X C H A N G E IN W A T A TMENT H. B. GUSTAFSON InJilco Incorporated, Chicago, I l l . practical water treatment generally is directed toward purifying the water to the degree that the use requires and seldom toward complete purification or total removal of all of the impurities. B, means of ion exchangers i t is possible to remove dissolved mineral matter in part, or in entirety, or to qualify the characteristics of any water. Thus ion exchangers applj to water treatment not on11 i n the process of complete removal of impurities, but also in the much larger field of partial purification. In one application, ion exchangers may compete against precipitation methods of treatment, in another against distillation, i n a third against an alternative ion exchange procedure, and
T
HE period extending approximately from 1920 t o 1930 was a period of active development in the United States of a number of naturally occurring base-exchange materials, as well as a number of sgnt,hetics, into practical exchangers useful primarily as water softeners. The important exchangers developed during this period mere inorganic in nature. At the present time, there is another surge of interest in ion exchangers, and t,his time the stimulus is supplied by resinous ion exchangers which are proving t o be much more versatile products than the inorganic exchangers developed during the twenties. The currently expanded interest in ion exchangers as evidenced in technical jourrials by scores of papers and profuse adTTertising may suggest to the relatively inexpert that, the ion exchange field is uricommonly large and lucrative. On occasions, t,he writer has endeavored without notable success to obt,ain figures on total United States exchanger production---that is, the combined total cubic feet of greensand, sulfonated coal, sodium aluminum silicate, cat,ion and anion exchanging resins in the year 1946, which appears to have bcen a peak year. F i t h scant information, plus guess work, a figure of 450,000 cubic feet seems reasonable. This is presented LTith the hope t,hat someone has a more reliable cstimate, and the trust that sometinic an accurate figure wili be available. .4t an average selling price of $10 per cubic foot, the total value amounts to $4,500,000. It appears to be agreed generally tha,t lwter treating processes took most of the material. that manufactmuredsodium aluminum silicate was the largest, volume product, that ion exchange softening was by far the largest application, and that the production of anion exchangers was a small fraction of the tot,al exchanger production of those companies which nianufacture both cation and anion exchangers and a much tinier fraction of the total United States exchanger production. Considerat'ions such as the above! however approximate they may be, can be of real value to the reader by serving as an anchor to his sense of proportion. There are a number of reasons for believing that resiris will not displace materials such as greensand, sulfonated coal, and sodium aluminum silicate as ion exchangers in x a t e r treatment; among these reasons is the important one that sodium aluminum silicate, for example, is the cheapest exchanger to produce per unit of exchange value. Incidentally, although it has been reported otherwise (1, 2), the capacity of this mat,erialis higher than that of sulfonated coal and equals or exceeds the capacity of several of the resins current,lyproduced in the United States.
in a fourth may coniplernent another method or methodv of treatment. Occasionally, ion exchangers are used as filters to remove suspended matter, but such an application is not generally sound or practicable. The expanded interest in ion exchangers in the past few years large15 seems due to the discovery of synthetic organic exchangers which apparently captured the imagination of resin manufacturers, as i t opened a new field for resin application. This discussion covers the niore important applications, with consideration to the importance or extent of the individual application, the costs, and comparisons with alternative methods of treatment.
In some quarters it is felt that cation and anion exchanging resins, owing t o their versatility, somctinic will displace completely, or to a large degree, precipitation methods of trealrneiit. The writer docs not share t,his optimism. It is the Triter's impression, and the impreasiou of many ot1iei.h in t'he industry, that the amount of effort and money being expended today on ion exchangers is much greater than the siec o! the field warrants. To justify t.hese efforts, exchangers, particnlarly resinous exchangers, of both the cation and anion types, niust find a much larger market either in water treatmerit or irt other industry. It is reasonable to expect the following developments: a continued expansion of resinous exchangers into thv water treating field particularly as neiv and more selective exchangers come out of the 1aborat)ories; tho expansion in use oi resinous exchangers to be somervhat more rapid than the exparision of the industry as a whole; and that this expansion which has been going 011 IIOW for several years soon will merge wit,h the cxpansion of the industry as a whole. There is nothing highly revolutiona,ry in this picture. Unless some new revo1utionar:and efficient techxiigue conies out of the laboratories, it loolts ah though resinous exchangers are not goiiig to t,alrc over the indus try, Production of aniori exchangers certainly is quite small relai i v c i to total ion exchanger production, probably about 2% of ihc. physical volume and 10% of the dollar volume. While anioii exchanging resins are int'rinsieally niore cxpcnsive to manufactuw than cation resins, the real explanation for the high price probably lies in the low volume which necessitates a high unit aqsesrment for p h t and development. Ion exchangers today, of \Thatever type or character, are a real bargain to any purchaser who can use them. The public is getting the benefit of extensive and expensive efforts on the part of t,hi~ rnanufac turers. The largest use of ion exchangers in 1946 possibly was in the. household softener. Excepting midget varieties of such soft,eners, the size of such household installations ranges from tanks 8 inches in diameter to tanks 24 inches in diameter and containing from 1.5 t o 10 cubic feet of base-exchange material. These softenerb either are bought outright by the householder and serviced by hiin or rented by t'he householder and serviced by the dealer. Use of ion exchangers in the home is confined to the sodium cycle, Practically all types of cation exchangers are used including greensand, sodium aluminum silicate, and resins. The home use of ion exchangers always is for the purposes of
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March 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
water softening and scale prevention and frequently for iron removal. Iron in water occurs partly i n suspended form as insoluble ferric iron and partly in solution as ferrous iron. Suspended iron can be removed by filtration and soluble iron by base exchange action; to successfully remove iron, the softening unit either must be so constructed that i t can serve the additional function of filtration or a separate filter must be provided. If the base exchange material is to be a filter also, it should preferably be of rather fine grain particle size and the operation should be downward. Moreover, a successful continuation of the iron removal process requires that the iron oxide entrapped in the bed be removed a t regular intervals by a thorough washing procedure. These requirements frequently are not met with the result that many, and perhaps most, installations do not do the job perfectly. Owing to modest service, i t is permissible to use a home softener for iron removal, but it usually is not wise and economical to use sodium zeolite for such a service in industrial and municipal plants. The average household uses about 30 gallons of water per day per person. Of this 30 gallons, 10 gsllons are hot water and 20 gallons cold water. An additional volume, usually under 10 gallons per day per person, is used for lawn sprinkling. With this breakdown, a significant difference between municipal water softening and softening water in the home is apparent. I n municipal water softening, all of the water that is consumed is softened; in home water softening, the hot water alone may be softened, or both the hot and cold water may be softened. Softening of the hot water, or rather the cold water going t o the heater, solves most of the household hard water problems. Economically, the production of soft water in the home frequently represents a pronounced saving in that as little as 25% of the total water used in and around the home need be softened. However, assuming a water of 20 grains hardness (a typical hard water) the mater cost by home rental service is about 2 dollars per 1000 gallons and the water cost by home-owned softener about 80 cents per 1000 gallons, not including the labor on the part of the owner, whereas, the same water municipally softened by ion exchange costs only 5 to 20 cents per 1000 gallons. Investment and chemical operating costs on home-owned softeners are much higher per unit volume of water softened than in municipal softeners. It can be concluded that even with the extreme burden of having t o soften four times as much water, the municipally softened water is the cheaper. While the use of exchangers in the home practically is confined to operation in the sodium cycle, industry, including publicly owned industry, uses these versatile materials in every conceivable and practically useful application. Moreover, industry works the exchangers harder. Each cubic foot of material used in industry treats somewhere between 10 and 100 gallons of water in the same period of time t h a t a similar cubic foot of material in the home treats 1 gallon of water. As a result, industry treats more water while homes use more material and treat less water. Certainly, the largest use by far that industry makes of ion exchangers is in the sodium cycle for softening water, Regeneration is with common salt which is used at a chemical efficiency which varies between the limits of 0.3 t o 0.6 pound of salt per 1000 grains of hardness removed as compared with the stoichiometric efficiency of 0.17 pound of salt. Industry may soften a raw water or a mater which has been given preliminary treatment t o remove suspended matter, colloids, iron and manganese, alkalinity, or silica. Choice of exchanger is governed by cost and conditions of use. Ordinarily, sodium aluminum silicate or greensand is used, but if silica, pickup must be avoided, or if temperature is above ordinary water temperature or p H is below approximately 7.0 or above 8.3, either sulfonated coal or a resin is chosen. All resins withstand low p H and some resins high pH. Inorganic exchangers can not safely be used at temperatures above 100" F., whereas a few of the organic materials can safely be used at a somewhat higher temperature.
465
Industry uses acid regenerated sulfonated coal or resin in t h e process of alkalinity reduction. Alkalinity reduction can be accomplished by chemical precipitation methods as well as by acid regenerated exchangers. The principal items which influence the choice of method are the composition of the raw water, relative cost of equipment and operation, and detail requirements connected with the use the treated water is to serve. The presence of considerable sodium bicarbonate alkalinity in a raw water which is to be treated for boiler feed water or for ice-making favors the selection of acid-regenerated cation exchangers. On the other hand, raw water which contains alkalinity due to calcium and magnesium bicarbonates and which is t o be used for boiler feed water, more frequently than not, is treated by the chemical precipitation method because chemical costs are less and because a n appreciable reduction in dissolved silica and practically complete removal of suspended impurities take place along with the removal of alkalinity. Ordinarily, the effluent from an acid-regenerated exchanger, commonly called hydrogen zeolite effluent, is substantially free of calcium, magnesium, and alkali metals, but contains dissolved carbon dioxide, sulfuric, and hydrochloric acids equivalent in quantity to alkalinity, sulfates, and chlorides originally present in the raw water. Usually, carbon dioxide is removed by aeration. Most uses will not allow the presence of mine a1 acids; consequently these either are neutralized or are removed bodily by means of anion exchanger. h'eutralization methods commonly employed consist in blending with alkaline raw water, zeolite softened water, or addition of a n alkali such as caustic soda. A strong mineral acid, usually sulfuric acid 1 to 15y0in concentration, is used to regenerate hydrogen exchangers. The chemical requirement for substantially complete exchange varies with different waters, exchangers, and equipment between 0.25 and 1.O pound of 66 O sulfuric acid per 1000 grains of cations removed, reckoned as calcium carbonate. Theoretic acid equals 0.15 pound per 1000 grains of calcium carbonate. Ion exchange methods generally are inefficient in chemical utilization compared with precipitation methods. When the raw water contains calcium, as it usually does, the capacity of a sulfuric acid regenerated resinous exchanger is lower than when calcium is absent. Frequently, this capacity reduction can not be tolerated and it is customary either t o soften the raw water or to apply sodium chloride brine t o the resin prior to the sulfuric acid regeneration ( 3 ) . Usually, the salt required in the preliminary regeneration amounts to 0.5 pound of sodium chloride per 1000 grains hardness present in the raw water. Sulfonated coal operating in the process of complete hydrogen exchange and regenerated with sulfuric acid does not appear to suffer capacity reduction when calcium is being removed. The explanation for this apparent difference in behavior is not clear Of course, sulfonated coal has relatively low capacity compared with the capacity obtained under favorable conditions of several resins currently available. As previously stated, hydrogen zeolite effluent is substantially free of calcium, magnesium, and alkali metals, but contains dissolved acid. Thompson ( 4 ) has shown that, in parallel flow cycling, the removal of sodium by hydrogen exchanger is a percentage proposition which is dependent on the quantity of regenerating acid used, as well as on other factors. The matter of percentage removal is not too important in the process of alkalinity reduction, but is important in the process of demineralization by ion exchangers. Industry occasionally requires water that is substantially frer of dissolved mineral matter. For example, a particular use may require a water containing not more than 5 p.p.m. dissolved solids. If the de-ashing is to be done by ion exchange and if the raw water contains 100 p.p.m. total solids, i t is clear that the cation exchanger reaction must be at least 95'% complete if the requirement, is t o be met. Likewise, if the raw water contains
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
500 p.p.m. total solids, the reaction must be 99% complete Typically, the chemical requirement for 95% reaction may be 0.5 pound of 66' sulfuric acid per 1000 grains ash in calcium carbonate equivalents, and for 99% reaction 0.8 pound 66 ' sulfuric acid. Thus, per 1000 gallons, approximately 3 pounds of sulfuric acid will be required for the water which contains 100 p.p.m. solids, whereas approximately 24 pounds of acid will be required for the water containing 500 p.p.m. solids. The second step in demineralizing involves passing the hydrogen exchanger effluent through an alkali-regenerated anion exohanger. The regenerating chemical may bc caustic soda; in t h a t case the quantity required will be about 0.15 pound solid caustic per 1000 grains calcium carbonate equivalent anions removed. Ion exchange materials gradually wear out in service. Ion exchanger replacement should be considered as a chemical operating charge along with regenerating chemicals, such as salt, acid. and alkali. The writer believes that, in so far as cation exchangers are concerned, when properly applied and selected materials are used the cost of exchanger replacement generally is minor compared with cost of regenerating chemicals. Tests made in the Infilco laboratory indicate that anion exchanger replacement may be an important part of the total chemical cost: These trsts n-cre started in June 1943 and continued on through the fall of 1944. The influent to the exchangers was Chicago tap water through hydrogen exchanger and further acidified by addition of sulfuric arid hydrochloric acids to 300 p . p m total mineral acidity F l o ~late through the evchangeis was 2 gallons per minute per cubic foot of exchanger. The cont a c t time provided by this rate of flow is within the practical range and is about the minimum contact period (or maximum flow rate) used in industrial size installations. Regeneration either was with caustic soda or with soda ash solutions after the recommendations of the manufacturers of the resins. Tests were so conducted that losses of material in backwashing were limited t o particles finer than U.S.70. Exchangers A, B, and C had the longest useful lives; they lost 50% of their initial capacity after treating 1,000,000 gallons per cubic foot of exchanger. Exchanger D lost 50% of its initial capacity after treating approximately 400,000 gallons per cubic
Vol. 41, No. 3
foot. Exchanger E lost 50% of its initial capacity after treating about 150,000 gallons per cubic foot. After 1,000,000 gallons per cubic foot the regenerated volume of exchanger A was 1.27 times its initial regenerated volume: of exchanger B, 1.12 tinies the initial regenerated volume; and of exchanger C, 1.22 times the initial regenerated volume. The regenerated volume of exchanger D after 400,000 gallons per cubic foot was 1.36 times the initial regenerated volume, and the regenerated volume of exchanger E after 150,000 gallons per cubic foot was 0.87 times the initial regenerated volume. Several of the exchangers showed a significant gradual increase in rinse water requirement-that is, water required to rinse out the excess of regenerating chemical. These tests were conducted under conditions approximating those which frequently prevail in the field. Assuming that the resin is replaced when capacity is 50% the original, the indicated cost of resin replacement a t current anion resin prices varies between about 6 and 40 cents per 1000 gallons of water treated, depending on choice of resin. The writer feels that the useful life of anion exchangers will vary widely depending on operating conditions. Temperature assuredly is highly important; low temperature is preferable to high temperature. Dissolved oxygen in the water supply, nature, and concentration of regenerating chemical may be other important factors. The purpose of this paper has been to show the size of the ion exchange field as it applies to water treatment in terms of collective volume of exchangers produced in a given year; to indicate which materials and applications are most widely used; to present generally the quantities of chemicals required by the various applications; to exhibit the present p o d i o n of organic exchangers in water treatment; and to speculate on the future of these versatile organic materials in this field. LITERATURE ClTED
(1) Bauman, W. C., J . Am. Water W o i k s Assoe., 37, 1211 (1945). ( 2 ) RIj-ers, K. J., Easte:, J. W., and Myers, F. J., IND. ENG.CHEM., 33, 097 (1941). (3) Rawlings, F. N., and De Geofroy, L., U. S. Patent 2,366,660 (Jan. 2 , 1945).
(4) Thompson, R. B., Proc. A m Water Conf., Engrs.Soc. West. Penn.
(1945).
[END OF SYMPOSIUM]
Tartrates from
astes
USE OF ANION EXCHANGER§ IN A CHLORIDE-TARTWATE CYCLE R . R . LEGAULT, C. C. NIMMO, C. E. HEKDEL, -4NU G. IC. KOTTER Western Regional Research Laboratory, Albany, Calijf. Tartaric acid niay be recovered from winery still slop b y exchange adsorption on an anion exchanger in the chloride form. No cation exchanger is needed. Common salt solution serves as the regenerant. Concentration of tartrate in solution is increased 15 to 18 times over that in the original slop. High purity calcium tartrate is the final product. Recovery data are presented from lnboratory and pilot plant scale experience. Fouling of exchanger and means for control of fouling are discussed.
K A previous publication (3)the recovery of tartaric acid from gIape n-astes by means of ion exchange materials operating in t h e acid-carbonate cycle Tvas described. That system employed both cation and anion exchangers, and the tartar was recovered
from the carbonate regenerant liquors of the anion exchanger. It was pointed out in the cited paper that other anion exchange cycles might prove better adapted to the purpose. Sussman, Nachod, and Wood (6) have reported the possibility of recovering metals which form complex ions by use of anion exchangers in the chloride or sulfate form. The present, paper describes a cycle whcrein tartaric acid recovery is accomplished through eschangc of h r trate for chloride ion. KO cation exchanger is needed, and rccovery of the acid as calcium salt is accomplished by prccipimtion from sodium chloride regenerant liquor after suitable clarification and adjustment of pH. It would clearly be advantageous to avoid the necessity of precipitation of calciuni salt and later reconversion to the acid. Means for direct rec0ver.y of the acid have been investigated and will be described in a later paper.
