ARR - Annual Report

Fundamental theoretical and experimental works about grinding and crack formation in solids has been done by Schönert [1]. Based on this work he estimates the possible minimum particle size for grinding purposes to be in a range of 10 to 100 nm depending on the physical properties of the material. Investigations of the comminution of fused corundum (Al2O3) should answer the question whether and in which particle size range a lower limit of the grindability exists. During the investigations the increasing particle- particle-interactions with increasing fineness are an essential problem. Due to this interactions the viscosity of the product suspension increases and often spontaneous agglomeration of product particles occurs.

This report shows the experimental setup which allows the measurement of the most important electrochemical properties and the analysis of the particle size distribution of the product suspension as well as an adjustment of the pH-value for stabilization during the comminution process.

First results for comminution of fused corundum with different grinding media materials and grinding media sizes are shown. In addition, the influence of the electrostatic stabilization on the grinding progress is discussed.

This year research on the rheological behavior of concentrated suspensions has (1) addressed theoretically the effect of interparticle forces on suspension rheology, (2) developed the Accelerated Stokesian Dynamics (ASD) method, and (3) used ASD to study the effects of surface roughness and sliding friction on rheological behavior. The goal has been to provide a quantitative microstructurally-based description of suspension rheology for a range of concentrations and particle-level interactions. This year most of the effort has been on the numerical side. Please note that not all of the work described here has been supported by IFPRI; for example, the work on ASD has been primarily supported by NASA.

The present annual report composes of three parts; (1) mechanochemical (MC) syntheses of rare earth oxyhalides (ROX, R=La, Sm, Nd, Pr, X=F, Cl, Br) to form functional materials from inorganic systems, (2) MC syntheses of solid state solutions such as LaOCl1-xBrx from mixtures of rare earth oxides (R2O3) and rare earth halides (RX3, X=F, Cl and Br) from the inorganic systems, and (3) MC synthesis of LaOF from an inorganic-organic system (La2O3 and polyvinylidene fluorine (PVDF)). Regarding the first part, the grinding the constituent components (R2O3 and RF3) enables us to form ROF monophase in the product, and the reaction proceeds with an increase in grinding time, and the crystallite size of the product formed is about 15~20nm. As for the second part, the MC reaction proceeds as grinding progresses, forming LaOCl1-xBrx. Unit cell dimensions, a, c and lattice volume of the solutions, evolve linearly with an increase in x in the LaOCl1-xBrx series. Comparing unit cell dimensions of LaOX synthesized by MC reaction to those of LaOX synthesized by solid state reaction at high temperature, there is not any significant difference in the length of c, while a is shortened slightly. This may be attributed to the complex cation layer of (LaO)nn+ with a close relationship to a of the cell dimensions, being affected by the intensive grinding. As for the final part, the reaction through the substitution of F- by O2- results in defluorination of PVDF forming LaOF. This reaction proceeds with an increase in grinding time and is almost completed by about 240min. Mean size of the synthesized LaOF particles is sub-micron order, and the particles look like agglomerates. By prolonged grinding, agglomerated fine particles, consisting of LaOF and resultant organic materials, are transformed to the composites of rupture-like shape and these particles are covered with residual fine particles, progressively. Bonds such as C-O-H and C=C, are formed in the resultant organic phase in the ground mixture. The reaction yield reaches about 98 % at 240min. From these investigations, they have found a new route as well as something that leads perhaps to high possibility of application of this MC reaction to pharmaceutical field.

The future work will be focused on the grinding a mixture of a pharmaceutical raw material like talc with a binder such as cellulose to investigate MC reaction and molecular design between the two materials. In addition, the MC reaction and effect for food materials such as lactose will be investigated. In this study, co-grinding of lactose with cellulose with or without additive such as paracetamol will be made.

Summary

In the particle technology, it is fundamentally important to know the interaction and adhesive forces between particles and to find the correlation of those forces with the microscopic characteristics of particle surface, because these forces are the origin of many phenomena which particles exhibit in industrial processes. The aim of this project is to clarify in-situ at the molecular level the microstructure of surfaces in solutions of industrial importance and the correlation with interaction and adhesive forces between surfaces, using not only an atomic force microscope (AFM) but also computer simulations.

It was planned to clarify the phenomena and mechanism on the subjects shown in the map of the following figure, which were considered to be fundamental in understanding the phenomena in industrial particle processes.

Executive Summary

Work is reported of relevance to the pharmaceutical, chemical, mineral and other manufacturing processes against our objectives of developing and demonstrating the use of multi-sensors for mapping of microstructure and flow properties of concentrated suspensions and dispersions. The goal is to use this information for control of particulate product formulation in which on-line measurement enables new or more consistent products to be manufactured. The measurement tools used have included electrical conductivity tomography, X-ray tomography and photography and ultrasound measurements. Principal achievements of the research and team are reported under the following headings:

Software Tools

• Development of a new algorithm for enhances data analysis and image reconstruction, that is more robust to fluctuations in the background electrical properties of the process mixture.
• Computation of local changes in axial and radial solid concentration profiles.
• Measurement of velocity of features/flow structures in the mixture.
• An example case study for hydraulic conveying.

Instrument and Technique Development

• Simpler (single) electrode arrays for pipe-based measurements.
• Alternative impedance measurements yielding absolute images for paste extrusion in barrel and die.
• Demonstration of 3d imaging and reconstruction from a limited electrode data set in a narrow bore die.
• Use of X-ray microtomography with a digital simulation approach to enable prediction of properties of complex structure materials.

Model Validation Case Studies

Here the goal has been to use the multi-sensor measurement method to validate models. Two examples have been progressed-

• Slurry hydrotransport modelling and measurement using electrical tomography (above)
• Slurry transport within a small diameter hydrocyclone, comparing CFD and tomographic measurement (electrical and XC-ray photographs)

The forward plan for 2002/3 is outlined with our focus on measurement and quantification of product microstructure in particulate materials and products.

Introduction

This research focuses on the dispersion of clusters of fine particles through the application of hydrodynamic shear. In many industrial applications, the processing goal is the production of breakdown of the particle agglomerate into small fragments (or possibly complete breakage of the agglomerate into its constituent particles) followed by distribution of the fragments throughout the liquid medium. Our ultimate goal for this work is to obtain a fundamental understanding of the various factors that influence the dispersion behavior of fine-particle agglomerates. Once such an understanding is achieved, chemical or mechanical strategies aimed at improving the outcome of dispersion processes or the design of more efficient dispersion equipment can be attempted.

Our general approach has been 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.

A major emphasis of the research supported under this IFPRI grant involves investigation of the role of certain time-dependent (dynamic) phenomena in governing dispersion. Such time-dependent effects are inherent in many aspects of the industrial practice of dispersion. For example, when agglomerates are contacted with processing liquids, wetting and spreading of liquid throughout the agglomerate structure occurs simultaneously with dispersion. Also, particle agglomerates are subject to complex, non-steady shear histories in practical processing equipment. Also, in some cases, dissolution of the particles or binder liquids may occur on a time scale comparable to that for dispersion itself.

In 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. In the second year, we performed additional modifications to the experimental device and refined our experimental procedures. This allowed us to complete additional experimental studies that highlight the dramatic influence of time-dependent effects on the outcome of dispersion. In addition, we performed a thorough modeling study of the flow and shear fields present within our experimental device. This modeling enables us to analyze the results of our experiments, and to understand the limitations of our dispersion experiments with oscillatory flow. In the third year, we continued to perform experimental studies that shed light on the fundamentals of dispersion processes. These studies are leading us toward a mapping of dispersion regime on which different types of dispersion behavior can be related to agglomerate and flow characteristics. In addition, we attempted to study the dispersion behavior of agglomerate samples that were obtained from IFPRI member companies, and found that our experimental methods divulged a significant non-uniformity in the agglomerate structure. Such non-uniformity makes difficult the interpretation of the outcome of dispersion experiments with these materials.

Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:

Approach 1: Impact Charging Experiment:

To apply the previous version of “impact charging experiment,” in which rather bigger particles were used as samples, to smaller particle sizes, the sensitivity of the charge measurement and trajectory control of the accelerated particles were required to be improved. In the first year project, we carried our the improvements, and new rig, which is available for hundreds micro- meter particles, was manufactured successfully. In the second year, actual experiments were performed, but we encountered with data scattering, while the order of magnitude of the data agreed well with that predicted by “charge relaxation model” which we have proposed as a scheme determining the amount of the impact charge.

In the third year, we tried to improve the rig with some points which could cause the scattering. In a point, particle trajectory control, was improved successfully, and the data accumulation was improved. However, the data scattering did not disappear: a question to be answered here was if there was a reasonable cause of this data scattering. Some discussions were done, and as its result, a localization of the initial charge was supposed. A top and a rear side localization against the contact point, gave two limitations. The scattering data were indeed held within the range. As a conclusion, the charge relaxation model works in the range of the particle size from 100 to 300 micro-meter. For the future study, a successive impact experiment for electrostatic charging of particles would be recommended.

Approach 2: Development of the Method of Electrostatic Adhesive Force Measurement:

This is the completely new approach to determine the amount of charge transferred onto a par- ticle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target.

In the third year project, we have actually launched AFM study. Now the experiments are still pilot status, but a force curve due to electrostatic charge was measured successfully for a 35 micro-meter particle. The amount of the electrostatic charge was estimated in the order of 0.1fC, and at this moment the sensitivity is enough for the order of magnitude of the amount of charge, which corresponds 1000 electrons (elementary charges). In the next year project, actual and detailed discussion will be available.

This report summarises progress in IFPRI project 37 in 2001. This is the first year of the continuation project. We have established new areas of investigation with new objectives. Thus, we report here a summary of work in progress at a fairly early stage.

Iveson and Page at The University of Newcastle

have continued to investigate the mechanics of partially saturated powders. For a range of powders, both spherical and non spherical, dimensionless strength is independent of strain rate for Capilliary No (Ca) less than 10-4 and strongly dependent on Ca above this value. Particle shape has a strong effect on yield stress due to its effect on particle packing, friction and particle interlocking. The failure mode also changes with strain rate. At low strain rate, brittle fracture is observed, while at high strain rate pellets deform plastically.

Forrest and Bridgewater at Cambridge University

have established a methodology for characterizing powder flow during granulation using Positron Emission Particle Tracking (PEPT). The technique is being used to study powder flow in a plough share mixer granulator throughout a batch granulation. Powder flow characteristics change significantly as liquid is added to the granulation. Changes in the granule velocity and local bed density are linked to regions of granule motion that are characterised by inter-granule contact time and force. Further experiments are required to determine values for the relaxation ratio and develop the relationship between granule motion and granule properties. Extension of this analysis to a greater range of experimental conditions will develop the relationship between granule motion and granule properties for a ploughshare mixer.

Wet Granule Breakage Research

A brief review of the status of wet granule breakage research is presented. Wet granule breakage is a relatively poorly studied phenomenon. Experimental evidence shows that it may play an important role in controlling granule size distribution in mixer granulators and can contribute significantly to binder distribution under some conditions. However, little attempt has been made to link breakage rates quantitatively to the mechanical properties of the granules (or even to characterise them). The theory of granule breakage and the mechanical characterisation of granules under dynamic conditions need further development in order to produce quantitative predictions of conditions for granule breakage. The work of Tardos and coworkers based on a Stokes deformation number analysis combined with the mechanical characterisation approach of Iveson and Page provide a good starting point for further development.

The objective of this work is systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance is to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we are seeking to develop new methods to control and manipulate the properties of their interfacial films.

Specifically, the dynamic mechanical properties of the resulting interfacial film are being studied directly with special emphasis on how they depend on both vibration frequency and strain rate. A homebuilt apparatus is employed to this purpose with the following unique properties:

• Surface-surface spacing are variable from thousands of Ångstroms to molecular contact. The surface force needed to produce this separation is measured while at the same time measuring shear nanorheology. The tip is imaged in situ directly during each experiment, therefore force can be normalized by area to produce stress.
• Oscillatory deformations with variable frequency in the range 0.01 to 103 rad-sec-1 can be applied, with deformations either in the shear direction or in the normal (pumping) direction.
• The amplitude of deformation can be varied from sub-Ångstrom to microns. The reason to use very small deformations is to produce a linear viscoelastic response (representative of the rest state, because this can be studied by methods of equilibrium statistical thermodynamics). The reason to use very large deformations is to produce strongly nonlinear deformations characteristic of very high shear rates.

To the best of our knowledge, no other instrument with these properties exists in any other laboratory in the world. We would like to take this opportunity to encourage IFPRI members to suggest new systems that would be interesting to study using these unique methods. The main finding during Year I was to develop criteria with predictive power to understand whether opposed particles will move past one another with intermittent stick-slip motion or with smooth sliding. We found that stick-slip motion occurred only when 2 thin films were deformed faster than their intrinsic relaxation time. The observation offered a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film. The main findings during Years II and III concerned methods to control suspension rheology with surface coatings. First, we found that because the interparticle potential is an equilibrium quantity, whereas nanorheological responses depend on rate-dependent processes, the interparticle potential failed to correlate directly with shear nanorheology when dealing with films that showed a viscoelastic response. We initiated studies concerning forces in a “tapping” mode – to hydrodynamics when particle surfaces come together and are pulled apart with fluid media in between. These studies focused on nonaqueous systems and some preliminary studies were undertaken in aqueous media. During Year IV, studies were initiated concerning the boundary condition, ‘no-slip’ or ‘partial slip’ of continuum hydrodynamics. Preliminary findings indicated massive breakdown of the conventional ‘no-slip’ assumption, but those conclusions were tempered by the fact that the experiments at that point had concerned only surfaces that were atomically smooth.

This year research on the rheological behavior of concentrated suspensions has focused on the connection between viscous suspensions and granular media – from wet to dry. The goal has been to understand the similarities and differences between these two classes of particulate materials. How the microstructure changes as one moves from viscous to rapid granular flow and how this microstructural evolution manifests itself in macroscopic properties such as stress. The effort has been numerical, as the ASD technique has been extended to incorporate particle inertia, which is necessary to model granular systems. Please note that not all of the work described here has been supported by IFPRI; for example, the development work for ASD has been primarily supported by NASA.

The major conclusions of this study are that there basically exist two distinct states as the effect of particle inertia is varied. At low particle inertia (or low Stokes number) – wet suspensions – viscous forces dominate, stresses scale linearly with the shear rate and the fluctuational motion of the particles as characterized by the suspension or granular temperature is small. This is the so-called ‘quenched’ state first predicted by Koch and co-workers (Koch 1990, Tsao & Koch 1995). At high particle inertia (high Stokes number) the fluctuational motion is large, corresponding to a large suspension temperature – the so-called ‘ignited’ state (Sangani et al 1996). At high Stokes number – dry suspensions – stresses scale inertially and are proportional to the shear rate squared. The transition from the quenched to the ignited state occurs at a Stokes number of roughly 10 over the wide range of volume fractions studied here, 0.01 ≤ φ ≤ 0.45.