The use of chemical additives for improving the efficiency of both wet and dry comminution is reviewed. Although such additives can affect breakage strength under general comminution conditions crack velocities are probably too great to involve adsorption effects. Additives in dry grinding systems appear to function by coating the particles and preventing reagglomeration. In wet grinding systems, their main influence on grinding is through their effect on slurry rheology. Such additives do not affect the specific energy for grinding but only regulate mill power draft and kinetics.
The measurement of particle size distributions and particle concentrations is an important task in laboratories and in technical processes that deal with suspensions. Especially the instruments designed for on-line applications in production processes mostly evaluate the interactions of light with the suspended particles, viz. extinction, scattering and diffraction. While the interaction of particles with ultrasonics is essentially the same as with light, the application of ultrasonics offers special technical advantages, which are due to the extremely broad range of wavelengths and the coherent detection of ultrasonic waves. Ultrasonic spectrometry makes possible the on-line measurement of the particle size distribution and particle concentration in suspensions at concentrations of up to 30% by volume without diluting the suspension. This paper is intended to introduce the reader to the fundamentals of ultrasonic particle sizing and to explain present efforts in this field. In the last section, some successful realizations of ultrasonic particle sizing instruments are presented.
This paper presents an overview of recent research performed at Rutgers University and the University of Connecticut on the synthesis and processing of nanostructured materials. Highlights of this collaborative research program include: (1) synthesis of carbide strengthened steel and hard cermet powders from aqueous solution precursors, (2) synthesis of ceramic powders and ceramic matrix composites from metalorganic precursors, (3) densification of powder compacts by liquid phase sintering, (4) formation of high quality coatings by thermal spraying, and (5) demonstration of superior hardness and wear resistance in bulk cermet materials and coatings. Nanostructured bulk materials with designed multifunctional coatings present unprecedented opportunities for advances in materials properties and performance for a broad range of structural applications.
The influence of chemical dispersants on the milling of dolomite was examined using a stirred ball mill. The effective degree of grinding was found to depend on the aggregation/dispersion state or the viscosity of the slurry, which can be controlled by the amount of dispersants and by the solids content. It has been indicated that a more viscous slurry should be preferred in the grinding operation to improve the grinding rate and specific energy input. The beneficial effect of the periodic addition of the polymeric dispersants on the grinding efficiency was quite pronounced at a 70% by wt. solids slurry. In addition, the size distribution of the product becomes steeper in the cases of periodic polymer addition. Viscosity control in the ultrafine grinding of dolomite is important from the viewpoints of grinding efficiency and specific energy input. The results were obtained from carefully conducted experiments.
Today, a world-wide interest exists in producing nanosized ceramic powders. One of the suitable techniques is based on the evaporation of solid primary materials by lasers. Although this technique has been known for nearly two decades, the literature has hitherto only rarely reported effective powder yields (a few grams per hour). We produced nanocrystalline zirconia and alumina powders by evaporation of oxides in the focus of a CO2 laser (transversal flow of the CO2 gas; PL = 0.75 ... 4 kW) and by recondensation of the oxides in a carrier gas stream (air, oxygen, argon). Most experiments were carried out in the continuous wave (cw) mode, but earlier ones were also done by means of a pulsed laser (pw). The powder yield depends strongly on the laser power, on the focusing and on the velocity of the moving oxide target. Maximum yields of more than 100 g h-1 were attained. The zirconia and alumina powders so prepared consisted of nearly spherical particles with diameters in the range of 10 to 100 nm. The particle distribution can be controlled by the parameters of the formation process. Features of the crystal phases and of the chemical composition of the produced powders are reported.
A variety of tests are currently used to determine surface area in different branches of engineering and industry. This paper analyzes the relationships between these tests based on the results obtained by each test for a group of nine red tropical soils collected in the north east of Brazil. The ethylene glycol monoethyl ether (EGME) method that showed a good correlation with direct measurement of particle size by electron microscopy, is taken as a base for the graphical representation of results. Conversion factors between the tests and the EGME test and the standard deviation of the conversion factors are calculated. The results show that test procedures can be divided into three groups in terms of increasing spread in the test results - polar molecules and direct measurement tests with a spread of less than 15%, methylene blue and BET tests with a spread of about 30% and Blaine, Fisher and granulometric tests with a spread of about 50% when plotted against EGME data. It is noted that the test methods that gave a larger spread are used in industry and process control, because they may be performed quickly at a modest cost. The more accurate methods are slower and in some cases more expensive.
The results from experiments with cohesive limestone powder in a Biaxial Tester are presented. The biaxial tester allows a plane homogeneous deformation and measurement of the complete stress-strain states. It was found that proportional strain paths lead to associated proportional stress paths. Strain paths with a sharp bend lead to asymptotic stress paths. Under pure shear deformation, the limestone exhibits steady-state flow in the critical state. The critical state stress was found to be a function of the volumetric strain, i.e. the porosity, but not of the consolidation procedure. The Cam-Clay model of Roscoe has been extended to include the cohesion exhibited by many fine powders. The cohesive strength is represented by anisotropic tensile stress measure, used to shift the yield curves into the tensile regime. Significant cohesive strength can only be achieved in fairly dense packings.
In this paper we describe experiments in which the equivalent of 100 tonnes of abrasive sand have been passed through a pilot-scale wedge-shaped hopper of half angle 10° and outlet width 1 cm. The walls were then cut up and the change in wall thickness accurately measured. The rate of wear was found to be greatest at the outlet. In interpreting these results, we used the simple abrasive model for wear recommended by Johanson and Royal1). In this model, the rate of wear is simply proportional to both the force on a particle pressing on the wall and the speed with which it scrapes down that wall. In order to calculate these parameters, we used a measurement of the velocity in a granular material at the wall of the hopper and a prediction for the stress which is a modification of the well-known method of Janssen. The constant of proportionality (the wear coefficient) for the model was measured in a pin-on-disc experiment. A prediction for the wear profile was thus obtained which shows reasonable (within a factor of two) agreement with experiment. The apparent success of the pin-on-disc method of measuring the wear coefficient suggests that it can be used instead of the more expensive methods advocated by others.
This paper presents four different studies from the French national research project that deal with the fine grinding of hydrargillite. These include pilot plant tests of dry grinding in an air jet mill and wet grinding in a stirred bead mill. Two other more fundamental studies are concerned with determining fragmentation schemes in fine grinding. The first investigation concerns theoretical predictions of breakage of complex agglomerate crystals such as hydrargillite, plus image-analysis-based methods of identifying different types of particle morphology. Finally, single-particle impact fragmentation is studied in a specially designed apparatus which allows observation of particle impacts on a target at up to 300 m/s. This has led to the identification of different regimes of breakage that affect complex crystals such as hydrargillite.
Bulk materials handling operations involving falling streams of the bulk material are common throughout industry. Proper design of any fugitive dust control system servicing such operations requires knowledge of the behaviour of the free-falling stream in terms of the air entrained by the falling material and the concentration of dust liberated. In this paper results of experiments on alumina powder free-falling from a hopper are presented. It was found that the material stream initially contracts in cross-section as it accelerates and a boundary layer of dusty air develops around the core stream of bulk material. Results of air entrainment per unit mass of parent material are also given. The authors describe a simple analytical model of the complex air entrainment process by treating the falling stream of material as a negatively buoyant plume of dusty air fully miscible with the ambient air. The experimental data are successfully correlated using this approach. The plume model provides a more realistic description for the situation considered here than previous theoretical treatments based on air movement induced by isolated particles independently falling through quiescent air.
Continuum theory-based constitutive models suitable for simulating the load response of dry cohesive powders have been critically reviewed. Based on a set of criteria, three candidate models for further investigation have been identified: Cam clay, Adachi and Oka model, and Krizek et al.'s endochronic theory based model. Since the modified Cam clay model is the foundation of many advanced constitutive models, it was chosen and applied to compaction load response of a dry cohesive powder (wheat flour). The constitutive equation parameters were determined using four types of conventional triaxial tests: drained, undrained, mean effective stress and isotropic tests. Based on these tests, the three Cam clay parameters for wheat flour were: 2.1 (slope of critical state line), 0.130 (loading index) and 0.022 (unloading index). Low pressure uniaxial compaction tests were performed with the powder in a thin-walled aluminum column. A finite element model (FEM), with the modified Cam clay constitutive model for dry cohesive powder's response, was used to predict the compression behavior. The FEM calculated values compared favorably with measured wall strains. The FEM predicted stress distribution in the powder mass identified troublesome regions with large shear stresses and tensile stresses.
Planetary mills are gaining importance in the area of ultrafine grinding. The charge motion inside the mill is complex and previous works have considered force balance on a single particle for motion analysis. The inherent difficulties of single particle force balance are avoided in the multiparticle numerical scheme called the discrete element method (DEM) presented here. Many interesting motion patterns develop as the operating parameters such as percent critical speed, ball load, etc., are varied. The application of the DEM code for the qualitative analysis of the charge motion is demonstrated.
Contamination of planetary milled alumina powder has been studied and correlated to sinterability. Ground powders contain Si and N contaminations derived from the grinding media and mill pot made of Si3N4. Strong shear stress generated during planetary milling induces a mechanochemical reaction of worn-out Si3N4 with H2O contained in methanol used as a liquid medium forming hydrated SiO2 and provides an amorphous layer consisting of Si3N4, hydrated SiO2 and A12O3 on the surface of alumina particles. This suggests a possibility of applying planetary milling to surface modification of ceramic powders despite the problem of contamination. The surface layer containing the contaminations results in the formation of a mullite phase at the grain boundary during sintering. The low diffusion coefficient of mullite degrades sinterability even at a contamination as low as 400 ppm-Si. When the contamination increases to 0.5 mass%-Si, irregular grain growth and the formation of closed pores take place, suggesting the degradation of mechanical properties.
†This report was originally printed in J. Soc. Powder Technology, Japan. 30(8), 548-555 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Small and spherical granules were made from CaCO3-powder by the simultaneous process of granulation, grinding and separation in a continuous rotating conical vessel. The shape characteristics of the granules were evaluated by a pair of shape indices: φ is a measure of sphericity, and ζ is that of surface roughness. φ, ζ and granule size χ were plotted on three-dimensional graphs for various operating conditions of the granulator. The result was that the increase in rotation speed of the vessel n was found to be more effective in obtaining smaller and spherical granules with smooth surface. The increase in ball charge B and feed ratio W produced small and non-spherical granules. Longer residence time θ produced more spherical granules with smooth surface with χ remaining unchanged.
†This report was originally printed in J. Soc. Powder Technology, Japan. 30(8), 563-569 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Composite microspheres of polystyrene or silica coated with a double layer of Fe3O4/SiO2 were prepared via mechanical routes using a shear – compressing type machine, specially designed for the purpose of surface coating. The binding energy of Fe2p3/2 measured by XPS increased, while that of Si2p decreased after composite formation. These synchronized shifts increased with increasing intensity of mechanical stress. After heat treatments, chemical interaction during mechanical treatment at the Fe3O4/SiO2 interlayer in a composite particle was verified. The fine structure of the interlayer boundary was also examined in detail by electron microscopy.
†This report was originally printed in J. Soc. Powder Technology, Japan. 31(1), 18-24 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
†This report was originally printed in J. Soc. Powder Technology, Japan. 31(3), 202-206 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The effect of operating conditions on the removal efficiency of deposited particles using high- speed air jet was studied with particular attention to the humidity of surrounding air during particle deposition and air-jet removal. The experimental results showed that the removal efficiency of particles deposited in a low humidity atmosphere, decreased with increasing drying time after the deposition. In the case of a deposition under high humidity, the removal efficiency increased at the beginning of drying and decreased rapidly after reaching the maximum efficiency. For drying over 80 hrs, however, the humidity of the surrounding air during the particle deposition didn't affect the removal efficiency. The efficiency also depended on the humidity during the air-jet removal, and reached the maximum value at 67% relative humidity. Such a variation of the removal efficiency has been well explained by the change of Hamaker's constant with the adsorbed layer-thickness of water molecules and the variation of liquid surface tension with the thickness of liquid bridge.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 20(2), 205-212 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
A new theory to estimate the state variables of flowing particles in a bin-hopper system was presented. The flowing behaviour of particles in the system was simulated by the particle element method. It was suggested that convergence of the flow path of particles in a hopper leads to a change in the bulk density of flowing particles, whereby particles in a hopper flow intermittently. On the basis of the microscopic behaviour of flowing particles observed through the simulation, a flow model was derived to analyse the flow of particles in a conical hopper. The values and fluctuation of dynamic pressure acting on the wall during the flow of particles in a bin-hopper system were estimated and the generation mechanism of over pressure during flowing of particles was also presented on the basis of the model.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 20(3), 397-404 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Water bridges formed among particles in industrial process often contain soluble impurities such as salts. When such bridges are exposed to dry air, water in the bridges vaporizes and the salts crystallize to form solid bridges. Solid bridges between two glass spheres were formed from NaCl, KCl, KNO3 and Na2SO4 solution by controlling the humidity, and their tensile strength was measured. It was found from these experiments that 1) the adhesion force of a solid salt bridge which was uniformly formed in the gap between two spheres was one to two orders of magnitude larger than that of a water bridge without crystallization; 2) the average adhesion force of solid bridges was proportional to the 1/2 power of the product of salt volume and particle radius; and 3) the adhesion force of bridges of NaCl and KCl was influenced by the surrounding humidity, which it was below their deliquescent points, because they adsorbed water until complete recrystallization took place.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 20(4), 542-548 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
To investigate the process of creep failure phenomena in fine powder beds, the change in the tensile failure strength and the displacement of powder beds during the creep failure process was analyzed. The increasing displacement is divided into three sections (I, II, III). The increasing rate of displacement slows in the first stage (section I), remains almost constant in section II, and rapidly increases and falls in section III. By using the experimental equation to relate the creep failure life to the increasing rate of displacement in section II, the change in tensile strength of powder beds can be estimated during the creep failure process. No decrease in tensile strength is observed during the first stage (section I). However, in the second and third stages (section II and III), the strength decreased remarkably with increasing creep time. The increasing displacement can be fitted by a Voigt model in the region of section I, in which the tensile strength is almost constant. According to the elastic/viscous parameters of a Voigt model decrease in relation to the decrease in fracture strength of powder beds, the complex increase of displacement in the region of sections II and III can be quantitatively estimated. The deformation resistance of powder beds was quantitatively related to the fracture strength.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 20(4), 556-563 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.