Chlorine : Chlorine Industry

On the large scale, hydrochloric acid is made by the action of fairly concentrated sulphuric acid on common salt (saltcake process). The acid may be mixed in the gaseous state with air, and the mixture passed over a heated mass containing a copper salt, which acts as a catalyst, so as to produce chlorine (Deacon process): 4HCl + O₂ <=> 2H₂O + 2Cl₂. The gas may also be dissolved in water, and the solution (spirit of salt) decomposed by heating with manganese dioxide (Weldon process): 4HCl + MnO₂ = 2H₂O + MnCl₂ + Cl₂. Large quantities of chlorine are now prepared directly from common salt by electrolysis.

The first step in the manufacture of alkali by the Leblanc process is to decompose common salt with sulphuric acid, with the production of sodium sulphate, Na₂SO₄, known as salt cake. The reaction is carried out in two stages, the first, which proceeds at moderate temperatures, giving mainly acid sodium sulphate, NaHSO₄; the second is carried out by heating the resulting mixture of acid sulphate and common salt at a dull red heat, when the normal sulphate, Na₂SO₄, is produced:

2NaCl + H₂SO₄ = NaHSO₄ + NaCl + HCl = Na₂SO₄ + 2HCl.

Half a ton of coarse-grain salt is charged into the large hemispherical cast-iron saltcake pan, A,

Fig: Saltcake muffle furnace

and an equal weight of sulphuric acid, sp. gr. 1.7, run on. A copious evolution of hydrochloric acid occurs, the gas being led off through p. When this slackens, the pan is heated by flue gases admitted by means of the dampers, f₁ and f₂. When the first reaction is complete, the pasty mass is raked into the closed firebrick box, or muffle, B, heated externally by flames from the fireplace, C, which functions as a gas producer. The hydrochloric acid gas passes out through the pipe d. Saltcake is left in the muffle.

In the Hargreave's process, hot gases from pyrites burners, consisting of sulphur dioxide, oxygen and nitrogen, are passed together with steam over porous pieces of common salt in large iron cylinders. Sodium sulphate is slowly formed, and hydrochloric acid gas evolved:

4NaCl + 2SO₂ + 2H₂O + O₂ = 2Na₂SO₄ + 4HCl.

The gas from the saltcake furnaces is cooled by passing through cast-iron pipes (which are not attacked by the gas if the temperature is kept above the point of condensation of the accompanying moisture), and then passes to a tower 60 ft. high, composed of sandstone slabs boiled in tar and clamped together with iron bands, which is packed with lumps of hard coke. Water is run down, and the hydrochloric acid is almost completely absorbed. To produce strong acid (about 33 per cent. HCl) the liquid is recirculated over the coke packing by acid-pumps of stoneware or ebonite. Efficient absorption depends chiefly on keeping the tower cool, and presenting a large wetted surface to the gas. The latter is provided by the irregularly-shaped lumps of coke, which retains water in its pores.

Protein crystallography
Chemistry guide
Chlorine
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