Pressure precompaction

For fine powders that tend to bridge or stick and are of low bulk density, some form of forced feed, such as the tapered screw feeder shown in Figure 9, must be used to deaerate, precompact, and pressurize the feed into the nip. Large machines are available with up to five screw feeders to spread the flow across the roUs, and vacuum hoppers are also used to remove air when densifying low density feeds. 

Ultimately, the feed system is not only responsible for conveying powder to the rolls, but also aiding in an initial deaeration of the blend. Weggel7 found that the torque on the feed system screw(s) is directly related to the precompaction pressure. He concluded that by maintaining a constant back pressure, the resulting compact quality can be better controlled. 

Miller noted that the total compaction power requirement is the sum of the power required for the feed screw(s) and the roll drive. Feed screws not only convey the powder material from the compactor storage hopper but they also help deaerate the powder in the process. The deaeration of the powder acts as a mini compactor by precompacting the material just prior to roll compaction. Optimum compaction pressures and feed screw designs vary widely for different powder material properties. Changes in bulk powder density and feed screw speeds will affect the roll gap, the compaction pressure, the throughput, and the quality of the compact. ... 

Three basic types of pressure application are used in commercial densification processes (1) Straight compression in a die (2) Extrusion through a constriction and (3) Shear of precompacted material to produce heat and flow under pressure. Approximate energy consumptions supplied by the manufacturers are compared to the laboratory tests reported here in Table IV but it must be stressed that these figures are only approximate, depending critically on type of material, size, temperature, etc. 

Figure 241. Effect of precompaction pressure on roller diameter. Briquetting pressures 1. 100 000 lb in-2 (approx. 6.9 x 10 N m 2). 2. 50 000 lb in (approx. 3.4X10 N m- ). 3. 100001bin 2 (approx. 6.9xlO Nm-2)...

As discussed earlier (Figure 236), the solids pressure p iax will be influenced by the precompaction pressure of the feeder pQ, Based on work by Johanson, Harris described the effect of feeder pressure with the following mathematical relationship ... 

Reductions in the roll force and diameter accompanying the increase in precompaction pressure lower the size, weight, and cost of roller presses. In contrast, roll drive requirements remain almost unchanged " if the production rate is kept constant. This is due to increases in the roll torque factor T in the roll drive power equation (106). 

Feed screw precompaction pressures up to and exceeding 4001b/in2 (approximately 2.8xlO N/m ) have been reported. In normal operation the pressures are probably in the range of 1-lOOlb/in (approximately lO -lO N/m ). 

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

Weggel indicated that feed screw torque varies directly with the precompaction pressure (32). He suggested that by maintaining a constant feed screw pressure, the compactor operator can control the compact quality. Variations in precompaction pressure and in the compacting pressures are directly related to feed screw amperages and the roll drive motor. The compaction pressures are dependent on a continuous flow of powder into the feed screw area. If the powder feed flow is intermittent or... 

As shown in Table 3.7, the extrusion could be achieved at the pressure of precompaction. However, no significant decrease was found in the specific volume it was 1.112 0.007 cmVg before and 1.116 cmVg after the extrusion. Perhaps, the shear rate of the extrusion was too low, 0.03-0.05 s to demonstrate the effect of shear on compaction. 

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