Cold Lime Soda Process 2d6c1z

APPLIED SCIENCE CHEMISTRY

Cold lime soda proces

Chemistry of Precipitation Softening In almost every raw water supply, hardness is present as calcium and magnesium bicarbonate, often referred to as carbonate hardness or temporary hardness. Reactions are as follows : H2CO3

CaCO3 ¯

+

carbonic acid CO2

carbon dioxide

=

calcium bicarbonate

calcium carbonate

+

H2O

water

Ca(HCO3)2

=

H2CO3 carbonic acid

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 Mg(HCO3 )2 magnesi um bicarbon ate

+

2Ca(OH)2

calcium hydroxide

=

Mg(OH)2 ¯

magnesium hydroxide

+

2CaCO3 ¯

calcium carbonate

+

2H2O

water

Ca(HCO3)2

+

calcium bicarbonat e

CO2 carbon dioxide

Ca(OH)2

=

calcium hydroxide

+

Ca(OH)2 calcium hydroxide

2CaCO3 ¯

2H2O

+

calcium carbonate

=

CaCO3 ¯

calcium

carbonate

water

+

H2O

water

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

MgSO4

Ca(OH)2

+

magnesium sulphate

MgCl2 magnesium chloride

Mg(OH)2 ¯

=

calcium hydroxide

+

Ca(OH)2

calcium

magnesium hydroxide

=

CaSO4

+

calcium sulphate

Mg(OH)2 ¯

magnesium hydroxide

+

CaCl2

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 sodium aluminate

+

4H2O water

=

2Al(OH)3 ¯ aluminum hydroxide

+

2NaOH sodium hydroxide

Mg

mag nesi um

[

SO4 Cl2 sulfate/ chloride

]

+

2NaOH

=

Mg(OH)2¯

+ [

Na2SO4 2NaCl

sodium hydroxide

magnesium hydroxide

sodium sulfate/ chloride

]

noncarbonate magnesium hardness reduction in cold process softening requires added lime. The reactions are as follows: MgSO4

+

magnesium sulfate

MgCl2 magnesium chloride

Ca(OH)2

+

calcium hydroxide

+

Ca(OH)2 calcium hydroxide

Na2CO3

sodium carbonate

+

Na2CO3 sodium carbonate

=

Mg(OH)2 ¯

+

magnesium hydroxide

= Mg(OH)2¯ magnesium hydroxide

CaCO3 ¯

+

calcium carbonate

+

CaCO3 ¯ calcium carbonate

Na2SO4

sodium sulfate

+

2NaCl sodium chloride

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