AU2009200702A1

AU2009200702A1 - Acid recovery

Info

Publication number: AU2009200702A1
Application number: AU2009200702A
Authority: AU
Inventors: Geoffrey Motteram; Michael Ryan
Original assignee: GME Resources Ltd
Current assignee: GME Resources Ltd
Priority date: 2008-02-25
Filing date: 2009-02-23
Publication date: 2009-09-10
Anticipated expiration: 2029-02-23
Also published as: AU2009200702B2

Description

ACID RECOVERY FIELD OF THE INVENTION The present invention relates to a process for regeneration of sulphuric acid from leach 5 liquors generated by atmospheric leaching of nickeliferous ore, wherein the ore includes impurities in the form of metals other than nickel and cobalt, and wherein the ore is subjected to atmospheric leaching using sulphuric acid to dissolve nickel and cobalt from the ore and produce a leach liquor. 10 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 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 15 nickel sulphide deposits being mined out, making the more complex laterite ores economically viable to process. The nickel and cobalt metals present in the laterite ore can be recovered using leaching to take nickel and cobalt into solution for subsequent recovery. Traditionally, the laterite ores have been treated via an expensive, energy intensive and 20 highly corrosive high pressure sulphuric acid leaching, smelting or the Caron processes. More recently, leaching circuits conducted at atmospheric pressure have been proposed. For example, heap leaching is an operation that involves low investment and circuital costs compared with high pressure leaching, is very well known, and is widely applied mainly for copper, uranium, and gold ores. Heap leaching is conducted at atmospheric pressure and 25 ambient temperature. Dilute acid solution is irrigated over the top of the heap and leaching proceeds for an extended period of time as the nickel and cobalt are dissolve into a pregnant leach liquor which is collected from the base of the heap. Similarly, it is known that laterite ores can be leached in slurry form at atmospheric pressure and elevated temperature, typically above 70*C. In either case, the resultant pregnant leach liquors can 30 be treated to recover the dissolved nickel and cobalt values. In all cases of such atmospheric acid leaching, excess acid is consumed in dissolving iron, aluminium, chrome, copper, and manganese which are also present in the laterite ores in various concentrations. 35 -2 Prior art processes have been developed to separate the iron and aluminum impurities from pregnant leach liquors by causing the impurities to precipitate out of the pregnant liquor by adding neutralizing reagents to alter the pH of the pregnant leach liquor (see, for example US Patent 3,720,749; US Patent 3,991,159; US Patent 4,097,575 and US Patent 5 4,547,348). The most commonly used neutralizing agent are magnesia (MgO) and calcium carbonate rich reagents such as limestone or coral mud, which is typically added to the pregnant liquor in a sufficient quantity to adjust the pH of the liquor within the narrow pH range of about 3.5 to 4.5 at a temperature of about 50 to 100*C to at atmospheric pressure. Following the precipitation and separation of the aforementioned impurities, the pH of the 10 pregnant or leach liquor is then raised to at least 7 by the further addition of magnesia to cause the nickel and cobalt present in the pregnant leach liquor to precipitate out, allowing the nickel and cobalt-rich precipitates to be separated from the liquor. It is also known to attempt to minimize consumption of both sulphuric acid and/or the 15 neutralizing agent by separating the nickel bearing ore into high and low magnesium fractions prior to leaching, and subjection only the low magnesium fraction to leaching, whilst using the high magnesium fraction or raw ore itself as some or all of the neutralizing agent (see, for example, US Patent 4,097,575; US Patent 3,804,613; US Patent 3,991,159; and, US Patent 4,044,096). 20 At the conditions under which these prior art processes are operated, aluminum and iron values precipitate out primarily as gelatinous hydroxides and basic sulfates. These precipitates are not very dense and tend to be voluminous, with the result that particular care must be taken to separate these precipitates from the pregnant leach liquor using 25 solid-liquid separation techniques well known in the hydrometallurgical art. These prior art methods are expensive in terms of the costs associated with the consumption of the neutralising reagents. More importantly, such prior art processes only exacerbate the problems associated with excessive consumption of the acid by the impurities present in the ore. 30 With atmospheric leaching methods becoming more attractive for processing nickel laterite ores, there remains a need for a process which reduces fresh acid consumption to overcome or at least ameliorate the above-identified problems with the prior art. 35 -3 SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method of recovering nickel from a nickeliferous ore, wherein the ore includes impurities in the form of metals other than nickel and cobalt, and wherein the ore is subjected to atmospheric leaching using 5 sulphuric acid to dissolve nickel and cobalt from the ore and produce a leach liquor, the sulphuric acid is regenerated using a method comprising: a) rapidly heating the leach liquor to a temperature in excess of 1200C to produce a slurry comprising sulphuric acid and precipitated oxides of impurities present in the leach liquor; and, 10 b) separating the precipitated oxides from the slurry of step a) to produce a clarified acid rich liquor stream for recycle to leaching. Step a) may be conducted at a temperature in the range of 120 to 2200C or in the range of 180 to 2200C to initiate the precipitation of oxides of impurities by hydrolysis. 15 Advantageously, leach liquors with high iron contents can be treated in that the leach liquor of step a) may have at least 10g/L or at least 25 g/L of dissolved iron. Preferably, the precipitated oxides are hematite or alunite. 20 In one form, the leach liquor of step a) has an initial free acid concentration of at least 1Og/L free acid. To achieve rapid heating, step a) may be conducted using an autoclave which operates in 25 combination with one or more pre-heating stages arranged to receive steam that is generated from cooling or "flashing" when the slurry discharged from the autoclave is depressurized. The autoclave may include one or more steam injection points arranged received fresh steam to control the temperature within the autoclave in use. 30 In one form of the present invention, the leach liquor of step a) is a pregnant leach liquor. The pregnant leach liquor may be subjected to a metals recovery process to recover nickel and cobalt from the pregnant leach liquor and form a barren leach liquor. The metals recovery process may be selected from the group consisting of sulphide precipitation with hydrogen sulphide gas, solvent extraction, ion exchange, or a combination thereof. When 35 the metals recovery process is sulphide precipitation, sulphide precipitation may be -4 conducted on a pregnant leach liquor having a level of acidity of less than 10 g/L free acid at a temperature in the range of 80 to 120'C. If required, a neutralizing agent may be added to a bypass stream of the clarified acid-rich liquor stream of step b) to achieve a target free acid content in a neutralized pregnant liquor stream within the range of 1 - 1Og/L 5 of free acid prior to conducting sulphide precipitation. In this form of the present invention, the neutralized pregnant liquor stream may then be directed to a metals recovery process. In one form, the neutralizing agent is calcium carbonate. The portion of clarified acid-rich leach liquor of step b) which forms the bypass stream may be determined by ensuring that the target amount of nickel recovered using the metals recovery process is equal to the 10 amount of nickel being leached per pass during atmospheric leaching. Preferably, acid recovery is conducted after metals recovery and the leach liquor of step a) is a barren leach liquor. 15 In one form of the present invention, atmospheric leaching may be conducted using a heap leaching process. According to a second aspect of the present invention there is provided a method of regenerating sulphuric acid substantially as herein described with reference to and as 20 illustrated in the accompanying drawings. 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 25 only, with reference to the accompanying drawings, in which: Figure 1 is a schematic flowchart illustrating a first embodiment of the acid recovery process of the present invention in which metals recovery occurs prior to acid recovery; and, 30 Figure 2 is a schematic flowchart illustrating a second embodiment of the acid recovery process of the present invention in which metals recovery occurs after acid recovery. 35 -5 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Particular embodiments of the method of regenerating sulphuric acid for re-use in heap leaching to recover nickel and cobalt from a nickeliferous ore. The present invention is equally applicable to the recovery of acid from leach liquors produced using atmospheric 5 leaching operations other than heap leaching. 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 and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. In the drawings, it is to be understood that like reference 10 numbers refer to like parts. The abbreviation " 0 C" as used throughout this specification refers to "degrees Celsius". The term "leaching" as used throughout this specification refers to the selective dissolution of a target metal contained in an insoluble solid phase using a solvent or "leaching solution". 15 For leaching to occur, an ore to be leached is brought into contact with the solvent, taking the target metal value(s) into solution. Any metals other than the target metal which dissolve in the leach solution are referred to throughout this specification as "impurities". By way of example, during leaching of laterite ores, the target metals are nickel and (optionally) cobalt, whilst iron, aluminium, chromium, copper and manganese are considered to be 20 impurities. Leaching results in the formation of "a pregnant leach liquor", which is a term that is used throughout this specification to refer to a leach liquor which is rich in target metal values. After leaching has been completed, target metals are extracted from the pregnant leach 25 liquor using a "metals recovery process". Downstream of the metal recovery process, after the target metals have been removed, the leach liquor is referred to throughout this specification as "a barren leach liquor". "Atmospheric Leaching" is a process which is conducted at ambient pressure (in contrast to 30 a high pressure leaching process which requires specialist equipment such as pressure vessels or autoclaves). "Heap Leaching" is a process in which the ore is piled up to form a "heap" and the target metals are taken into solution by percolation of the leaching solution through the heap. 35 -6 One embodiment of a process (10) for regeneration of acid for use in leaching nickel containing laterite ores is now described with reference to Figure 1, in which a pregnant leach liquor (12) for treatment is obtained using a leaching circuit (14), in this example, a heap leaching circuit. Agglomerated pellets of nickel-containing laterite ore are directed 5 from a stacker to form one or more heaps, each heap having a base and a top. Each heap 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 leaching solution being introduced at the top of the lag heap. 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 10 distribution means, for example a spraying apparatus, is used to apply a leaching solution or solvent at the top of the heap. The leaching solution is then allowed to percolate down through the heap under the influence of gravity towards the base of the heap. A suitable leaching solution is any reagent capable of selectively dissolving a target metal 15 from an ore whilst remaining chemically inert to gangue minerals. The leaching solution preferably contains sulphuric acid. When sulphuric acid is used, the leach solution typically contains between 0.1 and 20% sulphuric acid. Optimum recovery of nickel during heap leaching is achieved when about 400 to about 1000 kilograms of sulphuric acid per tonne of dry ore is consumed, without acid regeneration. The acid solution may be applied by 20 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. The acid solution is applied at an irrigation rate or flux in the range of 5 to 30 litres per hour per meter squared, preferably 10 to 25 litres per hour per meter squared. 25 Pregnant leach liquor (12) is collected using a collection means arranged at the base 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 metals recovery circuit (16) to recover one or both of the nickel values and the cobalt values present in the pregnant leach liquor. After leaching has been completed, the heap may be rinsed with water (either fresh 30 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. If desired, this effluent stream can be directed to the metals recovery circuit (16) in addition to the pregnant leach liquor to improve overall recovery of nickel and/or cobalt values. 35 -7 Any suitable metals recovery process known in the art, such as precipitation, solvent extraction or ion exchange, can be used to in the metals recovery circuit (16) to separately recover dissolved target metal values such as nickel and cobalt from the pregnant leach liquor at a satisfactory purity. By way of example, the nickel and cobalt present in the 5 pregnant leach liquor (12) can be removed through the addition of a source of sulphide ions, for example by introducing a hydrogen sulphide gas stream or through the addition of sodium hydrosulphide (NaHS) in slurry form. Other metal recovery methods can equally be used, for example, hydroxide precipitation, solvent extraction or ion exchange. The addition of the sulphide ions under certain conditions causes the nickel and cobalt metal values 10 present in the pregnant leach liquor (12) to precipitate. Precipitation occurs at temperatures in the range of 80 to 120 0 C provided that, prior to the introduction of the source of sulphide ions, the acidity of the pregnant leach liquor (12) is maintained at a level of acidity of less than 10 g/L free acid. 15 Precipitation of the nickel or cobalt occurs according to the following simplified general equation, where "M" is a divalent metal:

MSO

4 (aq) + H 2 S -> MS (s) + H 2 SO4 ----------------------------------------- (1) 20 It is clear from equation (1) above, that the acidity of the leach liquor increases as this precipitation reaction progresses. For a pregnant leach liquor with an initial acidity of 1g/L, the final acidity after nickel and cobalt precipitation can be as high as 10g/L. The precipitated solids of nickel and cobalt are removed from the leach liquor after precipitation using any suitable solid/liquid separation equipment (which forms part of the metals 25 recovery circuit (16)) to produce a clarified barren liquor stream (18). In the embodiment illustrated in Figure 1, the clarified barren liquor stream (18) from the metals recovery circuit (16) is directed to an acid recovery circuit (20). The clarified barren leach liquor stream (18) contains dissolved impurities selected from the group consisting of 30 iron, aluminum, chrome and manganese, in various concentrations depending on the starting composition of the laterite ore that was subjected to leaching. The most significant impurities from the point of view of acid consumption and regeneration in the leaching circuit (14) are iron and aluminium. Using the process of the present invention, the sulphuric acid which was consumed when the impurities were taken into solution in the leaching circuit 35 (14) is regenerated from the leach liquor using the acid recovery circuit (20) by causing -8 these impurities to precipitate as oxides. Acid recovery is achieved by rapidly heating the leach liquor to a temperature in the range of 120 to 220 0 C, preferably in the range of 180 to 220 0 C to initiate the rapid precipitation of iron and aluminum from the leach liquor by hydrolysis. Under these conditions, the precipitates formed are predominately oxides of the 5 impurities. It is important to note that the target metal values of nickel and cobalt do not precipitate from the leach liquor under these conditions. However, some loss of nickel and cobalt can occur by entrainment during subsequent solid/liquid separation operations, when the metals recovery circuit (16) is located downstream of the acid recovery circuit (20) as described below in relation to an alternative embodiment of the present invention which is 10 illustrated in Figure 2. By way of example, sulphuric acid is regenerated when iron and aluminium impurities present in the leach liquor precipitate as oxides according the following simplified reactions: 15 Fe 2 (SO4) 3 (aq) + 3H 2 0 -> Fe 2 0 3 (s) + 3H 2 SO4 ------------------------------ (2) 3Al 2

(SO

4

)

3 (aq) + 13H 2 0 -> 3Al 2 0 3 .4SO 3 .8H 2 0 (s) + 5H 2 SO4 ----------------- (3) It is clear from equations (2) and (3) above, that when the oxides are caused to form, 20 sulphuric acid is recovered from the leach liquor. By way of example, when the acid level of the leach liquor being treated has an initial free acid concentration of approx 10g/L free acid, the final free acid concentration after causing rapid precipitation of the impurities from the leach liquor increases to over 50g/L free acid. The practical outcome of this is that up to half of the acid used to conduct leaching can be recovered and made available for return to 25 the leaching circuit (14). The precipitated oxides are removed using a solid/liquid separation circuit (22) to produce a clarified acid-rich liquor stream (40) which is available for re-cycle to the leaching circuit (14). The oxides removed from the liquor stream using the solid/liquid separation circuit 30 (22) are disposed of. Solid / liquid separation in the solid/liquid separation circuit (22) can be achieved using any suitable process known in the hydrometallurgical art, for example; thickening, filtering, pressure filtering, centrifugal separation, gravity separation, counter current decantation, or cyclones. Unlike the hydroxide and basic sulphate precipitates of the prior art, the oxides which precipitate in the acid recovery circuit (20) of the present 35 invention form dense solid particles which settle well and are readily filterable.

-9 Rapid heating in the acid recovery circuit (20) can be achieved in a number of ways. In the embodiments illustrated in Figures 1 and 2, rapid heating is achieved using an autoclave (26) which operates in combination with one or more pre-heating stages (24) arranged to receive steam that is generated from cooling or "flashing" when the slurry discharged from 5 the autoclave (26) is depressurized. By way of example, the autoclave (26) can be a brick lined or titanium lined pressure vessel with multiple compartments, each compartment being fitted with an agitator (28) to improve reaction kinetics. The autoclave (26) has one or more steam injection points (30) arranged to control the temperature within the autoclave (26) in use within the predetermined temperature range referred to above to encourage acid 10 regeneration through the precipitation of oxides such as hematite (Fe 2

O

3 ) and alunite (A1 2 0 3 .4SO 3 .8H 2 0) in accordance with equations (2) and (3) respectively. After precipitation of the oxides from the liquor stream, a slurry stream (32) is discharged from the autoclave (26) into one or more stages of pressure let-down using a series of 15 cooling vessels or flash tanks (34). Within each of the flash tanks (34), the hot pressurised slurry stream (32) is caused to pass through a choke valve (not shown) causing rapid acceleration which leads to a drop in pressure. This rapid drop in pressure causes some of the water present in the slurry to flash to steam. The steam that forms within each of the flash vessels (34), apart from the final flash vessel (35), is ducted to the pre-heating stages 20 (24) for heat recovery. Two such flash tank (34) / pre-heating stages (24) are illustrated in each of the embodiments of Figure 1 and Figure 2, but this number can vary. The specific number of stages will depend on the cost of energy versus the capital cost of the equipment. The final flash vessel (35) is operated at atmospheric pressure and the steam from this final flash vessel (35) is vented to atmosphere as it is unsuitable for heat recovery. 25 A stream of fresh steam (37) is used maintain temperature control, with a portion of the fresh stream being directed to the final pre-heating stage (25) and a portion of the fresh steam (37) being directed to the steam injection points (30). The cooled slurry stream (36) downstream of the flash tanks (34), at a nominal temperature 30 in the order of 100 0 C, is then discharged to the solid / liquid separation circuit (22). The precipitated oxides of the impurities (38) removed from the cooled slurry stream (36) using the solid/liquid separation circuit (22) are discarded. The clarified acid-rich liquor stream (40) which is recovered from the cooled slurry stream (36) using the solid/liquid separation circuit (22) has a high free acid content which makes it suitable for recycle to the leaching 35 circuit (14).

-10 The acid recovery circuit (20) can be located either upstream or downstream of the metals recovery circuit (16). In the flowchart of Figure 1, the acid recovery circuit (20) is located downstream of the metals recovery circuit (16) with the result that the leach liquor being treated in the acid recovery circuit (20) is a barren leach liquor. In the flowchart of Figure 2, 5 the acid recovery circuit (20) is located upstream of the metals recovery circuit (16) with the result that the leach liquor being treated in the acid recovery circuit (20) is a pregnant leach liquor. For best results, the metals recovery process (16) is conducted prior to the acid regeneration process (20). One of the reasons for this is that conducting the metals recovery process (16) first minimizes the loss of nickel and cobalt values which can occur 10 due to entrainment when the precipitated oxides are removed from the leach liquor in the solid-liquid separation circuit (22). In the embodiment illustrated in Figure 2 for which like reference numerals refer to like parts, the acid recovery circuit (20) is arranged to receive the pregnant leach liquor stream 15 (12) produced by the leaching circuit (14). The clarified acid-rich liquor stream (40) produced from the solid/liquid separation circuit (22) has a high free acid content of at least 50 grams per litre, which makes the clarified acid-rich liquor stream (40) unsuitable for sulphide precipitation without first taking steps to pre-neutralize the liquor stream (40) prior to the addition of the source of sulphide ions. The reason for this is that the sulphide 20 precipitation method which proceeds according to equation (1) above is only effective in causing the precipitation of nickel and cobalt from a pregnant leach liquor which has a relatively low free acid content in the range of approximately 1 - 1 Og/L free acid. With reference to Figure 2, a bypass stream (50) of the acid-rich liquor stream (40) 25 downstream from the solid-liquid separation circuit (22) is directed to a pre-neutralization circuit (52). In the pre-neutralization circuit (52), a neutralizing agent, for example, limestone is added to achieve a target free acid content in a neutralized pregnant liquor stream (54) within the range of 1 - 10g/L of free acid. Addition of limestone in this manner causes the precipitation of gypsum from the neutralized pregnant liquor stream (54) 30 according to the following simplified equations: CaCO 3 + H 2

SO

4 -> CaSO 4 (aq) + CO 2 + H 2 0 --------------------------------- (4) 35 - 11 When the neutralized pregnant liquor stream is saturated with aqueous calcium sulphate, the calcium sulphate precipitates as gypsum in accordance with the following equation: CaCO 3 + H 2 0 -> CaSO 4 . H 2 0 ------- ------------------ (5) 5 Following separation of the gypsum solids, a neutralized pregnant liquor stream (54) is directed to the metals recovery circuit (16). A suitable metals recovery process, such as sulphide precipitation, is then used in the metals recovery circuit (16) to recover nickel and cobalt values from the neutralized pregnant liquor stream (54). The remaining portion of the 10 clarified acid-rich leach liquor (40) from the solid-liquid separation circuit (22) has a high free acid content, making it suitable for recycle to the leaching circuit (14). In the embodiment illustrated in Figure 2, the clarified acid-rich leach liquor (40) is low in impurities such as iron and aluminium, which have been removed as precipitated oxides in 15 the acid recovery circuit (20), but still contains significant levels of nickel and cobalt in solution because it has not been subjected to a metals recovery process. The portion of clarified acid-rich leach liquor (40) which forms the bypass stream (50) is determined by ensuring that the target amount of nickel recovered during the metals recovery circuit (16) is equal to the amount of nickel being leached from the heap per pass. For example if the 20 clarified leach liquor (50) from the solid-liquid separation circuit (22) contained 3 g/L of nickel, and 2 g/L of nickel was leached from the heap per pass during leaching circuits (14), giving a total resultant grade of 5 g/L, the portion of the clarified acid-rich leach liquor (40) to be subjected to pre-neutralisation and metals recovery circuits (52) and (16), respectively, would be around 40% of the clarified acid-rich leach liquor (40). 25 The present invention is particularly suited to the treatment of leach liquors containing at least 1 Og/L of dissolved iron and preferably at least 25 g/L of dissolved iron. The advantages of the various aspects and embodiments of the present invention are 30 further described and illustrated by the following examples and experimental test 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 equipment described in the following examples. 35 - 12 Examples A heap leach column test using sulphuric acid and laterite ore was conducted to produce a leach liquor containing target metal values in the form of nickel and cobalt in solution. The 5 leach liquor also contains dissolved iron, aluminium, chrome, magnesium and manganese impurities. Approximately 3000 grams of the leach liquor was placed in a titanium Parr autoclave, which was then sealed and heated to 220 OC. After achieving this temperature, samples of 10 the solution were taken after 15 minutes. Thereafter, the autoclave was allowed to cool. After cooling the pulp was weighed and filtered to recover precipitated solids. The initial leach liquor from heap leaching, the solids recovered during filtering and the leach liquor after precipitation were analysed to determine the free acid levels and composition of each, with the results being presented below in Table 1. 15 Table I Feed Solution Filtered Product % Precipitated Solution Free Acid 9.8 80.5 (g/I) Nickel 4379 4259 0.33 (mg/I) Cobalt 327 297 0.70 (mg/I) Iron 33220 4084 81.7 (mg/I) Aluminium 7791 893 84.4 (mg/I) Chrome 732 287 53.5 (mg/I) Magnesium 21970 22830 0.00 (mg/I) Manganese 1522 1436 0.07 (mg/I) 20 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. For example, a portion of the steam generated by the flash tanks (34) can be directed to one or all of steam injection points (30) of the autoclave (26) if desired. All such modifications and variations are 25 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.

- 13 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 the common general knowledge in the art, in Australia or in any other country. In the 5 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 further features in various embodiments of the invention. 10

Claims

1. In a method of recovering nickel from a nickeliferous ore, wherein the ore includes impurities in the form of metals other than nickel and cobalt, and wherein the ore is 5 subjected to atmospheric leaching using sulphuric acid to form dissolve nickel and cobalt from the ore and produce a leach liquor, the sulphuric acid is regenerated using a method comprising: a) rapidly heating the leach liquor to a temperature in excess of 120 *C to produce a slurry comprising sulphuric acid and precipitated oxides of impurities present in the leach 10 liquor; and, b) separating the precipitated oxides from the slurry of step a) to produce a clarified acid rich liquor stream for recycle to leaching.

2. The method of regenerating sulphuric acid of claim 1 wherein step a) is conducted at a 15 temperature in the range of 120 to 220 0 C to initiate the precipitation of oxides of impurities by hydrolysis.

3. The method of regenerating sulphuric acid of claim 1 wherein step a) is conducted at a temperature in the range of 180 to 220 0 C to initiate the precipitation of oxides of impurities 20 by hydrolysis.

4. The method of regenerating sulphuric acid of any one of claims 1 to 3 wherein the leach liquor of step a) has at least 10g/L of dissolved iron. 25

5. The method of regenerating sulphuric acid of any one of claims 1 to 3 wherein the leach liquor of step a) has at least 25g/L of dissolved iron.

6. The method of regenerating sulphuric acid of any one of claims 1 to 5 wherein the precipitated oxides are hematite or alunite. 30

7. The method of regenerating sulphuric acid of any one of claims 1 to 6 wherein the leach liquor of step a) has an initial free acid concentration of at least 1Og/L free acid. 35 -15

8. The method of regenerating sulphuric acid of any one of claims 1 to 5 wherein step a) is conducted using an autoclave which operates in combination with one or more pre-heating stages arranged to receive steam that is generated from cooling or "flashing" when the slurry discharged from the autoclave is depressurized. 5

9. The method of regenerating sulphuric acid of claim 8 wherein the autoclave includes one or more steam injection points arranged received fresh steam to control the temperature within the autoclave in use.

10 10. The method of regenerating sulphuric acid of any one of claims 1 to 9 wherein the leach liquor of step a) is a pregnant leach liquor.

11. The method of regenerating sulphuric acid of claim 10 wherein the pregnant leach liquor is subjected to a metals recovery process to recover nickel and cobalt from the 15 pregnant leach liquor and form a barren leach liquor.

12. The method of regenerating sulphuric acid of claim 11 wherein the metals recovery process is selected from the group consisting of sulphide precipitation with hydrogen sulphide gas, solvent extraction, ion exchange, or a combination thereof. 20

13. The method of regenerating sulphuric acid of claim 11 wherein the metals recovery process is sulphide precipitation conducted on a pregnant leach liquor having a level of acidity of less than 10 g/L free acid at a temperature in the range of 80 to 120 0 C. 25

14. The method of regenerating sulphuric acid of claim 10 wherein a neutralizing agent is added to a bypass stream of the clarified acid-rich liquor stream of step b) to achieve a target free acid content in a neutralized pregnant liquor stream within the range of 1 - 10g/L of free acid. 30

15. The method of regenerating sulphuric acid of claim 14 wherein the neutralized pregnant liquor stream is directed to a metals recovery process.

16. The method of regenerating sulphuric acid of claim 14 wherein the neutralizing agent is calcium carbonate. 35 - 16

17. The method of regenerating sulphuric acid of any one of claims 14 to16 wherein the portion of clarified acid-rich leach liquor of step b) which forms the bypass stream is determined by ensuring that the target amount of nickel recovered using the metals recovery process is equal to the amount of nickel being leached per pass during 5 atmospheric leaching.

18. The method of regenerating sulphuric acid of any one of claims 1 to 9 wherein the leach liquor of step a) is a barren leach liquor. 10

19. The method of regenerating sulphuric acid of any one of claims 1 to 18 wherein atmospheric leaching is conducted using a heap leaching process.

20. A method of regenerating sulphuric acid substantially as herein described with reference to and as illustrated in the accompanying drawings and examples. 15