WO2007095689A1

WO2007095689A1 - Hematite precipitation at elevated temperature and pressure

Info

Publication number: WO2007095689A1
Application number: PCT/AU2007/000210
Authority: WO
Inventors: Michael Rodriguez; Bruce James Wedderburn
Original assignee: Murrin Murrin Operations Pty Ltd
Priority date: 2006-02-24
Filing date: 2007-02-23
Publication date: 2007-08-30
Also published as: AU2007219059A1; EP1994190A1; CA2641919A1; EP1994190A4; AU2007219059B2; BRPI0707021A2; ZA200807098B

Abstract

A hydrometallurgical method (10) for precipitating iron as hematite at elevated temperature and pressure from a pregnant leach solution ('PLS') (12) containing nickel, cobalt and iron, the method comprising the steps of: (i) leaching a low to medium grade nickel laterite ore to produce a PLS (12) containing nickel, cobalt and ferric iron; (ii) subjecting the PLS (12) to elevated temperatures and pressures for a time sufficient to precipitate iron as hematite; (iii) passing the product of step (ii) through a solids/liquid separation circuit (26) to substantially remove the hematite precipitate, and produce a substantially iron-free, acid containing solution; and (iv) recovering nickel and cobalt from the final substantially iron-free, acid containing solution.

Description

"Hematite Precipitation at Elevated Temperature and Pressure"

Field of the Invention

The present invention relates to hematite precipitation from solutions containing nickel, cobalt and ferric iron at elevated temperature and pressure. In particular, the present invention relates to a hydrometallurgical method for co-treating a pregnant leach solution ("PLS") resulting from an atmospheric leach, with a typical slurry for a high pressure acid leach ("HPAL") of a sulphide concentrate, sulphide ore or laterite ore. More particularly, the method of the present invention is intended to allow the precipitation of iron as hematite from the PLS of an atmospheric leach, whilst potentiating the leach of a nickel laterite and/or sulphide in a HPAL circuit.

Background Art

To date, nickel laterite and sulphide ores and sulphide concentrates have typically been leached under conditions of elevated temperature and pressure. The HPAL process involves the use of specialised equipment resulting in a substantial capital outlay, in addition to costly energy requirements.

Alternatively, US Patent 4,548,794 teaches that the atmospheric leaching of laterite ores has been found to consume higher amounts of sulphuric acid making this process even less economical when compared to the HPAL circuit. This is dominated by the readily extractable iron and aluminium achieved under atmospheric pressure and temperature.

It has been found that leach solutions generated from an atmospheric leach operation are a valuable source of readily available ferric iron (used as an oxidant) with the mutual benefit of releasing free acid when the solution is discharged to an autoclave treating high grade laterite ore or sulphide ore or concentrates via a typical HPAL processing route. This significantly improves the overall economics of treating a nickel containing ore utilising atmospheric technology and allows for the co-treatment of sulphide ores or concentrates. In the nickel industry, iron is most often rejected as a ferric oxyhydroxide (typically as a goethite) and as a hematite product from the high pressure acid leaching process for nickel laterites. In some situations iron is also rejected as a jarosite product.

Unfortunately, the rejection of iron as a ferric oxyhydroxide requires the addition of considerable quantities of neutralising agent, such as limestone, which neutralises the freely available sulphuric acid plus the acid formed when the ferric sulphate is converted to ferric oxyhydroxide. This effectively results in the loss of valuable sulphuric acid, which is not economic to recover from the neutralised solutions.

In one form, the present invention economically addresses the problem of acid regeneration resulting from hematite precipitation by recycling the product solution to an atmospheric leach process, or back into the HPAL circuit. Additionally, the requirement for a neutralising agent in the precipitation of iron from an atmospheric leach solution is substantially overcome, and the ferric iron present can be utilised as the oxidant when treating sulphide ores.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

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.

Throughout the specification, the term "atmospheric" when used with reference to leaching is to be understood to refer to any one or more of a vat, heap, thin-layer, tank, dump or in-situ leach, unless the context requires otherwise. Disclosure of the Invention

In accordance with the present invention there is provided a hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a pregnant leach solution ("PLS") containing nickel, cobalt and iron, the method characterised by the steps of:

(i) leaching a low to medium grade nickel Iaterite ore to produce a PLS containing nickel, cobalt and ferric iron;

(ii) subjecting the PLS to elevated temperature and pressure for a time sufficient to precipitate iron as hematite;

(iii) passing the product of step (ii) through a solids/liquid separation step to substantially remove the hematite precipitate, and produce a substantially iron-free, acid containing solution; and

(iv) recovering nickel and cobalt from the final substantially iron-free, acid containing solution.

Preferably, the ferric iron is in the form of ferric sulphate.

Still preferably, hematite precipitation results in the regeneration of sulphuric acid.

Preferably, the PLS directed to the precipitation step (ii) is maintained within the range of about 100⁰C and 26O⁰C in order to convert substantially all of the ferric sulphate to hematite.

Still preferably, the temperature of the PLS is maintained within the range of about 12O⁰C and 26O⁰C, during the precipitation step (ii).

The residence time required for conversion of substantially all of the ferric sulphate to hematite is preferably within the range of about 5 minutes to 180 minutes. The pressure during hematite precipitation is preferably maintained within the range of about 100 kPa and 4500 kPa.

More preferably, the pressure during hematite precipitation is maintained within the range of about 200 kPa and 4500 kPa.

In one form of the present invention the precipitation step (ii) is carried out in a pipe reactor.

In a further form the present invention further comprises the method step of recirculating at least a portion of the substantially iron-free, acid containing solution of step (iii) to the leach circuit of step (i), to facilitate further leaching.

Preferably, the concentration of nickel, cobalt and iron in the PLS directed to the precipitation circuit of step (ii), is within the range of about 1 to 20 g/L, 0.1 to 5 g/L and 1 to 40 g/L, respectively.

The free acid concentration after the precipitation of hematite is preferably in the range of about 20 g/L to 120 g/L.

More preferably, the free acid concentration after the precipitation of hematite is within the range of about 30 g/L to 100 g/L.

In one form of the present invention, the PLS results from a heap leach of a low to medium grade nickel ore.

Further, in one form of the present invention at least a portion of the substantially iron-free, acid containing solution of step (iii) is recirculated to the precipitation circuit of step (ii) at elevated temperature and pressure.

In accordance with the present invention there is further provided a hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a leach solution containing nickel, cobalt and iron, and regenerating acid for application in a further leaching process, the method characterised by the steps of:

(i) leaching a low to medium grade nickel laterite ore to produce a pregnant leach solution ("PLS");

(ii) directing the PLS of step (i) containing nickel, cobalt, and ferric iron to a high pressure acid leach ("HPAL") circuit for the treatment of a laterite ore and/or sulphide ore or concentrate, maintaining this solution at a required temperature and residence time, to precipitate iron as hematite, and regenerate acid, thereby producing an autoclave discharge slurry;

(iii) passing the autoclave discharge slurry through a solids-liquid separation circuit to remove the hematite precipitate, and produce a substantially iron-free, acid containing solution; and

(iv) recovering nickel and cobalt from the solution of step (iii).

Preferably, the PLS directed to the HPAL is heated to within the range of about 160⁰C and 260⁰C in order to convert substantially all of the ferric sulphate to hematite.

Still preferably, the PLS directed to the HPAL is heated to within the range of about 24O⁰C and 26O⁰C in order to convert substantially all of the ferric sulphate to hematite.

Still further preferably, the temperature of the PLS is heated to within the range of about 255⁰C and 26O⁰C. The residence time required for conversion of substantially all of the ferric sulphate to hematite in the HPAL circuit is preferably within the range of about 5 minutes to 120 minutes.

Still preferably, the residence time required for conversion of the majority of ferric sulphate to hematite in the HPAL circuit is within the range of about 30 minutes to 90 minutes.

The pressure in the HPAL circuit is preferably maintained within the range of about 61 OkPa and 450OkPa.

The pressure in the HPAL circuit is more preferably maintained within the range of about 330OkPa and 450OkPa.

Still further preferably, the pressure for the HPAL conditions is maintained within the range of about 430OkPa and 450OkPa.

Preferably, the concentration of nickel, cobalt and iron in the PLS is within the range of about 1 to 20 g/L, 0.1 to 5 g/L and 1 to 40 g/L, respectively.

The free acid concentration in the HPAL circuit after the precipitation of hematite is preferably in the range of about 50 g/L to 120 g/L.

More preferably, the free acid concentration in the HPAL circuit after the precipitation of hematite is within the range of about 50 g/L to 100 g/L.

The PLS is preferably preheated using one ore more heat exchangers before entering the HPAL circuit, thereby reducing energy requirements.

Still preferably, the temperature of the PLS achieved by heat exchange prior to entering the HPAL circuit is preferably within the range of about 6O⁰C and 12O⁰C. Preferably, the autoclave discharge slurry is cooled by passing the solution back through a heat exchanger.

The cooled autoclave discharge slurry is preferably within the range of about 8O⁰C to 14O⁰C after passing through the heat exchanger.

In one form of the present invention there is provided the additional method step of recycling at least part of the substantially iron-free, acid containing solution of step (iii) to the leach circuit of step (i) to facilitate further leaching.

In one form of the present invention the leach of step (i) is provided in the form of a heap leach circuit.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to a first and second embodiment thereof and the accompanying drawings, in which;

Figure 1 is a diagrammatic representation of a flow sheet depicting a hydrometallurgical method for the precipitation of iron in the form of hematite at elevated temperature and pressure from a pregnant leach solution containing nickel, cobalt and iron in accordance with a first embodiment of the present invention;

Figure 2 is a diagrammatic representation of a flow sheet depicting a hydrometallurgical method for the precipitation of iron in the form of hematite at elevated temperature and pressure from a pregnant leach solution containing nickel, cobalt and iron in accordance with a second embodiment of the present invention, the PLS being a product of a heap leach; Figure 3 is a graph showing the change in iron concentration, free acid concentration and hematite precipitation from a column leach solution, wherein the leach liquor was heated to 140⁰C and held at 450 kPa in an autoclave;

Figure 4 is a graph showing the change in iron concentration, free acid concentration and hematite precipitation from a column leach liquor wherein the leach liquor was heated to 200⁰C and held at 1600 kPa in an autoclave; and

Figure 5 is a graph showing the change in iron concentration, free acid concentration and hematite precipitation from a column leach liquor wherein the leach liquor was heated to 24O⁰C and held at 3100 kPa in an autoclave.

Best Mode(s) for Carrying Out the Invention

In Figure 1 there is shown a hydrometallurgical method 10 for precipitating iron in the form of hematite at elevated temperature and pressure from a pregnant leach solution 12 ("PLS") containing nickel, cobalt and ferric iron in accordance with a first embodiment of the present invention.

The PLS 12, containing between 1 to 20g/L nickel, 0.1 to 5 g/L cobalt, and 1 to 40g/L iron, is the result of an atmospheric leach 14 of a low to medium grade nickel laterite ore. The PLS 12 is then directed to a reactor vessel, for example a pipe reactor 20 in which it is heated to within the range of 100⁰C and 26O⁰C, for example 12O⁰C to 26O⁰C, and maintained at a pressure within the range of 100 kPa and 4500 kPa, for example 200 kPa to 4500 kPa, for a residence time of between 5 and 180 minutes, such that hematite is precipitated and acid regenerated.

It is envisaged that the concentration of acid in a reacted PLS 18 resulting from hematite precipitation will be within the range of 20 to 120 g/L, for example 30 g/L to 100 g/L. The reacted PLS 18 then proceeds to a solid liquid separation circuit 26 before the acid containing solution resulting therefrom is redirected to the atmospheric leach 14 to facilitate further leaching and/or being directed to the recovery circuit 30.

In Figure 2 there is shown a hydrometallurgical method 40 for precipitating iron in the form of hematite at elevated temperature and pressure from a pregnant leach solution 12 ("PLS") containing nickel, cobalt and ferric iron in accordance with a second embodiment of the present invention. The method 40 is substantially similar to the method 10 described hereinabove and like numerals denote like parts/steps.

The PLS 12 is collected from an atmospheric leach in the form of a heap leach 14 and is directed to a first heat exchanger 16 where it is preheated to between about 6O⁰C and 120⁰C by an autoclave discharge slurry 18 exiting a high pressure acid leach ("HPAL") circuit 20. The preheated PLS 22 is then directed to the HPAL circuit 20 where it is integrated into the leach of a nickel sulphide, or high grade nickel laterite, or both. The ferric iron already present in the PLS 14 can be utilised as the oxidant, thus reducing the requirement for adding an oxidant to the HPAL circuit 20.

The slurry in the HPAL circuit 20 is then maintained at an elevated temperature of between about 16O⁰C and 26O⁰C, for example 240⁰C and 26O⁰C, or preferably 255⁰C and 26O⁰C, and pressure of between about 610 kPa and 4500 kPa, for example 3300 kPa and 4500 kPa, or preferably 4300 kPa and 4500 kPa, for the required residence time, which is dependent on the operating conditions adopted, generally ranging between about 5 minutes and 120 minutes, for example between 30 minutes to 90 minutes.

The autoclave discharge slurry 18 from the HPAL circuit 20 is cooled to between about 8O⁰C and 14O⁰C by passing it back through the heat exchanger 16. The cooled slurry 24 then undergoes a solid/liquid separation 26 to remove the precipitated hematite from the solution. It is understood by the inventors that the process of hematite precipitation generates acid according to the following equation:

2Fe₂(SO₄J₃ +3H₂O <→- Fe₂O₃ +3H₂SO₄

The concentration of free acid in the separated solution 28 after the hematite precipitation is generally within the range of about 50 g/L up to 120 g/L sulphuric acid, for example 50 g/L to 100 g/L. Thus the solution may be returned to the heap leach 14 to aid further leaching, and/or it may proceed to the recovery circuit 30.

The precipitation of hematite also at least reduces or may eliminate the requirement for a neutralising agent, as is typically needed for the removal of iron as ferric hydroxide or ferric oxyhydroxide, under atmospheric conditions.

The present invention is further illustrated by way of the following non-limiting examples:

EXAMPLE 1

A pregnant leach solution containing high iron levels in the form of ferric sulphate was treated at 14O⁰C and at 450 kPa to reduce the ferric sulphate to hematite. The composition of the feed solution is set out in Table 1 below:

Table 1: Composition of Pregnant Leach Solution 1.

Solution 1 was treated heated to 14O⁰C with a pressure of 450 kPa and held for 120 minutes, as the iron in ferric form was converted to hematite. In addition the free acid concentration increased from 14.2 g/l to 32.1 g/l as the ferric sulphate was converted to hematite.

The composition of the resultant solution is set out in Table 2 below:

Table 2: Composition of Reduced Leach Solution 1.

The change in iron concentration, free acid concentration and hematite precipitation under these conditions are shown in Figure 3.

EXAMPLE 2

A pregnant leach solution containing high iron levels in the form of ferric sulphate was treated at 200⁰C and at 1 ,600 kPa to reduce the ferric sulphate to hematite. The composition of the feed solution is set out in Table 3 below:

Table 3: Composition of Pregnant Leach Solution 1.

Solution 1 was treated heated to 200⁰C with a pressure of 1 ,600 kPa and held for 120 minutes, as the iron in ferric form was converted to hematite. In addition the free acid concentration increased from 14.2 g/l to 68.1 g/l as the ferric sulphate was converted to hematite. The composition of the resultant solution is set out in Table 4 below:

Table 4: Composition of Reduced Leach Solution 1.

The change in iron concentration, free acid concentration and hematite precipitation under these conditions are shown in Figure 4.

EXAMPLE 3

The pregnant leach solution containing high iron levels in the form of ferric sulphate was treated at 24O⁰C and at 3,100 kPa to reduce the ferric sulphate to hematite. The composition of the feed solutions is set out in Table 5 below:

Table 5: Composition of Pregnant Leach Solution 2.

Solution 1 was treated heated to 24O⁰C with a pressure of 3,100 kPa and held for 120 minutes, as the iron in ferric form was converted to hematite. In addition the free acid concentration increased from 18.3 g/l to 105.8 g/l as the ferric sulphate was converted to hematite. The composition of the resultant solution is set out in Table 6 below:

Table 6: Composition of Reduced Leach Solution 2.

The change in iron concentration, free acid concentration and hematite precipitation under these conditions are shown in Figure 5.

It can be seen from the above examples that significant quantities of iron can be removed from an atmospheric leach solution using the method hereinbefore described. Acid has also been shown to be regenerated in sufficient quantities to aid further leaching, be it leaching to first generate the leach solution containing ferric iron or leaching at elevated temperature and pressure.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

1. A hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a pregnant ieach solution ("PLS") containing nickel, cobalt and iron, the method characterised by the steps of:

(i) leaching a low to medium grade nickel laterite ore to produce a PLS containing nickel, cobalt and ferric iron;

2. A hydrometallurgical method according to claim 1 , wherein the ferric iron is in the form of ferric sulphate.

3. A hydrometallurgical method according to claim 1 or 2, wherein the precipitation of hematite results in the regeneration of sulphuric acid.

4. A hydrometallurgical method according to any one of claims 1 to 3, wherein the PLS directed to the precipitation step (ii) is maintained within the range of about 100⁰C to 26O⁰C in order to convert substantially all of the ferric sulphate to hematite.

5. A hydrometallurgical method according to any one of the preceding claims, wherein the temperature of the PLS is maintained within the range of about 120oC and 260oC, during the precipitation step (ii).

6. A hydrometallurgical method according to any one of the preceding claims, wherein the residence time required for conversion of substantially all of the ferric sulphate to hematite is within the range of about 5 minutes to 180 minutes.

7. A hydrometallurgical method according to any one of the preceding claims, wherein the pressure during hematite precipitation is maintained within the range of about 100 kPa and 4500 kPa.

8. A hydrometallurgical method according to any one of the preceding claims, wherein the concentration of nickel, cobalt and iron in the PLS directed to the precipitation circuit of step (ii) is within the range of about 1 to 20 g/L, 0.1 to

5 g/L and 1 to 40 g/L, respectively.

9. A hydrometallurgical method according to any one of the preceding claims, wherein the free acid concentration after the precipitation of hematite is within the range of about 20 g/L to 120 g/L.

10. A hydrometallurgical method according to any one of the preceding claims, wherein the PLS results from a heap leach of a low to medium grade nickel ore.

11. A hydrometallurgical method according to any one of the preceding claims, wherein the precipitation step (ii) is carried out in a pipe reactor.

12. A hydrometallurgical method according to any one of the preceding claims, wherein at least a portion of the substantially iron-free, acid containing solution of step (iii) is recirculated to the leach of step (i).

13. A hydrometallurgical method according to any one of the preceding claims, wherein at least a portion of the substantially iron-free, acid containing solution of step (iii) is recirculated to the precipitation step (ii) at elevated temperature and pressure.

14. A hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a leach solution containing nickel, cobalt, and iron, and regenerating acid for application in a further leaching process, the method comprising the steps of;

(ii) directing the PLS of step i) containing nickel, cobalt and ferric iron to a high pressure acid leach ("HPAL") circuit for the treatment of a laterite ore and/or sulphide ore or concentrate, maintaining this solution at a required temperature and residence time, to precipitate iron as hematite, and regenerate acid, thereby producing an autoclave discharge slurry;

(iii) passing the autoclave discharge slurry through a solid/liquid separation circuit to remove the hematite precipitate, and produce a substantially iron-free, acid containing solution; and

(iv) recovering nickel and cobalt from the solution of step (iii).

15. A hydrometallurgical method according to claim 14, wherein the PLS directed to the HPAL circuit is heated to within the range of about 16O⁰C to 26O⁰C.

16. A hydrometallurgical method according to claim 14 or 15, wherein the residence time required for conversion of substantially all of the ferric sulphate to hematite in the HPAL circuit is within the range of about 5 minutes to 120 minutes.

17. A hydrometallurgical method according to any one of claims 14 to 16, wherein the pressure in the HPAL circuit is maintained within the range of about 61O kPa to 450O kPa.

18. A hydrometallurgical method according to any one of claims 14 to 17, wherein the concentration of nickel, cobalt and iron in the PLS of step i) is within the range of about 1 to 20 g/L, 0.1 to 5 g/L and 1 to 40 g/L, respectively.

19. A hydrometallurgical method according to any one of claims 14 to 18, wherein the free acid concentration in the HPAL circuit after the precipitation of hematite is within the range of about 50 g/L to 120 g/L.

20. A hydrometallurgical method according to any one of claims 14 to 19, wherein the PLS is preheated using one or more heat exchangers before entering the HPAL circuit.

21. A hydrometallurgical method according claim 20, wherein the temperature of the PLS achieved by heat exchange prior to entering the HPAL circuit is within the range of about 60⁰C to 120⁰C.

22. A hydrometallurgical method according to any one of claims 14 to 21 , wherein the autoclave discharge slurry is cooled by passing the solution back through a heat exchanger.

23. A hydrometallurgical method according to claim 22, wherein the temperature of the autoclave discharge slurry is within the range of about 8O⁰C to 14O⁰C after passing through the heat exchanger.

24. A hydrometallurgical method according to any one of claims 14 to 23, wherein at least part of the substantially iron-free, acid containing solution of step (iii) is recycled to the leach circuit of step (i).

25. A hydrometallurgical method according to any one of claims 14 to 23, wherein the leach circuit of step (i) is a heap leach.

26. A hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a leach solution containing nickel, cobalt and iron, substantially as hereinbefore described with reference to Figures 1 or 2.

27. A hydrometallurgical method for precipitating iron as hematite at elevated temperature and pressure from a leach solution containing nickel, cobalt and iron, substantially as hereinbefore described with reference to any one of Examples 1 to 3.