Fourier-transform infrared spectrophotometric study of adsorbed aurocyanide species on activated carbon

Hydrometallurgy, 31 ( 1 9 9 2 ) 111-120

111

Elsevier Science Publishers B.V., A m s t e r d a m

Fourier-transform infrared spectrophotometric study of adsorbed aurocyanide species on activated carbon M.D. Adams Mintek, Process Chemistry Division, Randburg, South Africa (Received February 22, 1991; revised version accepted July 27, 1991 )

ABSTRACT Adams, M.D., 1992. Fourier-transform infrared spectrophotometric study of adsorbed aurocyanide species on activated carbon. Hydrometallurgy, 31 : 111 - 120. Evidence from Fourier-transform infrared spectrophotometric studies confirms that aurocyanide species adsorb onto activated carbon without undergoing chemical change. However, the results do not enable any distinction to he made between aurocyanide in the form of M n+ [ A u ( C N ) £ ]n ion pairs and Au(CN)~- adsorbed onto ion-exchange sites. In spectra of each of the Li +, K +, Cs + and Ca 2+ salts of aurocyanide loaded onto carbon, a CN stretch band occurred at 2140 cm -~, which is characteristic of the Au (CN)~- ion. No bands due to AuCN (2200 to 2250 cm-~ ) were observed. Cs+Au(CN )y displayed the 2140 cm-~ band, as well as a band at 2156 cm -~, due to the CsAu(CN)2 salt, which becomes the dominant species after the carbon has been dried at 120°C. The presence of crystalline CsAu(CN)2 on the dried carbon was confirmed by scanning electron microscopy.

INTRODUCTION

The adsorption of aurocyanide onto activated carbon has become an allimportant aspect of gold processing, with the current widespread use of the carbon-in-pulp (CIP) process. However, the mechanism of adsorption and the nature of the adsorbed species are issues that are only now beginning to be understood. There are two schools of thought regarding the adsorption mechanism, each of which may be further subdivided as follows: ( 1 ) Adsorption of aurocyanide without chemical change: (a) as M n+ [Au(CN)~- ]n ion pairs [1-4]; (b) via ion exchange of A u ( C N ) y [5-7]. (2) Adsorption of aurocyanide with decomposition to other species: Correspondence to: M.D. Adams, M I N T E K , Private Bag X3015, R a n d b u r g 2125, South Africa.

0 3 0 4 - 3 8 6 X / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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M.D. ADAMS

(a) decomposition to AuCN [ 5,6,8,9 ]; (b) reduction of gold (I) either to metallic gold [10], or to a partially reduced state [ 2 ] between gold (I) and gold (0). The lack of agreement may be partly explained by the fact that different mechanisms have been shown [ 4,11 ] to operate under various conditions of ionic strength, pH value, and oxygen concentration. Most of the evidence indicates that, under the conditions pertinent to the operation of a CIP plant, the aurocyanide is adsorbed without undergoing chemical change. This evidence includes the results of direct techniques such as X-ray photoelectron spectroscopy (XPS) [9,12] and M6ssbauer spectroscopy [ 13,14]. Previous work [4,11 ] suggests that, under conditions of high ionic strength, such as are present in CIP plant solutions, aurocyanide adsorbs predominantly in the form of ion pairs; whereas under conditions of low ionic strength a greater proportion of the aurocyanide adsorbs onto ion-exchange sites. However, evidence for this statement is inferred from the results and the direct identification of the gold species by use of a technique such as Fourier-transform infrared spectrophotometry (FTIR) is desirable. The only reported infrared spectroscopic study of gold-loaded carbons [8 ] suffered from the drawback that the spectrum of the virgin carbon was not included for comparison. Further work by the same authors has shown that the spectral peaks of the virgin carbon are not reproducible (P.F. Van der Merwe, Univ. Stellenbosch, personal communication, 1988 ). This gives rise to problems in the assignment of v (CN) bands to Au ( C N ) y and AuCN, since these peaks are lower in intensity than most of the background peaks. Clark et al. [ 15] reported v(CN) values for carbons loaded to a very high concentration with CuCN, and concluded that the species was present in the form of a surface-stabilized Cu + Cu (CN) 7 species. The present paper presents the results of an infrared spectroscopic study of aurocyanide species loaded onto activated carbon. The results highlight the problems that are encountered in both experimental procedure and interpretation. EXPERIMENTAL

PROCEDURE

Reagents and chemicals The KAu (CN) 2 was supplied by Johnson Matthey (Pty) Ltd. All other chemicals were of analytical reagent grade, and all solutions were made up with deionized water. The activated carbon used was Le Carbone G210, which was purified by continual washing with cold deionized water. Le Carbone G210 is a granular activated carbon derived from coconut shells, with an average particle diameter of about 2 mm.

FTIR S T U D Y O F A U R O C Y A N I D E SPECIES O N A C T I V A T E D C A R B O N

1 13

Infraredspectrophotometry Samples of activated carbon for infrared spectrophotometry were prepared by the contacting o f 0.5 g of carbon with 25 ml o f solution (containing typically 0.003 mole of Au as K A u ( C N ) 2 and 0.03 mole of either LiC1, KC1, CsC1, or CaCI2) in a rolling bottle for 24 h. Carbon samples were crushed with a mortar and pestle prior to drying. Approximately 0.8 mg of this dried carbon powder was mixed with 200 mg o f dry KBr by manual shaking. This ratio of 0.4% carbon in KBr was chosen from the results shown in Fig. 1, for spectra 0.1°7"0 C in KBr

0.898

0.897 0.2% C in KBr 0.896

0.895 0.4% C in KBr

0.894

0.893

0.7°7o C in KBr 1.0% C i n KBr

3.0% C in KBr

2250

2200

2100

2000

W a v e n u m b e r , cm - I

Fig. 1. Effect of the concentration of activated carbon in KBr on the CN stretch band of KAu(CN)2 on activated carbon (transmission mode; Au on carbon 18.7%).

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M.D. ADAMS

obtained in transmission mode. At low concentrations of carbon in KBr, low absorbances due to Au (CN)~- were obtained; whereas, at high concentrations of carbon in KBr, the high absorbance due to the carbon matrix tended to mask the comparatively small absorbance due to Au (CN)~-. The carbonKBr mixture was made into a disk 13 m m in diameter by placing it under 7534 kg/cm 2 of pressure for about 10 s. The transmission mode was found to produce reasonable spectra, as shown in Fig. 1. The diffuse reflectance (DRIFT) mode was also tested at various concentrations of carbon in KBr, but without success. Infrared spectra of activated carbons were obtained with a Perkin-Elmer 1725X Fourier Transform infrared spectrophotometer. Extreme care was taken to minimize the disturbance of carbon dioxide and water-vapour levels in the instrument during measurement. For each spectrum, 200 scans were made at a resolution of 4 cm -~, and spectra were obtained by use of the instrument in the transmission mode. The accumulation of 1000 scans was found to have no noticeable effect on the signal-to-noise ratio of the spectrum. No mathematical smoothing functions were performed on the spectra; however, the ordinate of each spectrum was normalized to facilitate comparisons between spectra. All spectra were baseline-flattened so that spectral detail could be enhanced.

Scanning electron microscopy Scanning electron micrographs were obtained by use of a Jeol 840A scanning electron microscope. RESULTS AND DISCUSSION

To obtain absorption bands with significantly high signal-to-noise ratios, it proved necessary to load carbons to somewhat higher levels (10-20% Au) than those normally achieved in practice ( 1-2% Au). Jones et al. [ 12 ] have shown that a consistent adsorption isotherm is obtained over a range of between 2% and 60% Au, which includes both of these concentration ranges. This suggests that the results of the present investigation may also be relevant to lower loadings of gold on carbon. The XPS technique [ 15 ] has also been used to examine carbons loaded with gold cyanide over this wide range of concentration, and it is of interest to compare the results of the two techniques.

EfJect of gold concentration of the carbon The infrared spectra of carbons loaded from K A u ( C N ) 2 solution in the presence of excess KC1 are shown in Fig. 2. The background spectrum of the

FTIR S T U D Y OF AUROCYAN1DE SPECIES O N ACTIVATED CARBON

1 15

2140 0.924

0.920 13.2% Au

~= .Q

0.911

~

0.941

~.5% Au

0.947

No Au

0.946 J 0.950t 0.948 2300

t 2200

I 2100

I 2000

1900

Wavenumber, cm 1

Fig. 2. Infrared spectra showing the CN stretch bands of KAu(CN)2 orl activated carbon at various concentrations (Drying conditions: 25 °C in vacuo). virgin carbon is clearly evident in the spectra of the loaded carbons. The 2140 c m - 1 adsorption band (Tables 1 and 2 ), being characteristic of Au ( C N ) ~-, could be assigned to the KAu ( C N ) 2 salt ( 2141 c m - 1), the K + Au ( CN ) f ion pair, or Au ( C N ) ~- adsorbed onto ion-exchange sites. (The free aurocyanide absorbs at 2147 cm -1, and Au (CN)~- adsorbed onto anion-exchange resins absorbs at 2142 cm-1, as shown in Tables 1 and 2.) Morever, it is evident that similar spectra are obtained over this range o f loading concentrations. Scanning electron microscopy (SEM) did not reveal the presence of any precipitated K A u ( C N ) 2 salt on the carbon containing 13.2% gold, even after it was dried at 120°C overnight. The absence of any band in the 2250 to 2200 c m - 1 region suggests that no A u C N is present.

Effects of cation and drying conditions Infrared spectra o f an activated carbon loaded with aurocyanide from Cs + solution and dried under various conditions are shown in Fig. 3. The strong band at 2140 c m - 1 that is evident after drying at 25 °C decreases in intensity, compared with the band at 2156 cm -1, when the carbon is dried at 120°C. The 2156 cm -1 band is assigned to the solid C s A u ( C N ) 2 salt (see Table 1 ). The scanning electron micrograph o f the carbon that was dried at 120 °C is shown in Fig. 4. The material appearing as white particles in the macropores

M.D. ADAMS

116 TABLE 1 Infrared spectral data for solid aurocyanide salts Salt

u ( C N ) (cm - l )

Reference

HAu(CN)2

2146 2212 2152 2159 2154 2140 2141 2142 2159 2134 2139 2140 2165 2162 2142 2239 2209 2143 2136 2125

17 18 19 20 19,20 19 21 20 20 20 19 20 20 20

LiAu(CN)2 NaAu ( CN ) 2 KAu(CN)2

CsAu (CN) 2 NH4Au (CN) 2 Bu4Au (CN)2 Be[Au(CN)2]2 Ca[Au(CN)z]2 AuCN AgAu(CN)2 TIAn(CN)2

22 19 19

TABLE 2 Infrared spectral data for aurocyanide species in solution and adsorbed on ion-exchange resins Condition

u (CN) ( c m - l )

Reference

In aqueous solution: HAu(CN)2 KAu(CN)2

2147 2147

17 21

In 1-methyl-2-pyrrolidinone solution: NaAu(CN)2 2143 KAu(CN)/ 2142 Adsorbed on ion-exchange resin: Dowex A- 1 +Au(CN)~2138 IRA-400+Au ( C N ) £ 2142 Dowex X F - 4 1 4 9 + A u ( C N ) ~ 2142

19 19 21 20 20

was analysed by use o f energy-dispersive spectroscopy (EDS), and was confirmed to be solid CsAu (CN)2. The fact that this material was only observed in the carbon that was dried at 120°C, and not in the carbon that was dried at 25 ° C, is consistent with the infrared assignments. These results are consistent with those obtained by Duke et al. [23], who detected both adsorbed and crystalline K2Cu ( C N ) 3 on alumina at high loadings using FTIR and SEM

FTIR STUDY OF AUROCYANIDE SPECIES ON ACTIVATED CARBON

1 17

2156 0925 1

~2141

;\

o.~oh o.9'oF

\

\

o.92t

120°C

i 25 °C No Au

0.948 2300

I

2200

I

I

21O0 2000 Wavenumber, cm 1

1900

Fig. 3. Infrared spectra showing the CN stretch bands of CsAu (CN)2 on activated carbon after drying under various conditions (Au on carbon 18.9%).

Fig. 4. Scanning electron micrograph of activated carbon surface after loading with CsAu (CN) 2 and drying at 120°C (Au on carbon 18.9%).

118

M.D. ADAMS

techniques, with the crystalline KzCu (CN)3 becoming evident after the sample had been heated. The 2140 cm-1 band corresponds to the single band in the KAu(CN)2 spectrum, and can be assigned to the presence of aurocyanide ions. However, no further conclusions can be drawn regarding the nature of the adsorbed species. The fact that a band attributable to the solid salt phase was observed only in the case of CsAu (CN)2, suggests that this salt is less soluble than the other adsorbed species that were studied (LiAu(CN)2, KAu(CN)2, and Ca[Au(CN)2]2). The results for several aurocyanide species, drying conditions and gold loadings are presented in Table 3, which shows that, at a somewhat lower concentration of gold on the carbon, only one band, at 2140 cm-1, can be observed after the carbon has been dried at 120°C for 24 h. The absence of the band at 2156 c m - 1 (attributed to the solid CsAu(CN)2 salt) suggests that the formation of that salt occurs to a significant extent only at very high gold concentrations. A carbon loaded with aurocyanide from Li + solution and dried at 25°C and 120 ° C shows a strong band at 2142 c m - ~ in both of the spectra, which can be assigned to adsorbed aurocyanide. The spectra are also not easily attributable to the formation of the solid LiAu (CN)2 salt, since no precipitated gold-bearing particles were revealed by SEM. Moreover, LiAu (CN)2 is soluble [25] to the order of 7 mol/1 in water, and is very hygroscopic. A carbon was also loaded with aurocyanide from a Ca 2+ solution and dried at 25°C and 120°C. Once again, the spectra are dominated by a band at about 2143 c m - ~, which can be attributed to adsorbed aurocyanide. The presence of the band at 2156 cm-~, in the carbon that was dried at 25 °C (apparently due to the Ca [ Au (CN) 2 ] 2 salt ), is not easily explicable, since no precipitated maTABLE 3 Infrared spectral data for aurocyanide species adsorbed on activated carbon Species

Drying temperature

Gold on loaded carbon (%)

u (CN) ( c m - ~)

19.7 19.7 18.9 18.9 18.9 18.9 9.2 21.8 21.8

2142 2143 2140 2140 2140 2141; 2156 2142 2156 2143

(oc) LiAu(CN)2 KAu(CN)2 CsAu (CN)2

Ca [Au(CN)2]2

25 120 25 120 25 120 120 25 120

FTIR STUDY OF AUROCYANIDE SPECIES ON ACTIVATED CARBON

1 19

terial was detected using scanning electron microscopy. However, residual salt may have been present on the exterior of the particle. CONCLUSIONS

FTIR spectrophotometric evidence confirms that aurocyanide species adsorb onto activated carbon without undergoing chemical change. The results do not enable any distinction to be made between aurocyanide in the form of M n+ [ Au (CN) ~- ] n ion pairs and Au (CN) ~- adsorbed onto ion-exchange sites. In spectra of each of the Li+, K +, Cs +, and Ca 2+ salts of aurocyanide loaded onto carbon, a CN stretch band occurs at 2140 cm -1, which is characteristic of the Au(CN)~- ion. No bands due to AuCN (2200-2250 cm -~ ) were observed. Cs + Au ( CN ) ~- displays the 2140 c m - l band as well as a band at 2156 c m - 1, attributable to the CsAu (CN) 2 salt, which becomes dominant after the carbon has been dried at 120 ° C. The presence of crystalline CsAu (CN)2 on the dried carbon was confirmed by SEM. The FTIR technique, despite having some measure of success, is not ideally suited to the present application, as a result of the extremely high gold loadings required, and the poor signal-to-noise ratios that are generally obtained. Moreover, the interpretation of the results remains a problem. For example, negative shifts in frequency are encountered [25] for v(CN) values for both Hg (CN) 2 and KAg (CN) 2 on activated carbon, but not for M n + [ Au (CN) 2 ] on carbon. These drawbacks are similar in nature to those experienced in the application of other techniques, such as XPS, to the study of metal cyanides on carbons [ 16 ]. ACKNOWLEDGEMENTS

This paper is published by permission of Mintek. The author wishes to thank Mr. P. Ellis for running the scanning electron microscope.

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8 Van der Merwe, P.F. and Van Deventer, J.S.J., Chem. Eng. Commun., 65 (1988): 121138. 9 Cook, R., Crathorne, E.A., Monhemius, A.J. and Perry, D.L., An XPS study of the adsorption of gold (I) cyanide by carbons. Hydrometallurgy, 22 ( 1989 ): 171-182. 10 Grabovskii, A.I., Ivanova, L.S., Kovostysherskii, N.B., Shirshov, R.K., Stovozhuk, R.K., Matskevich, E.S. and Arkadakskaya, N.A., Zh. Prikl. Khimii, 49 ( 1976): 1379-1381. 11 Adams, M.D., The mechanism of adsorption of aurocyanide onto activated carbon, 1. Relation between the effects of oxygen and ionic strength. Hydrometallurgy, 25 (1990): 171184.

12 Jones, W., Klauber, C. and Linge, H.G., In: Proc. Perth International Gold Conference. Randol Int., Golden, COlo. (1988), pp. 243-248. 13 Cashion, J.D., McGrath, A.C., Volz, P. and Hall, J.S., Trans. Inst. Min. Metall. Sect. C, 97 (1988): 129-133. 14 Kongolo, K., Bahr, A., Friedl, J. and Wagner, F.E., Metall. Trans. B., 21B (1990): 239249. 15 Clark, J.H., Duke, C.V.A., Brown, S.J. and Miller, J.M., Spectrochim. Acta, 42A ( 1986): 811-814. 16 Klauber, C.,Surf. Sci.,203 (1988): 118-128. 17 Penneman, R.A., Staritzky, E. and Jones, L.H., J. Am. Chem. Soc., 78 ( 1956): 62. 18 Evans, D.F., Jones, D. and Wilkinson, G., J. Chem. Soc., 1964 (1964): 3164-3167. 19 Chadwick, B.M. and Frankiss, S.G., J. Mol. Struct., 31 (1976): 1-9. 20 Kyriakakis, G., Studies on Activated Carbon. MSc. Dissertation, Univ. Witwatersrand, Johannesburg, S. Africa (1984). 21 Jones, L.H. and Penneman, R.A., J. Chem. Phys., 22 (1954): 965-970. 22 Penneman, R.A. and Jones, L.H., J. Chem. Phys., 28 (1957): 169-170. 23 Duke, C.V.A., Miller, J.M., Clark, J.H. and Kybett, A.P., Spectrochim. Acta, 44A ( 1988): 1207-1213. 24 Adams, M.D., The Chemistry of the Carbon-in-Pulp Process. PhD. Thesis, Univ. Witwatersrand, Johannesburg, S. Africa ( 1989 ), 419 pp. 25 Penneman, R.A. and Staritzky, E., J. Inorg. Nucl. Chem., 7 ( 1958): 45-50.