Granule formation:
As mentioned above, in chemical granulation, there is an acid and ammonia reaction plus addition of some salts in solid form, whereas in physical granulation all raw materials are in theirs solid form.
In physical granulation the granule grows by the process of agglomeration, wherein solids come and stick together because of the wetted surface of the soluble salts, and when heated, the water on the surface of the salts evaporates to form a strong bond leading to agglomeration of solid particles.
In chemical granulation if the entire process was limited to an acid and ammonia reaction, one would have an onion skin like granule formation, wherein one layer of salt grows over the lower layer, like in an Ammonium Nitrate granule. However as some salts are also added, the growth of granule is a mix of onion skin type layering and agglomeration.
No acid, no ammonia:
In physical granulation one does not have to handle acid and ammonia, and hence the costs (and even logistics) of sourcing, handling, and storage of these liquids is completely avoided. This is the single greatest advantage of the physical granulation process over chemical granulation process.
And as there are no acid and ammonia reactions there are no emissions of any NOx gases, hence pollution control equipments in physical granulation are simpler and not as expensive.
Lower production costs:
Logic and management theory tells us to go for vertical integration of manufacturing process to produce a cheaper product. This is not always true, for in the open free world of today, sourcing materials from another country may present a more economical solution. Also in the chemical granulation plants the capital cost of the plant is very high, primarily because of the storage and handling of ammonia, that it is now well established by many published technical papers that the cost of NPK produced by the steam granulation route is cheaper than by the chemical granulation route.
Produce a variety of grades:
Physical granulation of NPK allows one to produce a variety of grades, an option not available with chemical granulation plants. One can therefore manufacture grades to suit crop and soil requirements.
Further it allows fast change over of grades and extremely low start and stopping times; hence small quantity (special grades) NPK formulations orders can be met. In a physical granulation plants, typically grades are changed twice a week. Imagine the market flexibility!
