primary rock textures are preserved, but as the extent of destruction of primary minerals increases, relict primary textures are lost by collapse and compaction of the fabric resulting in a textureless massive goethite. The mineralogical transformation involving loss of Mg and residual concentration of Fe results in the obvious and familiar chemical trend in laterites of Mg decreasing upwards and Fe increasing upwards through the laterite profile (Fig. 3).
Ni and Co behave differently to the major elements. Nearly all of the original Ni and Co in the ultramafic bedrock occurs in solid solution in olivine and olivine-derived serpentine. As these minerals break down, the released Ni and Co ions have a chemical affinity for the newly-formed poorly-crystalline Fe hydroxides and are incorporated and concentrated into their structure by a combination of adsorption and replacement of Fe3+ (Gerth, 1990). Contents of 1.5% Ni and 0.1%
Co are seen in massive goethite developed from olivine containing 0.3% Ni and 0.02% Co. Ni and Co are also incorporated strongly into Mn oxides (asbolanes) where these are precipitated by redox reactions as veins and surface coatings on minerals and in fractures. Mn oxide minerals containing up to 12% Ni and 8.5% Co are known (Elias et al, 1981).
Mineralogical trends in the pedolith reflect the gradual transformation of goethite to hematite.
The first-formed Fe hydroxides resulting from the breakdown of ultramafic minerals are amorphous or poorly crystalline. Their crystallinity improves with time to well-structured goethite with a characteristic yellow-brown colour, which is progressively replaced by red-brown hematite is the goethite dehydrolises. The colour change is reflected in the commonly-used terminology of “yellow limonite” and “red limonite” for the lower and upper parts of the
“limonite” zone respectively. The transformation of goethite to hematite is accompanied by a loss of Ni, as hematite cannot accommodate in its lattice the Ni formerly contained in the goethite. At the very top of the profile, a nodular fabric develops in the red limonite, which develops further to an indurated crust as the nodules coalesce and harden. The crust is known as ferricrete, iron crust, or by the French term cuirasse.
Oxide-dominated deposits can pass directly from totally altered goethitic saprolite down into fresh bedrock over a distance of only a few centimetres. More generally there is an interval between goethite saprolite and the fresh bedrock interface, as shown in Figure 3, which comprises a mixture of goethite, oxidised ultramafic material and fresh ultramafic rock boulders.
Important examples of oxide deposits as described above are Moa Bay and Pinares (Cuba) (Linchenat and Shirokova, 1964), and Goro and Prony (southern New Caledonia) (Golightly, 1981).
A particular variety of oxide deposit that is formed over dunite bedrock is composed of goethite and minor clay with abundant free chalcedonic silica in forms ranging from fine-grained particles to coarse veins and discontinuous lenses and masses. Examples of this silica-oxide laterite can be found in association with clay laterites developed over peridotites, and it appears the lack of Al in the dunite precursor precluded development of secondary clay. Cawse and Ravensthorpe in Western Australia are examples of silica-oxide laterites (Brand et al, 1998).
Clay laterites: In less severe conditions of weathering, e.g., cooler or drier climates, silica is not leached as it is in humid tropical environments, and instead combines with Fe and a small amount of Al to form a zone where the smectite clay nontronite predominates, in place of Fe oxides.
Nontronite plays a similar role to goethite in fixing Ni ions within its lattice where they substitute for Fe2+ and are fixed in inter-layer positions (Brand et al. 1998). Nontronite clays typically contain 1.0-1.5% Ni in mineralised laterite. Silica in excess of that required to form nontronite can be deposited as opaline or chalcedonic nodules in the clay. Clay laterite profiles are also preferentially developed where there is restricted groundwater movement such as in broad areas with low topographic relief (Golightly, 1981). The clay horizon may be overlain by a thin zone of more Fe-rich oxide material which is generally low in Ni, and is underlain by partially weathered saprolite containing serpentine and nontronite. Clay-dominated nickel laterite profiles are extensively developed in Australia; e.g, Murrin Murrin (Monti and Fazakerley , 1996), Bulong (Elias et al, 1981) and Marlborough (Golightly 1981) and in Brazil.
Figure 4: Schematic comparison of principal laterite profile types
0
20
40
DEPTH (m)
SILICATE (eg New Caledonia)
CLAY (eg Murrin Murrin)
OXIDE (eg Moa Bay)
Cuirasse Red limonite Yellow limonite
Earthy ore Ore with boulders Rocky ore Bedrock
Bedrock Saprolite (Serpentine, chlorite, smectite)
Smectite zone
Ferruginous zone
Colluvium
Bedrock Saprolite Limonite Limonite overburden Iron cap
Silicate laterites: In situations where there is slow continuous tectonic uplift and the water table is kept low in the profile, weathering over long periods can result in the development of a thick saprolite zone, which may be overlain by a thin limonite zone depending on the intensity of erosion at the top of the profile (Golightly, 1981). Silicate laterites are characterised by an absolute enrichment or concentration of Ni in the saprolite zone which comprises altered primary minerals such as secondary serpentine, and neoformed goethite, smectite clays and “garnierite”
(garnierite is a term for mixed-structure hydrous Ni-Mg silicates of low crystallinity with affinities to serpentine, talc and chlorite; see Pelletier, 1996). Much of the nickel is derived from that released by the recrystallisation of goethite to hematite further up in the profile. Nickel is reprecipitated within the saprolite by substituting for Mg in secondary serpentines (which can contain up to 5% Ni) and in garnierite which can grade over 20% Ni (Pelletier, 1996). Average content of Ni in silicate laterites is typically 2.0-3.0%. Examples of silicate profiles are the economically important laterites on the massifs of New Caledonia (Golightly, 1981), which contain a large proportion of the world’s lateritic nickel resources. Silicate laterites are also the ore source of most of the nickel currently produced from laterites.
A schematic comparison of the three profile types is shown in Figure 4.
Regional and local geological setting of nickel laterite deposits
On a global basis, nickel laterite deposits are found in two tectonic settings (Brand et al, 1998):
Accretionary terrains – these are tectonically active zones often associated with oceanic or continental plate boundaries and collision zones. Thrust faulting has obducted slabs of upper mantle peridotites and associated rocks forming ophiolite complexes with extensive areas of exposure at surface. Tectonic processes (i.e., uplift) play a large part in influencing the type of nickel laterite deposits formed. Ages of host ultramafics and lateritisation range from Cretaceous through to the late Tertiary. These terrains are typical of active and inactive island arc settings of Indonesia, the Philippines and New Caledonia.
Cratonic terrains – laterites are developed on komatiites and the ultramafic phases of layered mafic complexes of any age from Archaean through the Palaeozoic. Relative tectonic stability allows peneplanation and the laterites are developed in moderate to subdued relief. Restricted drainage often results in smectite formation instead of oxides. Tectonic stability allows continuous lateritisation over extended time periods giving rise to deep weathering, and extension of laterite formation into cooler or less humid climate zones. Examples include the nickel laterites and bauxites of the Yilgarn craton, Western Australia, and parts of Brazil, West Africa and the Urals in Russia/Ukraine.
On a local or deposit scale, laterites may be classified according to their topographic setting into plateau, slope and terrace deposits (Troly et al, 1979).
Plateau deposits are affected by active drainage process but less by erosion, and hence tend to show complete profile development and form a thicker saprolite zone. The Thio Plateau and Koniambo deposits in New Caledonia are good examples of plateau deposits
Slope deposits are affected more by erosion and the oxide zone is poorly developed or absent.
Silicate laterite development can also be thinner than plateau deposits as the groundwater movement has a greater lateral component, i.e. downslope. However, the increased lateral groundwater flow can cause higher Ni grades to be developed.
Terrace deposits are relics of earlier peneplains or erosion surfaces and indicate a temporary stop of tectonic uplift. They tend to show development of complete profiles and of thick saprolite zones. Terraces can include the products of erosion of laterite from surrounding plateaux (particularly oxide material which is readily eroded) and multiple cycles of lateritisation are possible with the possibility of increased grades. Good examples of terrace deposits are found in southern New Caledonia, where the Goro deposit is the best known.
Global resources of nickel in laterites
Total global resources of nickel contained in laterites are about 160 million tonnes (Fig. 5).
When classified into limonitic (combined oxide and clay) and silicate-dominant deposits depending on the predominant profile zone (or in some cases the focus of a development project), limonitic or low-Mg deposits host about 70% of the total lateritic nickel resource. Because of their lower average grade, “ore” tonnages in limonitic deposits are closer to 80% of total tonnes.
When viewed on a regional basis (Fig. 6), the importance of just three countries, and in particular of New Caledonia, becomes very clear – New Caledonia, Indonesia and Australia together account for some 60% of the total resources of lateritic nickel, and New Caledonia, despite its small size, lays claim to 26% of the total. Nickel grade and proportion of resources in limonite vary between the three, with New Caledonia showing higher grades in both limonite and silicate ores. Its proportion of nickel in limonites is close to the global average of 70%, but these are nearly entirely in the small southern region (Massif du Sud). Australia, by contrast, has almost no saprolitic resources, and the grade of its limonitic laterites is considerably lower than other countries.
Figure 5: Global lateritic nickel resources (by contained nickel) Limonite
70%
Silicate 30%
9569 Mt ore 1.17% Ni 112.2 Mt Ni
2712 Mt ore 1.79% Ni 48.5 Mt Ni Total 160.7 Mt Ni
The range of individual deposit sizes and Ni grades for both silicate and limonitic (oxide and clay) laterites is seen in Figure 7. Limonitic laterites have a maximum deposit grade of 1.6% Ni and large tonnages, whereas silicate laterite deposits are smaller (mostly less than 50 Mt) and grades of 2.4-2.6% Ni predominate. A number of outstanding deposits in terms of size and grade are named in Figure 7.
Figure 6: Lateritic nickel resources by region based on contained Ni tonnes, divided into “limonite” (oxide plus clay) and silicate deposits.
Figure 7: Tonnage-grade plot of individual laterite deposits. Limonite deposits include oxide and clay laterites. See Table 4 for further information on named deposits.
Australia
Indonesia New Caledonia
Cuba Philippines
Brazil
Other Balkans Americas
Africa
silicate limonite 1.25%
2.49%
1.38%
1.88%
1.36%
0.86%
1.38% Average Ni grade
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0 50 100 150 200 250 300 350 400
Tonnes (millions)
Ni grade (%)
Soroa ko Bahodopi
Konia mbo
Goro Sam pala
Murrin
Silicate Limonite
Part 2 – Exploitation of nickel laterite resources
From the earliest nickel production in 1875 until the end of the 19th century when the vast sulphide nickel deposits of Sudbury came into production, laterites from New Caledonia were essentially the sole source of nickel in the world. Despite the fact that around 70% of global nickel resources are hosted by laterite, production of nickel from lateritic sources has lagged behind that from sulphides. Currently about 40% of nickel production comes from treatment of laterite ores, and this proportion has been in the range of 30-40% for the last decade (Fig. 2).
Before the late 1950s lateritic nickel production was about 10% of total nickel, but strong demand for nickel at that time driven largely by US requirements for the Korean War effort saw a number of laterite operations developed which quickly doubled the supply from laterites to 20%
of total nickel produced.
The reasons why the production of lateritic nickel is not at a level commensurate with its resource position relative to sulphide nickel are essentially economic, and are summarised in Table 2. The average weighted cost of production (fully allocated costs net of by-product credits) of nickel currently produced from laterites is greater than that for sulphide nickel by some 20%. The mining cost component of lateritic nickel production costs is considerably less than for sulphides, which are mostly produced from underground mines. Processing of nickel laterites is considerably more expensive than for sulphides, and more than offsets the mining cost advantage.
Table 2: Summary of issues relevant to the economics of nickel sulphide and nickel laterite deposits.
Sulphides Laterites
Treatment costs lower than for laterites - nickel in metastable minerals, which contain latent energy in the form of S
Nickel in stable oxide mineralogy which requires energy and/or aggressive chemical attack to disassociate.
Nickel-bearing minerals can be easily separated from barren (gangue) material at the mine site. Only the concentrate (higher grade, lower volume) undergoes further treatment.
Only rare opportunities to separate and concentrate nickel-bearing minerals, and must apply treatment processes to all the ore. Requires handling large quantities of material with low nickel concentrations, and large volumes of consumables which must be transported to the plant.
Ore and concentrate have generally
consistent characteristics within a deposit – easy to manage metallurgy.
High short-range variability of mineralogy and chemistry, requires careful planning, mining and blending to produce consistent, predictable feed for the treatment plant.
High recovery of nickel from ore possible with only one process route
Variable composition through profile restricts recovery of nickel from only part of the system with one process route.
High value in by-products (eg. Cu, PGE) helps to offset treatment costs of Ni
Few opportunities for by-product credits (only Co).
Moderate capital cost per unit Ni capacity for processing facilities.
Very high capital cost of processing
facilities – more so if the energy source has to be part of the capital cost.
Processing of nickel laterites
Methods currently in use on a commercial scale to extract nickel from laterites comprise three main processing routes:
• Smelting to produce ferro-nickel or matte,
• Caron process (reduction roast – ammoniacal leach), and
• High pressure acid leaching (HPAL) using sulphuric acid
Table 3 shows nickel production from laterites in 2001 by each of these process routes.
Due to the requirements of each process and the wide range in chemical and mineralogical composition in the laterite profile (see Fig. 3), each of these processes is only suited to a part of the profile.
The pyrometallurgical smelting route is the oldest and most widely used process. The process route treats the more nickel-rich silicate fraction of the profile and produces ferro-nickel (a reduced Fe-Ni alloy which can be directly used for stainless steel production) and sulphide matte (which can join a conventional sulphide treatment route) in an electric furnace. The technology is relatively simple, well-tried and reliable, and has the advantage that the composition of the ore makes it essentially self-fluxing. The process has high Ni recoveries (90%), but there is no Co recovered when ferro-nickel is produced. The process economics are most heavily dependent on the cost of power and the successful projects (e.g. Sorowako) have their own hydroelectric power plants.
The Caron process is applicable to oxide laterites (“limonite”) with tolerance for some silicate laterite, but excessive silica and Mg leads to decreased Ni recoveries. The process involves drying and roasting in a reducing atmosphere, followed by low-pressure ammonia leaching. Ni and Co are recovered by solvent extraction, and further refined to product stage by calcination and reduction. Recoveries of Ni are around 80% and of Co only about 40-50%. Ore drying and reduction roasting consume considerable energy, and process economics are heavily dependent on fuel prices. Caron plants were developed before the oil crises of the 1970s, and struggled to remain viable after fuel prices rose. None has been built since then, nor is there likely to be more built in the future.
The HPAL process was first used commercially in 1959 at Moa Bay (Cuba), which remained the only operating HPAL plant until the three Western Australian plants (Bulong, Cawse and Murrin Murrin) were developed in the late 1990s. The HPAL process involves leaching ore with sulphuric acid in an autoclave at about 250°C and extracting Ni and Co from the leach liquor by various methods such as sulphide precipitation using H2S, or solvent extraction and electrowinning. HPAL is used to treat predominantly the oxide fraction of the laterite and has high recoveries of both Ni and Co (over 92% in the leach stage). The process economics are largely dependent on the cost of sulphur and the conversion of the sulphur to sulphuric acid (sulphur-burning acid plants can generate much of the energy requirements for a HPAL plant).
As the consumption of sulphuric acid is determined mainly by the Mg level in the ore, the latter becomes a critical factor in managing mining and blending.
Table 3: Lateritic nickel production for 2001 by process route. Ores come from local deposits except where plants are marked with an asterisk, indicating production is from imported ores.
Plant Company Country Tonnes
Ni
% of total production
Smelting – Ferro-nickel
Doniambo SLN New Caledonia 45,912
Hachinohe Pamco* Japan 35,700
Hyuga Sumitomo* Japan 19,800
Oheyama Nippon Yakin* Japan 13,600
Falcondo Falcondo Domin. Republic 21,662 Cerro Matoso BHPBilliton Colombia 38,500
Larymna Larco Greece 18,600
Morro/Codemin Anglo-American Brazil 6,000 Loma de Niquel Anglo-American Venezuela 11,600
Pomalaa Aneka Tambang Indonesia 10,300
Various (4-5) Various Russia / Ukraine 21,600
S/T 243,274 51.2%
Smelting – Matte
Doniambo SLN New Caledonia 13,061
Sorowako PT Inco Indonesia 62,600
S/T 75,661 15.9%
Caron Process (Ammonia leach)
Yabulu Queensland Ni* Australia 28,500
Nicaro Cuban Govt Cuba 16,400
Punta Gorda Cuban Govt Cuba 26,900
Tocantins Votorantim Brazil 16,700
S/T 88,500 18.6%
HPAL (Acid leach)
Moa Bay Sherritt/Cuban G. Cuba 29,226
Bulong Preston Res. Australia 6,262
Cawse OM Group Australia 7,200
Murrin Murrin Anaconda Nickel Australia 24,991
S/T 67,679 14.2%
Total Laterite Production 475,114 100%
Total Sulphide Production 674,886
Total Nickel Production 1,150,000
The HPAL process can also be applied with high recoveries to clay laterites, but the presence of silica in the ore from both nontronite and serpentine can create slurry handling problems in the autoclave and subsequent steps; these require increased operating costs to overcome, and result in reduced efficiencies such as through-put rates in various parts of the plant. Pure oxide ores with low silica such as at Moa Bay are most efficient in the HPAL circuit.
Ore beneficiation
Certain types of lateritic ores can be beneficiated before being fed to the process plant.
Beneficiation is the process whereby a low-grade component of the mineralisation is separated from the rest and rejected, leaving a component with a higher grade to be treated. This is analogous to making a concentrate from a sulphide ore, but the concentration factor is much smaller for laterite ores. In the beneficiation process, some of the nickel is lost to the reject component but this is outweighed by the improved economics which result from the higher feed grade.
The practice has been applied for many years to silicate ores, where coarser boulders and fragments of hard, less-altered rock have much lower Ni content than the matrix of softer altered material in which they occur. Projects where optimisation of this type of beneficiation is integral to their economic viability include Sorowako (Indonesia) and Kopéto (New Caledonia), although the practice of screening out coarser lumps and boulders is carried out at all laterite mines. At Kopéto, a grade increase of 25% is achieved between run-of-mine ore and product finally shipped to the smelter.
Figure 8: Beneficiation of silica-oxide ore from Western Australia. Plot shows parameters of material passing screen sizes shown on the x-axis. Vertical dashed line is at 212 microns. “% mass passing” is the weight of the undersize fraction at the relevant screen size as a percentage of the total bulk sample weight, and Ni grade % is the grade of the undersize fraction. “% Ni recovery” is the proportion of total nickel in the bulk sample that is reported in the undersize fraction.
0 10 20 30 40 50 60 70 80 90 100
0 0.5 1 1.5 2 2.5
Screen size (mm)
%
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Ni grade %
% Mass passing
% Ni recovery Ni grade %
Beneficiation with more effective results is possible with silica-oxide laterite. The silica component is essentially devoid of nickel and easily separated from the associated goethite by simple screening after crushing. Figure 8 shows an example of how screening a crushed laterite of bulk grade 1.04% Ni at 212 microns can produce a fine fraction comprising 53% of the original mass, containing 81% of the original nickel and with a grade of 1.44% Ni. This form of beneficiation is carried out at the Cawse operation in Western Australia.
Clay ores are not amenable to upgrading, as Ni is uniformly distributed and there is no discrete low-nickel fraction which can be separated, except minor secondary silica in places.
Structure of the nickel laterite industry
Table 3 shows that two-thirds of lateritic nickel is produced by smelting. Smelters range in size and have up to 60,000–70,000 t/yr Ni capacity (e.g. Sorowako, Doniambo). Most smelters draw ore from nearby mines (often more than one) in integrated operations. The three Japanese smelters (Table 3), however, import ore purchased from mines in New Caledonia, Indonesia and the Philippines. In the case of Pamco and Sumitomo, the smelter owners have an equity interest in some of the mining operations that provide their ore.
The Yabulu refinery in Townsville, Australia, using the Caron process was built to treat ore from the Greenvale laterite deposit in Queensland, but now treats only ores imported from New Caledonia, Indonesia and the Philippines after the reserves at Greenvale were depleted. The Caron process plants in Cuba treat predominantly oxide ore from local mines.
The advent of new HPAL projects has been the most significant factor affecting the nickel laterite industry in the last decade. For many years the Moa Bay operation in Cuba was the sole HPAL producer. Over this time little was known about its performance as the plant was nationalised by the Castro regime immediately after commissioning in 1959. When information became available after the collapse of the Soviet economy in the 1990s, the Moa plant was found to be operating remarkably well despite poor maintenance. This observation, supported by a number of other factors, led to increased global industry interest in the potential of HPAL. These factors included a significant drop over the last decade in the price of sulphur and sulphuric acid (due to environmental pressures on smelters and resulting increased supply), technical advances in solvent extraction and improved reagents to selectively separate Ni and Co from leach solutions, and improved design and experience in autoclave technology from the gold industry.
On the back of the renewed interest in HPAL, some 15-20 lateritic nickel projects were proposed globally and all were subjected to some form of study ranging from scoping to bankable feasibility studies. Three greenfields plants were built in Western Australia: Bulong and Cawse are essentially large demonstration plants of about 9500 t/yr Ni capacity which were intended to be expanded after commissioning, and Murrin Murrin is a 45,000 t/yr Ni plant. All three operations were brought on stream in 1999 and suffered severe commissioning difficulties, engineering failures, longer than expected ramp-up times and higher costs, and all are currently operating below capacity and are in severe economic circumstances. As a consequence of these difficulties, most other project studies have been put on hold. An exception is Inco’s Goro project in New Caledonia which is under construction and expected to start producing in 2004-5 with a capacity of 54,000 t/yr Ni.
The economics of laterite operations vary widely. To better understand the economic drivers in the industry, it is helpful to refer to a list of 20 nickel laterite projects around the world (both producers and non-producing projects in the exploration or development stage) selected on the basis of the largest published resource size measured in contained Ni (Table 4), and consider them in the light of the process descriptions given above.
The more profitable laterite operations (Sorowako and Cerro Matoso) are notable because of their large size, and because they have cheap energy supplies. In the case of Sorowako, the operation is supported by a fully dedicated hydroelectric scheme. The economics of SLN’s operations in New Caledonia are improved by the high ore grades (over 2.6% Ni) and available hydroelectric energy, but some high-cost, oil-generated power is required. Falconbridge’s Falcondo smelter in the Dominican Republic relies totally on oil for energy, and the smelter has been closed a number of times when high oil prices have coincided with low Ni prices (most recently for three months in late 2001). The Exmibal operation in Guatemala operated for only three years from 1977 before being closed and has not operated since; it barely reached half its production capacity, and was penalised by small production capacity, low grade (1.8% Ni) and its dependence on oil- generated power.
In contrast, a significant number of laterite operations are marginal or loss-making and rely heavily on forms of Government support to remain in operation, thereby protecting reliability of strategic supply, protecting the national economy, or maintaining high employment levels (these are sometimes referred to as “social producers”). In the case of the Japanese smelters, the assistance is in the form of tariff protection which offsets the high costs of ore purchase, ore transport and energy. Greek producer Larco has poor economics due mainly due to poor ore grade (1.1% Ni), high energy costs and high smelting temperatures of the Fe-rich ores, and has received extensive Government subsidies.
The economic problems faced by the three Western Australian operations have resulted mainly from capital over-runs and underestimation of operating costs, as well as lost production due to engineering failures in pumps, pipes and pressure vessels unable to withstand the severely corrosive conditions of hot, concentrated sulphuric acid slurries. The experience has shown that in order to reduce technical risk, feasibility studies have to be more thorough, capital costs will be high and projects will have to be large in order to achieve economies of scale, leaving the development of HPAL projects in the domain only of the major mining houses. The capital and operating cost over-runs of the two small Western Australian HPAL producers has exacerbated their already precarious ability to repay capital.
Despite these events, most new development projects being studied or planned for construction are HPAL plants (ten in Table 4). It is considered by the promoters of these projects and some industry observers that the experience of the three Western Australia HPAL projects has shown where the effort has to be made to overcome the operational difficulties, and that the fundamental economics of HPAL are still sufficiently attractive for these projects to be progressed. Of the development projects listed, Sipilou, Musongati and Moramanga have location disadvantages away from the coast, and a number of projects are situated in countries that have political factors not in their favour. The latter issue is one which becomes important when the high capital cost is considered.
Table 4: Summary of features of twenty large nickel laterite projects (producers, ex-producers and development projects) selected on the basis of contained Ni metal in resources.
Ingredients of a successful laterite project
The experience of history has shown that new nickel laterite development projects have a very patchy record of success. Many have suffered from construction cost overruns, unforeseen technical difficulties and inability to reach nameplate capacity, and that the three new Western Australian HPAL projects so far seem to be suffering a similar fate is a clear indication that laterite projects require exceptionally high standards of engineering and technical excellence to be successful. However, there are a number of natural attributes of nickel laterite deposits that, if they applied in new projects, would improve their chances of successful development and becoming profitable operations. These attributes can be described in the four categories of quality, scale, location and infrastructure.
Project Country Ownership
Predom inant ore type
Contain ed Ni in resourc
e
% Ni grade
% Co grad e
Project status
Product ion capacit
y (t/yr Ni)
Processing route
Goro New
Caledonia INCO-BRGM Oxide >5 Mt 1.56 0.18 Feas. Study HPAL
Sorowako Indonesia INCO-Sumitomo Silicate >5 Mt 1.80 Producer 68000 Matte smelting Sampala Indonesia Rio Tinto Oxide >5 Mt 1.34 0.10 Exploration HPAL
Koniambo FeNi New Caledonia
Falconbridge-
SMSP Silicate 3-5 Mt 2.58 0.07 Feas. Study FeNi
smelting
Sipilou Cote d'Ivoire Falconbridge- SODEMI
Oxide-
silicate 3-5 Mt 1.48 0.11 Feas. Study HPAL
Murrin Murrin Western Australia
Anaconda-
Glencore Clay 3-5 Mt 0.99 0.06 Producer 45000 HPAL
Gag Island Indonesia BHP Billiton- Aneka Tambang
Oxide-
silicate 3-5 Mt 1.35 0.10 Feas. Study HPAL
Bahodopi Indonesia INCO Silicate 3-5 Mt 1.77 Feas. Study Matte
smelting SLN
Operations
New
Caledonia Eramet-SLN Silicate 3-5 Mt 2.40 Producer 60000 Smelting
Weda Bay Indonesia Weda Bay- Aneka Tambang
Oxide-
silicate 2-3 Mt 1.37 0.12 Exploration HPAL
Pinares de
Mayari Cuba Cuban
Government Oxide 2-3 Mt 1.07 0.12 Exploration HPAL
Pomalaa East Indonesia INCO Silicate 2-3 Mt 1.83 Exploration FeNi
smelting
Camaguey Cuba BHP Billiton Clay 2-3 Mt 1.30 0.05 Exploration HPAL
Musongati Burundi Argosy Oxide-
silicate 2-3 Mt 1.31 0.08 Exploration HPAL
Moramanga Madagascar Phelps Dodge Oxide 2-3 Mt 1.11 0.10 Feas. Study HPAL
Prony New
Caledonia
New Caledonian
Government Oxide 2-3 Mt 1.40 0.14 Exploration HPAL
Euboea Island Greece Larco Oxide 2-3 Mt 1.00 Producer 20000 FeNi
smelting
Exmibal Guatemala INCO Silicate 1-2 Mt 1.83 ex-producer 11300 Matte
smelting Cerro Matoso Colombia BHP Billiton Silicate 1-2 Mt 2.35 Producer 55000 FeNi
smelting Falcondo Dominican
Republic Falconbridge Silicate 1-2 Mt 1.23 Producer 34000 FeNi
smelting
Ore quality
It is an old adage that good mines are made from good orebodies. Ore quality for lateritic nickel deposits depends on factors such as:
Grade – the highest possible grades of both Ni and by-products, principally Co, improve efficiency of plant utilisation and decrease the effect of internal waste included in the ore stream. The ability to beneficiate ores can be an advantage, although it must be weighed up against the cost of mining more ore than is needed for the mill.
Consistency – continuity and consistency in grade and other physical and chemical properties allows for less variability in composition of material sent to the plant.
Efficiency in the plant relies heavily on maximum control and minimum variation in feed composition.
Ore and overburden thickness –greater ore thickness and less overburden improves stripping ratio of overburden to ore.
Mineralogy – in HPAL, oxide mineralogy is preferable to clay mineralogy. Although Ni recoveries for the two ore types are similar, in clay laterites the presence of colloidal silica in slurries and solutions released by breakdown of the clay causes problems with high pressure pumping and solid-liquid separation.
Higher slurry densities can be achieved with oxide mineralogy, increasing
through-put rates.
In smelting, the Si:Mg ratio in the feed is critical to controlling melt temperatures and slag reactivity and viscosity. The ratio is strongly influenced by mineralogy, particularly the occurrence of serpentine.
Deleterious elements – in HPAL, Mg and Al are strong acid consumers, and high levels of Al (as can be found in overburden) can cause the formation of alunite scale in
the autoclave.
Free silica occurs irregularly in places as veins and boxworks in all types of nickel laterite. If it occurs in smelter feed, it can cause major variations in the Si:Mg ratio, and it is therefore to be avoided.
Scale
The high capital cost of laterite plants requires large capacity treatment plants to achieve economies of scale, and a long mine life to allow payback of capital. As a rule of thumb, a minimum plant capacity of about 45,000 t/yr Ni is required for a viable greenfields HPAL plant to keep unit cost of capital to a minimum (a capital cost of US$10 per pound of annual nickel capacity is sometimes referred to as a benchmark). For a smelter, about 20-25,000 t/yr is the minimum viable size, but this depends mostly on Ni grade and cost of energy. Resources which show the potential to allow the definition of reserves sufficient for a mine life of 30 years are considered necessary: this would in most instances require several hundred million tonnes of ore for low-grade deposits, and at least 50-100 million tonnes for high-grade silicate ore.
Location
Because of the large amounts of consumables required for the operation of HPAL and Caron plants, a coastal location for the plant is preferred. Where possible, the plant should be located
close to the minesite to minimise the transport and handling of ore. Smelting operations in some cases are located close to energy sources or markets, with the ore being imported to the smelter.
In that case, coastal locations for the ore sources are preferable. Murrin Murrin suffers economically from the cost of transporting 500,000 t/yr of sulphur some 800 km from the seaboard to the minesite whereas this will not be the case for Goro, Gag Island and Weda Bay.
Infrastructure
The three main infrastructure requirements for laterite operations are water, power and access.
Water consumption of hydrometallurgical processes (Caron and HPAL) is high, but in tropical climates water availability is often not a problem. The problem of both water availability and quality exists in more arid locations such as inland Australia. Smelters are heavy users of power and nearby potential sources of low-cost energy are advantageous, such as hydroelectricity or natural gas. Access issues are important particularly in areas of rugged topography and uplifted terrain where ore has to be transported to a plant or shipping terminal on the coast.
The need for provision of infrastructure can add greatly to the already-high capital cost pf laterite operations. The location of the three Western Australian laterite projects close to a natural gas pipeline has been to their advantage, and the two smaller projects, Cawse and Bulong, are located close to the WMC Kalgoorlie Nickel Smelter from which they derive their sulphuric acid. A coastal location alone for a plant is not necessarily an advantage by itself, unless there is a port developed to handle materials and freight. A port can cost tens of millions of dollars to construct.
Environmental considerations
Environmental issues that need to be considered when developing new laterite projects include mining, processing, waste disposal and closure issues (Dalvi and Poetschke, 2000). Mining of laterite deposits is shallow (generally less than 50 metres deep) but develops a large “footprint”
and therefore large areas must undergo post-mining rehabilitation. In tropical areas re-vegetation is less of a problem than in arid areas. Processing issues relate to the disposal of tailings, effluent and emissions to the environment. Placement of tailings and disposal of effluent can be a problem in tropical climates due to high rainfall and low evaporation rates. Deep sea tailings disposal is technically feasible where the coast is near and sea-floor topography is suitable, but encounters opposition from environmental groups and is in some cases not permitted by governments.
Summary and conclusions
Nickel laterite deposits form where olivine-rich rocks are exposed to chemical weathering in humid climatic conditions over a sufficient time to allow the concentration of nickel in various parts of the laterite profile. Deposits form at all scales of size and degree of nickel enrichment, but the right combination of geological and climatic factors can allow giant deposits to develop.
In summary, these are:
- large areas of exposed olivine-rich ultramafic (especially dunite and harzburgites), such as are found in ophiolite complexes in current or former island arc and oceanic plate collision settings,
- warm, seasonally humid tropical climatic conditions over periods in excess of one million years,
- tectonic processes allowing a balance between rates of erosion and downward advance of the weathering front, and development of a topography that provides for a low water table and free drainage of the profile, and
- jointing and fracturing in the bedrock allowing penetration of groundwater.
Commercial development of nickel laterite projects is a high risk undertaking due to the high capital costs involved and the need for the application of the highest standards of technology and engineering. It helps to have a giant, quality orebody, but successful projects require a favourable combination of geological, mineralogical and mining factors, technical and engineering factors related to the process flowsheet, infrastructure-related factors and environmental considerations (Dalvi and Poetschke, 2000). Although currently lagging behind sulphides as sources of nickel, laterites are well positioned to increase their production levels and lower their costs due to their huge resource position and continuing improvements in processing technology and engineering
.
Acknowlegements
Numerous discussions with Dr C R M Butt (CSIRO, Australia) over many years have helped to develop the ideas expressed in this paper, and I look forward to more. Dr N Brand (now with Anglo American Exploration, Perth) is also thanked.
Comments from reviewers have also substantially improved the manuscript.