WO1999045158A1

WO1999045158A1 - Gold recovery process

Info

Publication number: WO1999045158A1
Application number: PCT/AU1999/000126
Authority: WO
Inventors: William Harold Jay; Paul Leslie Breuer
Original assignee: Arton (No 001) Pty. Ltd.; Everett & Goodall Electrical Pty. Ltd.
Priority date: 1998-03-02
Filing date: 1999-03-02
Publication date: 1999-09-10
Also published as: AUPP208498A0; ZA991672B

Abstract

The present invention relates to a gold recovery process, in particular a process for recovering gold values from carbonaceous materials having gold adsorbed thereon. A process for the recovery of gold from carbonaceous materials comprising oxidizing the gold-containing carbonaceous material at a temperature in the range of 400 °C to 1200 °C to form an ash and recovering the gold values from the ash.

Description

GOLD RECOVERY PROCESS

The present invention relates to a gold recovery process, in particular a process for recovering gold values from carbonaceous materials having gold adsorbed thereon. In particular, the present invention relates to a process for recovering gold values from fines resulting from the use of activated carbon particles in the recovery of gold using processes such as Carbon-in-Pulp (OP) or Carbon-in-Leach (CIL) processes.

In most gold mining operations, ore is reduced in particle size by crushing and then grinding in open-circuit autogenous (SAG) mills with final milling in ball mills. The ball mills are normally in closed circuit with hydrocyclones designed to allow oversize particles to be returned to the ball mill and particles of the desired size to proceed to the leaching circuit.

An aerated allcaline sodium cyanide solution is then employed in the leaching circuit to dissolve the gold present in the ore, forming gold cyanide. The gold cyanide is then recovered from the slurry using activated carbon in the processes such as the Carbon-in-Pulp (CIP) or Carbon-in-Leach (CIL) processes.

Organic materials such as tree roots, timber, etc. may also enter the milling circuit and while it may be partially removed by screening, much of it may also enter the leaching circuit with the ore. The presence of this carbon can lead to gold cyanide being adsorbed onto it resulting in further gold losses to tailings. Even if these organic materials are removed by screening from the circuit, any gold cyanide adsorbed on it is lost as there are currently no economic processes available for its recovery.

In CIP/CIL circuits, after the recovery of the activated carbon from the pachucas or stirred tank reactors, it is stripped of its adsorbed gold cyanide by the conventional Zadra or AARL processes. The wet, stripped carbon, is then regenerated (reactivated) by heating it in an inert atmosphere to about 720°C, then either screened to remove undersize carbon and then - 2 -

quenched in water, or, quenched in water and then screened to remove any undersize carbon. The coarse, reactivated carbon is then returned to the leach circuit. The undersize material is either stockpiled for resale, or pumped to tailings, resulting in further gold losses in the residual unrecovered gold in the undersized materials.

Hydrocyclones do not have a perfect separation efficiency. Thus, in the milling of the ore, some coarse or oversize particles may enter the leaching circuit. When these particles are sufficiently large, they settle to the bottom of the leach tanks and effectively reduce the working capacity of these tanks. As a consequence of this settling, some activated carbon particles can also be trapped with this coarse material. Periodically, the tanks need to be cleaned to remove this oversize material thereby returning the tanks to their full working capacity. The combination of inorganic gangue mineral particles and activated carbon which, when removed from these tanks is either stockpiled or sent to tailings. Again, this represents a financial loss to the gold mine as the activated carbon contains high concentrations of gold cyanide which remains unrecovered.

To date, the economic recovery of this gold which has up unto now been unable to be economically recovered because of various problems such as gold cyanide adsorbed in the micro- and macro-pores of the carbon being occluded, by high loadings of inorganic salts, particularly calcite and silicates, and by the thermal regeneration process reducing the gold cyanide to gold metal and sintering it with the inorganic deposits. It has also been found that these various waste carbonaceous materials contain variable concentrations of gold, and this has acted as a further disincentive to the economic recovery of this gold.

We have now found that the coarse carbon and the gold on fine carbon may be economically recovered. Accordingly there is provided a process for the recovery of gold from carbonaceous materials comprising oxidizing the gold-containing carbonaceous material at a temperature in the range of from 400°C to 1200°C to form an ash and recovering the gold from the ash. In the process of the present invention the carbonaceous matter is preferably separated from the gangue minerals prior to oxidisation.

Gold-containing carbonaceous materials suitable for use in the process of the present invention include undersized carbon resulting from the use of activated carbon in Carbon-in- Pulp and Carbon-in-Leach processes, waste carbonaceous materials removed from gold leaching processes including residues of tree roots, timber and the like, waste carbonaceous materials recovered from combinations of gold-containing activated carbon and gangue mineral particles, and any other gold-containing carbonaceous material.

Any suitable method for cleaning or separating the carbon from the gangue minerals may be adopted. For example, the carbon may be separated from the gangue minerals by washing the mixture in a suitably sized trommel to remove fines; the oversized materials dried in a rotary drier or steam dried in a fluidised bed and the particles separated by density on an air table. Alternatively, the carbonaceous materials and the gangue minerals may be separated in an elutriation column. The carbonaceous material may then be oxidised and the gold subsequently be recovered.

The carbonaceous materials are oxidised to remove the carbon from the metal values and from the inorganic minerals previously adsorbed on its surface. The oxidation is conducted at temperatures between 400°C and 1200°C, preferably below 850°C to minimise the possibility of sintering the inorganic residues associated with the organic matter which could then entrap the metallic gold. Furnaces of various designs, rotary kilns, fluidised bed roasters, fluidised bed gasification reactors, are typical of the types of equipment which may be used to conduct the oxidation step. The powdered inorganic residue, or ash, which is obtained from this oxidation procedure contains the gold residues originally present in the carbonaceous materials.

Recovery of the gold from this ash may preferably be conducted by hydrometallurgical means, although, pyrometallurgical methods are also suitable. In this process, any suitable 4

lixiviant may be used to dissolve the gold including cyanide, thiocyanate, thiourea, halides and mixed halides, and also thio sulphate-based reagents including the LSSS (Lime-Sulphur- Synthetic Solution) system. Depending upon the lixiviant, ion exchange processes, activated carbon, cementation, precipitation, solvent extraction, electrowinning, and/or any combination of the above recovery methods may be used to recover the gold from the solution.

The gold recovery process of the present invention is simple and readily portable and it is capable of being operated at sites remote from the CIL/CIP plant. This however may introduce the problem of controlling the toxicity associated with the use of cyanide solutions in the leach circuit and in disposed. Non-cyanide containing solutions are therefore preferred.

Particul.arly desirable non-cyanide lixiviants are based upon either chlorine/chloride-based solutions, acid-thiourea, or thiosulphate-based solutions. Recovery of the gold in metallic form may be by any method well-known to those skilled in the art.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising" , will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The present invention will now be described with reference to the following non-limiting examples. All percentages are by weight unless otherwise stated.

EXAMPLE 1.

A fine carbon residue generated in a carbon regeneration kiln was obtained from a gold mine located in North Queensland. Analysis of this carbon indicated that it contained 245 g/t Au and when ashed at 650°C in a muffle furnace it was found to contain 37% of inorganic matter. (a) The residue was then leached at room temperature in an oxygenated solution containing lg/1 NaCN plus 0.01 M NaOH for 1 hour to give a 94% recovery of the gold present on the carbon. After 5 hours leaching, the gold recovery increased to 95% and after 24 hours 96% recovery.

(b) The ash when leached for 5 hours under the same conditions, but using 5 g/1 of sodium cyanide gave a 96% recovery of the gold in the original carbon sample.

When the carbon was finely ground prior to the ashing and leaching step as in example 1(a), a 96% recovery of the gold was achieved after 5 hours.

EXAMPLE 2.

The sample of carbon used in Example 1 was ashed under the same conditions and then subjected to leaching at room temperature in a solution containing IM NH₃ + 0. IM [S ² O³]^2" + 0.005M Cu ²⁺ for 1 hour to give an 83% recovery of the gold in the original carbon sample. After 5 hours, the recovery increased to 91 % , and after 20 hours to 95% .

EXAMPLE 3.

(a) The sample of carbon used in Example 1 was ashed under the same conditions and then subjected to leaching at room temperature in a solution containing 0.14M NaOCl + 100 g/1 NaCl and adjusted to pH 1 HC1. After a 5 hour leach a 99.3% recovery was obtained.

(b) The pH conditions were modified to be 3.8, all other conditions remaining the same. After 30 minutes 99% of the gold in the original carbon sample was recovered. After 1 hour and 24 hour periods, no further leaching was achieved. EXAMPLE 4.

Carbon from a regeneration kiln located in Western Australia was assayed and found to contain 425 g/t Au. Ashing at 650°C in a muffle furnace gave an ash content of 33 % .

(a) A sodium cyanide leach of the sample, under the same conditions as in Example

1(a), gave a gold recovery of 98.5%.

(b) A thiosulphate leach under the conditions given in Example 2 gave a gold recovery of 68% recovery.

(c) A chlorine/chloride leach under the conditions given in Example 3(b) but the NaCl concentration reduced to 10 g/1, a 95% Au recovery of the gold was achieved.

EXAMPLE 5.

A sample of gangue miners containing carbon was obtained from the bottom of the leach tanks at a gold mine located in the Leonora district of Western Australia was cleaned by washing in a trommel screen to remove particles below 0.5mm in size, dried in a rotary drier and the dried material fed onto an air table to separate the carbon from the coarse gangue minerals. The carbon was then sized into a + 1.4mm fraction and a -1.4mm fraction. The fraction below 1.4mm in size when assayed was found to contain 155 g/t Au. After ashing at 650°C in a muffle furnace it gave an inorganic content of 17% .

The ash was then leached under the conditions described in Example 4(c) to give a gold recovery of 95% after 30 minutes. - 7

EXAMPLE 6.

A sample of gangue mineral containing carbon was obtained from the bottom of the leach tanks at a gold mine located in the Kalgoorlie district of Western Australia was cleaned by washing in a trommel screen to remove particles below 0.5mm in size, dried in a rotary drier and the dried material fed onto an air table to separate the carbon from the coarse gangue minerals. The carbon was then sized into a + 1.4mm fraction and a -1.4mm fraction. The fraction below 1.4mm in size when assayed was found to contain 540 g/t Au. After ashing at 650°C in a muffle furnace gave an inorganic content of 10% .

The ash was then leached under the conditions described in Example 4(c) to give a gold recovery of 98 % after 30 minutes.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims

1. A process for the recovery of gold from carbonaceous materials comprising oxidizing the gold-containing carbonaceous material at a temperature in the range of from 400┬░C to

5 1200┬░C to form an ash and recovering the gold values from the ash.

2. A process according to claim 1 wherein the oxidation of gold-containing carbonaceous materials is conducted at a temperature below 850┬░C.

0 3. A process according to claim 1 wherein the gold-containing carbonaceous material is selected from the group consisting of undersized carbon resulting from the use of activated carbon in Carbon-in-Pulp and Carbon-in-Leach processes, waste carbonaceous materials removed from gold leaching processes, and waste carbonaceous materials removed from combinations of gold-containing activated carbon and gangue mineral particles. 5

4. A process according to claim 1 wherein the carbonaceous matter is separated from gangue minerals prior to oxidisation.

5. A process according to claim 4 wherein the gold-containing carbonaceous materials are 0 separated from the gangue minerals by washing the mixture in a suitably sized trommel to remove fines; the oversized materials dried in a rotary drier or steam dried in a fluidised bed and the particles separated by density on an air table.

6. A process according to claim 4 wherein the carbonaceous materials and the gangue 5 minerals may be separated in an elutriation column.

7. A process according to claim 1 wherein the oxidation is conducted in a furnace selected from the group consisting of rotary kilns, fluidised bed roasters, and fluidised bed gasification reactors. 0

8. A process according to claim 1 wherein the recovery of gold values from the ash is conducted by hydrometallurgical or pyrometallurgical methods.

9. A process according to claim 8 wherein a lixiviant is used for dissolving the gold values from the ash.

10. A process according to claim 9 wherein the lixiviant is selected from the group consisting of cyanide, thiocyanate, thiourea, halides and mixed halides, the LSSS (Lime- Sulphur-Synthetic Solution) systems and other thiosulphate-based reagents.

11. A process according to claim 9 wherein the gold is extracted from the lixiviant solution by a process selected from the group consisting of ion exchange processes, activated carbon process, cementation, precipitation, solvent extraction, electro winning, and any combination thereof.