It is the purpose of this study to obtain the fundamental information of physico-chemical properties of commercial NiO, ZnO and CaCO3 powders in the grinding process. The effects of dry grinding by ball-milling for hundreds of hours on the structure of the powders have been studied by X-ray diffraction, spectroscopic reflection, and some other methods. The following results have been obtained. (1) During the early stage of grinding, the crystallite dimension of these powders decreases considerablly, but in further grinding it maintains almost constant value between 400∼700Å which is independent of the crystallite dimension of the original powders. (2) In the early stage of grinding, the lattice distortion of these powders increases with the decreasing crystallite dimension, but a structural disordering on the surface and in the interior is increasing gradually during the further stage of grinding. (3) As the grinding proceeds, no non-crystalline substance is produced in NiO particles. Considerable decrease in crystallinity occurs only on the surface of a CaCO3 particles in the early stage of grinding, but gradual decrease in crystallinity occurs from the surface of ZnO particles towards the interior as the grinding proceeds. (4) As the lattice distortion increases, the catalytic activity of the ground NiO increases correspondingly, but that of the ground ZnO decreases. (5) The absorption in the visible region of the reflection spectrum of ZnO powder increases considerablly as the grinding proceeds, and the change of the spectrum is similar to that of NiO powder. When the ground powder is heated at various degrees of temperature for about 3 hours, the reflection spectrum of NiO powder becomes perfectly the same as the original powder, but that of ZnO powder is imperfect. It seems reasonable to conclude that the phenomenon with the ZnO powder depends on the change in crystallinity.
Apparent adsorption isotherm equations have been derived from the solution on the ultrafine powders on the basis that the solution is of perfect solution, that the component molecules have sizes different from each other, and that there is no interaction between the admolecules in the adphase. The shape of these calculated isotherms exhibits the general features of experimental apparent adsorption isotherm. The surface areas obtained from these equations show reasonable agreement with the BET surface areas from the separate experiment on the vapour phase adsorption with single component.
The expansion of the compressed tablets in diameter and thickness due to their sorption of moisture has been studied, using starch and microcrystalline cellulose tablets. Of the starch tablets, the values of equilibrium volume expansion were very low as compared with the equilibrium moisture contents at low relative humidities below 20%. Above this relative humidity, the values of volume expansion were approximately at 1cm3 per gram of the sorbed water. In the kinetic measurement under different conditions of relative humidity, the rate constant of moisture sorption increased with increasing relative humidity. The values of rate constants of volume expansion were approximately equal to those of moisture sorption. For the microcrystalline cellulose tablets both the rate of moisture sorption and the rate of volume expansion decreased with increasing compaction pressure in tablet preparation.
Using two-dimensional model powder particles such as of small wooden rods and of small vinyl straw pipes, their consolidation and consolidation-shearing tests have been carried out in the two-dimensional testing container vessels. The momentary alternating state of consolidation or shearing that took place during the testing period can be observed directly in these observations as presented in Figs. 10, and 13. We can also decide the amount of strains induced by the external pressing stress by observing the variation of labeled marks on the piled surface of the two-dimensional model powder particles as can be seen in Fig. 14. The relations between the consolidating external stress σ and shearing stress τ induced in the shearing surface can be illustrated as linear and curved relations as shown in the Eq. (6) in the tests of ordinary three-dimensional powder particles. These relations can be observed in the two-dimensional model powder particles as illustrated in Fig. 7, in which the curved characteristics can be illustrated by the deformability of the piled powder as shown in Fig. 8. The method to calculate the stress-distribution obtained from the strain-distribution which was observed momentarily is presented in this paper though its detailed calcultions are reserved for another paper.
Influence of the shapes of particle on the angle and the packing properties of granular materials was studied experimentally by measurement of the reposed angle φr, the angle of internal friction φi and porosity ε, using wood particles of various shapes as non-cohesive materials. It is assumed in the present paper that tan φr and tan φi are apparent friction factors on the plane AB of reposed pile and the plane HH of shearing tests, respectively, as shown in Fig. 1, and the spherical unevenness as shown in Fig. 2, characterizing those planes. The shape factor ψ is defined by the ratio of the diameter of inscribed sphere to the diameter of circumscribed sphere of a particle for simplicity. Under the above assumptions, it is supposed that the experimental curves of φr and φivs. ψ have maximum values as shown in Fig. 5. These results are not necessarily identical to that shown by Crosby2)and Otsubo4). The experimental curve of ε vs. ψ in Fig. 4 shows the minimum value. Nor is this result identical to the tendency of the data shown by Shirai3) and Crosby2). From the above results and authors' considerations, it is suggested that particles differ from the symmetric to the un-symmetric in the angle and in the packing properties.
The measurement of tensile strength of the powder bed having various water contents has been carried out, using fine powder with its specific surface diameter below 10μ, and the effect of the water content on the adhesion force among the particles discussed. The adhesion force caused by the pendular rings of the water at the points of contact increases as the water content increases. It is considered that the pendular ring of the water formed at every contact point of the particles is of uniform size at relatively high void fraction. In this case, the relationship between the non-dimentional adhesion force of one contact point and the water content is to be expressed by the equation fc/πdpT=Kwc1.4. On the other hand, it is considered that at low void Fraction the pendular rings of the water blend into one another, or form a sort of bridges between the nearby particles. The adhesion force is considerably influenced by the true void of the powder bed. The present experimental result does not agree with the theoretical analyses of the two spheres of equal size hitherto reported by many investigators.
In this experiment, the effects of apparatus factors, such as the base ring diameter, the pouring rate, etc. on the angle of repose φr of the poured powder have been studied. By using the apparent fraction of the void ε_??_ of the poured powder and its adhesion property, the authors have tried to explain the dependency of φr on the apparatus factors. In this case, the apparatus factors for the angle of repose of the poured powder are as follows: (1) The base ring diameter Dr (mm) (2) The pouring rate Q (g/min) (3) The spatter height Hs (mm) (4) The base ring height hr (mm) Magnesium hydroxide, garnet, talc, artificial stone (white marble), glass bead, and lycopodium were used as sample powder. The results obtained are summarized as follows: (1) As Dr increases, φr of all sample powder decreases and ε_??_ of talc, glass bead, and artificial stone decreases, but ε_??_ of lycopodium, garnet, and magnesium hydroxide is constant. (2) As Q increases, φr and ε_??_ of all sample powder decrease. (3) As Hs increases, φr of magnesium hydroxide and artificial stone increases, and φr of lycopodium decreases, but ε_??_ of all these powder decreases. (4) As hr increases, in the case of the artificial stone, φr and ε_??_ take the maximum value and then they decrease again. For the dependency of φr on these apparatus factors, the following are matters of great importance. (1) Packing of the poured powder by its own weight. (2) Break down of the top of the heap of the poured powder by falling powder. (3) The pile up effect on the top of the heap of the poured powder by its adhesion property. (4) In fine powders their coagulated secondary particles moved as flowing units through the spatter, but, in the case of the large Hs, these coagulated particles were broken into primary particles during the falling.
The flow properties of some kinds of granular masses have been investigated with a double cylinder type viscometer which, in principle, is a Couette viscometer. Recently, observing the behavior of coarse sands in this viscometer, the present authors found that a flow pattern was expressed in a very simple form. In the present paper, is a report made of the experiments performed of the plastic yield stress of a few samples of sand of various particle diameter, and also of the functional dependence of the plastic yield stress on the rate of revolution of the inner cylinder and other variables. The following results have been obtained. (1) When the diameter of the outer cylinder is larger than about 20cm, the torque T∞ does not depend upon the diameter of the cylinder, but upon the particle diameter of dry sand. (2) The final torque decreases with the increasing rate of revolution of the inner cylinder in the range of rate of revolution, 30∼400rpm, but takes almost constant value at lower rates of revolution than about 30rpm. (3) The plastic yield stress S0 is related to the vertical pressure P of packed sand by the equation S0=cP+K where c is a constant depending upon the coefficient of internal friction, μ, and K is a constant accounting for the intergranular cohesion-like forces. (4) Using the constant value c obtained experimentally, the coefficient, μ can be calculated by the equation (4). The coefficient, μ decreases with the increasing rate of revolution. Then, at lower rates of revolution the coefficient, μ increases with the decreasing particle diameter of sand, but it can be considered that there is no effect of the diameter of the particle on the coefficient, μ at higher rates of revolution. The cohesion-like force increases with the decreasing internal friction, and depends on the wetting fluid saturation, but the physical problem of these phenomena has yet been left unsolved.
The bulking properties of fine powder in centrifugal fields were investigated, using CaCO3, Al2O3 and other pharmaceuticals having different particle size. The cylindrical tube filled with a known weight of powder was set to the rotor driven by a variable speed electric motor, and whirled until the level of the powder ceased to change. The apparent volume of the powder was varied with the magnitude of centrifugal force. The relation between ε, the porosity, and F, the average centrifugal force acting on a single particle, was given by the following equation for each material ε=k(1/F)n+0.26 where k and n are constants. Tapping experiments were also carried out on the same powder. The final porosity was varied with the magnitude of tapping impact which is the function of the falling height H. The data for each material were correlated by ε=h(1/mgH)n'+0.26 where m is the mass of a single particle, g is the gravitational acceleration, and h and n' are constants. These results support the assumption that the bulkiness of a fine powder is affected by the balance between the adhesion force of particle and the external force acting on an individual particle.
In planning a feeder of fine powder it is necessary to study the characteristics of the powder to supply in order to ensure stable feeding. Especially it is the case with sticky or agglomerative powder which will often bring up feeding problems, and of which feeding in minute portions with high degree of accuracy is extremely difficult. This prototype apparatus has been developed with a view to coping with the fact that neither the conventional method of stabilizing bulk density, nor the method of packing the powder relying on gravity into feeder mechanism, is adequate by itself for processing the supply of powder in minute portions. By means of this prototype apparatus the powder is compressed into the groove on the roller by the rotating blade, and then it is scraped off by means of a scraper. This mechanism is employed to reduce or eliminate a large fluctuation of bulk density by packing the powder with constant rate of force. As a result a very fine feeding rate as low as from one tenth gram to hundreds of grams per minute with accuracy of 1-3% has been obtained. Much finer and more accurate feeding rate will be achieved with further improvement of mechanism.
The particle diameter at 50% partial collection efficiency is usually used as critical cut size to evaluate a particle separator. However, such assumption is not adequate for exact estimation of the overall collection efficiency, because the latter depends on the inlet particle size distribution too. The theoretical correlations between overall collection efficiency and equilibrium (real) cut size are herein derived, assuming the Rosin-Rammler or the logarithmic normal distribution for the inlet particle size. The ratio of the equilibrium and an arbitrary cut size is also calculated for any R-R distribution, assuming an equation of partial collection efficiency. As an example, this calculation method is applied to the measurement of size distribution of monodisperse aerosol by a two-stage cascade impacter.
Performance of a micro-cyclone, mainly its pressure drop and collection efficiency, has been measured, using monodisperse aerosol by means of a Sinclair-LaMer generator. Even in a micro-cyclone, the modified pressure drop is found as proportional to the square of inlet velocity in its wide range. Over 90% collection efficiency is obtained for one-micron particles (particle density is about 1g/cm3) at the normal inlet velocity (20m/sec), and for 0.25 micron particles at 50m/sec inlet velocity too. The collection efficiency can be correlated to the inertia parameter with a modified factor of particle size for all experimental conditions.
In coating paper with kaolin mineral it is particularly important to pay attention to the distribution of its particle sizes. For measurement of particle sizes the sedimentation methods based on Stokes' Law have hitherto been generally adopted. But since the measurement obtained by the gravitational sedimentation methods is relatively inaccurate when applied to kaolin particles below 2 micron in mean spherical diameter, the centrifugal procedure is recommended for measurement of particles in so extremely fine range. The present investigation was conducted in order to check the existing centrifugal photo-extinction apparatus and the effect that this centrifugal photo-extinction method of measurement would have on the particle size distribution of kaolin mineral in respect of the reproducibility and the number of rotations of the sample cells. The comparative studies were made also of various effects that other methods of measurement would have. It may be said in conclusion that the centrifugal photo-extinction method is adequate for the determination of the particle size of kaolin mineral with considerable accuracy.
Measurement of particle size in the test of distribution particle sizes in powder by the sedimentation method, is often attended with difficulty by reason of coarse particles and of powder with high sedimentation velocity, therefore with high specific gravity. In order to cope with such a drawback it has been a common practice to carry out the measurement in the optimum conditions fittest for the instrument employed for controlling sedimentation velocity with high viscosity medium. This, however, would entail a number of problems concerning the dispersion of samples. The experiment described below shows the characteristics of the Shimadzu Recording Sedimeter based on the specific gravity balance method, together with the experimental results showing the influence of high viscosity in the medium on the particle size distribution.
Some properties of powder beds, i.e.“viscosity”of powder in a vibrating state, their packing density, the angle of repose and shearing stress, were measured at a range from room temperature to about 300°C. α-alumina powders with several kinds of particle sizes and spherical glass powder were used as the samples. Increasing the temperature in the range higher than 100°C, in which moisture was considered not to be effective upon the adhesive properties of these powders, the“viscosity”and the angle of repose increased and the packing density decreased. These results suggest that the attractive force between the particles increases with the increase of temperature as has been verified by the shearing tests. The theoretical calculation of sedimentation volume, which was studied by Nakagaki et al. for the packing density in relation to the adhesive probability of particles, P, was applied to the packing density in a heated state in order to obtain the quantitative relation between the attractive force and temperature. On the basis of an assumption that P was given as a function of kαD/mv, where k, α, D, m and v were respectively shape factor, temperature factor, particle size, mass of the particle and sedimentation velocity of particle in hot air, the relation between α and absolute temperature T was obtained in the following equation. logα=aT/Tm-b In this equation, Tm was the absolute temperature of melting point of the sample. The constant a was found to be the same one for α-alumina and glass powder. The effects of temperature upon the angle of repose and the“viscosity”of powder were systematically interpreted in terms of the temperature factor α obtained from the above equation.
The mechanism of deagglomerant and glidant additives on the physical characteristics of powdered material has been investigated experimentally by means of several kinds of measuring methods. Limestone, sillica sand and fused alumina have been selected as powdered materials, and colloidal silica, calcium stearate and magnesium oxide as additive agents. The results of measurement by split cell type tensile strength method reveal that the additives decrease the adhesion force of powder when the powder is strongly adhesive (limestone, Figs. 7, 8), and increase the force when it is less adhesive (fused alumina, Fig. 9). On the contrary, the results of measurement by shear strength method make it clear that the mixing of additive agent increases the internal frictional coefficient of powdered materials regardless of the kinds of additives (Fig. 10, Table II). From these two measuring methods, the mechanism of additive agents is assumed as follows. Ultra fine solid particles of additives are attached to the surface of larger particle, making it rough enough to increase friction coefficient and to decrease the adhesion force of large particle. In the latter case, the authors have already pointed out in a preceeding report on the measurement by centrifugal separating method3)11). The effects of additive agent on the angle of repose and on the packing property, which are complex compound of the two fundamental characteristics of powder mentioned above, are naturally neither clearly shown nor thought to be so simple, according to the experimental results as are shown in Figs. 2∼6, Table I.
The production of compressed materials, such as tablets, is often jeopadized by capping. Special phenomenalistic studies were made of the factors of capping, and the method to prevent capping was devised. Using a tableting machine and a compression testing instrument, the capping tendency under various compressing and ejecting conditions was studied. As the result of the investigation the following conclusion was reached. Capping occurs mainly within the range of about 3mm from the starting point of ejection. Application of suitable pressure within that range is effective for prevention of capping, and the magnitude of pressure is calculated from the quantity of the wall friction between the compressed material and the die in the ejecting process.
It is the major objective of this research to promote the development of new markets for pulverized coal burnt ashes in Japan and to advance its use in the existing markets. Many different research projects have been undertaken to accomplish these aims. In the present report it is discussed whether it is possible to utilize pulverized coal burnt ashes, particularly cinder-sand and clinker, to make them marketable. Since the flyash-cement process in manufacturing flyash-based structural materials appears to be a good partial solution of the problem concerning the disposal of pulverized coal burnt ashes, it is considered to be desirable to make further researches for further utility of pulverized coal burnt ashes, particularly cinder-sand and clinker. It is proposed to investigate for this purpose, to begin with, whether pulverized coal burnt ashes is fit to be used as source material for production of mineral wool, and take place among other already established materials for industry. Our inquiries are centered on the following two questions. (1) Assuming certain knowledge concerning the factor to equip substance to be suitable as raw material for mineral wool, whether such knowledge will endorse pulverized coal burnt ashes to be used for the purpose. (2) Assuming that mineral wool can be made from pulverized coal burnt ashes, whether it will be qualified as marketable. As necessary procedure to answer these two questions examination has been conducted of the viscosity of pulverized coal burnt ashes in its melted condition, and qualification of its products ascertained to be marketable as mineral wool. From these researches it is concluded that pulverized coal burnt ashes in Japan can be used as suitable raw material for mineral wool in so far as its physical and chemical properties are concerned.