CONTACT PROCESS HAS MANY ADVANTAGES

Press, Volume CVI, Issue 31341, 11 April 1967, Page 21

CONTACT PROCESS HAS MANY ADVANTAGES

The adoption by Kempthorne Prosser and Company of the contact process of making sulphuric acid is an example of the worldwide swing from the lead chamber process in vogue with many developments end refinements for just over 100 years. The more obvious features of the contact process are a greatly diminished space requirement, elimination of the use of nitrate of soda (which gave rise to the characteristic brown fumes formerly discharged into the atmosphere) and the recovery of the heat evolved in the chemical processes in the form of steam.

Although removal of heat was always one of the main

problems in the now obsolete lead chamber plant, retention of heat in a useful form has been so successfully carried out in the contact process that the weight of high pressure super

heated steam is 20 per cent greater than the weight of sulphuric acid produced. Although a certain amount of steam is used in the process. itself, the greater quantity is used to drive a steam turbine which in turn drives a generator to produce electric power.

Features of the plant are the two yellow painted acid towers, one using the acid produced to dry the air required to burn the sulphur, and the other being the acid production tower. The whole process must be carried out in the absence of moisture. Not only is the sulphur completely dried by raising it to a temperature

above its melting point, utilising steam obtained as a by-product of the process, but even the air used for burning the sulphur is dried, using some of the sulphuric acid produced as the drying agent. After the molten and filtered sulphur has been burnt the gases are cooled by passing through a 4501 b

boiler, so raising steam. This is the highest pressure boiler in the South Island. The partly cooled gases then pass to the converter where in effect there is a second stage of combustion. The chemical reaction with more air raises the temperature to 600 degrees C.

Cooling of the gases is carried out in stages, partly using the steam previously produced and so superheating it, partly by heating the water feeding the boiler, and partly by diluting the gas with cold dry air. The cooled gases are absorbed in the acid production tower.

By far the greater quantity of sulphuric acid produced will be used within the company’s chemical fertiliser works at Hornby for the manufacture of superphosphate. Canterbury soils require phosphatic fertilisers more than any other type. Superphosphate is made by grinding phosphate rock into a fine powder and mixing it with sulphuric acid.

The original phosphate rock, being insoluble, cannot be used by the plant as a food. The reaction with sulphuric acid converts this insoluble form into a soluble phosphate with the forma-

tion also of calcium sulphate or gypsum, in itself a valuable plant food. In fact, much superphosphate produced for Canterbury is now being fortified with additional sulphur.

Other uses for sulphuric acid are in freezing works, for the preparation of hides, as battery acid in electroplating and as a general chemical.