Ion Exchange - Simple to Understand

ION EXCHANGE

INDEX

Ion Exchange2ION EXCHANGE BASICS3Ion exchange resin types16Feed water and some recommended limits for ion exchange systems19Basic ion exchange processes in water treatment25Regeneration methods for ion exchange units36Ion exchange resins applications A general overview48Ion exchange capacity64Ion exchange columns71Ion exchange plant design87Glossary of ion exchange91Water analysis details98Concentration and capacity units105Ion exchange resin structure106Ion exchange resin properties114Approximate selectivity scales - cation exchange resins125Approximate selectivity scales -SBA resins127DRINKING WATER Ion exchange processes129Ion exchange reactions131Limits of use of anion exchange resins138

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This document wascreated from the following website:http://dardel.info/IX/

Ion ExchangeUpdate 7 June

Ion exchange is a powerful technology to soften and demineralise water to extremely good quality. This technology is well proven, as it developed initially in the 1950s, and today, it is still the best to produce ultra-pure water, i.e. to remove all traces of contaminants.In addition to water treatment, ion exchange is also used in a variety of industrial and domestic applications. It can for instance: Remove colour from cane sugar syrups to make white sugar Purify antibiotics and other pharmaceuticals Extract uranium from ores Separate metals Remove harmful substances from solutions Be used as an excipient in pharmaceutical formulations Catalyse reactions to make anti-knocking agents for petrol Produce clear and pure water for the tea or coffee you make at homeThis site covers essentially water treatment. Click one of the buttons on the right to see more details. The mark (N) indicates new or updated (U) pages or documents. Other pages are shown in the site map ().If you don't find here the answer to your question, I may offer you some help.Any suggestion and comment regarding these ion exchange pages are welcome. Just drop me a line at the e-mail address below.

Ion exchange basicsIntroductionIon exchange is a powerful chemical technology, little known to the general public. This simple page attempts to show what ion exchange is to those who are not chemical experts. When my friends ask me what is my professional activity, I tell them "ion exchange". Most of them have no clue. In Western Europe, the majority of my friends have one or two ion exchange devices in their household. So, I return a question: "Why do you think you put salt in your dishwasher?" Very few know, and if you are in this case, you will discover why below.

WaterWater looks simple: it is made of water molecules (formula H2O). You know however that this apparent simplicity is more complex in reality: otherwise, bottled water producers would not make such a fuss about its mineralisation. All natural waters contain some foreign substances, usually in small amounts. The water in the river, in a well or from your tap at home is not just H2O, it contains a little of: Solid, insoluble substances, such as sand or vegetal debris. You can in principle filter these solid substances out. Soluble substances, that you most often cannot see and that cannot be filtered out. These substances can be inorganic or organic, they can be ionised (electrically charged) or not ionised. The soluble, non-ionised substances are present in the water in form of molecules of various sizes and formulas, for instance: Carbon dioxide is a small molecule with a simple formula: CO2. Sugar is a larger molecule with a complicated formula abbreviated as C12H22O11. Want to see the 3 D formula? Sugars are not removed by ion exchange, though. You may want to remove these foreign substances from the water. You can remove the ionised substances by ion exchange.

IonsThe soluble, ionised substances are present in water as ions, which are electrically charged atoms or molecules. The positively charged ions are called cations, and the negatively charged ions are called anions. Because water is globally neutral electrically (otherwise you would get an electric shock when you put your hand in water) the number of positive charges is the same as the number of negative charges. Ions can have one charge or more, the most usual range being 1 to 3. Ions can be made of one atom only (monoatomiic ions) , or several atoms linked permanently together, like molecules (polyatomic ions). Examples: A monovalent monoatomic cation: the sodium ion Na+ A divalent monoatomic cation: the calcium ion Ca++ A monovalent polyatomic cation: the ammonium ion NH4+ A monovalent monoatomic anion: the chloride ion Cl A monovalent polyatomic anion: the nitrate ion NO3 A divalent polyatomic anion: the carbonate ion CO3= Another divalent polyatomic anion: the chromate ion (metallic complex) CrO4= The trivalent monoatomic aluminium cation Al+++ exists only in very acidic solution, not in normal water. Similarly, there are no monoatomic di or trivalent anions in normal water Ions are able to move around in water, they are not fixed, and they are not attached to ions of the opposite charge. Only the sum of the charges is the same for all cations and all anions. See figure 1 for a schematic representation of ions in water.

Figure 1: Ions in water are not attached to each other. The sum of charges is constant. Salts are crystallised substances containing a fixed proportion of cations and anions. For instance, table salt has exactly the same number of sodium cations (Na+) and chloride anions (Cl). Its formula is given as NaCl. When you dissolve a salt into water, its cations and anions are dissociated, and free to wander as seen on figure 1. The dissolved ions are surrounded by water. They are said to be hydrated. They are loosely connected to water molecules, cations attracted by the O atom, anions by the H atoms of the water molecule, as shown in figure 2. Ionic compound dissolved in water

Figure 2: Hydrated ions in water e.g. Na+ and Cl (table salt NaCl) Magnesium sulphate is a salt with exactly the same number of magnesium cations (with double charge: Mg++) and sulphate anions (also with double charge, SO4=) so that the formula is MgSO4. Calcium chloride is made of calcium ions (with 2 charges, Ca++) and chloride ions (with 1 charge only, Cl). You need 2 chloride anions to balance each calcium cation. Therefore the formula of calcium chloride is CaCl2. Similarly, in sodium carbonate you have sodium cations Na+ and carbonate anions CO3=, so that you need 2 sodium ions for each carbonate ion, and the formula is Na2CO3. When you boil and evaporate water for a long time, you are left with a dry residual which is made of salts and possibly other residues, such as silica and organic compounds. Only in sea water do you have a sizeable quantity of dry residual, 35 to 40 g dry residual for one litre of sea water. In river or tap water, the dry residual is usually very low, ranging from 50 to 500 mg/L. The dry residual is also called Total Dissolved Solids and abbreviated as TDS. You may want to remove these foreign substances from the water. You can remove the ionised substances by ion exchange. See details of the water analysis and units of concentration used in ion exchange.

Ion ExchangeImpurities in waterWater, as we have seen, contains small amounts of foreign substances. In many cases, these substances cause no problem. Drinking water containing some salinity is much better for health than ultra-pure water. For specific applications, however, these foreign substances are regarded as impurities and must be removed from water. Insoluble substances (sand etc.) can be removed by filtration. There are many different sorts of filtration technologies, down to ultrafiltration that can remove sub-micron particles. For soluble substances other techniques must be used. Soluble ionised substances can be removed by ion exchange. Ion exchange resinsThese are very small plastic beads, with a diameter of about 0.6 mm. These beads are porous and contain invisible water inside the beads, measured as humidity or moisture content. The structure of the resin is a polymer (like all plastics) on which a fixed ion has been permanently attached. This ion cannot be removed or displaced; it is part of the structure. To preserve the electrical neutrality of the resin, each fixed ion must be neutralised with a counterion. This counterion is mobile and can get into and out of the resin bead. Figure 3 shows a schematic cation exchange resin bead. The dark lines represent the polymeric skeleton of the resin bead: it is porous and contains water. The fixed ions of this cation exchange resin are sulphonates (SO3) that are attached to the skeleton. In this picture, the mobile ions are sodium (Na+) cations. Cation exchange resins such as Amberjet 1000 are often delivered in the sodium form.

Figure 3: Schematic cation and anion resin beads The anion resin bead has a very similar skeleton. The functional groups are here quaternary ammonium cations shown in the picture as N+R3; a more accurate formula would be CH2-N+-(CH3)3. Each ion going into the bead has to be replaced by an ion getting out of the bead, again to preserve electrical neutrality. This is what is called ion exchange. Only ions of the same electric sign are exchanged. You cannot make a resin that can exchange cations as well as anions, because the fixed cations inside the resin beads would neutralise the fixed anions and no exchange with the outside world would be possible. Therefore you need separate cation exchange resins and anion exchange resins. Details about resin structure are given in a separate page.

Water softeningAmong the substances dissolved in water, hardness is very commonly found. Hardness is a popular word to represent principally calcium and magnesium dissolved in the water; these ions can precipitate under certain conditions and form the scale that you may have seen in your boiling pan, and that can obstruct pipes and damage water boilers. The softening of water is the exchange of the hardness cations (Ca++ and Mg++) for another cation that cannot form scale because it is much more soluble: the sodium ion Na+. To soften water, you take a cation exchange resin in which the mobile ion inside the beads is sodium (Na+) and you pass the hard water through a column filled with the sodium form resin. The hardness ions Ca++ and Mg++ move into the resin beads and each of these divalent cations is replaced by two sodium ions getting out of the resin. The exchange reaction can be written as: 2 RNa + Ca++ R2Ca + 2 Na+Figure 4 illustrates the reaction: the resin beads are initially loaded with sodium (Na+) ions. As shown schematically, each calcium or magnesium ion entering the resin bead is compensated by two sodium ions leaving it. Anions from the water cannot enter the resin bead because they would be repelled by the fixed sulphonate (SO3) anions inside the beads.

Figure 4: Softening (sodium exchange) in a single resin bead This cation exchange can only take place efficiently because the cation exchange resin has a higher affinity for the hardness ions than for sodium. In plain English, the resin prefers calcium and magnesium over sodium. The result of the softening process is not a net removal of the hardness ions from water, it is the replacement of the hardness ions by sodium ions. The salinity of the water has not changed, only the constituents of the salinity are different at the end of the softening process. Obviously, this exchange is not unlimited: when the resin has removed so much hardness from the feed water that no room is left on the resin for removing more, the exhaustion run has to be stopped. At this stage, the resin will be replaced by a fresh resin, or regenerated.

DemineralisationIf you replace all cations dissolved in water by H+ ions and all anions by OH ions, these will recombine and form new molecules of water. To do this, you need a cation exchange resin in the H form and an anion exchange resin in the OH form. All cations and anions will be exchanged, and in this case the net result is a complete disappearance of the ionic contaminants. The cation exchange reactions will be: 2 RH + Ca++ R2Ca + 2 H+RH + Na+ RNa + H+In these equations, R represents the cation exchange resin. This is shown on figure 5. The resin is initially in the hydrogen (H+) form. In this picture the anions in water are not shown, but the sulphonic functional groups SO3 are. You can see that one Ca++ ion getting in causes two H+ ions to leave the resin, whilst one Na+ cation is exchanged for one H+ ion.

Figure 5: Decationisation (all cations replaced by H+) Similarly, an anion exchange resin initially in the OH form can remove all anions. The anion exchange reactions will be: ROH + Cl RCl + OH2 ROH + SO4= R2SO4 + 2 OHwhere R represents the anion exchange resin. All anions are replaced by hydroxide (OH) ions. There is no picture for this anion exchange, as it is very similar to the cation exchange picture in figure 5 above. At the end of the exchange process, the resin beads have loaded all cations and anions from the water and released H+ and OH ions. The resin beads are nearly exhausted (fig. 6). These H+ and OH ions will immediately combine and form water: H+ + OH HOH H2O The ionic contaminants are now sitting on the two resins (Na and Ca on the cation resin, Cl and SO4 on the anion resin) and the water has been completely demineralised. Its salinity is reduced to almost nothing, a few ions that have escaped from the resin columns, and that are called ion leakage.

Figure 6: Resin beads are exhausted. H+ and OH ions have been released into the water

Water demineralisation can thus be summarised in a small single picture:

Figure 7: Demineralisation summary!

RegenerationWhen the resins are exhausted, you can bring them back to the fresh state and start over again. Regeneration of ion exchange resins is a reversal of the exchange reactions shown above. Regeneration of a water softener The softening resin is regenerated with sodium (Na+) ions supplied by a salt (common salt: NaCl) solution. The regeneration reaction is: R2Ca + 2 NaCl 2 RNa + CaCl2Regeneration can only be performed when the concentration of the regenerant is high, typically 1000 times higher than the concentration in normal water. For instance, salt is used as a brine with 10 % (about 100 g/L) concentration. At this stage, you will have understood why you put salt in your dishwasher: the salt is diluted with water and regenerates the invisible softening cartridge usually located at the bottom of the machine, out of sight. Regeneration of a demineraliser In the case of demineralisation, strong acids such as hydrochloric acid (HCl) or sulphuric acid (H2SO4) are fully dissociated and can supply H+ ions to replace the cations that have been exchanged and are sitting in the cation exchange resin beads at the end of the exhaustion run: RNa + HCl RH + NaCl Similarly, strong alkalis, of which in practice only caustic soda (NaOH) is used, can supply OH ions to replace the anions sitting on the anion exchange resins beads at the end of the run: RCl + NaOH ROH + NaCl As can be seen from the regeneration reactions, the regeneration step produces saline waste. This is the principal disadvantage of ion exchange. See a page with co-flow and reverse flow regeneration methods.

How resins look likeClick on the picturesA sample of AmberliteTM FPC23AmberjetTM mixed bed resinsAmberjetTM 4400

There is a full page with many other resin pictures under the microscope.

Column operation

In the laboratory as well as in industrial plants, ion exchange resins are used in columns. The water or solution to be treated flows through the resin. On the picture at the right, you see the fresh resin, then you see how the resin gets progressively loaded with the ions from the feed solution. Ions from the resin not shown on the picture are released into the treated solution. At the end some of the ions from the feed escape into the pure solution, and operation is stopped. The next pictures show a typical laboratory column, a simple industrial column and a photograph of an existing Amberpack plant.

Laboratory setupIndustrial columnAmberpackTM column

The jug showed at the top of this page contains a small filter filled with activated carbon and ion exchange resin. The quantity of resin is around 150 ml. For comparison, a large industrial ion exchange column can contain 20'000 L of resin, sometimes more.

Ion exchange capacityTotal capacityThe number of "active groups", or "functional groups" in an ion exchange resin is its total capacity. As there are billions of individual active groups in a single bead of resin, the capacity is usually expressed in equivalents per litre of resin. One equivalent is 6.021023 active groups. You don't have to remember this very large number called Avogadro number. A typical strong acid cation exchange resin has a total capacity of 1.8 to 2.2 eq/LA typical weak acid cation exchange resin has a total capacity of 3.7 to 4.5 eq/LA typical weak or strong base anion exchange resin has a total capacity of 1.1 to 1.4 eq/L

Operating capacityIn the "column operation" picture above, the resin is 100% regenerated at the beginning of the run, and not completely exhausted at the end of the run. The definition of operating capacity is: the difference of regenerated sites between the beginning and the end of the ion exchange run. It is also measured in equivalents per litre. In operation, the operating capacity of the resin amounts to about half the total capacity. The actual range is 40 to 70 % of the total capacity depending on the operating conditions. See other details in a specific page. It is the number of ions and their charge (one, two, or three charges per ion), not their mass or weight, that is important for ion exchange. Therefore all feed water analyses must have the mass of ions converted to equivalents.

Why the resin quantity is expressed in volume, not weightWhen an ion exchange resin filter is designed and built, it is the volume of filtering media that is important to determine the column size, not its mass. Ion exchange resins have different density values (see resin properties), so the resins are sold by volume, in litres or cubic metres, or in cubic feet in the USA. Many of the resin properties are also related to the resin volume.

Treated water qualityIn a typical demineralisation system regenerated in reverse flow (see regeneration methods), the treated water quality, expressed in water conductivity, is below 1 S/cm. Considering that feed water from rivers and deep wells has a conductivity of 100 to more than 1000 S/cm, the efficiency of ion exchange ranges from 99 to more than 99.9 %. Other processes, such as reverse osmosis, are far from this high salt rejection number.

Limits of ion exchangeFor ion exchange to be efficient there must be a difference in affinity between the ion in the resin and the ion or ions you want to remove from solution. The resin must have a higher affinity for the ion in solution compared to the ion in the resin. The ion exchange technology is a perfect tool to remove or exchange contaminants present in low concentrations. In such a case the running time until the resin column is exhausted can be very long, ranging from a few hours to several months. When however the concentration of contaminants is high, say several grams per litre of water, the ion exchange cycles become exceedingly short and the quantity of regenerants increases to uneconomical levels. In the case of brackish water (underground water with high salinity as often found in arid countries) or sea water, ion exchange is not suitable and other technologies must be used, such as reverse osmosis or distillation. Also, any contaminant that is not ionised cannot be removed by ion exchange. Other technologies are available for this purpose, using activated carbon, polymeric adsorbents, molecular sieves and other media.

Selective ion exchangeThanks to differences of affinity for different ions, common ion exchange resins can be used to remove selectively ions from water. One of the most obvious examples is softening. You cannot soften water effectively with reverse osmosis i.e. remove only Ca++ and Mg++ ions: RO is not selective and will remove Na+ ions as well; only ion exchange can soften water with a cation exchange resin in the Na+ form. Similarly, you can remove fairly selectively other ions from water, such as nitrate of sulphate, using an anion exchange resin in the chloride form. This works because the anion exchange resin has more affinity or a better selectivity for the nitrate or sulphate ion than for the chloride ion, the order of affinity being: SO4= > NO3 > Cl > HCO3 > OH > FThere is thus no practical way with any technology to remove only chloride from water without removing other anions as well. For cation resins used in softeners, the affinity is Pb++ > Ca++ > Mg++ > Na+ > H+The Pb (lead) is shown here to indicate that any dissolved lead will be removed efficiently by a softening resin, as well as many other (but not all) heavy metals. See tables of selectivity values for cation and anion exchange resins. Some contaminants are not easy to remove by conventional ion exchange resins. In many cases, very specific resins have been developed for these contaminants. Selective resins are available today for the removal of: Boron Nitrate Perchlorate Nickel Chromate and some other contaminants. See the drinking water page.

Outside of water treatmentThere is an incredible number of applications in fields other than water treatment. Let us mention a few of them: Softening of beet sugar juices before evaporation Colour removal from cane sugar syrups Chromatographic separation of glucose and fructose Demineralisation of whey, glucose and many other foodstuffs Recovery of polyphenols for use in the food industry Recovery of uranium from mines Recovery of gold from plating solutions Separation of metals in solution Catalysis of anti-knocking petrol additives Extraction of antibiotics and other compounds from fermentation broths Purification of organic acids Powdered ion exchange resin is used in tablets in the pharmaceutical industry See a separate page with details of the above applications.

ConclusionIon exchange is a very powerful technology to remove impurities from water and other solutions. Many industries depend on ion exchange for the production of extremely pure water. Examples are: Nuclear and thermal power stations Semiconductor, computer chips and display panel production Selective removal of toxic contaminants from drinking water There are also many applications in areas other than water treatment, as mentioned above. Go to the site map for several detailed pages on applications, processes, resin properties and more.

Ion exchange resin types

Srongly Acidic Cation Exchange Resins (SAC)Functional groups SO3 H+Sulphonic acid

What they doIn sodium form, they remove hardness (essentially calcium and magnesium) from water and other solutions In hydrogen form, they remove all cations They are also used as acidic catalysts

Examples(uniform)AmberjetTM 1000 Na (uniform)DowexTM Marathon C (uniform)LewatitTM Monoplus S100 (conventional)AmberliteTM IR120 Na (conventional)LewatitTM S100

Typical total capacity1.9 to 2.2 eq/L [Na+]

Weakly Acidic Cation Exchange Resins (WAC)Functional groupsCOOHCarboxylic acid

What they doIn hydrogen form, they remove preferentially divalent ions (e.g. calcium and magnesium) from solutions containing alkalinity

ExamplesAmberliteTM IRC86 DowexTM MAC3 LewatitTM CNP80

Typical total capacity3.7 to 4.5 eq/L [H+]

Strongly Basic Anion Exchange Resins (SBA)Functional groupsN(CH3)3+ OHQuaternary ammonium

What they doIn hydroxyl form, they remove all anions In chloride form, they remove nitrate, sulphate and several other ions

Examples(uniform)AmberjetTM 4200 Cl (uniform)DowexTM Marathon A (uniform)LewatitTM Monoplus M500 (conventional)AmberliteTM IRA402 Cl (conventional)LewatitTM M500

Typical total capacity1.0 to 1.5 eq/L [Cl]

Weakly Basic Anion Exchange Resins (WBA)Functional groupsN(CH3)2Amines

What they doAfter cation exchange, they remove chloride, sulphate, nitrate, and other anions of strong acids, but they do not remove weak acids (SiO2 and CO2)

ExamplesAmberliteTM IRA96 DowexTM Marathon MWA LewatitTM Monoplus MP64

Typical total capacity1.1 to 1.7 eq/L [free base]

Selective and chelating resinsFunctional groupsMany different types

What they doThey remove metals, boric acid, perchlorate or other ions selectively

ExamplesFunctionResin typeRemoves

TriethylammoniumAmberlite PWA5NO3

ThiolAmbersep GT74Hg, Cd etc.

AminophosphonicAmberlite IRC747Ca from brine

IminodiaceticAmberlite IRC748Lewatit TP208Ni, Cu etc.

Methyl glucamineAmberlite IRA743Amberlite PWA10H3BO3

Bis-picolylamineDowex M4195Metals at low pH

ThioureaLewatit TP214Hg, Cd etc.

See details inResin structure.

Further reading: Laboratory photographs of various resins. Structure of the matrix and functional groups of ion exchange resins. Resin properties Selectivity Selectivity tables for cation and anion exchange resins. Ion exchange capacity

Feed waterand some recommended limits for ion exchange systems

IntroductionIon exchange resins exchange ions. Not a surprise, but the composition of the feed water affects plant performance. It is therefore essential to know precisely the water composition of the feed to the ion exchange system. The following components and characteristics should be known: Salinity (see also the separate page on water analysis details) Suspended solids and turbidity Temperature pH value Organic substances in the water Other impurities, such as iron, manganese, aluminium, oil, polyelectrolytes... We will examine the effect of all above parameters and try to set practical limits for each.

Salinity (water analysis)This is the single most important item to estimate the performance of an ion exchange system. It is also one of the first things to check when plant performance deteriorates. You cannot rely on an analysis that was made months or years ago. Some effects of a change in salinity are: Type of changeEffect

Higher salt contentShorter runs, lower throughput, sometimes lower quality of the treated water

Lower salt contentLonger runs, higher throughput

Change in ionic balance (e.g. less bicarbonate, more chloride)Change in treated water quality. The resin volumes become unbalanced, the degasifier has less or more carbon dioxide to handle

Higher ratio of silica to total anionsThis may increase silica leakage and require a change in regeneration conditions.

The picture below is a schematic representation of a water analysis, with cations and anions. A good water analysis must be balanced.

Click on picture to see it enlarged with more details.See also a detailed description of the water analysis, with the concentration units to use and a table of the most common ions in water. If the water analysis varies according to season, plant performance should be re-assessed, and perhaps operating conditions re-adjusted, to reflect the seasonal variations. If you don't analyse the water yourself, give a sample to a reputable laboratory for testing. If your feed water is city water, you should be able to obtain an accurate analysis from your municipality.When re-assessing the performance of a plant, or optimising it, it is recommended to use the most probable analysis for the basic calculation, then to re-run the calculation with seasonal analyses to estimate plant throughput under various conditions. All the water analyses should be real, not maxima, averages or minima.We strongly recommend that you should update the expected performance of the plant based on actual operating conditions. You should collect the necessary data: Water analysis (after pre-treatment) Resin types and volumes Regeneration method (co-flow, reverse flow, packed beds) Regenerant quantities and concentrations Salinity limitsIon exchange is the perfect technology for low concentrations. At high salinity, the cycles become very short, regenerant consumption increases and in extreme cases the water required for regeneration may exceed the volume of treated water. As a guideline, a salinity of 20 meq/L (1000 ppm as CaCO3) seems to be the high limit, with some exceptions. Higher salinity water is probably best treated with RO. Sea water cannot be demineralised by ion exchange, as the resins would be exhausted in less than 3 bed volumes.

Suspended solids and turbidityIdeally, the feed water to an ion exchange vessel should be perfectly clear and free of suspended solids. It is essential to ensure that mechanical filters installed ahead of an ion exchange system operate properly. Insufficient filtration resulting in excessive suspended solids may cause: Channeling of the resin bed, resulting in high leakage and short runs. High pressure drop values, sometimes resulting in flow reduction, and requiring frequent backwash of the unit. Suspended solids are traditionally measured by filtration on a 0.45 m filter and expressed as dry mass. The tolerated amount of suspended solids varies according to the ion exchange technology and to the run length. If the resins can be easily backwashed and cleaned, a higher quantity of suspended solids is acceptable. As co-flow regenerated vessels can be backwashed before each regeneration, they are not very sensitive to suspended solids, and several mg/L (ppm) are usually acceptable. In all cases, if the system has long cycles, the accumulated suspended solids may cause pressure drop problems even if the amount of suspended solids in the feed is relatively low. Reverse flow regenerated vessels are not backwashed at the end of every cycle, and the pressure drop should be monitored closely to determine when a resin backwash is necessary. Packed bed units are more sensitive to suspended solids, as they cannot be backwashed in situ. In general, the tolerated suspended solids should be well below 1 mg/L (1 ppm). In Upcore and Amberpack Reverse the suspended solids land on the surface of the resin bed, and some are backwashed away during regeneration. In Amberpack and floating bed, the suspended solids enter in a slightly fluidised part of the bed and accumulate there. A higher quantity is tolerated because it migrates partially upward, but this quantity cannot be removed until the resin is taken out to the backwash tower. Turbidity (cloudiness or haziness) is measured in NTU (Nephelometric Turbidity Units). There is no fixed relation between turbidity and suspended solids.Limits for suspended solidsThere is no simple number here: the most sensible way is to calculate the load of solids during one cycle and to express the result per square metre of vessel (cross-section). Here some suggestions: SystemMax. load per cycle

Co-flow6 kg/m2

Split-flow6 kg/m2

RFR hold-down2 kg/m2

Condensate2 kg/m2

UpcoreTM & similar0.5 kg/m2

AmberpackTM & similar0.2 kg/m2

ADITM, ADNTM0.1 kg/m2

Suspended solids

Turbidity limitsTurbidity is not used much in conjunction with ion exchange systems. See suspended solids above. For floating bed systems without a backwash tower, it was found that 1 NTU is more than what the columns can tolerate.

TemperatureThe temperature of the feed water (and of the regenerants) can affect plant performance.Some effects of a change in temperature are: At low temperature, the operating capacity of all resins decreases. There is an exception to the above rule: at high temperature, the silica removal capacity of a SBA resin decreases, to become virtually zero if the temperature exceeds about 60C. Styrenic SBA resins of type 2 (e.g. Amberjet 4600) and acrylic SBA resins (e.g. Amberlite IRA458) should not be operated or regenerated at a temperature higher than 35C. High temperatures may result in problems of rinse and a loss of strong base capacity, which will cause a higher silica leakage and shorter runs. Cation resins can operate at high temperature, sometimes in excess of 100C. However, the presence of oxygen and trace metals can cause slow oxidation of the resin. Temperature limitsSee the table with limits of temperature for all anion exchange resins. Cation resins can withstand 100C or even more. Product data sheets give details for all resins.

pH valueIon exchange resins can tolerate any pH value (0 to 14) without suffering damage, provided strong osmotic shocks due to rapid change of pH or concentration are avoided. In service however, resins operate only within pH limits: cation resins cannot operate at very low pH, or anion resins at very high pH, because they would be permanently regenerated and unable to exchange other ions. Similarly, the resins are normally not used in very concentrated solutions. This is why in practice the table below should only go up to pH 12 and down to pH 2, which would be 10 meq/L of NaOH or acid respectively. pH limitsType of resinpH range

WAC6 to 14

SAC4 to 14

WBA0 to 7

SBA0 to 9

Operating pH range

OrganicsOrganic matter in water can interfere with ion exchange. The main effect of organics is irreversible fouling of anion exchange resins.Some problems caused by organics are: Low pH (< 6) of the treated water when organic acids slip through the plant. High conductivity of the treated water. Increased silica leakage. Increased time for rinsing and high volume of waste water. Shorter runs. The traditional measurement of organics (COD) in natural water uses the potassium permanganate oxidation method, and its result is expressed in mg/L as KMnO4.Unfortunately, there is no direct correlation between this method and the more modern analysis of TOC (Total Organic Carbon). However, experience has shown that as a rule of thumb, 1 mg/L TOC (1 ppm as C) can be roughly translated into 5.5 mg/L (5.5 ppm) as KMnO4.Limits of organic loadSee the table for all anion exchange resins (same as temperature table).

Other impuritiesOther impurities can also interfere with ion exchange. Some of them are listed below with their effect and possible remedies.

EffectsPrevention/TreatmentLimits

Iron and manganese

Pressure drop Short cycles (capacity loss) Bad quality (high leakage) Oxidation and filtration Resin cleaning with HCl Limits for FeSoftening and nitrate removal: 1 mg/L Demineralisation HCl: 15 mg/L Demineralisation H2SO4: 0.5 mg/L Condensate polishing: 0.1 mg/L (up to 2 mg/L at startup)

Aluminium

Precipitation of Al(OH)3(at neutral pH) Al dissolves in acid or alkali Limits for aluminiumAluminium usually does not foul resins unless it is a large proportion of the cationic load.

Barium

Precipitation of BaSO4 Regenerate cation resins with HCl only! Limits for bariumWhen Ba is more than 0.1 % of total cations, H2SO4 should be avoided.

Oil

Short cycles (capacity loss) Bad quality (high leakage) Check pumps for oil leakage Resin cleaning with non-ionic surfactant Limits for oilVirtually zero0.05 mg/L maximum

Oxidants, chlorine or ozone

Short cycles (capacity loss) Sodium leakage from anion resins Pressure drop when resin gets "soft" Adjust (reduce) dosage Use activated carbon as pre-treatment Scavenge excess oxidant with bisulphite Limits for oxidantsSee table with acceptable limits.

Polyelectrolytes

Short cycles (capacity loss) Bad quality (high leakage) Adjust dosage Clean resin with 4 % NaOH Limits for polyelectrolytesNo known limits. Caution recommended. In doubt, polyelectrolyte supplier should be asked for harmlessness.

Basic ion exchange processes in water treatmentIntroductionThe ion exchange technology is used for different water treatment applications: Softening (removal of hardness) De-alkalisation (removal of bicarbonate) Decationisation (removal of all cations) Demineralisation (removal of all ions) Mixed bed polishing Nitrate removal Selective removal of various contaminants You will find here a description of the above processes, the exchange reactions and the changes in water. Resin types are described in another page, as well as regeneration methods. See also the general introduction to ion exchange, and an overview of ion exchange column designs in other pages.SofteningNatural water contains calcium and magnesium ions (see water analysis) which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove. Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt. Strongly acidic cation exchange resins (SAC, see resin types) used in the sodium form remove these hardness cations from water. Softening units, when loaded with these cations, are then regenerated with sodium chloride (NaCl, table salt). Reactions Here the example of calcium: 2 R-Na + Ca++ R2-Ca + 2 Na+R represents the resin, which is initially in the sodium form. The reaction for magnesium is identical. The above reaction is an equilibrium. It can be reversed by increasing the sodium concentration on the right side. This is done with NaCl, and the regeneration reaction is: R2-Ca + 2 Na+ 2 R-Na + Ca++What happens to the waterRaw waterSAC (Na)

Softened water

The water salinity is unchanged, only the hardness has been replaced by sodium. A small residual hardness is still there, its value depending on regeneration conditions. Uses Examples for the use of softeners: Treatment of water for low pressure boilers In Europe, most dishwashers have a softening cartridge at the bottom of the machine Breweries and soft drink factories treat the water for their products with food grade resins Softening the water does not reduce its salinity: it merely removes the hardness ions and replaces them with sodium, the salts of which have a much higher solubility, so they don't form scale or deposits.

De-alkalisationThis particular process uses a weakly acidic cation resin. This resin type is capable of removing hardness from water when it also contains alkalinity. After treatment, the water contains carbon dioxide, that can be eliminated with a degasifier tower. The cation resin is very efficiently regenerated with an acid, usually hydrochloric acid. Reactions Here the example of calcium: 2 R-H + Ca++(HCO3)2 R2-Ca + 2 H+ + 2 HCO3and the hydrogen cations combine with the birarbonate anions to produce carbon dioxide and water: H+ + HCO3 CO2 + H2O What happens to the waterRaw waterWAC (H)

Decarbonated water

Recombination of hydrogen and bicarbonate and removal of carbon dioxide with the degasifier:

Decarbonated waterDEG

Degassed water

The salinity has decreased. Temporary hardness is gone. Uses De-alkalisation is used: In breweries In household drinking water filters For low pressure boilers As a first step before the SAC exchange in demineralisation De-alkalisation reduces the salinity of water, by removing hardness cations and bicarbonate anions.

DecationisationThe removal of all cations is seldom practiced, except as a first stage of the demineralisation process, or sometimes in condensate polishing where the decationiser precedes a mixed bed unit. A strongly acidic cation exchange resin (SAC) is used in the H+ form. Reactions Here the example of sodium, but all cations react in the same way: R-H + Na+ R-Na + H+The equilibrium reaction is reversed for regeneration by increasing the hydrogen concentration on the right side. This is done with a strong acid, HCl or H2SO4: R-Na + H+ R-H + Na+What happens to the waterRaw waterSAC (H)

Decationised waterDEG

Decat + degassed water

In the second step, a degasifier is used again to remove the carbon dioxide formed by combining the bicarbonate anions and the released hydrogen cation. The water salinity is reduced, and the water is now acidic. A small sodium leakage is shown.

DemineralisationFor many applications, all ions in the water must be removed. In particular, when water is heated to produce steam, any impurity can precipitate and cause damage. As there are cations and anions in the water, we must use two different types of resins: a cation exchanger and an anion exchanger. This combined arrangement produces pure water, as presented in the general introduction. Demineralisation is also called deionisation. The cation resin is used in the hydrogen form (H+) and the anion resin in the hydroxyl form (OH), so that the cation resin must be regenerated with an acid and the anion resin with an alkali. A degasifier is used to remove the carbon dioxide created after cation exchange when the water contains a significant concentration of bicarbonate. The cation resin is usually located before the anion resin: otherwise if the water contains any hardness, it would precipitate in the alkaline environment created by the OH form anion resin as Ca(OH)2 or CaCO3, which have low solubility. Layout SAC (DEG) SBALet us first consider a simple demineralisation system comprising a strong acid cation exchange resin in the H+ form, a degasifier (optional) and a strong base anion exchange resin in the OH form. The first step is decationisation as shown above: RSAC-H + Na+ RSAC-Na + H+With calcium insead of sodium (also valid for magnesium and other divalent cations): 2 RSAC-H + Ca++ (RSAC)2-Ca + 2 H+In the second step, all anions are removed with the strong base resin: RSBA-OH + Cl RSBA-Cl + OHThe weak acids created after cation exchange, which are carbonic acid and silicic acid (H2CO3 and H2SiO3) are removed in the same way: RSBA-OH + HCO3 RSBA-HCO3 + OHAnd finally, the H+ ions created in the first step react with the OH ions of the second step to produce new molecules of water. This reaction is irreversible: H+ + OH H2O What happens to the water1: Cation exchange removing all cations (as in decationisation) followed by degassing:

Raw waterSAC (H)

Decationised waterDEG

Decat + degassed water

2: Anion exchange removing all anions (strong and weak acids):

Decat + degassed waterSBA (OH)

Demineralised water

Demineralised water is completely free of ions, except a few residual traces of sodium and silica, because the SAC and SBA resins have their lowest selectivity for these. With a simple demineralisation line regenerated in reverse flow, the treated water has a conductivity of only about 1 S/cm, and a silica residual between 5 and 50 g/L depending on the silica concentration in the feed and on regeneration conditions. Note that the pH value should not be used as a process control, as it is impossible to measure the pH of a water with less than say 5 S/cm conductivity. Regeneration The SAC resin is regenerated with a strong acid, HCl or H2SO4: R-Na + H+ R-H + Na+And the SBA resin is regenerated with a strong alkali, NaOH in 99 % of the cases: RSBA-Cl + OH RSBA-OH + ClLayout WAC/SAC DEG WBA/SBABecause weakly acidic and weakly basic resins offer a high operating capacity and are very easy to regenerate, they are used in combination with strongly acidic and strongly basic resins in large plants. The first step with the WAC resin is dealkalisation (removal of bicarbonate hardness), and the second step with the SAC removes all the remaining cations. A WAC resin is used when both hardness and alkalinity are present in large relative concentrations in the feed water. WBA resins remove only the strong acids after cation exchange. They are not capable of removing the weak acids such as SiO2 and CO2. In the regenerated, free base form, they are not dissociated, so no free OH ions are available for neutral anion exchange. On the other hand, their basicity is enough to adsorb the strong acids created after cation exchange: RWBA + H+Cl RWBA.HCl In the last step, a SBA resin is thus required to remove the weak acids, as shown in the preceding section: RSBA-OH + HCO3 RSBA-HCO3 + OHWhat happens to the water1 & 2: Cation exchange beginning with the removal of temporary hardness (WAC, as in dealkalisation) followed by the removal of all remaining cations (SAC):

Raw waterWAC (H)

Decarbonated waterSAC (H)

Decationised water

3 & 4: Anion exchange begining after degassing with the removal of strong acids (WBA) followed by the removal of weak acids (SBA):

Decat + degassed waterWBA (FB)

Partially demineralisedSBA (OH)

Demineralised water

A full demineralisation line is shown below, with a cation exchange column (WAC/SAC), a degasifier, an anion exchange column (WBA/SBA), and a polishing mixed bed unit. The use of a weakly acidic resin and the degasifier column are conditioned by the presence of hardness and alkalinity in the feed water, as explained in the previous sections.

A demineralisation line (click to enlarge)Regeneration Regeneration is done in thoroughfare, which means that the regenerant first goes through the strong resin, which requires an excess of regenerant, and the regenerant not consumed by the strong resin is usually sufficient to regenerate the weak resin without additional dosage. The cation resins are regenerated with a strong acid, preferably HCl, because H2SO4 can precipitate calcium. The anion resins are regenerated with caustic soda.

Regeneration of the demineralisation line (click to enlarge)The quality obtained is the same as in the simple SAC-SBA layout, but because the weak resins are practicallly regenerated "free of charge", the regenerant consumption is considerably lower. Additionally, the weak resins have a higher operating capacity than the strong resins, so the total volume of ion exchange resins is reduced. Uses Examples of demineralisation: Water for high pressure boilers in nuclear and fossil fuelled power stations and other industries Rinse water used in production of computer chips and other electronic devices Process water for many applications in the chemical, textile and paper industries Water for batteries Water for laboratories

Mixed bed polishing

Mixed bed unit in serviceand in regenerationThe last traces of salinity and silica can be removed on a resin bed where highly regenerated strong acid cation and strong base anion resins are mixed. Mixed bed units deliver an excellent treated water quality, but are complcated to regenerate, as the resins must first be separated by backwashing before regeneration. Additionally, they require large amounts of chemicals, and the hydraulic conditions for regeneration are not optimal. Therefore, mixed beds are usually only used to treat pre-demineralised water, when the service run is long. What happens to the waterPractically nothing is left:Demineralised waterSAC (H) + SBA (OH)

Nothing is left

Mixed bed polishing produces a water with less than 0.1 S/cm conductivity. With sophisticated design and appropriate resins, the conductivity of pure water (0.055 S/cm) can be achieved. Residual silica values can be as low as 1 g/L. The pH value should not be used as a process control, as pH meters are unable to operate at 1 S/cm conductivity or below. Uses Treatment of water pre-demineralised with ion exchange resins Polishing of reverse osmosis permeate Polishing of sea water distillate Treatment of turbine condensate in power stations Treatment of process condensate in various industries Production of ultra-pure water for the semiconductors industry Service de-ionisation (with off-site regenerated columns)

Nitrate removalNitrate can be removed selectively from drinking water using strong base anion resins in the chloride cycle, i.e. regenerated with a NaCl brine. The reaction is: RSBA-Cl + NO3 RSBA-NO3 + ClWhat happens to the waterRaw waterSBA (Cl)

Denitrated water

Conventional SBA resins can be used, but they also remove sulphate from water. See the selectivity table. Depending on the resin type, some (selective resins) or all (non-selective) sulphate is removed. Bicarbonate is only removed partially at the beginning of the service run. Uses Mainly municipal water treatment

Selective removal of various other contaminantsSelective removal of metals and other contaminants is mainly used for drinking water and for waste. Many of these applications require special resins: chelating resin making stable metal complexes, for instance. Examples Removal of boron (boric acid) from drinking water Removal of nitrate from drinking water (shown above) Removal of perchlorate from drinking water Removal of heavy metals from waste: Cd, Cr, Fe, Hg, Ni, Pb, Zn In many of these applications, a residual concentration in the g/L range is possible. Some contaminants are difficult to remove with ion exchange, due to a poor selectivity of the resins. Examples: As, F, Li. See the periodic system of the elements with some ion exchange data. See also the page about resin types (selective resins) and a separate page about ion exchange processes for drinking water.

Other informationAbbreviationsResin types are usually abbreviated in these pages: SAC: strongly acidic cation exchange resin WAC: weakly acidic cation exchange resin SBA: strongly basic anion exchange resin WBA: weakly basic anion exchange resin See a table with a complete list of abbreviations and units.

WaterSee details about the water analysis as required for the above processes.A special page is available about drinking water applications.

Ion exchange columnsVarious column types are described in a separate page. Degasifiers as well.

RegenerationSee details about regeneration processes, quantities and concentrations of regenerants.

Ion exchange reactionsA full page describes reaction equilibrium and chemical reactions of these resins.

Regeneration methodsfor ion exchange unitsIntroductionMost ion exchange resins are used in columns. Ion exchange operation is basically discontinuous: a loading phase, called service run, is followed by regeneration of the exhausted resins. There are two main methods for the regeneration process: Co-flow regeneration, where the fluids are flowing from the top to the bottom of the column both during the service run as well as during regeneration. Reverse flow regeneration, where the fluids are flowing alternatively upwards and downwards during service and regeneration. We will also give information about MB regeneration, regenerant quantities (regeneration ratio), thoroughfare regeneration, and regenerant types and concentrations. Separate pages give information about the water quality required for regeneration, and regenerant neutralisation. See also the page about capacity. Co-flow regeneration (CFR)This regeneration technique has been used used at the beginning of ion exchange: the solution to treat flows from the top to the bottom of the column, and the regenerant uses the same path. The problem is that strongly acidic and strongly basic resins are not completely converted to the H or OH form at the end of the regeneration, because this would require too large an excess of chemical regenerant. As a result, the bottom layers of the resin bed are more contaminated than the top layers at the end of regeneration, so that when the next loading run begins the leakage is high due to the displacement of the contaminating ions by the H+ (or OH) ions produced in the exchange.

The dark zone in the picture above represents the proportion of exhausted resin, the yellow zone the proportion of regenerated resin. The small picture on the right explains what I mean: for instance, at level A, the resin is 50% exhausted and 50% regenerated. Above the exchange zone, the resin is fully exhausted, and below it is fullly regenerated. With co-flow regeneration, the only way to reduce this permanent leakage is to increase the quantity of regenerant so as to leave less contaminating ions at the outlet of the column.

Reverse flow regeneration (RFR)This is also called "counterflow regeneraton". In the past, it was called counter-current regeneration, but the term is not strictly correct as the resin bed does not move. With reverse flow regeneration the regenerant is injected in the opposite direction of the service flow. There are two sub-cases: 1. Upflow loading and downflow regeneration, as in the floating bed and AmberpackTM processes. 2. Downflow loading and upflow regeneration, as in the UFDTM and UpcoreTM processes. In this case, the regenerant doesn't have to push the contaminating ions through the whole resin bed. The layers which are less exhausted will be regenerated first and will be the cleanest when the next loading run (exhaustion) starts.

Or with upflow loading:

Reverse flow regeneration offers two significant advantages: 1. The treated water has a much higher purity than with co-flow, due to a very low leakage. 2. Less regenerant is required, as the contaminating ions don't have to be pushed through the whole bed, and the leakage is almost independent of the regenerant dosage.

Treated water quality

At the end of regeneration, the exit layer of the column regenerated in CFR has the highest concentration of impurities, whereas in RFR the exit layer contains the most highly regenerated resin. This is why in CFR the contaminants at the bottom find their way into the treated water, more at the beginning than in the middle of the run, due to a "self regeneration" effect, whereas in RFR any displaced contaminant from the inlet layer gets immediately removed from a layer underneath. The graph shows the typical leakage profile during the loading phase (e.g. conductivity in S/cm but it can be any other leakage depending on the process). The ionic leakage obtained with reverse flow regeneration is usually so low, that it does not depend on the amount of regenerant used. With co-flow, low leakage values are obtained only with high regenerant dosage.

No backwash with RFRThe whole effect of reverse flow regeneration relies on undisturbed resin layers. The resin with the highest degree of regeneration should always be at the column outlet. Therefore, the resin bed should not be backwashed before regeneration, and should not be allowed to fluidise at any time. So either the columns are completely filled with resin (packed beds) or the bed is held down during regeneration. See the "column design" page for the concepts of holddown and packed beds.

Regeneration stepsThe general regeneration procedure for ion echange vessels is as follows: 1. Backwash resin bed (co-flow regeneration only) to remove suspended solids and decompact the bed. 2. Inject regenerant diluted in appropriate water quality. The injection is at a low flow rate, so that the contact time is 20 to 40 minutes. 3. Displace the regenerant with dilution water at the same flow rate. 4. Rinse the bed at service flow rate with feed water until the desired treated water quality is obtained. The above is valid for most ion exchange columns, e.g. softening, nitrate removal, de-alkalisation. For demineralisation, the cation column is regenerated first with acid, then the anion column with caustic soda; alternatively, both are regenerated at the same time. Additional steps may be required in some special applications (see below).

Mixed bed regeneration

Internal regeneration of a mixed bed unit is more complicated. The steps are: 1. Backwash resin bed to separate the cation from the anion resin. 2. Let the resins settle. 3. Optionally: drain the water down to the resin bed surface. 4. Inject caustic soda diluted in demineralised water. 5. Displace the caustic with dilution water. 6. Inject acid diluted in demineralised water. 7. Displace the acid with dilution water. 8. Drain the water down to the resin bed surface. 9. Mix the resins with clean compressed air or nitrogen. 10. Refill the unit slowly with water. 11. Do the final rinse with feed water at service flow rate until the desired treated water quality is obtained. Note 1: If no NaOH distributor is available, caustic "rains" from the top of the column down to the water level. This creates some dilution and the distribution is not as even as with a dedicated distributor. Note 2: Cation and anion resin can be regenerated simultaneously to save time. Otherwise, always start with the anion resin. Note 3: In condensate polishing, mixed bed units are usually regenerated externally.

Regeneration efficiencyThe three pictures on the left show the conversion of totally exhausted resins (in the Na+ or Cl form), as a function of regenerant dosage. The y axis "% Regeneration" represents the percentage of conversion of the resins to the H+ and OH form respectively. We can observe the following things:Hydrochloric acid is more efficient than sulphuric acid to regenerate a strongly acidic cation exchange resin (SAC) initially in the Na+ form.With 50 g HCl per litre of resin, a conversion of 60 % to the H+ form is achieved.With 50 g H2SO4, a conversion of only 40 % is achieved.Even expressed as equivalents, hydrochloric acid is more efficient: 36.5 g HCl (1 eq) will convert the resin to 45 %, whereas 49 g H2SO4 (1 eq) convert only 39 %.To obtain total conversion, i.e. 100 % in the H+ form, we need about 6.5 eq HCl (240 g/L) but 8 eq H2SO4 (400 g/L).This is due to the fact that the second acidity of sulphuric acid is considerably weaker than the first acidity.Regeneration of a strongly basic anion exchange resin (SBA) initially in the Cl form with caustic soda is more difficult:With 50 g NaOH per litre, only 37 % of the resin are converted; with 40 g (1 eq) only 32 %. As much as 37.5 eq NaOH (1500 g) are required to convert the SBA resin to about 100 % in OH form.The reason why SBA resins of type 1 are more difficult to regenerate than SAC resins is the selectivity coefficient:K(Cl/OH) = 22 whilst K(Na/H) = 1.7.

In practice, SAC and SBA resins are not regenerated to a high conversion level, which would be uneconomical in view of the high regenerant consumption.On the other hand, weakly functional resins (WAC and WBA) have a near-linear regeneration curve: the can be regenerated with a dosage close to the stoichiometric value, so they are fully converted (see below) at the end of each regeneration.Note: all regenerant values are expressed as grams of pure chemical (100 %) per litre of resin.

Regeneration ratioDefinition:

Introduction The regeneration ratio or regenerant ratio is calculated as the total amount of regenerant (in equivalents) divided by the total ionic load (also in equivalents) during one cycle. It is is also equal to the number of eq/L regenerant per eq/L of resin operating capacity. A (theoretical) regenerant ratio of 1.00 (i.e. 100 %) would correspond to the stoichiometric quantity. All resins need a certain excess of regenerant above the stoichiometric quantity.Example Amberjet 1000 regenerated with 55 g HCl per litre operating capacity : 1.20 eq/L 55 g/L HCl = 55/36.5 = 1.507 eq/L Regenerant ratio = 1.507/1.20 = 1.26 = 126 %

ExcessThe difference between ionic load and regenerant quantity is called excess regenerant.Excess [in eq]= regenerant [eq] - ionic load [eq]

Excess [in %] = 100 x (regenerant ratio 1)Minimum values WAC resins require just above the stoichiometric quantity. A safe number is 105 to 110 %. WBA resins require 115 to 140 %, because most of them they have some strongly basic functional groups. When regenerated with ammonia or sodium carbonate, WBA resins require a regenerant ratio of 150 to 200 %. These regenerants can be used for WBA only, not for SBA resins. SAC and SBA resins require a larger excess than their weak counterparts. Co-flow regenerated SAC and SBA resins require more than those regenerated in reverse flow. SAC resins regenerated in reverse flow with hydrochloric acid need an absolute minimum of 110 % regeneration, but a safer value is 120 %. If the water contains high hardness or low alkalinity, the minimum value must be increased. SAC resins regenerated with sulphuric acid require a larger excess than those regenerated with HCl. At least 40 % more. For SBA resins, there is no easy way to estimate a minimum, as it depends on the type of SBA resin (styrenic type 1 vs type 2 or acrylic resins). Important note: when calculating the regenerant ratio for SBA resins, one must take 2 equivalents of NaOH for each equivalent of CO2 or SiO2. WAC/SAC couples can be regenerated with a global ratio of about 105 %. WBA/SBA couples can be regenerated with a global ratio of 110 to 120 %. More is required if the silica level is high in the feed water. The regenerant ratio for silica should be at least 800 %. This should be calculated separately as the quantity of NaOH (in eq) divided by the load of silica (in eq) during one cycle. One equivalent of silica is taken as 60 g as SiO2.

Thoroughfare regenerationWhen a weak and a strong resin are used in series, the following two rules must apply: 1. The feed water must pass first through the weak, then only through the strong resin. 2. The regenerant must pass first through the strong, then through the weak resin. Separate columns in serviceSeparate columns in regeneration

Why is it so? 1. The weak resin has a high capacity and good regeneration efficiency, but does not remove all ions. Therefore it must be placed first, and the strong resin will be used to remove whatever the weak resin has not removed, albeit with a lower efficiency. 2. The strong resin requires a high excess of regenerant. The weak resin requires almost no excess. Therefore the regenerant passes through the strong resin first, and the weak resin is regenerated with the excess regenerant coming out of the strong resin. The above pictures are for old-fashioned, separate columns with co-flow regeneration. Below the same for an Amberpack double compartment column. Amberpack in serviceAmberpack in regeneration

All the above applies equally to a couple of weak acid and strong acid cation exchange resins.

Regenerant types and concentrationsTypes of regenerant Sodium chloride (NaCl) is normally used to regenerate SAC resins in the softening process, and SBA resins used for nitrate removal. For softening, potassium chloride (KCl) can also be used when the presence of sodium in the treated solution is undesirable. In some hot condensate softening processes, ammonium chloride (NH4Cl) can be used. For nitrate removal, the SBA resin can be regenerated with other compounds providing chloride ions, such as hydrochloric acid (HCl). For decationisation the first step of a demineralisation process SAC resins must be regenerated with a strong acid. The most common acids are hydrochloric and sulphuric acids. Hydrochloric acid (HCl) is very efficient and does not cause precipitations in the resin bed. Sulphuric acid (H2SO4) is sometimes cheaper and easier to store and to handle in general, but less efficient than hydrochloric acid: the operating capacity of the SAC resin is lower. Additionally, its concentration must be carefully adjusted to prevent calcium sulphate precipitation (see below). Once a CaSO4 precipitate is formed, it is very difficult to remove from the resin bed. Nitric acid (HNO3) can also be used in principle, but is not recommended as it can cause exothermic reactions; explosions have been observed in some cases, so that the use of nitric acid is considered dangerous. For dealkalisation, the WAC resin is best regenerated with hydrochloric acid (HCl). When using sulphuric acid, the concentration must be kept under 0.8 % to avoid calcium sulphate precipitation. Other, weaker acids can also regenerate WAC resins, such as acetic acid (CH3COOH) or citric acid, a molecule containing three COOH groups: (CH2COOH-C(OH)COOH-CH2COOH = C6H8O7). Have a look at the 3-dimensional formula. SBA resins are always regenerated with caustic soda (NaOH) in the demineralisation process. Caustic potash (potassium hydroxide KOH) is in principle also applicable, but usually more expensive. WBA resins are usually also regenerated with caustic soda, but other regenerants weaker alkalis can also be used, such as: Ammonia (NH3) Sodium carbonate (soda ash, Na2CO3) A lime (calcium hydroxide, Ca(OH)2) suspension In general, WAC resins can be regenerated with an acid having a pKa lower than that of the resin itself. The pKa value of most WAC resins is 4.4 to 4.8. Thus acetic acid (pK 4.8) can just regenerate WAC resins, and citric acid (pK 3.1) is efficient for this purpose, whilst carbonic acid (pK 6.4) is not. In most cases, however, HCl or H2SO4, usually cheaper, are used. In general, WBA resins can be regenerated with an alkali having a pKa higher than that of the resin itself. The pKa value of styrenic WBA resins is around 8.5, that of acrylic WBAs is around 9.5. Thus ammonia, with a pKa of 9.3, can regenerate styrenic WBA resins. In most cases, however, NaOH is used, being often cheaper and easier to handle. SAC and SBA resins can only be regenerated with strong acids or strong bases respectively. ConcentrationsThe most usual concentrations are: NaCl (softening and nitrate removal): 10 % HCl (decationisation, de-alkalisation and demineralisation): 5 % NaOH (demineralisation): 4 % H2SO4: for SAC resins, the acid concentration must be carefully selected between 0.7 and 6 % as a function of the proportion of calcium in the feed water (which is the same in the SAC resin). For WAC resins, the concentration is usually 0.7 %. Too high a concentration may cause calcium sulphate precipitation. For SAC resins, stepwise concentrations are often used: after a first step at a low concentration, a second step is carried out at a higher concentration once a great part of the calcium on the resin has been eluted. In rare cases, three steps are used. The steps at higher concentrations reduce the quantity of dilution water and increase the sulphuric acid efficiency. There are cases where different concentrations (often lower, rarely higher) must be selected.

Quality of water for regenerationThe required water quality for each regeneration step is shown in a separate page.Neutralisation of the regenerantsSee another page on the way to neutralise regenerants and increase ion exchange capacity.Special applicationsSweetening-off and -onIn several applications other than water treatment, the solution treated by resins must be displaced before regeneration, to recover as much treated solution as possible, avoiding losses in the regeneration waste. This additional step is called "sweetening off" because it was first introduced in sugar treatment processes. Similarly, a "sweetening on" step is carried out after regeneration and rinse before feeding the raw solution to be treated, to avoid dilution of the treated solution. The complete regeneration process consists then of the following steps: 1. Backwash with the raw solution (optional) 2. Sweetening off: displacement of the solution with water 3. Regenerant injection 4. Displacement of the regenerant with water 5. Fast rinse with water 6. Sweetening on with the solution to be treated The sweetening-off fraction is sometimes recycled, particularly when the solution contains valuable components (precious metals, expensive chemical compounds). When the solution has a high density, which can be higher than the resin, the service run which often takes place at a low flow rate can be carried out upflow to pack the resin bed and thus avoid its floating and fluidisation. This procedure is often used in the treatment of sugar solutions. Merry-go-round

To increase the concentration of the eluate and the operating capacity of the resin, a system consisting of at least three columns can be used, where two columns are in service, in series, and the third in regeneration, as shown in the picture on the left. The "lead" column is exhausted past its leakage breakthrough, whilst the "lag" column acts as a polisher and guarantees a low leakage in the treated solution. When the eluate contains a valuable compound removed on the resin, this compound is eluted at a higher concentration than with a single column.

Ion exchange resins applicationsA general overviewContents1. Water treatment1.1. Softening1.2. Dealkalisation1.3. Demineralisation1.4. Mixed bed polishing1.5. Drinking water2. Sugar industry2.1. Softening of water used for sugar extraction2.2. Softening of sugar juices before evaporation2.3. The NRS softening process2.4. The Gryllus softening process2.5. Demineralisation of sugar juices before evaporation2.6. Colour removal from sugar syrups after evaporation2.7. The Quentin process2.8. Sugar recovery from molasses2.9. Sucrose inversion2.10. Chromatographic separation2.11. Glucose treatment3. Other applications in the food industry3.1. Dairy products3.2. Beverages3.3. Fruit juices3.4. Recovery of polyphenols3.5. Citric acid3.6. Aminoacids3.7. Sorbitol demineralization3.8. Gelatine demineralisation4. Applications in the chemical industry4.1. Recovery or removal of metals4.2. Caustic soda and chlorine production4.3. Phenol4.4. Hydrogen peroxide purification4.5. Selective removal of various elements

5. Catalysis5.1. Alkylation5.2. Condensation5.3. Esterification5.4. Etherification5.5. Dehydration5.6. Hydrogenation6. Pharmaceutical industry6.1. Extraction and purification of antibiotics6.2. Slow-release formulations6.3. Resins used as drugs6.4. Taste-masking6.5. Production chromatography7. Miscellaneous applications7.1. Mining industry7.2. Enzyme immobilization7.3. Hydroculture

1. Applications of ion exchange resins in water treatmentWater softening and demineralisation are also described with chemical reactions in the IX basics page. And regeneration methods are in another page.

1.1. SofteningA strongly acidic cation exchange resin is used here in the sodium form. The ions forming hardness, essentially calcium and magnesium, are exchanged for the sodium ions of the resin, and the softened water can be used for several purposes: Laundries Domestic water boilers Low pressure industrial boilers TextileResins used: AmberliteTM IR120 Na, AmberjetTM 1000 Na Amberlite SR1L Na for drinking waterTreated water quality:Residual hardness < 0.02 meq/L (1 mg/L as CaCO3) with reverse flow regenerationRegeneration: brine (NaCl as a 10 % solution)

1.2. De-alkalisationIn a water containing bicarbonates most waters in Western and Central Europe do calcium and magnesium associated with bicarbonate ions are exchanged for hydrogen ions from a weakly acidic cation exchange resin. This is called removal of temporary hardness. The treated water contains carbon dioxide that can be removed with a degasifier. The salinity of the treated water is lower than that of the feed water. Dealkalisation is used: To treat water used to make beverages in breweries and soft drink plants To soften drinking water supplies in municipalities At home, to filter, soften and partially demineralise the water you use to make tea or coffee As a first demineralisation step For certain industrial processesResins used: Amberlite IRC86 for industrial water Amberlite PWC13 for municipal drinking water ImacTM HP333 and HP335 for household filter cartridgesTreated water qualityResidual alkalinity = very low (endpoint at 10 % of the raw water alkalinity)Residual hardness = permanent hardness (TH Alk)Regeneration : Acid (preferably HCl at 5 % concentration)

1.3. DemineralisationAll ions must be removed from water. Therefore the water passes first through cation exchange resins in the hydrogen form, then through anion exchange resins in the hydroxyl or free base form. All cations are replaced by ions from the cation resin, and all anions for the ions of the anion resin. These H+ and OH ions recombine to create new water molecules (H2O). The treated water contains only traces of sodium and silica.Resins used: Amberlite IRC86 (weakly acidic resin) Amberlite IR120 or Amberjet 1000 (strongly acidic resin) Amberlite IRA96 or IRA67 (weakly basic resin) Amberlite IRA402 or Amberjet 4200 or 4600 (strongly basic resin)The use of weakly functional resins depends on the raw water analysis and plant size.Treated water qualityConductivity: 0.2 to 1 S/cm with reverse flow regenerationResidual silica 5 to 50 g/L depending on the silica concentration in the feed water and the quantity of caustic regenerant.These values are lower than those obtained with other technologies, such as reverse osmosis or distillation.Note that the pH value should not be used as a process control, as it is impossible to measure the pH of a water with less than say 5 S/cm conductivity.Regeneration Cation exchange resins: strong acid (HCl or H2SO4)Anion exchange resins: caustic soda (NaOH)

1.4. Mixed bed units1.4.1. Polishing mixed bedsWhen an even better treated water quality is required, close to that of totally pure water, a polishing vessel is installed after a primary demineralisation plant. It is filled with cation and anion exchange resins, which must be mixed during the loading run, but separated for regeneration. The separation is carried out with an upflow backwash step, and requires resins with appropriate particle sizes and densities.Resins used: Amberjet 1000 or 1500 (strongly acidic resin) Amberjet 4200 or 4400 (strongly basic resin)For specific applications, such as ultrapure water or circuits in nuclear power plants, other highly purified grades are also used.Treated water qualityConductivity: 0.055 to 0.1 S/cmResidual silica: 1 to 10 g/L.Note that the pH value should not be used as a process control, as pH meters are unable to operate at 1 S/cm conductivity or below.RegenerationCation exchange resins: strong acid (HCl or H2SO4)Anion exchange resins: caustic soda (NaOH)1.4.2. Working mixed bedsFor low salinity waters, or when only moderate demineralised water volumes are required, mixed bed units can be installed and fed directly with city water or reverse osmosis permeate. These units are called "Working MBs". The resins used are essentially the same as those for polishing mixed bed units. A special case is Service De-Ionisation (SDI) using mixed bed columns or cartridges regenerated off-site described in a separate page.

1.5. Drinking waterIon exchange is a valuable technology for the selective removal or certain contaminants from underground water. See details in a separate page.

2. Use of ion exchange resins in the sugar industry2.1. Softening of water used for sugar extractionThe process it that described in point 1.1 (water softening).

2.2. Softening of beet sugar juices before evaporationThe hardness of beet sugar juices results in scaling of the heat exchanger in the evaporators. To prevent it, increase the thermal efficiency and save energy, it is usual to soften the sugar juice. The plant can then operate continuously, without frequent interruptions required for de-scaling the equipment.In this process, the type of resin used is the same as that for water softening, but the resins must be approved for use with food and resist specific stress due to the temperature and concentration of the juice.The calcium and magnesium ions present in the sugar juice are exchanged for sodium ions from the resin. The process is applied to thin juice, i.e. after carbonation. In general, several columns operate in parallel to ensure continuous operation.Resins used: Amberlite FPC14 Na

2.3. The NRS processThis is a clever process where the resin is regenerated with a solution of caustic soda diluted in thin juice. The basic idea is that whilst calcium hydroxide is insoluble in water, the calcium ions make a soluble complex with sucrose. The spent regenerant is recycled upstream, before the carbonation step, so that the production of waste is negligible. Moreover, the juice is not diluted in water as in the traditional softening process, because the NRS process does not include sweetening-off and sweetening-on steps. The energy balance is favourable and results in steam saving.Resins used: Amberlite FPC14 Na

2.4. The Gryllus processThis is an older process in which the softening resin is regenerated with thick juice, which contains high concentrations of sodium. The salt consumption is thus reduced, and again, no waste is produced, since the spent regenerant is recycled.Resins used: Amberlite FPC22 Na

2.5. Demineralisation of sugar juices before evaporationIn this process, "non-sugars" are removed from thin juice to increase the efficiency of crystalllisation, i.e. the sugar yield. In general, each kilogram of removed non-sugar produces 1.4 kg of additional sugar. Otherwise, the process is similar to water demineralisation: a strongly acidic cation exchange resin and a weakly basic anion exchange resin are used, regenerated respectively with acid and caustic soda.Resins used: Amberlite FPC14 Na (strongly acidic) Amberlite FPA53 (weakly basic)

2.6. Colour removal from cane sugar syrups after evaporationCane syrups usually contain many organic compounds imparting colour to the crystallised sugar and reducing the crystallisation yield. The colour removal process uses strongly basic anion exchange resins, regenerated with a sodium chloride solution. These resins are macroporous, so that high molecular mass compounds can be removed. The most efficient method uses two columns in series, the first one filled with acrylic resin, the second, polishing column with styrenic resin.Resins used: Amberlite FPA98 Cl (acrylic) Amberlite FPA90 Cl (styrenic)

2.7. The Quentin processCrystallisation of beet sugar is partially inhibited by the potassium and sodium ions contained in the juice, so that large quantities of sugar remain in the molasses after crystallisation. Magnesium being less "melassigenous" than sodium or potassium, the idea is to pass the thin juice through a column of strongly acidic cation exchange resin in the magnesium form. This increases the production of whit sugar and reduces the quantity of molasses.Resins used: Amberlite FPC23 H (must be first converted to the Mg++ form with magnesium chloride)

2.8. Sugar recovery from molasses This process is based on ion exclusion, a kind of ion exchange chromatography using fine mesh, uniform particle size resins. It separates sugar from non-sugars and increases the recovery of sugar contained in the molasses.Resins used: Amberlite CR1220 K

2.9. Sucrose inversionSucrose (common sugar) is a di-saccharide. In an acidic environment, the sucrose molecule splits into two mono-saccharides: glucose and fructose, in equal proportions. Invert sugar has a more powerful sweetening power than sucrose (1.15 vs. 1.0) , and a lower tendency to crystallise, an important feature for some industrial food products. Inversion is produced by passing sugar syrup through a low cross-linked strongly acidic cation exchange resin in the H+ form.Resins used: Amberlite FPC12 H

2.10. Chromatographic separation As fructose has a higher sweetening power than glucose (1.3 vs. 0.7), invert sugar syrups can be enriched with fructose by passing the syrup through a fine mesh, very uniform strongly acidic cation exchange resin in the calcium form. As the syrup stream moves down the column, fructose moves more slowly than glucose. This results in separated bands of higher purity of each component within the column. The fructose fraction is recovered separately in view of its commercial value. The glucose fraction can be either sold as a glucose syrup, or isomerised enzymatically to produce more fructose.Resins used: Amberlite CR1320 Ca

2.11. Glucose demineralisationGlucose syrups are demineralised to increase purity. The principle is identical to that of water or sugar demineralisation. In view of the high concentration and high temperature of the syrups, resins with a good resistance to these stresses must be used.Resins used: DowexTM 88 (strongly acidic resin) Dowex 66 (weakly basic resin)

3. Examples of other applications in the food industry3.1. Whey demineralisationWhey, a by-product of cheese production, contains valuable proteins and is used in the food industry. It is demineralised to increase purity. Again, the principle is the same as that of water or sugar demineralisation.Resins used: Amberlite FPC14 (strongly acidic resin) Amberlite FPA51 (weakly basic resin)

3.2. BeveragesThere are several applications in this area: Treatment of the water used to make beer or soft drinks (see chapter 1) De-acidification of beverages with Amberlite FPA51 (weakly basic anion resin) Removal of metals Removal of bad taste or smell Colour and turbidity removal with non-ionic adsorbents

3.3. Treatment of fruit juices Acid removal with Amberlite FPA51 (weakly basic anion resin) Removal of bitterness from orange juices with a non-ionic adsorbent resin, Amberlite FPX66 Colour removal with an adsorbent resin

3.4. Recovery of polyphenolsPolyphenols are praised today for their anti-oxidant properties. They are found in many types of fruit, such as berries or red grape. Anthocyanins are polyphenols that can be recovered from grape must.Resins used: Amberlite FPX68 (non-ionic adsorbent resin)

3.5. Citric acidThis acid is used as a preservative in many industrial food products. It is produced by fermentation. Its purification requires ion exchange demineralisation.Resins used: Amberlite FPC22 H (strongly acidic) Amberlite FPA51 (weakly basic)

3.6.AminoacidsL-lysine and other essential aminoacids (not produced by the human body) are produced by fermentation. Lysine is recovered from the fermentation broth with a cation exchange resin in ammonium form.Resins used: Amberlite FPC14 (strongly acidic)

3.7. Sorbitol demineralisationSorbitol is a polyol, a powerful sweetener and emollient used for instance in chewing gum. It can be produced by hydrogenation of glucose or by enzymatic processes. The final product often requires demineralisation.Resins used: Amberlite FPC22 (strongly acidic) Amberlite FPA51 (weakly basic) Amberlite FPC52 and FPA90 in a polishing mixed bed

3.8. Gelatine demineralisationGelatine is produced from the collagen present in pig skin and bones. To produce high purity gelatine, demineralisation is required.Resins used: Amberlite FPC14 or FPC22 (strongly acidic) Amberlite FPA53 (weakly basic acrylic)

4. Some applications in the chemical industry4.1. Recovery and removal of metalsIn surface finishing and plating shops, metals can be recovered or removed: Gold recovery from industrial jewelleries as cyanide complexes, with Amberlite IRA402 Recycling of various rinse water streams in plating shops, with Amberlite 252 (for cation removal), IRA96 (for chromate), and IRA410 (for cyanide) Copper and iron removal from chromium plating shops with Amberlyst 15Wet Chromic acid recovery in plating shops with Amberlite IR120 and Amberlite IRA96 Removal of iron from zinc baths with Amberlite IRC748 Purification of pickling baths, removing iron and zinc as chloride complexes with Amberlite IRA402. Elution is done simply with water.Other examples: Recovery of silver as a thiosulphate complex from photographic baths with Amberlite IRA67 or IRA402 Selective mercury removal in various industries with AmbersepTM GT74, a resin with thiol functionality. Cadmium can be removed with the same resin Recovery of vanadium and copper catalysts in the production of adipic acid (a precursor of nylon) with AmberlystTM 40Wet

4.2. Production of chlorine and caustic sodaThese chemicals are produced by electrolysis of saturated brine. In the production process, the absence of divalent metals is critical. A selective chelating resin is thus used to remove them (principally calcium), which reduces the initial calcium concentration from 10 20 mg/L down to a very low level, smaller than 20 g/L.Resins used: Amberlite IRC747 when strontium removal is not necessary Amberlite IRC748 when strontium must also be removed

4.3. PhenolTwo applications: Removal of sulphuric acid and organic acids from process streams in phenol production. A special weak base resin with a phenol-formaldehyde matrix is used. Removal of phenol from industrial waste. Phenol is removed on a non-ionic adsorbent resin. Regeneration is done with acetone.Resins used: Amberlyst A23 for acid removal Amberlite XAD4 for phenol removal from waste

4.4. Hydrogen peroxide purificationResins are used in two different processes: Removal of anthraquinone derivatives. These organic compounds can be removed on a non-ionic adsorbent. Regeneration is done with methanol. Removal of metal traces such as iron, with a strongly acidic resin. The treatment is done at a very high specific flow rate.In both cases, the product quality is excellent, with residuals of just a few g/L. Caution: hydrogen peroxide (H2O2) is a powerful oxidant, and serious steps must be taken in both processes to avoid accidents.Resins used: Amberlite XAD4 for organic contaminants Amberlyst 15Wet for metals

4.5. Selective removal of various elementsI have built up a periodic system of the elements (Mendeleev table) with brief information about the removal of several ions (mostly metals) with resins.

5. CatalysisA catalyst is a substance that increases the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction.In the majority of processes where a mineral acid was previously used as a catalyst notably in the petrochemical industry a strongly acidic cation exchange resin in the H+ form is now used instead. These resin must operate under stressful conditions often at temperatures between 130 and 170 C and display an acidity as high as possible.A few typical examples are shown below.

5.1. AlkylationProductOctylphenol

ReactantsOctane + phenol

CatalystAmberlyst 15Dry

Temperature100 120 C

5.2. CondensationProductBisphenol A

ReactantsAcetone + phenol

CatalystAmberlyst 131

Temperature60 80 C

5.3. EsterificationProductDimethyl maleate

ReactantsMaleic anhydride

CatalystAmberlyst 46

Temperature110 C

5.4. EtherificationProductMethyl-ter-butyl ether (MTBE)

ReactantsIsobutylene + methanol

CatalystAmberlyst 35

Temperature40 80 C

5.5. DehydrationProductIsobutylene

ReactantIsobutanol

CatalystAmberlyst 35

Temperature70 80 C

5.6. HydrogenationProductMethyl isobutyl ketone (MIBK)

ReactantAcetone

CatalystAmberlyst CH28 (palladium-doped catalyst)

Temperature130 140 C

6. Pharmaceutical industryThere are various and complex applications. As the pharmaceutical industry is intrinsically secretive, few details are known. Nevertheless, let us mention a few examples:

6.1. Extraction and purification of antibioticsVarious antibiotics use ion exchange and adsorbent resins in their production process. The objective is to purify them after extraction from fermentation broths. Examples: streptomycin, gentamycin, cephalosporin, tetracyclin.Resins used: Amberlite XAD1600 (non-ionic styrenic adsorbent) Amberlite XAD7HP (non-ionic acrylic adsorbent)

6.2. Slow-release formulationsPowdered, highly purified ion exchange resins are used as excipients in pharmaceutical formulations. The active ingredient is adsorbed on the resin and is released more slowly in the body than it would if it were present in their original state.Resins used: Amberlite IRP64 (weakly acidic) Amberlite IRP69 (strongly acidic) Amberlite IRP88 (weakly acidic in potassium form) DuoliteTM AP143 (strongly basic)

6.3. Resins used as drugsThe same resin types can be used as active substances in the medicine. It is obvious that they must meet very stringent specifications and be approved by health authorities. Let us mention two examples: Cholestyramine, a drug used to reduce the cholesterol level, is a powder based on a strongly basic anion resin in the choride form. Polacrilin potassium, a medicine used to regulate the potassium level in the blood, is a powder based on a weakly acidic resin with a methacrylic matrix.Resins used: Duolite AP143 (cholestyramine) Amberlite IRP88 (polacrilin potassium)

6.4. Taste-maskingSimilar resins are used to mask the unpleasant taste oir smell of a drug.

6.5. Production chromatographyThe chromatographic separation of various molecules used as active ingredients can be done with very fine particle size resins instead of silica gels or other media.Resins used: A whole range of products available as Amberchrom resins.

7. Miscellaneous applications7.1. Mining industryThe most significant application, involving thousands of cubic metres of resin, is uranium extraction. The crushed ore is treated with sulphuric acid, which brings the uranium in solution as uranium sulphate. The pregnant solu