ARR - Annual Report

INTRODUCTION

Agglomerate strength is an important property in a number of industries. In some processes agglomerates may limit the strength and uniformity of final or intermediate products, thus requiring nearly total breakdown of agglomerates. Conversely, agglomerates may be required to enhance flow ability and thus the agglomerated product must be capable of resisting breakage during the handling and processing stages.

In processing of ceramic powders, there are some type of agglomerates which are more detrimental than others. It is generally accepted that weak or “soft” agglomerates i.e. which disintegrate during green forming of compacts, do not impede densification. On the other hand strong or “hard” agglomerates are not broken down during compaction and can lead to incomplete densification and/or strength limiting flaws.

Hard agglomeration often results during certain stages of powder processing (such as filtration and drying) when the dispersed particles are subject to large hydrostatic or capillary forces which overcome the repulsive inter-particle forces. Under these conditions, a number of reaction pathways can lead to the formation of metal-oxygen-metal bonds between these individual primary particles. For small particle size, this can lead to a dissolution-reprecipitation process removing material from the particle surface and depositing it in the toroidal region at the particle-particle contact. Once these oxide bridges form, it is difficult to disperse (or break the agglomerate bonds) the resulting hard agglomerate by either chemical or mechanical means.

The inter-particle forces that can contribute to the cohesive strength of agglomerates can be separated into those which act independently of the metal-oxide bridges and those that are a result of such bridges. The Van der Waals force, which is always present, an electrostatic force and a magnetic force are examples of the former, while the force due to metal-oxygen-metal bond and a mechanical force that arises from the interlocking of irregularly shaped particles are examples of the latter.

The problem with the rational design of powder processing/treatment to avoid oxide bridging and subsequent hard agglomerate formation is the lack of knowledge concerning the kinetics and equilibrium of the various reactions associated with powder processing. These reactions depend upon a number of processing variables, including the Metal (M), the particle size, the degree of condensation/coordination of M, saturation level (defined as volume of liquid per unit void volume), temperature (i.e. drying conditions), contact angle, surface tension, pH, and the alkyl group, R.

In our previous IFPRI project it was demonstrated that the addition of a alcohol (EtOH) wash step during titania processing was able to reduce the formation of hard agglomerates. Although various alcohols have been used previously to control agglomeration, little work has been reported on the mechanism(s) involved. It has been suggested that the formation of surface ethoxy groups during alcohol washing has a direct influence on the strength of, resulting agglomerates.

The present study deals with the effect of various parameters including alcohol washing on the formation of oxide bridges during powder processing. Changes in both the chemical and physical structure of both titania and silica during processing will be assessed using low field NMR, high field MAS NMR, small angle x-ray scattering, TGA, FTIR, particle size analysis, and agglomerate strength distribution. These changes will be correlated with the dispersibility of the final dried powder and the processing conditions.

Summary

This report describes a twodimensional computer model that is suitable for studying flows of dry particles in which the particles are allowed to break. The model is based on discrete particle computer simulations. Here, macroscopic polygonal particles are constructed by ugluingn together small elements (hereafter referred to as “elementsn). Depending on the stress conditions the glued bonds can respond elastically, undergo plastic failure or break, allowing cracks to propagate across the macroscopic particle along the boundaries between their microscopic constituents.

In essence, this process creates a simulated material. The manner in which the microscopic elements are organized has an effect on the mechanical behavior in much the same way that the organization of molecules affects the behavior of a real solid. For example, the material must have internal, crystal-like slip planes in order for the resultant material to exhibit plastic behavior. Similarly, the elastic behavior of the bulk material depends on both on the the microscopic element shape and on the spring constants used to model the interactions of the microscopic elements. Consequently, many element shapes and arrangements of elements have been investigated.

Some examples are presented, including compression failure of a rectangular sample, the impact of particles with a plate or binary impacts of particles. Some preliminary simulations of the Utah ball-drop experiments have also been performed that show good qualitative agreement.

Executive Summary

1991-1992 began the first year of our three year study into the fundamental mechanisms responsible for effervescent atomization. Three issues were addressed during this period.

Early in the year we focused on spray behavior in the transition region where the two-phase flow that exits the nozzle as discrete gas bubbles in a continuous liquid is transformed into a continuous gas stream containing discrete liquid drops. Single-pulse holography was employed to observe the liquid breakup phenomena characteristic of effervescent atomization. A qualitative explanation of the mechanisms responsible for effervescent atomization is presented, based on this data. A quantitative model allowing calculation of spray mean drop size from nozzle geometry, operating conditions, and fluid rheology will be developed this year.

At mid-year we broadened our efforts to include an investigation of the interactions between the spray and its surroundings. In particular, we were interested in methods for improving the already superior energy efficiency of effervescent atomizers by minimizing the impact of the major energy loss mechanisms. A computational study was therefore performed to identify these major loss mechanisms and suggest methods for reducing their impact. We discovered that turbulent dissipation was the largest loss, followed by transformation of bulk kinetic energy into turbulent kinetic energy, and then entrainment of surrounding air. We concluded it was unlikely that turbulent dissipation and transformation of bulk kinetic energy into turbulent kinetic energy could be reduced. However, entrainment might be minimized by forming a more uniform distribution of smaller bubbles in the two-phase jet as it exits the nozzle. We will investigate spray-surroundings interactions more fully in 1992-1993.

Our most recent efforts have focused on the relationship between fluid rheological properties and spray mean drop size. This work was motivated by our previous study into the influence of polymer addition on nozzle performance. One result of that study was the conclusion that spray mean drop size was independent of changes in either consistency index or flow behavior index for a power law fluid. This year’s results show that it is fluid viscoelasticity that degrades nozzle performance. We will next develop a quantitative model that describes the performance of an effervescent atomizer when operating with viscoelastic fluids.

Executive Summary

• Previous annual reports concentrated on the presentation of experimental data relating to the behaviour of fluidized beds of catalyst (Group A) powders. These related to the influence of mean particle size, addition of fines, type of gas, and temperature, on bubbling, bed expansion/density, minimum fluidization and mimimum bubbling velocities/voidages, and bed collapse characteristics.
• In this report we focus attention on the theoretical background of non-bubbling fluidized/aerated beds of powder in order (a) to provide a sound foundation for the interpretation of our experimental data and (b) to allow generalisations to be made for other powders and operational conditions.
• Both hydrodynamic and interparticle forces play a role in the behaviour of fine particle fluidization, with the latter assuming increasing importance as the mean particle size of the catalyst powder is reduced below about 70um. The van der Waals forces are evaluated with respect to particle size, particle roughness, and, in particular, gas adsorption. It is shown, theoretically and experimentally, that the use of CO2 and other strongly adsorbing gases at room temperature causes a considerable increase in the interparticle forces, to the extent that the catalyst can not be fluidized. As expected, the effect of adsorption disappeared at temperatures above about lOOoC, and this finding has consequences for experimental work in cold models, especially at high pressures.
• The trends shown by the experimental data are, on the whole, in accord with the theoretical predictions; however, because of the lack of fundamental data on, for example, the size of asperities on the particles, the Hamaker constant for FCC, (and the change in the value, if any, with temperature), it is not possible to make accurate quantitative predictions.
• By using experimental data, theoretical equations, and dimensional analysis, semi-empirical correlations which include powder cohesion have been developed. Work to improve these is continuing but much remains to be done.

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.