ARR - Annual Report

Introduction

Attrition is a ubiquitous problem in processing and handling of particulate solids. It causes dust formation, and has a detrimental effect on product quality and the reliable operation of process equipment. The research coordination of IFPRI identified the need for a better understanding of the various mechanisms involved in attrition in order to provide a fundamental base on which to address the problem. The objective of our research programme is therefore to elucidate the mechanisms of attrition of particulate solids, and to relate the rate of attrition to the material properties and to the loading conditions to which the particles are subjected.

A predictive model of impact attrition of particulate solids having a semi-brittle failure mode was developed in the previous IFPRI research programme. The model has been applied to the analysis of attrition propensity for several species of ionic crystal. This report summarises the results obtained in the past year on the effect of particle size on the attrition rate. Furthermore, the approach developed for the analysis of impact attrition has been extended to modelling wear of single particles. A summary of the literature survey is also included in this report.

The mechanism of attrition considered here is a chipping process, where small quantities of material are removed from the surfaces around the comers and edges of the particle. Our previous work has shown this to be an important process in the impact attrition of ionic crystals with a semi-brittle failure mode in the velocity range up to about 40 m/s. Fragmentation of the whole particle occurs at higher impact velocities or for materials with a relatively low value of toughness. However, this mechanism has not been addressed so far in our work.

In the chipping process, material removal is caused by the initiation and propagation of sub-surface lateral cracks. These cracks form readily during the unloading stage of an elastic-plastic deformation, and are driven by the residual tensile stresses produced by the plastic flow. Therefore, the analysis of impact attrition is based on the fracture mechanics of sub-surface lateral cracks. A dimensionless parameter has been derived from this analysis, which represents the volume fraction of material lost from a single particle by the formation of such cracks:

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where H is the hardness, p is the density, U is the impact velocity, 1 is the linear dimension-of particle, Kc is the critical stress intensity factor, and + is the constraint factor defined as the ratio of the hardness to the yield stress. The parameter IJ quantifies the attrition propensity, and it includes all the relevant material properties and impact conditions. The fractional loss per impact 4 is considered to be some function of q. In the first instance the existence of a simple linear relationship has been explored:

where a is the proportionality constant.

To verify the theoretical predictions, ionic crystals with a cubic habit such as MgO, NaCl and KC1 have been used as model materials because their structure and properties are well-characterised. The dependence of fractional loss per impact on material properties and impact velocity was verified previously for 2 mm melt-grown crystals (Ghadiri and Zhang, 1992). A linear relationship between the fractional loss per impact and the particle size is expected, but this could not be shown unambiguously in the previous work. To examine the effect of particle size on attrition, it is necessary to measure the fractional loss per impact for several particle sizes while keeping other parameters constant. The earlier work on the size effect used commercial solution-grown PDV salt crystals in the size range of 355-500 pm. This material contains a large number of polycrystalline particles which split into individual crystals on impact, hence obscuring the mechanism of chipping under verification. To combat this problem, a special experimental procedure involving repeated impacts was developed in order to quantify an asymptotic value of fractional loss per impact. The asymptotic was considered to represent chipping, as at that stage all the polycrystalline particles had split into smaller individual crystals and been removed from the sample by sieving. This procedure has been reported in the Final Report (FRR 16-03) of the previous IFPRI project (Ghadiri and Zhang, 1992). The repeated impact technique was however considered unsatisfactory because the number of impacts could also influence the fractional loss due to work-hardening of the comers and edges of the crystals, or purely as a result of the gradual change in particle size as the test proceeds.

The choice of solution-grown crystals rather than large melt-grown crystals for the tests was at that time associated with the fact that the bore of the air-eductor used in our experimental device was too narrow to allow particles larger than 3 mm to pass through. Recently, a new attrition rig with a larger bore was constructed to overcome this shortcoming. The results of the experimental work investigating the effect of size on attrition rate for relatively perfect, large melt-grown crystals is described in this report.

In the past year some work has been carried out to verify the existing models of lateral crack formation in ionic crystals as well as other materials of interest. A new microscopic technique, the Confocal Laser Scanning Microscope has been used for this purpose, and the results are currently being analysed. This new technique is now available at the University of Surrey, and it will allow us to carry out further characterisation of sub-surface damage arising from impact or quasi-static contact.

Executive Summary

This is the second annual report of the IFPRI Suspension Rheology project 1991-1994 at the K.U.Leuven (Belgium). Whereas the two previous projects at K.U.Leuven dealt with the flow properties of stable colloidal dispersions, the present one concentrates on flocculated systems. In particular it is being attempted to generate some basic information on how to manipulate the complex rheology of reversibly flocculated dispersions. This latter term refers here to colloidal systems in which floes develop at rest, which however can be broken down reversibly during flow. Such systems occur frequently in materials processing operations.

During the first year a locally built dielectric device was used to probe the flow-induced changes in microstructure during flow and after stopping the flow. This led to the identification of a peculiar relaxational phenomenon. In addition a start was made with upgrading the instrument. This work has been continued and finished during the second year.

The application of the new set-up is demonstrated here for structure probing during flow and for following structural recovery after stopping the flow. Systematic measurements to study the relationship between structure and flow history are scheduled for the next year. In parallel with the dielectric approach, rheological experiments on reversibly flocculated dispersions are being performed. Work has been continued to develop suitable model systems, starting from sterically stabilized dispersions. By using suitable mixtures of suspending fluids and by adjusting the temperature as well, this has proved possible. Some preliminary rheological data are available and are being used to compare with the results of theoretical approaches based on square well potentials.

Thixotropy, time-dependent and reversible decrease of viscosity caused by flow, accompanies reversible flocculation. Non-aqueous dispersions of fumed silica have been formulated to study this phenomenon. Comparative transient measurements under constant stress and under constant shear rate are scheduled with these samples to elucidate the basic flow factors that govern the flow-induced structural changes.

Abstract

Further data are reported on the formation of filter cakes, and data are analysed through the two consecutive mechanisms, filtration and consolidation. Process design parameters for each mechanism are obtained. The magnitude and dependence of the constitutive parameters on solid/liquid mixture properties and operating parameters are shown. It was found that increasing the proportion of finer particles in the feed increased both the cake specific resistance and the equilibrium voids ratio; flocculation at the correct dosage causes higher filtration rates and lower equilibrium voids ratios; an optimum liquid conductivity (and hence ionic strength) exists to maximise the filtration rate; increasing the filtration pressure causes an increase in specific cake resistances and a reduction in equilibrium voids ratios; and a minimum cake resistance (and maximum filtration rate) occurs at the isoelectric point of the suspension. Forms of equations have been developed to calculate equilibrium voids ratios and specific cake resistances of binary mixtures of particles, such as occurs when filter aid is used as a body feed.

The third year project of the spherical reference materials is to manufacture opaque particles having unimodal distribution in the size and also to produce transparent particles in the size range of 1 to 1Opm (MBPl-10) according to the range of 10 to IOOpm (GCPlO-100), BCR’s request.

1) Opaque Particles

The opaque particles are glassy carbon beads denoted by GCPlO-100, and more than 95% in weight of these particles are in the size range of 10 to 100 pm. The product materials, GCPlO-100, of 20kg were sent to the AEA Technology, England for the certification in June, 1993.

2) Transparent Particles

The transparent particles are barium titanate glass beads denoted by MBPI-10, and more than 95% in weight of these particles are in the size range of 1 to 1 Opm. The product materials, MBPl-10, of 1 Okg were also sent to the AEA Technology, England in April, 1993.

The physical characteristics of the above product materials, especially the size distributions, were measured in several Japanese company-laboratories, and are given in this report.

We also studied the size segregation of larger particles, 150 to 650pm, by feeding them into a container, resulting in a significant segregation. Therefore, complete mixing and/or careful splitting is necessary before utilizing them as the reference materials.

Next Year Project

The next year project will be to manufacture following two kinds of spherical materials.

• LBPl50-650: Transparent soda-lime-silicate glass beads having the size range of 150 to 650p.m. The quantity is 20kg.
• GCPl50-650: Opaque glassy carbon beads having the size range of 150 to 650pm. The quantity is 20kg.

Summary

During the past year the main object of our work has been to refine our model for fully developed turbulent flow in vertical tubes, improve the numerical algorithm for its solution, and explore its predictions over wide ranges of gas and particle fluxes.

For engineering purposes it is important

• (a) to be able to predict the relation between the gas pressure gradient and the fluxes of gas and particles, over wide ranges of these fluxes, and for tubes of widely varying diameters, and
• (b) to be able to predict the cross-sectional profiles of particle concentration, and the velocities of both particles and gas, and hence deduce quantities important for chemical reactions, such as residence time distributions.

In this Annual Report we demonstrate that the model meets both these requirements and test the agreement between its predictions and some limited experimental data. In order to establish confidence in its predictive capability it is now important to generate experimental data over as wide as possible a range of operating conditions. We hope that a suitable body of results will be generated by a related experimental exploration of riser flow, to be initiated by IFPRI.

Executive Summary

This report summarizes research supported by I.F.P.R.I. at Stanford University on the use of high speed optical polarimetry measurements to characterize the dynamics and structure of fine particle suspensions. This reporting period is the second year of the funding period. During the first year, the research effort considered application of optical rheometry to several fundamental problems in particulate dynamics. These included:

• the characterization of single particle properties, such as size and shape distributions, and dipole moments,
• the structure of dense suspensions subject to electric fields,
• the structure/property relationships of shear thickening suspensions.

These projects have each yielded publications and served to introduce the I.F.P.R.I. community to the measurement techniques available in our laboratory.

Our efforts this past year have been directed towards developing these methods in a form that will facilitate their use within industrial applications. This objective has involved two activities:

• the development of oblique transmission as a means of analyzing optically dense materials,
• collaboration with scientists in several member companies to investigate specific applications.

The principal impediment of the use of optical methods is the opacity of most industrial suspensions. Although some systems will never be accessible to optical methods, many materials can be studied if their optical density is sufficiently lowered. This normally requires thin specimens, which makes it difficult to fully characterize their dynamics and structure in different planes of the applied flow field. The development of the method of oblique transmission is aimed at circumventing this problem. Ultimately, this method may make it possible to apply this technique “on-line” or “in-line” to provide a non-intrusive prove of structure in industrial processes.

During the past year, the principal investigator has collaborated with three member companies to identify areas of application of the techniques, and particularly for potential on-line applications. The companies included:

• Hosokawa Micron Corp. (contact: Mr. Toyokazu Yokoyama),
• 3M Company (contact: Dr. Caroline Ylitalo),
• Procter and Gamble Company (contact: Dr. David Githuku).

In each case, a variety of samples were analyzed using the optical rheometric system developed over the past year. In two of these cases, the results were sufficiently promising to encourage the development of similar instrumentation in-house.

In this IFPRI funded project we investigate the role of short range forces in controlling the morphology of sub-micron precipitates. Our progress can be summarized as:

1. Silicotungstic acid (STA) has been chosen as a model colloidal particle to mimic the properties of primary particles (nuclei) in precipitation reactions involving metal oxides.
2. STA forms crystal hydrates where the number of waters of hydration per STA molecule varies with the relative humidity of the vapor with which it is in equilibrium.
3. The waters of hydration of STA crystals are lost in discrete steps in solid/solid phase transitions involving changes in the crystal lattice.
4. The locations of the phase transitions can be related to the osmotic pressure which would have to be applied to hold the crystal at a particular solid’s volume fraction if it were equilibrated with pure water. These osmotic pressures are large (the highest hydrate is not formed until an osmotic pressure of approximately 300 atmospheres is applied at 25°C).
5. These studies demonstrate that large driving forces must be applied to squeeze solvent from between small metal oxide particles with a strong affinity for the continuous phase. Resulting from the small particle size and the particle/continuous phase affinity is a strong, short range repulsion which will completely dominate over electrostatic and Van der Waals forces.
6. Future studies will involve developing measures of the spatial dependence of the solvation interaction energy and the effects of varying continuous phase/particle affinity.
7. Our findings suggest that for precipitation reactions producing particles with a strong affinity for the continuous phase, classical nucleation events may never occur. The implications of this concept require better understanding of the role or solvent/particle interactions in controlling particle interaction potentials and molecular attachment rates to growing particles.

Summary

A quantitative theory developed herein for the rheology of concentrated colloidal dispersions accounts for the nature and effect of potential, as well as hydrodynamic, interactions. A configuration-space consemation equation for the pair density P2 provides a fundamental basis for calculating the nonequilibrium microstructure under shear, including three-body couplings due to the pairwise additive interparticle potential. The resulting many-body forces depend on the three-particle distribution function, necessitating an additional equation to completely specify P2. A nonequilibrium closure based on the hypernetted chain (HNC) equilibrium closure relates these forces to the interparticle force and pair distribution function completing the formulation. A computational algorithm exploiting Fast Fourier Transforms solves the resulting integro-differential equations for weak flows, yielding the perturbed pair density as a function of the volume fraction o and the inter-particle potential. Calculation of the stresses is then straightforward.

First, we present the low-shear limiting viscosity as a function of o for hard, soft, and charged spheres without hydrodynamic interactions and demonstrate satisfactory agreement with the limited results available from computer simulations and experiment. Second, we incorporate rescaled hydrodynamic mobilities and the viscous stress, based on extant results for the short-time self-diffusion coefficient and the high frequency limiting viscosity, tb account for hydrodynamic interactions. The corresponding predictions of the low shear viscosity for hard spheres lie within 20% of extensive experimental data for 0 < o < 0.60.

ABSTRACT

The effect of solvent surface tension, pH of water and surface chemistry of particles on agglomerate strength was studied using silica and titania agglomerates washed with different solvents. Aprotic solvents of various surface tension and water at various pH (with and without electrolyte) were employed to separate the effects of capillary pressure and particle-particle condensation reactions. The wetting behavior of the solvent was studied using both the imbibition method and the modified wilhelmy plate technique. The two techniques were compared to determine their limitations when applied to spherical particles. Agglomerate strength was determined using a calibrated ultrasonic field, mechanical shear and isostatic compaction. Organic groups present on the particle surface were identified using infra-red adsorption and 13C NMR and the effect of the groups on agglomerate strength was discussed. The strength and strength distribution of the agglomerates formed was found to depend on the solvent surface tension, wetting characteristic, pH and surface chemistry of the particle. The presence of electrolyte in the solvent was found to effect the extent and strength of agglomeration, specially at higher pH. Agglomerate strength was also found to increase with increasing capillary stresses, and pH of water.

Executive Summary

The goal of developing a unified field theory for bubble control in a fluidized bed is reviewed. Included in this unified theory is a single inter-particle force model that will handle the limiting cases of high as well vanishing field frequencies (ic. the ac and dc limits). The electrostatic bubble model utilizes a perturbation theory to relate force relationships at the particle level to the continuum equations of mass and momentum at the bulk bed level. An extension of the Davidson bubble model gives information on the local stresses induced around a bubble in terms of the far distant electric field strength.

For the first time in this IFPRI work we introduce a relationship between bubble control and elutriation control through an understanding of the various interparticle forces. Two mechanisms relating to electrostatics effects in elutriation are postulated by which the entrainment of fine particles can be diminished with electric fields:

• that bubble formation is inhibited with the application of electric fields, and
• that the bonding strength of fine particles to bulk bed particles is increased with electric fields.

To elaborate on these two mechanisms, various permanent and electrostatic interparticle forces are evaluated. For the electrical forces, current constriction is shown to be the dominating cohesion force that influences both bubble control and elutriation control, with the force of induction charging found to be less important. For the permanent forces, van der Walls and triboelectric particle forces contribute over different size ranges of fine particles depending on the surface roughness of the particle. Gravitational forces affect mainly bubble action through the larger bulk bed particles.

In a separate experiment it is now confirmed that elutriation control with electric fields is a consequence of field action within the bed and not simply a precipitator effect in the bed freeboard. The test was carried out on a specially designed fluidized bed in which the high voltage electrode entered through the bottom of the bed rather than from above so that the possibility of precipitator action was largely reduced or eliminated.

Our present correlations now include bed expansion, superficial velocity, particle diameter, and electric field strength for glass spheres. Previously, particle diameter had not been included in these correlations. The new results should be useful in helping to predict scale-up of beds as well as for predicting the delay of minimum bubbling conditions with fields.

The future plans for this research call for scale-up studies utilizing a larger bed experiment for bubble control and for developing a new high temperature facility. Numerical modeling studies will also be introduced. Our present facility has provided data up to 500 degrees C.