NZ233186A - Extraction of gold or silver from ore using an aqueous alkaline cyanide solution containing a poorly soluble divalent metal peroxide - Google Patents

Description

/ m 23 3 1 8 6 Priority Date;s): .k-' , CempJ^V* rpen'ivicf iicu F:!-.:"l: C;sss: $)....(Z.X2r<&\\./.Q.&t CO.I&I.S'./.Oi^'^. 28 APR.M ,/j:CO • Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION MANUFACTURE OF PEROXIDES i~3Apf?199G21 f v WE, Interox Chemicals Pty. Limited, of 20-22 McPherson Street, Banksmeadow 2019, New South Wales, AUSTRALIA hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: " i The present invention relates to the manufacture of peroxides, especially calcium peroxide arid its use in the extraction of precious metals from ores and other solids-containing such metals by the cyanidation process.

More specifically the invention relates to manufacture on-site of calcium peroxide for direct use in leaching of gold and/or silver from slurries containing aqueous alkaline cyanide.

In traditional times, precious metals such as gold have been obtained in native form by panning. In various of the oifis that contain gold and /or silver;, it is present in only a small proportion that is distributed throughout the ore, so that in modern times efficient extraction of the gold and/ox- silver has entailed c. ushing the ox-e to a very fine particle size, such as below 100 mosh and leaching the particles in a vessel with an aqueous alkaline solution of cyanide, particularly sodium cyanide 'ar.d usually whilst agitating and blowing air through the ^liquor.

■This process is termed variously by those in the art as "cyanidation" or "alkaline cyanide leaching". The times required to achieve quantitative extractions of precious 'metals by this process are quite long - 24 to 48 hours being typical. Accordingly say means which speeds up extraction time would be beneficial s.i 'ice ore throughput 'could be increased without the costly expenditure 'required to extend plant.

IPI 2 3 3 18 6 • -3~ In thG cyanidation process air or oxygen gay is used as n the oxidant- A number of reactions are possible. 2P + 4CN" + 02 + 2H20—-> 2P(CN)2" + 20H" + K>02 - (1) 4P + 8CM" + 02 + 2H,0 —>4 P(CN)a" + 40H~ - (2) 2P + 4CN" + H202 > 2P( CN) 2" + 20H' - (3) where P represents Au or Ag.

In summarising the research literature, A, R. Burkin (The Chemistry of Hydrometallurgical Processes, Span, London, . 1966 page 51) asserts that the predominant equation for I 0 • the dissolution of gold in cyanide solution is equation • (1), whilst for silver, the reaction proceeds in : analogous fashion to equation (2). Thus in both cases : the predominant oxidising species is oxygen, : In conventional cyanidation processes using air-, problems ' may be encountered in providing sufficient dissolved 'oxygen (DO) in the leaching process due to the limited | solubility of. oxygen in water. For instance, the maximum ]dissolved oxygen concentration achievable in water by :aeration is about 9mg/L at 20°C and 1 atmosphere pressure. { 2.0 .Even using pure oxygen under ideal conditions it : s i unlikely that DO will exceed 70% of saturation, or about '31tug/L (H.L. Day, Canadian Mining Journal, August 1967 Page 57). If the ore being leached contains appreciable Oi quantities of oxygen consuming species, for" example Jsuli •'C'-is, then DO may be consumed at a rate faster than it can be supplied by the aeration means commonly used in Vne art. Accordingly, insufficient DO may be available for the gold and/or silver dissolution reactions. Peroxygen c-omoounds provide a solution to these problems I 'as being liquids or solids they are easily incorporated into the leaching solution and they are a ready source of oxygen when decomposed. 23 3 1 Tha use of soluble peroxides, especially hydrogen peroxide '.s known in the art. Hydrogen peroxide1, decomposes in leaching solutions to give oxygeii which then acts to oxidise the precious metal into a soluble state, where it forms a complex with cyanide. Disadvantages are evident with use of hydrogen peroxide in this fashion however- Firstly, the rate of decomposition of hydrogen peroxide is dependent on the composition of the slurry matrix. Accordingly, the rate of oxygen release from hydrogen peroxide is variable. Furthermore, cyanide consumption is increased as hydrogen peroxide, expecially at higher concentrations, oxidises cyanide: H,02 -t- CN" —> H20 + CNCr - (4) Australian Patent Acceptance No. 589813 to Degussa ^ktiengesellschaft describes a process for leaching gold and/or silver from ores or ore concentrates using ail aqueous cyanide-containing leaching solution having a pH value of from 8 to 13 with addition of an aqueous H202 solution characterised in that the addition of the aqueous H202 solution is regulated through the measured concentration of the oxygen dissolved in the leaching solution, the leaching solution containing from 2 to 20mg 02 per litre. The benefits claimed for that process include x'educed cyanide consumption, increased gold yield and shortened leaching time, when compared to aeration processes.

The dissolved oxygen levels selected by that applicant are known fr^m the prior art. For instance E. L. Day (Canadian Mining Journal, August 1967 figux^e 4, page 59) shows that for cyanide concentrations of 0.1% and below ,(0.02 moles/Xi and below) which are levels normally used in practice, maximum rate of solution of gold occurs at DO's in the range of about 13 to about 22mg/L. y x T 1 ^ vj J I Whilst Australian Patent Acceptance No. 589818 specifics only aqueous hydrogen peroxide as the oxygen source, solid peroxygens which yield hydrogen peroxide in aqueous solution may have a similar effect but would certainly not be expected to exhibit any further advantages. For example sodium perborate and sodium carbonate peroxyhydrate are both known to yield hydrogen peroxide, readily in solution. There would bo no economic benefit in using these materials over hydrogen peroxide since being manufactured from hydrogen peroxide they are obviously more expensive than it.

Poorly soluble divalent metal peroxides have found application in he.ip leaching of go3d. For example, Australian Patent Application AU~A~3'i073/89 to interox Chemicals Ltd describes the use of these components to enhance gold extraction by incorporating them into the heap of solids being leached. According to AU-A-34073/89 the benefits of poorly soluble divalent metal compounds in heap leaching stem from them providing a long lasting source of oxygen within the heart of the heap. In this respect, tnoy are far superior to hydrogen peroxide. Despite the suitability of poorly soluble divalent metal peroxides as < :ygen source for heap leaching they would not be expected to offer any advantages over hydrogen peroxide in agitation leaching.

It has now been surprisingly foun-1 that poorly soluble divalent metal peroxides, especially calcium peroxide can give a faster rate of extraction of precious metal than hydrogen peroxide when used in alkaline cyanide agitation leaching processes. A further benefit of the present invention is that cyanide consumption is lower than when hydrogen x-^roxide is used, thus offering the miner economies in chemical usage.

VJhilst not wishing to be bound bv any theory as to how t?ie invention works, the present applicant suggests that : whilst under acidic conditions calcium peroxide yields ' hydrogen peroxide in solution it appears that under alkaline conditions this peroxide decomposes directly to yield oxygen. The fact that hydrogen peroxide is not formed means that consumption of cyanide via the mechanism of equation (4) above is eliminated.

IThe reduced consumption of cyanide when poorly soluble T / 0 \divalent metal peroxides are used is clearly shown in an ,experiment conducted by the present applicant. Simulated ;leaching solutions were prepared containing 0.35 g/L of copper (I) cyanide, 0.96 g/L o" sodium cyanide and ,adjusted initially to a pH of 10.5. Feroxygen compounds ;were added at a rate corresponding to 0.2 g/L of hydrogen peroxide (100% b?':is) and residual free cyanide [determined in the leaching solution over time. Results are shown in table 1.

Compound mg/L free residual cyanide at elapsed time (hrs) 0 4 8 22 G 1,Hydrogen peroxide 296 ! 2.Sodium carbonate t peroxyhydrate OXYPISR* 303 3.Sodium perborate tetrahydrate 288 4.Calcium peroxide, 1XPER* 60c 296 121 109 113 263 112 97 93 253 74 77 71 237 Registered Trade Marks*.

TABLE 1 23 3 1 8 -y- Compounds 2 and 3 each yield hydrogen peroxide in solution, so consumption of cyanide is similar to that with H202. Addition of calcium peroxide however sees a marked decline in cvanido consumption. Additionally, the l.caching process can be x-un at higher alkalinities than is possible with hydrogen peroxide as oxygen source. Accordingly loss of cyanide as HCM gas is minimised.

It is an object of the present invention to provide a I 'means whereby oxygen can be provided to metallurgical processes in order to enhance the rate of extraction of gold and/or silver from ores and other solids containing them whilst avoiding the disadvantages mentioned above, I | In accordance with a first aspect of the present invention there is provided a process for the extraction of gold and/or silver from ores or other solids containing them wherein ores or other solids are brought into contact with an aqueous alkaline cyanide leaching solution and held in a suitable vessel optionally with 'aeration or oxygenation, until at leas™ a fraction of the gold and/or silver is taken into solution, which process is characterised by the addition of an effective amount of at least one poorly soluble divalent metal peroxide.

The process by which gold and/or silver is extracted from ores or other solutions by contacting them with an 'aqueous alkaline leaching solution is well known in the 'art. j For instance, skilled persons will readily select ores or other solids, e.g. scrap, suitably rich in the metals to f be extracted by the process of the invention. The need for reduction of particular size of ores or other solids to a suitably fine mesh would also be understood by .skilled workers.

' I j '■ ' ' O i V ; t I - 8 By aqueous alkaline cyanide leaching solution is understood to comprise a solution of a cyanide, usually but not exclusively an alkali metal cyanide such as sodium or potassium cyanide. The pH of such alkaline solutions 5 is generally maintained in the range of from about pH 9 to about pH 13, preferably from about pH 10 to about pH 12. Lime is commonly used to maintain pH in these ranges, but clearly other suitable alkalis may be employed. Herein, the concentration of cyanide in such solutions often is 10 chosen in the range of 0.003 moles/litre to 0.05 moles/litre, and in many solutions in the range of 0.005 moles/litre to 0.02 moles/litre.

It will be understood that in the context of the present invention, the fashion in which the ores or other solids 15 are brought into contact with the aqueous alkaline cyanide leaching solutions, or the manner in which the leaching solutions is prepared, is not an essential feature.

For example, the skilled person will often choose to add alkali and cyanide in a dry form to the dry ore or other 20 solids prior to grinding, the bulk of water then is added after the grinding stage.

A suitable vessel is usually selected in light of the overall ore or other solid characteristics and the operational requirements of the processing plant. 25 Continuous stirred tank reactors are commonly used and therse may be operated either singly or in cascade. The term vessel in the context of this specification and claims will thus be understood to include the situation where more than one vessel is used. Vessels may be 30 operated at atmospheric pressure or at pressures above atmospheric dependent on their design. 2 3 3 1 Agitation may be supplied to the vessel by mecnanical means such as stirrers, but alternatively, aeration may give satisfactory agitation. The skilled worker may also supply dissolved oxygen to the process by aeration or oxygenation wiithout departing from the spirit of the invention, but this is optional.

In some processes encompassed within the scope of the present invention, extraction j.:, conducted in the absence of agitation. For example, in vat leaching ores or other jO solids are placed in a vessel provided with outlet means, covered with aqueous alkaline cyanide leaching solution and a slight vacuum applied to said outlet means whereby aqueous alkaline cyanide leaching solution is drawn through said ores or solids.

The extracti.on processes to which the present invention relate are normally operated at ambient temperature or slightly above. h skilled person may however operate the process at other temperatures if he decides that these are _ advantageous.

J.0 The skilled person will determine the most effective amount of poorly soluble divalent metal peroxide by plant optimisation studies. In a typical optimisation an initial level of poorly soluble divalent metal peroxide will be selected somewhere in the range of from about 0.1 to about 20 parts w/w per 1000 parts of ore or other solids. The level of poorly soluble divalent metal peroxide is then gradually reduced whilst measuring gold and/or silver recoveries to a point wherein recovery starts to drop away. In the event that recovery drops ^0 immediately on reduction of poorly soluble divalent metal peroxide dose, the dose is then increased while monitoring recovery till the point is reached where further additions do not result in any improvement in recovery. 2 3 3 1 -10..

It may be necessary to frequently re-optimise the level of addition of poorly soluble divalent metal peroxide in order to cope with changing quality of ore or other Solids.

Optimisation studies may also be directed to finding the most suitable point for addition of the peroxide. For instance, where a cascade of vessels is employed, the Skilled operator may determine that a single addition point is satisfactory, or it may be found that addition at two or more points thr-oughout the cascade gives superior recoveries of gold and/or silver.

The poorly soluble divalent metal peroxide will often he added continuously in slurry form by means commonly used in the art e.g. a pump. In some instances the skilled operator may add the peroxide intermittently , ox- as staged additions to one or more leaching vessels.

The present invention also encompasses the addition of divalent metal peroxides to an aqueous precious metal ox*e slurx-y as pretreatment prior to alkaline cyanide leaching. The function of pretreatment is to remove oxygen - consuming species from the slurry, so as to minimise oxygen demand in the leaching stage.

Pretreatment conditions vary according to the ore being processed. The quantity of poorly soluble divalent 'metal peroxide required is variable and dependent on the .nature of oxygen consuming imparities. Pretreatment is normally conducted at a pH in the alkaline range.

Dy the term poorly soluble in the context of peroxides is meant one that dissolves to an extent not significantly greater than the most soluble alkaline earth metal peroxide in water at 25° C. Such peroxides include magnesium peroxide, strontium peroxide, barium peroxide, zinc peroxide and calcium peroxide, of which calcium peroxide is most preferred in view of its combination of its good performance and its ecological acceptability.

The poorly soluble divalent metal peroxides used in the present invention are available commercially in Australia. For example, the. IXPER (Registered Trade Mark) range of inorganic peroxides marketed by Interox Chemicals Pfcy Ltd, Sydney. These peroxides are sold as dry powders.

Poorly soluble divalent metal peroxides are used in the present invention at a level of from about 0.1 to about 20 parts w/w per 1000 parts o£ ore or other solid and mere preferably from about 0.1 to about 5 parts v;/v; per 1000 part.-; of ore or other solids. In a specific embodiment of the present invention the poorly soluble divalent metal peroxide is used at a level of from about 0.1 to about 1 part w/w per 1000 parts of ore or other solid. 1 o A significant barrier to the practical use of these peroxides is related to the economics of manufacturing and transport. These metal peroxides are manufactured in slurry form by reacting the metal hydroxide, oxide or a suitable salt capable of generating the oxide or hydroxide with hydrogen peroxide. The water present in the slurry is romovod and the product dried and bagged for transport. This drying stage is a major expense. Furthermore, in tho case of calcium peroxide high quality lime may need to be used in order to prevent deterioration of the product during storage and transport.

In addition the use of manufactured peroxide precludes the use. of locally available lime. A?; most mining sites are in remote locations, considerable transport economies may be realised if the lime can be sourced locally.

A further object of the present invention is the . provision of means of manufacturing these peroxides on-! site in response to demand from a leaching process j.O \ whereby the composition of the peroxide slurry can be instantaneously varied in accordance with extraction conditions.

In accordance with another aspect of the. present invention there is provided a process for tho m:\nufactur© of poorly soluble divalent metal peroxides which process comprises a) mixing an aqueous solution of hydrogen peroxide to a stoichiometric excess of at least one suitable divalent metal compo^ nd to form an aqueous slurry of poorly soluble divalent metal peroxide, b) adding the aqueous slurry of poorly soluble divalent metal peroxide to an aqueous alkaline cyanide leaching process, n o o c) monitoring the levels of dissolved oxygen and pJT in said aqueous alkaline cyanide leaching process and d) using said levels of dissolved oxygen and pH to control the respective rates at which aqueous hydrogen peroxide and divalent metal compound are each added to form said aqueous slurry of poorly soluble divalent metal peroxide.

In this specification the term divalent metal compound is understood to include divalent metal oxides, hydroxides or mixtures thereof, either in the dry state or slurried with water and optionally combined with additives or adjuvants. It can also include compounds capable of generating oxides or hydroxides.

The term aqueous slurry of a poorly soluble divalent metal peroxide is understood to be the reaction product from step a) above and accordingly may contain, in addition to divalent peroxides, undissolved divalent metal compounds and water.

Commercial solutions: of hydrogen peroxide are suitable for use in the process of the invention. These are typically of strength 35-70% w/w, but the skilled worker could select a strength outside this range. The stoichiometric equation for formation of the divalent metal peroxide is of the form: MO + H202 -> M02 + H2 (5) or M(0H)2 + H202 -> M02 + 2H20 (6) An excess, on a molar basis of the metal oxide or hydroxide should be present in the reaction mixture. This ensures complete conversion of hydrogen peroxide to the divalent metal peroxide. Typically from about 1.1 to about 20 moles of divalent metal compound are employed per mole of hydrogen peroxide. Preferablty, from about 2 moles to about 10 moles of divalent metal compound are employed.

TV \ \ * 9 3 3 1 The reactor for formation of the divalent metal peroxide is typically agitated and may be cooled. The divalent metal confound can be ndded to the reactor as a solid or more preferably as an aqueous slurry. it is highly preferred that the compound be slurried in water prior to addition to the reactor. For the manufacture of calcium peroxide, calcium oxide is the preferred compound, however addition of this material to water results in considerable heat evolution. The skilled person, if IO using calcium oxide, should therefore provide equipment for cooling of the slurry prior to addition to the x^eactor. The divalent metal compound can be present in the slurry at any convenient level.

The term aqueous alkaline cyanide leaching process encompasses processes wherein the ore or other precious metal-containing solid is slurried or suspended in a leaching solution. These processes have been described earlier in this specification and may include conventional agitated leaching, pressure leaching, vat ,2) 2o leaching or other methods known in the. art. Precious mecals, especially gold and/or silver are present as ores or other solids containing them.

The levels of dissolved oxygen and pH in the leaching process can be monitored by methods known in the art. For example dissolved onygen and pH can be monitored by electrodes. The skilled person will select the levels Of dissolved oxygen and pH within which the process should be controlled based on tests carried out on the particular- ore body being processes. "7 A v j I A specific embodiment of the invention will now be illustrated with reference to the drawing. The drawing shows a continuous leaching vessel (1) into which an aqueous slurry of a divalent metal peroxide is dosed via a line (2), from a reactor (3). Dissolved oxygen and pH in the leaching vessel are monitored via electrodes (4 and 5). Signals from the electrodes are relayed to a controller (6) which then operates control valves (7 and 8) to vary the addition rate of hydrogen peroxide I q solution (9) and a suitable divalent metal compound slurry (10) into the reactor (3).

The invention will be i'ui char illustrated with reference to the following examples. Example 1, 2 and <5 are .included for purposes of comparison and are in accordance with the prior art. Examples 3 and 5 are in accordance with the present invention and clearly illustrate enhanced rate of gold recovery achievable over the prior art. Examples 3 and 5 whilst illustrating particular embodiments of the present invention should not be 1-0 construed as limiting on the scope theron.

Examnle 1 200 Kg of gold and silver bearing ore was obtained from an Australian mine site. Analysis of duplicate samples of ore showed it to contain 7.71 g/t of gold, 535 g/t of copper and 27.25 g/t of silver. Sample was crushed to 100% - 2mm and rotary split into about 60 X 1 kg samples, and the excess put aside. 3 x 1 kg samples wex*a slurried with water and milled in a laboratory rod mill to 80% - 45 micrometres. The milled slurry was 3 O transferred to a 7 L perspex vat and slurry density adjusted to 40% solids vz/w using minesite water* 233 18 5 Initially sodium cyanide was added at 0.05% based on the aqueous phase and liirie added during leaching to maintain pH at 10 1 pll unit. Leaching was conducted for 24 hours with aeration. Agitation was by flat blade paddle sufficient to keep the slurry in suspension.

Table 2 shows analyses of gold, silver and NaCN in solution during the course of the leach.

Time DO pH NaCN Au Ag Total hours mg/L % g/t g/t gold Extracted% 0 4.6 .0 0 .050 0 2 7.9 9.4 0.040 3. 56 8.93 56.00 4 8.9 9.5 0.038 4.81 8.67 75.67 8 8.3 9.8 0.035 .56 8.71 87.47 12 7.9 9.7 0.035 .50 8.59 86.52 24 .2 9.9 0.015 6.04 9.5 95.02 TABLE 2 23 3 1 Exampla 2 Fresh milled ore from example 1 was leached in identical fashion to that example but hydrogen peroxide (as 3% w/v solution) was employed during the leach in place of aeration with three single additions of lOmL each at 0, 2 and 4 hours. Tabic 3 shows results of solution analyses conducted during the course of leaching.

Time DO pH NaCN Au AG Total Gold hours mg/L % g/t g/t Extracted% 0 27.6 9.9 0.050 0 2 .0 9. o 0.035 4.40 7.80 74.95 4 .8 9.8 0.035 .20 8.05 8 8.57 8 9.2 9. B 0.035 .40 7. 80 91.98 12 9.1 9.9 0.030 .40 7.70 91.98 24 9 .1 .0 0.020 . 50 7.40 93.68 48 9.1 9.9 0.020 . 60 7.70 95.33 TABLE 3 ' Example 3 Fresh milled ore from example 1 was leached in identical fashion to that example but solid calcium peroxide 60% ■active - IXPER 60C (Registered Tx-ade Mark) was added as 3 single additions of 0.75g each at 0, 2. and 4 hours.

.Table 4 shows results of solution analyses conducted during the course of leaching • Time DO PH NaCN AU Ag Total Gold hours iug/L % g/t g/t Extracted % 0 6,0 9.9 0.050 0 2 13.6 9.6 0.035 .2.0 7.85 86.33 4 14.2 9.9 0.035 r.o 7. 50 91.31 8 9.3 9,9 0.038 . 60 7.40 92.97 12 9.0 9.9 0.035 ,70 7.25 94.63 24 9.1. .0 0,033 .70 7.10 94.63 48 9.2 9.8 0.030 .70 7.20 94.63 TABLE 4 Example 4 200 Kg of another Australian ore was obtained and milled according to the procedure in example 1. Analysis of duplicate samples of ore showed it to contain 7.66 g/t of gold, 10175 g/t of copper and below 0.5 g/t of silver. Leaching by aeration was conducted according to the method of example 1 except sodium cyanide was added a level of 0.2% on aqueous phase and Perth tap water used in place of mine water. Table 5 shows results of solution analyses conducted during the course of leaching.

Time DO pH NaCN Au Total Gold hours mg/L % y/t Extracted % 0 3.8 .2 0.200 2 6.7 .2 0.008 0.17 3.79 4 7.7 .1 0.020 0.46 .27 8 7.9 .0 0.033 1.30 29.02 12 7. 5 9.9 0.003 1.38 .81 24 8.6 9.9 0.033 1.60 .72 TABLE 5 23 3 186 Example 5 Fresh milled ore from Example 4 was leached in identical fashion to that example but in place of aeration, solid calcium peroxide 60% active - IXPER 60C (Registered Trade Mark) was added as 3 single additions of 1.50g each after leaching for 4, 8 and 24 hours. Table 6 shows results of solution analysis conducted during the course Of leaching.

Time DO pH NaCN Au Total Gold hours mg/L % g/t Extracted % 0 4.4 10.7 0.200 0 2 6.9 10.5 0.008 0,20 4.48 '4 7.8 10.4 0.020 0.52 11.64 8 9.3 10.9 0.020 1.80 40.30 12 9.2 11.2 0.030 1.92 42.99 '2.4 8.3 11.2 0.025 2,08 46.57 48 8.3 11.0 0.023 2.32 51.94 O TABLE 6 { Xx u t

Claims (24)

<) i. \) What We Claim Is:

1. A process for the extraction of gold and/or silver from ores or other solids containing them wherein ores or other solids are brought into contact with an aqueous alkaline cyanide leaching solution and held in a suitable vessel optionally with aeration or oxygenation until at least a fraction of the gold and/or silver is taken into solution, which process is characterised by the addition of an effective amount of at least one poorly soluble divalent metal peroxide.

2. A process according to claim 1 wherein the vessel is agitated and operated at atmospheric pressure.

3. A process according to claim 1 wherein the vessel is agitated and operated at a pressure greater than atmospheric pressure.

4. A process according to claim 1 wherein ores or other solids are placed in a vessel provided with outlet means, covered with aqueous alkaline cyanide leaching solution and a slight vacuum applied to said outlet means whereby aqueous alkaline cyanide leaching solution is drawn through said ores or solids.

5. A process according to any one of claims 1 to 4 wherein the concentration of cyanide is from 0.003 moles/litre to 0.05 moles/litre.

6. A process according to claim 5 wherein the concentration of cyanide is from 0.005 moles/litre to 0.02 moles/litre.

7. A process according to any one of claims 1 to 6 wherein sodium cyanide is employed as the source of cyanide.

/

../ H f*T £ '« r;9 'j '< >'.<:■;i-. Dlllutl;- 22 -;8. A process according to any one of claims 1 to 7 wherein the pH of the aqueous alkaline cyanide leaching solution is held in the range of from pH 9 to pH 13.;9. A process according to claim 8 wherein the pH of the aqueous alkaline cyanide leaching solution is held in the range of from pH 10 to pH 12.;

10. A process according to any one of claims 1 to 9 wherein poorly soluble divalent metal peroxide is added at a level of from 0.1 to 20 parts w/w per 1000 parts of ore or other solid.;

11. A process according to claim 10 wherein poorly soluble divalent metal peroxide is added at a level of from 0.1 to 5 parts w/w per 1000 parts of ore or other solid.;

12. A process according to claim 11 wherein poorly soluble divalent metal peroxide is added at a level of from 0.1 to 1 part w/w per 1000 parts of ore or other solid.;

13. A process according to any one of claims 1 to 12 wherein the poorly soluble divalent metal peroxide is calcium peroxide.;

14. A process according to any one of claims 1 to 12 wherein the poorly soluble divalent metal peroxide is selected from the group comprising magnesium, barium, strontium and zinc peroxides.;

15. A process according to any one of claims 1 to 12 wherein two or more poorly soluble divalent metal ^ t n peroxides selected from the group comprising calcium,;- 23 -;<--) 0 ^ : • : > r.l ( 1;magnesium, barium, strontium and zinc peroxides are added.;

16. A process for the manufacture of poorly soluble w* divalent metal peroxides which process comprises: a) mixing an aqueous solution of hydrogen peroxide with a stoichiometric excess of at least one suitable divalent metal compound to form an aqueous slurry of poorly soluble divalent metal peroxide, b) adding the aqueous slurry of poorly soluble divalent metal peroxide to an aqueous alkaline cyanide leaching process, c) monitoring the levels of dissolved oxygen and pH in said leaching process and, d) using said levels of dissolved oxygen and pH to control the respective rates at which aqueous hydrogen peroxide and divalent metal compound are each added to form said aqueous slurry of poorly soluble divalent metal peroxide.

17. A process according to claim 16 wherein the divalent metal compound is calcium oxide and/or calcium V j' hydroxide.

18. A process according to claim 16 or claim 17 wherein from 1.1 to 20 moles of divalent metal compound are employed per mole of hydrogen peroxide. O

19. A process according to claim 18 wherein from 2 moles to 10 moles of divalent metal compound are employed per mole of hydrogen peroxide.

20. A process according to any one of claims 16 to 19 wherein the levels of dissolved oxygen and pH in said _ leaching Drocess are in the range of from 2 mg/L to 20 mg/L and from pH 9 to pH 13. 20 MAR 1992$) o «') 1. O i > 9 '■> '< ' c

21. n

22.

23.

24. VJ A process for the extraction of gold and/or silver which process is substantially as hereinbefore described with reference to example 3 or example 5. Gold and/or silver whenever obtained by a process according to any one of claims 1 to 15 or claim 21. A process for the manufacture of poorly soluble divalent metal peroxides which process is substantially as hereinbefore described with reference to the drawing. An apparatus for the manufacture of poorly soluble divalent metal peroxides substantially as hereinbefore described with reference to the drawing, whenever used in a process according to any one of claims 16 to 20 or claim 23. INTEROX CHEMICALS (PTY)LIMITET By their Attorneys BALDWIN, SON & CAREY