SULPHIDE OXIDATION IN LEACHING OF MINERALS

Abstract

A process for leaching minerals that contain metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of a first leaching step to leach the minerals under oxidative conditions at a pH of less than 4 to form a slurry or pulp, the slurry or pulp comprising a solid phase containing unreacted components, solid reaction products and elemental sulphur, and subjecting the slurry or pulp or solid residue from the first leaching step to a second leaching step comprising oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.

Claims

1. A process for leaching minerals, the minerals containing metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of leaching the minerals under oxidative conditions at a pH of less than 4 to form a slurry or pulp, the slurry or pulp comprising a solid phase containing unreacted components, solid reaction products and elemental sulphur, and subjecting the slurry or pulp to oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.

2. The process for leaching minerals, the minerals containing metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of leaching the minerals under oxidative conditions at a pH of less than 4 to form a pregnant leach liquor containing dissolved metal and a solid residue containing unreacted components, solid reaction products and elemental sulphur, separating the solid residue from the pregnant leach liquor, and subjecting the solid residue to oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.

3. The process as claimed in claim 1, further comprising separating a leach liquor from the step of oxidative leaching at pH of at least 9.0 and recovering precious metals from the leach liquor.

4. The process as claimed in claim 1 wherein the minerals are subjected to a first leaching step conducted under oxidative conditions at a pH of less than 4, or at a pH of less than 3, or at a pH of less than 2.0 and the solid residue from the first leaching step is subjected to a second leaching step under oxidative conditions and under alkaline conditions, a pH of greater than 9.0.

5. The process as claimed in claim 1 wherein elemental sulphur is formed in the first leaching step and the elemental sulphur is separated from pregnant leach liquor with the solid residue and in the second leaching step, the elemental sulphur reacts with oxygen at the alkaline pH to form thiosulphate in-situ and the precious metals are leached into solution in the second leaching step and can be recovered from the leach liquor arising in the second leaching step.

6. The process as claimed in claim 1 wherein the slurry formed in the first leaching step is sent to the second leaching step without requiring solid/liquid separation.

7. The process as claimed in claim 6 wherein an intermediate neutralisation step is used to increase the pH of the slurry or pulp prior to feeding the slurry or pulp to the second leaching step.

8. The process as claimed in claim 1 wherein the leaching liquor from the second leaching step is separated from the solids and dissolved precious metals are recovered therefrom.

9. The process as claimed in claim 1 wherein unreacted sulphides that are present in the solid residue fed to the second leaching step are also oxidised in the second leaching step and breakdown of solid reaction products formed in the first leaching step also occurs in the second leaching step, whereby precious metals are leached into solution in the second leaching step and can be recovered from the leach liquor and elemental sulphur that is present will be destroyed.

10. The process as claimed in claim 1 wherein the first leaching step is conducted at a pH of less than 4, or at a pH of less than 3, or at a pH of less than 2.0, or at a pH of less than 1.5, or at a pH of 1.0 or less.

11. The process as claimed in claim 1 wherein the minerals that are fed to the first leaching step are finely ground.

12. The process as claimed in claim 11 wherein the minerals that are fed to the first leaching step are ground such that they have a P.sub.80 of 20 μm or less.

13. The process as claimed in claim 1 wherein the minerals comprise a sulphide mineral composition, ore or concentrate.

14. The process as claimed in claim 1 wherein the minerals are selected from one or more of chalcopyrite, bornite, enargite, pyrite, covellite, sphalerite, chalcocite, pentlandite, cobaltite, pyrrhotite or mixtures of any two or more thereof.

15. The process as claimed in claim 1 wherein sulphide minerals fed to the first leaching step are subject to fine grinding in a mill and ground to a maximum average particle size of 80% passing size of 20 microns as measured with a laser sizer, or ground to a particle size distribution having a P.sub.80 of 12 micron or less.

16. The process as claimed in claim 1 wherein the first leaching step is conducted at atmospheric pressure and at a temperature up to the boiling point of the mixture.

17. The process as claimed in claim 1 wherein oxidative leaching conditions are obtained in the first leaching step by sparging with an oxygen containing gas.

18. The process as claimed in claim 1 wherein the mixture of solid residue and pregnant leach solution from the first leaching step is separated using known liquid/solid separation technique, selected from filtration, sedimentation, clarification and the solid residue is optionally washed with wash water to remove any residual leach liquor therefrom.

19. The process as claimed in claim 18 wherein the solid residue is treated in the second leaching step in which oxidative leaching at pH of at least 9.0 occurs and unreacted sulphides that would be slow leaching in the first leaching step are also oxidised and elemental sulphur forms thiosulphate to leach precious metals into solution.

20. (canceled)

21. The process as claimed in claim 1 wherein a leach liquor containing dissolved precious metal is separated from the solid residue from the second leaching step and precious metal is recovered from the leach liquor.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0032] Various embodiments of the invention will be described with reference to the following drawings, in which:

[0033] FIG. 1 shows a flowsheet of a process in accordance with one embodiment of the present invention;

[0034] FIG. 2 shows a flowsheet of the process in accordance with another embodiment of the present invention; and

[0035] FIG. 3 shows a flowsheet of another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0036] It will be understood that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, the skilled person will appreciate that the present invention should not be considered to be limited solely to the features as shown in the drawings.

[0037] Turning to FIG. 1, the first leaching step 1 of the process involves feeding a sulphide material to an acid Albion Process, as described in WO 96/29439. After oxidation of sulphides and extraction of the soluble metals into solution, iron is removed from solution through an iron control step 2. Iron removal can be achieved using methods that are known to those skilled in the art. For example, limestone can be added to cause precipitation of insoluble iron compounds. Some gypsum may also be formed in the iron control step 2. Arsenic can also be removed in the iron control step 2, as will be known to those skilled in the art, to thereby avoid or minimise the amount of arsenic entering the second leaching step that comprises an elevated pH oxidation stage.

[0038] After iron removal a separation step occurs to separate solids from liquid. At this stage, the liquid contains valuable dissolved metals such as copper, zinc, nickel and cobalt. Any common method for solid/liquid separation technique can be employed to those skilled in the art including thickening and filtration or with a counter-current decantation (CCD) circuit (Step 3). Solid/liquid separations is important because dissolved metals will be precipitated when elevating the pH in the next step, lost to tails and potentially consume cyanide in a precious metals recovery step.

[0039] It should be noted that the iron removal step can be partially performed before and after separation of Albion Process™ leach residue solids and liquids or entirely after separation of Albion Process™ leach residue solids and liquids. The version of the flowsheet where iron removal occurs after the solid/liquid separation of the Albion Process™ leach residue is shown in FIG. 2.

[0040] The leach residue solids, now separated and washed from process liquor containing the majority of dissolved metals, is re-pulped in process water to around 30% solids and fed to the HAAL circuit which comprises between one and six Oxidative Leach Reactors (Step 4). The slurry density can be optimised to ensure the correct concentration of thiosulphate is formed in solution.

[0041] The pH is raised to at least pH 9.0 but more favourably pH 10.0 with any known alkali, with a calcium based alkali typically being most economical. For example, in the second leaching step 4, lime is added to increase the pH to at least 9.0.

[0042] Oxygen is injected to the base of the HAAL reactors with oxygen, more favourably with the HyperSparge™ supersonic gas injector to maximise oxygen utilisation. The oxygen injection and elevated pH serves a number of duties in the HAAL circuit.

[0043] The first is to oxidise any slow leaching sulphides hosting precious metals such as pyrite.

[0044] The second is to oxidise elemental sulphur to form thiosulphate which will in turn leach precious metals from the leach residue.

[0045] The third is for the breakdown of iron complexes formed as reaction products of the Albion Process™ which lock precious metals from leaching with thiosulphate or downstream cyanidation such as jarosites.

[0046] The fourth is for the breakdown of refractory compounds which lock precious metals from leaching with thiosulphate or downstream cyanidation such as tellurides.

[0047] The residence time in the process is typically 6 to 48 hours depending on the quantity of elemental sulphur generated in the Albion Process™ and the leaching kinetics of precious metals in the presence of thiosulphate. The HAAL leaching train may comprise a single or several Oxidative Leach Reactors.

[0048] The process will operate autothermally with the heat of reactions driving the operating temperature. No external cooling or heating is required.

[0049] Once the precious metals are dissolved in solution, they may be passed to a process for the recovery of the precious metals from solution such as adsorption to carbon, precipitation with known precipitating agents or adsorption to an ion exchange resin.

[0050] Additionally, the HAAL process can be performed in the presence of adsorbents in a similar way to the CIL process. This means activated carbon or ion exchange resin would also be present in the HAAL circuit to adsorb precious metals as they are solubilised. Precious metals are then recovered from adsorbents by those skilled in the art.

[0051] If the thiosulphate generation from the reaction of elemental sulphur is insufficient for complete leaching of the precious metals, then conditions can be generated where more thiosulphate is generated or added from an external source. Alternatively, the slurry can be directed to a cyanidation circuit to maximise recovery of precious metals.

[0052] In embodiments of the present invention, a separated and washed solid residue from and acidic oxidative leaching process is slurried with process water in an Albion Process™ Leach Reactor and the pH increased to at least 9.0 with an alkali. The solids density is adjusted in the re-slurrying process and with makeup water addition to ensure the thiosulphate concentration is sufficient for precious metals leaching. Oxygen is injected into the base of the reactor with a supersonic oxygen injector. Slow leaching sulphides are oxidised, resulting in liberation of precious metals for in-situ leaching and downstream leaching and recovery. Thiosulphate is generated in situ by the oxidation of elemental sulphur and will leach precious metals out of the leach residue. Precious metals locked in Albion Process™ reaction products are liberated for in-situ leaching and downstream leaching and recovery. Precious metals locked in compounds that remain refractory in the first leaching step are liberated for in-situ leaching and downstream leaching and recovery. The resulting system can be supplemented with thiosulphate by creating conditions for in-situ formation or addition of extra thiosulphate from an external source of thiosulphate, or cyanide, or directed to a cyanidation process. The process can be run in the presence of an adsorbent such as activated charcoal or an ion exchange resin to adsorb the precious metals leached into solution with thiosulphate generated from the oxidation of elemental sulphur, external supplement thiosulphate or with cyanide. The precious metals can be recovered from the adsorbents by techniques that are known to those skilled in the art.

[0053] FIG. 3 shows a process flowsheet of another embodiment of the present invention. In the flowsheet of FIG. 3, the slurry in the first leaching step 1 is not separated into separate solid and liquid fractions (as occurs in the flowsheets of FIGS. 1 and 2). Rather, the slurry from the first leaching step 1 is sent to the second leaching step 3 without undergoing solid/liquid separation. The slurry may be subject to an intermediate neutralization step 2, in which the pH of the slurry is raised by using an inexpensive neutralizing agent or alkali, such as limestone, prior to feeding the slurry to the second leaching step 3. This may reduce alkali costs in the flowsheet of FIG. 3.

[0054] In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0055] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0056] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.