362 



ANALYSIS OF ALUM ORES. 



Lixiriation. 



Oxidation of 

 the iron. 



Preparation of The aluminous schists, from which alum maybe obtained, 

 must undergo a process preparatory to their lixiviation. In 

 the aluminous schists properly so called, which are hard, 

 of a stony texture, and contain a great deal of pyrites, the 

 preparatory process consists in roasting. But for the softer 

 alum ores, such as that of Freienwald, exposure to the 

 air is sufficient. When the ore is extracted from the mine, 

 it is placed in large heaps, sloping to a ridge like the roof 

 of a house, and left exposed to the open air for a year or 

 more. When its decomposition, which is particularly pro- 

 moted by damp air, is sufficiently advanced, it is distributed 

 into long flat troughs, and lixiviated. When the water is 

 sufficiently saturated with the salts, which are sulphate of 

 alumine and sulphate of iron, it is carried to the manufac- 

 tory, and boiled in leaden caldrons, till the, proof liquor 

 taken out becomes on cooling a crystalline mass of the con- 

 sistence of honey. During the long boiling of the lixivium, 

 the greater part of the sulphate of iron is decomposed, the 

 iron passes to a higher degree of oxidation, in which state 

 so much of it cannot be dissolved in sulphuric acid, and it 

 is deposited in the form of brown oxide. When the lixivi- 

 um is sufficiently boiled down, it is carried to the settling 

 troughs, and as soon as it has grown clear by standing a 

 little, it is drawn oft" into other troughs, where it is mixed 

 with the quantity of potasli necessary for making it into 

 alum. 

 Thi<; obtained At Freienwald, as at most alum works, they use for 

 from soap-boil- supplying the alum with this potash the saline mass obtained 

 from soap manufactories, where soap is made with an alka- 

 line lie and muriate of soda, by boiling the spent lie to dry- 

 ness. The muriate of potash contained in this saline masS 

 is decomposed the instant it is mixed with the aluminous 

 lixivium : the potash unites with the sulphate of alumine, 

 and forms alum, which can no longer continue in solution 

 in the concentrated lixivium, and is precipitated in the form 

 of small crystalline grains, known by the name of alum 

 meal. The muriatic acid, thus set free, lays hold of the 

 oxide of iron, and prevents its falling down with the alum. 

 Other matters Instead of the saline mass from the soap-makers, matters 



Potash added. 



might be used. 



containing 



sulphate of potash might be employed, as the 



residuuH) 



