Ind. Eng. Chem. Process Des. Dev. 1985,2 4 , 236-238
236
use in three-phase systems and on the development of a comprehensive model to predict mass transfer in vertical bubble flow.
BorWMnskiy, V. V.; Andreyevskly, A. A,; Bykov, G. S.; Zaletnev, A. F., Fokin, B. S.; Voiukhova, T. G. FluklMech.-SovletRes. 1977, 6 , 51. Butterworth, D.; Hewtlt, G. F. "Two Phase Flow and Heat Transfer"; Oxford university Press; Oxford, 1977. Clark, N. N. "Air Lift Pumps for the Hydraulic Transport of Solids"; Powder Advisory Centre: London, 1984. Clark, N. N.; Fiemmer, R. L. C. CHEMSA 1983, 9 , 62. Clark, N. N.; Flemmer, R. L. C. Chem. Eng. Sci. 1984a, 3 9 , 170. Clark, N. N.; Fiemmer, R. L. C. AIChE J . 1964b, In press. Cox, G. C.; Lewin, V. H.; West, J. T.; Brlgnal, W. J.; Redhead, D. L.; Roberts, J. G.; Shah, N. K.; Waller, C. B. Water Pollut. Control 1960, 79, 70. Danckwerts, P. V. "Gas-Liquid Reactions"; McGraw-HIII; New York, 1970. Danckwerts, P. V.; Rlzvl, S. F. Trans. Inst. Chem. Eng. 1971, 49, 124. Govter, G. W.; Azlz, K. "The Flow 01 Complex Mixtures in Pipes"; Van Nostrand-Reinhoid; New York, 1972. Grifflth, P.; Wallis, G. B. J . Heat Transfer 1981, 3 5 , 58. Harmathy, T. 2. AIChE J . 1960, 6 , 281. Hines, D. A.; Bailey, M.; Ousby, J. C.; Roesler, F. C. 1.Ch.E. Conference, York, England, April 1975. Hsu, Y.; Dudukovic, M. P. I n "MuRlphase Transport"; Veziroglu, T. N., Ed.; Hemisphere: Washington, 1980. Hsu, Y.; Graham, R. W. "Transport Processes in Boiling and Two Phase Systems"; McGraw-Hill; New York, 1976. Hughmark, G. A.; Pressburg, B. S.AIChE J . 1961, 7 , 677. Iida, Y. Bull. Soc. Mech. Eng. Jpn. 1980, 3 , 247. Jagota, A. K.; Rhodes, E.; Scott, D. S. Chem. Eng. J . (Lausanne) 1973, 5 , 23. Keitel, G.; Onken, V. Chem. Eng. Sci. 1961, 3 6 , 1927. Keitel, G.; Onken, V. Chem. Eng. Sci. 1982, 37, 1635. Kubota, H.; Honsono, Y.; Fujle, K. J . Chem. Eng. Jpn. 1978, 7 7 , 319. Lamont, J. C.; Scott, D. S. Can. J . Chem. Eng. 1966, 4 4 , 201. Linek, V.; Vacek, V. Chem. Eng. Sci. 1981, 36, 1747. Lockhart, R. W.; Martinelll, R. C. Chem. Eng. frog. 1949, 45, 39. Mangartz, K. H.; Pilhofer, T. H. Cbem. Eng. Sci. 1961, 36, 1069. Nassos, G. P.; Bankoff, S. G. Chem. Eng. Sci. 1967, 2 2 , 661. Orklzewski, J. J . Per. Technoi. 1967, 19, 829. Oshinowo, T.; Charles, M. E. Can. J . Cbem. Eng. 1974, 5 2 , 25. Robinson, R. E.; James, G. S.; Van Zyl, P. C. N.; Marsden, D. D.; Bosman. D. J. I n t . Conf. Peaceful Uses At. Energy, 1955. Scott, D. S.; Hayduk, W. Can. J . Chem. Eng. 1966, 44, 130. Serizawa, A,; Kataoka, I.; Michiyoshi, I. Int. J . Multiphase Flow 1975, 2, 235. Shah, A. K.; Sharma, M. M. Can. J . Chem. Eng. 1975, 5 3 , 572. Shlllmkan, R. V.; Stepanek, J. B. Chem. Eng. Sci. 1977, 3 2 , 1397. Spedding, P. L.; Nguyen. van T. Chem. Eng. Sci. 1980, 3 5 , 779. Wales, C. E. A I C M J . 1968. 72, 1166. Zuber, N.; Findlay, J. A. General Electric Co. Report GEAP-4592 (1964): for less detailed account, see J . Heat Transfer 1965, 8 7 , 453. Zuber, N.; Staub, F. W.; Bijwaard, G.; Kroeger, P. G. General Electric Co. Report GEAP 5417 (1967).
Acknowledgment The authors are grateful to David Wright of Natal University for his invaluable assistance in determining residence times in the DSR.
Nomenclature a = interfacial area per unit volume, m2/m3 Co = profile constant in drift-flux model
d = diameter of bubble, m d b = Sauter mean bubble diameter, m F = frictional losses, Pa/m g = acceleration due to gravity, m/sz
k , = liquid side mass transfer coefficient, m/s n = exponent in pressure loss correlation P = pressure, Pa R e = Reynolds number U,, = bubble drift velocity, m/s V = incremental volume, m3 W = superficial velocity, m/s x = length along pipe, m X, Y = dimensionless regime map multipliers 6 = gas voidage p = density, kg/m3 u = surface tension, N/m Subscripts 0 = at atmospheric pressure 1, 2 = in first and second incremental length a = air property g = gas property
1 = liquid property sp = single phase tp = two phase w = water property
Literature Cited
Received for review June 24, 1983 Revised manuscript received April 20, 1984 Accepted April 30, 1984
Ardron, K. H.; Hall, P. C. J . Heat Transfer 1980, 702, 3. Barnea, D.; Shoham, 0.; Taitel, Y. Chem. Eng. Sci. 1962, 3 7 , 741.
Separation of Gold Cyanide Ion from Anion-Exchange Resins Henry H. Law,' Wllson L. Wilson,+ and N. Elise Gabriel* AT&T Bell Laboratories, Murray Hill, New Jersey 07974
The separation of gold cyanide ions from anlon-exchange resins is enhanced when organidwater mixtures are used as eluent solvent. Mixtures of water and dimethylformamide, dimethylacetamide, acetone, N-methyl-2pyrroliiinone, dimethyl sulfoxide, hexamethylphosphoramide, or tetrahydrofuran show better elution than aqueous solutions. The elution rate Is faster at 45 OC than at 23 O C with the preferred eluent (5 M KSCN in 50 vol % dimethylformamide/water).
Introduction It is desirable to recover gold even from very low concentration sources because of ita high price. Strong base anion-exchange resins have been used to remove gold Department of Chemistry, Stanford University, Stanford, CA 94305. t Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139. 0196-4305/85/1124-0236$01.50/0
cyanide ion from plating rinse solutions. However, it is difficult to recover the gold from the resins by elution. The current method of recovering the precious metals from resins is by incineration, even though this is not environmentally sound and could result in substantial convective losses. Recently, Law (1982, 1983) found that a mixture of dimethylformamide (DMF) and water enhanced the separation of gold cyanide ions from Amberlite IRA-400 resins. These encouraging results prompted us to investigate what other solvents have properties similar to those 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24,
Table I. Donor and Acceptor Numbers of Some Selected Solvents solvent DN AN HMPA 38.8 10.6 33.1 14.2 pyridine 27.8 13.6 DMA NMP 27.3 13.3 THF 20.0 8.0 DMF 26.6 16.0 29.8 19.3 MezSO acetone 17.0 12.5 18 54.8 water
of DMF and what parameters influence the desorption. The use of organic solvents as eluting agents for ionexchange resins has been investigated for a long time. Organic solvents considered (Korkisch, 1966;Panse and Khopkar, 1975) have included alcohols, organophosphorus compounds, acetone, tetrahydrofuran, and dioxane. In the separation of gold from Amberlite IRA-400 resins, Burstall et al. (1953)reported that mixtures of organic solvents and mineral acids would elute gold more completely than aqueous eluting agents. The solutions investigated were 5-10% HC1 or HN03 in methanol, ethanol, acetone, or ethyl acetate. In some experiments, an additional 50% of water was added. Although a high percentage of gold was desorbed, their results are not practical. With the best mixture (acetone+% HC1-5% water), it took 100 mL of eluent to remove 96% of the gold (5 mg in 1 g of resin). This means that the effluent solution would have a gold concentration of 50 ppm. One obvious problem is the complete recovery of gold from this solution. Another problem is that AuCN can be precipitated from Au(CN)~by the acid. Burstall et al. (1953)reported the presence of a yellow precipitate in one experiment, and gold cyanide (AuCN) is a yellow compound. The fact that the present method of recovering gold is to incinerate the resins suggests that the approach of using acetone and mineral acids might have some scale-up difficulties. The selection of DMF/water as the eluent in Law's work (1982)was based on the solvation effects of DMF observed in AlCl, organic solutions. To search for other solvents that also elute gold cyanide well, we need to understand the ion-solvent interactions. A good solvent may influence the elution in two ways. It may directly weaken the bond between the quaternary ammonium group on the resin and the AU(CN)~ion so that another anion could exchange with Au(CN), ion. It may affect the solution equilibrium and thus change the distribution of Au(CN), between the resin phase and the solution. Currently, it is not clear what the mechanism is. One approach to estimate quantitatively the essential solution properties of solvents is the use of donor and acceptor numbers (Gutmann 1976,1978).Donor numbers (DN) are based upon the enthalpy of the reaction of a basic solvent with a 1,2-dichloroethanesolution of SbC1, and the DN of a solvent represents its nucleophilic character. Acceptor numbers (AN) pertain to anion bonding and are based upon the relative 31Pchemical shifts of (C2H,),PO in various solvents. Although this two-parameter method may have some shortcomings (Drago, 1980), it can still serve as a useful guide for selecting organic solvents to test whether they elute gold from resins. For this application, a solvent of high DN and low AN is desirable because the anions would be less solvated and the RESIN+AU(CN)~ionic bond could be weakened. Table I shows a list of organic solvents of relatively high DN and low AN: hexamethylphosphoramide (HMPA), pyridine, N,N-dimethylacetamide (DMA), N-methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF), N,N-dimethylformamide
No. 2, 1985 237
Table 11. Effect of Eluent Reagent on the Percent Gold Desorbed eluent % Au desorbed 2.44 M NH,SCN/50% DMF 39 2.44 M KSCN/5O% DMF 37 4 M NaOH/50% DMF 1.4 3.25 M thiourea/2 M KOH/75% DMF 8.2
(DMF), dimethyl sulfoxide (Me2SO),and acetone. Water is included for comparison. In this study, we investigated the influence of organic/water mixtures and the effect of temperature on the elution of gold cyanide ion from Amberlite IRA-400 and 900 resins. Besides NH4SCN, other reagents were considered. The organic solvents studied were those listed in Table I with the exception of pyridine. Pyridine was excluded because its influence is expected to be within the range of other solvents and its offensive odor would not make it as a likely candidate for process application. Experimental Section There are two approaches to determining the influence of the experimental parameters on the elution process: measuring the time when the breakthrough of an ion-exchange column occurs or measuring the amount of gold desorbed from the resin after the mixing of resin with eluent reaches equilibrium. Operating columns takes more time than the batchwise mixing of resin and eluent. With so many organic solvents and parameters to consider, the most efficient approach was chosen-the batch mixing of resin and eluent. The experiment was simplified as follows. Eight grams of gold-containing dried resin was mixed with 20 mL of eluent by a magnetic stirrer. A t the appropriate time, the stirrer was shut off and a sample of the eluent (0.1 mL) was taken after the resin settled (about 45 s). In some experiments, particularly those with THF and acetone, the resin was floating and made it difficult to take the samples. It was necessary to use disposable pipets with a fine tip (1 mm diameter) for sampling. At the end of the experiment, the resin was filtered and the volume of eluent was measured so that the total amount of gold desorbed could be calculated. The gold-containing resins were prepared by mixing about 250 g of resin (in the hydroxide form as received) with an appropriate amount of potassium-gold cyanide solution. The resin was then dried in a vacuum oven for 2 or 3 days at 50 "C. For this study, two batches were needed and the gold contents were 12.8% and 16% by weight. A different moisture content of the resin in the as-received stage or in the fiial dried state could cause the difference. In order to have meaningful comparison, the data were normalized as the percent of gold desorbed. The resin used was Amberlite IRA-400 with the exception of the experiment in which the elution performance from IRA-400 and 900 resins was compared. All solvents and chemicals were of reagent grade. The gold concentration were determined by atomic absorption spectroscopy. The accuracy of the data is estimated to be f5%. Results and Discussion The eluent reagents used in the initial study (Law, 1982) of gold elution with DMF/water solutions were NH4SCN and NaOH. The selection was based on the work of Vitkovskaya et al. (1977). Although the results are good, it is not clear that the combination of NHISCN and NaOH is the best for our present system. Table I1 shows the percent gold desorbed for several eluent reagents. The hydroxides do not elute gold readily by themselves. Thiourea helps but not significantly. As expected, KSCN
238 Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985 6 0 ' : : : : : : : : : c X.
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Figure 2. The elution rate of gold in 4.88 M KSCN 50% DMF solution at different temperatures. Table 111. Effect of Mixed Solvent on the Percent Gold Desorbed % Au desorbed solvent, vol % water 30 25% HMPA 38 50% HMPA 38 50% THF 36 50% acetone 52 25% DMF 45 50% DMF 60 25% DMA 48 50% DMA 53 75% DMA 52 25% MezSO 40 50% MezSO 39 75% MezSO 41 25% NMP 45 50% NMP 47 48 75% NMP
performs as well as NH4SCN. KSCN is preferred over NH4SCN for two reasons: it provides the potassium ions which make crystallization of potassium gold cyanide possible and eliminates the offensive ammonia odor. I t is necessary to determine the optimal KSCN concentration. Figure 1 depicts the nonlinear dependence of the percent gold desorbed on the concentration of KSCN in 50% DMF solution and water. The advantage of 50% DMF mixture over water is also shown. The highest percent gold desorbed is with 4.88 M. Therefore, this is the KSCN concentration used for investigating the effects of solvent and temperature. Before proceeding to such a study, we decided to work with only one type of resin. Because the Amberlite IRA400 and 900 resins are similar, their elution performance should not be greatly different. This is confirmed by the fact that the amount of gold eluted from these resins with 4.88 M KSCN 50% DMF solution was the same. Amberlite IRA-400 resin was chosen for the remainder of the work. Table 111depicts the effect of organic/aqueous mixtures on the percent of gold desorbed. All mixed solvents show improvements over the aqueous solution. DMF, DMA, NMP, and acetone perform better than Me,SO, HMPA, and THF. Although the 50% DMF is better than the other mixtures, the difference in the percent gold desorbed is not large, given the experimental error. In a process application, other factors would have to be considered, such as safety and health hazards. For example, HMPA should not be used because it is classified as a potential carcinogen. So are THF and acetone, which are also extremely flammable. Thus the selection of the solvent has to be based on the final design of the process. These results indicate that the donor-acceptor numbers should not be used as the only criterion for selecting
solvents for the elution of gold. Despite its high DN and low AN, HMPA did not show good desorption. It is likely that its large molecular structure leads to slow diffusion inside the resin. In addition, acetone elutes gold well though its DN is lower and its AN is higher. The temperature dependence of the gold desorption is illustrated with the 4.88 M KSCN in 50% DMF (Figure 2). Temperature does not influence the elution equilibrium. The desorption rate is faster at 45 and 62 OC than a t 23 "C. Figure 2 also indicates that the equilibrium concentration was not limited by the solubility of potassium gold cyanide in the 50% DMF eluent. In summary, the separation of gold cyanide ions from anion-exchange resins (Amberlite IRA-400 and 900 OH) is enhanced when organic/water mixtures are used as the eluent solvent. Mixtures of water with DMF, DMA, NMP, acetone, Me2S0,HMPA, or THF show better elution than aqueous solutions. The KSCN is found to perform as well as NH4SCN,and it is preferred because the ammonia odor is offensive and the presence of potassium ions makes the crystallization of potassium gold cyanide possible. The elution equilibrium is not affected by temperature, but it is reached in a shorter time at 45 OC or higher. Based on this equilibrium study, the recommended eluent for the gold recovery from anion-exchange resins is 4.88 M KSCN in 50% DMF. The "optimal eluent" has to be determined in pilot-plant scale studies. Acknowledgment The resins used in the this study were kindly provided by the Rohm and Haas Co., Philadelphia, PA. Registry NO. DMF, 6812-2; DMA, 127-19-5; NMP, 872-50-4; HMPA, 680-31-9; THF, 109-99-9; Me,SO, 67-68-5; KSCN, 33320-0; Au, 7440-57-5; acetone, 67-64-1.
Literature Cited Burstali, F. H.; Forrest, P. J.; Kember, W. F.; Wells, R. A. I d . Eng. Chem. 1953. 45. 1648. Drago, d. S.'Pure Appl. Chem. 1960, 52, 2261. Gutmann, V. Electrochim. Acta 1976, 21, 661. Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions"; Plenum Press: New York, 1978. Korkisch, J. Sep. Sci. 1966. 7 , 159. Law, H. H. "Recovery of Gold from Ion Exchange Resins with Dimethylformamidelwater Mixture"; Proceedings of the International Precious Metals Institute Conference, Newport Beach, CA. June 1982. Law, H. H. US. Patent 4372830, 1983. Panse, M.; Khopkar, S. M. J . Sci. I d . Res. 1975, 3 4 , 612. Vltkovskaya, A. P.; Kuznetsov, V. I.; Zaitseva, V. Tsvet. Mtal. 1977. 5.77.
Received for review September 13, 1983 Accepted April 11, 1984 This paper was originally presented at the 1982 Annual Technical 'Conference of the American Electroplaters' Society, which has given the authors permission to publish in this journal.
