AU2007216761A1 - A pressure oxidation leach process - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION Standard Patent Applicant(s): BHP BILLITON SSM DEVELOPMENT PTY LTD Invention Title: A PRESSURE OXIDATION LEACH PROCESS The following statement is a full description of this invention, including the best method for performing it known to me/us: -2- A PRESSURE OXIDATION LEACH PROCESS The present invention relates to a process for leaching nickel from nickel-containing solid materials, such as nickel-containing ores and concentrates of such ores that contain nickel in sulphide minerals.

The present invention relates particularly, although by no means exclusively, to a pressure oxidation leach process for leaching nickel from nickel-containing materials that contain nickel in a specific sulphide mineral, namely millerite (NiS). The millerite-containing materials may include other sulphide minerals in addition to millerite. There may be nickel in one or more than one of the other sulphide minerals.

The nickel-containing materials may also include oxide minerals. There may be nickel in one or more than one of the oxide minerals.

In addition to nickel, the nickel-containing materials may include other valuable metals, such as cobalt and copper.

The nickel-containing materials may be derived from a specific ore body that contains sulphide minerals and/or oxide minerals.

Alternatively, the nickel-containing materials may be derived from a blend of a number of different ore bodies and contain different minerals.

Materials of interest to the applicant include millerite, heazlewoodite (Ni 3

S

2 godlevskite (NisFeSe), pentlandite ((Ni,Fe)gS) violarite (Ni 2 FeS 4 pyrrhotite (FeS), pyrite (FeS 2 and gersdorffite (NiAsS).

N \Ntclbworne\Casesqatent\59OOOSgq099\J'q47 AU I\Specis\HTP X FINALdoc 13/09/07 The present invention was made during the course of a research program carried out by the applicant on feed materials containing a range of sulphide minerals, including millerite, pentlandite, and violarite.

The applicant realized during the research program that pressure oxidation leaching of nickelcontaining sulphide minerals is a viable option for leaching nickel from these materials.

The applicant also realized that one consequence of the above finding is that, depending on the sulphur concentration in the feed materials, the leached slurry from a pressure oxidation leach step may have sufficient free acid for use in leaching other nickel-containing materials, for example by way of heap leaching.

During the course of the research program the applicant unexpectedly found poor leachability of some slurries of feed materials containing blends of the minerals millerite, pentlandite, and in some instances other minerals such as violarite, under acidic conditions.

The poor leachability occurred with pressure oxidation acid leaching under conditions that the applicant expected would achieve reasonable recoveries of nickel (and cobalt) from the nickel-containing materials.

The poor leach results were found to be due to a number of factors. Specifically, the applicant found that there was unexpected passivation of millerite at temperatures under 165 0 C that caused low nickel extraction. The applicant also found that at higher temperatures, i.e. above 180 0 C, millerite passivation was not an issue. However, there was low nickel extraction due to precipitation of nickel from solution.

Specifically, the applicant found that as the leach temperatures increased there was decreasing solubility of N McIbune\Cascs\Patent\59OO.-59999\P59847 AU I\SpccisUITPOX FINAL doc 13/09/07 -4nickel and magnesium in process liquors, with leached nickel precipitating from solution. The applicant also found that the decreasing solubility issue was not confined to millerite-containing feed materials.

The applicant found that, as a result of the above-mentioned competing factors, under certain conditions, there was practically no improvement in nickel leach extraction for slurries with the same pulp density when leaching at high temperatures (180 0 C, 200 0 C, and 220 0 C tested) compared to low leach temperatures under 165 0 C and, in overall terms, there was poor nickel recovery under the conditions tested.

In the context of millerite, the present invention is based on a realisation that pressure oxidation leaching of millerite-containing feed materials at high temperatures can be an effective option for achieving high extraction of nickel of these feed materials.

In more general terms, the present invention is based on a realisation that the decreased solubility of nickel and magnesium at high temperatures resulted in precipitation of a nickel/magnesium sulphate salt that is readily leachable in downstream processing of a leached slurry.

In particular, the applicant has realised that the following situations apply.

If the feed material is leached under conditions in which the total concentration of nickel and magnesium in solution is below 35 g/L and the concentration of free acid in solution is above 10 g/L, there will be a high leach extraction of nickel in the autoclave (at least 95%) and, therefore, a correspondingly N \Mllbourn\Cases\Plaenl\59000-5999\P59847 AU I\Specis\HTPOX FINAL doc 13/09/07 O low amount of nickel in leach residues in this situation, no further leaching is required.

If the feed material is leached under conditions in which the total concentration of nickel and magnesium in solution is above 35 g/L and the IDconcentration of free acid in solution is above 10 g/L, Ssome nickel will report to the residues resulting in Sreduced nickel extraction. In this situation, the acid in the autoclave discharge may be sufficient of itself to leach the remaining solid nickel downstream of the autoclave under atmospheric pressure conditions.

Alternatively further acid addition may be required to leach the remaining solid nickel downstream of the autoclave under atmospheric pressure conditions.

If the feed material is leached under conditions in which the total concentration of nickel and magnesium in solution is above 35 g/L and the concentration of free acid in solution is below 5 g/L, a relatively low leach extraction (70-90%) of nickel in the autoclave results due principally to precipitation of nickel from solution in the autoclave. The leach residues contain a correspondingly high amount of nickel in this situation, without the addition of more acid to the autoclave discharge slurry only partial re-dissolution of the nickel from the leach residues is achieved in the atmospheric leach stage. Further acid addition is required to the downstream acid leach under atmospheric pressure in order to increase nickel recovery to solution.

Thus, for millerite-containing feed materials, the present invention eliminates millerite passivation by leaching a nickel-containing solid material, such as nickel-containing ores and concentrates of such ores, in a pressure oxidation leach step at high temperatures (>180 0

C)

N Nei bournc\Cases\Patent\59000- 59999\P 59847 AU I\SpccislVITPOX FINAL doc 1 3/09/07 -6- In addition, for feed materials with or without Smillerite, the presnt invention maximises nickel recovery Sby appropriate selection of downstream processing steps M depending on the levels of magnesium, nickel and free acid in solution, with one option including leaching nickel that precipitates in the pressure oxidation leach step in Sa subsequent atmospheric leach step.

In broad terms, the present invention provides a t1 0 process for leaching nickel from a nickel-containing solid feed material, such as nickel-containing ores and concentrates, that contain nickel in sulphide minerals, which process includes leaching nickel from the nickelcontaining material in a high temperature pressure oxidation leach step at a high temperature of at least 180 0

C.

Preferably the process includes forming a slurry of the nickel-containing feed material and supplying the slurry to the pressure oxidation leach step.

Preferably the slurry has a pulp density of at least 15% solids.

More preferably the slurry has a pulp density of at least 20% solids.

The quality of water for producing feed slurries for the present invention is not a limiting factor for the invention. In particular, the feed slurries may be produced from water supplies having high levels of chlorides including levels that are in the hyper-saline range, i.e. more saline than sea water.

Preferably the process includes forming the slurry of the nickel-containing feed material by mixing the nickel-containing feed material with water and/or one N \Ihelborne\CasesPaenrS9000Sq99)q9ps947 AU I\Spccis\JITPOX FINAL dc 13/09/07 -7or more than one process liquor recycled within the process.

Preferably the process includes grinding the nickel-containing feed material prior to supplying the material to the pressure oxidation leach step.

There are no special requirements for fine particle size for the ground material in relation to the present invention.

Typically, the process includes grinding the nickel-containing material to a particle size of P 8 0 of micron prior to supplying the material to the pressure oxidation leach step.

In a situation in which the nickel-containing material includes millerite, preferably the process includes carrying out the pressure oxidation leach step at a temperature that is sufficiently high so that there is minimal passivation of millerite in the feed material.

Preferably the pressure oxidation leach step is carried out at a temperature of less than 240 0

C.

Preferably the pressure oxidation leach step is carried out at a temperature of less than 220 0

C.

Preferably the pressure oxidation leach step is carried out at a temperature in the range of 180-220 0

C.

Preferably the pressure oxidation leach step is carried out at a temperature in the range of 190-200 0

C.

Preferably the pressure oxidation leach step is carried out at an oxygen pressure of at least 400 kPa.

N \Mclborne\Cases\Jalen\5900059Q99\PS9847 AU I\Specis\ITPOX FINAL doc 1 3/09/07 V More preferably the pressure oxidation leach step CL is carried out at an oxygen pressure of at least 600 kPa.

Preferably the pressure oxidation leach step is carried out at an oxygen pressure of less than 1200 kPa.

O Typical conditions for the pressure oxidation leach step include an oxygen pressure of 600 to 1000 kPa C and a temperature in the range of 180-220'C.

SPreferably the pressure oxidation leach step includes leaching nickel from the nickel-containing feed material under acidic conditions.

Preferably the pressure oxidation leach step includes leaching nickel from the nickel-containing feed material under acidic conditions at a pH of less than 2 and more preferably less than 1.

Preferably the acidic conditions are produced by acid generated by oxidation of sulphur in sulphidic minerals present in the feed materials.

Preferably the process includes flashing to atmosphere the leached slurry from the pressure oxidation leach step.

Preferably the process includes thickening the first leached slurry from the pressure oxidation leach step.

The process may also include a downstream atmospheric leach step to leach nickel from solid residues in the leached slurry from the pressure oxidation leach step, with or without additional acid being supplied to the atmospheric leach step.

N \lclbowie\Cases\Pateng\59000.5g9995g9847 AU I\Spcis\HTPOX FINAL doc 1109/07 The term "atmospheric leach" is understood herein to mean that the leach is carried out at atmospheric pressure.

Depending on the concentrations of magnesium (if present) and sulphur in the feed material, there may be sufficient acid generated in the pressure oxidation leach step to re-leach the precipitated nickel in the downstream leach step and additional acid may not be required.

Preferably the atmospheric leach step is carried out at a temperature in the range of 65-95 0

C.

In situations where a downstream atmospheric leach step is not necessary, for example where nickel extraction rates in the pressure oxidation leach step are high (typically greater than 95%) and the feed material has high concentrations of sulphur and thereby generates high amounts of acid, the process may include a partial neutralization step to control the pH of the process liquor for downstream processing.

Preferably the process includes recovering nickel from process liquors produced in the process.

Preferably the process leaches at least 90% by weight, more preferably at least 95% by weight, of nickel from the nickel containing material.

The present invention is described further by way of example with reference to the accompanying drawings, of which: Figure 1 is a flowsheet of one embodiment of a process in accordance with the present invention; Figure 2 is a flowsheet of another embodiment of N \'Melbirn\CasesP aent\590O-59q99959847 AU I\Spccis\JITPOX FINAL doc 1309107 a process in accordance with the present invention; and Figure 3 is a flowsheet of another, although not the only other, embodiment of a process in accordance with the present invention.

The process options shown in the flowsheets of Figures 1 to 3 are described in the context of leaching nickel-containing materials in the form of concentrates of nickel ores that include millerite and other nickelcontaining minerals. The present invention is not confined to these nickel-containing materials.

The flowsheet of Figure 1 is suitable particularly for processing feeds with a high concentration of reactive magnesium and a low-medium concentration of sulphur and is described in this context.

With reference to Figure 1, a concentrate filter cake is repulped in a repulp tank 3 to about 50% solids in a mixture of process water, typically containing g/l chloride ions, and recycle process liquor.

The resultant slurry is fed from the repulp tank to a milling circuit 5. The target mill product size is passing 25-30 micrometers.

The mill product (about 50% w/w solids) is fed to a carbonate destruction tank 7 and recycle process liquor and acid are added to the slurry as required.

The carbonate destruction discharge slurry is fed to an autoclave 9 and the nickel-containing feed material in the slurry is subjected to a high temperature pressure oxidation leach under acidic conditions in the autoclave.

N VlMelborne\Cases\Patent\59000.59999\P59847 AU I\SpccUITPOX FINAL doc 13/09/07 I The autoclave 9 operates at a temperature of 190- 200 0 C and at an oxygen pressure of 800 kPa and a pH of less than 1. Under these conditions, >98% of the sulphide sulphur in the concentrate is converted to sulphate.

The autoclave discharge is flashed to atmospheric pressure in a flash tank 11 and pumped to a discharge tank 13 and then to a thickener 15. Based on batch test results, a significant proportion of nickel that precipitates in the autoclave re-dissolves in the thickener.

The thickener overflow is pregnant process liquor with low acid and low iron (and low arsenic, if present) that is supplied to a metal recovery circuit (not shown) and nickel is recovered from the process liquor.

The underflow from the thickener, which has been thickened to about 40% solids (to minimise acid consumption in the re-leach stage), is pumped to a releach tank 17 where further acid is added to recover precipitated nickel in an atmospheric leach.

The acid addition to the releach tank 17 is kept at a minimum to limit re-dissolution of iron and arsenic (if present) from the autoclave residue.

The re-leach discharge slurry is pumped to a thickener 19 supplied with wash water.

The thickener overflow is fed to the feed repulp tank 3 as the recycle process liquor and the washed leach residue is disposed to the tailings dam.

Batch tests carried out by the applicant as part of the program confirmed that two-stage leach process comprising the high temperature pressure oxidation leach N \Melbournc\Cascsalent\OO5909999TP9847 AU I\Specjs\HTPOX FINAL doc 13/09/07 -12and the atmospheric leach of autoclave leach residues in the releach tank 17 in the flowsheet of Figure 1 are required to achieve high nickel (and cobalt) extractions from concentrates with variable mineralogical composition.

More particularly, batch tests on concentrate containing nickel as millerite and pentlandite, a high concentration of reactive magnesium in a range of acid soluble minerals, and a low-medium concentration of sulphur demonstrated that nickel and cobalt recoveries in excess of 94% were achievable with the above-described two stage leach process of the high temperature pressure oxidation leach in the autoclave 9 and the atmospheric leach of autoclave leach residues in the releach tank 17 in the flowsheet of Figure 1.

As is discussed above, in general terms, the applicant found that the atmospheric leach in the releach tank 17 allows for the dissolution of a complex nickel/cobalt/magnesium salt that is formed in the autoclave when the solution concentrations of nickel and magnesium exceed a solubility limit for these elements at high temperature as is invariably the case with high concentrations of reactive magnesium. The re-solution of this precipitate is possible due to the reverse solubility of this salt observed at lower temperature. The addition of sulphuric acid to the re-leach step is helpful to increase the solubility of metals and maximise the overall metal recovery.

The flowsheet of Figure 2 is suitable particularly for processing feeds with a moderate concentration of reactive magnesium and medium-high concentration of sulphur and is described in this context.

There are substantial similarities between the Figures 1 and 2 flowsheets and the same reference numerals N \\iclbrn\Case\PaIcnl\S5900.59999S9847 AU I\Specs\HTPOX FINAL dx 13/09/07 -13are used to describe the same features in both flowsheets and the following discussion focuses on the differences.

One difference between the flowsheets is that the Figure 2 flowsheet does not have a thickener, such as thickener 15 in the Figure 1 flowsheet, between the flash tank 11 and the releach tank 17. The reason for this is that this flash tank 11 would not result in the same benefits in the Figure 2 flowsheet as it provides in the Figure 1 flowsheet. The flashed autoclave solution in the Figure 1 flowsheet has a comparatively lower concentration of free acid (and consequently lower concentrations of iron and arsenic) than the flashed autoclave solution in the Figure 2 flowsheet. The comparatively low free acid concentration in the Figure 1 solution means that additional acid is required in the subsequent releach step. The comparatively low iron and arsenic concentrations in the Figure 1 flowsheet mean that the solution is relatively clean and very suitable for treatment to recover nickel. Thus, the thickener 15 in the Figure 1 flowsheet makes it possible to minimize the amount of water going forward to the releach tank and thereby reduces the amount of additional acid required in the releach tank and takes advantage of the relatively clean solution for nickel recovery..The higher acid concentration in the flashed autoclave solution of the Figure 2 flowsheet can be beneficially used in the releach tank and, hence, there is no benefit removing water from the solution upstream of the releach tank.

A further difference between the flowsheets is that the overflow from the thickener 19 downstream of the releach tank 17 in the Figure 2 flowsheet is split into two streams, with one stream forming a pregnant leach stream that is transferred for nickel recovery and the other stream forming the recycle process liquor stream to the repulp tank 3.

N \1eclboumc\CasesvPaTn t\59000.59999\P59847 AU \Spccirs\iTPOX FINAL dc 13/09/07 -14- The flowsheet of Figure 3 is suitable particularly for processing feeds with a low concentration of reactive magnesium and a high concentration of sulphur and is described in this context.

There are substantial similarities between the Figures 2 and 3 flowsheets and the following discussion focuses on the main difference between the flowsheets. The same reference numerals are used to describe the same features in all three flowsheets.

This difference is that the Figure 3 flowsheet includes a partial neutralization tank 25 rather than a releach tank as shown in the Figures 1 and 2 flowsheets.

The reason for this is that the lower concentration of reactive magnesium in the Figure 3 feed concentrate compared to the Figure 2 (and Figure 1) concentrate means that there is higher solubility of nickel in the autoclave for a given pulp density. Thus, there are lower amounts of nickel in the residue from the autoclave. In addition, the higher concentration of sulphur in the Figure 3 concentrate compared to the Figure 2 (and Figure 1) concentrate means that the flashed autoclave discharge has a relatively high free acid content. As a result, the relatively small amount of nickel that precipitates in the autoclave is effectively re-leached in the discharge tank 13 and a dedicated re-leach tank is not required. The Figure 3 flowsheet covers a situation where the concentrate is a significant net acid producer and the free acid needs to be neutralised (in this case with limestone) prior to recycling the pregnant leach solution to the repulp tank and to solution treatment.

Many modifications may be made to the flowsheets described above and shown in the drawings without departing from the spirit and scope of the present N 'elborn\Cases\Patre \5900-59999\P59847 AU I\Specis\JiTPOX FINAL doc 13/09/07 invention.

By way of example, the re-leach liquor produced in the flowsheets of Figures 1 to 3 can contain relatively high concentrations of iron and free acid and may also contain some arsenic during periods when high arsenic concentrates are treated. Accordingly, the process may include an additional step of contacting the leach liquor with the feed concentrate to remove the iron, arsenic and to utilise the free acid.

N \Mclbournc\Cases\Paent\59000-59999\P59847 AU I\Specis\HTPOX FINAL doc 13/09/07

Claims (8)

16- CLAIMS 1. A process for leaching nickel from a nickel- containing solid feed material, such as nickel-containing ores and concentrates, that contains nickel in sulphide minerals, which process includes leaching nickel from the nickel-containing material in a high temperature pressure oxidation leach step at a high temperature of at least 180 0 C. 2. The process defined in claim 1 includes forming a slurry of the nickel-containing feed material and supplying the slurry to the pressure oxidation leach step. 3. The process defined in claim 2 wherein the slurry has a pulp density of at least 15% solids. 4. The process defined in claim 2 wherein the slurry has a pulp density of at least 20% solids. The process defined in any one of claims 2 to 4 includes forming the slurry of the nickel-containing feed material by mixing the nickel-containing feed material with water and/or one or more than one process liquor recycled within the process. 6. The process defined in any one of the preceding claims includes grinding the nickel-containing feed material prior to supplying the material to the pressure oxidation leach step. 7. The process defined in any one of the preceding claims includes grinding the nickel-containing material to a particle size of P 80 of 30 micron prior to supplying the material to the pressure oxidation leach step. 8. The process defined in any one of the preceding N \Mclbounmc\Cascs\Pcaenl\59000.59999P59847 AU I\Spccis\ITPOX FINAL doc 13/09/07 -17- claims wherein, when the nickel-containing material includes millerite, the process includes carrying out the pressure oxidation leach step at a temperature that is sufficiently high so that there is minimal passivation of millerite in the feed material. 9. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at a temperature of less than 240 0 C. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at a temperature of less than 220 0 C. 11. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at a temperature in the range of 180-220 0 C. 12. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at a temperature in the range of 190-200 0 C. 13. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at an oxygen pressure of at least 400 kPa. 14. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at an oxygen pressure of at least 600 kPa. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at an oxygen pressure of less than 1200 kPa. 16. The process defined in any one of the preceding claims wherein the pressure oxidation leach step is carried out at an oxygen pressure of 600 to 1000 kPa and a N 'MeIbourne\Cases\Pacentl59000-59999xP 9847 AU I\SpectsUTPOX FINAL doc 10/09/07 -18- temperature in the range of 180-220"C.

17. The process defined in any one of the preceding claims wherein the pressure oxidation leach step includes leaching nickel from the nickel-containing feed material under acidic conditions. 18 The process defined in any one of the preceding claims wherein the pressure oxidation leach step includes leaching nickel from the nickel-containing feed material under acidic conditions at a pH of less than 2 and more preferably less than i.

19. The process defined in claim 18 wherein the acidic conditions are produced by acid generated by oxidation of sulphur in sulphidic minerals present in the feed materials. The process defined in any one of the preceding claims includes flashing to atmosphere a leached slurry produced in the pressure oxidation leach step.

21. The process defined in claim 20 includes thickening the leached slurry from the pressure oxidation leach step.

22. The process defined in claim 20 or 21 includes a downstream atmospheric leach step to leach nickel from solid residues in the leached slurry from the pressure oxidation leach step, with or without additional acid being supplied to the atmospheric leach step.

23. The process defined in claim 22 wherein the atmospheric leach step is carried out at a temperature in the range of 65-95 0 C.

24. The process defined in any one of the preceding N 'Alelb ln\C5Se\Paent\59000-5999959847 AL I\Spcis\HTPOX FINAL doc 13/09/07 -19- claims wherein in situations where a downstream atmospheric leach step is not necessary, for example where nickel extraction rates in the pressure oxidation leach step are high (typically greater than 95%) and the feed material has high concentrations of sulphur and thereby generates high amounts of acid, the process includes a partial neutralization step to control the pH of the process liquor for downstream processing.

25. The process defined in any one of the preceding claims includes recovering nickel from process liquors produced in the process. N \Mlelbourne\Cases\Paen(\59000-59999x59847 AU I\Specs\HTPOX FINAL doc 13/09/07