156 ALUM. —Manufacture from Alum Shale.
amount of potassa in the rock, and the remainder,particularly the excess of alumina, is separated. Thisis effected by burning the stones in heaps, or fur-naces similar to those used in preparing gypsum—Fig. 1—particular care being taken that the tempera-ture does not rise too high.
The inner chamber of this kiln is divided into twounequal portions by an arch, p p, situated about a footfrom the bottom. The upper part, into which thealum rock is introduced, partly through the door, G,and partly through the mouth, H, is provided witheight draught holes, J J, the ninth hole being formedby the tube in the covering plate, m. The lowerchamber, or fire-space, is in connection with the fire,E, which is situated in front of the kiln, and which isreplenished with fuel through the door, D. Thedraught-channel, C, terminates in the ash-pit underthe grate, A, on which the fire is made. The flameenters at x, below the perforated arch, p, where it isuniformly disseminated over the whole area of thekiln, and passes through the fissures, e e, and themass of materials, in an upward direction, escaping atthe apertures, J J. If the heat is not uniform through-out the kiln, the draught-holes, J J, are opened on"that side where it is least intense, and where it isdesirable to lead the flame, while those on the otherside are closed. The aperture, L, is used for clearingthe fire-chamber, and is closed, as well as G and h,during the firing. Particular attention is given tothe maintenance of a proper temperature. 0 0 re-presents the exterior walls of the kiln, made of non-conducting materials to secure the retention of theheat.
At a red heat aluminium sulphate is decomposed,yielding partly anhydrous sulphuric acid, and partlyoxygen and sulphurous acid. As soon as thesevapours appear the burning is stopped, and the massis transferred to walled cisterns, where it is repeat-edly moistened with water, and is allowed to disin-tegrate for three or four months, at the expirationof which period it is converted into a soft mud,tasting perceptibly of alum, which may then be dis-solved out with water. If the alum stone containedan excess of aluminium hydrate this would infalliblyreact upon the alum, and form with it an insolublecompound containing basic sulphate—the burningexpels the water from the hydrate of alumina, andthus renders it chemically inactive ; the excess ofalumina is thus separated from the compound, whichthen yields an alum soluble in water. During theevaporation—until the spec. grav. is 1-114 at 113°Fahr. (45“ C.)—the lie still holds a fine ferruginousrose-red powder in suspension, which feebly coloursthe crystals, but is left behind when these are redis-solved. The crystals contain potassa, but no am-monia, and are known in commerce under the nameof Roman alum.
Production from Alum Ore. —The productionof alum from alum shale and alum earth is system-atically carried out or divided into three distinctoperations: the production of aluminium sul-phate ; the addition of sulphate or chloride of thealkali to the concentrated cold solution of the
former; and lastly, the purification of the alum byrecrystallization.
Alum shale is a kind of clay slate impregnatedwith sulphide of iron and bituminous matters—alliedto real clay slate by its firmness, appearance, slatystructure, and great extent. Although it is not soabundantly disseminated as many other species ofrocks and minerals, it forms, nevertheless, beds ofconsiderable extent in many localities, particularlyin the Scandinavian peninsula; in Bohemia ; in theHartz, in Upper Bavaria ; in Voigtland, in the moun-tainous districts of the Lower Rhine; in England,near Whitby; in Scotland , at Hurlet and Campsie,near Glasgow ; in the Uralian Mountains. It maybe also met with in many other districts, but notin sufficient quantity to be available for practicalpurposes.
The following analyses show the composition ofthe rocks:—
Alum Shale from Sieiida, by Lampadius.
Aluminium sulphate,. 2.68
Potash alum,. 0-47
Ferrous sulphate,. 0-95
Sulphate of lime,. 1-70
Silica,. 10-32
Alumina,. 9-21
Magnesia.traces
Sesquioxide of iron. 2-30
Oxide of manganese,. 0-31
Sulphur,. .... 7-13
Water,. 33-90
Carbon, &c.,. 31-03
100 00
Alum Shales, by G. Kersten.
Hermannsschachte. Gluckaufgang. Blucherschaciita
Carbonaceous matters,. 41-10 ...
. 27-92 _
34-20
Silica,.
.... 44-02 ...
. 51-32 ...
50-21
Sesquioxide of iron,... 6*23 ...
. 8-40 ....
0-42
Alumina,.
.... 5-60 ...
7-62 ....
5-21
Magnesia,.
.... 0-32 ...
0-26 ....
0-53
Oxide ot manganese,.. 0*12 ...
traces .... traces
Sulphur,.
.... 1*25 ...
2-89 ....
1-72
Sulphate of lime,.
traces ....
races
Loss,.
.. .. 1*36 ...
1-59 ....
7-71
100-00
100-00 100-00
Alum Shales, by Erdmann.
Soluble in acid. Gai-nsdorff.
Wezelstein.
Iron bisulphide,..
7*533
.... 10-166
Silica,.
0-0G0
.... 0-100
Sesquioxide of iron
0*966
.... 2-466
Alumina,.
1-833
.... 3-166
Lime,.
0*400
_ 1-000
Magnesia,.
trace
_ 1-022
- 10-792
--
17-920
Insoluble in acid.
Silica,.
50-066
.... 52-200
Alumina,.
8-900
.... 17-900
Sesquioxide of iron
1-300
.... 3-366
Magnesia,.
1-000
_ 1-133
Lime,'.
trace
.... trace
Coal,.
22-833
.... 0-803
84-099
....-
75-402
Loss,.
. 5-109
6-678
100-000
100-000
Subjoined is the composition of several shales whichare sometimes used in the manufacture of alum