Pressure agglomeration equipments

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. 

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...

The general conclusions of investigations into the effects of operating conditions of pressure agglomeration equipment are that density variation ... 

Two important phenomena limit the speed of compaction and, therefore, the capacity of any pressure agglomeration equipment compressed residual gas (air) in the pores and elastic spring-back. Both cause different, equipment specific cracking and a weakening or, sometimes, total destruction of the products (see also [B.12b B.42 B.56] and Sections 8.4.1 through 8.4.3). 

In medium-pressure agglomeration equipment (Fig. 8.42), the perforated support (die) can be either flat (a), concave (b), or convex (c). For all three designs, intermeshing, toothed executions have been proposed in the patent literature [B.42] to avoid slippage and improve extrusion as well as extrudate quality, but only the so called gear pelleter , in which the convex dies (Fig. 8.42c) are hollow gears with extrusion channels between the teeth, has reached commercial importance (see below). 

Pellet-Mills With Hollow Perforated Cylinder and Feed From the Inside This design represents the most commonly installed medittm-pressure agglomeration equipment (see Chapter 6, Fig. 6.4b.5). It is primarily used in feed mills for the granulation of animal feed and associated products. A large number of vendors in many countries manufacture and offer these machines. 

Fig. 6.2-48 Diagram of low-pressure agglomeration equipment using gravity feed and screens or thin perforated sheets ...

Equipment for pressure agglomeration with two rolls. Relative amount of voids in an agglomerate filled with a liquid or solid substance e.g. liquid saturation, binder saturation. 

Particulate matter Since pressure agglomeration between counter-currently rotating rollers deals with particulate matter, results of this theory are only applicable in a general way. If smooth rollers are used, a very close correlation can be obtained. Today, however, roller surfaces for agglomeration are in most cases equipped with some sort of a profile in order to improve the bite on the material which is a never-ending problem because of the noncontinuity of particulate matter. In the case of surface profiliations zones (2) and (3) cannot freely develop due to interlocking between the material and roller surface. This is most pronounced for briquetting. 

Peripheral equipment Roller presses also require peripheral equipment to accomplish the complete task of pressure agglomeration. 

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. 

Constant product quality requires an even feed rate, homogeneous bulk density of the material to be treated, uniform densification, and reproducible maximum pressure. This statement is true for all pressure agglomeration methods. However, while these conditions can be met relatively easily in die and roller presses with proper feed preparation and specific equipment parameters, it is rather difficult to achieve in extrusion. The reason for this is that densification and maximum pressure depend on the resistance to flow in the die channel or holes. Small variations in feed homogeneity or frictional properties can yield major differences in equipment performance and product quality. Wear or buildup in the extrusion die are among the most important parameters influencing the back-pressure which, in turn, is responsible for the amount of densification prior to extrusion. 

Therefore, it was the author s intent to cover—for the first time in a textbook on agglomeration—the fundamentals in considerable detail and to introduce the multitude of agglomeration techniques as well as applications that have been developed during the past 100 years and, more specifically, during the most recent four to five decades. In Chapter 4 (Industrial Size Enlargement Equipment and Processes) as well as Chapter 5 ([Some Selected] Industrial Applications of Agglomeration) pressure agglomeration and, explicitly, roller presses have been covered in particular detail because of the author s past and present involvement as an expert in this area. 

Fig. 6.4 Schematic representation of equipment for (a) low- and (b) medium-pressure agglomeration, (a.l) Screen,...

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). 

In the following four subchapters the technologies and the equipment for the beneficial agglomeration by pressure will be described. As already mentioned in Chapter 8, in pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in differently shaped and operating dies. 

Because of product and equipment design considerations, the forces that can be exerted on the mass to be pelleted are higher than those in low-pressure agglomeration but are still relatively small. Therefore, binders and lubricants (see Section 5.1.2) play an important role for the technology and the product is normally not highly den-sified. 

In spite of the problems that are associated with this design, it is often applied, particularly in clean environments and for easily deformable materials which require small extrusion pressure. Tab. 8.2 lists the standard models offered by one manufacturer and summarizes their most important technical details. Among the medium-pressure agglomerators or pellet mills, the Alexanderwerk moist granulator represents equipment that operates with the lowest forces. In Fig. 8.48 three examples of products are shown. As can be easily seen, granules or pellets can be well formed (a and c) or somewhat crumbly (b). If the latter is not acceptable, the extrusion characteristics of the feed may be adjusted by changing (in this case increasing) the moisture, binder, and/or lubricant contents. 

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