GB2219474A - Gold leaching - Google Patents

Abstract

Leaching of gold-bearing solids, such as crushed lean ores or material recovered from very old waste tips can be carried out with a dilute aqueous alkali cyanide solution, but the extent and/or rate of recovery of gold is impaired by the anaerobic or reducing conditions that can exist in such heaps, and especially towards their base and or by poor supply of oxygen in agitation leaching. The rate and/or extent of gold recovery in heap leaching can be increased and the supply of oxygen simply controlled by slow release in agitation leaching, using otherwise identical conditions by employing an effective amount of a slow dissolving poorly soluble solid metal peroxide, either by incorporating it within the heap or in the mixture of gold-bearing solids and lixiviant. The peroxide is advantageously calcium peroxide because it is not only very effective but also is environmentally suitable. The amount employed is preferably from 1 to 10 parts w/w per 1000 parts of gold-bearing solids for the low grade material, usually containing from 1 to 10 g gold per tonne solids.

Description

Gold Leaching The present invention relates to gold leaching and in particular to improvements in or relating to the extraction of gold from gold-containing solids using aqueous alkaline lixiviants.

In traditional times, gold has been obtained in native form by panning. In various of the ores that contain gold, it is present in only a small proportion that is distributed throughout the ore, so that efficient extraction of the gold has entailed crushing the ore to a very fine particle size, such as below 100 mesh and leaching the particles in an agitation vessel with an aqueous alkaline solution of an alkali metal cyanide, particularly sodium cyanide and usually whilst air or oxygen is blown through the liquor.

The crushing or grinding of ore to a very fine particle size can prove to be an expensive operation, so that there has been increased interest within the gold-extraction industry of methods that do not require such extensive grinding operations. One consequence is that in recent years interest has been shown in heap leaching, which in principle is a simple, low cost operation that does not need sophisticated equipment and which requires the solids to be ground no finer than coarsely, although finer ground material can be likewise treated. As such, heap leaching has been considered favourably by minerals processors for use with low grade ores or waste materials.

In heap leaching, a heap of solids material containing the desired metal is formed on top of an impervious base and a lixiviant solution is introduced onto or near the top of the heap, usually by spraying or pouring. The lixiviant solution percolates through the heap under the influence of gravity, and is directed by the base, which is usually appropriately inclined slightly, to a collection point and the resultant pregnant liquor is then subjected to further processing to separate and isolate the desired metal. The heap is often frusto-conical or frusto-pyramidal in shape, having a flattish top surface and sloping sides, and in practice it is often several metres high, containing several hundreds or thousands of tons of solids, although the precise dimensions and shape are at the discretion of the metal extractor and determined in part by the physical constraints of the site.

In heap leaching gold from solids material, an obvious candidate for the lixiviant solution comprises aqueous alkaline cyanide solutions, in view of their successful application in agitation leaching. However, most effective use of such a lixiviant solution arises from the presence of oxygen. The residence time of the lixiviant in the heap is long, though depending obviously on the size of the heap, and the porosity of the solids amongst other parameters.

The solublity of oxygen in such aqueous cyanide solutions is relatively low so that the amount which is present in solution at the time of its introduction is rapidly consumed as the solution percolates through the heap. Consequently, the effectiveness of the lixiviant for gold extraction becomes progressively poorer at increasing depth in the heap, whereas the risk of gold re-precipitation is undiminished. Moreover, the possibility that the the solution could retain its oxygen content until it percolates through to gold-bearing mineral in the heap decreases at greater depths as a result of other oxygen-consuming reactions with other constituents of the ore occuring en route. The practical result is that the overall extent or rate of leaching of gold from the heap is impaired.Thus, the heap can be replaced whilst a significant proportion of extractable gold remains, thereby being both wasteful and entalling a higher relative materials handling cost.

Some ccnsideration has already been given to employing soluble hydrogen peroxide as an additive. It was advocated in a paper by Norris, Brown and Caropreso entitled "the use of peroxygen chemicals in the heap leaching of gold and silver ores" presented at the SME-AIME Annual Meeting in March 1983, in order to improve the rate and/or extent of metal leaching, but the test work was carried out by the roll jar method in which extracting conditions inevitably differ markedly from heap leaching conditions. Accordingly, at best, this recommendation was not properly proven. The matter was clarified at the subsequent meeting of the SME AIME in October 1983 in a paper presented by Smith and Craft, entitled "Pilot scale heap leaching at the Pinson Mine, Humboldt County, Nevada. n The trials were carried out on heaps of about 14 feet high.The addition of hydrogen peroxide to the sodium cyanide solution at a level of 1200 ppm of 35% solution was stated to show no apparent effect on solution grade (sic. of gold) nor of recovery rate. These trials confirm that peroxide has little applicability in standard size heap leaching of gold.

It is an object of the present invention to provide means whereby the overall extent and/or rate of leaching of gold from solid gold-bearing material can be improved and including the provision of means for use in heap leaching processes.

According to the present invention, there is provided a process for the extraction of gold from solids materials containing it in which a dilute aqueous alkaline solution of an alkali metal cyanide is brought into contact with the solids material containing gold in the presence of a substance which generates oxygen in situ, and contact is maintained until at least a fraction of the gold is taken into solution which is characterised by employing an effective amount of a poorly soluble divalent metal peroxide as the substances which generates oxygen in situ.

Advantageously, the divalent metal peroxide releases oxygen slowly into the aqueous alkaline cyanide lixiviant, thereby either delivering the oxygen more effectively to its point of consumption or maintaining inexpensively and simply the continuing production in situ of oxygen to augment or replace atmospheric oxygen without the need to employ expensive monitoring equipment and apparatus to control the supply of an alternative material, such as the highly soluble hydrogen peroxide.

According to an especially suitable aspect of the present invention, there is provided a process for the extraction of gold from solids materials containing it in which a dilute aqueous alkaline solution of an alkali metal cyanide is introduced onto the top surface of a heap of said solids materials, is permitted to percolate-through said solids materials whereby gold passes into solution and the solution is collected thereafter, which process is improved by incorporating into the heap of said solids material an effective amount of a poorly soluble solid divalent metal peroxide.

By the term effective amount is meant an amount which results in an increase in at least one of the total amount of gold that is extractable from the solids materials or the amount extractable within a convenient period of leaching, in both cases the comparison being drawn with a process that does not employ such an amount of divalent metal peroxide, but under otherwise identical process conditions.

By 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 250C. 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 effectiveness of the peroxide solids depends to some extent upon the manner of their distribution throughout the body of gold-bearing material that is being leached. By virtue of the non-mobile characteristics of solids particles in heap leaching, it is especially beneficial to spread the diva lent metal peroxides throughout the heap during its construction, and particularly to ensure that they are present in that portion or zone of the heap that is most remote from top or sides of the heap. There are various ways of effecting such a distribution, of which by far the most convenient is to incorporate the peroxide, often as a dry solid, into the heap during its construction. The distribution can comprise a particulate mixture of the ore and peroxide or alternate layers of the two substances, the ore usually providing the lower layer.In practice, the distribution will probably resemble a compromise between the two previously mentioned types, there being areas rich in the peroxide interspersed within the ore solids that constitute the remainder of the heap. For the avoidance of doubt, the poorly soluble peroxides need not be incorporated into the heap as a dry solid, but instead can be incorporated as an aqueous slurry, either directly onto the heap or via agglomeration with at least part of the goldbearing solids. In situ dewatering of the slurry can easily occur if the ambient temperature is high during the period of construction of the heap. Whilst in a mobile condition, the slurry can penetrate to some extent beneath the subsisting surface of the heap, thereby assisting in distributing the peroxide throughout the heap.It will be further recognised that a reasonably even distribution of peroxide through the heap is potentially beneficial and also that unevenness can be accomodated to some extent. It will be recognised that the particle mobility in agitation or vat leaching means that the lixiviant and ore particles are continuously being mixed tbgether with the effect of distibuting any solid throughout the mixture.

Whilst the mechanism is not fully understood by which the presence of the poorly soluble peroxide enhances gold extraction, it is believed that the peroxide either decomposes slowly in situ, thereby releasing oxygen within the heap which can be absorbed by the solution as it percolates through or slowly dissolves into the liquor as it passes over the solid peroxide with the result that when the liquor subsequently comes into contact with more ore, it can either release oxygen at the point of leaching or in some other way take part in the cyanidisation reaction.

Possibly, both mechanisms could each contribute. In either case, it will be seen that the benefit accrues by providing a means for better oxidising conditions such as within the heart of the heap that is long lasting, a means that can be of comparable duration to the period during which it would be desired to leach the heap. It will be further recognised that in the instant process, there is at least qualitatively some degree of matching between the style of the process, a long lasting steady matter, and the rate of decomposition and/or dissolution of the selected peroxides. By such a choice of peroxide, it is possible to retain the benefit of enhancing conditions for extraction throughout a substantially greater proportion of the leach time than if no such peroxide is used.Moreover, by the choice and distribution of such peroxides, it is possible to achieve enhancement that has not been practical for soluble peroxides to attain.

In the course of investigations resulting in the present invention, using tall columns of material simulating heap leaching conditions, it has been found that the increase in the amount of gold extracted within a given period of extraction varied in accordance with the proportion of peroxide employed.

It is often desirable to select the amount of added peroxide within the range of 1 to 20 parts w/w per 1000 parts of gold-bearing material, and especially the choice can be made in the range of 2 to 10 parts per 1000 parts.

Such ranges are contemplated for the low grade gold-bearing solids for which heap leaching processes are often considered suitable. In those solids, the gold content is usually within the range of 1 to 10 ppm (w/w). It will be recognised, though, that the use of a higher ratio of peroxide to gold-bearing solids is not precluded on chemical grounds, and would bear close consideration if particularly accelerated extraction of gold is desired, especially if commercial circumstances warrant the additional cost of such larger amounts of peroxide.

The amount of peroxide that can be employed desirably can be related directly to the gold content of the material bearing it. Expressing the former as above in parts w/w peroxide per 1000 parts gold-bearing material and gold content in the conventional manner of ppm, also w/w, the ratio of peroxide : gold is preferably selected in the range of 0.2:1 to 4:1, and in many instances from 0.4:1 to 2:1. Very effective results have been obtained at a ratio of 0.5:1 to 1.2:1.

A second factor of some importance relating specIfically to the use of peroxide in the leach process, as compare with factors that are of similar importance in both peroxide using and non-peroxide using processes, is the pH of the lixiviant solution. The solution often has a pH from about pH 9 to about pH 10. It is preferable for environmental reasons to use a pH in excess of pH 9, and from the point of view of effective use of the peroxide with the leach liquor, it is particularly desirable to select the pH in the range of about pH 10 to about pH 12.5. This is a practical range of benefit for use of calcium peroxide, and also for the other peroxides.

Various other factors associated with leaching are common to both peroxide-free and peroxide-using processes and can usefully be chosen or controlled employing the same criteria in the invention process as in corresponding peroxide-free prior art processes, ie those using atmospheric oxygen as the oxidant. Such factors include such practically important matters as the concentration of cyanide in the lixiviant, the ratio of lixiviant to goldbearing solids, the particle size of the gold-bearing solids, the manner of introduction of lixiviant into the heap, the flow rate of lixiviant through the heap, and the temperature of operation.

The lixiviant solution preferably contains sodium as the alkali metal. The concentration of cyanide in the lixiviant is normally at least 0.005 moles per litre and often is not higher than 0.2 moles/litre. In many processes, it will have a concentration of from 0.01 to 0.1 moles/litre, for example on introduction into the heap. It will be recognised that the effluent pregnant solution will have suffered some reduction of free-cyanide, for example by absorption or reaction, arising from its interaction with the gold-bearing solids during the gold extraction process, for example during passage through the heap. In practice, there tends to be a correlation between the rate of extraction of gold and the concentration of cyanide in the lixiviant.In consequence, it is a matter of choice for the process operator as to whether he chooses a relatively high concentration and faster gold recovery, but risking thereby higher reagent losses or employs a comparatively low cyanide concentration with the consequential need to permit the leach to continue for longer in order to obtain a similar extent of gold recovery as with the higher concentrations.

In either case, though, the inclusion of slow-dissolving peroxide, such as calcium peroxide, with the gold-bearing solids increases the rate and/or extent of gold recovery.

In view of the fact that group 7B metals are commonly recognised as the best catalysts for peroxidic destruction of cyanide ions in aqueous alkaline conditions, typically at around pH 10 or higher, it might be expected that the presence of added metal peroxide would lead to substantial or even excessive additional loss of cyanide from the lixiviant liquor during its passage through the gold-bearing heap of solids. In fact, some increase in cyanide losses is observable, but the losses are approximately proportionate to the increase in the rate of gold extracted and accordingly the expected impairment does not occur when considered from the over-riding viewpoint of consumption of reagent per unit production of product.

Heap leaching processes are usually conducted out of doors and are therefore to some extent at the mercy of the climate. n the present invention, it is convenient to employ the lixiviant at a temperature from 0 to about 450C which usually exists within the body of the heap, even if the air temperature fluctuates outside that range, ie at similar operational temperatures to that employing simply atmospheric oxygen. Agitation leaching is often conducted within a similar temperature range.

A further factor of general importance in leaching gold, which the instant invention shares with non-peroxidec processes is the particle size of the gold-bearing solids.

This factor is fully under the control of the process operator, when he seeks to optimise the Increased rate of gold extraction, and possibly some increased total extent of gold recovery that obtained as the solids are crushed or ground to smaller particle sizes against the increased cost of carrying out such crushing or grinding processes, until the point is reached at which the particles are too fine for efficient heap leaching. The practical balance is often struck by crushing the mined product nominally to below a figure in the range of about 5mm to about 25mm diameter, so that the overwhelming proportion of the solids (by weight) still have a diameter of over lmm.Naturally even within the range of 5 to 25mm nominal maximum, the average particle size decreases in line with the nominal maximum diameter and hence the rate and extent of gold extraction tends to increase. Particles of diameter below lmm, and especially below 100 mesh (150 microns), are more conveniently agitation leached. If the process is contemplated for heap leaching fines, tailings or for reworking material that had previously been ground for and subjected to an inefficient agitation leach, it is preferable that the particles be agglomerated to the aboveidentified particle sizes, known techniques including binding with lime. The rate of gold leaching from any size particles is, however, enhanced under otherwise identical conditions by the employment of an effective amount of slow-dissolving divalent metal peroxide.

In heap leaching, the introduction of the lixiviant is normally effected by spraying it, typically continuously, onto the top of the heap, or by forming ponds on the top of the heap which are periodically topped-up. It is usual for the rate of lixiviant introduction to be so controlled that the heap itself is not flooded, but instead the lixiviant percolates through the solids and/or over their surfaces.

Such rates must be determined for each ore or other material being leached, since it depends upon the physicochemical characteristics of that material. The employment of the peroxide under otherwise identical conditions increases the gold extraction and/or rate-thereof, irrespective of the particle size.

As a simple rule of thumb, the types of gold-bearing material that are suitable for use in the instant invention process are the same types that would be suitable in the absence of the slowly soluble peroxide. Such materials can include quartzitic minerals and metal sulphides. Where the material is excessively acidic, it can be pre-treated with a neutralising solution, that is often a dilute alkali and possible for reasons of cost includes or consists of lime.

The slow-dissolving peroxides employed herein can be produced off-site and incorporated in the process, such as in the heap in-the form of preformed solid particles. In an alternative and more cost effective process, the peroxides are formed on site by reaction between the corresponding metal oxide or hydroxide and aqueous hydrogen peroxide employing an approximately equimolar ratio, the relevant formula being respectively: MO + H202 = MO2 + H2O or M(OH)2 + H2O2 = MC2 + 2H2 in which M represents the metal. It is especially appropriate to employ calcium as the metal in such a process, because lime is often a readily available and cheap substance at the remote locations in which gold ores seem to be found.Such manufacture on site can be carried out using a small continuous flow-through reactor producing an aqueous slurry that can be pumped directly onto the desired part of the heap during its construction or premixed with part of the gold-bearing solids. Thereby, the costs are avoided of drying the product which otherwise would be necessary.

Indeed, if desired, an excess of the metal oxide or hydroxide can be employed, so as to maximise the utilisation of the more expensive reagent, the hydrogen peroxide. The resultant solid peroxide product can accordingly contain residual and unreacted metal hydroxide. The slurry also contains water. When diluent is present, a correction (pro rata) should be made in the total weight of peroxidecontaining material used to allow for the dilution of the peroxide in calculating its weight ratio to the gold-bearing solids and/or their gold content. Thus, if for example the slurry contained 25% w/w calcium peroxide and it was desired to employ 4kg CaO2 per tonne of ore containing 5 ppm gold, then the appropriate amount of slurry is 16kg per tonne.

The pregnant liquor which is separated from the golddepleted solids, eg the liquor seeping out from the base of the heap is collected and then is usually passed to a current method for recovering the gold from solution, such as passage through a carbon column or pulping with carbon, or the older method of precipitation with zinc dust. The gold-depleted cyanide solution, possibly after further purification steps, is available for recycling through the heap or into the vat, as the case may be, and naturally, in practice, its alkalinity and cyanide concentration have each been restored to the level desired by the operator for fresh solution.

The advantage of the invention process is that it enables to enhance the extraction of gold from gold-bearing solids in leaching processes. In effect, the benefit can be taken in one of two ways. In one way, the daily and/or total production of gold from a given amount of solids materials, such as a heap, can be increased and in the second way, substantially the same daily and/or total production of gold can be leached but under less favourabie conditions, such as larger particle size (hence less crushing costs) or lower cyanide concentration.

Having described the invention in general terms, it will be further and more fully exemplifiedy the following descriptions.

Examples 1 to 16 In these Examples, those numbered 3, 4, 7, 8, 11, 12, 15 and 16 are in accordance with the instant invention whereas those numbered 1, 2, 5, 6, 9, 10, 13 and 14 are included for comparative purposes.

All these Examples were carried out by recycling a dilute alkaline solution of sodium cyanide at laboratory ambient temperature constantly through a column 2.5m high having an internal diameter of 0.lm, packed with approximately 30 kg of a gold- bearing ore. When the slow dissolving peroxide, CaO2 was employed, as in all the above identified Examples that are according to the invention, the fine particles of calcium peroxide were distributed evenly throughout the ore by blending the two solids together in a rotary mixer. The lixiviant was allowed to drip onto the top surface of the ore via a filter paper to simulate spraying. The leaching was permitted to continue for up to a maximum of 28 days, and the liquor analysed periodically to assess the amount of gold that had been extracted from the ore.If the analysis showed that the amount of gold extracted was no longer increasing, the run was halted. The results shown below are either the amount at the end of the run, be it after 28 days or earlier.

A different combination of factors was employed in each Example, and these variations in the composition of the lixiviant, the ore and its content are listed in Table 3.

The ore was a pyritic ore containing approximately 5.7ppm gold and additionally containing sphalerite, chalcopyrites, pitchblende and gangue minerals, that was reasonably typical of ore from the Witwatersrand reef.

The particle size distribution of the ore encorporated in the column was either of the following designated by respectively - and + in Table 3. The -ve ore sample was obtained by roughly crushing the ve ore (nominai -15mm diameter) to below a nominal 5mm diameter.

Table 1 Particle size mm wt % of -ve wt % of +ve nominal -5mm nominal -15mm + 13.2 0 4 +9.5 -13.2 6 28 +6.3 -9.5 22 32 +3.35 -6.3 16 17 +1.7 -3.35 12 6 +0.85 -1.7 5 3 +0.425 -0.8 7 4 -0.425 Balance The five factors varied in these Examples were the nominal pH of the lixiviant, the presence or absence of Calcium peroxide, the concentration of cyanide, the rate of flow of lixiviant and the particle size of the ore. The levels designated -ve or +ve for ease of reference are summarised in Table 2.

Table 2 Factor Effect -ve +ve A pH 12 10 B CaO2, g/kg ore 0 3.75 C (CN-, mol/l 0.01 0.05 D flowrate, ml/min 1.6 10 E ore size, mm -5 -15 Table 3 Example Factor Amount of gold Relative No A B C D E extracted ppm Extraction 1 - - - - + 2.3 80 2 + - - - - 1.6 55 3 - + - - - 2.13 73 4 + + - - + 2.55 88 5 - - + - - 1.83 63 6 + - + - + 2.5 87 7 - + + - + 2.9 100 8 + + + - - 2.3 80 9 - - - + - 1.7 58 10 + - - + + 2.2 76 11 - + - + + 2.58 89 12 + + - + - 2.2 76 13 - - + + + 2.48 86 14 + - + + - 1.6 56 15 - + + + - 2.2 76 16 + + + + -+ 2.6 90 By comparing the results listed in Table 3, it is possible to rank the effectiveness of the combinations of factors at extracting gold under simulated heap-leach conditions and to determine which of the factors show particular benefits.The relative amounts are included in the furthest right hand column in order to facilitate such comparisons. The ranking of the results, the best first, is: 7; 16; 11; 4, 6, 13; 1 = 8; 10 = 12 = 15; 3; 5; 9; 14 and 2.

Of the five factors, both factor B and E can be seen to be particularly influential in determining the amount of gold extracted. This can be seen either from a detailed statistical study of the results to demonstrate the extent to which each of the variables is significant or more crudely by a simple side by side comparison of various of the Examples. The four best results were obtained when both B and E were +ve and the four worst results when both B and E were -ve.This demonstrates that factor B, the presence or absence of calcium peroxide was not only significant, but if the leeser effect of factor C is considered also, that the effect of distributing the calcium peroxide in the ore during leaching is of greater significance than the variation in cyanide concentration, a factor which the impartial person would have expected to be a particularly relevant factor. The difference between its presence or absence gan be gauged from the average of each set of 8 results. Those without calcium peroxide averaged 2.03ppm gold extraction, ie 70% of that achieved by the best result, whereas those with calcium peroxide averaged substantially more, namely 2.44ppm gold, ie 84% of that achieved in the best result.The potential benefit from adding the calcium peroxide was greater than that actually demonstrated, in that the increase in the extraction of gold was still continuing for many of those Examples employing calcium peroxide, whereas for most of those Examples not employing calcium peroxide increase in the extraction of gold had stopped.

Thus, the eventual difference between the two sets of results would have been even greater.

The results in Table 3 also show that variation in pH in the range of pH10 to pH12 of the lixiviant did not change markedly the amount of gold extracted, and that when the calcium peroxide was also present, very good extraction occurred.

The average amount of gold extracted in the 16 Examples after 5 days was 1.77ppm (+/-0.006ppm) and the calculated effect of the calcium peroxide was +0.36ppm +/-0.012ppm, so that within a 99.5% confidence limit, its presence can be said to have a significant effect. Likewise, after 20 days, the average amount of gold extracted was 2.15ppm +/-0.03ppm and the measured effect of calcium peroxide being present was +0.42ppm, +/-0.06ppm, so that within the 99% confidence limit the calcium peroxide has a significant effect.

Examples 17 to 19 In these Examples, all of which are according to the present invention, the apparatus and procedure of the preceding Examples was repeated, using a 50/50 weight mixture of the -5mm and -15mm ore samples, at pH 12, a flow rate of l0ml/min and a cyanide concentration of 0.02moles/litre. The amount of calcium peroxide employed in Example 17 was 1.0g/kg ore, in Example 18 was 1.5g/kg ore and in Example 19 was 2.5 g/kg ore. In Example 17, the calcium peroxide was distributed evenly in the lower half of the ore only, whereas in Examples 18 and 19, it was distributed throughout the ore.The gold extraction was measured after 5 days for each, and found to be intermediate in effect between having no calcium peroxide as in Examples 9/10 and 5g/kg calcium peroxide present as in Examples 11/12 with which direct comparison can be made. The result in Example 17 was an increase in extraction of 0.07ppm gold, in Example 18 of 0.12ppm gold and in Example 19 of 0.2lppm gold. This set of results indicates that the extent of increase in gold extraction varies in line with the increase in the relative amount of calcium peroxide present.

In a modification to Example 19, after 30 days, a further amount of solid calcium peroxide, 10g, was sprinkled onto the top of the column, ie equivalent to an extra 0.3g/kg ore. The amount of gold extracted jumped.by about 0.1/0.2 ppm compared with the predicted level indicating that at least for a short while it is possible by top dressing to augment beneficially gold extraction from a column in which the level of calcium peroxide has become depleted.

These Examples demonstrate that when an alkali metal cyanide solution is brought into contact with gold-bearing solids in the presence of a poorly soluble alkaline earth metal peroxide, the extraction of gold into the lixiviant is improved.

Claims (13)

1. Claims:

   1A process for the extraction of gold from solids materials containing it in which a dilute aqueous alkaline solution of an alkali metal cyanide is brought into contact with the solids material containing gold in the presence of a substance which generates oxygen in situ, and contact is maintained until at least a fraction of the gold is taken into solution which is characterised by employing an effective amount of a poorly soluble divalent metal peroxide as the substances which generates oxygen in situ.
2. 2. A process according to claim 1 in which the aqueous alkaline solution of the alkali metal cyanide is introduced onto the top surface of a heap of said solids materials, is permitted to percolate through said solids materials whereby gold passes into solution and the solution is collected thereafter.
3. 3. A process according to claim 1 or 2 characterised in that the alkali metal cyanide solution has a concentration of from 0.005 to 0.02 moles/litre.
4. 4. A process according to any preceding claim characterised in that the alkali metal cyanide is sodium cyanide.
5. 5. A process according to any preceding claim in which the alkali metal cyanide solution is employed at a pH of from about pH 9 to about pH 13.
6. 6. A process according to any preceding claim characterised in that the amount of metal peroxide employed is selected in the range of from 1 to 20 parts w/w per 1000 parts of gold-bearing material.
7. 7. A process according to claim 6 in which the amount of metal peroxide is from 1 to 10 parts w/w per 1000 parts of gold-bearing material.
8. 8. A process according to any one of claims 1 to 5 in which the ratio of a) the amount of metal peroxide per 1000 parts w/w of gold-bearing material to b) the gold content of the material expressed in ppm w/w is selected in the range of from 0.5:1 to 1.2:1.
9. 9. A process according to any preceding claim in which the metal peroxide employed is calcium peroxide.
10. 10. A process according to any preceding claim characterised in that the metal peroxide is incorporated into the heap during the course of construction of the heap.
11. 11. A process according to claim 10 in which the metal peroxide is incorporated into the heap in the form of an aqueous slurry that has been produced adjacently by reaction between aqueous hydrogen peroxide and the corresponding metal oxide or hydroxide or salt that generates either.
12. 12. A process for extracting gold from gold-bearing material substantially as described herin with respect to any one of Examples 3, 4 7, 8, 11, 12, 15, 16, 17, 18 or 19.
13. 13. A leaching process for extracting gold employing a poorly soluble metal peroxide in conjunction with an alkaline cyanide solution and substantially as described herein with respect to any novel feature or novel combination of features.

    1+. Gold-containing solutions whenever obtained by a process according to any preceding claim, and solid gold recovered from such solutions.