0
INDUSTRIAL APPLICATIONS OF ION EXCHANGE RESINS1 R. M. WHEATON and R. E. ANDERSON The Dow Chemical Company, Midland, Michigan
THE
softening of water has provided the primary outlet for ion exchange materials throughout the approximately 50-year usage of these products. At first this use was of small scale and was primarily at the industrial level. However, as the homeowner realized the value of soft water the trend towards a disproportionate sale to him commenced. The desirability of soft water in the home is continuing to increase the market for ion exchange softeners a t a very marked rate, which has been greatly prodded by new, high-capacity, highly stable cation-exchange resins during the last 15 years. During the last decade industry has realized that ion exchange resins have tremendous potentiality for accomplishing jobs other than what might now he called conventional water softening. Thus industrial water treating now extends to a large number of variations including deionization (in which essentially all ionic species may be removed from solution), dealkalization (in which pH is lowered by exchange of carbonates for chloride ion), and hot-process zeolite softening (in which calcium and/or magnesium present as "permanent hardness" is removed). Ion exchange resins have great value to industry beyond water treatment. A host of new applications are now in laboratory, pilot plant, or plant stage involving chemical processing. There is one serious drawback to writing a paper such as this, viz., that it must by necessity he somewhat outof-date. Many plant processes are characteristically confidential and, not until everyone knows that information has "leaked out" or until it is felt that disclosure can do no harm because of time or patent, will the details of the process be released. Thus this paper must be principally a review article, hut perhaps tying together the facets of information in a somewhat different manner. A number of types of applications will be discussed, and in each case a representative example will be presented briefly. Ion exchange equipment of the presently utilized and more promising new types will be discussed. Something also will be said of the economy of ion exchange and of the projected growth of industrial applications. DEIONIZATION OF AQUEOUS SOLUTIONS OF ORGANICS BY ION EXCHANGE
This is one of the largest of the chemical processing applications for ion exchange resins, one which finds its widest use in purification of glycerin (19) and sugars
' Presented as part of the Symposium on Recent Developments
in the Use of Ion Exchange Experiments as a Means of Separating Substances before the Division of Chemical Education at the 129th Meeting of the American Chemical Society, Dallas, April, 1956.
VOLUME 35, NO. 2, FEBRUARY, 1958
( 5 ) . I n general it may be extended to the removal of salts, acids, or bases from aqueous solutions of any non-ionized solute. This i~ouldinclude a large range of alcohols, aldehydes, and ketones. Properly handled, weakly ionized acids or bases as well as ionic species of molecular weight too high (i.e., above 300-500) to permit appreciable penetration into the resin phase may also he freed of salt contaminants by these methods. Soap-lye crude glycerin may be purified by ion exchange to give products of extremely high purity. This soap-lye crude glycerin is obtained after evaporation of soap lye to 80% glycerin concentration to precipitate out the bulk of the salt previouqly added to decrease the solubility of the soaps. A typical "crude"
P R l M l R I STAGE
SECONDARY STAGE
TERTIARY STAGE ~ w i f i e dglycarin
li Ovop0,otor
Pigum 1.
Ion Exshanw Unit for R - m o d of Salt from Glycerin (2)
composition: 80% glycerin and 8% sodium chloride with small amounts of organic acids and color bodies. Based on the reports from a typical plant, operating to produce 26,600 lh. of 82% glycerin/day (Z),this crude is first diluted with water to about 20y0 glycerin and then passed through a series of cation and anion resin beds, and finally the last traces of contaminant are removed in the scavenger mixed bed (containing both resin types) (Fig. 1). I n this unit equipment has been designed to require regeneration of each of the resins during the same shutdown period, thus giving a longer uninterrupted cycle. Regeneration of the anion beds in this case is with caustic, of the cation beds with sulfuric acid. The cost of deionization of crude glycerin containing 8y0 NaCl by this method has been estimated t o he about 1.5#/lb. glycerin (19). DEIONIZATION OF AQUEOUS SOLUTIONS OF ORGANICS BY ION EXCLUSION
Ion exclusion is a process for the fractionation of low-molecular-weight, non-ionic solutes from highly ionized materials. Though involving ion exchange resins as a separating tool, the controlling factor is not ion exchange but the differential distribution of ionic 59
and non-ionic solutes between the resin and liquid phases. Because of the high degree of ionization of the resin phase, the equilibrium concentration of the ionic component is less within the resin thau in the surrounding environment, and thus, operating column wise, this component is accelerated in its progress through a resin bed as compared to a non-ionic component. Although no commercial ion exclusion operation has been described in the literature, several are known to be in use. One of these involves a unit containing about 1000 cubic feet of resin. An extensive pilot plant study using ion exclusion to remove salt from soap-lye crude glycerin has been presented (14). A unit containing 4.5 cubic feet of fine-mesh cation resin was operated to give a 99.4% recovery of the glycerin fed with 90% salt removal (16). It appears that on the industrial scale ion exclusion will be called on to tackle the big job of removing the bulk of the ionic contaminant a t low cost, and that where complete deionizatiou is required a scavenger ion exchange bed will still be used. The results of the glycerin pilot plant operation indicate a total purification cost of l.O$/lb. 95y0 C.P. glycerin starting with the soap-lye crude. (Ion exclusion: O.63b; ion exchange 0.126; evaporation: 0.25b.) It is realized that solutions containing low electrolyte concentrations are most economically deionized by ion exchange. At some higher concentration ion exclusion becomes more economical. Perhaps some day a rather clear line of demarcation may be determined. In any case the usefulness of ion exclusion to industry is assured. ION INTERCHANGE
The conversion of a product from one ionic form to another by ion exchange has not been carried out extensively in industry. Stable silica sols are produced by hydrogen exchange of sodium silicate. A portion of the calcium in milk is replaced by sodium to produce a sofecurd milk and in the preparation of evaporated milk. It is very possible that some fine chemicals are being prepared by ion exchange metathesis, but published accounts are meager. Applications in the pharmaceutical field include the conversion of penicillin from the sodium salt to the potassium salt and the conversion of thiamine bromide to thiamine nitrate or chloride. The latter has been described in the patent literature (11). Certain commercial syntheses of the vitamin produce the compound in the form of the bromide. The ion interchange is carried out by passing an aqueous solution of the thiamine bromide through a weakly basic resin in the nitrate (or chloride) form. The process is continued until bromide is detected in the effluent. The bromide-free effluent is crystallized to yield the desired salt. The economic advantage of this process is apparent when it is compared with the previous commercial method of effectingthis metathesis which employed precipitation with the appropriate silver salt, filtration, and subsequent silver recovery. Most metathesis reactions are potential ion exchange applications. Many can be carried out by ion exchange that cannot becarried out by conventional methods. The degree to which any of these reaches industrial scale is largely dependent upon the economic
factors involved. A good example is a patented process for the production of caustic from lime and salt by use of an anion exchange resin (1). While such a process cannot compete cost-wise with electrolytic caustic in this country, it appears much more promising in certain other localities. SEPARATIONS BY ION EXCHANGE
Nowhere have the feats of ion exchange resins been more spectacular than in the field of ion exchange chromatography. Paper after paper over the last ten years has told of amazing separations of such similar substances as the rare earths, the actinides, amino acids, carbohydrates, proteins, polyphosphates, complex ions, and even isotopes. Ion exchange techniques have become standard methods in isolating trace amounts of the transuranium elements and in the study of protein hydrolysates. With this record, why is the technique not yet used on a large industrial scale? The answer is twofold: the huge capital cost per unit of product and the undeveloped market for the possible products. Ion exchange chromatography is a powerful but cumbersome tool. The process consists of loading a small sample of the mixed materials onto the top of a column or columns of resin and eluting with another electrolyte. If the materials to be separated are very similar, only a small fraction of the total exchange capacity of the bed can be used per cycle and the permissible flow rate is usually many times slower thau in conventional ion exchange procedures. Thus the yield of product per cubic foot of resin per year may be quite low. I n addition a considerable volume of eluant is required which may in itself be a fairly expensive chemical. To the authors' knowledge the separation of the rare earths is the only application of this type that has been carried t o industrial scale. Spedding and co-workers developed the general process and carried it on into a pilot plant scale which in 1951 could produce two to three kilograms of pure neodymium per month and smaller amounts of the rarer rare earths (17). The gross separations of the rare earths are made by conventional precipitation and crystallization techniques. The more difficult fractionations are then carried out by ion exchange chromatography. A small volume of the mixed salts is loaded on the top of a column of sulfonic acid cation exchange resin. The column is then eluted with a buffered solution of citric acid, ethylenediamine tetraacetic acid, or some other compound which is capable of forming complexes with the various rare earth cations. These complexes travel through the column a t different rates and if all the conditions are properly controlled the individual rare earths appear in various portions of the effluent. Figure 2 shows the purification of neodymium by such a process (18). The center portion of the graph represents a cut of neodymium better than 99y0 pure. The process has been adopted by several chemical concerns, and rare earths so purified are now on the market (5). The future scale of this application obviously depends on the market for one or more of the pure rare earths. It is not beyond possibility that a second industrial scale operation of this type may occur in the field of JOURNAL OF CHEMICAL EDUCATION
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VOLUME OF ELUATE (liters)
biochemistry. Ion exchange chromatography has certainly proved its ability to make amazing separations in this field. The day that one of these products comes into demand, and synthesis fails, the separation will be carried out on an industrial scale. As the ions to be separated become more dissimilar chemically, the separation becomes easier and may he accomplished by more conventional methods-ion exchange or otherwise. CATALYSIS BY ION EXCHANGE
A recent and certainly non-conventional application of ion exchange has been in the field of catalysis. The fact that the hydrogen-form sulfonic acid resins have catalytic activity comparable to strong mineral acids has been recognized since early work on the demineralization of sugar solutions in which inversion was observed. Several dozen papers have been published during the last four or five years describing various reactions which have been carried out with ion exchange resins as catalysts. However, judging by published information, this application of resins only recently has reached industrial scale. This development is in the epoxidation of unsaturated animal fats, vegetable oils and their derivatives (4). Epoxidation is the reaction of an organic peracid with an olefinic compound to give an epoxy compound as shown.
The epoxide ring is easily cleaved by a numher of reagents and a wide variety of products can be obtained. Generally, mild conditions and the absence of any strong acid are necessary for isolation of the epoxide. The peracid required in the epoxidation reaction was previously prepared by reacting hydrogen peroxide with an organic acid in the presence of a strong mineral acid. This required the handling and storage of the hazardous peracid. In addition the mineral acid used to catalyze the peracid formation complicated the isolation of the epoxy compound in the subsequent reaction. The substitution of the sulfonic acid resin for the mineral acid allows the preparation of peracid free of mineral acid, or, in the preferred method, allows the VOLUME 35, NO. 2, FEBRUARY, 1958
formation of the peracid in situ during the epoxidation process. Three methods of carrying out the reactions have been proposed. I n the first, sulfonic acid cation exchange resin is added equal to approximately 25% of the weight of the reactants (unsaturate, organic acid, and hydrogen peroxide). The mixture is held a t 60° to 80°C. for several hours. The resin is removed by filtering and may be reused in subsequent epoxidations. The second procedure eliminates the reuse of the resin catalyst, with the accompanying handling difficulties, by reducing the amount used to about 2% which can be considered expendable. Equivalent results can he obtained by running the reaction a t a higher temperature and for a longer time. I n the third method the resin is held in a fixed bed and the reactants are circulated through it. This technique allows a high ratio of resin catalyst to unsaturate a t any given time with increased reaction efficiency. A water-wet resin can actually be used in this method. A numher of advantages have been claimed for the use of resin catalyst in this reaction: (1) A higher epoxy content is obtained. (2) Byproduct formation is reduced. (3) The hydrogen pwaxide can be converted to peracid almost
quantitatively. (4) Peracid preforming is eliminated, thus no storage or handling problems. (5) Raw material costs are reduced as the resin catalyst can be used as msnv as 25 times. In addition, neutralizing agents are not required for the product. (6) Lower mole ratios of glacial acetic acid can be used. The excess used is more easily recovered. (7) There is a decided safety advantage over the preforming and storage of peraeids.
Obviously not all of these advantages can be claimed for the use of resin catalysts in every organic reaction. However, other advantages are possible which are not listed above. A decided advantage of the resin catalysts when used in a fixed bed is the rapidity with which the product is separated from the catalyst. Thus a reversible reaction which is highly dependent on special conditions of temperature or pressure can be carried out in the resin bed, and the product stahilized, since no catalyst is present once it leaves the reaction zone. Resin catalysis is not limited to those reactions in which the active component is the hydrogen ion. Other possibilities are the use of hydroxide-form quaternary-ammonium anion exchange resins for hydroxide catalyzed reactions, the use of cation resins carrying particular metallic ions, or specific effects of the functional groups of the resins themselves. ION EXCHANGE IN THE METAL FINISHING INDUSTRIES
The application of ion exchange to various solutions arising in the metal finishing industries has received considerable attention in the last few years. These developments have been initiated in most cases by tightening stream pollution regulations. I n many cases, in addition to solving the disposal problem, the use of ion exchange has reduced the cost and/or improved the quality of the finishing process. A variety of processes are used in metal finishing. Examples are plating, pickling, and anodizing, and each of these may he broken down further according to
the particular job being done. Likewise a wide variety of chemicals are involved. However, despite this diversity of processes, the use of ion exchange in the metal finishing field can be broken down into two main categories. The first of these is the treatment of an exhausted or contaminated solution to make it suitable for reuse and the second is the recovery and concentration of valuablematerialsfromdilutesolutions. The first of these two applications arises whenever a metal is treated in an acidic solution. As the treatment hath is used, metal is dissolved, and the acid is gradually neutralized. As the dissolved metal concentration in the hath builds up and the p H rises, the behavior of the bath changes. The reaction usually slows, the finish produced becomes poorer in quality, and a t some critical dissolved-metals concentration the hath must be discarded, or a t best refortified with additional strong acid, even though there may be a considerable amount of free acid still present. The application of ion
fi
, , , q PICKLING TANK
H y
7%
RECLAIMPICKLE
SEWER l i g v l r 3.
Simplified Flow Sheet of the Reclsmation of Acetic Acid Pickla
exchange in such cases is for the removal of the contaminating metallic ion. The solution is passed through a column of sulfonic acid cation exchange resin in the hydrogen form. The metallic ions are retained and the original acid re-formed. This process has found commercial use in a variety of metal finishing plants using chromic acid and chromates and case histories are to be found in the literature (6). Another announced application has been in the phosphoric acid pickling of steel ( l d ) . A recent application is for the recovery of acetic acid used in the pickling of magnesium. The magnesium sheet produced by the Dow rolling mills a t Madison, Illinois, is freed of surface contaminants by spray pickling with amixture of acetic acid and magnesium nitrate (Fig. 3). As the bath is used the acetic acid is converted to magnesium acetate and, as outlined above, the solution eventually becomes unusable even though it may still contain as much as 8% acetic acid. The large continuous pickling unit was designed as a series of cascading tanks so that fresh acid could be fed into one end of the line of tanks and flow countercurrent to the direction of the metal until it was discarded a t the opposite end. The function of the ion exchange unit is to take the discarded acid. strip out the magnesium, and return the re-formed acid to the pickling line. The continuous pickling process and the hatch ion exchange are tied together through two storage tanks.
When enough used pickling solution has accumulated in the first of these tanks, the ion exchange cycle is started. The solution is passed through a column of Dowex 50 where the magnesium acetate and magnesium nitrate are converted to the free acids. The product portion of the effluent is run into the second storage tank where the somewhat diluted product is fortified by the addition of concentrated acetic and nitric acids and from where it is fed back into the pickle line as needed. The Dowex 50 bed is regenerated with 20y0-21% sulfuric acid giving a waste stream of magnesium sulfate. The entire operation, including the analysis of the used and reclaimed pickle solutions, initiation and control of the exchange cycle, selection of product cut, make-up or regenerant, and withdrawal and addition of solution to the pickle line, is automatic. This unit promises to pay a triple dividend. It gives an economy in cost of chemical raw materials as some 90voof the acetic acid and nitrates used in the process are recovered at the cost of slightly more than an equivalent amount of comparatively inexpensive sulfuric acid. A considerable economy in operating time is realized as the operating crew is freed of the problem of manually maintaining the proper acid concentrations in the line. Lastly, improved quality control can be realized as a constant acid gradient is maintained along the pickling line. This acetic acid solution is particularly well suited to the ion exchange treatment since the low ionization of the acetic acid allows an efficient pick-up of magnesium in the resin column, and the nitric acid produced reacts with magnesium acetate as soon as it is returned to the pickle line to give magnesium nitrate and more acetic acid. Also, acetic acid, in common with chromic acid and phosphorus acid, is a comparatively expensive acid, and the amount recovered is worth several-fold the value of the sulfuric acid used to regenerate the resin. That this comparative value of acids is all-important can he seen in attempts to apply ion exchange recovery to the huge volumes of sulfuric acid pickle solutions currently being produced in the steel industry. While the process is chemically feasible, the economics depend upon finding a way of recirculating the regenerant acid or producing a valuable hyproduct. The use of ion exchange for the recovery and concentration of valuable materials from dilute solutions is finding wide acceptance in the various chrome finishing operations for the stripping of rinse solutions. Practically all metal objects that are treated in a hath are subsequently rinsed and inadvertently carry out solution. The large volumes of contaminated rinse waters which result present a serious disposal problem in many areas and also represent a considerable loss of chromium. This problem is solved by the use of a two-bed deionization system. The rinse water is passed through a hydrogen-form, sulfonic acid resin which strips out the metallic cations. I t is then passed through a strongly basic, anion exchange resin which picks up the chromate and any other anions which may be present. The effluent is a good grade of demineralized water which can he returned to the rinse operation. When the cation bed is exhausted it is regenerated with a strong mineral acid and the effluent sewered after any necessary JOURNAL OF CHEMICAL EDUCATION
neutralization. When the anion bed is exhausted it is regenerated with sodium hydroxide. The effluent of sodium chromate and caustic is passed through the regenerated cation bed and the product is chromic acid suitable for return to the treating bath. Here again detailed case histories are to be found in the literature (15). Among the benefits cited from such installations are: (1) elimination of chromate waste problem, (2) water conservation by recirculation of rinses, (3) improved rinsing due to use of demineralized water, (4) saving in processing costs. Package units are now available for both the treatment of chromium baths and the recovery of chromium from rinse waters. The recovery of a valuable component from dilute solution by ion exchange is not limited to the metal finishing industries. A good commercial example is the isolation of streptomycin from culture broths. EXTRACTION OF METALS FROM ORES
Probably the fastest growing industrial application of ion exchange during the last few years is in the recovery of uranium from uranium-bearing ores. Advantage is taken in this operation both of the selective properties of particular resins for particular ions and the concentrating ability of resins. The earliest ion exchange work on uranium was carried out with cation exchange resins. However, the resins exhibited little specificity for this metal as related to many others also present in the ores. The discovery that uranium could he picked up as an anionic complex (U02(S04)~)-' by anion-exchange resins in presence of excess sulfate led to the rapid commercial development of the process. In the conventional vrocess (7) , , the ore. after crushine and grinding operations, is leached with sulfuric acid. The filtered acid leach is then passed through columns of anidn exchange resins where more than 99% of the uranium plus small amounts of other materials are removed from solution. After the first of a pair of columns operating in series is exhausted, it is taken off stream and regenerated with one molar chloride or nitrate solution containing about a tenth molar concentration of the corresponding acid. As a result of this operation, the uranium may be concentrated from less than one gram of U308 per liter of solution to about ten grams per liter, and simultaneously freed of most other metals. The uranium oxide is then precipitated as a yellow cake containing low quantities of impurities by raising the pH. In general, the strongly basic anion exchange resins are preferred in these operations, primarily because of greater stability. However, mixed weak base-strong base resins and
weakly basic resins have been, and perhaps still are being considered. Capital expenditures for these ion exchange plants run high, hut probably no higher than the many-step processes used prior to ion exchange. Ion exchange offers great advantage in simplicity of operation and quality of product and has been accepted for the over-all economy it offers to the extent that the hulk of the free world's uranium is now collected by ion exchange. The outstanding success of this process gives added impetus to the suggestion of similar applications in metal extraction, recovery, and concentration. TYPES OF EQUIPMENT
A discussion of industrial applications of ion exchange would be quite incomplete without reference to the types of equipment now in operation for bringing the resin and solution into contact. Batch Reactor. The simplest ion exchange reactor is
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F I L T E R MEDIA
BATCH RErlCMR
8 4 TCH COLUMN
COLUMN
rovring
Figure 4.
ion Exeheng. Equipmant
VOLUME 35, NO. 2, FEBRUARY, 1958
Pigur. 5.
Continuous Moving Bed (8)
the hatch reactor which involves simple contact of resin and solution in single stage such as in a stirred vessel (Fig. 4). The only instance in which this may be effectivelyused, however, is one in which the reaction goes essentially to completion in one direction. The best example of this is the neutralization of an acid or base. This may be accomplished by the addition of an acid-form cation exchanger to a base, or a hydroxideform anion exchanger to an acid. Or complete deionization of a salt may be accomplished by proper addition of mixed resins. Such reactions require subsequent removal of the spent resin from the reaction mixture and are most adaptable where only trace quantities of ionic materials are involved, such as catalysts. I n general the resins in these cases are considered as process materials and discarded from further use. Batch-Fixed Bed Reactor. There are cases where a batch type reaction is preferred but recovery of resin for re-use is required. Such is the case where larger excesses of acid or alkali than referred to above must be removed and the discarding of resin is too costly. A novel combination batch-fixed bed reactor has been designed for this type of operation as indicated in the sketch (Fig. 4). The bottom of a stirred vessel may be constructed of a porous filter media. After reaction with the resin is complete, the treated liquor may be removed by filtration and rinsing. The resin may then be regenerated for re-use as described below. I n one
example of this type of operationinfull plant production, the sodium salt of a weakly ionized organic acid is "titrated" with Dowex 50.H+ to remove the sodium ion from solution and produce the organic acid in high purity. Fixed Bed. The most commonly used type of ion exchange equipment is the column or fixed bed in which the solution phase is passed through the resin phase which in turn remains in a fixed position (Fig. 4). For any operation where low concentrations of ions are to be removed or exchanged, improvement on the fixed bed will be difficult to attain. Two inherent disadvantages of the fixed bed are (1) inefficient use of the resin and (2) variable feed and product compositions. In the fixed bed as normally used, and depending on the equilibrium constants and Emetics of the exchange reaction, a large portion of the bed is inactive. Only the resin zone in which exchange of one ion for another takes place may be called active. The zone above this is exhausted with the eluting ion, the zone below this with the regenerant ion-neither adding anything to the exchange reaction. On the other hand, both the shell housing the resin and the resin itself may be considered as capital investment and in themselves add little to operating cost. I t is generally much more costly to operate with very shallow beds which require frequent regeneration, particularly where manual operations are involved. A pseudo-continuous fixed bed unit may he devised by using a series of fixed beds. These may be placed in a circle over which feed and take-off lines pass successively, giving near uniform product composition. The success of the unit depends on the use of a large number of sections, perhaps twelve or more, and valving becomes very costly and complicated. One point which should also be considered here is that the exchange-band width will change for loading and regeneration (unless the equilibrium constant for the reaction is 1.00). If a resin is specific for a given regenerant ion, the band width will be narrow, but the band width on exhaustion will then he wide assuming no change in the reaction equilibrium constant. The chances for commercial adoption of such a unit seem slim. The arguments against the fixed bed because of variable product and feed composition diminish in merit as the exhausting solutions decrease in concentration and thus as the ratio of down time to operating time becomes small. I t seems unlikely that any continuous ion exchange unit will be widely accepted in preference to the fixed bed for water treatment, a t least until it has been proved outstanding for treatment of solutions of much higher ionic content. Surprisingly, however, the only existing commercial continuous unit known to the authors is being successfully applied to municipal water softening. Continuous Moving Beds. Perhaps the greatest amount of effort to date on continuous ion exchange has been devoted t o continuous moving heds. As stated earlier, the only known operating unit is used in municipal water softening (8). However, unit designs have been investigated in great detail. I n principle, resin falls by density difference through an ascending stream of the solution to be treated. The product liquor is removed from the top of the column, the spent
resin from the bottom (Fig. 5). Two of the primary problems of this operation are: (1) rather low flow rates because of small density differential between resin and solution and (2) scale-up problems of resin and liquid distribution. It is likely that resin manufacturers could solve the first problem; the second appears to he a greater challenge. The advent of intermittently moving heds and of fixed beds containing finer mesh resins has diverted attention away from the strictly continuous operation as described here. Intermittent Moving Beds. With the announcement of two units radically different from anything earlier described, first the Higgins contactor (9) and more recently the Porter contactor (IS), continuous ion exchange has been given a "shot in the arm." In each of these units the flow of resin and solution is intermittently co-current and counter-current. The time cycles are so set up that for the bulk of the cycle the resin is in fixed position with solution flowing in one direction through it. For short intervals, however, resin and solution are moved a short distance in the opposite direction. Thus, though the cycle is intermittent, the "down time" is very small and a standing concentration wave can he maintained. Basket UnitsResin-in-Pulp Process. The difficulty encountered in filtering the leach solutions from uranium ores prior to ion exchange has led to the development of a novel contactor. The technique, called resin-in-pulp, or RIP, has been developed under A.E.C. contract to permit contact of resin and unfiltered ore slurries (10). This contactor is simply a series of wire-mesh baskets of sieve-size coarse enough to permit ready passage of the slurry solids but fine enough to retain the resin particles. The standard mesh used is 9 X 35, the ore is ground to minus 200, and the resin to he used is a plus 20 grade. The slurry is passed through a series of tanks housing the baskets which in turn are raised and lowered t o give intimate contact between resin and slurry and to prevent plugging. A train of such baskets operate on both the exhausting and regenerating cycles. Each basket must represent about one equilibrium stage, and thus resin usage is less efficient than in column methods -perhaps one-fourth as efficient. This inefficiency is balanced by the elimination of prior filtration which can be very costly with certain slimes. The R I P units are being adopted to alarge extent on the Colorado Plateau. It should be noted here that columns may also be adapted to handling slurries. In recent work the Higgins contactor has been used in this manner without any plugging. Also Weiss (20) bas shown that a solids slurry may he passed through a pulsating fixed bed without loss of resin efficiency. I t seems very safe to say that new ion exchange contactors will he devised as the need arises. Units other than fixed beds have found their proper place, hut it is doubtful that the fixed bed mill be completely replaced or even much improved for treatment of solutions containing small amounts of electrolyte. ECONOMICS
A number of factors must always be considered in the evaluation of a new process and would certainly include these: (1) product yield, (2) produrt quality JOURNAL OF CHEMICAL EDUCATION
(assuming reflection in sales price), (3) operating costs, (4) investment. A proper balance of these must he weighed against the balance of the same factors with competitive processes. The process looking the most favorable, over-all, should normally he chosen. The primary cause for deviation from this rule results from lack of experience with a new process which adds an additional caution factor, particularly with the smaller operators who cannot afford the risk of change. Ion exchange resins are generally outstanding for producing high quality products a t good yield. Operating costs are moderate and can be very low with the use of automatic equipment. Capital expenditures often run higher than with competitive routes. Generalizations beyond this are impossible without referring to specific examples. Ion exchange resins themselves often can ice considered as investment items rather than process chemicals. The function of resins in an ion exchange column is very comparable to that of packing in a distillation column. The write-off time, however, is a difficult thing to determine as based on laboratory experienre. Actual operating experience will determine that the period will vary from weeks to years depending upon the process. I n any case, the approach should be conservative, realizing that the effective life of the resin is probably not indefinite. It is usually possible to gain a large' amount of know-how in this respect from other operating units treating similar solutions. Industrial ion exchange equipment (exclusive of water softening) will generally costsomewhere between $100 and $500 per cubic foot of installed resin. The variation of course reflects the complexity of the installation, the extent of automation, material of construction, and location. In weighing the merits of ion exchange it is essential that the process be fully evaluated and that all of the variables be considered. A numher of these are shou-n in the table. Resins and solution pairs must he chosen Variables i n Ion Exchange Process Work Em~ililirZum
Functional group PH Ion valence Ion size Cross linking
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Resin v h a s ~
Cross linking
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Rate
Particle size Ion size Temperature Ionization degree
Solt
