WO2009146485A1

WO2009146485A1 - Multi-stage leaching process

Info

Publication number: WO2009146485A1
Application number: PCT/AU2009/000680
Authority: WO
Inventors: Marjorie Valix
Original assignee: The University Of Sydney
Priority date: 2008-06-06
Filing date: 2009-05-29
Publication date: 2009-12-10
Also published as: AU2009253834A1; CA2726655A1; CN102057064A; CN102057064B

Abstract

A process of leaching a metal value such as nickel and/or cobalt from a metal laden solid, for example an ore such as laterite, comprises the steps of (a) contacting the metal laden solid with a leach solution comprising a mineral acid and an organic acid such as malic acid to provide a leachate including the metal value, (b) adding further organic acid to the leachate including the metal value, and (c) using the leachate including the metal value from step (b) as at least a portion of the leach solution in step (a). Steps (a) to (c) can be repeated any number of times, for example two or three times or more.

Description

MULTI-STAGE LEACHING PROCESS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from AU 2008902891, which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to the leaching of a metal value from a metal laden solid over multiple leaching stages. The process of the present invention is particularly, but not only, suited to the leaching of a metal value from an ore.

BACKGROUND

The removal of a metal from a solid laden with that metal can be desirable when the metal has commercial value. Ore obtained from a mine site is a metal laden solid that typically comprises one or more metal values of commercial interest. For example, laterite ore contains nickel and cobalt which are metal values that can attract a high price on the commodities market. The main uses of nickel include the production of stainless steel, rechargeable NiCad batteries and the production of electronic and computer equipment. Titaniferous magnetite ores contain metal values such as vanadium, iron and titanium, all of which are commercially desirable. Ilmenite ore is a fraction of magnetite ore that contains titanium, which can be oxidised to titania. Titania is a valuable material in many industrial and consumer products. The major use of titania is as the white pigment in paints, plastics, and paper. Aluminium is a soft, durable, light-weight metal that can be extracted from bauxite. Structural components made from aluminium are used in the aerospace industry and in other areas of transportation and building. The reactive nature of aluminium also makes it valuable for use as a catalyst or as an additive in chemical mixtures, including ammonium nitrate explosives to enhance blast power.

In some cases, the removal of a metal from a metal laden solid can be to improve the disposability of that solid. For example, the removal of a metal from catalyst waste can mean that the bulk catalyst (in the absence of the metal) is easier to dispose of since it no longer contains a metal that may be deleterious to the environment. For example, chromium is a toxin and a carcinogen, so any waste products comprising chromium need to be controlled i.e. by removing the chromium, before they can be disposed of responsibly.

The efficient removal of a metal value from a metal laden solid has clear economical benefits. For example, the lower the removal processing costs, the higher the return on the metal value removed. Some metal laden solids comprise the metal value in a stable form or comprise very small amounts of the metal value, which can make the removal process difficult and expensive. For example, although there has been modest investment in new plant, there is widespread concern in industry about the technical and economic viability of nickel laterite processing. Laterites contain relatively low levels of nickel and cobalt and, to make matters worse, almost all reserves are impossible to concentrate, thus requiring all of the ore to be processed to extract the nickel and cobalt. Nickel laterite is also highly stable, so requires aggressive processing treatments adding further to the costs and technical difficulties. Processing all of the ore is expensive and introduces technical complexities inherent in processing large amounts of other minerals (referred to as gangue or waste) at the same time. Current titania processing has problems of feedstock supply, cost and the generation of toxic waste, hi particular, only some titanium ores are suitable as feed materials for processing. Low-grade ilmenite, the most plentiful source of titania, has little or no economic value because it is unsuitable for processing with conventional methods. Not only is the ilmenite unviable, but the opportunities for the economic recovery of other valuable minerals present in the low-grade deposit are lost.

Generally, current commercial extractions of metal values from ores are energy intensive and operational costs are high. Despite these difficulties, processing of ore deposits continues to grow strongly. The sustainability of the production of many metals depends upon the development of new processes that allow the vast deposits of low-grade ore in particular to be commercially exploited. To extract a metal from a solid such as ore, the ore can be leached according to known techniques wherein the valuable metal is taken up into a leach solution. The pregnant leach solution or leach liquor can be termed "leachate". From a commercial perspective, it would be advantageous if leachate resulting from a leach process could be re-used or recycled in a further leach process. Re-use or recycle of leachate represents a significant saving in the cost of the leaching reagents and/or in the operational costs associated with heating and pumping of the leach solution.

When leachate is re-used, however, metals and other contaminants in solution can affect the ability of the leachate to further leach metals from a solid. Accordingly, the metals and other contaminants are removed prior to re-use of the leachate. Feeding the metal laden leachate to the metal recovery stage (e.g., ion exchange) to remove the metal values and/or other contaminants before re-using the leachate adds to the operational costs of a process. However, if the metal recovery is not undertaken the metal concentration in the leachate may reach saturation point at which point desirable metal values may be lost through co- precipitation. Before this occurs, however, the activity of the leach solution will likely decrease to such an extent that the leaching process will essentially stop.

Methods which improve leaching efficiency or reduce the associated cost of leaching a valuable metal from a metal laden solid are desirable.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a process of leaching a metal value from a metal laden solid, the process comprising the steps of: (a) contacting the metal laden solid with a leach solution comprising a mineral acid and an organic acid to provide a leachate including the metal value;

(b) adding further organic acid to the leachate including the metal value;

(c) using the leachate including the metal value from step (b) as at least a portion of the leach solution in step (a). In one embodiment, the metal laden solid is ore, preferably laterite ore, and the metal value is nickel (which includes nickel and/or cobalt). In some embodiments, there may be more than one metal value leached from the solid. In another embodiment, the metal laden solid is bauxite and the metal value leached is aluminium. Other embodiments comprise different metal laden solids and different metal values of interest.

According to the invention, leachate resulting from a leach process can be re-used as a leach solution without the need to remove the metal value leached into the leachate. However, in some embodiments, the method can further include the step of removing at least some of the metal value from the leachate prior to re-use. Once the multiple stage leaching process is complete, the resultant leachate can be recovered and subjected to process(es) to recover the metal value(s) from solution.

The re-use of the leachate is possible because the leach solution comprises organic acid which acts to sequester the metal value in solution. By "sequestering" it is meant that a chemical reaction occurs between the organic acid and the metal value resulting in the metal value being bound into a stable, soluble compound or complex. Effectively, the organic acid acts as a ligand to chelate or bind the metal value. The organic acid can be referred to as a chelating agent, a sequestering agent or a complexing agent. The dissolved metal ions complexed with the organic acid are substantially inhibited from adversely affecting the activity of the leach solution. Accordingly, the leachate can be re-used and during that re-use, the leach solution is able to extract further metal value(s) from a solid.

Equations 1 and 2 show the reactions of a metal value (M), with a leach solution comprising a mineral acid, e.g. sulphuric acid and an organic acid (L).

M²⁺ + H₂SO₄ →MSO₄ + 2H⁺ (1)

M²⁺ + H₂L = MHL¹⁺ + H⁺ (2)

In one embodiment, L = C₄H₄O₅ (malate) The equilibrium constant for the reaction of Equation (2) is greater than the equilibrium constant for the reaction of Equation (1). This means that the metal value is more likely to form the organo-metallic complex of Equation (2) than the metal sulphate of Equation (1). Accordingly, the mineral acid remains available to leach metal from the ore.

The leachate comprises a mineral acid and an organic acid. The mineral acid can be any mineral acid known for use in a leach process, for example, sulphuric acid or hydrochloric acid. Any organic acid (or combinations of organic acids) can be used in the present process provided at least one of the organic acids is able to sequester the metal value of interest as described above. In other words, any organic acid that has an equilibrium constant in a reaction similar to Equation 2 that is greater than the equilibrium constant of a reaction similar to Equation (1) may be used. Such organic acids could include malic acid, lactic acid, gluconic acid, pyruvic acid, succinic acid, ketoglutaric acid, oxalic acid, fumaric acid and citric acid or any combination thereof.

In one embodiment, the organic acid is malic acid and the metal value of interest is nickel.

Step (a) in the invention is considered a first leach stage or stage 1. Re-use of the leachate from the first stage in step (c) in the invention as at least a portion of the leach solution in a further leach process is a second leach stage or stage 2. Re-use of the leachate from the second stage as at least a portion of the leach solution in a further leach process is a third stage or stage 3, and so on. Any plural number of leach stages undertaken one after the other can be referred to as a multi-leaching or multi-stage leaching process. The number of stages undertaken can depend upon how much solid there is to leach and how effective the leach solution is at each stage. Advantageously, steps (a) to (c) are repeated to provide three leach stages or repeated a first time and a second time (i.e. repeated twice) to provide four leach stages. In some embodiments, there are at least 3 leach stages and there may be 5 stages or more e.g. 8 stages.

As mentioned above, any plural number of stages may be undertaken for as long as the metal value recovery from the metal laden solid remains acceptable. An acceptable percentage of metal leached in any one leach stage may be at least 80 % of the total available metal value in the solid or less, for example, 40 % or any percentage in between. Preferably, at least 90 % of the total available metal value is leached from the solid, more preferably, 98 %.

Where the solid still contains metal values after a leach stage, it can be re-leached alone in a second or further stage. Alternatively, the solid still containing metal values can be combined with fresh solid or fresh solid can be used alone in a second or further leach stage. For ore, whether the same solid needs to be leached over more than one stage may depend on the grade of the ore. With low grade nickel laterite ore (e.g. comprising about 1 % nickel), if half the nickel has been removed in a first stage, it may not be economical to process this ore further and fresh ore will be supplied in a further stage i.e. step (c). In some circumstances; however, lower percentage recovery, e.g. 40 %, may be acceptable because the economics of the process may make it viable to re-use the leachate rather than process the leachate to remove the metal values.

After a leach stage, the amount of organic acid in the leachate is topped-up to provide ideal leach conditions, or conditions within e.g. 5 or 10 % of ideal. The ideal amount of organic acid is an amount at which an acceptable percentage of metal value is leached in any one leach stage (discussed above). To determine how much organic acid to add after a leach stage, the amount of organic acid remaining in the leachate after a leach stage can be measured using, for example, ion chromatography. Advantageously, organic acid is then added to provide the same level of organic acid as in the leach solution prior to the leaching stage. Alternatively, a fixed amount of organic acid can be added. The fixed amount can be any amount; but is preferably determined based on prior test work to reveal how much of the organic acid is likely to remain in the leach solution after a leach stage.

It may be necessary to add further mineral acid to the leachate prior to reuse of the leachate in a leach stage in order to reduce the pH. The pH of the leachate may increase during leaching, for example as a result of acid neutralisation by alkaline minerals or other content. In a continuous system, extra mineral acid may be added^" continuously. The organic acid itself is generally not sufficiently acidic to maintain a desired pH. In some embodiments the pH may be adjusted to below about 1.0 to achieve optimum recovery of the metal value (>80 %, more preferably >90 %), for example about 0.8. The amount of mineral acid added to the leachate may be, for example, in the range of about 0.15 to about 0.25 kg/kg of acid to ore mass.

As discussed below in greater detail, the oxidation reduction potential (ORP) of the leachate may be advantageously adjusted prior to or during reuse, for example by the same addition of mineral acid as for adjustment of the pH.

Thus, the method can further include the step of adjusting the pH and/or ORP of the leachate before re-use of the leachate in step (c). The adjustment can be undertaken before or after the addition of organic acid in step (b). The pH and/or ORP of the leachate can be measured and adjusted, if necessary, to bring the pH and/or ORP into line with ideal conditions for leaching the metal value of interest. The optimum or ideal pH and ORP conditions to most effectively leach the metal value(s) of interest can be pre-determined by prior test work on the metal laden solid of interest, since different mineralogy will present different requirements. The ideal or optimum leach conditions are conditions under which the acceptable amount of metal value is extracted.

In one embodiment, in which nickel is the metal value of interest in a laterite ore, the pH is preferably below about 0.8 to achieve optimum recovery and the ORP is preferably below about 320 mV, more preferably within the range of from about 200 mV to about 320 mV. Alternatively, the ORP is in the range of about 450 mV to about 550 mV, more preferably 470 mV to 510 mV. If the pH and/or ORP of the leachate deviate from these parameters, the pH and/or ORP can be adjusted as necessary to bring them into line. It is believed to be preferable to maintain the ORP within the lower range of about 200 mV to about 320 mV, since this may facilitate the control of optimum dissolution.

Advantageously, the pH and the ORP are adjusted to be the same as for the original leach solution fed to stage 1 (assuming the conditions in the original leach solution were selected or otherwise adjusted to be ideal). In some embodiments, however, the ORP of the original leach solution is not ideal initially, but during the leach process the ORP is allowed to adjust to more favourable conditions. The ORP may adjust during leaching because organic acid in the leach solution is a reducing agent and there are also organic components of ore that can act as reducing agents. Furthermore, the mineral acid in the leach solution is an oxidising agent, so the overall ORP of the resultant leachate is the sum of the effects of the reagents and the ore or other metal laden solid itself.

During the leach process, it has been found that at least for some ores, the ORP decreases from a starting ORP of about 370 to 460 mV to an ORP of the leachate in the range of from about 350 to -165 mV depending upon quantities of reducing/oxidising agents present.

In some cases, where the original leach solution does not have optimal pH and/or ORP values and the solids and other components of the leach solution do not provide sufficient reducing agents, the ORP may not decrease enough during the leach process. Accordingly, the leachate may have an ORP value above the pre-determined optimal ORP range, e.g. above 320 mV for nickel. Under these circumstances, a reducing agent may be added to the leachate to decrease the ORP. Examples of suitable reducing agents include sodium metabisulphite and sulphur dioxide. On the other hand, if the ORP of the leachate is found to be too low and the ORP needs to be increased before re-use, the leachate can be oxidised with an oxidant such as hydrogen peroxide (H₂O₂), oxygen (O₂), ozone (O₃), calcium hypochlorite or bleach (Ca(ClO)₂). Preferably, the reducing agent or the oxidant is gradually added to the leachate. The concentration added will depend upon the starting ORP and the desired ORP.

As mentioned above, in some embodiments, from some leach stages, the pH and/or ORP of the leachate may not require adjustment, or only one of the pH and the ORP may require adjustment. It should be understood that if the pH and/or the ORP are not adjusted and the pH values and ORP values deviate from the pre-determined ideal leach conditions, some metal values may be leached from the solid, but the total amount leached will likely be less than under ideal conditions.

The total amount of leachate recovered from a stage can be re-used as leach solution in further stages, or a portion can be re-used. In some embodiments, a portion of the leachate can be separated from a stage and subjected to further processing to recover the metal values therein. Where the leachate is separated, the pH and/or the ORP can be adjusted before or after the separation step. The remaining portion of leachate not subjected to further processing can be re-used as leach solution as described above.

BRIEF DESCRIPTION OF THE FIGURES

Preferred embodiments of the invention will now be described with reference to the following drawings, which are intended to be exemplary only, and in which:

FIGURE 1 is a Table showing percentage consumption of the acids in the leach solution during an exemplary single stage leaching processes involving High Grade Goethite (HGG) ore;

FIGURE 2 is a graph showing the multi-leaching behaviour of sulphuric acid compared with the behaviour of a leach solution comprising a mix of sulphuric acid and organic acid, the latter is in accordance with an embodiment of the present invention;

FIGURE 3 is a graph showing the effect of total metal concentration in the leachate on percentage nickel extraction from laterite ore;

FIGURE 4 is a graph showing the amount of aluminium leached as a percentage of total available aluminium in the ore over five leach stages;

FIGURE 5 is a graph showing the amount of chromium leached as a percentage of total available chromium in the ore over five leach stages; FIGURE 6 is a graph showing the amount of iron leached as a percentage of total available iron in the ore over five leach stages;

FIGURE 7 is a graph showing the amount of nickel leached as a percentage of the total available nickel in the ore as the ORP is adjusted;

FIGURE 8 is a graph showing the amount of nickel leached as a percentage of the total available nickel in the ore as the pH is adjusted; and

FIGURE 9 is a graph showing acid usage in multi/single leaching of High Grade Saprolite ore and corresponding nickel recovery.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The process of the invention is used to leach metal values from metal laden solids. In some embodiments, the process is used to leach metal values from electronic waste or catalyst waste comprising metal solids of value. However, the process is especially suited to the leaching of metal values from metal oxide ores.

Ores which could be subjected to the process include oxide ores such as ilmenite which can contain appreciable quantities of iron. The present process could also be used to leach other oxide ores, for example, ore containing any of uranium, copper and gold. In one embodiment, the process is used to leach nickel from a laterite ore.

The metal laden solid must be contacted with a leach solution in order to leach the metal value(s) into solution. The solid can be brought into contact with the leach solution by any means. For example, the leach solution could be trickled onto solid piled in a heap (i.e. heap leaching of ore). Alternatively, the solid can be mixed with leach solution in a container. This process when undertaken with ore is known as vat leaching.

hi order to achieve effective multi-leaching, the leach solution comprises an organic acid. Before or during the leaching process, the organic acid is advantageously combined with a bulk mineral acid to provide a leach solution that more effectively leaches the metal value(s). The bulk mineral acid may be any acid known for use as a leach solution, for example sulphuric acid (H₂SO₄) or hydrochloric acid (HCl).

The concentration of mineral acid in the leach solution can be in the range of 50 g/1 to 300 g/1. However, this value could be more or less depending upon the amount of acid neutralising components in the solid to be leached and also upon the surface chemistry of the solid. The total amount of mineral acid required to effectively leach the desired metal from the solid can be referred to as the mineral acid to solid ratio (kg / kg).

Typically, in a standard leach process, the mineral acid to solid ratios used range from 0.5 to 2 (i.e. a range from 0.5 kg acid : 1 kg solid to 2 kg acid : 1 kg solid), for example, a ratio of 1 : 1 (1 kg acid : 1 kg solid). From an economic and environmental perspective, the less acid used in the leaching process the better. Using the present process, in some cases, the acid to solid ratio can be decreased to be in the range of from 0.16 to 0.3, which is less than the range given above thereby illustrating the advantages of the invention.

The amount of mineral acid used in vat leaching will depend upon the required pulp density of the ore (grams of ore per 100 ml of total leach solution). In vat leaching, the pulp density can vary and values of from about 10 to 20 % provide acceptable flow rates, although lower values are possible and higher values could be used in some cases, for example, up to 30 %. Higher pulp densities can inhibit the ability to stir and pump the material, hi a typical single stage leach processes to effectively leach at 20 % pulp density (20 g of ore per 100 ml) at a ratio of 1 kg acid : 1 kg ore, 200 g/1 of mineral acid will be used. Using the present multiple stage process, at 20 % pulp density a ratio of 0.16 kg acid : 1 kg ore can be used. This means as little as 32 g/1 of mineral acid is required to leach the metal value from the ore, a notable reduction from 200 g/1.

It is thought that the mineral acid provides the necessary acidity to break down or react with the solid during the leach process. The greater quantities of H⁺ (hydronium ion) available to attack the metal value host minerals, the better. The mineral acid can also be used to overcome re-adsorption of the dissolved metals by promoting a surface charge on the solid that would repel the metal complexes formed with the acid in solution. Effectively, a quantity of mineral acid and organic acid are combined in order to adjust the solution pH of the leach solution to below the iso-electric point of the solid to be leached. The iso-electric point of the solid can be determined by prior experiments. In one embodiment, an equal volume of mineral acid and bio-acid can be combined to sufficiently reduce the pH of the leach solution to the pre-determined optimum pH. The mineral acid therefore supports the effectiveness of the organic acid in complexing with the metal components of the solids. In one embodiment, the mineral acid is provided in concentrated form and mixed with the organic acid before the acids are diluted with water to form the leach solution.

The Table of Figure 1 shows that in a single leach process, under some conditions only between about 10 % and about 40 % of the mineral acid is consumed during a single stage of leaching. The Table also shows that in the same leach process, in some cases only about 70 % of the organic acid component of the leach solution is consumed during a single stage of leaching. Accordingly, in some cases, significant proportions of the active leach solution acid remain in the leachate and could be re-used.

In order to demonstrate that organic acid facilitates the re-use of leach solution, multi- leaching of nickel from High Grade Saprolite (HGS) ore at 20 % pulp density was conducted with a leach solution consisting of sulphuric acid (i.e. in the absence of organic acid) and a leach solution comprising sulphuric acid in combination with organic acid. The effect of adding the organic acid is shown in Figure 2.

Figure 2 shows that in a first stage (or stage 1), leaching in the absence of organic acid resulted in about 30 % extraction of nickel from the ore as a percentage of the total available nickel in the ore. When a leach solution was used comprising mineral acid and organic acid, the percentage nickel leached increased to about 100 %. The leachate from stage 1 was recovered and the pH and ORP adjusted as necessary. More organic acid was added (step (b)) before the leachate was re-used as the leach solution for the second stage of leaching (stage 2) (step (c)). The leachate from stage 2 was recovered and regenerated by adding further organic acid before re-use as the leach solution for a third stage of leaching (stage 3), and so on. Figure 2 shows that in the absence of organic acid, the percentage of nickel leached from the ore decreases to O % after five stages. This implies that, as the nickel tenor in the solution increases, the effectiveness or activity of the leach solution decreases. This is in accordance with what one of ordinary skill in this area of technology area would expect.

However, when the leach solution comprises organic acid, Figure 2 shows that the percentage of nickel recovered over multiple stages remains at about 100 %. There is no observable decrease in the effectiveness of the leach solution over at least eight stages despite the increasing nickel tenor.

Over multiple leach stages using leach solution including organic acid, the total dissolved metal concentration inevitably increases. Organic acid in the leach solution reduces the deleterious effect of the dissolved metals on the efficacy of the leach solution. However, as the total dissolved metal concentration increases beyond certain limits, the activity of the leach solution may decrease despite the presence of the organic acid.

As the number of stages is increased, the kinetics of metal dissolution may slow and a longer period of time may be required to leach the metal value. For example, a first or second leach stage may require about 3 or 4 hours to leach the metal value in solution, whereas a fifth, sixth, seventh or eighth stage may require up to 50 % longer, e.g. 6 hours to leach the metal value into solution. Despite this reduction in the kinetics of leaching a large percentage of the metal value can still be recovered after a number of stages as shown in Figure 2.

The effect of dissolved metal concentration on the ability of the leachate to dissolve nickel is shown in Figure 3. As shown, in general, higher total dissolved metal concentrations result in a decrease in nickel extraction. Eventually, the total dissolved metal concentration reaches a point at which the leach solution can no longer be effectively recycled using the process. At this point all or at least a portion of the leachate should be subjected to further processing to remove the metal values from solution.

The data of Figure 3 indicate that at a total dissolved metal concentration below about 40 g/L, in most cases, the multi-leaching process leaches about 80 % to 100 % of the total available nickel from the ore. However, the data indicate that, generally, the leaching solution becomes less effective when the total dissolved metal increases above about 60 g/L. In order to control the total dissolved metal levels to an extent, one option is to select or adjust the pH and ORP of the leach solution to only leach a selected metal value from the solid. Currently, the total number of stages is limited by the total dissolved metal concentration. By minimising the dissolution of other major metal components from the solid, such as iron and magnesium, the total dissolved metal concentration could be lowered. This would allow the re-use of the leaching solution to be extended to a greater number of stages, enabling more efficient use of the mineral acid in the leach solution.

Metals other than nickel can be leached as valuable metals using the present process. The leach efficiency for other metals also depends on the total dissolved metals in the leaching solutions. As such, for some other metals, multi-leaching is only effective at lower pulp densities, i.e. 20 % pulp density.

Figure 4 shows the multi-leaching process when used for leaching aluminium from ore. Figure 5 shows the process when applied to chromium and Figure 6 shows the multi- leaching of iron. These graphs demonstrate the effectiveness of the process over a number of stages for metal values other than nickel.

The leach solution can comprise any organic acid (or any combination of organic acids) that complex with the metal value and allows leaching over multiple stages. In one embodiment, at least the major proportion of organic acid in the leach solution is malic acid. Other organic acids that can be used in isolation or combination include lactic, gluconic, pyruvic, succinic, ketoglutaric, oxalic, fumaric and citric acid. It has been found that malic acid is particularly effective at preferentially leaching a metal value, such as nickel, from a solid such as laterite ore at high pulp densities.

In the first leach stage, preferably, the amount of organic acid used for leaching is at least 3 % (w/w) of the weight of the metal laden solid to be leached. It has been found, however, that increasing the amount of organic acid in the leach solution can increase the metal recovery and in some embodiments the amount of organic acid is in the range from 5 to 45 % (w/w) of the weight of the solid to be leached. The concentration of organic acid used in the leach solution for the first leach stage is preferably at least 5 g/1. However, the concentration can advantageously be in the range of 1 g/1 to 50 g/1, for example, 5 g/1 to 20 g/1. However, these ranges could alter depending upon the amount and type of solid to be leached.

As noted above, the amount of organic acid required for leaching is partly dependent upon the weight of solid to be leached. In vat leaching, this means the amount of organic acid required depends upon the pulp density of the ore. For example, with a pulp density of 2 %, only 1 g/1 of organic acid in the leach solution may be required. At a pulp density of 10 %, 5 g/1 may be required; and at 30 %, 15 g/1 may be required.

The optimum or ideal amount of organic acid used for a specific solid can be tailored in the light of prior experiments. The leachate is adjusted to top-up the amount of organic acid in the solution to bring the organic acid substantially to the level in the leach solution before the leach stage.

The organic acid can be generated ex situ in the absence of the metal laden solid either microbially or synthetically. If the organic acid is produced microbially, the selected acid or acids can be isolated from the total bio-acid produced in the Krebs Cycle by a microorganism. A combination of organic acid produced synthetically and organic acid produced microbially may be used in the leach solution. The leachate from any stage can contain any amount of metal value before the metal loading is removed by further hydrometallurgical processing. This removal can be undertaken by any processes known in the art, including for example, ion exchange or precipitation. The metal value containing leachate may be transported to another site for further processing by the same or a third party.

Embodiments of the invention will now be described with reference to the following non- limiting examples.

EXAMPLES

Example 1 - Determining optimum leach solution pH and ORP

The effect of ORP on nickel dissolution from HGS and HGG ores in multi-leaching tests is shown in Figure 7. Two pulp densities were considered in these tests, 20 % and 30 %. Figure 7 shows that optimum nickel leached is achieved in the range of from about 200 to 320 mV. A second optimum region can be found at about 450 to 550 mV, more preferably 470 to 510 mV. Exceeding these ranges of ORP values results in decreased nickel extraction.

Figure 8 shows the effect of increasing pH on nickel extraction. A solution pH of below about 1.0, preferably below about 0.8, provides optimum nickel extraction.

Example 2 - Preparing the ore

Selective fermentation of malic acid generated a solution comprising 45 g/L of malic acid. The ore to be leached was at 20 % pulp density, which requires 200 grams of ore per 1 litre of leaching solution. The leach solution was prepared ensuring the required acid : ore ratios were met. For example, 20 % HGS pulp density required a ratio of malic acid : ore of 0.05 and H₂SO₄ : ore of 0.5. Accordingly, to make up the leach solution, 220 ml of the 45 g/L fermented malic acid and 102 grams of (98 % H₂SO₄) were combined together and made up to a litre with water. Example 3- Multi-Leaching of HGG

The results of multi-leaching HGG with mineral acid and malic acid for four hours at 90 °C are shown in Table 1. The pH was adjusted and malic acid was added to the leachate, but ORP was not adjusted between stages.

The results show that adequate metal recoveries were achieved in the first three leach stages. However, in stages 4 and 5, both nickel and cobalt recoveries decreased significantly.

Examination of the final ORP in stages 4 and 5 (334.3 and 370.1 mV) shows these stages are outside the region found optimal in Figure 7. These results emphasise the advantage of controlling the ORP and pH conditions during leaching.

Table 1 - Multi-leaching of high grade goethite ore

Total Pulp Density (%): 96 Sulphuric Acid: Ore

(g/g): 0.3

Malic Acid: Ore (g/g): 0.024

Example 4 - Multi-Leaching of HGS Multi-leaching of HGS ore was conducted using mineral acid and malic acid four hours at 90 ⁰C using approximately 20 % pulp densities (see Table 2). The pH and ORP were adjusted following each stage. Table 2 - Multi-leaching of high grade saprolite ore

Total Pulp Density (%): 83

Sulphuric Acid: Ore (g/g): 0.092

Malic Acid: Ore (g/g): 0.029

In both Tables 1 and 2, the acid : ore ratios reflect the cumulative acid : ore ratios rather than the absolute ratios for each stage. The acid : ore ratios were calculated as follows:

Stage 1 : acid (stage 1) / ore (stage 1) Stage 2: acid (stage 1 + stage 2) / ore (stage 1 + stage 2)

Stage N: acid (sum used for all through to stage N) I ore (sum used for all through to stage N)

As shown appropriate adjustment and control of the ORP and pH resulted in favourable nickel recovery and cobalt recovery.

Multi-leaching of HGS ore. in this test allowed a total pulp density of up to 160% (see also Table 4) to be reached. This is perhaps the highest density that has been reported, considering typical leaching is only able to achieve up to 30% pulp density in a single stage. The additional advantage of this process, lower overall acid usage, particularly mineral acid, compared to typical acid to ore ratios for sulphuric acid only of 0.5 to 2.0, is evident from Table 2. This advantage is also evident for HGG ore from Table 1. These results are further highlighted in Figure 9 which compares the ratio of H₂SO₄ : ore (g/g)% and the subsequent nickel extraction at each of the multi-leaching stages. The acid : ore ratio reflects the cumulative acid and ore used. As shown, multi-leaching allows the acids to be used more efficiently whilst still maintaining high metal recoveries.

Example 5 - Multi-leaching HGS

The following data were obtained by leaching at 100 °C with two malic acid concentrations 20 g/L and 10 g/L which provided 0.1 and 0.05 malic acid : ore ratios respectively.

In both of these tests, high nickel recoveries were achieved at an ORP in the range of from about 470 to 51O mV.

In Table 3, there were constant conditions at each stage: the temperature of leaching was about 100 °C; the period of leaching was about 6 hours; the pulp density was 20 (g/ml)% at a malic acid concentration of 20 g/L.

Table 3 - Multi-leaching of high grade saprolite ore

Total Pulp Density I 100 Sulphuric Acid: Ore (g/g): 0.27 Malic Acid: Ore (g/g): 0.1 or the results in Table 4, the conditions were the same as for Table 3. Table 4 - Multi-leaching of high grade saprolite ore

Total Pulp Density (%): 160 Sulphuric Acid: Ore (g/g): 0.23 Malic Acid: Ore (g/g): 0.05

The results in Table 4 demonstrate the ability of malic acid to maintain the activity of the leaching reagent in dissolving both Ni and Co metal values from the saprolitic ore. The multiple recycling of pregnant acidic solutions in the leaching process with malic acid enables significant advances to be achieved in leaching Ni laterite ores. Foremost is leaching with a pulp density of 160%. Additional significant advances are the reduction in the mineral acid: ore ratio to 0.23 (kg/kg) (compared to ratios of 0.5-1:1 typically achieved in atmospheric acid leaching and 0.3 in the HPAL processes) and the selective leaching achieved (see Figures 4-6).

The amount of sulphuric and malic acids consumed in these multi-leaching stage tests are shown in Tables 5 and 6. Table 5 - Summary of acid consumption in multi-leaching HGS at 100 ⁰C with 10 g/L malic acid (leaching data shown in Table 3) compared to HPAL and atmospheric leaching

Table 6 - Summary of acid consumption in multi-leaching HGS at 100 ⁰C with 20 g/L malic acid (leaching data shown in Table 4) compared to HGAL and atmospheric leaching

Tables 5 and 6, and other data herein, demonstrate advantages of two embodiments of process in accordance with the invention over both HPAL and atmospheric leaching. Although the mineral acid consumption in the embodiments of the process according to the invention is similar to that in the HPAL (but much less than in the atmospheric leaching process), the advantage of the embodiments of the invention is the use of lower energy (100 ⁰C) compared to that required in HPAL (250-270 °C). Because the process of the invention can use temperatures as low as 90 °C or less, the capital expenditure cost is lower compared to HPAL, which requires more expensive titanium-clad autoclaves, hi addition the maintenance cost is lower and plant availability higher, hi comparison to the atmospheric leaching process, the embodiments of the process according to the invention give good Ni and Co recoveries (93-99%), significantly lower mineral acid consumption, faster kinetics (3-6 hours in comparison to typically 24 hours in AL) and lower iron dissolution (see Figure 6).

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

1. A process of leaching a metal value from a metal laden solid, the process comprising the steps of:

5 (a) contacting the metal laden solid with a leach solution comprising a mineral acid and an organic acid to provide a leachate including the metal value;

(b) adding further organic acid to the leachate including the metal value;

(c) using the leachate including the metal value from step (b) as at least a portion of the leach solution in step (a). O

2. The process of leaching according to claim 1, wherein steps (a) to (c) are repeated.

3. The process of leaching according to claim 2, wherein steps (a) to (c) are repeated twice or three times. 5

4. The process of leaching according to any one of claims 1 to 3, wherein the metal value is nickel and/or cobalt.

5. The process of leaching according to any one of claims 1 to 4, wherein the metalf> laden solid leached by the process is ore.

6. The process of leaching according to claim 5, wherein the ore is laterite ore.

7. The process of leaching according to any one of the preceding claims, wherein the$ organic acid comprises malic acid.

8. The process of leaching according to any one of the preceding claims, wherein the amount of organic acid in the leach solution is at least 3 % of the weight of the solid to be leached b

9. The process of leaching according to any one of claims 1 to 7, wherein the amount of organic acid in the leach solution is at least 5 g/1.

10. The process of leaching according to any one of the preceding claims wherein the method further includes the step of adjusting the pH and/or ORP of the leachate before the leachate is used in step (c).

11. The process of leaching according to claim 10, wherein the step of adjusting the pH and/or ORP comprises adjusting the pH and/or ORP to be the same as the pH and/or ORP of the leach solution from which the leachate was derived.

12. The process of leaching according to claim 10, wherein the step of adjusting the pH and/or ORP comprises adjusting the pH and/or ORP to provide a leach solution that leaches at least 80 % of the total available metal value from the ore.

13. The process of leaching according to claim 1, wherein the metal value is nickel and the metal laden solid is laterite ore and the pH is adjusted to be below about 1.0 and the ORP is adjusted to be below about 320 mV.

14. The process of leaching according to any one of the preceding claims further including the step of recovering the leachate and processing the leachate to remove at least some of the metal value.