IMPROVED PROCESS FOR PRODUCING FEED MATERIAL FOR A LEACHING PROCESS

FIELD OF THE INVENTION

The present invention relates to a process and plant for producing a feed material for a leaching process, and, in particular, a nickel leaching process.

BACKGROUND TO THE INVENTION

The worldwide demand for nickel is increasing greatly. As a result, the price of nickel on world markets is at historical highs and nickel producers are struggling to meet the demand for nickel. As a result of these market forces, nickel producers are looking more and more toward the exploitation of nickel laterite deposits. In the past, the exploitation of laterite deposits has been both technically difficult and economically unattractive. However, recent advances in technology have made the exploitation of these deposits more feasible.

Heap leaching is becoming an important process in recovery of metals from ores and concentrates. Heap leaching typically involves forming piles or heaps of the ore or concentrate, passing a leaching solution over the piles or heaps and recovering the solution that now contains dissolved metals. The solutions can then be treated to recover the dissolved metals therefrom. Heap leaching processes normally involve leaching the ores or concentrates for extended periods, such as from 6 months to 2 years. Heap leaching has allowed for metal recovery from low grade ores that would otherwise be uneconomic to treat.

Heap leaching involves the percolation of the leaching solution through the piles or heaps of ore or concentrate. The leach solution must be able to access the ore or concentrate and also be able to percolate through the piles or heaps in order to access the metal values in the ore or concentrate and thereby dissolve the metal values. To improve percolation of the solution through the piles or heaps, the ores or concentrates may be formed into agglomerates or pellets to enable the piles or heaps to have sufficient bulk porosity.

United States patent number 6,312,500, assigned to BHP Minerals International Inc., issued on 6 November 2001, describes a process for heap leaching of nickel- containing ores, particularly nickel-containing ores having a tangible clay content. In this process, the ore is size reduced to below one inch (25 mm). The particles are then formed into pellets by mixing them with a concentrated acid (preferably sulphuric acid) and agglomerating them in, for example, a rotary disk, drum or other suitable apparatus. The amount of acid added is generally that amount required to immediately neutralize the ore. In other words, the amount of acid is generally the amount required to neutralize the readily available acid consuming minerals, particularly iron and magnesium. The agglomerated pellets may then be cured for a suitable period of time ranging from as little as one hour to several days. The pellets are then formed into a heap and leached with an acidic solution.

Greek patent number 1001555 in the names of Styliani Agatzini-Leonardou and

Dimaki Dimitra, filed on 31 May 1991, describes a method for heap leaching of nickel- containing ores using dilute sulphuric acid as a leach solution. In one embodiment, this patent describes wetting the ore with water or the leaching solution before the deposition of the ore into the heap.

Both of these patents suffer from a number of technical disadvantages in that experience has demonstrated that the agglomerations conditions disclosed in each patent are not universally applicable. That is, agglomerations with concentrated sulphuric acid as in the US patent, or merely by wetting with water as in the Greek patent, will only produce stable agglomerates suitable for heap leaching with specific nickel laterite ores.

It is an object of the present invention to provide a method for producing a feed material for the heap leaching of nickel laterites that is generally applicable to a wide range of laterite feedstocks.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a method for producing a feed material for a leaching process comprising:

(a) mixing ore or concentrate particles with water and optionally other additives, said particles including relatively fine particles and relatively coarse particles;

(b) agglomerating the mixture to form pellets; and (c) curing the pellets.

In a preferred embodiment, the method produces a feed material for a nickel leaching process, and, in particular, a nickel laterite leaching process.

In the process of the first aspect of the present invention, it is believed that the relatively coarse particles act as "seed" particles in the pelletising step and that the relatively fine particles tend to coat the relatively coarse particles to form the pellets. In a number of ores and concentrates, the concentration of metal to be recovered is significantly higher in the relatively fine particles than in the relatively coarse particles. As a result, the pellets may be described as generally having a seed or core of low metal content surrounded by an outer layer of particles having a relatively higher metal content. This means, of course, that the leaching solution can readily access the region of relatively higher metal concentration in the outer part of the pellets.

The process of preparation of a run of mine ore for leaching under this methodology may require crushing and screening of the ore to a maximum size between 25 and 40 mm. The relatively coarse particles may have particle sizes in the range of 25 mm +/-10 mm. The relatively fine particles may constitute all particles below the size range of the coarse particles. The relatively fine particles may include extremely fine particles(<50 microns), such particles containing clays and iron minerals.

In some embodiments, the relatively coarse particles may be added to the relatively fine particles. In other embodiments, the ore forms the relatively coarse particles and the relatively fine particles during grinding.

- A -

The mode of addition and type of seed or matrix material is unique to the specific ore type being processed. The seed addition in terms of its mass proportion to the feed rate and particle size is defined by empirical testing for each ore type.

In one embodiment of the present invention, an acid is added to the mixture in step (a). Alternatively, an acid may be later mixed with the pellets. The acid may be a mineral acid, such as sulphuric acid or hydrochloric acid. The acid may be a dilute acid.

Without wishing to be bound by theory, it is believed that the acid initiates the release of the nickel from the clay matrix, a process which is continued by longer term acidic solution irrigation of the ore once stacked on the heap. The mixing step during agglomeration is the only opportunity to intimately mix acid with the ore, as during the heap irrigation the contacting of ore and acid relies on slow diffusion of fluid in and out of the pellets or agglomerates.

Other additives may optionally be added in step (a). The optional additives may include one or more additives that act to improve the strength and stability of the pellets or to increase the permeability of the pellet to the leach solution subsequently applied. The additives may be selected from inorganic materials, such as quartz, sand, gravel or fly ash, or organic materials such as surfactants, straw, cane mulch, other fibrous material, bagasse or molasses, or vegetation, such as cleared local vegetation (e.g. twigs, leaves, thin branches, bark) that has been chipped, shredded or pulped to form matted fibrous material.

In a preferred embodiment of the invention, curing of the pellets is carried out by air drying the pellets for a given period of time.

The pellets may be subjected to an acid leaching process such as vat heap leaching and vat leaching. The water and (in some embodiments) acid addition varies with ore type. In general the moisture content required for successful pellet or agglomerate formation is typically 25 - 30% by weight, however the water addition required depends on the inherent moisture of the ore which is variable (for example, from 5 - 15% by weight for dry Australian laterite ores). Some tropical laterite ores have moisture contents that fall within the range of from 30 to 45% and these ores may also be pelletised or agglomerated

in accordance with the present invention. The moisture variability will depend on the season, the mining depth, the water table level in the deposit and the specific mineral (clays) assemblage being mined. The acid addition must be carefully monitored as an excess causes a loss of pellet/ agglomerate strength. Generally a maximum of around 25% of the total acid consuming capability of the ore is added in the preparation stage. Higher additions can cause structural failure of the pellet.

Acid may be added to the particles to exploit the opportunity of intimately mixing the acid with the particles to optimize the contact of the two phases for the purpose of leaching the nickel and cobalt. This contacting becomes progressively more difficult once the ore is stacked on the heap and is being irrigated. Irrigation is an imperfect method of solid-liquid contacting, and hence the rate of leaching slows once the initial acid addition achieved during preparation of the pellets/agglomerates is consumed.

In a second aspect, the present invention provides a method for producing a feed material for a leaching process comprising: (a) mixing ore or concentrate particles with water and acid and optionally other additives, said water and acid being added in amounts sufficient to obtain a moisture content of 25 to 30% by weight;

(b) pelletising the mixture; and

(c) curing the pellets. The amounts of added acid and water appropriate to the specific ore being processed may be determined by empirical trials of pellet/agglomerate stability under a range of additions of each phase.

In a third aspect, the present invention provides a method for producing a feed material for a leaching process comprising: (a) mixing ore or concentrate particles with water and additives selected from one or more organic materials;

(b) pelletising the mixture; and

(c) curing the pellets.

Acid may also be added in step (a). This acid may be a dilute acid such as sulphuric acid

In step (a) above, the one or more organic materials may include straw, sugar cane bagasse, cane mulch, other fibrous material, or molasses. Generally, the additives are sticky substances and/or fibrous material. Such additives assist with physical binding of the pellets or agglomerates. The additives are desirably of relatively low cost and of local source.

Other features of a process that can be used in various embodiments of the present invention will be described hereunder with reference to the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 shows a flow sheet of a process in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

It is to be understood that the drawing is provided for the purpose of illustrating embodiments of the present invention. Thus, it will be appreciated that the invention should not be considered to be limited to the features as shown in the drawing.

Figure 1 shows a flow sheet of a process used for recovering nickel and cobalt from an ore or concentrate containing nickel and cobalt. In Figure 1 , a run of mine ore, which may comprise a lateritic ore containing nickel and cobalt, is delivered by trucks 10 and blended at 12. The blended ore is reclaimed to a hinged grizzly 14 over a feed bin from where it is delivered to a sizer 16. The sizer may, for example, be set at a 35 mm gap. The sizer may be a Stamler sizer that is provided with self-cleaning engaging tools, and two sizers may be employed in sequence to obtain a final maximum particle size around 20 to 25 mm. The sizer discharge is transferred via conveyor 18 to either a stacker conveyor 20 to build a crushed ore stockpile 22 for use during periods of pelletising shutdown or to a pug mill 24 that discharges directly into a pelletising drum 26. Water 28 is added to the pug mill feed to achieve a selected or optimum moisture level. For example, the moisture

level in the pug mill may be approximately 20 - 25% by weight. The pug mill functions to ensure that adequate moisture is well dispersed throughout the ore prior to passing to the pelletising step.

Diluted acid may also be added to the pellets 26. For example, acid additions of between 15% and 25% of the total acid required to achieve target nickel extraction may suitably be added to the pelletiser. This has been found to increase the leach kinetics - the rate at which nickel is extracted provided that the water and acid solution is between

20 and 30% by weight of the mix of ore and solution, depending on ore type.

The concentration of the dilute acid addition is linked to the water requirement to achieve target moisture. The acid is suitably premixed with the water for safety reasons so its concentration will change, depending upon the ore type.

Preferably, in order to maintain the physical properties of the pellets at optimum moisture and acid content levels, control systems may be used with sensors to monitor moisture and mass addition. Additives may be used to optimise the strength and permeability of the pellets.

The additives may be neutral or chemically active materials. For example, fibrous or solid material may add strength and create internal solution pathways inside the pellet to enhance solid liquid contacting. Liquid additives may increase the ionic conductivity of the moistened ore. If or where required, the mass addition of the additives will typically be in the order of 1-5%. In the case of fibrous additives, the additives may be added in amounts of 1-3% per tonne ore. This represents a significant volume ratio as the fibre mass is low relative to the ore. The effect of addition of fibrous additives is to provide binding strength analogous to glassfibre in epoxy for fiberglass products, and to provide solution pathways for the diffusion of acid into and nickel out of the particles during the irrigation process on the heap.

The pellets can be formed in a specialised pelletising device that builds a rounded pellet on a moving surface (e.g. a rotating drum or a revolving disk) as dispersed particulate matter comes into contact with other particles and adheres, then gains strength by compaction. Alternatively, pellets may be formed by the extrusion of the pugged ore, and the formation of pellets by cutting the extrusion to required length and/or section.

Pellet shape, size and strength are key factors in successful leaching, affecting achievable heap height, stacking performance, life of heap over leach cycle and stability of heap. The shape can influence the surface area available for contacting the pellet with acidic solution introduced into the void space by irrigation or flooding, the length of the internal diffusion path in the pellet for these acidic solutions (this applies to the transfer into the particle of the acid, and the transfer out of the particle of metal salts that are collected for subsequent refining to saleable products), and the general permeability of the aggregate of pellets to bulk solution flow.

The desired size range of the pellets is 40 mm ranging down to 4 mm. This provides:

• good surface area for contacting of solution with ore pellets/agglomerates,

• short path lengths for internal diffusion of fluids,

• good permeability over the height of the heap to irrigation solutions applied at the top and flowing down through the stacked pellets/agglomerates.

• material that can be readily handled by conventional conveyors and chutes for stacking

• pellets or agglomerates that are intrinsically strong - at larger sizes (>40 mm) the pellets/agglomerates lose strength under compressive load The pellets from pelletiser 26 may then be transported by a series of conveyors 28 to a stacking conveyor 30 on a leach pad. The stack conveyor 30 is used to form stacks or heaps of the pellets. Some of the heaps are shown at 32, 34 and 36.

Suitably, once the pellets have been formed they are stacked either on an impermeable membrane (such as HDPE or PVC sheet) for the purpose of heap leaching or in a vat (in lined excavations or manufactured containment) for the purpose of vat leaching. The pellets may also be used in other leaching processes.

The process of the present invention allows for the formation of pellets of good strength, which maximises the geotechnical stability of a heap for heap leaching in terms

of compaction due to vertical load and side slip failure and also maximises the ability of the pellets to remain physically stable when flooded in terms of compaction due to vertical load during vat leaching.

The flow sheet shown in Figure 1 utilises diluted sulphuric acid as a leachant and additive in preparation of the pellets/agglomerates. The sulphuric acid is delivered by delivery trucks 38 and stored in concentrated sulphuric acid tank 40. The sulphuric storage tank 40 has appropriate pumps and lines to transfer the acid to where it is required in the flow sheet. For example, line 42 transfers sulphuric acid to pelletiser 26.

The flow sheet shown in Figure 1 utilises heap leaching to form the pregnant leach solution. However, it will be appreciated that other leaching processes may be used in the process of the present invention. For example, the leaching process shown as part of the flow sheet of Figure 1 may be replaced by vat leaching. Further, although diluted sulphuric acid has been described as the leachant used in the embodiment shown in

Figure 1, other leaching agents (diluted or in concentrated form) may also be used. Some examples of other leaching agents include hydrochloric acid and ammonium sulphate.

The pellet heaps 32, 34, 36 are leached with diluted sulphuric acid, typically having a concentration of 20 grams to 80 grams per litre, using a counter-current technique, where ore older than about 30 pad days is irrigated with fresh sulphuric acid solution provided via line 44 and return solution pond 46. The newer ore heaps are irrigated with a proportion of the intermediate leach solution drained down from old ore panels. The intermediate leach solution is recovered in pond 48 and a proportion of the intermediate solution is returned via line 50 to irrigate the new heap 32.

The pregnant leach solution from the heap leaching is recovered in pond 51. The pregnant leach solution typically contains dissolved nickel and cobalt, as well as dissolved iron and magnesium, and small amounts of dissolved aluminium, chromium, and manganese.

Every ore type is subjected to a series of tests involving bench scale formation of pellets or agglomerates over a range of moisture additions and mass of acid additions per tonne ore

This is followed by measurement of the slump (shrinkage) with respect to change in the height of the pellets or agglomerates in a column under irrigation, and then evaluation of the pellet or agglomerate column slump/shrinkage when fully flooded ie pellets or agglomerates submerged in acid solution for a period of several days. Any shrinkage of the stack of pellets or agglomerates in the column container that exceeds 10% of the total height within 7 days is generally deemed to be unacceptable.

Selected pellets or agglomerates are also evaluated by load permeability testing, in which an artificial load is applied to a column of pellets or agglomerates to simulate the bearing loads experienced in a full scale heap. After each successive application of the load the permeability of the column of pellets or agglomerates to applied irrigation solution is assessed

The person skilled in the art will understand that the present invention may be subject to variations and modifications other than those specifically described. It is understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.