May 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
The slightest leakage or exhaustion of the cation bed is immediately indicated by increased p H and canductance of the anion bed effluent. The silica breakthrough develops rapidly and coincides closely with that of the carbon dioxide. Sometimes a drop of p H from around 8 t o 7.5 precedes the appearance of silica in the finished water, but this cannot be relied upon as a warning. Water will have to be checked frequently for silica when beds a p proach the exhaustion point.
day, using the anion‘ exchange resin and the regenerating procedure described. Silica in the finished water has averaged less than 0.05 p.p.m. and the total solids less than 0.5 p.p.m. Total exchange capacity has continued closely at 16,000 grains per cubic foot. This is slightly better than that of the pilot plant, owing to more complete removal of carbon dioxide by cold water deaeration. LITERATURE CITED
(1) Wheaton, R. M., and Bauman, W. C., IND. ENO.CHEM.,43, 1088 (1951).
SUMMARY
Since the presentation of this paper a plant has been in operation for 5 months, demineralizing 2,500,000 gallons of water per
r
1079
RECEIVED June 8, 1950. Presented before the Division of Water, Sewage, CHEMICAL and Sanitation Chemistry at the 117th Meeting of the AMERICAN SOCIETY,Detroit, Mich.
Metal Recovery by Cation Exchange A. B. MINDLER, M. E. GILWOOD, The Permutit Co., New York, N. Y.
A N D G.
H. SAUNDERS
T
HE chemical literature is rich in reports concerning the recovery of valuable metals by cation exchange (S,5,6,19, 17)
and anion exchange ( 2 1 , 16). Inorganic types of cation exchangers were used in some of this work, but in most of the experiments the organic types of ion exchangers such as the sulfonated coals and synthetic resins were employed. The organic cation exchangers are far more stable to the acid solutions necessary for the efficient recovery of the metal from the exhausted cation exchanger. In all the work reported previously the metal was present in substantially neutral solutions, and the metal ion or its complex was exchanged for other cations or anions on an ion exchanger. This paper reports two unique processes for metal recovery by cation exchange: The first is the recovery of tin from sodium stannate solutions by passing the solutions through a hydrogen ration exchanger which converts the salt to the insoluble metastannic acid. The precipitate may be concentrated by the sludge filtration principle. The dense flocculent material may then be converted to relatively concentrated sodium stannate which may be utilized directly or electrolyzed for the recovery of metallic tin. The second process described is the recovery of the metal cations of zinc from strongly acidic solutions utilizing a eulfonated crosslinked polystyrene type cation exchange resin. Solutions of these two types constitute a portion of the wastes of plating and textile industries. These recovery processes are of interest because of the monetary value of the metals and reduction of stream pollution. RECOVERY OF T I N FROM SODIUM STANNATE SOLUTIONS
Experimental. The general techniques of employing ion exchangers in laboratory work have been described in considerable detail (10, 18). However, in order to obtain maximum concentrations of recovered metal in tin recovery by this ion exchange process, it was necessary to employ a special procedure consistin of “sludge filtration.” This process has been widely employe$ in water treatment by both cold and hot processes (4, 8, 18) and in white water recovery in the paper industry (9). The ion exohange equipment emplo ed in the laboratory for the first of these processes is illustratedrin Figure 1. The e q u i p ment consisted of a glass tube, l l / a inches inside diameter, containing a 100-ml. bed of Permutit &. Regeneration waa canducted with 15% sulfuric acid employing the equivalent of 10 pounds per cubic foot and a contact time of 25 minutes. The bed was then rinsed substantially free of mineral acidity with demineralized water. The sodium stannate solution was passed upflow through the Permutit Q bed a t a flow rate sufficient t o raise the bed and maintain it in sus ension but insufficient to wash resin particles out of the tube ($0 to 150 ml. per minute). T h e tube was enclosed in a hot water jacket in order to maintain
This work was undertaken to contribute to the solution of waste disposal problems, conservation of strategic metals, water, and heat values. The results of this work indicate that it is possible to employ ion exchange for the recovery of tin, presently going to waste, from electroplating operations using alkaline baths. Furthermore, the detinned rinse water will be suitable for further use in rinse operations. The paper also outlines some preliminary data on removing and recovering zinc from relatively concentrated sulfuric acid solutions by using the sulfonated polystyrene cation exchange resin, Permutit Q. This preliminary report describes two more applications for ion exchange in industry. The tin recovery process comprises an interesting technique of employing ion exchange-namely, to cause precipitation of a floc which may then be removed from the rinse water by sludge blanket filtration. The zinc recovery includes use of a unique cation exchange resin for removing metal cations from strong acids, and the process lends itself to application with other metals and other acids.
operating temperatures of 170’ F. The effluent-carryin metastannic acid floc was handled by either of two methots: (a) settling or (b) sludge filtration utilizing apparatus similar to that described by Carpenter and Porter (9). This consisted of a 5gallon carboy fitted as shown in Figure 2. Analytical Procedure. The tin content in the influent and effluent was determined colorimetrically by a modification of the cacotheline method (1.4). A 10-ml. sample containing tin is reduced by adding magnesium metal and concentrated hydrochloric acid and agitating to ensure contact between hydrogen and stannic tin. After evolution of gas is completed, 0.1 ml. of 0.25% aqueous solution of cacotheline iu added. The presence of a violet color indicates tin. Suitable blanks were also run to establish that the demineralized water and reagents contained nothing that would interfere with the color determination. The nccuracy of this method is 1 p.p.m. of tin. Tin Recovery. Sodium stannate wastes are derived from the rinses of continuous sheet metal tinning mills of the electrolytic type. The waste water has a temperature of about 170” F. It contains approximately 600 p.p.m. of sodium stannate in addition to the salts in the water used for rinsing; the p H is 8.1. A typical tin plating bath of sodium stannate was diluted t o 675 p.p.m. of tin with demineralized water and used as influent solution in all the work reported.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1080
Vol. 43, No. 5
exchangers split the stannate salt to a small degree because the p H rose from 8 t o 11.5. However, less than half the tin war r+ moved by either of the anion exc’hangers. An ammonium cation exchangei was utilized to convert the sodium stannate to ammonium stannate which in turn would produce weakly basic ammonium hydroxide after anion exchange of hydroxyl ion for stannate ion. Na2Sn03 (K€€&Z e(NH&Sn03 Na2Z (NH&Sn03 2rZXOH +2NHaOH (A2&SnO7
.
+
+
+
+
n here XHJ is an ammonium regenerated cation exchanger, and AXOH is a highly basic anion exchange resin. Although thir procedure did remove the tin completely, the anion exchanger capacity vi a9 low. This two-step cation-anion exchange process wa? followed by another ration exchanger operating in the hydrogen cycle t o remove the ammonium ions so that the water could be re-used for rinsing the tin plat^. The second cation exchanger bed could then he employed as the Ammonium exchanger after it had become exhausted.
S e t t l i n g Rate of Metastannic Acid i n EfRuent
Table I.
Time, rnm. 1 2 4 5 6 8 12 25 30
____
From Cation Exchanger Water,
Sludge,
5 10 20 30
95 90 80 70
ml.
40
50 60 70 75
ml.
60 50 40
30 25
Cation Exchange. In view oi the incomplete removal of tin by the anion exchanger, it was decided to employ a hydrogen cation exchanger t o convert the sodium stannate to insoluble metastannic acid:
H2Z Figure 1.
Laboratory Ion Exchange E q u i p m e n t
During the preliminary work on this project, a number of different processes were investigated briefly t o determine the best type of ion exchange process. Among the possibilities investigated were both anion exchange and cation exchange processes and combinations of these. Both wealcly and highly baEic anion exchangers were tested to recover the tin from the sodium stannate waete solution. Anion Exchange. The m w t obvious ion exchange process t o employ for recovering stannate anion without recovering water for re-use is t o exchange it for a harmless anion from a cheap salt. For example, a weakly basic anion exchanger may be exhausted with chloride anions which are then exchanged for the stannate anions.
R3N
2(R3?U”)Cl
+ HCl -++(R8NH)CI
+ S a e S n 0 3a (R3WH)2SnO8 + 2NaCI + 2KaOH +R3N + 2Hz0 + NaaSnOs
(R,SH2)SnO3
where R& is a weakly hasic anion exchange resin (De-Acidite) However, only about 80% of the tin was exchanged, and the capacity of the anion exchange resin was very low. Two types of Rtrongly basic anion exchanger, the guanidine type and the quaternary ammonium type, were then used in an effort t o conduct the foIIowing reartion: 2AXOH
+ Na~Sn03
(.4X),SnO3
+ 2NaOH
whpie AXOH is a highly basic anion exchange resin.
Both ion
+ Xa2SnOl --+ HnSnOs + NazZ
This ieaction is driven to completion by the formation of insoluble products. A samplci of effluent was collected in a 100-mI. graciuated cylinder and allowed to settle. The settling rate is shown by- the data in Table I. The sludge, after settling for 8 hours, contained 1.69oJ, solids by weight and had a specific gravity of 1.014 by the pycnometer method. A sludge of this character settles wvell and niay be separated by the sludge filtration method. The concentrated sludge may be dissolved in hot 20% sodium hydroxide forming Ka26nO8,which is useful for make-up t o the electroplating bath or may be electrolyzed to reclaim metallic tin anodes. In a typiral run, 22 liters of rinse water containing 075 p.p.111.of sodium stannate were passed through a 100-ml. Permutit Q bed and filtered by the sludge filtration principle before more than a trace of tin w a b present in the effluent. This is equivalent to a capacity of 1400 me. per liter. Several times, during the courCie of a number of runs, when slow upflow rates werc investigated, the beads of resin became coated with a white precipitate of metastannic acid, but this did not appear t o interfere with high capacity or mode of operation of the bed, and the precipitate could be removed by increasing the flow rate t o the optimum. The clarified water, after separation of the precipitated metastannict acid, possessed a turbidity of about 3 p.p.m. (APHA) and about 30 p.p.m. sodium; the pH was about 3 Such water can be neutralized and re-used for rinsing tinned sheet metal; thus both the water and the heat values in the rinse mater are conserved. REMOVAL OF METAL CATIONS FROM STRONG ACIDS
Until the development of sulfonated cro-linked p o l y s t ~rene ration exchange resins, it was impossible to remove mrtall~c
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
May 1951
E F F L lJENT FROM PERMUTIT-a
COLUMN
F
/---
UBBER DEFLECTOR
HzS04, % 0.05 0.1
BUSHING
OOURTEBY PAPER TRADE JOURNAL
Laboratory Apparatus for Sludge Filtration
cations from strongly acid solutions by ion exchange. The Donnan effect in the diffusion of ions into resin particles has been discussed by Bauman (u“) and others. The concentration of electrolyte inside the ion exchange resin particle is always less than t h a t of the solution outside the particle. This effect is utilized in the removal of metal cations from acid solutions because a t equilibrium the hydrogen ion concentration inside the particle is not high enough t o disturb the ion exchange equilibrium. The effect of the Donnan membrane equilibrium is not nearly as marked for other types of organic cation exchangers such as sulfonated coals and sulfonated phenolic resins, and therefore it is impossible to remove metal cations from strongly acid solutions with these substances.
Experimental. For the second process in question, the removal of metal cations from strongly acid solutions, conventional downflow operation was used in 15-mm. inside diameter tubes containing 100-ml. beds of Permutit Q, a sulfonated cross-
500
400
V
E
300
N
z
a P 200
IO0
0
0
Figure 3.
2
4 6 EFFLUENT VOLUME
linked polystyrene cation exchange resin. Regeneration was accomplished by employing 25 t o 30% sulfuric acid and a dosage of 10 pounds per cubic foot; contact time was 25 minutes at room temperature. The zinc content was determined by potassium ferrocyanide titration in concentrations of about 50 p.p.m. (16)and colorimetrically below 50 p.p.m. (1). Discussion. Zinc may be removed from relatively strong sulfuric acid solutions by a sulfonated polystyrene cation exchangrr. .It sulfuric acid concentration below O.l%, the removal is virtually complete and at concentrations of 0.5% t o 1.0% sulfuric acid the removal is greater than 95% from solutions containing 500 p.p.m. of zinc. These data are shown in Figure 3. The capacities t o 10% breakthrough of zinc are:
HEADBOX AND AI
Figure 2.
-
e
1081
10
LITERS
P e r m u t i t Q Removal of Zinc f r o m Sulfuric Acid Influent
0.5 1.0
Zinc Removed, AIe./Liter 1430 1320 1040
830
At higher concentrations, the effluent from the Prrmutit Q bed contains high quantities of zinc slippage. Recovery. By employing known methods of regeneration of ion exchangers, it is possible t o recover*the zinc from the exhausted cation exchanger as a relatively concentrated solution (8,7 ) . By proper separation of the regeneration effluent into several portions, fortification of partially neutralized regenerant portions, and re-use of these fractions, it is possible t o recover concentrations of over 10% zinc sulfate containing relatively small amounts of sulfuric acid. These recovered solutions may often be re-used directly in the processes from which the wastes arise or they may be roncentrated for the production of the zinc salts LITERATURE CITED
(1) American Public Health Association, New York, N. Y., “Standard Methods for Examination of Water and Sewage,” 9th ed. p. 58, 1946. (2) Bauman, W. C., and Eichhorn J., J . Am. Chem. Soc.. 69, 2830-6 (1947). (3) Beaton, R. H., and Furnas, C. C., IND.ENG.CHEM.,33, 1501-13 (1941). (4) Beohner, H. L., Proc. Engrs.’ SOC.West.Penn., 1st Annual Water Conferenco (1940). (5) Beohner, H. L., and Mindler, A. B., IND. ENG.CHEM.,41,448-52 (1949). (6) Bliss, H., Chem. Eng. Progress, 12, 887-94 (1948). (7) Buck, R. E., and Mottern, H. H., IND.ENG.CHEM.,39, 1087-90 (1 947). (8) Burns, R. E., and Wood, J., Power Generation, 53, 70-3 (September 1949). (9) Carpenter, C., and Porter, C. C., Paper Trade J . , 121,40-4 (August 1945). (10) Myers, R. J., Eastes, J. W., and Myers, F. J., IND.ENG.CHEM., 33,697-706 (1941). (11) Nachod, F. C., “Ion Exchange-Theory and Application,” 1st ed., 235-59, New York, N. Y., Academic Press, Inc., 1949. (12) Pattock, K., Bitterfeld, K., and Wassenegger, H., U. S. Patent 2,184,943 (Dec. 26, 1939). (13) The Permutit Co., “Suggestions for Experimental Use of Ion Exchangers in Process Industries” (1949). (14) Snell, F. D., “Colorimetric Methods of Analysis,” Vol. I, p. 258, New York, N. Y., D. Van Nostrand, 1936. (15) Snell, F.D., and Biffen, F. M., “Commercial Methodsbf Analysis,” 1st ed., p. 505, New York, N. Y . .McGraw-Hill Book Co., 1944. (16) Sussman, S., Nachod, F. C., and Wood, W., IND.ENG.CHEM., 37,618-24 (1945). (17) Tiger, H. L., and Goetz, P. C., U. S. Patent 2,397,575 (April 2, 1946). (18) Yoder, J. D., Proc. Eng. SOC.West. Penn., 9th Annual Water Conference (1948). RECEIVEDJsnuary 11, 1951. Presented before the Division of Colloid Chemistry, Symposium on Applications of Ion Exchange, a t the 118th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, I11
