Table Of ContentButterworths Monographs in Chemical Engineering
Butterworths Monographs in Chemical Engineering is a series of occasion*
texts by internationally acknowledged specialists, providing authoritative
treatment of topics of current significance in chemical engineering.
Series Editor
J W Mullin
Professor of Chemical Engineering, University College, London
Published titles:
Solid-Liquid Separation
Liquids and Liquid Mixtures, Third Edition
Forthcoming titles:
Mixing in the Process Industries
Fundamentals of Fluidized-Bed Chemical Processes
Diffusion in Liquids
Introduction to Electrode Materials
Butterworths Monographs in Chemical Engineering
Enlargement and Compaction of
Particulate Solids
Editor
Nayiand G. Stanley-Wood, PhD., BPharm., C.Eng., M.I. Chem. Eng., M.P.S.
Lecturer, Schools of Studies in Chemical Engineering
and Powder Technology
University of Bradford
Butterworths
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First published 1983
© Butterworth & Co (Publishers) Ltd 1983
British Library Cataloguing in Publication Data
Compaction of particulate solids.—(Butterworths
monographs in chemical engineering)
1. Compacting 2. Bulk solids
I. Stanley-Wood, Nayland G. II. Series
660.2'8429 TP156.C/
ISBN 0-408-10708-1
Filmset in Monophoto Times by Northumberland Press Ltd, Gateshead
Printed in Great Britain at the University Press, Cambridge
Preface
Sir Isaac Newton stated in his opus on Opticks', London 1721, that:
'The parts of all homogeneal bodies which fully touch one another stick together very
strongly ... There are therefore agents in nature able to make particles of bodies stick
together by very strong attractions and it is the business of experimental philosophy to find
them out
Since British Patent 9977 for the 'shapping of pills, lozenges and black
lead by pressure dies', which is accredited to Brockenden (1843), the ex
perimental philosophy, initiated by Newton to understand why particles can
form coherent bodies, has been assiduously investigated. The pressing of
particulate materials to produce a wide variety of blocks, pellets, tablets and
compacts is nowadays extensively employed in many industrial processes.
The demise of skilled craftsmen, who produced tablets and compacts by
empirical control methods, resulted in a need for the development of more
sophisticated technological measurement and control. The awareness and the
desire to be in the vanguard of such technological changes created a demand
for post-experience courses specializing in the methodology of compaction
and size enlargement. In 1979, 1980 and 1982 post-experience courses on the
Compaction of Particulate Solids were held at the University of Bradford
sponsored by the Institution of Chemical Engineers in their programme of
continuing education. The policy of the Continuing Educational Sub-
Committee is to encourage development of such courses to meet the
continuing educational needs of the chemical engineering profession and
associated professions and industries. The courses which were attended by
post-experience delegates from all over Europe, were divided into the follow
ing topics: Fundamentals of Powder and Compact Characterization; Compac
tion and Agglomeration Techniques; and Industrial Practice and Application
of Compaction and Densification. Because of the success of these courses it
was decided to adopt the same organization of subject matter for this book.
Each chapter has been contributed by an industrial or an academic worker
actively engaged in research and development or the industrial production of
compressed or size enlarged particulate materials. The subject matter included
in the book, characterization of powders and granules before and after
compaction, mixing, shear testing, fluidized-bed granulation, mechanisms of
size enlargement and compaction, and the instrumentation of industrial
vi Preface
presses and industrial processes cannot, however, claim to be completely
comprehensive in such a diversified and wide-ranging field as compaction.
This interdisciplinary approach to the densification of materials, however,
draws upon chemical engineers, physicists, powder and pharmaceutical
technologists, ceramacists and metallurgists in an attempt to achieve a cross
fertilization of knowledge and 'know how' between various industries.
I should like to express my thanks and gratitude to all the lecturers who
participated in the post-experience courses and contributed to this book and
for the time and effort taken to write their chapter. Without such endeavour
this book would not have been accomplished.
Nayland Stanley-Wood
Contributors
J. J. BENBOW
Formerly, Imperial Chemical Industries pic, Agricultural Division, Billing-
ham, Cleveland
I. K. BLOOR
The British Ceramic Research Association, Queens Road, Penkhull, Stoke-
on-Trent
R. D. BRETT
The British Ceramic Research Association, Queens Road, Penkhull, Stoke-
on-Trent
N. HARNBY
Schools of Studies in Chemical Engineering and Powder Technology, Univer
sity of Bradford, Bradford
B. HUNTER
Imperial Chemical Industries pic, Pharmaceutical Division, Macclesfield,
Cheshire
D. E. LLOYD
The British Ceramic Research Association, Queens Road, Penkhull, Stoke-
on-Trent
P. J. LLOYD
Department of Chemical Engineering, Loughborough University of Tech
nology, Loughborough
H. M. MACLEOD
United Kingdom Atomic Energy Authority, Windscale Nuclear Power
Development Laboratories, Seascale, Cumbria
A. W. NIENOW
Department of Chemical Engineering, University of Birmingham, Birming
ham
H. S. THACKER
Manesty Machines Limited, Speke, Liverpool
J. C. WILLIAMS
School of Studies in Powder Technology, University of Bradford, Bradford
CHAPTER 1
Particle characterization by size, shape
and surface for individual particles
N. G. Stanley-Wood
Schools of Studies in Chemical Engineering and Powder Technology, University
of Bradford, Bradford
1.1 Scope
In the compaction of powders in a rigid mould to produce a coherent mass
the fabrication process can be considered to occur in three stages.
1. The powder or assembly of particles must flow or be fed to the mould.
2. On application of a compressive pressure the particles are forced into
intimate contact and areas of contact are formed between particles to
produce bonds.
3. Ejection of the compacted material and the stress release in the com
pact to produce a coherent compact is dependent upon the resultant
area of contact, fracture or deformation of the particles in addition to
the shape and dimensions of the discontinuities between particles.
In all three stages outlined above information on the size, shape, surface
and porosity of the material before and after compaction is required to
aid characterization of compaction as well as to quantify the degree and
properties of the compaction process and compacted material.
This chapter deals with the measurement of size and shape of individual
particles or collections of individual particles, both spherical and non-
spherical. Because the individual size and shape of particles, after compress
ion, is altered and difficulties arise in the distinction of bonded particles in
terms of size and shape, the alternative parameter of particle surface area
may be used to characterize the properties of compacts or particles in
contact with each other. This surface characterization can be achieved by
measurement of the amount of gas adsorbed on to the unbonded solid
surface by low temperature adsorption or mercury penetration techniques.
Since some materials are porous the adsorption and penetration methods
described can be adapted and adopted to measure the internal porosity of
compressed and non-compressed solids. Applications of these surface topo
graphical characterization methods to compacted material are discussed and
appraised in Chapter 2.
2 Individual particle characterization by size, shape and surface
1.2 Characterization of individual particles
1.2.1 Particle size
The term powder particle usually refers to an object which has a physical
boundary regardless of the finite size. The upper limit of an individual
particle is therefore difficult to define because a particle is a discrete portion
of matter which is small in relation to the space in which it is considered.
Thus planets, boulders and electrons can be encompassed within this spe
cification. For practical purposes however an arbitrary definition of the size
of powder particles has been chosen and, as stated in BS 2955, can be
defined as:
4 A powder shall consist of discrete particles of any material with a maxi
mum dimension of less than 1000 micrometre (= 1 mm)'
The lower limit is commonly taken, these days, to be in the sub-micrometre
to colloidal dimension (0.001 micrometre) range. The range normally en
countered in compaction technology is however 1000—10 micrometres.
However, with the advent of micronized comminution techniques and
vapour phase condensation, sizes in the nanometre range are being en
countered more and more.
Because no single method of particle measurement can be expected to be
applicable over the entire size spectrum many diverse techniques and in
struments are available for particle sizing and counting. The choice of
method for particle characterization is therefore dependent upon the use for
which collected information on size or shape is intended. Since in particle
characterization the physical dimensions of regular and irregular particles
are required, the methods and instruments for sizing and counting have
been classified on a physical criterion.
Table 1.1
Physical criterion Diameter measured Size range applicability/μπ\
1. Image Projected area d> Optical 0.8—800
Electron 0.002—15
2. Mechanical Sieve diameter d Dry 40—1000
x
Wet 1—40
3. Dynamic Free-fall diameter d Gravity 1—100
{
Drag diameter d Centrifugal 0.05—25
d
Stokes diameter 4
4. Attenuation Projected area d> X-Ray 0.05—100
Stokes diameter d Centrifugal 0.05—50
%x
5. Scattering Surface volume 0.3—50
diameter
dSy
6. Electrical Volume diameter d 0.5—300
v
7. Surface Surface diameter d* Permeametry 0.1—50
(mean)
Surface volume d Adsorption 0.005—50
sv
diameter (mean)
Characterization of individual particles 3
Table 1.2. Equivalent measurements
Key Description Symbol
i A circle having the same projected area as the K
profile of a particle
11 The width of a minimum square aperture through d
t
which a particle will pass
111 A sphere having the same volume as the particle d,
IV A sphere having the same surface area as the particle ds
V A sphere having the same velocity of fall in a fluid ds,
if in the Stokes region
vi A sphere having the same ratio of surface area to d
sv
Table 1.1 shows seven arbitrary classifications and the diameters measured
by these physical methods. In addition Table 1.1 shows the size range of
applicability for each method to aid the scientist or technologist in the
selection of the appropriate and desired measurement technique.
An irregularly shaped particle has no unique dimension and its size may
be expressed in terms of the diameter of a circle or of a sphere which is
equivalent to the particle in some stated property.
The more common equivalents are shown in Table 1.2. All these equi
valent diameters differ numerically except for a spherical particle. The particle
size measured, except for spherical particles, depends to some extent upon
the physical principles employed in the measurement process.
1.2.2 Image-forming instruments and techniques
Measurements by microscope are not solely limited to the sizing of solid
particles but can also be used to size oil or liquid globules in emulsions or
metal inclusions in alloys. Visual measurement of particles is recommended
in many instances because it is the only absolute method of sizing. Since
individual particles are measured it is at times used as the basic and stan
dardization method for many other indirect techniques. In addition to the
parameter of size, particles seen with either an optical or an electron micro
scope can be characterized by shape or surface topography. Direct measure
ment in the microscope range can be achieved either with a low power
microscope for particles between 1000 and 100 micrometre or with a high
power, good resolution microscope, for particles between 100 and 0.2 mi
crometre. The British Standard 34061 does however recommend that the
minimum size of particles measured with an optical microscope should be
limited to 0.8 μιη. If there are only a few and regularly shaped particles to
be measured and counted then a glass eyepiece disc engraved with a fine
line divided into equal parts2 can be used. This must be calibrated against a
stage or slide micrometer3 to give the real size of the particles under investi
gation. Instead of measuring the uniaxial linear dimensions of particles in
terms of the number of equally spaced divisions, irregularly shaped particles
can be compared with circles or discs, sometimes called globes, on an
4 Individual particle characterization by size, shape and surface
eyepiece graticule. This method classifies particles into size groups rather
than measuring each particle. In this way many more particles can be
sized and the labour required for measuring particles and determining part
icle size distributions reduced. The size of the circles or globes on the
graticules are calibrated in the same way as the linear eyepiece scale by using
a stage micrometer. Numerous types of ocular graticule have been prod
uced for specialized sizing4-6, but the graticules in general use are either the
Fairs' graticule7'8 in which each circle, and therefore group, increases in size
by a root two progression or the Integrating discs I, II, III or IV graticules9
which have 25, 100, 400 or 9000 equidistant graduations per inscribed
square respectively.
Manual or semiautomatic sizing and counting aids are available. The
size of particles can be measured directly from eyepiece images or by being
projected on to a screen as with a projection microscope3,10,11. The images
from a photomicrograph and a 35 mm slide can be sized and counted by
using either a Zeiss-Endter Particle Size Analyser12, or a Chatfield Particle
Size Analyser13.
With all the microscope instruments so far discussed the particle image
must be seen with the human eye and the size of the particle either meas
ured or placed into an appropriate size group which is subsequently
counted. To remove the fatigue of peering down a microscope or at a
projected image, instruments were produced in which the presence and size
of particles were detected with a scanning light spot14,15 or slit16. These
instruments have now however been superseded by image analysers which
use the electronic scanning beam of a television camera. One of the first
particle size analysers using the scanning spot of a television screen was the
Metal Research Particle Analyser17.
It is now possible with hardware computer modules, as seen with the
Quantimet 72018, to obtain not only a statistical diameter, a count and a
distribution, but also a distribution by area, projected length and chord
size. Many other mathematical particle functions such as particle perimeters
and shapes can now also be rapidly evaluated.
An alternative method, still using the scanning beam of a television
camera, is to process and data reduce the Videosignal by software computer
analysis. One of the first generation software analysis image analysers, the π
MC, was developed by Millipore19 in conjunction with Bausch and Lomb.
Bausch and Lomb have recently introduced the second generation software
image analysers termed the Omnicon20 which has a more powerful software
programme and versatility.
In size analysis of sub-micrometre particles, electron microscopy was,
until light scattering instruments became available, the only method of
characterization. With an accelerating voltage of 60 kV the wavelength of
radiation corresponds to 0.005 nm and gives a resolution of about 2.5 nm.
As the order of accelerating voltage increases to 105 V then the resolution
power of a transmission electron microscope (TEM) can increase to a re
solution value of 0.1—0.4 nm.
