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Although calcium is not regulated in water discharge, often its concentration needs to be reduced to soften the water before it can be recycled or used in other processes. The softening can be done in some case by reverse osmosis or ion exchange. But for high concentrations of calcium the preferred method of reduction is by lime softening or carbonate precipitation.
In lime softening, the intrinsic bicarbonate is converted to carbonate by raising the pH with lime and precipitating calcium carbonate. One mole of lime will precipitate two moles of calcium carbonate providing sufficient bicarbonate is available. If there is insufficient carbonate than additional carbonate can be added using sodium bicarbonate or carbon dioxide.
Ca(OH)2 + Ca++ + 2HCO3- è 2 CaCO3 + 2 H2O
Ca++ + CO2 + 2 NaOH è 2 CaCO3 + 2 Na+ + H2O
These reactions must be carried out between a pH of 9 and 11.
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