Caking is a common condition that happened during production and storage of compound fertilizer largely affecting the transportation and application. Then why the caking of compound fertilizer comes into being? Here we reveal the various reasons for you.
1. The Formula of Compound Fertilizer
Between the constituents on the surface of fertilizer granules, some chemical reactions will happen, giving rise to the phenomenon of recrystallization. Each of the raw materials has certain degree of hygroscopicity. When their critical relative humidity (CRH) is lower than the air’s relative humidity, they will absorb water from the air. The lower the value of CRH, the higher the hygroscopicity is, while higher moisture content brings about chemical reaction easily, and thus the phenomenon of caking is heavier. When different proportions of raw materials are mixed, the CRH will drop prominently. Usually fertilizer that has higher content of nitrogen is more easily to cake. When the air’s relative humidity is not stable, the change of humidity will cause recrystallization, powder cake will arise.
2. Fertilizer Granules
* The higher the moisture content of the granules, the easier the process of dissolution and crystallization is going on. On the other hand, moisture will soften the granules, and make them deformed under pressure, so that the contact area among granules is enlarged, and the granules are more easily to be bonded to cake.
* Small granules are more easily to cake than large ones. When the size gets larger, the specific surface area will decrease, then the contact area of adjacent granules is reduced, the tendency of caking is lowered.
* If the surface of the granules is smooth and the size is uniform, then the contact area is reduced.
* If the granules are not uniform, or there are too many small particles, or the shape is not so round, then the particle strength is low, the granules are prone to efflorescence or deformation under high pressure. The fine power largely reduces the contact area, and causes caking.
3. Packing Temperature and Storage Temperature
* If the environment temperature during storage is too high, the CRH of compound fertilizer will decrease, its hygroscopicity will increase, the fertilizer is easy to cake.
* If packing and keeping the fertilizer in storage while the dried granules are not fully cooled, especially when the storage temperature is high, the internal water of fertilizer will evaporate again. When the vapor meets the cooled external fertilizer, they will condense to water, and recrystallization will occur.
* Some materials, such as ammonium nitrate and ammonium sulfate, create double salt when granulating. During drying process, the thermal decomposition of the double salt takes place, and then the constituents revert to the double salt when cooling. In this process, the granules are likely to effloresce and cake.
4. Stack Height of the Bag
The higher the stack, the higher the stack pressure, and the larger is the plastic deformation of compound fertilizer granules as well as the contact area, creating good conditions for chemical reaction and recrystallization.
5. Storage Time
With the extending of storage time, the recrystallization and dissolution process takes place again and again. Long time under pressure make the deformation of fertilizer larger, the tendency of caking is more obvious.
So long as you adopt right solutions, you can ease the problem of caking. One way is to use rotary drum coating machine the special equipment for anti-caking, to coat the granules with oil and/or powder anti-caking agent. In such case, caking is prevented, meanwhile the brightness of the granules is improved.