Lime-soda softening

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) ... 

One of the main problems associated with lime or lime-soda softening is the disposal of the sludge. Depending upon the ratio of calcium to magnesium removed and upon the amount of soda ash used, the sludge produced is 2.8—3.6 times the weight of the lime added. The principal methods of... 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

The lime-soda softening process reduces the temporary hardness (icarbonate hardness) content of the RW, and often some of the permanent hardness (noncarbonate hardness) and some silica is also removed. 

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

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. 

The most widely used techniques for removing dissolved inorganic solids are boiling, addition of washing soda, lime-soda softening, complexation, sodium ion exchange, demineralization, reverse osmosis, electrodialysis, adsorption onto suspended solids, and aeration. 

Lime-soda softening. Lime-soda softening involves removal of the temporary hardness by adding the calculated amount of hydrated lime (Section 11.1) ... 

So, we need to add Ca(OH)2 equivalent to the temporary hardness plus the magnesium hardness (which is just the total hardness, if noncarbonate Ca2+ is absent), and Na2C03 equivalent to the permanent (i.e., total minus temporary) hardness. Clearly, if lime-soda softening is to be effective, accurate analyses for Ca2+, Mg2+, and temporary hardness are needed, and the lime must be accurately weighed out accordingly. 

Lime and lime-soda softening (high rate—upflow units) 1.5 2... 

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). 

Primary treatment processes, such as clarification, lime-soda softening, ion-exchange, and RO... 

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 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... 

Precipitation and dissolution phenomena are extremely important in both natural waters and water treatment processes. Dissolution of minerals is a prime factor in determining the chemical composition of natural waters. Natural water chemical composition can be altered by precipitation of minerals and the subsequent sedimentation of these solids from supersaturated solutions. Water and wastewater treatment processes such as lime-soda softening, iron removal, coagulation with hydrolyzing metal salts, and phosphate precipitation are based on precipitation phenomena. 

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