The first alkali works were established in Great Britain in 1814. The total amount of soda now made in this country every year is about 1,000,000 tons, of which nearly one-half is still made by the Leblanc process.
Salt Cake. The first stage of the Leblanc process consists in mixing a charge of salt weighing some hundredweights with the requisite amount of “chamber” sulphuric acid. The operation is carried out in a circular cast-iron pan (D, [Fig. 12]) about 9 ft. in diameter and 2 ft. deep. The pan is covered over with a dome of brickwork, leaving a central flue (E) for the escape of hydrochloric acid gas which is produced. At first, the reaction takes place without the application of heat, but towards the end the mass is heated for about one hour. The contents of the pan are then raked out on to the hearth of a reverberatory furnace (a, b) and more strongly heated. More hydrochloric acid gas is given off, and the reaction is completed. The solid product which remains is impure Glauber’s salt (sodium sulphate), and is known in the trade as “salt cake.”
Black Ash. In the second stage of the Leblanc process, salt cake is converted into black ash. The salt cake is crushed and mixed with an equal weight of powdered limestone or chalk and half its weight of coal dust. This mixture is introduced into a reverberatory furnace ([Fig. 13]) by the hopper K, and heated to about 1000° C. by flames and hot gases from a fire at a. During this operation, the mass is kept well mixed, and after some time it is transferred to h where the temperature is higher. The mixture then becomes semi-fluid and carbon monoxide gas is given off.
Fig. 13. BLACK ASH FURNACE
The formation of carbon monoxide within the semi-solid mass renders it porous. This is an advantage, because it greatly facilitates the subsequent operation of dissolving out the soluble sodium carbonate. The appearance of the flames of carbon monoxide at the surface of the black ash indicates the end of the process. The product is then worked up into balls and removed from the furnace.
The chemical changes which take place in making black ash are probably as follows: Carbon (coal dust) removes oxygen from sodium sulphate, which is thus changed to sodium sulphide. This substance then reacts with the limestone (calcium carbonate), forming sodium carbonate (soda) and calcium sulphide.
Extraction of Soda. It now only remains to dissolve out the soda from the insoluble impurities with which it is mixed in the black ash. It is evident that the smaller the amount of water used for this purpose the better, because the water has subsequently to be got rid of by evaporation. The process of extraction is, therefore, carried out systematically. The black ash is treated with water in a series of tanks which are fitted with perforated false bottoms. The soda solution, which is heavier than water, tends to sink to the bottom and, after passing through the perforations, is carried away by a pipe to the second tank, and so on throughout the series. The fresh water is brought first into contact with the black ash from which nearly all the soda has been extracted.
The method of finishing off the black ash liquor differs according to the final product which the manufacturer desires to obtain, for the liquor contains caustic soda as well as mild soda. For the present, we will suppose that the end product is to be washing soda. In this case, carbon dioxide is passed into the liquor to convert what caustic soda there is into mild soda.
The clarified soda liquor is then evaporated until crystals of soda separate out. The first part of this process is carried out in large shallow pans (P. [Fig. 13]), using the waste heat of the black ash furnace, and finally in vats containing steam-heated coils. As the crystals separate out, they are removed, drained, and dried.