AU2012225192A1 - Process for leaching metals from laterite ores - Google Patents

Abstract

A process for recovery of one or more base metal values from a laterite ore is disclosed The laterite ore may comprise limonite and/or saprolite, and the process has the steps of contacting the laterite ore with an organic acid to form a slurry, allowing the slurry to react at an elevated temperature of from 60°C to 200°C for a period not exceeding six hours, and separating the aqueous solution containing metal values from the solids in the slurry.

Description

WO 2012/119187 PCT/AU2012/000205 1 "Process for Leaching Metals from Laterite Ores" Field of the Invention This invention relates to a process for leaching metals from laterite ores. This invention in particular relates to a process for leaching nickel and/or cobalt from 5 laterite ores containing such metals. Background Art The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material 10 referred to was part of the common general knowledge as at the priority date of the application. Throughout the specification unless the context requires otherwise, the word ''comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion 15 of any other integer or group of integers. Nickel metal is sourced from two distinct ore types: nickel sulphides and nickel laterite. Sulphide nickel has dominated world production throughout history. However, current production trends suggests that production from nickel laterites will soon dominate world production as fewer high grade sulphide nickel deposits 20 are being discovered and existing sulphide reserves are being depleted. Known nickel laterite resources far exceed known nickel sulphide resources. However the costs of the recovery of nickel from laterite ores are significantly higher than the costs of recovery of nickel from sulphide ores. The additional costs are associated with a more complex processing flow sheet for laterite ores 25 than sulphide ores. However, given the shallow surficial nature of nickel laterite deposits and the relative ease of mining compared with mining of deep hard rock sulphide nickel ores, nickel laterites are becoming a sought after source of nickel. This is particularly true with the development of alternative technological approaches in metallurgical processing of nickel laterite ores.

WO 2012/119187 PCT/AU2012/000205 2 Nickel laterites are formed by weathering processes over ultramafic rocks containing abundant olivine and orthopyroxene with high nickel contents. The alteration of these minerals is by hydration to amorphous silica, hydrated iron oxides (goethite) and hydrous magnesium silicates. Some nickel concentration is 5 associated with the goethite, but the highest nickel concentrations are typically associated with close spaced fractures or jointing in the bedrock, which is where maximum ground water circulation and fluid-rock interaction takes place. Lateritic nickel deposits generally consist of two main zones: the limonite zone is the shallower part of the laterite profile and is dominated by goethite (FeOOH) 10 and manganese typically containing up to 1.2% nickel and 0.1% cobalt; and the lower saprolite zone which is characterized by high magnesium silicates, and less goethite. The nickel grade within the saprolite zone is typically higher than within the limonite zone, with nickel typically ranging from 1% to 2%; however the cobalt is often less than 0.04%. The saprolite horizon consists up an upper clay rich 15 transition layer and the lower portion which is dominated by less weathered bedrock remnants known as corestones which have resulted from the increased weathering along joints or fractures within the bedrock. The higher nickel grades are associated with the outer weathered rim of these corestones. The clay content of the saprolite profile decreases with depth towards the less weathered 20 bedrock. The typical chemical concentrations of tropical laterite ore types are shown below in Table 1. The transition zone has an MgO content of greater than 6% and is hence typically considered part of the saprolite profile. Ore type % Ni % Co % Fe % MgO % SiO 2 Limonite zone 0.5-1.5 0.08-0.15 35-50 < 6 <20 Transition zone 0.5-2.0 0.05-0.10 10-30 6-35 20-40 Saprolite zone 0.2 -2.5 0.01 - 0.04 5 - 10 >35 >40 25 Table 1: Chemical composition of the main horizons within a typical nickel laterite deposit. The nickel grade in the less weathered ultramafic material within the corestones is typically 0.2 to 0.4%, however the nickel grade of the outer weathered rim of the WO 2012/119187 PCT/AU2012/000205 3 corestone and the interstitial clay material between the corestones in some instances can exceed 2.5%. Both pyrometallurgical and hydrometallurgical processes are used in the recovery of nickel and cobalt from laterite ores. Pyrometallurgical techniques are typically 5 used for the processing of saprolite ores, which involves smelting of the ore in an electric furnace to produce a ferro-nickel (Fe-Ni) metal. Due to the high associated capital and operating cost of this process, high grade nickel ores (ideally 2.2 - 2.4%Ni) are required for economic viability. During recent years the higher nickel price has led to the production of nickel pig iron using low grade 10 limonite ores (<1.5% Ni), however the high operating costs of this process mean that this process is only viable during high nickel prices. Hydrometallurgical processes are more applicable to limonite ores with high pressure acid leaching (HPAL) utilizing sulphuric acid being the most common hydrometallurgical processes being commercially utilized. This process is not 15 suitable for the treatment of the saprolite ore as the high magnesium content (>6% MgO) results in a high acid consumption. MgO consumes sulphuric acid preferentially to the nickel and cobalt. As a result, processing of high MgO saprolite is not economically viable due to the high cost of sulphuric acid. As the associated capital and operating costs of an HPAL process plant are high, 20 economies of scale are required in order to reduce unit costs. The majority of HPAL plants built within the last few years have been designed for 30,000 to 60,000 tonnes per annum nickel production at a capital cost of US$2 to 4 billion, with an estimated operating cost of US$3.00 to US$4.50 per pound of nickel. Operating costs are highly sensitive to ore grades and acid costs. 25 Throughout the specification unless the context requires otherwise, the word "include" or variations such as "includes" or "including", 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.

WO 2012/119187 PCT/AU2012/000205 4 Disclosure of the invention The process developed by the inventors provides a hydrometallurgical process for bioleaching nickel and other soluble metals from laterite ores utilizing a bio renewable citric acid. The process has applicability in laterite ores containing, in 5 particular, nickel and cobalt. The process is carried out within a temperature range of 600C to 200C, and at a pH that is sufficiently low to keep the soluble moieties in solution, which would typically be a pH of less than 2. In the case of nickel bearing laterites, the process can be applied to both the goethitic (limonite) and magnesium rich (saprolite) portions of a nickel bearing 10 laterite ore. The pH conditions of the slurry should be monitored and stabilized at less than 2, for maximum extraction. The recovery of the soluble metals from the citrate solution can be by a number of common techniques used in the industry such as solvent extraction and ion exchange. 15 Specifically in accordance with one aspect of the invention there is provided a process for recovery of one or more base metal values from a laterite ore containing the same, said process having the steps of contacting the laterite ore with an organic acid to form a slurry, allowing the slurry to react at an elevated temperature of from 600C to 2000C for a period not exceeding six hours, and 20 separating the aqueous solution containing said metal values from the solids in said slurry. The period that the slurry is allowed to react may in practice be more or less than one hour, depending upon the laterite ore being processed, but enhanced extraction rates are achieved, compared with conducting an equivalent process at 25 ambient temperature. Depending on the ore type, the reaction time may be from as little as 5 minutes, but is typically from 30 minutes to two to three or four hours, or from 45 to 60 minutes to 90 to 120 minutes. The organic acid may advantageously be a carboxylic acid. In one embodiment the carboxylic acid may be a chelating agent such as a dicarboxylic acid, a 30 tricarboxylic acid such as citric acid, or a tetracarboxylic acid such as EDTA. The most preferred carboxylic acid is citric acid.

WO 2012/119187 PCT/AU2012/000205 5 It is preferred that in the process, a slurry having a pH of 2.5 or lower. In the most preferred arrangement, a slurry formed in the process has a pH of 2 or lower. It has been found that this leads to greater yields of extracted metal values. 5 The elevated temperature may be above 100 0 C, and particular advantage has been found at temperatures of from 1300C to 1800C, with a nominal 1650C showing particularly high yields with certain lateritic ores. In fact, with limonite alone, a temperature of 1650C shows very high yields whereas limonite is amenable to leaching at ambient temperatures and not particularly amenable to 10 leaching at temperatures below 1000C. Saprolite is amenable to leaching at temperatures above 60'C, and has been found to be particularly amenable to leaching at temperatures above 100'C, including at 165 0 C. Since saprolite leaches well at temperatures from 600C to 1000C there had never in the past been reason to explore superheated temperatures. However, with the realisation 15 that both limonite and saprolite leach well at temperatures above 1000C and particularly at 1650C, there has come the realisation through experimental data that a mixture of these ores may be effectively leached at superheated temperatures, without any adverse yield effects being noted, arising from the mixture of ores. 20 Nevertheless, it is appreciated that there may be ores that due to chemistry reasons require a two-step process, and thus, in accordance with a preferred feature the process may performed in more than one step, with a first step comprising contacting the laterite ore with an organic acid to form a slurry, allowing the slurry to react at a superheated temperature and at elevated 25 pressure (above atmospheric pressure) for a period not exceeding six hours, and separating the aqueous solution containing said metal values from the solids in said slurry; then subjecting the recovered solids to a second step in which said solids are contacted with an organic acid to form a further slurry, allowing the further slurry to react at elevated temperature from 600C at atmospheric pressure, 30 and separating the aqueous solution containing said metal values from the solids in said further slurry.

WO 2012/119187 PCT/AU2012/000205 6 Alternatively, in accordance with a preferred feature the process is performed in more than one step, with a first step comprising contacting the laterite ore with an organic acid to form a slurry, allowing the slurry to react at elevated temperature from 600C at atmospheric pressure for a period not exceeding six hours, and 5 separating the aqueous solution containing said metal values from the solids in said slurry; then subjecting the recovered solids to a second step in which said solids are contacted with an organic acid to form a further slurry, allowing the further slurry to react at a superheated temperature and at elevated pressure (above atmospheric pressure), and separating the aqueous solution containing 10 said metal values from the solids in said further slurry. The period that the slurry is allowed to react may in practice range be more or less than one hour, depending upon the laterite ore (or solids) being processed, but enhanced extraction rates are achieved, compared with conducting an equivalent process at ambient temperature. 15 The organic acid may advantageously be a carboxylic acid. In one embodiment the carboxylic acid may be a chelating agent such as a dicarboxylic acid, a tricarboxylic acid such as citric acid, or a tetracarboxylic acid such as EDTA. The most preferred carboxylic acid is citric acid. It is preferred that in the process, a slurry having a pH of 2.5 or lower. 20 In the most preferred arrangement, a slurry formed the in process has a pH of 2 or lower. It has been have found that this leads to greater yields of extracted metal values. The superheated temperature is above 1000C, and particular advantage has been found at temperatures of from 130'C to 1800C, with a nominal 1650C showing 25 particularly high yields with certain lateritic ores. The base metal contained in the laterite ore may predominantly comprise nickel. There may also be some cobalt present and possibly aminor amount of magnesium. The process is particularly suited to extraction of base metals from saprolite and 30 limonite.

WO 2012/119187 PCT/AU2012/000205 7 Brief Description of the Drawings Two preferred embodiments of the invention will now be described with reference to the drawings, in which: Figure 1 which is a brief schematic flowchart for extraction of metal 5 values including nickel, Figure 2 which is a more detailed schematic flowchart for a plant for extraction of metal values including nickel from limonite and saprolite ores according to a first embodiment; and Figure 3 which is a detailed schematic flowchart for extraction of metal 10 values including nickel, according to a second embodiment. Best Mode(s) for Carrying Out the Invention The embodiments provide a hydrometallurgical process of leaching nickel and other metal values from ores utilizing bio renewable citric acid to extract the metals. 15 Referring to figures 1 and 2, lateritic ore comprising a mixture of limonite and saprolite is mined and stockpiled. Coarse material is rejected using a wet trommel, and the fines are thickened before being subject to a pressure leach at a temperature of 165*C with citric acid sufficient to maintain the pH at 2 or lower, for a nominal period of one hour. 20 On completion of the pressure leach, solids are separated using a counter current decantation wash, and the recovered citrate solution is neutralised to a pH of 3 using limestone slurry. Iron oxide residue is separated and the citrate solution undergoes solvent extraction to recover nickel and cobalt hydroxides. The remaining solution, comprising citric acid and magnesium citrate is subjected to 25 electrolysis to recover magnesium and the citric acid is recycled for re-use. Citric acid is a relatively benign acid and is readily produced from the fermentation of an organic carbohydrate source such as cassava. Citric acid is a bio renewable acid and is relatively inexpensive to produce. The use of citric acid is preferred WO 2012/119187 PCT/AU2012/000205 8 over mineral acids such as sulphuric acid, as it presents a lower risk to the environment. In the process soluble metals, in particular nickel and cobalt, are leached from laterite ores, within a temperature and pH controlled enclosed vessel using citric 5 acid. The temperature is elevated to between 60*C to 2000C and the pH of the slurry is controlled at less than 2 during the leaching process. Where the temperature is below 100*C, it will be understood that the leaching process is not a pressure leach, and indeed the leaching vessel need not be closed in such circumstances. However at temperatures in the range of from 600C to 1000C, 10 only saprolite ore is amenable to leaching with any reasonable recovery of metal values. The elevated temperature and the controlled pH conditions are the key aspects of the leaching process which enables metal extraction from both the limonite and saprolite ores. Recovery of the soluble metals from the citrate solution can be by a number of 15 methods including pH controlled precipitation and filtration, solvent extraction, by methods such as electrowinning, or by ion exchange using resins, all of which are common techniques used in the industry. Leaching at ambient temperatures [20 to 300C] achieved low nickel extractions for limonite ore types and moderate to low nickel extractions for saprolite ore types 20 using similar pH conditions. Increasing temperature in the range 60 to 1700C, and possibly higher, had a significant increase in nickel extractions for both limonite and saprolite ore types for similar pH conditions, pH less than 2. The tables below show a comparison of leaching at ambient temperatures [20 to 30'C] with pressure leaching at 1650C, with a pressure vessel residence time of one hour. Ni Ore Type Head Assay Ambient Leach High Temperature Extraction Leach Extraction Limonite 0.76 6 50 Limonite 1.08 7 55 WO 2012/119187 PCT/AU2012/000205 9 Limonite 1.37 13 72 Limonite 1.51 14 75 Saprolite 1.21 16 93 Saprolite 0.95 18 78 Saprolite 0.93 20 84 Saprolite 0.84 21 90 Co Ore Type Head Assay Ambient Leach High Temperature Extraction Leach Extraction Limonite 0.14 39 100 Limonite 0.13 54 100 Limonite 0.09 67 100 Limonite 0.08 58 100 Saprolite 0.05 60 89 Saprolite 0.03 78 94 Saprolite 0.02 71 88 Saprolite 0.04 55 79 Mg Ore Type Head Assay Ambient Leach High Temperature Extraction Leach Extraction Limonite 0.51 2 16 Limonite 0.43 2 17 WO 2012/119187 PCT/AU2012/000205 10 Limonite 2.21 7 33 Limonite 5.41 5 26 Saprolite 12.42 4 43 Saprolite 17.91 7 74 Saprolite 20.45 7 65 Saprolite 16.95 9 70 In order to leach the nickel laterite ores at temperatures above 1000C, the leach vessel may be an agitated autoclave operating in either a batch or continuous basis. Leaching in the temperature range of 600C to 1000C could be undertaken 5 in agitated tanks, in either a batch or continuous basis. Materials of construction for these vessels or tanks could be a range of stainless steels or other suitable materials to handle citric acid solutions. Leaching in the temperature range of 60'C to 1000C is suitable only for saprolite since limonite is not amenable to leaching in this temperature range. Saprolite 10 has been found to leach with greater than 80% recovery of nickel values within 4 hours at 900C. An alternative arrangement for leaching may be a train of pressurised agitated slurry tanks that operate on a continuous basis and subject the slurry to a progressively lowering temperature and pressure as the slurry proceeds through 15 the train. In such an arrangement, the final stages operating at ambient pressure may be open agitated tanks. The residence time for one ore type, limonite, tested using the process, was 60 minutes. However, other ore types may take longer or shorter periods to achieve the necessary extraction. Particularly, saprolite has been found to leach with 95% 20 recovery of metal values in 30 minutes at 165 C. Referring to figure 3, an alternative embodiment is shown which differs in the leach parameters, with leaching being carried out in a batch autoclave process, at 1650C with a residence time of ninety minutes. With these parameters greater than 80% Ni recovery from limonite and greater than 95% Ni recovery from WO 2012/119187 PCT/AU2012/000205 11 saprolite was achieved. Figure 3 differs also in a minor way, in the recovery of the Ni Co and Mg values after the initial leaching using citric acid, showing electrowinning of Ni and Co. After leaching using citric acid the leached slurry is separated into leachate, 5 solution containing the nickel and cobalt citrate solution, and tailings. This may be undertaken using one or a number of known solid liquid separation techniques other than that described above, such as thickeners, filters or other devices. The tailings can be neutralised, if required, and disposed of in a suitable tailings facility. 10 The leaching process as described has been developed in order to address a number of issues found in known techniques and methods. The most important of these is the inability of the metallurgical technologies to process both the limonite and saprolite horizons within the laterite deposit. There is also the prohibitive cost of existing metallurgical process options, and environmental 15 concerns of using sulphuric or similar highly reactive acids in remote greenfield locations. Having described the invention, it will be appreciated that changes may be made to the process without departing from the spirit and scope of the invention. It should be appreciated that the scope of the invention is not limited to the specific 20 embodiment disclosed.

Claims (18)

1. A process for recovery of one or more base metal values from a laterite ore containing the same, said process having the steps of contacting the laterite ore with an organic acid to form a slurry, allowing the slurry to react at an elevated 5 temperature of from 600C to 2000C for a period not exceeding six hours, and separating the aqueous solution containing said metal values from the solids in said slurry.

2. A process as claimed in claim 1 wherein the period that the slurry is allowed to react is no more than four hours. 10

3. A process as claimed in claim 1 wherein the period that the slurry is allowed to react is no more than three hours.

4. A process as claimed in claim 1 wherein the period that the slurry is allowed to react is from five minutes to no more than two hours.

5. A process as claimed in claim 1 wherein the period that the slurry is allowed to 15 react is from 30 minutes to 90 minutes.

6. A process as claimed in claim 1 wherein the period that the slurry is allowed to react is from 60 minutes to 90 minutes.

7. A process as claimed in any one of the preceding claims wherein the organic acid may is a carboxylic acid. 20

8. A process as claimed in claim 7 wherein the carboxylic acid is a chelating agent selected from the group comprising a dicarboxylic acid, a tricarboxylic acid and a tetracarboxylic acid.

9. A process as claimed in claim 7 wherein the carboxylic acid is citric acid.

10. A process as claimed in any one of the preceding claims wherein in the 25 process, the formed slurry has a pH of 2.5 or lower.

11. A process as claimed in claim 10 wherein in the process, the formed slurry has a pH of 2 or lower.

12. A process as claimed in any one of the preceding claims wherein the elevated temperature is above 100*C. WO 2012/119187 PCT/AU2012/000205 13

13. A process as claimed in claim 12 wherein the elevated temperature is from 1300C to 1800C.

14. A process as claimed in claim 13 wherein the elevated temperature is about 165 0 C. 5

15. A process as claimed in any one of the preceding claims wherein the base metal contained in the laterite ore predominantly comprises nickel.

16. A process as claimed in claim 15 where there is some cobalt present and a minor amount of magnesium.

17. A process as claimed in any one of claims 1 to 14 wherein the laterite ore 10 comprises saprolite and limonite.

18. A process substantially as herein described, with reference to the drawings.