Note: Descriptions are shown in the official language in which they were submitted.
CA 02270744 1999-04-29
BACKGROUND OF THE PRESENT INVENTION
The present invention is directed to an improved method for
the separation of gold from an aqueous liquid containing a gold-
cyanide complex.
The recovery of gold from gold-bearing ores has been generally
accomplished by heap-leaching of the ore. In such a method, a pile of
low grade ore is contacted (by spraying, trickling, pouring on, etc.)
with an aqueous alkaline cyanide solution whereby the solution is
caused to permeate through the ore. Water-soluble gold cyanide
complexes (e.g., as defined by the formula [Au (CN) 4 ] -) are formed
upon such contact, with the now-pregnant gold-cyanide complex-
contairung leach liquor being recovered for separation of the gold.
This process is well described in the literature.
A number of methods have been employed in an attempt to
separate the gold from the leach liquor. See, for example, U. S. Patent
Nos. 2,753,258 (anion exchange resin); 3,215,524 (water-soluble
aldehydes); 3,816,587 (reverse osmosis); 3,935,006 (activated
carbon); 4,273,579 (metallic hydroxide); 4,289,531 (proteinaceous
material); 4,372,830 (anion exchange resin); 4,528,166 (activated
carbon); 4,540,435 (organic phosphorus and sulfur oxides); 4,762,556
(diallylamine polymer); 4,814,007 (guanidine reagent); 4,894,407
(organic oxides of sulfur and phosphorus on resin substrate);
5,019,162 (activated carbon); 5,023,060 (activated carbon); 5,073,354
(activated carbon); 5,156,825 (anion exchange resin); 5,158,603
CA 02270744 1999-04-29
(quaternary amine); 5,290,525 (activated carbon); and 5,643,456
(metal chelating polymer).
Such processes have not been altogether satisfactory. First,
complexes of gold with cyanide are very stable and the gold is not
easily separated therefrom. Once separated by means such as
activated carbon or chelating or anion exchange resins, additional
process steps must still be undertaken to separate the gold from this
additional material. Such multi-step recovery processes are time-
consuming, expensive, result in undesirable waste products, and are
generally inefficient. A fiirtller disadvantage of such processes is that
a cyanide-containing waste product is created upon recovery of the
gold from the cyanide-containing leach liquor. It is desirable to avoid
contaminating the environment with such cyanide-containing by-
product or waste solutions. It is also frequently found that a high
percentage recovery of the gold values from the solution cannot be
achieved. As the price of gold decreases, it is all the more important
to maximize the amount of gold removed from the leach liquor while
minimizing the cost of such removal. It is also important to provide a
gold product having the highest possible degree of purity to enhance
the value of the final product in the marketplace.
OBJECTS AND SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a
cost-effective process for the separation of gold from a cyanide-
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containing aqueous solution.
It is further an object of the present invention to provide a
process for the separation of gold from a cyanide-containing aqueous
solution which enables a high percentage of the gold to be separated.
It is still further an object of the present invention to provide a
process for the separation of gold from a cyanide-containing aqueous
solution whereby the amount of cyanide-containing waste products is
reduced.
It is also an object of the present invention to provide a process
for the removal of gold from cyanide-containing aqueous solution
which enables a gold product of high purity to be produced with high
efficiency and at minimal expense.
In accordance with the present invention, there is accordingly
provided a method for the recovery of gold from an aqueous liquid
containing gold-cyanide complex comprising the steps of
(a) providing an aqueous solution which comprises a gold-
cyanide complex;
(b) contacting said solution with a quaternary ammonium
hydroxide extractant of the formula:
R' R3 +
N OH'
RZ / ~ R4
wherein R' and Rz are C,o-,8 alkyl (preferably C,o_,4 alkyl), R3 is C,~
alkyl, and R4 is methyl, whereby a complex of said extractant with
gold in said solution is formed; and
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(c) separating said complex formed in step (b) from said
solution.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention is directed to the use of a quaternary
ammonium hydroxide extractant to separate gold from a gold
containing aqueous cyanide solution by liquid-liquid extraction.
The quaternary ammonium hydroxide extractant which may be
used in the method of the present invention is defined by the
following formula:
R' R3 +
N OH-
Rz~ \ R4
wherein R' and Rz are C,o_,8 alkyl, R3 is Cl_9 alkyl, and R4 is methyl.
Preferably, Rl and Rz are C,o_,4 alkyl, most preferably C14 alkyl.
Preferably, R3 is methyl.
Quaternary ammonium hydroxide compounds of the type
employed in the present invention are known in the literature. See, for
example, U.S. Patent Nos. 2,234,548; 3,169,983; 3,429,694;
3,506,828; 4,929,454; 5,399,762; and 5,559,155. However, none of
such compounds have been used to separate gold from aqueous
cyanide-containing solutions by solvent-solvent extraction.
Quaternary ammonium compounds have been previously
taught for use in the separation of metals from aqueous solution as
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evidenced by U.S. Patent Nos. 3,194,652; 3,203,968; 3,238,127;
3,429,694; 4,289,530 and S,l 58,603. However, the teachings of such
references do not encompass the use of the specific quaternary
ammonium compounds of the present invention in the removal of gold
front aqueous cyanide solutions in the manner claimed, especially in
llllsllpporteCl forrll.
The present invention may be practiced as follows. A
quaternary ammonium hydroxide extractant of the present invention is
admixed with au aqueous cyanide leach liquor prepared by the
leaching of gold-bearing ore with an aqueous alkaline cyanide
solution. Tlle extractant is permitted to remain in contact with the
leach liquor for a period of time ranging from about 1 to 60 minutes,
upon which gold in the leach liquor (in the form of [Au (CN) 4 ] -)
will be extracted from the solution according to the following reaction
scheme:
R ~ R3 .E
/
N OH- + [Au (CN) a ~ -'
Rz / ~ Rn
R' R3 +
\ N / [Au (CN) 4 ] - + OH
~. R
The resulting gold-bearing quaternary ammonium complex is
insoluble in the aqueous leach liquor and forms a insoluble gold-
bearing phase therein. Tlle insoluble gold-bearing phase may be
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recovered fiom the treated aqueous leach liquor by conventional
means such as by decanting/filtration. Once recovered, the gold-
bearing quaternary ammonium complex may be dried to remove
bound water (i.e., at a temperature less than 100 °C.) to yield a waxy
solid material. This material is then smelted or fired under
conventional conditions and procedures (i.e., at temperature in excess
of the melting point of gold) to recover the gold as substantially pure
metal, with the quaternary ammonium complex being burnt off during
the smelting step. Advantageously, a conventional flux such as borax
may be added during the smelting or firing step to assist in the
removal of impurities.
The process of the present invention enables gold of high
purity to be obtained from the cyanide leach liquor at lower cost than
has previously been possible. Cyanide by-product is also reduced due
to the fact that the cyanide by-product which is extracted together
with the gold is destroyed during the smelting process.
The quaternary ammonium extractant of the present invention
may be admixed with the cyanide leach liquor in the form of a
solution in a suitable diluent solvent in order to enhance the
dispersibility of the extractant in the leach liquor. Suitable solvents
which may be used in combination with the compounds of the present
invention include but are not limited to water-immiscible organic
solvents such as alcohols, xylene, methyl isobutyl ketone, etc.
Ethanol is a preferred diluent solvent. One skilled in the art can
readily determine which diluent solvents may be employed. A 1:1
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dilution (by volume) of extractant in the water-immiscible solvent is
generally acceptable, although the degree of dilution is not critical and
may suitably range from 1:10 to 10:1 (by volume). The extent of
dilution is generally dependent upon the viscosity of the extractant at
the conditions employed in order to achieve acceptable dispersibility
of the extractant in the leach liquor.
The conditions of extraction of the gold from the cyanide leach
liquor are not critical. The pH of the liquor will preferably range form
about 7.0 to 10.5. Extraction temperatures within the range of from
about 20 to 3 5 ° C. are generally employed, although temperatures
within the range of about 20 to 25 °C. are preferred.
EXAMPLE
In order to demonstrate the advantages of the present invention,
a quaternary ammonium hydroxide extractant of the following
formula is provided:
R~ R3 +
~ N ~ OH-
R2 / ~ Ra
wherein R' and RZ are C14 alkyl, R3 is methyl, and R4 is methyl.
The extractant compound is diluted with an equal volume of
ethanol on a volume basis and used to treat synthetic cyanide leach
solutions. The conditions of extraction included cyanide
concentrations of lg/1, gold concentration ranging from 10 to 50 mg/l,
extractant concentrations ranging from 12 to 48 mg/1, extractant
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temperatures ranging from ambient to 3 5 ° C. , pH ranging from 8. 5 to
10.5, and iron and copper concentrations in the leach solution ranging
from zero to 590 mg/1.
Experiments were carried out by contacting 170 ml of leach
liquor with the diluted solution of extractant in a conical flask The
above extractant in undiluted form is a light brown organic liquid.
Mixing of the respective phases was achieved by mechanically
shaking the flasks in an incubator, which enabled the temperature to
be controlled. Samples (10 ml) were taken after 5, 10, 30 and 60
minutes of shaking. The samples were immediately filtered through
coarse filter papers to remove the insoluble extractant phase. The
gold concentration of the aqueous phase was measured by atomic
absorption analysis upon the removal of each sample.
In most experiments it was found that the extraction of gold
after 5 minutes of contact was quite high. Depending upon the
conditions of extractions (pH ranging from 8.5-10.5 and extraction
temperatures ranging fiom ambient to 35 °C.), the degree of
extraction of gold from the aqueous solution may but did not
necessarily increase over time. For example, gold extraction after 5
minutes contact varied from 40% to 99 % with a mean value of 77%.
After 60 minutes, the degree of gold extraction ranged from 29% to
99.5 %, but the mean value increased to 83.9%, showing that the
longer contact time was in general more effective. The presence of
iron and copper in the leach solution did not adversely affect the
extraction of gold from the solution. The above trials demonstrate
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that, even under the most adverse conditions, substantial recovery of
gold is achievable. Under optimum conditions, substantially all of the
gold is recovered.
Modifications and variations of the method may be undertaken
by those skilled in the art upon practice of the claimed invention.
Such modifications and variations are intended to come within the
scope of the invention defined by the appended claims.
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