Genji Jimbo

A proposed new characterization of particle shape and its application

Abstract In order to identify particles morphologically nine new shape indices, using three kinds of powder particles which include circularity and elongation as reltionships between characteristic diameters, have been defined. Shape indices have been examined on their numerical lines. The nine shape indices were divided into three groups which characterize the concavities and/or convexities of a particle, mainly characterize slimless and do not characterize specific shape. Shape identification has been tried on two kinds of diagrams which were obtained by combining shape indices of different character. Particle shape without surface texture is characterized on the diagrams and it is also suggested that morphological identification of powder particles is possible with statistical treatment.

Theoretical analysis of the tensile strength of a powder bed

Abstract The authors have proposed an experimental equation which correlates the tensile strength of a powder bed measured by split cell methods with the porosity of a powder bed. In this report, the authors discuss the relationships between the porosity and the pre-compressive stress and also between the tensile strength and the pre-compressive stress. As the results of this discussion, the physical base of the proposed experimental equation is proved theoretically. Furthermore, it has also been proved that the pre-compression of a powder bed brings not only the decrease of the porosity but also the increase of compressive force at the contact point of particles, and therefore the increase of tensile strength by increasing the-compression force cannot be explained simply by the change of the porosity, and then another factor, such as the compressive force at the contact point, must be considered.

Development of a split-type tensile-strength tester and analysis of mechanism of increase of adhesion behavior of inorganic fine powder bed at high-temperature conditions

To analyze the phenomenon of the increase in cohesive force between fine particles under elevated temperature conditions, a new split-type tensile-strength tester of powder beds for high temperature was developed. Fused silica glass with low thermal expansion coefficient was used as the material for the suspended cell of a commercial split-type tensile-strength measuring device at room temperature. The powder samples used were pure fine silica and two kinds of fly ash powders collected in a pulverized coal (PC) combustion and a pressurized fluidized coal combustion systems. Under conditions of relatively low temperatures below 1000 K, the adhesive force of all powders increased gradually in proportion to the temperature. Rapid increases of tensile strength of both fly ash powder beds were observed in the high-temperature range (above 1100 K). Based on the results of thermo-mechanical analysis (TMA), it was estimated that a viscous flow sintering mechanism controlled this rapid increase phenomenon of adhesive behavior of ash powders at high-temperature conditions above 1100 K. On the other hand, using FT-IR analysis, it was demonstrated that the increase of the van der Waals force with the change of surface molecular structure controlled the stickiness of pure silica and both ash powders at relatively low-temperature conditions below 1000 K.

Simulation of ball behavior in a vibration mill in relation with its grinding rate: effects of fractional ball filling and liquid viscosity

Abstract The motion of balls in a two-dimensional vibration mill was investigated by means of a simulation method called Discrete Element Method, which made it possible to calculate the intensity and frequency of ball collisions. The effects of vibration conditions and fractional ball filling on the characteristics of ball collisions and the grinding rate were investigated by the simulation as well as the grinding experiment. Taking account of the strength of particles to be ground, effective breaking collision frequency was defined as the frequency of ball collisions greater than the particle breakage strength and compared with the grinding rate experimentally obtained. These results from the simulation and the grinding experiment agreed well regardless of the grinding conditions. Furthermore, the effects of the viscosity of liquid in the mill pot on the movement of balls, intensity and frequency of ball collisions were quantitatively discussed for a special case where the mill pot was completely filled with the liquid and balls.

Effect of chemisorbed water on bed voidage of high temperature fluidized bed

Abstract Minimum fluidization voidage in the beds of three kinds of silica is investigated at elevated temperatures up to 1223 K experimentally and theoretically. The bed voidage depends not only upon the operating temperature but also upon the humidity of fluidizing gas. Experimental results and theoretical consideration on the change in the bed voidage with temperature and/or humidity demonstrate that this change is caused by the adhesion force of silica particles originating from the change in the concentration of siloxane groups on the surface of the particles.

The Identification of Particles Using Diagrams and Distributions of Shape Indices

Abstract Using five kinds of real powder paricles, we obtained their shape indices from their pictures, attempted to identify the powder particles morphologically and studied the distribution functions of particle shapes and their relationships. The results were indicated as the follwing. It is possible to identify powder particles on the diagrams of the shape indices. The distributions of the shape indices are expressed by some distribution functions. The sample material and the particle size of sample particles affect the distribution of the shape indices in a different way. The distributions of two characteristic diameters and the distribution of shape index defined by ratio of these diameters are accurately connected by an equation.

Effect of aeration rate on flow rate of granular materials from a hopper attaching a standpipe

Abstract Air was injected close to the standpipe inlet to control the particle flow rate from a hopper, and the relationships between the particle flow rate, the aeration rate, the particle diameter and the hopper angle were examined. A semi-theoretical method is presented to calculate the aeration rate at the point where discharge stops. Using the calculated aeration rate, an estimation method for the mass flow rate of a granular material with air injection is also presented, and compared with the experimental results.

Wear of vibration ball mill in wet grinding at constant temperature

Abstract The characteristics of wear of a laboratory vibration mill with steel balls during wet fine grinding were investigated under various grinding condition

The effect of grinding media on the breakage rate in a planetary ball mill

Grinding experiments on the sieved-size fraction of silica sand in a planetary ball mill were carried out. It was confirmed that the breakage of a relatively fine size fraction by large balls in the mill can be described by the first-order law, but the law breaks down when the ball size is too small. The reasons for this abnormal breakage were experimentally investigated and the relationship between the ball size and the critical size of material was given in this paper. For the normal breakage, the specific rate of breakage of materials was described by the equation S1 = axiaQ(z), where Q(z) is the probability function ranging from 1 to 0, which has been used by Austin in a tumbling ball mill. In this equation, S has a maximum value, and the particle size of the maximum can be related to the ball size by xm = 0.05dB, that is, a 20:1 ball diameter/particle size ratio gives the optimum condition for the planetary milling. This optimum condition is the same as that in a stirred ball mill, but different from tumbling ball milling and vibration ball milling. Finally, the effect of the diameter and the density of balls on the breakage was expressed by the form a = k3(ρB/dB0.35).

Effect of fineness of wear particles from steel balls on the rate of hydrogen generation during wet grinding of hard materials in water

Abstract The rate of generation of hydrogen in wet grinding with steel balls in water, which may cause an explosion hazard but gives some information about the