Geology, Resources, & Exploitation of Ni.pdf

8/9/2019 Geology, Resources, & Exploitation of Ni.pdf

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Nickel laterite deposits – geological overview, resourcesand exploitation

M. Elias

Mick Elias AssociatesCSA Australia Pty LtdPO Box 139Burswood WA 6100Australia

In: Giant Ore Deposits: Characteristics, genesis and exploration, eds DR Cooke and JPongratz. CODES Special Publication 4, Centre for Ore Deposit Research, University of

Tasmania, pp 205-220.

Abstract

This paper reviews the nature and genesis of nickel laterite mineralisation, and describes therelationship between deposit characteristics (both geological and non-geological) and thesuccessful development of lateritic deposits as commercial nickel producers. The importance ofnickel laterites lies in their huge resource base, which could potentially provide a much greatershare 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 ofintense 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 theirchemical components into groundwater, the leaching of mobile components, the residualconcentration of immobile or insoluble components, and the formation of new minerals which arestable in the weathering environment. The combined effects of these processes is to produce avertical succession of horizons of differing chemistry and mineralogy (the laterite profile), theoverall structure of which is governed by the differential mobility of the elements in theweathering zone. The detailed structure of the profile varies greatly, and in any one place is theresult 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 thealteration 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 leachingare used to extract nickel and cobalt from the upper, low-magnesium part of the profile, andsmelting is used for the high-magnesium silicates lower in the profile. The economics of nickellaterite processing are strongly dependent on grade and composition of ore feed, economies ofscale, location, availability of low-cost energy and well-developed infrastructure. Historically,nickel laterite projects have proven difficult to develop and reach their nameplate capacity, butthe enormous surface resources of lateritic nickel provide compelling incentive to overcomeengineering challenges inherent in their successful treatment. The outlook is for a greater

proportion of nickel production in the future to come from lateritic sources.


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Introduction

Laterites are the residual products of chemical weathering of rocks at the surface of theearth, 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 theenvironment. 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 veryefficient in concentrating some elements. Well-known examples of important lateriticore deposits are aluminous bauxite and enriched iron ore deposits, but lesser knownexamples 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 extentkomatiites 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 thecharacter of the resulting laterite, are controlled on regional and local scales by the

dynamic interplay of factors such as climate, topography, tectonics, primary rock typeand structure.

Figure 1: Global distribution of sulphide and laterite nickel deposits

Most lateritic nickel resources occur within a band about 22 degrees of latitude either sideof the equator (Fig. 1) and the giant, and in some cases highest grade, deposits areconcentrated in tectonically active plate collision zones (eg Indonesia, the Philippines and New Caledonia) where extensive obducted ophiolite sheets are exposed to aggressive

CUBA

INDONESIA

AUSTRALIA

LATERITES SULPHIDES

NEW CALEDONIA

PHILIPPINES22

oN

22oS


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chemical weathering in tropical conditions of high rainfall and warm temperatures, andthere is the greatest opportunity for supergene enrichment. Resources in cratonic settingscan 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 andformer Yugoslavia) (Valeton et al, 1987) and the Yilgarn craton in Western Australiaoccur at higher latitudes. The latter two examples are “fossil” deposits, currently situated

in temperate or arid climates quite different from the warm, humid conditions underwhich they formed.

Nickel laterites play an important part in the global nickel industry and currently accountfor around 40% of the total nickel production of about 1 million tonnes. About 70% ofall continental or terrestrial nickel resources are contained in laterites. Production ofnickel 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 morerapid growth in lateritic nickel production are the high capital cost of processingfacilities, high energy requirements in the pyrometallurgical process routes, and thetechnical challenges of making hydrometallurgical processing more efficient.

0

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1988 1989 1990 1991 19921993 1994 1995 1996 1997 1998 1999 2000 2001

t o n n e s N i ( ' 0 0 0 )

30.0%

32.0%

34.0%

36.0%

38.0%

40.0%

42.0%

44.0%46.0%

48.0%

50.0%

% L

a t e r i t i c N i

Sulphide

Laterite

% Laterite

Figure 2: World nickel production by ore type, 1988-2001.

This paper is divided into two sections. Part 1 describes the processes by which lateriticdeposits enriched in nickel are developed over ultramafic rocks, the environmentalfactors controlling the processes, and the nature of the lateritic profile formed as a resultof 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 anddescribes the factors that characterise commercially successful operations.


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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 placein seasonally humid climates over long periods of time in conditions of relative tectonicstability, allowing the formation of a thick regolith with distinctive characteristics(Trescases, in Butt and Zeegers, 1992). Table 1 lists the main effects of chemicalweathering of rocks in general, and how these processes are manifested in the weatheringof ultramafic rocks. In summary, the process of lateritisation involves the breakdown of primary minerals and release of some of their chemical components into groundwater, theleaching of mobile components, the residual concentration of immobile or insolublecomponents, 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 materialoverlying the parent rock from which it was formed, which is generally referred to as the“laterite profile”.

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 secondaryminerals: 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 climates4. Mobilisation and partial

reprecipitation of redox-controlledconstituents: Fe, Mn

Precipitation of Mn oxides andadsorption of Ni and Co from solution

5. Retention and residualconcentration of resistant minerals:

zircon, chromite, quartz

Residual chromite concentration

Table 1: Main processes of chemical weathering and their effects in ultramaficrocks (after Butt and Zeegers, 1992, p. 10)

The process is dynamic and gradual, and the gross stratification that one sees in thelaterite profile (Fig. 3) is essentially a snapshot of lateritisation in progress. The lowestlayer reflects early stages of weathering of the bedrock, and each layer further uprepresents a transformation of that lying immediately below it, displaying progressivelyadvancing stages of the process. In the lowermost part of the profile ( saprock ),weathering takes place at contacts between minerals and at fracture boundaries and thereis abundant fresh rock and little alteration product. Further up the profile, the proportionof surviving primary minerals decreases, and more strongly fractured zones are

completely altered eventually leaving detached boulders of intact bedrock “floating” in amixture 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


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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 fromthe dominant mineralogy (goethite and hematite) in this zone in oxide laterites.

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%


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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 indetail from place to place in thickness, chemical and mineralogical composition, and therelative 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 solublecomponents. In addition to the amount, the effectiveness ofrainfall (the extent to which water is allowed to pass down throughthe 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 whichwater passes into the soil, and water table level.

Drainage: Drainage affects the net water budget available for leaching fromthe whole landscape.

Tectonics: Tectonic uplift increases erosion of the top of the profile, increasestopographic relief and lowers the water table. Tectonic stabilityallows 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 thecharacteristics of a profile at any one place can best be described as due to the combinedeffect 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 ofchemical 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 permillion 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 permillion 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 subduedand 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


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minerals are amorphous or poorly crystalline. Their crystallinity improves with time towell-structured goethite with a characteristic yellow-brown colour, which is progressivelyreplaced 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 ofgoethite to hematite is accompanied by a loss of Ni, as hematite cannot accommodate inits 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 crustas 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 downinto fresh bedrock over a distance of only a few centimetres. More generally there is aninterval 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 ofgoethite 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 thissilica-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 Feand 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 depositedas 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 withlow 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 (Montiand Fazakerley , 1996), Bulong (Elias et al, 1981) and Marlborough (Golightly 1981) and

in Brazil.

Silicate laterites: In situations where there is slow continuous tectonic uplift and thewater table is kept low in the profile, weathering over long periods can result in thedevelopment 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


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laterites are characterised by an absolute enrichment or concentration of Ni in thesaprolite 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 therecrystallisation of goethite to hematite further up in the profile. Nickel is reprecipitatedwithin 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). Averagecontent 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 lateritesare 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.

Figure 4: Schematic comparison of principal laterite profile types

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 oceanicor continental plate boundaries and collision zones. Thrust faulting has obducted slabs of

upper mantle peridotites and associated rocks forming ophiolite complexes withextensive 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


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typical of active and inactive island arc settings of Indonesia, the Philippines and NewCaledonia.

Cratonic terrains – laterites are developed on komatiites and the ultramafic phases of

layered mafic complexes of any age from Archaean through the Palaeozoic. Relativetectonic stability allows peneplanation and the laterites are developed in moderate tosubdued relief. Restricted drainage often results in smectite formation instead of oxides.

Tectonic stability allows continuous lateritisation over extended time periods giving riseto 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, WesternAustralia, 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 topographicsetting into plateau, slope and terrace deposits (Troly et al, 1979).

Plateau deposits are affected by active drainage process but less by erosion, and hencetend to show complete profile development and form a thicker saprolite zone. The ThioPlateau 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 orabsent. Silicate laterite development can also be thinner than plateau deposits as thegroundwater movement has a greater lateral component, i.e. downslope. However, theincreased lateral groundwater flow can cause higher Ni grades to be developed.

Terrace deposits are relics of earlier peneplains or erosion surfaces and indicate atemporary 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 fromsurrounding plateaux (particularly oxide material which is readily eroded) and multiplecycles of lateritisation are possible with the possibility of increased grades. Goodexamples of terrace deposits are found in southern New Caledonia, where the Gorodeposit is the best known.

Global resources of nickel in laterites

Total global resources of nickel contained in laterites are about 160 million tonnes


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(Fig. 5). When classified into limonitic (combined oxide and clay) and silicate-dominantdeposits depending on the predominant profile zone (or in some cases the focus of adevelopment 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.

Figure 5: Global lateritic nickel resources (by contained nickel)

Figure 6: Lateritic nickel resources by region based on contained Ni tonnes, divided into“limonite” (oxide plus clay) and silicate deposits.

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 andAustralia 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 showinghigher 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

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

Limonite

70%

Silicate

30%

9569 Mt ore1.17% Ni112.2 Mt Ni

2712 Mt ore1.79% Ni48.5 Mt Ni

Total 160.7 Mt Ni


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region (Massif du Sud). Australia, by contrast, has almost no saprolitic resources, and thegrade of its limonitic laterites is considerably lower than other countries.

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 gradeof 1.6% Ni and large tonnages, whereas silicate laterite deposits are smaller (mostly lessthan 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 7: Tonnage-grade plot of individual laterite deposits. Limonite deposits includeoxide and clay laterites. See Table 4 for further information on named deposits.

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 Caledoniawere 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 lateriticsources has lagged behind that from sulphides. Currently about 40% of nickel productioncomes 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 wasabout 10% of total nickel, but strong demand for nickel at that time driven largely by USrequirements for the Korean War effort saw a number of laterite operations developedwhich 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 itsresource position relative to sulphide nickel are essentially economic, and are

summarised in Table 2. The average weighted cost of production (fully allocated costsnet 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

0.6

0.8

1.0

1.2

1.4

1.61.8

2.0

2.2

2.4

2.6

2.8

3.0

0 50 100 150 200 250 300 350 400

Tonnes (millions)

N i g r a d e ( % )

SoroakoBahodopi

Koniambo

Goro

Sampala

Murrin

Silicate Limonite


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production costs is considerably less than for sulphides, which are mostly produced fromunderground mines. Processing of nickel laterites is considerably more expensive thanfor sulphides, and more than offsets the mining cost advantage.

Sulphides Laterites

Treatment costs lower than for laterites -nickel in metastable minerals, whichcontain latent energy in the form of S

Nickel in stable oxide mineralogy whichrequires energy and/or aggressive chemicalattack to disassociate.

Nickel-bearing minerals can be easilyseparated from barren (gangue) material atthe mine site. Only the concentrate (highergrade, lower volume) undergoes furthertreatment.

Only rare opportunities to separate andconcentrate nickel-bearing minerals, andmust apply treatment processes to all theore. Requires handling large quantities ofmaterial with low nickel concentrations,and large volumes of consumables which

must be transported to the plant.Ore and concentrate have generallyconsistent characteristics within a deposit –easy to manage metallurgy.

High short-range variability of mineralogyand chemistry, requires careful planning,mining and blending to produce consistent, predictable feed for the treatment plant.

High recovery of nickel from ore possiblewith only one process route

Variable composition through profilerestricts recovery of nickel from only partof 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 capacityfor processing facilities.

Very high capital cost of processingfacilities – more so if the energy source hasto be part of the capital cost.

Table 2: Summary of issues relevant to the economics of nickel sulphide and nickellaterite deposits.

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.

Plant Company CountryTonnes

Ni% of total

production


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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 producesferro-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, andhas 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 andthe successful projects (e.g. Sorowako) have their own hydroelectric power plants.

The Caron process is applicable to oxide laterites (“limonite”) with tolerance for somesilicate laterite, but excessive silica and Mg leads to decreased Ni recoveries. The process involves drying and roasting in a reducing atmosphere, followed by low-pressureammonia 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), whichremained 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 andextracting 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 costof 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 theconsumption of sulphuric acid is determined mainly by the Mg level in the ore, the latter becomes a critical factor in managing mining and blending.

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 increasedoperating 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 mostefficient 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 isseparated from the rest and rejected, leaving a component with a higher grade to betreated. This is analogous to making a concentrate from a sulphide ore, but theconcentration factor is much smaller for laterite ores. In the beneficiation process, someof the nickel is lost to the reject component but this is outweighed by the improved

economics which result from the higher feed grade.


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The practice has been applied for many years to silicate ores, where coarser boulders andfragments 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) andKopéto (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.

Beneficiation with more effective results is possible with silica-oxide laterite. The silicacomponent is essentially devoid of nickel and easily separated from the associatedgoethite by simple screening after crushing. Figure 8 shows an example of howscreening a crushed laterite of bulk grade 1.04% Ni at 212 microns can produce a finefraction comprising 53% of the original mass, containing 81% of the original nickel andwith 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 nodiscrete low-nickel fraction which can be separated, except minor secondary silica in places.

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 isat 212 microns. “% mass passing” is the weight of the undersize fraction at the relevantscreen size as a percentage of the total bulk sample weight, and Ni grade % is the gradeof the undersize fraction. “% Ni recovery” is the proportion of total nickel in the bulksample that is reported in the undersize fraction.

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

0

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100

0 0.5 1 1.5 2 2.5Screen size (mm)

%

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

N i g r a d e

%

% Mass passing

% Ni recovery

Ni grade %


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smelters draw ore from nearby mines (often more than one) in integrated operations. Thethree Japanese smelters (Table 3), however, import ore purchased from mines in NewCaledonia, 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 importedfrom New Caledonia, Indonesia and the Philippines after the reserves at Greenvale were

depleted. The Caron process plants in Cuba treat predominantly oxide ore from localmines.

The advent of new HPAL projects has been the most significant factor affecting thenickel laterite industry in the last decade. For many years the Moa Bay operation in Cubawas 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 in1959. When information became available after the collapse of the Soviet economy inthe 1990s, the Moa plant was found to be operating remarkably well despite poormaintenance. This observation, supported by a number of other factors, led to increasedglobal industry interest in the potential of HPAL. These factors included a significantdrop 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 solventextraction 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 Nicapacity which were intended to be expanded after commissioning, and Murrin Murrin isa 45,000 t/yr Ni plant. All three operations were brought on stream in 1999 and sufferedsevere commissioning difficulties, engineering failures, longer than expected ramp-uptimes and higher costs, and all are currently operating below capacity and are in severe

economic circumstances. As a consequence of these difficulties, most other projectstudies 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 of54,000 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 aroundthe 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.


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Project Country OwnershipPredomi

nant oretype

Contained

Ni inresource

% Ni

grade

% Co

grade

Project

status

Production

capacity(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 68000Matte

smelting

Sampala Indonesia Rio Tinto Oxide >5 Mt 1.34 0.10 Exploration HPAL

Koniambo FeNi New CaledoniaFalconbridge-SMSP

Silicate 3-5 Mt 2.58 0.07 Feas. StudyFeNismelting

Sipilou Cote d'IvoireFalconbridge-

SODEMI

Oxide-

silicate3-5 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. StudyMattesmelting

SLNOperations

New Caledonia Eramet-SLN Silicate 3-5 Mt 2.40 Producer 60000 Smelting

Weda Bay IndonesiaWeda Bay-

Aneka Tambang

Oxide-

silicate2-3 Mt 1.37 0.12 Exploration HPAL

Pinares de

MayariCuba

Cuban

GovernmentOxide 2-3 Mt 1.07 0.12 Exploration HPAL

Pomalaa East Indonesia INCO Silicate 2-3 Mt 1.83 ExplorationFeNi

smelting

Camaguey Cuba BHP Billiton Clay 2-3 Mt 1.30 0.05 Exploration HPAL

Musongati Burundi ArgosyOxide-

silicate2-3 Mt 1.31 0.08 Exploration HPAL

Moramanga Madagascar Phelps Dodge Oxide 2-3 Mt 1.11 0.10 Feas. Study HPAL

Prony New CaledoniaNew Caledonian

GovernmentOxide 2-3 Mt 1.40 0.14 Exploration HPAL

Euboea Island Greece Larco Oxide 2-3 Mt 1.00 Producer 20000FeNismelting

Exmibal Guatemala INCO Silicate 1-2 Mt 1.83 ex-producer 11300 Mattesmelting

Cerro Matoso Colombia BHP Billiton Silicate 1-2 Mt 2.35 Producer 55000FeNismelting

FalcondoDominicanRepublic

Falconbridge Silicate 1-2 Mt 1.23 Producer 34000FeNismelting

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.


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The more profitable laterite operations (Sorowako and Cerro Matoso) are notable becauseof 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 dependenceon 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 protectingreliability of strategic supply, protecting the national economy, or maintaining highemployment levels (these are sometimes referred to as “social producers”). In the case ofthe Japanese smelters, the assistance is in the form of tariff protection which offsets thehigh costs of ore purchase, ore transport and energy. Greek producer Larco has pooreconomics due mainly due to poor ore grade (1.1% Ni), high energy costs and highsmelting temperatures of the Fe-rich ores, and has received extensive Governmentsubsidies.

The economic problems faced by the three Western Australian operations have resultedmainly 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 towithstand 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 toachieve economies of scale, leaving the development of HPAL projects in the domainonly of the major mining houses. The capital and operating cost over-runs of the twosmall Western Australian HPAL producers has exacerbated their already precarious

ability to repay capital.

Despite these events, most new development projects being studied or planned forconstruction 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 AustraliaHPAL 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 attractivefor these projects to be progressed. Of the development projects listed, Sipilou,

Musongati and Moramanga have location disadvantages away from the coast, and anumber 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.

Ingredients of a successful laterite project


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The experience of history has shown that new nickel laterite development projects have avery 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 engineeringand technical excellence to be successful. However, there are a number of naturalattributes of nickel laterite deposits that, if they applied in new projects, would improve

their chances of successful development and becoming profitable operations. Theseattributes can be described in the four categories of quality, scale, location and

infrastructure.

Ore quality

It is an old adage that good mines are made from good orebodies. Ore quality for lateriticnickel 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 internalwaste included in the ore stream. The ability to beneficiate ores can bean advantage, although it must be weighed up against the cost of miningmore ore than is needed for the mill.

Consistency – continuity and consistency in grade and other physical andchemical properties allows for less variability in composition of materialsent to the plant. Efficiency in the plant relies heavily on maximumcontrol 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 lateritesthe presence of colloidal silica in slurries and solutions released by breakdown of the clay causes problems with high pressure pumping andsolid-liquid separation. Higher slurry densities can be achieved withoxide mineralogy, increasing through-put rates.

In smelting, the Si:Mg ratio in the feed is critical to controlling melttemperatures 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 ofalunite scale in the autoclave.

Free silica occurs irregularly in places as veins and boxworks in all typesof 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.


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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 greenfieldsHPAL plant to keep unit cost of capital to a minimum (a capital cost of US$10 per poundof 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 costof 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 mostinstances require several hundred million tonnes of ore for low-grade deposits, and atleast 50-100 million tonnes for high-grade silicate ore.

Location

Because of the large amounts of consumables required for the operation of HPAL andCaron plants, a coastal location for the plant is preferred. Where possible, the plantshould 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, withthe ore being imported to the smelter. In that case, coastal locations for the ore sourcesare preferable. Murrin Murrin suffers economically from the cost of transporting 500,000t/yr of sulphur some 800 km from the seaboard to the minesite whereas this will not bethe case for Goro, Gag Island and Weda Bay.

Infrastructure

The three main infrastructure requirements for laterite operations are water, power andaccess. Water consumption of hydrometallurgical processes (Caron and HPAL) is high, but in tropical climates water availability is often not a problem. The problem of bothwater 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 areadvantageous, such as hydroelectricity or natural gas. Access issues are important

particularly in areas of rugged topography and uplifted terrain where ore has to betransported 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 toa 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 theyderive 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 projectsinclude mining, processing, waste disposal and closure issues (Dalvi and Poetschke,

2000). Mining of laterite deposits is shallow (generally less than 50 metres deep) but


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develops a large “footprint” and therefore large areas must undergo post-miningrehabilitation. 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 istechnically feasible where the coast is near and sea-floor topography is suitable, butencounters 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 weatheringin humid climatic conditions over a sufficient time to allow the concentration of nickel invarious parts of the laterite profile. Deposits form at all scales of size and degree ofnickel enrichment, but the right combination of geological and climatic factors can allowgiant deposits to develop. In summary, these are:

- large areas of exposed olivine-rich ultramafic (especially dunite andharzburgites), 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 thehigh capital costs involved and the need for the application of the highest standards oftechnology and engineering. It helps to have a giant, quality orebody, but successful projects require a favourable combination of geological, mineralogical and miningfactors, technical and engineering factors related to the process flowsheet, infrastructure-related factors and environmental considerations (Dalvi and Poetschke, 2000). Althoughcurrently lagging behind sulphides as sources of nickel, laterites are well positioned toincrease their production levels and lower their costs due to their huge resource positionand continuing improvements in processing technology and engineering

.

Acknowlegements

Numerous discussions with Dr C R M Butt (CSIRO, Australia) over many years havehelped to develop the ideas expressed in this paper, and I look forward to more. Dr NBrand (now with Anglo American Exploration, Perth) is also thanked.

Comments from reviewers have also substantially improved the manuscript.


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