Sodium hydroxide, commonly called caustic soda, is produced commercially by two basic methods: electrolytic cells and chemical process. Most caustic soda is produced from electrolytic cells. There are three types of electrolytic cells: diaphragm, mercury, and membrane (to be described later). Chemical process caustic soda is produced by reacting sodium carbonate (Na2CO3) with hydrated lime (Ca[OH]2) to form sodium hydroxide (NaOH) and calcium carbonate (CaCO3).
Caustic soda is one of the basic building blocks of chemistry and as such it finds diversified applications. A few of the broad use categories are: chemical manufacturing; pulp and paper manufacturing; cleaning products; petroleum and natural gas; cellulose film; cotton textile process; and water treatment.
Caustic soda, especially when used for pH control, neutralization of waste acids, and similar applications, competes with other alkalies, particularly sodium carbonate (soda ash). The common factors for selecting caustic soda are its stronger basicity and easier storage and handling.
One of the major chemical uses of caustic soda is in the manufacture of alumina from bauxite, where it dissolves the alumina so that insoluble impurities can be separated. In addition, caustic soda performs a variety of functions in chemical manufacturing including waste acid neutralization, pH control, caustic scrubbing of off-gases, catalysis, and caustic extraction.
Caustic soda is an important commodity chemical for the pulp and paper industry. Principal uses in pulp and paper production include the cooking/processing of Kraft pulps, the extraction of lignin during the pulp bleaching sequences, and the on-site manufacture of sodium hypochlorite. The general pulp bleaching procedure involves a bleaching sequence during which impurities and colored matter in the pulp are oxidized and/or converted to alkali-soluble forms, and an extraction sequence during which the impurities are removed. Extraction stages almost invariably use caustic soda.
Caustic soda is used to some extent in all of the chemical pulping processes: sulfate, sulfite, and soda. In the sulfate and soda processes, the pulping liquors are solutions of caustic and sodium sulfide, and caustic, respectively. In the sulfate process, which is by far the most important pulping process, maximum recycling is accomplished by concentrating the solids in the waste pulping liquor, burning the concentrated liquor, and then re-causticizing the dissolved smelt from the furnace with a lime causticizing system. Additionally, caustic soda is used in the acid sodium sulfite pulping process for softening and swelling wood prior to making certain types of mechanical pulps, for water treatment, and for scrubbing off-gases.
Caustic soda is used in mercerizing and scouring cotton fabrics and fibers. About 90% of all cotton undergoes the scouring treatment, which renders the fabric more absorbent to bleaching agents. Approximately 35% of cotton is mercerized to improve fiber strength and affinity for dyes.
Two other uses of caustic soda are in water treatment and food processing. Municipal water treatment facilities use caustic soda for pH adjustment, ion exchange regeneration, and on-site generation of sodium hypochlorite. Caustic soda is used in several food processing applications. An example is in removing skins from potatoes, tomatoes and other fruits. It is also used in processing edible fats and oils.
The cleaning products market includes a very large number of caustic soda uses, either as a reactant to produce chemical compounds employed in these applications, such as sodium salts of organic sulfonic acids and sodium hypochlorite, or as a direct component of products such as drain pipe and oven cleaners. Caustic consumption in cleaning agents includes the use of caustic to make soaps, synthetic detergents and alkaline detergents for household and non-household use, and various other cleaning products. Most household liquid bleaches are manufactured by the reaction of chlorine and caustic soda solution.
Caustic soda is used in several applications by the petroleum and natural gas industry for production (and exploration) and in the processing of products. It is an important component of most drilling muds, and can be used in alkaline flooding of oil fields for enhanced petroleum recovery.
Caustic soda is consumed in the production of rayon, cellophane, and other cellulose products.