Granulation of Complex Fertilisers

Keywords: Granulation, complex fertiliser, control, drum, pugmill, spherodiser.

Granulation of fertilisers is one of the most important applications of agglomeration and accretion processes. Larger particles are preferred for centrifugal spreaders, as spreading distance has increased over the centuries. The term ‘granular’ implies a controlled size range and is derived from the Latin ‘granus’, suggesting a size close to cereal grains. The granulation of complex NPK fertilisers has so far remained an art rather than a science. Whilst operating a granulation plant, operators tend to select conditions that provide them with more flexibility to correct and react to changes in conditions.

As the liquid phase is fundamental for good granulation the solubility of raw materials is of key importance: the solubility of fertiliser salts increases as the temperature is raised. The best granulation and final product qualities are, often obtained in a relatively narrow granulation zone illustrated by the temperature – moisture relationship. As granulation is an equilibrium process good control of recycled material is essential to keep flows constant i.e. constant granulation and drying conditions. Buffering recycled material with product size return is useful as the temperature-moisture relationship can be kept constant. Hot recycled material helps to stabilise the granulation temperatures.

Often an incorrect selection of screens and crushers is made, to reduce initial capital costs. Cleaning of screens needs to be done frequently and the environment around the crushers should be clean. Adjusting the granule size from crushers can increase or decrease the number of seeds introduced into the granulation. Correct filling rates of the granulation equipment are needed for optimised granule compaction, to achieve high density as well as granule strength. Similarly the correct speed of the drum, pan or pug mill is important for best product quality. Granulation often continues in a drier, this can be promoted by selecting the right drying temperatures. Temperature selection relates to the air flow and relative humidity through the drier. Using water or steam in the granulator may help to bring granulation back, but should be avoided as the best final product quality is obtained with a low final product moisture content. The spraying of melt/slurry/solution into the granulator should be located in the right positions and preferably should be divided. The fineness of the spray affects granulation. In truth, granulation is a science, and because of this highly automated plants are possible.

H. T. Kiiski and A. G. Kells, Saudi Basic Industries Corporation, Riyadh, KSA

63 pages, 25 figures, 12 tables, 130 references