WO2000015856A1 - A process for gold extraction - Google Patents

Abstract

A process for the extraction of gold and/or silver from gold and/or silver bearing ore particles or concentrates, said process including the stages of: A. filling a reaction vessel with said ore particles or concentrates, and B. circulating a gold and/or silver leaching solution through the reaction vessel such that a fluidised bed of said ore particles or concentrates is achieved.

Description

A PROCESS FOR GOLD EXTRACTION TECHNICAL FIELD

This invention relates to the field of gold and silver extraction, and particularly the extraction of gold and silver from coarse crushed and milled ore and gold and silver that has been concentrated from ore by gravity separationr flotation and any other concentration technique. BACKGROUND TO THE INVENTION

Conventional gold processing facilities use a number of standard technologies in order to recover gold and any associated silver from the as- mined ore.

Conventionally, the ore and its associated waste rock is first crushed and then ground to a size at which most or all of the gold or the minerals containing the gold are liberated from the other minerals, or the gold is sufficiently exposed to be dissolved during the subsequent leaching process. The size to which the ore is ground is dependent on the liberation size of the gold and associated silver but, commonly, 80% of the ground ore would be finer than 75 microns. Water is added during the grinding stage and the gold and silver are leached from the ground pulp under controlled chemical conditions normally with cyanide solution, although other lixiviants, such as thiourea, are or could be used. The solubilised gold and silver are recovered from the leach solution by either adsorption onto carbon or by zinc precipitation. The carbon adsorption methods of recovering gold are known as the carbon-in-leach (CIL) and carbon- in-pulp (CIP) processes. The zinc precipitation method is known as the Merrill- Crowe process. Many, but not all, gold bearing ores contain particles of native gold, native silver or electrum which, after they have been liberated during grinding, are at a sufficiently coarse size such that they may not be completely dissolved by cyanide in the leaching time available. Therefore, it is common practice to remove these coarse particles of precious metals before the leaching stage using gravity concentration techniques. The amount of gold recovered from ground ore by gravity concentration can vary from as low as 5% to above 50% of the total gold in the ore. Many gravity concentration techniques are employed for this purpose including, but not limited to: strakes, jigs, spirals, shaking tables, Reichert cones, Johnson drums and Knelson concentrators and other centrifugal concentrating devices. Gravity concentration is normally used within the grinding circuit, but may also be applied to the ground product and is sometimes used after the leaching process. Gravity concentration is also used to recover free gold from base metal and other flotation concentrates, such as copper concentrates.

Where gold is locked at a relatively fine size within sulphide minerals, such as pyrite and arsenopyrite, the froth flotation process is used to recover the gold bearing sulphide minerals into a flotation concentrate. Depending on the size at which the gold occurs within the sulphide minerals, further grinding of the sulphides to an ultrafine size may release more gold for leaching. In this event, ultrafine grinding of the concentrates is employed followed by high intensity cyanidation. The gravity concentration process separates mineral particles with a high specific gravity from those with lower specific gravities. Therefore, the primary gravity concentration stage recovers all particles with a high specific gravity as well as the precious metals. These particles will comprise grinding steel, and any sulphide minerals or other heavy minerals present in the ore. These unwanted particles may release undesirable or unsafe gases, vapours and dust during smelting of the gold and silver or affect the quality of the bullion produced. Consequently, they must be removed by further upgrading of the primary gravity concentrate. This upgrading is commonly performed by a single process or a combination of processes that include tabling and other secondary gravity concentration devices, magnetic separation, calcining, acid digestion and amalgamation with mercury. As an example, the current process used for upgrading the primary gravity concentrate produced by a Knelson concentrator at the Union Reefs mine in Australia is shown in Figure 1. The magnetic separation stage is used to remove grinding steel and calcining is used to remove the sulphide minerals. The multiplicity of the processing stages is needed to ensure that the final product is suitable for direct smelting. Some of the problems that have been identified with this particular upgrading process include: the low efficiency of the first tabling stage due to the wide size distribution of the gold, the platelike nature of some of the gold which has been flattened during grinding, and the high grinding steel and sulphide mineral content of the Knelson concentrate; each Gemini tabling stage, magnetic separation stage and the calcining stage all result in gold losses which, in combination, are substantial; the process is labour intensive and very time consuming; - the extended exposure of material rich in coarse, free gold is a- considerable security risk; the daily estimate of gold recovered by gravity concentration can be very inaccurate for daily metallurgical accounting purposes; the calcining stage results in fumes containing sulphur dioxide, arsenic trioxide and lead oxide all of which can cause considerable discomfort at low concentrations and significant health and safety concerns at high concentrations or with prolonged exposure; cleaning out and handling of the dust from the calcining fume and dust collection baghouse creates similar health and safety concerns; - incomplete calcining results in similar fume generation during the smelting stage.

Most of these problems are common to all process flowsheets that upgrade primary gravity concentrates by similar means. Significant health and safety concerns also exist where amalgamation of the gold with mercury or acid digestion of the sulphide minerals are employed.

In summary, the methods most commonly employed for upgrading primary gravity concentrates to direct smelting quality results in a reduction in the amount of gold recovered by gravity concentration and create health hazards. Similar problems arise in respect of other gold and silver concentrates produced from gold and silver bearing ores, and not just in relation to the extraction of gold and silver from gravity concentrates. In fact, it has been observed that concentrates which have been derived from the treatment of gold and silver bearing ore by flotation normally contain significant amounts of sulphides and gold that may be too large to effectively dissolve in the standard CIL process. Consequently, extraction of gold and silver from these concentrates can encounter the same problems such as poor gold recovery and health hazards from the presence of arsenides and sulphides.

An alternative method of extracting gold and silver from concentrates that is less commonly used is the High Intensity Cyanidation (HIC) process. This employs elevated concentrations of cyanide to increase the dissolution rate of gold which may be slow due to its coarse particle size or because of interference from other mineral species such as sulphides. To assist the dissolution kinetics the process can be performed at elevated temperatures and pressures to maintain high dissolved oxygen concentrations which, if lower, can be rate limiting at the high cyanide levels employed (Marsden, 1992). The HIC process is used at the Stawell gold mine in Victoria, Australia to upgrade gravity concentrates produced by a Knelson concentrator that contain very high levels of various sulphide minerals. While this leaching process results in a high level of gold extraction from the gravity concentrates, better security of the gold bearing material, and reduces some of the health risks associated with the upgrading techniques previously described, it is performed in stages over a four day period and involves agitated vessels, transfer pumps and recovery of the gold bearing pregnant solution using decantation and repulping techniques (Mason, 1996).

This results in a process which is complicated and very time consuming. Other disadvantages of the HIC process using agitated vessels include: awkward transfer of solids and solution; difficulty in maintaining solids in suspension; large volumes of wash water are required; relatively long cycle times; and - reagent costs. Accordingly, it is an object of the fluid bed cyanidation process invention to provide a process for the improved extraction of gold and silver from gold and silver bearing ore particles or concentrates, in particular gravity concentrates.

It is also an object of this invention to provide a system whereby the extraction of gold and silver from gold and silver bearing ore particles ot concentrates, in particular gravity concentrates, is achieved.

It is a further object of this invention to provide apparatus by which the improved extraction of gold and silver from gold and silver bearing ore particles or concentrates, in particular gravity concentrates, occurs. It is also an object of this invention to provide both gold and silver which is high in grade and low in contamination. STATEMENT OF INVENTION

Accordingly, a first aspect of the invention provides a process for the extraction of gold and/or silver from gold and/or silver bearing ore particles or concentrates, said process including the stages of:

A. Filling a reaction vessel with said ore particles or concentrates, and

B. Circulating a gold and/or silver leaching solution through the reaction vessel such that a fluidised bed of said ore particles or concentrates is achieved.

A related aspect of the invention provides a process for the extraction of gold and/or silver from gravity concentrates, said process including the stages of:

A. Filling a reaction vessel with said gravity concentrates, and B. Circulating a gold and/or silver leach solution through the reaction vessel such that a fluidised bed of said concentrates is achieved.

Preferably, said process also includes the stage of pre-washing said reaction vessel with a pre-wash solution to fluidise the concentrate bed and removing fine particles by elutriation. Also preferably, said process includes the stages of post-washing said reaction vessel with a post wash solution on completion of stage B. Most preferably, the process includes the stage of extracting the gold and/or silver from the leach solution and the post wash solution. This is preferably performed using a dedicated electrowinning cell. Zinc precipitation can also be used. Preferably the pre-wash and post-wash solutions consist of water. -

Further preferably, the ratio of solids being leached to leaching solution is 1 :2 or less.

Preferably, the leaching solution includes cyanide or any other leaching agent that dissolves gold and silver. More preferably, the gold/silver leach solution includes a cyanide solution such as sodium cyanide (NaCN) and and a hydroxide such as sodium hydroxide (NaOH). Most preferably, the cyanide concentration is of high strength and in sufficient quantity to dissolve the gold and silver content in the particles concentrates being treated.

Preferably, leaching is performed using a solution flowrate which gives adequate fluidisation of the bed.

The instant fluid bed cyanidation process invention is a highly compact and efficient process that has a large number of advantages including much greater security, rapid gold extraction, high gold extraction and reduction of goldroom labour requirements. A specific advantage of the process is the elimination of toxic fumes and dusts from the goldroom that are generated during the pyrometallurgical processing of upgraded gravity and other concentrates containing arsenopyrite and other sulphides.

Further, it has been found that the above process advantageously reduces the costs associated with the treatment of gravity concentrates. Also, the substantially higher recovery achieved from the treatment of gravity or other concentrates by the above process will result in potential cost savings and recovery improvements during the subsequent extraction of the remaining gold and silver in the ore by the CIP or CIL processes.

It has been demonstrated that gold recovery from the gravity concentrates produced at the Union Reefs Gold mine using the process and apparatus of the instant invention is well above the estimated 77% achieved by the current methods employed. Gold recoveries in excess of 95% have been achieved in less than 24 hours.

Finally, this new fluid bed cyanidation process invention eliminates many of the problems associated with traditional HIC in agitated vessels as summarised above and allows the HIC process to be performed in a time efficient manner using simple and compact equipment. -

A major advantage of extraction using the instant fluid bed process invention of extraction is the degree of agitation that is achieved. Under conditions of uniform leach solution distribution, the solid particles are free to move independently and are constantly exposed to the vertically moving leach solution.

The gold and silver dissolution rates are preferably enhanced through the use of a leach accelerator reagent. LeachWell™GC is a reagent supplied by Mineral Process Control Pty Ltd which is preferably added to the leaching solution at a concentration of around 0.25% or less. The gold and silver leaching rates may also be enhanced by increasing the dissolved oxygen level in the leaching solution by air or oxygen injection or the addition of peroxide or other oxidants.

In accordance with a further aspect, the present invention also provides a system for removing gold and/or silver from gold and/or silver bearing ore particles or concentrates, said system including a reaction vessel for holding a volume of gold and/or silver bearing ore particles or concentrates, means for introducing gold and/or silver bearing ore particles or concentrates into said reaction vessel, a leach solution inlet to said reaction vessel and a leach solution distribution means located in a lower region of said reaction vessel whereby leach solution can be introduced into said reaction vessel to pass through said distribution means and fluidise the gold/silver bearing ore particles or concentrates, recirculation means to recirculate leach solution after passing through said gold and/or silver bearing ore particles or concentrates back to said leach solution inlet to said reaction vessel, means for removing the gold and/or silver bearing ore particles or concentrates from said reaction vessel after leaching of the gold and/or silver into the leach solution, and gold and/or silver recovery means for removing gold and silver from said leach solution. Conveniently, the system further includes a leach solution holding vessel and pumping means to pump leach solution from the holding vessel to the leach solution inlet to said reaction vessel.

Preferably, a leach solution return means is provided to return leach solution from said reaction vessel to the holding vessel. _

Conveniently, the gold and/or silver recovery means comprises electrowinning apparatus with the leach solution being pumped through said electrowinning apparatus by the aforesaid pumping means upon completion of a leaching process stage with the leach solution being returned to the holding vessel after passage through said electrowinning apparatus.

In accordance with a still further aspect, the present invention also provides a leach solution distribution means for use in the fluidised reaction vessel for gold and/or silver bearing ore particles or concentrates, said distribution means including a first means for supporting a particle layer while allowing a leach solution to pass therethrough, a second means located above said particle layer configured to allow a leach solution to pass therethrough, said first and second means being configured to retain particles of said particle layer therebetween and third means located upstream of said first means arranged to distribute leach solution flow substantially across the full flow surface of said first means.

The distribution means is designed to preferably meet the following requirements:

Uniform leach solution distribution to eliminate channelling and dead zones in the fluidised bed; - Minimal ingress of solids into the distributor system;

A low pressure drop across the distributor system; and

Simple and inexpensive manufacture and ease of removal for maintenance. The total cycle typically comprises the following stages:

1. Column Fill

2. Pre-wash

3. Leaching 4. Post-wash -

5. Solids Discharge

6. Gold Recovery

For example, typically the reaction vessel is first filled to a pre-set level with the ore particles or concentrates to be treated. The pre-wash stage preferably eliminates fine slime material from the gold and silver bearing ore particles or concentrates and ensures the bed has been completely fluidised for optimum leach-solution distribution during the leaching stage. The flowrates for all stages minimise entrainment losses of gold and/or silver particles in solution to those below a size which will easily be recovered in the subsequent CIL or CIP processes.

The leach solution typically comprises cyanide, caustic soda and LeachWell™ GC at the reagent concentrations required. Other strong oxidants may be substituted for LeachWell™GC.

During the leaching stage, the leach solution is preferably circulated by a pump up through the reaction vessel and back to the leach solution holding tank for the specified leaching period. If the slime particles have been removed during the pre-wash stage, the clarity of the leach solution is excellent for the duration of the leaching stage.

At the completion of the leaching stage, supernatant pregnant solution may be first drained from the reaction vessel back to the leach solution holding tank. The bed of ore particles or concentrates can then be washed with water to remove any solubilised gold and silver. This post wash stage can be achieved by displacement washing of the bed from above by flow of water through the bed. Post wash solution is combined with the pregnant leach solution or alternatively can be used as the make up solution for the next leach cycle.

One of the principal advantages of the fluid bed cyanidation process over the use of HIC in agitated vessels is that the washing of the solids can be undertaken with substantially improved efficiency. This reduces the final quantity of solution for electrowinning and maintains a high gold/silver solution tenor in the pregnant solution.

Discharging of the residue solids is achieved by the incorporation of an outlet located just above the distributor and isolated with a valve. The solids are fluidised with water and discharged through the outlet.

Gold recovery is performed using the electrowinning process or other alternative processes such as zinc precipitation. An advantage of the electrowinning process is that due to the small volume of solution, high gold/silver tenor and as a consequence high current efficiencies, allows installation of a small electrowinning cell.

The consumption of LeachWell™GC reagent and possibly cyanide can be reduced by increasing the amount of dissolved oxygen in the leach solution using air or oxygen sparging or peroxide or other strong oxidants such as sodium perborate.

DETAILED DESCRIPTION OF THE DRAWINGS

Now follows a detailed discussion of the preferred embodiments of the various aspects of the invention. In this discussion, reference is made to the following Figures: Figure 2a) which provides an overview of one preferred system according to the invention,

Figure 2b) which provides a preferred reaction vessel design, and Figure 2c) which provides a preferred Distributor design and plan view of a preferred distributor base. From Figure 2a) it can be seen that in the preferred full scale plant structure, a settling cone (1) is retained to receive the gravity or other gold/silver concentrates.

The settling cone discharge is piped to the reaction vessel (3). A decant outlet (5) positioned above the settled solids allows for the decantation of supernatant solution from the disengagement zone within the reaction vessel

(3). At the top of the reaction vessel is the overflow outlet (6) which is a launder arrangement. Both the decant and overflow outlets are valved to direct flow either to the reaction vessel leach solution tank (7) or to the reaction vessel discharge pump (8).

At the base of the reaction vessel (3) is an outlet (9) piped directly to reaction vessel discharge pump (8) to facilitate the return of the leached solids to the main processing plant (10). -

Delivering to the fluidising distributor (21) at the base of the reaction vessel (3) is a variable speed centrifugal pump (12). The pump (12) is connected to water storage tank (1 1) outside of the goldroom for the pre-wash stage and the post-wash stage. Connected to the reaction vessel leach solution tank (7) is a variable speed pump (13). The pump (13) discharge is valved to direct either leach solution via the distributor (21 ) to the reaction vessel (3) or pregnant solution

(17) to a dedicated electrowinning cell solution feed tank (14).

Connected to the electrowinning cell solution feed tank is a variable speed pump (15). The pump discharge is valved to direct pregnant solution to the electrowinning cell (18) or barren solution to the solution transfer tank (16).

Connected to the solution transfer tank (16) is a variable speed pump (19). The pump discharge is valved to direct barren solution to the reaction vessel leach solution tank (7) as make up solution or to the main processing plant (10).

Post-washing done by the down flow displacement method via spray water added at the top of the reaction vessel (3) above the leached solids and is allowed to drain through the distributor (21) into reaction vessel leach solution tank. The reaction vessel leach solution tank (7) is fitted with an agitator (22) to facilitate mixing of reagents prior to leaching.

From Figure 2b) it can be seen that the reaction vessel (3) is preferably designed to keep the rise velocity at the top of column below 6 mm/s which has been found to minimise entrainment losses. A rise velocity at the base of the column of around 22 mm/s is achieved using the preferred design dimensions. The capacity of a reaction vessel (3) is designed to suit the process requirements. Two simplified equations are used to describe the flow in a vessel of this description and are shown below (Levenspiel, 1986). The first calculates the minimum fluidisation velocity and the second gives the settling velocity for any given particle. The minimum fluidisation velocity is that velocity at which the weight of the bed is just balanced by the pressure of the solution passing through it.

Where

Umf minimum fluidisation velocity

Ut terminal settling velocity dsph effective spherical particle diameter (after accounting for sphericity) density of solids

P, density of solution

g = acceleration due to gravity 1 = viscosity of solution

Using these equations and the rise velocities described above, native gold particles below 400μm diameter will be fluidised and gold particles above 30μm will not be lost through entrainment in the reaction vessel overflow. Gold particles above 400μm diameter, although not fluidised, will be subjected to good contact with the leaching solution since other particles of the same size, but of lower density, will be fluidised leaving the coarse gold at the base of the reaction vessel. The size range of the gold/silver particles retained and fluidised in the reaction vessel can be altered by adjusting the dimensions of the reaction vessel as required. From Figure 2c) it can be seen that the distributor (21 ) preferably consists of a bed of sized inert material (25) approximately 30 mm thick contained between two layers of screen mesh (26, 27) with slotted 700μm apertures to minimise blinding. The sized inert material is in the size range 1.2 to 2 mm to ensure it is retained by the screen and to give the required voidage characteristics. In addition, the sized inert material should be cubic to avoid losses through the slotted mesh. The bottom mesh (27) is supported underneath by a heavy duty mesh constructed of 5 mm steel bar (28).

At the bottom of the distributor is the distributor base (29) which receives the leach solution or wash water from the pump. The leach solution or wash water are first deflected by two deflector plates (30) to ensure uniform distribution around the base. A baffle plate (31) above the deflector plates (30) converts the turbulent solution flow into uniform vertical flow for entry into the distributor. The pressure drop across the distributor under a range of flows has been measured in a pilot plant at around 5-10 kPa which is remarkably low. The bed of concentrate resulted in a further pressure drop of around 5 kPa. These would need to be scaled up for a plant installation.

Traditional problems of fluidised bed equipment associated with poor fluid distribution and channelling have been eliminated with the new distributor design. The design also virtually eliminates solids migration into the distributor and therefore allows it to act as a filter which results in excellent post-washing efficiency at the completion of leaching using the down flow displacement post- washing method. The most preferred process for the extraction of gold and/or silver from gold and/or silver bearing ore particles or concentrates is now described with reference to Figure 2a).

Preferably, before filling the reaction vessel with the ore particles or concentrates, water should be pumped into the reaction vessel to just cover the distributor. This is to protect the distributor and minimise the ingress of solids into the distributor system. A key requirement in the successful operation of a fluidised bed system treating a wide range of particle densities and sizes is to ensure that the bed has been completely fluidised. This requires the segregation of the bed into layers of particles with equal minimum fluidisation velocities so that no channelling or dead zones will result. To achieve this, the pre-wash stage is used which alse eliminates slime material from the solids in the bed. This stage begins with water being pumped into the reaction vessel at a rate which results in fluidising and stratifying the solid particles according to size and specific gravity. Slime is washed from the solids and leaves the top of the reaction vessel via the overflow launder. The solids are allowed to settle and water is removed by decant and then by drainage through the distributor.

Once the leach solution in the holding tank has been adjusted to contain the required concentrations of reagents, the leaching cycle commences. The leach solution is pumped continuously into the reaction vessel via the distributor at a flow rate which fluidises the concentrate bed and discharges via the solution overflow launder back into the holding tank. The solution is continuously recycled in this manner until dissolution of the gold and silver in the concentrate is complete.

At the completion of the leaching stage the decant outlet is used to drain the pregnant solution from the reaction vessel disengagement zone into the pregnant solution tank. The post-washing stage then commences. This is used to remove solubilised gold and silver from the bed of leached concentrates by the downflow displacement post-washing technique.

The remainder of the solution is drained from the column through the distributor prior to the addition of water above the solids. The water addition for this technique is in the form of a spray such that the water is distributed evenly. The post-wash solution is collected and combined with the pregnant solution in the pregnant solution tank.

The drained residue solids is fluidised with water and discharged from the reaction vessel. Vigorous fluidisation may be required to remove the last of the solids. The recommended final stage of gold and silver recovery involves electrowinning to recover the gold and silver from the pregnant solution and post-wash water that are now contained in the pregnant solution tank. The electrowinning cell would be sized to ensure that the spent electrolyte at the end of the electrowinning stage can be discarded. -

The very high gold grade of the pregnant solution that has been achieved in test work indicates that high cell currents can be used in the electrowinning stage to achieve rapid deposition of the gold and to take full advantage of the high current efficiencies at such concentrations. The applied current may have to be reduced over the duration of the electrowinning stage to ensure the gold deposit remains cohesive and the quantity of side-reactions is minimised as the gold tenor of the electrolyte decreases with time.

It is intended that the leaching and gold recovery processing cycles be carried out as independent batch processes and installed equipment sized such that each should be completed within a 24 hour period so that the equipment is available for processing the next daily batch of gravity or other concentrates. EXAMPLE A

A pilot plant was constructed at the Union Reefs Gold Mine, N .,

Australia, to test and tune the fluid bed cyanidation process of the instant invention and to generate sufficient design information for a full scale installation. A schematic diagram of the pilot plant is shown in Figure 3a) and a schematic diagram of the pilot plant distributor is shown in Figure 3b).

The leach reaction vessel consisted of a 260 mm ID poly pipe column (32) with a sand filter distributor (33) at the base of the reaction vessel to distribute solution evenly and to prevent solids ingress into the distributor system.

The reaction vessel was fed by a variable speed centrifugal pump (34) which drew from one of two drums (35, 36). One drum contained the leach solution (35), the other fresh water (36) for washing. The pump was installed with a variable speed drive to enable suitable flowrates to be established for the process. In order to raise the fluidisation rise velocity without causing solids to be lost to the column overflow, a conical section (37) was installed in the base of the reaction vessel. This increased the rise velocity at the base of the reaction vessel by two and a half times. Overflow from the reaction vessel flowed through a hose (38) and during the leaching stage returned to the leach solution drum (35).

Three full scale trials were performed using the pilot plant. The first was used to verify that the process was practicable, and the last two were used to tune the process. The feed material used for the trials was a gravity concentrate produced by a Knelson concentrator. This was collected from the discharge of the settling cone during normal upgrading of the concentrate. The leached residue from each pilot plant trial was dumped from the reaction vessel, collected and dried. Each residue was then thoroughly mixed and split into half by riffling. One half was then split into four samples by riffling. Each sample was fire assayed in duplicate.

Solution samples were collected from the reaction vessel discharge at 0.5, 1 , 2, 3, 4, 5, 6, 7, 21 and 24 hours and assayed for gold by solvent extraction followed by A.A. analysis. Trial One

The reaction vessel was used in this trial without the addition of the conical section. Some of the key criteria for this trial are shown below:

Feed: 35 kg solids;

Leach solution: 105 litres of water; giving a solid/liquid ratio of 1 :3: - Reagent additions: 175 g NaOH, 1750 g NaCN, 525 g LeachWell™KC;

Initial cyanide concentration: 1.67%;

Initial LeachWell™KC concentration: 0.50%; and

Leach solution flowrate: 0.30 litres/sec.

The gold extraction curve is shown in Figure 4. Key results from the trial include:

77% gold extraction after 8 hours;

94% gold extraction after 20 hours; Head grade: 1.36% Au;

Pregnant solution: 4250 ppm Au;

Electrowin Feed: 2125 ppm Au; and

Residue grade: 812 ppm Au. Comparison of the gold extraction curve, as shown in Figure 4, with those obtained in bench scale bottle roll tests indicated that the kinetics of the process were relatively low, probably to due physical reasons. Consequently, the bottom section of the reaction vessel was then fitted with the conical section to significantly increase the fluidisation velocity and eliminate possible dead zones in the fluidised bed. Trial Two

In addition to the inclusion of the conical section in the column, a second trial was performed using 50% higher reagent concentrations.

Key criteria for this trial were as follows: - Feed: 35 kg solids;

Leach solution: 105 litres of water, giving a solid/liquid ratio of 1 :3;

Reagent additions: 263 g NaOH, 2625 g NaCN, 788 g LeachWell™KC;

Initial cyanide concentration: 2.5%;

Initial LeachWell™KC concentration: 0.75%; and - Leach solution flowrate: 0.32 litres/sec.

The extraction curve is shown in Figure 5a) and the post-washing profile in Figure 5b). Key results from the trial include:

97% gold extraction after 7 hours;

99.4% gold extraction after 24 hours; - Head grade: 1.04% Au;

Pregnant solution: 3440 ppm Au

Electrowin Feed: 1720 ppm Au; and

Residue grade: 66 ppm Au .

This trial was successful in proving the fast leach kinetics achievable with the fluidised bed equipment design. The very slow leaching rate after the first eight hours, which is suggested in the extraction curve of Figure 5a), is thought to be the result of leach solution evaporation losses which were not measured. This would have increased the gold solution assays and given the impression that leaching was still occurring.

It is therefore likely that the final extraction of >99% was achieved in less than 12 hours. A small quantity of elutriated material totalling 85 g was recovered during the pre-washing cycle with an estimated grade of 4000 ppm Au. Microscopic examination showed that the free gold was fine (<30 microns in size). This resulted in a very small loss of extraction (less than 0.1 %) and has not been included in the overall results. However, the fine nature of the gold suggests it would have been easily recovered in the subsequent CIL circuit.

The post-washing technique used in this trial involved pumping fresh water at a low flowrate (0.13 l/s) up through the column. This resulted in a washing efficiency in excess of 99.9% with 105 litres of water. Traditional High

Intensity Cyanidation uses up to three times as much water and produces lower washing efficiency.

Further, the post-washing flowrate used in Trial 2 was not optimised and there is significant scope for a reduction in the quantity of water required for post-washing. Trial Three This trial was used to test a lower leach solution volume whilst maintaining the same reagent concentrations. In addition a weir was placed ahead of the column overflow pipe. This was thought to improve the flow characteristics of solution at the overflow and reduce solids entrainment losses.

In addition, dissolved oxygen, cyanide concentration and LeachWell™KC activity were monitored throughout the trial.

Key criteria for this trial were as follows:

Feed: 31 kg solids;

Leach solution: 70 litres of water, giving a solid/liquid ratio of 1 :2;

Reagent additions: 175 g NaOH, 1750 g NaCN, 525 g LeachWell™KC; - Initial cyanide concentration: 2.5%;

Initial LeachWell™KC concentration: 0.75%; and

Leach solution flowrate: 0.30 litres/sec. The extraction curve and other results are shown in Figures 6a)-e). Key results from the trial include:

95% gold extraction after 7 hours;

97.1 % gold extraction after 24 hours;

Head grade: 0.61 % Au; - Pregnant solution: 2700 ppm Au;

Electrowin Feed: 1800 ppm Au; and

Residue grade: 177 ppm Au.

The feed for this trial contained substantially less coarse material, particularly sulphides, than the previous two trials. This was due to the segregation of material that occurs within the gravity concentrate settling cone and the variable nature of the discharge that inevitably results. This had a number of implications:

The mass used (as measured on the dried residue) was lower than the expected 35 kg despite a higher volume being used. Bulk density for this material was measured to be 1.8 t/m3.

The gold feed grade was lower than previous trials and was thought to be lower than the average grade of the Knelson concentrates normally produced.

Although the overall gold extraction was lower, the extraction curve in Figure 6a) was very similar in shape to that of the second trial with only marginally slower dissolution kinetics. In addition, the trays used for holding the residue whilst drying were not properly cleaned due to operator error and so the higher residue grade was very likely to be due to contamination (The trays had previously been used for calcination of gravity concentrates). The range of residue assays from Trial Two was 63.5 to 70.1 ppm Au, whereas for Trial Three the range was 152 to 202 ppm Au. The greater variability is indicative of the occurrence of coarse gold which could only be present through contamination.

The earlier bottle-roll test results indicated that an increasing amount of sulphides generates higher gold residue assays and so the lower quantity of such sulphides in the feed for this trial suggests that the residue grade should have been comparable, or lower, than that achieved in Trial Two. The lower gold head grade in this trial would also have tended to reduce the percentage gold extraction.

Approximately 240 g of elutriated solids were recovered during the pre- wash stage. However, examination suggested that it was of lower grade than in the previous trials. The higher quantity was due to the lighter nature of the feed material, as discussed above. The overall loss of gold extraction through the elimination of this material was low.

Although the amount of solids entrained in the overflow was higher, observations suggested that the use of the overflow weir in this trial was successful in reducing the quantity of heavy material in these losses. The residual cyanide and dissolved oxygen concentrations are charted and shown in Figures 6c) and 6d) respectively.

The LeachWell™KC activity was determined by measuring the time taken for a piece of gold leaf of a specific thickness to dissolve in the leach solution, collected at various times during the leaching stage. In 5% NaCN and 2% LeachWell™KC the gold leaf should dissolve in around 15 seconds. In a solution with a similar cyanide concentration but without LeachWell™KC, the leaf should take around 10-15 minutes to dissolve. The concentrations of NaCN and LeachWell™KC used in the testwork are below these reference levels but the Leach Well™ KC activity can be interpreted relative to the dissolution rate of the gold leaf in the leach solution at the commencement of the leaching stage. An additional measurement was taken at the completion of the leaching stage whereby the residual cyanide concentration was measured and then made up to the original 2.5% concentration so that the LeachWell™KC activity could be measured in isolation. The results are shown in Table 1.

Table 1 : LeachWell™KC Activity Results The results indicate that the dissolution kinetics of the gold leaf quickly reduced from its maximum at time zero. However, the cyanide measurements showed that the cyanide concentration quickly dropped to 1.6% in the first hour which may be responsible for a significant part of this reduction. Although at the completion of the leach test a significant loss of LeachWell™KC activity was evident, the results do show that the dissolution kinetics were favourable throughout the trial. Some of the cyanide and possibly some of the LeachWell™KC will be regenerated during the electrowinning stage.

The dissolved oxygen level, the cyanidation concentration and the LeachWell™KC activity all show a very substantial reduction in the first hour. Consequently, it is believed that if the dissolved oxygen level were to be increased during this period, the loss of LeachWell™KC activity and cyanide concentration may be reduced and gold dissolution kinetics would also be increased. Among the methods available for increasing the dissolved oxygen level are air or oxygen sparging or peroxide or an other strong oxidant, such as perborate, addition.

Some oxygenation of the leach solution would be effected by the return of the leach solution to the holding tank. As the return solution breaks the liquid surface of the tank, air would be entrained and distributed throughout the solution within the holding tank. Measurements taken during Trial 3 indicated that this increased the dissolved oxygen levels by around 1 to 2 ppm. Lower volumes of leach solution would favour an increase in the dissolved oxygen levels since the number of times the total leach solution is recycled would be higher.

Post-washing of the leached residue was accomplished by downflow displacement in Trial 3. First, the pregnant liquor was allowed to drain from the column. Fresh water was then introduced into the column and allowed to drain through the bed under gravity flow. Exiting solution was collected and sampled every 10 litres. The first 30 litres was added in 10 litre increments. This would have resulted in some back mixing of the water with the pregnant solution before passing through the solids, resulting in inefficient washing. The last 20 litres was added in small increments. In practice water should be preferably added continuously at a rate matching the flow through the bed. The rate of drainage from the column was measured to be approximately one litre per minute. The washing profile is shown in Figure 6b). A washing efficiency of 99.9% was achieved after about 40 litres of wash water addition. This is less than 40% of the volume used for the fluidisation (upflow) washing technique in Trial 2 for a similar washing efficiency. Further reductions in the wash water requirements for this technique are thought to be possible with a slow continuous addition of water. In addition the base of the distributor was not designed to facilitate efficient drainage. A distributor base plate using an angled base rather than a flat base will reduce liquid holdup and increase the drainage rate. Furthermore, such a high washing efficiency as achieved in Trial 3 may not be required in practice and this would lead to further reductions in the post-wash water volume.

A fluidisation curve was generated which examined the pressure drop across the pilot plant column for a range of solution flowrates or rise velocities.

The results are shown in Figure 6e. The curve follows the shape expected from the literature with slight departures for two reasons. Firstly, the conical section at the bottom decreases the superficial (or rise) velocity as the solution moves up the column. Secondly, the material has a wide distribution of both particle size and density. This results in a material that has range of minimum fluidising flowrates. Inspection of the curve reveals two important aspects for the operation of the process:

1. The curve for decreasing flow contains an inflection at around 21 mm/s superficial velocity indicating that the majority of the solids are fluidised above this point. The pilot plant testwork was conducted with a maximum solution velocity of 16 mm/s and so there may be benefits in operating with a higher solution velocity.

2. As previously noted it is important that the bed be fully fluidised before leaching commences so that the bed is segregated into layers of solids with equal fluidisation velocities. The fluidisation curves for increasing and decreasing flow show a substantial difference at lower flows which is the result of this effect. The curves indicate that the bed must be fluidised at above 21 mm/s superficial velocity prior to the start of the pre-wash stage.

The results from Trial 3 have shown that the leach solution volume and the post-wash water volume can be reduced. Further reductions in the leach solution and post-wash water volumes may be possible. These would increase the dissolved oxygen levels, reduce the volume to electrowinning and reduce reagent consumptions to even greater extents.

It is believed that there is little scope for a reduction in the initial cyanide concentration in the leach solution. However, improvements in solution oxygenation may allow a reduction in the quantity of LeachWell™KC added. This is because the LeachWell™KC reagent has both oxidising and catalytic components. The catalytic component continues to function at lower concentrations. Higher concentrations of LeachWell™KC are used merely to enhance the dissolved oxygen level in solution and the gold dissolution reaction. Thus, alternative means of oxygenation would allow lower addition rates of LeachWell™KC to be used.

Due to the much higher cost of the LeachWell™KC reagent relative to cyanide, it may be possible to increase the cyanide level and decrease the LeachWell™KC addition and still maintain good gold dissolution kinetics.

The efficiency of the post-washing technique in terms of the volume of water used has a substantial influence on both the gold grade and quantity of solution delivered to electrowinning and the proportion of reagents that can be recycled. It is very important therefore that the post-washing technique is optimised. Of the two post-washing techniques used in the pilot plant trials, the downflow displacement technique was shown to be far superior in terms of the quantity of wash water required. However, to allow gravity flow back to the holding tank, the fluidising column would need to be raised to a higher level. Alternatively, the wash solution could be collected in a dedicated sump and returned to the holding tank by pump.

The recycling of leach solution may result in the build up in solution of material that is soluble in cyanide but which is not electrowon. This may require some or all of the spent electrolyte (other than that necessary to discard the volume of post-wash water) to be periodically bled from the circuit back to the main plant where any solubilised gold would be recovered in the CIP or CIL circuit.

The process has resulted in a substantial improvement in the recovery of gold from Knelson gravity concentrates produced at the Union Reefs mine compared to the methods currently employed. _

This improvement in gold recovery from the gravity concentrates has a number of potential benefits. These include:

Reduced losses of coarse gold;

Improved security; - Reduced residence time requirement in the CIL circuit;

Lower gold and carbon inventories in the CIL circuit;

A reduction in the number of elutions required on activated carbon; and

Lower gold and silver losses in solids and solution in CIL tailings.

Other benefits are: - Simplified and less labour intensive gold room operations;

Greater ease of smelting the higher quality gold bearing material produced in the electrowinning stage with reductions in flux and crucible usage; and

The elimination of toxic fumes and hazardous dust created during the pyrometallurgical treatment of upgraded gravity concentrates, thereby creating a more comfortable and safer workplace.

Most of these potential benefits would also accrue from the use of HIC in agitated leach tanks. The benefits of this invention relate to the improvements to the HIC process itself. These are: - Shorter cycle times;

Smaller volumes of solution to electrowinning;

Greater washing efficiency of leached residue;

Faster dissolution kinetics due to improved solution/solids contact;

Elimination of the problems associated with maintaining solids in suspension;

Reduced labour requirements;

Improved daily metallurgical accounting; Reduced capital cost due to the use of smaller and more compact equipment; and

The process equipment can be more easily totally enclosed to provide improved security. In addition, the process could be completely automated to further reduce labour requirements and increase operating efficiency. EXAMPLE B

A trial portable treatment plant was developed and used to pilot-scale test a batch fluidised bed leaching procedure utilising LeachWell™KC as a gold dissolution accelerator to maximise the recovery of gold from 1.0m3/day of gravity-won concentrate without further physical concentrating operations. The recovery of the gold from the pregnant solution was not investigated but the concept was to use electrowinning.

The design of the transportable treatment plant incorporated the following:

The concentrate was to be treated in batches with the leach cycle completed within 24 hours and the electrowinning cycle completed with the following 24 hours-

The operation of the plant was not to interfere with normal Gold Room operations;

All gravity concentrate originating from the Gold Room was to be returned as plated cathode to the Gold Room as leach residue and electrowinning tail to the Gold Room or a particular nominated area of the main plant; and - The transportable plant was to be secure and include measures to ensure full accountability.

The plant consisted essentially of a reaction vessel used for leaching the concentrate, three solution tanks and one water tank, five pumps and an electrowinning cell. With the exception of the water tank, the equipment is arranged in two transportable flat racks which are assembled on site as upper and lower modules. The equipment was designed to be operated as a section external to the Gold Room of any operation. Electric power, process water, potable water and compressed air are drawn from the test site.

The reaction vessel was an inverted truncated conical vessel fitted with a distributor and filter at the base and through which a rising stream of water of leaching solution could be flowed to give rise to a diminishing rising velocity as the fluid passed up the conical section.

Raw or process water is used for concentrate movements and pretreatments and potable grade water was used for leaching and residue washing.

The concentrate was transferred daily to the reaction vessel and accumulated to approximately 1.0ιτι3 of settled solids. The reaction vessel was mounted on load cells with a digital readout to permit approximations of weight to be made. Water was introduced through the distributor at a rate which resulted in fluidising and stratifying the concentrate particles according to size and Specific Gravity. Slime was washed from the concentrate and left the top of the vessel via an overflow launder.

The concentrate was allowed to settle and water was removed by decanting and then by drainage through the filter and was replaced by leach solution which contained sodium cyanide, sodium hydroxide and LeachWell.

This solution was circulated for a specific period of time or until a major percentage or all of the free gold dissolved.

The pregnant solution was recovered by decant and then drained through the filter followed by a draining wash of potable water. The drained residue was fluidised with raw water and discharged from the reaction vessel.

The pregnant solution and wash water was transferred to a tank mounted on load cells and the solution circulated on the electrowinning cell for a period of time or until a major percentage of the gold was recovered on the cathodes.

The design criteria for the transportable treatment plant was as follows: Units

Settled Concentrate Capacity rτι3 1 .00

Concentrate SG 3.03

Settled Concentrate Bulk Density t/m3 1 .80

Settled Concentrate Voids % 40.7

Settled and Drained Concentrate Moisture % 10

Fluidised Concentrate Pulp Density t/m3 1.40

Fluidised Concentrate Voids % 53.9

Distributor Fluidising Velocity mm/s 22

Distributor Stratification Velocity mm/s 44

Distributor Prewash Velocity mm/s 24

Overflow Leach Velocity mm/s 6

Concentrate Transfer and Prewash Water Raw

Concentrate Transfer and Prewashing 1.05-1.10

Water SG (assume groundwater) Concentrate Grade Au g/t 10000 Leach Extraction Au % 95 Minimum Leach Time hours 16 Minimum Leach Solution Temperature °C 20 Leaching Solution and Wash Water Potable Leaching Make-up and Wash Water SG 1 .00 Column Distributor Drainage Rate 1/min/m2 77

Washing Efficiency Au% 99

Reagent Additions -

NaCN kg/kg Au 5.00

NaOH kg/kg Au 0.50

H₂O₂ (100 volume) mL/kg Au 1500

LeachWell™ GC kg/kg Au 1 .50

Leach Solution SG 1 .03

Minimum Electrowinning Time hours 1 6

Electrowinning Cell Temperature(max) °C 60 Electrowinning Tail Grade Au g/rrι3 <10 Process Description

Transfer of Concentrate from the Gold Room

Knelson concentrators were set to discharge concentrate at time intervals and the concentrate was held in a storage vessel in the Gold Room for subsequent treatment on day shift. The settled concentrate in the storage vessel was fluidised with water and air and transferred daily by pumping to the reaction vessel at the FBHIC. Successive transfers were made to accumulate about 1.Orris of settled material in the reaction vessel. A cushion of water in the bottom of the reaction vessel assisted in minimising forced ingress of concentrate into the filter medium and passage through into the distributor. Stratification of the Concentrate

The success of fluidised bed leaching is dependent upon segregating the concentrate into layers of particles with equal minimum fluidisation velocities such that channelling of solution will not occur. This was achieved by flowing a limited volume of water through the distributor at a rate considerably higher than the fluidising flow rate. The flow was terminated by a level cut-out switch before the reaction vessel was full to avoid the carry-over of concentrate particles in the overflow. The concentrate was allowed to settle and the supernatant water decanted through a nozzle located immediately above the level to which the design charge of concentrates settled. This operation, referred to as "stratification," was repeated if necessary. Prewashing of the Concentrate

To ensure that the leach overflow solution did not carry suspended solids that would interfere with the subsequent electrowinning of the gold and that water and pregnant solution can be recovered by drainage through the settled solids, further raw water was flowed for about 30 minutes, at a velocity slightly above that determined as the velocity required for fluidisation, through the reaction vessel and was discharged via the overflow launder. This operation eluted some minor fine solids from the concentrate charge. The concentrate was allowed to settle, the supernatant water was decanted and the remaining free water was drained from the reaction vessel through the filter. All water and solids recovered either by decantation, overflow or drainage through the filter were transferred by the Reaction Vessel Discharge Pump to the No. 2 Mill

Discharge Sump.

Mixing of Leach Solution Reagents

The leach solution was made up in the reaction vessel feed tank which was fitted with a side-mounted mixer. Potable water was added to the tank to -a depth of 1.20m (3.2m3) and NaOH, NaCN and LeachWell were added as solids in that order and mixed. Leaching of the Concentrate

The leach solution was pumped from the reaction vessel feed tank via the distributor through the reaction vessel at a rate sufficient only to fluidise the concentrate. This ensured that no fine solids which would interfere with subsequent electrowinning were carried out in the pregnant overflow.

The leach solution was circulated on the reaction vessel for about 16 hours or until the dissolution rate of gold was determined to be minimal. The reaction vessel feed tank was fitted with three 4 kW heaters which raised the solution to about 20°C when necessary. Recovery of Pregnant Solution and Washing of the Residue

At termination of leaching, the reaction vessel overflow was sampled for assay, the flow was stopped and drainage of the solution through the distributor to the reaction vessel feed tank was commenced. After about 15 minutes, when the solids in the reaction vessel had settled, pregnant solution was also taken off to the reaction vessel feed tank via the decanted supernatant. This reduced the pregnant solution recovery time.

When the surface of the settled residue was exposed, the drainage valve was closed and potable water was applied with a hand-held spray to the surface of the residue solids to provide a cushion of water before the remainder of the wash was applied.

The minimum quantity of wash applied was approximately equal to the voids in the residue which were about 0.5m3 but up to 1.0ms could have been added without exceeding tankage capacities. Any reduction might be justified by the assays on samples of recovered wash as the residue drains. The degree of washing must be balanced against the time required and the need to transfer concentrate for the next batch treatment within scheduled Gold Room attendance hours. Any soluble gold retained in the residue was eventually recovered in the CIL circuit.

The depth to which the potable water wash must be applied can be determined approximately by weight using the load cell digital indicator. -

Discharging of the Leach Residue

Raw Water was pumped via the distributor through the reaction vessel at the same rate as for prewashing, fluidising the residue. The residue discharge valve was fully opened and the fluidised residue was discharged to the grinding section floor sump pump.

The residue was sampled for assay as it discharges. Electrowinning

The pregnant solution in the reaction vessel feed tank was mixed and pumped to the electrowinning cell feed tank where it was sampled and the depth of the solution is measured. The weight of the solution could also be established from the load cell digital indicator.

The electrowinning cell was operated with eight cathodes and nine anodes.

The solution in the electrowinning cell feed tank was pumped to the EW Cell where the flow rate was controlled. Solution was circulated on the cell for a specific period of time or until the cell tail assay was low.

When the cathodes were to be removed for calcining and smelting, the cell was carefully drained to prevent cathode slime from entering the electrowinning cell feed tank. The cathodes were removed from the cell, all cathode slime was recovered from the bottom of the cell and all material recovered was calcined and smelted.

The depth and weight of EW Tail in the EW Cell Feed Tank was measured and the solution was pumped to the Solution Transfer Tank. This solution was sampled for assay as it discharged to the Transfer Tank. Three 22 kW heaters were installed in the EW Cell Feed Tank to raise the temperature of the solution to about 80°C as necessary. Partial Recycling or Disposal of Electrowinning Tail

The EW tail was transferred in total to the CIL circuit via the leach feed sump or, after transferring a volume equal to the wash applied to the residue, to the leach feed sump. The remainder was transferred to the reaction vessel feed tank, makeup reagents were added and used to leach the next batch of KnelsoA Concentrate. Batch Treatments

Throughout the operation of the process, pH of solutions and HCN levels in air in the Upper Module were monitored. The maximum HCN level recorded was 4 ppm but repeat readings failed to confirm this level. Most readings even immediately above the solution tanks, the Reaction Vessel and the EW Cell even at 60°C were 2 ppm HCN or less.

The operating and production data for 33 days of batch process treatments are summarised below: Units

Production days 33

Concentrate treated m3 11.1

Estimate dt 20

Cathode gold refined kg Au 62.555

EW Tail to Leach Feed Sump kg Au 6.502

Residue contents kg Au 1.417

Calculated Head kg Au 70.473

Au g/t 3528

Leach Extraction Au % 98.0

Recovery as bullion Au % 88.8

URGM reported Recovery by

Gravity

December 1998 Au % 12.6

January 1999 Au % 10.7

February 1999 Au % 24.7

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for the extraction of gold and/or silver from gold and/or silver bearing ore particles or concentrates, said process including the stages of:

A. filling a reaction vessel with said ore particles or concentrates, and

B. circulating a gold and/or silver leaching solution through the reaction vessel such that a fluidised bed of said ore particles or concentrates is achieved.

2. A process according to claim 1 wherein the concentrate is a gravity concentrate.

3. A process according to claim 1 or claim 2 wherein the process also includes the stages of pre-washing said reaction vessel with a pre-wash solution to fluidise the concentrate bed and removing fine particles by elutriation.

4. A process according to any of the preceding claims which further includes the stage of extracting the gold and/or silver from the leach solution.

5. A process according to claim 4 which further includes the stage of post- washing said reaction vessel with a post wash solution on completion of stage B.

6. A process according to claim 5 which includes the stage of extracting the gold and/or silver from the post wash solution.

7. A process according to claim 4 or claim 6 wherein the extractionof gold and/or silver is performed using a dedicated electrowinning cell or zinc precipitation.

8. A process according to claim 3 or claim 5 wherein the wash solution consists of water.

9. A process according to any of the preceding claims wherein the ratio of solids being leached to leaching solution is 1 :2 or less.

10. A process according to any of the preceding claims wherein the leaching solution includes cyanide.

11. A process according to claim 10 wherein the leaching solution further includes a hydroxide.

12. A process according to claim 11 wherein the leaching solution includes sodium cyanide and sodium hydroxide.

13. A process according to any of the preceding claims wherein the leaching solution further comprises a leach accelerator reagent.

14. A process according to any of the preceding claims wherein the dissolved oxygen level in the leaching solution is increased by air or oxygen injection or the addition of peroxide or other oxidants.

15. A system for removing gold and/or silver from gold and/or silver bearing ore particles or concentrates, said system including:- a reaction vessel for holding a volume of gold and/or silver bearing ore particles or concentrates; means for introducing gold and/or silver bearing ore particles or concentrates into said reaction vessel; a leach solution inlet to said reaction vessel and a leach solution distribution means located in a lower region of said reaction vessel whereby leach solution can be introduced into said reaction vessel to pass through said distribution means and fluidise the gold/silver bearing ore particles or concentrates; recirculation means to recirculate leach solution after passing through said gold and/or silver bearing ore particles or concentrates back to said leach solution inlet to said reaction vessel; means for removing the gold and/or silver bearing ore particles or concentrates from said reaction vessel after leaching of the gold and/or silver into the leach solution; and - gold and/or silver recovery means for removing gold and silver from said leach solution.

16. A system according to claim 15 wherein the system further includes a leach solution holding vessel and pumping means to pump leach solution from the holding vessel to the leach solution inlet to said reaction vessel.

17. A system according to claim 15 or claim 16 wherein the system further includes a leach solution return means to return leach solution from said reaction vessel to the holding vessel.

18. A system according to any one of claims 15 to 17 wherein the gold and/or silver recovery means comprises electrowinning apparatus with the leach solution being pumped through said electrowinning apparatus by the aforesaid pumping means upon completion of a leaching process stage with the leach solution being returned to the holding vessel after passage through said electrowinning apparatus.

19. A leach solution distribution means for use in the reaction vessel of the system of any one of claims 15 to 18, said distribution means including:- a first means for supporting a particle layer while allowing a leach solution to pass therethrough; a second means located above said particle layer configued to allow a leach solution to pass therethrough, said first and second means being configured to retain particles of said particle layer therebetween; and a third means located upstream of said first means arranged to distribute leach solution flow substantially across the full flow surface of said first means.

20. A process according to claim 1 and as herein described with reference to the trial examples;

21. A system according to claim 15 and as herein described with reference to the drawings.

22. A leach system distribution means according to claim 19 and as herein described with reference to the drawings.

AMENDED CLAIMS

[received by the International Bureau on 14 January.2000 (14.01.00); original claims 1 and 2 amended; remaining claims unchanged (lpage)]

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A batch process for the extraction and recovery of gold and/or silver from goid and/or silver bearing ore particles or concentrates, said process including the stages of:

A. filling a reaction vessel with said ore particles or concentrates,

B. circulating a gold and/or silver leaching solution through the reaction vessel such that a fluidised bed of said ore particles or concentrates is achieved; and

C. recovering the extracted gold and/or silver solution thus formed, by decantation and percolation.

2. A process according to claim 1 wherein the concentrate is a gravity concentrate, with a maximum particle size of 3mm.

3. A process according to claim 1 or claim 2 wherein the process also includes the stages of pre-washing said reaction vessel with a pre-wash solution to fluidise the concentrate bed and removing fine particles by efutriation.

4. A process according to any of the preceding claims which further includes the stage of extracting the gold and/or silver from the leach solution.

5. A process according to claim 4 which further includes the stage of post washing said reaction vessel with a post wash solution on completion of stage B.

6. A process according to claim 5 which includes the stage of extracting the gold and/or silver from the post wash solution.

7. A process according to claim 4 or claim 6 wherein the extraction of gold and/or siiver is performed using a dedicated electrowinning cell or zinc precipitation.

8. A process according to claim 3 or claim 5 wherein the wash solution consists of water. STATEMENT UNDER ARTICLE 19 (1)

In response to the citations in the International Search Report, amendments have been proposed to define the present invention more precisely.

In this regard, we enclose retyped page 32 to replace page 32 as originally filed. New page 32 contains amended claims 1 and 2 which we propose as replacements for the original claims 1 and 2. Furthermore, it is requested that the new claims be published with the PCT application under Article 21 of the PCT.

The processes of the cited patents are continuous and do not teach the use of a batch process in combination with recovery of an extracted gold and/or silver solution by declaration and percolation.