UNITIV WATER AND ITS TREATMENT HARDNESS OF WATER

UNIT-IV WATER AND ITS TREATMENT

HARDNESS OF WATER Hardness is the property of water which prevents lather formation with soap. This is due to the presence of certain salts of Ca and Mg. These dissolved Ca and Mg ions do not produce lather but form insoluble white ppt. 2 C 17 H 35 COONa + Ca. Cl 2→(C 17 H 35 COO)2 Ca + 2 Na. Cl Other metal ions like Fe 3+, Al 3+, Mn 2+ also contribute to hardness but these ions are present in natural water only in traces. However the hardness of a sample of water is usually taken as a measure of its Ca 2+, Mg 2+ ions content. Water can be classified as: HARD WATER Thus water which does not produce a ready and permanent lather with soap solution is called hard water. Eg. Sea water, River water etc. SOFT WATER Water and which produces a ready and permanent lather with soap solution is called soft water. Eg. Distilled water, Rain water etc.

TYPES OF HARDNESS 1. Temporary Hardness It is due to the presence of dissolved bi-carbonates of Ca, Mg and other heavy metals. Presence of bicarbonates causes alkalinity to water. Temporary hardness can be removed by simple boiling of water. 2. Permanent Hardness It is due to the presence of dissolved chlorides and sulphates of Ca, Mg and other heavy metals it can not be removed by simple boiling, chemical treatment is required. DISADVANTAGES OF USING HARD WATER A. In domestic purpose i. Washing and bathing When hard water is used it does not lather freely with soap. Only after all the Ca and Mg ions are removed from water as insoluble salts, lather will be produced. So a lot of soap will be wasted. ii. Drinking and Cooking Hard water causes bad effect on the digestive system. The possibility of formation of calcium oxalate stones in the urinary tracts is also increased. More fact and time are required since the boiling point of water is increased due to the presence of dissolved salts.

B. In industrial purposes 1. Textile Industry Consider quantities of soap are wasted during the washing of cloth. Moreover the insoluble salts of Ca and Mg may adhere to the fabric. 2. 2. Paper industry Ca 2+, Mg 2+ and Fe 2+ ions may react to the chemical used and affect the smoothness and colour of paper. 3. Sugar Industry Dissolved salts cause problems in the Crystallisation of sugar. 4. Dyeing Industry The dissolved salts may react with the functional groups present in dyes causing precipitation of dye or changing the colour. 5. Alcoholic distillaries Alkaline water tends to utilize the fruit acids and changes the taste of beverage. 6. Dairies and Pharmaceuticals These industries require highly soft water as salt present in hard water may react with chemicals to form undesirable products. C. In steam generation boilers Most of the water used for industrial purpose is in the form of steam. Boilers are used for steam generation. If hard water is used in boilers, the following boiler troubles may arise.

SCALE AND SLUDGE FORMATION IN BOILERS As water evaporates continuously in boilers, the concentration of dissolved salts increases. When their concentration exceeds the saturation point they get precipitated on the inner walls of the boilers. If the ppt formed is soft, loose and non-adherent it is called sludge. If the ppt formed is hard, thick and strongly adhering on the inner walls of the boiler, it is called scale. Sludges are formed by substances like Mg. Cl 2, Mg. CO 3, Mg. SO 4 and Ca. Cl 2 which have greater solubilities in hot water than in cold water. Sludges are generally formed at the colder parts of the boiler. They can be easily removed by scrapping off with a wire brush. Scales are the firmly sticking hard substances which cannot be easily removed. Scales are the main source of boiler troubles. Disadvantages of sludge formation 1. Heat loss because of poor conductance of sludge. 2. Disturbs the proper working of boiler. 3. Deposits on the colder parts of the boiler like bend, joint etc. Disadvantages of scale formation 1. Waste of fuel 2. Lowering of boiler safety 3. Decrease in efficiency 4. Boiler explosion

CAUSTIC EMBRITTLEMENT Caustic embrittlement is a type of boiler corrosions caused by using highly alkaline water in the boiler and also due to stress. Water softened by lime soda process may contain Na. OH which is formed by the hydrolysis of Na 2 CO 3 + H 2 O→ 2 Na. OH + CO 2 The Na. OH containing water flows through small pores in the stressed areas like bends, joints hair-cracks etc. When water evaporates the concentration of Na. OH increases and it corrodes the surrounding area by forming sodium ferrate. This causes embrittlement of boiler parts particularly stressed parts causing even failure of the boiler.

WATER SOFTENING The process whereby we remove or reduce the hardness of water is termed as softening of water. It is very essential process since hard water is unsuitable for domestic as well as industrial use. The hardness causing salts can be removed from water by following two ways. a. Internal treatment The internal treatment of boiler feed water refers to the conditioning of water in the boiler itself by the addition of chemicals. The following conditioning methods are used in the internal treatment. Colloidal, phosphate, calgon and carbonate conditioning b. External treatment The external treatment of water is carried out before its entry in to the boiler. This treatment prevents boiler problems. It can be done by lime soda process, zeolite or ion exchange process.

i. Lime – Soda process The basic principle behind this method is the chemical conversion of all the soluble hardness causing impurities in to insoluble precipitates which may be removed by settling and filtration. This method consisted of simultaneous addition of calculated quantities of both lime and soda in to hard water followed by removal of the precipitates. Both temporary and permanent hardness can be removed by this process. Ca(OH)2+CO 2→Ca. CO 3↓+H 2 O Ca(OH)2+Ca(HCO 3)2→ 2 Ca. CO 3↓+2 H 2 O Ca(OH)2+Mg(HCO 3)2→Mg. CO 3+Ca. CO 3↓+2 H 2 O Ca(OH)2+Mg. CO 3→Ca. CO 3↓+Mg(OH)2↓ Ca(OH)2+Mg. SO 4→Ca. SO 4+Mg(OH)2↓ Na 2 CO 3 + Ca. SO 4 → Na 2 SO 4 + Ca. CO 3↓

II. Zeolite Process Zeolite are hydrated sodium alumino siticates Na 2 Al 2 Si 2 O 8. x. H 2 O. They work as water softness by replacing the calcium and magnesium ions in water with the sodium ions in zeolite. In this process hard water is allowed to pass through a bed of zeolite at a specified rate. Then the sodium ions present in the zeolite bed continuously replace the calcium and magnesium ions present in water and hence the water becomes soft. Na 2 Ze + Ca(HCO 3)2 → Ca. Ze + 2 Na. HCO 3 Na 2 Ze + Mg(HCO 3)2→ Mg. Ze + 2 Na. HCO 3 When the zeolite bed becomes exhausted it requires regeneration. This is achieved by passing 10% Na. Cl solution through it. Ca. Ze+2 Na. Cl→Na 2 Ze+Ca. Cl 2

III. Ion Exchange Method Ion exchange resins are insoluble cross linked long chain organic polymers having a microporous structure where acidic or basic functional groups attached to the chain are responsible for the ion exchange capacity. Cation exchange resins contain acidic functional groups like –COOH, -SO 3 H etc. while onion exchange resins contain basic functional groups like –OH, -NH 2 etc. The hard water is allowed to pass through a cation exchange column to remove all the cations like Ca 2+, Mg 2+ etc. 2 RH+ + Ca 2+ → R 2 Ca 2+ + 2 H+ 2 RH+ + Mg 2+ → R 2 Mg 2+ + 2 H+ Afterwards the water is allowed to pass through an anion exchange resin column to remove anions like SO 42 -, Cl- etc. ROH- + Cl- → RCl- + OHThe H+ and OH- ions so produced from the cation and anion exchange resins combine to become water H+ + OH- → H 2 O Water thus coming out of the ion exchanger will be free from both cations and anions and hence called demineralised water.

INTERNAL TREATMENTS OR INTERNAL CONDITIONING In this method appropriate chemicals are added to water in the boiler when the hardness producing ions are converted in to either more soluble salts or insoluble ppts. The following are the important internal treatment methods done inside the boiler. 1. Colloidal Conditioning Certain organic substance like tannin is added to the boiler water they get absorbed over scale forming substances thus preventing scale deposition. 2. Phosphate conditioning In high pressure boilers, scale formation is prevented by adding sodium phosphate to boiler feed water. This reacts with Ca 2+ and Mg 2+ ions in water forming non adherent and easily removable soft sludge of Ca and Mg phosphate. The commonly used phosphate are Na. H 2 PO 4, Na 2 HPO 4 and Na 3 PO 4. 2 Na 3 PO 4 + 3 Ca. Cl 2 → Ca 3(PO 4)2 + 6 Na. Cl

The choice of phosphate depends up on alkalinity of boiler feed water. For eg. Ca can not be precipitated below PH 9. 5. Hence Na 3 PO 4 is selected to remove Ca 2+ ions. 3. Carbonate Conditioning In low pressure boilers, scale formation can be prevented by adding Na 2 CO 3 to boiler feed water. Ca salts are precipitated as loose sludge of Ca. CO 3 which can be removed by blow down operation. 4. Calgon Conditioning Addition of sodium hexameta phosphate called calgon prevents scale formation by converting Ca 2+ and Mg 2+ ions in to soluble complexes. 5. Aluminium conditioning Boiler water is treated with sodium meta aluminate when. Na. OH and Al(OH)3 are formed. Na. OH precipitates Mg salt as Mg(OH)2. Both Al(OH)3 and Mg(OH)2 ppts formed entraps all the scale forming materials. These loose ppt formed can be removed by blow down operation. Na. Al. O 2 + 2 H 2 O → Na. OH + Al(OH)3 2 Na. OH + Mg. Cl 2 → Mg(OH)2 + 2 Nacl

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