Pressure agglomeration mechanism

The binding mechanisms between powder particles are applied to build agglomerates with specific product characteristics. The two most widely used agglomeration processes are tumble and pressure agglomeration. The latter also includes pelleting . In this book, all other agglomeration techniques will be covered under other processes . 

In pressure agglomeration a more or less stationary bed of particles is consolidated by pressure, bringing about various binding mechanisms. 

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). 

There is a great variety of pressure agglomeration methods, each corresponding to one or more of the binding mechanisms of agglomeration (see Section 5.1,1, Tab. 5.1, Fig. 

Corresponding to the previous discussion, pressure agglomeration can be carried out by a number of techniques. Each method results in the manufacture of different types of products with respect to size, shape, and physical properties. However, all have in common a basic compaction mechanism. 

If the mechanism of densification is considered (refer to the sketches in the upper part of Fig. 8.1), it becomes clear that the pores in the feed for a pressure agglomeration process of any kind must not be filled completely (saturated) with a liquid. An example of such a material would be a normal filter cake, i.e. one that has not been blown dry or otherwise further dewatered. Since liquids are incompressible, the pressing force would increase quickly and mechanical dewatering would have to occur, which further reduces the speed of densification. It would also require an effective separation of solids and liquid during the densification process this is a task which, so far, has not been solved satisfactorily. Therefore, with increasing pressure applied to the particulate solids, which typically results in higher densification or lower porosity, the moisture content of the feed must diminish. In high-pressure a lomeration the feed must be essentially dry ... 

Medium Pressure Axial Screw Extruders Axial screw extruders normalty operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.1 (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. 

While, originally, hot isostatic pressing was developed and used to remove defects and/or produce parts with minimum porosity and, consequently, ultimate density from powders and preforms, the study and understanding of the mechanisms of pressure agglomeration also led to a modification of the process which is then actually used to produce parts with a controlled high porosity [B.61] (see also Section 5.3.2). 

A more recent development in instantizing utilizes compaction/granulation. One of the most important binding mechanisms of high-pressure agglomeration (see Section... 

As a final example of the use of pressure agglomeration for minerals, various applications of roller presses in the salt (sodium chloride) industry shall be discussed. The technology is used for the size enlargement of rock salt fines and of crystallized byproduct salt from the concentration of potassium chloride and for potassium chloride itself and for other salts. All salts deform easily under pressure and, while becoming corrosive in the presence of water, do not cause much mechanical wear unless hard solid impurities or contaminants are present, which happens rather infrequently. 

The mechanisms discussed above are universally present advantages of products obtained from high-pressure agglomeration. They may be applied for all materials destined for similar uses and requiring these characteristics. The most important properties of briquettes or granules produced vith high pressure agglomeration methods are ... 

The variable production of oxygen is determined by the dimensions, shape, and chemical composition of the core. To make sure that the reaction proceeds uniformly, the core is made by pressure agglomeration. For that purpose, different mixtures of chemicals, which may be pre-agglomerated to avoid segregation and improve metering, are filled in layers into the die of a mechanically or hydraulically operated punch-and-die-press. After compaction, the chemical components of the blend are in very dose contact so that the influences of the different layers are only visible as little defined steps in the curve representing the actual production of oxygen in Fig. 6.1 l-7b. Since a major... 

Contrary to tumble/growth agglomeration, where individual particles or clusters of particles coalesce with each other and successively build a structure by material addition on the surface of a growing mass, in pressure agglomeration, a defined volume of loose, sometimes pre-processed feed material is introduced into a more-or-less closed or closing die and mechanically compressed by external forces that are caused by the... 

Fig. 10.15 Sketches explaining the mechanisms of pressure agglomeration (see also Fig. 5.9, Chapter 5)...

Fig. 5.7 Processes occurring between particles of a tumbling powder bed after wetting with droplets of a binder liquid [5.1] Fig. 5.8 Diagram of particle and droplet distribution in a low-density fluidized bed and of particle penetration into a liquid [5.1] Fig. 5.9 Sketches explaining the mechanism of pressure agglomeration Fig. 5.10 Diagram of equipment for ...

---