502

CHLORIDE OF ALUMINUM.

is conveyed nearly to the bottom of this retort by means of a porcelain tube, f, luted into the tubulure: the gas reacts upon the mixture in the retort, forming carbonic oxide and chloride of

aluminum; and the chloride of aluminum condenses in the gasjar, h, which is placed for its reception: the open mouth of this jar is closed by means of a funnel, luted on with a strip of pasted paper; and the carbonic oxide escapes through the open tube, i, into the chimney. In order to purify the crude chloride of aluminum from the small quantity of volatile ferric chloride which usually accompanies it, the compound is redistilled from iron wire, by which the ferric chloride is converted into ferrous chloride which is much less volatile, and the chloride of aluminum sublimes nearly in a state of purity.

Deville prepares this chloride on a large scale from a mixture of coal tar and alumina, which is heated in a clay retort, such as is used in gas making; a current of chlorine is sent over the ignited mass, and the product of the operation is received in a chamber lined with glazed brickwork. (See also par. 660)

If chloride of aluminum be heated in considerable mass, it melts at a dull red heat, and near its fusing-point sublimes rapidly; when exposed to the air it emits fumes of hydrochloric acid: it is very deliquescent, and when thrown into water hisses from the heat developed by the violence of the combination. This solution, when concentrated by a very moderate heat, yields crystals with the formula Al,Cl. 12 H2O. It is soluble in alcohol. By

FLUORIDE AND SESQUISULPHATE OF ALUMINUM.

503

subliming chloride of aluminum in a current of sulphuretted hydrogen it forms a combination with this gas: this compound is decomposed by resublimation, or by solution in water. The chloride may so be made to combine with phosphuretted hydrogen, and with ammonia.

(665) FLUORIDE OF ALUMINUM occurs native, combined with fluoride of sodium, forming cryolite (3 NaF,AlF). It may be obtained in large quantity from Greenland, and as it is easily decomposed by sodium, it has been employed as a source of metallic aluminum, of which it contains 13 per cent. Another highly prized aluminous mineral, containing fluorine, is the topaz, which is extremely hard; the colourless variety of it has a lustre which has sometimes caused it to be mistaken for the diamond. Its composition may be represented by the formula [2 (Al‚Ð ̧‚SiO„) Al Ꮎ ,ᎦᎥF ] .

3

4

3

It

(666) SESQUISULPHATE OF ALUMINUM (Al, 3 SO. 18 H2O =343 + 324), or Al2O3,3 SO ̧ . 18 HO = 171'5+162.-This salt is formed by dissolving alumina in sulphuric acid. It is now manufactured on a large scale in the north of England, by mixing finely-powdered clay or shale, after it has been gently roasted, with about half its weight of crude sulphuric acid from the chambers, heating it gradually until fumes of acid begin to escape; this digestion is continued for 3 or 4 days, after which the mass is lixiviated, and the solution thus obtained is freed from iron by the addition of ferrocyanide of sodium so long as it occasions a blue precipitate; the clear liquid is decanted and evaporated, and the residue is sold under the name of concentrated alum. crystallizes in thin flexible scales which are soluble in twice their weight of cold water: this solution may be used as a test for potassium, for by mixing it with a solution containing a salt of this metal, and evaporating, octohedral crystals of alum are deposited. Sulphate of aluminum has a strong tendency to form double salts with monobasic sulphates, of which those with the sulphates of potassium and ammonium, constituting potash- and ammonia-alum respectively, are the most important. A remarkable anhydrous sulphate of aluminum which assumes the form of a white mealy powder, insoluble in cold water, but which may be rendered soluble, and converted into the ordinary sulphate, by prolonged boiling, is obtained by boiling either cryolite, or ordinary alum, with from three to ten times its weight of oil of vitriol, and distilling off about three-fourths of the sulphuric acid; the acid sulphate of potassium or of sodium, may be removed by washing, and the anhydrous sulphate is left as a

504

MANUFACTURE OF ALUM.

white powder, analogous to the corresponding modification of the ferric and chromic sulphates (Persoz).

A basic sulphate of aluminum, soluble in water, and of a yellow colour, may be obtained; the yellow is not due to the presence of ferric sulphate (Siewert).

(667) ALUM; Sulphate of Aluminum and Potassium (KAI 2 SO. 12 H2O,=258°5+216), or (KO,SO,Al2O3, 3 SO ̧ . 24 Aq): Sp. Gr. anhydrous, 2228; crystallized, 1726.-This valuable salt is occasionally found native in volcanic districts, in the form of a white efflorescence, produced by the action of the sulphuric acid of the volcano upon the alumina and potash contained in the lava and trachytic rocks. For the purposes of commerce, however, alum is manufactured artificially. Three principal methods are adopted:

1. In the first the alum is procured by the addition of sulphate of potassium to the crude sulphate of aluminum prepared from clay by the process just described.

2.-A still simpler method is practised in Italy, where, especially in the neighbourhood of Civita Vecchia, the alum-stone is abundant. This rock contains the elements of alum, with an excess of hydrate of alumina, mixed with a variable proportion of siliceous matter. The ore is first roasted at a gentle heat in kilns, avoiding direct contact with the fuel: water is thus expelled, and the mass is rendered spongy; the hydrate of alumina is decomposed, and the formation of a basic sulphate of aluminum and potassium, which is insoluble in water, is thereby prevented: the roasted ore is then arranged in long heaps or ridges upon a firm clay floor, where it is frequently moistened with water: in the course of two or three months the mass crumbles down into a sort of mud, which is lixiviated: and the solution when evaporated yields crystals of alum, which after a second crystallization are fit for the market. This variety of alum, known as Roman alum, crystallizes in opaque cubes, which retain basic sulphate of aluminum.

3.-A third process is resorted to in England and Germany for the purpose of turning alum schist, or alum ore as it is termed, to good account. This mineral is abundant at Whitby, in Yorkshire, and in the neighbourhood of Glasgow: it is a bituminous shale, found amongst the lower beds of the coal-measures, and it contains a large quantity of very finely divided iron pyrites, disseminated through its mass, which is composed chiefly of a siliceous clay. The mineral is decomposed either by exposure to the air, or, as is more usually practised, by a slow roasting, conducted

MANUFACTURE OF ALUM.

505

upon the ore arranged with alternate layers of fuel in long heaps or ridges, which are covered more or less completely with spent ore, in order to regulate the heat and to absorb the excess of sulphuric acid. In this operation the pyrites, or bisulphide of iron, is converted into the protosulphide of iron, losing half its sulphur, which absorbs oxygen and is converted into sulphuric anhydride; this at the moment of its formation unites with the alumina, while the protosulphide of iron, gradually combining with more oxygen, is converted into ferrous sulphate, or green vitriol : 2 FeS2+32=2 FeS +2 SO3; and FeS+20,=FeSO4. Great care is required to prevent the temperature from rising too high, a circumstance which would be attended with decomposition of the sulphate of aluminum and loss of sulphuric acid. By the time that the roasting is complete, the mass has become greatly reduced in bulk, and is rendered porous and freely permeable to the air; in this condition the heap is allowed to lie exposed to the atmosphere, and is moistened from time to time; it is then lixiviated, the liquor is digested on metallic iron to reduce any ferric salt to the state of ferrous sulphate, and the green sulphate of iron is separated from the sulphate of aluminum by crystallization of the liquor. The mother-liquors often yield sulphate of magnesium when concentrated further.

In the Whitby alum works, in which the quantity of the sulphate of aluminum much exceeds that of the sulphate of iron in solution, the concentration is completed in leaden pans; being carried so far as that the liquid shall, when cold, be perfectly saturated, but shall deposit no crystals. The liquid is then run off into the precipitating tank, where it is mixed with a saturated solution of sulphate of potassium, or, what is better, of chloride of potassium, in quantity sufficient (as found by trial on the small scale) to yield the maximum proportion of alum. The mixture is briskly agitated, and the double sulphate of aluminum and potassium, which is sparingly soluble in cold water, is deposited in minute crystals, technically termed alum meal or flour. When chloride of potassium is used the sulphate of iron is decomposed, sulphate of potassium is produced, and the very soluble ferrous chloride is retained in the liquor; 2 KCl+FeSO1=FeCl2 + K2SO4• To produce 100 parts of crystallized alum, between 18 and 19 parts of sulphate of potassium are required, or about 16 parts of the chloride of potassium. The mother-liquor is drained off and preserved, and the crystals, which have a reddish-brown colour from adhering iron, are twice washed by subsidence with a small quantity of cold water, being well drained after each washing. The crystals

are then dissolved by heat in as small a quantity of water as possible, and the solution is run off into crystallizing barrels, which in ten days or a fortnight are taken to pieces; the crystalline mass is broken into fragments, drained, and sent into the market.

In the Scotch alum works at Campsie, in the neighbourhood of Glasgow, alum meal is not formed; but the hot liquor from the evaporating pan is run into a stone cooler, in which the necessary quantity of dry chloride of potassium has been placed. The liquid is thoroughly agitated and left to cool; on the sides of the vessel large crystals of alum are formed in four or five days. The mother-liquor is then drained off, and the crystals are afterwards washed and recrystallized twice.

Where sulphate of ammonium can be obtained sufficiently cheap, it is substituted for sulphate of potassium' in the manufacture of alum, as the double salt which it forms with sulphate of aluminum crystallizes with almost as much facility as the potassium salt; it constitutes what is known as ammonia alum. In England at present the greater part of the alum which is made is ammonia alum. Indeed, for the purposes to which alum is applied, neither the sulphate of potassium nor that of ammonium is essential; the object proposed in the manufacture of alum being to obtain a salt of aluminum which, by the facility with which it crystallizes, can be freed from iron and from earthy impurities.

:

A number of other salts may be procured which have the same crystalline form as potassium alum, and are similar to it in constitution thus, sulphate of potassium may be displaced by sulphate of sodium, and a sodium alum may be formed, but the compound is much more soluble than potassium alum: in like manner the place of the sulphate of aluminum may be supplied by ferric, chromic, or manganic sulphate, forming a remarkable series of isomorphous compounds, some of which are enumerated in the annexed table :—

Besides these true alums, a number of double salts of aluminum may be formed with the sulphates isomorphous with that of mag