WO2009149522A1

WO2009149522A1 - Rheological method for the hydrometallurgical recovery of base metals from ores

Info

Publication number: WO2009149522A1
Application number: PCT/AU2009/000755
Authority: WO
Inventors: Michael Rodriguez
Original assignee: Murrin Murrin Operations Pty Ltd; Poseidon Nickel Ltd
Current assignee: Murrin Murrin Operations Pty Ltd; Poseidon Nickel Ltd
Priority date: 2008-06-13
Filing date: 2009-06-15
Publication date: 2009-12-17
Anticipated expiration: 2010-12-13
Also published as: AU2009257204A1; CA2727557A1; EP2288735A1; AU2009257204B2; BRPI0909875A2; EP2288735A4

Abstract

A rheological method for the hydrometallurgical recovery of base metals such as nickel from ores comprises the steps of combining a sulphide ore or concentrate (10) with a laterite or other oxide ore (12) and milling them together in a milling circuit (14) to form a combined slurry with improved rheological characteristics. The milled combined ore from the milling circuit (14) is subjected to a screening step in the screening circuit (18). Undersize ore is fed from the screening circuit (18) to a slurry tank (19), and the combined slurry is then pumped to a pressure acid leach circuit.

Description

"RHEOLOGICAL METHOD FOR THE HYDROMETALLURGICAL RECOVERY OF BASE METALS FROM ORES"

Field of the Invention The present invention relates to a method for the improved recovery of base metals from sulphide and/or oxide ores. More particularly, though not exclusively, the invention relates to a rheological method for the improved application of a hydrometallurgical process for leaching of nickel from a combination of nickel sulphide ores or concentrate and nickel oxide ores. Background to the Invention

Nickel sulphide ores have traditionally been treated via a pyrometallurgical smelting process, in order to recover nickel as a high grade nickel matte. The nickel content of the matte can range from 60 to 80% nickel as a sulphide. In Western Australia flash smelting and converting has been commercially applied to produce a high grade nickel matte 70% nickel, from nickel sulphide concentrates. The nickel sulphide concentrate is typically 12 to 18% nickel. The high grade matte is subsequently refined utilising the Sherritt Gordon process.

Hydrometallurgical processes such as leaching have historically not been applied to nickel sulphide ores or concentrates, as smelting is commercially competitive when compared to hydrometallurgical processes. Unlike the

Activox or Albion hydrometallurgical processes, smelting unlocks significant energy credits that is converted to electrical energy and produces sulphuric acid or sulphur as by-products. This co-generation approach improves the overall competitiveness of pyrometallurgical process when compared to hydrometallurgical processes.

Furthermore hydrometallurgical processes such as the Activox or the Albion Process typically require fine grinding P90 or minus 10 microns, which consumes energy. The energy released via the leaching process is lost to cooling towers or a simple flash system that does not capture any of the energy released during leaching.

It is well documented that hydrometallurgical treatments such as High

Pressure Acid Leach (HPAL) plants operating in Western Australia have added sulphides as either "transition" sulphide ore or non-smeltable concentrates that cannot be treated via a conventional concentrator or smelter. However, these plants are limited in their ability to add sulphides due to the reducing nature of the sulphide ores or concentrates. Typically these hydrometallurgical plants are hydraulically limited and therefore unless the slurry density or the ore grade is increased these hydrometallurgical plants remain bottlenecked.

Furthermore as these hydrometallurgical plants are hydraulically limited, increasing the slurry density or solids specific gravity has a significant impact on the plant capacity, unlocking sunk capital and more importantly reducing the unit cost of production.

There are several commercial examples where high density or paste thickening has been retrofitted to existing HPAL plants to improve the existing plant capacity by increasing slurry density though the application of improved thickening technology. The typical improvement in density achieved by the installation of high density or paste thickening is within a range of 2 to 4 w/w% solids increase. The use of indirect heating is also a well known technically and commercially proven method for increasing slurry density. Indirect heating can be retrofitted into an existing hydrometallurgical plant to unlock capital, introduced in the initial design to reduce the capital intensity of new plants. The unit cost of production in both instances is also reduced, improving the plant competitiveness. However both of the above methods of increasing slurry density are capital intensive and energy expensive.

The present invention aims to de-bottleneck existing hydrometallurgical plants or reduce the capital intensity of proposed new plants by combining sulphide ores or concentrate with oxide ores (such as laterite ore) in a milling environment. 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 rheological method for the hydrometallurgical recovery of base metals from ores, the method comprising the steps of: providing a sulphide ore or concentrate and a laterite or other oxide ore; combining the sulphide ore or concentrate with the laterite or other oxide ore and milling them together to form a combined slurry with improved rheological characteristics.

Preferably the ratio of sulphide ore or concentrate to laterite or other oxide ore in the combining and milling step is in the range of about 1 :1 to 1 :40.

Preferably a sulphide concentrate is used in the combining and milling step as the specific gravity of sulphide concentrate is about twice that of a typical laterite or other oxide ore.

Preferably water and/or pregnant leach solution (PLS) is added to the sulphide ore or concentrate and the laterite or other oxide ore to form the combined slurry in the combining and milling step.

Preferably the method further comprises the step of leaching the combined slurry in a pressure acid leach circuit. Preferably the pressure acid leach circuit comprises a series of pressure Pachuca tanks.

Typically the base metal is selected from the group consisting of nickel, cobalt, copper, lead and zinc. Preferably the sulphide ore or concentrate is a - A - nickel sulphide or concentrate and the laterite or other oxide ore is a nickel laterite or other nickel oxide ore.

According to another aspect of the present invention there is provided a rheological method for the hydrometallurgical recovery of nickel from ores, the method comprising the steps of:

providing a nickel sulphide ore or concentrate and a nickel laterite or other nickel oxide ore; combining the nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together to form a combined slurry with improved rheological characteristics.

Preferably the step of combining the nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together involves milling the ores together.

The step of combining nickel sulphide ore or concentrate with nickel laterite or other nickel oxide ore and mixing them together to form a combined slurry with improved rheological characteristics allows higher overall slurry densities to be achieved. This may allow for a reduction in capital and operating costs in hydrometallurgical nickel processing plants.

The ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining and mixing step may be anywhere in the range of about 1 :1 to 1 :40. More typically the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining step is in the range of about 3:7 to 3:97.

Preferably the method further comprises the step of leaching the combined slurry in a pressure acid leach circuit.

The method preferably further comprises a step of directing a nickel laterite or other nickel oxide ore to an atmospheric leach process and providing the PLS from the atmospheric leach process to the combining and mixing step. The nickel sulphide ore or concentrate typically has a nickel concentration within the range of about 1 to 10% Ni. Preferably the nickel sulphide ore or concentrate has a nickel concentration within the range of about 2 to 4% Ni. Typically the nickel laterite or oxide ore has a nickel concentration within the range of about 0.8 to 5% Ni. Preferably the nickel laterite or oxide ore has a nickel concentration within the range of about 1 to 2% Ni.

Preferably the free acid concentration achieved in the pressure acid leach circuit is maintained within the range of 30 to 80 g/l. Preferably, the temperature within the acid leach circuit is maintained between about 160° and 26O⁰C. More preferably, the temperature within the acid leach circuit is maintained at about 220° to 25O⁰C. Preferably, the oxygen over pressure within the acid leach circuit is maintained between 100 to 1000 kPag.

According to a still further aspect of the present invention there is provided a method of improving the rheological characteristics of a laterite or other oxide ore, the method comprising: providing a sulphide ore or concentrate and a laterite or other oxide ore; combining the sulphide ore or concentrate with the laterite or other oxide ore and mixing them together to form a slurry with higher density relative to a slurry formed from the laterite or other oxide ore by itself. Preferably the sulphide ore or concentrate is a nickel sulphide or concentrate and the laterite or other oxide ore is a nickel laterite or other nickel oxide ore.

Preferably the step of combining nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together involves milling the ores together. Preferably the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining step is in the range of about 1 :1 to 1 :40. Typically, the ratio of nickel sulphide ore or concentrate: nickel laterite or other nickel oxide ore in the combining step is in the range of about 3:7 to 3:97. 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 several specific embodiments of the rheological method for the hydrometallurgical recovery of a base metal according to the invention, given by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a process circuit of a preferred rheological method for the hydrometallurgical recovery of nickel in accordance with the present invention;

Figure 2 is a graphical presentation of rheology test results for laterite, sulphide and a combined slurry in process water;

Figure 3 is a graphical presentation of rheology test results for laterite slurry in process water and two types of PLS;

Figure 4 is a graphical presentation of rheology test results for sulphide slurry in process water and two types of PLS;

Figure 5 is a graphical presentation of rheology test results for combined slurry in process water and in PLS with first and second types of chemistry; Figure 6 is a graphical presentation of rheology test results for three different blends of combined slurry in PLS with a first type of chemistry;

Figure 7 is a graphical presentation of rheology test results for three different blends of combined slurry in PLS with a second type of chemistry; Figure 8 is a graphical presentation of rheology test results for combined slurry in process water and in PLS with a first type of chemistry with and without shear; and,

Figure 9 is a graphical presentation of rheology test results for laterite slurry in process water and PLS.

Detailed Description of Preferred Embodiments

A preferred embodiment of the rheological method for the hydrometallurgical recovery of a base metal according to the invention, as shown in schematic form in Figure 1 , relates to the leaching of nickel. The method preferably comprises the step of combining nickel sulphide ore or concentrate 10 with nickel laterite or other nickel oxide ore 12 and milling the combination in the milling circuit 14 with a pregnant leach solution (PLS) and/or water as the case may be to form a combined slurry. The nickel sulphide ore or concentrate 10 preferably has a nickel concentration within the range of about 1 to 10% Ni. Preferably, the nickel laterite or other nickel oxide ore 14 should have a nickel concentration within the range of 0.8 to 5% Ni. More typically the nickel sulphide ore or concentrate 10 has a nickel concentration within the range of about 2 to 4% Ni, and the nickel laterite or other nickel oxide ore 12 has a nickel concentration within the range of about 1 to 2% Ni. The method preferably further comprises the step of directing a nickel laterite or other nickel oxide ore to an atmospheric leach process 16, which in the embodiment of Figure 1 is a first heap leach process (not illustrated). The clarified PLS from the first heap leach process is then directed to the milling circuit 14. The PLS is preferably heated prior to injection into the milling circuit 14. The PLS may be derived from any suitable atmospheric leach process and it not limited to heap leaching. However in the event that a suitable source of PLS from an atmospheric leach process is not available, water may be substituted for the PLS that is directed to the milling circuit.

Preferably the PLS from the first heap leach process 16 has a nickel concentration of more than 4 g/l. Hence a significant benefit of adding the

PLS to the milling circuit 12 is that the head grade of ore passing through the plant is doubled. This, together with acid credits, greatly improves the economies of scale and efficiency of the plant.

Preferably the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining step is in the range of about 1 :1 to 1 :40. More preferably the ratio of nickel sulphide ore (or concentrate) to nickel laterite ore (or other nickel oxide ore) is in the range of about 3:7 to 3:97. Preferably the nickel laterite or nickel oxide ore for the atmospheric leach is a saprolite smectite ore and the laterite or oxide ore used for the combined leach is a limonite ore. The viscosity of laterite ores is impacted by additives such as free acid or total dissolved solids. Limonites typically exhibit a reduction in viscosity when solutions from a heap leach operation are slurried with limonite ores. That is, for a given weight percent, solids milling in PLS reduces the viscosity of the pulp. However with saprolite or smectite ores slurrying in PLS will increase the viscosity for a given weight percent solids. Adding sulphides to all laterite ores, whether limonite, saprolite or smectite, acts to significantly reduce the viscosity and is considered innovative. By appropriate selection of the relative proportions of both kinds of minerals in the combined ores, milling at optimum density can be achieved. Therefore saprolite or smectite is the preferred laterite ore for the atmospheric leach, and limonite is the preferred laterite ore for milling in atmospheric PLS due to the improvement in slurry density achieved.

The milled combined ore from the milling circuit 14 is then subject to a screening step in screening circuit 18. Oversize ore is directed from the screening circuit 18 back to the first heap leach process 16. Undersize ore is fed from the screening circuit 18 to a slurry tank 19, and the combined slurry is then pumped by high pressure slurry pumps to a pressure acid leach circuit (not illustrated). The pressure acid leach circuit may comprise a series of pressure Pachuca tanks. Wash from the screening circuit 18 is returned to the milling circuit 14.

Figures 2 and 9 in the accompanying drawings clearly highlight the positive impact that adding sulphide ore has on laterite rheology. The improvement in the rheological characteristics of the combined slurry has a significant effect on the economics of the mineral recovery process. The density (% solids) of the slurry is a measure of the ore per unit volume. The higher the density the more ore can be processed per unit volume. Therefore the aim is to maximise the density of the slurry without increasing the viscosity to such an extent that the slurry cannot be pumped through the process plant. A typical slurry pump may be rated, for example, to a maximum slurry viscosity of 75 Pa. If the slurry viscosity exceeds this figure the pump may fail.

Milling of the combined nickel sulphide ore or concentrate 10 with nickel laterite or other nickel oxide ore 12, preferably in the proportions specified above, has a dramatic effect on the rheology of the combined slurry. The step of combining nickel sulphide ore or concentrate with nickel laterite or other nickel oxide ore and milling them together to form a slurry with improved rheological characteristics allows higher overall slurry densities to be achieved. This allows for a reduction in capital and operating costs in hydrometallurgical plants.

The milling is typically carried out using the PLS from the first heap leach process instead of, or in addition to, water. The clarified PLS from the first heap leach process preferably has a ferric iron concentration within the range of 10 to 60 g/l. Preferably the PLS from the first heap leach process 10 has a free acid concentration of less than 30g/l.

Rheology Tests

Tests were conducted on the rheology of the combined slurry using various ratios (blends) of nickel laterite ore to nickel sulphide ore or concentrate with process water (PW) and PLS. Slurries formed using two different chemistries of PLS were also tested. Chemical analyses of the ores and liquors used and the results of the rheology tests are summarised below:

Chemical Analysis:

Analysis of the major elements in the respective laterite and sulphide ores and the various blends used in the tests are as follows: Laterite: Ni 1.29%, Fe 12.4%, Si 20.4%, Mg 5.22, Al 4.35% Sulphide: Ni 2.05%, Fe 18.3%, Si 22.1 %, Mg 3.49, Al 2.02% L/S Blend 80:20: Ni 1.46%, Fe 13.3%, Si 21 %, Mg 4.88, Al 4.02% L/S Blend 70:30: Ni 1.54%, Fe 14.4%, Si 21 %, Mg 4.64, Al 3.64% L/S Blend 60:40: Ni 1.67%, Fe 15.1 %, Si 21.6%, Mg 4.42, Al 3.48% Calculated analyses of the above blends are very close to actual analysis

Liquor Analysis:

PLS: Free acid 19 g/L, Ni 4.5 g/L, Fe 41.6 g/L, Mg 19.5 g/L, Al 9 g/L, Na 2.8 g/L, Ca 0.36 g/L Process Water: Mg 1.2 g/L, Ca 0.38 g/L, Na 10.8 g/L, Cl 19.4 g/L The following chemistries 1 and 2 of PLS were employed in the tests:

Chemistry 1 : Fe 41.6 g/L and Free acid 19 g/L Chemistry 2: Fe 41.6 g/L and Free acid 34 g/L

Analysis of all the other major elements are very much the same as the PLS analysis.

Mineralogical Analysis:

The mineralogy of the nickel laterite and nickel sulphide ores employed in the tests was as follows:

Laterite: Smectite (nontronite) is the major phase Maghemite, goethite, hematite, chlorite, hornblend, quartz are minor to moderate phases

Sulphide: Quartz, pyrrhotite, feldspar, hornblend, chlorite are major phases

Pentlandite, pyrite, chalcopyrite, muscovite talc are minor to moderate phases

Rheology:

(i) Laterite, sulphide and L/S blend 70:30 in PW

Yield stress at 100Pa: Laterite in PW -51 % w/w Sulphide in PW -82% w/w

Blend in PW -56.6%

(ii) Laterite in PW, PLS Chemistry 1 and PLS Chemistry 2

Yield stress at 100Pa: Laterite in PW -51 % w/w Laterite in PLS Chem 1 -42.5% w/w Laterite in PLS Chem 2 -45% w/w

(iii) Sulphide in PW, PLS Chemistry 1 and PLS Chemistry 2 Yield stress at 100Pa: Sulphide in PW -82% w/w

Sulphide in PLS Chem 1 -77.8% w/w Sulphide in PLS Chem 2 -78.3% w/w

(iv) L/S Blend 70:30 in PW and Chemistries 1 and 2 Yield stress at 10OPa: Blend in PW -56.6% w/w

Blend in PLS Chem 1 -48.5% w/w Blend in PLS Chem 2 -51.8% w/w

(v) Laterite, sulphide and L/S blends 80:20, 70:30 and 60:40 in PLS Chem 1 Yield stress at 100Pa: Laterite in PLS Chem 1 -42.5% w/w

Blend 80:20 in PLS Chem 1 -46.2% w/w Blend 70:30 in PLS Chem 1 -48.5% w/w Blend 60:40 in PLS Chem 1 -56% w/w Sulphide in PLS Chem 1 -77.8% w/w

(vi) Laterite, sulphide and L/S blends 80:20, 70:30 and 60:40 in PLS Chem 2

Yield stress at 10OPa: Laterite in PLS Chem 2 -45% w/w

Blend 80:20 in PLS Chem 2 -46% w/w Blend 70:30 in PLS Chem 2 -51.8% w/w Blend 60:40 in PLS Chem 2 -55.5% w/w

Sulphide in PLS Chem 2 - 78.3% w/w

The rheological results illustrate the significant improvement in density of the combined slurry (blend) that can be achieved by the use of sulphides to modify the viscosity (as measured by the yield stress) of the laterite ores. With each of the blends there is a substantial increase in the density of the slurry compared to the laterite by itself in slurry. The more sulphide is added to the blend the greater the density. Since the primary objective is the leaching of nickel from laterite or other nickel oxide ores, a compromise between preferred density and the proportion of sulphide ore added is necessary. Preferably a blend of laterite and sulphide in the ratio of about 70:30 or 7:3 achieves an acceptable compromise, i.e. blend comprising 70% laterite ore and 30% sulphide ore. However the blend of nickel sulphide to nickel laterite may typically vary within the range 1 :1 to 1 :40. More typically the ratio of nickel sulphide to nickel laterite varies within the range of about 3:7 to 3:97.

A more detailed comparison of the rheology of the laterite, sulphide and L/S combined slurry using various liquors and blends may be gained from the graphical presentations of the test results in Figures 2 to 9. Figure 2 illustrates the improvement in density for the same viscosity that can be achieved using a 70:30 blend of combined slurry compared to laterite by itself in process water. The much higher densities of sulphide slurry by itself is also illustrated for comparison. Figure 3 illustrates the change in density for the same viscosity that occurs using PLS (Chemistries 1 and 2) to form slurry using the laterite ore by itself compared to using process water (PW) to form the slurry. These results show that for a moderate reduction in density, simply combining the laterite with the PLS can achieve a significant improvement in the head grade of ore passing through the process. Figure 3 also shows that increasing the free acid concentration (Chemistry 2) in the PLS results in an increase in the density of the laterite slurry with the same viscosity.

Figure 4 illustrates the change in density for the same viscosity that occurs using PLS to form slurry from the sulphide ore by itself compared to using PW. The results are similar to that shown in Figure 3, except at the higher densities of the sulphide slurry. Figure 5 illustrates the change in density for the same viscosity that occurs using PLS to form combined slurry with a 70:30 L/S blend compared to using PW. These results show that for a moderate reduction in density, combining a 70:30 L/S blend with the PLS can achieve a significant improvement in the head grade of ore passing through the process. Figure 5 again shows that increasing the free acid concentration in the PLS (Chemistry 2) results in an increase in the density of the combined slurry with the same viscosity.

Figures 6 and 7 are similar to Figure 2 and illustrate the improvement in density for the same viscosity that can be achieved using three different blends of a combined slurry in PLS compared to laterite by itself and sulphide by itself. The three blends employed for the combined slurry are L/S 80:20, L/S 70:30 and L/S 60:40. In Figure 6 the results relate to a slurry formed in a PLS with Chemistry 1 (see Liquor Analysis above), and in Figure 7 the results relate to slurry formed in a PLS with Chemistry 2. A comparison of Figure 6 with Figure 7 reveals that increasing the free acid concentration in the PLS in most cases has the effect of increasing the density of the combined slurry for a specified viscosity. However the effect is most marked in the combined slurry with a L/S 70:30 blend and L/S 80:20 blend.

Figure 8 is similar to Figure 5 except that only the results for a combined slurry with a 70:30 L/S blend using PLS with Chemistry 1 are shown. Figure 8 illustrates the affect that shearing has on the density of the combined slurry ( a marked reduction) for the same viscosity. Figure 9 is similar to Figure 2 except that is also includes the results for Laterite in PLS and a 70:30 blend in PLS for comparison. Figure 9 illustrates the positive impact that adding sulphide ore has on laterite rheology. Combining the sulphide with the laterite in a 70/30 blend results in a marked increase in the density of the combined slurry with the same viscosity, whether in PW or PLS.

Now that preferred embodiments of a hydrometallurgical method for leaching nickel in a combined pressure acid leach have been described in detail, it will be apparent that the embodiments provide a number of advantages over the prior art, including the following:

(i) Combining sulphide ore or concentrate with oxide ore in a milling environment produces a combined slurry with modified viscosity and higher density relative to slurry formed from a laterite or other oxide ore by itself. (ii) The increased slurry density achieved by combining sulphide ore or concentrate with oxide ore significantly reduces the unit cost of production and improves the competitiveness of the hydrometallurgical process plant.

(iii) As existing or new hydrometallurgical plants are hydraulically limited, adding a sulphide concentrate (with a typical concentration of 8% nickel) to a laterite or other oxide ore results in an immediate 15 to 20% increase in throughput on a displaced volume for volume basis due to the differences in specific gravity.

(iv) With this increased throughput, compounded by the improved rheological properties typically >4% w/w and nickel grade 8% versus 1.5%, nickel the benefits of the process are significant.

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.

Claims

The Claims defining the Invention are as follows:

1. A Theological method for the hydrometallurgical recovery of base metals from ores, the method comprising the steps of: providing a sulphide ore or concentrate and a laterite or other oxide ore; combining the sulphide ore or concentrate with the laterite or other oxide ore and milling them together to form a combined slurry with improved Theological characteristics.

2. A Theological method for the hydrometallurgical recovery of base metals from ores as defined in claim 1 , wherein the step of combining the nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together involves milling the ores together.

3. A rheological method for the hydrometaliurgical recovery of base metals from ores as defined in claim 1 or claim 2, wherein a sulphide concentrate is used in the combining and milling step as the specific gravity of sulphide concentrate is about twice that of a typical laterite or other oxide ore.

4. A rheological method for the hydrometallurgical recovery of base metals from ores as defined in any one of claims 1 to 3, wherein water and/or pregnant leach solution (PLS) is added to the sulphide ore or concentrate and the laterite or other oxide ore to form the combined slurry in the combining and milling step.

5. A rheological method for the hydrometallurgical recovery of base metals from ores as defined in any one of the preceding claims, wherein the method further comprises the step of leaching the combined slurry in a pressure acid leach circuit.

6. A rheological method for the hydrometallurgical recovery of base metals from ores as defined in claim 5, wherein the pressure acid leach circuit comprises a series of pressure Pachuca tanks.

7. A rheological method for the hydrometallurgical recovery of base metals from ores as defined in any one of the preceding claims, wherein the base metal is selected from the group consisting of nickel, cobalt, copper, lead and zinc.

8. A method for the hydrometallurgical recovery of base metals from ores as defined in any one of the preceding claims, wherein the sulphide ore or concentrate is a nickel sulphide or concentrate and the laterite or other oxide ore is a nickel laterite or other nickel oxide ore.

9. A rheological method for the hydrometallurgical recovery of nickel from ores, the method comprising the steps of: providing a nickel sulphide ore or concentrate and a nickel laterite or other nickel oxide ore; combining the nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together to form a combined slurry with improved rheological characteristics.

10. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in claim 9, wherein the step of combining the nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together involves milling the ores together.

11. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claim 9 or claim 10, wherein the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining and mixing step may be anywhere in the range of about 1:1 to 1 :40.

12. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in claim 11 , wherein the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining step is in the range of about 3:7 to 3:97.

13. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 9 to 12, wherein the method further comprises a step of directing a nickel laterite or other nickel oxide ore to an atmospheric leach process and providing the PLS from the atmospheric leach process to the combining and mixing step.

14. A Theological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 9 to claim 13, wherein the method further comprises the step of leaching the combined slurry in a pressure acid leach circuit.

15. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 9 to 14, wherein the nickel sulphide ore or concentrate typically has a nickel concentration within the range of about 1 to 10% Ni.

16. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in claim 15, wherein the nickel sulphide ore or concentrate typically has a nickel concentration within the range of about 2 to 4% Ni.

17. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 9 to 16, wherein the nickel laterite or oxide ore has a nickel concentration within the range of about 0.8 to 5% Ni.

18. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in claim 17, wherein the nickel laterite or oxide ore has a nickel concentration within the range of about 1 to 2% Ni.

19. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 14 to 18, wherein the free acid concentration achieved in the pressure acid leach circuit is maintained within the range of 30 to 80 g/l.

20. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 14 to 19, wherein the temperature within the acid leach circuit is maintained between about 160° and 26O⁰C.

21. A rheological method for the hydrometallurgical recovery of nickel from ores as defined in claim 20, wherein the temperature within the acid leach circuit is maintained at about 220° to 25O⁰C.

22. A Theological method for the hydrometallurgical recovery of nickel from ores as defined in any one of claims 14 to 21 , wherein the oxygen over pressure within the pressure acid leach circuit is maintained between 100 to 1000 kPag.

23. A method of improving the rheological characteristics of a laterite or other oxide ore, the method comprising: providing a sulphide ore or concentrate and a laterite or other oxide ore; combining the sulphide ore or concentrate with the laterite or other oxide ore and mixing them together to form a slurry with higher density relative to a slurry formed from the laterite or other oxide ore by itself.

24. A method of improving the rheological characteristics of a laterite or other oxide ore as defined in claim 23, wherein the sulphide ore or concentrate is a nickel sulphide or concentrate and the laterite or other oxide ore is a nickel laterite or other nickel oxide ore.

25. A method of improving the rheological characteristics of a laterite or other oxide ore as defined in claim 24, wherein the step of combining nickel sulphide ore or concentrate with the nickel laterite or other nickel oxide ore and mixing them together involves milling the ores together.

26. A method of improving the rheological characteristics of a laterite or other oxide ore as defined in any one of claim 24 or claim 25, wherein the ratio of nickel sulphide ore or concentrate to nickel laterite or other nickel oxide ore in the combining step is in the range of about 1:1 to 1 :40.

27. A method of improving the rheological characteristics of a laterite or other oxide ore as defined in any one of claims 24 to 26, wherein the ratio of nickel sulphide ore or concentrate: nickel laterite or other nickel oxide ore in the combining step is in the range of about 3:7 to 3:97.

28. A rheological method for the hydrometallurgical recovery of base metals from ores substantially as herein described with reference to and as illustrated in any one or more of the accompanying drawings.

29. A Theological method for the hydrometallurgical recovery of nickel from ores substantially as herein described with reference to and as illustrated in any one or more of the accompanying drawings.

30. A method of improving the Theological characteristics of a laterite or other oxide ore substantially as herein described with reference to and as illustrated in any one or more of the accompanying drawings.