Cascade Separation of Powders

479 Pages·2006·8.342 MB·English

by  E Barsky, M Barsky

Michael Barsky was born in 1936. He graduated in Mechanical C
Engineering from the Ural State Technical University
a
of Katerinburg, Russia in 1960. He received his Ph.D.
s
degree in 1964 and D.Sc. degree in 1971. In 1973 he c
was appointed the full professor. In 1990 he joined a
the staff of Ben-Gurion University of the Negev, Beer- d
Sheva, Israel. Professor Barsky’s scientific interests lie e
in mass processes, separation of free-flowing materials in air and
s
gaseous streams, dynamics of two-phase flows in critical regimes
e
and physical foundations of flows of this type. Professor Barsky is
p
the author of three books and of more than 200 scientific papers.
a
r
Eugene Barsky was born in 1974. He graduated in 1993 from a
Ben-Gurion University, Beer-Sheva, Israel, with a B.Sc. t
i
degree in mathematics. Thereafter, he received his
o
M.Sc. degree in 1998 and Ph.D. degree in 2001 in
n
Industrial Mathematics. In 2002, he became a staff Cascade

member of the Negev Academic College of Engineering, o
Beer-Sheva, Israel. His scientific interests lie in the f

mathematical modelling of technological processes, optimisation p
separation
and combinatorics. Dr. Barsky has published 15 articles. o
w
d
of powders
e
r
s
E. Barsky and
Cambridge International Science Publishing Ltd.
E M. Barsky
7 Meadow Walk, Great Abington
.
Cambridge CB1 6AZ MB
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wwwU.cnisitpe-dp uKbilnisghdionmg.com . Brs
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ry
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ISBN 1904602002 ka
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Cambridge International Science
Publishing Ltd.
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E. Barsky and M. Barsky
CAMBRIDGE INTERNATIONAL SCIENCE PUBLISHING
iii
Published by
Cambridge International Science Publishing
7 Meadow Walk, Great Abington, Cambridge CB1 6AZ, UK
http://www.cisp-publishing.com
First published 2006
© E Barsky and M Barsky
© Cambridge International Science Publishing
Conditions of sale
All rights reserved. No part of this publication may be reproduced or
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in any form or by any means, electronic or mechanical, including photocopy,
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British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British
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ISBN 1-904602-002
Cover design Terry Callanan
Printed and bound in the UK by Lightning Source Ltd
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Industry imposes stringent requirements on the quality of powder
materials used in many areas of technology. To satisfy these
requirements, it is necessary to overcome technical problems and
find solutions of the problems, in most cases by the application of
highly efficient separation processes.
The following aims are followed in the fractionation of powder
materials:
– the production of dust-free products in relation to the given boundary
grain size. Small amounts of the fine classes are permitted in these
products.
– the production of pure fine products as a result of the removal
of coarse particles. This task is inverse to the first task. The apparatus
facilities in the process greatly differ from the devices used in
the first task. Usually, the first task is realised with a loss of part
of the course product together with dust, and the second task with
the loss of the final product with the coarse particles. In most
cases, these losses are large. The situation has been aggravatedby
the absence of highly efficient separation systems:
– separation of bulk (loose) material on the basis of the density of
particles irrespective of particle size;
– the separation of the polydispersed material with a wide range
of the grain size composition into fractions with a narrower range
of the grain size;
– the production of powders whose grain size characteristic is specified
in advance for the entire grain size range of the initial material.
At present, the fractionation of materials in technology is carried
out using different systems and different classification methods. These
methods may be divided into the following groups:
– screening and vibroseparation on flat surfaces (perforated and smooth)
– Hydraulic classification in a moving or stationary liquid medium;
– dry classification in gas flows.
In most cases, fractionation carried out by screening methods. In
this case, classification sections are represented by different technological
lines with a large number of hoppers, feeders and conveyeres. In most
cases, they do not ensure that the required efficiency of the process.
The main shortcoming of screening is that its separating capacity
v
greatly decreases when the boundary grain size of separation approaches
1 mm and practically approaches zero in the regions of separation
in respect of classes finer than 0.5 mm which are mostly characteristic
of modern industrial technology. The powders of this grain size can
be separated most efficiently in moving flows.
At present, the methods of hydraulic classification are used most
widely. These methods have been developed and studied for a long
time and the available experience creates favourable conditions for
extensive application of the methods. However, the application of
hydraulic classification results in difficult-to-solve technological
problems. The main problems are associated with the disruption of
the principles of environmental protection and also with a high
consumption of water which causes considerable difficulties. In addition
to this, a relatively large number of materials can be separated by
the wet method owing to the fact that in wetting they changed their
physical properties or bonder together.
It should also be mentioned that the technology using hydraulic
method of fractionation is characterised by high energy consumption
because after the separation operations the powders must be often
dried because further processing (dosing, mixing, shaping, etc.) is
possible only in the dehydrated condition.
Dry separation methods are more efficient. These methods are realised
in most cases in equipment with air flows or, if necessary, flows of
inert, flue and other gases. Their efficiency is indicated by the current
tendency of transition to the dry methods of production.
Therefore, without reducing the significance of the hydraulic
classification methods and discussing methods of improving them,
in the book special attention is paid to the analysis of the results
of investigations and main relationships of the dry fractionation methods.
It should be mentioned that there is no principal difference in the
physics of the process of hydraulic and pneumatic classification.
Naturally, the difference in the density and viscosity of water and
air assumes different orders of the rates of the process and this is
reflected in the design features of separation equipment. However,
in both cases, the process is based on the ratio of the forces of natural
or artificial, for example centrifugal, gravity of particles to the value
of their hydrodynamic resistance in a moving medium. Regardless
of the design of equipment and the separation medium, only this factor
predetermines the nature of phenomena taking place during fractionation.
The presence in the Arsenal of modern technology of sufficiently
reliable dust cleaning systems and also the possibility of carrying
out separation in a closed cycle create favourable conditions for the
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extensive application of air classification methods.
Until recently, it was generally recognised that pneumatic
classification does not result in acceptable efficiency of the process,
and equipment used for this method should be very large. These
assumptions are basically confirmed only for the conventional methods
of organisation of the process based on the principle of equalisation
by the gas flows of the particles of the boundary separation size.
At the same time, the possibilities of pneumatic classification are
not exhausted only by this principle. In the examination of the
mechanism of separation of bulk materials in the flows, it has become
possible to use this method with high efficiency.
The role of the processes of separation of bulk materials increases
at the present time owing to the fact that, firstly, the requirements
on the quality of powders and intermediate products continuously
increase and, secondly, because of the increase of the volume of
production larger and larger quantities of low-quality starting material
are used in processing.
It should be mentioned that, regardless of the extensive application
of classification systems used for the separation of bulk materials,
no significant advances have been made in the design of these systems
with the exception of, possibly, the construction of cascade separation
systems in the last couple of decades.
The main but not only reason explaining the given situation is
that no accurate methods of comparison of the separation capacity
of the classification systems and qualitative parameters of the process
have been developed. This prevents the effective definition of advanced
design of separation systems and suppresses the tendency in the
development of this group of systems.
Work on the development of the criteria of quality for the evaluation
of the separation processes started at the beginning of the 20th century.
However, in addition to correct concepts, these developments have
been based erroneous concents which prevented a solution of the given
problem. More than 100 years have passed since the publication of
Hancock’s studies in which the generalised quality criteron was
formulated for the first time. In this period, approximately 100 different
dependences for expressing the efficiency of classification have been
proposed. The large number of the criterial methods, the absence
of unity in the problem of the method of evaluation and optimisation
of separation have resulted in an uncertain situation in the selection
of classification systems and evaluation of the quality of their operation.
Therefore, in the majority of cases the design of new production
processes and optimisation of existing equipment have not been carried
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out on a strictly scientific basis but on the basis of experience obtained
in the service of related equipment, intuition and the ‘courage’ of
designers.
Attempts have been made to systematise the entire range of the
methods of optimisation of separation. However, investigators could
not link clearly the criteria parameters of the process with its physical
nature and, in the majority of cases, they did not even formulate this
task.
In the middle of the 30s of the previous century, methods of
optimisation of separation, based on the Tromp curve, were introduced
in enrichment practice. The curve was used for formulating a group
of parameters which, however, also have significant shortcomings.
The investigations carried out in recent years have shown that the
methods of objective and unambiguous evaluation of the classification
processes must be based on the relationship between the separating
capacity of the system and the physical fundamentals of the investigated
processes.
Recently, it has become necessary to develop new methods of
organisation of separation of powders. They include the separation
of powders in a single system into more than two products, and the
production of bulk media with the defined grain size characteristic.
Multiproduct separation differs principally from two-product separation
in both the methods of physical organisation and the methods of
evaluating the quality of realisation.
New concepts are also being proposed in the solution of the problem
of the special purity two-product separation of powders. The main
concept is based on the application of combined classification schemes.
All these problems are reflected in the book in which special attention
is given to the examination of cascade principles of organisation of
separation.
It should be mentioned in particular that the main relationships
obtained for the cascade processes are of the phenomenological nature.
They may be used for various cascade processes, such as rectification,
extraction, isotope separation, cascade drying of bulk materials, etc.
In the book, the authors generalise the results of experimental and
theoretical investigations carried out by them in Russia (Ural
Polytechnical Institute, Department of Silicate Technology) and in
Israel (Institute for Applied Research and Department of Mathematics
of the Ben Gurion University of the Negev and Department of Industrial
engineering of the Sami Shamoon College of Engineering).
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CONTENTS
Preface v
Chapter 1. GRAIN SIZE COMPOSITION OF BULK
MATERIALS 1
1. Methods of determination of the particle size 1
2. Size distribution of particles 5
Chapter 2. METHODS OF OPTIMISATION
OF THE SEPARATION OF BINARY MIXTURES 13
1. Determination of the efficiency of separation 13
2. Simplified optimisation indicators 19
3. Unique indicators of the process 25
4. Analysis of the criteria of quality of separation
processes, differing from the Hancock method 30
5. Analysis of the applicability of the
Hancock dependence in cases of changes
in the composition of the initial product 41
6. Methods of direct optimisation of separation
processes 44
7. Fraction separation curves 52
8. Relationship between separation curves and the
quantitative indicators of the classification
process 61
9. The quantitative criterion of quality
based on separation curves 67
Chapter 3. PHYSICAL FUNDAMENTALS OF THE PROCESS
OF SEPARATION OF BULK MATERIALS IN MOVING
FLOWS 76
1. The general characteristic of the current state
of theory 76
2. Special features of the movement of continuous
flows 84
3. Settling and hovering of single particles 98
4. Special features of the formation of the two-phase
flow in the separation conditions 123
CHAPTER 4. STATISTICAL FUNDAMENTALS OF THE PROCESS 137
1. Justification of the statistical approach 137
2. Numerical evaluation of the state of the statistical
system 141
3. Main statistical characteristics of the
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