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 1000oC 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 commercial scale: CaO + H2O → Ca(OH)2. The calcium oxide is broken down into fine particles of calcium hydroxide in the hydration reaction which occurs in a continuous hydrator. The particle sizes can then 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 sewage water which causes the waste materials in the water to form colloids that can then be removed. The pH of water can also 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. It is used in the steel industry as a flux, in gold mining and also in fertilizer industry.