ALUM 119 



mineral dissolved out by sulphuric acid, and used in the manufacture of alum, but 

 the phosphoric acid set free forms a highly valuable by-product. 



The following analysis may be cited as that of a good sample of the Eedonda 

 phosphate : 



Phosphoric acid 42-6 



Alumina . 26*1 



Peroxide of Iron 3-5 



Silica 2*1 



Water 25-2 



9lF6 



The mineral is first calcined at a red heat, whereby the water is expelled, and tho 

 material rendered porous, so as to be more freely acted on by sulphuric acid. If 

 desirable, the calcination may be dispensed with, and the raw material merely pre- 

 pared by grinding. In either case the phosphate is digested in leaden vessels with 

 sulphuric acid of specific gravity T6. 



The proportion of sulphuric acid varies with the richness of the mineral in alumina ; 

 thus, if it contain 20 per cent, of alumina, an equal weight of acid will be used ; if 

 only 12 per cent, of alumina, about three-fifths its weight of acid ; and so forth. To 

 facilitate solution of the mineral by the acid, heat is applied most conveniently by 

 blowing steam into the vessel. As solution proceeds, the density of the liquid rises, 

 and the strength of the liquor is then reduced by adding water, or weak liquors from 

 subsequent stages of the process, and the liquid is boiled until its specific gravity 

 ultimately reaches 1'45, or 90 Twaddle. This liquid is then transferred to a closed 

 leaden vessel, and ammoniacal vapour from gas-liquor is distilled into it. Tho 

 sulphate of ammonia thus formed combines with the sulphate of alumina to form 

 ammonia-alum. From 600 to 900 gallons of gas-liquor are used to each ton of 

 phosphate. After introduction of ammonia, the liquor is allowed to settle, and the 

 clear solution, which has now a specific gravity of 1'4, or 80 Twaddle, is run into 

 leaden coolers, where the alum crystallises. When the phosphate contains 20 per 

 cent, of alumina, it yields about a ton-and-a-half of alum per ton. One of tho great 

 difficulties in this process consists in completely freeing the alum from the associated 

 phosphoric acid. 



Whilst the alumina of the phosphate is thus utilised for the production of alum, 

 the phosphoric acid, which is at the same time eliminated, forms a product of high 

 value to the agriculturist. Indeed the mother-liquor from which the alum is de- 

 posited consists chiefly of a solution of phosphoric acid, with a small proportion of 

 sulphate of alumina, iron, and sulphate or phosphate of ammonia. This liquid may 

 be used as' a fertilising agent either directly or made into an artificial manure by 

 being absorbed in sawdust and dried at a low heat. 



We believe that Mr. Spence's new process is now carried out on a large scale at 

 the Pendleton Alum Works. 



Other Methods of Alum Manufacture. In addition to the four principal processes 

 of manufacture already described, numerous other methods have from time to timo 

 been proposed, and in some cases worked to a limited extent. The more important 

 of these suggested improvements are included in the following notices, which are 

 arranged in chronological order. 



In 1743, Ambrose Newton wished to economise the manufacture by boiling the 

 scum of the alum works, tlio muddy cleposit in Yorkshire, and adding to the concen- 

 trated solution of 45 pennyweights, stale urine, which is ammonia, until the solution 

 became 27 pennyweights. The liquor stands 'for four days, and strikes out into 

 small allom, and afterwards melted and reached into casks, which stand 14 days, and 

 are taken down and the allom is finished.' 



Another patent in 1765, by Holme, Cropper, and two Nicholsons, uses stale urine 

 and kelp liquor. They seem to use, by a mistake in names, iron pyrites only for their 

 alum, but no doubt it contained both iron and alumina. They took advantage of the 

 potash, and perhaps also of the soda, of the kelp. 



In 1780, Matthew Sanderson patented a plan for making alum by burning tho 

 metallic sulphurets, obtaining the sulphuric acid, and uniting it with aluminous earth 

 a far-seeing plan, not till long after adopted. 



In 1794 Lord Dundonald patented a process for 'washing aluminous, vitriolic, or 

 pyritous schist or materials with sea-water or solutions of salts containing muriate of 

 soda,' or mixing muriate of soda with aluminous or vitriolated salts or pyritous 

 substances. He also proposed tho use of muriatic acid. It is probable, then, that 

 both a soda- and an ammonia-alum have been manufactured when the whole method 

 was not very clearly understood. 



