ALUM 115 



The cistern may be 4 or 6 feet wide, 2 or 3 feet deep, and 30 or 40 feet long, and it 

 is covered in by an arch of stone or brickwork. At one extremity of this tunnel, or 

 covered canal, a fire-grate ia set, and at the other a lofty chimney is erected. The 

 cistern being filled to the brim with the alum ley, a strong fire is kindled in the re- 

 verberatory grate, and the flame and hot air are forced to sweep along the surface of 

 the liquor, so as to keep it in constant ebullition, and to carry off the aqueous parts in 

 vapour. The soot which is condensed in the process falls to the bottom and leaves 

 the body of the liquor clear. As the concentration goes an, more of the rough lixi- 

 vium is run in from the settling cistern, placed on a somewhat higher level, till the 

 wholo gets charged with a clear liquor of a specific gravity sufficiently high for 

 transferring into the proper lead boilers. 



At Whitby, the lead pans are 10 feet long, 4 feet 9 inches wide, 2 feet 2 inches 

 deep at the one end, and 2 feet 8 inches deep at the other. This increase of depth 

 and corresponding slope facilitates the decantation of the concentrated lixivium by 

 means of a syphon applied at the lower end. The bottom of the pan is supported by 

 a series of parallel iron bars placed very near each other. In these lead pans the 

 liquor is concentrated, at a brisk boiling heat, by means of the flame of a flue beneath 

 them. Every morning the pans are emptied into a settling cistern of stone or lead. 

 The specific gravity of the liquor should be about 1'4 or 1'5, being a saturated solu- 

 tion of the saline matters present. The proper degree of density must vary} however, 

 with different kinds of lixivia, and according to the different views of the manufac- 

 turer. For a liquor which consists of two parts of sulphate of alumina, and one part 

 of sulphate of iron, a specific gravity of 1*25 may be sufficient; but for a solution 

 which contains two parts of sulphate of iron to one of sulphate of alumina, so that the 

 green vitriol must be withdrawn first of all by crystallisation, a specific gravity of 

 1*4 may be requisite. . 



The construction of an evaporating furnace well adapted to the concentration of 

 aluminous and other crude lixivia is described under SODA. The liquor basin may 

 be made of tiles or flags puddled in clay, and secured at the seams with a good 

 hydraulic cement. A mortar made of quicklime mixed with the exhausted schist in 

 powder, and iron turnings, is said to answer well for this purpose. Sometimes over 

 the reverberatory furnace a flat pan is laid, instead of the arched top, into which the 

 crude liquor is put for neutralisation and partial concentration. In Germany, such a 

 pan is made of copper, because iron would waste too fast, and lead would be apt to 

 melt. From this preparation-basin the under evaporating trough is gradually supplied 

 with hot liquor. At one side of this lower trough, there is sometimes a door, through 

 which the sediment may be raked out as it accumulates upon the bottom. Such a 

 contrivance is convenient for this mode of evaporation, and it permits, also, any 

 repairs to be readily made ; but, indeed, an apparatus of this kind, well mounted at 

 first, will serve for many years. 



In the course of the final concentration of the liquors, it is customary to add some 

 of the mother-waters of a former process, the quantity of which must be regulated by 

 a proper analysis and knowledge of their contents. If these mother-waters contain 

 much free sulphuric acid, they may prove useful in dissolving a portion of the 

 alumina of the- sediment which is always present in greater or less quantity. 



4. The Precipitation of the Alum, by adding Alkaline Salts. As a general rule, it is 

 most advantageous to separate, first of all, from the concentrated clear liquors, the 

 alum in the state of powder or small crystals, by addition of the proper alkaline 

 matters, and to leave the mingled foreign salts, such as the sulphate of iron or mag- 

 nesia, in solution, instead of trying to abstract those salts by a previous crystallisation. 

 In this way wo not only simplify and accelerate the manufacture of alum, and leave 

 the mother-waters to bo worked up at any convenient season, but we also avoid the 

 risk of withdrawing any of the sulphate of alumina with the sulphate of iron or mag- 

 nesia. On this account the concentration of the liquor ought not to be pushed so far 

 as that, when it gets cold, it should throw out crystals, but merely to the verge of 

 this point. This density may be determined by suitable experiments. The powder 

 of alum is also called flour. 



The clear liquor should now be run off into the precipitation cistern, and have the 

 sulphate of potash or chloride of potassium, or impure sulphate or carbonate of am- 

 monia, added to it. The sulphate of potash, which is the most direct, forms 18 '34 

 parts out of 100 of crystallised alum ; and therefore that quantity, or an equivalent 

 in chloride of potassium, or other potash, or ammoniacal salts, must be introduced 

 into the aluminous liquor. Since sulphate of potash takes 10 parts of cold water 

 to dissolve it, but is much more soluble in boiling water, and since the precipi- 

 tation of alum is more abundant the more concentrated the mingled solutions are, it 

 would be prudent to add the sulphate solution as hot as may be convenient : but> as 

 chloride of potassium is fully three times more soluble in cold water, it is to be pro* 



