Recent Advances in Liquid–Liquid Extraction

585 Pages·1971·8.992 MB·English

by  C. Hanson (Eds.)

Schlieren Photography Showing Mass Transfer From A Drop In
The Turbulent Regime — Photograph by courtesy of Dr. H. Sawis-
towski.
RECENT ADVANCES IN
LIQUID-LIQUID EXTRACTION
EDITED BY
C. HANSON
University of Bradford, U.K.
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First edition 1971
Library of Congress Catalog Card No. 70-127322
Printed in Hungary
08 015682 7
Preface
LIQUID extraction has been recognized for many years as a powerful
separation technique in the chemistry laboratory. However, its applica-
tion on an industrial scale has been much more limited and it appears
only to have been considered when more conventional methods such as
distillation have failed. It is difficult to know the extent to which this
contrast between laboratory and industrial use should be ascribed to
conservatism, to lack of basic knowledge on the technology of liquid
extraction or to fundamental limitations of the technique itself. The
spectacular successes of liquid extraction in the nuclear industry and in
the manufacture of antibiotics, to name two examples which illustrate
the possible diversity of the technique, have stimulated a much closer
examination of the subject during the last decade, both from a fundamen-
tal aspect and a consideration of possible industrial applications. On the
whole, the possible fruits of this research have still to be seen.
The basic principles of liquid extraction were developed many years
ago and are admirably covered in standard texts on the subject such as
that of Professor R. E. Treybal. Recent research has done nothing to
change these principles or reduce the value of such texts. When approach-
ed with the suggestion of preparing a new book on extraction, I could
see no justification for another basic text, which would inevitably du-
plicate much of what is already available. While not detracting from the
value of these basic works, the intense research of the last decade has
greatly enlarged our knowledge of certain fundamental aspects as well
as leading to major developments in the technology of extraction. Publi-
cation of these results has been by diverse means in all parts of the world
(some have not even been published) and the time did appear to me
opportune for a review of the important developments which have taken
place since the appearance of the established texts on the subject. The
result is presented here. It is not intended to replace the standard texts,
but rather to complement them. I hope it will do something to stimulate
further use of the technique and exploitation of the results of the research
undertaken during the last decade.
With a subject as diverse and interdisciplinary as solvent extraction,
xiii
xiv Preface
it is difficult if not impossible for one person to remain abreast of all
developments. It was therefore decided to restrict the scope of this vol-
ume to selected topics in which significant recent developments have
been made and to seek individual expert authorship for each of these.
Coverage has been limited to liquid-liquid extraction. Many advances
have been made in the allied field of solid-liquid extraction—both ion
exchange and leaching—but these merit separate treatment.
The successful development of new processes demands an interaction
between chemists and chemical engineers and this is nowhere more vital
than in solvent extraction. The last few years have seen, in my opinion, an
unfortunate tendency for relatively little contact between the two dis-
ciplines. A similar divergence is apparent between academic and industrial
research. In considering the contents and authorship for this volume,
I have tried to integrate these different interests in the hope of producing
an interaction which will enhance the value of the whole. Thus research
on the chemistry of solvent extraction is obviously basic to any future
expansion in use of the technique. One expects to find the expertise for
such a subject in the universities. The sequel to research into the basic
chemistry should be new processes. Since economic and technological
considerations become paramount at this stage, one must turn to in-
dustry for an expert assessment. Having developed a new process, the
natural question to ask is what type of equipment it would be best to use:
again, the field of the industrial expert. Once on a plant scale, the process
has to be controlled, an aspect which has undergone rapid change with
the advent of on-line computers. In addition to control, the design engi-
neer must calculate the number of stages required to effect the desired
separation. Research over recent years has demonstrated the vital im-
portance of backmixing in limiting the performance of liquid-liquid
contactors. Workers in both industry and the universities have tackled
this problem to evolve design methods allowing for the phenomenon and
also types of contactor in which it is eliminated.
Research in universities over the last decade has greatly advanced our
knowledge of the basic transfer processes taking place in liquid extrac-
tion, both in the presence and absence of simultaneous heat transfer or
chemical reaction. Direct liquid-liquid contact has also been recognized
as highly efficient for heat transfer. All these processes demand intimate
interdispersion of the two phases. Upon their conclusion, however, the
phases must be separated again and the difficulties experienced by in-
dustry with this final stage has stimulated extensive research, primarily
in the academic world, on drop coalescence. It was with these consider-
Preface xv
ations in mind that I chose the subjects to be covered in this volume and
decided whether to seek the authors from industry or the universities.
As Editor, I would like to record my appreciation for the ready co-
operation I have received from both the individual authors and the pub-
lishers, which has greatly eased my task. This has been performed partly
here in Bradford and partly in the U.S.A. at Purdue University, where
I was fortunate to spend a delightful year. My thanks are due to the
chairmen of the Schools of Chemical Engineering at both Universities
for their encouragement and help.
University of Bradford, C. HANSON
Bradford 7,
U.K.
CHAPTER 1
Solvent Extraction: the Current Position
C. HANSON
University of Bradford, U.K.
Classification of Extraction Processes
Separation of the constituents of a homogeneous mixture is one of
the most frequently met problems in chemical technology. It is invariably
solved by the creation or introduction of a second phase, immiscible or
only partially miscible with the original. For this to effect a separation,
the various components must have different distribution coefficients be-
tween the two phases. Liquid extraction is used to separate constituents
of homogeneous liquid solutions. It involves the addition of a second
liquid solvent, which is immiscible or only partially miscible with the
first, and distribution of the components of the mixture between the two
phases. For such distribution to give a degree of separation, the ratio of
the concentration dependence of the chemical potentials of the two key
components must be different in the two phases since, at equilibrium,
the chemical potential of a given component is the same in each phase,
assuming the temperature and pressure to be constant. Two broad cate-
gories of extraction system can be distinguished depending on the origins
of this differential solubility.
In the first, it arises from purely physical differences such as polarity.
Many separations in the organic field belong to this category. As would
be expected of a process depending only on the physical effects of differ-
ences in molecular structure, such separations are rarely very specific
and the separation factors obtained are usually only modest. In the
second category, the differential solubility is due to one of the solutes
interacting chemically with the solvent to form a complex. This is exploit-
ed for many metallurgical separations.
The distinction between the two categories is significant in influencing
both the physical changes which occur during extraction and the method
1
2 Recent Advances in Liquid-Liquid Extraction
of solvent recovery. In systems where the differential solubility arises
purely from physical factors, the relative miscibility of the two phases
will usually be a function of solute concentration. On the other hand,
when definite chemical compound formation is involved, the mutual
solubilities of the two solvents often do not vary significantly and so
the flow rates of the solvents, calculated on a solute-free basis, do not
change in passing through an extractor. This simplifies both design cal-
culations and the mathematical representation of the overall process,
which can be important for considerations of control.
When an extraction process depends upon chemical interaction, sub-
sequent solute-solvent separation demands reversal of the complex form-
ing reaction, usually by chemical means. In the case of systems exploiting
only physical forces, solute-solvent separation is achieved by physical
means, usually distillation. Thus the final solute-solvent separation stage
may introduce either a chemical or a thermal cost item into the overall
economics of a process. As will be seen later, this stage is one which
repays careful consideration when solvent extraction is first contemplated
as it can easily hold the key to the economic success or otherwise of a
process.
Inorganic Separations
The development of the nuclear industry gave tremendous impetus to
research on complex formation between metal salts and organic solvents.
Such solvents can be divided into three broad groups according to whether
their action is by cation exchange (as with carboxylic acids), anion
exchange (as with amines) or adduct formation (typified by ethers and
neutral organophosphorus compounds). Recent developments in the
chemistry of such systems are reported in Chapter 2 and current industrial
applications in Chapter 3. All involve chemical interaction between the
solute and solvent.
It has been suggested that solvent extraction will be extensively used
in the future for most metals in the periodic table with a value down to
that of copper. Certainly copper appears to be both a challenge and a
turning point for solvent extraction. Several solvents have appeared to
show the selectivity and stability necessary, only to have these hopes
dashed by economic considerations, usually the cost of chemical reagents
necessary for conditioning and for solvent recovery. However, it only
seems a matter of time before these problems will be solved and it has
been suggested already that the solvent LIX 64 offers an economic
process for the recovery of copper from leach liquors.(1)
Solvent Extraction: The Current Position 3
After copper, the separation and purification of cobalt and nickel
appear a natural goal for solvent extraction. The specificity and selectivity
of solvent extraction also make it an obvious method to adopt for high
purity metals. In view of its commercial success for separation of the rare
earths, its extension to the platinum metals seems only a matter of time.
Thus tri-n-butyl phosphate already has been suggested for this purpose(2)
and also for the extraction of palladium.(3)
The conventional approach to a separation problem is to seek a solvent
which will extract in a pure state the desired solute. When the latter is
already present at fair concentration, the alternative approach of seeking
a solvent which will extract the impurities should not be neglected as it can
offer economic advantages. This will be discussed later.
In looking to the future, the possible use of solvent extraction for the
recovery of acids and metal values from effluents should not be forgotten,
particularly with the passage of tighter legislation on the discharge of
effluents in many parts of the world. It can also be used for concentration,
having obvious attractions for systems forming azeotropes or having low
thermal stability. It has been suggested in this context for hydrogen
fluoride.(4)
Organic Separations
The organic chemical industry saw the earliest large-scale applications
of solvent extraction: in the coal-tar field and for the separation of aro-
matics from aliphatics in the petroleum industry. On the other hand,
its use in the organic field does not appear to have expanded as dramati-
cally as in the inorganic. This may be an erroneous impression and the
use of solvent extraction, if measured in terms of tonnages processed, is
certainly greatest on the organic side.
Whilst there are important exceptions, many separations in the organic
field exploit physical rather than chemical interaction between the solute
and solvent. Developments in physical chemistry have greatly increased
our knowledge of these interactions so that we are much better placed
now than in the past to select solvents scientifically instead of by trial
and error.(5)
Solvent extraction processes in the organic field which have reached
industrial application during the last few years are described in Chapter 4.
These show a surprising variety, illustrating the versatility of the tech-
nique. Further extension of its use will obviously depend largely on the
development of new processes.
One field in which solvent extraction may well find considerable future
4 Recent Advances in Liquid-Liquid Extraction
use is the separation of isomeric compounds. Distillation is the method
most commonly employed at the moment. However, the small structural
differences between isomers rarely produce an appreciable relative vola-
tility so that separation by distillation is difficult and expensive. These
same structural differences can be exploited for effecting a separation by
extraction. In the first place, they usually result in the isomers having
different polarities so that their relative distributions between polar and
non-polar solvents vary. Thus Hanson and Patel(6) have shown that a
separation factor of 1-8 can be obtained for o- and ^-chloronitrobenzene
distributed between n-heptane and aqueous methanol (10% water). Sec-
ondly, if the isomers contain an acidic or basic group, the dissociation
constant for this group will depend on its position relative to other
substituents. This is exploited in dissociation extraction.
Details are given in Chapter 4 of the industrial use of dissociation
extraction for separation of the phenol homologues. Laboratory work(7)
has shown that an exactly analogous procedure can be used for aromatic
bases, e.g. to separate 3- and 4-picoline, o- and /?-toluidine, and o- from
/7-chloroaniline. The first comprehensive theoretical treatment of disso-
ciation extraction was presented by Wise and Williams.(8) However, this
has a number of deficiencies. pH is indicated as a critical parameter.
In actual fact, control is best based on the amount of mineral acid or
alkali present in the aqueous phase since the method depends upon there
being a stoichiometric deficiency compared with the total isomers present.
There is then competition between the isomers for the available acid or
alkali, leading to a concentration in the aqueous phase of the isomer
with the greater dissociation constant. The aqueous phase will always
contain isomer stoichiometric to the acid or alkali present. The separa-
tion is really based on an exchange process, the isomers exchanging
between the two phases, rather than normal solvent extraction principles.
For this reason, reflux plays a different role in dissociation extraction from
solvent extraction.
Equipment
The multistage counter-current contacting of two immiscible liquids
is an intriguing problem which has attracted the attention of many
workers. In consequence, the number of contactors which have been
described in the literature is disproportionately large in comparison with
the current industrial use of solvent extraction. The emphasis in Chapter 5
is on those designs which have withstood the test of adoption for use on
an industrial scale.