Rare Metal Extraction by Chemical Engineering Techniques

379 Pages·1963·26.201 MB·English

by  W.D. Jamrack

OTHER TITLES IN THE SERIES ON
CHEMICAL ENGINEERING
Vol. 1. WILKINSON—Non-Newtonian Fluids
RARE METAL
EXTRACTION
by Chemical Engineering
Techniques
by
W. D. JAMRACK, B.Sc, A.R.I.C.
Formerly Research Manager at U.K.A.E.Â.
Springfields, Lancashire
PERGAMON PRESS
OXFORD LONDON · NEW YORK · PARIS
1963
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Set in Times No. 327, 10 on 12 pt. and printed in Great Britain at
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PREFACE
THE aim of this book is to describe processes which are applicable to the
extraction and purification of rare metals and to illustrate by fairly detailed
examples from the metals uranium, thorium, zirconium, hafnium, titanium,
niobium, tantalum, beryllium and vanadium. The techniques are not neces­
sarily limited to this range of metals, and it is indeed the author's hope that
this work may stimulate thought on their application over a wider field.
The chemical engineering approach is adopted, in that rare metal ex­
traction and purification is divided into a number of "unit processes", each
of which is discussed individually in a separate chapter. Much of the sub­
ject matter has been taken from those branches of extraction metallurgy
and chemistry which can contribute, since in many places they are closely
linked with chemical engineering. Further metallurgical processing, after
the initial production of the metal in ingot, powder or sponge form, is not
included, since this is not typically the province o fthe chemical engineer. In
addition to the examples in each chapter, demonstrating the application of
individual unit processes to particular metals, a selection of complete flow­
sheets has been included in the last chapter. These flowsheets are typical of
modern processes, but their number is by no means comprehensive. Others
can be pieced together to suit individual circumstances, from the unit pro­
cess examples quoted, or from the fairly extensive lists of references. The
reference titles are given in full so that they may also be used as a biblio­
graphy, to aid a wider or more detailed study of any particular topic.
Besides published research papers, review articles and works of reference,
material for a volume of this type normally arises from an author's own
background and experience. In this connection, I wish to acknowledge the
work of those of my colleagues and associates in the U.K. Atomic Energy
Authority whose work has contributed to the design of various Authority
processes which have been discussed. I also wish to thank those outside the
Authority, concerned with rare metal extraction, whose factories I have had
the privilege of visiting.
Thanks are due to Mr. J. C. C. Stewart, the Managing Director of the
Production Group, for permission to publish U.K.A.E.A. material. I am
also grateful to the librarians, information officers, and photographers of
the Springfields and Windscale Works; Mrs. B. Withrington, Mr. F. Cross,
Mr. E. Moore, Mr. G. R. Taylor and Mr. H. Stocks, for the supply of
published literature, U.K.A.E.A. reports and illustrations. Mrs. S. Strong
and Mrs. H. J. D. Coulter deserve special mention for their patient typing
of a none too legible manuscript.
W.D.J.
xi
CHAPTER 1
INTRODUCTION
THIS book is concerned with the extraction of a number of rare metals.
They do not fall into any well-defined chemical or metallurgical category,
but are selected because they have all progressed from being almost labora­
tory curiosities to the stage of industrial production on a moderate scale in
approximately the last two decades.
Two of the metals, uranium and thorium, have an obvious connection
with the atomic energy industry which has arisen during and since the
1939-45 war, and have no major uses, in metallic form, outside that field.
They are required mainly as fuels for nuclear reactors and their standards
of purity are probably greater than those of any other metals which have
been produced on a similar scale. This arises partly because trace impurities
such as silicon or iron might confer adverse metallurgical properties, but is
chiefly because of the necessity to exclude minute quantities of elements
with high neutron-capture "cross-sections". Specification limits for boron,
cadmium and some rare earth elements, for example, may be quite realisti­
cally fixed at a fraction of 1 ppm.
Other metals such as beryllium, hafnium, niobium, vanadium, and
zirconium are known to have nuclear and other properties which make
them desirable materials of construction in various designs of nuclear
reactor1, but also they have, or may have in the future, important uses
outside that field. All these metals except hafnium have been used or pro­
posed for "canning materials" to clad and protect the nuclear fuel metals
from corrosion by the reactor coolants or moderators, air, carbon dioxide,
water, heavy water, graphite or molten sodium, etc. In some cases the speci­
fications for neutron-absorbing impurities are of the same order as for the
fuel metals uranium and thorium. Hafnium, however, with a high neutron-
capture cross-section, is a useful material for reactor control rods and ex­
hibits favourable metallurgical properties under irradiation.
Titanium and tantalum are normally extracted for quite different pur­
poses. Titanium metal has a high strength-to-weight ratio, a good hot
strength up to 500 °C and will stand prolonged exposure to air at about this
temperature. Hence its value as a material of construction in gas turbines
and for certain parts of aircraft which are exposed to high temperatures.
1
2 INTRODUCTION
It has also been employed for the construction of items of chemical plants
where use can be made of its corrosion resistance towards hydrochloric
acid, sulphuric acid and other aqueous reagents. Tantalum also finds appli­
cations based upon its corrosion resistance towards nitric acid and other
acids. It is also used to some extent in surgery owing to it not being attacked
by body fluids.
The various chemical extraction stages from ore to metal are discussed :
some of the intermediates arising from these stages have major industrial
uses or potential uses of their own. Pure thorium oxide, for example, has
been employed for many years in the manufacture of incandescent gas
mantles and this is at present probably still the major outlet for thorium in
any form. It is anticipated that new uses will be found for most of these
metals or their pure intermediates in the near future and as a result many of
the process stages described will achieve a wider application.
The classical metallurgical processes of smelting the oxides with carbon
in the presence of a fusible slag, such as are used for the production of
many of the commoner metals, are not applicable to the range of rather
rare elements about which this book is written, if the metals are required in
pure condition. The latter are all fairly strongly electropositive elements
whose oxides are not readily reduced. In addition, in many cases, it is im­
portant to produce the metal from a halide rather than an oxide, and other­
wise exclude oxygen or air from the system, because of the deleterious effect
of traces of oxygen upon the metallurgical properties. The selection of a
particular pure compound of an element, together with a suitable reactant
which will allow the element to be produced in metallic form, is based upon
thermodynamic and chemical data.
Similarly, the production of the compounds in a pure condition is essen­
tially by means of a series of chemical reactions. First, use is made of the
differences in the chemical properties of the desired element and those of
any unwanted impurities, and then the desired compound is prepared by
reaction with a suitable chemical reagent or reagents. The stage of trans­
formation of the known chemistry from the laboratory to an industrial
scale process is accomplished by chemical engineering methods, and a
breakdown can be made into unit operations of the same general types as
are used throughout the whole chemical industry. Problems arise involving
materials of plant construction, both metallic and non-metallic, the flow of
materials, the relationship of one process to another and the economic
optimization of the processes, which are entirely typical of the traditional
chemical engineering field. This should not imply that rare metal extraction
processes are solely the preserve of the chemical engineer, but on the con­
trary it offers the suggestion that the typical chemical engineering approach
might prove useful to those metallurgists, chemists, or others who are
interested in the extraction of rare metals.
INTRODUCTION 3
The fundamental chemical engineering processes are essentially of general
application and can usually be discussed without reference to particular
elements, although admittedly much of the interest and progress arises
from the application of these processes to novel materials processed under
unorthodox conditions. Hence the division of this book into a series of
individual processes which can be applied to a range of metals, rather than
an arrangement in terms of specific metals. Numerous detailed examples
are included in the text for those interested only in particular metals rather
than in general appUcations however.
Mineral dressing techniques and the physical beneficiation of ore shave
not been covered in this volume, although they have appUcations in rare
metal extraction, since it is a large field with an adequate literature of its
own. Also, the novelty associated with the use of mineral dressing tech­
niques belongs generaUy to an earlier period than, for example, ion-
exchange, solvent extraction, or Kroll type reduction processes. An up-to-
date description of mineral dressing plant is given by Cremer and Davies.2
This book does not deal with chemical engineering principles but only
with those appUcations of the chemical engineering technique which can be
of use in a particular field. The aim is therefore to provide factual informa­
tion on existing types of process exemplified by a range of rare metals. It
is hoped that this might assist in guiding the design of other processes in­
volving similar problems in the future.
REFERENCES
1. MCINTOSH, A. B. Metallurgy of nuclear power production. The Engineer 200, 759
(1955).
2. CREMER, H. W. and DAVIES, T. Chemical Engineering Practice, vol. 3, Solid Systems.
Butterworths (1957).
CHAPTER 2
ORE BREAKDOWN PROCESSES
THE first metallic iron to be extracted was undoubtedly obtained by smelting
a high-grade ore consisting of a relatively pure iron oxide mineral. Most of
the other common metals which are extracted today in vast tonnages
throughout the world, e.g. copper, lead, zinc, antimony, tin and nickel, can
similarly be obtained directly by the smelting of high-grade mineral ores
with some form of carbon. A certain degree of physical beneficiation is how­
ever carried out in some cases, e.g. copper and tin, to remove inert gangue
material or other useful by-product minerals, before smelting. With some
of the more electropositive of the common metals, e.g. magnesium and
aluminium, it may even be necessary to convert a high-grade ore to a more
tractable compound by chemical means before production of the metal by
electrolysis.
Some of the rarer metals, e.g. beryllium, thorium, vanadium and zir­
conium, are electropositive in character and usually occur in nature as
fairly refractory minerals. Silicates and phosphates are common examples,
but the compounds concerned are often much more complex than this.
Consequently the ores cannot be broken by smelting processes since the
minerals are not always readily converted to oxide and the oxides in any
case cannot easily be reduced to the pure metals by carbon. It is in fact
necessary to separate the ore breakdown stage from subsequent metal pro­
duction, and in most cases a considerable degree of purification may be
necessary after breakdown and before the final conversion to metallic
form. Breakdown of these refractory ores is often accomplished by reaction
with powerful chemical reagents such as concentrated acids, alkalis,
fluorides, or gaseous chlorine at high temperature and specialized chemical
engineering plant is usually required. Since these reagents are reactive
towards other mineral impurities in the ore, it is important, for reasons of
reagent economy, to employ a reasonably high grade of ore, or possibly an
ore which has first been beneficiated by physical means. Fortunately, the
metals concerned have at present only limited application in industry and
it is usually still possible to mine them in relatively high grades. In some
cases, however, e.g. beryllium, this situation is unlikely to continue for
many years.
4
DILUTE ACID LEACHING 5
Other rare metals are available as low-grade ores, but in chemical forms
such as oxides which can sometimes be converted to soluble compounds
by leaching with dilute acids or alkalis. Uranium is the prime example of
this, but certain vanadium and thorium ores can also be treated by dilute
acid or alkali leaching processes. In the case of uranium particularly, the
type of process and equipment employed for leaching is becoming increas­
ingly complex and is amenable to the application of chemical engineering
techniques. Consequently, with improved processes, the grade of ore which
is regarded as workable on an economic basis is becoming extremely low
and is, at the present time, for uranium, often between 0·01 and 0·2 per
cent. The choice of "cut-off " grade of ore and selection of the reagent,
whether acid or alkali, depends to a high degree upon local conditions.
The relative cost and availability of the two classes of reagent, their relative
leaching efficiency with the particular ore, and the waste involved in the use
of reagents for the extraction of associated minerals from the ore, must all
be taken into account. The presence of high concentrations of limestone in
an ore, for example, inevitably leads to the use of an alkaline rather than
an acid reagent.
It is convenient to discuss dilute acid leaching first. Much of the equip­
ment and some of the chemical principles are common with dilute alkali
leaching. Other breakdown techniques are only employed when these
cheaper processes have been found unsuccessful.
DILUTE ACID LEACHING
Batch leaching
Many acid leaching processes are carried out as simple batch operations
in which measured quantities of acid, water, and ore are added to suitably
agitated vessels and contacted for an appropriate time under known con­
ditions of temperature and acid concentration, etc. The acid concentration
would normally be expected to fall throughout the process and relative
quantities would be such that the final acidity would be appropriate for
the subsequent treatment stages. In a process of this type, charging and
discharging operations generally require more labour than if they were
performed in a continuous manner and, in addition, provision has to be
made for intermediate storage of at least one batch before separation of
the liquid from the solid phase.
Continuous co-current leaching
It is frequently possible to design the leaching process on a continuous
co-current basis, where acid, water and ore are fed into a vessel continu­
ously, or at regular intervals, and discharged in a similar manner. In these
circumstances the agitation system is usually designed to suspend the solids
sufficiently in the liquid phase so as to allow them both to overflow from
2
6 ORE BREAKDOWN PROCESSES
the vessel in the same proportions as in the feed. It will be appreciated that
with single-vessel, co-current, continuous leaching of this type the chem­
istry of the system can be considerably different from that of a batch system
even with the same proportions of acid and ore. In co-current leaching,
for example, the acidity of the process is essentially controlled at the fairly
low value of the outgoing product solution, whereas a high range of acidity
might be present from start to finish of a batch process. This feature may
be a disadvantage if the rate of solution of the desired mineral decreases
markedly at low acid concentrations, since the residence time, and hence
the capacity of the leaching vessel, may become prohibitively large. How­
ever, in other cases a considerable gain in acid economy might be possible
if, for example, the desired component is freely soluble in acid of all con­
centrations above a threshold value, but if the solubility of unwanted
minerals, particularly the common siliceous ones, can be decreased by the
lower acidity. In this manner the whole chemical constitution of the leach
liquor might be changed, with advantage to the process.
Another feature of co-current leaching in a single vessel is always
deleterious : this arises from the fact that each portion of ore is not leached
for a constant time, but an infinite range of leaching times exists. This
range is distributed about a mean value T = V/v where V is the volume
9
capacity of the vessel and v is the volumetric flow rate through it. Clearly,
"by-passing" of a small proportion of the ore will take place, i.e. some
of it will pass directly from the feed to the overflow point in almost
zero time without appreciable leaching. A small proportion at the other
extreme will have a residence time tending to infinity. With an inefficient
system of agitation, the denser or coarser particles might in fact remain in
the vessel for an infinite period, or until the vessel "sanded up" to an
extent which necessitated cleaning out.
Assuming steady-state conditions and efficient agitation in a single co-
current leaching vessel, it is easily shown1 that the probability of a particle
remaining in the vessel after time t is equal to exp( — t/T).
The probability of any particle first remaining in the vessel for time t and
then leaving during an additional interval δ t is 1/Γβχρ( — tjT) St.
The function 1 jT exp( — tjT) is plotted as a continuous function in Fig. 2.1
(N = 1 for a single vessel) in units of the mean residence time T. The pro­
portion of material with a residence time between any two values is given
by the area under the curve lying between the two values. It is clear that a
high proportion of the ore is leached for periods which are widely different
from the mean residence time.
In order to achieve a continuous co-current system in which a higher
proportion of the ore is leached for a period near to the mean residence
time, several vessels may be used in cascade. The ore, acid and water are
fed to the first vessel as in the single-stage system but the overflowing slurry