Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group ib metal
Reexamination Certificate
1999-12-21
2001-05-08
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group ib metal
C423S027000, C423S029000, C423S041000
Reexamination Certificate
active
06228334
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to methods of gold recovery. The invention relates in particular to a method of recovering gold from fine carbon residue produced during a process in which gold is recovered from its ores by leaching and absorption onto coarse activated carbon.
DISCUSSION OF BACKGROUND ART
Activated carbon is widely used for recovering precious metals including gold and silver, throughout the precious metals industry. In one generally preferred prior-art gold recovery method, a reagent solution of potassium or sodium cyanide or some other chemical is used to dissolve a precious metal from its ores. Coarse carbon, for example, about 20-mesh or greater, is added to the gold-containing solution so formed. The dissolved precious metal can be effectively adsorbed on to the coarse carbon. The coarse carbon is then separated from the solution by passing the mixture of carbon and solution through 20-mesh screens.
The gold is then stripped from the gold-loaded carbon with a further reagent solution of sodium or potassium cyanide to form a second gold-containing solution. The gold is then generally recovered from the solution by an electrochemical process such as electrowinning.
One disadvantage of this carbon adsorption method is a loss of carbon, and gold adsorbed thereon, during the various steps of the process. This loss occurs as a result of a percentage of coarse carbon being reduced to fine carbon particles, for example less than about 20 mesh, during the above-described process steps. The particles are sufficiently fine that they can not be retained by the 20-mesh screens typically used for carbon separation. Because of this the fine carbon may be deposited in the tailing along with the adsorbed gold. A global survey of 36 selected gold producing companies indicated that the average carbon consumption in this process was 0.025 pound per ton of processed ore (D. Seymour, Carbon consumption in Precious Metal Recovery—An Industry Survey, Randol Proceedings, pp319-326, Vancouver, Canada, 1992). Yearly carbon consumption at the 36 mines surveyed is approximately 2,000 tons. This carbon can contain from about 4 to 50 ounces (oz) of gold per ton.
Due to the potential loss of gold on the fine carbon, most precious metal producing companies employ some method, for example, flocculation and filtration, to recover at least a part of the residual fine carbon generated during the process steps. In one industry standard procedure, once sufficient fine carbon has been recovered, the recovered fine carbon is shipped off site to be processed (Rescan Engineering LTD., World gold Survey, Section 4, May 1998) for recovery of any precious metal adsorbed thereon. The typical off-site process includes grinding and smelting the fine carbon, followed by recovering precious metals from the ash with further stages of cyanide dissolution and electrowinning.
The average cost of shipping and processing of the fine carbon can be sufficiently high that about five to six ounces of gold must be recovered per ton of fine carbon for the recovery process simply to break even. Clearly, there is a need for a simple and inexpensive on-site process for recovering precious metals from the fine carbon produced during the coarse carbon process for precious metal recovery.
SUMMARY OF THE INVENTION
The present invention is based on a discovery that gold can be transferred from gold-loaded fine carbon to coarse carbon in a slurry containing a suitable transfer reagent. This permits that gold can be recovered from fine carbon residue from a prior-art coarse carbon recovery process at the prior-art process site without the expensive grinding and smelting steps of prior-art methods for recovering gold from fine carbon residues.
In one aspect the method of the present invention comprises mixing the gold-loaded fine carbon residue with coarse carbon and an aqueous solution of a transfer reagent to form a coarse-carbon-containing slurry. The coarse carbon is retained in the first slurry for a time period sufficient that gold is transferred from the gold-loaded fine carbon to the coarse carbon, whereby the coarse-carbon becomes gold-loaded and the fine carbon becomes gold-depleted. The gold-loaded coarse carbon is separated from the coarse-carbon-containing slurry, thereby leaving a coarse-carbon-free slurry including gold-depleted fine carbon. The transferred gold is recovered from the gold-loaded coarse carbon.
The transfer reagent solution preferably includes one or more reagents selected from the group of reagents consisting of potassium cyanide, sodium cyanide, potassium sulfite, sodium sulfite, potassium thiosulfate, sodium thiosulfate, and thiourea. Potassium and sodium cyanide in a concentration of about 0.01% or greater have been found particularly effective.
Transfer of gold from the gold-loaded fine carbon to the coarse carbon progressively decreases as the gold content of the fine and coarse carbon become equal. Transfer thus becomes more effective the higher the coarse carbon to fine carbon ratio in the coarse-carbon-containing slurry. More gold can be transferred from the gold-depleted carbon in the coarse-carbon-free slurry by further addition of coarse carbon thereto and repetition of the above described transfer and separation stages.
In one example, wherein 1% solution of sodium cyanide was used as a transfer reagent solution and the coarse carbon to fine carbon ratio was about 1/1, 66% of gold was transferred from the fine carbon to the coarse carbon in 7 hours in a single cycle of the inventive gold recovery method. About 89% and 96% or gold could be transferred from the fine carbon to coarse carbon in respectively second and third cycles (repetitions) of the inventive gold recovery method.
REFERENCES:
patent: 4528166 (1985-07-01), McDougall
patent: 4981598 (1991-01-01), Komadina
patent: 5019162 (1991-05-01), Suzuki et al.
patent: 647074 (1991-08-01), None
patent: 139056 (1985-05-01), None
patent: 177293 (1986-04-01), None
Zadra, J.B., “A Process for the recovery of gold from activated carbon by leaching and electrolysis,” Bureau of Mines Report 4672, Apr. 1950.*
D. Seymour, Carbon consumption in Precious Metal Recovery—An Industry Survey, Randol Proceedings, pp319-326, Vancouver, Canada, 1992, no month.
Rescan Engineering Ltd., World gold Survey, Section 4, May 1998.
Hill Eric M.
Lin Hsing Kuang
Austin R. Russel
Bos Steven
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