Introduction:
The main subject to be discussed will be that of the caking of granular fertilizers, but linked with it will be a study of the problem of manufacturing a product having a high proportion of its granules in a fairly narrow size range.

The problem of caking was the first one tackled by our new Research Department. The paper is a record of experimental work done in the years 1952 and 1953 and of the use made of it subsequently on the production scale. Much of this early work was done by research workers without a good background of fertilizer experience and without fully adequate facilities. In some cases, therefore, the approach to individual problems cannot be recommended as being good.

When granulation of mixed fertilizers based on superphosphate was commenced, it was the hope of those concerned that a single straight-through process of manufacture would yield a merchandisable article of quality very superior to that of the powder product manufactured by the old two-stage process. In the old process caking took place as a stage in the manufacturing operation, and the product was re-milled as close as possible to the time of use. Re-caking in the bag could, of course, take place if there was a delay before use, but this could be reduced to some extent at least by the incorporation in the product of appropriate anti-caking agents such as bonemeal. The essence of the granulation process was thought to be that setting would take place within the granules themselves, facilitated by the water added in the process. This water would hasten the reactions between the ingredients, and the internal setting would give strong granules. Once packed the material would not cake, partly because of the greater degree of reaction occurring during granulation, but mainly because of the greatly diminished number of inter-particle contacts. It was on this point that hopes were not fulfilled. As is well known, granular fertilizers can cake very hard in the bag and, even though they are usually easier to break down than are powder fertilizers, caking has caused great trouble and expense to manufacturers and users.

Dr Bernard Raistrick, Scottish Agricultural Industries Ltd, UK

46 pages, 10 figures, 12 tables.