The present invention relates to a method for the recovery of nickel, other base metals, and precious metals from sulphide ores, sulphide concentrates, and sulphide metallurgical wastes using nitric acid. The invention has particular application to the recovery of nickel from sulphide metallurgical wastes such as nickel sulphide tailings.

Background to the Invention Nickel resources in the world occur principally as either nickel sulphide or nickel laterite (oxide) deposits. The chemistry of the two ores is quite differentTresulting ^" in ^~ quite ^'" different ^~ extra ^' ction and ^" treatment processes required in each case.

Nickel sulphide ores are typically easier to process, with techniques of conventional mining, smelting and refining being used to extract the nickel and other metals, usually referred to as pyrometallurgy. Nickel laterite ores require more rigorous hydrometallurgical extraction techniques such as high pressure acid leaching (HPAL). Due to the easier processing of nickel sulphide ores, historically most nickel production has been derived from sulphide ores.

Given the continuing worldwide demand for nickel, particularly for the steel industry, there is an ongoing need to find improved ways of extracting nickel from deposits. In addition there is a need to find improved ways of extracting nickel from already processed components and waste, such as tailings, nickel matte and so on. Given that the bulk of historic nickel production has been derived from sulphide ores, there is a need to find an economic and environmentally effective way to extract nickel from the existing waste components resulting from such sulphide extraction processes, for example sulphide tailings. Furthermore, some of these nickel sulphide tailings also include other metals such as precious metals and platinum group metals (PGMs). It is therefore also desirable to find a recovery process for these precious metals and PGMs, in particular in view of the high value of some of these metals. Prior art acid extraction processes for nickel, such as leaching, have principally used sulphuric acid. However international application PCT/US2008/005608 by Drinkard Metalox, Inc. describes an improved method for processing nickel laterite ores using nitric acid as the solvent. This disclosure describes the leaching of laterite ores at temperatures above about 70°C and higher.

The previous discussion of the background to the invention is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any.of.the.material,referred-to is. or was part of the common general knowledge as at the priority date of this application. References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a method for the recovery of nickel and other metals from nickel bearing sulphide ores, nickel bearing sulphide concentrates, and nickel sulphide metallurgical waste, the method comprising the steps of: i) adding nitric acid to the sulphide ore, sulphide concentrate or sulphide metallurgical waste to form a slurry; ii) agitating the slurry at ambient temperature and atmospheric pressure to allow at least partial oxidation of the sulphide ore, sulphide concentrate or sulphide metallurgical waste to form a treated slurry; and iii) recovering nickel from the treated slurry. Preferably the method also includes the step of grinding of the sulphide ore, sulphide concentrate or sulphide metallurgical waste prior to the nitric acid addition step. Preferably the sulphide ore, sulphide concentrate or sulphide metallurgical waste is ground to a particle size of less than 250 microns. Preferably the slurry comprises more than about 50% w/w solids. More preferably the slurry comprises about 50% w/w solids.

Preferably the nitric acid is of a concentration between about 1 and 90%. More typically the concentration of the nitric acid is about 70%. Water may be added in the nitric acid addition step to form the slurry. Preferably the concentration of the nitric acid after water addition is around 20-50%, and more typically about 30-40%.

Preferably the agitation step is carried out for about 1 to 4 hours. Typically -the ambient-temperature-oHhe-agitation s^ range of about 18°C to 60°C. More typically, the ambient temperature in the agitation step commences at a temperature of about 24°C to 28°C, and increases during the agitation step to about 50°C to 60°C.

Preferably the recovery step involves separating the treated slurry into a pregnant nickel solution and an insoluble residue in a separation step.

Preferably the recovery step also comprises treating the pregnant nickel solution to precipitate the nickel, the high value base metals and the precious metals as sulphides, hydroxides or carbonates. Alternatively other methods may be used to treat the pregnant solution to extract the nickel, base metals and precious metals in metallic form.

Preferably the recovery step includes a step of heating the treated slurry to increase the pH to form the pregnant nickel solution and the insoluble residue, the insoluble residue containing precipitated iron and other base metals.

The heating step is preferably carried out at about 80°C to 100°C. Preferably the heating step is carried out for about 30 minutes to 3 hours. Typically the pH of the solution rises to between 1 and 2 during the heating step so that iron and other low value base metals, such as aluminium and chromium are precipitated as the insoluble residue.

The pregnant nickel solution typically also comprises other high value base metals in addition to nickel such as cobalt and copper, as well as some precious metals and PGMs, for example gold and platinum.

Preferably the nickel sulphide metallurgical waste is nickel sulphide tailings. Typically the nickel sulphide tailings comprise less than 10% nickel. More typically the nickel sulphide tailings comprise about 0.2 to 1.0% nickel, and more typically about 0.5% nickel.

The method may further comprise a nitric acid recycle step in which NOx gases formed during one or more of the steps of the method are removed ^" and ^" Converted ^_ into ^~ recycled ^" nitric ^" acid ^" for ^" further ^~ use ^~" in the method. Alternatively the NOx gases or recycled nitric acid may be collected and used for other purposes. Typically the NOx gases formed during the agitation step may be removed and converted into nitric acid.

According to a further aspect of the present invention there is provided a method for the recovery of nickel and other metals from a nickel sulphide metallurgical waste, the method comprising the steps of: i) adding nitric acid to the nickel sulphide metallurgical waste to form a slurry; ii) agitating the slurry at ambient temperature and atmospheric pressure to allow at least partial oxidation of the sulphide ore or sulphide concentrate to form a treated slurry; and iii) recovering nickel from the treated slurry.

Preferably the nickel sulphide metallurgical waste is nickel sulphide tailings.

Preferably the method also includes the step of grinding of the nickel sulphide metallurgical waste prior to the nitric acid addition step. Preferably the sulphide ore, sulphide concentrate or sulphide metallurgical waste is ground to a particle size of less than 250 microns.

Preferably the method also comprises the step of heating the treated slurry to increase the pH to form a pregnant nickel solution and an insoluble residue containing precipitated iron and other base metals.

According to a still further aspect of the present invention there is provided a method for the recovery of nickel and other metals from nickel sulphide tailings, the nickel sulphide tailings having a nickel concentration of less than 10%, the method comprising the steps of: i) adding nitric acid to nickel sulphide tailings to form a slurry; ii) agitating the slurry at ambient temperature and atmospheric pressure to allow at least partial oxidation of the nickel sulphide tailings to form a treated slurry; and iii) recovering nickel from the treated slurry. Typically nickel sulphide tailings comprise about 0.2 to 1.0% nickel, and more typically about 0.5% nickel.

Preferably the slurry comprises more than about 50% w/w solids. More preferably the slurry comprises about 50% w/w solids.

Preferably the method also includes the step of grinding of the sulphide ore or sulphide concentrate prior to the nitric acid addition step. Preferably the sulphide ore or sulphide concentrate is ground to a particle size of less than 250 microns.

Water may be added in the nitric acid addition step to form the slurry. Preferably the nitric acid is of a concentration less than 1 tonne nitric acid to 1 tonne tailings. Preferably the nitric acid is of a concentration in the range of about 0.1 - 0.95 tonne nitric acid to 1 tonne of tailings.

Preferably the agitation step is carried out for about 1 to 4 hours. Typically the ambient temperature of the agitation step is a temperature in the range of about 18°C to 60°C. More typically the ambient temperature in the agitation step commences at a temperature of about 24°C to 28°C, and increases during the agitation step to about 50°C to 60°C.

Preferably the recovery step includes separating the treated slurry into a pregnant nickel solution and an insoluble residue in a separation step.

Preferably the recovery step also comprises treating the pregnant nickel solution to precipitate the nickel, the high value base metals and the precious metals as sulphides, hydroxides or carbonates. Alternatively other methods may be used to treat the pregnant solution to extract the nickel, base metals and precious metals in metallic form.

Preferably the recovery step includes a step of heating the treated slurry to increase the pH to form the pregnant nickel solution and the insoluble residuerthe nsoluble-residue ^~ containin^

metals. The heating step is preferably carried out at about 80°C to 100°C. Preferably the heating step is carried out for about 30 minutes to 3 hours. Typically the pH of the solution rises to between 1 and 2 during the heating step so that iron and other low value base metals, such as aluminium and chromium are precipitated as the insoluble residue. The pregnant nickel solution typically also comprises other high value base metals in addition to nickel such as cobalt and copper, as well as some precious metals, for example gold.

According to yet another aspect of the present invention there is provided a method for the recovery of one or more metals from metal bearing sulphide ores, metal bearing sulphide concentrates, and metal sulphide metallurgical waste, the method comprising the steps of: i) adding nitric acid to the sulphide ore, sulphide concentrate or sulphide metallurgical waste to form a slurry; ii) agitating the slurry at ambient temperature and atmospheric pressure to allow at least partial oxidation of the sulphide ore, sulphide concentrate or sulphide metallurgical waste to form a treated slurry; and iii) recovering one or more metals from the treated slurry.

Preferably the one or more metals are selected from the group comprising copper, zinc and lead.

Preferably the metal bearing sulphide ores, metal bearing sulphide concentrates, and metal sulphide metallurgical waste is one or more of copper sulphide ores, copper bearing sulphide concentrates, and copper sulphide metallurgical waste, zinc sulphide ores, zinc sulphide concentrates, and zinc sulphide metallurgical waste, lead sulphides, lead sulphide concentrates and-lead sulphide metallurgical Waste. ^"

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of a specific embodiment of a method for the recovery of nickel and other metals from nickel bearing sulphide ores, nickel bearing sulphide concentrates, and nickel sulphide metallurgical waste according to the invention, given by way of example only, with reference to the accompanying drawing in which: Figure 1 is a schematic flow diagram of a preferred method for the recovery of nickel and other metals in accordance with the present invention.

Detailed Description of Preferred Embodiments A preferred embodiment of a method for the recovery of nickel and other metals ^' from nickel bearing sulphide ores, nickel bearing sulphide concentrates, and nickel sulphide metallurgical waste (such as tailings) is illustrated in schematic form in Figure 1.

The method 10 for the recovery of nickel and other metals from nickel bearing sulphide ores, nickel bearing sulphide concentrates, and nickel sulphide metallurgical waste (tailings) 12 comprises the step of adding nitric acid 14 to the ore, concentrate or tailings 12 to form a slurry 16. The method then involves step of agitating 18 the slurry 16 at ambient temperature and atmospheric pressure to allow at least partial oxidation of the sulphide ore or sulphide concentrate to form a treated slurry 20. Nickel may then be recovered from the treated slurry 20 in a recovery step 22.

The method may include the step of grinding 24 of the sulphide ore, or concentrate or tailings 12 prior to the nitric acid addition step 14. Preferably the ore, concentrate or tailings 12 is ground to a particle size of less than 250 microns.

Preferably the slurry 16 comprises more than about 50% w/w solids. More preferably the slurry 16 comprises about 50% w/w solids.

Preferably the nitric acid 14 is of a concentration between about 1 and 90%. More typically the concentration of the nitric acid is about 70%. Water 26 may be added in the nitric acid addition step 14 to form the slurry. Preferably the concentration of the nitric acid after water addition is around 20-50%, and more typically about 30-40%. In the case of sulphide tailings 12 the nitric acid is of a concentration less than 1 tonne nitric acid to 1 tonne tailings. Preferably the nitric acid is of a concentration in the range of about 0.1 - 0.95 tonne nitric acid to 1 tonne of tailings. Since a low level of nitric acid may be used in the method of the invention, it should be noted that a recycle of the nitric acid in the overall process is not usually required for economic functioning of the invention. Preferably the agitation step 18 takes place for about 1 to 4 hours. Typically the ambient temperature of the agitation step 18 is a temperature in the range of about 18°C to 60°C. More typically the ambient temperature in the agitation step 18 commences at a temperature of about 24°C to 28°C, and increases during the agitation step to about 50°C to 60°C, due to the heat generated in the reaction of sulphides (sulphide tailings) with nitric acid. This is an important advantage of the preferred method of the invention and is specific to the reaction of nitric acid with nickel sulphides. It allows for an increase in temperature with no external heating, thereby assisting in the leaching ^~ of ^~ nickel (as well as cobalt and copper) into the nitric acid. By further pH adjustment (usually by heating as explained below), lower value metals such as iron, chromium and aluminium may be collected as an insoluble residue from the pregnant solution.

Preferably the treated slurry 20 is separated into a pregnant nickel solution 30 and an insoluble residue 32 in a separation step 36. Preferably the recovery step 22 includes an optional step of heating 28 the treated slurry 20 to increase the pH to form the pregnant nickel solution 30 and the insoluble residue 32 containing precipitated iron and other base metals.

The heating step 28 is preferably carried out at about 80°C to 100°C. The heating of the treated slurry to a temperature less than or equal to about 100°C is an important advantage of the preferred method of the invention, since prior art methods (for example as described in PCT/US2008/005608) generally require a much higher temperature, making the present invention more economic. Preferably the heating step 28 is carried out for about 30 minutes to 3 hours. Typically the pH of the solution rises to between 1 and 2 during the heating step 28 so that iron and other low value base metals, such as aluminium and chromium, are precipitated as the insoluble residue.

The pregnant nickel solution 30 typically also comprises other high value base metals in addition to nickel such as cobalt and copper, as well as some precious metals, for example gold. By appropriate treatment of the pregnant nickel solution the present invention also may provide a way of recovering precious metals and PGMs, such as gold and platinum in particular from nickel tailings.

Preferably the recovery step 22 also comprises further treatment 34 of the pregnant nickel solution 30 to precipitate the nickel, the high value base metals and the precious metals as sulphides, hydroxides or carbonates. During precipitation of the nickel/ copper/ cobalt solution with H ₂ S or Na ₂ S, metal sulphides and nitric acid are formed, .and .the sulphides-are-then- insoluble in the resultant nitric acid. Alternatively other methods may be used to treat the pregnant solution 30 to extract the nickel, base metals and precious metals in a metallic form.

As already discussed, this invention has particular application to nickel sulphide tailings. Typically nickel sulphide tailings comprise less than 10% nickel, and typically in the range of about 0.2 to 1.0% nickel, and more typically about 0.5% nickel.

The method may further comprise a nitric acid recycle step in which NOx gases formed during one or more of the steps of the method, such as the agitation step or the heating step, are removed and converted into recycled nitric acid for further use in the method. Alternatively the NOx gases or recycled nitric acid may be collected and used for other purposes.

TEST SAMPLE 1 : NICKEL TAILINGS

A sample of nickel sulphide tailings was treated according to the method of the invention as follows. 500g of solids (nickel sulphide tailings) were mixed with 179g of 70% nitric acid and 321 g of water to produce a 50% w/w solids slurry. The slurry was agitated for 120 minutes at ambient temperature and pressure over a temperature range from 26-51 °C, the temperature change being caused by the heat of reaction. The slurry was then heated in a heating step to a temperature of 100°C and held for 30 minutes. The slurry was filtered and the filter cake washed to leave a pregnant metal solution of composition:

5.5g/INi, 95mg/l Co, 0.2mg/l Fe, 40.3g/IMg, 0.6g/l Cu, 17mg/l Si, 1.5g/l S and 9g/l Ca.

Further data is provided in Table 1.

Table 1

TEST SAMPLE 2: NICKEL TAILINGS The method of the invention was applied to a number of further samples of nickel sulphide tailings which were treated using the method of the invention. However parameters such as temperature and acid concentration were varied in each of the test samples. The results are set out below in Tables 2 and 3. Note that acid addition is in tonne of nitric acid/tonne tailings.

Table 2

ACID

TEST * ADDITION TEMP Ni Co Mg Mn

278 1.6S I 110 99.6* 95.3% I 45.3% 87.9% 98.4% ; 39.8% 95.7% ! 96.7% !

334 1.00 110 98.3» 92.8 17.7% 5S.8% ! 92.1% 16.0% 87.3% 95.4%

336 1.2 110 98.9% ^' 91.7% 28.4 68.9 94.5% 24.6% 92.3% 93.5% :

338 1.4 ; 110 93.9% 33.1% ^' 75.2% _: 96.5% 26. : 92.7% 95.6

371 0.93 RT 93.6% ; 92.9% 6.9% = 46.6% 37.7% 11.4 ^" 74.8% 94.1%

385 0.55 : 110 93.2% 92.3% 0.9% ^! 0.0% 52.2% 0.9%; 76.2% 94.6% ;

389 ^' O.SS RT 93.5% : ^" 93.7% 7.0% 41.8% 41.7% 12.6% 75.6% ; 92.8% j

392] 0.1 ; 29 68.0% 56.9% 2.1% 11.1 6.8 5.3% 59.2% 81.8%

393 ^* 0.25 40 82.0% 90.1 3.6 27.5% ^Γ 14.3%T 8.1% 64.4% : 88.1% .

394 ^*"' 0.55 63 95.0% ^' 93.1% 7.9% ^" 47.6% 46.7%) 11.5 77.3% ^' 94.6

401 : 0.25 _. 51 84.8% : 85.9 3.9% 30.1% , 17.7 8.8 65.7% 74.9%

404 ^' 0.35 ^' 56 89.1% i 92.4% ^{s 2%} 37.7% ^' 27.0% 9.4% 71.3% : 84.0%

415 ^' 0.17 56 78.2% 65.0% 3.1 21 9% 10.9% 6.7% ^" 60.8 ^' SS.2%

416 ! 0.17 26 82.1% 69.8 3.2% 24.2% 12.8% 7.0 64.2% 50.S%

429 ,

430 0.17 5 : 89.4% 82.4% ^" 5.9% 34.9% ^" 23.1% .14.4 78.2% 78.7%

5

Table 3

ACID IEACH FINAL

TEST * ADDITION TEMP TEMP Ni Co Al fe Mg O Mn Ca

278 1.65 i 110 110 99.6% : 95.3% 4S.3% 87.9%i 98.4% 39.8% . 95.7%! 96.7% ;

33 1.00 110 110 98.3% : 92.8% 17.7% 55.8 92.1 16.0% 87.3 95.4%

336 1.2 110 110 93.9% 91.7% ; 28.4% 68.9% ! 94.5 24.6% · 92.3%; 93.5%

338 1. no 110 ^' 99.1 _{93 9¾} r 33.1% 75.2% ; 96.S 26.4% 92.7 9S.6

38S 0 S5 ; _. 110 110 93.2 92.3% 0.9 0.0 52.2% 0.9% 76.2%] 94.6%

392 0.1 29 ^" 100 9.0% 2.9% Γ ^' 0.0 0.0% 12.1% 0.1% 402*7 64.9

393 0.25 ? 40 100 6S.7% ^' 51.7% 0.0% o7o%> ^{' ""} 27.3% 0.1% ^" 65.1% 73.4%

394 0.55; 63 100 95.2 92.4% 5.7%. ^' 0.1% SS.3% 6.4 81.2% ; 95.4

401 0.25 51 100 68.9% 58.9% 0.0% 0.0 24.0% 0.0 80.1%

404 0.35 56 100 85.8 91.4% 0.0%. 0.0%i 34.8 0.0% 73.0% Γ 88.7%

430 ^*"" ~ 0.17 5 100 ^' 81.8% 73.0% 0.0% 0.0% 16.8% 0.0% 66.2% 57.6%

In particular it can be noted that at low temperature ranges, 29°C to 63°C (Table 2) and 40°C to 63°C (Table 3) about 65-95% nickel is extracted.

10 For example, at a temperature of 56°C (Table 2), two readings (tests 404

and 415) were obtained for nickel extraction as follows: 89.1% and 78.2% (at 0.35 and 0.17t/t nitric acid concentration respectively).

Thus the results show that the method of the invention works satisfactorily without the optional heating step, providing an advantageous method over prior art methods where significant process costs are incurred in heating steps.

In Table 2, it has been demonstrated that sulphides leach at ambient temperature under the conditions of the present invention. The heat of the reaction raises the temperature to around 63°C (at 0.55 t/t acid), well short of 70°C (see tests 371 , 389, 392-430).

These results also show that when the nitric acid addition is reduced from say 0.93 tonne of 100% acid/ tonne of tailings (0.93t/t) to 0.1t/t, then there ^' is some reduction in nickel leaching but a substantial reduction in Fe, Al and Mg (see Table 2, tests 371 and 392). Thus the leaching part of this "sulphide invention" takes place outside the laterite invention range ρΓ ^β νίου8^ ^' Γβροι1β^Ίη ^" ΡΟΤ/ϋ82008/005608 Further PCT7DS^00 /0O5608 ^" notes that the slurry temperature should be heated to 125-200°C, preferably 165°C, to remove Fe and Al. As seen in Table 3, when the acid addition is constrained to 0.55t t and below, the leach is carried out at < 60°C (with no external heating, since heat is generated by the reaction itself) - see tests 393 to 430 in Table 3 where 65%-95% of the nickel is extracted. The slurry is then heated to <100°C, and the Fe, Al and Cr are stripped out of solution.

TEST SAMPLE 3: NICKEL SULPHIDE ORE

Two samples (Sample A and Sample B) of nickel sulphide ore of differing sulphurnickel ratios were tested using the method of the invention and along the lines as outlined above for Test Sample 1 (Nickel Tailings).

Samples A and B were ground to a particle size of about 150 micron. 450g of Sample A was mixed with 321g of 70% nitric acid and 47g of water to produce a 55% w/w solids slurry. The slurry was agitated at room temperature and atmospheric pressure for about 3 hours. Similar steps were carried out in the case of Sample B.

The results of these tests are provided in Table 4 below.

Table 4

Referring to Table 4 above, Sample-A- has a -sulphur: nickel- ratio ^" Of T.26 ^~ indicating the nickel is present largely as pentlandite (NiFeS ₂ ) and there are few other iron sulphides. The extraction after leaching for three hours at ambient temperature show high extractions of Ni, Co, Cu and S and a moderate amount of Fe and Mg. Following two hours of heating at 95°C the Ni, Co and Cu remain in solution and much of the Fe and sulphur is removed by precipitation.

Sample B has a sulphur.nickel ratio of 6.7:1 indicating that in addition to pentandite there is a significant amount of other iron sulphides. Despite this difference the results after leaching at ambient temperatures and then heating to 95°C confirm the trends of this invention, Ni and Co stay in solution and iron is totally removed and sulphur is also precipitated.

TEST SAMPLE 4: NICKEL SULPHIDE CONCENTRATE

Whilst no results are available as yet for nickel sulphide concentrates, it is envisaged that the method of the invention will work in a similar way to that described for Test Sample 3: Nickel Sulphide Ore, because of the similar chemistry and mineralogy of nickel sulphide concentrates to nickel sulphide ores.

The results of the experimental work on the method of the invention showed that leaching of nickel sulphide ores (and nickel sulphide tailings) with nitric acid occurs at room temperature. This is an important advantage of the method of the invention, since it allows nickel (and other metals) to be extracted from nickel sulphide ores and concentrates at low temperature thereby resulting in cost savings and environmental benefits. A temperature reduction of only one degree in these sorts of processes can have a large impact on the overall processing costs. A further benefit of this method is that the leaching reaction generates heat which further assists in the leaching process.

It J as also been. found, that. by. limiting-the-amount-of-aeid -added -to-the - solids, it is possible to preferentially leach nickel, cobalt and copper from iron, chromium, aluminium and magnesium, i.e. nickel, cobalt and copper are extracted in the pregnant solution, and iron, chromium, aluminium and magnesium are precipitated in the insoluble residue.

After leaching at ambient temperature with low acid addition, if the solution is then heated to 90-100°C a shift in pH occurs and at around 1.0-1.5 the iron, chromium and aluminium precipitate leaving a clean metal solution with less than 50ppm iron. This clean metal (pregnant) solution is then very suitable for downstream precipitation by pH change or sulphide addition.

NO and NO2 are evolved during the leach process even at ambient temperature, and this can be recovered using the nitric acid recycle system of Drinkard Metalox, Inc (US patent 6,264,909).

It has also been found that during the treatment of nickel sulphide ores and concentrates with nitric acid (possibly in the presence of chloride ions), gold may also be leached from the sulphide ores, and this gold is precipitated with the sulphides during H ₂ S or Na ₂ S treatment. This can be a useful benefit of the method especially where gold occurs in a reasonable quantity in the sulphide ores or concentrates.

Now that preferred embodiments of a method for the recovery of nickel and other metals from nickel bearing sulphide ores, sulphide concentrates or sulphide metallurgical waste (such as nickel sulphide tailings) has been described in detail, it will be apparent that the preferred method provides a number of advantages over the prior art, including the following:

(i) The process is suitable for treating all forms of nickel sulphide ores, sulphide concentrates and sulphide wastes, and has particular application to sulphide tailings.

(ii) The method may be operated successfully at jimbienj temperature and atmospheric pressure resulting in many cost and economic benefits over prior art methods.

(iii) The low temperature of the invention is of particular importance due to the significant economic and environmental benefits.

(iv) The low temperature of operation of the invention allows for the use of lower cost equipment such as equipment made of plastics, due to the less corrosive environment, again reducing costs.

(v) When nitric acid is mixed at room temperature with nickel sulphide materials as described _. in the specification, the temperature gradually increases due to the internal reaction which takes place thereby generating heat, and speeding up the leaching process.

(vi) The recovery of nickel exceeds at least 75% for an operating temperature of about 56°C. It is possible to precipitate iron and aluminium from the leach liquor at temperatures at or below 100°C without the use of a neutralant. This is not possible in a sulphate system (such as a sulphuric acid leach system) where it is necessary to use a neutralant at temperatures below 00°C.

There are many benefits of the present invention over existing high pressure acid leach (HPAL) methods. For example, not all of the Fe and Al is removed from the liquor in an HPAL system. The HPAL circuit usually achieves say 90% hydrolysis of Fe and Al, whilst the present invention removes 99% of ail the Fe and Al in the combined leach/hydrolysis stage. Furthermore to remove the remaining Fe and Al from an HPAL system, requires a pH adjustment usually using lime or ^" limestone In hen iethod Tof ^" the ^" invention we do not need either as we simply use a low temperature hydrolysis around 85°C to precipitate Fe and Al successfully to trace levels.

The method allows leaching of sulphide tailings containing approximately 0.5% nickel using nitric acid, as well as recovery of the acid, simply by heating the slurry to temperatures well below 100°C.

Since platinum group metals (PGMs) may be leached into the nitric acid using the method of the invention, the invention also provides for a relatively easy way to extract such PGMs simultaneously with the extraction of nickel, allowing the PGMs to be extracted using appropriate techniques.

The NOx gases liberated during various stages of the method, and in particular during the low temperature leaching step, may be recovered to produce nitric acid which may be recycled in the present method, or used in separate processes. (xii) However due to the low level of nitric acid used in the method of the invention (and therefore economic cost), it is usually not necessary to go to the added cost of using recycled nitric acid in the process of the invention.

It will be readily apparent to persons skilled in the relevant art that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.