Caustic soda is primarily used because of its excellent organic dissolving and saponifying powers, its considerable germicidal activity, and its comparative cheapness. Liquid caustic soda and various additives have been used for many years in a wide variety of applications. In general, caustic soda “cleans” because of its strong alkalinity and is widely used due to its low cost.
Over the years, compounders have formulated a wide variety of “cleaning compounds” and the end user is sold a trade name product. When this occurs, many people are unaware as to how the additives are used. For this reason, it is important to fully understand why certain additives are used.
Liquid Caustic Soda
Commercially,there are several methods of producing liquid caustic soda. Sodium chloride (salt) brine is reacted in a cell with a strong direct current. This electrolytic reaction with water and salt yields caustic soda (NaOH), hydrogen (H₂), and chlorine (Cl₂) in accordance with the following formula:
2NaC1 + 2H₂O 2NaOH + H₂ + C1₂
The industry terminology is known as diaphragm (commercial grade), mercury cell (high purity, rayon, etc.), membrane (equivalent to mercury, etc.), and chemical grade. Please note chemical grade is manufactured by a different process and does not produce chlorine as a byproduct. The specifications vary by process with mercury and membrane prices at a premium because they produce low salt, low carbonates, and low iron.
The market is generally sold on a “dry ton basis” and is generally billed on the active NaOH content. Commercial grade liquid caustic soda is manufactured at 50%.
Caustic Soda Additive Packages
The word “chelant” is from the Latin word “chela” meaning “claw.” Chelants are materials that work in aqueous solutions to tie up metal ions so they are no longer effective. The term “chelating agent” refers to a molecule that contains two or more complexing sites (or claws) capable of coordinating around a metal ion. In technical terms, chelation is a kind of sequestration process whereby the chelating agent forms a non-ionic ring structure with a di- or polyvalent cation by ionic valence bonding, supplemented by coordinate covalent bonding. Sequestration, on the other hand, is a generic term which covers all the ways in which chemical species may be protected from unwanted chemical reactions in its solution environment.
Scale is defined as a “deposit” adhering to a surface. In the ethanol industry, scale can be formed with the presence of calcium, magnesium, and other trace minerals. The caustic soda scale inhibitor virtually modifies and attacks the crystal growth by changing the structure and hence prevents the scale from forming.
Dispersants attack the undesired species by using a high negative charge. This works much like two magnets with like charges, the magnets repel from one another.
Surface active agents are used in formulations primarily to reduce surface tension in caustic soda. The uses of surfactants give the working solution a reduction in surface tension and, hence, are called wetting agents. The wetting agent increases the ability to wet and aid in rinsing.