Acids, Alkalis and Salts

by George Henry Joseph Adlam

Fig. 4. SULPHUR TRIOXIDE—THE CONTACT PROCESS

Platinized asbestos is made by soaking long-fibred asbestos in a solution of platinum chloride. The material is then dried and subjected to a gentle heat. In this way, metallic platinum in an exceedingly fine state of subdivision is deposited on the asbestos fibre, which merely serves as a convenient support.

Catalytic or Contact Action. The influence of the finely divided platinum is a very important factor in the reaction. It cannot, however, be said to cause the union of sulphur dioxide with oxygen, for the gases combine to a very slight extent when it is not present. What the platinum actually does is to influence the rate of formation to such a degree that, under favourable conditions, practically the whole of the sulphur dioxide is changed to sulphur trioxide instead of an exceedingly small fraction of it.

The most interesting, and at the same time the most perplexing, feature of the reaction is that the platinum itself does not appear to undergo any change. It is not diminished in quantity, for only a very small amount is necessary for the conversion of a very large amount of the mixed gases. Its activity lasts for a very long time, and even when it does become inactive, it can be shown that this is due to some external cause, such as the presence of dust and certain impurities in the gases.

Many other similar cases are known in which the presence of a small quantity of a third substance greatly influences the course of a chemical reaction without appearing in any other way to be necessary to the reaction. These substances, which are often metals in a very fine state of subdivision, are called catalytic or contact agents.

The Contact Process for making sulphuric acid is nothing more nor less than the simple laboratory operation which we have described above, carried out on a larger scale.

The sulphur dioxide is produced as in the lead chamber process by roasting iron pyrites in a current of air. This gas, together with the excess of air, is passed into the contact furnace, which consists of four tubes, each containing platinized asbestos, supported on perforated plates. The union of the two gases is said to be almost complete: an efficiency of 98 per cent. of the theoretical value is claimed for this process. The sulphur trioxide, or “sulphuric anhydride”[1] is either condensed in tin-lined drums or absorbed in ordinary concentrated sulphuric acid.

The proposal to manufacture sulphuric acid by this method was first made in 1831 by Peregrine Phillips, of Bristol. The early attempts were not successful, and it was not until about forty-four years later that the difficulties arising in the working of the contact process were overcome sufficiently to enable the sulphuric acid produced in this way to be sold at the same price as that made by the lead chamber process. Since 1890, the total quantity of acid made by the contact method has increased very rapidly, so that it now furnishes about one-half of the world’s supply, and seems likely in time to displace the lead chamber process altogether.

The history of the rise of the contact process is interesting because it illustrates in a striking manner the very great difference that there is between a successful laboratory process and a successful manufacturing process, though seemingly identical.