WO1998005569A1 - Gold extraction from ores with chemicals regeneration - Google Patents

Abstract

Methods for extracting gold from ores are disclosed. A solution of iodine and potassium iodide (102) dissolves the gold from the ore in a reaction pond (104). A filter unit (120) is positioned between the solution supply and the pond. Liquid solution product is withdrawn by pump (162) and includes unreacted KI3 and KI and a dissolved gold iodide complex. The liquid product passes to a heat exchanger (166) and then to a crystallization tank (170) which uses an evaporative cooling unit (171) to lower the temperature in the tank. In the tank a precipitate Au.KI3 (180) forms which is removed by a vacuum filter (182). The precipitate is combined with water until it is dissolved and as a result AuI crystals (202) are formed which are then separated from the remaining solution (204) which is dissolved KI and I2.

Description

GOLD EXTRACTION FROM ORES WITH CHEMICALS REGENERATION

FIELD OF THE INVENTION

The invention generally relates to the extraction of gold from ores. More particularly, the invention relates to the dissolution of the gold from ore and then recovering the dissolved gold from the solution, while minimizing the consumption of chemicals and of energy, minimizing the production of undesirable toxic waste, ^"and minimizing the toxicity to the operators and to the environment.

BACKGROUND OF THE INVENTION

At the present time, about fifty million ounces of gold are produced yearly worldwide. Some of the gold is obtained as a by-product of smelting and refining processes during the production of base metals. The bulk of gold production is from mined ore. In free milling ores, the gold is in its metallic form, finely distributed throughout the ore. In refractory ores, the gold is combined with other elements in the form of gold compounds. The metallic gold of free milling ores can be leached out of the ore by dissolution with suitable chemicals. Refractory ores are roasted at high temperature to reduce the gold to its metallic form, finely distributed throughout the ore from which it is then leached.

Ores from veins or lodes usually are broken down to a finely divided condition by crushing and/or milling. The high density gold then is mechanically concentrated. The gold of ore obtained by mining is then leached out into a liquid solution using suitable chemicals.

Historically, gold was leached out of the finely divided ore by exposure to mercury, forming a gold amalgam. The mercury then was distilled out of the amalgam by heating, leaving the gold behind. The mercury was recovered for reuse. The toxicity of mercury lead to the banishment of this process worldwide.

At the present time, the gold is typically leached out from the finely divided ores with a dilute aqueous solution of sodium cyanide. With reference to Figure 1 , a gold leaching system 10 as presently used in the industry is schematically illustrated. The leaching system 10 includes a supply 12 of leaching solution. The leaching solution is a dilute aqueous solution of sodium cyanide. The supply 12 of leaching solution is routed via conduit 22 into a pond 24. The pond 24 consists of a natural or dug out depression in the ground that is lined with a plastic to minimize leakage to the ground. The pond 24 is filled with finely divided gold ore. The leaching system 10 may have one or more filter units, such as filter units 20 and 26, that are designed to remove particulates.

Immediately upon contact of the leaching solution with the ore's gold, leaching proceeds. As leaching proceeds, a liquid product is produced which includes unreacted sodium cyanide and a dissolved gold cyanide complex. The liquid solution product may be withdrawn slowly by a centrifugal pump 30 via a conduit 28. The withdrawal rate of the liquid solution depends on the pond size and geometry, and is designed to optimize the use of the leaching chemical, i.e. sodium cyanide. As the leaching solution slowly flows in the pond from routing-in conduit 22 to withdrawing conduit 28, more sodium cyanide reacts with and dissolves more gold. Thus, the etching solution increasingly is depleted of sodium cyanide in the pond from conduit 22 to conduit 28, and is loaded with dissolved gold cyanide complex. As a result, the leaching power of the solution from conduit 22 to conduit 28 in the pond decreases and the leaching rate decreases.

The gold is recovered from the resulting liquid solution product by treatment with excess zinc or aluminum. The base metal reduces the gold to metal while forming zinc or aluminum cyanide. The gold metal with the excess of zinc or aluminum is filtered out, and the toxic cyanide solution is discarded. The excess base metal is then removed with an acid, yielding the product gold. When the ore in the pond 24 is nearly depleted of gold, it is rinsed with fresh leaching solution before being removed and discarded. The pond is filled with a new load of ore and the previous rinsate is returned to the pond for further use (circuit not shown).

Note that the discarded leached ore contains toxic sodium cyanide.

A variation of the process of recovering the gold from the dilute cyanide solution consists of withdrawing the liquid solution product via the conduit 28 and routing it through an ion exchange bed 32. The ion exchange bed 32 extracts nearly all the cyanides from the liquid solution product. The ion exchange bed 32 is then burned to yield the gold. In the burning process, the cyanide is also burned to produce C0₂ and N0₂.

In all cases, the gold metal thus obtained contains metal impurities which may be removed by further treatment, mostly by electrochemical refining.

The gold metal extraction and recovery processes involve severe pollution of the environment by leakage (mercury, cyanide) and effluent (N0₂, cyanides from the wet depleted ore and from incomplete combustion), and a substantial consumption and cost of chemicals and/or energy. Thus, there is a significant need for a non-polluting process to extract and recover the gold from ores.

While conventional techniques of gold metal extraction from ores are toxic to the operators and to the environment, they are also energy intensive and consume significant quantities of chemicals. Specifically, the conventional process of gold metal extraction from ore involves the application of dilute aqueous solution of sodium cyanide (NaCN) to the ore in a manner that loads the solution with dissolved metal reaction products. In turn, the production of metal reaction products results in a depletion of active ingredients in the solution. As the loading of the solution with metal increases, the extraction rate slows to an ineffective level. Once this level is reached, the loaded solution must be removed and treated in a separate facility for metal recovery and toxic waste disposal.

Accordingly, conventional extracting processes are not conducted at an optimum rate, with the rate decreasing due to:

(1 ) loading of the extracting solution with dissolved metal materials; and

(2) the depletion of active ingredients.

This continuous decrease in extraction rate results in a corresponding decrease in production output, increased costs, and the increased generation of undesired waste materials.

J. Leibovitz, D. Miller, M. Cobarruviaz, J. Scaiia, H. Nakano, V. Nagesh, and C. Chao, Controlled Etching Process for Forming Fine Geometry Circuit Lines on a Substrate, U. S. Patent No. 5,221 ,421 (22 June 1993) disclose an etching method for producing fine geometry gold circuit structures. To remove the dissolved gold, one method involves cooling the etchant to precipitate a gold complex therefrom. The remaining recovered etchant is then heated and made available for continued etching. Another method involves a cathode/anode assembly which is immersed in the etchant. Activation of the assembly recovers metallic gold and regenerates the etchant. These methods, when used continuously or periodically in a dip or spray etching system, maintain a constant etching rate. As a result, fine geometry circuit structures are disclosed to be accurately produced while minimizing material costs {e.g. etchant use) and minimizing the production of undesirable waste products and disposal expenses associated therewith. The prior art thus provides methods for gold metal extraction from ores, but with a high cost in energy, chemicals consumption, and toxicity to both operators and the environment. The development of a cost-effective, cyclic process for the extraction of gold from ores, which minimizes the use of energy and of expensive chemicals, while reducing the production of undesirable waste products, would constitute a major technological advance.

SUMMARY OF THE INVENTION

The invention involves the application of unique cyclic processes to leach out the gold from the ore, extract the gold from the leaching solution, and regenerate the leaching chemicals. Specifically, the invention involves the application of a process in which a chemical is used to leach out gold metal from finely divided ores. In addition, the gold leaching processes herein disclosed enable rapid recovery of the leached metal, and minimize leaching chemical use.

Leaching of the gold metal from finely divided ores in accordance with the invention is achieved using a chemical aqueous solution consisting of dissolved KI and l₂ as described below. The finely divided ore containing gold metal is placed in contact with the leaching solution in a pond. The leaching proceeds faster at higher temperatures, but the vapor pressure of l₂ also increases with temperature. However, in hot or temperate climates, it is not necessary to heat the leaching solution. In cold climates it is necessary to maintain the solution above the freezing point. As the leaching solution comes in contact with gold metal in the finely divided ore, the gold is dissolved into the solution. As a result, a liquid product is produced which consists of unreacted reagent (dissolved l₂ and dissolved KI) combined with a gold reaction product

(e.g. a dissolved gold complex) which may be written as follows: Aul.K l₃ (aq.).

In one embodiment of the invention, the liquid product containing the ore that exits the pond is cooled to a temperature sufficient to cause precipitation of the dissolved gold complex from the solution {e.g. about 0-4 degrees C). The resulting black precipitate is then treated to recover metallic gold therefrom using a variety of methods and techniques. For example, an exemplary treatment method involves contacting the precipitate with water in an amount sufficient to produce solid Aul and a solution containing dissolved l₂ and dissolved KI. The amount of water suitable for this purpose is about 5-50 ml/g of precipitate. The remaining solution containing dissolved l₂ and dissolved KI is then returned to the leaching system for the continued leaching of gold from the finely divided ore. The solid Aul is thereafter heated at a temperature of about 100-150 degrees C. for a time period of about 60-200 minutes to produce metallic gold and l₂ vapor. The l₂ vapor is likewise returned to the leaching system for the continued leaching of gold from the ore.

An alternative method for recovering metallic gold involves the following steps:

1. Heating the resulting black precipitate described above at about 150- 300 degrees C. for about 60-200 minutes to produce l₂ vapor, KI crystals and pure gold powder;

2. Returning the l₂ vapor to the leaching system for reuse;

3. Dissolving the KI crystals by combining the crystals (and gold powder) with deionized water;

4. Recovering the gold powder using conventional filtration techniques; and

5. Returning the KI aqueous solution prepared as described above in step 3 to the leaching system for reuse.

The resulting solution, which now contains substantially less dissolved gold (due to removal of the dissolved gold complex as indicated above), is then heated to a temperature of about 10-50 degrees C and returned to the leaching system for additional leaching of gold from ore. The return of treated solution in this manner enables the constant removal of gold reaction products from the solution and, in combination with the other procedures described above, enables the solution to function continuously without being hindered by excessive amounts of gold by-products dissolved therein. As a result, there is no consumption of appreciable amounts of chemicals.

A second, equally preferred method of the invention uses a leaching solution consisting of dissolved KI and l₂. To implement the second method, finely divided gold ore, as described above in the first method, is placed in contact with the leaching solution. During leaching, it is preferred that the solution be maintained at a temperature of about 10-50 degrees C. The resulting liquid product (consisting of unreacted dissolved l₂ and dissolved KI in combination with substantial amounts of aqueous gold by-products, e.g. Aul.K l₃ (aq.), is then treated using an electrodeposition process.

Specifically, a cathode/anode assembly is provided for this purpose. In a preferred embodiment, the cathode is made from gold or a gold-plated substrate (e.g. ceramic), with the anode being made from platinized platinum or a platinized substrate (e.g. titanium or niobium mesh). In a preferred embodiment of the invention, an electric current source (e.g. a current regulated power supply) is connected to the cathode and anode to place the cathode and anode under an electrical potential.

Thereafter, the cathode and anode are immersed within the liquid product, with the electric current source causing the passage of a current of about 65-120 mA/cm² through the cathode and anode. As result, the dissolved gold complex is decomposed into metallic gold and a recovered/regenerated leaching solution consisting primarily of dissolved KI and dissolved l₂. The metallic gold is plated onto the cathode, which is withdrawn from the recovered leaching solution before the metallic gold falls off of the cathode. The metallic gold is then physically removed from the cathode after withdrawal as indicated above. The recovered/regenerated leaching solution (in combination with any initially unreacted chemical) is then used to continue the leaching of gold from the ore. As a result, the leaching process proceeds at a constant, optimum rate while avoiding any reductions in leaching efficiency caused by the use of chemical solution having excessive amounts of gold byproducts dissolved therein.

The invention therefore represents an advance in the art of gold extraction from ores.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of the conventional art of extraction of gold from ores;

Figure 2 is a schematic illustration of one preferred embodiment of the invention;

Figure 3 s a schematic illustration of another preferred embodiment of the invention; and

Figure 4 is an enlarged schematic illustration of a representative cathode/anode assembly used to implement the process shown on Figure 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides methods for extracting gold metal from ores, which involve the application of a gold leaching solution to the ore in a manner that loads the leaching solution with dissolved metal reaction products. To extract the gold metal from ores at an optimum rate, with most efficient use of chemicals and of energy, at reduced cost, and without generation of undesired waste materials, two methods are provided. Both of these methods involve control of the leaching solution to remove gold reaction products therefrom and regenerate the leaching solution. Loading of the extracting solution with gold reaction products substantially and continuously decreases the leaching rate, thereby decreasing throughput and production. Without regeneration of the leaching solution, chemicals are consumed, adding to cost, and undesirable wastes are continually produced, creating an additional cost for disposal.

It should be noted that the invention as described in the following methods is not limited to any specific equipment units and/or chemicals which may be described herein. The invention may be implemented using a variety of components and materials other than those specifically recited below.

Method 1

The gold extraction from ores system 100 produced in accordance with the first method of the invention is schematically illustrated in Figure 2. The extraction system 100 first includes a supply 102 of chemical leaching solution. The solution is known in the art of gold etching and typically consists of a solution of dissolved KI and dissolved l₂. By way of example, a one liter sample of the Kl/I₂ leaching solution may be prepared as follows:

1 ) Heat one liter of deionized H₂O to a temperature of about 50 degrees C and maintain the H₂0 at that temperature. 2) Stir the H₂O while maintaining the above temperature.

3) Add 166.0 g of solid KI to the H₂0 while continuing to stir until all of the KI is dissolved.

4) Pour 80 g of l₂ crystals into the solution formed after the completion of step number 3 and continue to stir until all of the l₂ crystals have dissolved. In the alternative, the solution (immediately after addition of the l₂ crystals) may be transferred to an ultrasonic bath which is energized until all of the l₂ crystals are dissolved.

Commercially, the foregoing mixture of chemicals is available from Film Micro- Electronics, Inc. of Burlington, Mass. under the designation "C35". It is indicated by this manufacturer that the foregoing solution has a nominal gold etching rate of about 1.0 microinch/minute. The commercially available chemicals mixture and the solution produced in accordance with the steps listed above have an l₂:KI mole ratio of about 2:7, with the actual concentrations of l₂ and KI being about 0.3 mole/liter and about 1 mole/liter, respectively.

Next, the supply 102 of chemical leaching solution is routed into a reaction pond 104. The reaction pond 104 preferably consists of a natural or dug out depression in the ground that is lined with a material of an iodine-resistant composition (e.g. high density polyethylene). A movable curtain of iodine resistant material may cover the pond to control evaporation. The curtain may be sliding and foldable.

At this point, it should be noted that the extraction system 100 may have one or more filter units designed to remove particulates and chemical contaminants from the solution passing into and/or out of the pond 104. The number and exact position of these filter units in the etching system 100 may vary, with the embodiment of Figure 2 including two filter units 120, 122. Filter unit 120 is positioned within conduit 124 which connects the supply 102 of chemical reagent with the pond 104. Filter unit 122 is positioned within conduit 130 which leads out of the pond 104 as illustrated. Filter units 120, 122 (and any other filter units described herein) are of a type known in the art which may include activated carbon units in combination with conventional polypropylene filter cartridges capable of removing particles as small as 0.5 micron. Conduits 124 and 130 (as well as the other conduits described herein) are produced of an iodine-resistant material, including but not limited to polyethylene, polypropylene, or other comparable inert compositions known in the art.

Also, the extraction system 100 may involve one or more valves for connecting and/or shutting off various units and/or flow circuits. The number and exact position of these valves in the extraction system 100 may vary, with the embodiment of Figure 2 including seven valves, 110, 112, 254, 256, 262, 264 and 267. Valve 110 opens or shuts the supply 102 of fresh leaching solution. Valves 112 and 254 open or shut the recirculating flow of the leaching solution through the pond 104 and the regenerating circuit to be described further below. Valve 256 controls the flow to or from tank 258 to be described later. Valves 262 and 264 open or shut a second flow circuit involving valve 267 and tank 269, also to be described later.

The pond 104 is filled with finely divided gold ore. The leaching solution routed to the pond through conduit 124 contacts the ore filling the pond 104. Immediately upon contact of the solution with the gold of the ore, leaching of the gold occurs. The primary active ingredient in the reagent is dissolved l₂ which functions as a powerful oxidant in the aqueous KI solution. Gold oxidation proceeds in accordance with the following reaction sequence:

2Au (metal) + l₂(aq.) > 2Aul(aq.) (1) Both l₂ and Aul are insoluble in water, but soluble in aqueous solutions of KI by forming complex compounds. The complexing reactions may be written as follows:

l₂ (solid) + KI (aq.) > Kl₃(aq.) (2)

Aul (solid) + Kl₃(aq.) > Aul.KI₃(aq.) (3)

It is important to consider the two electrochemical reactions which form the underlying basis for reaction (1 ) as follows:

2Au (metal) > 2Au⁺(aq.) + 2e- (4)

l₂ (aq.) + 2e' > 21" (aq.) (5)

Reaction (4) represents the anodic oxidation of the gold metal. Reaction (5) represents the cathodic reduction of the l₂ (aq.). The sum of reactions (4) and

(5) yields reaction (1 ) (e.g., the gold leaching process). For reaction (1) to proceed, the equilibrium electrode potential (E(c)) of reaction (5) must be higher than (e.g., positive relative to) the electrode potential (E(a)) of reaction (4) as follows:

E(c) - E(a) > 0 (6) For condition (6) to be satisfied, the molar concentration of Au⁺ ions must be less than 5x10"¹⁹. Accordingly, for the etching process to proceed in an efficient manner, it is necessary to complex the Au⁺ ions to maintain the concentration limit thereof in the reagent solution below the requisite limit described above. This requirement is satisfied by complexation with, e.g., Kl₃

(aq.). The electrochemical reaction summarizing this process may be written as follows:

2Au (metal) + 2KI₃ (aq.) + 2ϊ (aq.) > 2Aul.KI₃ (aq.) + 2θ^' (7)

Similarly, the cathodic process including the complexed iodine may be written as follows:

Kl₃ (aq.) + 2θ^" > KI (aq.) + 21^" (aq.) (8)

Accordingly, the entire etching process is best represented by the sum of reactions (7) and (8) as follows:

2Au (metal) + 3KI₃ (aq.) > KI (aq.) + 2Aul.KI₃ (aq.) (9)

Thus, as leaching proceeds, a liquid product is produced which includes unreacted chemical agents (containing residual dissolved l₂ and dissolved KI) having a dissolved gold reaction product complex (e.g. Aul.KI₃ (aq.)) therein.

There are many factors which control the leaching rate in the foregoing process as follows:

A. Dissolved gold concentration in the solution. Etching rates decrease as gold concentration increases. B. Solubility of gold in the reagent. This is a function of dissolved l₂ and dissolved KI concentrations in the solution. Higher l₂ and KI concentrations increase the gold solubility level, with increased gold solubility causing a corresponding increase in leaching rate.

C. Etching solution temperature. Etching rates increase with increasing temperature.

D. Gold Properties. The properties of the gold in the ore (e.g. purity, porosity, etc.) affect etching rates.

E. Hydrodynamic conditions of the leaching system. For example, the degree of solution agitation and flow velocities can influence the leaching rate. Regarding agitation and flow speed, the stronger the agitation and the faster the flow, the higher the leaching rate.

A key factor in the leaching rate is item "A" above, which involves the concentration of dissolved gold concentration in the solution. Increasing amounts of dissolved gold materials in the solution greatly decrease the gold leaching rate in the system. In fact, gold dissolution tests conducted with the leaching solution have shown that the dissolution rate decreases linearly as dissolved gold concentration increases.

Accordingly, it is necessary to control dissolved gold concentration in the solution in order to ensure that an optimum leaching rate is achieved. The use of solution having substantial amounts of dissolved gold materials (e.g. Aul.Klg

(aq.)) therein causes a significant decrease in leaching rate in accordance with the principles indicated above.

Today's production methods do not include any procedures for controlling the leaching solution composition and the corresponding rate at which gold is leached from ore. Typically, ores are flooded with dilute aqueous sodium cyanide solution. Immediately, the leaching proceeds. However, as the amount of dissolved gold in the solution increases, the leaching slows down. When the leaching rate becomes too slow, the solution is withdrawn for treatment to recover the dissolved gold. The actual leaching time is a compromise between time (throughput) and the efficient use of the chemicals. A slow flowing solution through a long pond is more efficient than a stagnant pond. Nonetheless, the above compromise principles remain effective. Finally, the typical process produces undesirable waste products such as cyanide, of which the disposal is costly, or NO₂ with a cost penalty from heating energy.

To achieve an optimum leaching rate in accordance with this embodiment of the invention, a number of unique procedures are followed.

As leaching proceeds, a liquid product is produced which includes unreacted Kl₃ and KI, and a dissolved gold iodide complex. The liquid solution product may be withdrawn by a centrifugal pump 162 via conduit 130. The withdrawal rate depends on the pond size and geometry and is designed to optimize the leaching rate. As the leaching solution flows in the pond from routing-in conduit 124 to withdrawing conduit 130, more iodine reacts with and dissolves more gold. Thus, the etching solution is increasingly depleted of iodine in the pond from conduit 124 to conduit 130 and loaded with dissolved gold iodide complex. As a result, the leaching power of the solution from conduit 124 to conduit 130 in the pond decreases and the leaching rate decreases.

The decrease in leaching rate is controlled by the added amount of dissolved gold resulting from the leaching process, which in turn is controlled by the leaching solution flow rate. The higher the flow rate the lesser the decrease in leaching rate, and the faster the leaching of the gold from the ore. This apparently paradoxical result is understood by considering that the gold leaching rate Q (amount of gold extracted per unit time, say per minute) is equal to the product F x DC of the increment DC in the concentration C of gold in the leaching solution upon passing through the pond, by the solution flow rate F through the pond. So, while the product Q increases with the flow F, Q is now distributed over a larger volume of liquid (per unit time) to which corresponds a lesser increase in concentration.

The liquid product withdrawn through conduit 130 includes unreacted Kl₃ and

KI, and a dissolved gold iodide complex. This dissolved gold iodide complex product as described in method 1 may be written as follows: Aul.KI₃ (aq.).

In accordance with the invention, the gold extraction from ores proceeds in a cyclic process where the chemicals are continuously regenerated and re-used. Therefore, the issue of the efficient use of the chemicals is resolved independently of the leaching rate. Thus, there is no conflict and no compromise is required between chemical consumption and leaching rate. The concentration of dissolved gold in the leaching solution can be optimized for leaching rate and recovery rate from the solution.

After the leaching solution contacts the ore's gold, a liquid product is generated which consists of unreacted reagent (containing residual dissolved l₂ and dissolved KI) combined with a gold reaction product. This product is a dissolved gold complex which is currently understood to have the formula Aul.KI₃ (aq.) as indicated above. As increased amounts of the gold complex are generated, the solution becomes increasingly loaded therewith. Likewise, the increased production of gold complex results in a corresponding depletion of dissolved KI and l₂. These events substantially slow the leaching rate.

To remove the gold complex from the solution, the liquid product containing both unreacted reagents and gold complex is continuously withdrawn from the pond 104 through filter unit 122 and conduit 130. Withdrawal of the liquid product is accomplished through the use of a conventional centrifugal pump 162 which is positioned within and/or in fluid communication with the conduit 130. In a preferred embodiment of the invention, withdrawal of the liquid product from the reaction pond 104 is continuous, as noted above.

The liquid product (e.g. unreacted solution and gold complex) which leaves the reaction pond 104 has a gold concentration level of above 12 g of gold/liter of reagent (typically about 12.5 to 19 g of gold/liter of leaching solution, depending on temperature, pond design, solution flow, and ore properties such as porosity, purity, distribution, and presence of contaminants) due to the dissolved gold complex therein. The liquid product thereafter passes into a counter-current heat exchanger 166 which is of a type well known in the art. After passing through the heat exchanger 166, the liquid product enters a crystallization chamber or tank 170 where it is cooled to a temperature of about 0-4 degrees C (about 2 degrees C is optimum). At this temperature, the solubility of gold within the solution decreases to a level of about 12 g of gold/liter of solution. Thus, if the solution has more than 12 g/liter of gold dissolved therein, the excess gold materials precipitate therefrom upon cooling, as described below.

Cooling may be accomplished using a number of known techniques. For example, cooling may occur and/or be enhanced through the use of a conventional refrigeration or evaporative cooling unit 171 operatively connected to the tank 170 (schematically illustrated in Figure 2) which is designed to lower the temperature of the liquid product in the tank 170 to the desired level.

In addition, the liquid product may be pre-cooled prior to entry into the tank 170 by the counter-current heat exchanger 166 described above. Specifically, once the initial batch of liquid product is cooled within the tank 170 and the desired precipitation reaction occurs as described below, the cooled, residual solution is withdrawn from the tank 170 through a conduit 172 which passes through the counter-current heat exchanger 166. As a result, cooled solution leaving the tank 170 is able to pre-cool the warm liquid materials entering the tank 170 by the mutual passage of both materials through the counter-current heat exchanger 166. Pre-cooling in this manner substantially decreases the time and energy needed to cool the liquid product in the tank 170 to the desired temperature. It should also be noted that there are a number of alternative conventional methods which may be used to cool and/or pre-cool the liquid product within the tank 170. Accordingly, the invention shall not be limited to any specific cooling methods.

Once the liquid product is cooled within the tank 170, a precipitation reaction occurs. This reaction takes place because the liquid product (prior to entry into the tank 170) has a dissolved gold concentration level of above 12 g/liter as noted above. As cooling of the liquid product occurs within the tank 170, the solubility of gold within the liquid product drops to about 12 g/liter as noted above. Thus, excess dissolved gold materials precipitate from the liquid product until a dissolved gold level of about 12 g/liter is reached. The excess dissolved gold materials precipitate in the form of a solid, black gold complex (e.g. precipitate) determined to have the formula Aul.KI₃ (solid) as noted above.

In addition, the foregoing precipitation reaction produces a supply of leaching solution having a consistent composition which is characterized by a substantially reduced amount of dissolved gold therein (e.g. approximately 12 g of gold/liter of solution at about 2 degrees C, assuming an initial solution concentration of about 0.3 mole/liter of l₂ and about 1.0 mole/liter of KI).

The precipitate (shown at reference number 180 in Figure 2) is physically removed from the remaining reagent materials, and is conventionally decanted (e.g., using a standard vacuum filtration system 182 or other comparable apparatus known in the art). Liquid 184 recovered from this step (consisting primarily of leaching materials) is routed via conduit 186 into conduit 172 for combination with the reagent materials leaving the tank 170 as described in greater detail below. The precipitate 180 is then combined with a supply 200 of deionized H₂O and agitated until the precipitate 180 is completely dissolved. The amount of H₂0 required for this purpose is about 5 ml H₂O/g of precipitate 180. As a result, a supply 202 of yellowish Aul crystals are formed which are conventionally separated from the remaining solution 204, which consists primarily of dissolved KI and l₂. The solution 204 is routed via conduit 206 into line 172 for reuse, as further described below.

The water added to the solution is insufficient to make up for water loss during evaporation. Therefore, enough water is used in this stage of the process to maintain a desired solution level in pond 104. The supply 202 of Aul crystals is then air dried and heated at a temperature of about 140 degrees C, thereby forming a supply 220 of pure, finely divided metallic gold and l₂ vapor 222. The l₂ vapor 222 is routed via conduit 122 into the conduit 172 for reuse, as described below.

With reference to the tank 170, the recovered solution (having a dissolved gold level of about 12 g/liter at about 2 degrees C.) is removed therefrom using a centrifugal pump 230 or the like which draws the solution into and through the counter-current heat exchanger 166 from conduit 172, where it is combined with the KI and l₂ materials described above which are received from conduits

186, 206, and 122. Within the heat exchanger 166, the temperature of the solution is increased, thereby resulting in an increased gold solubility level (e.g. the capacity to retain gold therein).

The solution is then passed through a filter unit 236 and routed through a heater 250 of a type known in the art (e.g. a conduit or cylindrical vessel surrounded by conventional resistance heating tape). Within the heater 250, the temperature of the leaching solution is raised to about 10-50 degrees C to produce a supply of heated, recovered solution which is ready for reuse/recirculation. Within this temperature range, the solution gold solubility level is further increased to between about 15-22 g of gold/liter of solution, thereby suitably "reactivating" the solution (and rendering it "unsaturated") so that it may be redirected via conduit 252 back into the pond 104 for continued leaching of the gold ore.

Periodically, the addition of minimal quantities of fresh solution to the system 100 from supply 102 is necessary because some of the initial solution materials are lost (e.g. by drag out) in the foregoing process. This may be accomplished by periodic additions of fresh solution from the initial supply 102 as described above. The addition of fresh solution materials is undertaken in view of numerous factors, including the quantity and properties of the processed ores. Accordingly, the amount and frequency of fresh solution addition is experimentally characterized for each different leaching system..

The foregoing method enables fresh, unsaturated solution to be delivered to the reaction pond 104 in a continuous cycle at a constant composition so that an optimum leaching rate may be achieved (e.g. at a rate of about one micron of gold/minute). More specifically, the foregoing method involves the constant delivery of "regenerated," unsaturated leaching solution to a pond loaded with gold ore to avoid excessive loading of the solution and a progressive slowing of the leaching process. As a result, optimum-rate leaching is achieved which enables a high production rate.

When the load of ore in pond 104 is exhausted of its gold, the pond 104 and the ore in it are drained of the leaching solution by opening valve 256 and closing valve 254, thus diverting the regenerated leaching solution to tank 258. When the draining of the pond 104 and of the exhausted ore therein is completed, valve 112 is closed. However, the exhausted ore is wet by a liquid film containing unreacted KI and l₂ , and a dissolved gold complex, about 12 g of gold per liter of wetting liquid. Because the effective surface area of the finely divided ore is very large, the amount of dissolved gold in the wetting liquid film is appreciable. Therefore, before the exhausted ore is discarded, it is exposed to a dilute solution of KI in water, about 17 g of KI per liter of solution. For that purpose, the extraction system 100 involves a supply 260 of the dilute KI solution. Valves 262 and 264 now are opened, and the supply 260 of dilute KI solution is routed to the exhausted wet ore via valve 262 and conduit 124. In the pond 104, the dilute KI solution progresses through the exhausted wet ore, from inlet conduit 124 to outlet conduit 130, and is withdrawn back to its supply 260. While in contact with the exhausted wet ore, the dilute KI solution extracts most chemicals from the liquid wetting film, leaving instead a wetting film of diluted KI solution with negligible amounts of iodine and gold.

The dilute KI solution then is drained from pond 104 by closing valve 262 and opening valve 267, thus diverting the dilute KI solution to tank 269. When the draining is completed, valves 264 and 267 are closed, pump 162 is switched off, the exhausted rinsed ore load is removed from pond 104 and discarded, a new load of ore is loaded, and the cycle applied to the new load.

The metal gold extracted in accordance with this embodiment of the invention may be refined by the known art used in the refining of gold extracted from ores by prior art techniques. The foregoing method readily enables the recovery of the metallic gold, while reducing the consumption of materials and minimizing toxic waste generation.

Method 2

This extraction method is schematically illustrated in Figure 3. It should be noted that the invention is not limited to any specific equipment units and/or chemicals which may be described below. The extraction system 300 includes a supply 302 of chemical leaching solution. The chemical leaching solution in the supply 302 is the same as the chemical leaching solution listed above in method 1 (e.g. a solution of dissolved KI and dissolved l₂ ).

Next, the supply 302 of chemical leaching solution is routed into a reaction pond 304, also of same type as the reaction pond 104 described above in method 1. Specifically, the reaction pond 304 preferably consists of a natural or dug out depression in the ground that is lined with a material manufactured of an iodine-resistant composition (e.g. high density polyethylene). A movable curtain of iodine resistant material may cover the pond to control evaporation. The curtain may be sliding and foldable.

System 300 of method 2 may have one or more filter units designed to remove particulates from the solution passing into and/or out of the pond 304. The number and exact position of these filter units in the system 300 may vary, with the embodiment of Figure 3 including a filter unit 320 positioned within conduit 324 which connects the supply 302 of chemical leaching solution with the pond 304.

Filter unit 320 (and any other filter units described herein) are of a type well known in the art which may include activated carbon units in combination with conventional polypropylene filter cartridges capable of removing particulates as small as 0.5 micron. Conduit 324 (as well as the other conduits described herein) is produced of an iodine-resistant material, including but not limited to polyethylene, polypropylene, or other comparable inert compositions known in the art.

The extraction system 300 may involve one or more valves for connecting and/or shutting off various units and/or flow circuits. The number and exact position of these valves in the extraction system 300 may vary, with the embodiment of Figure 3 including seven valves, 110, 112, 254, 256, 262, 264 and 267. Valve 110 opens or shuts the supply 102 of fresh leaching solution. Valves 112 and 254 open or shut the recirculating flow of the leaching solution through the pond 104 and the regenerating circuit to be described further below. Valve 256 controls the flow to or from tank 258 to be described later. Valves 262 and 264 open or shut a second flow circuit involving valve 267 and tank 269, also to be described later.

Upon contact of the leaching solution with the ore's gold, leaching of the gold occurs. The main leaching agent in the leaching solution as previously described is dissolved l₂ which functions as a powerful oxidant in an aqueous

KI solution. Gold oxidation in this method again proceeds in accordance with reactions (7)-(9) described above in method 1. Thus, as leaching proceeds, a liquid product is produced which consists of unreacted leaching chemicals (including residual, decreased amounts of dissolved l₂ and dissolved KI) having a dissolved gold reaction product/complex (e.g. Aul.KI₃ (aq.)) combined therewith.

There are many factors which control the leaching rate, with such factors being specifically listed in method 1. It is necessary to maintain an optimum leaching rate to ensure high throughput and production. A key factor which must be controlled to achieve an optimum leaching rate is the dissolved gold concentration level in the leaching solution, as previously discussed. The uncontrolled accumulation of dissolved gold reaction products within the leaching solution greatly reduces the leaching rate in the system. In fact, gold dissolution tests conducted with the leaching solution have shown that the dissolution rate decreases linearly as dissolved gold concentration increases. Accordingly, it is necessary to control dissolved gold concentration in the solution to ensure that an optimum leaching rate is achieved. The use of solution having substantial amounts of dissolved gold materials (e.g. Aul.KI₃ (aq.)) therein causes a significant decrease in leaching rate. To achieve an optimum leaching rate in accordance with this embodiment of the invention, a number of unique procedures are followed.

The pond 304 is filled with finely divided gold ore. Immediately upon contact of the leaching solution with the ore's gold, leaching proceeds. As leaching proceeds, a liquid product is produced which basically includes unreacted Kl₃ and KI, and a dissolved gold iodide complex therein. The liquid solution product may be withdrawn by a centrifugal pump 162 via conduit 130. The withdrawal rate depends on the pond size and geometry and is designed to optimize the leaching rate.

As the leaching solution flows in the pond from routing-in conduit 324 to withdrawing conduit 130, more iodine reacts with and dissolves more gold. Thus, the etching solution increasingly is depleted of iodine in the pond from conduit 324 to conduit 130, and loaded with dissolved gold iodide complex. As a result, the leaching power of the solution decreases from conduit 324 to conduit 130 in the pond and the leaching rate decreases.

The decrease in leaching rate is controlled by the added amount of dissolved gold resulting from the leaching process, which in turn is controlled by the leaching solution flow rate. The higher the flow rate the lesser the decrease in leaching rate, and the faster the leaching of the gold from the ore. This apparently paradoxical result is understood by considering that the gold leaching rate Q (amount of gold extracted per unit time, say per minute) is equal to the product F x DC of the increment DC in the concentration C of gold in the leaching solution upon passing through the pond, by the solution flow rate F through the pond. While the product Q increases with the flow F, Q is now distributed over a larger volume of liquid (per unit time) to which correspond a lesser increase in concentration.

The liquid product withdrawn through conduit 130 includes unreacted Kl₃ and KI, and a dissolved gold iodide complex therein. This dissolved gold iodide complex product as described in method 1 may be written as follows: Aul.KI₃(aq.).

In this embodiment of the invention, a unique process is used for removing the dissolved gold reaction product/complex from the unreacted leaching solution. This process involves the electrodeposition of gold therefrom. Specifically, it has been discovered that electrolytic procedures may be used to extract metallic gold from the gold complex in the liquid product while simultaneously reconverting other portions of the complex back to the original dissolved leaching materials. This process involves the use of a cathode/anode assembly 370 schematically illustrated in Figure 4.

The assembly includes a plurality of cathodes 372 and a plurality of anodes 374 arranged in an alternating relationship as illustrated. While Figure 4 illustrates two cathodes 372 and two anodes 374, the invention as described herein may use any number of cathodes 372 and anodes 374, with the number of cathodes 372 and anodes 374 being approximately equal. In a preferred embodiment, each cathode 372 consists of a planar ceramic substrate 380 having a thickness of about 0.1 cm with a layer 382 of metallic gold on each side having a thickness of about 6.0 microns. Such a structure is commercially available from Kyocera, Inc. of San Diego, Calif. In the alternative, other materials suitable for constructing each cathode 372 include but are not limited to elemental gold, gold-plated glass, and gold-plated plastics.

A preferred material used to construct each anode 374 consists of platinized titanium wire mesh 383 which is known in the art and commercially available from the Englehard Co. of East Newark, NJ. This material consists of titanium mesh coated with a porous platinum layer having an effective surface area which is far greater than its apparent (e.g. visually observable) surface area. As a result, the effective current density in each anode 374 is much lower than the apparent current density, thereby allowing each anode 374 to operate at quasi-equilibrium conditions even when the apparent current density is high. Alternative materials suitable for use in constructing each anode 374 include but are not limited to platinized niobium and platinized tantalum. While platinized titanium may be used for anode 374 construction, it should not be used to construct the cathodes 372 of the invention because, at the current densities described below, platinized titanium cathodes catalyze the cathodic decomposition of water and adversely affect the leaching solution composition.

The configuration and shape of each cathode 372 and anode 374 must be designed for uniform current distribution. In this regard, the planar structures oriented in a parallel relationship as illustrated in Figure 4 provide effective results. The cathodes 372 and anodes 374 may be either plane-parallel as illustrated or curved-parallel in the alternative.

The attainment of uniform current distribution in this embodiment of the invention is of substantial importance. Traditionally, in the electrodeposition of metal materials, uneven current distributions yield a corresponding uneven metal deposit thickness. In the present method, uneven current distributions can cause the formation of gold deposits on high current density areas, with gold dissolution occurring at areas of low current density. This problem is not solved by merely increasing the total current. For this reason, the plane- parallel cathode 372 and anode 374 configuration described herein is preferred.

In addition, the total cathodic surface area (involving all of the cathodes 372 in combination) preferably is about 200 cm² for each liter of product solution (e.g., about 0.02 m² for about 1 m³), based on a preferred gold removal rate of about 0.004 g/cm² per hour. Using these parameters, a cathode system having 3 m² total surface area should be able to remove at least about 4 oz of gold/hour from the liquid product.

In addition, it is preferred that all of the cathodes 372 and anodes 374 be oriented in a vertical position as illustrated in Figure 4. Also, it is preferred that steps be taken to avoid cathodic "edge effects" which involve current densities at the edges of the cathodes 372, which are substantially different from the current densities at other portions of the cathode 372. To accomplish this, the cathodes 372 and anodes 374 may be positioned at least partially within an optional retaining chamber 385 schematically illustrated in Figure 4.

In the embodiment of Figure 4, the chamber 385 includes side walls 386 and bottom wall 387, although the invention is not limited to this specific configuration. The chamber 385 may be manufactured of any insulating material which is iodine-resistant including but not limited to glass, polypropylene, and high density polyethylene.

In addition, it is preferred that the chamber 385 have an internal width about equal to the width of the cathodes 372/anodes 374, with a height equal to about 0.5 inch less that the height of the cathodes 372/anodes 374. Small outwardly-extending projections (not shown) may optionally be used on the inside of the walls 386 to hold the cathodes 372 and the anodes 374 in the vertical orientation shown in Figure 4, with the cathodes 372 and the anodes 374 being spaced apart from each other by about 1.0 cm.

In addition, the side wails 386 have a plurality of openings 389 therein which are preferably positioned so that they are between the cathodes 372 and anodes 374 of the system. The openings 389 enable the product solution to fill the chamber 385 during operation of the system, and also allow product solution drainage from the chamber 385 after the termination of system operation. The chamber 385 may also optionally include at least one handle 390 to facilitate removal and transport of the chamber 385 (and cathodes 372/anodes 374) when desired. A chamber 385 suitable for optional use in the invention as described herein is commercially available from Fluoroware, Inc. of Chaska, Minn. With continued reference to Figure 4, each cathode 372 is connected to a cathode clamp 400 which, in turn, is attached to an elongate cathode connector 402. In a preferred embodiment, the cathode clamp 400 and cathode connector 402 are constructed of titanium. Each cathode connector 402 is operatively attached to a cathode supply bus 404, preferably made of copper. The cathode supply bus 404 electrically connects all of the cathodes 372 together as illustrated in Figure 4.

Likewise, each anode 374 is connected to an anode clamp 408 which, in turn, is attached to an elongate anode connector 410. The anode clamp 408 and the anode connector 410 are also preferably made of titanium. In addition, each anode connector 410 is operatively attached to an anode supply bus 412 which is constructed of copper in a preferred embodiment. It should also be noted that each cathode 372 is separated from each anode 374 in the assembly 370 of Figure 4 by a preferred distance of about 1.0 cm as noted above.

The electrodes assembly 370 described above and illustrated in Figure 4 works well in various locations. For example, the assembly works well in the pond 304 in contact therein with the liquid product 348 of the leaching operation. As described earlier, that liquid product essentially contains water, unreacted Kl₃ and KI, and a gold iodide complex product of reaction of gold with the leaching chemicals. Figure 3 illustrates another embodiment of this second preferred method of the invention. The liquid product is withdrawn from the pond 304 through pump 162 and routed through conduit 130 to vessel 380 and returned to the pond 304 via conduit 252 and 324. The electrodes assembly 370 are located in the vessel 380 where they work well.

Following contact between the leaching solution and the ore, the liquid product in the reaction pond 304 is treated in accordance with the electrodeposition process described herein. Specifically, the cathode/anode assembly 370 is operatively connected to an electrical current source in the form of a low voltage, current regulated power supply unit 414 known in the art. For safety purposes, the power supply unit 414 should have an upper output limit of about 10 volts and a current capacity not less than about 20 amps for each liter of liquid product being treated. For example, about 400 amps is required for treating about 20 liters of liquid product at a current density of about 0.1 amp/cm² over about 200 cm² (e.g. the preferred cathodic surface area) for each liter of liquid product. As illustrated in Figure 3, the "-" end of the power supply unit 414 is operatively connected to the cathode supply bus 404 using conductive line 415, and the "+" end of the power supply unit 414 is operatively connected to the anode supply bus 412 using conductive line 416.

To remove gold from the liquid product contained with the pond 304, all electrical connections are made between the power supply unit 414 and the cathodes 372 and anodes 374 using the cathode connector 402, the anode connector 410, the cathode supply bus 404, and the anode supply bus 412. After connection of the power supply unit 414 to the cathodes 372 and anodes 374, the power supply unit 414 is activated, with the cathodes 372, anodes 374, and chamber 385 if used) being thereafter immersed within the liquid product containing dissolved gold. If the chamber 385 is used to retain the cathodes 372 and anodes 374 therein, the immersion depth of the chamber 385 should be limited so that the upper edge 430 of the chamber 385 (FIG. 4) is not submerged within the liquid product. This avoids the formation of increased current densities at the edges of the cathodes 372.

In addition, the power supply unit connected to the cathodes 372 and anodes 374 should be activated (e.g. turned on) prior to immersion of the cathodes 372 and anodes 374 into the liquid product so that an electrical potential is applied to the cathodes 372 and anodes 374 prior to immersion. This is important because immersion of unenergized cathodes 372 in the liquid product (which contains significant amounts of leaching solution) causes the etching of gold therefrom. Furthermore, the cathodes 372 and anodes 374 (especially the cathodes 372) should be removed from the liquid product before the power supply 414 is deactivated.

In a preferred embodiment, the applied voltage from the power supply unit 414 should be about 3-5 V (about 4 V is optimum). Likewise, the resulting current density applied to the cathodes 372 and anodes 374 of the cathode/anode assembly 370 should be about 65-120 mA/cm² (about 85 mA/cm² is optimum).

As a result, metallic gold particles form on the surfaces of the cathodes 372 at a rate of about 4-16 mg/cm² per hour. It is preferred that the gold particles be removed from the cathodes before they fall off and back into the liquid product. Otherwise, any particles which fall into the liquid product dissolve therein.

Also, removal of the collected gold particles from the surfaces of the cathodes 372 increases the deposition of new gold particles thereon. In a preferred embodiment, the cathodes 372, anodes 374, and chamber 385 (if used) are withdrawn from the liquid product at a pre-seiected time interval (e.g. about every 15 minutes) and rinsed with deionized water. The rinse water (which includes gold particles received/removed from the cathodes 372 and the chamber 385 (if used) is then conventionally filtered (by vacuum filtration or the like) to isolate the gold particles therefrom.

The electrolytic process described above decomposes the dissolved gold complex at the cathodes 372 and restores the iodide (21^') to iodine (l₂) at the anodes 374 so that the original leaching solution (containing dissolved KI and dissolved l₂) is regenerated and recovered. More specifically, the process of this method recovers metallic gold from the dissolved gold complex, e.g., Aul.KI₃ (aq.), and automatically reconverts the corresponding iodide to dissolved iodine, thereby restoring the reagent to its selected controlled point composition in accordance with the following reactions: At the cathodes 372:

2Aul.KI₃ (aq.) + 2e^" > 2Au (metal) + 21^" (aq.) + 2KI₃ (aq.) (10)

At the anodes 374:

KI (aq.) + 2I- (aq.) > Kl₃ (aq.) + 2e^" (11 )

The overall result of these reactions is shown as follows:

2Aul.KI₃ (aq.) + KI (aq.) > 2Au (metal) + 3KI₃ (aq.) (12)

Thus, in accordance with this embodiment of the invention, metallic gold may be readily collected, along with simultaneous leaching solution regeneration. Such regeneration offers numerous benefits with respect to material recovery and the promotion of optimum-rate leaching. Accordingly, in a gold extraction process, continuous operation of the leaching system 300 and cathode/anode assembly 370 as described herein enable the dissolved gold concentration level in the leaching/liquid product to be maintained at a consistent value, thereby ensuring that the leaching rate is optimum. As a result, throughput and production can be maximized, while chemicals and energy consumption are minimized, and toxicity to the operators and the environment are also minimized.

In the invention, the continuous use of the electrodeposition procedure of this embodiment results in a low dissolved gold concentration of about 6 g of gold/liter of solution, which is substantially below the solution saturation point, which is approximately 12-22 g of gold/liter of solution (at a temperature of about 2-50 degrees C).

In the alternative, the cathode/anode assembly 370 of the extraction system 300 may be cycled on and off as desired so that once a selected gold concentration level is reached (e.g. as determined by experimental time trials), the cathode/anode assembly 370 is turned on until a lower level is reached. For example, the assembly 370 could be activated when about 9 g of gold/liter of solution is reached, and deactivated when a concentration level of about 6 g of gold/liter of solution is achieved.

Finally, it should be noted that method 2 as described above has an additional advantage in that only very minimal amounts of fresh solution need to be added to the system 300. This is possible because of the direct regeneration of the components used in the original leaching solution (e.g. dissolved l₂ and dissolved KI). The addition of very minor amounts of fresh solution may be periodically needed in view of minor losses by drag out which accompany the removal of leached ore upon leaching completion, as well as the removal of other components (e.g. cathode/anode assemblies) which are used in the extraction process. In addition, certain minor losses occur because of a cathodic side reaction which proceeds at a very small rate as follows:

2H₂0 + 2e- > H₂ (g) + 2OH^" (13)

In addition, the balancing current at the anodes 374 stoichiometrically drives the following anodic reaction:

3KI (aq.) > Kl₃ (aq.) + 2K⁺ + 2e^" (14)

Furthermore, the following additional reactions are taking place:

Kl₃ (aq.) > KI (aq.) + l₂ (gas) (15)

20H- + 2K⁺ > 2KOH (aq.) (16)

The net result is a summation of reactions (13)-(16) as follows: 2KI (aq.) + 2H₂ O > 2KOH (aq.) + H₂ (g) + l₂(g) (17)

Accordingly, the loss of l₂ (g) (unless trapped and reintroduced into the system) requires the introduction of small amounts of fresh reagent solution from the supply 302, although such amounts are minimal, and do not affect the unique economy of operation which is inherent in the process of method 2.

The net result of reaction (17) also involves the conversion of KI (aq.) to KOH (aq.) as illustrated above. Unless neutralized or eliminated, increasing amounts of KOH (aq.) can cause slight decreases in gold solubility and the overall leaching rate. Thus, depending on the amount of KOH (aq.) generated (which, in turn, depends on how long the system is operating and how much leaching has been completed), it may be necessary to chemically eliminate the KOH (aq.). Single pH measurement with a pH-meter will indicate when to eliminate KOH. This can be readily accomplished through conversion of the KOH (aq.) to KI (aq.) by neutralization with HI (aq.) in accordance with the following reaction:

2KOH (aq.) + 2HI (aq.) > 2H₂0 + 2KI (aq.) (18)

When the load of ore in pond 104 is exhausted of its gold, the pond 104 and the ore in it are drained of the leaching solution by opening valve 256 and closing valve 254, thus diverting the regenerated leaching solution to tank 258. When draining the pond 304 and the exhausted ore therein is completed, valve 112 is closed. However, the exhausted ore is wet by a liquid film containing unreacted KI and l₂ , and a dissolved gold complex, about 6-19 g of gold per liter of wetting liquid.

Because the effective surface area of the finely divided ore is very large, the amount of dissolved gold in the wetting liquid film is appreciable. Therefore, before discarding the exhausted ore, it is exposed to a dilute solution of KI in water, about 17 g of KI per liter of solution. For that purpose, the extraction system 300 involves a supply 260 of the dilute KI solution. Valves 262 and 264 now are opened, and the supply 260 of dilute KI solution is routed to the exhausted wet ore via valve 262 and conduit 124.

In the pond 104, the dilute KI solution progresses through the exhausted wet ore, from inlet conduit 124 to outlet conduit 130, and is withdrawn back to its supply 260. While in contact with the exhausted wet ore, the dilute KI solution extracts most chemicals from the liquid wetting film, leaving instead a wetting film of diluted KI solution with negligible amounts of iodine and gold. The dilute KI solution then is drained from pond 104 by closing valve 262 and opening valve 267, thus diverting the dilute KI solution to tank 269. When the draining is completed, valves 264 and 267 are closed, pump 162 is switched off, the exhausted rinsed ore load is removed from pond 104 and discarded, a new load of ore is loaded, and the cycle applied to the new load.

After the metal gold is extracted from the ore in accordance with this embodiment it may be refined by the same known art used to refine the gold extracted from ores using prior art technology.

The invention represents an advance in the art of gold extraction from ores technology. Not only does it enable the efficient production of metal gold from ores, but it accomplishes this objective using minimal amounts of chemicals and of energy while minimizing the production of chemical wastes and minimizing the toxicity to the operators and to the environment.

Although the invention has been described in detail with reference to particular preferred embodiments, the invention is not limited to the use of any specific hardware, system configurations, and components. Persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.

Claims

CLAIMSWhat is claimed is:

1. A method for the extraction and recovery of gold from ores comprising the steps of:

providing a pond for containing finely divided ore containing gold metal;

adding a leaching reagent solution to said pond, producing a liquid product containing unreacted reagent solution and dissolved gold; and

extracting the gold from said liquid product and regenerating the leaching reagent solution.

2. The method of Claim 1 wherein the leaching reagent solution is of dissolved KI and dissolved l₂ and the extraction of gold metal from the liquid product produced in the leaching step, and the regeneration of the leaching solution comprise the steps of:

removing said liquid product from said pond;

cooling said removed liquid product to a temperature sufficient to cause precipitation of said dissolved gold complex from said liquid product; said cooling resulting in the formation of a gold precipitate;

separating said gold precipitate from said liquid product;

heating said liquid product after said separating of said gold precipitate therefrom in an amount sufficient to generate a supply of heated, recovered solution; and redirecting said heated, recovered solution back to pond to continue said extraction and recovery of gold from ore.

3. The method of Claim 2, wherein said leaching reagent solution is maintained at a temperature of about 10-50 degrees C during said production of said liquid product containing said unreacted reagent solution and said dissolved gold complex.

4. The method of Claim 2, wherein said cooling of said removed liquid product comprises the step of cooling said liquid product to a temperature of about 0-4 degrees C.

5. The method of Claim 2, wherein said heating of said recovered solution after said separating of said gold precipitate therefrom comprises the step of heating said recovered solution to a temperature of about 30-50 degrees C.

6. The method of Claim 2, further comprising the steps of:

combining said gold precipitate with water in an amount sufficient to form a plurality of Aul crystals; and

heating said Aul crystals in an amount sufficient to produce metallic gold therefrom.

7. The method of Claim 6, further comprising the steps of:

separating said Aul crystals from any remaining liquid materials left over after said combining of said gold precipitate with said water; and redirecting said remaining liquid materials back to pond to continue said extraction and recovery of gold from ore.

8. The method of Claim 2, further comprising the steps of:

combining said gold precipitate with water in an amount sufficient to form a plurality of Aul crystals; and

heating said Aul crystals to a temperature of about 100-150 degrees C and produce metallic gold therefrom.

9. The method of Claim 8, further comprising the steps of:

separating said Aul crystals from any remaining liquid materials left over after said combining of said gold precipitate with said water; and

redirecting said remaining liquid materials back to pond to continue said extraction and recovery of gold from ore.

10. The method of Claim 1 , wherein said pond comprises a depression in the ground, said depression lined with an iodine-resistant material.

11. The method of Claim 1 wherein the leaching reagent solution is of dissolved KI and dissolved l₂ and the extraction of gold metal from the liquid product produced in the leaching step, and the regeneration of the leaching solution comprise the steps of:

removing said liquid product from said pond;

providing at least one cathode and at least one anode; immersing said cathode and said anode within said removed liquid product;

passing an electrical current through said cathode and said anode immersed within said liquid product in an amount sufficient to cause said dissolved gold complex to be converted to both metallic gold and a regenerated reagent solution comprising dissolved KI and dissolved l₂, said metallic gold collecting on said cathode during said passing of said electrical current therethrough; and

redirecting said regenerated reagent solution back to pond to continue said extraction and recovery of gold from ore.

12. The method of Claim 11 , wherein said electrical current is about 65-120 mA/cm².

13. The method of Claim 11 , further comprising the steps of:

providing an electrical current source;

connecting said electrical current source to said cathode and said anode; and

activating said electrical current source prior to said immersing of said cathode and said anode within said liquid product to apply an electrical potential to said cathode and said anode prior to said immersing thereof within said liquid product.

14. The method of Claim 11 , further comprising the steps of: withdrawing said cathode from said liquid product after said collecting of said metallic gold thereon and before said metallic gold falls off of said cathode; and

removing said metallic gold from said cathode after said withdrawing of said cathode from said liquid product.

15. The method of Claim 11 , wherein said leaching reagent solution is maintained at a temperature of about 10-50 degrees C during said production of said liquid product containing said unreacted reagent solution and said dissolved gold complex.

16. The method of Claim 11 , wherein said pond comprises a depression in the ground, said depression lined with an iodine-resistant material.

17. The method of Claim 11 , wherein said cathode comprises a planar ceramic substrate having a layer of metallic gold.

18. The method of Claim 11 , wherein said anode comprises a titanium mesh coated with a porous platinum layer.

19. The method of Claim 11 , wherein said cathode and said anode are plane-parallel to each other.

20. The method of Claim 11 , wherein said cathode and said anode are curved-parallel to each other.