114 ALUM 



course of the year, or at least during the severity of the winter season, when the 

 calcination may be suspended, and the lixiviation becomes unsatisfactory. The beds 

 are known to be sufficiently decomposed by the efflorescence of the salt -which appears 

 upon the stones, from the strong aluminous taste of the ashes, and from the 

 appropriate chemical test of lixiviating an aliquot average portion of the mass, and 

 seeing how much alum it will yield with solution of sulphate of potash or chloride of 

 potassium. 



2. The Lixiviaticn. The lixiviation is best performed in stone-built cisterns ; those 

 of -wood, however strong at first, Are soon decomposed, and need repairs. They 

 ought to be erected in the neighbourhood -of the calcining heaps, to save the labour 

 of transport, and so arranged that the solutions from the higher cisterns may spon- 

 taneously flow into the lower. -In this point of view, a sloping terrace is the best 

 situation for an alum work. In the lowest part of this terrace, and in the neighbour- 

 hood of the boiling-house, .there ought to be two or more large tanks, for holding the 

 crude lixivium, and they should be protected from the rain by a proper shed. Upon 

 a somewhat higher level the cisterns,of the clear lixivium may be placed. Into the 

 highest range of cisterns .the calcined mineral is to be put, taking care to lay the 

 largest lumps at tho bottom, ,and to cover them with lighter ashes. A sufficient 

 quantity of water is now to be run over it, and allowed to rest for some time. The 

 lixivium may then bo drawn off, by a stopcock connected with a pipe at the bottom of 

 the cistern, and run into another cistern at a somewhat lower level. Fresh water 

 must now be poured on the partly exhausted schist, and allowed to remain for a 

 sufficient time. This lixivium, being weak, should be run off into a separate tank. 

 In some cases a third addition of fresh water may be requisite, and the weak lixivium 

 which is drawn off may be reserved for a fresh portion of calcined mineral. In .order 

 to save evaporation, it is always requisite to strengthen weak leys by employing them 

 instead of water for fresh portions of calcined schist. Upon the ingenious disposition 

 and form of these lixiviating cisterns, much of the economy and success of an alum work 

 depends. The hydrometer should be always used to determine the degree of concen- 

 tration which the solutions acquire. 



The lixiviated stone, being thus exhausted of its soluble ingredients, is to bo 

 removed from the cisterns, and piled up in a heap in any convenient place, where it 

 may be left, either spontaneously to decompose, or, after drying, subjected to another 

 calcination. 



After calcining and washing the Campsie ores, the residue had the following com- 

 position : 



Silica ......... 38-40 



Alumina 1270 



Peroxide of iron 20-80 



Oxide of manganese traces 



Lime 2-07 



Magnesia . . 2-00 



Potash 1-00 



Sulphuric acid . . . . . '.' 10 '76 



Water 12-27 



100-00 



It is, therefore, very far from being a complete process ; but it is not considered 

 profitable to remove tho whole of the alumina. In some places the exhausted ore is 

 burnt a second time with fresh ore, as at Campsie, but wo are not told the estimated 

 exhaustion. 



Tho density of the solution may be brought, upon an average, up to the specific 

 gravity of from 1 -09 to 1-16. The latter density may always be obtained by pumping 

 up the weaker solutions upon fresh calcined mine. This strong liquor is then drawn 

 off, when the sulphate of lime, the oxide of iron, and the earths are deposited. It is 

 of advantage to leave the liquor exposed to air for some time, whereby the green 

 Vitriol may pass into a persulphate of iron with the deposition of some oxide, when 

 the acid will act better on tho clay present, so as to increase the quantity of sulphate 

 6f alumina. Tho manufacture of alum is the more imperfect, as the quantity of sul- 

 phate of iron loft undecomposod is greater; and therefore every expedient ought to be 

 tried to convert the sulphate of iron into sulphate of alumina. 



' 3. The Evaporation of the Schist Lixivium. As the aluminous liquors; however 

 well settled at first, are apt, on tho great scale, to deposit earthy matters in the course 

 of their concentration by heat, they are best evaporated by a surface fire, such as that 

 employed at Hurlet and Campsie. A water-tight stone cistern must be built, haying 

 a layer of well-rammod clay behind tho flags or tiles which line its bottom and sides. 



