Ion exchange for water softening

The main uses of natural zeolite as adsorbent are as drying agents, deodorants, adsorbents for air separation, ion exchangers for water purification especially for removing ammonium ion and heavy metal ions and for water softening, soil upgrading and so on. 

Water is softened by removing calcium and magnesium ions from hard water in exchange for sodium ions at sites on cation-exchange resin. Water softeners typically use a gel polystyrene sulfonate cation-exchange resin regenerated with a 10% salt brine solution (25). 

Precipitation softening processes are used to reduce raw water hardness, alkalinity, siHca, and other constituents. This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. Waters with moderate to high hardness and alkalinity concentrations (150—500 ppm as CaCO ) are often treated in this fashion. 

Supply of MU water for a medium-pressure (450 psig) WT boiler, from a surface water source with very variable suspended solids and hardness (sugar refinery, South Africa). The process used is a. carbonate removal using hot-lime precipitation softening coupled with silica adsorption by magnesia addition b. clarification in anthracite filters and c. cation ion-exchange for the balance of hardness removal. 

A volume measure of the permitted flow rate of water through ion exchange resins and sometimes other media, such as sand. Typically, ion-exchange resins used for water softening will tolerate 8 to 40 BV/hr. 

Ion exchange is effective for the removal of cationic or anionic heavy metal contaminants. It can also be used for water softening. Ion-exchange resins are usually regenerable with salt.65... 

As early as the middle of the nineteenth century it was observed that certain naturally occurring substances could extract and concentrate ions from their environment.341 342 However, 50 years elapsed before ion exchange, as the process became known, was used on an industrial scale for water-softening,343 and another 50 years before ion exchange came of age as a metallurgical process with the advent of the atomic age and the worldwide demand for uranium that accompanied it. 

The most frequently used resin type for water softening is the Na type. A typical ion-exchange reaction is described by Eq. (29). Owing to the presence of NaHCOg, treated effluent pH is always weakly caustic. The regeneration of resin performed with sodium chloride solution is described by Eq. (30) ... 

Conventional softening treatment plant may not remove the impurities in water to the recommended permissible level. Demineralised or reverse osmosis technique is needed for removal of TDS from water but is costly. Water purification in the process house normally consists of floculation, sedimentation, filtration and ion-exchange. Hard water is normally softened using one or combination of methods the details of each process is given in many text books. 

Ion exchange is becoming used more extensively in water and wastewater treatment. Ion exchange is primarily used for water softening (Ca + and Mg +) and for water demineralization. For water softening, Ca + and Mg + are replaced with Na to prevent scale formation. For complete water demineralization, all cations and anions are replaced with H+ and OH , respectively. This approach is also used in wastewater treatment. It is important to note that not all dissolved ions are removed equally and/or completely. Ions that are low on the selectivity preference order (described in Section 8.5) may not be completely removed. 

The first truly successful use of ion exchange was in 1905, when R. Gans used a natural zeolite for water softening (Ger. Patent 174,097, 1906). 

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