Over the last ten years or so the inclined rotating drum has become the standard apparatus for the granulation of compound fertilisers by continuous methods.
In this period it has replaced the batch-type pan granulator, characterised by the Eirich machine, because of its simplicity, high output for a given capital cost, improved quality of granule, and reduced labour requirements with continuous operation.
In the same period of time there has been a big swing to more concentrated fertilisers, and this has frequently meant an increase in the salts/superphosphate ratio of the mixings to be granulated. The result has been increased difficulty in the granulation of many compounds, already complicated by the number of variables associated with rotary-drum granulators – e.g. speed of rotation, slope, position number and type of water sprays, etc.
We started to experience these difficulties many years ago, and felt that there was a need to study the effect of certain variables under controlled conditions, which could be done best on pilot scale plant. On none of our works were facilities ideal or sufficient for accurate investigational work, and as a result, an experimental pilot plant rotary granulating unit was constructed, operating on the batch principle.
Most of the developments to be described in this paper were the direct result of work carried out on the batch pilot unit or resulted from deductive thinking promoted by the batch pilot plant work. Typical of these were our investigations into fines and oversize circuits. The effect of recycled material on granulation is so pronounced that it cannot be excluded from a practical study of granulation.
Other developments, such as the use of steam for granulation, were tried from the first on full scale, because it was more appropriate to do so; indeed if steam had been tried first on pilot-scale, we might well have been discouraged from proceeding further.
It is only fair to point out that the implications of many results obtained on the pilot batch plant were not always appreciated at the time, and much tribute is due to the careful experimental procedure and record-keeping of our experimental staff, which enabled previous experiments to be re-assessed in the light of later information or thinking.
Alex T Brook, Fisons Ltd, UK.
79 pages, 28 figures, 9 tables.