Richard Hogg

Fine grinding of aggregated powders

In many cases, including natural ores as well as synthetic powders, fine grinding involves the breakage of bound aggregates rather than solid particles. The characteristics of breakage in such systems have been investigated by experimental studies of grinding kinetics, in a model system of partially sintered alumina particles, ground in a laboratory centrifugal ball mill. The effects of aggregate strength (extent of sintering) and energy input (mill speed) on the breakage rates and breakage distributions have been evaluated. Breakage appears to occur primarily through splitting of the aggregated mass into two or three smaller aggregates accompanied by release of the primary particles, leading to strongly bimodal breakage distributions.

MEASUREMENT OF AGGLOMERATE DENSITY IN FLOCCULATED FINE PARTICLE SUSPENSIONS

ABSTRACT An experimental procedure has been developed for estimating agglomerate densities in flocculated suspensions. The basic technique involves simultaneous measurement of floe size and free-settling velocity in the suspending fluid. Floc density is calculated using standard expressions for free settling under the appropriate flow conditions. Experiments have been carried out using a variety of solids flocculated with commercial, polymeric flocculants. The results indicate that floc density generally decreases with increasing floc size and is limited at the smallest sizes by the density of the solid particles and at the coarsest sizes by the solids concentration in the original suspension. The experimental data have been evaluated statistically in order to establish confidence limits on the observed density-size relationships. The results are compared with previously reported experimental measurements and floc simulation studies and a simple model is proposed in which it is postulated that large flocs...

Characterization of Airborne Particles and Droplets

Water sprays have been commonly used to suppress airborne dust. Water is doped with surface-active agents to enhance the dust capture efficiency through a reduction of surface tension. Nevertheless, dust collection efficiencies have been less than satisfactory historically.

Coal Flotation Kinetics - Effect of Particle Size and Specific Gravity

To determine the flotation response for individual size-specific gravity fractions, the products of flotation were analyzed in detail for a high volatile A coal. Flotation rates for each size-specific gravity fraction were obtained using a classical first-order rate equation to fit the data. The results were evaluated to determine the effect of size and specific gravity of the coal particles on the flotation response.

The role of particle shape in size analysis and the evaluation of comminution processes

The shape of the particles produced by comminution is affected by the kind of breakage action employed. Furthermore, the procedures used to determine product particle size are themselves influenced by shape. Illustrative examples of the effects of shape on particle size analysis and of shape variations in comminuted material are presented. Light scattering size measurements on narrow sieve fractions were found to indicate size distributions that were significantly broader than the sieve size range, especially for irregularly shaped particles. The distributions were also shifted to coarser sizes for the irregular particles but not for spheres. The apparent size shift is attributed to differences in the definition of size, while it is proposed that the apparent broadening of the distribution is a result of orientation effects. It is shown that broadening is relatively insignificant for broad, continuous distributions due to compensation effects. Applications to the determination of composite distributions by combining sieving and light scattering data are described. Distributions of particle shape descriptors, obtained by image analysis of comminution products, are shown to depend on the comminution method used. A slight variation with size was observed for coal particles but not for quartz. The implications of the size/shape interaction on the evaluation of comminution process performance are discussed.

Evaluation of Procedures for Production of Dust Samples for Biomedical Research

Abstract There are several logical objectives for dust samples used in biomedical research. Obviously, the dusts used should closely resemble natural mine dusts with respect to particle size distribution, particle shape, and composition. These characteristics must be consistent from sample to sample. Usually the relative freshness of the dust is also of primary concern. A generalized procedure for producing dust by single- or multiple-stage grinding and classification has been developed. Three laboratory-scale grinding devices—a Bleuler rotary mill (ring and puck grinder), a Retsch high speed planetary mill, and a Sweco vibratory mill—have been tested. The results have been evaluated in terms of the product size distribution, particle shape, and contamination introduced in the grinding process. All three devices have been found to be capable of producing appropriate size distributions at acceptable contamination levels. Multistage grinding with intermediate classification steps generally provides angular ...

Control of Particle Characteristics in the Production of Fine Powder by Grinding

ABSTRACT The development of improved methods of grinding and separation of fine particles is becoming increasingly important in a wide variety of industries. The requirements on the powder production differ widely, and depend on both the material and the specific application. This paper describes the effects of a variety of parameters on the production of fine powder by dry grinding; these parameters include particle agglomeration, the classification method employed, and air moisture in the mill. It is also demonstrated that particle shape is affected by the type of machine, by the specific breakage mechanism in a grinding device, and by the time spent in the grinding environment.

The Electrostatic Charge on Particles and Its Relation to Agglomeration in Air

Abstract Electrostatic charges on particles of quartz and coal of different ranks were measured and the results were correlated with the agglomeration behavior of these materials. It was observed that the particles in ambient air carried both positive and negative charges. The extent of charging was found to be a function of material type, coal rank, degree of aging in air, and humidity. The total charge on particles increased with an increase in the humidity for quartz and coal samples, whereas it decreased with an increase in the aging time. The numbers of positively and negatively charged particles were approximately equal for all the conditions studied. For quartz particles, the numbers were a function of humidity, however. The extent of agglomeration of particles in air was also found to be a function of material type and humidity. The charging mechanisms and the role of electrostatic charge on the extent of agglomeration were discussed.

Problems in Sink-Float Analysis of Fine Coal

A procedure for determining size and specific gravity distributions of the total quantity, and the sulfur and ash content of fine coal is described. The procedure involves cumulative fractionation by specific gravity followed by size separation by sieving. An averaging procedure is used to minimize experimental error and a regression method is employed in order to provide best estimates of the individual size-specific gravity fractions. An example of the application of the procedure to analyze froth flotation products is presented.

A PROCEDURE FOR CORRECTING MEASURED PARTICLE SIZE DISTRIBUTIONS FOR UNDETECTED FINE PARTICLES

ABSTRACT A simple procedure is described which can be used to correct size distributions obtained fron measuring devices which do not detect particles smaller than some limiting size. The method involves fitting the measured distribution to the Gaudin-Schuhmann Law but involves no assumptions regarding the size distribution outside of the measurement range. Application of the procedure is illustrated using simulated size data.