SAR - Review

This report summarizes the technologies that are emerging as potential on-line sensing tools for particle size discrimination and volume fraction assessment for feedback control in the chemical and pharmaceutical industries. Specifically, the physics and the inversion algorithms associated with ensemble measurements of turbidimetry, dffiaction or angular scatter, dynamic light scattering, Fiber optical dynamic light scattering, diffising wave spectroscopy, diffirse reflectance and/or transmittance, photon migration, acoustic spectroscopy and electroacoustic spectroscopy are described with examples of their deployment in industrial settings as found in the literature, In addition, non-ensemble techniques such as particle imagery and back scatter and reflection are also described, Since ensemble techniques must include particle interaction effects when interrogating size information in suspensions with volume fractions greater than ten percent by solids (or dispersed phase), a section of the review is devoted to “static” and “dynamic” structure and how these interactions can seriously impact size distributions 11 not properly accounted for, Since vendor information for particle sizers and analyzers is found elsewhere in the literature, this review does not survey this aspect of the particle size characterization.

There are two difficulties associated with the integration of measurements for control. The Jirst dtifficulty resides in the fact that the first principles models for the process are often unknown or not known accurately enough, thereby limiting the eflectiveness for measurement input, X,,,, into the inverse of the process model to predict in closed or open loop fashion, the corrective action, AY, for maintaining the desired process output as close to the setpoint as possible. Secondly, the ensemble measurement model which relates the process output, X, to the measurement, X, may also be inaccurate or unknown restricting the usefulness of the input into the inverse process model. In the review of “ensemble ” techniques, we attempted to carefully point out the assumptions of the measurement model which cause the measurement, X,, to be less than a realistic predictor of process output X. As described above, there has been no demonstration for the inversion of$rst principles model to obtain size distribution in the presence of particle interactions. while optical techniques promise the opportunity of Jirst principle modeling with simple Percus-Yevick theory for hard-sphere interactions, actual measurements and inversions to obtain f(x) and # remain to be performed to establish practicality of the approach. In so far as these ensemble measurement models can accurately reflect changes in the internal state of the process, the model mismatch in non-spherical, concentrated and interacting systems may not be serious. On the otherhand, it is imperative that process disturbances that result in off- specification product are sensitively detected by these ensemble measurements.

Owing to the uncertainties of process and ensemble measurement models, the error associated with the inversion of the ensemble measurement model to obtain X,,, (i.e., f(x) and 4), and the magnification of the error in X,,, upon inversion of the process model to obtain AY, it may be advantageous to feed measurement data directly to an empirical or semi-empirical process model. Thus, it is important to be able to employ raw ensemble measurement signals directly as measured process outputs, even though their physical significance has not been defined by a measurement model.

Size reduction is a widely used process in industry. In the mineral industry it finds application on a large scale, but in other industries like the chemical, pharmaceutical and pigment industry, where it is performed on a much smaller scale, it is a big issue. There is even wider interest in the closely associated problem of attrition, inadvertent breakage, which occurs in almost all particle transport processes and can lead to extensive costs to control the fine dust produced, as well as interfering with the flow properties and subsequent processing of the main product.

Key questions arise:

• Can we understand what is happening during breakage?
• Can we quantitatively predict the breaking process?
• What can be done with the available knowledge in industrial situations?

These and other questions have been addressed in an extensive IFPRI program of research over the past ten years. This review attempts to bring together the achievements of this program and to identify what questions may now be posed or remain unanswered for any reason.

This review is divided into seven chapters, covering Breakage Phenomena, Problems and solution methods available, Material Properties, Breakage in Industrial Equipment, Industrial Requirements and concludes with a summary of achievements and remaining questions.

Fracture mechanics, Discrete Element Simulation, materials measurement as well as improved experimental techniques have done much to improve our understanding of the processes involved in Comminiution and Attrition. The IFPRI program has contributed a major element in this improvement. Attempts at quantification, whilst much improved have been unable to leap the barrier between laboratory measurement and industrial equipment. The main reason is that we have an inadequate knowledge of the forces developed in industrial machinery nor yet any successful method for predicting a successful comminution event in a multi particle situation in real machinery. Additionally our materials property assessment assumes uniformity of material property and idealised fracture behaviour, which is contrary to practical experience. Changes in property for nominally the same material brought about by a change in previous processing of raw material source will add to the variability of processing and unpredictability of output.

There are reasons, therefor, as a long-term requirement to improve on our ability to predict particle breakage from material properties. This needs to be extended to include agglomerated and porous materials and to define more precisely existing flaw structures, inhomogeneity and shape effects. The role of repeated collisions also need clarification, as do the breakage mechanisms brought about by shear and compression.

A major problem is the definition and description of the processes that occur in industrial equipment. This needs to include flow regimes, the occurrence (scale and frequency) of breakdown events, and the type of comminution or attrition event. At this time, the best way forward may be by simulation to provide a guide to what events to look for, followed by carefully planned experiments for confirmation. It remains an open question as to whether simulation can provide the hoped for guidance and the experimental program will not be cheap.

Until these further objectives are achieved, empirical methods will still be required, but even here improvements will be slow unless the problems posed above are tackled vigorously.

The importance of studying the electrostatic charging problem of particles, especially in dry powder processes, is self-evident for practitioners in particle technology.

In this review, the problems of contact or impact electrification of particulate materials are focused and described in detail. The major subject or the initial problem of this review could be specified here as a question of “What process dominates the amount of electrostatic charge? (at least for atmospheric conditions)” This was based on the fact that the typically observed charge density is comparable to that by which gaseous discharge in the atmosphere occurs. If gaseous discharge takes place, there is a possibility that the charges due to impact with a metal plate by particulate materials are observed as residual charges which as a result of charge relaxation during the separation process of the contact surfaces. This charge determining scheme is called “The CHARGE RELAXATION MODEL,” and to review the model in detail was the major object of this review.

Section 2

Section 2 shows the principal results of the “impact charging experiments.” They are:

• (i) the impact charge depends proportionally on the initial charge,
• (ii) the equilibrium charge, which is the initial charge when no net charge transfer occurs, is independent of the impact conditions such as the impact velocity and angle, and
• (iii) the impact charging is influenced not only by the impact velocity but also the impact angle.

Hereafter, the models or schemes determining the impact charge should be tested by whether they can account for these results.

Section 3

Section 3 describes the “simple condenser model” in detail. This model is the traditional and most simple understanding of the generation of contact electrification. It provides a first physical insight, and shows us qualitatively that there is a linear dependency of the impact charge upon the initial charge. However, the model has some essential difficulties, e.g., a quantitative prediction for the impact charging cannot be provided by this model.

Section 4

Section 4 reviews the “charge relaxation model” in detail. This is a new model proposed as the mechanism dominating the charge generated on a particle due to impact. In the model, the charge relaxation process due to gaseous discharge dominates the impact charging of a particle in atmospheric conditions, and as a result, some difficulties of the simple condenser model are avoided. The conclusions derived from the model are summarized as follows:

1. The model predictions are in good agreement with the “equilibrium charges” given as the results of impact charging experiments. Furthermore, the impact charging experiments conducted in Ar gas indicated that the impact charging characteristics change corresponding to the breakdown limit potential of the environmental gas; this was also predicted quantitatively by the model.
2. It was also shown that the model explains not only the “equilibrium charge,” but also the linear dependency of the impact charge on the initial charge.
3. The measured impact charge increased and assumed the saturation value with increasing impact velocity and angle. This saturation value fell within the range around 110% of that predicted by the model.
4. The two-dimensional relaxation on the particle surface in the discharge relaxation process predicted that charge would relax completely in the case of impact charging between a conducting particle and a metal plate. This was in accord with experimental results.

It should be emphasized here that the model provided good agreement with the experimental results without recourse to any empiricism.

Section 5

Section 5 briefly reviews the mechanism of the charge generation. The charge generation process at contact and charge relaxation process which determines the actual impact charge need to be discussed independently. Our review on the mechanism of the charge generation shows that there are many models which are applied to this problem, without the problem being clearly understood at present.

Section 6

Section 6 is the summary section of this review, and a direction for the additional research which is recommended.

Additionally the Appendices provide, in detail, the precise mathematical procedures for the potential of a charged spherical particle located in the vicinity of a conducting plane.

Introduction

When a colloidal particle is brought into contact with an aqueous electrolyte environment, the orientation of water molecules at the solid - solution interface induces electrical charging of the surface. Mechanisms for this process may include dissociation or ionisation of surface sites, specific ion adsorption, differential dissolution of surface- based ions and, in the case of clay minerals, exchange of lattice cations with others of lower valence in a process known as isomorphous ion substitution. The presence of the charged surface promotes redistribution of the ions in the surrounding solution, with counter-ions preferentially attracted towards and co-ions repelled away from the surface. The result is an ionic neutralisation of the surface charge over a short distance into the neighbouring aqueous environment. The combination of the charged surface and the unequal distribution of counter-ions and co-ions in the surrounding solution is referred to as the electrical double layer.

This review concerns systems that contain rigid particles packed to high concentrations. Either dry powders or in a mixture with a fluid phase. Colloids (sub- micron particles, fluid saturated) at one extreme of length scale and dry powders (larger than 100 FM, no fluid) at the other extreme. Suspensions and pastes are a broad range of materials lying between these extremes. Often these are highly poly- disperse, in a visco-elastic matrix and sometimes of three phases. Scientists in soil- mechanics, chemical engineering, physics and physical chemistry research these systems. Each bring their own approach and there is a need for unification of understanding.

The scope of the report is on the flow and mechanical properties of such systems in regimes where inertia can be neglected. In the context of dry powders this will be limited too systems in either static equilibrium or in quasi-static flow. In the context of colloids and suspensions this will be limited to dense and concentrated systems.

We lack tools to relate particle shape, polydispersity, Ph, ionic strength and polymeric components in suspensions to their bulk flow properties. Despite the evident fact that we have learnt much as to how to process and design such materials, technology would greatly benefit from such tools. Particle base simulations will be shown to provide a potentially powerful route to this. The report will review both techniques (Section 4) and results (Section 5) for flow curves in concentrated colloids. Many microscopic models of inter-particle motion, structure and kinetics exist for colloids and suspensions, but with little validation or observational support. The review will point out that simulations of colloids in shear flow are giving many new insights, in particular as to the nature of states at high shear rates. Section 7 will discuss issues and prospects for future simulations.

By reviewing certain aspects of suspensions, paste and powders together the report will try to identify those features which are common due to their particulate nature and which made lead to a unified understanding. This is done in section 6.

In recent years a new debate has emerged on the basic physics of force transmission in dry powders. In part computer simulations have prompted this. The debate is reviewed. Many new ideas and new arguments are being floated and some of these do hold prospects for a unified approach. Partly in response to this report, the author has added to this debate by showing features of force transmission common between simulations of granular media and colloids.

A more precise study has begun of the transition between a visco-elastic liquid and visco-elastic solid on increasing packing. Understanding this is identified as a key step in any unified view. Recent results are reviewed and questions set.

Flow instabilities such as shear banding, paste fracture, and shear thickening are common in suspensions and paste, but little is understood. The report will identify issues and recent work.

Given the increasing role of particle based simulations the report will review in section 7 the uses and prospects for these.

Many ideas, methods and testers exist to measure the flowability of bulk solids. The primary intent is the characterization of bulk solid‘s flow properties, but most often the measured data are used to design equipment for storage, transportation or general handling of bulk solids. Flowability testing is also needed to compare the flowability of similar or competing bulk solids, to determine whether a product fulfills the requirement of quality control, to model processes with the finite element method or to judge any other process in which the stiength or flowability of bulk solids plays an important role. Many testers are available which measure some value of flowability, only some of these shall be mentioned in this introduction: Jenike’s shear cell, annular shear cells, triaxial tester, true biaxial shear tester, Johanson Indicizers, torsional cell, uniaxial tester, Oedometer, Lambdameter, Jenike and Johanson’s Quality Control Tester, Hosokawa tester and others.

The scope of this report is as follows:

1. First, flow properties shall be defined.
2. Secondly, applications are mentioned. Here the properties which are needed for design are described.
3. In the main chapter the known testers for flow properties measurement are listed and described.
4. Following, the suitability of these testers to actually measure the required properties is discussed.
5. Finally, a comparison with regard to applications will be made.

It is beyond the scope of this report to describe all testers very detailed or to compare all of them one by one.

The objective of this paper is three fold. First, the intention is to review the porous structure of particle systems, especially those porous pastes and gels formed during powder processing operations. Second, this report critically analyses some deficiencies and unknowns in our knowledge of porous particle structuring processes. Third, the objective is to suggest future work which could resolve these problems and lead to more rational approaches to the processing of fine particles.

Since there are several levels of porosity to be found in particle systems; namely atomic and molecular pores ie nanopores less than about 1 nm in size, micropores around 1 pm in size and macropores above 1 pm; it is necessary to split part 5 of the report into three sections. The section on nanopores, which are important in catalysts, absorbents, zeolites etc has been written with the help of Dr Zholobenko. The section on micropores, where Brownian movement is a strong factor, has been compiled with the help of Dr Stainton. Large macropores are more readily measured, for example by light scattering and optical microscopy, and are dealt with in the final section.

The conclusion is that the main opportunities for novel advance are in the micro and nanopore regimes. The structuring of nano and micro-particles is dominated by molecular forces which can be measured by Atomic Force Microscopy. New computer models can begin to simulate structures resulting from these forces. Novel experimental methods, such as micro-focus X-ray, are emerging to compare with such models.

Problems and suggestions associated with a progress of particle technology toward better-defined materials – engineered particulates – are presented.

In the first part of this review, coating technology was categorized from various angles, e.g., purpose, principle, methodology or instrumentation.

Dry, physical methods are divided into mechanical and sputtering, and spray coating inbetween. Typical machines and techniques are displayed. Emphasis is laid on the mechanochemical effects involved in the mechanical treatments. Deliberate introduction of mechanochemical reaction during coating is explained with some concrete examples.

Wet chemical methods are then described in detail. After explaining surface modification by surfactants or coupling agents, various colloid chemical routes are explained. Use of in situ reactions and related micro encapsulation are then referred.

Importance of characterization from non-conventional points of view, e.g., homogeneity of the coating between a lot, within a lot, within a treating chamber or vessel in a machine or even within a single particle is emphasized with concrete examples of analyses. Evaluation of chemical interaction at the host-guest interface and within a guest or shell layer are refined as well.

Examples of particle coating application are given from various area including electronics, optonics, magnetic materials and pharmaceutics. Representative case studies carried out in the author’s own laboratory are introduced. They will cover most of the categories of coating referred above.

Throughout the entire review, the author pays special attention on the microscopic and chemical views to fabricate and to evaluate the well-defined coated products to fulfill the concept of the modern engineered particulates.

Executive Summary

This review may contribute to improve process engineer’s understanding of rheo-mechanical properties of wet-mass powders by means of a suitable combination of molecular mechanics, particle contact models and continuum mechanics.

If a non-drained paste is sheared then the shear stress will increase with applied normal stress (Coulomb friction between particle contacts) for a certain water content below the pore saturation. During this shearing, the packing is consolidated and the porosity decreases. At the point of pore saturation a sharp transition is often assumed to change immediately the previous dominant particle friction into viscous shearing of the liquid. Now, the shear stress remains constant and the paste flow state is achieved without any influence of external pressure. But by means of intensive pre-stressing, large shear strain, spatial drainage with pore liquid flow, liquid pressure drop and comparatively small moisture contents the reversal transition between initial viscous paste flow to Coulomb friction is obtained. The wet-mass powder is expanding, pore saturation is reduced, the capillary pressure increases and consequently the particle contacts are loaded and more and more deformed. This produces an undesired crumbly structure of the mass, like green pellets. An increased normal stress is the consequence to press the mass through moulds and matrices. Considering the stiffening by capillary pressure distribution as an attractive internal pressure and, consequently, remarkable soft particle contact deformation within the shear zone plus softening by a hydrodynamic repulsion force, flow of pore liquid relative to the particle shear velocity, long-range particle-particle interactions and lubrication effects between particle surface asperities, one may expect a certain metastable range of transition in a compressed wet-mass powder or paste. This term “metastability” may be used to describe serious transition problems as follows: Either the wet-mass powder is exposed dominant Coulomb friction during “supersaturation” of pore liquid instead of expected viscous flow, or the paste is sheared with dominant viscous flow during “supersaturation” of particles instead of expected Coulomb friction. This may be equivalent to jamming in rheology as well as caking, or contrary, avalanching and flooding effects in powder mechanics.

Nevertheless the fundamentals of this metastable transition range are not completely understood. A basic research in detail seems to be necessary to understand what is really happened in the packing during these transitions of dominant Coulomb friction ←→ viscous paste flow. These facts lead to the necessity of closing the huge gaps between particle mechanics, powder mechanics and paste rheology. This complex combination of theoretical work, computer simulations, experimental evaluation and full-scale practical application may be sufficient to ensure the success of this joint research and development project. Unfortunately the reality of technological conversion processes in powder technology is complicated enough to discover the adequate constitutive model which requires an amount of practical experience, sure instinct, theoretical knowledge, experimental skills, cleverness and sometimes fortune as well.