Demineralization

Ton-exchange systems vary from simple one-column units, as used in water softening, to numerous arrays of cation and anion exchangers which are dependent upon the appHcation, quaHty of effluent required, and design parameters. An Hlustration of some of these systems, as used in the production of deionized (demineralized) water, is presented in Figure 7. 

Demineralizers are often used to treat raw makeup water or condensate where high purity is required, such as in large central station boHers that operate at high steam pressures. Demineralizers employ a combination of cation and anion exchange to remove additional material, including sodium and ammonium cations. VirtuaHy aH salt anions, such as bicarbonate, sulfate, and chloride, are removed and replaced by hydroxide ions in the demineralizer. 

The manufacture of silver nitrate for the preparation of photographic emulsions requires silver of very high purity. At the Eastman Kodak Company, the principal U.S. producer of silver nitrate, 99.95% pure silver bars are dissolved in 67% nitric acid in three tanks coimected in parallel. Excess nitric acid is removed from the resulting solution, which contains 60—65% silver nitrate, and the solution is filtered. This solution is evaporated until its silver nitrate concentration is 84%. It is then cooled to prepare the first crop of crystals. The mother Hquor is purified by the addition of silver oxide and returned to the initial stages of the process. The cmde silver nitrate is centrifuged and recrystallized from hot, demineralized water. Equipment used in this process is made of ANSI 310 stainless steel (16). 

The principal commercial use of stannous fluoride is in toothpaste formulations and other dental preparations, eg, topical solutions, mouthwash, chewing gum, etc, for preventing demineralization of teeth (21—23) (see Dentifrices). 

SAC resins can neutralize strong bases and convert neutral salts into their corresponding acids. SBA resins can neutralize strong acids and convert neutral salts into their corresponding bases. These resins are used in most softening and hiU demineralization appHcations. WAC and WBA resins are able to neutralize strong bases and acids, respectively. These resins are used for dealkalization, partial demineralization, or (in combination with strong resins) hiU demineralization. 

Demineralization. Softening alone is insufficient for most high-pressure boiler feed waters and for many process streams, especially those used in the manufacture of electronics equipment. In addition to the removal of hardness, these processes require removal of all dissolved soHds, such as sodium, sihca, alkalinity, and the mineral anions (Cl , and NO ). 

Demineraliza tion of water is the removal of essentially all inorganic salts by ion exchange. In this process, strong acid cation resin in the hydrogen form converts dissolved salts into their corresponding acids, and strong base anion resin in the hydroxide form removes these acids. Demineralization produces water similar in quaHty to distillation at a lower cost for most fresh waters. 

The standard cation—anion process has been modified in many systems to reduce the use of cosdy regenerants and the production of waste. Modifications include the use of decarbonators, weak acid and weak base resins. Several different approaches to demineralization using these processes are shown in Figure 1. 

The quahty of feed water required depends on boiler operating pressure, design, heat transfer rates, and steam use. Most boiler systems have sodium zeohte softened or demineralized makeup water. Feed-water hardness usually ranges from 0.01 to 2.0 ppm, but even water of this purity does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary. 

High Pressure Boiler Water Treatment. High pressure boilers usually have feed water composed of demineralized makeup water and a high percentage of condensate returns. Because of these conditions, high pressure boilers are prone to caustic and acid attack. Low pressure boilers that use dernineralized water and condensate as feed water are also susceptible to caustic and acid attack. 

The success of EDR ia water demineralization has appareatly resulted from its greater tolerance of particulate and fouling matter compared to reverse osmosis greater forgivingness of process upsets greater tolerance for unskilled operators simplicity ia desiga and constmction of EDR stacks compared to reverse osmosis modules the abiHty to iaspect, clean, or replace one membrane at a time the existence of a comprehensive global sales and... 

Other paved areas Secondary water treatment—deionization, demineralization, pH and... 

Shell side Demineralized water, sodium nitrite, 90-120°F (32-49°C), pressure 50-75 psig (345-517 kPa)... 

It would not be expected that crevice corrosion could easily occur in demineralized water. However, this exchanger was subject to extended outages with no lay-up procedures. Under such conditions, crevice attack is more likely. 

The water used should be demineralized. The use of nondemineralized water would harm the turbine. 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

Suifate (S04>-2 Adds to solids content of water, but, in itself, is not usually significant combines with calcium to form calcium sulfate scale Demineralization, distillation, reverse osmosis, electrodialysis... 

Siiica S1O2 Scale in boilers and cooling water systems Insoluble turbine blade deposits due to silica vaporization Hot process removal with magnesium salts adsorption by highly basic anion exchange resins, in conjunction with demineralization distillation... 

Ammonia NH3 Corrosion of copper and zinc alloys by formation of complex soluble ion Cation exchange with hydrogen zeolite, chlorination, deaeration, mixed-bed demineralization... 

Conductivity Expressed as micromhos, specific conductance Conductivity is the result of ionizable solids in solution high conductivity can increase the corrosive characteristics of a water Any process which decreases dissolved solids content will decrease conductivity examples are demineralization, lime softening... 

Dissolved Solids None Dissolved solids is measure of total amount of dissolved matter, determined by evaporation high concentrations of dissolved solids are objectionable because of process interference and as a cause of foaming in boilers Various softening processes, such as lime softening and cation exchange by hydrogen zeolite, will reduce dissolved, solids demineralization distillation reverse osmosis electrodialysis... 

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