Sulfate of aluminium
Aluminum sulfate in anhydrous state is a white powder with a specific gravity of 2.71. It crystallizes from aqueous solutions in the form of a crystalline hydrate containing 18 molecules of crystallization water at a normal temperature. Hydrate A12 (SO4) 3 ∙ 18H2O forms colorless needle crystals with a specific gravity of 1.62 and a sour astringent taste. A12 (SO4) 3 is highly soluble in water.
Due to partial hydrolysis, the aqueous solution has an acidic reaction:
A12 (SO4) 3 + 6HOH ↔ 2Al (OH) 3 + 3H2SO4
Products of hydrolysis of aluminum sulfate can also be obtained in the crystalline state. For example, the basic sulfate [A1O] 2SO4 ∙ 9H2O occurs naturally in the crystalline state in the form of aluminate (websterite). On the contrary, from solutions, containing a large excess of acid, the acid salt А1Н (SO4) 3 ∙ 1,5H2O crystallizes.
The main double salt А12 (SO4) 3 ∙ 6Са (ОН) 2 ∙ 26Н2О, crystallizing in the form of hexagonal needles, is found in nature in the form of the mineral ettringite. It can also form when sulphate-containing cement hardens, significantly reducing its hardness. When heated, a 16-hydrate (rhombic prism) usually crystallizes from a solution of aluminum sulfate. In addition, there are hydrates with 27.10 and 6H20; when heated above 340 ° C, complete dehydration occurs. The elimination of SO3 begins at about 600 ° C.
Aluminum sulphate is widely used in industry, mainly in the production of paper. When added together with sodium chloride to paper pulp, they serve for the so-called paper sizing. The aluminum chloride formed as a result of the exchange reaction glues the paper fibers. Aluminum sulfate used in this production should not even contain traces of iron. This is even more important in those cases when it is used for tanning leather (white tanning), as well as as a mordant when dyeing fabric. Aluminum sulfate also serves as a precursor for the preparation of other aluminum salts, which can be conveniently obtained from it by reaction with the corresponding lead salts. The use of aluminum sulphate as a mordant is based on the fact that the extremely dispersed aluminum hydroxide formed as a result of hydrolysis in an aqueous solution is absorbed and firmly held by wool fibers. In turn, aluminum hydroxide can bind organic substances (with the formation of so-called dye varnishes). Other easily hydrolysable salts act in the same way, for example, chromium sulfate, tin chloride, etc. Wool fibers treated (etched) with such salts can, due to the ability of the metal hydroxides adsorbed by them to absorb dyes, to be colored by such substances that would not otherwise be retained on the fiber. Paper fibers, like wool fibers, cannot directly absorb aluminum hydroxide from a solution (hot). Therefore, when crushing paper fabrics, a precipitate of aluminum hydroxide is obtained inside the fiber, impregnating it first with a solution of aluminum sulfate and then acting on it with alkali (soda, etc.).
The industrial production of aluminum sulfate is best done by dissolving pure (no iron impurities) aluminum hydroxide in hot concentrated sulfuric acid. You can also treat bauxite or clay directly with sulfuric acid; but at the same time there are difficulties associated with the purification of the obtained aluminum sulfate from iron in a fairly simple way.