WATER TRETMENT: COLD LIME SOFTENING
Precipitation softening accomplished at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca(OH)2, is added to the water being treated, the following reactions occur:
CO2 + Ca(OH)2= CaCO3 ¯+ H2O
carbon dioxide + calcium hydroxide = calcium carbonate + water
Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 ¯+ 2H2O
Calcium Bicarbonate + calcium hydroxide = calcium carbonate + water
Mg(HCO3)2 + 2Ca(OH)2= Mg(OH)2 ¯+ 2CaCO3 ¯+ 2H2O
magnesium bicarbonate + calcium hydroxide = magnesium hydroxide + Calcium carbonate + Water
If the proper chemical control is maintained on lime feed, the calcium hardness may be reduced to 35-50 ppm. Magnesium reduction is a function of the amount of hydroxyl (OH-) alkalinity excess maintained. Figures 7-1 and 7-2 show these relationships.
Noncarbonate or permanent calcium hardness, if present, is not affected by treatment with lime alone. If noncarbonate magnesium hardness is present in an amount greater than 70 ppm and an excess hydroxyl alkalinity of about 5 ppm is maintained, the magnesium will be reduced to about 70 ppm, but the calcium will increase in proportion to the magnesium reduction.
For example, in cold lime treatment of a water containing 110 ppm of calcium, 95 ppm of magnesium, and at least 110 ppm of alkalinity (all expressed as calcium carbonate), calcium could theoretically be reduced to 35 ppm and the magnesium to about 70 ppm. However, an additional 25 ppm of calcium would be expected in the treated water due to the following reactions:
MgSO4 +Ca(OH)2 = Mg(OH)2 ¯+ CaSO4
Magnesium sulfate + Calcium hydroxide = magnesium hydroxide + calcium sulfate
MgCl2 + Ca(OH)2 = Mg(OH)2 ¯+ CaCl2
Magnesium chloride + calcium = magnesium hydroxide + calcium chloride
To improve magnesium reduction, which also improves silica reduction in cold process softening, sodium aluminate may be used. The sodium aluminate provides hydroxyl ion (OH-) needed for improved magnesium reduction, without increasing calcium hardness in the treated water. In addition, the hydrolysis of sodium aluminate results in the formation of aluminum hydroxide, which aids in floc formation, sludge blanket conditioning, and silica reduction. The reactions are as follows:
Na2Al2O4 + 4H2O = 2Al(OH)3 ¯ + 2NaOH
sodium aluminate + water = aluminum hydroxide + sodium hydroxide
Mg + [SO4 - Cl2 ] + 2NaOH = Mg(OH)2¯ + [Na2SO4 - 2NaCl ]
Magnesium + sulfate chloride + sodium hydroxide = magnesium hydroxide + sodium sulfate chloride
Soda ash (Na2CO3) may be used to improve hardness reduction. It reacts with noncarbonate calcium hardness according to the following:
CaSO4 + Na2CO3 = CaCO3 ¯ + Na2SO4
Calcium sulfate + sodium carbonate = calcium carbonate + Sodium Sulfate
CaCl2 + Na2CO3 = CaCO3 ¯ + 2NaCl
Calcium chloride + sodium carbonate = calcium carbonate + Sodium Chloride
However, noncarbonate magnesium hardness reduction in cold process softening requires added lime. The reactions are as follows:
MgSO4 + Ca(OH)2+ Na2CO3= Mg(OH)2 ¯+ CaCO3 ¯+ Na2S - O4
Magnesium su lfate + calcium hydroxide + sodium carbo Nate = magnesium hydr oxide + calcium carbo nate + sodium sulfate
MgCl2 + Ca(OH)2+ Na2CO3= Mg(OH)2¯+ CaCO3 ¯+ 2NaC I
magnesium chl oride + calcium hydro xide + sodium carbo nate = magnesium hydr oxide + calcium carbo nate + sodium chloride
In these reactions, dissolved solids are not reduced because a solution reaction product (sodium sulfate or sodium chloride) is formed.
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