In this process (Figure 1) mechanical force is used to form dense particles (granules) from finely divided (nongranular) solid raw materials. With compaction granulation the dry materials are first proportioned and thoroughly mixed (blended) to achieve the desired nutrient ratio (fertiliser grade). This step of the process is identical to that normally used in the steam/water granulation process. The mixture is then continuously fed to a pressure-roll compaction machine that forms the powdered mixture into a dense, hard sheet of material usually about 2-3 cm thick and 60-100 cm wide. The compaction machine consists of two horizontally opposed. rollers that turn inward toward each other. The rollers may be as much as 100 cm wide and about the same diameter. The shaft of one roller has a stationary shaft while the other is movable. Pressure is applied by a hydraulic system to the movable roller shaft and bearing assemblies, which in turn determines the pressing force applied to the fertiliser material fed to the machine. The compacted sheet of material is crushed in a controlled manner to form smaller particles, The crushed material is screened to obtain the desired product-size fraction. The oversize material is crushed and recycled to the screens and the undersize material is returned (recycled) to the compaction machine after first being thoroughly mixed with the fresh raw material feed. No chemical reaction is required for the process although the raw material properties (especially chemical compatibility, thermal characteristics, and plasticity) have a major influence on the performance of the process. The pressing force of the compaction rollers required to produce an acceptable granular product is measured in terms of tonnes of force per centimeter of roller width. Refer to Table 1 for typical pressing forces required for various fertiliser materials and mixtures.

Granular fertilisers prepared by compaction have a more irregular particle shape as compared with the more smooth and rounded (spherical) granules produced by other granulation processes. The irregular particle surface and shape characteristics of compacted products may promote caking during storage because of the generation of fines during handling and because of the increased surface area of the irregular particles compared with the more rounded granules produced by other granulation processes. The irregular shape of compacted fertiliser granules may cause some resistance among users who are accustomed to the more traditional spherical fertiliser granules. However, experience has shown that market resistance to compacted products can be largely overcome through refinements in the process, for example, tumbling/abrasion, to round the rough edges and produce more spherical granules and through promotion and customer education.

Links to related IFS Proceedings

238, (1985), Granulation by Extrusion and Compaction Methods,
A Stephenson, R Zisselmar.

380, (1996), Ammonium Sulphate: An Innovative Process for Production,
G N Brown.

386, (1996), Fertiliser Supply from Factory to Farm: a Global Review,
K F Isherwood.

351, (1994), Experiences with Compacting Fertilisers,
T Koivumäki, T Heiska, I Tähtinen.