SAR - Review

Introduction

This report is concerned with the characterisation of the surfaces of solids, and with the measurement and interpretation of interactions at the solid/gas and solid/liquid interfaces insofar as they are relevant to particle technology. We are particularly concerned with how the chemistry of the surface affects the surface and interfacial chemistry of the solid. We review the techniques currently in use, and some that were never developed to their full potential. We also make recommendations for future work that could improve our understanding of the role of the surface in the behavior of fine particles.

SUMMARY

The literature on the subject of the flow of aerated powders has been reviewed and is presented in this report under 5 headings.

1. THE FLOW OF COARSE MATERIALS AND THE EFFECT OF INTERSTITIAL PRESSURE GRADIENTS.

It is concluded that the flow of unaerated coarse materials is understood and that correlations and theories exist from which it is possible to predict the discharge rate with precision. The effect of deliberately imposed pressure gradients on the flow of a coarse material is also understood. Some progress has been made in the understanding of the effects of self-generated pressure gradients on the flow of materials of mean size less than 500 um.

CONSOLIDATION

The expulsion of pore water from clays has been well studied in the soil-mechanics literature. The case of deaeration of a loosely fitted material has been analysed by Jenike and by Murfitt using similar techniques. Methods are discussed.

5. FLOW OF AERATED POWDER DOWN CHUTES

Only one paper specifically on this topic has been found and this is discussed. T. Rathbone Under the Supervision of Dr. R.M. Nedderman Cambridge November 1983 The case of deaeration.

2. FLOODING

The empirical work of Carr seems to provide a criterion for identifying which materials are liable to flood, though the mechanism of flooding is not understood. Jenike and co-workers maintain that flooding is more likely to occur in core flow hoppers and that materials liable to flood should therefore be stored in mass flow hoppers.

3. THE FLUIDISATION OF FINE POWDERS

It is concluded that the Geldart classification of powders is the most reliable available for predicting the type of fluidisation that will occur with a given material.

4. CONSOLIDATION

The expulsion of pore water from clays has been well studied in the soil-mechanics literature. The case of deaeration of a loosely fitted material has been analysed by Jenike and by Murfitt using similar techniques. The approximations in their methods are discussed.

5. FLOW OF AERATED POWDER DOWN CHUTES

Only one paper specifically on this topic has been found and this is discussed. T. Rathbone Under the Supervision of Dr. R.M. Nedderman Cambridge November 1983 The case of deaeration.

Complementary to the already existing surveys offered by myself to IFPRI on "Air Classification" and the "Sorting by Sieving and Air Classification" a third survey report has been prepared on "Sorting in Gas Fluidised Beds". Sorting in a fluidised bed may be achieved either in a counterflow or a crossflow sorting machine. The present report describes in detail the relevant theoretical background necessary for the precalculation of a fluidised bed sorting process. Most of it belonging to the standard theories used in fluidised bed reactor design. It was clearly found, that the existing theories describing the behaviour of a fluidised bed with respect to changes in the size distribution of the particles forming the bed, their shape or the influence of the void ratio is inadequate and needs further theoretical and experimental consideration. The precalculation of a fluidised bed used for sorting furthermore suffers from the fact, that the settling behaviour of the heavy that is the sink product cannot be described accurately enough. This is mainly due to the fact, that the influence of the above mentioned variables on the apparent density of the fluidised bed is not very well known. Therefore in actual practice bench scale experiments must yield the relevant data. Bench scale experiments have been performed with four different bed materials and several different materials to be sorted. The last part of the report summarises equipment as invented and suggested in the literature for the sorting in gas fluidised beds. This report finalises my work in classification and sorting.

Executive Summary

A literature review of precipitation from the liquid state has been undertaken. Two areas of concern to precipitation technologies were highlighted i) factors controlling precipitate size, morphology and agglomeration, and ii) parameters influencing the crystalline phase precipitated.

WHAT IS KNOWN

A wide variety of materials have been precipitated as particles of uniform size in the micron and submicron size range. No limitations on preparing narrow size distribution precipitates appears to be imposed by the chemical nature of the material of interest. The importance of colloidal interactions in the control of size distribution cannot be over emphasized.

Equilibrium thermodynamic factors as well as solution and surface reaction rates are important in determining the phase and morphology of precipitates. An understanding of these factors at small length and short time scales is required for morphological control. Impurities and additives strongly affect particle growth rates and the phase precipitated. Today, shape and phase control are more art than science.

Many of the technologies currently available for the preparation of uniform precipitates have been developed at the bench scale and have yields in the milligram to gram per liter range. With the exception of silver halides and emulsion polymers, no work has been reported in the literature describing scale-up to industrial quantities. Poor understanding of surface reactivity of freshly prepared particles and their colloidal nature currently limits development of separation schemes when unagglomerated particles are required. Due to the lack of published information on large scale processes, no attempt was made to quantify limitations on post precipitaton processing steps.

Agglomeration of nanometer sixed particle nuclei appears to be a critical step in the formation of many uniform precipitates. Heterogeneous size distributions arise when aggregation/agglomeration is not halted at the required size.

Little information was found on the cost of generating uniform particles in the sub-ten micron size range. Much of this information resides in industry and not academia.

WHAT IS UNKNOWN

No general rules for controlling the precipitate phase or morphology have been developed despite the industrial need.

Growth mechanisms of precipitates are poorly understood for most materials. The chemistry occurring in supersaturated solutions far from equilibrium impacts strongly on precipitate phase and morphology but is not well characterized. This poor understanding limits attempts to scale-up current precipitation recipes.

Mechanisms governing nucleation and growth of uniform particles have seen surprisingly little study. In the early stages of precipitation, growth mechanisms are difficult to assign. Often, even the size of the growth unit is poorly characterized.

Scale-up of precipitation chemistries resulting in uniform particles has not been extensively studied. As a result little is known about the magnitude and size of the scale-up difficulties.

WHAT IS PROPOSED

Critical review of the current state of the precipitation of uniform particles has led to the formulation of two key areas of opportunity where fundamental research could lead to enhanced understanding and thus to better precipitation technologies. These opportunities are discussed on pages 26-29 of the review and are briefly described below.

Particles Fundamental Studies of Nucleation and Growth During Precipitation of Uniform Particles

While the chemistry of precipitation reactions is very complex, broad themes important to preparing uniform precipitates may be more general. This program would search for these themes. Three areas have been identified as being of critical importance. First, prenucleation chemistry and what species form nuclei would be explored. Secondly, the colloidal interaction of nuclei would be studied in order to establish factors which act to give colloidal stability to growing precipitate particles. Thirdly, growth mechanisms of the particles would be determined to establish the importance of aggregation on final particle morphology. The goal of this work would be to develop an understanding of the physical chemistry leading to uniform particles and to determine the generality of the mechanisms such that they can be applied to new systems.

Fundamental Studies in the Scale-up of Precipitation of Narrow Size Distribution Particles

Few attempts have been published detailing the scale-up of precipitation technologies leading to uniform particles. As a result, little is known about the difficulties in increasing the size or the yield of current precipitaton technique. The thrust of these studies would be: i) to explore various scale-up scherncs, ii) develop quantitative models from a basic understanding of the reaction pathways, and iii) provide a means of assessing the difficulties in scale-up operations. The general goal of the research would be to determine the difficulties in scaling up precipitation reactions that result in uniformily sized particles.

SUMMARY

This report is an extension of the paper given at the AMU Meeting held at Princeton and includes further analysis on the application of Fracture Mechanics to the chipping experiments reported at the Annual Meeting by the workers at the University of Birmingham. In addition it has been found that the initial pessimism of finding published work on fracture under applied compressive loads was not fully justified.

Some of the ideas of Fracture Mechanics are briefly reviewed and fracture criteria for both Linear Elastic Fracture Mechanics (LEFM), which is predominantly brittle, as well as for ductile failure in the presence of extensive plastic deformation are presented. The role of plastic deformation is stressed: it governs toughness and brittle/ductile failure, characteristics. The effect of size is considered and it is shown that the observed ductility of particles of materials which may be brittle in large scale components is a logical consequence of the theory. Kendall’s work on this topic is briefly described.

It is argued that fracture mechanics can be usefully applied to particle breakage, although LEFM will be of less use than for fracture of larger samples and plastic deformation must be incorporated in the treatment. Attrition is not seen to differ mechanistically from gross fracture and will depend on the same parameters, particle shape being a significant factor in determining whether small fragments are chipped off or not. Some ideas concerning the dependence of abrasion on yield stress are discussed and it is concluded that fracture toughness and yield stress are both important governing parameters.

Finally, some suggestions for future work are presented.

This review was undertaken to evaluate existing information about the fundamentals and engineering aspects related to particle formation from the gas phase.

What is Known:

• Particle production mechanisms in the gas phase are fairly well understood for dilute systems, but not for concentrated systems.
• The three major mechanisms are nucleation, condensation and coagulation.
• Coagulation is the most detrimental mechanism as it can lead to increased polydispersity and undesired aggregation. Control of coagulation is essential for powder manufacture.
• In dilute systems, the average primary particle size can be controlled by the correct choice of available vapor and the number of nuclei produced.
• The width of the particle size distribution is determined by the residence time and temperature distribution of both the reactants and the condensing vapor.
• Coagulation rate is a function of the particle concentration, relative particle motion and turbulence. Coagulation leads to broader size distributions.
• Particle shape depends on the temperature and the environment.
• Primary particle shape can be spherical or facetted depending on the precursors. Continued gas reactions can produce porous particles, the porosity being eliminated by high temperature.
• Aggregates and agglomerates have differing morphologies depending on such factors as field gradients and particle dielectric constants.
• Gas produced particles are very pure, and purity can be ensured by the use of inert gas sheaths to prevent wall contact.
• Powder yields can be high - depending on the reactor type - because particle production can be continuous.
• The product quality and economics are often a trade-off between reactor throughput and the acceptable degree of agglomeration.

What is Unknown

• The relative importance of reactor design, growth process competition and coagulation suppression are not well understood for concentrated systems.
• The importance of additives, applied fields and reactor design criteria have not been widely studied.
• Dilution systems are not widely applied because of the additional gas volume involved in the final gas solid separation process, but strategic dilution, i.e. dilution at key reactor locations or at key residence times has not been explored and might provide a solution with minimal extra gas volume.
• Sensors for monitoring real time size and concentration information are not available for high solids gas loadings and hostile environment.
• Methods of producing pure non-aggregated powders of size distribution greater than 0.5 micrometer on high concentration gas systems are not available.

What is Needed

A fundamental research program which will:

• study the relative importance and interplay of the three major mechanisms using a simple design of laboratory reactor.
• study the effect of reactor geometry and design on particle size distribution, aggregation, purity and yield.
• develop a suitable reactor to study the role of additives and strategic dilution. With the overall objective of extending the useful size range of gas/solid formation processed to coarser sizes while maintaining high yield, purity, morphology and no aggregation.