Ind. Eng. Chem. R e s . 1987,26, 1716-1719
1716
i -
-1
-
r
m
(30-200 mg). By use of samples from sink-float separation procedures, a wide range of organic and inorganic contributions can be studied. Results for spent Green River oil shales at 978 K agree reasonably well with those derived from classical bomb calorimetry, but there are important questions remaining on the contributions of mineral reactions to the enthalpy.
7
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Acknowledgment
-2-
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-4
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2
3
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I
4
5
6
7
8
9
10
ORGANIC CARBON ( % OF SPENT S H A L E )
Figure 2. Comparison of our high-temperature combustion values to values calculated from the work of Burnham et al. (1982): (- - - -1 assumes 0% silicates and (-) assumes 100% silicates.
initive statement on the occurrence of silicate reactions in our calorimeter, since some of the points in Figure 2 support one assumption and some the other. Conclusion High-temperature Calvet calorimetry is a powerful means of determining accurate enthalpy values for oil shale processes at high temperatures, using very small samples
We thank Charles J. Vadovic for providing us with the valuable series of sink-float samples and James E. Hardy for the suggestion of humidifying the oxygen stream. We also thank the many colleagues at Exxon who have offered valuable comments and suggestions during the course of this work. Literature Cited Burnham, A. K.; Crawford, P. C.; Carley, J. F. Znd. Eng. Chem. Process Des. Dev. 1982, 21, 485. Burnham, A. K.; Stubblefield, C. T.; Campbell, J. H. Fuel 1980,59, 871. Mraw, S.C.; Keweshan, C. F. J. Chem. Thermodyn. 1984,16,873. Mraw, S. C.; Keweshan, C. F. Ind. Eng. Chem. Fundam. 1985,24, 269. Mraw, S. C.; Keweshan, C. F. Fuel 1986, 65, 54. Mraw, S. C.; Kleppa, 0. J. J. Chem. Thermodyn. 1984, 16, 865. Vadovic, C. J. In Geochemistry and Chemistry of Oil Shales; Miknis, F. P., McKay, J. F., Ed.; ACS Symposium Series 230; American Chemical Society: Washington, D.C., 1983; pp 385-396. Receiued for review October 1, 1986 Revised manuscript received May 15, 1987 Accepted June 3, 1987
COMMUNICATIONS Regeneration of Ion-Exchange Resins Used for Gold Recovery Ion-exchange resins are currently used at gold-plating facilities to collect gold from waste solutions. T h e gold-containing feed solution is passed through an anion-exchange resin column where gold is adsorbed as AU(CN)~-.The present method of recovering the adsorbed gold is by drying and incineration of the resin. An alternative method involves eluting gold from the resin by use of a concentrated KSCN solution in a mixed solvent of water and DMF, followed by crystallization as potassium gold cyanide. A two-step process for regeneration of ion-exchange resins from which gold has been extracted has been developed. This process involves conversion from the initial thiocyanate form of the resin to the chloride form, followed by conversion to the hydroxide form. The operating temperature range has been determined to be 45-55 "C. Changes in flow rate and eluent concentration show little effect on the rate of thiocyanate elution. 1. Introduction
Ion-exchange resins are currently used at gold-plating facilities to collect gold from waste solutions. The goldcontaining feed solution, obtained from process rinse water, waste plating solutions, plating dragout solutions, and waste washing solutions, is passed through an anion-exchange resin column where gold is adsorbed as AU(CN)~-. Typically, these columns are run until they contain 10-20 wt 7% gold. While the high affinity for Au(CN)< makes it easy to adsorb gold-onto the resin, it also makes it extremely difficult to elute. The present method of recovering the 0888-5885/87/2626-1716$01.50/0
adsorbed gold is by drying and incineration of the resin. The gold-containing resin is fed to an incinerator where the organic components are burned off. The residue is then converted in several steps to potassium gold cyanide. However, this process has two major drawbacks. Firstly, there is the potential for substantial loss of gold, both through stack loss and also through the multistep conversion to potassium gold cyanide. Secondly, environmental hazards are encountered due to the incineration of the organic resin material. In an effort to fiid a simpler and more environmentally sound method for recovering gold from ion-exchange re0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1717 sins, Law (1982,1983) and Law et al. (1985) developed a process by which gold can be eluted from the resin and recovered directly as potassium gold cyanide (Law and Gabriel, 1986). The eluent, consisting of a concentrated KSCN solution in a mixed solvent of water and DMF, is fed to the gold-containing resin column. Under these conditions, thiocyanate ion has a greater affinity for the resin and the gold is eluted quickly and completely. The concentrated effluent is then passed to a low-temperature crystallizer where rapid precipitation of highly pure KAu(CN), occurs. While this process affords simpler gold recovery, it also leads to an accumulation of the thiocyanate form of the resin to be disposed of. A more efficient operation would involve a process for regeneration of the resin so it could be used for many cycles. This would reduce waste and make the gold recovery more environmentally and economically attractive. The OH- form of Amberlite IRA-SOOC ( R o b and Haas Co., Philadelphia, PA) is the resin of choice for gold collection, making direct conversion from the SCN- form to the OH- form the most desirable regeneration process. However, experiment shows this equilibrium to be unfavorable, and the conversion is impractical. According to Kunin (1958), 2 M HCl has been used to regenerate the chloride form from the perchlorate form of anion-exchange resins. It is also known that conversion from the chloride form to the hydroxide form is possible, with commercially available anion-exchange resins of the hydroxide form being prepared by this process. Since it has been found that perchlorate ion has an even higher affinity for the resin than thiocyanate ion (Kunin, 1958), it should be possible to convert the thiocyanate form to the chloride form. Therefore, a two-step regeneration process is proposed, with initial conversion of the SCN- form to the C1form, followed by conversion of the C1- form to the OHform. Since the initiation of this work, a process has been developed (Fleming, 1986) in which a solution containing ferric ions is used to elute thiocyanate from strong-base resins with approximately 90% efficiency. However, unless flow conditions are adequately maintained, there is a possibility of forming anionic ferric thiocyanate complexes which may cause readsorption of the eluted thiocyanate. Also, introduction of ferric ions into the system provides a source of possible contamination of the KAU(CN)~ produced in the gold recovery process. A regeneration process utilizing only potassium salts would virtually remove any risk of cationic contamination. The objective of this work is to demonstrate the feasibility of using KC1 to convert the resin from the thiocyanate form to the chloride form and then using KOH to regenerate the resin to the hydroxide form. In addition, the effect of the operating parameters on the regeneration process was determined. 2. Experimental Section Amberlite IRA-9OOC ion-exchange resin, with an effective particle size of 0.68 mm (Rohm and Haas Co., 1981) was used for all experiments. Equilibrium data were obtained for the following systems: SCN-/OH-, SCN-/Cl-, and Cl-/OH-. The C1- form of the resin was used as received, while the SCN- form was prepared by passing the appropriate amount of 1 M KSCN through a column containing the C1- form. Equilibrium experiments were performed by the batch method of mixing resin and eluent. All experiments involved adding 2 M eluent solution to an appropriate amount of resin and mixing with a magnetic stirrer at 45
"C for 60 min. The resin was then filtered, and the filtrate was analyzed for the concentration of the appropriate desorbed species. Solid-phase concentration, q (mequiv of solute/g of resin), and fluid-phase concentration, c (N), were determined from these data. q is calculated from q = (qrefw
-W)/W
(1)
where qmfis the capacity of the resin for the particular ion, w is the weight of resin (g), u is the volume of solution (corrected for liquid sorption by the resin), and c is the fluid-phase concentration. Both small- and large-scale columns were used for the regeneration process. The small-scale column had dimensions of 38-cm height X 1-cm diameter and held 20 g of wet resin for all experiments. The operating temperature was 45 "C, and flow rates were 0.5 and 3.0 mL/min for 2 M KC1 and 2 M KOH and 0.5 mL/min for 4.88 M KSCN in 50% DMF. The large-scale column had dimensions for 60-cm height X 5-cm diameter and held 200 g of wet resin for all experiments. I t was run at temperatures of 45, 55, and 60 "C with flow rates of 3.5 and 7 mL/min for the KC1 and KOH eluents. KC1 concentrations of 1,2, and 3 M were used. KOH concentrations were 1 and 2 M. A 5 g/L KAu(CN12solution was used to load the resin with gold and was run at a flow rate of 20 mL/min at 25 "C for both columns. Operating temperatures were maintained by use of heated jackets on the columns. Samples were taken at appropriate time intervals and were analyzed for the concentration of the appropriate desorbed species in order to determine the various breakthrough curves. The low initial effluent concentrations were caused by dilution from the mixing of eluent with the water present in the column. Analysis of gold was performed by atomic absorption spectroscopy. Analysis of C1- was performed by titration according to Volhard's method. Analysis of SCN- was performed by oxidation to Sod2with bromine-water, followed by gravimetric determination as BaSO, (Vogel, 1978).
3. Results and Discussion Results of equilibrium experiments for the systems resin-SCN- + C1- = resinC1-
+ SCNresimC1- + OH- = resimOH- + C1resimSCN- + OH- = resimOH- + SCN-
(2)
(3)
(4)
are shown in Figure 1as plots of solid-phase concentration ( q ) vs. liquid-phase concentration (c). High values of q correlating with low values of c indicate high affinity of the ion in question for the resin and therefore poor equilibrium for desorption. It can be seen that in the case of both SCN-/Cl- and SCN-/OH-, thiocyanate ion has a much greater affinity for the resin. It is obvious, however, that the equilibrium is more favorable with chloride ion (qma = 1.6) than with hydroxide ion (qmax= 2.1). It can also be seen that conversion from the chloride to the hydroxide form is even more favorable (qmar = 1.2). These results support the decision to pursue a two-step regeneration process, with initial conversion from the thiocyanate form to the chloride form, followed by conversion to the hydroxide form. The small-scale column was run through three cycles of gold collection, desorption, and regeneration. Figure 2 depicts the breakthrough curves for desorption of thiocyanate for the first two cycles. The plots shown are of
1718 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
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6
8
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Eluent V o l u m e (ml/g wat r e s i n ) 2M KCl. 45'C. 3 ml/min
Figure 2. Thiocyanate concentration of effluent vs. eluent volume/gram of wet resin for two regeneration cycles of small column: (*) cycle 1; (+) cycle 2.
effluent concentration vs. volume of eluent per gram of wet resin. It was found that 75% of the thiocyanate could be removed from the resin in 80 min with use of 2 M KC1 eluent at 45 "C. Since there is no significant change in the shape of the curve upon repeated regeneration, it can be inferred that there is no negative effect on the resin. Breakthrough curves for desorption of chloride from this column are shown in Figure 3. It was found that 100% of the chloride form was converted to hydroxide form by 2 M KOH eluent after approximately 1h, and again there was no significant change in the breakthrough curve upon regeneration. The column was also checked for changes in gold capacity. After one regeneration cycle, the capacity was 2.9 mequiv of Au/g of wet resin, and this value showed no decrease upon subsequent regeneration. Resin containing 10-30 wt % gold corresponds to capacities of 0.6-2.2 mequiv of Au/g of wet resin, so the gold capacity of the resin
I
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c (N)
Figure 1. Solid-phase concentration vs. fluid-phase concentration for equilibrium systems involved in resin regeneration, 45 OC, 2 M eluent solution: (*) [SCN-] in SCN-/Cl- system; (0)[Cl-] in C1-/ OH- system; (+) [SCN-] in SCN-/OH- system.
0
+
Figure 3. Chloride concentration of effluent w. eluent volume/gram of wet resin for same column as in Figure 2: (*) cycle 1; (+) cycle 2.
upon regeneration is more than adequate for practical use for gold recovery. Once the two-step regeneration process had been proved viable, the large-scale column was used to study the effect of various process conditions on the elution rate of thiocyanate. The first condition studied was temperature. The optimum operating temperature for the original gold recovery process had been determined to be 45 "C (Law et al., 1985), and since elution rate generally increases with increasing temperture, there was no need to attempt regeneration at a lower temperature. Although the maximum operating temperature of the IRA-9OOC resin is quoted as being 60 "C for processes involving hydroxide (Rohm and Haas Co., 1981), running the column at this temperature for regeneration produced an unacceptable capacity loss and a physical texture change which was attributed to breakdown of the resin. Thus, the operating temperature range should be below 60 "C. Running the column at 55 "C showed no negative effects on the resin, but swelling as a result of this high a temperature may be a problem in larger columns. The next condition examined was flow rate. The column was run using a 2 M KCl eluent at 45 "C and flow rates of 3.5 and 7.0 mL/min. No significant difference was found between the curves for these flow rates, indicating that doubling the flow rate does not slow down thiocyanate elution. Under these conditions, 50% of the thiocyanate can be desorbed in approximately 1h. This value is lower than that for the small-scale column but can probably be increased by passing more eluent. The last condition studied was eluent concentration. Figure 4 shows breakthrough curves for thiocyanate desorption using l , 2, and 3 M KC1 eluent at 45 "C and a flow rate of 7 mL/min. One would expect to see a significant improvement in elution rate with increasing eluent concentration, since the presence of a greater amount of chloride should favor desorption of thiocyanate in the SCN-/Cl- equilibrium. However, this is not the case, as evidenced by Figure 4. There appears to be a trend toward better elution with more concentrated eluent, but no significant differences in the breakthrough curves are seen. This may be due to the fact that the rate-determining step of the elution process is the pore diffusion of thiocyanate out of the resin. Therefore, this regeneration process is
Ind. Eng. Chem. Res. 1987,26, 1719-1721
eluent concentration do not appear to have a significant effect on the rate of thiocyanate elution. There is no decrease of gold capacity of the resin with successive regeneration cycles. There is also no decrease in the efficiency of regeneration with successive regeneration cycles. This regeneration process should be valid for Amberlite IRA-400 resin as well as IRA-SOOC, due to the similarities in the chemical properties of the two. Registry NO,Au, 7440-57-5; KCl, 7447-40-7; KOH,1310-58-3; amberlite IRA-SOOC, 59979-80-5.
0
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45'~.
1719
(ml/g wet r n s ~ n ) 7 ml/min
Figure 4. Thiocyanate concentration of effluent vs. eluent volume/gram of wet resin for various eluent concentrations in large column: (*) 1 M KCl; (+) 2 M KC1; (0) 3 M KCl.
feasible with an eluent concentration as low as 1 M. In summary, regeneration of the ion-exchange resin (Amberlite IRA-9OOC) used for gold recovery is possible by a two-step process. The first step involves use of KCl eluent to convert the thiocyanate form resin to the chloride form. The second step similarly converts the chloride form to hydroxide form using KOH eluent. The recommended operating temperature range is 45-55 "C. Flow rate and
Literature Cited Fleming, C. A. U.S. Patent 4608 176, 1986. Kunin, R.Zon-Exchange Resins; Wiley: New York, 1958. Law, H. H. Proceedings of the Sixth International Meeting of the Precious Metals Institute Conference, Newport Beach, CA, June 1982; p 503. Law, H. H. U.S. Patent 4372830, 1983. Law, H. H.; Gabriel, N. E. Znd. Eng. Chem. Process Des. Deu. 1986, 25, 352. Law, H. H.; Wilson, W. L.; Gabriel, N. E. Znd. Eng. Chem. Process Des. Dev. 1985, 24, 236. Rohm and Haas Co. "Amberlite Ion Exchange Resins, Fluid Process Chemicals and Apparatus", May 1981. Vogel, A. I. Textbook of Quantitatiue Inorganic Analysis, 4th ed.; Longmans: New York, 1978; p 510. 'Present address: Division of Math and Science, Northeastern State University, Tahlequah, OK 74461.
Joyce Sapjeta,* Henry H. Law, Jack R. Caseboldtt AT&T Bell Laboratories Murray Hill, New Jersey 07974 Received for review December 29, 1986 Accepted May 29, 1987
Measurement of Particle Concentration by Sampling from Liquid Fluidized Beds A device for withdrawing samples isokinetically from liquid fluidized beds is described. Sampling experiments on single- and two-component fluidized beds are reported, and volumetric concentrations of solid are compared with results generated by independent means. The sampling technique could be used with other fluid particle operations such as sedimentation or crystallization . The study of mixing and segregation in liquid fluidized beds continues to arouse significant research interest. A model which describes such phenomena in terms of an eddy diffusion coefficient was developed some 20 years ago (Kennedy and Bretton, 1966): this model is still often used as a basis for interpreting and describing experimental observations. Formulae or correlations which would allow a priori prediction of the diffusivity, or dispersion coefficient, in the model in terms of the physical properties of the solid and liquid components of the fluidized bed do not as yet exist. One common approach to studying mixing is to measure the axial concentration profile of a particular solid component within a bed and then to evaluate the diffusivity which most closely matches an experimentally observed profiie with that of its theoretically predicted counterpart. For instance, AI-Dibouni and Garside (1979) were able to section their fluidized bed, effectively instantaneously, by introducing fine meshes across the column cross section at a number of discrete axial locations. Average compositions within each zone were then determined by manual separation and weighing of the two solid components which were present in the samples. In a program of similar work (Juma and Richardson, 1983), samples were physically 0888-5885/87/2626-1719$01.50/0
removed from fluidized beds using a device which passed through the bed wall. The samples were then washed, dried, weighed, and seived in order to determine the relative proportions of each size particle present. This method means that samples can only be taken at a discrete number of axial locations and a correction factor had to be applied to the results in order to account for nonideal effects inherent in the technique: the consequence of the latter was to sample small particles preferentially because of their lower inertia. Van der Meer et al. (1984) review a number of other studies of mixing in liquid fluidized beds and in their own work measured a particle axial dispersion coefficient by a photographic technique. For beds consisting of a binary mixture of solid components, the mixing volume which separated bed zones containing essentially a single pure solid component was photographed. If combinations of opaque and transparent particles were picked, the negatives which resulted could be scanned by laser and a light transmission curve obtained. When expressed in an appropriate logarithmic form, these data could be presented as a linear graph, the slope of which was related directly to the particle dispersion coefficient. Unfortunately this elegant technique may be unduly sensitive to wall effects 0 1987 American Chemical Society
