Bruce Firth

Flow properties of coagulated colloidal suspensions: I. Energy dissipation in the flow units

Abstract The flow properties of electrically charged, but unstable, colloidal sols are investigated in the light of three possible models. Models which treat the ultimate flow unit at high shear rate as either a single particle or a hard (nondeformable) floc are shown to be incapable of providing a satisfactory description of the behavior. On the other hand an elastic floc model provides a consistent physical interpretation of both the linear region of the flow curve and the Bingham yield value for these plastic-pseudoplastic systems.

Flow properties of coagulated colloidal suspensions: III. The elastic floc model

Abstract The viscous flow behavior of a colloidal sol can be described in terms of the various contributions to the energy dissipation during flow. When this is done for a coagulated sol it is found that a satisfactory description of the known features of the basic shear diagram can be obtained by attributing most of the energy dissipation to two processes: (i) the viscous flow of the suspension medium around the flocs, which are the basic flow units, and (ii) the energy involved in stretching the flocs in order to break floc doublets apart so that the amount of structure in the system is preserved in spite of floc-floc collisions. This model can be analyzed in a simple manner and is able to generate all 10 of the known relationships between the flow parameters (Bingham yield value, critical shear rate, and plastic viscosity) and the colloidal properties (particle size, volume fraction, and interaction energy). It also predicts a linear dependence of critical shear rate on particle radius, but this has not yet been tested.

Flow properties of coagulated colloidal suspensions: II. Experimental properties of the flow curve parameters

Abstract The basic shear diagram (shear stress versus shear rate) for coagulated colloidal suspensions is plastic-pseudoplastic. This flow curve has three definable experimental parameters: the Bingham Yield Value, the shear rate at which the linear portion of the flow curve begins, and the gradient of the linear part of the flow curve. The relationships between the experimental flow parameters and the colloidal parameters (volume fraction, particle size, and interactive forces) are examined. Ten such relationships can be established and these can be used to test the validity of possible models of the flow of coagulated suspensions. For some systems there appears to be a critical radius at which a change occurs in the dependence of the flow parameters on the particle size.

Flotation circuits for poorly floating coals

Abstract A number of flotation circuits for the recovery of a poorly floating coal were investigated by laboratory batch testing. Analysis of the size and ash distributions of the products and tailings showed that the circuits which would allow an equitable distribution of collector between the coarse and fine size fractions were superior. These circuits were two-stage reagent addition, reflotation of classified tailings and split feed flotation. The reflotation of classified tailings circuit not only gave the best metallurgical performance, it also was the least affected by pulp density variations and imperfect size classification.

Prediction of dense medium cyclone performance from large size density tracer test

Abstract A modified Suspension—Partition Model has been developed to predict partition curves of the smaller particle size fractions from experimental data of density tracer test at large particle size in dense medium cyclones. In this model, the sedimentation flux caused by a centrifugal force is balanced by the turbulent diffusion flux. In addition, medium segregation and the dependence of particle diffusivity on particle size are also taken into account in the model. Three model parameters are estimated by fitting experimental data for a large particle size fraction to the model, and the model can be then used to calculate partition curves of the smaller particle size fractions. The capacity to predict the partition coefficients of finer particle size is demonstrated in a number of examples. The benefit of using this improved mathematical model is a cost reduction in the experimental determination of the performance of a given dense medium cyclone.

Flow behavior of sterically stabilized suspensions: II. Brownian motion effects in laminar shear

Abstract Sterically stabilized latex supensions can exhibit, under some circumstances, an Ostwald-type flow curve at low shear rates, even though the suspension is thermodynamically stable. It is shown that the behavior can be explained by assuming that at low shear rates the formation of doublets by both brownian motion and the shearing process must be considered. Some doublets can separate slowly by brwnian motion, but the energy dissipation for the flow process (and, hence, the flow curve) must be calculated by determining the number of doublets which are separated by the shear field. The separation energy in this latter case is much higher than for brownian separation, since the shear field tears the particles apart in a time which is short compared to the relaxation time of the adsorbed polymer chains which confer stability on the system. Separation energies of about 5 kT are observed for PMMA systems stabilized by adsorbed polyethylene oxide.

Quantification of the data improvement produced by optimised metallurgical plant mass balances

Abstract The data improvement, as achieved by optimising a mass balance of inconsistent experimental data, was quantified statistically using a Monte Carlo technique. It was found that an increase in the number of levels in the mass balance problem produced further increases in the data improvement. However, the improvement was lost when, for example, the solids split value was numerically in the vicinity of the flow split value. Furthermore, accuracy in the data at one level of the mass balance was found to be transferred to only one adjacent level of the mass balance. For example, accurate assay data produced improvement in the solids and assay split values, but not in the flow split values. Similarly, accurate pulp density data produced improvement in the flow and solids split values, but not in the assay split values. And, accurate flow data produced no significant improvement in the solids split or assay split values. In general, the transfer of improvement only occurred in the direction of lower level data (e.g., from the assay to the solids level, or form the solids to the flow level).

Hydrocyclones in dewatering circuits

Abstract When hydrocyclones are used in dewatering circuits it is important to know the potential impact of trying to balance the flow of liquid to the overflow and underflow streams and the size distribution of the solids in these respective streams. The diameters of the vortex finder and spigot play important roles in this balance but neglecting the effect of the feed size distribution could also severely impact the final outcome. This is particularly the case where the transport of coarse particles through the spigot becomes a significant factor. This paper presents a model which accounts for these effects and examines its capability with a data set of over 40 experimental trials with a 225 mm cyclone with a broad range of operating conditions (solids concentration, vortex finder and spigot diameters and flow rates). The model’s capability was then subsequently subjected to a further examination by altering the feed size distribution so that it contained a significantly greater amount of material in the size range near the separation size (0.090–0.200 mm). Good results were obtained in both cases.

Porosity and water retention in coarse coking coal

At two Queensland mines producing high-rank coking coals, seam borecores and corresponding samples of product from the coarse coal centrifuges of the preparation plants were examined by mercury porosimetry. The dull (inertinite) particles were found to be much more porous than the bright (vitrinite), and to show greater variability in properties. Optical micrographs suggest that some of the variation is due to clays deposited in the pores of some inertinite particles. The pore water content predicted by assuming that all the particle porosity is filled with water was in good agreement with the internal moisture value determined experimentally by a centrifuge technique. The total porosity was estimated from the sum of the volume occupied by equilibrium moisture (micropores) and volume measured by mercury porosimetry (meso- and macropores). The mercury porosimetry results must be corrected for the compressibility of the coal.

Adsorption of quaternary ammonium ions at the mercury/solution interface: Part IV. Hydrophilic/hydrophobic character of the interface

Summary The differential capacitance of the mercury/solution interface shows a frequency dependent peak on the cathodic side of the p.z.c. in the presence of long chain quaternary ammonium ions. This peak has previously been ascribed to a rearrangement of the adsorbed quaternary ions from a head-in to a head-out configuration, which should cause a change from hydrophobic to hydrophilic character as the applied e.m.f. moves through the position of the peak. Static (contact angle) and dynamic (film rupture) studies of the interaction of the mercury drop with an air bubble confirm the change from hydrophobic to hydrophilic character on passing through the peak.