الالوم وانواعه واستخداماته

Alum
(pronounced /ˈæləm/) is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with theformula KAl(SO4)2.12H2O
Industrial
Alums are useful for a range of industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acidto litmus; and crystallize in regular octahedra. When heated they liquefy; and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.
Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are manufactured.
Alum is also used in purification of drinking water in industries. In a holding tank, some alum (phitkari) is added to the water so that the negatively charged light colloidal parts stick together and get heavy (flocculate) when alum makes the colloidal particles neutralized by making its aluminum ions get loaded with the colloidal parts . When the colloidal parts get heavy they can be easily separated from the tank for further chlorination of water in the process of purification of water .
Chemical flocculant
 Alum is used to clarify water by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them toflocculate (stick together). After flocculation, the particles will be 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 is an ingredient in some recipes for homemade modeling compounds intended for use by children.

Production
Alum from alunite
In order to obtain alum from alunite, it is calcined and then exposed to the action of air for a considerable time. During this exposure it is kept continually moistened with water, so that it ultimately falls to a very fine powder. This powder is then lixiviated with hot water and sulfuric acid, the liquor decanted, and the alum allowed to crystallize. The alum schists employed in the manufacture of alum are mixtures of iron pyrite, aluminium silicate and various bituminous substances, These are either roasted or exposed to the weathering action of the air. In the roasting process, sulfuric acid is formed and acts on the clay to form aluminium sulfate, a similar condition of affairs being produced during weathering. The mass is now systematically extracted with water, and a solution of aluminium sulfate of specific gravity 1.16 is prepared. This solution is allowed to stand for some time (in order that any calcium sulfate and basic ferric sulfate may separate), and is then evaporated until ferrous sulfate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40. It is now allowed to stand for some time, decanted from any sediment, and finally mixed with the calculated quantity of potassium sulfate, well agitated, and the alum is thrown down as a finely-divided precipitate of alum meal. If much iron should be present in the shale then it is preferable to use potassium chloride in place of potassium sulfate.

Alum from clays or bauxite
In the preparation of alum from clays or from bauxite, the material is gently calcined, then mixed with sulfuric acid and heated gradually to boiling; it is allowed to stand for some time, the clear solution drawn off and mixed with acid potassium sulfate and allowed to crystallize. When cryolite is used for the preparation of alum, it is mixed with calcium carbonate and heated. By this means, sodium aluminate is formed; it is then extracted with water and precipitated either by sodium bicarbonate or by passing a current of carbon dioxide through the solution. The precipitate is then dissolved in sulfuric acid, the requisite amount of potassium sulfate added and the solution allowed to crystallize.

Types of alum
Potash alum
Main article: Potash alum
Aluminum potassium sulfate, potash alum, KAl(SO4)2•12H2O is used as an astringent and antisepsis in various food preparation processes such as pickling and fermentation and as a flocculant for water purification among other things. A common method of producing potash alum is leaching of alumina from bauxite which is then reacted with potassium sulfate. As a naturally occurring mineral potash alum is known as kalunite. Other potassium aluminium sulfate minerals are alunite (KAl(SO4)2•2Al(OH)3) and kalinite (KAl(SO4)2•11H2O).
The molecular weight of potash alum is 474 g/mole.
Soda alum
Main article: Soda alum
Soda alum, NaAl(SO4)2•12H2O, mainly occurs in nature as the mineral mendozite. It is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C. Soda alum is used in theacidulent of food as well as in the manufacture of baking powder.
Ammonium alum
Main article: Ammonium alum
Ammonium alum, NH4Al(SO4)2•12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.
Chrome alum
Main article: Chrome alum
Chrome alum, KCr(SO4)2•12H2O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.
Selenate-containing alums
Alums are also known that contain selenium in place of sulfur in the sulfate anion, making selenate (SeO42-) instead. They are called selenium- or selenate-alums. They are strong oxidizing agents.
Aluminium sulfate
Aluminium sulfate is sometimes called alum in informal contexts, but this usage is not regarded as technically correct.i ts properties are quite different from those of the set of alums described above.




LIQUID ALUMINUM SULFATE
COMMERCIAL GRADE
CHEMICAL PROPERTIES
Specification Limits
Component Min. Max . Typical
Al2O3 (H2O Sol.) 8.0% 8.5% 8.3%
Fe as Fe2O3 - - 0.35% 0.080%
Basicity as Free Al2O3 0 0.5% 0 .15%
Insoluble - - 0.2% 0 .03%