8/2/2019 Process Aluminum Sulfate

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Process Aluminum Sulfate

Aluminum sulfate is produce by the reaction of sulfuric acid on bauxite, a naturally occurring

hydrated alumina ore having the approximate formula Al2O3.2H2O. the mined material varies

in composition and generally contains one to the three molecules of water plus impurities

such as iron, silica, titanium, and selenium. The soluble aluminum oxide ( Al2O3 ) content ofa typical bauxite varies from 52 to 57% and it may contain 1 to 10% ferric oxide ( Fe2O3 ).

The crude bauxite ore is ground to a fine powder ( 80% passes 200 mesh ) and charged into

open lead-lined steel reaction tanks. Sulfuric acid ( sp gr 1.7 ) is added, and the raw materials

are thoroughly agitated using paddle agitators, hot air, or life steam. The reaction mixture is

kept at a temperature of 105 to 110oC by the live steam or lead steam coils. An excess of

bauxite is fed to the reactor, so that there is an excess of 0.1 to 0.2% of soluble aluminum

oxide. From 15 to 20 h are required to complete the reaction. At the end of this time, a

reducing material is added to the reaction mixture to reduce the iron ( ferric sulfate ) to a

colorless ferrous condition. Barium sulfide in the form of black ash is commonly utilized,

although sodium sulfide, hydrogen sulfide, sodium bisulfate, or sulfur dioxide may also beused.

If the operation is performed in batches, the charge is allowed to settle in settling tanks.

Flaked glue or some similar coagulable substance is generally added to remove the finely

divided suspended material remaining in the supernatant liquid. This liquid is drawn off, and

the residue is washed several times. The washings are combined with the decanted liquor,

which is then sent to concentrators.

The process is generally operated in a continuous manner by using a battery of combined

reaction and settling tanks. A common variation of this process is the Dorr procedure, which

utilizes reaction agitators in mechanical agitators, air, and live steam. Black ash ( barium

sulfide ) is added to the last reactor to reduce the ferric sulfate. The reaction mixture is sent

through a series of thickeners, operating countercurrently, which remove the undissolved

material. At the same time the waste is washed thoroughly, so that it contains practically no

aluminum sulfate when discarded. Glue is generally added to the first thickener as a

coagulant.

The clarified aluminum sulfate solution, from the countercurrent decantation system, is

concentrated in open steam-coil-heated, lead-lined evaporators. Here, the specific gravity is

increase from about 1.3 to about 1.7. The concentrated solution is run into flat iron pans or

onto a cooling table. The liquid quickly and completely solidifies, and when cool is broken

up and ground to a uniform powder for shipment.

Commercial aluminum sulfate generally contains only about 13 or 14 moles of water instead

of the theoretical 18 moles. Also, it is usually in the basic form containing excess alumina.

Anhydrous aluminum sulfate may be obtained by dehydration. The yield of aluminum sulfate

based on the amount of aluminum oxide in the both the finished product and raw material is

90 to 95%.