Demineralized Water and Demineralization Process

Demineralized Water and Demineralization Process 

In industrial water treatment, demineralization (DM) process refers to the removal of dissolved salts and mineral ions from water and process streams. Demineralization can prove ideal for facilities needing ultra-pure water for a variety of applications. Demineralization is a type of water purification. While it can refer to any treatment process which removes minerals from water, the term demineralization is typically reserved specifically for ion exchange processes used for near total removal of ionic mineral contaminants.

The definition of demineralization is that all the minerals in the water, which consist of positive and negative ions, are removed, leaving pure water (H2O) molecules or trace quantities of these ions in water. The positive ions are defined as cations and can include magnesium, sodium, calcium, iron and copper. The negative ions are referred to as anions and can include chlorides and sulphates. Demineralization can help a plant meet its industrial water quality needs. Frequently, the terms demineralization and deionization are used interchangeably. Ion exchange demineralization process utilizes both cation and anion exchange resins, sometimes even in the same column or bed. After demineralization, the treated water is of a high level of purity comparable to distilled water, but typically at a much lower cost.

Demineralized water is normally made by using ion exchange, electrode-ionization, or membrane filtration technologies, which can be more efficient for creating ultra-pure water than processes such as distillation (where water is boiled in a still and condensed, leaving dissolved contaminants behind). The CAS number for the demineralized water is 7732-18-5.

Typical properties of the demineralized water are (i) pH – 7 – 7.5, (ii) hardness as CaCO3 – 0 milligrams per litre (mg/l), (iii) chloride as NaCl – 0 mg/l, (iv) electrical conductivity – less than 1 micro-siemens per centimetre, (v) total dissolved solids (TDS) – less than 1 ppm (parts per million), (vi)  boiling point – around 100 deg C, (vii) vapour pressure – 2.33 kilopascals at 20 deg C, (viii) viscosity – 1 millipascal-second at 20 deg C, (viii) specific gravity – 1 at 25 deg C, (ix) total silica (as SiO2) – less than 0.02 ppm, (x) total dissolved salt – 50 parts per billion (ppb), (xi) total ionized salts – 25 ppb, (xii) dissolved iron as Fe – 5 ppb (xiii) dissolved copper as copper – 5 ppb, (xiv) dissolved silica as SiO2 – 20 ppb, (xv) colloidal impurities – 5 ppb, and (xvi) total organic carbon – 60 ppb. Demineralized water does not conduct electrical current through it.

Demineralization by ion exchange, electro-deionization, or membrane filtration can produce water which is nearly 100 % free of minerals and salts including (but not limited to)  (i) alkalinity (CO3)2-, and (HCO3)–, (ii) calcium (Ca)2+, (iii) chloride (Cl)–, (iv) iron (Fe)3+, (v) magnesium (Mg)2+, (vi) manganese (Mn)2+, (vii) nitrate (NO3)–, (viii) potassium (K)+, (ix) silica (SiO2), (x) sodium (Na)+, and (xi) sulphates (SO4)2-.

Using ion exchange or other processes using resins does not remove uncharged molecules, so while demineralization can be counted on to remove almost all minerals and salts, the resulting water can contain some organic contaminants such as certain viruses or bacteria. Using reverse osmosis or nano-filtration to demineralize the water remove these contaminants.

Ion exchange demineralization is an application of ion exchange. Two main types of water treatment are exercised with the use of the ion exchange technology namely (i) water softening, and (ii) demineralization. Water softening is when minerals which give hardness to the water like calcium and magnesium are exchanged for sodium (which is a lighter molecule). Soft water is needed for several processes. Demineralization is when the ions in the solution are almost completely removed. This process is the basis on which the demineralization plant operates and produces demineralized water.

Demineralization of water typically refers to the removal of dissolved mineral solids through an ion exchange process. The basic principles of an ion exchange reaction are described here. In the presence of water, minerals and salts dissociate into their constituent ions. These dissolved solids consist of negatively-charged ions known as anions, and positively charged ions known as cations, each of which are attracted to counterions (or ions of an opposing charge). Within an ion exchange column, a resin is present. This resin consists of plastic beads to which an ionic functional group has been bound. These functional groups loosely hold ions of an opposing charge through mutual electrostatic attraction.

Fig 1 Ion exchange demineralization process

The columns or vessels are the containers in which these resins are there. In Fig 1, it can be seen that the fresh resin gets loaded progressively with ions from the feed solution until leakage / exhaustion occurs, that is, once the resins are fully loaded and the ions from the feed escape the ion exchanger. The resin is loaded in the direction of the flow. When the resin is exhausted, the next step is regeneration. Regeneration can only take place when the regenerant concentration is high, typically 1000 times the concentration in normal water.

During an active ion exchange cycle, water with dissolved ions is introduced to the resin. The ions in solution exchange places with the ions on the resin beads, clinging to the resin’s functional groups even as the resulting solution is drained away. Ion exchange happens when one ion has higher affinity for the functional group than the ion which is already present. The specific ionic contaminants present dictate whether anionic and / or cationic resin types are needed.

In a typical ion exchange reaction, the exchange of ions simply results in the replacement of contaminant ions with other, less objectionable, ions. In an ion exchange sodium softening system, for example, the objective is to remove hardness ions (e.g., Ca2+ or Mg2+) from solution by replacing them with sodium ions (Na+). As a result, the treated solution has little to no hardness, but it contains a higher concentration of sodium ions. While this is acceptable for several applications, some processes need near-total removal of dissolved solids. That is, where demineralization comes into picture. In demineralization, cations in the feed water are exchanged for hydrogen (H+) ions and cations are exchanged for hydroxyl (OH–) ions. The result is water (H+ + OH- = H2O).

Demineralization is typically reserved for applications needing higher levels of water purity, such as feed or makeup water for high pressure boilers. Purified water can be demineralized, deionized, and distilled water. The similarity in these waters is that they all are purified water which have undergone some treatment process to remove impurities and make the treated water suitable for a given use. While they are of similarly high quality, the distinction between the three lies in which treatment process is used to produce them. The terms ‘demineralization’ and ‘deionization’ are frequently used interchangeably, with both referring to ion exchange processes used to remove nearly all dissolved solids from a stream. Less commonly, ‘demineralization’ is used to refer broadly to any or all treatment processes used to remove dissolved solids from a liquid, encompassing not just ion exchange, but filtration processes such as reverse osmosis (RO) and nano-filtration (NF) as well.

Demineralized / deionized water and distilled water have similar applications. Demineralized / deionized water and distilled water alike can be used for applications needing high purity thresholds. Distilled water offers slightly higher purity than deionized water (bacteria, organics, and particulate), though because of its higher cost, it is normally reserved for only those applications with particularly stringent purity standards. It is also used as part of an evaporation process for zero liquid discharge.

Demineralized / deionized and distilled water offer similarly high purity. Still, there are a few characteristics which set them apart from one another. They are produced through different processes. The chief difference between demineralized / deionized and distilled water is the process by which they are produced. Demineralization and deionization consist of an ion exchange process where a stream is passed through one or more ion exchange resin beds. The specialized resins contained in the ion exchange unit(s) remove dissolved solids based on an electrostatic attraction between the ions in solution, and those on the resin.

Another form of deionization, called electro-deionization (EDI) is similar in principle to ion exchange deionization in that it removes dissolved solids based on their electrostatic charges. It differs in that it leverages electricity and semi-permeable ion exchange membranes rather than resins. Electro-deionization is used across a variety of industries for polishing post reverse osmosis process.

Distillation is a fundamentally different process from deionization. In distillation, a liquid is heated in a still to boiling, then the resulting water vapour is cooled in a condenser, and the purified liquid water is then captured in a sterile container. Hence, where deionization removes contaminants from the water, distillation effectively removes the water from the contaminants, which are left behind in the still after the water has evaporated away.

In a high-purity system, deionized or reverse osmosis product water is fed to the still as make-up. They have different operational costs. For the majority of industrial applications, deionization is far more cost-effective than distillation. This is since distillation needs considerable energy expenses for heating, circulation, and cooling, especially for the large volumes of water needed to support production on an industrial scale.

Over the last few decades, innovations such as vapour compression and multiple effect distillation set-ups have led to higher energy efficiency, but they are still costly to run comparative to other purification technologies. The ongoing operational costs of a deionization system are comparatively quite low. Deionization typically needs minimal energy costs, normally only to power pumps to circulate water, as well as chemical costs for resin regeneration. They remove different contaminants.

Deionization is highly effective for near-total removal of ionizable substances, however, because of the nature of the ion exchange reaction, deionization is not effective for removal of non-ionic substances such as organic or biological contaminants. For this reason, it can be necessary to use some form of filtration to pre-treat a stream prior to ion exchange. Distillation, on the other hand, is capable of removing nearly all contaminants, including ionic organic or inorganic substances as well as biological contaminants.

While distilled water offers very high purity, it can be contaminated when volatile materials rise away with the water vapour and end up in the distillate in what is known as carry-over. Distilled water also carry-over inorganic material in small quantities so the purity regarding total dissolved solids is higher than deionization. Additionally, distilled water is able to readily dissolve materials it is exposed to, and hence, it is to be stored carefully to prevent contaminants from leaching into the treated water from the air or storage container.

Demineralization, or the removal of virtually all ionic mineral contaminants from water, is crucial for producing water of sufficient purity for a variety of industrial applications. Demineralized process water is used to ensure the quality and consistency of various products, as well as to ensure consistent and predictable function of sensitive equipment. Demineralized water is useful for a range of applications.

There are some common demineralization system designs and there are some considerations for determining which technologies are best for demineralization. Hence, choosing of the best demineralization technologies to fulfill the needs of the plant and processes is necessary. By definition, demineralization is the near-total removal of inorganic salts from water. In most industrial process water treatment applications, this is accomplished through ion exchange. Given that ion exchange demineralization systems can be designed to meet different needs, there is no one best way to demineralize process water. In its place, choosing the best demineralization strategies comes down to matching ion exchange system design to the plant’s unique process conditions, purity specifications, and plant environment.

The underlying principles of the demineralization process are the same from one system to the next, but there are two main aspects where a system can be customized to fit a specific application. The first of these principles is system configuration. Ion exchange demineralization systems are typically available in either of two configurations namely (i) twin-bed ion exchange units, and (ii) mixed-bed ion exchange units.

Twin-bed demineralization systems consist of two columns or beds, one containing a cation exchange resin, and the other containing an anion exchange resin. Process water is cycled through the beds sequentially, first through the cation resin, where mineral contaminants are replaced by hydrogen ions, and next through an anion resin, where mineral contaminants are replaced by hydroxyl ions, which combine with the hydrogen ions to form pure water.

Mixed-bed demineralization systems are a single unit or column which houses a mixture of cation and anion exchange resins. As process water is cycled through the resins, ion exchange reactions take place repeatedly within the unit, resulting in comparatively higher removal of ionic contaminants comparative to twin-bed systems.

The second aspect is the selection of specific resin products which is to be used within the ion exchange unit. There are hundreds of ion exchange resins which are available today, several of which offer functional benefits for specific applications. Some key factors people are to consider in order to help them to navigate the key aspects of ion exchange system design, and ultimately zero in on the best demineralization solution for their needs are described below.

Process water quantity and consumption – Plants whose processes need large quantities of demineralized water are frequently better served by twin-bed ion exchange demineralization systems. The reason for this is two-fold. First reason is that dual bed ion exchange systems consist of multiple columns versus the single column mixed-bed configuration. Second reason is that twin-bed ion exchange systems offer a comparatively simpler resin regeneration process, allowing for potentially less downtime, and less consumption of regenerant chemicals and / or rinse water each time the ion exchange resins are consumed.

The operating personnel can get a rough estimate of the total volume of process water which the demineralization system can treat before a regeneration cycle is needed by simply dividing the exchange capacity of the ion exchange unit by the total dissolved solids present in a representative sample of the process water. Understanding of the capacity needs helps the operational personnel to predict the frequency of resin regeneration cycles and resin life, and to decide whether it is worth it to invest in a larger unit.

Process water purity specifications – Finding the best demineralization strategy also depends on the level of purity needed to support a given process or to protect downstream equipment from corrosion or other damage. Mixed-bed demineralization units produce water of a comparatively higher purity than produced by the twin-bed units, and are hence used for processes needing the most stringent purity standards, such as feed water for high-pressure boilers. Mixed-bed demineralization typically uses a mixture of strong acid cation (SAC) resins and strong base anion (SBA) resins, normally with a higher ratio of SAC resin relative to SBA resin. Typically, mixed-bed units are used for polishing to achieve extremely pure water.

For processes with slightly more forgiving purity standards, dual-bed demineralization systems use is a better choice, as they can be less costly to maintain. Twin-bed demineralization systems use sequential ion exchange columns, normally with a strong acid cation resin followed by a strong base anion resin. This general-purpose ion exchange system configuration produces demineralized water of a sufficient grade to support the majority of the industrial processes, such as low-pressure boilers.

Alternatively, twin-bed systems which use a strong acid cation resin followed by a weak base anion (WBA) resin are sufficient for applications needing a relatively lower grade of demineralized water. These systems are sometimes used for rinse or cleaning water in industrial settings, or where there is relatively little silica present in a feed stream.

Process conditions – There are a variety of other factors which are to consider in identifying an optimal demineralization strategy. These include the following.

Temperature is important for ensuring a longer operating life for the resins. Hence, those resin products are to be selected which accommodate the temperature of the process stream. The resin manufacturer gives recommended operating temperatures which are to be considered when selecting an ion exchange resin.

Parameters of hardness / total dissolved solids are to be considered. Streams with high hardness can put a strain on demineralization systems, forcing more frequent regeneration cycles and shorter overall resin life. This can be mitigated by employing a two-step ion exchange system with a weak acid cation (WAC) unit ahead of a strong acid cation unit or by employing some form of pre-treatment, such as reverse osmosis.

In case of variable stream content, mixed-bed units are more susceptible to resin fouling and inferior system function in the presence of unanticipated contaminants. Hence, if the contents of the process stream fluctuate over time, a twin-bed demineralization unit can be a better choice. Alternatively, different pre-treatment strategies can be used ahead of a demineralization unit to protect it from variability in stream content.

Maintenance and downtime are to be considered in identifying an optimal demineralization strategy. Ion exchange resins become exhausted through normal use cycles and need periodic regeneration and replacement. For lower-capacity systems, it is possible to contract with a service provider for offsite resin regeneration and replacement, as is the case for the majority of the modular demineralization systems. If the plant needs its own resin maintenance, then it needs to determine how much downtime is acceptable to permit periodic resin maintenance. For production lines with low tolerance for downtime, it is better to choose a design which permits removal of resin for external regeneration. This allows for the flexibility to rotate in a back-up quantity of resin while one batch is being regenerated. These are just some of the process conditions which determine which types of demineralization are best suited to a given process.

Choosing the best demineralization system for the plant is frequently a highly individualized undertaking which can be simplified by qualified water treatment personnel. A specialist can perform analyses to help people to understand the demineralization goals and challenges and can help in identifying the best water treatment technologies for the unique process water needs.

Preceding ion exchange, when water feed to the demineralization plant is the raw feed then there is normally some type of pre-treatment is needed. The pre-treatment can be micro-filtration or conventional clarification, The choice of pre-treatment depends on the contaminants in the plant water source. Pre-treatment needs to be considered carefully since, in some cases, it can add around 30 % to 50 % to the total cost. and double or triple the size of the footprint (especially if the project site needs clarification). Also, the sludge from the pre-treatment system is needed to be treated with filter presses.

Demineralization water system design

The specific design and components of an ion exchange demineralization water system can vary from one application to the next based on process conditions and composition of the stream to be treated. Still, majority of the demineralization water systems include several components consisting of (i) one or more ion exchange columns, (ii) regenerant dosing system, (iii) chemical feed storage tanks, (iv) PLC (programmable logic controller), (v) control valves, and piping, and (vi) ion exchange resins. There is some flexibility in the configuration of a demineralization system in order to optimally meet different process conditions and purity objectives.

In designing a demineralization water system, consideration is to be given to variability of the feed water, level of purity needed, system footprint, tolerance for ion leakage (in particular sodium and silica), and chemical feed requirements, among other factors.

In general, demineralization ion exchange systems are available in either two-bed or mixed-bed configurations. Two-bed or dual-bed exchange unit use two or more ion exchange resin beds or columns for the treatment of a stream. Each resin bed contains a specific type of ion exchange resin. In two-bed demineralization, a stream is first treated with a strong acid cation resin which captures the dissolved cations, and releases hydrogen (H+) ions in exchange. The resulting mineral acid solution is then routed to the strong base anion resin bed. This second step removes the anionic contaminants while releasing hydroxide (OH–) ions, which combine with the existing hydrogen ions (H+) to form water. The resulting stream is low in total dissolved solids and has a nearly neutral pH. While two-bed exchange units are effective for demineralization, sodium leakage can affect the quality of their output, especially for streams with high total dissolved solids and / or low pH.

Mixed-bed ion exchange units offer a higher water quality compared to twin bed systems. Mixed-bed ion exchange units hold a mixture of different ion exchange resins housed within a single ion exchange column. When a stream is introduced to the unit, the cation and anion exchange reactions take place simultaneously within the unit, which has the effect of addressing the sodium leakage issues which can compromise the quality of demineralized water produced by a twin-bed ion exchange system. While mixed-bed exchange units produce higher quality water, they also need a more involved resin regeneration process. Additionally, mixed-bed units are more susceptible to resin fouling and / or inferior system function because of the fluctuations in stream contents and are hence typically used downstream of other treatment measures.

Demineralization normally results in almost complete removal of minerals, and is hence typically reserved for applications needing a very high level of water purity, such as feed-water or make-up water for high-pressure boilers. For fresh water applications, demineralization process can be a good alternative to distillation process, since it is capable of producing water similar in quality to distilled water, but through a cost-effective ion exchange process.

Power stations receive raw water which needs to be purified to demineralized water. Raw water mostly comes from dams and rivers sourced in close proximity to the power stations. The properties of the raw water which need to be addressed before the water can be defined as demineralized water are (i) suspended solids like sand and silt need to be removed, (ii) the cations and anions, which are dissolved salts and minerals, needs to be removed, (iii) certain gases such as carbon di-oxide which are dissolved into the water are to be taken out, and (iv) living material such as micro-organisms (bacteria) need to be eliminated as far as possible. Fig 2 shows the water purification process from raw water to drinking water and demineralized water. In the figure, the demineralization plant area is bounded by the dashed line. It starts at the cation vessel and ends at the mixed resin bed vessel.

Fig 2 Flow diagram of demineralization process

After sand filter, the next stage of treatment is demineralization by ion exchange process. The cation exchange resin bed exchanges positive ions in the water for hydrogen ions found in the resin while the anion exchange exchanges negative ions for hydroxyl ions found in the anion resin. This exchange is based on the adsorption potential of the specific ion towards the resin bead. Resin beads are frequently spherical objects which have a network of fixed charges and are neutralized by a moveable ion.

The next process item in is the degasser. The degasser removes excess carbon di-oxide from the water. The final step is the mixed bed which polishes the water by removing the residual quantities of cations and anions left in the water. In the mixed bed ion exchange unit, ions are removed from the water by using either cation or anion resin.

The cation exchange resin bed unit contains strong acid cation exchanger in hydrogen form which converts all the salts present in feed water into acids. Alkaline salts are converted into carbonic acids (H2CO3) while the neutral salts are converted into mineral acids (HCl, H2SO4, HNO3). On exhaustion, the ion exchange resin is regenerated either by HCl or by H2SO4. Regeneration is carried out either in co-flow or counter-current-flow regeneration mode. Counter-current-flow regeneration mode is preferred since sodium slip from the unit can be reduced to less than 1 mg/l. Carbonic acid is a weak acid which dissociates into H2O and CO2. Degassing unit removes the carbon di-oxide by blowing air through a column packed with rasching rings (pieces of tube, around equal in length and diameter).

Anion exchange resin bed neutralizes the mineral acids present in the effluent from the cation unit. This unit contains a strong base anion exchange resin in hydroxyl form. It also removes the residual carbon di-oxide from the degassing unit and weakly ionized silica. The only impurity now present in treated water from the anion exchange resin bed is NaOH (sodium hydro-oxide) present because of the sodium slip. On exhaustion, the unit is regenerated by NaOH. Regeneration is carried out either in co-flow or counter-current-flow regeneration mode. Counter-current-flow regeneration mode is preferred since silica slip from the unit can be reduced to less than 0.2 mg/l as SiO2.

Mixed bed unit polishes the treated water from the anion exchange resin bed unit. It contains a mixture of a strong acid cation exchange unit and a strong base anion exchange unit, and produces high purity treated water with conductivity less than 1 micro-siemens per centimetre and silica less than 20 ppb (parts per billion). On exhaustion, the resin is separated by back-washing and regenerated separately.

In some plants ultra-filtration unit is also there. This unit removes colloidal silica present in water. Removal of colloidal silica results in reduced blow-down which in turn results in savings in fuel, water, and chemicals. Treated water is now highly pure and is stored in the demineralized water tank.

Polystyrene-di-vinyl benzene resins are used in majority of ion exchange applications. The resins which have ionic sites consisting of mobile charge of ‘SO-H’ radicals and mobile sodium cations (Na+) remove the cations present in water (hence called cation exchange resins). The resins which have tertiary or quaternary ammonium group as mobile cationic radicals and mobile chloride anions (Cl-) remove the anions including silicic and carbonic acids present in water (hence called anion exchange resins). Fig 3 shows ion exchange process.

Fig 3 Ion exchange process

Synthetic ion exchange resins of polystyrene group are normally used for deionisation purposes. For cation removal (e.g., calcium, magnesium, and sodium etc.) strongly acidic cation exchange resins are used: For anion removal (e.g., chloride, sulphate, and nitrate etc.) strongly basic anions exchange resins are used. The exchange reactions are RH+ + Ca(HCO3)2 / MgCl2 / Na2SO4 = R(Ca / Mg / Na) + H2CO3 / HCl / H2SO4, and ROH- + H2CO3 / HCl / H2SO4 = R(H2CO3 / HCl / H2SO4) + H2O.

After the exchange reactions, when resins are exhausted, they are recharged to work as before, by reactions with (i) dilute acids preferably HCl, in of cation exchange resins and (ii) dilute alkalis preferably NaOH, in case of anion exchange resins. The recharging resin reactions are R(Ca / Mg / Na) + HCl = RH+ + (CaCl2 / MgCl2 /NaCl), and R[H2CO3 / HCl / H2SO4] + NaOH = ROH- + [NaHCO3 / NaCl / Na2SO4].

In the case of demineralization, strong acids such as hydrochloric acid and sulphuric acid are fully dissociated and can supply H+ ions to replace the cations which have been exchanged. Almost the whole process takes place with OH- ions in the anion column. In practice, caustic soda (NaOH) is used and supplies the OH- ion to replace the anions sitting on the anion exchange resin beads at the end of the run. For the ion exchange to be efficient there is needed to be a difference in affinity between the ion in the resin and the ion in the solution. The resin needs to have a higher affinity for the ion in solution compared to the ion in the resin.

Ion exchange resins – Ion exchange resins are synthetic polymers which are insoluble in all solvents. They are capable of reacting like acids, bases, or salts. The resins, however, differ from acids, bases, and salts in one way. Only the cations (in cation exchange resins) or anions (in anion exchange resins) are free to take part in chemical reactions. Those exchange units in which the anionic portions are able to react are called anion exchange units whereas, the ones in which the cationic portions are able to react are called cation exchange units. In aqueous media and sometimes in non-aqueous media, cation exchange resins are able to exchange their cations with other cations and similarly anion exchange resins are able to exchange their anions with other anions.

The basic polymeric structure or solid support for ion exchange resins is called the matrix. The matrix is a three-dimensional co-polymer on which acidic or basic sites are situated. These acidic or basic sites are called functional groups or functionality of the ion exchange resin. The matrix along with these functional groups is called a functional polymer or ion exchange unit or simply resin. The mechanism of ion exchange is reversible and hence ion exchange resins can be repeatedly converted from one ionic form to another and reused. The process of converting the ion exchange resins to the desired ionic form at the end of cycle of its intended use is known as ‘regeneration’.

Ion Exchange resins can be broadly classified into following four categories namely (i) strong acid cation exchange resin, (ii) weak acidic cation exchange resin, (iii) strong base anion exchange resin, and (iv) weak base anion exchange resin.

All the major strong acid cation exchange resins involved in industrial water treatment applications have a chemical matrix consisting of styrene and di-vinyl-benzene. The functional groups are sulphonic acid radicals. These resins differ mainly in di-vinyl-benzene content or matrix structure (gel / iso-porous / macro-porous). For demineralization, the main resin which is used is the cation resin in hydrogen form.

Weak acid cation exchange resin is used primarily where there is a high degree of hardness and a high degree of alkalinity. This resin has the capacity of exchanging all cations associated with alkalinity to a much greater degree than strong acid cation exchange resins. A weak acid cation exchange resin consists of poly-acyrlic acid di-vinyl-benzene matrix with carboxylic functionality and gel structure. The major advantage of this resin is that it can be regenerated with stoichiometric quantities of regenerant, and is hence, much more efficient.

Strong base anion exchange resins can be divided into two categories called type 1 and type 2. The type 1 resin has the highest overall basicity, and, hence, gives better effluent quality. The type 2 resin also removes anionic constituents, but has as lower basicity, and hence, needs less caustic during the regeneration cycle. In general, a type 2 strong base anion exchange resin is recommended where silica effluent quality is not as critical, and also where a relatively high chloride and / or sulphate content prevail in the raw water.

In the field of deionization, weak base anion exchange resin is used primarily to remove strong acids such as hydrochloric and sulphuric acids. Unlike strong base anion exchange resin, weak base resin does not have the capability to remove bi-carbonate and silica. But it has a much higher capacity for the removal of chlorides and sulphates. The resin can be of gel or macro-porous structure. For treatment of water which does not present organic fouling problems, the gel type weak base resin is used. For treatment of water containing organic contaminants (humic and fulvic acids), macro-porous weak base anion resins are preferred.

The characteristics of ion exchange resins can be broadly classified into three categories namely (i) physical characteristics, (ii) chemical characteristics, and (iii) operating characteristics. Physical characteristics are particle size distribution (typically, ion exchange resins have effective size between 0.4 mm to 0.5 mm, uniformity coefficient below 1.7, and fines content below 1 %), cracks and pieces, density (anion exchange resins have typically a true density of 1.05 gram per millilitre, g/ml to 1.15 g/ml while cation exchange resins are heavier, having true density of around 1.2 g/ml to 1.3 g/ml), apparent density (bulk density), surface area (gel type or iso-porous resins have negligible total surface area of less than 0.01 square meter per gram, sq-m/g, macro-porous resins have very large surface area, at least 5 sq-m/g and frequently around 30 sq-m/g to 40 sq-m/g), and colour-throw (colour-throw is undesirable not only because it shows inferior quality of cation resin, but also since it has an adverse effect on the down-stream anion exchange bed) .

Chemical characteristics are water regain (defined as the quantity of water chemically bound with one gram of dry resin), total exchange capacity (TEC) (indicates the total number of exchange sites available in the resin), sodium chloride value or salt splitting capacity (a cation exchange resin gives the number of milli-equivalents of exchangeable hydrogen ions in the resin which are sufficiently acidic to split neutral salts), and strong base capacity (an anion exchange resin is a measure of the strong base, i.e., quaternary ammonium groups in it).

Operating characteristics are operating capacity (capacity obtained under actual plant operating conditions and is not proportional to the total capacity of the resin, but a function of several characteristics and operating parameters), resin fouling (contaminants fouling such as water fouling, iron / heavy metal fouling, organic fouling, oil fouling, and poly-electrolyte fouling), and precipitation.

Even though utmost care is normally taken for the minimization of the impurities at the source of water or the regenerants, still there can be a slippage of these down through the demineralization plant. In such cases, few treatments become necessary to restore the resin back to its near original state. If such maintenance is not done, it can result in poor treated water quality, reduced plant capacities and higher expenditure, for regeneration of the resins.

A resin which has become dirty can be cleaned by a thorough backwash given for an extended period. Open manhole backwashes given regularly also help. The resin bed is to be maintained in a clean condition. Air scouring of the bed is a further aid to cleaning and helps improve resin condition considerably when backwash alone does not help.


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