THE EFFECT OF PROCESS PARAMETERS ON DRUM GRANULATION OF FERTILIZERS WITH BROAD SIZE DISTRIBUTIONS

The effect of moisture content, particle size distribution and binder properties on the granulation of ammonium sulphate (AS), monoammonium phosphate (MAP) and diammonium phosphate (DAP) were investigated using experiments carried out in a 0.31 m diameter laboratory granulation drum. Broad initial size distributions were used to simulate recycle size distributions in operating granulation circuits. Granulation extent increases with the available solution phase for a given fertilizer salt. However, differences in granulation extent for different salts cannot be explained by their differences in solubility alone. Other properties of the solution phase binder, especially viscosity, are also important. At low moisture content and for low binder viscosity (MAP), an equilibrium size distribution is reached in less than 5 min. At higher moisture contents and binder viscosities (DAP), granulation continues up to at least 15 min before an equilibrium size distribution is reached. The initial size distribution has a strong and complex effect on the granulation process. This extent of granulation goes through a maximum as the proportion of fine (minus 1 mm) in the initial size distribution increases. This effect is attributed to the changes in granule porosity and deformability. A two-stage granulation mechanism is proposed based on the analysis of granulation regimes given by Ennis et al. In the first stage (non-inertial regime), successful coalescence is independent of particle sizes. An equilibrium granule size distribution is quickly attained. In the second stage (inertial regime), at higher moisture contents, preferential coalescence predominates, leading to a significant growth of the large granules. The size distribution is subsequently widened. This mechanism is consistent with the experimental results. The analysis of granulation regimes given by Ennis et al. successfully explains the effects of process parameters (liquid content, binder viscosity and initial size distribution) on the extent of granulation and the shape of the granule size distributions.