Note: Descriptions are shown in the official language in which they were submitted.
CA 02736680 2011-04-08
PROCESS FOR THE RECOVERY OF GOLD USING
MACROPOROUS RESINS
The present invention relates to a process for the recovery of gold using gold
selective ion exchange resins.
Numerous ion exchange resins have been proposed for the selective extraction
of
gold from leach solutions. Weak-base resins were generally considered to be
the most
advantageous in view of the anticipated ease of elution with aqueous sodium
hydroxide.
However, such weak-base resins were accompanied by various disadvantages.
Work has been conducted in addition on commercially available strong-base
resins, such
resins having a higher capacity than do weak-base resins in the typical gold-
cyanide
liquor. Also, the chemistry involved in adsorption of ions onto strong-base
resins is less
complex than in the case of weak-base resins. This is because the charge on
the resin is
fixed and the effect of pH can generally be disregarded.
In spite of these advantages, the selectivity of such commercially available
strong-
base resin towards gold has been inadequate.
Test work has been carried out on certain experimental strong base resins
having
various different functional amine groups. (See article entitled "The
Extraction of Metals
from alkaline cyanide solutions by basic ion exchange materials" by A. A.
Buggs et al-
Published by Department of Scientific and Industrial Research, National
Chemical
Laboratory, Teddington, Middlesex 1963). These resins, however, whilst having
a
satisfactory loading and selectivity towards gold, exhibited poor elution
characteristics
and were accordingly unsuitable for commercial exploitation. Effective elution
could
only be achieved when the resin was further modified by the inclusion of weak-
base
groups and furthermore a methanolic solution of thiourea or thiocyanate had to
be used as
eluant. Aqueous solutions of thiourea, which are better suited to commercial
application
required about 40 bed volumes of eluant to effectively remove adsorbed gold
from a
structure modified with weak-base groups. This volume of eluant is undesirable
in a
commercial operation.
While improved strongly basic anion exchange resins, such as the commercially
available product from The Dow Chemical Company, XZ-91419 resin, are regarded
as
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CA 02736680 2011-04-08
alternatives to activated carbon in cyanidation gold mining, they still are
not selective
enough to eliminate the need for an additional expensive and cumbersome split
elution
process. Split elution processing entails the use of multiple sequential
eluants and
regenerants, to first elute unwanted base metals (primarily Cu and Zn), and
then elute the
desirable gold and platinum group metals.
It has now surprisingly been found that such drawbacks are satisfactorily
alleviated by properly selecting the resin matrix. The present invention
solves the
problem by providing a class of strong base anion exchange resins that are
hyper-
selective for gold cyanide complexes as opposed to base metal cyanide
complexes; such
resin do not require further elution processing.
In accordance with this invention there is provided a process for separating
gold
from a leachate comprising:
i) providing a leacheate wherein the leachate comprises gold;
ii) providing a macroporous resin comprising alkylamine functional groups
wherein the resin: a) is between 3% and 12% crosslinked; b) comprises a
functional group content of from 0.02 mmol/g to 1.0 mmol/g ; c) has a
water retention capacity of at least 30%; and d) has a surface area in the
range of 400-1200 m2/g; and
iii) contacting the leachate with the macroporous resin.
All ranges provided herein are inclusive and combinable.
This invention is the use of a unique resin copolymer matrix to produce strong
base anion exchange resins in which nearly all charged functional groups are
separated
within the resin matrix such that there are essentially no functional group
pairs in close
enough physical proximity to concurrently bind a single multivalent ion. This
efficient
site separation eliminates the need for exotic hydrophobic amines and results
in strong
base anion exchange resins with ultra high gold/copper selectivity.
The present invention provides a process for the extraction of gold from
solutions
thereof, in particular cyanide solutions thereof, which comprises contacting a
solution
containing gold with a resin herein defined, separating the resin and barren
solution, and
recovering adsorbed gold by elution.
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For purposes of describing this invention, the resin is a macroporous
copolymer
that is broadly defined to include copolymers prepared by suspension
polymerization of a
monomer composition under conditions conventionally used to prepare ion
exchange
resins, in the presence of one or more porogenic diluents, or swelling
solvent, using
quantities sufficient to cause phase separation of the prepared copolymer from
the diluent.
Although, it should be noted that there are many other polymerization
techniques known
in the art for preparing copolymers which could be useful in polymerization
herein.
When a macroporous copolymer is contacted with a swelling solvent, such as
chloromethyl methyl ether, its structure is characterized by the presence of
regions of
densely packed polymer chains separated by pores, often referred to as
mesopores (50 to
200 A and macropores ( > 200 A). The nonuniformity of the internal structure
of a
swollen macroporous copolymer causes the copolymer to appear opaque because of
its
ability to refract light. If inert diluents or swelling solvents are removed
from the
macroporous copolymer, for example by subjecting the copolymer to vacuum or
steam
distillation, then in many instances the pores will collapse from the stress
of internal
pressures created by increased attractive forces among the regions of packed
polymer
chains, and the copolymer would then appear transparent or translucent. A
class of
macroporous copolymers has been developed which retains its porous structure
even
upon removal of inert diluents or swelling solvents. Such macroporous
copolymers are
referred to as "macroreticular" copolymers and are described in U.S. Pat. No.
4, 382,124.
They are characterized by their opaque appearance, regardless of whether or
not the
copolymer is examined in the presence or absence of inert diluents or swelling
solvents.
Processes for preparing macroreticular copolymers of a monovinyl aromatic
monomer and a crosslinking monomer, which have been post-crosslinked with a
polyfunctional alkylating or acylating compound in a swollen state in the
presence of a
Friedel-Crafts catalyst, are disclosed in U.S. Pat. Nos. 4,191,813 and
4,263,407.
Such macroreticular copolymers are referred to as "macronet
polymeric adsorbents". A macronet polymeric adsorbent can be functionalized
with
hydrophilic groups using conventional methods for functionalizing copolymers
which are
prepared via suspension polymerization with ion exchange groups. For example,
the
polymeric adsorbent can be functionalized by aminating a chloromethylated
polymeric
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CA 02736680 2013-04-24
adsorbent with an alkylamine species such as for example a dimethylamine,
trimethylamine, or dimethylethanolamine, depending on whether weak base or
strong
base functionality is desired. The alkylamines of the present invention will
have alkyl
groups of one to six carbon atoms in length. Similarly, the macronet polymeric
adsorbent
can be functionalized by sulfonation. Alternatively, a chloromethylated
polymeric
adsorbent can be functionalized by solvolysis at elevated temperatures. The
functional
group content of the resins of the present invention range from 0.02 mmol/g to
1.0
mmol/g.
The most preferred process for preparing adsorbent resins which have been post-
crosslinked in a swollen state in the presence of a Friedel-Crafts catalyst is
described in
East German Pat. No. DD 249,274 A1. This patent
describes post-crosslinking a "solvent-free", chloromethylated macroporous
copolymer of
styrene and divinylbenzene. After chloromethylation, the copolymer is first
contacted
with a washing agent, such as methanol, and then the washing agent is removed
by either
drying the washed copolymer or extracting the washing agent with the swelling
solvent
used for the subsequent post-crosslinking reaction. After post-crosslinking
the
chloromethylated copolymer, the copolymer can be functionalized with
hydrophilic
groups in the conventional manner, thereby producing a useful adsorbent resin.
If it is
desirable, functionalization could also be performed before post-crosslinking
the
copolymer.
Although the East German patent only describes a process for preparing
adsorbent
resins from macroporous copolymers of styrene and divinylbenzene, the process
can be
used to prepare other macroporous copolymers of a monovinyl aromatic monomer
and a
crosslinking monomer. These copolymers can be used to produce other adsorbent
resins
which can be employed in mining applications of the present invention.
Preferably, the macroporous copolymer is functionalized by first
chloromethylating the copolymer, post-crosslinking the copolymer and then
aminating
the chloromethylated post-crosslinked copolymer with tributyl n-amine,
isopropyldimethyl amine, triethylamine, tripropylamine, dimethylamine,
trimethylamine
or dimethylethanolamine. Most preferably, the post-crosslinked macroporous
copolymer
is functionalized by aminating the chloromethylated copolymer with
trimethylamine.
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Preferred monovinyl aromatic monomers are styrene and its derivatives, such as
cc-
methylstyrene and vinyl toluene: vinyl naphthalene; vinylbenzyl chloride and
vinylbenzyl
alcohol. Crosslinking monomers broadly encompass the polyvinylidene compounds
listed
in U.S. Pat. No. 4, 382,124. Preferred crosslinking monomers are
divinylbenzene
(commercially available divinylbenzene containing less than about 45 weight
percent
ethylvinylbenzene), trivinylbenzene, and ethylene glycol diacrylate.
The invention achieves efficient functional site separation through catalytic
methylene bridging of the chloro-methylated styrene-divinylbenzene copolymer.
Essentially all chroro-methyl groups in close proximity to one another are
destroyed via
the bridging reaction. The remaining chloro-methyl groups are physically
isolated from
one another. These well spaced chloro-methyl groups are aminated to product
highly
gold selective anion exchange resins.
The preferred macroporous copolymer is a copolymer of up to about 99. 75
weight percent styrene with the balance divinylbenzene. Another preferred
macroporous
copolymer is a copolymer of about 40 to about 60 weight percent styrene, about
40 to
about 60 weight percent vinylbenzyl chloride and about 1 to about 20 weight
percent
divinylbenzene. The macroporous copolymers may contain minor amounts of other
monomers, such as the esters of acrylic and methacrylic acid, and
acrylonitrile.
The crosslinker serves to increase the physical stability of the adsorbent
resin. The
amount of crosslinker required depends significantly on the process conditions
used to
prepare the copolymer and can range anywhere from about 1 to about 45 percent
by
weight of total monomer, preferably from about 3 to about 12 percent by
weight.
Post-crosslinking in a swollen state displaces and rearranges polymer chains,
causing an increase in the number of micropores ( < 50A diameter) and
mesopores. This
increases porosity and surface area and decreases average pore size. Just as
significantly,
post-crosslinking also imparts rigidity to the polymer, which reduces its
tendency to
shrink or swell upon contact with an aqueous solution (often referred to in
the ion
exchange art as the "shrink/swell") and reduces its dry weight capacity when
functionalized, which is an indication of its ion exchange capacity.
The amount of post-crosslinking required for any given application is an
amount
effective to achieve the adsorbent resin properties described above to the
extent desired.
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The adsorbent resin preferably has a surface area of about 400 to about 1200
square
meters per gram of dry adsorbent resin (m2 /g), more preferably about 600 to
about 1000,
most preferably from 800-950 m2 /g. Surface area is measured by BET nitrogen
adsorption techniques. Porosity ranges from about 0.10 to about 0.70 cubic
centimeters of
pore volume per cubic centimeter of resin (cc/cc), preferably about 0.43 to
about 0.58
cc/cc, as calculated from BET nitrogen adsorption techniques. The porosity
contributed
by micropores ranges from about 30 to about 100 percent, preferably about 30
to about
50 percent, depending on the resin characteristics. Percent shrinldswell
ranges below
about 15 percent, more preferably below about 7 percent, and most preferably
below
about 4 percent. Percent shrink/swell is determined by measuring the volume
expansion
or contraction of the adsorbent resin when subjected to hydration or a change
in ionic
form. The dry weight capacity, determined according to conventional methods
used for
characterizing ion exchange resins, ranges from greater than zero to about 4 0
milliequivalent per gram (meq/g), preferably from greater than zero to about
2.0 meq/g. If
the macroporous copolymer is functionalized by solvolysis, for example by
contact with
water or an alcohol, then the dry weight capacity is essentially zero.
The adsorbent resin can be used in the form of beads, pellets or any other
form
desirable. If the adsorbent resin is used in the form of beads, bead size
ranges from about
to about 1000 microns (g), preferably from about 100 to about 800 IA, and more
preferably from about 300 to about 800 u.
The macroporous resin afore described is then contacted with a leachate
solution
containing gold. The gold in the leachate solution may be present in levels in
the range
of parts per trillion or alternatively, more commercially in ranges form 0.5
parts per
million (ppm) to 50000ppm, preferably 30-1000 ppm or more preferably 30-
300ppm.
The leachate solution is preferably a cyanide solution.
The various processes by which the macroporous resin and leachate may
contacted are known to those of ordinary skill in the art. These methods
include but are
not limited to resin-in-leach, resin-in-pulp, or column. These methods are
merely listed
as examples and do not intend to further limit the invention. Any method of
contacting
known to those of skill in the art would be envisioned under this invention.
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Once contacted with the macroporous resin, the gold is then adsorbed to the
resin
and becomes separated from the leachate. To recover the gold from the
leachate, the gold
is then eluted from the resin.
It is believed that another advantage of the invention is that a single stage
elution
should yield high quality gold solutions, eliminating the need for further
processing by
costly split elution. The gold may be eluted form the macroporous resin by
contacting
the gold/resin complex with an eluate such as acidic thiourea. The macroporous
resin in
the invention does not catalyse the decomposition of thiourea, thereby
creating no
significant degradation and excellent elution characteristics. The invention
therefore
provides advantageous gold selective ion exchange resins which can be eluted
effectively
without the resin becoming poisoned or fouled to any appreciable extent.
EXAMPLES
EXAMPLE 1: PREPARATION OF FEED SOLUTIONS
The resin loading test work was conducted using two synthetic gold cyanide
leach
solutions. Both solutions contained the same concentrations of gold, silver
and base
metals (zinc, nickel, cobalt, iron and copper). The concentrations of un-
complexed, free
cyanide were 20 mg/L for Solution 1 (see Table 1) and 110 mg/L for Solution 2
(see
Table 1). The solutions were prepared by dissolving the required amounts of
the metal
salts in de-ionized water, and making up the volume to 80 liters. The
solutions were then
adjusted to approximately pH 11 with sodium hydroxide before use for testing.
Table 1
REF. # pH CN Free Gold Iron Zinc
Nickel Copper Silver
Solution 1 11.0 144 2.33 10.50 _ 19.30 4.93 19.93
1.08
Solution 2 11.0 56 2.25 10.60 _ 18.90 4.89 20.30
1.06
Solution 3 11.0 200 5.10 8.60 8.50 14.20 18.60
NA
The target and actual concentrations of metals and free cyanide of the two
feed solutions
are listed in Table 2.
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Table 2: Synthetic gold cyanidation solutions for resin testing
Metals Metai Reagent 1.7sett.
C.0114X311t2Itt*(01407)
and Cyanide Nam Amount Target Actual
CN(F) Speiee 80 I-
Feed Solution
Au Au((N)2 KAint.24), 3.35 0.27 13 2.25
Ag AWN), g(N), 1.85 0.15 1.0 1,06
_
Zs ZaitCt,), ZnS0, H-0 M.9 4.39 20 18.9
Ni - ...-
Ni(CN), NINO, f:iho n.4 1.79 5.0 4.89
Co CotCN), Coti(), 711,0 9 94 0.76_ 2 0
2.13
Fe FeiCno KõPc((.N)e 46 6.05- -10 -
10.8
Cu QgcN), 2.25 20 203
_
CN(F) (7N
NaCTI _ 152 1220 20 56*
Feed Solution 2
Ac Au(CN)2 1r, 0.27 3 .,_2.3
t, 145 Ø15 ..c 171-$
it :NcNi: ti.5 9 19.3
.. 7 .. _ ............. .
NO,CNL (l1410 :2 4 1.79 ,0 4.93
co CotrN12 CoSQ, 7kizO 9 '4 0,76 2,0 2.11
1.ekt-Nts IcTe(C7N). 3 Up 75.6 6.05 10 10.5
Cu Cit(C1,4, Coal 28.2 125 20 19,9
CN(F) . CN NaCN 322 22.78 no 144.
Cto determined by ciliation with AgNO, (including CN associated with Zn)
EXAMPLE 2: RESIN ADSORPTION TESTWORK
Resin adsorption was carried out by contacting 7mL of each resin sample (in
the CN-
form) with both feed solutions. At solution-to-resin volume ratios of 200-to-1
and 1000-
to-1, for 24 hours. The loading tests at solution-to-resin volume ratios of
200 (1.4 L
solution) were conducted in a rolling bottle, while those at the 1000-to-1
solution-to-resin
volume ratio (7 L solution) were carried out in an agitated pail. Following
loading, the
loaded resin was recovered, dried and prepared for analysis of Au, Ag, Cu, Fe,
Ni, Co
and Zn by SGS Minerals Laboratory Method Number 9-8-50. A sample of the barren
solution was submitted for the same analyses, plus a cyanide titration using
silver. The
results are summarized in Tables 3 and 4. Dow XZ-91419 resin is a commercially
available product from The Dow Chemical Company. DOW HSGR is a hyper-selective
gold resin of the present invention that is a strong base anion exchange resin
prepared
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,
,
,
according to the process disclosed herein and functionalized with a
trimethylamine.
AURIX is a commercially available strong base anion exchange resin from
Cognis.
Table 3
*DOWEX and AMBERLITE are Trademarks of The DOW Chemical Company
RESIN TEC SSC Capacity
(g/L) Select. Select Select
Feed Solution (Tbl 1) (eq/L) (eq/L) gold Iron zinc
nickel copper Au/all Au/Cu Au/Cu+Fe
Dow XZ-91419
(Solution 3) 0.30 0.30 4.62 _ 2.32 16.15 3.09
1.42 0.201 3.26 1.24
Dow HSGR
(Solution 2) 0.09 0.09 _ 5.72 0.36 0.62 0.77
0.03 3.200 168.09 14.51
, _
Dow HSGR
(Solution1) 0.09 0.09 6.03 0.09
0.34 1.04 0.01 4.087 602.90 60.29
Table 4
Equilibrium Resin Loading Capacity mg/L
Low Free Cyanide 56 ppm 200:1 fluid to resin vol ratio Solution 3- Table 2
All
Au Ag Cu Fe Ni Co Zn
Metals
AURIX 1202 527 5600 1098 2059 187 7774 18447
XZ-91419 1273 536 7818 461 2593 444 10405 23530
_
HSGR 1524 524 1172 414 959 <200 32.41 4625
Low Free Cyanide 56 ppm 1000:1 fluid to resin vol ratio Solution 1-Table 2
All
Au Ag Cu Fe Ni Co Zn
Metals
AURIX _ 4130 778 4559 1743 2977 375
9655 . 24217
XZ-91419 4400 600 4286 1712 3143 <200 14286 28427
HSGR 5749 643 4396 2029 1792
237 4396 19242
High Free Cyanide 144 ppm 200:1 fluid to resin vol ratio Solution 2 - Table
2
All
Au Ag Cu Fe Ni Co Zn
Metals
AURIX 1241 522 4337 604 1993 390 7741 16828
XZ-91419 1277 535 6526 502 2415 508 10375 22138
HSGR 1541 512 <20 337 693 74 3096 6253
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'
High Free Cyanide 144 ppm 1000:1 fluid to resin vol ratio Solution 2- Table
1
,
All
Au A9 Cu Fe Ni _ Co Zn
Metals
AURIX 4378 757 1892 2757 3108 <200 7297 20189
XZ-91419 4619 737 1417 2324 3089 <200 16154 28340
HSGR 6029 938 <20 <500 1038 <200 335
8340
