Lime soda process

In this process lime and sodium carbonate are added to precipitate the calcium and magnesium salts as temporary hardness. 

Ca(HC03)3 + Ca(OH)3 CaCOj + 2Hp Mg(HC03)3 + Ca(OH)3 - MgCOH) + 2CaC03 + 2HjO For permanent hardness the reactions are  

The softened water is usually slightly alkaline with 1-4 residual hardness. 

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The lime—soda process is practiced mainly in isolated areas in some process operations, in the Kraft recovery process, and in the production of alurnina. It is not as efficient a route as electrolytic production. 

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

Lime and Lime-Soda Processes. The first softening plant in the early 1900s used the lime softening process with fill and draw units. Later, continuous-treatment units, which greatiy increased the amount of water that could be treated in a faciUty of given size, were developed. More than 1000 municipahties soften water. Most are in the Midwest and in Florida. However, concern for the adverse effect of soft water on cardiovascular disease (CVD) may limit the number of plants that introduce softening. 

The lime or lime—soda process results in the precipitation of calcium as calcium carbonate and magnesium as magnesium hydroxide. The solubiUties of these compounds are shown in Figure 4 as functions of pH. When lime is used alone, only the carbonate hardness is reduced. The carbonate hardness is present as calcium or magnesium bicarbonate. The additional use of soda ash can reduce the noncarbonate hardness by providing additional carbonate ion. The reactions involved in the various steps of the process are Hsted below ... 

Porter-Clark The original name for the cold lime-soda process. A water-softening process using sodium carbonate and calcium hydroxide. It removes the non-carbonate, as well as the bicarbonate, hardness. Developed by J. H. Porter. See Clark. 

Lime slurry, 15 21 Lime soda process, 22 795 Lime-soda sintering process, for alumina, 2 354-355... 

It is not known whether this process would be competitive with the lime/soda precipitation process for primary treatment. For new plants, the main issue is capital cost. The lime/soda process is relatively inexpensive for large-scale plants. For small plants, the BDFI system may be competitive, particularly for packaged turn-key plants, if installation and start-up costs are taken into consideration. The BDH system is very compact and can be quickly started and easily operated with minimal labour requirements. 

Caustic soda is manufactured by the lime-soda process. 

In water at ambient temperature, the solubilities of MgC03, CaCOs, Mg(OH)2, and Ca(OH)2 are about 106, 15.3, 9.0, and 1600 mg kg-1, respectively. Show that stoichiometric application of the lime-soda process should leave a residual hardness of some 31 ppm CaC03 equivalent. 

Lime-soda process. Combination of the use of slaked lime for the removal of temporary hardness and sodium carbonate for the removal of permanent hardness constitutes the lime-soda process for softening natural waters. This method is commonly used in municipal water-treatment plants and is a cheap and yet fairly effective process. If sufficient time can be allowed, the insoluble carbonates may be permitted to settle out in settling basins, or they may be more rapidly removed by means of filters. Frequently, iron or aluminum salts are added, and these hydrolyze to form gelatinous precipitates of ferric or aluminum hydroxides. As these precipitates slowly settle, they carry with them the insoluble normal carbonates, as well as any other suspended matter such as sand, clay, or organic matter which is sometimes slow in settling otherwise. 

This metaphosphate is used in water treatment in the form of sodium hexametaphosphate [(NaP03)s], which is known by the trade name Calgon, to prevent the precipitation of small quantities of calcium and magnesium salts not removed in the lime-soda process and to dissolve scale that has formed in boilers, water pipes, and so forth. 

The various commercial lime-soda processes for softening water are based on the foregoing principles, and differ essentially only in their mechanical details. Their underlying principle is more commonly applied than any other for the purification of water for boiler-feed purposes. 

In the lime-soda process, which is part of the chlor-alkali industry, converting Na2C03 to NaOH or vice versa... 

Slaked lime is also widely used in water treatment plants for softening hard water, which involves the removal of ions such as Ca2+ and Mg2+, ions that hamper the action of detergents. The softening method most often used in water treatment plants is the lime-soda process, in which lime (CaO) and soda ash (Na2C03) are added to the water. As we will see in more detail later in this chapter, the C032- ion from soda ash reacts with water to produce the HC03 ion. When the lime is added to the hard water, it forms slaked lime,... 

Lime-soda process a water-softening method in which lime and soda ash are added to water to remove calcium and magnesium ions by precipitation. (7.6)... 

As shown by the various chemical reactions above, the chemicals soda ash and lime may be used for the removal of hardness caused by calcium and magnesium. Thus, the lime-soda process is used. This process, as mentioned, uses lime (CaO) and soda ash (Na2C03). As the name of the process implies, two possible sets of chemical reactions are involved the reactions of lime and the reactions of soda ash. To understand more fully what really is happening in the process, it is important to discuss these chemical reactions. Let us begin by discussing the lime reactions. 

Calculation of Stoichiometric Lime Required IN THE Lime-Soda Process... 

The solids produced in water softening plants, if not put to use, pose a disposal problem. Conceptually, because of their basic nature, they can be used in absorption towers that use alkaline solutions to scrub acidic gas effluents. These solids, as far as the lime-soda process is concerned, come from the solids produced in the removal of (1) the carbonate hardness and (2) the noncarbonate hardness. 

Example 10.4 For the raw water of Example 10.2, the lime-soda process in the split treatment mode is used to remove the total hardness to 120 mg/L as CaC03 containing magnesium hardness of 30 mg/L as CaCOs. Calculate the chemical requirements in the first stage. 

Lime-soda process—A process of removing hardness ions that uses lime and soda ash. 

Amount of soda ash required in the lime-soda process Mass of soda ash needed to precipitate the noncarbonate calcium hardness removed... 

Mass of solids produced in the lime-soda process Total mass of noncarbonate hardness of calcium Total mass of Ca(HC03)2... 

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