Caustic soda, also known as sodium hydroxide, is an inorganic alkali salt which is hygroscopic in nature and is soluble in both water and polar solvent. It is mainly used in textile in the form of caustic soda flakes or pearls. It has a slippery feel and absorbs carbon dioxide in the atmosphere to form sodium carbonate.
Being a basic compound, it undergoes neutralization reaction with acid and form the corresponding salt and water. It will also corrode metals such as aluminium, producing hydrogen gas which behaves as an explosive gas. Caustic soda has a decomposing effect on proteins and may penetrate deep tissues, hence caution must be exercised when handling.
By reacting sodium carbonate with calcium hydroxide, sodium hydroxide and calcium carbonate is produced. These products can be easily separated by filtration as calcium carbonate is not soluble whereas sodium hydroxide is highly soluble. The solution will then be purified, processed and standardized for industrial consumption.
With the electrolysis of sodium chloride solution, solid sodium hydroxide is manufactured and obtained with the evaporation of water. Chlorine gas is also produced as the main by-product. This is done through three main types of cell, mainly mercury cell, diaphragm cell and the membrane cell.
Caustic soda is used in the manufacturing of textile. Caustic soda is used in the scouring process, mercerization process, as well as dyeing process. During the process of scouring, it aids the removal of natural oil, wax and fat. During the process of mercerization, it aids in improving the tensile strength, smoothness and dye substantively of the cellulosic. Caustic soda also serves as a pH regulator and controls the pH of dyeing bath, which is required to be precise and constantly maintained throughout the process.
Caustic soda is used in pulp and paper industry during the cooking process and oxygen delignification process. Both sulfate and sulfite pulps are purified by removing lignin compounds in the caustic extraction stages of multistage bleach plants. In some kraft mills, caustic soda is also used as a makeup chemical. It is also used as the initial treatment in deinking secondary fibers.
It is also used to manufacture soap and detergents, applied during the saponification process in soap industry and neutralization process in detergent industry. Caustic soda saponifies fats into water soluble sodium-based soaps.
In addition, caustic soda helps to extract alumina from bauxite ore via Bayer process and is employed to produce an array of organic and inorganic chemicals.