AU7889500A - The elution of gold from anion exchange resins - Google Patents

Description

WO 01/23626 PCT/AUOO/01203 TITLE "THE ELUTION OF GOLD FROM ANION EXCHANGE RESINS" FIELD OF THE INVENTION This invention relates to an improved process for the elution of gold from anion exchange resins. More particularly, the improved process of the present invention may be used in the recovery of gold from thiosulphate leach pulps and solutions. BACKGROUND ART Precious metals, including gold, have traditionally been leached from ore using cyanide-containing solutions. The precious metal is then recovered from that solution or pulp. Importantly however, several factors have made cyanide unsuitable for precious metal recovery. The most important of these are the increasing environmental concerns with the use of cyanide and the increasing proportion of so-called refractory gold ores which do not respond favourably to cyanidation. For these reasons, alternative lixiviants such as thiosulphate have been proposed. The conventional method for the recovery of gold from cyanide leach pulps is based on the use of activated carbon granules. However, activated carbon does not adsorb gold from thiosulphate solutions or pulps. For this reason, ion exchange resins have been suggested as alternatives to activate carbon. If looking to recover gold from a thiosulphate leach pulp using an anion exchange resin where that leach pulp also contains copper which is used as a catalyst in the leaching process, a number of particular problems arise. For example, known procedures for the elution of gold thiosulphate from resins involve the use of 300 g/L ammonium thiosulphate or 200 g/L potassium thiocyanate. Copper has been shown to be selectively eluted using initially 200 g/L ammonium thiosulphate solution followed by elution of the gold using 200 g/L potassium thiocyanate.

WO 01/23626 PCT/AUOO/01203 -2 These eluants are relatively costly. Eluant concentrations at this level are relatively high and may prove uneconomical in a commercial plant setting. A major problem with thiosulphate leaching is the high concentrations of thiosulphate needed and the high rate of lixiviant losses during a leach. The thiosulphate can be lost from solution through oxidation to tetrathionate, complexation with other noble metal ions or solution losses to the tails, which adds to the cost of the process. The improved process for the recovery of gold from anion exchange resins of the present invention has as one object thereof to overcome the above-mentioned problems of the prior art. The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. DISCLOSURE OF THE INVENTION In accordance with the present invention there is provided an improved process for the recovery of gold from anion exchange resins the method comprising the step of: * Eluting with a nitrate salt to displace gold from the resin. Preferably, the gold has been loaded on to the resin from a thiosulphate leach solution or pulp.

WO 01/23626 PCT/AU00/01203 -3 Preferably, the nitrate salt is provided in the form of ammonium nitrate. Still preferably, the ammonium nitrate is provided at a concentration of about 2M (160 g/L). Where the method of the present invention is conducted in the presence of copper, before eluting with a nitrate salt the method preferably comprises the additional step of: Exposing of the resin to excess oxygenated ammonia which displaces the copper, loaded on the resin as copper (I) thiosulphate, as copper (11) ammine. Preferably, a buffer is provided to maintain pH at about 9.2. This buffer is preferably ammonium sulphate. In accordance with the present invention there is further provided a process for the recovery of gold from thiosulphate leach solution or pulp, the method comprising the method steps of: * Loading gold from a thiosulphate leach solution on to an anion exchange resin; " Eluting with a nitrate salt to displace gold from the resin; and * Recovering thiosulphate from the leach solution or pulp using a resin. The method may further comprise the method step of recycling the recovered thiosulphate to a leaching process from which the thiosulphate leach solution was obtained. Preferably, the elution of thiosulphate from the resin is performed with either a nitrate or sulphate solution.

WO 01/23626 PCT/AUOO/01203 -4 The resin is preferably either a gel type resin or a macroporous type strong-base anion exchange resin. BRIEF DESCRIPTION OF THE DRAWINGS The improved process of the present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which: Figure 1 is a graphical representation of the elution of gold from a commercial strong base anionic exchange resin by ammonium nitrate, ammonium thiocyanate and ammonium thiosulphate; Figure 2 is a graphical representation of the elution of gold from the same resin by two concentrations of ammonium nitrate in accordance with the present invention; Figure 3 is a graphical representation of gold elution from the same resin by different nitrate salts in accordance with the present invention; Figure 4 is a graphical representation of the total recovery and separation of copper, then gold, for a commercial gel type resin (A) and a commercial macroporous resin (B); Figure 5 is a graphical representation of the elution of thiosulphate from resin by either a nitrate or sulphate solution in accordance with a further embodiment of the present invention; Figure 6 is a graphical representation of the equilibrium loading of gold onto the resins (A) and (B) in the presence of varying ammonium nitrate concentrations; Figure 7 is a graphical representation of consecutive equilibrium loadings onto resin with a simulated leach solution containing gold, silver and copper; WO 01/23626 PCT/AUOO/01203 -5 Figure 8 is a graphical representation of the rate at which gold is eluted from resins (A) and (B); and Figure 9 is a graphical representation of the elution of copper and gold from a loaded resin from a small scale resin-in-pulp operation by ammonium sulphate and ammonium nitrate solutions respectively. BEST MODE(S) FOR CARRYING OUT THE INVENTION The improved process for the recovery of gold from anion exchange resins of the present invention will now be described with reference to a preferred embodiment. However, it is to be appreciated that the following description of a preferred form of the invention is not to limit the above generality of the invention. The strong base anion exchange resins used in the embodiment include a gel type resin (A), being Vitrokele 911, and a macroporous resin (B), being Amberjet 4200. The resins used were of a particular size fraction, being 0.85<x<0.6 mm in diameter. The inventors have found that gold loading rates onto each of these resins is similar and followed a simple first-order rate equation for absorption to equilibrium. The kinetics of loading of gold onto these resins is largely unaffected by the presence of thiosulphate. After trials of methods of the prior art and several other known eluants, nitrate salts have been found to be the preferred eluant for gold. A concentration of 2M (160 g/L) ammonium nitrate was determined to be the most efficient at eluting the gold. Figure 1 shows results for tests of the gold elution characteristics of ammonium nitrate, ammonium thiocyanate and ammonium thiosulphate. A result of 99% recovery of gold was achieved. 1 M (80 g/L) ammonium nitrate was also found to account for 99% of the gold. However, the elution curve was broad compared to that of the 2M ammonium nitrate. Figure 2 shows the results for the elution of gold from the gel resin (A) by the two concentrations of ammonium nitrate.

WO 01/23626 PCT/AUOO/01203 -6 To confirm that the active species is the nitrate ion, a number of nitrate solutions of the same concentration, but from different salts were used, as shown in Figure 3. It is apparent that it is the nitrate ions in solution which are eluting the gold from the resin and the cations of the salt play no part in the elution process. The presence of copper in the system was determined to be problematic as copper simultaneously loads onto the resin and does not selectively elute off the resin. For example, it was found that treatment with a lower concentration of ammonium nitrate (0.1M) did not elute any of the copper or gold off the resin. Further, treatment with 0.5M ammonium nitrate eluted 70% of the copper and 15% of the gold. Whilst this demonstrates some selectivity, gold was lost in the initial strip (with 0.5M ammonium nitrate) and there was carry over of copper into the final strip for the gold using the 2M ammonium nitrate. The inventors have consequently observed that the copper thiosulphate complex behaves on the resin in a similar manner to the gold thiosulphate complex. A clue to the solution for the above problem may be found in the chemistry of the leaching of gold with thiosulphate in the presence of copper and ammonia. It is to be noted that the exact mechanism of this process is presently uncertain. However, it is known that there is a cyclic system that reverts from copper (1l) to copper (1) and which promotes the dissolution of gold. The copper (11) ammine drives the cathodic reaction for the dissolution of gold. As such, it should be desirable to have as high a concentration of copper (11) ammine present within a leach solution. However, a high copper (II) ammine concentration would result in increased degradation of thiosulphate. The most likely cycle of copper during the dissolution of gold in a thiosulphate leach solution is copper (II) ammine to copper (1) thiosulphate. Copper (II) ammine is the more stable complex in aerated solutions. As such, the copper (1) thiosulphate that loads onto the resin should be displaced off the resin by converting back to the copper (II) ammine complex with the addition of excess oxygenated ammonia solution. Figure 4 shows the separation of copper and gold for both resins under this process.

WO 01/23626 PCT/AUOO/01203 -7 A buffer of ammonium sulphate is desirable when ammonia is added to the system so as to maintain the desired pH at 9.2. Accordingly, once gold and copper have loaded onto the resin, the process of the present invention is able to selectively elute these species by initially displacing the copper as the copper (1l) ammine with ammonia, then finally eluting with ammonium nitrate for the displacement of the gold. A means to minimise the loss of thiosulphate during thiosulphate leaching may be to recover as much of the thiosulphate from the leach solution before it leaves to the tail, and then to recycle it back into the leaching process. Resin could be used to absorb the anionic species (which would include all the poly-thionate compounds) from leach solutions. The elution of thiosulphate from the resin could be performed with either a nitrate or sulphate solution, see Figure 5. Since the eluted thiosulphate would be reused in the leaching process, ammonium sulphate would be the prefered eluant since nitrate ions may prevent gold loading onto resin. Another good reason for using ammonium sulphate is that it would not elute any gold so any absorbed anions other than gold would be eluted prior to using nitrate solution for gold. To obtain a thermodynamic understanding of the elution process, equilibrium loadings of gold in the presence of ammonium nitrate were performed on the two resins (A) and (B). Both resins gave similar gold loadings for a particular ammonium nitrate concentration and the data in Figure 6 contains the results from both resins. The results show that nitrate ions are very good at preventing the gold from loading onto the resin. The results also show that as the ammonium nitrate concentration decreases the gold loading increases greatly, indicating that nitrate ions absorbed to the resin after elution could have minimal effect on the gold loading onto the resin if reintroduced to the absorption process. Therefore there is no need for regeneration of the resin from the nitrate form.

WO 01/23626 PCT/AUOO/01203 -8 To examine the effect of consecutive loading/elution cycles on equilibrium loadings a simulated leach solution was prepared consisting of 0.01 M trithionate, 0.05 M thiosulphate, 0.2 M ammonia at pH 9.5. Trithionate was added to the solution to mimic a real pulp solution. To this solution 10 ppm Cu, 10 ppm Ag or 10 ppm Au was added to investigate the effect of copper and silver on the loading of gold. Although it is known that gold, silver and copper will load onto the resin it was considered desirable to investigate the capacity of the resin after many cycles in order to assess how practical a resin-in-pulp (RIP) process would be for the recovery of gold from thiosulphate leach solutions. In the elution cycle the resin was initially eluted with 0.5 M ammonia buffered with 1 M ammonium sulphate, then further eluted with 2 M ammonium nitrate. The ammonia solution was buffered to prevent any precipitation of metal complexes onto the resin during elution and to recover the loaded thiosulphate. The copper that was loaded as the copper(l) thiosulphate [Cu(S 2 0 3

)

2

]

3 was eluted by converting it to the copper(II) tetrammine [Cu(NH 3

)

4

]

2 + with the addition of excess oxygenated ammonia solution. The gold was then eluted with a nitrate solution. The resin was then dried and the loading and elution cycles repeated. Evaluation of the elution data is presented in Table 1. Although the conditions were not optimised for the silver and gold elution, the low overall recovery for silver and gold would suggest some precipitation of these metal ions onto the resin surface over time. It can also be noted that there will be some carry over of gold and silver back to the leaching circuit in the ammonia eluate. Table 1. Averaged metal recovery over eight cycles. Metal ion Ammonia elution Nitrate elution Overall recovery Copper 98 8 110 Gold 2 91 93 Silver 12 65 77 WO 01/23626 PCT/AUOO/01203 -9 The equilibrium loadings from eight cycles showed very little difference in the equilibrium loading for any metal ion, see Figure 7. Although there was no effect of elution on the equilibrium loading, the resin darkened when cycled. At the conclusion of these experiments, some of the resin was washed in nitrate solution and some in thiosulphate solution (2M), both were then fire assayed to determine what was precipitating onto the resin, see Table 2. The fire assay results showed there was about 7000 ppm silver and about 3000 ppm gold. There was very little copper precipitated on the resin. The thiosulphate removed 99 % of the silver, 70 % of the copper and 30 % of the gold. Therefore, it is apparent that the resin may be successfully cleaned with thiosulphate when needed. Table 2. Fire Assay of resin for gold, silver or copper. Experiment Metal ion Concentration Concentration Average (ppm) (ppm) Concentration Nitrate Copper 111 111 111 Elution Gold 2760 2980 2870 Silver 6640 6710 6675 Thiosulphate Copper 38 38 38 Elution Gold 1790 1820 1805 Silver 38 50 44 To improve the understanding of the rate at which the gold is eluted from the resin, a loaded resin was added to a solution of ammonium nitrate and samples taken over time. Both resins, the macroporus resin (A) and the gel resin (B) gave similar elution profiles. There was an initial fast elution of the gold from the resin followed by a slower elution, see Figure 8. This slower elution is expected as the nitrate solution now needs to diffuse into the pores of the resin before releasing the gold into solution. The rate of elution was very fast for 1.0 and 2.0 M ammonium nitrate with both solutions reaching the expected equilibrium loading calculated from the thermodynamic study. The macroporus resin (B) eluted faster since having greater rate of prenetration of the eluant into the pores of the resin.

WO 01/23626 PCT/AUOO/01203 -10 A fixed-bed elution of some of the resin from the counter-current adsorption run showed that there is fast and effective elution of copper and gold when using the elution cycle of 1 M ammonium sulphate and 2 M ammonium nitrate solutions respectively, see Figure 9. The volume of solution required by either eluant was low with only 5 bed volumes necessary to elute 95 % copper and 98 % gold from the resin. It is envisaged that the process of the present invention may be used to ultimately provide a cost effective and viable process for the recovery of gold from thiosulphate leach pulp. Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims (12)

1. A process for the recovery of gold from anion exchange resins, the method comprising eluting with a nitrate salt to displace gold from the resin.

2. A process according to claim 1, characterised in that the gold has been loaded on to the resin from a thiosulphate leach solution or pulp.

3. A process according to claim 1 or 2, characterised in that the nitrate salt is provided in the form of ammonium nitrate.

4. A process according to claim 3, characterised in that the ammonium nitrate is provided at a concentration of about 2M (160 g/L).

5. A process according to any one of the preceding claims, where the method is conducted in the presence of copper, characterised in that before eluting with a nitrate salt the method comprises the additional step of exposing the resin to excess oxygenated ammonia which displaces the copper, loaded on the resin as copper (I) thiosulphate, as copper (II) ammine.

6. A process according to claim 5, characterised in that a buffer is provided to maintain pH at about 9.2.

7. A process according to claim 6, characterised in that the buffer is ammonium sulphate.

8. A process for the recovery of gold from thiosulphate leach solution or pulp, the method comprising the method steps of: * Loading gold from a thiosulphate leach solution on to an anion exchange resin; * Eluting with a nitrate salt to displace gold from the resin; and WO 01/23626 PCT/AUOO/01203 -12 Recovering thiosulphate from the leach solution using a resin.

9. A process according to claim 8, characterised in that the method further comprises the method step of recycling the recovered thiosulphate to a leaching process from which the thiosulphate leach solution was obtained.

10.A process according to claim 8 or 9, characterised in that the elution of thiosulphate from the resin is performed with either a nitrate or sulphate solution.

11.A process for the recovery of gold from anion exchange resins substantially as hereinbefore described with reference to the examples.

12.A process for the recovery of gold from thiosulphate leach solution or pulp substantially as hereinbefore described with reference to the examples.