Note: Descriptions are shown in the official language in which they were submitted.
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47793-2
GOLD EXTRACTION APPARATUS
This invention relates to an apparatus and to a
method for extracting gold from a gold source. Typically
the gold source will be an ore from a mine or, more
usually, a mill.
Because of its chemical inertness gold frequently
occurs in elemental form and may be separated relatively
easily. This is particularly so in placer mining where
loose surface material is washed for gold. The high
density of the gold is of value in this regard. The
placer deposit can be concentrated in a concentrator
whose function is simply to isolate from the ore the
gold, silver or other heavy, valuable metal.
However, the majority of the world's gold is now
recovered by deep mining, particularly in South Africa.
In this process mined ore is milled to reduce the
particle size. The milled ore is then subjected to
chemical extractants. These chemical extractants are
relatively complicated because of the inertness of gold.
Recovery processes include amalgamation with mercury, the
cyanidation process, gravity concentration, flotation,
roasting and a combination of any of the above. Enzyme
extraction is also coming into use.
The cyanidation process, although of some antiquity,
is still widely practised. It hinges on the ability of
gold to react with alkaline cyanides to form the
aurocyanide ion, which is soluble in water. In
cyanidation ore, or perhaps tailings from previous
attempts at gold extraction, are leached with dilute
sodium cyanide solution, typically of a concentration of
0.02 to 0.05 molar. Calcium cyanide and calcium
hydroxide may also be used. The reaction is assisted
greatly by aeration. The gold is separated from the ore
as sodium aurocyanide, which goes into solution.
The gold is recovered from solution by using zinc
dust or aluminum or, in a relatively new technique, by
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extraction on activated carbon. Typically 9 to 12
kilograms of gold can be recovered by about a ton of
activated carbon. When exhausted the carbon can be
stripped of its gold with ethanolic sodium hydroxide.
The carbon is reactivated by roasting.
As indicated this is a relatively recent development
and there are still a number of improvements that would
be desirable. It would, for example, be desirable to
improve extraction and to improve the stripping of the
gold from the carbon.
A disadvantage of the carbon process is that it is
demanding of space. The processes are essentially
sequential. The ore is taken from step to step and the
reagent solution, used to extract the gold, is treated in
a series of steps. It would be desirable to make the
apparatus more compact.
An attempt to simplify gold extraction is heap
leaching where an extractant is percolated through a heap
of ore. Some extractants, especially cyanide, raise
environmental concerns.
The present invention renders the process of gold
extraction more compact, economical and free of
environmental concerns. Accordingly, the present
invention, in a first aspect, is an apparatus to extract
gold from a gold source comprising:
a reactor vessel;
means to rotate the reactor vessel;
an inlet for the gold source;
an outlet for the treated gold source;
an inlet for extracting reagent;
an outlet for used extracting reagent; and
a plurality of perforate bodies in the vessel
adapted to receive an absorbent for the gold.
In one embodiment the vessel may be divided into a
plurality of compartments, for example three
compartments. In such an embodiment there will be a
first compartment to communicate with the inlet and to
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conduct a first leaching, a second compartment to receive
material from the first compartment and to conduct solids
liquid separation and the third compartment to receive
material from the second compartment. The perforate
bodies in this embodiment are positioned in the second
and third compartments.
The present invention is also a method of extracting
gold from a gold source using an extracting reagent able
to react with gold to form a soluble gold compound, the
method comprising:
containing the ore and the extracting agent in a
rotating vessel; and
extracting the gold on an absorbent contained within
the rotating vessel.
In a further method aspect the extraction of the
gold is carried out in a rotating vessel having a
plurality of compartments.
Aspects of the invention are illustrated in the
accompanying drawings, in which:
Figure 1 is a schematic view of a gold extraction
using the apparatus of the invention;
Figure 2 is a section of an apparatus according to
the present invention;
Figure 3 is a section on the line 3-3 in Figure 2;
Figure 4 is a section through a further embodiment
of the apparatus of the present invention;
Figure 5 is a section on the line 5-5 in Figure 4;
and
Figure 6 is a section on the line 6-6 in Figure 4.
Figure 1 shows a process for extracting gold from an
ore or other gold source. First the ore is treated in a
jaw crusher 2. For alluvial deposits, removal of coarse
boulders using a screen may be all that is required.
The jaw crusher 2 reduces the rock to a size of
about minus 3" - 4". The crushed ore is fed by conveyor
4 to a roll crusher 6 that reduces the ore to the final
size.
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Crushed ore is fed to a hopper 8 between the
crushing components of the system and the remaining
components. Hopper 8 provides surge capacity.
From the hopper 8, the ore is fed by a conveyor 10
5 to a reactor vessel 12 according to the present
invention. The vessel 12 has an inlet hopper 14 that
receives the ore from conveyor 10. Lime is added to the
ore at 16. The extracting reagent, usually cyanide, is
introduced into hopper 14 through line 18. An airline 20
is provided as air sparging is essential to the cyanide
extraction of gold.
As indicated generally in Figure 1, but as shown in
more detail in Figure 2, the reactor vessel 12 is
provided with a series of perforate bodies in the form of
15 baskets 22.
From the vessel 12 the treated ore is fed to screen
24 and to a vacuum filter 26 for dewatering. Cyanide
reagent is extracted by pump 28 and recirculated to the
vessel 12. Tailings can be treated in conventional
20 manner.
In Figure 1 the jaw crusher 2 may not be necessary.
Furthermore a roll crusher 6 is illustrated but a cone
crusher could be used. Generally speaking roll crushers
are cheaper and lighter, features that are of advantage
25 in a small, portable crushing plant.
If a finer feed is required than that achieved by
the use of a jaw crusher and roll crusher, a small closed
circuit grinding mill can be incorporated into the
crusher circuit. Such equipment is well known.
The reaction vessel 12 is more fully shown in
Figures 2 and 3. The reactor vessel is a generally
cylindrical vessel 30 mounted on rollers 32 and inclined
from inlet end 34 to outlet end 36. The degree of
inclination will vary depending on dwell time required
and the characteristics of the ore. The vessel 12 has a
peripheral gear wheel 38 driven by the output shaft of a
motor (not shown). This arrangement is conventional in
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the art. It may be replaced by a drive motor and drive
chain, again conventional in the art. The latter
provides greater flexibility in positioning the drive
motor relative to the drum than gear wheel 38. This can
5 be an advantage in a portable plant.
There is an inlet for the gold source in the form of
the hopper 14 communicating with the interior of the
vessel 30. There is a discharge chute 40 at the other
end of the vessel. In this regard hopper 14 and chute 40
are mounted in respective end plates 42 and 44 which are
fixed. Vessel 30 rotates on end plates 42 and 44. Seals
46 are provided.
There are means to agitate and lift the gold ore
comprising paddles 48 extending from the inner wall of
15 the vessel 30, as shown most clearly in Figure 3. Air is
fed to the vessel from line 20 through a rotary valve 50
which feeds to perforate airlines 52, for example three
lines, positioned on the inner wall of the vessel 30.
The airlines 52 rotate with vessel 30 and the rotary
valve 50 ensures that air is not fed to a perforate
airline 52 that is above the level of the liquid within
the vessel 30.
A plurality of perforate bodies 22, typically
cylinders, are located in the vessel 30. The bodies 22
25 each have a threaded hatch 54. The bodies 22 may be
punched plate or heavy wire screen. Each perforate body
22 contains an inner canister 56 made of a finer mesh
screen containing activated charcoal. These canisters 56
may be removed from the perforate bodies 22 through
30 hatches 54. At the outlet end of the vessel there is a
plurality of lift buckets 58 located on the inner surface
of the vessel 30 and rotating with that vessel 30. These
buckets 58 align with the mouth of the discharge chute 40
so that slurry carried to the top of the drum by a bucket
35 58 is spilt into the discharge chute 40. From there the
effluent vessel passes to the screen 24 as shown in
Figure 1.
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The gold dissolves in the cyanide solution, in
conventional manner. When the cyanide solution
containing the gold in the form of aurocyanide (the so-
called pregnant cyanide solution) contacts the activated
5 carbon the gold is absorbed on to the carbon. After an
appropriate time, determined by routine experiment, the
feed is turned off and the canisters 56 removed and
replaced by another set of canisters. The typical period
between changes would be about 3 to 10 days.
As indicated in Figure 1, the roll recovery and
recycling of the cyanide solution is conventional. The
cyanide solution deteriorates in use due to the formation
of cyanide complexes and cyanates and a small cyanide
make-up solution is therefore added through line 60.
The carbon, containing gold, is removed from the
drum and gold is stripped from the carbon using caustic
soda wash, again in conventional manner. The carbon can
be regenerated, for example by heating to about 600~C.
Gold can be removed from the caustic soda solution
20 by electrowinning. Again this is conventional. Steel
wool cathodes are used. When loaded with gold they are
removed, heated in a furnace and the gold poured off as
bullion.
A second embodiment of the invention is illustrated
25 in Figures 4, 5 and 6. The second embodiment differs
from that of Figure 1 principally by the provision of a
plurality of compartments. Where appropriate, common
reference numerals, are used.
Figure 4 shows a reactor vessel 12. The vessel can
30 be rotated, as indicated by the arrow A in Figure 5, but
the means of rotation is not shown in this drawing. The
vessel 12 is divided into a plurality of compartments.
Inlet 60 receives feed from the feed chute 14. The
material is fed to a first compartment 62 of the vessel
35 12. Compartment 62, as shown in Figures 4 and 5, is
equipped with lifters 64, that function to agitate and
lift the solids. There is a second compartment 66 to
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- 7 -
receive material from the first compartment 62 to conduct
solids liquid separation. There is a partition 68
between the first and second compartments.
A third compartment 70 receives material from the
second compartment 66. Perforate bodies are positioned
in the second and third compartments and take the form of
annular bodies 72 positioned (a) between the second and
third compartments and (b) the third compartment and
outlet 74. There is a lifter 76 between the second and
third compartments. As shown particularly in Figure 6
the lifter includes a screen 78 and restricts the size of
the ore to a particular, predetermined particle size.
Similarly the third compartment has lifter 80 to move
material to the outlet. Again a screen 82 is provided to
15 restrict the size of the particles.
The apparatus of Figures 4 to 6 is particularly
appropriate, although not restricted to, the processing
of relatively coarse ore. The inter-stage screening
allows for differential solid and solution retention
times in the various compartments 62, 66 and 70. The
first compartment 62 provides a majority of the retention
time for gold leaching by leaching solution, usually
cyanide solution. The second and third compartments have
increased solution retention time due to the rapid
25 removal of the coarser fractions of the ore. This
increased retention time provides enhanced recovery of
gold onto the activated carbon. Carbon may be advanced
from the third to the second stage to provide counter
current carbon movement. This is illustrated
30 diagrammatically in Figure 4 where carbon transfer piping
84 moves carbon from the downstream cage 72 the upstream
cage 72. A typical ratio of carbon in compartment 66 to
that in compartment 70 is approximately 1 to 4. This
ratio accelerates carbon loading in compartment 66.
In general each compartment has the following
metallurgical function:
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Direct leaching of gold from the crushed ore to
achieve a majority of gold extraction to solution is
achieved in compartment 62. In general the volume in
this compartment will be about 43% solid rock, 13 - 23%
solution and the remainder open space above the leaching
ore. The ore characteristics in compartment 62 details
will influence the exact percentages of the volume
useful.
Compartment 66 is designed to maximize gold loading
on the carbon as well as to provide solid/liquid
separation by removal of coarse ore to prevent carbon
attrition.
Carbon is included in compartment 70 to minimize the
solution losses and to provide the final required leach
time. Compartment 62 provides the majority of the
leaching retention time. Gold loading of the solution,
that is leaching, will depend on the ore grade, stage 1
extraction and relative feed rates of all solution.
The solution in ore retention time in each of the
stages will differ and solution hold-up in various stages
would be a benefit to leaching and carbon loading.
Solution hold-up in various stages may be achieved by
dewatering the ore prior to its transfer from one stage
to the other. Efficiency of the dewatering need not be
high to have an impact on the solution and carbon
loading.
Internal weirs or partitioning between compartments
62 and 66 and, to a lesser extent, between compartments
66 and 70, provides solution hold-up while the lifter and
screen arrangement moves dewatered solids to the
subsequent stage. Solution pumping between stages is not
normally required as screen inefficiency, or the
overflowing of the partition, will allow the movement of
solution from stage to stage. The retention times of
solids and liquid in compartment 66 differ. Compartment
66 is used primarily to achieve high gold loadings on the
carbon held in the cage. This location of carbon allows
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g
the maximum contact between solution and carbon and also
prevents the inclusion of grit in the carbon baskets.
Grit wears the carbon and leads to loaded carbon losses
and thus gold losses.
The present invention has a number of advantages
over the prior art. The equipment is compact. The
invention is environmentally friendly in that there is no
leachate. The closed loop system means that extracting
solution is maintained. Furthermore the notion of
dissolving of the gold and then immediately absorbing the
dissolved gold is an advantage. Unlike the placer
extraction process finds are not loss but are, in fact,
more readily dissolved, thus providing increased
extraction efficiency.
Although the present invention has been described in
some detail by way of example for purposes of clarity and
understanding, it will be apparent that certain changes
and modifications may be practised within the scope of
the appended claims.
