Calcium hydroxide is an inorganic
compound with the formula Ca(OH)2. It is a colourless crystal or
white powder. It is also known as hydrated lime or slaked lime.
It has a boiling point of more than
1000 ◦C and dissolves sparingly in water to give a highly alkaline solution of
pH 12 to 13.
Limestone undergoes thermal
decomposition at high temperatures to form calcium oxide or burnt lime. Calcium
oxide is treated or slaked with water to produce calcium hydroxide on a
CaO + H2O → Ca(OH)2.
The calcium oxide is broken down into
fine particles of calcium hydroxide in this hydration reaction and this occurs
in a continuous hydrator. The particle sizes can be modified through milling to
ensure consistent particle size.
Hydrated lime is used to recover
sodium hydroxide for reuse in the Kraft process of paper manufacturing. After
converting wood into wood pulp, spent liquor is obtained. The spent liquor
undergoes recovery process to recover sodium sulphide and sodium carbonate
solution, which is diluted and treated with hydrated lime. The sodium carbonate
is causticised into sodium hydroxide for use. It is sometimes used to produce
calcium hypochlorite used to bleach paper. It also acts as a coagulant aid to
clarify plant process water. It is used to make precipitated calcium carbonate
as filler in papers by bubbling carbon dioxide through lime.
Waste Water Treatment Industry
Hydrated lime is used as a flocculant
to treat water and sewage, to allow the undesirable materials in the water to
form colloids and clumps that can then be removed from water. The pH of water
can be increased by using hydrated lime which prevent corrosion of water pipes
due to acidic water
Hydrated lime is used to treat animal
hides and stabilise roads. It can also be used to remove acidic gases by
injecting it into the flue gas, such as sulphur dioxide. It is used in the
steel industry as a flux and in gold mining and also in fertilizer industry
especially in acidic soils to improve crop yield.