Field of the Invention

The present invention relates to a hydrometallurgical method for leaching base metals. More particularly, the method of the present invention is intended to provide a single process capable of use in the leaching of nickel from a variety of nickel ore compositions.

Background Art

To date, high grade ("smeltable") nickel sulphide ores have been treated pyrometallurgically, via smelting, in order to recover nickel. These ores have generally been considered to be unsuited to hydrometallurgical treatments, such as High Pressure Acid Leach (HPAL), as their highly reducing nature lowers the oxidation reduction potential (ORP), which is believed to damage the autoclave lining in cases where the lining comprises titanium dioxide. Traditionally, an operator has had to be very selective about any sulphide material entering a HPAL autoclave. In particular, the magnesium content, together with the ORP potential, needs to be carefully monitored.

Alternatively, the treatment of ores through pyrometallurgical methods requires significant capital expenditure due to the plant infrastructure required to support the conditions required for such processes.

Low grade ("non-smeltable") nickel sulphides are typically unsuitable for smelting as they generally contain high quantities of magnesium (Mg), often in the form of magnesia (MgO). If the MgO content of the slag produced in smelting is too high, it becomes very viscous and can be difficult to remove from the furnace. Further, these ores contain arsenic at levels which render them generally unsuitable to pyrometallurgical treatment due to the potential health and safety issues.

Australian Innovation Patent 2008100563 addresses the treatment of sulphide ores and/or concentrates in a high pressure acid leach (HPAL) circuit by combining the sulphide ore or concentrate with a HPAL feed comprising oxide ore material and, importantly, adding an oxidant to the HPAL circuit. To date it has been understood that the addition of an oxidant, such as manganese dioxide (MnO ₂ ), was necessary to maintain the ORP in a range suitable to ensure sufficient conditions for oxidation of the sulphide ore, whilst minimising damage to the titanium lining in a HPAL autoclave.

The present method has as one object thereof to substantially overcome problems associated with the prior art, or to at least provide a useful alternative to those prior art methods.

Throughout the specification and claims, 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.

The above discussion of the background art 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 the application.

Throughout the specification and claims, 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.

Disclosure of the Invention

In accordance with the present invention there is provided a hydrometallurgical method for leaching base metals, said method comprising the method steps of:

I) combining an oxide ore material with a sulphide ore or concentrate in the presence of acid to form a slurry; and

ii) leaching the slurry in a pressure acid leach (PAL) circuit to produce a feached autoclave discharge liquor, whereiή the acid concentration in the slurry is sufficient to ensure some iron remains in its oxidised (ferric) form in the presence of the sulphide ore or concentrate.

Preferably, the presence of some ferric iron is maintained by the acid dissolution of hematite at one or more points in the PAL circuit.

The free acid concentration measured in the autoclave discharge liquor (after cooling) is preferably within the range of about 10 to 80 g/L.

More preferably, the free acid concentration measured in the cooled autoclave discharge is preferably within the range of about 30 to 60 g//L.

The sulphide ore or concentrate is preferably a nickel sulphide ore or concentrate.

Preferably, the oxide ore material comprises a nickel oxide ore or nickel laterite.

The sulphide ore or concentrate may comprise a smeltable sulphide ore or concentrate, a non-smeltable ore or concentrate, or a blend thereof.

The PAL circuit may preferably comprise a plurality of autoclaves arranged in parallel.

The at least one autoclave is preferably operated within the temperature range of about 200 ⁰ C to 27O ⁰ C

More preferably, the at least one autoclave is operated within the temperature range of about 23O ⁰ C to 26O ⁰ C.

The pressure of within the autoclave is preferably maintained within the range of about 350OkPa to 650OkPa.

More preferably, the pressure within the autoclave is maintained within the range of 400OkPa to 550OkPa.

The oxide ore material preferably has an iron content within the range of about 10% to 50%. More preferably, iron content in the oxide ore material is preferably within the range of about 15% to 50%.

The ferric iron concentration in the autoclave is preferably at least about 0.02 g/L.

More preferably, the ferric iron concentration in the autoclave is preferably at least about 0.1 g/L.

Preferably, acid is provided in the form of one or more of a pregnant leach solution (PLS) exiting an atmospheric leach circuit, or an aqueous acid solution comprising acid and water, or concentrated acid. Where a PLS is utilised, acid levels may be supplemented by the addition of aqueous acid solution.

Still preferably, the acid is sulphuric acid.

The sulphide ore or concentrate preferably has a nickel content within the range of about 2% to 30%.

More preferably, the sulphide ore or concentrate and has a nickel content within the range of about 5% to 20%.

The sulphide ore or concentrate preferably comprises about 1 % to 30% (w/w) of the slurry.

More preferably, the sulphide ore or concentrate preferably comprises about 1% to 20% (w/w) of the slurry.

The oxide ore preferably has a nickel content within the range of about 0.5% to 3%.

More preferably, the oxide ore has a nickel content within the range of about 0.9% to 2%.

The residence time within the autoclave is preferably within the range of about 15 to 120 minutes. More preferably, the residence' time within the autoclave is within the range of about 30 to 90 minutes.

The base metals may include but is not limited to, any one or more of nickel, copper and zinc.

Preferably, the oxidation/reduction potential (ORP) of the cooled autoclave discharge is within the range of about 250 to 90OmV (measured against an Ag/AgCI reference).

More preferably, the ORP of the cooled autoclave discharge is within the range of about 300 to 500 mV (Ag/AgCI reference).

Preferably, the PAL circuit is in the form of a high pressure acid leach circuit (HPAL).

In accordance with the present invention there is further provided a hydrometallurgical method for leaching nickel, said method comprising the method steps of:

i) combining a nickel oxide ore material with a nickel sulphide ore or concentrate in the presence of acid to form a slurry; and

ii) leaching the slurry in a pressure acid leach (PAL) circuit, to produce a leached autoclave discharge liquor,

wherein the acid concentration in the slurry is sufficient to ensure some iron remains in its oxidised (ferric) form in the presence of the nickel sulphide ore or concentrate.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawing, in which:- Figure 1 is a diagrammatic representation of a flow sheet depicting a hydrometallurgical method for leaching nickel in accordance with the present invention.

Best Mode(s) for Carrying Out the Invention

In Figure 1 there is shown a hydrometallurgical method 10 for the leaching of base metals in accordance with the present invention.

A sulphide ore, for example a nickel sulphide ore 12, comprising a smeltable sulphide ore or non-smeltable ore, or a blend thereof, undergoes a concentration step 14 to produce a nickel sulphide concentrate 16. The nickel sulphide concentrate 16, having a nickel content within the range of about 2% to 30%, for example about 5% to 20%, is then combined with an oxide ore, for example a nickel oxide or nickel laterite ore 18, together with water and/or acid 19, to form a slurry 20. The acid 19 may comprise a pregnant leach solution (PLS) exiting an atmospheric leach, for example a heap leach, or an aqueous acid solution comprising acid and water, or it may comprise concentrated acid, for example sulphuric acid. The content of nickel within the nickel laterite ore 18 is within the range of about 0.5% to 3%, for example 0.9% to 2%. The nickel sulphide ore 12 or concentrate 16 and nickel laterite ore 18 are combined such that the nickel sulphide ore 12 or concentrate 16 comprises about 1 % to 30% of the slurry 20, for example 1 % to 20%.

The slurry 20 is then directed to an autoclave 22 of a pressure acid leach circuit, for example a high pressure acid leach (HPAL) circuit. The HPAL circuit may comprise a plurality of autoclaves arranged in parallel. The slurry 20, is then leached in the autoclave 22 at a temperature within the range of about 200 ⁰ C to 27O ⁰ C, for example, about 230 ⁰ C to 26O ⁰ C and a pressure within the range of about 350OkPa to 650OkPa, for example about 4000kpa to 550OkPa, for a residence time within the range of about 15 to 120 minutes, for example, 30 to 90 minutes. The free acid concentration within the autoclave 22 is sufficient to ensure that the ferric iron in solution remains at or above at least about 0.02 g/L, for example at least about 0.1 g/L. The resulting autoclave discharge liquor 24 exiting the autoclave 22 is then subjected to known recovery processes. It will be noted that the process of the present invention does not require the addition of an oxidant. This is because provided the free acid levels are within the range of about 10 to 80g/L as measured in the cooled autoclave discharge liquor, for example about 30 to 60 g/L, the ferric iron concentration can be maintained, albeit at low concentrations. This is understood to result from the dissolution of hematite (which is precipitated from solution under autoclave conditions), in the presence of acid. Small quantities of ferric iron (as low as about 0.02g/L) remain in solution, and this has been found to be sufficient to oxidise sulphide in the ore to sulphate. Thus, as the ferric iron is consumed, more ferric is subsequently generated by the dissolution of hematite. It is important to note that unlike previous processes, the process of the present invention does not require ORP to be controlled. Again, this results as provided there is sufficient acid present, the ferric iron will be present and oxidising conditions maintained.

It is understood that the sulphide ore 12 need not necessarily undergo a concentration step 14 in order to be utilised in the method of the present invention. It is also understood that the sulphide ore 12 or concentrate 16, and the oxide ore 18 could be added directly to the autoclave 22, together with water and/or acid 19 without the requirement of blending first.

It is envisaged that the ability to treat smeltable concentrates, non-smeltable concentrates and laterite ores in an autoclave, without the requirement for the infrastructure employed for pyrometallurgical processes will have applications for both laterite and sulphide producers.

It is further envisaged that the process of the present invention helps to overcome the problems associated with treating sulphide ores having high Mg content, which can cause problems in pyrometallugical methods.

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

Example 1

Investigation of ORP in a High Pressure Acid Leach (HPAL) autoclave under moderately reducing conditions The aims were to determine soluble metal contents and examine the effect of high ferrous iron concentrations upon the ORP of the leach liquor inside the autoclave. This required separation of the solid and liquid phases to produce "at temperature" liquor samples. Separation was effected by employing a porous graphite filter element fitted to the end of a dip tube.

Five tests were conducted during the experimental program. The first employed a synthetic laterite liquor with the remaining tests conducted as HPAL runs using a smectite/saprolite ore sample. For the HPAL tests various acid additions were employed in order to target final liquor acidities of 30-40 g/L and 50-60 g/L H ₂ SO ₄ .

A sample of smectite/saprolite ore was pulverised, milled using a SaIa Cone Crusher and dry screened to -710 μm. Sub-samples were taken for High Pressure Acid Leach (HPAL) tests. A head analysis of the sub-sample is provided in Table 1 :

Table 1

Leaching experiments were conducted using a vertical reactor constructed from Grade 705 Zirconium and fitted with a titanium grade 2 dip tube to which was attached a porous 4842 moel temperature controller via electrical elements housed within a stainless steel enclosure and electrically switched water stream passing through a serpentine cooling coil. Agitation was provided by a magnetically driven twin impeller.

Slurry feeds were made up from 577g ore, 1100 g tap water and 51.25 g ferrous sulphate heptahydrate. These were charged to the reactor, the head space displaced 3-4 times to an overpressure of 10OkPa with industrial nitrogen to minimise oxidation of ferrous ion and heating applied to bring the mixture to the acid addition temperature of 235 ⁰ C. The required acid charge was injected using excess pressure supplied from a cylinder of industrial nitrogen using an actuated valve system. The amounts of acid used were targeted to leave i) 30-40g/L free acid and ii) 50-60 g/L free acid, in the final liquors.

A test was also conducted using synthetic liquor with a composition as depicted in Table 2:

Table 2

Sulphuric acid was also added such that the concentration was 22 g/L in the synthetic liquor, in order to obtain a target free acidity of 30 g/L after ferric iron hydrolysis.

After the last sample at 250 ⁰ C was taken, the head space of the sample container was flushed with nitrogen. The Oxidation/Reduction potential (ORP) measurement was made at room temperature within 15 minutes of taking the sample using a TPS smartChem-pH-mV-Temperature meter with platinum electrode (Ag/AgCI, saturated KCI reference) calibrated against a fresh mixture of ZoBell's solution (Eh 236 mV at 22 ⁰ C). The ferrous iron content of the liquor was determined by titrating 10 mL aliquots, to which was added 10 mL of 50:30 sulphuric: phosphoric acid mixture and 4-5 drops of barium diphenylamine sulphonate indicator, against 0.05N potassium dichromate solution.

The "at temperature" filtrates and final leach liquors, collected after cooling the autoclave to room temperature, were submitted for elemental analysis by ICP- AES.

The free acidity of the final leach liquors were determined after 1 :10 dilution and titration of 10 mL aliquots, to which was added 20% excess calcium EDTA masking solution, against a 0.05N sodium hydroxide solution to an endpoint of pH 5.7. Analytical data for a series of "at temperature" leach liquors is provided in Table 3 hereinafter. From the results obtained it can be concluded that hydrolysis of the ferric iron produces hematite and decreases the synthetic leach liquor Eh. Furthermore, most of the iron in the "at temperature" liquor samples is seen to be in the ferrous state. At higher final acidity (54g/L and 83g/L) the data clearly indicates the presence of small amounts of ferric iron (<0.5g/L) with a small increase in ORP.

The ferrous iron concentrations in the final HPAL liquors are approximately half of those in the feed slurry, consistent with oxidation by manganese in the smectite/saprolite ore.

Significantly, for the HPAL tests, the change in ORP is approximately 30 mV when the final acidity changes from about 40 g/L to about 85 g/L. The higher ORP at higher acidity is consistent with the presence of greater amounts of ferric iron arising from the acid dissolution of hematite.

Furthermore, it is observed that the ferrous/ferric concentrations in solution remain relatively constant under autoclave leaching conditions ("at temperature").

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Table 3

N/A = Not Applicable R2-R5 refer to HPAL tests with various acid additions

N/D = <10 mg/L Filtrate = "at temperature" leach liquor.

N/M = Not Measured Final = cooled leach liquor

Note that ferrous iron concentrations were obtained some days after the experiments were completed.

Example 2

Three batch leaching tests were conducted in an autoclave at 25O ⁰ C. The tests were conducted with a) nickel laterite only, b) 1 % nickel sulphide blend and c) 5% nickel sulphide blend, without addition of further oxidant. The nickel laterite ore composition is provided in Table 4 below:

Table 4

The nickel sulphide ore composition is provided in Table 5 below:

10 Table 5

Al Pb Mg Co K Mn Ca Ti Mo V Cr S Ni Na Ce Fe Zn As Cu

% ppm % % ppm ppm % ppm ppm ppm ppm ppm % ppm ppm % ppm ppm ppm

0.99 I 165 I 2.42 I O.23 | 24Oθ| 273 | 0.84 I 38il 14 I 20 I 200 I 1871001 19.1 1 14801 14 I 25.8 I 335 I 340019250

The leach test results, provided in Table 6 hereinafter, clearly show that even in the absence of reducing conditions caused by the presence of sulphide, and at 15 high ORP (about 800 mV), the solubility of ferric in solution is limited. The 5% sulphide test shows significantly greater levels of ferrous iron in solution, lower ORP (<400 mV), yet ferric iron is still shown to be present.

That is, under reducing conditions, and in the absence of oxidant additions, the presence of ferric iron is maintained provided there is acid present. The ORP will 20 drop due to the absence of additional oxidant (which in turn would oxidise ferrous iron). However, good oxidising conditions are maintained, as evidenced by the continued extraction achieved under reducing conditions. The data also shows that generally, there is more ferric iron present in solution at higher free acid concentrations, which is consistent with hematite dissolution.

As can be seen from the above examples, sulphide ores and concentrates can be processed in a HPAL circuit without the need to add further oxidant to the system. Provided the acid levels are maintained at a suitable level, low levels of ferric iron remain in solution. These low levels of ferric iron concentration are sufficient to provide oxidising conditions within the autoclave sufficient to oxidise the sulphide content.

Oxidising conditions in the autoclave also minimises damage to the autoclave lining as a result of the reduction of titanium dioxide.

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Table 6

Example 3

Table 7 provides a sample of plant data for a leaching trial conducted and good extractions obtained. Importantly, it should be noted that it was anticipated that extractions would drop slightly if ORP increased. Therefore, if an increase in ORP was observed at any time, additional acid was added to compensate and to ensure extraction levels were maintained.

In Table 7 it can be seen that at lower ORP and lower free acid concentrations, good extractions (up to about 94%) were observed. Similar extractions were observed at higher ORP, as a result of increased free acid concentration. It can be concluded from this data, that in the absence of the addition of further acid, extractions at higher ORP could be expected to drop to levels below those observed at lower ORP.

Table 7

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