ALUM.— Roasting.

159

COMPOSITION OJ? NATURAL ALUMINIUM SULPHATE—FEATHER ALUM (Al 2 (S0 4 ) 3 ,18U a 0).

Saldana, Pasto.

Columbia.

Pyromoni,

in

Island Milo.

Coquimbo,

Cliili.

Kolosorak,

Bohemia .

FriersdorfF,

near

Bonn .

Potscliappel,

near

Dresden .

Freienwaldej Ararat

Andes.

Campsie,

Glasgow .

Sulphuric acid,.

36-400

35-68

40-31

36-97

35*82

37*380

35-710

35-637

58-58

35*872

40*425

Alumina,.

16-000

14-98

14*98

14-63

15-57

14*867

12-778

11-227

38-75

14*645

10-482

Sesquioxide of iron,....

0*004

—

2-58

—

—

—

0*500

8*530

Protoxide of iron,.

—

—

—

—

—

2-463

0-667

•718

leo} 2 ' 78

—

—

Protoxide of manganese,

—

—

—

—

—

—

1-018

0-307

—

_

Potassa, .

—

—

0*26

—

—

0*215

0*324

0*430

—

—

1*172

Soda,.

—

—

1*13

—

—

—

—

—

—

2-262

Lime,.

0-002

—

—

—

—

0*149

0*640

0*449

—

—

—

Magnesia,.

0-004

—

0-85

0*14

—

—

0*273

1*912

—

—

—

Hydrochloric acid,.

—

—

0*40

■—

—

—

—

—

—

—

—

Silica, .

—

—

M3

1*37

—

—

—

0-430

—

o-ioo

—

Water,...

46-600

49-34

40-94

44-64

48-61

45-164

47*022

48*847

—

46-375

3 6-295

Loss,...

0-990

—

_

—

—

—

1*568

*043

0-246

3-096

100-000

100-00

100-00

100-33

100-00

100-238

100-000

100-000

100-11

100-000 100-000

Boussh

gault.

IlartwcU.

II. ltose.

Gobel.

T. Thomson.

The manufacture of alum from alum schists maybe distributed under the six following heads:—■

Preparation of the alum shale.

Lixiyiation of the roasted shale.

Evaporation of the lixivium.

Addition of the saline ingredients to precipitatethe alum.

Washing of the aluminous salts.

Crystallization.

Preparation of the Alum Shale,—Boasting or Ustu-lation .—Some alum slates are of such a nature that,when piled in heaps in the open air, and moistenedfrom time to time, they become hot spontaneously,and by degrees fall into a pulverulent mass, which isfit for lixiviation. The greater part of the ores,however, require the roasting process. By roastingthe cohesion of the dense slates is so much impairedthat their disintegration becomes more rapid ; thedecomposition of the pyrites is quickened by theexpulsion of a portion of the sulphur; and the sul-phate of iron already formed is partly decomposedby the heat, and transfers its sulphuric acid to theclay, producing aluminium sulphate.

Such alum shales as contain too little bitumen, orcoal, for the roasting process, must be interstratifiedwith layers of small coal or brushwood. The fuelbeing kindled, the whole slowly ignites. More rockis piled upon it, until in some instances a vast heapof inflamed material, 100 feet high and 200 feetsquare, is raised, which continues to burn for months.When the heap is fired with brushwood, the ash,containing potash, gives rise to the formation ofsmall quantities of alum.

Alum is manufactured at Whitby by the combus-tion of the schists of the upper lias, which contain aquantity of iron pyrites and bituminous or carbon-aceous matter. The temperature being regulatedand water occasionally supplied, decompositionoccurs, producing sulphates of alumina, iron, andmagnesia, as follows :—

4FeSo + Al s Oo + MgO + 0. 28 + 32H„0 = 4(FeS0 4 ,7H 2 0)+' A1 2 (S0 4 ) 3 18H 2 U + MgS0 4 ,7H 2 0.

In Rhenish Prussia , especially at Salzweiler, roast-

ing is effected by the aid of a stratum of brown coalbeneath it, which has continued in a state of restrictedcombustion ever since 1660, when it was accidentallyignited.

A clay bottom is best adapted for the erection ofa heap, as it prevents any of the salts being carriedby the moisture into the soil; the heap is also some-times constructed under a shed. As a general rule,the slower and more uniform the heat during thecalcination the better the produce.

The Scotch alum-works are those of Ilurlet andCampsie—the former six miles south from Glasgow ,the latter about nine miles north of the city, at thefoot of the Campsie Hills. The aluminous shalefound in this locality is interstratified between thecoal bed and the limestone, and is nearly as dark-coloured as the former; it is occasionally foundmixed with native crystals of sulphate of iron. Afterthe shale has been exposed to the air for some time,the black colour is replaced by a grey, owing to theaction of the air causing it to throw out a whiteefflorescence of alum.

A striking appearance is presented by the alum-field, which is covered throughout with ridges orelongated mounds of the ore for calcination, oralready calcined, and these assume a reddish-brownhue from the effect of the heat. The heaps vary insize, and contain from 6000 to about 20,000 tonseach. At one of the works under consideration,there are about 15 of the mounds or ridges of shale,each being 120 to 180 feet in length, with a base ofabout 20 feet, and a height of 15.

Fig. 2 shows the arrangement of the field.

The mounds are commenced by making a few firesof coal along the intended length, and covering themover with stones or bricks in any convenient manner,leaving lateral ducts, or passages, for the air toenter. The shale is then thrown on the fires, thecoarsest first; and as it ignites, and communicatesheat to the outer portions, more of the mineral isthrown upon it successively, until the heap is con-sidered large enough, the whole being kept at thatstate of ignition which practice leads the manager to