Resins

Water treatment The annual production of ion exchange resins has been estimated at 500 000 m3, of which at least 90% goes to industrial water treatment. The various end uses in this area will now be described.

Softening The removal of calcium and magnesium ions from water supplies is a requirement for many industries. Laundries, dye-houses and cleansing plants are examples of specific industries but the need can be generalized to many hot water circuits, heat exchangers and low pressure boilers. Strong sulfonate styrene gel cation resins, in sodium form with 8-12% cross-linking, are the usual choice, but macroporous resins may well be used when the process is demanding in terms of attrition, elevated temperatures or in the presence of oxidizing agents. An example of such an aggressive environment is in the modern ChlorAlkali membrane electrolysis cell for the production of chlorine gas and sodium hydroxide. This process also provides an example of the use of a chelating style resin (iminodiacetate, or aminophosphinate) to scavenge alkaline earth ions (see Table 1).

Dealkalization In dealkalization, weakly acidic car-boxylic resins, in hydrogen form, are used to meet the limits of calcium and magnesium concentrations needed in feed water to medium pressure boilers. Hydrogen ions released are neutralized by the HCO3~ and CO2 3 anions present to give carbon dioxide, which remains in solution as carbonic acid. The softening is only partial and sometimes a sodium resin treatment, as above, is added. There may be an ancillary need to attain some demineralization as well which would involve an additional column treatment.

Single-stage dealkalization finds wide application in the treatment of cooling water and water used in the food and drinks industries. Desalination is a deal-kalization process. Modern plants use membranes made from ion exchange resins in an electrodialysis cell.

Demineralization Demineralization involves the use of both cation and anion resins to produce 'de-ionized water'. This can be achieved by a two-stage process in which the raw water is first passed through a column containing a strong cation resin (H) form, and then through a strong anion resin (Type I or II). Some ion exchange plants use weak anion resins, and multistage processes or countercurrent variants are available as standard plant.

An alternative, and common, option is to use mixed beds that contain uniform mixtures of strong cation resins (H) and anion resins (OH). Variants of this approach are widely used to 'polish' water previously demineralized by strong cation/anion beds. Now the admixture of resins can be adjusted to cope with the expected load of residual ions from a specific process water, and so meet the heavy demands of water purity essential, for example, for high pressure steam production in power generation. This is the technology of 'condensate polishing' pervasive through this and similar industrial circumstances. It is appropriate to mention here the production of resins to meet the special needs of the nuclear industry. This manifests itself as 'nuclear grade' resins which are themselves of high purity to reduce potential problems that otherwise would arise from leaching of materials capable of being neutron-activated in reactor circuits. These add to the radiation fields in the reactor, create additional decontamination and waste disposal considerations, and do not help in the general aim to inhibit corrosion. The nuclear power industry constantly strives to reduce potential corrosion in its steam/water circuits. In pressurized water reactors with 'recirculating steam generators' some operators have a target of low ng levels of ionic impurities per kg water used. These reactors use additions to the primary coolant of (a) boric acid to help moderate the fission process and control corrosion and (b) lithium hydroxide, also to inhibit corrosion. They depend on a sophisticated use of resin beds to achieve the desired coolant water chemistry. In the semiconductor manufacturing industry the demands for 'ultra pure' water are even more stringent with targets in the range pg kg-1. (Note 'ultra pure water' is usually taken to mean water with less than Igkg-1.) Water used in the production of pharmaceuticals also involves the use of high purity water.

In both dealkalization and demineralization the choice between a gel or macroporous resin is again conditioned by the relative aggressiveness of the feed. In some instances additional factors such as high column pressure differentials and the need for fast kinetics will lead to the use of macroporous materials. Additional treatment of demineralized water with a sulfite-based exchanger removes oxygen via the following route:

Removal oforganics In this context 'organic' means the complex anions arising from decay products of organic matter and peaty soils. They are mainly derivatives of fulvic and humic acids and can be removed by employment of a macroporous strong anion resin. This was one of the main uses envisaged from the development of resins of this type. Typical waters requiring treatment contain a total organic carbon (TOC) of 2-20 mg of carbon per litre, which can be more than halved by resin exchange.

Nitrate removal The presence of nitrates in water intended as a potable supply, or for the food industry, is a major environmental concern, arising from the use of nitrate fertilizers. The suggested limit is 50 mg NO3 L_1, and the options open to attain this include microbiological, reverse electrodialysis using nitrate selective membranes, and traditional ion exchange methods.

Ion exchange is the cheapest and most reliable approach, not least because of the wealth of experience and established technology available. It makes use of nitrate-selective resins based on triethylam-monium as functional groups. It is thought that the selectivity may arise from a size/shape exclusion effect that encourages the uptake of the small, flat, nitrate anion in preference to the larger sulfate, and perhaps chloride, ions, with their spherical nature also being a factor.

Waste effluent treatment Prevention of the release into the environment of toxic metals arising from industrial processes has long been a useful application of ion exchange. Typical examples come in the metal finishing industries whereby wash solutions containing chromium and zinc, for example, can be rendered suitable for discharge. Many similar examples can be found in the photographic, paper and metal pickling industries. Some progress has been made in the employment of resins in the recovery of water from sewage treatment plants.

Another area of concern is in the treatment of aqueous nuclear wastes. The use of ion exchange offers the prospect of vastly reducing some problems by volume minimization of the waste form. Resins have high capacities for the trace quantities of hazardous isotopes arising from nuclear fuel production, reprocessing, reactor water circuits, decommissioning, and in 'pond' waters. The latter can be used as a major example where ion exchange materials can be used to scavenge the caesium and strontium radioisotopes that leak into the ponds in which spent fuel rods are stored prior to decladding and reprocessing (see Table 1).

In all nuclear applications the ultimate safe storage of the waste form is of prime importance. Previous practice has often been to store the highly radioactive resins in a concrete pit. Clearly this is undesirable, due to the limited radiation stability of the organic resins, and new approaches are now preferred, either by using inorganic exchangers or by encapsulating the spent resins in concrete. Even encapsulation should be regarded as temporary in the timescales recommended for nuclear waste disposal. The search for suitable inorganic materials has only been realized in a limited area because of reasons that will be considered later. The resin manufacturers are continuing to synthesize nuclear resins, and seek to provide products that can take up radioisotopes with high degrees of selectivity, often by chelating action. An example of this area of work is in the potential use of novel strong base polyvinylpyridine anion resins to remove americium and technetium from nuclear wastes.

Metal recovery Resins often form part of metal recovery processes, both in primary ore processing and from process waste streams. The second of these is becoming of increasing economic importance with the need to create efficient metal 'winning' from low grades ores, tailings, mine wastewater and

Table 5 Metals recovered and purified by ion exchange

Uranium Thorium Rare earths

Plutonium (and other trans-uranics such as neptunium and americium) Gold Silver

Platinum metals

Copper

Cobalt

Nickel

Zinc

Chromium

Rhenium

Molybdenum mineral dumps. Ion exchange competes with liquid-liquid extraction in these areas, with varying success, and can be used in combination with this process in some instances. Macroporous resins are popular for these applications. Table 5 provides a list of metals that can be recovered on a commercial basis by ion exchange.

Note should be made of the essential role played by resins in aiding the separation of uranium from its ore in nuclear fuel production, whereby uranyl sulfate is loaded onto anion resins from which it is leached prior to solvent extraction to complete the separation process. Solvent extraction is the major separation technique in the process and the same is true of reprocessing, but both anion and cation resins are an essential part of the method whereby purified uranium and plutonium are obtained.

Other applications of ion exchange resins The reader is reminded of the wide use of resins in catalysis, and in the purification of antibiotics, vitamins, nucleotides, amino acids, proteins, enzymes and viruses that have been excluded from this review. One application in the purification of natural substances is the use of a finely sized cation resin to replace sodium and potassium ions by magnesium in sugar refining. Sodium and potassium promote the deleterious formation of molasses that has to be discarded.

Historically the best studied area of ion exchange application has been the use of exchange materials to perform separations to aid quantitative analysis. Originally, in the majority of cases the intent was either to remove interfering ions or to scavenge trace ions onto an exchanger so as to preconcentrate sufficient material for analysis. This work forms the basis of ion exchange chromatography, which has evolved into one of the most useful analytical techniques ever developed, namely high performance ion chromato-graphy (HPIC). In HPIC low capacity ion exchange materials are used in pellicular form on stable micro-spheres. Fast uptake of trace quantities of ions is guaranteed and differential elution provides an adequate separation so that individual ions can be recognized by their diagnostic column residence times. This presupposes that a suitable detector can monitor the column effluent to detect the ions in the eluent. Conductivity changes prove to be adequate for most purposes.

An important stage in the development of the technique was the use of suppressor columns that were able to remove the ions present as a consequence of the use of eluents by retaining them on suitable resins, thus greatly improving the detection sensitivity. Highly sophisticated instruments are now able to revolutionize the quantitative trace analysis of cations and anions, both organic and inorganic. The method is highly reproducible, rapid, and able to carry out routine analyses on a diverse range of analytes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment