Note: Descriptions are shown in the official language in which they were submitted.
~ 5 5257
GOLD RECOVERY PRO~ESS
This invention relates to a process for the recovery of precious
metals, e.g. gold, from solution.
Ore bodies or other materials containing gold can be leached with
dilute cyanide solutions to obtain a liquor containing dissolved gold
compound(s). It is known that such a liquor can also contain
dissolved compo~md(s) of other precious metal(s), e.g. silver and/or
platinum group metals such as palladium.
One known process for recovering gold from this material
comprlses adding at least one base metal, e.g. zinc, to the liquor to
precipitate gold as metal particles. Particles of other precious
metal(s) can also be precipitated with the gold particles~
Another kno~n process for recovering gold from the liquor
comprises an adsorption stage and a stripping stage. In the
adsorption stage, particles of activated carbon (i.e. granular
activated carbon) are connected with the liquor so that dissolved
compound(s) of gold are adsorbed onto those particles. Dissolved
compound(s) of other precious metal(s) can also be adsorbed by those
particles. In the stripping stage, the resultant particles of carbon
loaded with adsorbed gold compound(s) and any adsorbed compound(s) of
other precious metal(s) are treated with a liquid stripping
composition comprising cyanide(s), to give a solution containing
dissolved gold compound(s), and optionally dissolved compound~s) of
oth~r precious metal(s). The stripping composition may be a hot,
dilute aqueous mixture of sodium hydroxide and sodium cyanide.
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The adsorption and stripping stages can be carried out i~ columns
or vats and are slow~ Also, because of the physical nature of the
granular particles of actlvated carbon, those stages can be labour
intensive. A fur~her problem is that pieces of carbon can break off
from the bulk of activated carbon and cause gold loss in the
adsorption stage.
The granular activated carbon used in the adsorption stage is
often prepared from wood or coconut husks.
The particles of carbon obtained from the stripping stage can be
reactivated by heating them, e.g. at 650 to 750C, usually in the
absence of air. They may then be re-used in the adsorption stage.
The solution obtained from the stripping stage can be further
processed, e~g. in an electrolytic stage using a suitable cathode,
e.g. steel, on to which metallic gold is deposited and from which it
may be recovered.
According to the present invention there is provided a process
for the recovery of a precious metal from solution, particularly gold,
by adsorbing a dissolved compound of the precious metal from the
solution onto activated carbon wherein the activated carbon is in the
form of a body comprising fibres of activated carbon.
The gold may be associated with one or more additional precious
metals.
The precious metal solution may be obtained by leaching an ore or
similar material.
The fibrous body can have any suitable structure, e.g. it may be
woven or knitted or be in the form of a felt, mesh or pad. It is
preferably in the form of a cloth consisting of fibres containing
substantially 100% by weight of activated carbon.
The precious metal may be recovered by subsequently treating the
fibrous body containing adsorbed precious metal with a liquid
stripping composition to give a solution containing dissolved precious
metal. The metal can be recovered from this solution by conventional
work-up techniques.
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The stripping composition may comprise at least one solute and/or
at least one solvent. Preferably the stripping composition contains a
cyanide. An a~ueous solution of sodium hydroxide and sodium cyanide
forms a particularly suitable stripping composition.
Alternatively, the precious metal may be recovered by subjecting
the fibrous body containing adsorbed precious metal to electrolysis.
The loaded fibrous body can conduct electricity and be used as an
anode from which the precious metal can be removed by electrolysis and
deposited onto any suitable cathode. At high current densities,
however, the precious metal may not plate onto the cathode but may be
recovered in the form of a solid powder.
In processes of the prior art involving the use of beds of
granulated active carbon~ the beds were not suitable for use as
anodes. Thus the adsorbed gold could not be directly recovered by
electrolysis and additional treatment stages were necessary.
In one modification of a process according to ~he present
invention an activated carbon fibre cloth may be mounted within a
filter press for contact with the precious metal solution. This
enables the cloth to be subsequently treated in situ with the
stripping composition.
According to another modification, the cloth may be used in the
form of a belt, preferably an endless belt, which ~oves from the
adsorption stage to the recovery stage and returns to the adsorption
stage and so on. This configuration is applicable both to solvent
stripping and electrolytic recovery. An endless belt permits the
opertion of a continuous process.
If desired, additional strength can be conferred on an activated
carbon fibre cloth by bonding it to a suitable material such as a
porous natural or synthetic textile.
A cloth of fibres of activated carbon termed "Charcoal Cloth" is
a known material available from Charcoal Cloth Limited. It is
manufactured by a continuous process developed and patented by Che
Chemical Defence Establishment of the British Government. The process
permits the manufacture of a flexible fabric consisting of highly
adsorbent fibres which are substantially 100~ by weight activated
5~
carbon. Reference is made to British Patents 1310011 and 1376888 in
connection with this fabric. One example of the preparation of a
Charcoal Cloth is a process whlch involves dipping rayon cloth in
chemicals, drying the dipped cloth, carbonising the dried cloth at
350C and heating the carbonised cloth at 900C to activate ito When
a cloth of fibres of activated carbon is contacted with a solution of
a precious metal, the cloth presents greater surface area to the
solution per unit weight of carbon, and considerably higher quantities
of precious metal, e.gO gold compound(s) can be adsorbed, contrasted
with the known adsorption using particles of activated carbon. A
further advantage of the cloth is that it will require less handling
than particles of activated carbon, and there will be a considerable
reduction in pieces of carbon breaking away from the main bulk of
carbon. Indeed, the nature of the cloth can act as a trap to retain
at least some of those pieces.
In a typlcal mine leaching operation, the present invention
enables the adsorptive stages and stripping stages to take place more
quickly and thereby provide better overall efficiency.
The body of activated carbon fibres can be periodically
reactivated by heating to a suitable temperature in an oven e.g.
250C,to remove surface contamination and regenerate adsorptive
activity~
The invention is illustrated with reference to Figures 1-4 o~ the
accompanying drawings wherein Figure 1 is a schematic representation
of a continuous process according to the invention and Figures 2-4 are
graphs in which the performance of activated carbon cloths is compared
with that of granular activated carbon.
With reference to Figure 1, an endless woven web W of a Charcoal
Cloth (see above) extends in a path contacting a variable drive roller
1 and rollers 2 to 14. Rollers 2, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14
are plastics (e.g. nylon) rollers. Rollers 3 and 5 are positively
charged. A first portion of the web's path is through Chamber A,
containing any suitable cathode 15, e.g. steel, and an electrolyte.
Chamber A is used for subjecting to electrolysis the Charcoal Cloth
when loaded with an adsorbed gold compound, the metallic gold being
deposited onto cathode 15. Chamber B contains an aqueous wash bath for
the web.
Chamber C contains a liquor obtained by treating a gold containing ore
with a leaching composition comprising sodium hydroxide, sodium
cyanide and water. Chamber D contains a further aqueous wash bath for
the web. Any chamber can be replaced by a plurality of chambers, to
allow plural treatments. The web can pass through an optional oven 16
between chambers B and C~ which reactivates the web by heating.
ExamRle 1
Equilibrium Adsorption Trials - Gold
l'he adsorption capacity of two activated carbon fibre cloths was
compared with granular activated carbon.
The cloths were purchased from Charcoal Cloth Limited and had the
following weights per unit area:
Cloth B 12.4 mg/cm2
Cloth D 12.9 mg/cm2 (Used in Ex. 2)
Cloth F 14.5 mg/cm2
Cloth discs of 2 cm diameter were used in the tests.
A number of gold solutions were prepared from pure gold potassium
cyanide at strengths calculated to produce equilibrium concentrations
in the range 0 to 75 ppm. gold.
Discs of Cloths B and F were placed in contact with the gold
solutions in plastic containers and mechanically agitated for 48
hours. Samples of granular activated charcoal were similarly treated.
The results depicted graphically in the accompanying Figure 2
were obtained. ~hese equilibrium adsorption isotherms show that at
any particular equilibrium gold concentration, the uptake o~ gold
achieved by the cloths is greater than that by the granular material.
This effect is particularly marked in the low concentration ranges
which are most likely to be found in industrial applications.
Exam~le 2
Kinetic Trials - Gold
As in Example 1~ gold solutions of various concentrations were
prepared froM potassium gold cyanide.
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S5
700 ml of start solutions were used and pieces of Charcoal Cloths
B,D and F weighing between 0.2 and 0.5 g were held in a sintered glass
crucible~ The temperature was held at 21~C throughout and the pH at
10.5.
The gold solution was circulated through the cloth and 5 ml
samples were withdrawn at intervals of 5 minutes. Gold concentrations
were determined by atomic absorption spectra.
Granulated activated charcoal was similarly treated~
The results shown graphically in the accompanying Eigure 3 were
obtained.
The rate of adsorption using cloth samples was extremely fast.
Steady state corditions were reached after 20-25 minutes.
With the granular carbon, much less adsorption and a much slower
adsorption rate were found. In order to magnify these effects a
test was carried out in which a relatively large sample of granular
carbon was used, 2.56g. This took about 2~ hours to reach
equilibrium.
Because the adsorption rate of the cloth i2 greater it follows
that a lesser quantity of activated carbon in this form is required to
be in contact with the solution at any given time.
Example 3
Kinetic Trials - Silver
Example 2 was repeated with a sample of Cloth B and granular
charcoal, using solutions obtained fro~ silver potassium cyanide
instead of gold.
The results are set out graphically in the accompanying Figure 4
and prove similar to Example 2 in establishing the superiority of the
cloth. This adsorbed more silver than the granular material and
reached equilibrium more quickly, again in about 20-25 minutes as
compared with 2~-2~ hours.
Example 4
Electrolytic Recovery - Gold
A sample of Charcoal Cloth was contacted for 2 hours with a
solution of gold cyanide. At the end of this period the cloth had
adsorbed 55.20 mg gold.
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It was then placed in an electrolyte containing l.O% by wt NaOH
and 0.1% NaCN to form an anode, the cathode being a pre-weighed
titanium electrode. A current was passed for 45 minutes equivalent to
a current density on the carbon cloth anode of 10 amps/ft2 (about
S 100 amps/m2).
At the end of the trial, the electrolyte was found to contain
0.45 mg gold and 35.9 mg gold had deposited onto the cathode.
This experiment shows that gold can be removed from the cloth at
low current densities and deposited electrolytically onto a suitable
cathode.
Example 5
Electrolytic Recovery - Gold - Hi~h Current Density
A sample of Charcoal Cloth was contacted for 2 hours with a
solution of gold cyanide. At the end of this period the cloth had
adsorbed 28.05 mg gold.
It was then placed in an electrolyte containing 1.0% by wt NaOH
and 0.1% NaCN. A titanium cathode was also placed in the electrolyte
and the cioth (the anode) and the cathode were electrically connected
to a DC power source.
A current was passed through the system equivalent to a current
density on the carbon cloth anode of 100 amps/ft2 (about 1000 amps/m2)
for 5 minutes.
No gold could be detected on the cathode but 4.18 mg gold were
found to be dissolved in the electrolyte and a fine powder was formed
which, when filtered off and redissolved, was found to contain 1 mg
gold.
This experiment shows that at relatively high current densities,
gold can be swiftly removed from the cloth.
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