Pressure Agglomeration Compaction

In contrast to tumbling agglomeration where no external forces are applied, in pressure agglomeration pressure forces act on a confined mass of particulate solids, which is then shaped and densified (Engelleitner, 1994). Pressure 

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Pressure agglomeration (compaction) methods for size enlargement of solid fertilizers... 

Fig. 6.1-13 Block diagram of a powder granulation system by pressure agglomeration (compaction/granulation) for general applications also showing various optional features...

For accurately shaped compacts with extreme demands on tolerance only the confined die approach of pressure agglomeration is applicable. Such requirements exist, for example, in the pharmaceutical industry where tabletting machines are used to carry out the task. None of the growth agglomeration methods can yield products with these specifications. 

Shape is often also an important characteristic. In many cases spherical products of size enlargement are desired. This approximate shape can be obtained with all growth agglomeration methods. On the other hand, unless extremely accurate feed control can be established in some tabletting machines, spherical products cannot be produced with pressure agglomeration equipment. The nearest approximation would be lens- or almond-shaped compacts. 

The most versatile methods of pressure agglomeration use high forces (Figure 179) to compact particulate matter into tablets and briquettes of specific size and shape or compacts. 

Figure 179. Schematic representation of high pressure agglomeration equipment, (a) Reciprocating piston and die (eccenter) press, (b) rotary tabletting machine, (c) ram extrusion or plunger press, (d) roller compacting machine, (e) roller briquetting machine...

These variations are present in products from all pressure agglomeration techniques and lead to a weakening of the compact. If a sintering step follows, distortion is possible due to differences in the amount of contraction occurring at the positions of different density (see also sections 4.2.2.3.5 and 4.2.2.5). 

If the feed is made up of several components or of only one raw material and recycle, a mixer may be installed in front of the roller press. Selection of this piece of equipment plays an important role in pressure agglomeration. In dry, binderless applications, a high intensity mixer may excessively aerate the material and prohibit successful compaction because the roller press is not capable of sufficiently deaerating the blend. A too severe mixing action may also destroy primary agglomerates in the recycle which are necessary to obtain the required product density. 

Figure 296. Diagram of two typical flow sheets for pressure agglomeration with roller presses, (a) Briquetting or compacting (solid lines) (a) + (b) compacting/granulating...

The following chapter will exclusively deal with the use of pressure agglomeration or compaction for the size enlargement of fertilizers because this is the modern and still little known alternative to conventional fertilizer granulation . 

Principally, all pressure agglomeration methods (see Section 4.1.2) can be used for the compaction of solid, particulate fertilizers. 

High pressure agglomeration methods compact essentially dry particulate solids. 

High pressure agglomeration uses two fundamentally different principles confined volume compaction (Section 4.2.2.3) and roll pressing (Section 4.2.2.4). 

The most economical pressure agglomeration method, which is also applicable for a wide range of capacities, uses roller presses (see Section 4.2.2.4). Depending on the surface configuration of the rollers, either compacted sheets are produced which are subsequently crushed and screened into a granular product or, if the rollers are pocketed, briquettes are formed. For waste treatment and the production of secondary raw materials (recirculation of valuable components) the latter is often preferred. Briquettes are usually pillow shaped and almost monosized, featuring a volume that may typically range between 20 and 100 cm3. 

In pressure agglomeration, the distance between the surfaces of compacting tool sets specifically in extrusion, the distance between the pressure generating device and the die plate, in roller presses, the closest distance between the rollers. Double-roll pellet mill in which the rollers are in the shape of coarse, intermeshing gears with bores at the root sections between the gear teeth. (Also gear pelletizer.) Distribution plate in which the perforations are manufactured such that they produce a directional flow of gas. See melt solidification. 

In high-pressure agglomeration, another interlocking mechanism may occur if a mixture of rigid and plastic materials is compacted. In this situation, the plastic component flows into recesses and, more generally, envelopes the exterior structure of harder particles, thus producing a strong structural bond that resembles the effect of a matrix binder (see also Section 8.1). 

In pressure agglomeration, lubricants also reduce the coefficient of friction between the material to be compacted and the tooling. This results in a more uniform structure of the compact and in less density variation (see also Section 8.2). During ejection from a die or release from a mold lower forces are required for separation and, therefore, higher survival rates are obtained. 

Fig, 6.5 Schematic representation of equipment for high-pressure agglomeration, Ram press (upper left), punch and die press (upper right), roller presses for compaction (lower left) and briquetting (lower right). 

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