Note: Descriptions are shown in the official language in which they were submitted.
2043654
Process For Leaching Of Precious Metals
The present invention relates to a process for leaching of
precious metals (e. g., gold and/or silver) from particle-
shaped, solid materials, particularly ores and ore
concentrates. The process includes the use of a cyanide-
containing alkaline leach solution in the presence of a
peroxo compound releasing oxygen.
Leaching of precious metals involves the formation of cyano
complexes (particularly complexes with gold and/or silver)
from ores, ore concentrates, and other particle-shaped,
solid materials. Such solid material may be available, for
example, from mining waste material (or tailings), from
previously incomplete leaching operations, or from
electronic scrap. A leaching process using a cyanide-
containing alkaline leach solution and an oxidizing agent,
normally atmospheric oxygen, has long been known. Although
air is widely used as an oxidizing agent for so-called
leaching by agitation, as well as for heap leaching, there
have been many attempts to increase leaching speed and the
yield of precious metals (i.e., the degree of extraction)
by using agents for releasing oxygen or other oxidizing
agents.
Hydrogen peroxide has proved to be a suitable agent for
increasing the oxygen concentration of the leach solution,
thus accelerating leaching and increasing the degree of
AX'S 11
204365
extraction. Oxygen in dissolved form is released by the
decomposition of the hydrogen peroxide during leaching (see
for example U.S. Pat. Nos. 732,605 issued June of 1903 to
Thede, and 3,826,723 issued July 30, 1974 to Waods et al,
and Canadian Patent 1,221,842 issued May 19, 1987 to Coburn
et al; see also Japanese Kokai Ol-270512). These processes
were not considered viable solutions for a long time due to
the large amount of hydrogen peroxide and sodium cyanide
being used.
The problem that prevented a practical utilization of these
processes (i.e., the excessive consumption of cyanide and
hydrogen peroxide) was solved by the process mentioned in
German Patent DE-PS 36 37 082 published May 19, 1988 to
Knorre et al. In this patent, a process is described
wherein the addition of the aqueous H202 solution is
adjusted via the concentration of the oxygen dissolved in
the cyanidic leach solution, whereby the 02 concentration
should be situated in the range of 2 to 20 mg per liter
(preferably 7 to 13 mg per liter).
In order to solve the same problem, German Patent DE-PS 38
Ol 741, published August 15, 1989 to Bahr, (U.S. Patent No.
4,971,625) suggests a different approach. In this patent,
special molar ratios of hydrogen peroxide to cyanide are
maintained, and specific pH-ranges are maintained, along
with the addition of the total amount of hydrogen peroxide
at the beginning of the leaching.
2
Q4~~
When using the process described in German Patent DE-PS 36
37 082 in gold mines, an unexpected high consumption of
chemicals and/or an insufficient increase in the gold yield
and/or a reduction of the leaching period occurred in some
cases. This happened in spite of careful adjustment of the
hydrogen peroxide dosage. An improvement could be obtained
in some instances by simultaneous use of dissolved or solid
decomposition catalysts for hydrogen peroxide (as described
in European Patent Application EP-A 0 358 004 published
March 14, 1990 to Knorre et al). The concentration of free
hydrogen peroxide could thereby be kept sufficiently low,
whereby the consumption of cyanide and hydrogen peroxide
could be lowered.
The chemical consumption, the maximum degree of extraction,
and the pertinent leaching period apparently depend upon
the chemical and physical properties of the material to be
leached in some manner that is not immediately predictable.
Although the prior art processes which use hydrogen
peroxide as an oxygen releasing source are often superior
to the process using conventional air or oxygen-Basing
techniques, a great interest persists in trying to further
decrease chemical consumption and, during the shortest
possible time, to obtain a maximum degree of extraction.
~~ .. 3
~Q4~~54
In British Patent Application GB-A 2,219,474 publishecl
December 13, 1989 to Mobbs et al, a further process of
leaching gold-containing materials by using a diluted
aqueous alkaline cyanide solution in the presence of an in
situ compound releasing oxygen (e.g., a peroxide of a
divalent metal, especially calcium peroxide) is described.
The peroxide compound is applied either as a solid or a
slurry produced from aqueous hydrogen peroxide and a
divalent metal oxide or hydroxide. It is added to the ore
pulp for leaching by agitation and it is added to the
leaching material during filling up of the heap for heap
leaching.
During reproduction of the process in British Patent
Application GB-A 2,219,474, it appeared that, due to the
relatively high chemical consumption, the plausibility of
this process is rather restricted. More particularly, the
consumption of the peroxo compound (in this instance the
moles of calcium peroxide per ton ore for leaching by
agitation) was significantly higher than the amount
required for leaching according to the process of German
Patent DE-PS 36 37 082 which utilizes hydrogen peroxide.
One object of the present invention is to make available a
process for leaching gold or silver or mixtures thereof
from at least one of the particle-shaped solid materials
containing these metals, especially ores and ore
concentrates. The process includes bringing the above-
4
~Q43~54
mentioned solid, finely divided particulate materials into
intimate contact with an aqueous leach solution having a pH
in the range of 8 to 13 and containing cyanide in an amount
of 0.005 to 2.5o by weight. The leach solution and metal
are brought into contact in the presence of an oxygen-
releasing peroxo compound in an amount of from 1 to 100
equivalents peroxoborate per ton of material to be leached.
The materials are allowed to remain in contact for a period
long enough to achieve the desired degree of extraction.
As a result of this process, the subsequent separation of
the formed cyano complexes of gold and silver from the
leach solution enriched thereby is improved in terms of
chemical consumption and/or the leaching period and/or the
degree of extraction. To attain this and other objects of
the invention, a feature of the present invention resides
in using at least one peroxoborate compound as the peroxo
compound in the leaching process.
The results of comparative tests of leaching by agitation
using different oxygen-releasing peroxo compounds on the
one hand, and atmospheric oxygen or pure oxygen on the
other, are discussed below. These results establish that
the degree of dissolved molecular oxygen is not the only
determining factor for the leaching kinetics. Without
being bound by any particular theory of operation, it
appears that the applied peroxygen compounds and/or the
secondary products produced from them, such as hydrogen
A ,s~ . 5
2043654
peroxide and peroxo anions, play an important role in the
leaching kinetics.
For an identical concentration of dissolved oxygen in the
leach solution, as determined with an oxygen electrode,
while maintaining the specified OZ value during leaching,
highly varying leaching results are obtained, as can be
seen from Examples 2 to 5. These examples show the use of
oxygen, hydrogen peroxide, calcium peroxide, or sodium
peroxoborate-tetrahydrate respectively, as the oxidizing
agents. Leaching in the presence of the added
peroxoborate, as shown in Example 5, led to an entirely
surprising result. Not only was the leaching accelerated
and the degree of extraction increased compared to the
other peroxo compounds, but the cyanide consumption (kg per
ton ore) and the consumption of peroxo compound (mol per
ton ore) could be lowered significantly. By using a
peroxoborate, the economy of the generic-type process could
be increased in a non-predictable manner in the specific
case of ordinary leaching techniques, especially leaching
by agitation and heap leaching.
As used herein, 1 to 100 equivalents peroxoborate
(expressed as B03) is defined to include 1 to 130
equivalents peroxoborate (expressed as sodium perborate
tetrahydrate (NaB03.4H20), 1 to 200 equivalents
peroxoborate (expressed as sodium perborate monohydrate
(NaB03.H20), and 1 to 255 equivalents peroxoborate
6
;-
., 204654
(expressed as Ca(B03)2) per ton of material to be leached
in leaching by agitation or heap leaching.
For leaching by agitation, the material to be leached is
brought into contact, in a finely divided form, with the
leaching solution and the oxidizing agent in one or more
leaching tanks. The thorough mixing during a leaching
period lasting several hours can take place mechanically or
through Basing with air. However, in case of Basing with
air, problems peculiar to this technique, such as increased
cyanide consumption resulting from hydrogen cyanide Basing
out, can be expected. After leaching is completed, the
fluid and solid phases of the leach slurry are separated
from one another by conventional separation techniques
known to those skilled in the art. Dissolved precious
metal cyano complexes are separated from the liquid phase
by means known to those skilled in the art (e. g.,
adsorption by coal or precipitation by means of zinc dust).
Conventional leaching by agitation equipment is used for
purposes of the present invention.
For the leaching technique characterized as heap leaching,
the material to be leached, which is particle-shaped and
may stem from a prior applied conventional agglomeration
process, is stacked in a large heap and sprinkled for
several days with an alkaline solution (known as barren
solution) containing cyanide. The solution gathering at
the bottom of the heap (pregnant solution) is fed back to
.,.
r_
7
zo4~6~~
the heap of particulate material after separation of the
cyano complexes and adjustment of the pH value and the
cyanide concentration in order to sprinkle the heap. This
process is described in J. H. Worstell, Mining Magazine,
May 1986, 405-411. Conventional heap leaching equipment is
used for purposes of the invention.
For the specific embodiments of the invention relating to
leaching by agitation and heap leaching, it is important
that an effective amount of peroxoborate be present in a
dissolved and/or most finely dispersed form. In carrying
out the invention, the peroxoborate can be added to the
system as a solid product, as an aqueous suspension, or as
an aqueous solution, before and/or during leaching.
In case of leaching by agitation, the addition of the
peroxoborate to the leach slurry is done in portions or
continually, at the beginning and/or during leaching. The
addition to the leach slurry, by portions or continually,
generally represents a more profitable specific embodiment
since it typically leads to less consumption of cyanide and
peroxo compound.
In the case of heap leaching, the peroxoborate may be
distributed as uniformly as possible in the material to be
leached during filling up of the heap by superposition of
the peroxoborate as an aqueous solution, aqueous
suspension, or as a powder. Naturally, with heap leaching,
8
2043654
the peroxoborate can also be added uniformly to the system
with the barren solution during leaching, especially if it
is sufficiently water soluble. This technique, however, is
less preferred since it cannot always be assured that
peroxoborate is available in an effective quantity in the
lower layers of the heap. Provided that the material to be
leached is converted to a granulate in an agglomeration
process before the filling up of the heap, the peroxoborate
can be added even during the agglomeration process.
One advantageous embodiment of this invention is
characterized in that immediately before leaching, a
solution or suspension of the peroxoborate in an aqueous
phase is produced by combining a borate with aqueous
hydrogen peroxide, whereupon the leach slurry of the
leaching by agitation or the barren solution of the heap
leaching is added to this peroxoborate solution or
suspension in one or preferably more portions or
continually. Starting from hydrogen peroxide and an alkali
or alkaline earth metaborate solution or suspension, the
alkali or alkaline earth peroxoborate is obtained in the
form of a solution or suspension in a quickly established
equilibrium reaction.
The term "peroxoborates" should be understood to include
the peroxoborates themselves, the hydrates thereof, the
peroxoborates of alkali and alkaline earth metals, and in
principle, even peroxoborates of other metals (e. g., zinc).
9
';
r,
2043654
Sodium and calcium peroxoborate are especially preferred.
Peroxoborates dissociate in an aqueous solution into the
metal cation and the peroxoborate anion. According to
Koberstein et al., in the Journal of Inorganic and General
Chemistry (1970) volume 374 pages 125-127, the peroxoborate
anion displays the following structure:
HO 0 - OH 2-
O
/ ~ /
B B
/ ~ /
HO O - OH
O
although normally only peroxoborate or perborate is being
considered. The peroxoborate anion together with the
metaborate anion and hydrogen peroxide are in equilibrium.
It is known that both hydrogen peroxide and peroxoborate
belong to the class of compounds known as active or
available oxygen compounds (i.e., they are able to release
oxygen). The release of oxygen can take place in a more or
less accelerated manner through the use of a decomposition
catalyst contained in the material to be leached or added
during leaching. The mechanism by which the peroxoborates
in accordance with the present invention take effect has
not yet been clarified. To the extent that preformed
peroxoborates are being used, commercial products (e. g.,
sodium perborate mono- or tetrahydrates) are suitable. The
superoxidized peroxoborates can also be added, as known
from German Published Patent Applications DE-OS 28 11 554
204354
published November 12, 1978 to Brichard et al and DE-OS 35
05 158 published July 21, 1987 to Werner et al. It is
advantageous to use calcium peroxoborate if a peroxoborate
which is less soluble compared to alkali peroxoborates is
desired.
In accordance with a preferred specific embodiment of the
present invention using leaching by agitation, an alkali or
alkaline earth peroxoborate is added during leaching in
such a quantity that an oxygen concentration in the range
of 5 to 20 ppm is set and maintained in the leach solution
(i.e., the fluid phase of the leach slurry). The oxygen
concentration can be determined in any known manner (e. g.,
by using an oxygen electrode). In this manner, an
overdosage of peroxoborate is avoided so that the overall
result is minimum chemical consumption. The optimal
correlation between the concentration of added peroxo
compound and oxygen must be adjusted individually for the
material to be leached. Such adjustment techniques are a
matter of routine experimentation to one of ordinary skill
in the art.
For heap leaching, preferably 0.05 to 100 mol alkali
peroxoborate or 0.025 to 50 mol alkaline earth peroxoborate
per ton of material to be leached (corresponding to 0.05 to
100 peroxoborate equivalents in both cases) is added to the
material to be leached during filling up of the heap or
during prior agglomeration. As can be seen from Examples 6
~.,2 ,; 11
2043654
and 7, by using sodium peroxoborate, which is added as
sodium peroxoborate tetrahydrate to the ore heap, it is
possible attain a higher OZ concentration in the leach
solution trickling through the heap than by using an
equimolar amount of calcium peroxide. When using calcium
peroxide, however, only an 02 concentration in the leach
solution identical to the one for conventional heap
leaching in the presence of atmospheric oxygen alone could
be observed. Leaching according to the present invention
is significantly favored by the presence of the
peroxoborate anion and/or its secondary products. This is
in contrast to leaching with the previously known process
using Ca02 (GB-A 2,219,474).
The leaching solution contains 0.005 to 2.5o per weight
cyanide (computed as CN) and preferably 0.02 to 0.2% by
weight cyanide. Alkali or alkaline earth cyanides may be
used as the cyanide. Preferably sodium cyanide and calcium
cyanide (also known as "Black-Cyanide") is used. The pH
value during leaching is typically 8 to 13 and preferably 9
to 12. The pH may be adjusted in a conventional manner,
preferably by adding soda liquor or lime milk. It should
be remembered that the alkalinity of the alkali or alkaline
earth peroxoborate to be added according to the invention
contributes to the pH adjustment, and the amount of soda
liquor or milk of lime can be reduced in accordance
therewith.
12
2043654
The concentration of the leach slurry according to the
present invention (i.e., the amount of material to be:
leached in the slurry) is situated within the normal range
known from leaching by agitation using air gassing (i.e.,
approximately 30 to 60o by weight). The slurry per ton of
material to be leached contains preferably 1.5 to 1 m3
leach solution (this term is understood to include the
entire liquid phase of the leach slurry).
The precious metal containing ore material to be leached is
treated in the form of particles. The finer the
particulate material, the faster the leaching occurs. In
practice, mostly material with a particle size within the
range 0.02 to 0.2 mm is treated by agitation leaching. For
heap leaching, the ore material having a particle size
within the range of 5 to 25 mm is treated. The materia l
also may include agglomerates of the most finely ground
material. The particle size distribution may also be
outside these ranges if it appears useful in terms of the
ore or ore concentrate to be leached, as well as of the
operational conditions of the mine.
Before leaching by agitation or heap leaching in accordance
with the present invention, the material to be leached,
mostly ore, ore concentrates, or mining waste material from
previous leachings, may be leached in the presence of
atmospheric air. For leaching according to the present
invention, atmospheric air may be present in addition to
13
D
the peroxoborate compound. For heap leaching, depending on
the process used, this is normally the case. In leaching
by agitation, depending on the process used, air frequently
serves not only as a source of oxygen but at the same time
it is used as a source of agitation to thoroughly mix the
slurry. In individual cases, before cyanidic leaching, it
may be expedient to subject the material to be leached to
an oxidative pretreatment (e. g., by using hydrogen
peroxide).
When leaching in accordance with the present invention, the
leaching by agitation or heap leaching technique may
include various known substances additionally being
contained in the leaching solution or the barren solution
in order to optimize the execution of the leaching and/or
to increase and/or to accelerate the yield of gold and
silver and/or to reduce the consumption of cyanide and
oxidizing agent. These additional substances may include
effective surfactants (e. g., alkali-stable tensides) which
serve to improve the wetting and penetration of the
materials to be leached. Foam inhibitors or defoamers,
which are useful for leaching of heavily foaming materials
(e. g., biologically pretreated ore concentrates), may also
be included. Finally, the addition of flotation chemicals
from the series of thiocarbonates, thiophosphates,
thiocarbaminates, or anionic polymers, particularly from
the series of polyacrylic acids, starches, carboxymethyl
cellulose, has proved to be advantageous for the depressing
14
,:.k_2v :~
2043654
or masking of iron and copper in ores containing such
metals, so as to minimize the need for cyanide and/or
oxidizing agent (see DE-PS 38 Ol 741). Such additives,
adjuvants, and auxiliary agents may be added to contribute
their expected function as will be apparent to those
skilled in the art.
To the extent that it is advantageous and desirable (which
may be tested in a normal leaching test), leaching
according to the invention may also take place in the
presence of substances additionally added in order to
accelerate the release of oxygen from the peroxoborate or
the intermediately formed hydrogen peroxide. The additives
may include decomposition catalysts from the series of
heavy metals acting destructively and/or solid substances
with destructively acting centers, preferably manganese(II)
compounds or active carbon, as described EP-A 0 358 004.
The process according to the invention may also be carried
out in the form of a CIL process (CIL = Carbon In Leach).
In order to separate gold and silver from their solutions
containing cyano complexes, conventional processes are
considered, particularly the CIP process (CIP = Carbon-in-
Pulp), the Merrill-Crowe process, and the Ion-Exchanger
process. Such techniques are known to those skilled in the
art.
za4~6~
The following examples, with peroxoborate as the oxidizing
agent for leaching by agitation and heap leaching relative
to previously known oxidizing agents (e. g., air, oxygen,
hydrogen peroxide, and calcium peroxide ) demonstrate the
superiority of the leaching process according to the
present invention. The superiority is evidenced in the
unexpectedly low consumption of cyanide and peroxoborate,
and also in the acceleration of the leaching and the higher
degree of extraction.
EXAMPLES
Examples 1 to 5 (leaching by agitation)
A gold ore from South Africa was leached. The ore was
bornite-containing and included 6.5 g gold per ton of ore.
The granularity was 80% less than 75 Vim. The ore was
leached in a leaching tank with mechanical intermixing.
The cyanide concentration of the leach solution, at the
beginning of the leaching, was adjusted to 0.1% by weight
NaCN by using sodium cyanide; additional sodium cyanide was
added as needed in order to maintain a minimum
concentration of 0.030 by weight NaCN. The solid-substance
content of the slurry was 50% by weight. The pH value was
adjusted and maintained at 10.8.
In Example l, air gassing was used (11/h and kg ore). In
Examples 2 to 5, a constant 02 level of 12 ppm was set and
16
,,:~ :;_
2043654
maintained in the leach solution. The oxidizing agent was
added, spread over the leaching period, as needed. the
results of the leaching, measured in the degree of
extraction as a function of time, the NaCN consumption (kg
per ton ore), and the consumption of oxidizing agent (mol
per ton ore) can be seen from Table 1. In Table l, Example
represents the process in accordance with the present
invention.
17
__ ,.
. 2043~~4
*
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lt1 t' d1 e-a N N
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2~~3654
Examples 6 to 8 (Heap Leaching)
A gold ore from Xapetuba (Brazil) with a gold content of 1
g/t ore was leached. Grain-size distribution of the ore
was 2 to 20 mm. In Example 6, calcium peroxide was used.
In Example 7, sodium perborate tetrahydrate, as per the
invention was used. The oxidizing agent was added in an
equimolar amount (i.e., 15.3 mol per ton ore). The
oxidizing agent was distributed evenly in the ore stack.
Subsequently, sprinkling with the barren leach solution was
done. The leach solution was added in the amount of 1 F
per day per 6 kg ore quantity. The pH value of the barren
solution was adjusted with calcium oxide to 10.5 and
maintained. The sodium cyanide concentration was set to
O.lo by weight. Leaching occurred during the entire test
period with the leach solution adjusted in this manner (in
other words, without feedback or recycling of the pregnant
solution). The results can be seen in Table 2.
19
2043654
3~i Ol 01 M O ~-i C CO
[Q ~e~ . . . . . . M
N M 'd' d' In O
O
N
x
V ~!' d' d' 1O N r-1 M d' M M
O M N In d' N t"~ O ~ O lf1
~-i N M d' ~ t~ CO c-I
f~
x
N
a
as
H
I~ O N O 00 ~ d' M M
f~ O c-~01 d' l~ Q1 01 O In
U v-i N N f~ d~ d~ 111 ,-i
W W
~ O O
o~
x
~,
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d-1 tt'S fd ftS(t~ (C t13 fa f0 O 'Cs +~
.~r ~ ~ b b '~ b T~ b r1
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ro
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0
20
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,m,~-.
' 2043654
In Example 8, air served as the oxidizing agent.
The Oz concentration of the leach solution in Examples 6
and 7 is also shown in Table 3.
TABLE 3
Example 6 7
Peroxy compound Ca02 NaB03 . 4H20
02 content (ppm)
after 8 hours 7.9 16
24 " 8.1 14
" 36 " 8.5 11.5
2 days 8.3 10.8
3 " 8.1 9.0
" 4 " 8.0 8.4
" 6 'r 7.8 7.9
" g " 7.5 7.5
" 10 " 7:8 7.8
" 15 " 7.2 7.6
21
