HYDROCHLORIC ACID, SALTCAKE, AND SODA
Manufacture.—The production of hydrochloric acid (HCl), sodium sulphate (Na₂SO₄), and sodium sulphide (Na₂S) forms part of the manufacture of soda (Na₂CO₃) by the Leblanc process. The products first named increase in importance, while the Leblanc soda process is being replaced more and more by the manufacture of soda by the Solvay ammonia process, so much so that on the Continent the latter method predominates and only in England does the Leblanc process hold its ground.
Health interests have exercised an important bearing on the development of the industries in question. At first, in the Leblanc process the hydrochloric acid gas was allowed to escape into the atmosphere, being regarded as a useless bye-product. Its destructive action on plant life and the inconvenience caused to the neighbourhood, in spite of erection of high chimneys, demanded intervention. In England the evils led to the enactment of the Alkali Acts—the oldest classical legislative measures bearing on factory hygiene—by which the Leblanc factories were required to condense the vapour by means of its absorption in water, and this solution of the acid is now a highly valued product. And, again, production of nuisance—inconvenience to the neighbourhood through the soda waste—was the main cause of ousting one of the oldest and most generally used methods of chemical industrial production. Although every effort was made to overcome the difficulties, the old classical Leblanc process is gradually but surely yielding place to the modern Solvay process, which has no drawback on grounds of health.
We outline next the main features of the Leblanc soda process, which includes, as has been mentioned, also the manufacture of hydrochloric acid, sodium sulphate and sulphide.
The first part of the process consists in the production of the sulphate from salt and sulphuric acid, during which hydrochloric acid is formed; this is carried out in two stages represented in the following formulæ:
- 1. NaCl + H₂SO₄ = NaHSO₄ + HCl.
- 2. NaCl + NaHSO₄ = Na₂SO₄ + HCl.
The first stage in which bisulphate is produced is carried out at a moderate heat, the second requires a red heat. The reactions, therefore, are made in a furnace combining a pan and muffle furnace.
This saltcake muffle furnace is so arranged that the pan can be shut off from the muffle by a sliding-door (D). The pan (A) and muffle (E) have separate flues for carrying off the hydrochloric acid developed (B, F). First, common salt is treated with sulphuric (Glover) acid in the cast-iron pan. When generation of hydrochloric acid vapour has ceased, the sliding-door is raised and the partly decomposed mixture is pushed through into the muffle, constructed of fire-resisting bricks and tiles, and surrounded by the fire gases. While the muffle is being raised to red heat, the sulphate must be repeatedly stirred with a rake in order, finally, while still hot and giving off acid vapour, to be drawn out at the working doors into iron boxes provided with doors, where the material cools. The acid vapour given off when cooling is drawn through the top of the box into the furnace.
