Gunn, R . D.. Yamada. T., A.I.Ch.E. J.. 17, 1341 (1971). Guggenheim, E. A.. J. Chem. Phys.. 13, 253 (1945). Harmens, A., Chem. Eng. Scb, 20, 813 (1965); 21, 725 (1966). Klosek, J., McKinley, C., Proceedings, First International Conference on LNG, Chicago, Ill., 1968. Lu, B. C.-Y., Ruether J. A,, Hsi. C., Chiu, C.-H., J. Chem. Eng. Data, 18, 241 (1973). Mollerup, J.. Rowlinson, J. S.,Chem. Eng. Sci., 29, 1373 (1974). Racket!, H. G., J. Chem. Eng. Data. 16, 308 (1971). Rodosevich. J. B., Miller, R. C.. A.I.Ch.E. J., 19, 728 (1973). Shana’a. M. Y., CanfieM. F. B., Trans. Faraday Soc., 64, 2281 (1968) Spencer, C. F.. Danner, R. P.. A.LCh.E. J., 18, 230 (1973). Yen, L. C., Woods, S.S.,A.I.Ch.€. J., 12, 95 (1966).
Nomenclature
k = binary interaction constant V = volume x = mole fraction cy, /3 = adjustable parameters p = density 9 = scaling volume fraction w = acentricfactor Subscripts c = critical property i j , k = component identification m = mixture r = reducedproperty rs = reduced saturation property sc = scaling critical property
Department of Chemical Engineering University of Ottawa Ottawa, Ontario, Canada K I N 6N5
Paul Yu Benjamin C.-Y. Lu*
Receiued for review June 19, 1975 Accepted July 24,1975
Literature Cited Chiu, C. H., Hsi. C., Ruether, J. A., Lu, B. C.-Y., Can. J. Chem. Eng., 51, 751 (1973). Chueh. P. L., Prausnitz, J. M., Ind. Eng. Chem., Fundam., 6, 492 (1967)
The authors are indebted to the National Research Council of Canada for financial support.
Differential-Bed Study of Cyanide Treatment with Activated Carbon
Sorption rates measured for total cyanides, in the presence of copper and dissolved oxygen, were 40-70 pmol/g-hr for the optimum pH range of 6.5-8, in qualitative agreement with earlier integral bed data. Cyanide/ copper molar ratios 1 3 result in precipitation outside the activated carbon bed. Rate data for oxygen and copper do not correlate with those for cyanides.
Activated carbon has been proposed for treatment of cyanides in waste waters from electroplating shops, steel mills, and coke plants. This could provide a reliable lowcost alternative to conventional biological treatment and chemical “destruction” methods. Published data for the proposed treatment are encouraging, but the process chemistry is not yet well defined. Also, the validity of cyanide analysis techniques used earlier is now open to question (see Chemical Analysis). Previous studies with packed beds were concerned mainly with breakthrough capacity for cyanide removal. A differential-bed study was undertaken to extend data to conditions of higher cyanide loadings. Process Concepts
Activated carbon was used for treatment of cyanide wastes either as a trickle-phase reactor open to the atmosphere (Thiele, 1966) or as a packed-column reactor, under pressure and with both dissolved oxygen (D.O.) and Cu2+ ions present (Battelle, 1971; Bernardin, 1973). Commercial activated carbons derived from lignite, bituminous coal, and other raw materials were used, with pH values in the range 3-10. Successive oxidation and hydrolysis reactions were postulated (Thiele, 1966; Bernardin, 1973). (Battelle had a different interpretation. They attributed the CNOto homogeneous oxidation of cyanide with reduction of copper. Also, they had to use extended contacting with aerated 10% H2S04 for quantitative elution of copper and cyanide.)
CN- (sorbed) CNO-
+ 2H20
+ KO2 --*
-
HCOs-
CNO-
+ NHs
(1) (2)
The role of copper was explained as a “catalyst” for reaction 2, removing the products as deposits of mixed copper salts precipitated on the carbon particles (Bernardin, 1973). In support of this, all previous studies reported: (i) finding CNO- in the treated effluents; (ii) obtaining cyanide sorption in excess of 100 mg/g (4 mmol/g) with both copper and D.O. present vs. values tenfold smaller otherwise; and (iii) recovering sorbed copper quantitatively by acidification. Reaction 1 seems to be rate limiting, since only about 10% of the cyanide sorbed from oxygenated feed in extended runs remained on the bed (Bernardin, 1973) vs. up to 60% for cyanide sorbed from aerated feed (Battelle, 1971). At steady state, the rate of cyanide sorption would equal that of the rate-limiting oxidation. Rate Data
Observed sorption rates are listed in Table I for total cyanides (TCN), copper and D.O., covering conditions of pH 3-10, aeration or oxygenation, near-ambient temperatures, and cyanide concentrations of about 100 mg/l. of TCN. Besides differential-bed data from the present study, Table I includes values inferred from the referenced integral-bed reactor studies for runs lasting longer than the breakthrough of cyanide in the treated effluent. Data for runs with fresh carbon are omitted, since they characteristically had very high sorption rates. Our rate data for cyanide sorption lie between the extremes of the integral bed values, but closer to the higher values. The recommended conditions (Bernardin, 1973) of pH 6.5-8.0 and molar TCN/Cu ratios 2 3 result in 40-70 wmol/g of cyanide removal. Low TCN/Cu ratios are found Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
223
Table I. Rate Data for Activated Carbon Treatment of Copper-Cyanide Wastes Feed solution conditions Sorption rates, pmol/g-hr TCN
Cu
DO
Concentrations, TCN/Cu mg/l. molar pH ratio TCN DO
A. Thiele (1966) data for “trickling filter” apparatus using
activated carbon made from sewage sludge or lignite; 46 ml/hr-cm2flow rate; 45-90 cm bed height. -a Xb 10 noCua 115 8 11.5-14.5 7.8-10.3 2.5-3.4 X 10 3.0 80 8 0.5-1.6 X X 10 >3 50-300 8 B. Batelle (1971) data for packed column apparatus using Type OL bituminous-coal base activated carbon (Calgon Corp.); 370-2600 ml/hr-cm2 flow rate; 25-76 cm bed height. 80-142 X x 7 2.4C 100 8 130-220 X x 7 2.3 98 8 45-75 X X 3 2.0d 64 8 C. Bernardin (1973) data for packed column apparatus using Filtrasorb 300 and 400 bituminous coal base activated carbons (Calgon Corp.); 150-730 ml/hr-cm2 flow rate; 76 cm bed height. 4.7 4.7 10 noCu 20 36 5.2 X 3.4 7 4.6 19 30 100 158 15 X x 7 2.4 D. Present study data for packed differential column using Filtrasorb 400 bituminous coal base activated carbon (Calgon Corp.); 350 ml/hr-cm2flow rate; 6 cm bed height; temperatures 29-31°C. Ce 19 12 6.0 2.6d 32 34.4 28 7.7 5.1 6.0 4.1 38 7.2 11 18 7.0 3.5 60 34.0 70 58 11 10f 7.0 6.5 20 7.3 64 11 13.f 7.8 1.7 40 7.5 0 13 8.0 3.2 70 35.6 42 70 3.8 2.9 8.0 3.0 55 7.9 Ce Ce 2.9 8.0 3.2d 25 7.5 22 0 1.4 10.0 3.7 66 7.5 a Indicates copper not present. b X denotes insufficient data. C Znz+ also present. d Precipitation of solids in feed. e Rate cannot be calculated due to constant effluent concentration. f Uncharacteristically high D.O. sorption due to fresh activated carbon.
to result in precipitation outside the AC bed, forming yellowish-brown Cu(CN)2 at pH 1 6 and bluish-white Cu(OH)2 for pH 18,as observed earlier (Battelle, 1971). However, the rate data for oxygen and copper do not correlate with those for cyanides. Oxygen sorption is seen to depend only on the D.O. concentration, and the average rate for aerated conditions is 3.2 pmol/g-hr, essentially equal to D.O. sorption rates in the absence of cyanides (Prober et al., 1975). The large variations in copper sorption rates are attributed to a saturation effect, since acidification between our runs typically eluted only 25-50% of the copper sorbed in the immediately preceding run. This would be consistent with the observation that after breakthrough of copper occurs, addition of Cu2+ to the feed could be halted without impairing cyanide sorption (Bernardin, 1973). Experimental Section The apparatus shown schematically in Figure 1 was described in detail in an earlier paper (Prober et al., 1975). The glass column used here had a 2.8-cm inside diameter packed heights up to 12.7 cm. Test solutions were circulated through it upflow at 35 ml/min. Rates of sorption were calculated from concentration changes across the carbon bed 224
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
(3) or, preferably from a standpoint of accuracy, from concentration changes in the recirculated influent solution
(4) Here, r accounts for processes occurring outside the carbon bed, such as gas-liquid oxygen transfer in the feed tank. Stripping of HCN into the sparge gas, however, was found to be negligible (Vr/W < 6 pmol/g-hr) in control runs with the usual procedure except for bypassing of the carbon bed. Test solutions were made in 7-1. batches before each run from separate stock solutions of reagent CuSO4 and NaCN in deionized (D.I.) water. (The D.I. water also was pretreated with a separate activated carbon bed to remove trace organics.) The carbon bed was pretreated before each run by an acid wash with 150 ml of 0.1 N HCI, a D.I. water rinse to pH 4, a wash with 0.1 N NaOH to pH 10, and a final D.I. water rinse to pH 8. Figure 2 is a typical composite concentration history for conditions of pH 6-8 and molar ratios of TCN/Cu > 2.5. For all solutes in such runs, the p values calculated by eq 3 and 4 agree within experimental error, i.e., processes outside the bed other than oxygen transfer are negligible ( r = 0). Weighted average P values are listed in Table I. For pH 8 or in runs with precipitation outside the carbon bed, copper and/or cyanide sorption appeared to approach an equilibrium, as indicated in Table I. Data on free cyanide are not included here, since it did not amount to more than 10%of the initial TCN in any run and generally was not detected in the carbon effluents after the initial loading. Chemical Analysis Samples (20 ml) were withdrawn at about %-hr intervals. In all cases of precipitation, the samples were filtered before analysis. Copper was determined by the atomic absorption technique (Standard Methods, 1971) calibrated for reagent CuSO4 and Cu(CN)2 solutions. The D.O. probe was calibrated in place with aerated or oxygenated solution as standard, Analysis for TCN was a two-step procedure, which was adopted to avoid systematic errors that could lead to reported concentrations being low by factors of 1.5-10 (Goulden et al., 1972). First, the copper-cyanide complexes were converted to NaCN by a modified ion exchange procedure (Frant, 1971). The 10-ml samples were treated in small columns containing 25 ml of strong acid cation resin (Dowex 50W-8X)in H+ form and eluted with 60 ml of D.I. water; all 70 ml were collected in 130 ml of 0.1 N NaOH solution via a submerged discharge tube from the column. Then, analysis for free cyanide was carried out by specific ion electrode techniques. This included both direct analysis with a CNprobe and indirect analysis by the method of known additions with a specific Ag+/S2- probe (Frant et al., 1972). For standard solutions containing Cu2+ and 0.5-15 mg/l. of TCN, our two-step procedure gave essentially 100% yield with about 8% standard deviation. Other procedures such as digestion in boiling H2S04, volatilization of HCN, and reabsorption into NaOH (Standard Methods, 1971) or digestion at pH 2 in the presence of EDTA (Frant et al., 1972) were subject to standard deviations of 20-35%. Closure Differential-bed sorption rate data from the present study confirm earlier published data for activated carbon treatment of cyanides in the presence of Cu and dissolved
I I
ACTIVATED CARBON COLUMN
I
I I I
I I 1
I
SPARGING FOTAMETER
ROTAMETERS
Figure 1.
Schematic flow diagram
SHUT-OFF VALVE s
9
FEED TANK
0 I
I
$ I
for c o l u m n apparatus. Dashed lines represent flows used t o r o u t e t h e column i n f l u e n t t h r o u g h t h e
probe.
I
5o
-
n
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I-
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1
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7
out
TOTAL C Y A i i I O E
30
I
C"
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I
along with characterization of the activated carbon functional groups responsible for cyanide and copper sorption. Acknowledgments
The authors acknowledge the assistance and advice of Dr. Peter Melnyk in developing the procedure for total cyanide analysis, and Gregory Manning in the determination of sample copper concentrations. Activated carbon samples for this research were supplied by the Calgon Corporation. Financial support for one of the authors (W.E.K.) was provided in part by Training Grant No. T-900067 of the U.S. Environmental Protection Agency, Office of Manpower and Training. Nomenclature
10
3
c = concentration in solution, mg/l. or wmol/l. Q = flow rate through carbon bed, l./hr P = rate of sorption, wmol/g-hr r = rate of homogeneous reaction (outside carbon bed), fimol/l.-hr V = volume of solution circulated through column, 1. W = weight of activated carbon in column, g
I
I
I
I
I
I
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I
1
2
3
4
5
6
7
C
I
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hours
Literature Cited
O
I
I
Figure 2. Composite
I
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I
concentration h i s t o r y curves; 20 g b e d of trasorb 400; 2.1 1Jhr flow, 31"C, pH 7.
I Fil-
oxygen. However, further data are needed to characterize formation of CNO- and elucidate the role of copper and dissolved oxygen in the mechanism; to determine the effects of temperature and particle size; and to consider longterm effects. Further work on these lines is in progress,
Battelle Memorial Institute, "An Investigation of Techniques for Removal of Cyanide from Electroplating Waste," U.S. Env. Protect. Agency, Water Poll. Control Res. Ser., No. 12010 EIE, Nov 1971. Bernardin, F. E.. J. Waterpoll. ControlFed., 45, 221 (1973). Frant, M. S . , Plating, 58, 686 (197i). Frant, M. S . , Ross, J. W.. Jr., Riseman, J. H., Anal. Chem., 44, 2227 (1972). Goulden. P. D., Afghan, 8. K., Brooksbank, P., Anal. Chern., 44, 1845 (1972). Prober, R., Pyeha, J. J.. Kidon. W. E., "Interaction of Dissolved Oxygen and Activated Carbon," paper submitted for publication to A.l.Ch.E. J., 1975. Thiele, H.. Fortschr. Wasserchem. lhrer Grenzgeb, 9, 109 (1966).
Chemical Engineering Department Case Western Reserve University Cleueland. Ohio 44106
R. Prober* W. E. Kidon
Receiued for reuieru July 7, 1975 Accepted August 26,1975 Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
225
