4. From Clay.—The process of obtaining alum from aulphate of alurnina prepared artificially from clays waa first practised by Alban and Chaptal. The claya selected should be altogether free from iron and carbonate of lime. Ordinary clays consist of silicate of alumina and free alumina; china or pipe-clay being much the beat fOr the above purpoae. The clay is first calciued in order to peroxidizo any iron which may be present, and to render it perfectly poroua ; it is then ground aud sifted, and digested with 45 per cent. of sulphurio acid at 45° B.; this digestion is conducted in a pan or cistern heated by the waste heat from the calcining furnace to about 70° (158° F.). When the mass haa become thick from separation of silica, it is removed from the pan and exposed for a long period to the atmosphere, by which means the conversion of the alumina into sulphate is completely effected ; it is then lixiviated in stone or brick cisterns aud precipitated by the addition of an alkaline sulphate in the manner previously described.
The following process was patented by H. D. Pochin, and is atill employed in his worka at Salford, Manchester :—Fino china clay, as free as possible from iron and carbonate of lime, is calcined in a furnace in order to remove the water and render the clay porous. It is then mixed with a suitable proportion of sulphuric acid, which must be considerably diluted (sp. gr. 1.52), in order to prevent ita action being too energetic. The mixture is then run into a cistern having movable sides, where in a few minutea it becomes hot and boils violeutly. It now becomes gradually thicker until it is converted into a aolid but very porous mass. In this furm it is called " alum-cake," or " concentrated alurn." It containa much water, although it appears perfectly dry, and also much of the silica contained in the original clay, in a state of very fine division. A solution of pure sulphate of alumina is readily obtainable from this substance by lixiviatiou ; the solutiou so obtained is allowed to deposit its silica before being further used, though for many purposes the presence of silica is not objectionable. Ao the potaah or ammonia contained in ordinary alum is a wholly unnecessary conatituent, being only added to facilitate the purification and crystallization of the ealt, the solutiou obtained by the above process may be adapted to all the processes of dyeing and calico-printing in which alum is so largely consumed. In order to convert the porous alum-cake into alum, a concentrated solution must be made and precipitated with potash or ammonia, or both, the process being then conducted as described above. If the proper quantity of aulphuric acid has not been used in the preparation of the cake, it must be added before preci pitating with the alkaline solution.
Pipe-elay has also been largely used by Taylor of Bristol, in the manufacture of alum. The clay is calcined in the ordinary way, and mixed with about an equal weight of sulphuric acid (sp. gr. 1.20). This mixture is run into a tub and heated for about twelve hours by means of a coil of leaden pipe, lying on the bottom of the tub, through which ateam is made to pass ; the heat attained should be about 90° to 100°, or rather below the boiling point of water. The solution
ehould then have a sp. gr. of about 1.3, and is run off tluough a pipe into other vats, where it is precipitated with an alkaline solution in the usual manner.
5. From Mineral Phosphates.—In the year 1870, patent was secured by Peter Spence for obtaining alum from a mineral consisting of phosphates of iron and alunaina, and-obtained from the ialand of Redonda, near Antigua, in the West Indies. The mineral is taken iu pieces, as it comes to hand, and c.alciued in kilna similar. to those used for lime, at a red heat, being previously mixed with coal or coke ; or it is ground until it passes through a sieve of, say, twenty meshes to the inch ; but the former plan is preferred by Spence, as it facilitates the aolution of the mineral substance, and renders a portion of the iron insoluble by oxidation. The mineral having been prepared by these or similar means, it is placed in leaden vessels together with an equal weight of sulphuric acid of sp. gr. 1.6 if the mineral contain 20 per cent. of alumina, but only three-fifths of its weight if it contain 12 per cent., and in similar proportions for other degreea of richness. Heat is then applied by blowing steam into the vessel containing the mixture. The mineral dissolves and the specific gravity risea. It is then cautiously reduced by water or weak liquors from subse quent parts of the process (especially the washings of the sediment hereafter to be mentioned), constantly boiling until all is dissolved except the insoluble sediment, and the strength becomes 90° Tw., or 1.45 sp. gr. The liquor is now passed into a close leaden vessel, and a vapour is distilled into it containing ammonia obtained from gaa ammoniacal liquor, subjected to boiling either by fire or steam injected into the gas-liquor. The quantity of gas-liquor employed is 600 to 900 gallons for every ton of the mineral, according to its richness. When all the ammonia has been distilled into the liquid, it is allowed to aettle for a few hours, and the clear solution is run off at a sp. gr. of 1.4, or 80° Tw., into leaden coolers to crystallize in the ordinary way. From a mineral containing 20 per cent. of alumina, Spence obtains a ton and a half of alum for every ton of the mineral used. The mother liquor consists chiefly of a solution of phosphoric acid, and with a small quantity of sulphate of alumina, iron, and sulphate or phosphate of ammonia. This solution may be used directly as a fertilizing agent ; or by adding aawdust in sufficient quantity to absorb the whole of the liquor, and then drying it at a low heat, ao as not to char the sawdust, an artificial manure may be prepared containing phosphoric acid and ammonia in such quantities and condition as to make it a valuable fertilizer. The phosphoric acid contained in the mother liquor may also he applied to the purposes of producing phosphorus or the commercial phosphates. The insoluble sediment alluded to above may be washed with water, and the washings used for the reduction of the dissolved mineral.