SODA 



551 



a blind-roaster or muffle-furnace, for making salt- 

 cake. A is the iron pan in which the charge of 

 common salt and sulphuric acid is first placed, 

 and B is the muffle in which the calcination of the 

 half-finished sulphate is completed, the charge 

 being raked from A into B. The fire (C) heating 



n- n 



I r~? r^ 



Fig. 1. Vertical Section of Furnace for making 

 Sulphate of Soda. 



the muffle is so placed that the fire gases do not 

 get inside, hut heat it by external flues. Usually 

 the pan is heated by a separate fire, but in some 

 instances by the waste heat from the muffle fire. 

 The gaseous hydrochloric acid evolved during the 

 time the reaction between the salt and 

 the sulphuric acid is going on in the 

 pan is sometimes led away to a separate 

 condenser ; but if a comparatively pure 

 acid is not required then the gases from 

 both pan and muffle go into the same 

 condenser. 



In the case of the open roaster, an 

 older kind still used, there is a direct 

 opening from the fire into the bed of 

 the furnace, which is then of an ordin- 

 ary reverberatory type (see COPPER, 

 fig. 1, and LEAD, fig. 1), with the pan 

 placed at the end as in the blind-roaster ; 

 but in this case the pan has always a 

 separate fire. Coke is generally used 

 for fuel in the open roaster, and with it 

 the pan gas is usually separately con- 

 densed. Each of these kinds of furnaces 

 has its advantages and disadvantages. 

 The usual charge for open roasters is 

 about 9 cwt. of common salt, and for 

 close roasters one-half more. With the equivalent 

 quantity of sulphuric acid 100 parts by weight of 

 pure chloride of sodium would theoretically yield 

 121-45 parts of sulphate of soda, but in practice 

 only about 110 are obtained, owing to the presence 

 of moisture in the common salt. The specific 

 gravity of the sulphuric acid employed varies from 

 1-700 to 1-720. 



Rotating furnaces for making sulphate of soda 

 have been introduced in recent years, the one 

 patented in 1875 by Jones and Walsh being the first 

 which came into actual operation. It nas since 

 been improved by the patentees and others, and is 

 now in use at several large alkali-works. Fig. 2 is 

 a diagram of this furnace. It consists of a rotat- 

 ing iron pan, 16 feet and upwards in diameter, 

 placed in a chamber arched over with firebrick. 

 From the fireplace at one side the fire gases pass 

 over the surface of the charge to the flue at the 

 other side. The salt is admitted by hopper 

 closed by a balanced cone, and there are lead-pipes 

 for admitting the sulphuric acid at the edge of the 

 pan. Fixed stirrers or ploughs are used for turning 

 over the charge, which, in the largest sized pans, 

 is now sometimes as much as 25 tons of common 

 salt, and this with 20 tons of sulphuric acid gives 

 27 tons of sulphate of soda every twelve hours. 

 Gas from a producer is sometimes used to heat the 

 furnace, and a self-acting method of emptying the 

 pan was patented by the inventors in 1880. There 

 IB a great saving of manual labour by the use of 



this furnace. Sulphate of soda, although chiefly 

 consumed as an intermediate product in alkali- 

 making, is used by itself in glass-making, in the 

 manufacture of ultramarine, for cold-producing 

 mixtures, and for making various chemicals. 



From any of these salt-cake furnaces the hydro- 

 chloric acid gas is led away by pipes or flues to 

 condensing towers sometimes 100 feet high, and 

 filled with pieces of coke. The acid vapour enters at 

 the bottom of the tower, and in ascending through 

 the piled coke meets with a descending stream 

 of water from the cistern on the top by which 

 it is absorbed, and flows away as liquid acid 

 through a pipe at the base of the tower to 

 stock-cisterns. Strong hydrochloric acid, which is 

 chiefly used in the manufacture of Bleaching 

 Powder (q.v.), is easily obtained by good condens- 

 ing appliances either from the rotatory pan furnace 

 or from the pan of a stationary furnace, but the 

 acid from the roaster gas is usually weak. The 

 Alkali Acts of 1863 and 1874 make the condensa- 

 tion of nearly the whole of the gaseous hydro- 

 chloric acid produced at soda- works imperative. 



Conversion of the Sulphate of Soda into Black- 

 ash. The sulphate of soda is roasted with coal 



ng; 



Fig. 2. Rotating Furnace for making Sulphate of Soda : 

 A, iron pan for containing charge ; B, hopper, with balanced cone for cha 

 C, fireplace ; D, one of the stirrers ; E, self-acting arrangement for disi 

 ing pan. 



and limestone to produce crude carbonate of soda. 

 In this operation the oxygen of the sulphate com- 

 bines with carbon furnished by the coal to form 

 carbonic oxide, which escapes into the air. The 

 remaining sodium sulphide interchanges combina- 

 The specific j tions with the carbonate of lime (limestone), form- 

 ing carbonate of soda, easily soluble in water, and 

 calcium sulphide, which is insoluble. The action 

 of the carbon in reducing the sulphate of soda to 

 the sulphide is shown by the equation 



NajSO, + C 4 = NajS + 4CO. 



The further change which occurs is thus repre- 

 sented : 



NazS + CaCOs = NaaCos + CaS. 



The proportions of the materials as now used are 

 the same as those first recommended by Leblanc 

 viz. sulphate of soda, 100 parts ; carbonate of lime 

 (limestone), 100 parts; carbon (charcoal), 55 parts. 

 But as coal is employed in England instead of char- 

 coal, the quantity used is generally 75 to 100 of each 

 of the other two ingredients. The ' balling furnace ' 

 used in this operation is shown in fig. 3. It has 

 two beds, the one being raised a few inches above 

 the other. F is the fireplace, the waste heat from 

 which is usually employed in boiling down the 

 tank liquor or soda-lye as indicated in the section. 

 The charge of about 8 cwt. of the above mixture ii 

 thrown into the bed, A, of the balling furnace, 

 after it has been raised to a bright red heat, and 



