ALUM.— From Felspar. 173

AlKSi0 4 ,2SiO, 2 —is decomposed by potassium sul-phate in a reverberatory furnace, and then fused withcarbonate of potash to a glass ; on treating this withboiling water decomposition follows, a soluble silicateof potash dissolves, while an insoluble double silicateremains. The insoluble portion, when treated withboiling sulphuric acid, is decomposed into alum, andsilica separates. The alum is dissolved out by water,and the solution evaporated for crystallization; thesoluble alkaline silicate may bo turned to account bycombining the silica with lime.

Turner was the first to cany out the propositionof Sprengel, of converting felspar into alum. Apatent was granted to him in 1842 for his process,which he describes in his specification as follows:—If desirous of making potash alum, the best substanceto operate upon is a potash felspar. This felspar isground in a common edgestone mill, till the powderis like fine sand, a process which is much assisted byheating it to redness, and then plunging it in coldwater; it is afterwards mixed with its own weight ofsulphate of potash, and placed in the upper part ofthe inclined bed of a reverberatory furnace, similarto the annexed Fig. 14 —known in the potteries as aJrit-furnace —previously brought to a red heat. When

the glass produced by the fusion of the powderedmineral and the alkaline sulphate has been observedto flow down to the bed of the furnace, there is addedgradually, at the lower end of the furnace, as muchcarbonate of potash as was previously used of sulphate.This operation of adding proportionate quantities ofcarbonate of potash to the molten mass as it flowsdown into the lower part of the furnace, is continuedtill the “sack” of the furnace is quite full, the mineraland sulphate being introduced at the upper end: theglass is then fit for the next operation.

The compound might be prepared in a furnacewith a fiat bottom, from which it can be separatedwith greater facility; in this case, however, no car-bonate should be used till the whole of the sulphatehas been seen to be decomposed, after which it is tobe introduced, as in the preceding instance, and fusedwith the mass. On boiling the glass produced in thismanner with water, the same quantity of potash aswas added to the felspar, and two-thirds of the silicacontained in the mineral, are dissolved, while theremaining one-third of silica, and the alumina, andan equal quantity of potash as the felspar originallycontained, are left in the form of a light porous sub-stance, which is carefully separated from the solution,and washed well with water to withdraw potassium

silicate; next, the porous precipitate is put into alarge leaden cistern or boiler, and acted upon withboiling sulphuric acid, of specific gravity T20, whichwill contain sufficient water for the solution andcrystallization of the alum formed by the decomposi-tion of the mass, As a general rule, as much acidshould be added in the diluted state as will contain160 lbs. of dry acid to every 285 lbs. of felsparemployed. The boiling liquor, after the sedimenthas subsided, is drawn into coolers, such as aregenerally used for crystallizing alum ; here aboutfour-fifths of the alum contained in the liquor willseparate into crystals. From this mother liquor theresidual alum is obtained by evaporating it to dry-ness ; by this means the silica present in the solutionis rendered insoluble, and the dry mass is acted uponeither with water or a further quantity of motherliquor, and the solution evajiorated to the crystalliz-ing point as before. When ammonium or sodiumsalts are used in the above process, ammonium andsodium alums are produced.

With the exception of the high temperature re-quired to decompose the felspar, Turner’s processis very successful, but this was found to be a seriousdrawback when working on the large scale.

Alum may likewise be obtained by treating a mix-ture of two parts of felspar and three of fluorsparwith sulphuric acid at a low red heat, until vapours ofhydro-fluosilieic acid cease to be evolved. The decom-position of the fluorspar by the sulphuric acid causesthe liberation of hydrofluoric acid, and this in turndecomposes the silicates, combining with their silicicacid to form the evolved hydro-fluosilicic acid. Sul-phate of lime is formed in the first stage of the pro-cess; but this is subsequently decomposed, andpotassium and aluminium sulphates formed.

The mass is lixiviated with water, and after theseparation of some sulphate of lime, the solution isevaporated, and on cooling it deposits an abundantcrop of nearly pure alum crystals.

From Cryolite or Greenland Sraij.— This min-eral has the composition:—

Aluminium,..13*0.

Fluorine,..54-5

Sodium,.32-5

1U0-0

and is represented by the formula, Al 2 Na 3 F 8 . Ithas of late years been much used for the productionof soda and alum. Its decomposition is effected inthe dry way by igniting it with calcium carbonate inthe proportion of 6 equivalents of Ca 2 CO s to 1 ofof cryolite, when

Cyrolite . Calcium Fluoride. Sodium Aluminate.

Al 2 Na 3 F 6 + 6Ca 2 C0 3 = 6CaF + Al„Na 3 0 3

The sodium aluminate is extracted with water de-canted from the heavy calcium fluoride. Carbonicacid gas is then passed into the solution, when sodiumcarbonate is formed, and gelatinous alumina is pre-cipitated. This precipitate is collected, dissolvedin dilute sulphuric acid, and evaporated to obtainaluminium sulphate, or treated with an ammon-ium or potassium salt to convert it into alum: