Sodium softeners calcium exchange

Sodium cycle softening (base-exchange softening) is used primarily to remove the risk of calcium- and magnesium-based crystalline scale formation and deposition. The general reaction is as shown in Figure 9.2. 

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. 

Scale control is complex the particular procedure depends on the composition of the feed water. Fortunately, calcium carbonate scale, by far the most common problem, is easily controlled by acidifying the feed or by using an ion exchange water softener to exchange calcium for sodium. Alternatively, an antisealant chemical such as sodium hexametaphosphate can be added. Antisealants interfere with the precipitation of the insoluble salt and maintain the salt in solution even when the solubility limit is exceeded. Polymeric antisealants may also be used, sometimes in combination with a dispersant to break up any floes that occur. 

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 ... 

Figure 8.13 Sodium softening reaction for calcium exchange.

In operation, metal ions (M ) in solution are preferentially bound to the insoluble matrix with the concomitant release of protons or another cation (M). The toxic metal ion (M ) is effectively held within the matrix. The most common examples of cation exchange are water-softening resins. Hard water (containing Ca ) can be treated with an ion-exchange matrix containing sodium ions. The sodium ions are exchanged with the calcium ions, with the resultant removal of calcium from solution (see Figure 14.3). 

To avoid these undesirable effects, some people install water softeners in their homes. These devices are charged with sodium ions, usually from sodium chloride, that exchange with the calcium and magnesium ions in hard water. Sodium ions do not form scaly deposits as calcium and magnesium do, and they do not react with soap. However, sodium does increase the risk of high blood pressure and must therefore be avoided by those who have high blood pressure or heart problems. 

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. 

Hard water contains metal ions, such as iron, calcium, manganese, and magnesium, which can form water-insoluble salts when used with cleaning solutions and leave a residue when evaporated. Soft water is water that is relatively free of metal ions that form water-insoluble salts. A type of soft water is produced in a water softener by exchanging the ions that can form insoluble salts with sodium ions from sodium chloride (NaCl). The NaCl is water-soluble however, it will leave a residue when the water is evaporated. After using soft water for rinsing, the surface should be rinsed with pure or ultrapure water for the best residue-free surface. 

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). 

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

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