Definition - What does Alum mean?

Alum is a common coagulant and as well as a generic name for a variety of aluminum compounds that are combinations of aluminum sulfate and group IA metal sulfates. It is a hydrated form of potassium aluminum sulfate and has the chemical formula KAl(SO4)2·12H2O. However, any of the compounds with the empirical formula AB(SO4)2·12H2O are considered to be alum. Alum is sometimes seen in its crystalline form, although it is most often a powder.

Corrosionpedia explains Alum

Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminum sulfate (potassium alum). More widely, alums are double sulfate salts, with the general formula A2(SO4).M2(SO4)3.24H2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminum or chromium (III).

Alum is used to clarify water by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them to flocculate. After flocculation, the particles are large enough to settle and can be removed. Alum may be used to increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily adherent and slows its rate of sedimentation.

Alum solution has the property of dissolving steels while not affecting aluminum or base metals, and can be used to recover workpieces made in these metals with broken toolbits lodged inside them.

Alum has several household and industrial uses, including:

  • Soap
  • Greases
  • Fire extinguishers
  • Textiles
  • Cosmetics
  • Plastics

Common coagulants used in conventional water and wastewater treatment include alum, ferric chloride, ferric sulfate, sodium aluminates, and various cationic polymers. Alum is acidic with a pH of 2 to 2.8 and is corrosive, but it is much less corrosive than ferric chloride. Alum is corrosive to soft steel. It dissolves in water, causing corrosion.

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