Briefly about: Granulation of fertilizers/soil improvers

Granulation is the process used for the production of pharmaceuticals, fertilizers, animal feed, various chemicals, also coffee, etc. obtaining granules which are dense grains of a substance. Granular fertilizers are usually 1-5 mm in diameter, produced from larger materials after they are crushed and sieved, or by bonding smaller particles during dry or wet granulation, thus obtaining a higher density almost round-formed particle material.

The granulation process depends on the properties of the material used – particle size, moisture content, temperature, hardness, porosity, density, pH, etc.

Types of granulation

Dry or solid granulation is the compression of a powder-like substance/particles under high pressure. For dry granulation compression between rollers is used (roller compaction granulation), the density of the granules depends on the applied compression force. These granules usually have a flatter shape from two sides. With dry granulation granules are formed faster, they can be packed immediately, and this process is simpler and cheaper than wet granulation. Dry granulation is more suitable for materials that are sensitive to heat and moisture.

Wet granulation is the process when powder-like particles are bound using a liquid as a binder – water, organic solvent, glue solution. The wet particles begin to bind, agglomerate, and form granules. The obtained granules are then dried, as a result, they cool down, lose moisture and harden. With the wet granulation method, it is possible to obtain more concentrated and homogeneous mineral fertilizers with the same shape. Harder granules can be obtained when longer powder wetting is performed. The most popular wet granulation techniques are rotary drum granulation technology, disc pan granulation, roller press granulation, spray granulation, fluid bed granulation. In many cases, the granulation solution is sprayed from above and the powder is dispensed as a foam or spray.

Which materials can be granulated

It is possible to granulate both inorganic materials – various minerals and rocks are used for granulation, and organic materials – various industrial by-products and waste, biological waste, compost, wood ash, bone meal, pet litter, manure (livestock, swine, poultry), especially bird droppings are often referred to as a material which is granulated.

Advantages of granules

Advantages of granules include their easier storage, transport, dosing and spreading (their use, especially in mechanized field cultivation), they are not blown away by the wind, they do not dust, and stable granules do not crush into finer particles, causing losses and dust that can pose environmental, health and economic risks. Granules are more concentrated, their content ratio is known, with granules it is possible to regulate the effect impact over time and a longer impact time is possible as the breakdown can be slowed. By fertilizing with granules, it is possible to place nutrients closer to the plant, ensuring a better interaction between fertilizer, soil and plant, reducing the leaching of nutrients and reducing nutrient transition to a plant inaccessible form, thus reducing the risk of environmental pollution. As granules are produced with the same physical properties (size, density), they can be evenly scattered and do not stratify (in a mixture of smaller and larger granules, they will layer from the top to the bottom of a pile). Granulated organic fertilizers are more concentrated, as organic fertilizers in the form of granules have lost moisture and are compressed, its volume is reduced up to 10 times, thus reducing the required amount and cost of fertilizer transportation.

Many complex fertilizers are available in the form of granules (mineral fertilizers containing at least two or all three main plant nutrients – N, P, K). Plant nutrients in a complex fertilizer granule are formed in one chemical compound (salt), their ratio is constant, the fertilizer is practically free of ballast materials.

Various granule coatings are used to improve their mechanical properties (to make them more stable and less abrasive) as well as to improve/control their effects (to break the granules down over a longer period of time; to break down the granules under certain weather conditions). For this purpose, for example, wax, polymer, a sulphur coating is used.