SODIUM 



513 



somewhat porous limestone), and 500 parts of small coal is charged 

 from above. This mixture is first heated in the portion of the furnace 

 which is furthest removed from the fire-grate ; it is then brought to the 



FIG. 69. Kovorberatory furnace for the manufacture of sodium carbonate. F, grate. A, bridge. 

 M, hearth for the ultimate calcination of the mixture of sodium sulphate, coal, and calcium car- 

 bonate, which is charged from above into the part of the furnace furthest removed from the fire F. 

 P, P, doors for stirring and bringing the mass towards the grate F by means of stirrers R. At the 

 end of the operation the semifused mass is charged into trucks C. 



portion nearest to the fire-grate, when it is stirred during heating. 

 The partially-fused mass obtained at the end of the process is cooled, 

 and then subjected to methodical lixiviation 1S to extract the sodium 



15 Methodical lixiviation is the extraction, by means of water, of a soluble substance 

 from the mass containing it. It is carried on so as not to obtain weak aqueous solutions, 

 and in such a way that the residue shall not contain any of the soluble substance. This 

 problem is practically of great importance in many industries. It is required to extract 

 from the mass all that is soluble in water. This is easily effected if water be first poured 



FIG. 70. Apparatus for the methodical lixiviation of black ash, &c. Water flows into the tanks 

 from the pipes r, r, and the saturated liquid is drawn off from c, c. 



on the mass ; the strong solution thus obtained is then decanted, then water is again 

 poured on, time being allowed for it to act ; then again decanted, and so on until the 

 water does not take up anything. But then finally such weak solutions are obtained 

 that it would be very disadvantageous to evaporate them. ' This is avoided by pouring 

 VOL. I. L L 



