CA2736680C - Process for the recovery of gold using macroporous resins - Google Patents

Abstract

Provided is a process for separating gold from a leachate comprising:
providing a leacheate wherein the leachate comprises gold; 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 separating the gold from the leachate by contacting the leachate with the macroporous resin such that the gold is adsorbed to the macroporous resin.

Description

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 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.

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 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.

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.

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.

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.

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 , , , 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 _ 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 High Free Cyanide 144 ppm 200:1 fluid to resin vol ratio Solution 2 - Table All Au Ag Cu Fe Ni Co Zn Metals HSGR 1541 512 <20 337 693 74 3096 6253 ' High Free Cyanide 144 ppm 1000:1 fluid to resin vol ratio Solution 2- Table , 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

Claims (7)

1. 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) separating the gold from the leachate by contacting the leachate with the macroporous resin such that the gold is adsorbed to the macroporous resin.

2. The method of claim 1 further comprising eluting the gold from the macroporous resin by contacting the macroporous resin containing the gold with an eluant.

3. The method of claim 2 wherein the eluant is acidic thiourea.

4. The method of claim 1 wherein the surface area of the macroporous resin is from 800-950 m2/g.

5. The method of claim 1 wherein the amount of gold in the leachate is from 30 to 300ppm.

6. The method of claim 1 wherein the alkylamine functional groups is an alkylamine selected from the group consisting of tributyl n-amine, isopropyldimethyl amine, triethylamine, tripropylamine, dimethylamine, trimethylamine and dimethylethanolamine.

7. The method of claim 1 wherein the alkylamine functional groups are trimethylamine.