522 PRINCIPLES OF CHEMISTRY 



bonate, the mixture of calcium oxide and sulphide forming the so-called 

 ' soda waste ' or ' alkali waster 14 



which has already been subjected to a first lixiviation by weak solutions. In this way the 

 fresh water gives a weak solution. The strong solution which goes to the evaporating 

 pan flows from those parts of the apparatus which contain the fresh, as yet unljxiviated, 

 mass, and thus in the latter parts the weak alkali formed in the other parts of the 

 apparatus becomes saturated as far as possible with the soluble substance. Generally 

 several intercommunicating vessels are constructed (standing at the same level) into 

 which in turn the fresh mass is charged which is intended for lixiviation ; the water is 

 poured in, the alkali drawn off, and the lixiviated residue removed. The illustration 

 represents such an apparatus, consisting of four communicating vessels. The water 

 poured into one of them flows through the two nearest and issues from the third. The 

 fresh mass being placed in one of these boxes or vessels, the stream of water passing 

 through the apparatus is directed in such a manner as to finally issue from this vessel con- 

 taining the fresh unlixiviated mass. The fresh water is added to the vessel containing 

 the material which has been almost completely exhausted. Passing through this vessel 

 it is conveyed by the pipe (syphon passing from the bottom of the first box to the top of 

 the second) communicating with the second ; it finally passes (also through a syphon 

 pipe) into the box (the third) containing the fresh material. The water will extract all 

 that is soluble in the first vessel, leaving only an insoluble residue. This vessel is then 

 ready to be emptied, and refilled with fresh material. The levels of the liquids in the 

 various vessels will naturally be different, in consequence of the various strengths of the 

 solutions which they contain 



It must not, however, be thought that sodium carbonate alone passes into the solution; 

 there is also a good deal of caustic soda with it, formed by the action of lime on the 

 carbonate of sodium, and there are also certain sodium sulphur compounds with which 

 we shall partly become acquainted hereafter. The sodium carbonate, therefore, is not 

 obtained in a very pure state. The solution is concentrated by evaporation. This is 

 conducted by means of the waste heat from the soda furnaces, together with that of 

 the gases given off. The process in the soda furnaces can only be carried on at a high 

 temperature, and therefore the smoke and gases issuing from them are necessarily very 

 hot. If the heat they contain was not made use of there would be a great waste of 

 fuel ; consequently in immediate proximity to these furnaces there is generally a series 

 of pans or evaporating boilers, under which the gases pass, and into which the alkali 

 solution is poured. On evaporating the solution, first of all the undecomposed sodium 

 sulphate separates, then the sodium carbonate or soda, crystals. These crystals as they 

 separate are raked out and placed on planks, where the liquid drains away from them. 

 Caustic soda remains in the residue, and also any sodium chloride which was not 

 decomposed in the foregoing process. 



Part of the sodium carbonate is recrystallised in order to purify it more thoroughly. 

 In order to do this a saturated solution is left to crystallise at a temperature below 80 

 in a current of air, in order to promote the separation of the water vapour. The large 

 transparent crystals (efflorescent in air) of Na 2 CO 3 ,10H 2 O are then formed which have 

 'already been spoken of (Chapter I.). 



14 The whole of the sulphur used in the production of the sulphuric acid employed in 

 decomposing the common salt is contained in this residue. This is the great burden 

 and expense of the soda works which use Leblanc's method. As an instructive example 

 from a chemical point of view, it is worth while mentioning here two of the various 

 methods of recovering the sulphur from the soda waste. Chance's process is treated in 

 Chapter XX., Note 6. 



Kynaston (1885) treats the soda waste with a solution (sp. gr. 1'21) of magnesium 

 chloride, which disengages sulphuretted hydrogen: CaS 4- MgCl 2 + 2H 2 O = CaCl 2 

 + Mg(OH) 2 + H 2 S. Sulphurous anhydride is passed through the residue in order to form 

 the insoluble calcium sulphite: CaCl 2 + Mg(OH) 2 + SO 2 = CaSO 5 +MgCl 2 + H s O. The 



