ATERIALS capable of ion exchange have been in commercial use for many years, but these zeolites and related products have generally been considered as water treating chemicals, and have found relatively little use in fields not related to the conditioning of water sup.plies. Originally this was due mainly to the limitations imposed by the general properties of siliceous exchangers. They tended to disintegrate both physically and chemically when used under alkaline or acid conditions, and were thus limited to a very narrow pH operating range. As a result they could not be converted to their hydrogen form, and thus only sodium cycle operation was possible. With the advent of the carbonaceous or sulfonated coal type of cation exchanger, hydrogen cycle operation became possildle, and some specialized uses were developed. However, until the resinous exchangers became available in 1935, as a result of the work of Adams and Holmes (g), little progress was made in the adaptation of exchange materials to special processes other than water conditioning.
M
EXCHANGER TYPES AND REACTIOXS
A comprehensive bibliography on the existing literature and the past history of exchangers has been presented by Myers and co-workers (19-15), and only a brief summary will be included in this paper. The following is a simplified classification of exchanger types: 1. Cation exchangers a. Zeolites (natural or synthetic siliceous compounds) b. Carbonaceous (sulfonated coals, etc.) c. Resinous (generally phenol-formaldehyde types) 2. Acid adsorbents (incorrectly called “anion exchangers”) a. Inorganic (dolomite, heavy metal silicates) b. Organic (amine-formaldehyde resins, etc.) I n so far as the special applications are concerned, the most important types are the carbonaceous and resinous cation exchangers, and the organic acid adsorbents. The typical reactions of exchangers are summarized as follows: SODIUMCYCLECATIONOPERATION.Regeneration is effected by an excess of sodium salt: NaX MR NaR MX
+
+
HYDROGEN CYCLECATIONOPERATION.Regeneration is effected by an excess of acid: MR HX HR MX
+
should also be noted that the resinous cation exchangers have proved eminently suitable for use in both of these wellestablished processes (12, I S ) . A third exchanger process for water conditioning is now available, and its success has been proved on a commercial scale. This process has been variously referred to as deionizing, demineralizing (I8),or merely two-step exchange. DEIONIZING.It was generally recognized that the only method of producing very pure water was by distillation, whether in specifically designed stills or by the use of industrial condensate. In many cases, however, it was realized that the degree of purity was greater than necessary, and that costs were invariably high. Furthermore, throughput per unit of equipment necessary was low, and if the quality of the raw water was poor, scaling and priming would occur to such an extent that still oapacity was appreciably reduced or the purity of the water produced was seriously affected. Deionizing processes now available are proving to be a valuable addition to distillation as a method of producing very pure water. Although fundamentally simple and discussed in detail by Adams and Holmes (8) in 1935, only recently has the process been adopted on a large scale. The soluble salts in the water being treated are converted to their corresponding acids by passage through the hydrogen exchanger, and the acids thus produced are removed by the acid adsorbent. Although earlier cation exchangers exhibited low capacity, they did function satisfactorily, but the acid adsorbents previously available were not physically stable and were invariably incapable of complete acid removal. Recent synthetic acid adsorbents, resinous in nature, overcome the difficulties previously encountered. It should be noted that this process removes only the salts and other ionic impurities, and soluble silica and nonionic organic impurities are unaffected. However, there are several processes (gd) which can be successfully used to pretreat deionizer feed water for the removal of silica and organic matter, so that a very pure deionizer effluent can be obtained. Analyses from a typical commercial unit are shown in Table I. The analysis of the condensate which the deionized water replaced is given for comparison TABLEI. COMPARISON OF RAWAND DEIONIZED WATERSAND CONDENSATE
+
Total dissolved solids p. p. m. Volatile matter,,p. p.’m. Inorganic material, p. p .m. Iron, p. p, m. Silica, p. p. m.
ACIDADSORPTION : * RNH,
+ HX .--f RNH,.HX
ANIONEXCHANGE : RNH,.HX HY
+
% RNH2.HY
EFnductivity, reciprocal ohms
+ HX
Raw Water
Deionized Water
117.9 39.6 78.2 0.07 2.0 6.9
6.9 2.5 4.4 0.01 2.0 3 x 6.7 10-
..
Condeneate 3.9 2.6 1.3 0.25
0.25 7 x 6 . 310-0
The purity of deionized water, in so far as U. S. Pharmacopoeia specifications are concerned, was discussed in detail by Harrison, Myers, and Herr (IO). They found that resinous exchanger systems would produce water equal in quality to that required by U. S. P. specifications, and biological tests proved that the pyrogen content of the water was unaffected by the exchange process. The success of this process for the economical production of large quantities of very pure water appears to be assured, and its adoption has solved many industrial water conditioning problems.
WATER CONDITIONING
Prior to the advent of satisfactory ion exchange resins, only two water conditioning exchange processes were being widely used. They were the softening of water (sodium cycle operation) and the reduction of alkalinity by the use of a cation exchanger opexating in the hydrogen cycle. These processes have been described in detail elsewhere (3, 4). It
SPECIALIZED DEIONIZING PROCESSES
Deionizing is now being used in other fields. Rawlings and Shafor (16) summarized the application of this process in the sugar indugtry. Because the nonsugar constituents of t h e
Photographs on the Opposite Page Show Top and Bottom of Semicommercial-Scale Amberlite Sugar Purification Unit at Burley Plant of the Amalgamated Sugar Company. 859
860
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 35, No. 8
Figure 1. Flow Sheet of Exchange Process for Recovering and Concentrating Magnesium in Sea Water Raw materials: sea water, CaCOa [Ca(OH)z and Cor], HzSOd; produots: MgSOI, MgClz, NaHCOs, pure water.
juice retard the crystallization of sucrose with subsequent loss in yield, many means of removing these nonsugar impurities prior t o crystallization have been studied so that purer sugar would result, and less sucrose would be retained in the low-price molasses. Rawlings and Shafor developed a resinous exchanger process which treats the partially purified beet or cane juice (approximately 12-14 per cent sucrose) prior t o evaporation and crystallization. This process has shown as high as 98 per cent removal of inorganic salts ("ash"), organic impuiity removal up to 83 per cent, color removal from 50 to over 90 per cent, and a high degree of colloid elimination from the sugar juice as determined by the ultramicroscope. The degree of impurity removal can be controlled by varying the operating conditions of the process and by changes in the regeneration steps. The extent to which impurities are removed is governed by economic considerations which are dictated by local operating costs and prevailing sugar prices. On a semicommercial scale the exchange process of sugar juice treatment has proved completely satisfactory; it has the advantages of increased yield of sugar, appreciable elimination of scaling, reduction in the amount of low-purity products to be reboiled, reduction in fuel consumption, and increased production of sugar without increase in evaporation or crystallization equipment. Although similar in theory t o deionizing as used in water purification, the application of the process to the sugar industry is further complicated by the numerous varieties of impurities present, the susceptibility of sucrose t o hydrolysis a t low pH, and many other peculiar limitations. Treatments for cane, beet, and refinery sugars present distinctive problems which are being actively investigated. Weitz (19) recently described the large-scale operation of an exchanger sugar purification plant; on the basis of all data obtained to date, it appears that the exchange process represents a real advance in sugar technology. Englis and Fiess (8) evolved a similar process in a related field. In the production of a palatable levulose sirup from
Jerusalem artichokes, their original process consisted in the acidification of the polysaccharides (predominantly inulin) with hydrochloric acid, hydrolysis, and salt and acid removal by dialysis or electrodialysis. However, as a result of the buffering action of the salts present, large quantities of acid 11 ere necessary for hydrolysis, and the purification of the hydrolysate was tedious and expensive. They found that treatment of the polysaccharide solution with a cation exchanger operating in the hydrogen cycle resulted in the production of sufficient acid to make hydrolysis possible without the use of additional acid. After autoclaving, the acidic levulose solution m s treated with Amberlite IR-4 t o effect acid removal. The expensive acid addition was avoided, and as a result of conversion of the salts, a quick and simple purification process could be substituted for dialysis. Furthermore, because of removal of organic acids, not removed by dialytic procedures, the exchanger-treated product was superior in quality and flavor t o sirups produced by the earlier process: I n this study complete elimination of nonsugar impurities was not attempted. However, further work on this problem is being conducted with the aim of effecting crystallization of levulose from highly purified sirups. A modification of the deionizing process, in which it is desired to recover the valuable acidic constituents removed, was developed by Matchett (11). The recovery of tartrates from wine residues has long been practiced, but the process was inefficient, and residues with low tartrate content were not amenable to treatment. The newly developed process is applicable t o either distillery slops or grape waste extracts. The aqueous extract, following clarification, is passed through a bed of Amberlite IR-1 (hydrogen form) which frees the tartaric acid. This tartaric acid is subsequently adsorbed and concentrated in the Amberlite IR-4 bed. The acid is flushed from the bed with sodium carbonate and purified by precipitation as the calcium salt. Beginning with an initial tartrate concentration of 0.28-0.30 per cent, a twelve to fifteen fold concentration of the tartaric acid is being effected. At present in the pilot plant stage, commercial scale application is
August, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
planned if preliminary results are substantiated by further operation. Adoption of this process by domestic wineries alone could add several million pounds of tartaric acid to the present annual production. Further, the Amberlite process makes possible the recovery of this valuable chemical which is being wasted at present. No discussion of the deionizing process would be complete without a consideration of the effects obtained at very high salt concentrations. I n so far as maximum concentrations are concerned, there are two main limitations-chemical inhibiting effect and economics, which are in some cases of secondary importance. Considering the former, the limiting factor is the lowest pH at which the particular cation exchanger being used will effect exchange. Studies on Amberlite IR-1 (hydrogen cycle) indicate that the lowest p H a t which exchange will occur is approximately 0.85. The following data were obtained on treating sodium chloride solutions of varying concentrations with Amberlite IR-1-H: Initial Salt Concn. 10.28 4.91 2.00 0.99 0.75 0.40
82U::t. 0.85 0.90 0.86 0.87 1.05 1.35
Salt Concn. of Effluent, % 4.60 1.50 0.69 0.05 0.03 0.00
As a result of these studies, it appears that the critical concentration a t which sodium chloride is completely converted to hydrochloric acid by Amberlite IR1 is approximately 0.5 per cent (5000 p. p. m.). However, it is obvious that complete salt removal from more concentrated solutions may be effected by stepwise treatment with cation and anion exchangers which can be carried out by a recycling process or multiple bed operation. Studies have also been conducted on the desalting of sea water. Although work on combinations of chemical and exchange processes continues, it does not appear a t present that any unmodified exchange process is the answer t o the problem. ACID REMOVAL
The development of acid adsorbents which are physically and chemically stable, and will effect complete removal of acid traces, has made another valuable unit process possible. Admittedly the use of an acid adsorbent eventually involves neutralization, but the neutralization is effected independently of the solution being treated for acid removal, and contamination of the product is thus avoided. Furthermore, the neutralized acids are handled separately, and can be purified or further treated if desired. Considering the acid-binding capacities of acid adsorbents, it would be expected that the capacity would depend on the chemical composition and molecular configuration of the exchanger being studied. This is true, but many other factors also influence capacity, and the relative importance of all are by no means clearly understood. Thus degree of ionbation of the acid, basicity, influent concentration, molecular size, operating and regenerating conditions, etc., all have their effect; and capacity values should be accompanied by a description of the conditions under which they were determined. Typical capacities of Amberlite IR-4 are shown in Table 11. A well-established use of the exchanger acid removal process is in the deacidification of formaldehyde. Most formaldehyde produced today is obtained by the catalytic oxidation of methanol, and may contain as much as 0.05-0.08 per cent formic acid. I n many of the uses of formaldehyde this impurity is objectionable; it is particularly so if the formaldehyde is to be used for the production of synthetic resins.
861
TABLE 11. ACID-BINDINGCAPACITY OF AMBERLITEIR-4
Acid
HC1
HNOs His04 HsPOi HaBOa HCOOH CHsCOOH CaHiCOOH
(as acid)
P. P. h?.
--Capacity Grains (as CsCOa) per ou, ft.
400 372 500 1500 64 500 2000 2140
30.000 49,500 50,000 98,000 117 1e.000 24,200 21,300
3.13 8.91 7.00 9.15 0.007 1.58 4.15 5.35
Influent Concn
a t Break-throu haLb. Eb. acid per Ib. per dry cu. f t . resin 0.174 0.495 0.389 0.508 0.0004 0.088 0.231 0.298
a Break-through capaoity as given is acid-binding oapacity until first appearance of acid in effluent. total acid-binding capacity (complete ssturation of resin) is in all cases’appreciably higher. I n all cases re enerant used was approximately three fimes theoretical requirement; norma7 operating rates were in the range of 8-5 gallons per square foot per minute.
Neutralization of the acid is laborious, and the salts are as objectionable as the free acid. Removal of the acid can be effected with activated carbon, but the process is cumbersome and expensive. Passage of acidic formaldehyde through a bed of Amberlite I R 4 effects complete and rapid removal of the acid, and high throughput is possible. Typical data on a commercial size Amberlite IR-4 unit follow : Diameter of resin unit, f t . 5.0 Surface area, sq. ft. 19.6 Bed de t h f t 3.0 Bed voyuulde, bu. ft. 59.0 Acid concentration (in formaldehyde) Per cent 0.05 Lb.(gal. 0,0045 1.58 (equivalent to 351 gal. formCapacity of IR-4, lb. formic acid per cu. ft. aldehyde) Total bed aapacity (between regen- 9 3 . 1 (equivalent t o 20,700 gal. eration) lb, formic acid formaldehyde) Throughpht, gal./hr. Approx. 2350 REMOVAL OF METALLIC 10NS
The removal of metallic ions may be desired because they constitute objectionable impurities, or because they are sufficiently valuable to be recovered. I n certain applications, both factors enter into consideration. The exchange process adopted for metal ion removal depends on the particular conditions. If sodium salts are not objectionable in the effluent, then sodium cycle operation will be satisfactory, and in unusual cases, salts other than sodium have been used as regenerant. Generally, however, more complete removal of cations is effected by the use of the cation exchanger in the hydrogen form. Beaton and Furnas (6),in applying exchangers to the purification of brass mill pickling liquor wastes and to the recovery of valuable constituents therefrom, have presented an excellent study of the Cu++-hydrogen cation exchanger system. The value of cation exchangers as a means of concentrating metallic ions is clearly shown, and in the recovery of copper from a dilute copper sulfate solution (0.005 N ) , concentration increases of over 220 fold were obtained. They further determined that one pound of sulfuric acid used for the regeneration of a cation exchanger could effect a concentration increase equivalent to the evaporation of 4200 pounds of water. With many special chemicals, heavy metal tolerances are low, and in some cases repeated recrystallization or distillation was the only method of purification possible. It has been found that resinous exchangers are unique in the completeness with which they remove traces of ions and are thus applicable to these special problems. The removal of calcium, iron, copper, and lead from organic acids (7), glycerol, carbohydrates, and wines are typical commercial applications. The experimental data on the removal of iron from aluminum sulfate solutions are as follows: A tower was charged with 800 grams Amberlite IR-1 (sodium form) and flushed with dis-
862
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 35, No. 8
the process is shown in Figure 1. Closer consideration of the process suggests that there are many details to be clarified, and it is not known Khether this process, or a modification thereof, is in actual commercial use. However, investigation of exchange processes in connection with the recovery of magnesium from sea water is under way in this country. PREFERENTIAL ADSORPTION
Preliminary data on this subject were presented by Myers, Eastes, and Urquhart (16),and the use of exchangers for the separation of sulfuric acid from hydrochloric acid and copper ion from zinc ion was clearly demonstrated. Further studies have been conducted; although additional data are available, all the factors which govern ion separation are not clearly understood and much work still remains to be done. A typical problem is the removal of copper from rayon wastes, in which the concentration of ammonium salts is high. The composition of the waste was as follows: Copper (as CuSO), P. P. m. Ammonia (as NH3zS04, P. P. m. PH
4.8 2750 5.02
Amberlite IR-1 was used, and the static adsorption technique rather than column operation was employed. Removal of copper was Typical Test Columns for Studying Special Exchanger Applications, for effected to a residual of approxiEvaluation of New Types, and for Production Control mately 2.00 p. p. m. even in the presence of this high ammonium ion c o n c e n t r a t i o n . However. t'ne capacity of the resin for copper under these conditions tilled water. After backwashing to classify the bed, a 33" Baume alum solution (approximately 25 per cent aluminum was only one third of that obtained when the concentration of other salts is negligible. As would be expected, a 50 per sulfate) containing 125 mg. ferric oxide per 100 ml. was allowed cent reduction in the concentration of copper and ammonium to flow through the column at the rate of 50 ml. per minute. ions in the original solution increased the efficiency of removal A total of 3300 ml. was passed through the bed before the exas well as the total capacity for copper. Further column periment was discontinued. The first 250-ml. portion of the studies have shown that copper removal from a waste such effluent contained no alum, since it was required to convert as this is entirely practical. the sodium derivative of the resin to the aluminum salt. The A study of the recovery of zinc from mine waters has also course of the iron removal is summarized as follows: been made, The mine water studied had the following compoSample Total Vol. FezOs, Grams sition (in parts per million) : zinc 137.0, calcium 147.7, magNO. Treated, Ml. "Baume per 100 M1. nesium 17.7, iron and aluminum 8.0, copper
