ARR - Annual Report

The objective of this part of the report is to present the lastest results on cornmunition under the IFPRI grant that started in 1993. We recall that the objectives of the research are to develop fundamental understanding and techniques to predict communition behaviour from a universal test. An experimental rig was built up to reproduce single impact on a target in an air jet mill. The influence of the material properties on breakdown was investigated in ultra- fine grinding.

Section 1

Section 1 presents the results of the single particle impact experiments performed with the single jet apparatus. The influence of the size of particle is shown with glass beads and two polymers.

Section 2

Section 2 gives the results of the study of repeated impacts. Two approaches are used to compare the efficiency with a single impact.

Section 3

Section 3 studies the influence of the process of different hydrargillites on their behaviour at impact.

Morphological analysis tools are used in relation with the classical study (fragmentation profiles).

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, Most investigations on these forces reported so far have been carried out using macroscopic and statistical techniques. Hence it is impossible to clarify the relationship of these forces with the local and microscopic characteristics of the surfaces by which the interaction forces are influenced greatly.

The aim of this research is to clarify on the molecular scale the interaction and adhesive forces between surfaces in various solutions and correlate those with the micro-structure of the surfaces, using an AFM. The following figure illustrates the whole map of experiments which we have done so far and we are going to do in this program.

The objective of this work is systematic understanding of particle-particle nanorheology based on a single particle-particle contact: two atomically-smooth solid surfaces in molecularly-thin proximity (ultimately 1 nm or less) and the dynamic mechanical properties of the resulting inter-facial film studied directly as they depend on dynamic mechanical frequency and on strain rate.

This was the first year of a 3-year grant. Motivation is provided by the realization, upon surveying the literature, that progress in understanding fine powder applications is impeded by difficulties in separating the overall rheology of a macroscopic-sized sample into various mechanistic subprocesses. Much is known about inter-particle forces: van der Waals, electrostatic, hydrogen bonding, steric. The information obtained in rheology experiments has usually been interpreted in these terms. Yet these attempts have been largely unsuccessful, for systems of realistic makeup, owing to insufficient appreciation that the inter-facial rheology may dominate because it is the weakest link. This is a critical issue because particles are almost always lubricated by condensed films of moisture; films of condensed ambient gases are ubiquitous, whether intentionally or not.

A breakthrough was obtained as concerns rate criteria to observe stick-slip rather than smooth sliding. Complications in the traditional definitions of static and kinetic friction were analyzed as they pertain to lubricated sliding (for example, in the presence of condensed gases). We found that stick-slip motion occurred only when thin films were deformed faster than their intrinsic relaxation time. The observation offers a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film.

The use of arrays of different types of sensors and the interpretation of signals derived from them provides a change in philosophy compared to traditional methods of particle characterisation and analysis. This change is necessary if measurements are to be made on line, in real process using sensor hardware that is inherently low cost, low maintenance and fault tolerant. This first report describes the objectives and deliverables of the overall programme of work, their significance, reviews the status and research development of new sensing methods that could be applied to flowing particulate systems.

Review (Part I)

In the review (Part I) attention is focused on concepts and technologies that might be used to enable measurement of the size and shape of particulates on-line in dilute and concentrated mixtures using multiple sensor arrays. The use of synergistic information from multiple sources in order to assist in the overall understanding of a phenomenon is deemed to be of considerable importance. However the application of such data fusion methods may rely upon the availability of a specific ‘model’ that describes the phenomena that are being investigated. Such a model is not always readily available, in which case more abstract methodologies based on multivariate statistical/chemometric analysis may be required.

The viability of using multiple sensors to gain more data (and hence more knowledge) about a particulate system is sensible, but the practical application is so system-specific that it is often not possible to make meaningful comments of a generalised nature. It is demonstrated that the two key factors include:

• translating the information that the user requires (or believes that they require) into a measurable parameter;
• defining the hierarchy of the measurements and/or analysis system so that it is sufficiently adaptable to tolerate loss of input from one or more sensor(s) or to allow new sensors inputs to be incorporated.

Some examples are given relating to the use of microelectrical resistance tomography applied to gas/paste mixtures.

Experimental Measurements (Part II)

Experimental measurements to establish the operational reliability of different sensing strategies to achieve the above goals are reported here (Part II). It is concluded that a system of continuous measurement using 4 multi-segment ultrasound sensors and an array of current/voltage electrodes can be utilised. For example, in the case of size measurement the electrical measurement system, current injection and voltage detection (from independent electrodes) will be made and interpreted using a hierarchy of models including combinations of direct parametric interpretation and particle timing information. The next stage is fabrication of such a sensor system. Useful contact has been made with several IFPRI members regarding the use of such a system.

It has hcen noticed that, the flows in risers of circula,ting fluidixed beds (CFI3s) is con- trolled by meso-scale structure of particles, that is clusters. The clusters largely changes fiow structure, consequently, they have considerable effect on the transport phenomena in risers of CFBs. Many experiments on the struct,ure of clusters have been performed, for example, Yerushalmi ct al. (1978), Rhodes ct, .al. (1992), Horio & Kuroki (1994), Hatano ct, al. (1994), zou et al. (1994), Li et al. (1995) and Tsukada ct al. (1997). Most of them intended to study the flows in the conditions of industriad applications, thus the conditions with respect, to the properties of gases aad solid particles are restrictfed to a small range. There are few experimental data on the structure of clusters in nonstandard conditions. For example, concerning the pressurised CFBs, it is difficult, to make observation. Therefore, it has been dcsircd to dcvclop a prediction method that is available for ra wide range of conditions.

Numerical simulation is a promising method to study these phenomena, because the presence of particles does not make the flow fields less acccssible, and it is easy to change parameters. Tsuo and Gidaspow (1990) calculated flow patterns in circulating fluid beds. They used a two-fluid model in which solid phase is rnodeled as a viscous fluid with constant effective viscosity, and predicted unstable flows with clusters. Our group proposed another simulation method for particle flows. We applied the Lagrangian approach coupled with direct simulation Monte-Carlo (DSMC) method, which was proposed for numerical simulations of rarcficd gas flows by Bird (1976), to the calculations of flow with clusters (Tanaka et al., 1995, 1996; Tsuji et al. 1997, 1998). The advantage of this method is that the properties of gas and particles can be easily taken into account. Tanaka, et, al. (1995) simulated two-dimensional flow under the same condition by Horio & Kuroki (1994) by using this method, and showed that the predicted structure and size of clusters are sirnilar to the experiments.

This project aims to produce numerical predictions that, are available to improve the understanding of the flows in CFB risers by using our method mentioned above. In this report, we present, a three-dimensional calculation that assumes the same conditions of gases and particles as Louge’s experiments (1997), in which riser flows of pressurised CFBs were studied by using high density gases. The effect, of the pressurised condition on the structure of clusters is discussed. Furthermore, correlations that appear in continuum rnodels are given.

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.