Acids, Alkalis and Salts

by George Henry Joseph Adlam

To return now to practical considerations. The electrolysis of salt solution appears to be an ideally simple method of obtaining caustic soda and chlorine from sodium chloride. As a manufacturing process, it would seem to be perfect, for the salt is broken up directly into its elements and a secondary reaction gives caustic soda automatically. There is no “waste” as in the Leblanc process, and it does not require the use of any expensive intermediary substance afterwards to be recovered, as in the Solvay process. But, as very often happens when working on a large scale, difficulties arise, and these up to the present have only been partially overcome.

Some of the chlorine remains dissolved in the liquid and reacts with the caustic soda, forming other substances which, though valuable, are not easy to separate from the caustic soda. It is possible to get over this difficulty to some extent by placing a porous partition between the anode and the cathode, and in that way dividing the cell into cathodic and anodic compartments. As long as the partition is porous to liquids, it will allow the current to pass, but at the same time it will greatly retard the mixing of the contents of the two compartments. Porous partitions or cells which are in common use for batteries are made of “biscuit” or unglazed porcelain.

It must be remembered, however, that porous partitions only retard the mixing of liquids; they do not prevent it. Moreover, a further difficulty arises from the fact that chlorine is a most active substance, and therefore it is difficult to find a material which will resist its corrosive action for any length of time, and the same difficulty arises in the case of the anode where the chlorine is given off.

Castner Process for Caustic Soda. The following is the most successful electrical process for the manufacture of caustic soda yet devised. It was introduced in 1892, and is known as the Castner process. It should be noted that the use of the porous partition has been avoided in a very ingenious way.

Fig. 16. THE CASTNER PROCESS

The cell (see [Fig. 16]) is a closed, rectangular-shaped tank divided into three compartments by two non-porous partitions fixed at one end to the top of the tank, while the other end is free and fits loosely into a channel running across the tank. The floor of the tank is covered with a layer of mercury of sufficient depth to seal the separate compartments. The two end compartments contain the brine in which are the carbon anodes; the middle compartment contains water or very dilute caustic soda in which the cast-iron cathode is immersed.

The current enters the end compartments by the carbon anodes and passes through the salt solution to the mercury layer which in these compartments are the cathodes. The current then passes through the mercury to the middle compartment, and then through the solution to the cathode, thence back to the dynamo. It is important to note that in the middle compartment the mercury becomes the anode.

Chlorine is liberated at the carbon electrodes, and when no more can dissolve in the liquid it escapes and is conveyed away by the pipe P. Sodium atoms are formed at the surface of the mercury cathodes in the outside compartments and dissolve instantly in the mercury, forming sodium amalgam.

While the current is passing, a slight rocking motion is given to the tank by the cam E. This is sufficient to cause the mercury containing the dissolved sodium to flow alternately into the middle compartment, and there the sodium amalgam comes into contact with water; the sodium is dissolved out of the mercury and caustic soda is formed. Water in a regulated stream is constantly admitted to the middle compartment, and a solution of caustic soda of about 20 per cent. strength overflows.