Note: Descriptions are shown in the official language in which they were submitted.
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Method of Leaching Metal From a Heap of Comminuted Ore
The invention relates to a method of extracting metal,
particularly precious metal, from ore material, according to
the preamble to Claim 1.
It is already part of the general prior art that precious
metals and other metals can be extracted from corresponding
ore material by comminuting this ore material and depositing
it on a heap, whereupon a suitable leaching fluid (e.g. a
cyanide solution) is distributed or sprayed over the heap so
that this leaching fluid penetrates this ore material heap and
in so doing breaks up the metal compounds, whereupon the laden
leaching fluid is collected and drawn off for further
processing.
Corresponding naturally occurring ores, residues in ore mining
as well as any kind of industrial residues which contain the
metals and in particular precious metal to be extracted are
regarded as ore material in this context. In this case the
expression "precious metal" includes principally gold, silver
and platinum. Furthermore, other soluble valuable substances
or metals, such as for example copper, can also be obtained by
this heap leaching process.
A method of obtaining precious metal from corresponding ore
material using material bed comminution and with heap leaching
is known for example from US Patent No. 4,960,461. In this
case it is preferred to add a binder to the starting ore
material before the material comminution, by which fines from
the comminution process should be bound to coarser particles
so that the ore material heap can be made permeable to the
leaching fluid. Thus the comminuted ore material coming from
the material bed comminution has the shape of round flat
agglomerates, so-called scabs, which can be deposited directly
on the heap, but in many cases can also be broken up somewhat
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in a disagglomeration stage in order to limit the scabs to be
deposited on the heap to a thickness of 10 to 30 mm and an
edge length of 30 to 90 mm. In any case, however, this scab
material can or will contain a very wide range of grain sizes,
that is to say it may also contain relatively large particles
which can have the maximum dimensions mentioned above.
In the practical execution of this known method it has been
shown that extracting metal from the starting ore material is
still most unsatisfactory.
The object of the invention, therefore, is to propose a method
of the type set out in the preamble to Claim 1 which permits
an increased extraction or yield of precious metal and other
metals from corresponding starting ore material.
This object is achieved according to the invention by the
characterising feature of Claim 1.
Advantageous embodiments and further developments of the
invention are the subject matter of the subordinate claims.
In the tests on which the invention is based it has been
demonstrated that comminuted ore material is only
insufficiently leached and thus the metals contained therein
can only be insufficiently dissolved by the leaching fluid
even when the comminution takes place in the form of material
bed comminution, that is to say in a material bed roll mill.
It is known that in the material bed roil mill some of the
brittle material can be completely comminuted and some of it
only scored by micro-cracks or partially comminuted by a
combined individual grain loading and material bed loading.
The ore material thus comminuted leaves the material bed roll
mill as more less severely compacted, round flat agglomerates,
so-called scabs, which can normally be broken up or
disagglomerated with relatively little exertion of force.
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However, if these scabs are rendered more stable in shape by
the addition of a corresponding binder to the starting ore
material, as occurs for instance in the known method described
above according to US Patent No. 4,960,461, then the
relatively large scab pieces will also largely retain their
shape and size in the leaching heap. Due to these relatively
large scab pieces relatively large interstices are indeed
created which favour the permeability of the heap to the
leaching fluid; however, the scabs, which are round and flat
or at least to some extent plate-shaped, can also often lie
close above one another in layers in the heap, greatly
reducing the permeability to leaching fluid. Overall, with
this known method it has been shown that the rate of yield of
precious metal from ore material leaves a great deal to be
desired, and moreover this known method can only be used with
oxidic ore.
The present invention likewise does utilise the advantages of
material bed comminution on a material bed roll mill for the
extraction of metal, particularly precious metal, from
corresponding ore material. However, this is done in such a
way that the comminuted ore material is deposited on the heap
with a maximum grain size of approximately 12 to 15 mm. Thus
the ore material to be deposited on the heap or leaching heap
is controlled after the material bed comminution and kept in a
grain size range with a maximum grain size of 12 to 15 mm.
The previously mentioned tests on which this invention is
based have also in fact shown that the yield of metals, and
particularly of precious metals, from the corresponding
starting ore material in heap leaching already deteriorates at
a grain or particle size of over approximately 6 to 8 mm
(maximum edge length), this deterioration in the metal
extraction increasing particularly markedly above a grain or
particle size of approximately 12 to 15 mm.
The previous observations could also be confirmed by a series
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of laboratory tests which were carried out with ore material
containing gold, in each case with the same leaching fluid
(cyanide solution) and the same ratio of leaching fluid to
quantity of ore. Thus in a test example the ore material was
comminuted to approximately 12.5 mm (= %") mesh size and
leached over a period of 120 hours; the yield of pure gold ore
was 31.3%. Coarser test grain sizes lead to somewhat lower
percentages in the gold extraction. With comminution of the
ore to approximately 6.35 mm (= !4") mesh size it was already
possible to achieve a dramatic rise in the gold extraction,
namely a percentage of 67,0% of the gold contained in the ore.
Thus according to the invention it is particularly preferred
to set the grain or particle size of the comminuted ore
material to be deposited on the heap to smaller than
approximately 8 mm and quite especially preferably to smaller
than approximately 6 mm in the case of substantially powder-
free grains. In this way an optimal yield of metal fractions
containing valuable substance from the corresponding starting
ore material can be brought about. This advantage is all the
more important if one considers that oxidic ore, i.e. ore
which is directly leachable, containing metal or precious
metal is available in ever decreasing quantities.
In this connection it should be explained that in this case
oxidic ore is characterised in that with direct leaching of
this ore general rates of yield of over 80% are achieved.
However, semi-refractory or refractory ores often require an
intermediate treatment or at least still finer comminution, so
that economically viable rates of yield of extraction can be
achieved.
The maximum grain size proposed according to the invention for
the comminuted ore material to be deposited on the leaching
heap has also proved favourable with regard to the
permeability to the leaching fluid within the heap. In fact,
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with this grain size a particularly uniform distribution of
the leaching fluid and thus to some extent a uniform washing
round of all material grains or particles is achieved, which
leads to the desired optimal release of the metals contained
in the ore material particles comminuted in the material bed,
that is it leads to an optimal extraction of precious metals.
Depending upon the mineralogical composition (e.g. high clay
fractions or the like) of the ore material to be treated, in
the case of certain ore materials it is also important that
the comminuted ore material to be deposited on the leaching
heap is substantially powder-free in the said grain size
range, i.e. no material particles in the form of dust or
powder should be deposited on the leaching heap. This would
clog the heap to some extent, so that the heap would lose its
necessary permeability to the leaching fluid. Thus with this
method according to the invention the optimal grain size range
of the ore material to be deposited on the heap can be
controlled or regulated quite deliberately.
In the simplest embodiment of the method according to the
invention the procedure is such that the ore material which
has been comminuted in the material bed comminution to a
maximum grain size of approximately 6 mm is deposited directly
on the heap. However, this procedure is only reasonable when
the ore material coming from the material bed comminution on
the one hand does not exceed the aforementioned maximum grain
size of approximately 6 mm and on the other hand is largely
powder-free or contains few pieces of material which could
cause partial blockage or uneven wetting of the entire heap.
Blockage should be understood here to mean that parts of the
stockpile-like heap become impermeable to the applied fluids
due to these fines, so that no or only insufficient yield of
valuable substance can take place from these regions. A
particularly good possibility for also binding the fine,
approximately powdery material fractions can be that either
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the fine material fraction from the classification stage
before being deposited on the heap or also the comminuted ore
material coming from the material bed roll mill is at least in
part, preferably in total, first of ail agglomerated in an
agglomeration stage and then deposited on the heap. Such
agglomeration can be carried out in an extremely controlled
manner, so that approximately rounded or approximately
spherical pellets, briquettes or the like can preferably be
produced. In contrast to the flat round scab pieces described
above, due to this agglomeration according to the invention of
the ore material comminuted in the material bed roll mill the
heap can be piled up particularly evenly with a
correspondingly uniform permeability to the fluid to be
applied to the heap, which in turn leads to optimum and
uniform wetting of all material particles resulting in an
increased yield of valuable substance.
Whereas the previously described procedure can be used above
all for leaching directly leachable oxidic ore materials by
the addition of leaching fluid on the heap, there is also,
however, the possibility of utilising this method according to
the invention for processing of so-called refractory ores or
ore materials, i.e. ore which is not directly leachable. In
this case the procedure can be such that the ore material with
a set grain size, that is to say in this case the refractory
ore material, is first of all subjected on the heap to an
oxidation treatment by the addition of a suitable oxidising
agent, whereupon the ore material which has thus been rendered
oxidic is then washed (and thus to a certain extent
neutralised) and then leached on the heap by the addition of
leaching fluid - as described above.
In this way non-oxidic, that is to say refractory or semi-
refractory ore material can be economically leached on a
stockpile. According to this embodiment of the method
according to the invention this comminuted ore material
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(typically above all gold and certain copper ores) is thus
subjected to an oxidation treatment directly on the stockpile
or heap. This oxidation can - as is known per se - be
achieved by the action of certain bacteria or by corresponding
chemicals. Following this oxidation treatment the ore
material is then first of all set to the desired chemical
properties in a washing process, i.e. is neutralised to a
certain extent, before the valuable substances can then be
extracted from this ore material by means of the said leaching
process.
Thus in the processing of refractory ore materials the
comminution of these types of ore in a material bed roll mill
offers the advantage of micro-cracks in the comminuted ore
particles, as a result of which the oxidation then also
progresses in the manner described above more quickly and with
higher degrees of oxidation, which overall can take place very
economically. In this case it also proves particularly
sensible in the case of the comminuted ore material coming
from the material bed roll mill - optionally after
disagglomeration of scabs which have formed - for the grain
size of this comminuted ore material to be limited or set in
the stated manner so as to favour the most complete oxidation
possible of the individual particles.
There are various possible ways of using the method according
to the invention in order to be able to regulate or control
the optimum range of grain sizes with which the ore material
coming from the material bed comminution is deposited on the
leaching heap. This is explained in greater detail below with
reference to some embodiments which are illustrated in the
drawings, in which:
Figure 1 shows a flow diagram (block diagram) of a first
embodiment for carrying out the method according to the
invention, in which a screen classifier is interposed between
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the material bed comminution and the leaching heap;
Figure 2 shows a flow diagram of a second embodiment with an
agglomerating conveyor belt disposed between the material bed
comminution and the leaching heap;
Figure 3 shows a flow diagram for an embodiment in which a
pelletising table is provided between the material bed
comminution and the leaching heap;
Figure 4 shows a flow diagram with variants of the embodiment
according to Figure 1.
In all the embodiments described below with reference to
Figures 1 to 4 it may be assumed that they are designed for
carrying out the method according to the invention of which a
far-reaching description has already been given above,
particularly for extracting precious metal, such as for
example gold, silver and platinum, from corresponding ore
material. However, other suitable ore materials, e.g. in
particular copper ores, can be processed in substantially the
same way to give corresponding metals. For the sake of
simplicity all machines or apparatus parts which are the same
or of similar construction are provided with the same
reference numerals, so that the corresponding apparatus parts
essentially only have to be explained in detail once.
The first embodiment according to Figure 1 contains a material
bed roll mill 1 which can be constructed in a manner which is
known per se and accordingly is equipped with two rolls 3, 4
which are pressed against one another under high pressure and
can be driven in opposite directions (cf. arrows 2) and
between which a grinding gap 5 is constructed. The material
bed roll mill 1 also has a material delivery shaft 6 in the
region above the grinding gap 5. Downstream of the material
bed roll mill 1 is provided a screen classifier 7 which for
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example contains a top screen 7a with suitable screen hole
size, and also a discharge 7b for the oversize material
fraction as well as a discharge 7c for the product falling
through or fine material fraction. Downstream of the
discharge 7c for the fine material fraction of the screen
classifier 7 there is provided a leaching heap 8 which can
likewise be of a construction which is known per se and which
accordingly has associated with it a delivery arrangement 9
for leaching fluid (broken arrows 10) - which arrangement is
optionally movable over the heap.
In the apparatus according to Figure 1 starting ore material
containing precious metal is delivered according to the arrow
11 to the material bed roll mill 1 via the material delivery
shaft 6 thereof. This starting ore material 11 is subjected
to material bed comminution in the grinding gap 5 between the
two rolls 3, 4. The comminuted ore material according to the
arrow 12 is then first of all delivered to the screen
classifier 7, the top screen 7a of which is constructed so
that only comminuted ore material with a grain size of smaller
than approximately 8 mm to 12 mm, preferably smaller than
approximately 6 mm to 10 mm, falls through downwards, so that
this product falling through the screen or the fine material
fraction formed thereby can be deposited and distributed
uniformly on the heap 8 according to the broken line 13 where
it can be leached by means of leaching fluid 10. The retained
product or the oversize fraction of the classification stage
formed by the screen classifier 7 is drawn off at the
discharge 7b and returned to the material feed shaft (feed
arrangement) 6 of the material bed roll mill 1 according to
the dash-dot line 14, so that it is again subjected to
material bed comminution with the fresh starting ore material
11. In the processing of the ore material in the material bed
roll mill 1 it may be mentioned that it is comminuted without
the addition of binder and thus the scabs obtained can be
disagglomerated relatively easily, at least partially on the
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top screen 7a of the screen classifier 7. Therefore the
maximum grain size of the comminuted ore material to be
deposited on the heap 8 can easily be set in a very controlled
manner.
In the second embodiment of the apparatus illustrated in
Figure 2 an agglomeration stage in the form of a conveyor belt
or agglomerating conveyor belt 15 is interposed between the
material bed roll mill 1 and the leaching heap 8. In this way
at least part but preferably all of the ore material 12
comminuted in the material bed roll mill 1 can be shaped into
pellets of any suitable size on this agglomerating conveyor
belt 15. In this case a suitable binder can then be added to
the comminuted ore material on the conveyor belt 15 above all
when the component consisting of very fine particles or
powdery particles is relatively large, so that these powdery
particles can then be incorporated into the pellets and cannot
impair the permeability of the heap.
As regards the resulting agglomerates in pellet form, it may
also be mentioned that due to the production on the
agglomerating conveyor belt 15 or another suitable pelletising
arrangement these agglomerates can be at least somewhat
rounded, but generally can be approximately spheroidal or
spherical. Moreover, they can also be somewhat larger than
the maximum grain size of the ore material particles, that is
to say for example larger than 6 to 8 mm. In contrast to the
depositing of larger round flat scab pieces - as in the known
method explained above - the desired quality and size of the
pellets to be deposited on the heap, in which the grain size
of the comminuted ore material in each case is kept to the
maximum grain size explained above or set to that size, can
be set in a very controlled manner by this agglomeration of
the ore material which has already been comminuted. When the
pellets or agglomerates produced in this way are deposited on
the heap, optimum preconditions are created for the fluid
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(leaching fluid) delivered to the heap to be very evenly
distributed and for the individuai pellets and thus also the
ore particles incorporated therein to be optimally wetted, so
that a particularly high degree of leaching and consequently a
very high rate of yield of valuable substances can be
achieved.
The embodiment according to Figure 3 to some extent represents
a variant of the example previously explained with reference
to Figure 2. In this case (Figure 3) the agglomeration stage
is formed by a pelletising table 16 which is known per se, to
which all of the comminuted ore material from the material bed
roll mill 1 is also delivered, so that it can be agglomerated
to the optimum grain size (that is to say with the optimum
pellet size) and can then be deposited on the heap.
It should also be pointed out at this point that other
suitable agglomeration arrangements can also be used for the
agglomeration stage if the sizes and shapes or properties of
the agglomerates or pellets as described above can be achieved
and maintained thereby. Thus instead of an agglomerating
conveyor belt and a pelletising table a pelletising drum can
for example be used. Moreover, it may also be advantageous to
carry out the agglomeration of the comminuted ore material in
a conventional briquette press, in which case the agglomerates
can advantageously also be passed to the briquette press - in
the desired briquette mould (in a similar manner to the
pellets) - without the addition of binders. As a result the
operating costs can be reduced, and depending upon the
leaching agent or the leaching fluid a better yield of
valuable substance can be achieved.
Also in connection with the agglomeration stage (e.g. 15 or
16) it should be pointed out that at the beginning of the
agglomerating or pelletising operation the scabs from the
material bed roll mill 1, which without binder are relatively
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loosely cohesive, can first of all be at least partially
broken up and only after that brought in a quite controlled
manner to the desired grain size.
Thus the two embodiments of apparatus according to Figures 2
and 3 are particularly suitable if the starting ore material
11 is comminuted relatively finely or largely comminuted in
the material bed roll mill 1 and contains a relatively large
amount of very fine material in the form of powder or dust.
Thus by the interposition of the agglomeration stage 15 or 16
it is possible to ensure in a reliably controlled manner that
the comminuted ore material to be delivered to the heap
leaching is kept substantially powder-free in the desired
grain size range.
Depending upon the nature or the properties of the starting
ore material 11 to be comminuted as well as the mode of
operation of the material bed roll mill 1, there are yet
further combinations or variants which are possible in
apparatus for carrying out the method according to the
invention.
Thus basically it may be helpful or necessary if the
comminuted ore material to be delivered to the classification
stage (Figure 1) or the agglomeration stage (Figures 2 and 3)
is previously intentionally disagglomerated at least to some
extent.
Accordingly Figure 4 shows a variant of the embodiment of the
apparatus according to Figure 1 in such a way that a suitable
disagglomeration stage 17 can also be disposed between the
material bed roll mill 1 and the screen classifier 7 if need
be. Thus if for example the comrninuted ore material coming
from the material bed roll mill 1 has been agglomerated in
relatively large round flat scabs, then these scabs can first
of all be at least partially or sufficiently broken up in the
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disagglomeration stage 17 so that afterwards a good
classification of the comminuted ore material 12 can take
place in the screen classifier 7. Also in this case the
oversize material fraction (screen residue) 14 is thereupon
returned to the material delivery shaft 6 of the material bed
roll mill 1, whilst the screenings or the fine material
fraction 13 can be deposited on the leaching heap 8.
As is indicated furthermore by a dash-dot line in Figure 4,
however, an agglomeration stage in the form of a pelletising
drum 18 or the like can also be disposed between the screen
classifier 7 and the leaching heap 8. Thus this measure also
provides the possibility that the fine material fraction 13
from the classification stage or the screen classifier 7 can
be set in the agglomeration stage 18 to an optimum grain size
for heap leaching before it is deposited on the heap 8. In
this way - as already described above - very fine fractions of
the comminuted ore material which are in the form of powder or
dust can be incorporated into the ore material pellets or
agglomerates, thus reliably preventing the pores or
interstices between the material grains in the leaching heap
from being clogged by powdery fractions.
The embodiments of apparatus or methods explained above with
reference to Figures 1 to 4 are suitable in the described form
above all for heap leaching of oxidic ore material. However,
increasing efforts have been made recently to leach non-oxidic
ore, so-called refractory or semi-refractory ore or ore
material economically on a heap or a stockpile. As has
already been explained above, this method according to the
invention can also be designed so that it is suitable for heap
leaching of non-oxidic or refractory ore materials,
particularly for example gold and certain copper ores. In
this case it must generally be ensured that the comminuted ore
material to be leached on the heap is previously subjected in
a suitable manner to oxidation treatment. This can preferably
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take place for example in such a way that the refractory ore
material, which has previously in a similar way been subjected
to material bed comminution and set to a predetermined maximum
grain size as described with reference to Figures 1 to 4, is
subjected on the heap first of all to oxidation treatment by
the addition of a suitable oxidising agent (e.g. bacteria or
other suitable oxidising agents). With this oxidation
treatment too, the optimum preparation, i.e. material bed
comtninution and setting to an optimum maximum grain size of
the ore particles, has an extremely advantageous effect on the
distribution of the oxidising agent and thus on the entire
oxidation of the ore material on the heap. This oxidation
treatment is followed by a washing operation in which the
residual oxidising agent is washed out and thereby the
oxidised ore material is neutralised to a certain extent
before the actual leaching can be initiated by delivery of
leaching fluid and carried out in the aforementioned manner.
In this case the washing out of the oxidation agent can take
place on the same heap. However, if a particularly intensive
washing of the heap is necessary, then this can also take
place due to the fact that the oxidised stockpile or the
oxidised heap is built up and in the course of that subjected
to a washing operation, whereupon the leaching of the valuable
substances is carried out in a heap or stockpile to be newly
erected. Accordingly the valuable substances can then also be
extracted from refractory or semi-refractory ore materials by
means of heap leaching with a high rate of yield.
With this method according to the invention there are also the
possibilities of adding leaching fluid or an oxidising agent
to the comminuted ore material as early as before or during
the agglomeration or pelletising or briquetting operation, as
is indicated by the arrow 19 for example in Figures 3 and 4 in
the case of the pelletising table 16 or the pelletising drum
18 respectively. In this way depending upon the type of
processing required or depending upon the type of ore material
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to be treated (oxidic or refractory) the operation of leaching
or oxidation of the ore material can already be initiated in
the region of the agglomeration stage.
