Synthetic Resins as Exchange Adsorbents ’
ROBERT J. MYERS, JOHN W. EASTES, AND FREDERICK J. MYERS The Resinous Products and Chemical Company, Inc., Philadelphia, Penna.
mechanical and chemical), to acids, alkali, and heat, uniformity of quality, low operating costs, freedom from “leakage”, and wider fields of application. Laboratory and field tests have proved the practicability of the use of resinous exchangers in the softening of water, the partial or complete removal of salts from water, sugar solutions, and protein solutions, in the recovery of traces of copper and other valuable metals, in the removal of iron and acids from industrial effluents and commercial products.
The recent development of synthetic resins which exhibit exchange-adsorbent properties has opened new fields of application, and promises unique uses in the purification of water and other fluids, in the recovery of valuable substances, in the removal of undesirable impurities, and in a multitude of special applications hitherto considered impossible or impractical. The resinous ion exchangers offer ’the advantages of high exchange capacity, high exchange velocity, excellent stability (both
T
H E discovery by Adams and Holmes ( I ) in 1935 that phenol-formaldehyde resins exhibited ion-exchange properties opened a new era in the chemistry of synthetic resins. Prior to this discovery the selection of a resin for a particular application had always been based upon physical properties exhibited by the resin, such as hardness, solubility, color, etc. Even in those cases where considerable effort was expended upon the chemistry of a resin designed, for example, for use in varnishes, the ultimate criteria of a resin’s worth were the physical properties imparted to the final product. However, the observation that synthetic resins of the phenolformaldehyde and amine-formaldehyde type could be used as exchange adsorbents has focused attention upon the chemical behavior as well as the physical and mechanical qualities of resinous substances. I n the field of base-exchange phenomena also, the work of Adams and Holmes led t o a new viewpoint. Thus, the first observations on base-exchange phenomena are generally accredited to Way (66‘),who studied the adsorption of cations by soils, and to Eichorn (80) who discovered that the bases were mutually exchangeable. Subsequent researches were devoted to base exchange occurring in both zeolites and humus materials. Lemberg (48), Eichorn (80), and Gans (86) studied the mineral exchangers exhaustively, and later Gans (84) made commercial applications of zeolites to water softening. The investigations of several workers on the role of humus compounds in soil acidity were followed by the suggestion of Fischer and Fuchs (83) that water could be softened by the sodium humates of brown coal, and by patents of Hepbuin (89) and Borrowman (IS) on the‘softeGng of water with treated peat, lignite, or brown coal. Whereas these investigations and applications were based upon naturally occurring organic substances of ill-defined chemical constitution, the materials of Adams and Holmes were synthetic 697
resins of definite composition and thus were the first truly synthetic organic ion-exchange materials. The further development of synthetic resins as ion-exchange adsorbents has been investigated in the laboratories of the I. G. Farbenindustrie and The Resinous Products and Chemical Company, who are licensees under the Holmes patents (1) on the Continent and in the United States, respectively. These researches have resulted in the synthesis of new and superior cation- and anion-exchange adsorbents, and in the successful application of several resins to water purification problems. Laboratory and field data have been collected on the comparative performance of synthetic resin ion-exchange adsorbents and the older base-exchange substances. This paper constitutes a summary of the available information concerning resinous exchangers and contains data on the performance of several typical resins. Two other papers (46, 46‘) cover other aspects of the ion-exchange resins.
Types of Exchange Adsorbents No general classification of exchange adsorbents appears t o have been made. Calleton Ellis (26) suggested that the term “organolites” be applied to all base-exchange substances of organic origin, while Bird (18) classified exchange adsorbents on a cation and anion basis. In view of the new chemical interpretation of synthetic resins mentioned above, the following classification of exchange adsorbents is suggested and will be used in this paper : I. Cation-Exchange Adsorbenta A. Inorganic 1. Natural origin a. Unmodified (example, natural greensand) b. Modified (example, fortified greensand) 2. Synthetic (example, synthetic gel zeolites)
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
B . Organic 1. Natural origin a. Unmodified (examples, peat, lignite) b. Modified (example, sulfonated coal) 2. Synthetic (chemical constitution fairly well dehed) a. Tannin-formaldehyde resins b. Phenol-formaldehyde resins 11. Anion-Exchange Adsorbents Inorganic 1. Natural origin (example, dolomite) 2. Synthetic (example, heavy-metal silicates) Organic 1. Natural origin a. Unmodified (examples, horn, wool) b. Modified (example, alkali-treated asphalt) 2 Synthetic (example, amine-formaldehyde resin)
Nonresinous Exchange Adsorbents Commercial water softening by means of greensand (68) has long been used, but owing to the low capacity of the natural product, commercial applications have been based on modifications such as glauconite heated in a reducing atmosand alkali-leached greensand (16). phere (67),baked clay (60), The search for materials of higher exchange capacity has produced a variety of patents covering synthetic gel zeolites and fused products, typified by those of Rudorf (65) based upon an aluminosilicate gel dehydrated under pressure, and of Gans (24) based upon a fusion product of alumina minerals and caustic alkali. The instability of siliceous exchangers in acidic media and the search for other cation exchangers stimulated studies on the cation exchange adsorbents based upon naturally occurring organic materials. Borrowman’s work (13) was followed by disclosuresthat acid-treated humic substances could be used for the removal of cations from water (65), and that sulfonated tannins ( $ I ) , coal, lignite (43) and other organic materials (30, 67) could be used for a similar purpose. I n such applications the material is regenerated with dilute acid, and the exchange is a replacement by hydrogen for the cations in the water being treated, or the material may be saturated with sodium ions and used in the sodium cycle as well, whereby sodium ions are exchanged for the heavy-metal ions present in the water. I n the removal of anions, research on nonresinous substances has proceeded along two lines: the use of basic inorganic or organic materials, and the modifications of acidic substances with heavy metal ions, such as aluminum and chromium, to induce formation of insoluble double salts in the removal of acids from water and other fluids. Barium carbonate (66),iron oxide and aluminum oxide gels (26,34), zinc and other metallic oxides (61), and basic dolomite (3) have been suggested for the removal of acids. Chlorinated coal as such (62)or treated with ammonia or hydrolyzed (61), activated carbon (M),and alkali-treated asphalt (61) have been similarly used. Horn, wool, silk, and leather scrap have been employed for this purpose (4.9). Compounds prepared from a metal chloride and silicic acid (62) undergo halide-ion interchange, as well as similar compounds based upon titanium, phosphorus, and similar elements (62). Fluorides may be removed from water by means of clays and silicates treated with aluminum salts (61). While many of these nonresinous exchange adsorbents have been and are now used in the purification of water and other fluids, their application has been handicapped by certain disadvantages. The siliceous exchangers, such as greensands and synthetic gel zeolites, have been limited in application t o waters of a definite pH range, since disintegration of the silicate lattice occurs in low-pH waters (do), and a marked decrease in the exchange capacity occurs in water even a t p H 6.0 (9). This difficulty has been overcome by the organic exohangers derived from natural substances, although many of
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them are sufficiently soluble t o throw color to the treated water, especially when alkaline water is being treated (28). Siliceous zeolites tend t o impart silica t o water of low silica content (41) or when the pH of the water is appreciably high. In the removal of anions, most of the available materials have been marked by general mechanical or chemical instability.
Resinous Exchange Adsorbents I n the development of synthetic resin exchange adsorbents, materials have been prepared which are free of the defects commonly associated with the older adsorbents. The original work of Adams and Holmes ( I ) indicated that polyhydric phenols, when condensed with formaldehyde, gave resins which exhibited exchange capacity. Monohydric phenols were not observed to give adsorption or exchange of cations, although Akeroyd and Broughton (g), as well as Bhatnagar, Kapur, and Puri (10) later noted that monohydric as well as polyhydric phenols showed exchange capacity. The polyhydric phenolic nucleus of naturally occurring tannins offered a cheap source of raw material, and Holmes’ investigations dealt largely with tannins condensed with formaldehyde. Burrell (16) showed that only tannins of the catechol type produce resins which undergo calcium exchange. Kirkpatrick (59) also produced gels from tannins and formaldehyde which had base-exchangeproperties. The exchange of calcium ions for hydrogen or sodium ions in simple phenolic resins may be considered to take place on the phenolic hydroxyl group. The exchange adsorbent, therefore, may be considered as an insoluble structure of the Bakelite or C-stage type of resin, in which active phenolic groups are bound to produce a nondiffusible substance capable of cation exchange. Later work showed that increased exchange capacity, particularly a t low pH, is obtained by the incorporation of strongly acidic groups such as alkyl or aryl sulfonic acids into the resin “lattice” which was effected by a condensation of the phenolic body with formaldehyde and sodium sulfite (1, 64) whereby methylene sulfonic acid groups were introduced, or by a condensation of sulfonic acids of aromatic hydroxy compounds with formaldehyde (66). Increased exchange capacity, particularly a t low pH, results from the substitution of the sulfonic acid group which may also be introduced by the use of aldehyde sulfonic acids (57), and the products may be hardened further by co-condensation with urea, thiourea, and hydroxybenzenes ($6). Synthetic resins which exhibited anion-exchange or acidadsorbent properties were prepared by Holmes ( I ) by condensing aromatic amines, such as aniline or m-phenylene diamine with formaldehyde. Aniline itself, as well as other aromatic amines, may be oxidized to insoluble dyes of the aniline black type which may be used to adsorb acids from water (22, 60),although such products are likely to throw color t o the treated water. The condensation of the amine with an aldehyde such as a monosaccharide (52,33)yields an anion exchanger which may be regenerated with alkali. As in the case of the cation exchanger, the anion-exchangeresin may be considered as an insoluble “lattice” containing “active spots” for acid adsorption. However, the condensation of aromatic amines with aldehydes proceeds both on the ring and through the amino groups, so that low values for the adsorption of sulfuric and hydrochloric acids by a rn-phenylene diamine resin, as found by Broughton and Lee (14), are to be expected. I n order to “fortify” the adsorptive capacity of m-phenylene diamine resins, there may be incorporated into the resin during preparation alkyl groups to form quaternary ammonium bases, or the amine-resin may be co-condensed with aliphatio polyamines or polyimines to give a more basic material (36). Treatment with cyanamide or dicyandiamide
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June, 1941
introduces the strongly basic guanidino group, while the aromatic amine may be eliminated altogether and resins prepared by the condensation of aliphatic polyamines with polyhalogen derivatives of a hydrocarbon (36). The hard impervious resins formed by the condensation of mphenylene diamine with aldehydes exhibit a pronounced effect of surface development on anion adsorptive capacity, and they may be improved by preparing the resin as a thin layer on a carrier body of coke or pumice (31).
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699
ous resin gels. As in silica gel, this feature combines a larger inner surface, assuring high reactivity, with mechanical strength. These few types of exchange resins now in commercial production on a small scale are to be regarded only as single examples in a scheme of the greatest variety. I n principle it is possible t o prepare a whole scale of resinous exchangers with any desired physical and chemical properties, provided that fundamental principles of resin’chemistry and preparative technique are employed.
32-CUBIC FOOTWATER-SOFTENING UXITS CONTAINING SYNTHETIC RESIN IOX EXCHANQERS IN A BOILERHOUSE INSTALLATION
Holmes (1) noted no cation exchange in resins of the g l y p tal, hydrocarbon-formaldehyde, urea-formaldehyde, and vinyl types. Burrell (16)also found no sodium-calcium base exchange in resins of the rosin-maleic anhydride type. The researches into synthetic resin exchange adsorbents in this country and abroad have led to superior products with increased mechanical and chemical stability, higher exchange capacity, higher exchange velocity, greater porosity, and improved regenerability. By due regard to fundamental principles of synthetic resin chemistry, success has been achieved in the production of “tailor-made” material from a broad range of raw materials. The well-defined chemical constitution of synthetic resins permits the preparation of individual substances designed best to fill a particular application, and the application of technical preparative principles leads to an optimum combination of chemical and physical properties. Researches conducted in the laboratories of The Resinous Products and Chemical Company have led to the development of many new synthetic resin exchange adsorbents. Four synthetic resins of different characteristics have been studied in detail and brought to commercial production and application. These resins are the “Amberlite-IR” products and consist of Amberlite IR-1 and IR-2 (cation exchangers) and Amberlite IR-3 and IR-4 (anion exchangers). Amberlite I R 2 and IR-3 are synthetic resins which have been highly condensed to give very hard resistant materials, which are also extremely insoluble and “throw” no color to the treated liquid. Amberlite IR4 and IR-1 are synthetic products with high exchange capacity and so constituted chemically that the quality of ion removal is excellent, with effluents free of ions being adsorbed. The Amberlite-IR resins have the structure of homogene-
Theory of Use of Resinous Exchange Adsorbents The ion-exchange and adsorption reactions of synthetia resins follow reactions such as the following:
+
+
2NaR CaSO, +CaRs; NaaSOi HR NaClNaR HC1 X HaSOa +X.HaSO4
+ +
+
(1)
(2) (3)
where NaR = sodium salt of a cation-exchange resin HR = hydrogen form of a cation-exchange resin X = anion-exchange (acid-adsorbent) resin I n the exchange of sodium ions for heavy metal and alkaline earth ions, the resin may be regenerated, after depletion, with sodium chloride solution. Regeneration of a hydrogen exchanger is accomplished by dilute sulfuric or hydrochloric acid. An anion exchanger may be regenerated by sodium carbonate or sodium hydroxide solution. It is apparent, then, that the use of a hydrogen exchanger results in the replacement of all cations by hydrogen ions, and the neutral salts are converted to the corresponding acids. Similarly, the use of an anion-exchange adsorbent results in the complete removal of free acids. Thus the passage of water through a double system of hydrogen and hydroxyl exchangers removes all dissolved salts and results in the production of water comparable to distilled water in quality. Furthermore, the phenomenon of base exchange permits the replacement of one metallic ion by another or the exchange of one anion for another, and the resinous exchangers are suitable for the production of salts by double decomposition. Specific applications of these principles are illustrated below
700
INDUSTRIAL AND ENGINEERING CHEMISTRY
Physical Properties of Resinous Exchangers
As Bird ( l a ) indicated, in the application of resinous exchangers to water softening, a field hitherto dominated by zeolites, the new products should at least meet the specifications now set for zeolites unless other considerations or advantages are more important. Accordingly, physical characteristics, such as the grain size, sieve analysis, apparent and true specific gravities, solubility, and attrition of resinous exchangers, have been evaluated in this study and compared to those of typical commercial exchangers of the zeolite type; the exchange capacity has been determined in accordance with accepted methods ( 6 ) . On the other hand, the applications of resinous exchangers as hydrogen exchangers and as acid adsorbents, as well as certain unique properties of these materials, have called for the development of new testing methods. Thus, chemical analysis, adsorptive capacity, swelling, neutral salt cleavage, and anion-exchange column technique have been developed for the determination of properties peculiar to the resins or their application. The grain size was determined on several typical resin exchangers and other products, and is listed in Table I, which also includes typical data from the literature on exchange adsorbents. The sizes were determined by dry- and wet-sieve analysis over standard screens (6). The carbonaceous exchangers were two commercial materials manufactured in this country. The tannin-formaldehyde resin was a preparation manufactured in England. The m-phenylene diamine resin preparation was a commercial preparation formerly manufactured in this country. The Amberlite resins were selected from typical semiplant-production batches,
TABLEI. GRAINSIZE O F EXCHANGE ADSORBENTS Material Greensand Greensand Gel zeolite Gel zeolite Carbonaceous exchanger Carbonaceous exchanger Carbonaceous exchanger Io Samed Carbonaceous exchanger IIC Samad Samed Cation-exchange E%%n-exchange resin Anion-exahange resin Tannin-HCHO resinc m-Phenylene diamine-HCHO resinc Amberlite IR-1C Samed ilmberlite IR-2d Amberlite IR-3e Amberlite IR-4d Same0
GrainEffective Uniformity Sise. Sizes c0Mm. Mm.' e5cientb Reference 0 . 1 -1.20 0.26 1.72 (2 7 ) 0.35 1.52 0 . 2 -1.20 (17) 0 . 2 -1.90 1.5s 0.64 (17) 0 . 1 -1.60 1.83 0.42 (17) 0 . 1 -1.0 (28) 0.25-1.2 O:i7 1:io (28) 0 . 2 -1.65 0.37 This work 2.13 0 . 2 -1.65 0.37 1.92 This work 0.32 2.17 This work 0 . 1 -1.20 0 . 2 -1.65 This work 0 . 5 -2.5 .. (88) 0 . 5 -2.5 ($8) This work 0 . 1 -0.59 oris 1:$5 0.15-1.65 0.25-1.20 0.26-1.98 0.25-1.20 0 . 2 -0.9 0.25-1. 65 0 . 2 -0.8
.. ..
..
0.29 0.39 0.38 0.44 0.30 0.38 0.32
3.27 1.51 1.9s 1.59 1.60 2.00 1.75
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5 T h a t size below which 10% of t h e sample is finer and 90% of t h e sample is ooarser (6). b T h e ratio of the size (in mm.) below which 60% of the sample is finer, t o the effective size ( 6 ) . c Dry. d Wet.
The resinous exchangers are obtained in the crude state in the gel form, and it is possible to have any desired grain size by suitable mechanical reduction. I n the case of greensands the dependence of capacity upon developed area and the character of the naturally occurring mineral have limited the grain sizes obtainable to materials with an effective size of 0.25-0.35 mm., with most of the commercial products a t the lower end of this range. Carbonaceous exchangers inevitably sufferpulverization during the vigorous sulfonation treatment, and the preliminary reduction of the carbonaceous or humus material before sulfonation (60) does not necessarily yield a directly usable product. Only the gel zeolites share with the resinous exchangers the property of a grain size variable at
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will. Since head losses mount rapidly when the effective size is reduced below 0.3 mm., the demands that commercial production of exchange adsorbents be held to close tolerances to reduce head losses can be met satisfactorily by the resinous exchangers. On the other hand, the greensands show a marked dependence of capacity upon particle size, whereas resinous exchangers may be prepared which exhibit no effect of developed surface upon exchange capacity. Thus, the grain size of resinous exchangers may be suitably adjusted to permit low head losses a t high operating rates without loss of exchange capacity. Apparent specific gravities were determined on several typical resinous exchangers and other products by determination of the volume occupied by a weighed sample in a graduated cylinder, tapped until no further settling occurred. Similar measurements on the samples covered with water or toluene were used in the calculation of the true densities. The values obtained are listed in Table 11, which includes data from the literature. The greensand used in this work was a typical unmodified product, manufactured in h'ew Jersey. The synthetic gel zeolite was also a typical highcapacity material, screened to a -20 +40 grading. The carbonaceous exchangers were two commercial products; carbonaceous exchanger I was in the hydrogen form, carbonaceous exchanger I1 was in the sodium form. The Amberlite resins were selected from typical semiplant production batches. The tannin-formaldehyde and m-phenylene diamine resins were the same as those used in the determinations of grain size and sieve analysis. The resinous exchangers are considerably lighter in weight (true density) than the greensands or synthetic gel zeolites. The lower true density of resinous exchangers permits complete turnover of the exchanger bed a t backwash rates which would be insufficient to expand a greensand bed. This is of advantage in areas of low water pressure, since booster pumps are eliminated. Also, when soft water is used in the backwash operation, considerable savings in the quantity of water required are effected, since lower flow rates suffice t o expand the resinous exchanger bed. On the other hand, the bed expansion on backwash or in upflow operation is greater with a resinous exchanger for a given rate of flow, and adequate freeboard should be provided in construction design. Thus, at 25" C. Amberlite IR-1 was found to give about 50 per cent bed expansion a t an upflow rate of 5.0 gallons per square foot per minute, and 25 per cent bed expansion a t a 3-gallon rate. At 10" C., 25 per cent bed expansion was observed at a 2.5gallon rate. Both carbonaceous exchangers and resinous exchangers imbibe water and swell when the dry product is placed in water. For convenience those Amberlite resins which swell to a considerable degree when wetted are finished in a damp condition in commercial production. I n spite of the swelling, the capacity of the wet resins on a volume basis is considerably higher than that of siliceous or carbonaceous exchangers; as a consequence, no additional floor space is required t o accommodate a resinous unit which will have a capacity equal to or greater than that occupied by units of other types of exchangers. The resinous exchangers are insoluble in acids, alkalies, and salt solutions as well as in hot and cold water, in so far as present methods are capable of measuring such solubilities. It has been found that solubility may be induced by loading the resin with additional exchangeable atoms or groups which impart hydrophilic qualities as well as increase the capacity. Thus the resin best suited for exchange applications must be carefully selected on the basis of a precise balance between those groups which impart exchange but also solubility, and those which promote the condensation to give an insoluble product.
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TABLE11. D E N S I T I E OF~ EXCHANGW ADSORBENTS Material
L
Greensand Greensand Processed greensand Synthetic gel zeolite Greensand (commercial) Synthetic gel zeolite Carbonaceous exchanger Carbonaceous exchanger Carbonaceous exchanger I Carbonaceous exchanger I1 Cation-exchange resin Anion-exchange resin Amberlite IR-16 Amberlite IR-26 Amberlite IR-36 Amberlite IR-46 Amberlite IR-1/ Amberlite IR-4/ Tannin-HCHO resin m-Phenylene diamine resin (I
0
Density in Airb Apparent True #./eo. L b . / c u . ft. G . / c c . L b . / c u . fl. 1.94 120 1:46 91 1.36 85 0.82 61 .. ... 1.51 94.2 ,. 0.53 33.0 ... *. .. i:ig 74 1.35 84 0:is 4a:7 0.71 44.3 1:io 75 1.25 0:8l 51 .. .78 0.74 46 ... 0.67 42 ... 0.60 37 ... 0.68 42.4 0.65 34.3 0.82 61.2 0.66 40.9 ,
..
.. ..
..
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Density in Waterc Density in Toluene Apparent True Apparent True G./cc. L b . / c u . ft. G./cc. Lb./cu. f t . Q./cc. L b . / c u . f1. G . / c c . L b . / c u . ff.
Reference
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1:57 0.61
9S:O 37.8
2172 1.81
169:s 113
1:63 0.58
9516 38.2
2:6l 1.90
162:s 118.5
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0:il 0.65
38:2 34.4
1:44 1.61 .
90.0 100.0
0179 0.63
49:3 39.6
1148 1.80
.., 92.3 112
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0:47 0.49 0.54 0.38 0.78 0.68 0.78 0.72
2913 31 33.7 24 48.6 42.7 48.7 44.4
1:47 1.62 1.30 1.26 1.24 1.11 1.49 1.27
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92 95 81.1 79 77.6 70 92.6 79.1
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Attrition losses noted with the older adsorbents were really tion, and slow disintegration produced by chemical reactions induced in waters of low pH. The exchange of a heavily hydrated ion for one less hydrated produced a strain on the silicate lattice, and disruption often occurred after a period of use. Contrasted to these effects, the flexible structure of synthetic resins readily permits the exchange of a large ion for a small ion without disruption of the resin “lattice”. Thus, in a particular instance, a n anion-exchanger resin was studied which showed a change in volume of over 25 per cent in the conversion of the resin from the free base to the sulfate (exhausted) condition. Upon regeneration with sodium carbonate solution, the swollen resin returned to its initial volume. I n spite of this remarkable stretching of the resin “lattice”, studies conducted over a period of one hundred successive cycles of regeneration and exhaustion produced no ob-
.. .. .. *.
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4f:4
Data calculated on as received basia unless otherwise noted. b Specific gravity measured as received. Material covered with water and swelling allowed to take place. d Mineral thinner in place of toluene.
a combination of mechanical effects, due to rubbing together of the exchanger granules during backwash or in upflow opera-
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servable disintegration of the exchanger. Under the same conditions, a siliceous exchanger would have been completely ruined by disruption of the silicate lattice. In addition t o this property of a n elastic structure, the resilient character of the synthetic resins should permit complete “teetering” of the bed during backwash or upflow with no mechanical disintegration, since the particles would rebound from collisions without harm, much as rubber spherules would rebound without attrition losses. A definitely established method for the determination of attrition does not appear to have been developed or accepted by zeolite workers in general (6). Fundamental studies are in progress in our laboratories on the determination of attrition, and significant results will be reported at a later date.
Determination of Chemical Properties
The exchange capacity of the resins has received the greatest attention in this work. The total exchange capacity and the usable (break-through) capacity were determined under a variety of conditions. The columns used in the investigation varied in internal diameter from 12.5 to 100 mm., with bed depths from 150 to 750 mm. The recent proposal (6) by the Committee on Testing Zeolites of the American Water Works Association that 25-mm. tubing and a 500-mm. bed depth be employed in laboratory testing does not alter the results or comparisons; the results on large-diameter and on small-diameter columns were almost identical, although the wider tubes were admittedly less troublesome in operation. Upflow as well as downflow operation was studied, but in view of the general prevailing practice of operating exchanger-bed downflow, most of the tests were conducted in that manner. In the examination Lojf“ various laboratory preparations and in studies on other exchanger adsorbents, a downflow rate of 5 gallons per square foot per minute was employed in operation, with the regeneration held downflow at a 1-2 gallon rate. Backwash following each cycle was maintained for a brief time at a rate sufficient to give 50 per cent bed expansion for the particular material under test. Rinse at a 2-3 allon rate was maintained until the effluent failed to give a (qualitative) ion in the uositive test for the significant ;e enerant used. %he calcium chloride solution used in the studies of sodium-calcium and hydro en-calcium exchange ww made up in %stilled water to contain 500 rng. er liter as calSMALL-SCALE EXCHANGE UNITSUSEDIN THE LABORATORY STLTDY OF PERcium carbonate. &,dim c&oride, 500 m FORMANCE OF SYNTHETIC RESINS per liter (as calcium carbonate), was similarfy
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prepared. The solutions used in the testing of anion exchangers consisted of hydrochloric acid, sulfuric acid, and a 1 to 4 mixture by weight of hydrochloric and sulfuric, all 500 mg. per liter as calcium carbonate. Sodium chloride (4-7 per cent), sodium carbonate (1-2 per cent), hydrochloric acid (1 per cent), and sulfuric acid (1 per cent) were umally made up in Philadelphia tap water which contained approximately 50 p. p. m. calcium ion, and 80 p. p. m. total solids. A 3-inch layer of -20 +30 mesh graded quartz sand was employed as bed sup ort above a very thin layer of glass wool. The effluent was sampyed periodically at such a rate that the breakthrough oint was known to within 3-5 per cent of the total volume water treated. The samples were tested qualitatively (soap test) for calcium ion, in sodium-calcium exchange. Breakthrough was, in general, very sharp and capacities were calculated up to a 10 p. p. m. effluent which usually occurred between single samples. Calcium-hydrogen exchange was followed by titration of the effluent with 0.1 N sodium hydroxide and bromothymol blue, after which a soap titration for hardness was made. Break-through was sharp, in general, and evidenced by a rapid fall in the titration value and a sudden increase in the soap titration value. Again, 10 . p. m. calcium was taken as a breakthrough limit. Sodium-gydrogen exchange was followed by titration with 0.1 N sodium hydroxide and by pH with the glass electrode; 10-15 p. p. m. sodium chloride was taken as breakthrough. The effluent from the anion-exchanger column was tested with methyl orange indicator, and pH was determined by the glass electrode. A t methyl orange neutrality (pH 4.0) the bed was cut out of service. The effluent samples were analyzed later for chloride ion by Mohr’s silver nitrate-potassium chromate method and for sulfate ion by the tetrahydroxyquinone-barium chloride method (66). In general, the sodium columns were regenerated with 4 per cent sodium chloride a t a salt ratio of 0.5 pound sodium chloride per kilograin calcium carbonate removed. This corresponds to 300 per cent chemical equivalency. The salt ratio was varied in some cases to study the effect on the capacity obtained. The hydrogen exchangers were similarly regenerated with hydrochloric or sulfuric acids (1 per cent solutions) at a ratio of 0.5 ound acid (as sodium chloride) per kilograin cation removed i s calcium carbonate). The anion exchangers were regenerated with a sodium carbonate solution (1 and 2 per cent solutions were used) at a ratio of 0.5 pound sodium carbonate (as sodium chloride) per kilograin anion removed (as calcium carbonate). The materials under examination were, in general, screened to a -20 +40 sieve grading on a wet basis in so far as possible, in order to equalize effects due to developed surface which were studied separately. Commercial materials and Amberlite batch sam les were examined as received, although in some cases gra&d fractions were also studied. The capacity data were calculated to a volume basis in place, using the settled volume following rinse as the most reproducible volume determination.
07
Exchange Capacity in Sodium-Calcium Cycle
Vol. 33, No. 6
a 0.3-0.4 pound ratio, and industrial installations at 0.4-0.5, while domestic units usually employ a 0.5-0.7 pound ratio to obviate frequent regeneration. It was of interest, therefore, to examine the resinous exchangers under a variety of salt ratios and to compare the capacities thus obtained with similar data in the literature on other exchange adsorbents. The details of such a study are given in another paper (45), and the tabulated results only are given here. I n Tables I11 and IV the capacity data on a 4-inch (100-mm.) column filled with a cation-exchange resin (Amberlite IR-1) operated a t various salt ratios are given and compared with similar data from the literature. The data in Tables I11 and IV indicate that the capacity of the cation-exchange resin (Amberlite IR-1) is substantially greater than that of greensands, synthetic gel zeolites, and carbonaceous exchangers at all salt ratios. The capacity of the cation-exchange resin, for example, is 8400 grains calcium carbonate per cubic foot at a salt regeneration ratio of 0.37 pound per kilograin calcium carbonate removed. The greensand examined (48) showed a capacity of only 2600 grains per cubic foot a t the same regeneration ratio. The values given in column 6 of Table I11 are based upon the values in column 2 and were obtained from Table IV directly or by interpolation. Higher capacity offers the industrial chemist savings in floor space (smaller beds required for equal volumes softened), in regeneration time and labor (longer runs for equal bed volumes), and in wash water requirements (fewer regenerations required). Furthermore, as Table V indicates, the capacity of the resin was found to be independent of particle size or developed surface. This permits the use of the exchanger in a relatively coarse grading, and the throughput capacity as well as the total capacity of existing beds may be increased by use of the resinous exchanger. Conversely, volume output and total capacity can be maintained, with a saving in floor space, by the use of smaller beds. Other studies (45) have indicated that the capacity of the cation-exchange resin, Amberlite IR-1, is constant and independent of the calcium-ion concentration over a range from 25 to 1500 mg. per liter, and is unchanged under operating rates as high as 10 gallons per square foot per minute. The influence of sodium ion has been investigated also and found t o exert no appreciable effect at the concentrations encountered in natural waters. The unique combination of high capacity, high exchange rate, and freedom from effects due to developed surface is characteristic of the cation-exchange resin described above. This may be pictured as due to a very porous, open resin “lattice” or gel structure. That this is not true of all resinous exchangers was shown by studies on other synthetic resin exchangers-a highly condensed phenolic-type cation exchanger
As a consequence of the mass-action principle which operates in the exchange equilibria, the usable capacity of a cation exchanger becomes a function of the quantity of regenerant employed. Thus, greensands, synthetic gel zeolites, and carbonaceous exchangers have been employed at salt ratios which range from 175 to 400 per cent of that required on the basis of chemical equivalence. The salt ratios employed in practice represent a balance between cost of TABLE 111. COMPARISON OF CAPACITY AND SALTRATIOFOR RESINOUS EXCHANGER salt per unit volume of water softened, AND OTHER EXCHLVQB ADSORBENTS IN SODIUM CYCLE time and expense of regeneration, cost CaCOa Capaoity of of floor space needed for large beds, NaCl R a t i o % , Resinous Exchanaera and initial investment involved. While Capacity (as C a C W Lb./kiio- Lb /cu, ChemiGrains/ Kg./100 grain ft. excal Grains/ K$./100 considerable variation exists in indusType of Exohanger cu. ft. liters CaCOa changer Theory cu. f t . litera Referenoea trial practice, greensand beds are gen222 8,400 0.96 0.594 0.37 Greensand 2600 1.91 (48) 270 11,400 0.45 1.26 Greensand 2800 0.640 2.60 (48) erally operated at 0.30-0.40 pound salt 215 8,400 0.36 1.07 2980 0,680 Greensand 1.91 (18) per kilograin calcium carbonate re210 7,200 0 35 1.40 0.913 Synthetic zeolite 4000 1.64 (48) 300 12,700 0.50 2.73 Synthetic zeolite 5500 1.255 2.90 (4, 8,48) moved (177 per cent of theory) and 1 12 240 10,000 0 639 0.40 Greensand 2800 2.28 (18) 2.45 210 7,200 1 .60 0.35 7000 1.64 (48) Carbonaceous zeolite synthetic gel zeolites from 0.45-0.70 407 16,000 3.65 This work 0.68 4.00 1.35 Carbonaceous zeolite I11 5900 pound sodium chloride per kilograin 280 12.000 2.58 1 26 0.47 Carbonaceous zeolite 6600 2.74 {%8) 300 12,700 2.90 This work 0.60 3.35 6700 1 . 6 3 Carbonaceous zeolite I calcium carbonate removed (260-410 300 12,700 2.90 This work 0.50 3.20 Carbonaceous zeolite I1 6400 1 4G per cent of theory). Large-scale softena Under similar conditions: value8 interpolated from d a t a i n Table IV, on “chemical theory” basis. ing plants, such as municipal estabCompare with values in columns 2 and 3. lishments, are generally operated at
June, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
703
a “decationized” water results, with the salts in the form of TABLEIV. CAPACITY IN SODIUM CYCLE AND SALTREGENERA- the free acids. Regeneration is conveniently effected by the TION VALUEFOR CATION-EXCHANGE RESIN(AMBERLITE lR-l)a use of 1-2 per cent hydrochloric or sulfuric acid solution. Lb, s a y % ChemiCapacity (as CaCOa) Conon. Bicarbonates and carbonates are converted to carbonic acid Kilograin cal Grains/ Kg./100 Lb. NaCl/ NaCl which may be removed by aeration; the result is a reducCaCOz Theory cu. ft. liters Cu. Ft. % tion of total solids, alkalinity, and hardness. The free 1.92 3.5 222 0.37 8,400 2.28 4.0 10,000 240 0.40 mineral acids may be neutralized with alkali or, preferably, 5.5 2.78 12,200 288 0.48 8.2 3.28 14,400 352 0.59 removed by an anion-exchanger adsorbent. 10.6 3.54 15,500 390 0.65 11.65 Several resins were examined in the hydrogen cycle, in 3.76 420 16,500 0.70 14.40 4.11 18,000 480 0.80 which both calcium-hydrogen and sodium-hydrogen ex81.0 29,800 6.80 2.72 1640 change were studied. The exchange is stoichiometric in Calcium chloride solution 500 m /liter (as CaCoa) used. Breakthrough taken as 5-10 mg./liter baCOa. %emp., 25O.C.; flow rate, 5 gal./sp. character and the titration with 0.1 N alkali of the effluent ft./ min. downflow; bed volume, 0.909 CU. f t (2.57liters). from an exchanger column is exactly equivalent to the salt concentration in the water being treated. I n fact, this beTABLEV. PARTICLE-SIZE AND CALCIUM-EXCHANGE CAPACITY havior has been utilized in the laboratory in the analysis of OF CATION-EXCHANGE RESIN(AMBERLITE IR-1) solutions of calcium, sodium, ammonium, and copper salts. Screen Capacity A portion of the solution is passed through a column of the Grading, Qrains CakOa/ Mesh Cu. Ft. Remarks resin in its hydrogen form, and the titration of the effluent 13,300 -16 +20 serves to determine the concentration of the cations in the 13,700 -20 +30 original solution. When only one cation is present, the pro13,500 -30 +40 12,720 -40 +50 cedure is of particular value in obviating tedious gravimetric 12 100 -50 +60 --Rn 4-7n i -R,...k m . methods. -76 +lo0 13,200 mm. deep bed. Break-through of the ion being adsorbed is indicated by a rapid fall in the titration value and may well serve for control of industrial units, to indicate when regeneration is necessary. If both sodium and calcium ions are present in and an aromatic diamine-formaldehyde anion-exchange resin. the original water, the titration value will fall to zero and reThe gross appearance of the two resins was that of hard glassmain there for some time, although no calcium ion appears in like substances, and it was reasoned that an effect of developed the effluent. The exchanger thus operates as a sodium exsurface on exchange capacity would be shown. Experimental changer and continues to remove calcium by sodium-calcium investigation verified the conclusion. Thus, the cation-exexchange until saturated with calcium. change resin based on phenol had a calcium-sodium exchange The exchange capacity of two cation-exchanger resins is capacity of 9600 grains calcium carbonate per cubic foot a t a listed in Table VI, together with data on other hydrogen ex-20 +30 screen grading, and a capacity of 11,400 grains at a changers. The resinous exchangers exchanged sodium, cal-40 +50 screen grading (both a t a salt ratio of 0.5 pound cium, ammonium, magnesium, and copper about equally well sodium chloride per kilograin calcium carbonate adsorbed). in the hydrogen cycle. The calcium-hydrogen exchange was Similarly, the acid-adsorptive capacity of the aromatic diindependent of developed surface in one case but somewhat amine resin was increased more than tenfold by a reduction influenced by particle size in another. The adsorption exin particle size from a -12 +20 to a -30 +40 screen gradchange capacity for calcium was approximately 1.1 to 1.2 ing. Hence, the development of a synthetic resin exchanger times that for sodium on an equivalent basis (total exchange suitable for industrial application must involve investigations capacity, determined on fully regenerated resin). The of capacity, exchange velocity, and the effect of developed removal of sodium ion was found to be much more effective surface on these properties, as well as the synthesis of a variety when sodium carbonate or sodium hydroxide was employed of substances designed for a particular purpose. (2.22 and 3.18 milliequivalents per gram, respectively) than I n addition to the laboratory and semiplant studies which when sodium chloride was treated (1.47 milliequivalents per form the basis of the above data, several field units were regram). Such effects as these are but a natural consequence cently installed for the collection of data under a variety of of the effects of valence and YHon base-exchange equilibria. actual operating conditions, The performance of these units will be reDorted a t a later date. Sufficient data have been collected abroad -1 (27, 66, 64) to justify the conclusion TABLEVI. EXCHANGE CAPACITY IN THE HYDROGEN CYCLE that the synthetic resin exchangers are Regenerant Ratio well adapted for use in water-softening Lb. acid % Capacity, (as NaCl)/ chem. applications and possess many ad1oae Grains Acid kilo rain e5vantages over the older exchange adMaterial Exchanged CaCOs/Cu. Ft. used. % ’ (as 8aCOa) oiency sorbents. Tannin-HCHO resin I, screened to: 5
b
*
Exchange Capacity in Hydrogen Cycle Resins which contain an aromatic (nuclear) or aliphatic (side-chain) sulfonic acid group may be converted to the hydrogen derivative by treatment with dilute acid, and the sulfonic acid hydrogen is then available for ion exchange. By the treatment of aqueous solutions of salts with such a material, all the cations, including sodium and potassium, are exchanged for hydrogen;
-10 +20 -30 +40 -40 +50 -50 +00 -60 4-70 Modified phenol-HCHO, soreened to: -20 +30 -40 +50 Carbonaceous zeoljtea Carbonaceous zeolite I Carbonaceous veolite Tannin-HCHO resin I1 Tannin-HCHO resin I
(I
b
Ca/H Ca/H Ca/H Ca/H Ca/H
8300 8650 7550 7880 8180
Ca/H Ca/H Ca/H Ca/H Ca/H Ca/H Na/H
7400 8iao 8000-9000 8500 8800 7100 73.7a 24,000 22,000 23,000
%
