Field of the Invention

The present invention related to the recovery of nickel and cobalt from a lateritic ore by means of leaching with sulphuric acid followed by downstream purification and separation steps in order to yield high value, marketable products

In some embodiments, the present invention relates more particularly but not exclusively to the leaching of nickel- and cobalt-containing lateritic ores with sulphuric acid under conditions that minimises the overall sulphuric acid consumption in order to achieve an economically and technically viable degree of nickel and cobalt dissolution from the lateritic ore feedstock

In some embodiments, the present invention relates more particularly but not exclusively to the separate mining of the high iron (limonite) and high magnesium (saprohte) components of the nickel- and cobalt-containing lateritic ore For convenience, the split between the limonite and saprolite fractions is typically based on their respective MgO contents, with the limonitic component typically having a MgO content of < 5% and the saprolite fraction typically having a MgO content of > 5% The corresponding iron (Fe) contents are typically >15% for limonitic ore and <15% for saprolitic ore These naming conventions will be used throughout this specification

In some embodiments, the present invention relates more particularly but not exclusively to the treatment of the pregnant reaction slurry derived from the sulphuric acid leaching nickel- and cobalt-containing lateritic ores under conditions that minimises the consumption of a suitable alkaline material in order to neutralise excess acid contained within the pregnant reaction slurry prior to the separation and recovery of high value, marketable products

In some embodiments, the present invention relates more particularly but not exclusively to the treatment of the pregnant reaction slurry derived from the sulphuric acid leaching nickel- and cobalt-containing lateritic ores under conditions that result in the precipitation an iron-containing leach residue that is readily separated from the nickel- and cobalt-containing pregnant leach solution and washed free of any entrained pregnant leach solution In some embodiments, the present invention relates more particularly but not exclusively to the precipitation and recovery of a mixed nickel-cobalt sulphide product from the clarified pregnant leach solution obtained subsequent to excess acid neutralisation and soluble iron precipitation, together with any other required impurity removal steps

Background to the Invention

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was, at the priority date

(a) part of common knowledge or

(b) known to be relevant to attempt to solve any problems with which this specification is concerned

Although nickel- and cobalt-containing lateritic ores constitute the major terrestrial sources of both nickel and cobalt, their processing is relatively complex and involves both high capital and operating costs This combination of complexity and high processing costs is a direct reflection of the mineralogical complexity of the lateritic ores themselves and the fact that there are no discrete nickel- and/or cobalt-containing minerals that can be selectively recovered in the form of a high-grade concentrate by conventional physical separation methods such as flotation or use of differential magnetic or density properties As a consequence it is necessary to process what is essentially run-of-mine ore The only run- of-mιne feed preparation that normally applies is generally limited to screening out coarse, over-size material and crushing/grinding of the product to a size suitable for the subsequent treatment steps

The subsequent treatment steps are largely dictated by the chemical/mineralogical content of the somewhat upgraded run-of-mine ore Ferronickel and matte smelting can be applied to high-grade lateritic ores that have relatively low iron and high magnesium contents, whereas leaching with sulphuric acid is generally applied to low-grade lateritic ores

There are two major processing problems associated with sulphuric acid leaching of lateritic ores The first relates to the relatively high acid consumption required to achieve economically acceptable rates of nickel and cobalt dissolution, say > 95%, since the high gangue component of the lateritic ore is also a high acid consumer The second relates to the fact that nickel and cobalt dissolution is not selective with respect to concurrent iron dissolution

Nickel and cobalt separation and recovery from an iron-containing pregnant solution is metallurgically difficult to achieve Almost without exception, it is necessary to remove the bulk of the soluble iron before the soluble nickel and cobalt can separated and finally recovered One way of removing the soluble iron is to carry out the initial leaching step at elevated temperatures Under such conditions the iron-containing precipitate is generally fairly coarse and crystalline, readily settles and can be washed free of entrained leach solution At temperatures above about 160 ⁰ C advantage can be taken of the fact that not only does the iron precipitate as a readily filtered/washed product, but sulphuric acid is regenerated as part of the hydrolysis/precipitation reaction

Fe ₂ (SO ₄ ) ₃ + 3H ₂ O → Fe ₂ O ₃ + 3H ₂ SO ₄

Advantage of this reaction is taken in the so-called HPAL (high pressure acid leach) process originally commercialised at Moa Bay and now incorporated into a number of other lateπte processing facilities such as those at Vermelho and Mumn Murrin

Apart from the production of a readily filtered/washed iron-containing product, the above high temperature hydrolysis/precipitation reaction yields a product which is generally regarded as environmentally benign and can be safely discharged into a well-maintained tailings disposal facility

The potential advantages of the generation of a pregnant leach liquor with a significantly lower soluble iron content are somewhat negated by the fact that additional alkaline material is required to neutralise the excess acid produced by the above hydrolysis/precipitation reaction In a conventional HPAL circuit this is achieved by addition of limestone or another suitable alkaline material Whatever its nature, the consumption of this alkaline reagent adds to the overall operating costs of the processing facility

Since sulphuric acid is a costly consumable, it would also clearly be advantageous if the excess acid produced by the above hydrolysis/precipitation reaction could in fact be recovered and recycled back upstream to the initial leach step

We have now discovered a way in which it is possible to maximise nickel and cobalt dissolution from the lateπtic ore under conditions that minimise both sulphuric acid and alkaline consumption and the generation of a readily filtered/washed iron-containing residue suitable for safe disposal in a conventional tailings disposal facility

Summary of the Invention

According to one aspect of the present invention there is provided a method for the sulphuric acid leaching of nickel and cobalt from a lateritic ore, the method comprising the steps of

(a) separating the nickel- and cobalt-containing lateritic ore into a limonite fraction and a saprolite fraction,

(b) subjecting part of the saprolite fraction to sulphuric acid leaching at atmospheric pressure,

(c) subjecting the limonite fraction to sulphuric acid leaching at elevated temperatures and pressures,

(d) combining slurry from step (c) with slurry from step (b) together with further of the saprolite fraction to cause further leaching of nickel and cobalt from the further saprolite fraction and to neutralise acid, and

(e) subjecting slurry exiting from step (d) to a solid/liquid separation step to produce a pregnant leach liquor and a solids stream

A key feature of the present invention is the splitting of the saprolite fraction of the lateritic ore into two separate streams The first stream is subjected to leaching with concentrated sulphuric acid at atmospheric pressure, for example, in a series of agitated tanks at a temperature close to the boiling point The product from this atmospheric pressure leaching step is then mixed with the slurry, which may have been flashed and cooled, from the high pressure acid leach circuit used to treat the limonite fraction, together with a further stream of the saprolite fraction Within this particular step, the so-called saprolite neutralisation step, the second stream of the saprolite fraction is used to neutralise the excess acid generated in the limonite high pressure acid leach and the saprolite atmospheric pressure leach steps At the same time, the amount of nickel and cobalt leached from the total saprolite fraction is maximised Moreover, the amount of alkali that is required to neutralise excess acid and precipitate any soluble iron ahead of nickel and cobalt is recovered from the final pregnant leach slurry is substantially reduced

In some embodiments of the present invention, the limonite fraction may be leached in step (a) at a temperature of from about 250-260 ⁰ C The limonite fraction may be leached in step (a) with a retention time of about 40 minutes The sulphuric acid addition rate in step

(a) may be about 300-350 kg/t ore The sulphuric acid addition rate in step (a) may be adjusted to yield a target discharge solution free acid composition of about 50 g/L (before flashing, calculated at 25 ⁰ C)

In some embodiments of the present invention an amount of saprolite fraction sent to step (b) may be about 50%, on a dry weight basis, of the amount of the limonite fraction subjected to high pressure acid leaching in step (c) Step (b) may be operated at a temperature in the range of 95-100°C Step (b) may be operated with a concentrated sulphuric acid addition rate of 900-1000 kg/t ore The free acid concentration of the leached saprolite fraction in step (b) may be of the order of 35-40 g/L (calculated at 25 ⁰ C)

In some embodiments of the present invention, the amount of further saprolite added to step (d) may be about 35% of the amount of limonite processed in the high pressure acid leach circuit in step (c) Step (d) may be operated at about 90 ⁰ C Step (c) may operated with a total retention time of about 6 h Step (c) is operated such that it yields a final pregnant leach slurry with a free acid concentration of about 5-10 g/L (calculated at 25 ⁰ C)

Step (c) is operated such that at least some of the dissolved iron that goes into solution in this leaching step re-precipitates as Fe ₂ O ₃ and sulphuric acid is regenerated Thus, step (c) comprises a high pressure acid leach (HPAL) process

Desirably, the slurry from step (c) is cooled prior to mixing with the slurry from step

(b) The slurry from step (c) may be cooled by passing it through a flash vessel to cause flash cooling thereof Other cooling methods known to the person skilled in the art may also be used

The solid/liquid separation in step (e) may utilise any suitable solid/liquid separation step known to be suitable to the person skilled in the art For example, the solid/liquid separation step may comprise a multistage countercurrent decantation Other solid/liquid separation steps, such as filtration, thickening, clarification, centπfugation or settling, may also be used

Step (e) of the present invention produces a pregnant leach liquor The pregnant leach liquor from step (e) may be further treated to recover a product containing nickel and cobalt This step may involve any treatment known to the person skilled in the art for recovering a product containing nickel and cobalt The product containing nickel and cobalt may contain nickel and cobalt compounds In one embodiment, the product containing nickel and cobalt may comprise a mixed precipitate

In one embodiment the present invention further comprises the steps of (f) subjecting the pregnant leach liquor to a neutralisation and impurity removal step, and (g) precipitating a mixed nickel-cobalt sulphide from a clarified solution exiting step

(f)

In some embodiments, step (f) may comprise adding a neutralising agent to the pregnant leach liquor, optionally in the presence of air or oxygen, to oxidise any ferrous iron to ferric iron The pH of the solution at that stage may be about 3 5 The solution may be further treated to remove any soluble copper and zinc and reduce any chromιum(VI) to chromιum(lll) and manganese(IV) to manganese(ll) This may be achieved, for example, by controlled injection hydrogen sulphide gas into the pregnant leach liquor Precipitates may then be removed from the treated pregnant leach liquor using a solid/liquid separation technique

The pregnant leach liquor may then be treated to precipitate a mixed nickel-cobalt precipitate The mixed nickel-cobalt precipitate may be a mixed nickel-cobalt sulphide For example, the leach liquor may have further hydrogen sulphide gas injected into it to cause precipitation of a mixed nickel-sulphide product Alternatively, sodium sulphide may be added to the pregnant leach liquor to cause precipitation of a mixed nickel-sulphide product

In another embodiment, the pregnant leach liquor may be treated by adding further neutralising agent, such as an alkaline material, to cause precipitation of a nickel-cobalt hydroxide precipitate The neutralising agent may be limestone, although other alkaline materials, such as lime or magnesia, may also be used should they be readily or inexpensively available

The mixed nickel-cobalt precipitate may be separated from the barren liquor The mixed nickel-cobalt liquor may be further treated to recover nickel and cobalt therefrom The mixed nickel-cobalt precipitate may be transported to a refinery to recover nickel and cobalt therefrom

In other embodiments, the pregnant leach liquor may alternatively be treated to recover nickel and cobalt therefrom The solids containing stream from step (e) is suitably treated for disposal, such as in a tailings disposal facility or a landfill The solids containing stream may be treated to increase its pH prior to disposal

Detailed Description of the Invention

The following detailed description of the presently preferred but non-limiting embodiment of the invention refers to the overall flowsheet outlined in Figure 1 All those skilled in the art will appreciate that the flowsheet outlined in Figure 1 does not include a number of supplementary unit steps and process streams that are required to maintain the overall process water balance, disposal of solid and waste streams, etc

In the embodiment shown in Figure 1 , the nickel- and cobalt-containing lateπtic ore is first separated into its limonite [1] and saprolite [2] fractions based upon a detailed geometallurgical analysis and selective mining of the ore resource and is typically based upon a split based on its MgO content - Ore classified with a MgO content < 5% is generally classified as limonitic ore, whereas ore with a MgO content > 5% is generally classified as saprolite ore

The limonite fraction [1] is first subjected to a pretreatment step [3] where it is first subjected to a primary scrubber/trommel stage, then screened with the oversize being cyclone classified to remove any coarse chromite before the undersize and chromite-free component are ground in a suitable comminution circuit, such as a ball mill The ball mill product will typically have a P ₁₀₀ of 150 micron and a P ₈₀ of 50 micron and is thickened to about 40% solids and transferred to a suitable storage tank [4] The thickened slurry is then heated [5] to about 195 ⁰ C using steam [6] produced in the downstream pressure leach flash vessels [7] before being pumped into the high pressure leach circuit [8] which consists of one or more multi-stage autoclaves Air, steam and controlled quantities of concentrated sulphuric acid are added to the high pressure leach autoclaves in order to achieve the designed extent of nickel and cobalt dissolution, typically > 97%, at the specified operating temperature, typically 250-260 ⁰ C and within the specified retention time, typically about 40 minutes The sulphuric acid addition rate is typically 300-350 kg/t ore and is adjusted as required to yield a target discharge solution free acid composition of about 50 g/L (before flashing, calculated at 25 ⁰ C) The saprolite fraction [2] of the lateritic ore is subjected to a pretreatment step [9] which includes a primary scrubber/trommel stage followed removal of coarse reject material by screening The screen undersize, typically -5 mm, is then ground in a suitable comminution circuit such as a ball mill [9], to yield a final product that typically has a Pi _O o of 500 micron and a P ₈ o θf 106 micron The mill product is then thickened to about 35% solids

A specified amount of the thickened saprolite fraction [10] is then pumped to an atmospheric pressure leach circuit [11] where it is reacted with concentrated sulphuric acid The amount of thickened saprolite fraction pumped to the atmospheric pressure leach circuit [11] is typically about 50%, on a dry weight basis, of the amount of the limonite fraction [1] subjected to high pressure acid leaching [8] The actual ratio is determined by the relative compositions of the two feed materials and the operating criteria of the high pressure acid leach circuit [8]

The atmospheric pressure acid leach circuit [1 1] typically consists of one or more leach trains, each tram consisting of one ore more reactor Typical operating temperatures are in the range 95-100 ⁰ C, with a concentrated sulphuric acid addition rate of 900-1000 kg/t ore and a retention time, typically 2 h, required to achieve the design extent of nickel and cobalt from the saprolite fraction Typically the free acid concentration of the leached saprolite fraction will be of the order of 35-40 g/L (calculated at 25 ⁰ C)

The product slurry from the atmospheric pressure leach circuit is pumped to a saprolite neutralisation circuit [12] consisting of one or more trains, each train having one or more reactors Typically the saprolite neutralisation circuit will consist of 4-6 reactors Here the product slurry is mixed with

(a) the flashed slurry [13] from the high pressure acid leach circuit [8] and

(b) a specified amount of fresh saprolite slurry [14] derived from the saprolite fraction pretreatment step [9] Typically the amount of fresh saprolite slurry [14] added at this stage is typically about 35% of the amount of limonite fraction [1] processed in the high pressure acid leach circuit [8]

Typically the saprolite neutralisation circuit is operated at about 9O ⁰ C with a total retention time of about 6 h and yields a final pregnant leach slurry [15] with a free acid concentration of about 5-10 g/L (calculated at 25°C) The final pregnant leach slurry [15] is pumped to a suitable solids/liquid separation, typically a series of counter current decantation (CCD) thickeners Typically a total of seven CCD thickeners will be used to ensure maximum recovery of a clarified pregnant leach solution with minimal losses of soluble nickel and cobalt through inefficient interstage washing The overflow [16] from the first CCD thickener [17] constitutes the pregnant leach solution that is forwarded to a final solution neutralisation and solid/liquid separation step [18] where limestone and air are used oxidise any ferrous iron and yield a final liquor pH of about 3 5

As part of the final solution neutralisation and solid/liquid separation step [18] it is advantageous to remove any soluble copper and zinc and reduce any chromιum(VI) to chromιum(lll) and manganese(IV) to manganese(ll) in order to improve the quality of the final mixed nickel-cobalt sulphide product of the overall process flowsheet These impurity removal/control steps are achieved by the controlled injection of hydrogen sulphide gas, recovered as vent gas from the downstream sulphide precipitation circuit [20] and subjected to an appropriate cleaning step [21] by suitable means, such as an in-line mixer Caustic soda (NaOH) may be injected into the hydrogen sulphide using an in-line mixer as part of the cleaning step

The sulphide precipitation circuit [20] includes injection of hydrogen sulphide gas or addition of sodium sulphide to cause the precipitation of a mixed nickel-cobalt sulphide precipitate

The mixed nickel-cobalt sulphide precipitation circuit [20] incorporates facilities for the recycling of fine product as seed material recovered by cycloning [22] of the product discharged from the sulphide precipitation circuit The coarse cyclone underflow is counter currently washed, dewatered and transferred to the product handling area

The barren liquor remaining after final product recovery is stripped of any remaining hydrogen sulphide and reacted with limestone slurry to produce a CCD wash liquor [23] with a target pH of about 2 Any excess barren solution is directed to the final neutralisation circuit [24] which treats the underflow from the last CCD thickener [25] Air and lime/limestone are added to the final neutralisation circuit to raise the pH of the final slurry to about 8 5 before it is discharged to the final disposal area

In the preceding description of the invention and in the claims which follow except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i e , specify the presence of the stated features, but do not preclude the presence or addition of further features in various embodiments of the invention

It is to be understood that this invention and the preferred embodiments are not limited to the particular materials and process conditions described, as these may vary It is also understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention in any way

It is also to be noted that, as used herein, the singular forms of "a", "an" and "the" include the plural unless the context requires otherwise Unless defined otherwise, all technical and scientific terms herein have the same meanings as commonly understood by one of ordinary skill in the art to which the invention belongs