ARR - Annual Report

This review highlights recent work on stable dispersions, specifically hard spheres and floculated systems, emphasizing the effects of structure created by many-body interactions. Predictions of weak flow behavior, e.g., low shear viscosity, linear viscoelastic response, and plastic/yield stresses, conform semi-quantitatively with experimental results and provide insight into the mechanisms responsible for shear thinning at higher rates.

This report covers the first year of the 1FPRI Suspension Rheology Project 1985-88 at the K,U.Leuven. It deals with the prediction and manipulation of the rheological properties of colloidal suspensions. Both colloidally stable and weakly flocculated systems are considered.

With respect to the stable systems, a procedure is described to predict viscosities over a wide range of conditions using a limited number of experiments. Suitable equations for shear rate and concentration dependency are given. The average error is shown to be less than 10% in most cases.

Data on concentrated systems are compared with recent theoretical predictions. The correlation is quite promising and might lead to a more scientific basis for predicting rheological properties. The present results suggest a method to derive the interparticle potential, which governs the physical behaviour of colloidal suspensions, from the modulus- concentration relation.

New samples are being prepared to study the transition from hard to soft particles. They will also be used to extend the previous results on monodisperse suspensions to systems with a particle size distribution. Making suitable materials is an even more important problem for the weakly flocculated systems. Various suspensions of silica and bentonite particles have been screened. It is being attempted to detect changes in structure with shear rate and time using rheological and dielectrical and/or neutron scattering methods. Preliminary results with these three techniques are shown.

Summary

The aim of our program research is to gain an accurate understanding of the role of surface-chemical and other possible forces which control separation behavior of fine particles. Specifically, we are studying the effect of selected chemical as well as mechanical pretreatments on flocculation and solid/liquid separation in such flocculated systems using certain gravitational, magnetic and filtration techniques.

During the last eight months (since 3/86), we have developed use of fractal techniques with computer models to characterize the shape and texture of particle aggregates formed by different mechanisms. We are working on the development of novel three dimensional imaging techniques by using CAT scan and gamma camera for characterization of floccs and sediments. A detailed investigation of double flocculants usage, in which different polymers and inorganics are added in series or parallel to enhance or control flocculation, is currently on the way. We have also initiated a study of the properties of colloids in non-aqueous systems.

Introduction

Fine particles, especially smaller than one micrometer in diameter, are expected to be valuable materials for new ceramics such as jet engines or motor engines. Even the dust particles exhausted from steel industries may be changed to a useful powder from which chemical catalysts are made through grinding and classification.

A piece of equipment has been devised in which the gap promoting attrition can be varied on a controlled manner with a view to carrying out studies on the relation between attrition in process equipment and in the annular attrition cell.

A modified attrition cell was fitted with fins to simulate flow of particles through a gap and preliminary trials at constant pressure showed that a small gap clearance gave more attrition than a larger one. Constant volume testing yielded a finer product. ProvIsionally, the particle breakage mechanism is unaltered in this type of equipment.

With the cell in its normal format using extruded particles the Gwyn kinetic formulation has been found to be true whatever attrition production size, testing stress and initial particle shape is selected. A shift in breakage mechanism from bodily fracture to surface abrasion has been found at low stresses. In general, mechanisms in operation are flexical bending (for long particles), fracture across a principal dimension, chipping (which is particularly noticeable for angular particles), and surface abrasion.

This report covers the contract year 1985/86 and describes the work performed to investigate the flow of fine materials from conical hoppers. Six materials - Kale seed and five different grades of sand ranging in size from 2,28mm to 150pm - are used in the experiments, Mass flow rates and interstitial pressure profiles adjacent to the orifice are measured simultaneously. Results have been obtained for four orifice diameters on a cone of half angle 9.5’ and for six orifice diameters on a cone of half angle 15”.

Previous work in the field has been extensively reviewed. Experiments on the kale seed and the coarsest sand (d = 2.28mm) provide further evidence for the applicability of the Beverloo (196l)correlation in describing the flow of coarse granular materials. For conical hoppers, the form of the correction factor proposed by Hose and Tanaka has been verified but the value of the exponent is still in doubt. They proposed a value of -0.35 which compares with the value of n m -0.2 indicated by the results obtained in this work.

Experiments on the different size ranges indicate that for particles in the range 50 to 600pm the measured flow rates are less than those predicted by the Beverloo correlation. It is shown that this retarded flow is caused by self-generated interstitial pressure gradients which arise as the material dilates on approaching the orifice. The Crewdson equation (1977), originally developed for air-augmented flows is found to be adequate in describing these retarded flows. Below 50um cohesive arching becomes an important factor and it may even prevent flow.

Crewdson et al. investigated a theory of stress induced dilation but their efforts proved inconclusive because of the difficulty in obtaining an accurate voidage-stress relationship at the low stress levels prevailing near the orifice. This line of thought is revived as the commercially available consolidometer can be used to obtain a voidage- stress relationship for stresses less than 10kN/ms. However, experimentally determined voidage changes exceed those predicted from the theory by a factor of almost 100. This suggests that it is necessary to look elsewhere for a suitable mechanism to explain the cause of the dilation.

Possible avenues of further work, both experimental and theoretical, are presented. An important element in the future is to be the direct determination of the voidage profile, which will be attempted using the r-ray tomography method developed by Seville et al. (1986) at the University of Surrey.

It is hoped to be able to develop a correlation for the flow of fine powders in the near term using the fractional retardation W/Ws. Meanwhile in the absence of any suitable alternatives, Carleton’s (1972) correlation can be used to predict the flow of fine powders from orifices greater than 20mm. The results presented in this work suggest that an overprediction in the range 20% to 50% can be expected.

Work performed on agglomeration of particle systems in fluidized beds at both low temperatures (granulation) and high temperatures (sintaring) is described in Part I and Part II of this report, respectively.

Part I

Part I includes a detailed review of the available literature on low temperature granulation as well as a description of original work performed on the behaviour of liquid bridges between two relatively moving particles. Experimental results are given for bridge strength measurements using two small spheres in a vibrational motion. These results are then compared to a modified form of a theoretical model developed from the well-known lubrication approximation. From both, the reviewed literature as well as from the experimental and theoretical work performed on moving liquid bridges, it is clearly concluded that the fluid (binder) viscosity and its rate of change with time are two of the most important binder characteristics which ultimately determine agglomerate growth in a granulator. It is also concluded that the instrument at CCNY used to measure bridge strength as a function of particle velocity (frequency, amplitude) and binder viscosity may be an efficient tool to characterize industrial binders. Based on these findings, future work is proposed to actually correlate bridge strength measurements with agglomerate growth rates in an experimental granulator.

Part II

Part II of the report includes the description of a large number of dilatometer and defluidization experiments performed at high temperatures with a great variety of amorphous and crystalline materials. Sometimes these experiments were complemented by differential scanning calorimeter (DSC) experiments to determine characteristic temperatures of recrystallization and/or phase change. The defluidization experiments were performed on the newly constructed fluidized bed agglomerator capable of operating at temperatures up to about 1150°C. Among the materials characterized during these experiments were different polymers and glass powders, sodium chloride, sodium bromide, sodium citrate and ferrous chloride crystals and a large number of more complex materials such as a titanium dioxide ore, FCC catalyst and fly ash samples. It was clearly established that the minimum sintering temperature (and other phase transition temperatures) can be determined using dilatometry. It was also shown that fluid bed defluidization (high temperature agglomeration) always occurs at temperatures somewhat higher than the minimum sintering measured in the dilatometer. Furthermore, the behaviour of the powder during high temperature fluidization can be reliably determined from the dilatometer experiments. Future work on this project will include upgrading both the dilatometer and the fluid bed agglomerator to withstand temperatures as high as 1500°C. The new dilatometer will also enable tests to be performed under a controlled atmosphere thereby allowing study of agglomeration due to chemical reactions.

The rheology of suspensions of submicron particles depends on the particle size, shape, and concentration and the nature and magnitude of the interparticle potentials. Indeed the rheological response differs qualitatively between rigid rods and hard spheres, between submicron and macroscopic particles, between dilute and concentrated suspensions, and between stable and flocculated colloidal suspensions. Hence a useful first step in understanding the subject is to classify systems according to the size and shape of the particles and the dominant interaction potential. Our work focusses on submicron, i.e. colloidal, particles which are spherical and strives to assess the effects of concentration and interparticle potential.

For colloidal systems the rheology is closely linked to the stability or phase behavior, which also reflects the nature and magnitude of the interparticle potentials. Dramatic rheological behavior is generally related to thermodynamic non-idealities, e.g. the solid-like rest state and plastic flow associated with flocculation due to van der Waals forces or phase separation induced by dissolved polymer, the elasticity of colloidal crystals generated by long range electrostatic repulsions, and the shear thinning and shear thickening phenomena associated with disorder-order transitions in stable dispersions near close packing. Thus an initial classification of colloidal systems might take the form.

Synopsis

Flocculation and sedimentation of fine particles are investigated here with emphasis on the role of conformation of adsorbed polymers on flocculation, theoretical modelling of aggregation and sedimentation processes, and structural characterization of sedimentation and floccs.

Polymer adsorption and, more importantly, the configuration of adsorbed polymer is considered critical in flocculation/dispersion processes. We have developed a fluorescence spectroscopic technique to investigate the conformational aspects of polymer adsorption. We have also successfully applied this technique to monitor coiled/stretched transformation of adsorbed polymers and to correlate the conformational information with the flocculation performance in the same system. Characterization of floccs is a major hurdle in flocculation studies and in this regard we have initiated the characterization of sedimentation processes and flocc structure by a CAT scan.

A Monte Carlo model has been developed and applied successfully to simulate sedimentation of fine particles by considering sedimentation as the result of competition between gravitation and the Brownian motion. In computer simulation of aggregation, the diffusion-limited aggregation model has been modified to generate sparse floccs as well as dense floccs by varying the step size of random walks.

This report covers the second year of the IFPRI Suspension Rheology Project 1985-88 at the K.U.Leuven. This project deals with the prediction, and manipulation of the rheological properties and the flow-induced structure in colloidal suspensions. It consists of two sub-projects. The first deals with colloidally stable systems, the second with weakly flocculated materials.

For the stable suspensions, the effect of particle size and that of stabilizer layer deformability is under investigation. Rheological measurements have been performed on suspensions, containing sterically stabilized PMMA particles of narrow size distribution and different particle sizes. This provides reference values for the projected measurements on systems with particle size distributions. In addition the data could be used to analyse the effect of stabilizer layer deformability on the rheology. It is concluded that this effect can be represented quite accurately by modifying two factors: the volume fraction at maximum packing and the factor expressing the shear stress dependence. The latter changes with concentration but it is shown that the shape of this curve hardly depends on softness. Quantitative results for the two parameters are given. Scaling relations are still under consideration.

Yielding was investigated, using silica suspensions. Their floe structure changes reversibly with shear. Yield stresses were measured with three different techniques. It is shown that, for some samples, identical results are obtained with the different techniques. However, in other cases deviations occur because of shear history effects during the measurement. Rational prediction of this phenomenon is not possible yet. For the silica under investigation yield stress and storage modulus change with concentration according to a power law. The data agree qualitatively with recent theoretical analyses.