Lime-soda water softening

Fig. 8.7 Vertical lime-soda water-softening plant...

Treated Water Quality. Predicted analyses of a typical raw water treated by various lime and lime—soda softening processes are presented in Table 3. Treatment by lime converts the sodium bicarbonate in the raw water to sodium carbonate as follows ... 

Factor Raw water Removal of calcium alkalinity cold-lime Lime—soda softening (cold) Lime—soda softening (hot) Lime softening (hot) ... 

The two principal methods of softening water for municipal purposes are addition of lime or lime-soda and ion exchange. The choice method depends upon such factors as the raw-water quaUty, the local cost of the softening chemicals, and means of disposing of waste streams. 

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. 

Chelants should not be used downstream of hot process or hot lime soda softening MU water treatment. 

Maximum values of specific conductance are often not achievable without exceeding maximum T alkalinity values, especially in boilers below 900 psig (6.21 MPa) with greater than 20.0% MU water whose alkalinity is >20% of TDS naturally or after pretreatment by lime-soda or sodium cycle ion exchange softening. Actual permissible conductance values to achieve any desired steam purity must be established for each case by careful steam purity measurements. The relationship between conductance and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values. 

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. 

In modern high-pressure systems, blowdown water is normally of better quality than the water supply. This is because plant intake water is treated using clarification, filtration, lime/lime soda softening, ion exchange, evaporation, and in a few cases reverse osmosis to produce makeup for the boiler feedwater. The high-quality blowdown water is often reused within the plant for cooling water makeup or it is recycled through the water treatment and used as boiler feedwater. 

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. 

All cooling water treatment programs, whether designed in-house or via a water treatment service company, continue to focus on the minimization of hard water crystalline scales and sludges in the system as a major criteria for success. Program techniques employed are either pretreatment processes, such as lime-soda softening or ion exchange, the use of sulfuric acid or polymer-based chemicals that operate in an alkaline environment, or combinations of some or all of these processes. 

Although the basic processes using lime (calcium hydroxide) and soda ash (sodium carbonate) to soften water by precipitating insoluble hardness salts have been known for 200 years, they are still relevant today. Many large industrial cooling systems around the world use lime-soda-softened makeup water, especially in some newly industrializing countries, where local water supplies may be particularly hard and the economics favor this process rather than, say, ion exchange. 

The fundamental reason for using lime-soda softening processes is to reduce the temporary hardness (carbonate hardness) content of the raw water in order to minimize risks of carbonate scaling in the user s cooling systems. Often some of the permanent hardness (noncarbonate hardness) is also removed, as is some silica. The principal temporary hardness salt is calcium bicarbonate, formed by dissolution of limestone (calcium carbonate) by water containing dissolved carbon dioxide. 

When extremely poor control is exercised over lime-soda softening, the pH of the treated water can often rise and fall, and may rise to exceed pH 11.0 to 11.5 on occasion. This is clearly unacceptable and will dramatically increase the risk of carbonate fouling, thus tending to make subsequent treatment and control of the cooling system extremely difficult (on the premise that chemical and services treatment programs can be developed for almost any quality of cooling system makeup water, as long as the water quality remains reasonably consistent). 

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. 

Cold lime and lime-soda softening is conducted in a solids-contact clarifier (see Chapter 8.1.1.). Cold lime softeners are typically followed by filtration and sodium softening to remove the balance of hardness from the water. 

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. 

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)... 

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. 

The principle of a simple continuous lime-soda softener is shown in Fig. 8.4 to 8.6. There are four essential features a chemical measuring device, a reaction chamber, a settling tank, and a filter. The hard water enters through a feed pipe into one of the compartments of the tipping bucket (Fig. 8.4). When the compartment is full the centre of gravity is... 

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