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Models for the adsorption of aurocyanide onto activated carbon. Part II: Extraction of aurocyanide ion pairs by polymeric adsorbents
HydrometaUurgy, 18 (1987) 139-154
139
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Models for the Adsorption of A u r o c y a n i d e onto A c t i v a t e d Carbon. Part II: Extraction of A u r o c y a n i d e Ion Pairs by P o l y m e r i c Adsorbents M.D. ADAMS*
Council for Mineral Technology, Private Bag X3015, Randburg 2125 (South Africa)
G.J. McDOUGALL
NationalChemicalProducts, P.O. Box344, Germiston1400(SouthA~ica)
and R.D. HANCOCK
University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001 (South Africa) (Received March 30, 1985; accepted in revised form September 3, 1986)
ABSTRACT Adams, M.D., McDougall, G.J. and Hancock, R.D., 1987. Models for the adsorption of aurocyanide onto activated carbon. Part II: Extraction of aurocyanide ion pairs by polymeric adsorbents. HydrometaUurgy, 18: 139-154. The adsorption of aurocyanide onto activated carbon has been known but not fully understood for many years. This is due to the difficulty involved in studying the nature of the adsorbed gold cyanide species on activated carbon by conventional techniques of chemical investigation such as infrared and ultraviolet-visible spectroscopy. A novel approach using model systems was adopted by the authors to shed further light on the adsorption mechanism. This study reports on the extraction of gold cyanide by polymeric adsorbents and ion-exchange resins and demonstrates that, as in the case of ion-pair solvent extraction (Part I), the adsorptive behaviour of Au(CN) 2 in the presence of various cations onto polymeric adsorbents is analogous to the adsorptive behaviour onto activated carbon (Part III). The effect of the cation is rationalized in terms of the type of ion pair viz., [ M n+ ] [ Au (CN) ~-] n formed between the Au (CN) ~ anion and the M" ÷ cation in the adsorption medium, and which is subsequently adsorbed.
INTRODUCTION This investigation of the extraction of aurocyanide by polymeric adsorbents and ion-exchange resins follows on from Part I of this series [ 1 ] dealing with *To whom correspondence should be addressed. 0304-386X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
140
the ion-pair solvent extraction of aurocyanide. These relatively simple systems have provided much insight into resolving the mechanism of extraction of aurocyanide by activated carbon reported on in Part III [ 2 ], which is a system significantly more complex, and less amenable to direct study. Polymeric adsorbents are porous polymeric beads that may have organic functional groups on the surface, the nature of which determines the hydrophobicity of the surface. This in turn determines the affinity of the adsorbent for various species. When charged functional groups are present, the adsorbent is termed an ion-exchange resin. The surface of a polymeric adsorbent provides an organic matrix not dissimilar to the organic phase in a solvent extraction system. Thus some comparison can be drawn between the two types of system. Two polymeric adsorbents of differing hydrophobicity, as well as two anionexchange resins are examined in this study and mechanisms for their extraction of aurocyanide are postulated. S-761 polymeric adsorbent contains phenolic hydroxyl groups on a polystyrene matrix and was chosen as a model because activated carbon is known [ 1 ] to contain these groups on an aromatic matrix. E X P E R I M E N T A L PROCEDURE
Reagents and chemicals The KAu(CN)2, KAuC14.0.5H20 and KAg(CN)2 used in this study were all supplied by Johnson Matthey (Pty) Ltd. The chloride salt (AR grade) of each cation was used, unless otherwise indicated. Acid solutions were standardised by titration with NaOH solutions previously standardised with 1.0 M HC1 solution, supplied by BDH. The other chemicals were of AR or CP grade. Table 1 briefly describes the polymeric adsorbents and ion-exchange resins which were studied. The polymeric adsorbents were washed with deionized water and dried at 90 °C before use. The ion-exchange resins were treated in a column with 0.05 M HC1 before washing and drying.
Equilibrium experiments For each system, a known mass of the extractant was contacted and agitated with solutions containing a known concentration of aurocyanide, as well as the appropriate mass of additive to attain a constant 0.1 mol/kg ionic strength throughout. Conditions for each extraction test, unless stated otherwise, are as indicated in Table 2. Samples of the solution were analyzed for gold or silver by atomic absorption spectrophotometry.
141 TABLE 1
Some physicaland chemical propertiesof the polymeric adsorbentsand ion-exchangeresinsstudied Extractant
Manufacturer
Functionalities
Duolite Duolite Rohm & Haas
-OH -C02R
Rohm & Haas Rohm & Haas Dow
-C02H -NMe~ CIBis-picolylamine
Polymeric adsorbents S-761 a ES-862 b XAD-7b
Ion-exchange resins IRC-50b IRA-400b XF-4195b'c
aPhenol-formaldehyde matrix. bpolystyrene matrix. CThis is marketed as a chelating resin, but in our system will act as a simple anion exchanger.
Temperature experiments The solution and extractant were placed in a round-bottomed flask with condenser, and agitated using a magnetic stirrer. The flask was placed in an oil-filled bath which was heated. Samples were taken and analysed for gold or silver after equilibrium had been achieved, which on the basis of other experiments, was taken to be three hours. The volume of solution used was 50 ml.
Infrared spectrophotometry A Perkin-Elmer 580B infrared spectrophotometer was used to study the nature of the aurocyanide species loaded onto the XF-4195 chelating resin. Fourier Transform Infrared Spectrophotometry (FTIR) was used in the study of aurocyanide-loaded S-761 polymeric adsorbent. TABLE2 Experimental conditions employed in the extraction tests
Initial gold concentration Ionic strength Volume of solution Mass adsorbent/resin Equilibrium period
(mg/l) (mol/kg) (ml) (g) (h)
Polymeric adsorbents
Ion-exchange resins (IRA-400 and XF-4195)
100 0.1 50 1.0a 3b
300 0.1 100 0.25 20
a2.0 g was employed in studying the effect of gold concentration, pH and type of cation. bThis is sufficient time for equilibrium to be achieved.
142
X-ray powder diffractometry A Philips X-ray diffractometer was used to identify crystalline compounds (e.g., AuCN and Au °) on polymeric adsorbents. RESULTS AND DISCUSSION
Factors influencing the extraction of aurocyanide by polymeric adsorbents and ion-exchange resins The distribution ratio for aurocyanide between the adsorbent phase and the aqueous phase can be defined as in eqn. (1) : D - mg Au/g adsorbent x/m mg Au/1 of s o l u t i o n - c
(1)
Effect of cation type The effect of alkali metal and alkaline earth cations on the extraction of aurocyanide by the hydrophobic ES-862 polymeric adsorbent (Fig. la) and the more hydrophilic S-761 polymeric adsorbent (Fig. lb) were investigated. The results are plotted in Figs. la and lb in the logarithmic form of the distribution ratio (log D) for aurocyanide extraction by the polymeric adsorbent, against the crystallographic radius of the particular cation ( r + ). All the cations investigated produced an enhanced distribution ratio. A similar trend to the cation effect on the ion-pair solvent extraction of aurocyanide by 1-pentanol [1] is evident, which indicates that a similar mechanism of extraction must be operative in both instances. Therefore a neutral ion-pair species is extracted into the polymer phase. F T I R on KAu (CN)2-loaded S761 confirms that no chemical change occurs during extraction, since the C-N stretch of the extracted gold complex occurs at 2145 cm -1, corresponding to that of Au (CN) 2" The enhanced distribution ratio for larger cations is believed to be due to a decrease in hydration with increasing ionic size [ 1 ]. This results in larger cations being more hydrophobic and having a lower aqueous solubility than small cations. In the presence of smaller cations such as Li+, which forms a highly soluble aurocyanide salt, the enhanced distribution could be explained in terms of an interaction between the strongly electrophilic H +, Li + and Be e+ ions and the electron-donating cyanide groups of the aurocyanide anion, by polarization of the surrounding water molecules. Another interpretation [ 1 ] of these effects is in terms of the changing balance between the enthalpic and entropic contributions to the free energy upon transfer of these ions from water. The values for H + and Be z+ are significantly less than expected. The Be 2+
143 -1.6 -
-1.8 -
\~:
.,4"~<,~+ l
Mg2+~
-2.0 -
rb+
-2.2K+ NO additives
o:~
,'.o
,:~
7.o
r+(.k) Li + _ fsJ'~ k
-1.5-
%//"
• C s+
',
Mg~,+/\
//
÷/
\
a S a2 ,- ~/_.+
,A-
F
\ /
K N o additives
-2.0"
0:5
1:0
1.5 '
2.0 '
r+(],) Fig. la. Effect of Group IA and IIA cations on the extraction of Au (CN)~- by ES-862 polymeric adsorbent (10/~= 1 nm). Fig. lb. Effect of Group IA and IIA cations on the extraction of Au (CN)~ by S-761 polymeric adsorbent (10 A = 1 nm).
solution was slightly more acidic (pH 4 ) than the other salt solutions (pH 7) due to the strong Lewis acidity of this cation. The loaded beads were observed to contain yellow AuCN, the identity of which was confirmed by X-ray did fractometry. Microscopy revealed that the insoluble solid was physically blocking the pores of the adsorbent, resulting in a lower distribution ratio than expected from the argument presented above. The reaction in acidic medium, H + +Au(CN)~ ~
HCN+AuCN
(2)
144
I I
,/
-0.5"
-,1-
l -1.0'
2 f J
+
NHMe3 7
Ol ._O -1,5
;
+
e4
A/
S-761
• "'4
/ /
-2.0// / /
/
•
E5-862
r+ {~,) Fig. 2. Effect of alkylammonium cations on the extraction of Au (CN) f by polymeric adsorbent resins (10 A = 1 nm).
is evidently catalysed by a hydrophobic surface, since this reaction occurs extremely slowly at room temperature in a glass container. It is interesting to note that the curves for the alkali metal and alkaline earth cations lie at similar distribution ratios for both polymeric adsorbents studied, whereas in the 1-pentanol case [ 1 ], the alkaline earth cations were extracted with significantly lower distribution ratios than the alkali metal cations. The reason for this effect is not clear. As can be seen in Fig. 2, the alkylammonium cations do not result in the Ushaped curve observed for the 1-pentanol system [ 1 ]. In this case, the hydrophobic effect clearly outweighs the hydrogen-bonding effect of, for example N H + . The ion pairs involving the large organic cations are much more compatible with the hydrophobic surface than the smaller, more polar N H + , NH2 Me~ and N H M e ~ ion pairs. This is also manifested in the more hydrophobic ES-862 having a larger affinity for the ion pairs involving the large
145 TABLE 3 Effect of the nature of the cation on the extraction of aurocyanideby the anion-exchangeresins, IRA-400 and XF-4195 Cation
H÷ Li ÷ Na ÷ K÷ Cs ÷ No additives
log D" IRA-400
XF-4195
0.562 0.571 0.544 0.509 0.561 2.045
0.958 0.574 0.517 0.521 0.470 1.097
"logD is the logarithm of the distribution ratio as defined in eqn. (1). cations, t h a n the more hydrophilic S-761 adsorbent. This situation is reversed for the smaller, more polar ion pairs. As expected, there is no significant cation effect on the extraction of aurocyanide by anion-exchange resins, as can be seen from the results presented in Table 3. IRA-400 is a strong-base resin with - N ( C H 3 ) ~ functionalities, whereas XF-4195 has bis-picolylamine chelating functional groups [ 3 ], which are protonated to some extent, resulting in it acting as a weak-base resin. Some increase in log D is consequently seen in this case in acidic solution. In both cases, each of the cations produce a depression of the distribution ratio, resulting from competition for anion-exchange sites by way of the presence of 0.1 M C1- anions in the solution. In acidic solution, no AuCN was visible on the surface of the resin beads. This was confirmed by infrared spectroscopy, which indicated the presence of Au (CN) ~ anions only. Although XF-4195 extracts ions such as Cu 2+ by a chelation mechanism [ 4 ], the extraction of Au (CN) ~, as shown above, involves only ion exchange.
Effect of aurocyanide and cation concentration Similar concentration effects to those in the solvent case [1] have been observed for S-761. Figure 3 shows t h a t at constant aurocyanide concentration, the distribution ratio increases with increasing Na + concentration, and levels out at higher concentrations. Portions of experimental isotherms for various adsorbate/adsorbent systems have been found to fit the Freundlich equation, which has the mathematical form [ 5 ] :
x / m = kc 1/"
(3 )
where x / m is the mass of adsorbate removed from solution per unit mass of
146 O.4-
D 0.3
021 0:1
0:2
0:3
0.'4
0,'5
I-No'] (M) Fig. 3. Effect of Na ÷ concentration on the extraction of Au(CN)~- by S-761polymeric adsorbent.
adsorbent ( mg Au/g adsorbent), c is the equilibrium aurocyanide concentration in solution (mg Au/1), and the terms k and n are constants. The term k is a measure of the fundamental effectiveness of the adsorbent under these conditions, and n is a measure of the change in the effectiveness of the adsorbent with relative dosage. In physical terms, the equation describes adsorption where the adsorptionsite energies decrease exponentially as the surface coverage increases. The logarithmic form of eqn. (3) can be written as:
log(x/m) = (1/n)log c+log k
(4)
So, if the extraction data fit the Freundlich equation, a plot of log x/m against log c should yield a straight line. Figure 4 shows that the Freundlich equation does describe the adsorption of aurocyanide onto S-761 in the range 0.08 to 4.45 mg Au/g. Values of n-- 1.33 and k--0.0525 are obtained.
Effect of temperature For both polymeric adsorbents and ion-exchange resins, the distribution ratio is found to decrease with increasing temperature. This effect is related to the tenfold increase in aqueous solubility of KAu (CN)2 which occurs when the temperature is raised from 25 °C to 100 ° C, as well as to the increased dissociation of ion pairs in the organic phase. Again, there is a distinct similarity with the behaviour involved in the extraction of aurocyanide by 1-pentanol [ 1 ]. A plot of log D against T -1 for both S-761 polymeric adsorbent resin and IRA-400 anion-exchange resin results in a straight line, and, as in the case of
147
E x ~0-
-1-
1
3
log
c
Fig. 4. Freundlichplot for the extractionof Au(CN) ~ by S-761 polymericadsorbent. the pentanol system, no hysteresis occurs on decreasing the temperature, see Figs. 5a and b. This again indicates that no chemical change, such as that shown in eqn. ( 2 ) occurs. An analysis similar to that for the 1-pentanol case [ 1 ] shows that for the reaction, M~-~q) + A u ( C N ) ~ . q ) ~-MAu(CN)2(r)
(5)
where (aq) denotes the aqueous phase and (r) denotes the resin phase, the enthalpy change is more favourable for S-761 than for 1-pentanol, see Table 4. This must be due to the fact that adsorption as well as ion-pair formation is occurring in the system. The enthalpy change for IRA-400 is similar to that for 1-pentanol. A similar plot for XF-4195 resin does not result in a linear relation, and hysteresis does occur, see Fig. 5c. The points for the initial low-temperature region coincide with the IRA-400 curve, but lower values for log D are obtained after heating to higher temperatures. This phenomenon could be due to decomposition of the aurocyanide ion, forming AuCN ( eqn. 2 ) at elevated temperatures. The proton required for this reaction is readily available, the resin being in the protonated form.
Effect of other anions In the case of the extraction of aurocyanide into 1-pentanol (Part I) [ 1], an excess of C1Oj anions resulted in a reduction in the distribution ratio. Table 5 shows that this effect also occurs for extraction of gold cyanide by the polymeric adsorbents, ES-862 and S-761. This effect is attributed [ 1 ] to the
148 - 1,6
-1.8-2.0-22. -2A 2.7
2.9
3.1
3.3
2.9
3,1
&3
3.5
3:1
32
3-~
0.6 ¸
o
0,4
0.2
0,5
OA
0.2
0
-02
2:9
T-l( IO-3K -I )
Fig. 5. Plot of log D versus T - t for the extraction of KAu ( C N ) 2 by (a) S-761 polymeric adsorbent; (b) IRA-400 anion-exchange resin; (c) XFS-4195 chelating resin ( [ KCI] = 0.1 M ) .
large and weakly hydrated perchlorate ion having a greater tendency to form ion pairs and thus providing competition for aurocyanide ion pair adsorption. It would also be expected that excess of C10~- anions would compete severely with the extraction of aurocyanide anions into anion-exchange resins such as TABLE4 Enthalpy change AH for the extraction of aurocyanide Extractant
z]H (kJ/mol)
IRA-400 1-pentanol S-761
- 17.07 - 20.00 - 28.36
149 TABLE 5 Effect of perchlorate relative to chloride on the distribution ratio for the extraction of aurocyanide by polymeric adsorbents Adsorbent
ES-862 S-761
Distribution ratio D NaC1
NaCl04
HCI
HCI04
0.0125 0.0214
0.0087 0.0120
0.0167 0.0236
0.0110 0.0028
IRA-400. This has been demonstrated by McDougall et al. [ 6 ]. A similar effect was observed by Burstall et al. [ 7 ], who report that sulphate has no effect on the extraction of aurocyanide, but the affinity of IRA-400 for aurocyanide is reduced to some extent by the presence of the large thiosulphate or thiocyanate anions in solution. The mechanism presented in Part I [ 1 ] for solvent extraction of aurocyanide correctly predicts the trend of greater affinity shown by anion-exchange resins for large monovalent anions such as Au ( CN ) ~, as observed by various workers [8-10]. Aveston et al. [10] ascribe the sequence solely to the polarizability of the anions, but it is unlikely that the polarizing power of the large quaternary a m m o n i u m group is large enough to polarize the anions significantly. Thus, the theory presented in Part I [ 1 ] explains some of the trends observed in ion exchange as well as in other types of extraction systems. However, the affinity of ion-exchange resins for ions also depends on various other factors such as cross-linking.
Effect of pH As previously observed, the proton produces a larger distribution ratio for gold adsorption than the potassium ion, and insoluble AuCN was found on the adsorbents at pH 1. One would expect that, in solutions with a constant ionic strength ([M+] =0.1 M), the distribution ratio would increase as the pH decreases. Table 6 shows this to be the case for ES-862, and further that the hydroxide ion has virtually no effect on the extraction. The S-761 polymeric adsorbent, which contains phenolic hydroxyl groups, shows a much more pronounced pH effect (Fig. 6). The amount of aurocyanide extracted at pH values higher than 9 is very small. Above this pH value, deprotonation of the functional groups occurs [ 11 ], effectively resulting in an ion exchange of H + for K + on the phenolate group. The poor extraction observed at high pH values is ascribed to the deprotonation of these phenolic groups,
150 TABLE 6 Effect of pH on the distribution ratio for the extraction of aurocyanide by ES-862 polymeric adsorbent pH
Medium
D
1 7 13
0.1 M HC1 0.1 M NaC1 0.1 M NaOH
0.0167 0.0125 0.0106
resulting in a much more hydrophilic and effectively negatively charged surface. To confirm that the observed effect is due to the surface properties of the polymeric adsorbent rather than the chemical properties of the hydroxide ion, a sample of S-761 was converted to the potassium form by treatment with KOH, and after washing to neutrality with deionized water, was tested for the extraction of aurocyanide. No extraction was observed to occur.
Effect of the hydrophobicity of the polymer matrix It is evident from the results discussed in the previous Section that the surface hydrophobicity plays an important role in the extraction of ion-pair species, as is the case in solvent extraction. Furthermore, evidence was presented 80 ~
60-
u
~a4o× i,u
20-
0
1'4 pH
Fig. 6. Effect of pH on the extraction of Au (CN)~ by S-761 polymeric adsorbent ([K +] = 0.1
M).
151 TABLE 7 Extraction of aurocyanideby polymericadsorbentsof varyinghydrophobicity Polymericadsorbent Functionalities
% Extraction No additives
ES-862 S-761 XAD-7 IRC-50
None (polystyrene) 8 R-OH, R-CH20H 21 R-CO2-R 36 R-CO2H 6
1 M HC1 30 40 51 8
in the section on cation effects which showed that the more hydrophobic ES862 polymeric adsorbent had a greater affinity for the [ NBu + Au (CN) ~ ] ion pair than the hydrophilic S-761. The latter showed a greater affinity for the smaller ion pairs such as [ NH + Au (CN) ~ ]. Table 7 shows the amount of gold extracted by equal masses of adsorbent resins of varying hydrophobicity from solutions containing gold cyanide in the absence of added electrolytes and in the presence of I M HC1. In both a 1 M HC1 solution, where yellow AuCN was observed in all cases, and a solution containing no additives, the adsorbents with intermediate hydrophobicity are observed to have the highest affinity for the gold cyanide species. IRC-50, which contains carboxylic acid groups, is quite hydrophilic and water molecules are more compatible with the surface than the ion pair. On the other hand, the hydrophobic ES-862 is relatively non-wettable and the polar ion pair has little affinity for it. A similar effect has been observed in the distribution of organic compounds between resins and water [ 12 ]. As the degree of sulphonation of a strong-acid cation-exchange resin increases ( expressed in terms of capacity as meq/g), so does the hydrophilicity. Figure 7 shows that the distribution ratio undergoes a maximum, which shifts to more hydrophobic resins as the adsorbate becomes more hydrophobic. Polarizability has also been shown to play an important role in the extraction of organics by polymeric adsorbents. Weber and Van Vliet [ 13, 14 ] show that the order of affinities of polymeric adsorbents for a range of organic compounds follows the order of aqueous solubilities at higher loadings, due to the hydrophobic effect mentioned earlier. At low loadings, molecules with a high polarizability are preferentially extracted by the adsorbents. This effect is ascribed to Van der Waals force interactions between the adsorbate and the benzene rings of the adsorbent resin. It may be expected that modification of the aqueous phase with a solvent would change the extractive behaviour by making the ion pair more compatible with this phase in a manner akin to solvent extraction. Thus it has been observed that AuCIj can be eluted from XAD-7 polymeric adsorbent with an
152 1.5-
1.0-
log D I).5-
O-
~
0
methanol
Capacity {meq/g )
Fig. 7. Effect of the degreeof sulphonation of a strong-acidcation-exchangeresin on the extraction of organic compounds (after Ref. [ 12]). acetone/hydrochloric acid mixture [ 15 ]. The acetone enhances the activity of C1- relative to that of AuC1j thus modifying the extractive behaviour in favour of elution. CONCLUSIONS The mechanism of extraction of aurocyanide by polymeric adsorbents such as ES-862 and S-761, is to a large extent similar to the solvent extraction of Au (CN) F by solvents such as 1-pentanol [ 1 ]. The mechanism involves the formation of an ion-pair species [M n+ ] [ A u ( C N ) ~ - ] n in the environment of the polymeric adsorbent, with subsequent adsorption of the ion pair onto the surface. The cation involved in the extraction shows the same remarkable trend as found in the extraction of aurocyanide by 1-pentanol [1]. Minima in the distribution ratios are found along the alkali metal and alkaline earth cation series. The hydrophobicity of the surface plays an important role in the extraction, with hydrophobic surfaces having a greater affinity for large ion pairs, while hydrophilic surfaces are superior when small cations are involved. An equation of the Freundlich type was found to describe the adsorption isotherm. The presence of an excess of the large perchlorate anion in the adsorption medium results in a decrease in the distribution ratio, as expected.
153
The hydroxide ion has little effect on the extraction of aurocyanide by a polymeric adsorbent with no functional groups, for example ES-862. However, in the case of S-761, which contains phenolic hydroxyl groups, a decrease in the extraction was observed at high pH values. This is ascribed to the deprotonation of these groups, resulting in a much more hydrophilic and effectively negatively charged surface. The adsorptive behaviour of gold cyanide, in the presence and absence of various electrolytes, onto polymeric adsorbents and anion-exchange resins parallels closely the behaviour established in the case of solvent extractants reported on in Part I of this series, and can be rationalized in terms of a single theory involving an ion-pair formation mechanism. In Part III of this series [ 2 ], the application of this knowledge to aspects of the extraction of gold cyanide by activated carbon is discussed. ACKNOWLEDGEMENTS
Gratitude is expressed to the Council for Scientific and Industrial Research, University Research Division and Sentrachem Ltd., for their sponsorship of this work. REFERENCES 1 McDougall, G.J., Adams, M.D. and Hancock, R.D., Models for the adsorption of aurocyanide onto activated carbon. Part I: Solvent extraction of aurocyanide ion pairs by 1-pentanol. Hydrometallurgy, 18 (1987) 125-138. 2 Adams, M.D., McDougall, G.J. and Hancock, R.D., Models for the adsorption of aurocyanide onto activated carbon. Part III: Comparison between the extraction of aurocyanide by activated carbon, polymeric adsorbents and 1-pentanol, Hydrometallurgy, (1987) to be published. 3 Grinstead, R.R., New developments in the chemistry of XFS 4195 and XFS 43084 chelating ion exchange resins, in: Naden, D. and Streat, M. (Eds.), Ion Exchange Technology, Ellis Horwood Ltd., 1984. 4 Jones, K.L. and Grinstead, R.R., Properties and hydrometallurgical applications of two new chelating ion exchange resins, Chem. Ind., (1977) 637-641. 5 Glasstone, S., Textbook of Physical Chemistry, Macmillan, London, 1956. 6 McDougall,G.J., Hancock, R.D., Nicol, M.J., Wellington, O.L. and Copperthwaite, R.G., The mechanism of the adsorption of gold cyanide on activated carbon, J.S. Aft. Inst. Min. Metall., 80 {1980) 344-356. 7 Burstall, F.H., Forrest, P.J., Kember, N.F. and Wells, R.A., Ion exchange process for recovery of gold from cyanide solution, Ind. Eng. Chem., 45 (1953) 1648-1658. 8 Diamond, R.M. and Whitney, D.C., Resin selectivity in dilute to concentrated aqueous solutions, in: Marinsky, J.A. (Ed.), Ion Exchange, Arnold, London, Vol. 1, 1966, pp. 277-351. 9 Kitchener, J.A., Ion-Exchange Resins, Methuen, 1961. 10 Aveston, J., Everest, D.A. and Wells, R.A., Adsorption of gold from cyanide solutions by anionic resins, J. Chem. Soc., {1958) 231-239. 11 Duolite polymeric adsorbents, technical notes, Dia-prosim, France. 12 Rieman, W., Salting-out chromatography. A review, J. Chem. Educ., 38 (1961) 338-343. 13 Weber,W.J. and Van Vliet, B.M., Synthetic adsorbents and activated carbons for water treatment: overview and experimental comparisons, Res. Technol., (1981) 420-426.
154 14 Weber,W.J. and Van Vliet, B.M., Synthetic adsorbents and activated carbons for water treatment: Statistical analyses and interpretations, Res. Technol., (1981) 426-431. 15 Haines, A.K., Edwards, R.I. and Te Riele, W.A.M., The separation of gold from acidic leach liquors with amberlite XAD-7, in: Streat, M. (Ed.), The Theory and Practice of Ion-Exchange, Society of Chemical Industry, London, 1976, pp. 40.1-40.12.
139
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Models for the Adsorption of A u r o c y a n i d e onto A c t i v a t e d Carbon. Part II: Extraction of A u r o c y a n i d e Ion Pairs by P o l y m e r i c Adsorbents M.D. ADAMS*
Council for Mineral Technology, Private Bag X3015, Randburg 2125 (South Africa)
G.J. McDOUGALL
NationalChemicalProducts, P.O. Box344, Germiston1400(SouthA~ica)
and R.D. HANCOCK
University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001 (South Africa) (Received March 30, 1985; accepted in revised form September 3, 1986)
ABSTRACT Adams, M.D., McDougall, G.J. and Hancock, R.D., 1987. Models for the adsorption of aurocyanide onto activated carbon. Part II: Extraction of aurocyanide ion pairs by polymeric adsorbents. HydrometaUurgy, 18: 139-154. The adsorption of aurocyanide onto activated carbon has been known but not fully understood for many years. This is due to the difficulty involved in studying the nature of the adsorbed gold cyanide species on activated carbon by conventional techniques of chemical investigation such as infrared and ultraviolet-visible spectroscopy. A novel approach using model systems was adopted by the authors to shed further light on the adsorption mechanism. This study reports on the extraction of gold cyanide by polymeric adsorbents and ion-exchange resins and demonstrates that, as in the case of ion-pair solvent extraction (Part I), the adsorptive behaviour of Au(CN) 2 in the presence of various cations onto polymeric adsorbents is analogous to the adsorptive behaviour onto activated carbon (Part III). The effect of the cation is rationalized in terms of the type of ion pair viz., [ M n+ ] [ Au (CN) ~-] n formed between the Au (CN) ~ anion and the M" ÷ cation in the adsorption medium, and which is subsequently adsorbed.
INTRODUCTION This investigation of the extraction of aurocyanide by polymeric adsorbents and ion-exchange resins follows on from Part I of this series [ 1 ] dealing with *To whom correspondence should be addressed. 0304-386X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
140
the ion-pair solvent extraction of aurocyanide. These relatively simple systems have provided much insight into resolving the mechanism of extraction of aurocyanide by activated carbon reported on in Part III [ 2 ], which is a system significantly more complex, and less amenable to direct study. Polymeric adsorbents are porous polymeric beads that may have organic functional groups on the surface, the nature of which determines the hydrophobicity of the surface. This in turn determines the affinity of the adsorbent for various species. When charged functional groups are present, the adsorbent is termed an ion-exchange resin. The surface of a polymeric adsorbent provides an organic matrix not dissimilar to the organic phase in a solvent extraction system. Thus some comparison can be drawn between the two types of system. Two polymeric adsorbents of differing hydrophobicity, as well as two anionexchange resins are examined in this study and mechanisms for their extraction of aurocyanide are postulated. S-761 polymeric adsorbent contains phenolic hydroxyl groups on a polystyrene matrix and was chosen as a model because activated carbon is known [ 1 ] to contain these groups on an aromatic matrix. E X P E R I M E N T A L PROCEDURE
Reagents and chemicals The KAu(CN)2, KAuC14.0.5H20 and KAg(CN)2 used in this study were all supplied by Johnson Matthey (Pty) Ltd. The chloride salt (AR grade) of each cation was used, unless otherwise indicated. Acid solutions were standardised by titration with NaOH solutions previously standardised with 1.0 M HC1 solution, supplied by BDH. The other chemicals were of AR or CP grade. Table 1 briefly describes the polymeric adsorbents and ion-exchange resins which were studied. The polymeric adsorbents were washed with deionized water and dried at 90 °C before use. The ion-exchange resins were treated in a column with 0.05 M HC1 before washing and drying.
Equilibrium experiments For each system, a known mass of the extractant was contacted and agitated with solutions containing a known concentration of aurocyanide, as well as the appropriate mass of additive to attain a constant 0.1 mol/kg ionic strength throughout. Conditions for each extraction test, unless stated otherwise, are as indicated in Table 2. Samples of the solution were analyzed for gold or silver by atomic absorption spectrophotometry.
141 TABLE 1
Some physicaland chemical propertiesof the polymeric adsorbentsand ion-exchangeresinsstudied Extractant
Manufacturer
Functionalities
Duolite Duolite Rohm & Haas
-OH -C02R
Rohm & Haas Rohm & Haas Dow
-C02H -NMe~ CIBis-picolylamine
Polymeric adsorbents S-761 a ES-862 b XAD-7b
Ion-exchange resins IRC-50b IRA-400b XF-4195b'c
aPhenol-formaldehyde matrix. bpolystyrene matrix. CThis is marketed as a chelating resin, but in our system will act as a simple anion exchanger.
Temperature experiments The solution and extractant were placed in a round-bottomed flask with condenser, and agitated using a magnetic stirrer. The flask was placed in an oil-filled bath which was heated. Samples were taken and analysed for gold or silver after equilibrium had been achieved, which on the basis of other experiments, was taken to be three hours. The volume of solution used was 50 ml.
Infrared spectrophotometry A Perkin-Elmer 580B infrared spectrophotometer was used to study the nature of the aurocyanide species loaded onto the XF-4195 chelating resin. Fourier Transform Infrared Spectrophotometry (FTIR) was used in the study of aurocyanide-loaded S-761 polymeric adsorbent. TABLE2 Experimental conditions employed in the extraction tests
Initial gold concentration Ionic strength Volume of solution Mass adsorbent/resin Equilibrium period
(mg/l) (mol/kg) (ml) (g) (h)
Polymeric adsorbents
Ion-exchange resins (IRA-400 and XF-4195)
100 0.1 50 1.0a 3b
300 0.1 100 0.25 20
a2.0 g was employed in studying the effect of gold concentration, pH and type of cation. bThis is sufficient time for equilibrium to be achieved.
142
X-ray powder diffractometry A Philips X-ray diffractometer was used to identify crystalline compounds (e.g., AuCN and Au °) on polymeric adsorbents. RESULTS AND DISCUSSION
Factors influencing the extraction of aurocyanide by polymeric adsorbents and ion-exchange resins The distribution ratio for aurocyanide between the adsorbent phase and the aqueous phase can be defined as in eqn. (1) : D - mg Au/g adsorbent x/m mg Au/1 of s o l u t i o n - c
(1)
Effect of cation type The effect of alkali metal and alkaline earth cations on the extraction of aurocyanide by the hydrophobic ES-862 polymeric adsorbent (Fig. la) and the more hydrophilic S-761 polymeric adsorbent (Fig. lb) were investigated. The results are plotted in Figs. la and lb in the logarithmic form of the distribution ratio (log D) for aurocyanide extraction by the polymeric adsorbent, against the crystallographic radius of the particular cation ( r + ). All the cations investigated produced an enhanced distribution ratio. A similar trend to the cation effect on the ion-pair solvent extraction of aurocyanide by 1-pentanol [1] is evident, which indicates that a similar mechanism of extraction must be operative in both instances. Therefore a neutral ion-pair species is extracted into the polymer phase. F T I R on KAu (CN)2-loaded S761 confirms that no chemical change occurs during extraction, since the C-N stretch of the extracted gold complex occurs at 2145 cm -1, corresponding to that of Au (CN) 2" The enhanced distribution ratio for larger cations is believed to be due to a decrease in hydration with increasing ionic size [ 1 ]. This results in larger cations being more hydrophobic and having a lower aqueous solubility than small cations. In the presence of smaller cations such as Li+, which forms a highly soluble aurocyanide salt, the enhanced distribution could be explained in terms of an interaction between the strongly electrophilic H +, Li + and Be e+ ions and the electron-donating cyanide groups of the aurocyanide anion, by polarization of the surrounding water molecules. Another interpretation [ 1 ] of these effects is in terms of the changing balance between the enthalpic and entropic contributions to the free energy upon transfer of these ions from water. The values for H + and Be z+ are significantly less than expected. The Be 2+
143 -1.6 -
-1.8 -
\~:
.,4"~<,~+ l
Mg2+~
-2.0 -
rb+
-2.2K+ NO additives
o:~
,'.o
,:~
7.o
r+(.k) Li + _ fsJ'~ k
-1.5-
%//"
• C s+
',
Mg~,+/\
//
÷/
\
a S a2 ,- ~/_.+
,A-
F
\ /
K N o additives
-2.0"
0:5
1:0
1.5 '
2.0 '
r+(],) Fig. la. Effect of Group IA and IIA cations on the extraction of Au (CN)~- by ES-862 polymeric adsorbent (10/~= 1 nm). Fig. lb. Effect of Group IA and IIA cations on the extraction of Au (CN)~ by S-761 polymeric adsorbent (10 A = 1 nm).
solution was slightly more acidic (pH 4 ) than the other salt solutions (pH 7) due to the strong Lewis acidity of this cation. The loaded beads were observed to contain yellow AuCN, the identity of which was confirmed by X-ray did fractometry. Microscopy revealed that the insoluble solid was physically blocking the pores of the adsorbent, resulting in a lower distribution ratio than expected from the argument presented above. The reaction in acidic medium, H + +Au(CN)~ ~
HCN+AuCN
(2)
144
I I
,/
-0.5"
-,1-
l -1.0'
2 f J
+
NHMe3 7
Ol ._O -1,5
;
+
e4
A/
S-761
• "'4
/ /
-2.0// / /
/
•
E5-862
r+ {~,) Fig. 2. Effect of alkylammonium cations on the extraction of Au (CN) f by polymeric adsorbent resins (10 A = 1 nm).
is evidently catalysed by a hydrophobic surface, since this reaction occurs extremely slowly at room temperature in a glass container. It is interesting to note that the curves for the alkali metal and alkaline earth cations lie at similar distribution ratios for both polymeric adsorbents studied, whereas in the 1-pentanol case [ 1 ], the alkaline earth cations were extracted with significantly lower distribution ratios than the alkali metal cations. The reason for this effect is not clear. As can be seen in Fig. 2, the alkylammonium cations do not result in the Ushaped curve observed for the 1-pentanol system [ 1 ]. In this case, the hydrophobic effect clearly outweighs the hydrogen-bonding effect of, for example N H + . The ion pairs involving the large organic cations are much more compatible with the hydrophobic surface than the smaller, more polar N H + , NH2 Me~ and N H M e ~ ion pairs. This is also manifested in the more hydrophobic ES-862 having a larger affinity for the ion pairs involving the large
145 TABLE 3 Effect of the nature of the cation on the extraction of aurocyanideby the anion-exchangeresins, IRA-400 and XF-4195 Cation
H÷ Li ÷ Na ÷ K÷ Cs ÷ No additives
log D" IRA-400
XF-4195
0.562 0.571 0.544 0.509 0.561 2.045
0.958 0.574 0.517 0.521 0.470 1.097
"logD is the logarithm of the distribution ratio as defined in eqn. (1). cations, t h a n the more hydrophilic S-761 adsorbent. This situation is reversed for the smaller, more polar ion pairs. As expected, there is no significant cation effect on the extraction of aurocyanide by anion-exchange resins, as can be seen from the results presented in Table 3. IRA-400 is a strong-base resin with - N ( C H 3 ) ~ functionalities, whereas XF-4195 has bis-picolylamine chelating functional groups [ 3 ], which are protonated to some extent, resulting in it acting as a weak-base resin. Some increase in log D is consequently seen in this case in acidic solution. In both cases, each of the cations produce a depression of the distribution ratio, resulting from competition for anion-exchange sites by way of the presence of 0.1 M C1- anions in the solution. In acidic solution, no AuCN was visible on the surface of the resin beads. This was confirmed by infrared spectroscopy, which indicated the presence of Au (CN) ~ anions only. Although XF-4195 extracts ions such as Cu 2+ by a chelation mechanism [ 4 ], the extraction of Au (CN) ~, as shown above, involves only ion exchange.
Effect of aurocyanide and cation concentration Similar concentration effects to those in the solvent case [1] have been observed for S-761. Figure 3 shows t h a t at constant aurocyanide concentration, the distribution ratio increases with increasing Na + concentration, and levels out at higher concentrations. Portions of experimental isotherms for various adsorbate/adsorbent systems have been found to fit the Freundlich equation, which has the mathematical form [ 5 ] :
x / m = kc 1/"
(3 )
where x / m is the mass of adsorbate removed from solution per unit mass of
146 O.4-
D 0.3
021 0:1
0:2
0:3
0.'4
0,'5
I-No'] (M) Fig. 3. Effect of Na ÷ concentration on the extraction of Au(CN)~- by S-761polymeric adsorbent.
adsorbent ( mg Au/g adsorbent), c is the equilibrium aurocyanide concentration in solution (mg Au/1), and the terms k and n are constants. The term k is a measure of the fundamental effectiveness of the adsorbent under these conditions, and n is a measure of the change in the effectiveness of the adsorbent with relative dosage. In physical terms, the equation describes adsorption where the adsorptionsite energies decrease exponentially as the surface coverage increases. The logarithmic form of eqn. (3) can be written as:
log(x/m) = (1/n)log c+log k
(4)
So, if the extraction data fit the Freundlich equation, a plot of log x/m against log c should yield a straight line. Figure 4 shows that the Freundlich equation does describe the adsorption of aurocyanide onto S-761 in the range 0.08 to 4.45 mg Au/g. Values of n-- 1.33 and k--0.0525 are obtained.
Effect of temperature For both polymeric adsorbents and ion-exchange resins, the distribution ratio is found to decrease with increasing temperature. This effect is related to the tenfold increase in aqueous solubility of KAu (CN)2 which occurs when the temperature is raised from 25 °C to 100 ° C, as well as to the increased dissociation of ion pairs in the organic phase. Again, there is a distinct similarity with the behaviour involved in the extraction of aurocyanide by 1-pentanol [ 1 ]. A plot of log D against T -1 for both S-761 polymeric adsorbent resin and IRA-400 anion-exchange resin results in a straight line, and, as in the case of
147
E x ~0-
-1-
1
3
log
c
Fig. 4. Freundlichplot for the extractionof Au(CN) ~ by S-761 polymericadsorbent. the pentanol system, no hysteresis occurs on decreasing the temperature, see Figs. 5a and b. This again indicates that no chemical change, such as that shown in eqn. ( 2 ) occurs. An analysis similar to that for the 1-pentanol case [ 1 ] shows that for the reaction, M~-~q) + A u ( C N ) ~ . q ) ~-MAu(CN)2(r)
(5)
where (aq) denotes the aqueous phase and (r) denotes the resin phase, the enthalpy change is more favourable for S-761 than for 1-pentanol, see Table 4. This must be due to the fact that adsorption as well as ion-pair formation is occurring in the system. The enthalpy change for IRA-400 is similar to that for 1-pentanol. A similar plot for XF-4195 resin does not result in a linear relation, and hysteresis does occur, see Fig. 5c. The points for the initial low-temperature region coincide with the IRA-400 curve, but lower values for log D are obtained after heating to higher temperatures. This phenomenon could be due to decomposition of the aurocyanide ion, forming AuCN ( eqn. 2 ) at elevated temperatures. The proton required for this reaction is readily available, the resin being in the protonated form.
Effect of other anions In the case of the extraction of aurocyanide into 1-pentanol (Part I) [ 1], an excess of C1Oj anions resulted in a reduction in the distribution ratio. Table 5 shows that this effect also occurs for extraction of gold cyanide by the polymeric adsorbents, ES-862 and S-761. This effect is attributed [ 1 ] to the
148 - 1,6
-1.8-2.0-22. -2A 2.7
2.9
3.1
3.3
2.9
3,1
&3
3.5
3:1
32
3-~
0.6 ¸
o
0,4
0.2
0,5
OA
0.2
0
-02
2:9
T-l( IO-3K -I )
Fig. 5. Plot of log D versus T - t for the extraction of KAu ( C N ) 2 by (a) S-761 polymeric adsorbent; (b) IRA-400 anion-exchange resin; (c) XFS-4195 chelating resin ( [ KCI] = 0.1 M ) .
large and weakly hydrated perchlorate ion having a greater tendency to form ion pairs and thus providing competition for aurocyanide ion pair adsorption. It would also be expected that excess of C10~- anions would compete severely with the extraction of aurocyanide anions into anion-exchange resins such as TABLE4 Enthalpy change AH for the extraction of aurocyanide Extractant
z]H (kJ/mol)
IRA-400 1-pentanol S-761
- 17.07 - 20.00 - 28.36
149 TABLE 5 Effect of perchlorate relative to chloride on the distribution ratio for the extraction of aurocyanide by polymeric adsorbents Adsorbent
ES-862 S-761
Distribution ratio D NaC1
NaCl04
HCI
HCI04
0.0125 0.0214
0.0087 0.0120
0.0167 0.0236
0.0110 0.0028
IRA-400. This has been demonstrated by McDougall et al. [ 6 ]. A similar effect was observed by Burstall et al. [ 7 ], who report that sulphate has no effect on the extraction of aurocyanide, but the affinity of IRA-400 for aurocyanide is reduced to some extent by the presence of the large thiosulphate or thiocyanate anions in solution. The mechanism presented in Part I [ 1 ] for solvent extraction of aurocyanide correctly predicts the trend of greater affinity shown by anion-exchange resins for large monovalent anions such as Au ( CN ) ~, as observed by various workers [8-10]. Aveston et al. [10] ascribe the sequence solely to the polarizability of the anions, but it is unlikely that the polarizing power of the large quaternary a m m o n i u m group is large enough to polarize the anions significantly. Thus, the theory presented in Part I [ 1 ] explains some of the trends observed in ion exchange as well as in other types of extraction systems. However, the affinity of ion-exchange resins for ions also depends on various other factors such as cross-linking.
Effect of pH As previously observed, the proton produces a larger distribution ratio for gold adsorption than the potassium ion, and insoluble AuCN was found on the adsorbents at pH 1. One would expect that, in solutions with a constant ionic strength ([M+] =0.1 M), the distribution ratio would increase as the pH decreases. Table 6 shows this to be the case for ES-862, and further that the hydroxide ion has virtually no effect on the extraction. The S-761 polymeric adsorbent, which contains phenolic hydroxyl groups, shows a much more pronounced pH effect (Fig. 6). The amount of aurocyanide extracted at pH values higher than 9 is very small. Above this pH value, deprotonation of the functional groups occurs [ 11 ], effectively resulting in an ion exchange of H + for K + on the phenolate group. The poor extraction observed at high pH values is ascribed to the deprotonation of these phenolic groups,
150 TABLE 6 Effect of pH on the distribution ratio for the extraction of aurocyanide by ES-862 polymeric adsorbent pH
Medium
D
1 7 13
0.1 M HC1 0.1 M NaC1 0.1 M NaOH
0.0167 0.0125 0.0106
resulting in a much more hydrophilic and effectively negatively charged surface. To confirm that the observed effect is due to the surface properties of the polymeric adsorbent rather than the chemical properties of the hydroxide ion, a sample of S-761 was converted to the potassium form by treatment with KOH, and after washing to neutrality with deionized water, was tested for the extraction of aurocyanide. No extraction was observed to occur.
Effect of the hydrophobicity of the polymer matrix It is evident from the results discussed in the previous Section that the surface hydrophobicity plays an important role in the extraction of ion-pair species, as is the case in solvent extraction. Furthermore, evidence was presented 80 ~
60-
u
~a4o× i,u
20-
0
1'4 pH
Fig. 6. Effect of pH on the extraction of Au (CN)~ by S-761 polymeric adsorbent ([K +] = 0.1
M).
151 TABLE 7 Extraction of aurocyanideby polymericadsorbentsof varyinghydrophobicity Polymericadsorbent Functionalities
% Extraction No additives
ES-862 S-761 XAD-7 IRC-50
None (polystyrene) 8 R-OH, R-CH20H 21 R-CO2-R 36 R-CO2H 6
1 M HC1 30 40 51 8
in the section on cation effects which showed that the more hydrophobic ES862 polymeric adsorbent had a greater affinity for the [ NBu + Au (CN) ~ ] ion pair than the hydrophilic S-761. The latter showed a greater affinity for the smaller ion pairs such as [ NH + Au (CN) ~ ]. Table 7 shows the amount of gold extracted by equal masses of adsorbent resins of varying hydrophobicity from solutions containing gold cyanide in the absence of added electrolytes and in the presence of I M HC1. In both a 1 M HC1 solution, where yellow AuCN was observed in all cases, and a solution containing no additives, the adsorbents with intermediate hydrophobicity are observed to have the highest affinity for the gold cyanide species. IRC-50, which contains carboxylic acid groups, is quite hydrophilic and water molecules are more compatible with the surface than the ion pair. On the other hand, the hydrophobic ES-862 is relatively non-wettable and the polar ion pair has little affinity for it. A similar effect has been observed in the distribution of organic compounds between resins and water [ 12 ]. As the degree of sulphonation of a strong-acid cation-exchange resin increases ( expressed in terms of capacity as meq/g), so does the hydrophilicity. Figure 7 shows that the distribution ratio undergoes a maximum, which shifts to more hydrophobic resins as the adsorbate becomes more hydrophobic. Polarizability has also been shown to play an important role in the extraction of organics by polymeric adsorbents. Weber and Van Vliet [ 13, 14 ] show that the order of affinities of polymeric adsorbents for a range of organic compounds follows the order of aqueous solubilities at higher loadings, due to the hydrophobic effect mentioned earlier. At low loadings, molecules with a high polarizability are preferentially extracted by the adsorbents. This effect is ascribed to Van der Waals force interactions between the adsorbate and the benzene rings of the adsorbent resin. It may be expected that modification of the aqueous phase with a solvent would change the extractive behaviour by making the ion pair more compatible with this phase in a manner akin to solvent extraction. Thus it has been observed that AuCIj can be eluted from XAD-7 polymeric adsorbent with an
152 1.5-
1.0-
log D I).5-
O-
~
0
methanol
Capacity {meq/g )
Fig. 7. Effect of the degreeof sulphonation of a strong-acidcation-exchangeresin on the extraction of organic compounds (after Ref. [ 12]). acetone/hydrochloric acid mixture [ 15 ]. The acetone enhances the activity of C1- relative to that of AuC1j thus modifying the extractive behaviour in favour of elution. CONCLUSIONS The mechanism of extraction of aurocyanide by polymeric adsorbents such as ES-862 and S-761, is to a large extent similar to the solvent extraction of Au (CN) F by solvents such as 1-pentanol [ 1 ]. The mechanism involves the formation of an ion-pair species [M n+ ] [ A u ( C N ) ~ - ] n in the environment of the polymeric adsorbent, with subsequent adsorption of the ion pair onto the surface. The cation involved in the extraction shows the same remarkable trend as found in the extraction of aurocyanide by 1-pentanol [1]. Minima in the distribution ratios are found along the alkali metal and alkaline earth cation series. The hydrophobicity of the surface plays an important role in the extraction, with hydrophobic surfaces having a greater affinity for large ion pairs, while hydrophilic surfaces are superior when small cations are involved. An equation of the Freundlich type was found to describe the adsorption isotherm. The presence of an excess of the large perchlorate anion in the adsorption medium results in a decrease in the distribution ratio, as expected.
153
The hydroxide ion has little effect on the extraction of aurocyanide by a polymeric adsorbent with no functional groups, for example ES-862. However, in the case of S-761, which contains phenolic hydroxyl groups, a decrease in the extraction was observed at high pH values. This is ascribed to the deprotonation of these groups, resulting in a much more hydrophilic and effectively negatively charged surface. The adsorptive behaviour of gold cyanide, in the presence and absence of various electrolytes, onto polymeric adsorbents and anion-exchange resins parallels closely the behaviour established in the case of solvent extractants reported on in Part I of this series, and can be rationalized in terms of a single theory involving an ion-pair formation mechanism. In Part III of this series [ 2 ], the application of this knowledge to aspects of the extraction of gold cyanide by activated carbon is discussed. ACKNOWLEDGEMENTS
Gratitude is expressed to the Council for Scientific and Industrial Research, University Research Division and Sentrachem Ltd., for their sponsorship of this work. REFERENCES 1 McDougall, G.J., Adams, M.D. and Hancock, R.D., Models for the adsorption of aurocyanide onto activated carbon. Part I: Solvent extraction of aurocyanide ion pairs by 1-pentanol. Hydrometallurgy, 18 (1987) 125-138. 2 Adams, M.D., McDougall, G.J. and Hancock, R.D., Models for the adsorption of aurocyanide onto activated carbon. Part III: Comparison between the extraction of aurocyanide by activated carbon, polymeric adsorbents and 1-pentanol, Hydrometallurgy, (1987) to be published. 3 Grinstead, R.R., New developments in the chemistry of XFS 4195 and XFS 43084 chelating ion exchange resins, in: Naden, D. and Streat, M. (Eds.), Ion Exchange Technology, Ellis Horwood Ltd., 1984. 4 Jones, K.L. and Grinstead, R.R., Properties and hydrometallurgical applications of two new chelating ion exchange resins, Chem. Ind., (1977) 637-641. 5 Glasstone, S., Textbook of Physical Chemistry, Macmillan, London, 1956. 6 McDougall,G.J., Hancock, R.D., Nicol, M.J., Wellington, O.L. and Copperthwaite, R.G., The mechanism of the adsorption of gold cyanide on activated carbon, J.S. Aft. Inst. Min. Metall., 80 {1980) 344-356. 7 Burstall, F.H., Forrest, P.J., Kember, N.F. and Wells, R.A., Ion exchange process for recovery of gold from cyanide solution, Ind. Eng. Chem., 45 (1953) 1648-1658. 8 Diamond, R.M. and Whitney, D.C., Resin selectivity in dilute to concentrated aqueous solutions, in: Marinsky, J.A. (Ed.), Ion Exchange, Arnold, London, Vol. 1, 1966, pp. 277-351. 9 Kitchener, J.A., Ion-Exchange Resins, Methuen, 1961. 10 Aveston, J., Everest, D.A. and Wells, R.A., Adsorption of gold from cyanide solutions by anionic resins, J. Chem. Soc., {1958) 231-239. 11 Duolite polymeric adsorbents, technical notes, Dia-prosim, France. 12 Rieman, W., Salting-out chromatography. A review, J. Chem. Educ., 38 (1961) 338-343. 13 Weber,W.J. and Van Vliet, B.M., Synthetic adsorbents and activated carbons for water treatment: overview and experimental comparisons, Res. Technol., (1981) 420-426.
154 14 Weber,W.J. and Van Vliet, B.M., Synthetic adsorbents and activated carbons for water treatment: Statistical analyses and interpretations, Res. Technol., (1981) 426-431. 15 Haines, A.K., Edwards, R.I. and Te Riele, W.A.M., The separation of gold from acidic leach liquors with amberlite XAD-7, in: Streat, M. (Ed.), The Theory and Practice of Ion-Exchange, Society of Chemical Industry, London, 1976, pp. 40.1-40.12.