AU2013325098A1

AU2013325098A1 - Method for the recovery of metals from nickel bearing ores and concentrates

Info

Publication number: AU2013325098A1
Application number: AU2013325098A
Authority: AU
Inventors: Graham BROCK; Fiona Mccarthy; Michael Rodriguez
Original assignee: POSEIDON NICKEL Ltd; Direct Nickel Pty Ltd
Current assignee: POSEIDON NICKEL Ltd; Direct Nickel Pty Ltd
Priority date: 2012-09-28
Filing date: 2013-09-18
Publication date: 2015-02-26
Anticipated expiration: 2033-09-18
Also published as: AU2013325098B2; WO2014047672A1

Abstract

A two stage method (10) for the recovery of nickel and other metals from nickel bearing laterite ore, sulphide ore, oxide ore or concentrate (14), and nickel sulphide ore or tailings (24) is described. The method (10) in a first stage leach (20) comprises the steps of: adding nitric acid (16) to the nickel bearing laterite ore, sulphide ore, oxide ore or concentrate (14) to form a 1st stage leach slurry (18); and, treating the slurry to allow at least partial oxidation of the ore or concentrate to form a 1st stage treated slurry (22). A second stage leach (28) in the method (10) comprises the steps of: adding nickel sulphide ore or sulphide tailings 24 to the 1st stage treated slurry (22); treating the slurry to form a 2nd stage treated slurry (26); and recovering nickel and other metals from the 2nd stage treated slurry (26).

Description

WO 2014/047672 PCT/AU2013/001066 1 "METHOD FOR THE RECOVERY OF METALS FROM NICKEL BEARING ORES AND CONCENTRATES" Field of the Invention The present invention relates to a method for the recovery of nickel, other 5 base metals, and precious metals from nickel bearing ores and concentrates, and sulphide metallurgical wastes, using nitric acid, and relates particularly, though not exclusively, to such a method for the recovery of nickel and other metals from nickel laterite ores and concentrates. 10 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 different resulting in quite different extraction and treatment processes required in each case. 15 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 typically require more rigorous hydrometallurgical extraction techniques such as high pressure acid leaching (HPAL). Due to the easier processing of 20 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 25 nickel from laterite ores and 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 laterite ores and the existing waste components resulting from such sulphide WO 2014/047672 PCT/AU2013/001066 2 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 5 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. 10 This disclosure describes the leaching of laterite ores at temperatures above about 70*C and higher. The present invention was developed with a view to providing a two stage acid leach process for the recovery of nickel and other metals from nickel bearing laterite and sulphide ores using nitric acid as the solvent, and 15 subsequently adding sulphide ore or tailings to improve the economics of metal recovery. 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 20 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 25 According to one aspect of the present invention there is provided a method for the recovery of nickel and other metals from nickel bearing laterite ore, sulphide ore, oxide ore or concentrate, and nickel sulphide ore or nickel sulphide metallurgical waste, the method comprising the steps of: in a first stage leach: WO 2014/047672 PCT/AU2013/001066 3 i) adding nitric acid to the nickel bearing laterite ore, sulphide ore, oxide ore or concentrate to form a slurry; and, ii) treating the slurry to allow at least partial oxidation of the ore or concentrate to form a 1st stage treated slurry; 5 in a second stage leach: iii) adding nickel sulphide ore or nickel sulphide metallurgical waste to the 1 s stage treated slurry; iv) treating the slurry to form a 2 nd stage treated slurry; and v) recovering nickel and other metals from the 2"n stage treated slurry. 10 Preferably the method also includes the step of grinding of the ore, concentrate or nickel sulphide metallurgical waste prior to the nitric acid addition step. Preferably the ore, concentrate or nickel sulphide metallurgical waste is ground to a particle size of less than 150 microns. Preferably the 1 st stage slurry comprises more than about 50% w/w solids. 15 More preferably the 1 st stage slurry comprises about 50% w/w solids in barren liquor recovered from a precipitation step. Preferably the nitric acid is of a concentration between about I and 90%. More typically the concentration of the nitric acid is about 70%. Barren liquor or water may be added in the nitric acid addition step to form the slurry. 20 Preferably the concentration of the nitric acid after water addition is around 20-50%, and more typically about 30-40%. Preferably the 1 st stage treatment step involves agitation carried out for about 1 to 3 hours. Typically the temperature of the slurry during the 1 st stage treatment step is heated to between 60 0 C to 100 0 C. More preferably the 25 temperature of the slurry during the 1 St stage treatment step is increased to about 90 0 C. Preferably barren liquor from a precipitation step or water is also added during the nickel sulphide metallurgical waste addition step to promote WO 2014/047672 PCT/AU2013/001066 4 hydrolysis during the 2 nd stage leaching. Typically the temperature of the slurry increases during the 2 nd stage treatment step. Typically the temperature of the slurry as it enters the 2 nd stage treatment step is in the range of about 85 0 C to 100 0 C. More typically the temperature in the 2 nd 5 stage treatment step commences at a temperature of about 90 0 C, and increases during agitation to about 95*C. Advantageously the 2 nd stage treatment releases metals, such as gold, nickel and cobalt, from the nickel sulphide metallurgical waste into solution. Preferably the 2 nd stage treated slurry is separated into a pregnant solution 10 and an insoluble residue in a solid/liquid separation step as part of the step of recovering nickel and other metals from the 2 nd stage treated slurry. Preferably the pregnant solution is subjected to further treatment to recover the metals in solution. Typically the pregnant solution is treated to extract the nickel, base metals and precious metals in metallic form. Alternatively the 15 pregnant solution is treated to precipitate the nickel, the high value base metals and the precious metals as sulphides, hydroxides or carbonates. Preferably the pregnant solution is further treated by the addition of Na 2 S, NaHS and H 2 S to form a cobalt-nickel mixed sulphide precipitate ("MSP"). Advantageously spent liquor from the step of recovering the metals in 20 solution is recycled to the 1 st and 2 nd stage leaching circuits. Preferably the step of recovering nickel and other metals from the 2 nd stage treated slurry includes a step of heating the treated slurry to increase the pH to form the pregnant solution and the insoluble residue, the insoluble residue containing precipitated iron and other base metals. 25 The heating step is preferably carried out at about 80 0 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.

WO 2014/047672 PCT/AU2013/001066 5 The pregnant 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. 5 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 10 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. Throughout the specification, unless the context requires otherwise, the word 15 "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 20 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 laterite ores or nickel bearing 25 oxide 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 30 invention; WO 2014/047672 PCT/AU2013/001066 6 Figure 2 is a graphical representation of test data showing the recovery of nickel and cobalt, iron an aluminium during a 2 nd stage leaching. Detailed Description of Preferred Embodiments 5 A preferred embodiment of a method 10 for the recovery of nickel and other metals from nickel bearing laterite ore, sulphide ore, oxide ore or concentrate, and nickel sulphide tailings is illustrated in schematic form in Figure 1. The method 10 preferably involves the step of grinding 12 the nickel bearing laterite ore, oxide ore or concentrate 14 prior to the 1 st stage of the 10 acid leach process. Preferably the nickel bearing laterite ore or concentrate 14 is ground to a particle size of less than 150 microns. Then the 1 st stage acid leach process comprises the step of adding nitric acid 16 to the ore or concentrate 14 to form a 1 st stage leach slurry 18. Preferably the slurry 18 comprises more than about 50% w/w solids. More preferably the slurry 18 15 comprises about 50% w/w solids in barren liquor recovered from a precipitation step. Preferably the nitric acid 16 is of a concentration between about 1 and 90%. More typically the concentration of the nitric acid is about 70%. Barren liquor or water 17 may also be added to the ore or concentrate 14 to form the 20 1 st stage leach slurry 18. Preferably the concentration of the nitric acid after water addition is around 40-70%, and more typically about 50-60%. In the case bf oxide ores the nitric acid is of a concentration less than 1 tonne nitric acid to 1 tonne ore. Preferably the nitric acid is of a concentration in the range of about 0.6 - 0.8 tonne nitric acid to 1 tonne of tailings. Typically 25 greater than 30% nickel extraction is achieved using nitric acid in the 1 st stage leach. The acid can be added to the ore quickly during this stage as the oxide ore is quite benign. Since a high level of nitric acid may be used in the 1 st stage leach process according to the method of the invention, it should be noted that recycling of WO 2014/047672 PCT/AU2013/001066 7 the nitric acid in the overall method is usually required for economic functioning of the invention. The 1 st stage leach process then involves agitating the slurry 18 in a 1 st stage treatment step 20 to allow at least partial oxidation of the nickel bearing 5 laterite (oxide) ore or oxide concentrate to form a 1 st stage treated slurry 22. Preferably the agitation during the 1 st stage treatment step takes place for between 2 to 3 hours. More typically the slurry is agitated for about 150 minutes. Typically the temperature of the slurry during the 1 s stage treatment step is heated to between 60*C to 100*C. More preferably the temperature of 10 the slurry during the 1 st stage treatment step is increased to about 90*C. A nickel sulphide ore or disseminated nickel bearing sulphide ore may also be used in the 1 st stage leach process. If sulphide ore is used as the primary leach up to 90% recovery of the nickel can be achieved. The 1st stage treated slurry 22 is then subject to a 2 nd stage leach process. 15 The 2 nd stage leach process comprises adding nickel sulphide or sulphide tailings 24 to the 1 st stage treated slurry 22 to form a 2 nd stage leach slurry 26. The nickel sulphide tailings 24 typically comprise less than 10% nickel. More typically the nickel sulphide tailings 24 comprise about 0.2 to 1.0% nickel, and more typically about 0.5% nickel. Preferably barren liquor from a 20 precipitation step or water is also added during the nickel sulphide tailings addition step to promote hydrolysis during the 2 nd stage leaching. Preferably the 2 nd stage leach process further comprises treating the slurry in a 2 nd stage treatment step 28 to form a 2 nd stage treated slurry 30. Typically the 2 nd stage treatment step 28 comprises adjusting the pH of the 2 nd stage 25 leach slurry 26 to maximise the concentration of free acid. Typically the 2 nd stage treatment step also comprises agitating the slurry. Preferably the agitation takes place for 1 to 3 hours. More preferably the agitation in the 2 nd stage treatment step 28 takes place for about 2 hours. Typically the temperature of the slurry as it enters the 2 nd stage treatment step 28 is in the 30 range of about 85 0 C to 100*C. More typically the temperature in the 2 nd WO 2014/047672 PCT/AU2013/001066 8 stage treatment step 28 commences at a temperature of about 900C, and increases during agitation to about 950C, due to the heat generated in the reaction of sulphides (sulphide tailings) with nitric acid. This is an advantage of the invention and specific to the reaction of nitric acid 5 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. Advantageously the 2 nd stage treatment step releases metals, such as gold, nickel and cobalt, from the 2 nd stage treated slurry 30 into solution at step 32. By further pH adjustment (usually by 10 heating as explained below), lower value metals such as iron, chromium and aluminium may be collected as an insoluble residue from the pregnant solution. Advantageously adjusting the pH of the 2 nd stage leach slurry 26 during the 2 nd stage treatment step 28 comprises incrementally adding nickel sulphide 15 tailings 24 to the slurry 26. Addition of the nickel sulphide tailings has to be performed in small increments due to the high reactivity of the sulphide. Preferably the pH is adjusted to > I by the incremental addition of nickel sulphide tailings 24. This has the further advantage of obviating the need for adding a neutralant to precipitate iron and other low value base metals, such 20 as aluminium and chromium, as well as releasing free acid. The iron cannot precipitate in the 1 st stage leach due to the high level of nitric acid required to leach nickel laterite or oxide ore. This is another significant benefit of the two stage leach process. Finally, the method comprises the step of recovering nickel and other metals 25 in solution 32 from the 2 nd stage treated slurry 30. Preferably the treated slurry 30 is separated into a pregnant nickel solution 36 and an insoluble residue 38 in a solid/liquid separation step 34. The pregnant nickel solution 36 typically also comprises other high value base metals in addition to nickel such as cobalt and copper, as well as some 30 precious metals, for example gold. By appropriate treatment of the pregnant nickel solution the present invention also may provide a way of recovering WO 2014/047672 PCT/AU2013/001066 9 precious metals and PGMs such as gold and platinum, in particular from nickel tailings. Preferably the step of recovering nickel and other metals from the 2 nd stage treated slurry also comprises further treatment 40 of the pregnant nickel 5 solution 36 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, NaHS or Na 2 S, a cobalt-nickel mixed sulphide precipitate ("MSP") 42 and free nitric acid are formed, and the sulphides are then insoluble in the resultant nitric acid. 10 Ni(N0 3

)

2 +NaHS -> NiS+Na(N0 3

)+HNO

3 The insoluble MSP is then sent to market after thickening and filtration at step 44. Alternatively other methods may be used to treat the pregnant solution 36 to 15 extract the nickel, base metals and precious metals in a metallic form. Advantageously the spent barren liquor from the step of recovering nickel and other metals from the 2 nd stage treated slurry with the free nitric acid is recycled to the 1 st and 2 nd stage leaching circuits to recover more acid, as shown in Figure 1. 20 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. 25 TEST SAMPLE 1: TWO STAGE LEACH A sample of nickel oxide ore was treated according to the method of the invention as follows. A brief overview of the test is provided in Table 1: WO 2014/047672 PCT/AU2013/001066 10 Design Criteria Woodbine Well Run 3 Blend Feed Sample (0.45% Ni Tails) Dilution Liquor Barren (MSP O/F) Pulp Density 50 % solids HN0 3 (100%) 700 kg/t Carbon Addition 30 g Temperature 95 *C Stage 1 Time 150 min Stage 2 Time 150 . min Table I 5 1kg of Woodline well was repulped to 50% in MSP overflow, and an equivalent of 700kg/t acid/woodline well was added in the 1 st stage leach. The temperature was increased to 95*C. At time t=150, 1kg of sulphide ore tailings (repulped to 50% in MSP o/f) was added in the 2 nd stage leach. This process is slow, as considerable frothing occurs - completed at t=210. 10 Additional tailings were then added at a ratio of -0.1:1 until the process acidity was >1 pH. All additional tailings were added as a 50%w/w slurry. Figure 2 provides a graphical summary of the % extraction of nickel and cobalt, as well as iron and aluminium, during the 2 nd stage leach. At t=240, a sample was taken. Its pH was <1 so an additional 11 5g of tailings 15 was added. At t=260, a sample was taken. Its pH was <1 so an additional 105g of tailings was added. At t=275, a further sample was taken. Its pH was <1 so an additional 100g of tailings was added. At t=300, a sample was taken. Its pH was <1 so an additional 91g of tailings was added. At t=320, a further sample was taken and its pH was >1 so no additional tailings were 20 added. The test was completed at t=360. Carbon was added at t=300. As can be seen from Figure 2, maximum nickel extraction (>80%) and cobalt extraction (>75%) occurred in the 2 nd stage leach at about t=275. At that time WO 2014/047672 PCT/AU2013/001066 11 most of the iron has dropped out of solution. Maximum aluminium extraction (>55%) during the 2 nd stage leach also occurred at this time. Further data is provided in Table 2. Pulp Samples Pulp Measured Calc'd Solid % Liquor Liquor Mass, density, density, Mass, Solids Mass, Volume Sample g kg/L kg/L p (w/w) g mL Feed (Woodline) 140 1.66 65 46.36 75 57 Feed(Run 3 Tails) -- -- -- -- -- t=60 162 1.65 1.68 43 26.48 119 81 t=120 102 1.64 1.67. 22 21.18 80 53 t=240 143 1.66 1.76 54 37.55 89 61 t=260 115 1.58 1.83 52 45.42 63 43 t=275 111 1.68 1.74 40 35.73 72 49 t=300 165 1.74 1.74 64 38.71 101 70 t=320 105 1.80 1.38 43 40.99 62 43 t=360 133 1.79 1.42 52 39.52 80 56 t=360 Bulk 4188 1.25 1593 38.03 2595 1825 5 Table 2 Now that preferred embodiments of a method for the recovery of nickel and other metals from nickel bearing laterite ores, disseminated sulphides or oxide concentrates and sulphide metallurgical waste (such as nickel sulphide 10 tailings) have 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 oxide and sulphide ores, oxide and sulphide concentrates and sulphide 15 wastes, and has particular application to nickel bearing laterite (oxide) ores and sulphide tailings. (ii) The two stage acid leach process means that previously uneconomic leaching of oxide ores can be successfully WO 2014/047672 PCT/AU2013/001066 12 accomplished, using sulphide metallurgical waste (such as nickel sulphide tailings) in the 2 nd stage leach. (iii) By mopping up the acid from the 1 st stage leach to drive the 2 nd stage leach process it is possible to commercialise this method 5 of treatment. If it were not possible to use the acid from the 1 5t stage leach to leach more nickel from either sulphide or sulphide tailings this process would not be economical. (iv) When nitric acid is mixed at room temperature with nickel sulphide materials as described in the specification, the 10 temperature gradually increases due to the internal reaction which takes place thereby generating heat, and speeding up the leaching process. (v) It is possible to precipitate iron and aluminium from the leach liquor at temperatures at or below 100 0 C without the use of a 15 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 1 00"C. (vi) The method allows leaching of sulphide tailings containing approximately 0.5% nickel using nitric acid, as well as recovery 20 of the acid, simply by heating the slurry to temperatures well below 100*C. (vii) 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 25 simultaneously with the extraction of nickel, allowing the PGMs to be extracted using appropriate techniques. (viii) 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 30 the present method, or used in separate processes.

WO 2014/047672 PCT/AU2013/001066 13 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 5 appreciated that the scope of the invention is not limited to the specific embodiments described.

Claims

1. A method for the recovery of nickel and other metals from nickel bearing 5 laterite ore, sulphide ore, oxide ore or concentrate, and nickel sulphide ore or nickel sulphide metallurgical waste, the method comprising the steps of: in a first stage leach: i) adding nitric acid to the nickel bearing laterite ore, sulphide ore, oxide ore or concentrate to form a slurry; and, 10 ii) treating the slurry to allow at least partial oxidation of the ore or concentrate to form a 1 st stage treated slurry; in a second stage leach: iii) adding nickel sulphide ore or nickel sulphide metallurgical waste to the 1 st stage treated slurry; 15 iv) treating the slurry to form a 2 nd stage treated slurry; and v) recovering nickel and other metals from the 2 nd stage treated slurry.

2. A method for the recovery of nickel and other metals as defined in claim 1, wherein the method also includes the step of grinding of the ore, concentrate or nickel sulphide metallurgical waste prior to the nitric acid 20 addition step.

3. A method for the recovery of nickel and other metals as defined in claim 2, wherein the ore, concentrate or nickel sulphide metallurgical waste is ground to a particle size of less than 150 microns.

4. A method for the recovery of nickel and other metals as defined in any 25 one of claims 1 to 3, wherein the 1 st stage slurry comprises more than about 50% w/w solids.

5. A method for the recovery of nickel and other metals as defined in claim 4, wherein the 1st stage slurry comprises about 50% w/w solids in barren liquor recovered from a precipitation step. WO 2014/047672 PCT/AU2013/001066 15

6. A method for the recovery of nickel and other metals as defined in any one of claims 1 to 5, wherein the nitric acid is of a concentration between about 1 and 90%.

7. A method for the recovery of nickel and other metals as defined in claim 5 6, wherein the nitric acid is of a concentration of about 70%.

8. A method for the recovery of nickel and other metals as defined in any one of claims 1 to 5, wherein barren liquor or water may be added in the nitric acid addition step to form the slurry.

9. A method for the recovery of nickel and other metals as defined in claim 10 8, wherein the concentration of the nitric acid after water addition is around

20-50%. 10. A method for the recovery of nickel and other metals as defined in claim 8, wherein the concentration of the nitric acid after water addition is around about 30-40%e 15 11. A method for the recovery of nickel and other metals as defined in any one of claims 1 to 10, wherein the 1 st stage treatment step involves agitation carried out for about 1 to 3 hours. 12. A method for the recovery of nickel and other metals as defined in any one of claims 1 to 11, wherein the temperature of the slurry during the 1 st 20 stage treatment step is heated to between 600C to 1000C. 13. A method for the recovery of nickel and other metals as defined in claim 12, wherein the temperature of the slurry during the 1 st stage treatment step is increased to about 900C. 14. A method for the recovery of nickel and other metals as defined in any 25 one of claims 1 to 13, wherein barren liquor from a precipitation step or water is also added during the nickel sulphide metallurgical waste addition step to promote hydrolysis during the 2 "d stage leaching. WO 2014/047672 PCT/AU2013/001066 16 15. A method for the recovery of nickel and other metals as defined in claim 14, wherein the temperature of the slurry increases during the 2 nd stage treatment step. 16. A method for the recovery of nickel and other metals as defined in claim 5 15, wherein the temperature of the slurry as it enters the 2 nd stage treatment step is in the range of about 85 0 C to 1 00*C. 17. A method for the recovery of nickel and other metals as defined in claim 16, wherein the temperature in the 2 nd stage treatment step commences at a temperature of about 90 0 C, and increases during agitation to about 95*C. 10 18. A method for the recovery of nickel and other metals as defined in claim 17, wherein the 2 nd stage treatment releases metals, such as gold, nickel and cobalt, from the nickel sulphide metallurgical waste into solution. 19. A method for the recovery of nickel and other metals as defined in any one of claims 1 to 18, wherein the 2 nd stage treated slurry is separated into 15 a pregnant solution and an insoluble residue in a solid/liquid separation step as part of the step of recovering nickel and other metals from the 2 nd stage treated slurry. 20. A method for the recovery of nickel and other metals as defined in claim 19, wherein the pregnant solution is subjected to further treatment to 20 recover the metals in solution.

21. A method for the recovery of nickel and other metals as defined in claim 20, wherein the pregnant solution is treated to extract the nickel, base metals and precious metals in metallic form.

22. A method for the recovery of nickel and other metals as defined in claim 25 20, wherein the pregnant solution is treated to precipitate the nickel, the high value base metals and the precious metals as sulphides, hydroxides or carbonates. WO 2014/047672 PCT/AU2013/001066 17

23. A method for the recovery of nickel and other metals as defined in claim 22, wherein the pregnant solution is further treated by the addition of Na 2 S, NaHS and H 2 S to form a cobalt-nickel mixed sulphide precipitate ("MSP").

24. A method for the recovery of nickel and other metals as defined in any 5 one of claims 20 to 23, wherein spent liquor from the step of recovering the metals in solution is recycled to the 1 8t and 2 nd stage leaching circuits.

25. A method for the recovery of nickel and other metals as defined in claim 20, wherein the step of recovering nickel and other metals from the 2 nd stage treated slurry includes a step of heating the treated slurry to increase 10 the pH to form the pregnant solution and the insoluble residue, the insoluble residue containing precipitated iron and other base metals.

26. A method for the recovery of nickel and other metals as defined in claim 25, wherein heating step is carried out at about 800C to 1000C.

27. A method for the recovery of nickel and other metals as defined in claim 15 26, wherein the heating step is carried out for about 30 minutes to 3 hours.

28. A method for the recovery of nickel and other metals as defined in claim 27, wherein 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. 20 29. A method for the recovery of nickel and other metals as defined in claim 28, wherein the pregnant solution 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.

30. A method for the recovery of nickel and other metals as defined in claim 25 28, wherein the nickel sulphide metallurgical waste is nickel sulphide tailings.

31. A method for the recovery of nickel and other metals as defined in claim 30, wherein the nickel sulphide tailings comprise less than 10% nickel. WO 2014/047672 PCT/AU2013/001066 18

32. A method for the recovery of nickel and other metals as defined in claim 31, wherein the nickel sulphide tailings comprise about 0.2 to 1.0% nickel.

33. A method for the recovery of nickel and other metals as defined in claim 32, wherein the nickel sulphide tailings comprise about 0.5% nickel. 5 34. A method for the recovery of nickel and other metals as defined in claim 1, wherein the method further comprises 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. 10 35. A method for the recovery of nickel and other metals as defined in claim 34, wherein the NOx gases or recycled nitric acid are collected and used for other purposes.

36. A method for the recovery of nickel and other metals as defined in claim 34, wherein the NOx gases formed during the agitation step are removed 15 and converted into nitric acid.