Water Tretment: Cold Lime Softening

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

MgCl_{2 +} 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.