AU2009200438A1 - Pellitization process - Google Patents

Description

PELLETIZATION PROCESS FIELD OF THE INVENTION The present invention relates to a process used to form pellets of a nickel-containing ore for 5 use in heap leaching. BACKGROUND TO THE INVENTION Nickel ores can be classified into two major types according to their composition, namely, sulfide and laterite (the latter being also known as "oxidized"). Until recently, the majority of 10 total nickel production came from processing of sulfide ores. In more recent times, laterite ores have started to be treated on a commercial scale as a consequence of higher-grade nickel sulphide deposits being mined out, making the more complex laterite ores economically viable to process. Traditionally, the laterite ores have been treated via an expensive, energy intensive and highly corrosive high pressure sulphuric acid leaching 15 process. Heap leaching is an operation that involves low investment and operational costs, is very well known, and is widely applied mainly for copper, uranium, and gold ores. Despite successful laboratory trials, there is currently still no commercial use of heap leaching 20 processes to extract nickel and cobalt from laterite ores. One of the reasons for this apparent lack of commercial viability is a failure in the past to successfully develop agglomeration methods to ensure that heap construction provides adequate percolation of the leach solution through the heap. This is particularly true for nickel laterite ores that contain a higher clay component and more fines that other types of ores treated in the past 25 using heap leaching. It is fundamental for the process that the heap has good permeability, with good contact between the ore and the leaching solution. During heap leaching, percolation rates can slow down as a result of migration of fines within the heap resulting in localized areas of 30 "channeling" through which the leaching solution preferentially flows. Channeling results in dormant or unleached areas within the heap. Alternatively, fines present in the ore can form a "slime mud" which can effectively seal the heap or parts of the heap causing the leach solution to run off the sides rather than percolate through. When this occurs, recovery of target metal values is reduced and mechanical reforming of the heap may be needed which 35 can be extremely costly and time consuming or ineffectual.

-2 It was proposed in the mid-1 970's that it is possible to remove fines by agglomeration prior to heap leaching, by mixing crushed ore with Portland Cement wetted with water. The addition of lime has been found to be less effective than cement in controlling clay fines present in various types of ores as it is understood that the lime needs to react with the clay 5 minerals present for binding to occur. The use of cement or lime is uneconomical for leaching ores that require acid leaching solutions as the cement or lime reacts with and consumes the acid. With heap leaching methods becoming more attractive for processing nickel laterite ores, 10 there remains a need for an improved agglomerating method to produce pellets for heap leaching thus removing fines to improve percolation through the heap. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a process to form 15 pellets of a nickel-containing ore for use in heap leaching, the process comprising the steps of: a) agglomerating mineral fines present in the nickel-containing ore by mixing a flocculant with the ore to form pellets incorporating the mineral fines; and, b) adding a sufficient quantity of a mineral acid to the ore during or after step a) to 20 neutralize clay minerals present in the nickel-containing ore prior to heap leaching. In one form, the flocculant is a long-chain water soluble polymeric flocculant with long chains being preferred to maximize the openness of the structure of the pellets as they form and water solubility being advantageous as this makes it easier to achieve a more even 25 distribution of the flocculant when mixed with the ore. The flocculant may be a straight chained polymeric flocculant and is preferably a polymer formed from acrylamide subunits having the nominal formula per unit of -CH 2

CHCONH

2 -. The process may further comprise the addition of a surfactant during step a) to improve wettability. 30 In one form the flocculant is mixed with the ore at a flocculant concentration in the range between 0.05 to 1.0 % by weight or at a flocculant concentration in the range between 0.2 to 0.6% by weight. When the flocculant is an anionic acrylamide polymer, the flocculant may be added to the ore at a rate in the range of 5 to 500 grams per tonne of dry ore or at a rate in the range of 80 to 120 grams per tonne of dry ore. 35 -3 In one form, the mineral acid may be selected from the group consisting of sulphuric acid, hydrochloric acid and nitric acid with the use of sulphuric acid being preferred for economic reasons. When the mineral acid is sulphuric acid, the amount of mineral acid added during step b) is concentrated sulphuric acid and the acid is added in the range of 50 to 200 5 kilograms acid per tonne of dry ore or in the range of 80 to 120 kilograms acid per tonne of dry ore. For optimum strength, sufficient water may be added during step a) to form a pellet having a target level of agglomeration moisture in the range of 35 to 65% or in the range of 40 to 10 55%. The water may be added prior to the addition the flocculant. The water may equally be added with the flocculant in form of an aqueous suspension. In one form, the flocculant is added to the ore particles before agglomeration with a mineral acid being added during agglomeration. The flocculant may be dripped or sprayed onto the 15 ore particles in step a). In one form, the nickel-containing ore is subjected to a screening operation to provide a particle size prior to agglomeration having a P80 of 10 - 50mm. After agglomeration, the pellets may have an average particle size in the range from 1mm to 50mm. 20 According to a second aspect of the present invention there is provided a process for percolation leaching of metal values from a nickel-bearing ore wherein the ore is first formed into pellets which are then formed into a heap and then leached by percolating a leaching solution through the heap which extracts nickel from the pellets for subsequent recovery, 25 the improvement in which the pellets are produced by: a) agglomerating mineral fines present in the nickel-bearing ore by mixing a flocculant with the ore to form pellets incorporating the mineral fines; and, b) adding a sufficient quantity of a mineral acid to the pellets formed during step a) to neutralize clay minerals present in the ore prior to heap leaching. 30 In one form, the leaching solution contains between 0.1 and 20% sulphuric acid. Optimum recovery of nickel is achieved when 400 to 1000 kilograms sulphuric acid per tonne of dry ore is consumed during leaching. 35 In one form, the leaching solution is applied at an irrigation rate of flux in the range of 5 to 30 litres per hour per metre squared or at a rate of 10 to 25 litres per hour per meter -4 squared. The heap may have a height in the range of 4 to 8 meters. The heap may be a static or a dynamic type of heap. In one form, heap leaching is conduced using a counter current series of heaps including a lead heap and a lag heap, with fresh leaching solution being introduced at the top of the lag heap. To optimize recovery of nickel and/or cobalt 5 values, the heap may be rinsed with water after leaching operations have been completed, with an effluent stream being collected from the base of the heap after rinsing, and the effluent stream is subjected to a metal recovery process. Alternatively, the effluent stream is reprocessed over the heap. 10 In one form, heap leaching is conducted using the pellets formed using the process according to the first aspect of the present invention. According to a third aspect of the present invention, there is provided a process to form pellets of a nickel-containing ore for use in heap leaching substantially as herein described 15 with reference to and as illustrated in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the present invention will now be described in detail, by way of example 20 only, with reference to the accompanying drawings, in which: Figure 1 is a schematic flowchart of one embodiment of the present invention in the context of heap leaching of nickel laterite ores; Figure 2 is a graphical representation of drain rates after 48 hours in percolation 25 tests as a function of acid addition illustrating the effect of addition of flocculant; and, Figure 3 is a graphical representation of drain rates after 98 hours in percolation tests as a function of acid addition illustrating the effect of addition of flocculant. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Particular embodiments of the process of forming pellets of the present invention are now 30 described, with particular reference to the use of sulphuric acid and polyacrylamide as agglomerating agents by way of example only. The present invention is equally applicable to the formation of pellets using other mineral acids or other flocculants. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical 35 and scientific terms used herein have the same meanings as commonly understood by one -5 of ordinary skill in the art to which this invention belongs. In the drawings, it should be understood that like reference numbers refer to like members. The term "leaching" as used throughout this specification refers to the selective dissolution 5 of a target metal contained in an insoluble solid phase using a solvent or "leaching solution". For leaching to occur, an ore to be leached is brought into contact with the solvent, taking the target metal into solution. The target metal is then extracted from the so called "pregnant leach solution" using a metal recovery process. "Heap Leaching" is a process in which the ore is piled up to form a "heap" and the target metal is taken into solution by 10 percolation of the solvent through the heap. The term "flocculant" or "flocculating agent" refers to a chemical which promotes the aggregation of suspended particles into larger clusters, referred to in the art as "flocs". The particles are held together by a physical bond or by Van der Waals forces, not by chemical 15 binding. One embodiment of a process for agglomeration of pellets for use in heap leaching nickel containing laterite ores is now described with reference to Figure 1. Run-of-mine ore (10) is subjected to a screening operation (12) to provide a particle size prior to agglomeration 20 having a P80 of 10 - 50mm. When necessary, the process includes subjecting run-of-mine nickel-containing laterite ore to a size reduction operation (14), such as crushing, to reduce the particle size to within a preferred size range prior to agglomeration. It will be understood by one of skill in the art, that where the ore already has a sufficiently fine particle size for agglomeration, this size reduction step will not be necessary. When size reduction is 25 necessary, crushing can be performed as a single stage process or in as many stages as required to achieve a particle size suitable for agglomeration, depending on the characteristics of the ore. The screened ore is then transported to a conventional agglomeration unit (16) using any 30 suitable transfer means, for example a conveyor (18). Water may be added to the ore during transport to reduce dust formation if the ore contains a high amount of fines, bearing in mind that the moisture content of the screened ore affects the strength of the pellets that are formed later in during agglomeration (as described in greater detail below). The agglomeration unit (16) may be a pelletizing drum, pelletizing disc, or another means for 35 pelletizing mineral ore particles, for example using multiple transfer points or mixing paddles -6 on a conveyor system. By way of example, a pelletizing drum typically comprises an inclined rotating elongated cylindrical drum. The ore particles are subjected to agitation as they travel along the length of the rotating drum, forming ball-like pellets which are substantially spherical. 5 The size and strength of the pellets is governed by a number of factors, including but not limited to, the efficiency of mixing achieved by the agglomerating unit, the moisture content of the ore, the wettability of the ore, the shape and size distribution of the mineral ore particles, and the type of agglomerating agent(s) added to the ore. Agglomeration can be 10 conducted on a batch or continuous basis. A key factor in achieving good recovery of nickel and cobalt values from nickel laterite ores is the construction of the heap (20) through which the solvent or leaching solution will pass. The agglomerating agent is applied to improve pellet strength and stability. Using the 15 process of the present invention, the sized particles of ore are mixed with an agglomerating agent comprising a flocculant in the presence of sufficient water to form a pellet having a target level of agglomeration moisture, nominally in the range of 35 to 65%, preferably 40 to 55%. The specific amount of water added is dependent on the physical and mineralogical characteristics of the ore. The water added can be raw or process water or seawater or 20 hypersaline water. The water is added first, followed by the flocculant. The flocculant may be added to the ore particles before or during agglomeration with a mineral acid being added during agglomeration. Traditionally, flocculants are added to a liquid to encourage gravity separation of finely 25 dispersed particles by forming larger clusters or "flocs" which either float to the top or settle to the bottom of a liquid. The flocculant does not react with the fines, as such, but relies rather on charge to encourage particles to aggregate. Using the process of the present invention, a water-soluble flocculant is added as an agglomerating agent to encourage fines to clump together into larger clusters or to encourage fines to become attached to existing 30 larger particles. Either way, the flocculant acts to reduce the number of fines present to improve percolation rates during heap leaching. To achieve a uniform distribution of the flocculant over the ore, the flocculant may be sprayed onto the ore particles which are being caused to rotate within the agglomerating unit, or at conveyor transfer points. Best results have been achieved through the addition of a long-chain water soluble 35 polymeric flocculant, for example, anionic polyacrylamide (a polymer formed from -7 acrylamide subunits having the nominal formula per unit of -CH 2

CHCONH

2 -), preferably in solid or liquid straight-chained form. The polymeric flocculant may be added alone or in combination with a surfactant to improve wettability. The concentration of anionic acrylamide polymer solutions added to the agglomerates may vary between 0.05 to 1.0 % 5 by weight and preferably 0.2 to 0.6%. The amount of anionic acrylamide polymers used to treat and agglomerate the ore fines may vary between 5 and 500 grams per tonne of dry ore, preferably 80 to 120 grams per tonne of dry ore. During agglomeration, a mineral acid is added to pre-neutralize basic clay mineral 10 constituents present in the laterite ore, predominately MgO or CaO. This is done to accelerate the extraction of the target metal values when heap leaching is commenced. The amount of mineral acid used during agglomeration of the pellets thus depends on the clay content of the laterite ore being treated and the type of mineral acid being used. Whilst is it possible to use nitric or hydrochloric acid for this step, concentrated sulphuric acid is the 15 most economically viable. The amount of mineral acid required to be added to the ore during agglomeration is in the range of 50 to 200 kilograms of concentrated (98 to 100%) sulphuric acid per tonne of dry ore, preferably 80 to 120 kilograms per tonne of dry ore, but is dependent on the chemical composition of the laterite ore, and in particular the Mg, Ca and Fe content. Sulphuric acid may be mixed with the ore by being added onto the ore 20 either before, during or after agglomeration. Without wishing to be bound by theory, the flocs which form during agglomeration have a loose or open network structure (compared with the more dense structure achieved when pellets are formed using binders or cements, such as bentonite as agglomerating agents). 25 In this regard, the agglomerated pellets have a void fraction of at least 50%. This more open structure allows for more efficient penetration of the mineral acid to pre-neutralize the basic clay minerals during agglomeration, as well as improving percolation of the leaching solution through the pellets when they are subsequently stacked for heap leaching. 30 After agglomeration, the pellets are directed to a stacker (22) to form a heap (20) having a base (24) and a top (26), the heap (20) having a height in the range of 4 to 8 meters tall. The pellets have an average particle size in the range from 1mm to 50mm. Alternatively, the pellets can be stacked into vessels or vats, which should be considered to consist of a heap having a limited, fixed wall size. A distribution means (28), for example a spraying 35 apparatus, is used to apply a leaching solution or solvent at the top (26) of the heap (20).

-8 The solvent is then allowed to percolate down through the heap (20) under the influence of gravity towards the base (24) of the heap (20). A suitable "leaching solution" or "solvent" is any reagent capable of selectively dissolving the target metal from an ore whilst remaining chemically inert to gangue minerals. Acids such as sulphuric, hydrochloric, and nitric acid 5 are the most suitable. For economic reasons, the leaching solution preferably contains sulphuric acid. When sulphuric acid is used, the leaching solution contains between 0.1 and 20% sulphuric acid and optimum recovery of nickel is achieved when about 400 to about 1000 kilograms sulphuric acid per tonne of dry ore is consumed. 10 The acid solution is generally applied by spraying it onto a layer of inert bedrock, which acts as liquid distributor on the heap, although any method to adequately and uniformly disperse the acid onto the heap can be used including dispersion directly on to the heap. The acid solution is applied at an irrigation rate or flux in the range of 5 to 30 litres per hour per metre squared, preferably 10 to 25 litres per hour per meter squared. The pregnant leach liquor is 15 subjected to a suitable metals recovery process such as precipitation, solvent extraction or ion exchange to separately recover target metal values such as nickel, cobalt and iron. Each heap (20) can be a static or dynamic ("on-off') type of heap. If desired, a counter current series of heaps can be used including a lead heap and a lag heap, with fresh 20 leaching solution being introduced at the top of the lag heap. Pregnant leach liquor is collected using a collection means (30) arranged at the base (24) of each heap. The pregnant leach liquor is either recycled for a second pass through the same heap or another heap in the series, or directed to a metal recovery process (32) to recover one or both of the nickel values and the cobalt values present in the pregnant leach liquor. After leaching 25 operations have been completed, the heap may be rinsed with water (either fresh or salt water), concentrated or dilute acid or a combination of both of these, with an effluent stream being collected from the base of the heap after rinsing. This effluent stream can be subjected to a metal recovery process to improve overall recovery of nickel and/or cobalt values. Alternatively, the effluent stream can be reprocessed over the heap by directing 30 the effluent stream to flow over the heap to allow for more use of any residual acid present in the effluent stream to improve metal recovery before the effluent stream is finally processed by neutralisation or subjected to metal recovery. Some of the advantages of the various aspects and embodiments of the present invention 35 are further described and illustrated by the following examples and experimental test -9 results. These examples and experimental test results are illustrative of a variety of possible implementations and are not to be construed as limiting the invention in any way. The present invention is also not limited by the particular number nor type of evaporation chamber, condenser, heater or pump described in the following examples. 5 Examples To allow comparison of the efficiency of the agglomerating agents of the present invention when applied to different ores, standardized percolation tests were conducted. The ore tested contained 0.6 to 2.5% Ni, 0.03 to 1.2 % Co, 2 to 10% Mg, 2 to 10% Al, 10-35% Fe 10 and 0-5% Ca. These procedures allow the efficiency of the various agglomerating agents to be compared. Initial percolation testwork was conducted on samples of lateritic nickel/cobalt ore sourced from the Leonora area of Western Australia. Column percolation tests were conducted on 15 agglomerated material, agglomerated using varying amounts of concentrated sulphuric acid, ranging from zero kg/t sulphuric acid and up to 150 kg/t sulphuric acid. The percolation rate of a predetermined volume of a leaching solution solution through a column of agglomerated ore was then tested with the results of these percolation tests being presented in Table I below: 20 TABLE 1: Percolation test results without flocculant Cumulative Slumping After 48 Hrs After 98 Hrs Sample ID (Agglom. Acid Addition) Slump, % Slump, % Drain Rate, Drain Rate, (after 48 hrs) (after 96 hrs) Um 2 /hr Um 2 /hr Sample 1 50 kg/t (no floc) 0.0 4.0 5,587 6,391 Sample 1 100 kg/t (no floc) 0.0 0.0 13,557 10,732 Sample 1 150 kg/t (no floc) 0.0 0.0 8,172 6,217 Sample 2 50 kg/t (no floc) 2.0 2.0 2,441 3,728 Sample 2 100 kglt (no floc) 3.6 4.0 695 192 Sample 2 150 kglt (no floc) 6.0 7.0 758 203 It is apparent from Table 1 that agglomerate strength and percolation rates improved with 25 increased acid addition up to 100 kg/ t. In a second series of percolation tests, five different flocculants were added, as a binder, in combination with sulphuric acid. The results of column percolation tests are presented -10 below in Table 2: TABLE 2: Percolation test results for various flocculant Slumping After 48 Hrs After 98 Hrs Sample ID Bulk Density, (Agglom. Acid Addition) Slump, % Slump, % Drain Rate, Drain Rate, (t/m 3 ) (after 48 hrs) (after 96 hrs) L/m 2 /hr Lm 2 /hr 0 kg/t, (Flocculant #1) 0.0 0.0 21,484 20,131 1.03 50 kg/t , (Flocculant #1) 0.0 0.0 75,402 71,901 1.01 100 kg/t , (Flocculant #1) 0.0 0.0 54,401 52,793 1.11 0 kg/t, (Flocculant #2) 0.0 0.0 17,042 16,680 1.04 50 kg/t, (Flocculant #2) 0.0 0.0 67,470 66,525 1.03 100 kg/t , (Flocculant #2) 0.0 0.0 29,776 28,663 1.09 0 kg/t , (Flocculant #3) 1.0 1.0 19,148 18,043 0.98 50 kg/t , (Flocculant #3) 0.0 0.0 72,038 67,585 1.07 100 kg/t , (Flocculant #3) 0.0 0.0 55,094 51,077 1.14 0 kg/t , (Flocculant #4) 0.0 0.0 13,634 12,281 1.07 50 kglt , (Flocculant #4) 0.0 0.0 50,242 47,268 1.08 100 kg/t , (Flocculant#4) 0.0 0.0 38,138 37,546 1.13 0 kg/t , (Flocculant #5) 0.0 0.0 16,786 16,639 1.02 50 kg/t, (Flocculant #5) 0.0 0.0 42,014 40,200 1.07 100 kg/t , (Flocculant #5) 0.0 0.0 70,670 69,147 1.07 5 It is apparent from Table 2 that the addition of the flocculant leads to formation of stronger agglomerates with better percolation characteristics. This improvement in percolation rates was similar for each of the eight different flocculants tested. In Figures 2 and 3, the results of percolation tests are illustrated, showing the improvement observed using flocculants in 10 combination with acid addition. Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. All such modifications 15 and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

- 11 All of the patents cited in this specification, are herein incorporated by reference. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of 5 the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of 10 further features in various embodiments of the invention.

Claims (31)

1. A process to form pellets of a nickel-containing ore for use in heap leaching, the process comprising the steps of: 5 a) agglomerating mineral fines present in the nickel-containing ore by mixing a flocculant with the ore to form pellets incorporating the mineral fines; and, b) adding a sufficient quantity of a mineral acid to the ore during or after step a) to neutralize clay minerals present in the nickel-containing ore prior to heap leaching. 10

2. The process of claim 1 wherein the flocculant is- a long-chain water soluble polymeric flocculant.

3. The process of claim 2 wherein the flocculant is a straight-chained polymeric flocculant. 15

4. The process of any one of claims 1 to 3 wherein the flocculant is a polymer formed from acrylamide subunits having the nominal formula per unit of -CH 2 CHCONH 2 -.

5. The process of any one of claims 1 to 4 further comprising the addition of a 20 surfactant during step a) to improve wettability.

6. The process of any one of claims 1 to 5 wherein the flocculant is mixed with the ore at a flocculant concentration in the range between 0.05 to 1.0 % by weight. 25

7. The process of any one of claims 1 to 5 wherein the flocculant is mixed with the ore at a flocculant concentration in the range between 0.2 to 0.6% by weight.

8. The process of any one of claims 1 to 5 wherein the flocculant is an anionic acrylamide polymer added to the ore at a rate in the range of 5 to 500 grams per tonne of 30 dry ore.

9. The process of any one of claims 1 to 5 wherein the flocculant is an anionic acrylamide polymer added to the ore at a rate in the range of 80 to 120 grams per tonne of dry ore. 35 - 13

10. The process of any one of the preceding claims wherein the mineral acid is selected from the group consisting of sulphuric acid, hydrochloric acid and nitric acid.

11. The process of any one of the preceding claims wherein the mineral acid added 5 during step b) is concentrated sulphuric acid and the acid is added in the range of 50 to 200 kilograms acid per tonne of dry ore.

12. The process of any one of the preceding claims wherein the mineral acid added during step b) is concentrated sulphuric acid and the acid is added in the range of 80 to 120 10 kilograms acid per tonne of dry ore.

13. The process of any one of the preceding claims wherein sufficient water is added during step a) to form a pellet having a target level of agglomeration moisture in the range of 35 to 65%. 15

14. The process of any one of the preceding claims wherein sufficient water is added during step a) to form a pellet having a target level of agglomeration moisture in the range of 40 to 55%. 20

15. The process of claim 14 wherein the water is added prior to the addition of the flocculant.

16. The process of claim 14 or 15 wherein the water is added with the flocculant in form of an aqueous suspension. 25

17. The process of any one of the preceding claims wherein the flocculant is be added to the ore particles before agglomeration with a mineral acid being added during agglomeration. 30

18. The process of any one of the preceding claims wherein the flocculant is sprayed or dripped onto the ore particles in step a).

19. The process of any one of the preceding claims wherein the nickel-containing ore is subjected to a screening operation to provide a particle size prior to agglomeration having a 35 P80 of 10 - 50mm. -14

20. The process of any one of the preceding claims wherein the pellets have an average particle size in the range from 1mm to 50mm.

21. In a process for percolation leaching of metal values from a nickel-bearing ore 5 wherein the ore is first formed into pellets which are then formed into a heap and then leached by percolating a leaching solution through the heap which extracts nickel from the pellets for subsequent recovery, the improvement in which the pellets are produced by: a) agglomerating mineral fines present in the nickel-bearing ore by mixing a flocculant with the ore to form pellets incorporating the mineral fines; and, 10 b) adding a sufficient quantity of a mineral acid to the pellets formed during step a) to neutralize clay minerals present in the ore prior to heap leaching.

22. The process of claim 21 wherein the leaching solution contains between 0.1 and 20% sulphuric acid. 15

23. The process of claim 21 or 22 wherein optimum recovery of nickel is achieved when 400 to 1000 kilograms sulphuric acid per tonne of dry ore is consumed during leaching.

24. The process of any one of claims 21 to 23 wherein the leaching solution is applied at 20 an irrigation rate or flux in the range of 5 to 30 litres per hour per metre squared.

25. The process of any one of claims 21 to 23 wherein the leaching solution is applied at an irrigation rate of flux in the range of 10 to 25 litres per hour per metre squared. 25

26. The process of any one of claims 21 to 25 wherein the heap has a height in the range of 4 to 8 meters.

27. The process of any one of claims 21 to 26 wherein the heap is a dynamic type of heap. 30

28. The process of any one of claims 21 to 27 wherein heap leaching is conducted using a counter-current series of heaps including a lead heap and a lag heap, with fresh leaching solution being introduced at the top of the lag heap. 35 - 15

29. The process of any one of claims 21 to 28 wherein, after leaching operations have been completed, the heap is rinsed with water, with an effluent stream being collected from the base of the heap after rinsing, and the effluent stream is subjected to a metal recovery process to recover nickel and/or cobalt values. 5

30. The process of any one of claims 21 to 28 wherein, after leaching operations have been completed, the heap is rinsed with water, with an effluent stream being collected from the base of the heap after rinsing, and the effluent stream is reprocessed over the heap. 10 30. The process of any one of claims 21 to 29 wherein the pellets are formed using the process of any one of claims 1 to 20.

31. A process to form pellets of a nickel-containing ore for use in heap leaching substantially as herein described with reference to and as illustrated in the accompanying 15 figures.