Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the recovery of gold from
sedimentary, carbonaceous gold-containing ores which are re-
fractory to standard cyanidation techniques. In the contextof this disclosure "carbonaceous gold-containing ores", or,
simply, "carbonaceous ores", denote refractory gold-containing
ores which also contain organic carbonaceous matter which
exhibits the property of inhibiting or substantially reducing
the extraction of gold from such ores by conventional cyanidation
technology. The invention relates, more particularly, to an
improved process for ~he treatment of these ores which makes
use of simultaneous cyanidation and carbon adsorption to
obtain consistently high gold recoveries.
2. The Prior Art
Sedimentary carbonaceous gold-bearing ores are found in
Nevada, Utah, California and other states.in the United States,
as well as other countries throughout the world. These ores
are not amenable to standard cyanidation techniques because the
carbonaceous impurities with which they are associated tend to
tie up the cyanide gold.complexes in chemical compounds from
which thè gold cannot be separated by standard methods. Also,
gold may be associated with organometallic complexes in the ore
which are not
, ~
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attacked by the cyanide complexing agents used in conventional
cyanidation techniques. Conventional cyanidation techniques can
normally recover only up to about 50 percent of the gold which is
present in these ores.
Prior art methods for dealing with the problem of carbonace-
ous impurities in gold-containing ores have been confined, for the
most part, to various treatments of the ore prior to cyanidation in
attempts to make it more amenable to the cyani~e leachlng action.
Thus, for example, U. S. Patent No. 1,461,807 discloses the use of
certain mineral oils for ~'blinding" the action of the carbonaceous
impurities on the cyanide complex formation; U. S. Patent No.
- 2,234,140 teaches that certain wetting agents can make the ore more
amenable to cyanidation; U. S. Patent No. 3,639,925 discloses the
use of sodium hypochlorite and calcium hypochlorite as agents for
oxidizing the carbonaceous material so as to prevent it from
absorbing the gold cyanide; and U. S. Patent No. 3,846,124 calls
for a chlorine pretreatment of the ore, in the absence of any alka-
line material, in order to decompose the organic carbonaceous com-
ponents and remove them prior to cyanidation. In addition, U. S.
Patent No. 3,574,600 teaches that certain acids can be used in con-
~unction with an ozone treatment prior to cyanidation to oxidize
the carbonaceous impurities; and U. S. Patent No. 4,038,362 discloses
a preoxidation technique, which is carried out in the absence of
extraneous alkaline material, for reducing the amount of chlorine
needed to pretreat the ore, as in the '124 patent. Alternatively,
calcining of the ore, prior to cyanidation, has also been suggested
as a way of oxidizing all of the organic constituents and thus pre-
vent the carbon from interfering with the cyanide leachin~ action.
Calcining operations, of course, tend to generate mercury, arsenic
and sulfur-containing gases, the release of which to the atmosphere
~.~.52'-~54
is extremely undesirable from an environmental standpoint.
Methods which involve treatment during cyanidation include
those of U. S. Patents No. 2,147,009 and 2,315,187, which cover the
use of finely divided charcoal during cyanidation to simultaneously
leach the gold values from the ore and adsorb them on the charcoal,
so as to maintain the solution continuously depleted of gold and
thereby improve cyanidation efficiency.
These prior art methods have proven satisfactory in some
cases, but have not been able to provide a solution to the problems
caused by the organic carbon in some of the more refractory ores
such as the ores found in the Jerritt Canyon and Marlboro Canyon
areas of Elko County, Nevada, and other equally refractory carbona-
ceous ores. Thus the methods of U. S. Patents No. 1,461,807 and
2,234,140 work well on ores where inorganic carbon predominates, but
do nothing or very little to alleviate the problems caused by
adsorption of the gold cyanide complex as a result of the presence
of organic carbon in ores such as those contemplated by the process
of this invention. Likewise, the method of U. S. Patent No. 3,639,925
provides only a partial solution to the problem of handling very
refractory ores because, even though the hypochlorite treatment makes
them more amenable to cyanidation, recoveries are not satisfactory
unless high levels of hypochlorite are used. Chlorination methods
such as those of U. S. Patent No. 3,846,124 do not yield very good
recoveries in spite of the fact that they call for very high consump-
tion of chlorine or hypochlorite and are, conse~uently, very expen-
sive, usually necessitating additional steps such as the preoxidation
step of U. S. Patent No. 4,038,362, which entails heating the slurry
to elevated temperatures while bubbling air for ]ong periods of time.
The method of U. S. Patent No. 3,574,oO0 consumes expensive
ozone and acid and has essentially the same effect as the method of
2754
U. S. Patent No. 3,63~,925 with its attendant shortcom:Lngs- In
additlon, it must be carried out at very low pH values, which makes
it unsuitable for treating calcium carbonate-containing ores such as
the sedimentary ores treated by the process of this invention.
Finally, the methods which deal with treatment during
cyanidation, that is, those of U. S. Patents No. 2,147,009 and
2,315,187, treat oxide type ores, not carbonaceous ores, and use
finely divided charcoal, not granular activated carbon. These
methods do not work on the carbonaceous ores of this invention,
partly because the carbonaceous material present in the ores of this
invention inhibits the dissolution of gold complexes and prevents
finely divided charcoal from removin~ the gold from the cyanide
leach solution. In addition, finely divided charcoal cannot be
separated by means of screening.
Obviously, a process is needed that can recover the gold
values from ores containing substantial quantities of organic car-
bonaceous matter without consumption of large amounts of expensive
rea~ents such as chlorine.
It is an important object of this invention to provide an
economical process for treating highly refractory, carbonaceous,
gold-containing ores which does not suffer from the disadvantages of
prior art processes and which, at the same time, results in high
yields of gold extraction from said ores.
SUMMARY OF THE INVENTION
The present invention provides a process whereby improved
recovery of gold is obtained from carbonaceous ores which are
otherwise refractory to gold extraction.
In accordance with the process of the invention an aqueous
slurry of a carbonaceous gold-containing ore is oxidized and then
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subjected to simultaneous cyanidation and granular activated carbon
adsorption in two or more solid-liquid extraction stages, where the
ore flows countercurrent with the carbon and the gold transfers to
the carbon. The gold-loaded carbon can be separated from the gold-
depleted slurry by means of screenin~, and the gold values may be
removed from the gold-loaded carbon by any number of conventional
operations such as stripping with hot caustic cyanide solution. The
carbon can then be conveniently recycled to the solid-liquid extrac-
`tion stages to be reused in the simultaneous cyanidation and granular
carbon adsorption operation.
BRIEF DESCRIPTION OF THE DRAWING
-
Figure l is a flowsheet diagram illustrating one mode of
operation of the process of this invention.
DETAILED DESCRIPTION
Referring to Fig. l, carbonaceous ore l is fed to crushing
operation 2 where it is crushed, for example, by a cone crusher.
Carbonaceous ore l is a gold-bearing ore, normally containing any-
where from 0.05 to 1.0 ounce of gold per ton and from about l to
about 8% by weight total carbon. The organic carbon content of the
ore is usually between about 0.5 and l~ by weight. Essentially all
of the inorganic carbon is in the form of carbonates. Sulfides of
arsenic, lead, mercury and other metals are also often found in this
type of ore.
Higher and lower gold and carbon contents are sometimes
found in the type of ores contemplated by the process of this inven-
tion, although for the most part the gold content is rarely lower
than 0.05 or higher than 2.0 ounces per ton, and the organic carbon
content is seldom below about 0.25 and rarely above 3~ by weight.
~hat is characteristic of this type of carbonaceous ores is that they
are not amenable to standard cyanidation techniaues, that is, less
Z75~
than about 50% gold extraction is obtainable from them when treated
by conventional straight cyanidation methods.
Crushing operation 2 reduces the ore to a size convenient
for wet grinding 7. An intermediate screening 4 may be used to
separate crushed ore 3 into fine fraction 5 and coarse fraction 6,
with the coarse fraction returning to crushing 2 and the fine frac-
tion advancing to wet grinding. Slurrying of the ore is done by
adding water 8 at a rate sufficient to provide about 60% solids in
discharge 10 from the grinding mill or mills. Preferably, the dis~
charge from the final grinding mill is diluted to 35-40% solids and
fed to a cyclone 11 for classification. The coarse ore in cyclone
underflow 13 is recycled to the mill or mills and the fine ore in
cyclone overflow 12 is concentrated to about 50% solids in thickener
14. Thickener overflow 15 is recycled to the grinding classi~ication
circuit and used to dilute the discharge from the grinding mill or
mills, as indicated above.
Alkaline material 9 may be added during wet grinding, but
is preferably added after thickening and prior to oxidation. It
may also be added during oxidation. The material is preferably
added to thlckener underflow 17 in an amount sufficient to provide
between about 10 and 100, and preferably between about 20 and 60,
pounds of alkaline material, expressed as Na2CO3, per ton of dry ore.
The material must be soluble in the liquid phase of the slurry, and
is selected from among any of the alkali metal basic salts, oxides
and hydroxides, and mixtures thereof. Preferably, soda ash (Na2CO3)
is chosen as the soluble alkaline material because of its low cost
and availability.
The grinding operation need not be wet grinding, that is,
the ore may be dry ground first and ~hen slurried in a separate
slurrying operation. If this is done then the alkaline material may
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be conveniently added with the slurrylng water as part of the
separate operation. Wet grinding is preferred, however, for con-
venience in the handling Or the ore.
It should be understood that, although the presence of the
soluble alkaline material in the aqueous ore slurry is an essential
part of the process of this invention, the particular method and
manner of forming the aqueous slurry, that is, the steps of crushing,
screening, grinding, thickening, etc., are not part of the invention.
Indeed, many variations and modifications of the various configura-
tions which exist of these particular techniques are known to those
skilled in the art and are available for use and contemplated by
the process of this invention as means for forming the slurry.
Referring to Fig. 1, again, underflow 17 is a slurry having
a solids content of about 50%, a temperature between about 50 and
120F and a pH between about 7 and 13, depending on the point and
manner of addition of the soluble alkaline material. Its gold con-
tent may be between about 0.05 and 1.0 ounce of gold per ton of dry
ore, and its organic carbon content about 0.25-3% by weight. This
slurry is fed to the oxidation operation, which is preferably
carried out in two steps: oxygenation and chlorination. In oxygen-
ation step 18 the slurry comes into contact with oxygen-containing
gas 19, e.g., air or oxygen, in one or more tanks or similar vessels,
for at least one hour, but preferably between about 4 and 20 hours
while under agitation. This step is best carried out in several
stages. The oxygenation reaction is exothermic, but externally-added
heat is provided to keep the temperature of the mixture inside the
oxygenation tanks between about 120 and 210F, and preferably around
140-190F. Enough oxygen-containing gas is fed into the tanks to
provide the equivalent of between about 1,000 and 10,000, and pre-
ferably between about 3~000 and 5,000, standard cubic feet of air
52~75~
per ton of dry ore. More preferably, the retention time during
oxygenation is about 8 hours. Oxygen, of course, can be used
instead of air.
The pH of the slurry tends to drop during oxygenation, and
its actual value depends on the amount, manner of addition and
point of injection of the allcaline material. If the alkaline material
is injected during oxygenation by, for example, in~ecting it in each
tank, or stage, the pH remains relatively constant throughout the
operation. If, on the other hand, the alkaline material is added
prior to oxygenation the pH may gradually drop to as low as about 8.
Preferably, soda ash is added immediately prior to the oxygenation
step in an amount sufficient to cause a gradual drop of the pH to
about 9-10 by the end of the last oxygenation stage and prior to
chlorination.
It is surmised that, during oxygenation, sulfides and other
sulfur compounds which are often found in the type of carbonaceous
ores treated by tne process of this invention are oxidized and/or
the nature of the organic carbon associated with them is altered in
some manner which causes an improvement in gold recoveries and a
decrease in chlorination requirements. While it has been determined
that oxygenation does not have the effect of decomposing the organic
carbon and driving off carbon dioxide, the exact mechanism by which
it works has not been precisely determined and is not quite well
understood.
Following oxygenation the slurry temperature is preferably
adjusted to about 70-140F, and more preferably to 80-120F, in heat
exchanger 16 and the resulting slurry 20 fed to chlorination step 21.
Chlorination is carried out in one or more agitated vessels, e.g.,
enclosed tanks provided with mechanical agitators, by injecting
chlorine gas, sodium hypochlorite, potassium hypochlorite or any
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- ~5Z7S~
other suitable source of hypochlorite lons 22 into the oxygenated
slurry prior to entering the vessels, or in the vessels themselves,
as shown in ~ig. 1. Chlorination is carried out for at least 1
hour and preferably between 1 and 6 hours at a temperature of about
70-140F. The amount of hypochlorite ion source 22 added shoùld be
between about 10 and 100 pounds, expressed as NaOCl, per ton of dry
ore. The pH during chlorination tends to drop as the slurry advances
from stage to stage, and may drop to as low as 5.
Following chlorination slurry 23 is preferably held for an
additional 2-3 hours in holding tank 24 to allow any excess hypo-
chlorite ions to be consumed and thereby avoid, or at-least minimize,
the presence of the hypochlorite ion during the subsequent simul-
taneous cyanidation and granular carbon adsorption operation. The
hypochlorite ion would tend to react with the cyanide ion and inter-
fere with this operation. Any suitable vessel, e.g., an enclosed
tank, or similar, can be used for this purpose. In addition, or
alternatively, air blowing through the slurry until all of the
hypochlorite ions are substantially removed may be employed as a
means for allowing excess hypochlorite to be consumed. No holding
or air blowing is needed if the amount of excess hypochlorite ion is
nil. From holding tank 24 chlorinated slurry 25, at about 70-140F,
is preferably fed to heat exchanger 26, where its temperature is
adjusted to about 40-100F to minimize decomposition of the cyanide
species in the operation which follows, and then fed, as slurry 27,
to the simultaneous cyanidation and granular carbon adsorption
operation of the process of this invention.
It is not absolutely necessary that the oxidation operation
be effected by first oxygenating with an oxygen-containing gas and
then chlorinating. Depending on the type and particular characteris-
tics of the ore being processed, it hasbeen fo~nd that the oxidation
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i2~54
operation may also be carried out under certain circumstances in
one step, by treatment wlth an oxygen-containing gas, without
chlorination, or by chlorination, without oxygenation. Thus, slurry
17 may be treated with an oxygen-containing gas, e.g., air or oxygen,
at a temperature of 120-210F for at least 6 hours, and preferably
for 8-12 hours, in one or more stages, then cooled to, for example,
40-100F, and sent to the simultaneous cyanidation and granular
carbon adsorption operation. Likewise, oxidation may be effected by
a chlorine treatment, without treatment with an oxygen-containing
gas, by, for example, bubbling chlorine gas or some other suitable
source of hypochlorite ion in an amount sufficient to provide between
about 15 and 150 pounds, expressed as NaOCl~ per ton of dry ore for
at least 4 hours at a pH between about 5 and 11. Preferably, between
100 and 150 pounds of NaOCl per ton of dry ore would be provided,
with a retention time of between 8 and 12 hours. The type and
composition of the ore being processed, the cost of the chlorine con-
sumed and other economic considerations would determine the best way
of carrying out the oxidation operation.
The simultaneous cyanidation and granular carbon adsorption
operation is carried out, in accordance with the process of this
invention, in a plurality of stages, by simultaneously contacting
the oxidized slurry with a cyanide solution and granular activated carbon, the
carbon movin~ countercurrent with the flow of the slurry. Thus,
in Fig. l, stream 27 enters first mixing tank 28, where it contacts
cyanide stream 29, containing cyanide in an amount sufficient to
provide between about 0.25 and 2.5 pounds of cyanide, expressed as
NaCN, per ton of dry ore. The cyanide may be added in solid form,
but it may also be added as a solution, for example, as a sodium
cyanide solution having between about 10 and 25% NaCN by weight, and
preferably about 15% NaCN. Other cyanide solutions, e.g., solutions
r5 2 7 5 4
of KCN, Ca(CN)2, etc. may also be used. The compositions and
strengths of cyanide solutions best sulted for cyanidation are
matters known to those skilled in the art of cyanidation processes
for recovering gold from ores, and their selection can be made
5 utilizing standard cyanidation techniques criteria. Lime 30 is pre-
ferably added to maintain the pH between about 9.5 and 10.5, in order
to decrease cyanide decomposition.
Also fed into tank 28 is carbon-containing stream 45, the
source of which is described below. The mixture of carbon, slurry
and cyanide is agitated in mixing tank 28 for a pe~riod of time long
enough to provide intimate contact between all phases. Usually,
between about 30 and 180 minutes of retention time, and preferably
around 90 minutes, will suffice. The resulting mixture 31 is fed
to screen 32 where it separates into carbon stream 33 and ore slurry
stream 34. Carbon stream 33 may contain between about 50 and 800
ounces of gold per ton of carbon and normally about 100-350 ounces
per ton. About 50 pounds of carbon per ton of dry ore fed to screen
32 are removed through stream 33. A substantial portion of stream
33, e.g., about 85-95%, is diverted, as stream 35, and mixed with
recycle stream 43 to make up stream 45 which is fed to tank 28, as
already described; another portion of stream 33, e.g., about 5-10%
is removed as gold-loaded carbon stream 36, carrying with it the
adsorbed gold, at a rate of about 5 pounds of carbon per ton of dry
ore. This granular carbon stream, pregnant with complexed gold, is
sent to one or more stripping towers, or any other suitable system,
to be stripped of its gold content.
Stream 34, now containing between about 0.03 and 0. 2 ounce of
gold per ton of dry ore, goes into mixing tank 37 which is part of
the second stage of the simultaneous cyanidation and granular carbon
adsorption operation. Agitation is provided in tanX 37 as in tank 28
.
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to mix the slurry with recycle carbon stream 42, and the screenlng
operation repeated as berore. Thus, slurry 38 exlts tank 37 at a
gold content of about 0.015-0.1 ounce per ton of dry ore, and goes
into screen 39 where it is separated into carbon stream 40 and
slurry stream 41. Stream 40 is divided into stream 43, which blends
with stream 35 5 as already described, and stream 44 which blends
with recycle stream 52 to make up stream 42 which is fed to tank 37.
Slurry 41 goes into the third and final stage via mixing tank 46
where it is mixed with recycle stream 47 and carbon stream 53, and
leaves tank 46 as slurry 48 which is separated in screen 49 into
gold-depleted slurry 50 and carbon stream 51. Stream 51 is divided
into recycle stream 52 and recycle stream 47 which make their way
to tanks 37 and 46, respectively.
Carbon stream 53 contains the fresh granular activated carbon
of the process of this invention. Any type of granular activated
carbon can be used as long as its particle size is substantially
larger than the particle size of the ore being treated so as to
allow for an efficient separation of the two via a screening operation
or similar type of separation operation based on particle size. Since
the ore particle sizes during simultaneous cyanidation and granular
carbon adsorption are usually smaller than 48 mesh, granular activated
carbon having particles, for example, in the 6 x 16 mesh size is quite
acceptable. One such type of carbon is manufactured by Westates
Corporation under the name of Westates Carbon Grade CC-321 G/S. This
carbon is made from coconut shells and activated tJith steam.
Other types and grades of activated carbons can also be used.
The carbon, however, must be granular carbon, that is, must not be
finely divided carbon, e.g., in pot^rder form. Thus, carbons t~rith
average particle sizes smaller than about 48 mesh are not acceptable.
In the context of this disclosure "mesh" refers to the Tyler standard
754
screen-scal~ sieve designation.
Enough carbon should be provided with stream 53 so as to
provide between about 1 and 10 pounds of carbon per ton of dry ore
being processed, and preferably between about 3 and 8 pounds per ton.
It is essential to the process of this invention that the ore
flow be countercurrent to the carbon flow. It has been found that
the use of granular activated carbon in this fashion, together with
the oxidation operation that precedes it, results in the recovery
of up to 97% of the gold values originally present in the ore, which
gold values are not usually recoverable in those quantities by con-
ventional cyaniding methods. Recoveries substantially higher than
those obtained by conventional cyanidation are always obtained when
the process o~ this invention is efficiently employed.
The cyanide solution is preferably fed to the system with
the ore slurry, for example, as shown in Fig. 1, into mixer 28, and
for the most part the bulk of it moves concurrent with the ore
slurry, although a portion of it distributes itself throughout all
stages and moves in the direction of flow of the carbon.
While, for ease of illustration, three stages are shown for
the simultaneous cyanidation and granular carbon adsorption opera-
tion in Fig. 1, it will be understood that more or fewer stages may
be usèd. In fact, we prefer to operate in eight such stages and use
interstage screening. Any suitable type of particle size separation
equipment may be used to separate the carbon from the slurry in
between stages, as already described.
The gold is recovered from gold-loaded carbon stream 36 by
conventional means for separating gold from gold-loaded carbon. One
such means is stripping with hot NaOH-ethanol-cyanide solution 55 in
one or more columns or towers 54 which are first packed with the
pregnant carbon, and where the hot NaOH-ethanol-cyanide solution
754
circulates whi:le remov:ing the golcl rrom the carbon. The rernoved
gold in product llquor 56 is then sent to electrolysis operation 57,
where it is recovered by electrolytic deposition in stream 58, and
further made into gold bullions 59 in refining furnaee 60.
Spent solution 61 from electrolysis operation 57 ean be re-
eycled, preferably after blending with make up solution 62, to
columns 54 as already indicated. The stripped carbon 63 leaving
eolumns 54 ean also be recyeled, after regeneration and blending with
make up earbon 64, as stream 53 to countercurrently eontact the ore
slurry, as required by the proeess of this inVentiOn, in, for example,
tank 46, as shown in Fig. 1.
It will be understood that the manner of reeovering the gold
values from the gold-loaded earbon product from the simultaneous
eyanidation and granular earbon adsorption operation of this invention
and the manner of further separating and purifying said gold values
are not part cr the invention. Indeed, many techniques for doing this
are known to those skilled in the art, espeeially those familiar with
the reeovery of gold values from loaded carbons obtained by the so-
ealled earbon-in-pulp teehnique used to reeover gold from gold-loaded
eyanlde solutions.
The following are examples illustrating the effeetiveness of
the proeess of the invention.
EXAMPLE 1
This example illustrates one mode of operation of the inven-
tion.
A sample of a earbonaeeous gold-containing ore from the area
known as Marlboro Canyon, in Elko County, Mevada,is prepared in the
laboratory by crushing, grinding to -100 mesh and slurrying with
water, and adding to it soda ash in an amount sufficient to provide
50 pounds per ton of dry ore. This slurry, at about 50~ solids,
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~27S4
100F and a p~l Or 10, has a gold content of 0.359 ounces of gold per
ton o~ ore and an organic carbon content of 0.49% by weight, and is
representative of stream 17 in the process flowscheme of Fig. 1.
The slurry is fed to oxygenation step 18 where it contacts 4,000
standard cubic feet of air per ton o~ dry ore while under agitation
in a tank. Retention time in this step is 8 hours, and the pH of
the slurry in the tank is 10. The slurry is heated so as to maintain
a temperature of 180F.
Following oxygenation the slurry is fed to indirect heat
exchanger 16 where its temperature is lowered to 120F. The resulting
cooled slurry 20 is fed to chlorination operation 21, which is carried
out in an enclosed vessel provided with mechanical agitation.
Chlorine gas 22 is injected into the tank at a rate sufficient to
provide 50 pounds of NaOCl per ton of dry ore. Retention time is 6
hours, and the temperature of the slurry is maintained at or below
120F.
Following chlorination, slurry 23 is held for an additional
2 hours in holding tank 24 to allow excess hypochlorite ions to be
consumed. Slurry 25 from holding tank 24, at 120F advances to heat
exchanger 26 where it is cooled to 80F so as to minimize decomposi-
tion of the cyanide in the subsequent simultaneous cyanidation and
granular carbon adsorption. Cooled slurry 27 is fed to the simul-
taneous cyanidation and granular carbon adsorption of the process of
this invention. Accordingly, stream 27 first enters mixer 28 where
it contacts cyanide stream 29 containing enough cyanide to provide
1 pound of NaCN per ton of dry ore. The cyanide is added in liquid
form as a sodium cyanide solution. Lime 30 is added to mixer 28, and
retention time is 1 hour. The resulting mixture 31 advances to
vibrating screen 32 where it separates into carbon stream 33 and ore
slurry stream 34. Carbon stream 33 contains 300 ounces of gold per
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-- ~1 5~7S~
ton of carbon; 30 pounds of carbon per ton of dry ore fed to screen
32 are removed through stream 33. 95% of' stream 33 is diverted, as
stream 35, and mixed with recycle stream 43 to make up stream 45
which is then fed to mixer 28, as already described, while 5% of
stream 33 is removed, as gold-loaded carbon stream 36, which carries
with it the adsorbed gold at a rate of 2 pounds of carbon per ton of
dry ore.
Stream 34 contains 0.1 ounce of gold per ton of dry ore and
is fed to mixer 37, which is part of the second stage of the simul-
taneous cyanidation and granular carbon adsorption operation. Agita-
tion is provided in mixer 37, as in mixer 28, to mix the slurry with
recycle carbon stream 42 and the screening operation repeated as
before. Slurry 38 exits mixer 37 having a gold content of 0.02 ounce
per ton of dry ore and goes into screen 39, where it separates into
carbon stream 40 and slurry stream 41. Stream 40 is divided into
stream 43 which blends with stream 35, as already described, and
stream 44 which blends with recycle stream 52 to make up stream 42
which is then fed to mixer 37. Slurry 41 goes into the third and
final stage via mixer 46 where it blends with recycle stream 47 and
carbon stream 53, and leaves mixer 46 as slurry 48 which enters
screen 49 and separates into gold depleted slurry 50 and carbon stream
51. Stream 51 is divided into recycle streams 47 and 52 which end
up in mixers 46 and 37, respectively.
Carbon stream 53 contains 3 pounds of Westates Grade CC-321
G/S granular carbon per ton of dry ore being processed.
The gold may be recovered from gold-loaded stream 36 by
stripping with hot NaOH-ethanol-cyanide solution 55 in to~Jers 54,
which are packed with the loaded carbon; the hot NaOH-ethanol-cyanide
solution moves in and out of towers 54 and in the process removes
the gold from the carbon. The recovered gold, in product liquor 56,
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is sent to electrolytic operation 57, wherc it is separated by rneans
of conventional electrolysis techniques. The spent solution 61 from
the electrolytic operation is recycled to columns 54. The stripped
carbon 63 may be reactivated and recycled in mixer 46. Make up
carbon 64 is added to carbon 63 to make up for carbon losses in the
system.
EXAMPLE 2
Comparative tests were made with ore samples from an area of
the Marlboro Canyon designated as "Pit 2" by subjecting the samples
to cyanidation with and without the method of this invention.
The feed for these tests was prepared by grinding and
thoroughly blending about 30 pounds of the Pit 2 ore and splitting
them into fifteen 900-~ram samples of a gold-containing carbonaceous
ore having 0.248 ounces of gold per ton of ore, o.67% by weight
organic carbon, 7.62% by weight total carbon and 1~71~J by weight
sulfur. The particle size of the samples was -100 mesh.
In Test A, 900 grams of this ore were slurried with 900 grams
of water (50% solids slurry) in a 5 inch-diameter, 9 inch-high
stainless steel vessel equipped with an agitator and several baffles,
and the resulting slurry subjected to oxygenation in the absence of
any alkaline materials. Oxygenation was carried out by bubbling air
at a rate of 600 cc/min for 8 hrs, at a temperature of 180F, in
the same stainless steel vessel. (This air rate is equivalent to
about 5,000 cubic feet per ton of dry ore.) The pH during oxygena-
tion ranged between 9 and 11.
A sample of the resulting liquor was then taken and analyzed
for total sulfur and sulfate~content in order to determine the
extent of oxidation. It contained o.6 gpl sulfur, indicating a
conversion of sulfur to sulfate of about 30%. The oxygenated
slurry was then allowed to cool to 120F in a separate vessel.
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Standard cyanidation leach:ing was then carried out by mixing
the cooled slurry with 0.9 grams of sodium cyanide. This is
equivalent to 2 pounds of NaCN per ton of dry ore. Cyanidation was
effected in rolling bottles at 75F for 24 hours, and using lime to
maintain the pH of the slurry at 9.6-9.8. After separation of the
two phases by means of filtration the solids were dried and analyzed,
and showed a gold content of 0.213 ounces per ton. This means that
only 14% of the gold originally in the ore was leached.
In Test B, 900 grams of ore were slurried, oxygenated9
allowed to cool and leached with sodium cyanide solution in exactly
the same fashion as in Test A, except that, ~ust prior to leaching,
Westates 5arbon Grade CC-321 G/S activated carbon was added to the
slurry in an amount sufficient to provide 20 grams of carbon per
liter of slurry. After separation and analyses the gold content of
the solids was 0.188 ounces per ton. This means that only 24% of
the gold originally in the ore was leached.
In Test C, a sample was subjected to the same procedure as
in Test A, except that soda ash was added during slurrying in an
amount sufficient to provide 50 pounds of Ma2C03 per ton of dry
ore. After separation and analyses the gold content of the solids
was 0.224 ounces per ton, indicating that only 10% of the gold
originally in the ore had been extracted.
In Test D, a sample was subjected to the same procedure as
in Test C, except that, just prior to leaching, Westates Carbon
Grade CC-321 G/S activated carbon was added to the slurry in an
amount sufficient to provide 20 grams of carbon per liter of
slurry. After separation and analyses the gold content of the solids
was o.o88 ounces per ton, indicating that 65~ of the gold originally
present in the ore had been leached.
In Test E, a sample was slurried as in Test A but was not
--lg--
Z754
subjected to oxygenation; instead, the slurry was oxidized by
bubbling chlorine gas through it for 4 hours at 120F so as to
provide 78 pounds of NaOCl per ton of dry ore. Oxidation was
carried out in a glass beaker equipped with a mechanical
agitator. No soda ash was used in this test priox to cyan-
idation. After chlorination the slurry was maintained under
agitation for 2 hours to allow excess hypochlorite ions to
be converted to oxygen and chloride ions. Cyanidation leaching
was then carried out in rolling bottles for 24 hours using the
same cyanide and the same procedure as in the previous tests.
No carbon was used in this test. After separation and analyses
the gold content of the solids was 0.198 ounces per ton,
indicating that only 20% of the gold originally present in the
ore had been leached.
In T~st F, the experiment of Test E was repeated except
that 20 grams of the Westates carbon ~er liter of slurry were
used during leaching as in Tests B and D. After separation
and analyses the gold content of the solids was 0.161 ounces
per ton, indicating that only 35% of the gold originally in
the ore had been extracted.
In Test G, a sample was slurried as in Tests C and D,
that is, in the presence of 50 pounds of Na2CO3 per ton of
dry ore, and then subjected to oxygenaiion by bubbling 600 cc
of air per minute for 8 hours at a temperature of 180F in
the same stainless steel vessel used for oxygenation in
previous tests. The oxygenated slurry was then allowed to cool
to 120F in a glass beaker e~uipped with a mechanical agitator,
and chlorinated with chlorine gas in the same amount and in
the same fashion as in Tests E and F. After holding the slurry
under agitation for 2 hrs to destroy excess hypochlorite ions
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~.~C~2754
and cooling to 75F, the oxidized slurry was subjected
to cyanidation leaching as in previous tests, but no carbon
was used. After separation and analyses the gold content
of the solids was 0.156
- -2Oa-
2~5~
ounces per ton of ore, indicating that only 38% of the gold originally
in the ore had been leached.
In Test H the experiment of Test G was repeated except that
20 grams of the Westates carbon per liter of slurry were used during
cyanidation in the same fashion as in previous Tests B, D and F.
After separation and analyses the gold content of the solids was
0.029 ounces per ton, indicating that 88% of the gold originally pre-
sent in the ore had been leached.
Tests I and J were performed on samples of a different gold--
containing ore, namely an oxide ore with the following composition:
gold: 0.286 ounces per ton; organic carbon: 0.14% by weight, total
carbon: 3.3% by weight; sulfur: 0.24% by weight.
In Test I 900 grams of the ore were slurried as in Test A in
the absence of any alkaline material and then subjected to cyanida-
tion leach using the same cyanide and the same procedure as in pre-
vious tests. After separation and analyses the gold content of the
solids was 0.061 ounces per ton, indicatiDg a gold extraction of 79%.
In Test J 900 grams of the ore were treated in the same manner
as in Test I except that 20 grams of the Westates carbon per liter of
slurry were added during cyanidation in the same fashion as in Tests
B, D, F and H. After separation and analyses the gold content of the
solids was 0.011 ounces per ton, indicating a gold extraction of 96%.
The results of these tests are summarized in the following
table.
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27S4
Simultaneous
Type Carbon
Of Adsorption and % Gold
Test Ore Na2C3 ~Y~ Chlorination Cyanidat;on Extraction
A Carb. No Yes No No 14
B Carb. No Yes No Yes 24
C Carb. Yes Yes No No 10
D Carb. Yes Yes No Yes 65
E Carb. No No Yes No 20
F Carb. No No Yes Yes 35
~ Carb. Yes Yes Yes No 38
H Carb. Yes Yes Yes Yes 88
I Oxid. No No No No 79
J Oxid. No No No Yes 96
It is evident from these results that good gold extraction
can be obtained from oxide ores by conventional cyanidation techni-
ques, as indicated by Tests I and J, without using the process of
thls invention.
It is also evident that, when dealing with carbonaceous ores,
~0 good gold extraction is obtainable when the process of this invention
is used, as ln Tests D and H, but is not obtainable when the process
of this invention is not used, as in the rest of the tests. More
important, these results show the importance of the three critical
requirements of the present invention, to wit, the presence of alka-
line material during oxidation, the oxidation itself and the simul-
taneous carbon adsorption and cyanidation, and the fact that gold
extraction falls when any one of these three is missing from the test.
The foregoing examples are considered representative of the
principles of the instant invention, but are given here as illustra-
tions only, and should not be interpreted as limiting the scope of
the invention. Obviously many modifications will be apparent to those
skilled in the art which will fall within said scope of the invention.
.
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