Calcium hydroxide


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  • One significant application of calcium hydroxide is as a flocculant, in water and sewage treatment. It forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and low toxicity of calcium hydroxide. It is also used in fresh water treatment for raising the pH of the water so that pipes will not corrode where the base water is acidic, because it is self-regulating and does not raise the pH too much.

It is also used in the preparation of ammonia gas (NH3), using the following reaction:

Ca(OH)2 + 2NH4Cl → 2NH3 + CaCl2 + 2H2O
  • Another large application is in the paper industry, where it is an intermediate in the reaction in the production of sodium hydroxide. This conversion is part of the causticizing step in the Kraft process for making pulp. In the causticizing operation burned lime is added to green liquor which is a solution primarily of sodium carbonate and sodium sulfate produced by dissolving smelt, which is the molten form of these chemicals from the recovery furnace.

Food industry

Because of its low toxicity and the mildness of its basic properties, slaked lime is widely used in the food industry to:

Native American uses

Dry treated corn (left), and untreated maize (right) after boiling in water with calcium hydroxide (15 ml, or 1 Tbsp, lime for 500 g of corn) for 15 minutes.
  • In Spanish, calcium hydroxide is called cal. Maize cooked with cal (nixtamalization) becomes hominy (nixtamal), which significantly increases the bioavailability of niacin, and it is also considered tastier and easier to digest.
  • In chewing coca leaves, calcium hydroxide is usually chewed alongside to keep the alkaloid stimulants chemically available for absorption by the body. Similarly, Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt mollusc shells to enhance the effects. It has also been used by some indigenous American tribes as an ingredient in yopo, a psychedelic snuff prepared from the beans of some Anadenanthera species.

Asian uses

  • Calcium hydroxide is typically added to a bundle of areca nut and betel leaf to keep the alkaloid stimulants chemically available to enter the bloodstream via sublingual absorption.
  • It is used in making naswar (also known as nass or niswar), a type of dipping tobacco made from fresh tobacco leaves, calcium hydroxide (chuna), and wood ash. It is consumed most in the Pathan diaspora, AfghanistanPakistanIndiaBangladesh. Villagers also use calcium hydroxide to paint their mud houses in Afghanistan, Pakistan and India.