Wednesday, 26 November 2008

Method of Gold Recovery from Thiosulfate Solution

W.T. Yen

Department of Mining Engineering Queen’s University, Kingston, Canada

Email: wy@mine.queensu.ca

K. J. Liu

DOWA Environmental Management Co. Ltd., Suzhou, China








ABSTRACT


Gold thiosulfate complex is difference with gold cyanide complex, which is easily adsorbed on activated carbon while gold thiosulfate complex cannot be satisfactorily recovered by activated carbon. However, Gold thiosulfate could be recovered by activated carbon if cyanide is added into the thiosulfate solution. To avoid the cyanide solution, this paper will review several possible processes to recover the gold from thiosuldate solution: such as the cementation with metals, ion exchange resin and solvent extraction. The gold cementation with the powders of zinc, copper, iron and aluminium will be discussed. The advantage and disadvantage of each are compared. The consumption of thiosulfate and copper and feasibility of solution recycling will be discussed. The gold recovery by some ion exchange resins will be suggested. The processing conditions, its applications and the resin elution methods will be presented. The gold recovery by using solvent extraction will be reviewed. The processing conditions of solvent extraction process are discussed and compared.



Keywords: Gold, Recovery, Thiosulfate, Cementation, Zinc, Copper, Iron, Aluminium, Ion Exchange, Solvent extraction, Electrowinning





INTRODUCTION


It was reported (Marsden, 2006) that agitated cyanide leaching accounted for approximately 50% to 55% of gold extraction around the world. The pressure to ban or limit the operation of cyanidation is increasing due to a negative public perception about the environmental impact of the cyanide toxicity. An increase in the research into alternative lixiviants has increased in last few decades. The alternative lixiviants include thiourea, chlorine, bromine, iodine, and thiosulfate etc. The ammonium thiosulfate is one of the most favored alternatives.



Before 1970’s, the zinc cementation (Merill-Crow Process) was the most popular technology to recover the gold from the gold- cyanide pregnant solution. In recent gold production operation, carbon-in-pulp (CIP) and carbon-in-leach (CIL) are the predominant recovery method in use, accounting for approximately 42% of worldwide production (Marsden, 2006). The Affinity of activated carbon towards all other gold complexes is difference. Gallagher et al. (1990) found that the effectiveness of activated carbon in removing gold complexes from aqueous solutions decrease in the following ligand order: SCN->SC(NH2)2>CN->> S2O32-. The gold thiosulfate complex has the lowest affinity to adsorb on the activated carbon. Kononova et al. (2001) found that the carbon adsorbents (charcoal, brown coal, anthracite, cocoa stone) do not have high exchange capacity for gold (no more than 5 mg Au/g), i.e., 15% -17% gold recovery. Jiexue and Qian (1991) recorded a 30% recovery of gold thiosulfate on activated carbon. Abbruzzese et al. (1995) found an increase in gold recovery from 22% to 43% when the carbon concentration was increased from 5 g/L to 60 g/L. Yen et al. (2002) reported that gold recovery from the thiosulfate solution was less than 20% by using 20 g/L or 40 g/L activated carbon. It requires 60 g/L carbon to recovery 95% gold with the loading of less than 2 mg Au/g, which is not feasible in the practical operation.



To circumvent the problem of low carbon affinity for gold thiosulfate, Lulham and Lindsay (1991) recommended to add nearstoichiometric amounts of cyanide to the gold thiosulfate liquor and then adsorb the newly formed gold cyanide complex onto carbon. The result demonstrated that more than 99% of gold was recovered from a solution contained 1.91 - 2.32 mg/L Au with 20 g/L carbon, leaving the barren solution contained 0.01 mg/L Au. It has to be noted that this process does not work with silver.

Excluding the carbon adsorption process, this paper will present other potential processes to recover the gold from the gold thiosulfate pregnant solution, such as metal cementation, ion exchange and solvent extraction processes.


METAL CEMENTATION


Cementation with zinc, copper and aluminium




Gold and silver can be recovered from the clarified copper-ammonia thiosulfate solution by the addition of finely divided zinc metal (Navarro et al., 2005; Arima, et al., 2003; Gelves et al., 1996; Ravaglia et al., 2000; Pappas, 1997; Panayoto et al., 1994; Berezowsky and Sefton, 1979), copper metal (Choo and Jeffrey, 2004; Perez and Galaviz, 1987; Ellwanger, 1986), and aluminium metal (Ravaglia and Barbosa, 2005; Arima et al., 2003). However there is limited information described the optimal conditions and the effects of the solution compositions.



The gold thiosulfat pregnant solution usually contains the components of S2O32-, NH3,

Cu(NH3) , [Au(S2O3)2]3- and [Cu(NH3)2]+. The overall reaction of gold cementation by zinc, copper and aluminium in the ammonium thiosulfate solution can be described by the following equations:





Above equations indicate that one mole of gold can be precipitated by one mole of zinc and also one mole of aluminium. But it needs three more of copper to precipitate one of gold. Again the fresh thiosulfate species, S2O32- is produced after the gold cementation with zinc and aluminium. There is no thiosulfate produced in the cementation of gold with copper.

Since there are thiosulfate, ammonia and copper ions in the pregnant solution, the actual amount of zinc, copper and aluminium required to precipitate the gold is much more than the stoichiometric amount. In a solution contained 8 mg/L Au, 1 mole/L NH4(OH), 0.01 mol/L CuSO4 and 0.4 mole/L (NH4)2S2O3 at pH 9.5. The effect of zinc, copper and aluminium powder (45 um) on the gold cementation is found as listed in Table 1. The metal/gold ratio (Zn/Au, Cu/Au, Al/Au) increases with the increase of ammonia and also copper concentration. The metal/gold ratio decreases with the increase of ammonium thiosulfate concentration.


Table 1 Effect of reagent concentration on metal/gold ratio requirement to recover gold







The comparison of three metal powders in the gold cementation is listed in the Table 2. The actual amount of zinc and aluminium required to precipitate 100% of the gold is 30 to 1. The ratio of 50 mole of copper to one mole of gold can precipitate 93% of the gold. It requires 200/1 ratio to recover 100% of gold by copper powder.

Table 2 Comparison of three metal powders in gold cementation.





Since the barren solution may be recycled to leaching circuit after zinc cementation, the thiosulfate concentration increases and copper concentration decreases to near zero. It requires to replenishing the copper concentration for recycling barren solution. The barren solution composition has changed after copper cementation, the thiosulfate concentration decreases and copper concentration increases, It requires to replenishing thiosulfate concentration for recycling barren solution. Both thiosulfate and copper concentrations decreases in the barren solution after aluminium cementation. It requires to replenishing both thiosulfate and copper concentration for recycling barren solution.



Cementation with iron


Yen et al. (2002) reported that more than 99% of the gold and silver are recovered from thiosulfate pregnant solution at pH 9.5 -11.0 with 4 g/L iron powder at 20°C. At lower pH range, i.e., 7.0 - 9.5, same amount of gold precipitation might obtained at temperature of 30°C. The co-precipitated copper is only 10% - 20%.



Xia (2007) reported the gold recovery by ferrous ion (ferrous sulphate solution).

ION EXCHANGE RESINS


Resin adsorption of gold


Gold and silver can be adsorbed on weak anion resins at very diluted thiosulfate concentrations (Wan et al., 1993). A strong base resin consisting of quatemary amine attached to a polymer backbone (e.g. polystyrene beads) is preferred rather than a weak base resin. The commercial strong base resin includes type I (triethylamine functional group) and type II (triethyl ethanolamine functional groups). The positively charged ammoniated site of aromatic nuclei has a function to adsorb anionic species. Generally strong base anion exchange resin should be conditioned to chloride form. The aurous thiosulfate complex is exchanged with chloride ion of anion resin:

3[CH2N(CH3)3]+Cl- + [Au(S2O3)2]3- — [CH2N(CH3)3]+ [Au(S2O3)2]3- + 3Cl- (4)



Table 3 summarizes the gold recovery from thiosulfate pregnant solution by using both weak and strong base anionic resins. It is obvious that the gold recovery from the strong base resin is higher than that from weak base resin. Also the gold recovery in a lower pH (below 9.5) is better than at a higher pH (above 11.0).



Nicol and O’Malley (2002) reported that the thiosulfate decomposed products, trithionate (S3O62-) and tetrathionate (S4O62-), strongly compete the active sites on the resin surface and greatly reduced the gold loading and recovery. Thiosulfate and sulphate ion have the least effect. The cationic species, Cu and Zn do not adsorb on the resin while lead and gold ions having strong affinity to adsorb on the resin.



Elution of gold


The aurous thiosulfate on a resin surface cannot be eluted by conventional hydrochloric acid due to the strong bonding. Table 4 shows the level of relative selective coefficient for various anionic species, which are potential eluant for gold thiosulfate complex. Table 3 shows that nitrate was used as an eluant by O’Malley (2001) and chlorate was successfully used by Arima et al. (2003).



Kononova et al. (2001) used 0.5 M thiocarbamide and 0.5 M sulphuric acid to elute the gold from the resin. Fleming et al (2001) used the trithionate and tetrathionate to elute the resin. Thomas et al. (1998) and Flaming et al. (2001) suggested two stages elution of copper and gold. In the first stage, ammoniacal ammonium thi- solution (ca. 100-200 g/L, 5-6 bed volume) or oxygenated ammonia buffered by ammonium sulphate solution was used to elute the copper. Gold was than stripped from the resin with a solution of ammonium, potassium calcium thiocyanate solution (ca. 100-200 g/L, 5-6 bed volume).


Table 4 Relative selectivity coefficient of strongly base anion exchange resins(Marsden and House, 1992)







SOLVENT EXTRACTION


extraction of gold


Alkyl phosphorus esters and amines were used for the extraction of gold from thiosulfate solutions (Zhao et al., 1997, 1998, 1999). Table 5 shows that 95 % of gold can be extracted with the combination of alkyl phosphorus esters and trialkylphosphine oxide at pH higher than 10 (Zhao et al., 1997). While 96%-97% of gold could be extracted with the combination of primary amine (N1923) with alkyl phosphorus esters (Zhao et al., 1998) or trialkylamine oxide (Zhao et al., 1999) at pH below 8 or 9. Kerosene and octane are used as diluent. Since many of optimal thiosulfate leaching pH range is 9 -10, the solution pH may be required some adjustment for the solvent extraction of gold with esters and amines.

Table 5 Solvent extraction of gold from thio sulfate solution




Liu et al. (2004) reported that more than 99% of gold could be extracted with trioctylmethylammonium chloride (TOMAC) at a pH range of 5.5 and 10.5, which is the optimal pH range for thiosulfate leaching. Thus, the solution pH is not required further adjustment. Octane is preferred as diluent. The gold extraction is not sensitive to the concentrations of copper and ammonia but sensitive to the gold concentration. A 0.18 mole/L TOMAC solution diluted with octane could extract more than 99% of gold from a thiosulfate solution containing 0.025x10-3- 1.65x10-3 mole/L Au. The extraction time is only 10 minutes. A higher concentration of TOMAC is required for a higher concentration of gold in the leach Sierakoski (2001) have patented the extractant of guanidine compound, quaternary amine and phenol to extract the gold in a pH range of 8-10. Unfortunately the metallurgical data was not available.


Solvent extraction of Ag, Cu, Pb and Zn


The thiosalfate leach solution contains not only gold but also silver and some base metals, such as Cu, Zn. All these species may also be extracted by solvent extractants. Table 6 shows that the N1923 and TRAO could extract 95% of gold along with 32% of silver and 12% of Zn, <2% of copper. While TOMAC extracts 99.9% of gold in pH range of 6-10.3 and 99.5% in pH3. The silver is extracted at the low pH range, i.e., 100% Ag extraction at pH 3 and 36.3% of silver extraction at pH 6. At pH 6, 14.4% of copper and 64.5% of lead are also extracted. In the alkaline solution, there is no silver extraction but 72.5% of lead and 3%-6% of copper are extracted. TOMAC do not extract zinc from the thiosulfate solution.

Table 6 Selective extraction of Au, Ag, Cu, Pb, Zn from thiosulfate solution


ELECTROWINNING


Aurothiosulfate ions in the leach solution will migrate to the cathode and form a metallic gold deposit (Gallagher et al., 1989, Abbruzese et al 1995, McPartland and Bautista 1999). The electro reduction of gold thiosulfate to gold is kinetically faster than the reduction of gold cyanide to gold (Sullivan and Kohl, 1997). Gold thiosulfate is reduced at -0.15 V vs SCE and is independent of pH for values of above 4. However the electrowinning is especially problematic in the presence of a great excess of unwanted cupric and cuprous ions. Also, the oxidation and reduction reactions of S2O32- occur at the anode and cathode surfaces. Thus, electrowinning process does not appear to be an attractive option for gold recovery.


CONCLUSIONS


After liquid-solid separation, the gold is recoverable by metal cementation or solvent extraction processes. De-aeration is not required in metal cementation. At Zn/Au ratio of 30, 100% of gold is recovered and the barren solution can be recycled with replenished with copper concentration. More copper powder is required for gold cementation. The choice of solvent to extract gold is esters, amines and TOMAC. TOMAC may extract the gold directly from the optimal thiosulfate leaching solutions. Ion exchange process can extract the gold from either slurry or clear solution with strongly base anion resins. The chlorate is a better choice to elute the loaded resin. The gold can be recovered from the eluted solution by electrowinning or metal precipitation.



Acknowledgements: Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged.



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2 comments:

Raja Das said...

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Raja Das said...

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