514 PRINCIPLES OF CHEMISTRY 



carbonate, the mixture of calcium oxide and sulphide forming the so- 

 called 'soda waste.' 14 



the fresh hot water destined for the lixiviation, not on the fresh mass, but upon a mass 

 already subjected to preparatory lixiviation by means of weak solutions. In this way the 

 fresh water gives a weak solution. The solution which is obtained flows to those 

 parts of the apparatus which contain the fresh, as yet unlixiviated, 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 intercommuni- 

 cating vessels are constructed (standing at the same level) into which in turn the fresh 

 mass is deposited which is intended for lixiviation ; the water is poured in, the alkali 

 drawn off, and the lixiviated residue cleared out. 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. If the fresh mass be placed in 

 one of these cases or vessels, the stream of water passing through the apparatus is 

 directed in such a manner as to finally issue from this vessel containing the fresh un- 

 lixiviated mass. The fresh water is added to the vessel containing the matter which is 

 almost completely exhausted. The fresh water passing through this vessel is conveyed 

 by the pipe (syphon passing from below the first case to the top of the second) communi- 

 cating with the second ; it finally passes (also through a syphon pipe) into the case (the 

 third) containing the fresh stuff. The water will extract all that is soluble in the first 

 vessel, leaving only an insoluble mass. This vessel is then destined to be emptied, and 

 refilled with fresh matter. The level of the liquids in the various vessels will naturally 

 be different, in consequence of the various strengths of the solutions which they con- 

 tain. 



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

 decomposing the common salt enters into this residue. This residue carrying off the 

 sulphur 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 describing here 

 one of the various methods of regaining the sulphur from the soda waste. 



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

 chloride, which disengages sulphuretted hydrogen : CaS + 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 3 + MgCl2 + H 2 O. The 

 solution of magnesium chloride obtained is again used, and the washed calcium sulphite 

 is brought into contact at a low temperature with hydrochloric acid (a weak aqueous 

 solution) and hydrogen sulphide, the whole of the sulphur then separating : 



CaS0 5 + 2H 2 S + 2HC1 = CaCl 2 + 3H 2 O + 88. 



It is necessary to turn once more to the lixiviation of the sodium carbonate from the 

 mass formed in the furnace (see also Note 25). 



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 on the carbonate of 

 sodium of the lime remaining from the first process of the action of the charcoal, 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 subjected to evaporation. The evaporation is conducted at the expense of the 

 waste heat from the soda furnaces, together with that of the gases given off. The 

 process in the soda furnaces is only carried on at a high temperature, and therefore the 

 smoke and gases issuing from the furnaces are inevitably 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 the soda furnaces there is generally a series of pans or evapo- 

 rating boilers, under which the gases from the furnace pass, and into these the alkali 

 solution obtained is poured; On evaporating the solution first of all the undecomposed 

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



