PROCESS FOR THE RECOVERY OF NICKEL AND/OR COBALT FROM HIGH FERROUS CONTENT LATERITE ORES

Field of the Invention

In general, the present invention relates to a new method for recovering nickel and cobalt from less weathered or partially oxidised laterite ore which contains substantial amount of ferrous ions (Fe ⁺² ). In a preferred embodiment, the present invention provides a new process for treating less weathered or partially oxidised laterite ore which contains a substantial proportion of its iron component in ferrous form, and which involves acid leaching, oxidation of ferrous ions (Fe ⁺² ) to ferric ions (Fe ⁺³ ) and precipitation of ferric ions at atmospheric pressure. This may lead to the manufacture of nickel and cobalt- containing intermediate products such as hydroxides, carbonates and sulphides and/or the final products of ferronickel, metallic nickel or cobalt powders and nickel or cobalt cathodes.

Background of the Invention

Laterite nickel and cobalt ore deposits generally contain oxidic type ores, limonites, which are normally found at the top layer of ore body, and silicate type ores, saprolites which are normally found at the bottom layer of ore body, as two layers in the same deposit, separated by a transition zone. The state of iron in limonite and saprolite are either ferric ions (Fe ⁺³ ) and/or ferrous

(Fe ⁺² ), dependent upon the geological history of weathering or oxidation. Mineralogy investigation indicated that the less weathered or oxidised limonite contains magnetite Fe ₃ O ₄ and defined the less weathered or oxidised saprolite as the lower saprolite with greenish colour. Table 1 illustrates the chemical compositions of laterites found in various deposits around the world. The

concentration ratio of ferric to ferrous indicates the extent of weathering or oxidation.

Table 1 : Chemical Composition of Laterite Ore

Laterite ore Tot. Fe Fe +2 Mg Ni Co

Indonesian limonite 40.8 nd ^* 1 .3 1 .53 0.10

Indonesian saprolite 8.5 nd ^* 14.6 3.37 0.03

New Caledonia limonite 47.1 nd ^* 0.4 1 .33 0.16

New Caledonia saprolite 7.7 nd ^* 23.3 1 .00 0.02

Western Australia low-Mg ore 25.4 nd ^* 4.9 2.50 0.07

Western Australia high-Mg ore 10.0 nd ^* 16.6 1 .38 0.02

Southern American partially 30. 60 8. 36 3. 98 1 .38 0 .10 oxidized limonite

Southern American partially 14. 38 4. 93 15 .53 0 .96 0 .04 oxidized saprolite

Southern American partially 22 .6 6 .6 6 .9 1 .30 0 .10 oxidized laterite composite of limonite and saprolite

nd ^* : Not detected

Conventionally the higher nickel content saprolites tend to be treated by pyrometallurgical process named as RKEF process involving roasting ore inside a rotary kiln, and electrical furnace smelting techniques to produce ferro- nickel, which is rarely dependent on the weathering or oxidation extent of iron in ore. This treatment normally involves a drying step, followed by a reduction

roast step to convert the nickel oxides to nickel, and smelting in an electrical furnace. This is a highly energy intensive process. It requires a high grade saprolite source to make it economic and specific SiO ₂ /MgO concentration ratio of the feed ore for slag making in furnace operation. It also has the disadvantage that financial value of any cobalt in the ore, which is recovered into the ferro-nickel, is not realised.

Due to the high iron/nickel concentration ratio of laterite and the enormous dissolution of iron in acidic hydrometallurgical leach, the state of iron inside the ore and relevant iron treatment have significant influence on process selection and process control. As most laterite deposits were prolonged weathered or fully oxidized, the published iron treatments of acidic hydrometallurgical processes focus to the dissolved ferric ions (Fe ⁺³ ) and its precipitation/separation as hematite, jarosite, goethite or para-goethite, ferrihydrite and ferric hydroxide. On the basis of iron treatment as ferric ions (Fe ⁺³ ) only, various hydrometallurgical processes have been developed, for example PCT/AU03/00309 in the name of QNI Technology Pty Ltd and PCT/USOO/41555 in the name of BHP Minerals International Inc.

In the heap leach area, US Patent. No. 5,571 ,308 (BHP Minerals International, Inc), describes a process for heap leaching of high magnesium containing laterite ore such as saprolite. US patent no. 6,312,500 (BHP Minerals International, Inc) also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component such as nickel-containing smectite and nontronite (greater than 10% by weight). The counter-current heap leach proposed by US patent no. 6,312,500 (BHP Minerals International, Inc) precipitated significant amount of dissolved iron as hydroxides inside the heap to decrease acid consumption and Fe/Ni concentration ratio in the Product Leach Solution (PLS).

The existence of iron as ferrous ions in laterite ore has the advantage of less acid consumption because the acid consumption to dissolve ferrous (Fe ⁺² ) in laterite ore is only two thirds of the acid consumption to dissolve the same amount of ferric ions. However it is known that the precipitation behaviour of

ferrous ions is similar with that of nickel and cobalt ions. This means that for iron/nickel separation purposes, the precipitation of ferrous ions may cause nickel and cobalt loss. The co-precipitation of nickel, cobalt and ferrous ions may also cause high consumption of neutralizer reagent such as limestone, lime, magnesium carbonate, MgO, soda ash and acoustic soda.

Australian provisional application 2007902546 teaches the use of ion exchange resins (IX resin), which have much less affinity to ferrous ions than nickel and cobalt ions to treat heap leach PLS. The nickel was selectively loaded by resin and the ferrous ions (Fe ⁺² ) was expelled into raffinate. After Ni/ferrous ions separation with IX technology, the Ni-containing elution was neutralized with neutralizers such as MgO, soda ash or acoustic soda to produce nickel hydroxide as intermediate product for ferronickel manufacture.

The present invention aims to find an economic and effective process for the recovery of nickel and/or cobalt from lateritic ores, particularly those laterite ores which are less weathered or partially oxidised that include a substantial proportion of iron in its ferrous state.

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Brief Summary of the Invention

The present invention provides a process for the recovery of nickel and cobalt from sulfate solutions that contain nickel and/or cobalt in solution together with other soluble impurities such as iron. In one particular embodiment, the process is suitable for the recovery of nickel and/or cobalt from the resultant product leach solution (PLS) of an acid leach process such as heap leach, atmospheric or pressure leach process, or a combination of such processes, where a nickel and/or cobalt containing lateritic ore has been processed. The process is particularly applicable to the processing of laterite

ores that are less weathered or partially oxidised laterite ores, and as a result have a substantial proportion of the iron present in its ferrous state.

The present invention resides in a process where the ferrous ions in solution are oxidised to ferric ions and precipitated as a ferric hydroxide such as ferric hydroxide, ferrihydrite, goethite or para-goethite. In a particular embodiment, the present invention resides in a process where the ferrous ions are continuously oxidised or oxidised progressively in more than one step, resulting in precipitation of the iron firstly as a ferric hydroxide or ferrihydrite, and then following further oxidation at higher temperatures, as goethite or para-goethite.

Accordingly, the present invention resides in a process for the recovery of nickel and/or cobalt from an acidic sulfate solution that also contains iron, wherein a substantial proportion of the iron present is in the ferrous state, said process including the steps of: a) oxidising the majority of the ferrous ions in the sulfate solution to ferric ions to precipitate iron as a ferric hydroxide, ferrihydrite, goethite or para-goethite; b) separating the precipitated iron from the solution; and c) recovering the nickel and/or cobalt from the resultant solution. In a particular embodiment, the invention resides in a process wherein the step of oxidising the majority of the ferrous ions in the sulfate solution includes progressively or continuously oxidising the ferrous ions to ferric ions, by: a) oxidising the ferrous ions to ferric ions at ambient temperature and atmospheric pressure to initially precipitate iron as ferric hydroxide or ferrihydrite; and b) further oxidising the remaining ferrous ions to ferric ions at higher temperatures and atmospheric pressure, to precipitate the iron as goethite or para-goethite. In a particular embodiment of the invention, the step of oxidising the ferrous ions to ferric ions is conducted in a progressive or continuous manner.

Initially, oxidation of the sulfate solution will take place at atmospheric pressure and ambient temperature at a pH of from 1 to 3 by sparging the solution with air or a mixture of air and sulfur dioxide under pressure. The iron will precipitate as ferric hydroxide or ferrihydrite under those conditions.

The temperature of the reaction may then be elevated to be in the range of from 50°C to 90°C and the solution neutralised as the acid is consumed, such that the pH will be within the range of from 2.0 to 4.5. This will precipitate the majority of any remaining iron as goethite or para-goethite under these conditions. The pH may be raised progressively, first to a range of 2.0 to 3.5, and then to a range of 3.5 to 4.5 to progressively precipitate the iron, firstly as iron hydroxide or ferrihydrite, which is precipitated at lower pH and temperature conditions, and then as goethite or para-goethite at the elevated temperatures. At the higher pH conditions, of 3.5 to 4.5, other impurities such as aluminium and chromium will precipitate with the iron.

Preferably, limestone (calcium carbonate) soda ash, acoustic soda, magnesium carbonate or magnesium oxide are used to assist in neutralising the solution as acid is produced with ferric iron precipitation, and to stabilise the pH within the range of from 2.0 to 4.5.

Any nickel and/or cobalt that is co-precipitated with the iron following the precipitation step may be recycled to the acidic PLS and processed to recover any entrained nickel and cobalt.

The solution is suitably the product leach solution (PLS) from a heap leach, an atmospheric leach, a pressure leach or a combination of any one of these processes, wherein the ore that is being processed is a less weathered or partially oxidised laterite ore.

Any copper that is present in the PLS may be removed either prior to or subsequent to the iron precipitation steps in order to avoid detrimental effects on the subsequent specification of the final nickel and/or cobalt products. In one embodiment, the copper may be removed by an ion exchange process by selecting a resin that is selective for copper in preference to other ions such as iron, nickel and cobalt.

Accordingly, in a further embodiment, prior to the iron removal steps, any copper present in the product leach solution may be removed by contacting the PLS with an ion exchange resin to selectively absorb any copper present from the solution leaving the nickel and cobalt in the raffinate.

In a further embodiment of the invention, any copper present in the PLS may be removed by treating the PLS with an organic reagent in a solvent extraction process to selectively extract any copper present leaving the nickel and cobalt in the raffinate.

In a further embodiment, the copper may be removed from the resultant iron free solution by treating the solution to an ion exchange or solvent extraction process. The copper is removed prior to nickel and cobalt recovery processing.

The nickel and cobalt may be recovered from the resultant solution, substantially free of iron and copper ions, by precipitation as a mixed nickel/cobalt hydroxide, a mixed nickel/cobalt carbonate, a mixed nickel/cobalt sulfide, or as a high grade hydroxide or oxide or carbonate, sulfide, metallic cathode or powders with purification/separation by multi-stage neutralisation, solvent extraction or ion exchange. For example, in order to precipitate the nickel and cobalt as a mixed nickel/cobalt hydroxide, the solution is generally neutralised with magnesium oxide, soda ash or caustic soda at a pH of from 7 to 10 and a temperature of from ambient to 80°C.

Detailed Description of the Invention

The process of the present invention provides a method for treating the ferrous ion content in a product leach solution (PLS), particularly a PLS resulting from the sulfuric acid heap leaching, atmospheric or pressure leaching or combinations thereof, of less weathered or partially oxidised laterite ore. Such ore will generally have a higher content of ferrous iron when compared to the fully oxidised laterites where the iron generally exists in its ferric state.

In a preferred embodiment, the PLS, which is an acidic sulfate solution resulting from the sulfuric acid leaching of the ore, is initially oxidised at ambient temperature and atmospheric pressure and at a pH of from 1 to 3, which oxidises the ferrous to ferric and precipitates the iron as ferric hydroxide or ferrihydrite, under these temperature and pH conditions. The oxidation preferably is initiated by aerating the PLS with air under pressure, however a combination of air and sulfur dioxide could be used.

The sulfate solution is then continuously oxidised, or progressively oxidised in stages, preferably with aeration in iron neutralisation/precipitation agitation tanks such that the temperature is allowed to raise to be in the range of from 50°C to 90°C and as acid is consumed, the pH will rise, following the oxidation of ferrous ions to ferric ions as shown in Eq. 1. As more acid is released during ferric ion precipitation, a neutraliser, such as a limestone slurry, is used to stabilize the pH in the range of from 2.0 to 3.5. The pH may also progressively raise up to 4.5 in order to complete precipitation of the iron and clean other impurities such as aluminium and chromium. Under these temperatures and pH conditions, iron will precipitate as goethite or para- goethite, as shown in Eq. 2, which entrains less water and has better settling and filtration behaviour than ferric hydroxide.

2Fe ⁺² + 0.5O ₂ + 2H ⁺ = 2Fe ⁺³ + H ₂ O Eq. 1

Fe ₂ (SO ₄ )S + 3CaCO ₃ + H ₂ O = 2FeOOH + 3CaSO ₄ + 3CO ₂ Eq. 2

One particular advantage of developing a system to precipitate iron as goethite or para-goethite rather than ferric hydroxide or ferrihydrite, is that goethite or para-goethite is better crystallised and less water is entrained, which improves the solid/liquid separation behaviour and less nickel loss.

Preferably, the neutralising agent is limestone (calcium carbonate), but may be soda ash, acoustic soda, magnesium carbonate, magnesium oxide, or any other agent suitable to assist in neutralising and stabilising the pH of the acid sulfate solution.

After solid/liquid separation with thickener or CCD, the nickel/cobalt may be recovered by any conventional means such as hydroxide, carbonate or sulfide precipitation, ion exchange, or solvent extraction. In order to precipitate the nickel and cobalt as a mixed hydroxide, a preferred means is to further neutralise the nickel/cobalt containing resultant solution with another neutralising agent, such as magnesium oxide, soda ash, or acoustic soda to raise the pH to about 7 to 10 and to precipitate the mixed nickel/cobalt hydroxide from the resultant solution. The temperature of the resultant solution for this precipitation step may be from ambient to 80 °C. The mixed nickel cobalt hydroxide is an intermediate product that may be used for products such as ferro-nickel, nickel or cobalt powder, nickel or cobalt cathode, or, may be recovered as a high-grade hydroxide or oxide, sulfide, metallic cathode or powders with purification by multi-stage neutralisation/precipitation.

The PLS obtained from either a heap leach, atmospheric leach, pressure leach or a combination of such leach processes may include quantities of copper, the presence of which may have detrimental effects on subsequent nickel and cobalt recovery and quality of final products. In order to remove the copper, the PLS may be treated by an ion exchange or solvent extraction process in order to remove any copper present. The copper may be removed from the PLS by a preliminary ion exchange or solvent extraction process prior to iron removal. Alternatively, the copper may be removed from the resultant solution by ion exchange or solvent extraction subsequent to iron removal but before nickel and cobalt recovery.

The preferred preliminary ion exchange resins for copper IX are Amberlite IRC748, Bayer TP207 or DOW 4195, but other suitable resins with selectivity for copper may be used. The copper may then be stripped from the resin by sulfuric acid and rejected if in small quantities. If there is sufficient copper in the PLS to economically justify recovery, the copper removal step may be a solvent extraction step, using reagents such as LIX84, or LIX984, followed by electrowinning or cementation to recover the copper.

Brief Description of the Drawings

Figure 1 illustrates profiles of Fe _To tai, Fe ⁺² , Fe ⁺³ and pH in PLS tank with aeration.

Figure 2 illustrates a concentration profile of Fe and Ni in counter-current heap leach.

Figure 3 illustrates a concentration profile of ferrous with aeration inside agitation tanks

Figures 4 to 6 are flowsheets of preferred embodiments of the invention.

Detailed Description of the Drawings

The present invention will now be described with reference to Figure 4.

Figures 5 and 6 represent alternative embodiments to this general flowsheet.

It should be understood that these illustrations are illustrative of preferred embodiments of the invention, and the scope of the invention should not be considered to be limited thereto.

In the embodiment shown in Figure 4, the run of mine ore (1 ), which is preferably a less weathered or partially oxidised laterite ore, and has a substantial proportion of iron in its ferrous state, is acid leached in a heap leach process with sulfuric acid (2). The ore may be treated by other acid leach processes such as atmospheric or pressure leach processes or combinations thereof. This produces a product leach solution pond (3) which is sparged with air (4) under pressure.

This initial sparging with air is conducted at atmospheric pressure and ambient temperature, and at a pH of from about 1 to 3. Under these conditions, the ferrous iron will be oxidised to ferric iron, and the iron will be precipitated as a ferric hydroxide or ferrihydrite.

The ferric hydroxide- or ferrihydrite-containing slurry is transferred to agitation tanks and continued to be sparged with air (5) and the pH will rise as the acid is consumed to a pH of from 2.0 to 3.5. The pH is stabilised with a neutraliser such as calcium carbonate (6). The temperature is maintained in the range of 50 °C to 90 °C. After solid/liquid separation with thickener or CCD (7), ferric iron precipitation (8) is discharged and the overflow solution is continued to be aerated by the addition of air (9), and the pH of the solution would generally rise to a pH of from about 3.5 to 4.5 with calcium carbonate (10). The temperature would remain to be within the range of from 50 °C to 90 °C. This would generally precipitate remaining ferric ion as goethite or para- goethite and other impurities such as aluminium and chromium at the higher pH.

With solid/liquid separation, any nickel and/or cobalt co-precipitated with the iron may be recirculated (1 1 ) to the acidic PLS pond (3) to enable recovery of the nickel and/or cobalt.

The resultant solution (12), which is now substantially free of iron is then further neutralised, for example with magnesium oxide (13) to precipitate the nickel and/or cobalt as a mixed nickel/cobalt hydroxide (27). This is conducted at a temperature, preferably in the range of from ambient to 80 °C, and at a pH of from about 7 to 10.

The nickel barren solution, which has a pH in the range of from 7 to 10 following nickel and/or cobalt recovery, is recirculated to the leach process (14).

Figure 5 illustrates an alternative embodiment wherein copper is removed by either ion exchange or solvent extraction (15) prior to iron precipitation. The copper is rejected as waste, or if there is sufficient copper present, it is converted to a copper product (16).

The nickel, cobalt, ferrous iron and any other acid soluble impurities will remain in the raffinate, whereby it will then be subjected to the continuous and/or progressive oxidation and neutralisation steps to oxidise the ferrous to

ferric ions and precipitate the iron as ferric hydroxide, ferrihydrite, goethite or para-goethite in the manner described with reference to Figure 4.

Any nickel that is co-precipitated with the iron may be recirculated (17) and redissolved in the raffinate at a pH of about 1 , and at a temperature of from ambient to 80 °C.

In this embodiment, the nickel and cobalt is recovered as a mixed nickel cobalt sulfide (18) following the addition of hydrogen sulfide, sodium sulphide or sodium bisulphide (19). The nickel and cobalt barren solution may then be recirculated (20) to the initial leach step.

Figure 6 illustrates a process similar to that of Figure 5, except that the nickel and cobalt is recovered from the solution in a solvent extraction (SX) process (21 ) using a solvent such as Versatic Acid. The cobalt is then stripped from the nickel and cobalt leach solution using a solvent such as Cyanex 272 (22) and then stripped from the cobalt leach solution with sulfuric acid (23) and recovered as a cobalt product (24).

The nickel is then recovered from the cobalt barren solution by nickel electrowinning (25). The spent electrolyte (26) is recycled back to Ni/Co SX (21 ) as the strip solution. With this process nickel is recovered as nickel cathode.

Examples

Example 1 : Ferrous ions oxidation and ferric ions precipitation at ambient temperature inside PLS storage tank

The column leaching PLS in store tank contained 1 ,910 mg/L Ni and 24,606 mg/L total iron which included 17,966 mg/L Fe ⁺² and 6,640 mg/L Fe ⁺³ . The pH was measured as 2.20 at ambient temperature. Pressure air was sparged into the tank to oxidise ferrous to ferric ions. With aeration the original clear PLS was turn to colloid. As shown in Figure 1 , the concentrations of total iron, ferrous ion and ferric ion dropped and the pH increased. This indicated that ferrous ion was oxidized to ferric and then precipitated. As acid was

consumed during this conversion according to Eq. 1 , a part of original existing ferric ion was also precipitated. These reactions approached equilibrium after 24 hours. No nickel loss occurred during ferrous oxidation/precipitation as shown in the horizontal nickel line in Figure 1.

Example 2: Iron Precipitation inside Heap with Counter-current Heap Leach

Two heaps, defined as secondary leaching heap and primary leaching heap, were built consecutively and operated with counter-current leaching style. The initial fraction of PLS of secondary heap with low acidity and iron concentration was directly fed to downstream plant to make Ni/Co mixed hydroxide precipitation (MHP). After acid and/or iron breakthrough, the PLS of secondary heap was defined as ILS (intermediate leachate solution) and fed into the primary heap. As shown in Figure 2, the iron including ferric and ferrous iron in ILS was precipitated inside the primary heap without nickel loss. The discharged PLS from primary heap was then sent to downstream plant to make Ni/Co mixed hydroxide precipitation (MHP). As the acidity and iron concentration was controlled at minimum level, the counter-current heap leach decreased the consumption of acid and neutralizer.

Example 3: Ferrous ions oxidation and ferric ions precipitation at temperature range of 50 ⁰ C -90 °C and pH 2.0-3.5 inside agitation tanks

With continuous operation heap leaching PLS was fed into a series of agitation tank namely CSTR (Completely Stirred Tank Reactor). The reaction temperature was controlled in the range of 50-90 °C. Aeration was applied in tank to oxidize ferrous to ferric. The pH was controlled in the range of 2.5-3.5 with limestone slurry to precipitate ferric ions as goethite or para-goethite. Figure 3 illustrates the concentration profiles of total feed iron, total out iron, feed ferrous and out ferrous. It was observed that more than half of the ferrous ions were converted to ferric ions and almost all ferric ions were precipitated. The nickel loss in this iron neutralization/precipitation operation was less than 3%.

The obtained ferric ion-free nickel/cobalt solution can be used to produce the mixed nickel/cobalt hydroxide precipitation (MHP) or mixed nickel/cobalt sulphide precipitation (MSP) with conventional neutralization and sulfidation respectively. It also can be used as feed solution for SX process in which the organic reagent can separate nickel and cobalt from ferrous ions and other impurities, or an ion exchange process for the recovery of nickel and/or cobalt.

Example 4: Production of Nickel-containing Hydroxide

The iron-precipitated slurry was sent to thickener for solid/liquid separation. The overflow of thickener was mixed with MgO slurry in a series of agitation tanks (CSTR) to precipitate nickel and cobalt as mixed hydroxide precipitation (MHP) at pH7-10. No extra heat was provided for this unit operation so that the reaction temperature was around 30-60° C. The MHP slurry was fed to thickener for solid/separation separation. The thickener underflow was sent to a pressure frame filter to produce MHP cake. Table 2 shows the chemical composition of MHP cakes. The Ni/Fe concentration ratio of MHP is flexible by controlling the iron precipitation efficiency depending upon the specification of final product e.g. ferronickel.

Table 2: Composition (%) of MHP (Mixed Hydroxide Precipitation)

Samples