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Sulphate of Alumina. —■ Aluminium sulphate,A1.,(S0 4 ) 3 + 18H 2 0.—This substance has of lateyears been very largely manufactured, and soldunder the name of concentrated alum. It occurs incommerce as four-sided plates about an inch in thick-ness, which are white and partially translucent,dissolve perfectly in water, exhibit no trace of crys-tallization, and possess the peculiar taste of alum ina more marked degree than even alum itself. Thecommercial salt is entirely free from iron, and canbe cut with a knife. It is very soluble in water,and has an acid reaction. Ileated to redness, itis decomposed, leaving pure alumina. The pro-cess for its preparation consists in treating clay,cryolite, bauxite, &c., with sulphuric acid in themanner before described ; and differs only in addingno potassium sulphate to the solution of alum-inium sulphate, and in precipitating any ironthat may be present by adding potassium ferro-eyanide, evaporating to the consistence at whichthe mass will solidify when cold, and then pouring itinto the “ melting vessel.” Mohr found this sub-stance, which is improperly called alum, to be per-fectly free from iron, and to contain—■
Alumina,. 13*91 per. cent.
Sulphuric acid. 30*24 “
Water,. 49*00 “
Potassium sulphate,. 1*50 11
The latter ingredient is due to the small portion ofpotassa naturally existing in all clays. The preced-ing composition corresponds exactly with the com-binations of
7*1 per. cent, of ordinary potassium alum, and92*9 “ of neutral aluminium sulphate.
The formula of which is AI 2 (S0 4 ) ;i + 181I a O.
Wiesmann’s process for the preparation of purealuminium sulphate consists in agitating the crudesolution of sulphate of alumina in a large woodenvessel with potassium ferrocyanide.
The vessel wherein the mixture is made is similarto A, in Fig. 16. After allowing the bulk of theprecipitated prussian blue to subside, the supernatantliquor is run off into the wooden vessels, B B B , whereit remains until perfectly clear and free from preci-pitate, after which the clear liquor is treated as hasbeen already described.
The precipitated prussian blue is repeatedlywashed with cold water to separate the whole of thealuminium sulphate, and the solutions are drawn offinto some of the vessels, b, for the above object.
The prussian blue is now treated with a lie ofcaustic soda to separate the iron, and thus the sameprecipitant serves for a long period with very littleloss.
The refuse matter which remains contains somesulphuric acid, and resists the solvent action ofwater.
Two samples of this residuary matter gave theannexed results:—
Fig. i«.
Virpmiii ; yTT"
MlwaMiMinilendnin
kpiiiiiiirSulSiSBlBli
Silica,.
41 85 .
.. 39*05
Alumina, with traces of sesquioxide of iron
, 18*40 .
. 25*55
Sulphuric acid,.
4*45 .
,. 8*47
Sulphate of lime,.
4*01 .
,. traces
Water,.
29*53 .
. 27*50
Loss,......
, 1*76 .
• —
100*00
100*57
The composition of aluminium sulphate, as metwith in commerce, varies very greatly. In foursamples analyzed by Vaurentrapp, he found:—
Alumina,.
Sulphuric acid,.
1.
. 15*3
. 38*0...
Theoretic amount ofSulphuric Acid .
.35-8
Alumina,..
Sulphuric acid,.
3
Alumina,.
Sulphuric acid,.
. 15*1
. 38*0...
4
.43*3
Alumina,.
Sulphuric acid,.
. 13*0
. 34*0...
.30*5
And also that the quantity of water varies even inthe same cake.
VOL. i.
Fleck found similar differences. Below are givenhis results for three samples .*—
i.
II.
III.
Aluminium sulphate,.
47*35
50*80
51*63
Sodium sulphate,.
4*35
1*21
0*77
Free sulphuric acid,.
0*73
0*27
—
Water,.
47*37
47*47
46*94
99*80
99*78
99*34
In prospect of aluminium sulphate being largelyused for the clarification of sewage, Sidney W. Richdevised a process of manufacture by which he statesthat this salt may be profitably manufactured on thelarge scale to sell at 30,s*. per ton. He makes use ofthe aluminous shale of Guisborough; after burning,and whilst still hot, he places it in a brick tower 18feet high. By withdrawing the burnt material fromthe bottom, and continually renewing the heap ofburning shale at the top of the tower, a continuouspassing of the material down the tower is effected.Whilst this is in progress sulphurous acid gas is intro-duced at the bottom of the tower from a pyritesburner, mixed with air and steam. Owing to the
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