How are Carbonates Reacted on by INternal Treatment?
Calcium bicarbonate entering with the feed water is broken down at boiler temperatures or reacts with caustic soda to form calcium carbonate. Since calcium carbonate is relatively insolu-ble it tends to come out of solution. Sodium carbonate partially breaks down at high temperature to sodium hydroxide (caustic) and carbon dioxide. When phosphates are used in internal treatment they react with calcium carbonate to form calcium phosphate and sodium carbonate (soda ash). In the presence of sufficeint hy-droxide (caustic) alkalinity, mahnesium bicarbonate will pre-cipitate as magnesium hydroxide or will react with any silica present to form magnesium silicate. The minerals precipitated from solution (calcium carbonate, calcium phosphate, magnesium hydroxide, magnesium silicate, ect.) form sludge in the water which must be conditioned to prevent its sticking to the metal. The conditioned sludge is removed from the boiler by blow-down.
