ARR - Annual Report

Zirconia (ZrO,) particles are one of the most important materials for structural ceramics used at high temperatures. However, commercially available ill-defined powders prepared by milling of calcined agglomerates or by reaction of ZrCl, and O, in gas phases are normally difficult in preparation of crack-free compact for sintering or need high sintering temperatures above 1700 degrees C. On the other hand, the monosized amorphous powders of a high sinterability prepared by hydrolysis of the alkoxides are not free from the economical problem of their low productivity (< 0.2 M in final concentration).

The objectives of our first project are to develop a new method for preparation of unagglomerated ultrafine spherical particles of a narrow size distribution with the mean diameter of the order of a few ten nanometers or less in large quantities on the basis of the gel-sol technique developed in our laboratory, to elucidate the formation mechanism, and to examine the sintering properties. The “gel-sol method” essentially differs from the popular sol-gel method. The standard procedure for the preparation of ZrO, particles and their growth mechanism have already been reported in detail in the annual report of the last year (ARR 31-02).

A mixture of CaO and silica-gel (SiO,) was heated to investigate the temperature for synthesizing para-wollastonite (CaO * SiO,) after dry grinding the mixture using a planetary ball mill. Mechanochemical treatment of the mixture brings about amorphous aggregates with almost homogeneous chemical composition. Para-wollastonite can be synthesized from the 2hours ground mixture by heating at 1273K for 2 hours, while it is normally synthesized from the mixture by heating at constant temperature of around 1400K for about one week. Heating the Shour ground mixture at 923K enables us to form a precursor of wollastonite, leading to its easy crystallization at higher temperature than about 1273K. Thus, dry mechanochemical grinding for the mixture before heating is quite effective operation for thermal synthesis of para-wollastonite.

Tricalcium aluminum hydrate (3CaO a AIZO, * 6H20 : C&H,) is synthesized mechanochemically from mixtures composed of calcium hydroxide (Ca(OH),) and pseudo-boehmite ( y - A l O ( powders by room temperature grinding using a planetary ball mill. Use of the boehmite sample with inferior crystallinity is more favorable for the mechanochemical synthesis rather than that with well crystalline one. The time required to form C&E& from the Ca(OH),- y - A l O mixture is much longer than that from the Ca(OH),-gibbsite (AI(O one. Adsorbed water from air during grinding plays a significant role in the formation of C,AH, from the former mixture. After water addition to Ca(OH),- y - A l O mixtures ground for various times, excess hydrated calcium aluminates such as &AH,, C&H,.,, and C,A, gH6 .i are formed in the starting and the short time ground mixtures, while a few amount of these compounds is formed in these hydrated mixtures after prolonged grinding. Formation of these excess hydrated compounds, which belong to layered structural materials, is enhanced in the presence of free Ca, Al compounds and water.

Anatase (TiO,, tetragonal) is transformed into rutile (Ti02, tetragonal) via brookite (TiO,, ot-thorhombic) within lhour by room temperature grinding using a planetary ball mill. This polymorphic transformation of anatase leads to mechanochemical synthesis of crystalline CaTiO, from the mixture with CaO easier than from the CaO-t-utile system. Grinding the CaO-anatase mixture for 2 hours or more enables us to produce very fine particles of about 20nm in the first order mean size. The amount of CaTiO, in the ground product increases with improving its crystallinity as the i grinding progresses. Many CaTiO, crystal grains of about km are formed in the mixture ground for Zhours, and they grow up in the prolonged grinding. The grain size of the CaTiO, crystals reaches about 20nm by about Shours of grinding, and the grain boundary and lattice fringe become clear as the grinding progresses.

The aims of this Lancaster University - Bradford University collaborative project arc to choose suitable particle compositions and ambient conditions, for work on the lack of reproducibility of powder flow behaviour; to measure force curves and investigate energy-dissipative contact processes; to clarify the role of ambient conditions (humidity) and of particle morphology; and to obtain complementary particle flow data. Our intention is thereby to assess how far such single-particle data are able to predict flow behaviour of real value to chemical engineers.

The purpose of the force curve measurements (direct measurements of force as a function of separation) is to charactcrisc the interaction between pairs of individual particles, and between single particles and a flat “wall” surface. The details of particle shape and Gne-scale morphology are revealed by scanning force microscopy (“SFM”, available within the same experimental setup as is used for the force curve measurements). In the previous year’s report we described the results obtained up to 14 November 1997, including the design and construction of a system that allows us to control the relative humidity in a glove box within which the force curves are measured. WC also described how the work at Bradford enabled us to develop improvements to the Warren Spring - Bradford cohesion tester (WSBC) and to standardise the method.

The importance of explicitly considering interparticle interactions in the rheology of dense suspensions and particle slurries is well established, although the exact relationships between particle-level quantities and macroscopic rheology and stability are at best qualitative. Most of the understanding has been developed for low shear rheological (linear viscoelastic) properties and/or for dilute dispersions. This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces (due to the presence of the solvent) on the moderate to high shear rheological properties and shear stability of dispersions that span the colloidal to particulate range (colloidal dispersions to slurries). Of particular interest is the shear thickening transition and dilatancy, and how that explicitly depends on the strategy used to stabilize the dispersion or slurry (i.e. steric, electrostatic, polymeric stabilization), as well as the hydrodynamic forces important at higher shear rates.

The research to date has the following components:

• Systematically explore the influence of the basic methods of particle stabilization on the shear thickening and dilatancy of well-characterized colloidal dispersions and slurries of non-colloidal particles. In this report in particular, particle concentration and the properties of the solvent have been varied to study the influence of hydrodynamic forces on the shear thickening and dilatant behavior.
• Develop the rheological method of stress-controlled parallel superposition rheometry for directly deconvoluting the relative influence of hydrodynamic to interparticle forces in colloidal dispersions and slurries.

We have examined the effects of powder cohesion and particle size distribution on mixing and segregation processes. As agreed in our Year 1 work plan, we have completed the development of experimental and computational procedures for this study and we have conducted an extensive characterization of non-segregating systems in order to provide a baseline for segregating mixtures (to be examined in years 2 and 3 of this project). We have met all of the stipulated milestones, summarized as follows.

1. We have built a computer-controlled lab scale double-cone blender.
2. We have developed and debugged discrete element simulation code for the double-cone blender.
3. We have simulated mixing in the double-cone over the course of up to 12 tumbler revolutions, using 15,000 monodisperse and 13,000 bidisperse particle blends.
4. We have measured the mixing rate as a function of fill level and vessel speed.
5. We have evaluated effects of filling level and vessel speed on mixing and segregation, both experimentally and computationally.
6. We have examined the effects on mixing and segregation of three-way interactions between particle size, fill level and vessel speed.

At this stage of the project, we have found the following.

1. Mixing in the double-cone occurs by a combination of radial-azimuthal convection and axial diffusion. The chief bottleneck to mixing in the double-cone is diffusion acrass the symmetry plane; we have demonstrated that judicious baffle placement can significantly improve mixing.
2. As particle sizes are reduced below about 200mu, steady and regular flow in tumbling blenders gives way to intermittent and chaotic mixing. This results in a dramatic improvement in mixing rates, overwhelmingly exceeding what would be possible by traditional mixing mechanisms. Moreover, this work demonstrates that traditional analysis cannot be applied, even qualitatively, to the study of flow and mixing of fine grains.
3. Several new segregation modes have been identified. We have charted the phase space of these modes, and we have begun an analysis of a preliminary model of segregational mechanisms which seems to show promise for developing a predictive understanding of flow and transport of polydisperse granular mixtures.

Executive Summary

This is the first annual report for the IFPRI project 37 Powder-Binder Agglomeration. The aims of this project are to:

• Define the controlling groups for each of the following classes of granulation process (1) binder dispersion, wetting and nucleation (2) consolidation and growth (3) attrition and breakage;
• Use appropriate models to link these groups to key product attributes eg. size, size distribution, density (porosity);
• In each case, qualify the models for the effects of complicating powder-binder interactions eg. dissolution, reaction, drying.

The report summarises progress in both wetting and nucleation, and consolidation and growth. Drop penetration time and dimensionless spray flux are proposed as two controlling groups for wetting and nucleation. The drop penetration time, which depends mainly depends on formulation properties, is a promising tool for studying nucleation processes. Preliminary studies show that penetration time varies widely, particularly with binder viscosity. Combining the existing models for drop penetration (Denesuk, 1993) and nuclei growth (Schaafsma, 1998b) will create a more complete picture of nuclei formation and morphology, and the effect of material properties. Ex-granulator experiments demonstrated different nucleation regimes from drop-controlled nucleation to caking. In the drop controlled regime, each drop forms a single nucleus and the nuclei distribution can be controlled by controlling the drop size distribution form the spray. A single dimensionless group, the dimensionless spray flux, characterises the main process parameters with respect to spraying. An experimental plan and methodology for studying wetting and nucleation is presented.

For granule growth and consolidation, a regime map of granule growth behavior is proposed based on granule deformation during collision and the granule liquid content measured as the maximum pore saturation. The granule deformability on collision is represented by a deformation number, which is a ratio of granule impact energy to the plastic energy absorbed per unit strain. Granule growth regimes such as steady growth, induction, nucleation, crumb, and slurry are defined. This regime map qualitatively explains the variations in granulation behavior. Laboratory drum granulation experiments were used to test the regime map. Increasing granule yield stress by decreasing particle size and increasing binder viscosity caused the system to move from steady growth to induction behavior as predicted by the regime map. Preliminary validation with literature data was also encouraging. More work, however, is required to better quantify the boundaries between different growth regimes and to investigate the effect of process agitation intensity. This regime map has great potential to help design and control granulation systems, because it is based on properties of the powder/binder system that can be measured or estimated without performing any granulation tests.

Following completion of the review, experimental work and design stages of the first year of our work, significant developments have since occurred in moving towards providing new sensing strategies to enable on line measurements in concentrated and, using different approaches, for dilute flowing mixtures. Over the last year we have been concerned with the two main tasks of designing and testing instrumentation for these purposes.

For concentrated systems - use of conventional electrical tomographic measurement method coupled with analysis of raw and reconstructed data using new statistical methods have been employed. This has been used with some success to quantify mixture properties (structural homogeneity, concentration fluctuations etc). Results are reported for solid-liquid pastes, liquid- liquid and gas-liquid mixtures. This has demonstrated that some interesting and, we believe, unique applications exist using such an approach to enhance quality control procedures in manufacturing plants and processes. It is proposed that direct feedback of the outputs from the measurement and interpretation software could be implemented as part of a control scheme.

For dilute systems - a new multi-sensor approach The Particle Gymnasium has been proposed and recently a test system has been built. Ultimately this is intended to provide a means of particle shape and size measurement in a rapidly flowing stream. Preliminary results show that the method appears to be viable and work can now continue to consider particulate systems of specific interest to IFPRI members working in crystallisation and controlled formation.

During the period under review, discussions and visits have taken place between several industrial members to consider and identify measurement needs. A forward project plan is in place. In future this will include an opportunity to assess the benefits to be gained by incorporating additional sensors (including ultrasound and magnetic resonance imaging). Further fabrication and testing of the two types of sensor systems for dilute and concentrated suspensions is planned in conjunction with industrial members in the year ahead. This will begin to define the practical performance of the sensing methods.

For fundamental discussions or understanding on contact/impact charging of particulate materials, it is essential to address measuring the charge generated due to a single impact or contact between a single particle and target- this is our basic concept at the study. Actually for that, we performed so-called “impact charging experiments,” in which a spherical plastic particle was launched one by one and made impact onto a metal target to measure the impact charge due to a single collision. To realize this, in our previous version of the experiments, the size of sample particles was limited to be big, the diameter of the particle used mainly was 3 mm. In this project, therefore, our major subject is to extend the concept into an actual powder size region. To realize the concept, we proposed two approaches in the direction of the enhancement:

The importance of explicitly considering interparticle interactions in Ihe rheology of dense suspensions and particle slurries is well established, although the exact relationships between particle-level quantities and macroscopic rheology and stability are at best qualitative. Most of the understanding has been developed follow shear rheological (linear viscoelastic) properties and/or for dilute dispersions. This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces (due to the presence of the solvent) on the moderate to high shear rheological properties and shear stability of dispersions that span the colloidal to particulate range (colloidal dispersions to slurries). Of particular interest is the shear thickening transition and dilatancy, and how that, explicitly depends on the strategy used to stabilize the dispersion or slurry (i.e. steric, electrostatic, polymeric stabilization), as well as the hydrodynamic forces irnport,ant at higher shear rates. The research to date has the following components:

• Systematically explore the influence of the basic methods of parrticle stabilization on the shear thickening and dilatancy of well-characterized colloidal dispersions and slurries of non-colloidal particles. In this report, in particular, the effect of added adsorbed polymer on the rheology is studied for explicit comparison to sirriulation results.
• Use the experimental data to develop simple force-balance based models for predicting the onset of shear thickening, as well as for use by formulators to prevent shear thickening.

Introduction

Agglomerates of fine particles are pervasive in industry. In many particle processing applications, the agglomerates are carried within a suspending fluid, and hydrodynamic shear is applied to break the agglomerate into fragments and to distribute the fragments throughout the suspending media. The underlying purpose of this research it to obtain a fundamental understanding of the various factors that influence this dispersion process. Such information can lead to the development of interfacial engineering strategies aimed at improving the outcome of dispersion processes, or to better design of dispersion equipment.

Our general approach is to study the dispersion behavior of well-characterized single agglomerates in controlled flow fields. This allows us to establish the links between the fundamental properties of an agglomerate and dispersion characteristics such as critical shear stress for dispersion, mode and kinetics of fragmentation, and the evolution of the fragment size distribution.

The specific focus of the work supported under the IFPRI grant involves investigation of how certain time-dependent (dynamic) behaviors can influence the outcome of dispersion processes. Dynamic effects can arise in several facets of the dispersion process. For instance, in practical processing equipment, complex shear histories are inherent. Also, the wetting and spreading of fluids associated with contacting particles and agglomerates with processing fluids are dynamic effects. Finally, for some materials, dissolution of the solids plays a significant role.

For the first year of this IFPRI grant, the bulk of the research effort was devoted to the development of a new experimental approach for the investigation of the influence of dynamic effects on dispersion behavior. This entailed the design (and redesign) of a dynamic dispersion chamber, and construction of it and the ancillary equipment. Preliminary experiments were done to validate the experimental techniques and to refine the analytical procedures.