Acids, Alkalis and Salts

by George Henry Joseph Adlam

The Solvay Process. Soda ash is one of the principal forms of mild alkali used in commerce. Large quantities of this substance are made by heating bicarbonate of soda. We shall now consider another alkali process in which this substance is the primary product.

For the greater part of the first century of its existence, the Leblanc soda process had no rival, although another method, known as the ammonia-soda process, was patented as early as 1838. In this case, however, as in many others, expectations based on the experiments carried out in the laboratory were not realized when the method came to be tried under manufacturing conditions. It was not until 1872 that Ernest Solvay, a Belgian chemist, had so far solved the difficulties, that a new start could be made. In that year, about 3,000 tons of soda were produced by the ammonia-soda or Solvay process, as it has now come to be known. Since then, however, the quantity produced annually has been steadily increasing, until at the present time it amounts to more than half of the world’s supply.

The Solvay process is very simple in theory. Purified brine is saturated first with ammonia gas and then with carbon dioxide. Water, ammonia, and carbon dioxide combine, forming ammonium bicarbonate, which reacts with salt (sodium chloride), producing sodium bicarbonate and ammonium chloride.

The principal reaction is carried out in a tower ([Fig. 14] (1), a, a) from 50 to 65 ft. in height and about 6 ft. in diameter. At intervals of about 3½ ft. throughout its length, the tower is divided into sections by pairs of transverse discs, one flat with a large central hole, and one hemispherical and perforated with small holes ([Fig. 14] (2)). The discs are kept in position by a guide rod G. [Fig. 14] (3) shows a better arrangement of the guide rods. In modern works, the space between the discs is kept cool by pipes conveying running water. The ammoniated brine is led into the tower near its middle point. The carbon dioxide is forced in at E in the lowest segment, and as it passes up the tower it is broken up into small bubbles by the sieve plates. Sodium bicarbonate separates out as a fine powder, which makes its way to the bottom of the tower suspended in the liquid.

The perforated plates are necessary for the proper distribution of carbon dioxide through the brine. They are, however, a source of trouble, because the holes quickly become blocked up with sodium bicarbonate, and every ten days or so it is necessary to empty the tower and clean it out with steam or boiling water.

Recovery of Ammonia. The production of 1 ton of soda ash by the Solvay process involves the use of a quantity of ammonia which costs about eight times as much as the price realized by selling the soda. It is evident that the success of the process as a commercial venture depends largely on the completeness with which the ammonia can be recovered.

During the process, ammonia is converted into ammonium chloride, which remains dissolved in the residual liquor. From this ammonia gas is set free by adding quicklime and by blowing steam through the mixture. It is now claimed that 99 per cent. of the ammonia used in one operation is recovered.

Soda Ash. The bicarbonate of soda produced by the Solvay process is moderately pure. For all ordinary purposes, it is only necessary to wash it with cold water to remove unchanged salt, and after drying, it is ready to be placed on the market if it is to be sold as bicarbonate. The greater part of the Solvay product, however, is converted into soda ash by the application of heat. If soda crystals are required, the soda ash is dissolved in water and crystallized.

In many ways, the Solvay process compares very favourably with the older method. It is an advantage to start with brine, for that is the form in which salt is very often raised from the mines. The end product is relatively pure; moreover, it is quite free from caustic soda, which for some purposes for which soda ash is used is a great recommendation. There is no unpleasant smelling alkali waste. On the other hand, the efficiency of the Solvay process is not high, for only about one-third of the salt used is converted into soda. This would make the process impossible from the commercial point of view were it not for the cheapness of salt.

The Leblanc process, too, has its advantages. In the next chapter we shall see that it is adaptable for the production of caustic as well as mild alkali. The chlorine which is recovered in the Leblanc process is a very valuable by-product. In the Solvay process, chlorine is lost, for hitherto no practicable method has been found for its recovery from calcium chloride.