Compacting roller press

Equipment used for high-pressure agglomeration (a) compacting roller press, (b) briquetting roller press. 

Dec, R.T., Zavaliangos, A., Cunningham, J.C., Comparison of various modelling methods for analysis of powder compaction in roller press, Powder Technology, 130, 265-271, 2003. 

Material is compacted as it is carried into the gap between a molding groove on the inner circumference of a ring and a press wheel of smaller diameter running in the groove. As above Ring roller presses... 

As a dry granulation technique, roller compaction has to be considered as a simple continuous process able to convert powder into granules (Fig. 6). The system comprises powder hoppers, feeding screws, a roller press, a mill, and, optionally, a recycling unit. 

A method of pressing whereby two or more compacts are produced simultaneously in separate die cavities. Area at which particles approach each other closely enough for binding mechanism(s) to become effective. In roller presses converging space (volume) between two counter-rotating rolls. 

A more or less continuous band of compacted material produced in roller presses featuring smooth or shallowly profiled rolls and a gap between those rolls. A method by which a particulate mass is pressed in a stationary die between one moving and one fixed punch. 

The particular merit of roller presses is their ability to compact large quantities of material at low cost. Other considerations may be the ease with which it can be made dust-tight for briquetting of, for example, toxic chemicals, or the great variety of materials from which the rollers can be constructed for those cases where high resistance to corrosion or wear is required. Of more recent importance is their usefulness for compacting hot materials with temperatures of up to 1000°C. 

The most widely used roller presses are double roll presses which achieve compaction by squeezing material between two counter-currently rotating rollers (Figure 222), much in the same manner as the operation of rolling mills. Pockets or indentations, which have been cut into the working surfaces of the... 

If rows of identical pockets are machined into the working surface and the rollers are timed such that the pocket halves exactly match, so-called briquettes are formed (Figure 224). Roller presses do not produce compacts with the same fine detail and uniformity as those made by tabletting machines or other die presses. The flashing or web, caused by the land areas around each briquette pocket, which is usually found on the edges of all briquettes from roller presses can not be removed completely and reliably and, therefore, may also be objectionable. 

The process occurring during compaction of particulate matter in roller presses is described and interpreted by different authors in a rather similar way. The feed mechanism is characterized by the pressure caused by gravity or a force feeder system and the friction between the material and roller surface. Compaction between two rolls may be explained by dividing the nip area into two zones the feed zone and the compaction zone. 

Figure 227 shows a smooth face roller press. The feed zone is defined by the two angles and In the feed zone, the material is pulled into the nip by friction on the roller surface. Densification is solely due to rearrangement of particles (Figure 181A). The density of the feed is characterized by the bulk density 70 and reaches the tap density 7t at the point . The peripheral speed w of the rolls is higher in this zone than the velocity u of the material to be compacted, ocq is the so-called angle of delivery which is defined by the width /Zo of the feed opening above the rollers as well as the material (flowability)...

One of the problems of counter-currently rotating double roller presses results from the quick change of cross section of the nip, i.e. a fast increase in densification, particularly between rollers of small diameter. Especially between essentially smooth rollers establishment of a compaction zone is difficult if no positive feed pressure is applied. 

In contrast, the briquetting or compacting of some other materials demands a degree of compaction that cannot be achieved by a single pass in a choke-fed roller press, irrespective of the ratio of pocket size (or gap width) to roll diameter. In addition, redistribution of material (which may be extensive) from the nip against the flow of material or from the rear of cups into following cups, e.g. due to the flow of displaced air, may further reduce the efficiency of compaction. 

The solids pressure p ax is the maximum pressure exerted by the rollers at approximately a = 0° that produces a compact of sufficient strength and density. It is determined in bench scale laboratory tests. In general, a pressure exists for each material above which compact strength no longer significantly increases. Similarly, there is usually a specific pressure at which optimum density is obtained. Since these conditions are often close to each other, they provide well-defined load limits of the roller press. 

Figure 243. Drive energy of roller presses with screw feeder as a function of precompaction pressure and influence of permeability during compaction...

A more detailed study of material motion in the actual compaction zone has been made in the roll press simulator using marker beads, 16 mm cinematographic photography, and stereoanalysis. The flow pattern of particles in the cups of the roller press was recorded which enabled determination of strain distribution. Figure 251 shows examples of bead positions before and after partial compaction. 

Therefore, it is likely that operating a briquetting roller press at the slowest possible speed consistent with economic throughput would be advantageous in reducing stress differences during compaction. Moreover, a slower roller speed will allow more time for any time-dependent recovery to attain equilibrium and plastic flow to reduce high stress concentrations. 

Roll timing In the case of roller presses for briquetting or if corrugated roller surfaces are applied for compaction, optimum product quality can be assured... 

Design of the roller surfaces In designing a roller press surface, some fundamental considerations do influence the possible shape, size, and distribution of the cups or indentations.Limitations include the relationships of circumferential pocket dimension/roller circumference and pocket depth/roller diameter as well as the pocket shape. Associated with these are problems of release, clam-shelling (splitting), extrusion, removal of air from the compaction zone, and uneven distribution of feed as well as compaction pressure. In spite of those limitations, a large variety of shapes can be produced. 

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...

Most briquettes or compacts leaving the roller press must be handled gently directly after pressure release. Therefore, it is essential to select correctly designed inclined chutes and rubber belt conveyors. Perfectly well formed and densified products can be destroyed by the use of shaker, drag chain, apron, screw, or... 

Flow sheet of fertilizer granulation plants utilizing roller presses for compaction... 

Figure 412 is the most versatile flowsheet of a fertilizer granulation plant using a roller press for compaction. 

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