Sulphide oxidation in leaching of minerals

Abstract

A process for treating minerals containing metal sulphide and a precious metal, the process comprising fine grinding the minerals and subjecting the minerals to a first leaching step conducted under oxidising conditions at a pH of from 5 to 7, and subjecting a pulp or suspension or solid residue from the first leaching step to a second leaching step conducted under oxidising conditions at a pH of at least 9.0.

Claims

1. A process for treating minerals containing metal sulphide and a precious metal, the process comprising: fine grinding the minerals such that they have a P80 of 25 m or less and subjecting the minerals to a first leaching step conducted under oxidising conditions at a pH of from 5 to 7; subjecting a pulp or suspension or solid residue from the first leaching step to a second leaching step conducted under oxidising conditions at a pH of at least 9.0 to provide an oxidised residue; and leaching the oxidised residue with cyanide or thiosulphate to recover the precious metal.

2. The process as claimed in claim 1, wherein limestone is used to maintain pH at 5 to 7 in the first leaching step.

3. The process as claimed in claim 1, wherein the first leaching step is conducted at a pH of between 5.5 to 6.

4. The process as claimed in claim 1, wherein the minerals have a P.sub.80 of 20 m or less.

5. The process as claimed in claim 1, wherein the first leaching step is conducted under atmospheric conditions.

6. The process as claimed in claim 1, wherein the first leaching step is conducted at a temperature up to the boiling point of a slurry formed in the first leaching step.

7. The process as claimed in claim 1, wherein the minerals that are treated in the first leaching step are transferred to a second leaching reactor and the second leaching step takes place in the second leaching reactor.

8. The process as claimed in claim 1, wherein conditions in a leaching reactor or a leaching train in which the first leaching step is conducted are adjusted by increasing the pH to thereby conduct the second leaching step.

9. The process as claimed in claim 1, wherein the pH of a slurry formed in the second leaching step is raised from a nominal pH of 5.5 in the first leaching step to at least 9.0 by the addition of an alkali.

10. The process as claimed in claim 1, wherein the second leaching step is conducted at atmospheric pressure.

11. The process as claimed in claim 1, wherein the second leaching step is conducted at a temperature up to the boiling point of a slurry formed in the second leaching step.

12. The process as claimed in claim 1, wherein an oxygen containing gas is injected in the second leaching step in order to provide oxidising conditions.

13. The process as claimed in claim 1, wherein a residence time of the second leaching step is from 6 to 48 hours.

14. The process as claimed in claim 1, wherein: in the first leaching step limestone is used to maintain the pH; and in the second leaching step lime is used to maintain the pH.

15. The process as claimed in claim 7, wherein a slurry from the first leaching step flows directly to the second leaching reactor.

16. The process as claimed in claim 7, wherein in the second leaching step, oxygen is injected into the second leaching reactor and the second leaching reactor is held at a pH of at least 9.0 to further oxidise unreacted sulphides and to liberate any precious metals locked within any reaction products arising from the first leaching step.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Various embodiments of the invention will be described with reference to the following drawings, in which:

(2) FIG. 1 shows a flowsheet of a process in accordance with one embodiment of the present invention

DESCRIPTION OF EMBODIMENTS

(3) 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.

(4) In the NAL system, sulphide material is fed to the Albion Process which is described in WO 00/17407 and illustrated in FIG. 1 (Step 1), albeit with the pH in the first leaching step (step 1) being at a pH of from 5 to 7, suitably about pH 5.5. After oxidation of sulphides at around pH 5.5 the slurry can flow directly to the HAAL circuit which comprises between one and six Oxidative Leach Reactors that are added onto the main NAL Albion Process leaching train (Step 2 in FIG. 1).

(5) In the second leaching step (step 2) 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 such as lime or limestone typically being the most economical.

(6) Oxygen is injected to the base of the HAAL reactors, more favourably with the HyperSparge supersonic gas injector to maximise oxygen utilisation.

(7) The oxygen injection and elevated pH serves a number of duties in the HAAL circuit, as follows:

(8) The first is to oxidise any slow leaching sulphides hosting precious metals such as pyrite.

(9) The second is for the breakdown of iron complexes formed as reaction products of the NAL Albion Process (step 1) which lock precious metals from leaching with thiosulphate or downstream cyanidation such as jarosites.

(10) The third is for the breakdown of refractory compounds which lock precious metals from leaching with thiosulphate or downstream cyanidation such as tellurides.

(11) The residence time in the process is typically 6 to 48 hours depending on the reaction kinetics specific to each system and the HAAL leaching train may comprise a single or several additional Oxidative Leach Reactors.

(12) The process will operate autothermally with the heat of reactions driving the operating temperature. No external cooling or heating is required.

(13) The resulting slurry is then directed to the downstream process for the recovery of precious metals from the residue with known methods to those skilled in the art. These processes may include a solid/liquid separation step to separate the solid residue from the liquor, followed by cyanidation treatment of the solid residue to recover precious metals therefrom.

(14) Preferred embodiments of the process of the present invention can provide at least the following advantages:

(15) The first advantage arises in that limestone can be used in the first leaching stage (NAL) where most of the oxidation is done. Limestone is cheaper than any other alkali source. The second advantage arises in that when limestone is used, the carbonate ions generated help to stabilize some Fe.sup.3+ in solution and the action of the Fe.sup.2+/Fe.sup.3+ couple facilitate the transfer of electrons and hence oxidation making the oxidation process more effective

(16) 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.

(17) 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.

(18) 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.