Note: Descriptions are shown in the official language in which they were submitted.
(`yanidation is conunonly employc~cl for extrac~ion of
gold from its ores. In this process, the crush~d ore is
disso]ved in a di.lute solution of sodi.um cyanide, or
calcium cyanide, and a small amo~nt of lime, in tile presence
of oxyyen, the gold dissolving in the form of the comp~.e~
Au(CN)2. Recovery of the gold has conventionally been
accomplished by treatment of the resulting cyanide solution
with zinc dust. Moxe recently, activated carhon has been
found to be an efficient adsorbent on the gold cyani.de
]0 complex, and a vari.ety of processes based on this reaction
have been d.e~l.oped. ~ffectiveness of these processes is,
however, dependent on development of efficient means for
desorbi~.g the gold from the loaded carbon. Stripping with
hot caustic sodium cyanide soluti.on at atmospheric pressure
: to desorb the gold, followed .by electrolysis to win the
metal values, has been employed. However, this method has the
disadvanta.~e that simultaneous strippincJ and electrolysis
is slow. Elevated pressure has. aJ.so been employe~, so that:
the temperatu~:e of th~ caustic cyanide solution can be rai.sed
to about 250~ without boiling, thereby accelerating tlle
desorption rate. This technique, however, requires the
use of a ho.i.ler-pressure reactor, which ma]ces the process more
complicated.
: ~ recent innovation, described in U.S. patent
application Ser.ial No. 551,941, filed March 19, 1975, employs
caustic-alcohol-water mixtures, containing about 75 percent
: alcohol by volume, a.t ambi.ent temperature and pressure for
desorbing gold from activated carbon. This method is very
cffective for de60rption of yo].d from activated carbon loaded
` 10 by treatment with synthetic NaAu(CN)2 solutions. I~owe~er, .it
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has not been found to be effective for desorption of gold from
activated carbon loaded by treatment with cyanide plant effluent5,
particularly when the effluents contain the gold in the form of
CaAu(CN)2, i.e., when the effluent is formed by treatment of ores
with calcium or sodium cyanide and lime. Investigations have
shown that there is a significant difference in the behaviour
of sodium and calcium regarding carbon adsorption of gold, the
aurocyanide complex being much more strongly adsorbed when calcium
is employed as the cation. Since lime is generally employed to
provide protective alkalinity in conventional cyanidation processes,
an efficient process for desorption of the aurocyanide complex
in the presence of calcium ion is essential for economi.cal recovery
of gold by means of activated carbon adsorption. In addition,
soluble extraneous matter, such as organics, silicates, and
ferrous-iron salts, in the pregnant cyanide effluents may play a
significant role in the desorption process.
According to the process of the present invention there
is provided a process for desorption of metal cyanide complexes
from the group gold, silver, or mixtures thereof, from activated
carbon loaded by treatment with a leach solution containing cal-
cium or sodium cyanide and lime comprising contacting the carbon
with stripping solution consisting essentially of solution com-
prising about 20 to 30 percent by volume of a water soluble alco-
hol and about 80 to 70 percent by volume of a strong base, the
operating temperature being about 70 to 160C. It has been found
that the use of the elevated temperature provides a much more
efficient desorption of the gold or silver, while at the same
time permitting the use of a relatively low concentration of
alcohol, whereby the economy of the process
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is subs~ln~ially improved. Recovery of the c~o]d or silver
from the strippin~3 solution is readily accomplis}led by
convelltiolla] mealls such as elect:rolysis.
Activateci carbon, suitable for use in the invention
is a widely available material that is convelltionally used
in adsoxption process, including yold adsorptioll. Typically,
it wi11 have a particle size of about 0.15 to 2 mm and a total
surface area of about G00-900i~12/y. It may be derivecl from any
of a variety of sources sueh as eoal, petroleum chars, c~conut
shell, or pulp mill blae~ ash, and is aetivated by
eonventional means sueh as heatlng i.n a steam air mixture at a
temperature of about 850C.
The aetivated carbon adsorbent is initially loe~ed
by eonventional means, i.e., by eontaeting a gold-eyanlde
and/or si].ver-eyanide solution, e.g., a eyanide plant effluent,
with the aetivated earbon for a time suffieient to permit
adsorption of a major amount of the gold-eyanide and/or
sîlver-eyanide eomplex. This may be aeeomplished by any
eonventional me~ans for eorltaetinc3 liquid3 wit:ll sol.id adsorbents,
e.g., by passin~ the gold-cyanide and/or silver-cyallide soluLion
throuyh a columnar unit conl:aininy a fixed bed of the
aetivated earbon.
The ~old and/or silver are then desorbecl from the
loaded earbon, aeeording to the proeess of the invention, by
treatment with a solution eomprisiny a water soluble aleohel
and an aqueous solution of a stron~3 base, at a temperature
of about 80 to 160 DC. As discussed above, and illustrated
in the examples below, this temperature range has been found
to be essential to aehievement of effieient desorption of
the gold, partieularly where the gold-cyanide solution is a
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cyanide p~ant effluent formed by treatment: of ores witl
calcium or sodium cyallide and lime.
l`he water soluble alcohol is prefera~ly a lower
aliphatic alcohol such as methanol, ethanol, propanol,
or isopropanol. As noted above, the use of an elevated
temperature permits the use of a relatively small proportlon
of alcohol, preferably about ~0 to 30 percel-t by volume of
the stripping solution, thereby substantially improving
the economy of the process.
The balance of the strlpping solution consists
essent:ially of a water solution of a strong base. Sodium
hydroxide is the preferred base, but potassium hydroxide may
be used. The base is employed in an amount of about
1 to 2 percent by weight of the water solution, or an amount
sufficient to provide the stripping solution with a pH of
about 11 to 14, preferably about 13 to 14. Use of a high
pH is essential since the activated carbon has a substantially
decreased capacity for adsorption of tlle go:Lcl-cyarl;de
complex at higher values of pH.
The strippiny solution may also contain a small
amount of sodium cyanide, e.g., about 0.02 to 1 percent hy
weight of the water solution, particularly where the
stripping solution is recycled after removal of the desorbed
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gold. Such recycled solutions often contain small amounts
of sodium cyanide as a contaminant from the stripping
operation. Alternatively, the sodium cyanide may be
added to the stripping solution and may result in somewhat
increased efficieny in some cases. It is not, however, an
essential ingredient of the stripping solution.
l'he stripping operation is carried out by
contactillg the loaded carbon with -the stripping solution of th~
invention, at tlle l-equired tempeLature. Again, contacting
may be by any conventional means such as passing the stripping
solution through a bed or column of the loaded carbon.
Optimum amounts and flow rates of the stripping solution
will depend on the amount of yold and/or silver adsorbed on
the loaded carbon, the composition of the gold-cyanide and/or
silver-cyanide solution used for loading and specific
composition and temperatures of the stripping solution and
are best determined empirically. Generally, however, treatment
of the loaded carhcl with about 5 to 30 bed volumes of
stripping solution, at a flow rate of about 2 to 8 bed
volumes of so].ution per hour givec good results.
Although the process of the invention has been
found to be particularly effective for desorption of gold, it
is also generally effective for desorption of silver, either
alone or in combination with the gold.
The following examples will more speci~:ica1ly
illustrate the practice of the .invention and the advantages
obtained thereby.
xample 1
~; In this example, a series o~ desorption tests
was conducted on loaded activated carhon from a heap leach-
carbon adsorption cyanide plant in which leaching was
accomplished by means of calcium cyanlde solution containing
sufficient lime to provide a pl-l in the range of 9-ll. The
loaded carbon carried 235 or Au/ton and 35 oz ~g/ton.
Desorption ~as conducted in a vertical column
24 lnches in length and 2 inches in diameter, cantaining an
;~ 30 ~ 18 inch deep bed o the loaded carbon. The stripping
5~7
solution consisted of a water solution of 1 ~ercent NaOII and
0.1 percent NaCN and varying proportions of ethallol.
Nineteen bed volumes of the stripping solution were pumped
upward through the carbon bed at a rate of 1 bed volume of
solution per 15 minutes. Operating temperature was 80C.
Results shown graphically in Figure 1. It will be seen
that an ethanol concentration as low as 20 percent gave
near-maximum gold desorption. Results of such tests have in
general shown a concentration of about 20 to 30 percent by
volume of the alcohol to be optimum.
In this example, a series of desorption tests was
conduct~l under conditions similar to those of Example 1,
except that all strip solutions contained 20 volume
percent of methanol as the alcohol component, and varying
operating temperatures were employed. Results are shown
graphically in Figure 2. It will be seen that the gold
desorption is highly temperature dependent and that a
temperature of about 80C or above is essential for
efficiellt desorption. Temperatures above about 160C,
however, have been found to result in minor, if any,
add1tional improvement in deeorption efficiency.
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