Aluminum sulfate [Al2(SO4)3, alum, filter alum, papermaker’s alum] is manufactured from aluminum oxide (Al2O3, alumina, bauxite). A mixture of the crude ore and sulfuric acid is heated at 105 to 110 degree C for 15 to 20 hours.
Filtration of the aqueous solution is followed by evaporation of the water to give the product, which is processed into a white powder. Alum has two prime uses. It is bought by the pulp and paper industry for coagulating and coating pulp fibers into a hard paper surface by reacting with small amounts of sodium carboxylates (soap) present. Aluminum salts of carboxylic acids are very gelatinous. In water purification it serves as a coagulant, pH conditioner, and phosphate and bacteria remover. It reacts with alkali to give an aluminum hydroxide floc that drags down such impurities in the water. For this reason it also helps the taste of water.
Pharmaceutically, aluminum sulfate is employed in dilute solution as a mild astringent and antiseptic for the skin. The most important single application of aluminum sulfate is in clarifying water; sodium aluminate, which is basic, is sometimes used with aluminum sulfate, which is acid, to produce the aluminum hydroxide. Aluminum sulfate is also used in sizing of paper, as a mordant in the dye industry, chemical manufacturing, concrete modification, soaps, greases, fire extinguishing solutions, tanning, cellulosic insulation, and in some baking powders.
Al2O3.2H2O + 3H2SO4 → Al2(SO4)3 + 5H2O
Filtration of the aqueous solution is followed by evaporation of the water to give the product, which is processed into a white powder. Alum has two prime uses. It is bought by the pulp and paper industry for coagulating and coating pulp fibers into a hard paper surface by reacting with small amounts of sodium carboxylates (soap) present. Aluminum salts of carboxylic acids are very gelatinous. In water purification it serves as a coagulant, pH conditioner, and phosphate and bacteria remover. It reacts with alkali to give an aluminum hydroxide floc that drags down such impurities in the water. For this reason it also helps the taste of water.
6RCO2–Na+ + Al2(SO4)3 → 2(RCO2–)3Al3+ + 3Na2SO4
Al2(SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4
Al2(SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4
Pharmaceutically, aluminum sulfate is employed in dilute solution as a mild astringent and antiseptic for the skin. The most important single application of aluminum sulfate is in clarifying water; sodium aluminate, which is basic, is sometimes used with aluminum sulfate, which is acid, to produce the aluminum hydroxide. Aluminum sulfate is also used in sizing of paper, as a mordant in the dye industry, chemical manufacturing, concrete modification, soaps, greases, fire extinguishing solutions, tanning, cellulosic insulation, and in some baking powders.
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