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Some finely divided solids, whether powdered, granulated or prilled may cake under certain conditions, losing their fludility and forming hard blocks or lumps.
This phenomenon is called "caking" and effects at solids with very diverse chemical constitution. The follow table show us some common characteristics of these products: |
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Characteristics |
Examples |
Physical Appearance |
Powder, crystals, granules. |
Powdered cacoa Powdered milk |
Physical Properties |
Water solubility, Hygroscopicity.
Sensitivity moisture,
pressure and
temperature variations |
Soluble coffee starch sugar
metalic powders, plastic powders,
plastic resins |
Chemical Properties |
Oxygen or water reaction.
Reactions of decomposition |
Minerals salts,
common salts,
Fetilizers |
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| The caking phenomenon uses to appear during the storage, handling or transport operations of the material. |
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Recently caking an its inhabitation have received great attention, due to its importance in the fertilizer industry, owing to:
- Return of lumps or caked products to the manufacturer.
- Reconditioning of rejected products.
- Extra transport, handling and packing costs.
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Dosing of Anticaking Agent |
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| Main factors inducing a caked condition |
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1. Chemical composition
when the chemical nature of fertilizer (formula, consentration, impurities, etc.) changes their hygroscopicity and their tendancy to cake can be modified. |
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2. Specific surface area
Caking tendency of fertilizer closely depend on the particle size and shape of the material. |
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a) Particle size
When the particle size increases, the specific surface area decrease, causing a reduction of the forces of attraction and the points of contact between adjacent particles and lowering their tendency to cake.
b) Particle Shape
If particle surface is smooth and well formed the caking will be delayed due to the limitation of contact points between the particles. |
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| 3. Humidity |
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a) Initial moisture content
Relatively small changes in moisture content have marked effect on the caking of fertilizer.
A high initial moisture content favour the atmospherical moisture absorption and consequently the formation of liquid bridges.
Below an initial moisture content of 0.5%, products do not show any tendency to cake under the pressure applied in storage, therefore a careful control of humidity is specially recommended in the fertilizer industry.
b) Relative atmospherical humidity
Caking depends closely on atmospherica relative humidity variations. Every salt or mixture of salts has a definitive critical relative humidity. Above it will have moisture absorption and below it moisture evaporation.
Relative humidity changes are usually more dangerous than a higher constant hygrometric level. |
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4. Temperature
The effect of temperature is an important one. If the product undergoes high temperatures or sudden thermic variations, it tends to cake badly due to:
• Water re-evaporation (recrystallisation and stabilishment of crystalline solid bridges)
• Development of internal reactions (double decomposition reactions)
• Crystal-habit changes (e.g. ammonium nitrate as a structure change at 32.3°C) |
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5. Pressure
Pressure tends to increase the number.of points of contact between particles, including stored material to cake. |
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6. Time of storage
The time of storage has a great effect on caking, which increases rapidly in the first fifteen days, going on slightly for 6 to 8 weeks, and finally reaching an almost constant value. |
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Product without the use of Anticaking Agent |
Final Products after using Anticaking Agent |
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SALIENT FEATURES
Addition of anticaking agent on fertiliser granules keeps them in free flowing form. Handling, packaging and many other operations are all made easier if the granules maintain their original shape and size.
Sometimes the fertilisers have to be stored in silos for long duration due to various reasons. The caking of
these products due to long exposure to varying humidity conditions and under high pressure due to storage makes handling of these products very difficult and necessitates the use of bull-dozers and other mechanical means which cuts into profits by reducing product appeal and creates second grade product stocks. The Anti-caking agent prevents caking and maintains free flowing properties of straight, mixed and complex fertilisers in granules or crystalline form. The range of anticaking agents are safe to use, provide excellent anticaking effects on urea and NPK products respectively which can hardly be expected from any other conventional anticaking agents.
In addition to crystal modifying property, the product has a surface activity which furnishes adequate anticaking effects on them. These properties of the Anti-caking Agent range of products enables crystal, prilled and granulated products to be handled, transported and stored easily without having caking problems in bulk or as packed. |
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