Since the general introduction in the late 1960s of gold-palladium catchment gauzes to recover platinum lost from the catalysts of ammonia oxidation plants (1, 2), and their incorporation into an efficient, reliable process in the mid 1970s (3), their application has become widespread. There are now very few nitric acid manufacturing plants in Europe that do not use them on a regular basis and recoveries in the order of 50 to 60 per cent of the gross losses are being achieved regularly. In the United States of America, a market that has traditionally used filters to recover platinum, their use is also spreading rapidly, the merits of the system having been recognised clearly from both a technical and an economic point of view. It is estimated that on a world wide basis there are in the order of one hundred nitric acid plants regularly using catchment gauzes, and few problems are encountered with their use.

When the most recent article on this topic appeared in this Journal in 1973 catchment gauze packs were generally less than 2.5 metres in diameter, and the maximum size then in use was 3.4 metres. However, since that time a general change has occurred in the design of nitric acid plants; outputs per burner have increased considerably and as a consequence reactor diameters and hence effective catalyst diameters have, generally, also increased. The largest are now 4.8 metres (4) and even larger reactors may be produced in the future.

The merit of the catchment gauze installation method described previously (5) has been vindicated on many occasions, particularly in large reactors and especially where unscheduled shutdowns are a common occurrence. The ability of the pack to expand and contract freely, illustrated in Figures 1 and 2, without damaging either the catalyst or the gold-palladium catchment gauzes has been, in a large number of cases, the key factor enabling the process to be used without operating problems. In addition noble metal losses experienced on removal of the gauzes have been minimised.

In contrast to this arrangement, separate base metal support gauzes, illustrated in Figure 3, inevitably tear the now fragile gold-palladium gauzes on removal, and if this is necessary for an exceptional reason, such as boiler problems, the damage caused when the gauzes are separated can be considerable. The gauzes may even become unfit for further use, to the detriment of operating economics. Furthermore handling and storing a partly used pack of gauzes is a difficult, if not impossible, task. Therefore on anything other than a completely reliable plant it is advantageous to use a composite or ‘biscuit’ pack. This consists of the normal arrangement of gold-palladium catchment gauzes and stainless steel separating gauzes stacked alternately, but with one larger than usual stainless steel gauze, coloured blue in Figure 1, wrapped round all the others making them into an integral unit.

For small plants a ‘biscuit’ pack can be delivered already made up by the manufacturer. Thus the advantages of such packs are fourfold; fast installation, tolerance of thermal expansion, stability on removal, and they facilitate secure storage during long shutdown periods.