100 parts by weight of cryolite yield 305 partsof alum.
Cryolite is also decomposed by boiling it, after hay-ing pulverized it very finely, with lime and water ina leaden pan. The result of the interchange of con-stituents is the same as that given above, aluminateof soda and fluoride of calcium being produced.When the heavy calcium salt hag settled down theclear liquor is decanted and the sediments washed,the first washings being added to the original liquor,and the second and third washings used instead ofpure water at a subsequent stage. The alumina isthen precipitated by adding finely pulverizd cryolitein excess, thus:— 1
Sodium Aluminate. Cryolite. Alumina. Sodium Fluoride .
Al 3 Na a 0 3 + Al 2 Na 3 Kg = 2Al 2 Na 3 Fg + tiNal 1 .
The gelatinous alumina is allowed to settle, andtreated by any of the methods previously described.
Spence of Manchester has recently patented aprocess for the manufacture of alum from Rodondaphosphates, a mineral from the West Indies con-taining alumina with phosphate of iron in variablequantity.
The treatment for the aforesaid purposes of thesaid minerals may be varied in details, but the fol-lowing is a description :—The raw mineral is calcinedin kilns (similar to those used for lime) by mixing itwith coal or coke and exposing it to a red heat, orelse it is ground sufficiently fine to pass through asieve of twenty meshes to the inch. The former planis, however, preferable, as it facilitates the solutionof the mineral substance, and renders a portion ofthe iron insoluble by oxidation. The mineral havingbeen prepared by these or similar means, is thenplace'd in leaden vessels, and an equal weight of sul-phuric acid of spec. grav. 1-6 added to it, if it contains20 per cent, of alumina, but only three-fifths ofits weight if it contain 12 per cent., and in similarproportions for other degrees of richness. Iloat isthen applied by blowing steam into the vessel con-taining the mixture. The mineral dissolves and thespecific gravity rises. This is cautiously reduced byaddition of water or weak liquors from subsequentparts of the process, the whole being kept constantlyboiling until all is dissolved except the insolublesediment, and the strength of the liquor becomes90 Twaddle, or 1-45 spec, g-rav. This liquor is nextpassed into a close leaden vessel, and vapour dis-tilled into it containing ammonia obtained from gasammoniacal liquor, subjected to boiling either by fireor steam injected into it. The quantity of gasliquor I used being from 000 to 900 gallons to eachton of the mineral, according to its richness. Whenall the ammonia has been distilled into the mineralliquor it is allowed to settle for a few hours, and allthe clear solution (now at a strength or spec. grav.of 1-4 or 80 Twaddle) is run off into lead coolers,where the alum crystallizes out in the usual way.The liquor remains in these coolers for some days,with frequent stirring, in order to obtain all thealum possible. Spence finds that when the mineralcontains 20 per cent, of alumina, that he obtainsabout 1.V tons of alum from 1 ton of the ore.
The mother liquors having deposited all the alumthat can be obtained, is now chiefly a solution ofphosphoric acid with a small quantity of aluminiumand iron sulphate, and ammonium sulphate or phos-phate of ammonia. This liquid may be used directlyas a fertilizing agent, but is usually mixed with drysawdust or other absorbing agent, in just sufficientquantity to absorb all the liquor. It is then driedat a low heat, so as not to char the sawdust; andwhen dry it forms an artificial manure containingphosphoric acid and ammonia, in such quantities andcondition as to make it a valuable fertilizer. Insteadof ammonia gas liquor used with the mineral solu-tion to produce alum, salts of potash may be usedeither alone or in combination with ammonia; ofthe former, the chloride of potassium of commerce,or preferably sulphate of potash, as although chlorideof potassium will yield a sufficient product of alum,the fertilizer would from its use have a tendency todeliquesce, but sulphate of potash will not have thateffect.
Wilson, of Glasgow , patented a process for themanufacture of alum, the chief novelty of 'whichconsisted in heating the liquids for the digesting theshale, and in applying the mother liquors after pre-cipitating or separating the alum, so as to make thesulphuric acid, either free or in combination withother bodies, available in subsequent operations.
The mixture of sulphuric acid and water is warmedin a separate vessel, and then run into the digestingtank containing the calcined shale, at a temperaturebetween 150° and 200° Fahr. (65° - 5 to 93°'3 C.), andthe heat maintained at the proper degree by con-ducting the waste heat from the furnace of theheating vessel below, through flues under and aroundthe digesters,
To prevent the loss of sulphuric acid, the ordinarymethod is varied in the following manner:—Thestrong mother liquors, left over after depositing thealum formed from the combination of the aluminiumsulphate with the ammonium sulphate, are not usedagain to dilute the acid, but are employed with freshammoniacal liquor from the gas works or elsewhere,to form sulphate of ammonia. The decompositionof the portion of alum which the mother liquor heldin solution is guarded against by adding the ammon-iacal liquor in such quantities, that a slight portionof the acid in the liquor is uncombined. A fewtrials enable the operator to judge, by means of testpapers, the proper point at which the neutralizingshould be suspended.
When the solution of the mixture of ammoniumsulphate and alum, after filtration or reposing toallow impurities to deposit, is sufficiently concen-trated by evaporation, it is run-in among the solutionof aluminum sulphate from the digesters, and thealum which it contains is deposited with that of thefresh liquor.
Richardson, ih 1850, sealed a patent for furtherimprovements in the manufacture of alum, the lead-ing parts of which are as follow:—In lixiviating thecalcined shale for the purpose of obtaining the rawalum liquor, a series of pits, such as is seen in the