Bulk Density Measurement by a New Tapping System

A new tapping system has been developed to measure bulk densities of powdery materials and to study their compaction characteristics. The new tapping system has an electronic monitor of impact acceleration to observe the peak value of impact acceleration, that is, the intensity of tapping action. The impact acceleration of a conventional tapping machine is not monitored quantitatively and generally higher than 200 G. The adjustable range of the new tapping system is from 3 to 500 G. Tapping height adjustment of a conventional tapping machine is done without any appropriate change of colliding materials. Such an adjustment causes very ambiguous covariation of tapping intensity and tapping energy quantity. Some confusions of these different factors have been making it difficult to analyse and discuss the effects of tapping height. The peak value of impact acceleration and the tapping energy quantity of the new tapping system can be varied independently through the combination of tapping height adjustment and cushioning material selection. Four limestone powders, five white alundum powders and two Kanto loam powders were used as testing powders. Their specific surfaces, Sw, were measured by an air permeability method. Tapping tests were done for each powder under various tapping conditions. Higher tapping energy in the sense of quantity causes faster compaction process but does not affect the terminal tapping density, p~, which varies widely with Sw and tapping acceleration, A. The terminal tapping density of finer powder is lower than that of coarser powder. Higher impact acceleration causes higher terminal density, but the effect of A seems to be saturated when A becomes sufficiently high. The density difference owing to the size difference becomes smaller with an increase in A. Tapping tests with lower impact acceleration are much more informative than those with higher acceleration.

1. Introduction aim of tapping height adjustment is to control or to vary the intensity of tapping, but few quantitative correlations of intensity and height have been reported.Furthermore, the intensity of tapping depends not only on the tapping height but also on the combination of colliding materials.A conventional tapping test gives therefore very ambiguous and relative informations.Any quantitative comparison of a test result with another one by a different tapping machine is almost impossible and generally in vain.
Tapping of powdery materials is a well known and conventional technique of bulk density measurement, but it has remained in a native state with little sophistication.The tapping height or the falling height of a powder container is almost only one adjustable parameter of a conventional tapping machine.The oratory has an electronic monitor of impact acceleration.The acceleration and the energy of tapping are adjustable independently.The adjustable range of acceleration is from 3 to 500 G.

New tapping system
The main part of a new tapping system is schematically shown in Fig. 1.A 100 cm 3 acrylic resin cylindrical cell is generally used as a powder container.Its ratio of depth to inner diameter is about 6.The cell is fixed to a tapping table by a screw and filled with powder to be tested.The powdery material is poured into the cell through a 24 mesh vibrating sieve.An impact accelerometer is attached to the tapping table to monitor the tapping acceleration.The tapping acceleration can be varied by tapping height adjustment and/or by selection of damper material and damper dimensions.Storage oscilloscopic observation of impact acceleration of tapping of a conventional tapping machine is generally from several millimeters to a few centimeters.
After our preliminary experiments, such tapping height range gives impact acceleration of tapping higher than 200 G when no special cushioning system is used.As described in the following chapters, tapping tests with such higher level of impact acceleration are less informative for many cases than those with lower impact acceleration.The new tapping system is, therefore, used mainly in the range of 3 to 100 G.By the combination of appropriate cushioning system and tapping height, the impact acceleration of tapping and the energy of tapping are adjustable independently.Higher tapping height gives always more tapping energy but the peak value of impact acceleration of tapping can be reduced by cushioning.
An inverval of tapping is 6 s.This interval is much longer than that of an ordinary tapping machine so that any motion or displacement of powder initiated by a tapping action is completed before the next tapping process.

Powders
Limestone powder (CaC0 3 ), white alundum (Al 2 0 3 ) and Kanto loam dust were used as testing materials.Many samples with different particle size distribution were prepared.Their particle sizes were measured by an air permeability method and evaluated by specific surface, Sw [ cm 2 /g].In the following tables, p P denotes true particle density and p 0 denotes the initial bulk density of each powdery sample poured into the 100 cm 3 tapping cell before tapping.Table 1 is the list of powdery samples of limestone ground by a small ball mill.Particle sizes were varied by grinding time adjustment.Table 2 is the list of white alundum powders, which were manufactured and distributed as polishing powder.Table 3 is the list of Kan to loam powders, which were prepared and supplied by APPlE (The Association of Powder Process Industry and Engineering, Japan) as a test dust after JIS Z 8901.

Experimental results
Tapping tests were done for each powdery sample described in the last chapter under various tapping conditions.Examples of compaction process by tapping are shown in Fig. 3 for limestone powder.The tapping height, H, was 10 mm and the impact acceleration of tapping, A, was 3.4 G. n denotes number of tapping and Pn denotes bulk density of powder tapped n times.The compaction process starts from the initial bulk density, Po, which is defined as the bulk density of powder poured into the tapping cell before the tapping.The compaction proceeds monotonically to the terminal tapping density, p~, which is defined as the bulk density of powder tapped sufficiently many times.In the case of Fig. 3, the terminal tapping density, p~, varies remarkably with specific surface of powder, Sw.
Fig. 4 shows the effects of the impact accel- eration of tapping, A, and those of the tapping height, H, on the compaction process.The terminal tapping density, p~, varies with the impact acceleration, A, but does not depend on the tapping height, H, as far as A is constant.Compaction proceeds faster by tapping of higher tapping height but the terminal density is the same.This means that the energy of tapping in the sense of quantity affects the rate of compaction but it has no effect on the terminal density.The terminal density of tapping seems to depend not on the quantity of tapping energy but on the intensity of each tapping action, that is, the impact acceleration of tapping.The terminal density is also independent of any preliminary compaction by tapping with higher impact acceleration, as far as the density after the preliminary compaction does not exceed the terminal density of final tapping.The terminal density of limestone powder at 40 G tapping, for example, was not affected by 80 G nor 200 G preliminary tapping.This fact gives a very useful experimental technique to shorten the time required for the terminal density measurement.
If a combination of colliding materials is not changed, higher colliding velocity causes not only higher impact acceleration but also higher impact energy in the sense of quantity at the same time.The tapping height variation of a conventional tapping machine means therefore ambiguous covariation of the quantity of tapping energy and the intensity of tapping action.
It may be said that some confusions of these different factors have been making it difficult to discuss and analyse the effects of tapping height clearly.For more successful discussions, the effects of tapping energy in the sense of quantity and the effects of tapping intensity should be distinguished clearly from each other and analysed separately.Descriptions and discussions in this paper are concentrated mainly on the influences of tapping intensity on the terminal tapping density.The other discussions KONA No.1 ( 1983) Fig. 6 Terminal tapping densities of Kanto loam powders will be done in the near future.
In Fig. 6, the terminal tapping densities, p=' of Kanto loam powders are correlated to the impact acceleration of tapping, A. As already shown in Fig. 4, larger impact acceleration causes larger terminal tapping density.In the case of Fig. 6, the density difference between No. 8 and No. 11 does not varies with the impact acceleration.The terminal density of Kanto loam No. II, of which specific surface is nearly 4 times larger than that of Kanto loam No.8, is about 0.3 g/cm 3 smaller than the density of No.8.As shown also in the other figures, p= of finer powder is, in general, smaller than that of coarser one.
In Fig. 7, the terminal densities of white alundum powders are correlated to the acceleration of tapping.It is clear in Fig. 7 that the influence of A on p= is almost saturated when A becomes sufficiently high.The critical value of A, beyond which p= is almost independent of A, seems to depend on specific surface of powder, Sw.The critical value of A for coarser powder is lower than that for finer powder.The terminal tapping density of finer powder, which is smaller than that of coarser powder, becomes larger still in the range of A where the density of coarser powder is already independent of A. The density difference between It may be estimated that the critical values of tapping acceleration for Kanto loam powders are larger than 500 G.In Fig. 8, p~ of limestone powders are correlated to A. When the impact acceleration of tapping becomes higher than about 150 G, the density differences among powders owing to their particle size differences are practically no more detectable.Tapping tests with lower impact acceleration are much more informative than those with higher impact acceleration.

Conclusion
Bulk densities of CaC0 3 , Al 2 03 and Kanto loam powders were measured by a new tapping system with a electronic monitor of impact acceleration of tapping.The peak value of impact acceleration was varied from 3 to 500 G independently of tapping energy in the sense of quantity.
Experimental results were, I) Energy quantity of a tapping action affected the rate of compaction but did not affect the terminal tapping density, p~ 2) Higher tapping acceleration caused higher terminal density.This effect was saturated in the range of sufficiently high acceleration.3) p~ of finer powder was lower than that of coarser one.The density difference owing to  It is very important to distinguish the effects of tapping energy in the sense of quantity and the intensity of tapping action.Tapping tests by the new tapping system are very informative especially with lower impact accelerations than a conventional tapping machine.Impact accelerations of a conventional tapping machine are generally higher than 200 G.
Fig. IMain part of new tapping system

Fig. 2
Fig. 2Storage oscilloscopic observation of impact acceleration of tapping Fig. 3Examples of limestone powder compaction process by tapping (Effect of specific surface of powder, Sw) Fig. 5Independency of terminal tapping density, p=' from tapping height, H density from the tapping height.The terminal density is also independent of any preliminary compaction by tapping with higher impact acceleration, as far as the density after the preliminary compaction does not exceed the terminal density of final tapping.The terminal density of limestone powder at 40 G tapping, for example, was not affected by 80 G nor 200 G preliminary tapping.This fact gives a very useful experimental technique to shorten the time required for the terminal density measurement.If a combination of colliding materials is not changed, higher colliding velocity causes not only higher impact acceleration but also higher impact energy in the sense of quantity at the same time.The tapping height variation of a conventional tapping machine means therefore ambiguous covariation of the quantity of tapping energy and the intensity of tapping action.It may be said that some confusions of these different factors have been making it difficult to discuss and analyse the effects of tapping height clearly.For more successful discussions, the effects of tapping energy in the sense of quantity and the effects of tapping intensity should be distinguished clearly from each other and analysed separately.Descriptions and discussions in this paper are concentrated mainly on the influences of tapping intensity on the terminal tapping density.The other discussions

Fig. 7
Fig. 7 Terminal tapping densities of white alundum powders coarser and finer powders becomes therefore smaller in such a range of tapping acceleration.It may be estimated that the critical values of tapping acceleration for Kanto loam powders are larger than 500 G.In Fig.8, p~ of limestone powders are correlated to A. When the impact acceleration of tapping becomes higher than about 150 G, the density differences among powders owing to their particle size differences are practically no more detectable.Tapping tests with lower impact acceleration are much more informative than those with higher impact acceleration. 18

Table 1
Limestone powder