Sodium zeolite softening

Sodium softeners are used to treated RO influent water to remove soluble hardness (calcium, magnesium, barium, and strontium) that can form scale on RO membranes. Once known as sodium zeolite softeners, zeolites have been replaced with synthetic plastic resin beads. For sodium softeners, these resin beads are strongly acidic cation (SAC) polystyrene resin in the sodium form. The active group is benzene sulfonic acid, in the sodium, not free acid, form. Figure 8.12 shows styrene-divinylbenzene gel cation resin. Equation 8.4 shows the softening reaction for calcium exchange ... 

SODIUM ZEOLITE SOFTENING - The process of removing scale forming ions of calcium and magnesium and replacing them with the equivalent amount of sodium ions. 

Another modification of the process hot-lime zeolite softening process (Fig. 11.12) is used to increase silica reduction and carried out at 98°C. Calcium carbonate is filtered from the solution. To reduce silica. Mg is precipitated as Mg(OH)z which acts as an absorbent for silica. The residual hardness is removed by sodium zeolite softener. The increased temperature improves the potential of the softener to exchange sodium for hardness ions. 

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

Hoi-Lime Zeolite Softening. In this process hydrated lime is employed to react with the bicarbonate alkalinity of the raw water. The precipitate is calcium carbonate and is filtered from the solution. To reduce silica, the natural magnesium of the raw supply can be precipitated as magnesium hydroxide, which acts as a natural absorbent fur silica. These reactions are carried out in a vat or tank that is located just head of the zeolite softener tank. The effluent from this tank is filtered and then introduced into the zeolite softener. There is always some residual hardness leakage from the hot-process softener to be removed in the final zeolite process. The hot lime process operates at about 220T (I04°C). At this temperature the potential for the exchange of sodium for hardness ions is greater than at ambient temperature, and the result is a lower hardness effluent than is achieved at ambient temperatures. This system is shown schematically in Fig. 2. 

If, therefore, a hard water containing a dissolved calcium or magnesium salt is passed through a layer of sodium zeolite, the calcium is abstracted and the corresponding sodium salt passes into solution. In this way the water is readily softened. 

A number of lime-softening treatments were used in the past, but these have given way to more sophisticated treatments. Probably the most common for boilers up to 2.8 to 4.0 MPa is Zeolite softening. In this treatment, a sodium salt of a long-chain polymeric organic molecule comprises the ion exchange bed. As the feedwater passes... 

Waters of intermediate hardness frequently contain fair amounts of other constituents and there is often a tendency for the scale to be loosely attached, permitting corrosion to occur irregularly underneath. In most waters the bicarbonate content is less than the hardness, but a few natural waters are known where the reverse is the case. These waters have been partially softened by the zeolite process which occurs underground, and then contain sodium bicarbonate which, together with the high concentration of chloride and other minerals, may accelerate attack. 

Some zeolites have the ability to exchange sodium for calcium and thereby function as water softeners by removing Ca2+. After the zeolite has become saturated with Ca2+, it can be renewed by washing it in a concentrated NaCl solution to restore the Na+ ions. Zeolites are also used to prepare ion exchange resins, as molecular sieves and as catalysts. 

The ion exchange technique uses either natural zeolites or synthetic resins. It is a familiar process used in water softening where a calcium ion is exchanged for a sodium ion. 

Sodium ion exchange. Sodium ion exchange on zeolites (Section 7.3) or on synthetic organic cation-exchange resins such as Dowex-50 (a sulfonated polystyrene Fig. 14.1), in most circumstances, is superior to the above softening methods.13 The exchange process favors binding of Ca2+ or Mg2+ over Na+ in the solid resin phase ... 

Sodium Cation Exchanger (Zeolite) Process. This is the most widely used water-softening process in industrial, commercial, institutional and household applications. Hard water is softened by flowing it, usually downward, through a bed (2 feet to over 8 feet in thickness) of a granular... 

Hot Lime Zeolite-Split Stream Softening. Many raw waters softened by the first two processes would contain more sodium bicarbonate than is acceptable for boder feedwater purposes. Sodium bicarbonate will decompose in (lie boiler water to give caustic soda. Caustic soda in high concentrations is corrosive and promotes foaming. The American Boiler Manufacturers Association has adopted the standard that the alkalinity content should not exceed 20% of the total solids of the boiler water. Split stream softening provides a means for reducing the alkalinity content. 

Chemical analysis of scale deposits present on the surface of the failed clamp by X-ray diffraction revealed the presence of predominantly sodium iron oxide, sodium carbonate sodium chloride ( 10%), iron oxide and iron sulfide. The scale composition was consistent with the evaporated residue from the 80% quality steam, which had been leaking from the joint prior to the failure. The high sodium concentration in the scale was attributed to the zeolite ion exchange system used to soften the boiler feedwater, while the chlorides and sulfides were naturally present in the feedwater. 

Detergency. The largest-scale industrial production of zeolites is that devoted to the production of the sodium form of zeolite A (LTA) for use in the detergent industry. This uses currently in excess of 0.5 million metric toimes per annum worldwide. Zeolite A (NaA) is added to washing powders and other household detergents or cleansing powders as a water softener (described by the detergent industry as a builder). It... 

In 1977, the German firm Hankel patented the use of synthetic zeolites as a partial replacement for phosphates. The sodium aluminum silicates as zeolites have a particular lattice structure capable of absorbing heavy metal cations through ion exchange process. The role of zeolites that were added to TPS was to soften water by rapid reaction with calcium at normal temperature [4]. 

Referring to other references if necessary, explain how zeolites containing sodium ions can be used to soften water. 

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