with increasing coating solution–dry matter concentration and atomisation pressure. Model validations showed adequate precision inside the parameter space on which the model was derived.
The impact strength model indicated increasing impact strength with increasing coating solution–dry matter concentration, atomisation pressure as well as bed temperature. The observed impact strength tendencies were concluded to be closely associated with the high extent of coating liquid droplet penetration into the Na2SO4 cores. Additional experiments indicated that the coating liquid droplet penetration is significant and that this droplet penetration has a significant importance regarding the morphology of the final coated granule. It appears thereby that the porosity of a core being coated has a significant but not yet fully understood influence on the properties of the final, coated granule. A reasonable theory is that depleted coating droplets with high contents of dry matter will form solid salt bridges inside the pores of the carrier particles upon drying, and that this will improve the impact stress resistance of the final, coated granule. This theory was partly verified by porosity measurements indicating a decrease in final coated granule porosity with increasing coating solution–dry matter contents. From viscosity experiments it was observed that addition of CMC to the coating solution has a beneficial influence on the impact strength and that the agglomeration tendency increases with increasing viscosity of the coating solution.
The validation of the impact strength model was slightly less successful, as the model in all cases predicts higher impact breakage values than the new experimental data, although the model does follow overall impact strength tendencies closely.
According to the derived models, there is no contradiction between high impact strength and low tendency of agglomeration. The models may be seen as a first step towards the design of fluidised bed processes leading to unagglomerated, coated, enzyme granules with high mechanical strength.