AU642087B2 - A process for leaching gold and silver - Google Patents

Abstract

In the so-called cyanidation-in-mill technique, the ores, ore concentrates and gangue material, which are to be leached, from previous incomplete leachings are treated with a cyanide-containing leaching solution already in the mill, gold and silver being leached out at least partially. As compared with the previously known cyanidation-in-mill technique in the presence of hydrogen peroxide, the gold yield can be substantially increased if the leaching is carried out in the presence of a peroxoborate compound. The peroxoborate can be present in a solid and/or dissolved form. Preferably 10 to 60 equivalents of peroxoborate are used per tonne of the material to be leached.

Description

S F Ref: 208852

AUSTRALIA

PATENTS ACT 19 4 28 COMPLETE FOR A STANDARD PATENT

ORIGINAL

I I o r s a Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Degussa Akt'engesellschaft 9, Weissfraunstrasse D-6000 Frankfurt am Main

GERMANY

Jurgen Lorosch and Annette Ziegler Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, Nev South Hales, 2000, Australia A Process for Leaching Gold and Silver S. S S

S.

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a

S

The following statement is a full description of this invention, including the best method of performing it known to me/us:- 555555 5845/3 1 91 136 PV A process for leaching gold and silver Description This invention relates to a process for leaching gold and silver from ores, ore concentrates and waste material from previous incomplete leachings by grinding of the material to be leached in the presence of a cyanide-containing leaching solution which has a pH value of 8 to 13 and which contains a peroxo compound.

The leaching of noble metals forming cyano complexes, particularly gold and silver, from ores, ore concentrates and other particnlate materials, such as for example waste material from previous incomplete leachings, using a cyanide-containing alkaline leaching solution and an oxidizing agent has long been known and has been practised on an industrial scale for more than 100 years.

In so-called heap leaching, the ore precrushed to a 15 particle size of 10 to 50 mm is heaped up and sprayed with an aqueous cyanide solution. Atmospheric oxygen is normally used as the oxidizing agent, although peroxo compounds may also be added to the ore heap or to the leaching solution (DE-PS 38 30 703, German patent application P 17 899.4, GB-A 2,219,474.

In so-called agitation leaching, finely ground ore is leached in tanks or other vessels. The gold recovery rate is higher in this process than in heap leaching, although the process costs are also significantly higher. In 25 agitation leaching, the ore to be leached has to be ground on average to a particle size of smaller than 100 Am. In most cases, grinding has to be carried out to the extent where at least 80% of the particles are smaller than 45 Am in size.

Known discontinuous and continuous wet mills, more particularly ball mills and rod mills, are used to grind the ore. Grinding is carried out in a single stage, but 2 91 136 PV mostly in two or more stages, any oversize particles always being recycled. Cyanide and, optionally, other leaching chemicals are then added to the resulting pulp of the ground ore and,. generally, water, the whole then being delivered to the leaching tanks.

In agitation leaching, the leaching time in cases where air is used for aeration is generally between 15 and hours, but can be considerably shortened by using peroxo compounds, such as hydrogen peroxide, calcium peroxide or peroxoborates, instead of or in addition to the air (DE-PS 36 37 082, DE-PS-38 30 703, German patent application P 17 899.4.

For some time now, certain mines have been using the so-called cyanidation-in-mill technique. In this method of leaching, cyanide is added before or during grinding and the residence time of the ore in the mill is used to shorten the leaching time in the tanks used for agitation leaching. Whereas passable results can actually be obtained in the mill in the leaching of fully oxidized ores, 20 this method is not really suitable for the leaching of sulfidic ores because, in their case, reductive conditions prevail in the mill and too little, if any, oxygen is available for leaching. To make sulfide-containing ores ,more accessible to the cyanidation-in-mill technique, 25 oxidizing agents have also been used by various mines.

Whereas no improvement could be obtained with pure oxygen, the gold yields in the mill could be increased and the consumption of NaCN reduced by using hydrogen peroxide (CA 102(14): 117216r; Smith, M.E. et al. in Proc. SME Fall S 30 Meeting 1983, 41, 43-49, ed. Hiskey, J. Brent, Soc. Min.

Eng. AIME Littleton, Colo.).

The problem addressed by the present invention was to improve the cyanidation-in-mill technique to the extent that an even higher gold yield could be obtained in relation to the use of hydrogen peroxide. Accordingly, it 91 136 PV would be possible not only further to shorten the overall leaching time (time required for grinding and subsequent agitation leaching) for a high gold yield, but also where possible to dispense with the need for agitation leaching.

This problem has been solved by a process for leaching gold and silver from ores, ore concentrates and waste material from previous incomplete leachings by grinding of the material to be leached in the presence of a cyanidecontaining leaching solution which has a pH value of 8 to 13 and which contains a peroxo compound, characterized in that at least one peroxoborate compound is present as the peroxo compound.

It is crucial to the process according to the invention that an effective quantity of peroxoborate be present in dissolved and/or very finely divided form. The total quantity of peroxoborate required may be present in the mill from the beginning of the leaching process. Alternatively, the peroxoborate is added during leaching in one or 20 more portions, for example before each grinding stage of a multistage grinding process, or is formed in situ from an added borate and hydrogen peroxide. The addition of a peroxoborate is preferred to formation in situ in the i leaching pulp. The peroxoborate may be added in powder S 25 form or as an aqueous solution or suspension to the material to be leached, to the cyanide-containing leaching solution or to the leaching pulp.

The effective quantity of peroxoborate, which depends to a large extent on the material to be leached, may 30 readily be determined by preliminary tests. A quantity of 1 to 100 equivalents peroxoborate, expressed as B0 3 per t of the material to be leached will generally be sufficient.

The quantity of peroxoborate added is preferably between and 60 equivalents. As active oxygen compounds, peroxoborates and the hydrogen peroxide formed by hydrolysis give 91 136 PV off oxygen. The oxygen, the hydrogen peroxide and the peroxoborate may serve as oxidizing agents in the leaching process.

Suitable peroxoborates, by which are also meant hydrates thereof, are those of the alkali and alkaline earth metals, although peroxoborates of other metals, such as zinc for example, may also be used in principle.

Commercially available peroxoborates, namely the so-called sodium perborate mono- and tetrahydrate and also calcium perborate, may be used with particular advantage. Socalled superoxidized perborates, which are known from DE- OS 28 11 554 and from DE-OS 35 05 158, may also be used.

In the same way as heap and agitation leaching, cyanidation-in-m'.ll leaching is also carried out at a pH value of 8 to 13 and preferably at a pH value of 9 to 12.

The pH value is adjusted in known manner with alkalis, such as in particular milk of lime and sodium hydroxide. It may be advisable during the grinding process to adapt the pH to values recognized as optimal.

20 The cyanide content, expressed as CN, in the leaching solution is normally between 0.005 and 2.5% by weight.

Cyanide contents of 0.02 to 0.2% by weight are preferred.

Cyanide is used in the form of alkali metal cyanide, more Sparticularly sodium cyanide, or in the form of calcium 25 cyanide, for example in the form of so-called black} cyanide. The cyanide may be added in solid or dissolved form to a water-based pulp containing the material to be leached or, alternatively, a cyanide-containing leaching solution is directly used for the preparation of the leaching pulp.

Known wet grinding units may be used for leaching in accordance with the invention, ball mills and rod mills being preferred. The grinding time depends on the material to be leached and the desired degree of extraction. The grinding time is normally between 15 minutes and 2 hours.

The solids concentration of the leaching pulp during 91 136 PV grinding may vary within wide limits and is normally between 25 and 60% by weight. The degree of grinding may be in the range typical of agitation leaching.

On completion of grinding, the leaching pulp may if necessary be subjected to agitation leaching or to pressure leaching in order further to increase the extraction level.

Otherwise the noble metal/cyano complexes present in dissolved form in the leaching pump may be separated off for the further extraction of noble metal by known processes, for example by the carbon-in-pulp (CIP) and resin-inpulp (RIP) processes and by the Merrill-Crowe process.

SWhereas the extraction kinetics observed during cyanidation-in-mill leaching are similar to those observed during agitation leaching in accordance with DE-PS 36 37 082 where hydrogen peroxide is used as the oxidation agent, extraction is surprisingly accelerated by using an equivalent quantity of a peroxoborate. The gold yield is thus increased for the same leaching time. In some cases, therefore, there is no need for agitation leaching after 20 leaching in the mill. In addition, the use of peroxoborate Sreduces the consumption of cyanide. The factors mentioned increase the economy with which gold and silver are leached from ores, ore concentrates and waste material from previous incomplete leachings, for example heap leachings.

S 25 The advantages of the peroxoborate are all the more surprising insofar as other peroxo compounds, including calcium peroxide, sodium percarbonate and ammonium peroxodisulfate, proved to be less effective than hydrogen peroxide in regard to the gold yield.

The following Examples illustrate the surprising effect of peroxoborates in relation to other peroxo compounds in cyanidation-in-mill leaching.

91 136 PV Examples A sulfide-containing ore, the so-called mill feed of the Vumbachikwe mine in Zimbabwe, was used in all the tests. With this ore, the cyanide-leachable gold, 80 to of the gold content, can be extracted in 4 to 6 hours where hydrogen peroxide is used in accordance with DE-PS 36 37 082.

The leaching tests using the cyanidation-in-mill technique were carried out in a stainless steel rod mill with a total capacity of 4.5 1. Twelve stainless steel rods (18.2 cm long, 2.5 cm in diameter) were used as the grinding elements, occupying 24% of the mill volume. The grindings were each carried out with 400 g ore and 600 g water (40% solids). The volume of the ore pulp formed occupies another 16% of the mill so that the total filling of the mill is oo. Other leaching parameters are: 20 NaCN addition: 0.5 kg/t ore CaO addition: 1.25 kg/t ore Grinding time: 1 h Grinding speed: 60 r.p.m.

pH at the end of grinding: 11.5 12.0 The various oxidizing agents and the lime were introduced into the mill at the beginning of grinding. On completion of grinding, the gold yield (leached gold, based on the total gold content of the ore), the 0 content (ppm) in the leaching solution, as determined with an 02 electrode, and the cyanide consumption in kg/t ore (cyanide used minus residual cyanide) were determined.

The results of Examples E 1 to E 5 according to the invention and those of Comparison Examples C 1 to C 7 are set out in the following Table, reflecting the superior 91 136 PV ef fect of added perborates (E 1 to E 4) although sodium perborate (E 5) formed in situ leads to substantially the same gold yield as H 2 0 2 the consumption of NaCN could be reduced.

S

*5 S S *S

S

S S 91 136 PV Table Test oxidizing No. agent Quantity of peroxo compound, expressed as mol H 2 0 2 /t ore Gold yield 02 content after grinding

PPM

NaCN consumption kg/t ore Cl1 H 2 0 2 20 64 0.8 0.30 C 2 H 2 0 2 40 66 1.3 0.35 E 1 NaBO 3 *41i 2 0 20 67 0.9 0.27 E 2 NaB0 3 -4H 2 0 40 84 2.7 0.20 E 3 NaB0 3

-H

2 0 40 76 1.9 0.25 E 4 Ca (B0 3 2 40 76 2.9 0.24 E 5 H 2 0 2 Na 2

B

4 0 7 40 65 0.5 0.26 (in situ perborate) CaO.

Na 2

CO

3 1.5 H.0,

(NH

4 2

S

2 0 8 CaroatO (Degussa AG) 0.8 0.8 0.3 0.1 0.37 0.22 0.37 Complete cyanide oxidation V 7 Air0.

0.7

Claims (8)

1. A process for leaching gold and silver from ores, ore concentrates and waste material from previous incomplete leachings by grinding of the material to be le .hed in the presence of a cyanide-containing leaching solution which has a pH value of 8 to 13 and which contains a peroxo compound, characterised in that at least one peroxoborate compound in dissolved or very finely divided form is present as the peroxo compound.

2. A process as claimed in claim 1, characterised in that the leaching solution contains an alkali metal and/or alkaline earth metal peroxoborate in dissolved and/or undissolved form.

3. A process as claimed in claim 1 or 2, characterised in that a powder-form peroxoborate is added in an effective quantity to the pulp of the material to be leached and the leaching solution at the beginning of and/or during the grinding step.

4. A process as claimed in any one of claims 1 to 3, characterised in that 1 to 100 equivalents peroxoborate, expressed as BO3, are used per t of material to be leached.

5. A process as claimed in claim 4, characterised in that 10 to 60 equivalents peroxoborate, expressed as B0 3 are used per t of material to be leached.

6. A process as claimed in any one of claims 1 to 5, characterised in that the leaching solution has a pH value of 9 to 12 and contains 0.02 to 0.2% by weight cyanide, expressed as CN.

7. A process for leaching gold and silver from ores, substantially as hereinbefore described with reference to any one of the Examples, but excluding the comparative Examples.

8. Gold or silver produced by the process of any one of claims 1 to 7. o: Dated 27 July, 1993 Degussa Aktiengesellschaft Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON eeo IG:\WPUSER\LIBZ100117:EAR 1 of 1 A PROCESS FOR LEACHING GOLD AND SILVER ABSTRACT In the so-called cyanidation-in-mill technique, the ores, ore concentrates and waste material from previous incomplete leachings which are to be leached are treated in the mill with a cyanide-containing leaching solution, gold and silver being at least partly leached. Compared with the known cyanidation-i-mill technique in the presence of hydrogen peroxide at a pH of 8 to 13, the gold yield can be considerably increased if leaching is carried out in the presence of a peroxoborate compound. The peroxoborate may be present in solid and/or dissolved form. 10 to 60 equivalents peroxoborate are preferably used per t of the material to be leached. sem s e a. e 0 o c *f h LMM/7596W