Silica Removal with Highly Basic Anion Exchange Resins ROY OSMUN
AND LOUIS WIRT'", J R . The Dow Chemical Co., Midland, Mich.
T
HE Dow Chemical Co., Midland, hlich., has been operating a plaht demineralizing 1500 gallons of water per minute for the past year and a half. This plant supplies water for 1200 pound-per-square inch boilers with 100% make-up. A two-bed system with a weakly basic anion exchange resin has been used with fluoride addition to the cation bed for silica removal. A second half of this plant will soon be in operation t o supply two new 1200-pound-per-square inch boilers with a steam capacity of 400,000pounds per hour, each also with 1 0 0 ~ make-up. o For this addition it was decided to use a strongly basic anion exchange resin which absorbs silica directly without the use of fluoride. This resin has been described as Dowex 2 or Nalcite SA4R(1). It was not developed a t the time that the first dennneralizing plant wm started, but it promises to deliver a water lomi in final silica content, lower in total solids, lower in cost, and at a higher flow rate than is possible with the present fluoride method Bccause this exchange material was new and relatively untried, Dow has had a pilot plant in operation for several months collecting as much data as possible on its operating characteristics. The information prebented here admittedly pertains t o a specifir plant using a specific water and perhaps reflects the plant operator's viewpoint, but the results are believed to be indicative of what may be expected with other wate-4 arid othv t p p l i vi1 ions.
.
WATER
The water is Lake Huron water which is now piped t o the cities of &lidland and Sayinaw from Whitestone Point Pome 12 miles south of East Tawas. I t is taken from a 5Gfoot depth nearly 2 lrules from shore. The temperature ranges from 35 to 65 F. and the turbidity is low enough to permit demineralizing without previous filtration. An average analysis of the raw water and of the cation exchanged water after blowing out carbon diovide is shown in Table I
Table 1.
Analysis of Lake Huron Water
Calcium aci Ca Magnesium aa Mg Chloride as C1 Sulfate a8 so& Bicarbonate as RCOJ Hardness as CaCOz Silica a8 SiOe CATION
E F b ' L U E S T Ak,'L'CH
P.P.M. 30 8 10 12
105 102 1.7
co?REMOVAL P.P.M. as CaCOs
Free mineral acid Total acid SiOn
25
36 3
The silica content of the water is lorn-, but the ratio of silica to other anions is relatively high and is comparable to that of a great many waters. An exhausted anion bed will carry a relatively high percentage of silica. Collecting data from even twenty cycles takes considerable time, as a bed 5 feet deep at ordinary flow rates will operate a &day week, 24 hours per day, without regenerating.
Highly basic anion exchange ~ e s i nw a s t o be ased fwr the first time on a large scale industrial appEcatiarF a$ the Iprrw Chemical Co. plant. Mica removal from boiIerfed water was necessary to permit 100% n~alie-irpae I2000 pc~wnds per square inch. A pilot plant was operated for several. months to determine %he crperating chirracteriszim d the resin. A finished water was produced containing less than 0.05 p.p.m. of silica and 1 p.p.m.of tofaldids. Highregenexating efficienciesresulted with sdZum hydrmide;,and rinse rates were low. Proper regenerating methads are indicated lo ensuse complete removal of silica from the resin. Separation and segregation of various anions take place in the resin be& as it becomes exhausted. AppIieaticm of the u&rx for anion separation may be indicated.
PILOT PLANT
The pilot plant for demineraliaing coiiists of two glass tubes 6; inches in diameter and 8 feet high in series. A unit of this i a e will take about 0.75 cubic foot of exchange material for a bed 4.5 feet deep, and this bed depth has been used. T h e flow rate is high enough to be measured by a standard water meter. Tho cation exchange resin was Xalcite HCR, a styrene-type resin takcn from a bed in the cornpiany's softening plant, where it had I I W I i n i i s e for 4 years on the sodium cycle. During this time it had operateu , : ~ o u g hsome 350 cycles and had softened 2,000,000 gallons of water per cubic foot. I t s operating exchange capacity mas still in the neighborhood of 30 kilograins per cubic foot, using 9 pounds of hydrochloric acid per cuhic foot, comparable to that of new resin. The water after acid exchange was pumped over R glass blowing-out tower 4 inches in diameter and 4 feet high filled with 0.75-inch Anthrafilt. Here the carbon dioxide xits reduced to 3 to 5 p.p.m. depending on the temperature. From the tower the water flowed by gravity through the anion exchanger which was regenerated with sodium hydroxide. A great deal of information as to the exchange characteristics of this material was revealed by progressive analysis of the spent caustic regenerant leaving the bed. All this solution was collected and each 2-gallon portion was analyzed for chloride, sulfate, carbonate, silica, and free caustic,. Corrections were made for small amounts of impurities in the sodium hydroxide used. The removal rate and sequence of the various anions during regeneration are shovvn by Figures 1 to 5 . The photometric method for silica determination was used with the Klett-Summerson photometer and molybdate solution and sodium sulfite as the reducing agent. The authors are indebted to the Kational Aluminate Co. for pointingout that this method aa usually described is not accurate for determining concentrations of silica higher than that found in natural waters when the solutions are not alkaline and colloidal silica may be present. The samples are boiled with a few sodium hydroxide pellets in platinum and then neutralized to the phenolphthalein end point. If this is not done, analysis of the first portion of apent caustic regenerant leaving an anion bed will account for only 25y0 of the silica present; 7501, may be colloidal silica. After free caustic appears, this digestion is not necessary and samples are neutralized with silica-free acid. ANION CLASSIFICATION
An interesting characteristic of this exchange resin is evident when operating in a glass tube with new resin. A greenish horizontal band appears near the top of the bed soon after a freshly regenerated bed is put in service. This band moves steadily down the bed aa the bed becomes exhausted, deepening as it
1076
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
0
l
l
!
l
l
l
l
l
l
0 600 1200l800
l
l
3000
GALLONS PER SO FT AT
Figure 1.
l
l
4200 5GPM
l
l
l
Figure 2.
Exhaustion of Dowex 2
Position of Ions i n a n Exhausted Dowex 2 Bed
SiOt
I
4800 3500
6000 600
804 c1
cos
Si09
so4 1
Cl C08 Si09
14
20
42
Lbr,Cu Fl 27a NOOH I 2
I, 95.F 3
Regeneration of Dowew 2
Silica removal by carbonic acid
(Approximate ion content of bed, grains/cu. foot as CaCOa)
so4 c1 co:
Cation EfTiuenf Downflar G o l f C u Ft at 95.E
42
l
moves down, and a t the point of silica breakthrough it is a foot or less above the bottom of the bed and 2 to 3 inches deep. This greenish color proved $0 be due t o silica in combination with traces of nickel introduced during the resin manufacturing process. It disappears after a few cycles. A logical conclusion from this observation was that a t least some anion was not being removed progressively from the water from the top of the bed downward, but was being constantly replaced and pushed down by something else. I n order t o substantiate this theory an exhausted bed was removed from the tube foot by foot without mixing and each foot was regenerated separately. An analysk of the spent regenerant showed some interesting results. More strongly absorbed anions are constantly replacing the more weakly absorbed and the silica is eventually concentrated near the bottom of the bed. The relative position of the various anions is shown in Table 11. Had it been possible t o divide the bed a t the right spot, a much sharper separation of anions might have been found. If the water ffow is continued beyond exhaustion, the anions appear in the effluent in successive peak concentrations as shown in Figure 1.
Table 11.
Cotlon Effluent Bockwash Gol/Cu FI at 95-E 14 28
10!77
5% 50%
6'7 1%
0 14% 94% 98%
The position of the anions after backwashing a bed is not known exactly, but some difference in specific gravity exists in an exhausted bed. The sulfate-bearing resin a t the top of the bed has been largely replaced by carbonate resin after mixing, and while there is some increase in silica a t the top of the bed it is probable t,hat the silica stays near the bot,tom.
Although this anion classification in the bed does not have too much significance in water treatment, it is mentioned here with the thought that other applications of the resin might be suggested, It indicates that silica must be removed completely in regenerating if leakage of silica is t o be avoided, inasmuch as any remaining silica will be a t the bottom of the bed where it will do the most harm. The concentration of silica in a relatively small bed area and moving steadily downward suggests that much higher concentrations of silica in the raw water should be removed equally well. CAUSTIC SODA REGENERATION
The removal of chlorides, sulfates, and carbonates from this exhausted resin presents no problems; they can be removed rapidly with a high caustic efficiency. The complete removal of silica was not so easy. Silica leakage of a few tenths of a part per million appearcd in the latter part of the runs and finally during all of the run. A prohibitive amount of time and sodium hydroxide was necessary t o remove a much silica in each regeneration as was added from the water during the run. Upflow regenerstion was tried, because regeneration countercurrent to the main water flow normally decreases leakage of weakly held anions. This proved to be inefficient, owing to short circuiting and jetting of the caustic up through the bed even a t very low flow rates. Silica could be removed completely by continuing the run after the silica breakthrough without carbon dioxide removal. I n this case the silica would be replaced and flushed out completely by the carbon dioxide, but this required a prohibitive amount of time and water. It was finally found that the temperature a t which the regeneration was carried out had R very pronounced effect on the speed of silica removal. The pilot plant was being operated a t temperatures seldom above 60" F. When the temperature of regeneration was increased to 95" F. and the contact time with caustic was 1 hour, or better 1.5 hours, silica was removed from the bed completely with 2.5 to 3 pounds of caustic per cubic foot of resin. With this regeneration the plant delivered silica-free water up t o the point of breakthrough at a total exchange from 14 to 16 kilw grains per cubic foot, depending on the extent of carbon dioxide removal. If the same amount of caustic and contact time were used a t 60" F. the silica removal would be above half that a t 95" F. This removal rate is shown in Figures 3 and 5. Stronger caustic a t these lower temperatures did not speed up the silica removal with the same contact time; 4% caustic might show about twice the silica content as 2% on leaving the bed
INDUSTRIAL AND ENGINEERING CHEMISTRY
1078 I
I
LBS 29.
N a O H l C U FT A T 60.F
2
I
3
100
I
3 6 0 ~-rt EXCHANGE OF PREVIOUS RUN SO4 4300
20 Si01 I I
'6
20
40
Figure 3.
60
I
930 14,830
100 I20 C O N T A C T TIME I N MINUTES
80
140
Or9
CaC03/Cu FI
160
180
Regeneration Characteristics of Dowex 2 Removal r a t e of a n i o n s a t 60° F.
with the same contact time, but because the volume was half as much the total removal wm about the same. The 2% caustic is preferred at the elevated temperature. The larger volume warms up the bed faster and maintains the higher temperature during regeneration. Prewarming the bed has not proved necessary. -4number of consecutive cycles have been carried out without silica leakage regenerating at 95' F. with 2.5 pounds of caustic per cubic foot and a 1-hour contact period. With this schedule an equilibrium appears to be reached where as much silica is removed at each regeneration as is added to the bed during the last run, but about 10% of the total silica is left in the bed a t the end of any absorption cycle. This residual silica may be removed by using 3 pounds of caustic per cubic foot, but such complete removal with Lake Huron water is not necessary to prevent silica leakage.
Vol. 43, No. 5
content up to 25 p,p.m. did not seem to be a factor. If the bed was soaked overnight in water containing up to 50 p.p.m. of carbon dioxide the maximum silica concentration in the water seemed always to stop a t 80 to 90 p.p.m. These facts seem inconsktent from an ion exchange standpoint. They indicate that in neutral 01 acid condition the silica may be present in a nonionic form. When conditions are such that the bed is exhausted of silica and carbon dioxide at nearly the same time, a raw water backwash at 95" to 100" F. is nearly as effective for silica removal. Some free carbon dioxide and a silica concentration of 40 p.p.m. were found in 20 gallons per cubic foot of raw water backwash applied at a rate of 5 gallons per square foot per minute. This represents about 15yoof the total silica in the bed. The water entered the bed a t a pH of 8 and left it at a pH of 6. Where water is cheap, perhaps enough silica can be removed in the backwash t o reduce the regenerating time or the caustic dosage. The authors have not given this much study, because their plant has high-priced 'i\ater and cheap caustic. QUALITY OF FINISHED WATER
Rinsing with 50 gallons per cubic foot a t a rate of 5 gallons per square foot per minute, the pH drops below 9 and the total solids as measured by conductance drop below 1 p.p.m. as sodium chloride. The conductance is due to a trace of caustic. Apparently a certain amount of caustic soaks into the resin grain and diffuses out slowly during the following run. Both the pH and conductance of the finished water continue to drop slightly as the run continues and reach a minimum just before the silica breakthrough (Figure 4). If the bed stands idle for several hours, the water content of the bed will show a slight increase in a1kalinity . Les 2% tioon/cu 6
0
12
FT AT 9 5 0 ~ 18 24
i 1
30
1
I I
I
REGENERATION B Y ACIDS
100%. A n o n r Exchanged---
At temperatures around 95" F. silica is removed from an exhausted bed by carbon dioxide much more rapidly than at lower temperatures (Figure 2 ) .
I40-
O F PREVIOUS (IUN C P 01 c n c o , / c u n
EXCHAMGE
so,
-
5200 4,200 5,100
CI COI
s102 ---+-
20
0
0
20
Figure 5.
40
-
BO 100 120 CONTACT TIME IN M I N U T E S
60
t,oM,
15,500
140
160
180
Regeneration Characteristics of Dowex 2 Removal rate of a n i o n s a t 95' F.
0
Figure 4.
2
4
6 8 10 12 E X C H A N G E IN KILOGRAINS
14
16
I8
Characteristics of Demineralized Lake Huron Water A n i o n e x c h a n g e w i t h Dowex 2
-
At a flow rate of 3.5 gallons per square foot per minute 60% of the silica in an exhausted bed has been removed by 80 gallons per cubic foot, using cation effluent containing all of the carbon dioxide when the temperature was 95" F. Silica removal was constant a t 60 p.p.m. of silica as Si02 in the water, with either downflow or upflow as a backwash. Variation in the carbon dioxide
A low concentration of hydroxyl ion in the finished water i s favored by low water temperat'ure and high flow rate. At ii water temperature of 40" F. and a flow rate of 4 to 5 gallons per square foot per minute a water is produced with a conductance as l o x as 0.5 micromho, equivalent to about 0.1 p.p.m. of sodium hydroxide. If necessary t o operate with 80' F. water, the conductance will be about five times that figure a t the same flow rate. It is certain that if flow rat,es were maintained at at least 6 gallons per square foot per minute, the alkalinity of the finished water would be lower and the anion removal not adversely affected. I t has not been possible to maintain that flow rate with this pilot plant. Water temperature cannot always be controlled, but the addition of waste heat to the water before demineralizing should be avoided.
May 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
The slightest leakage or exhaustion of the cation bed is immediately indicated by increased pH 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 to 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.
AND
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 to 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). The 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 to 675 p.p.m. of tin with demineralized water and used as influent solution in all the work reported.
