Title of Invention

"A PROCESS FOR THE RECOVERY OF GOLD AND SILVER FROM THE EXHAUSTED DROSS GENERATED DURING THE REFINING OF GOLD"

Abstract The present invention relates to a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold. This invention particularly relates to a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold, through the application of non-cyanide lixiviants like sodium thiosulphate and thiourea respectively . This invention will be useful to the gold producing and gold refining industries. The process gives a recovery of 94% of gold and 95% of silver from the exhausted dross generated during the refining of gold.
Full Text The present invention relates to a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold. It particularly relates to a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold, through the application of non-cyanide lixiviants which will be useful to the gold producing and gold refining industries.
BACKGROUND OF THE INVENTION
Usually, pyrometallurgical refining of impure gold is carried out in fire clay crucibles kept inside graphite guard crucibles. During this process a portion of the gold and silver enter into the walls of the crucibles. At the end of their life cycle, these crucible assemblies are crushed and ground giving rise to the dross. The dross is then washed on James tables to recover the entrapped gold and silver. The exhausted dross left after tabling still contains substantial quantities of gold and silver. To the best of our knowledge, no published literature or patent is available to address the problem of recovering gold and silver from the exhausted dross generated during the refining of gold. In the recent past, several investigations have been carried out by a number of researchers to find out a suitable alternative reagent to replace the century old cyanide for the leaching of gold and silver. Groenewald ( Groenewald. T, The dissolution of gold in acidic solutions of thiourea, Hydrometallurgy ,1976 , I (3), pp 277-290.) conducted fundamental studies on the dissolution of gold in acidic thiourea solutions, using rotating discs of pure gold as well as ground gold ores in presence of different oxidising agents. Reference may be made to the work of Deng and Liao (Deng, T and Liao, M, Gold recovery enhancement from a refractory flotation concentrate by sequentia I bioleaching and thiourea leach, Hydrometallurgy ,2002, 63 (3), pp 249-255.) who reported enhanced gold recovery from a gold flotation concentrate by sequential bioleaching and thiourea leach. Li and Miller (Li, J and Miller, J.D, Reaction kinetics for gold dissolution in acidic thiourea solution using formamidine as an oxidant, Hydrometallurgy , 2002, 63 (3), pp 215-223) studied kinetics for gold dissolution in acidic thiourea solutions using formamidine as an oxidant. Murthy et .al (Murthy, D.S.R, kumar, Vinod and Rao, K.V, Extraction of gold from an Indian low grade refractory gold ore through physical beneficiation and

thiourea leaching, hydrometallurgy, 2003, 68 (1-3 ), pp 125-130) studied the recovery of gold from a low grade refractory gold ore through physical beneficiation and thiourea leaching. Hisshion and Waller (Hisshion, R.J and Waller, C.G, Recovering gold with thiourea, Min.Mag,1984, 151 (3), , pp 237-243) described the operation of a gold pilot plant using thiourea as the leachant. Reference may also be made to a Canadian patent no. 1236980 in which a precious metal containing sludge was leached with hydrochloric acid and hydrogen peroxide to solubilize at least gold, palladium and platinum and silver was removed as insoluble silver chloride. Gold was selectively precipitated from the leach solution by reduction with SO2 in the presence of sodium ions.
However, there are no patents or published literature dealing with the recovery of gold and silver from the exhausted dross (in which the contents of gold and silver are much more than those present in the natural ores) generated during the refining of gold.
The main object of the present invention is to provide a process to recovery of gold and silver from the exhausted dross generated during the refining of gold, which obviates the drawbacks as detailed above.
In an object of the present invention is to provide a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold , through the application of non-cyanide lixiviants .
Another object of the present invention is to provide very fast process which can be used for recovering gold and silver from the secondaries of gold producing and gold refining industries.

Novelty of the process lies in use of non-cyanide lixiviants i.e sodium thiosulphate and thiourea for solubilising of silver and gold in this processStatement of invention
Accordingly, the present invention provides a process for the recovery of gold and silver from the exhausted dross generated during the refining of gold, which comprises:
(i) selecting the dross generated during the refining of gold from gold producing or refining industries and grinding it to 80% - 200 mesh size,
(ii) mixing the ground dross with an alkali chloride additive and roasting at a temperature in the range of 400-900°C,
(iii) leaching the said roasted dross with sodium thiosulphate solution having concentration in the range of 5 g/l to 70 g/l and wherein the ratio of the dross to the leachant varies from 1:3 to 1:10 in presence of ammonia at a temperature in the range of 40-90°C for 4-10 h, and separating the solids and solution by filtration,
(iv) precipitating silver from the leached solution obtained from step (iii) as silver sulphide and converting it into metallic silver by heating at a temperature in the range of 600-800°C,
(v) leaching the solid residue obtained from step (iii ) with acidic thiourea solution having concentration in the range of 0.05 M to 2.0 M in the presence of ferric sulphate at a pH of 1-2.5 in the temperature range of 20-50°Cfor4h and filtering,
(vi) mixing activated charcoal with the filtered leached liquor obtained from step (v) and agitating for 1-2 h for adsorbing the gold onto the activated charcoal,
(vii) filtering the activated charcoal loaded with gold and incinerating it at a temperature in the range of 700-850°C for 2-3 h,

(viii) smelting the incinerated charcoal along with the metallic silver obtained from step (iv) to get gold-silver bullion.
(ix) Parting gold and silver from the gold-silver bullion by known method.
According to a feature of the invention, the ratio of the exhausted gold dross to the leachant may vary from 1:3 to 1:10.
Thiosulphte solution used for the leaching of silver may be selected from sodium thiosulphate, ammonium thiosulphate etc and have a concentration in the range of 5 g/l to 70 g/l.
Thiouea solution used for the leaching of gold may have a concentration in the range of 0.05 M to 2.0 M.
The activated charcoal employed may be selected from wood charcoal, coconut shell charcoal and the like and the amount used may be in the range of 0.25 g/l to 10 g/l of the thiourea leached liquor.
The amount of ferric sulphate used along with the thiourea during the leaching of gold may be in the range of 0.5 g/l to 10 g/l.
The additive used in the roasting of the dross may be selected from NaCI, KCI and
the like.
All the reagents used in the present invention may be of commercial grade.
The process gives a recovery of 94% of gold and 95% of silver from the exhausted
dross generated during the refining of gold.
The novelty of the present invention is recovering gold and silver from the exhausted dross generated during the refining of gold, with the help of non-cyanide reagents.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
E X AM P L E -1
1000 g of the ground exhausted dross generated dross during the refining of gold containing 845 g/t of gold, 8250 g/t of silver, 0.32% Cu, 8.0% Fe203 , 23.5% AI2O3, 41.2% Si02, 4.5% MgO and 21.6% graphite was mixed with 100 g of the additive and roasted at 750 °C for 2 h, in a muffle furnace and then leached with sodium thiosulphate solution having a concentration of 40 gm/l in presence of ammonia at a temperature of 60° C for 7 h at a solid to liquid ratio of 1:4. After leaching, the solids
and liquid were separated by filtration and from the leached liquor, silver was precipitated as silver sulphide and then converted to metallic silver by heating at 750°C.
The leach residue left after sodium thiosulphate leaching was further subjected to leaching with acidic thiourea solution of 0.25 M concentration in presence of 2.5 g/l of ferric sulphate at a pH of around 1.5, for 4 h . After leaching, the solids and the thiourea leached liquor were separated by filtration and the leach liquor was mixed with 40 g of activated charcoal and agitated for 2 h in order to adsorb the gold onto the activated charcoal.
The activated charcoal loaded with gold was separated by filtration and was incinerated at 800°C for 2 h and then smelted along with the metallic silver obtained from the thiosulphate leaching and a gold silver bullion was obtained. The gold-silver bullion was parted with nitric acid and gold and silver were obtained. The recoveries of gold and silver were 93% and 95% respectively.
EXAMPLE-2
1000 g of the ground exhausted dross generated during the refining of gold containing 562 g/t of gold, 5340 g/t of silver, 0.42% Cu, 8.0% FezOs 23.5% AI2O3, 41.2% SiOa, 4.5% MgO and 21.7% graphite was mixed with 90 g of the additive and roasted at 750°C for 2 h, in a muffle furnace and then leached with sodium thiosulphate solution having a concentration of 25 g/l in presence of ammonia at a temperature of 60°C for 5 h at a solid to liquid ratio of 1:4. After leaching, the solids and liquid were separated by filtration and from the leached liquor, silver was precipitated as silver sulphide and then converted to metallic silver by heating at 750°C.
The leach residue left after sodium thiosulphate leaching was further subjected to leaching with acidic thiourea solution of 0.15 M concentration in presence of 2.5 g/l of ferric sulphate at a pH of around 1.5, for 4 h. After leaching, the solids and the thiourea leached liquor were separated by filtration and the leached liquor was
mixed with 25 g of activated charcoal and agitated for 2 h, in order to adsorb the gold onto the activated charcoal.
The activated charcoal loaded with gold was separated by filtration and was incinerated at 800°C for 2 h and then smelted along with the metallic silver obtained from the thiosulphate leaching and a gold - silver bullion was obtained. The gold-silver bullion was parted with nitric acid and gold and silver were obtained. The recoveries of gold and silver were 95% and 94% respectively.
EXAMPLE-3
1000 g of the ground exhausted dross generated during the refining of gold containing 734 g/t of gold, 7126 g/t of silver, 0.37% Cu, 8.0% Fe203, 23.5% AI2O3, 41.3% Si02, 4.5% MgO and 21.6% graphite was mixed with 110 g of the additive and roasted at 750°C for 2 h, in a muffle furnace and then leached with sodium thiosulphate solution having a concentration of 35 g/l in presence of ammonia at a temperature of 60°C for 6 h at a solid to liquid ratio of 1:4. After leaching, the solids and liquid were separated by filtration and from the leach liquor, silver was precipitated as silver sulphide and then converted to metallic silver by heating at 750°C.
The leach residue left after sodium thiosulphate leaching was further subjected to leaching with acidic thiourea solution of 0.2 M concentration in presence of 2.5 g/l of fen-ic sulphate at a pH if around 1.5, for 4 h. After leaching, the solids and the thiourea leached liquor were separated by filtration and the leached liquor was mixed with 35 g of activated charcoal and agitated for 2 h, in order to adsorb the gold onto the activated charcoal.
The activated charcoal loaded with gold was separated by filtration and was incinerated at 800°C for 2 h and then smelted along with the metallic silver obtained from the thiosulphate leaching and a gold - silver bullion was obtained. The gold-
silver bullion was parted with nitric acid and gold and silver were obtained. The recoveries of gold and silver were 94% and 96% respectively.
The main advantages of the present invention are:
(1) The process is very safe from the environmental point of view, because it uses only non-cyanide lixiviants for the leaching of gold and silver.
(2) Quantitative recoveries of gold (94%) and silver (95%) can be obtained.
(3) The process is very fast and can also be used for recovering gold and silver from the secondaries of gold producing and gold refining industries.






We claim:
1. A process for the recovery of gold and silver from exhausted dross generated during the refining of gold through the application of non-cyanide lixiviants which comprises:
(i) selecting the dross generated during the refining of gold from gold producing or refining industries and grinding it to 80% - 200 mesh size,
(ii) mixing the ground dross with an alkali chloride additive and roasting at a temperature in the range of 400-900°C,
(iii) leaching the said roasted dross with sodium thiosulphate solution having concentration in the range of 5 g/l to 70 g/l and wherein the ratio of the dross to the leachant varies from 1:3 to 1:10 in presence of ammonia at a temperature in the range of 40-90°C for 4-10 h, and separating the solids and solution by filtration,
(iv) precipitating silver from the leached solution obtained from step (iii) as silver sulphide and converting it into metallic silver by heating at a temperature in the range of 600-800°C,
(v) leaching the solid residue obtained from step (iii ) with acidic thiourea solution having concentration in the range of 0.05 M to 2.0 M in the presence of ferric sulphate at a pH of 1-2.5 in the temperature range of 20-50°C for 4 h and filtering,
(vi) mixing activated charcoal with the filtered leached liquor obtained from step (v) and agitating for 1-2 h for adsorbing the gold onto the activated charcoal,
(vii) filtering the activated charcoal loaded with gold and incinerating it at a temperature in the range of 700-850°C for 2-3 h,
(viii) smelting the incinerated charcoal along with the metallic silver obtained from step (iv) to get gold-silver bullion.
(ix) Parting gold and silver from the gold-silver bullion by known method.
2. A process as claimed in claims 1-4 wherein the activated charcoal employed is selected from wood charcoal, coconut charcoal and the like.
6. A process as claimed in claims 1-5 wherein The additive used in the roasting of the dross is selected from NaCI, KCI and the like.
7. A process as claimed in claims 1-6 wherein the amount of activated charcoal used is in the range of 0.25 g/l to 10 g/l of the leach liquor.
8. A process as claimed in claims 1-7 wherein the amount of ferric sulphate used along with thiourea during the leaching of gold is in the range of 0.5 g/l to 10g/l.
9. A process for the recovery of gold and silver from the exhausted dross generated during the refining of gold, through the application of non-cyanide lixivients substantially as herein described with reference to the examples.

Documents:

422-DEL-2004-Abstract-(30-07-2010).pdf

422-del-2004-abstract.pdf

422-DEL-2004-Claims-(30-07-2010).pdf

422-del-2004-claims.pdf

422-DEL-2004-Correspondence-Others-(30-07-2010).pdf

422-del-2004-correspondence-others.pdf

422-del-2004-correspondence-po.pdf

422-DEL-2004-Description (Complete)-(30-07-2010).pdf

422-del-2004-description (complete).pdf

422-del-2004-form-1.pdf

422-del-2004-form-18.pdf

422-del-2004-form-2.pdf

422-DEL-2004-Form-3-(30-07-2010).pdf

422-del-2004-form-3.pdf

422-del-2004-form-5.pdf


Patent Number 243904
Indian Patent Application Number 422/DEL/2004
PG Journal Number 46/2010
Publication Date 12-Nov-2010
Grant Date 10-Nov-2010
Date of Filing 11-Mar-2004
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110 001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 DAITA SREE RAMACHANDRA MURTHY NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR JHARKHAND, INDIA.
2 VINOD KUMAR NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR JHARKHAND, INDIA.
PCT International Classification Number C22B 11/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA