358 ALUM. —Manufacture from Alum Shales.
Alum earth.
Alumina,....
_ 40-00 . . .
From Putzberg.Bergmatm.
.. .. 10-90.. f 45-40
Majrnesia,. ..
Sulphur,..
_ 2-85 .. .
. 8-94
Carbon,.
.... 19-45
. 5-95
Protoxide of iron,.
.... 0-40 ....
.... 5-50 •
“ manganese.
,... — . ,.
.... 0’60
Protosulphate of iron,..
.... 1'80 ....
.... 5-73
Sulphate of alumina,..,.
—
.... 1-20
.... 1-50 ....
_ 1-71
<c potassa,....
... 0-50 _
.... 1-75
Chloride of potassium,.
.... 0-50 ....
.... 0-35
0-47
Water,..
... 1075 .. ..
.... 1G-60
100-00
100-00
The sulphur is in combination with the iron, andnot, as was formerly supposed, in the free state, oras sulphide of carbon. Although the constituents ofalum earth are nearly the same in specimens takenfrom other localities, the proportions are neverthe-less variable, as might have been expected from themode in which the deposits are formed; so great,indeed, is the difference in this respect, that theexamples given above can hardly be viewed as a fairaverage of these compounds. Pyrites and alum oresowe their property of being rapidly decomposed underthe influence of atmospheric air to the fine state ofdivision of the bisulphide, and probably, also, to theoccasional presence of the protosulphide of iron.Massive crystalline pyrites is under the same circum-stances but very slowly decomposed.
The combined action of air and moisture areessential to effect this change. The decomposition isoccasioned by 7 equivalents of oxygen and 8 equiva-lents of water being taken up by the pyrites, FeS 2 ,which convert it into ferrous sulphate and sulphuricacid, thus:—
FeS 2 + 70 + 8H 2 0 = FeS0 4 ,7H 2 0 + H 2 S0 4 .
The combination of the oxygen with the iron andsulphur is accompanied by a spontaneous rise oftemperature which results from the chemical union ;as the process proceeds, the iron sulphide is convertedas by roasting into monosulphide and sulphur, whichimmediately burn, the former being resolved intoferrous sulphate as shown above, and the latter intosulphurous acid, which is absorbed by the alumina.The aluminium sulphite thus formed is then byfurther absorption of atmospheric oxygen, transformedinto aluminium sulphate (A1 2 (S0 4 ) 3 ). The alumore, when first taken from the ridges—calcined heaps—and covered with water (which is removed after ashort interval), will be found to have yielded littlesulphate of alumina to the solution, but a great dealof sesquisulphate of iron ; while, after being allowedto remain for some time over the ore, the solutionwill be found to contain much sulphate of aluminaand protosulphate of iron; alum-makers know thatthe first washes are not those which afford the mostalum, unless the ore has been permitted to remain inthe steeps for a length of time; they are also awarethat more alum is procured by adding to the lie apoi tion of mothers, which are very rich in sesquisul-
phate of iron, than is obtained by pure water,per se.
The sulphides of iron are, therefore, only necessaryto the production of alum as affording sulphuric acidto unite with the alumina. Besides the acid pro-duced by the second portion of sulphur, an additionalquantity of sulphuric acid is furnished to the aluminaby the decomposition of the green vitriol, the ferrousoxide being speedily converted into ferric oxide(Fe 2 0 3 ) by the oxygen of the air, and precipitated inthe form of a basic salt. Potassa , which is neveraltogether wanting in the alum ores, sets free in asimilar manner sulphuric acid by the production ofpotassio-forric sulphate(iron alum, FeK(S0 4 ) 2 12H,0).Ferric sulphate is also decomposed by aluminiumhydrate, forming aluminium sulphate; this reaction is,however, very slow, and practically of small account.
Potash-iron alum is also always formed whenpotassium sulphate and ferric sulphate are togetherin acid solution, and by spontaneous evaporationcrystallizes out in octahedral crystals, which in formand taste are not to be distinguished from commonalum. These crystals are generally colourless, butsometimes have an amethyst tint, which arises fromtheir containing a trace of ferric acid.— IIeintz.Potash-iron alum is not stable at temperatures above32° Fahr. (0° C.), and is decomposed wholly at212° Fahr. (100° C.).
Lime in the ores is most prejudicial, since it deprives the aluminium and ferrous sulphates of theirsulphuric acid, and thus entirely stops the produc-tion of alum; ores containing any considerablequantity of lime cannot consequently be used in thismanufacture. Gypsum, however, is always found inthe crude lies. Magnesia is no less deleterious to theformation of alum, but the magnesium sulphate—Epsom salt—thus produced is in some of the English alum-works regarded as a most important obj’ect.Fresh alum ores contain no soluble salts of aluminiumor iron ; it is only where air has had access to them,either in the pit or at the surface, that efflorescencein the form of fine needles—feather alum—is observed,and this consists partly of real alum, partly of alumin-ium sulphate, or of combinations of the latter saltwith ferrous oxide, magnesium sulphate, or other salts.
An aluminous iron sulphate occurred abundantlysome years ago in the Hurlet and Campsie wrought-out coal beds, which had the following composition:—
Berthier.
Phillips,
It. ID. Thomson.
It. I). Thomson.
Sulphuric acid,
34-40
30-90
35-600
28-635
1 errous oxide,
12-00
20-70
13-560
19-935
Alumina,.
8-80
5-20
7-127
2-850
Magnesia,.
0-80
_
_
_
Water,..
44 00
43-20
43-713
48-580
100-00
100-00
100-000
100-000
The composition of this feather alum, or hair-salt,as it is sometimes termed, is shown by the subse-quent analyses. The term hair-salt, sometimes givento these natural effloresced bodies, belongs properlyto the magnesium sulphate often accompanying them.