U.S. patent application number 10/060580 was filed with the patent office on 2003-08-07 for method and apparatus for enhanced purification of high-purity metals.
This patent application is currently assigned to DOWA MINING CO., LTD.. Invention is credited to Hodozuka, Toshiaki, Tayama, Kishio.
Application Number | 20030145683 10/060580 |
Document ID | / |
Family ID | 29252201 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030145683 |
Kind Code |
A1 |
Tayama, Kishio ; et
al. |
August 7, 2003 |
Method and apparatus for enhanced purification of high-purity
metals
Abstract
A 99.99% pure indium feed is charged into crucible 8 and heated
to 1250.degree. C. by upper heater 6 in a vacuum atmosphere at
1.times.10.sup.-4 Torr, whereupon indium evaporates, condenses on
the inner surfaces of inner tube 3 and drips to be recovered into
liquid reservoir 9 in the lower part of tubular member 11 whereas
impurity elements having lower vapor pressure than indium stay
within crucible 8. The recovered indium mass in liquid reservoir 9
is heated to 1100.degree. C. by lower heater 7 and the resulting
vapors of impurity elements having higher vapor pressure than
indium pass through diffuser plates 12 in the upper part of tubular
member 11 to be discharged from the system whereas the indium vapor
recondenses upon contact with diffuser plates 12 and returns to
liquid reservoir 9, yielding 99.9999% pure indium while preventing
the loss of indium.
Inventors: |
Tayama, Kishio; (Akita-Shi,
JP) ; Hodozuka, Toshiaki; (Akita-Shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
DOWA MINING CO., LTD.
Tokyo
JP
|
Family ID: |
29252201 |
Appl. No.: |
10/060580 |
Filed: |
January 30, 2002 |
Current U.S.
Class: |
75/367 ; 266/208;
75/407 |
Current CPC
Class: |
Y10S 266/905 20130101;
C22B 9/02 20130101; Y02P 10/20 20151101; C22B 19/16 20130101; C22B
19/18 20130101; Y02P 10/234 20151101; C22B 58/00 20130101; C22B
17/06 20130101; C22B 9/04 20130101 |
Class at
Publication: |
75/367 ; 75/407;
266/208 |
International
Class: |
C22B 005/16 |
Claims
What is claimed is:
1. A method of enhanced purification of high-purity metals which
comprises purifying a metal feed by distillation in a vacuum
atmosphere to yield the desired metal with high purity, said method
further comprising a first thermal purification step in which said
metal feed in a feed crucible positioned in the upper interior of
an inner tube maintaining said vacuum atmosphere is heated and the
generated vapor of said desired metal is brought into contact with
the inner surface of said inner tube so that it is condensed and
recovered in a separate form from impurity elements that have lower
vapor pressure than said desired metal and which are allowed to
stay within said feed crucible, and a second thermal purification
step in which said desired metal as recovered is admitted into and
heated in a liquid reservoir in the lower part of a tubular member
positioned in the lower interior of said inner tube and the
generated vapor is passed through a diffuser positioned in the
upper part of said tubular member and guided by suction so that the
vapors of impurity elements having higher vapor pressure than said
desired metal are solidified in a separate form in a cooling trap
positioned below said tubular member and the vapor of said desired
metal is brought into contact with said diffuser so that it is
condensed and returned to said liquid reservoir.
2. The method according to claim 1, wherein said diffuser plate is
made of a carbonaceous material.
3. The method according to claim 1 or 2, wherein said liquid
reservoir is a recovery mold for casting said desired metal having
high purity after enhanced purification.
4. The method according to claim 1 or 2, wherein said desired metal
is indium, said metal feed is heated at 1100-1300.degree. C. in the
first thermal purification step and said desired metal as recovered
is heated at 900-1200.degree. C. in the second thermal purification
step.
5. The method according to claim 3, wherein said desired metal is
indium, said metal feed is heated at 1100-1300.degree. C. in the
first thermal purification step and said desired metal as recovered
is heated at 900-1200.degree. C. in the second thermal purification
step.
6. The method according to claim 1 or 2, wherein said desired metal
is at least one metal selected from the group consisting of
antimony, zinc, tellurium, magnesium, cadmium, bismuth and
silver.
7. The method according to claim 3, wherein said desired metal is
at least one metal selected from the group consisting of antimony,
zinc, tellurium, magnesium, cadmium, bismuth and silver.
8. An apparatus for enhanced purification of high-purity metals,
which comprises an inner tube in which a vacuum atmosphere is to be
formed, a first heating chamber provided in the upper interior of
said inner tube, a second heating chamber provided in the lower
interior of said inner tube, said first heating chamber
accommodating a feed crucible with an open top into which a metal
feed is charged and the desired metal in said metal feed is
evaporated for recovery while impurity elements having lower vapor
pressure than said desired metal are separated by being allowed to
stay within said feed crucible, said second heating chamber
accommodating a tubular member having in the top an inlet for
receiving said desired metal as recovered and an outlet through
which impurity elements that have higher vapor pressure than said
desired metal and which are evaporated in separate form upon
heating are discharged, as well as a liquid reservoir for heating
said desired metal which is formed in the lower part of said
tubular member, and a diffuser for condensing said desired metal as
evaporated which is installed across the upper part of said tubular
member.
9. The apparatus according to claim 8, wherein said inner tube is
surrounded by an outer tube of a larger diameter that permits said
vacuum atmosphere to communicate with said inner tube and which is
generally concentric therewith, said apparatus further including an
upper heater and a lower heater provided in the space between the
inner surface of said outer tube and the outer surface of said
inner tube, said upper heater being positioned in the upper part of
said space to heat said feed crucible and said lower heater being
positioned in the lower part of said space to heat said liquid
reservoir.
10. The apparatus according to claim 8 or 9, wherein said diffuser
consists of a plurality of generally parallel plates each having a
plurality of holes made through it.
11. The apparatus according to claim 8 or 9, wherein at least the
inner surface of the ceiling of said inner tube is domed or made
conical in shape.
12. The apparatus according to claim 10, wherein at least the inner
surface of the ceiling of said inner tube is domed or made conical
in shape.
13. The apparatus according to claim 8 or 9, wherein said desired
metal is at least one metal selected from the group consisting of
indium, antimony, zinc, tellurium, magnesium, cadmium, bismuth and
silver.
14. The apparatus according to claim 10, wherein said desired metal
is at least one metal selected from the group consisting of indium,
antimony, zinc, tellurium, magnesium, cadmium, bismuth and
silver.
15. The apparatus according to claim 11, wherein said desired metal
is at least one metal selected from the group consisting of indium,
antimony, zinc, tellurium, magnesium, cadmium, bismuth and silver.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to an enhanced purification method by
which a high-purity metallic indium feed with a purity of about
99.99% (4N) is further purified to give metallic indium with a
purity of about 99.9999% (6N) or higher and which is also
applicable for such enhanced purification of antimony, zinc,
tellurium, magnesium, cadmium, bismuth and silver (which are
hereunder referred to as similar metals). The invention also
relates to an apparatus for purification that is used to implement
the method.
[0002] Indium is generally produced as a minor amount component of
zinc concentrates, so in zinc metallurgy, it is recovered either as
flue cinder or as a concentrate obtained in an intermediate step
such as eletrowinning of zinc. In recent years, indium is also
recovered in pure form from waste compound semiconductors. To
purify the indium feed, three methods are commonly used and they
are electrolysis, vacuum distillation and zoning.
[0003] The metallic indium obtained by electrolysis or vacuum
distillation is about 99.99% pure and contains at least 0.5 ppm
each of impurities such as Si, Fe, Ni, Cu, Ga and Pb. The
purification from-waste compound semiconductors has the problem
that large equipment and prolonged time are needed to separate and
recover indium.
[0004] In the zone purification method, the purified indium mass
has to be cut and there is a potential hazard of contamination;
hence, the purification process inevitably suffers limited
throughput and lowered yield. In addition, when the purified indium
is cast into an ingot, impurities may enter during casting to cause
contamination.
[0005] With a view to solving these problems, the present inventors
previously developed an improved technology for purifying indium to
a purity of at least 99.9999% by vacuum distillation and proposed
it in Japanese Patent Application No. 8-294430. As it turned out
this technology had the problem that purification became more
difficult as the difference between the vapor pressures of the
metal of interest and impurity elements decreased. Hence, it was
desired to develop a purification technology that was capable of
producing indium of higher purity with higher efficiency and which
was also applicable to the similar metals mentioned above.
SUMMARY OF THE INVENTTON
[0006] An object, therefore, of the present invention is to provide
an enhanced purification method by which even an indium feed
containing many impurity elements can be purified consistently and
at high speed to a purity of 99.9999% or higher and which is also
applicable to the above-mentioned similar metals to yield equally
purified products.
[0007] Another object of the invention is to provide an apparatus
for purification that can be used to implement the method.
[0008] The present inventors conducted intensive studies in order
to attain the stated objects by a two-step process in which the
indium in an indium feed was evaporated and then condensed for
recovery in the first thermal purification step to be separated
from impurity elements of lower vapor pressure and in which the
recovered indium was then heated in the second thermal purification
step to evaporate away impurity elements of higher vapor pressure.
As a result, they found that not only the impurity elements having
lower vapor pressure than indium but also those having higher vapor
pressure could be separated in a consistent and efficient manner to
yield indium with a purity of about 99.9999% or higher. They also
found that by using graphite as the constituent material of areas
which were to be contacted by indium during the purification
process, in particular, the inner tube and by providing diffuser
plates in the pathway of distillation in the second thermal
purification step, recontamination could be prevented and the
purification speed could be markedly improved. The inventors also
found that this technology was applicable not only to indium but
also to other metals that could be purified by the difference in
vapor pressure, in particular, the similar metals mentioned
above.
BRIEF DESCRIPTION OF THE DRAWING
[0009] FIG. 1 is a schematic vertical section of an apparatus for
purifying indium according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The present invention provides the following methods
according to its first aspect:
[0011] 1. A method of enhanced purification of high-purity metals
which comprises purifying a metal feed by distillation in a vacuum
atmosphere to yield the desired metal with high purity, which
method further comprising a first thermal purification step in
which said metal feed in a feed crucible positioned in the upper
interior of an inner tube maintaining said vacuum atmosphere is
heated and the generated vapor of said desired metal is brought
into contact with the inner surface of said inner tube so that it
is condensed and recovered in a separate form from impurity
elements that have lower vapor pressure than said desired metal and
which are allowed to stay within said feed crucible, and a second
thermal purification step in which said desired metal as recovered
is admitted into and heated in a liquid reservoir in the lower part
of a tubular member positioned in the lower interior of said inner
tube and the generated vapor is passed through a diffuser
positioned in the upper part of said tubular member and guided by
suction so that the vapors of impurity elements having higher vapor
pressure than said desired metal are solidified in a separate form
in a cooling trap positioned below said tubular member and the
vapor of said desired metal is brought into contact with said
diffuser so that it is condensed and returned to said liquid
reservoir.
[0012] 2. The method according to item 1, wherein said diffuser is
made of a carbonaceous material.
[0013] 3. The method according to item 1 or 2, wherein said liquid
reservoir is a recovery mold for casting said desired metal having
high purity after enhanced purification.
[0014] 4. The method according to any one of items 1-3, wherein
said desired metal is indium, said metal feed is heated at
1100-1300.degree. C. in the first thermal purification step and
said desired metal as recovered is heated at 900-1200.degree. C. in
the second thermal purification step.
[0015] 5. The method according to any one of items 1-3, wherein
said desired metal is at least one metal selected from the group
consisting of antimony, zinc, tellurium, magnesium, cadmium,
bismuth and silver.
[0016] According to its second object, the present invention
provides the following apparatus:
[0017] 6. An apparatus for enhanced purification of high-purity
metals, which comprises an inner tube in which a vacuum atmosphere
is to be formed, a first heating chamber provided in the upper
interior of said inner tube, a second heating chamber provided in
the lower interior of said inner tube, said first heating chamber
accommodating a feed crucible with an open top into which a metal
feed is charged and the desired metal in said metal feed is
evaporated for recovery while impurity elements having lower vapor
pressure than said desired metal are separated by being allowed to
stay within said feed crucible, said second heating chamber
accommodating a tubular member having in the top an inlet for
receiving said desired metal as recovered and an outlet through
which impurity elements that have higher vapor pressure than said
desired metal and which are evaporated in separate form upon
heating are discharged, as well as a liquid reservoir for heating
said desired metal which is formed in the lower part of said
tubular member, and a diffuser for condensing said desired metal as
evaporated which is installed across the upper part of said tubular
member.
[0018] 7. The apparatus according to item 6, wherein said inner
tube is surrounded by an outer tube of a larger diameter that
permits said vacuum atmosphere to communicate with said inner tube
and which is generally concentric therewith, said apparatus further
including an upper heater and a lower heater provided in the space
between the inner surface of said outer tube and the outer surface
of said inner tube, said upper heater being positioned in the upper
part of said space to heat said feed crucible and said lower heater
being positioned in the lower part of said space to heat said
liquid reservoir.
[0019] 8. The apparatus according to item 6 or 7, wherein said
diffuser consists of a plurality of generally parallel plates each
having a plurality of holes made through it.
[0020] 9. The apparatus according to any one of items 6-8, wherein
at least the inner surface of the ceiling of said inner tube is
domed or made conical in shape.
[0021] 10. The apparatus according to any one of items 6-9, wherein
said desired metal is at least one metal selected from the group
consisting of indium, antimony, zinc, tellurium, magnesium,
cadmium, bismuth and silver.
[0022] The apparatus for enhanced purification of high-purity
metals according to the invention may be designed to have a layout
as shown schematically in vertical section in FIG. 1. To be more
specific, the apparatus has an outer tube 1 that is composed of a
stainless steel frame, water-cooled areas and heat insulators such
as alumina sheets and which has its inner surfaces made of a
heat-insulating carbon material. The inner space of the outer tube
1 maintains a vacuum atmosphere by means of a vacuum pump 2. A
smaller-diameter graphite inner tube 3 which is generally
concentric with the outer tube 1 is inserted into the latter and
the inner spaces of the two tubes communicate with each other so
that the inner space of the graphite inner tube 3 also maintains a
vacuum atmosphere. The ceiling of the inner tube 3 preferably has
at least its inner surface domed or made conical in shape. By this
design, the metal of interest evaporating from within the feed
crucible 8 contacts the inner surface of the ceiling of the inner
tube 3 and then condenses in the form of drops deposited on the
inner surface of the ceiling; the drops are pulled by surface
tension to run rapidly down the inner surfaces of the sidewalls
instead of just falling down from the inner surface of the ceiling
of the inner tube 3 to come back into the feed crucible 8. The
inner tube 3 has a first heating chamber 4 in its upper part and a
second heating chamber 5 in its lower part that communicates with
the first heating chamber 4. An upper carbon heater 6 for heating
the first heating chamber 4 and a lower carbon heater 7 for heating
the second heating chamber 5 are provided in the space between the
inner surface of the outer tube 1 and the outer surface of the
inner tube 3. The feed crucible 8 made of graphite is provided
within the first heating chamber 4 and a tubular member 11 is
provided within the second heating chamber 5; the tubular member 11
has a liquid reservoir 9 in the lower part and is open in the
center and periphery of its top to be fitted with a inwardly
funnel-shaped inlet 10.
[0023] Diffusers 12 are provided across the upper part of the
tubular member 11 to extend from its inner surface to the
funnel-shaped inlet 10. The diffusers 12 may be plates having
through-holes or they may be packed layers having large voids
penetrating through them. In short, while various impurity elements
are evaporated by heating in the tubular member 11 to generate
convecting vapors, the vapors of those impurity elements having
higher vapor pressure than indium pass through the diffusers 12 to
be discharged out of the second heating chamber whereas the vapor
of indium condenses on the diffusers 12 and drips back to the
liquid reservoir 9; in this way, the impurity elements having
higher vapor pressure than indium are removed. The diffusers 12 are
preferably made of a material that is not highly reactive with
metals and more preferably made of graphite throughout. The
required number of diffusers 12, the diameter and number of
through-holes in each diffuser plate, the spacing between adjacent
plates, etc. may be adjusted in accordance with the purification
speed, the concentrations of impurities, the heating temperature,
etc. The through-holes in each diffuser plate may be clogged by a
metal solidified from a vapor state if they are too small in
diameter or number. Hence, the through-holes are preferably at
least 2 mm in diameter. A cooling trap 13 is provided below the
inner tube 3 in the neighborhood of the suction port of the vacuum
pump 2; by means of this cooling trap 13, vacuum intake containing
the vapors of impurity elements having higher vapor pressure than
indium, namely, the vapors generated in the first heating chamber
but not condensed and the vapors discharged from the second heating
chamber are cooled to trap the residual vapors in separate
form.
[0024] The term "vacuum atmosphere" as used herein means a highly
evacuated state which is preferably represented by the degree of
vacuum not higher than a pressure of 1.times.10.sup.-3 Torr
(1.3.times.10.sup.-1 Pa), more preferably a pressure in the range
of from 1.times.10.sup.-3 to 1.times.10.sup.-6 Torr
(1.3.times.10.sup.-1.about.1.3.times.10.sup.-4 Pa). A suitable
amount of an indium feed (with a purity of about 99.99%) is charged
into the feed crucible 8 in the first heating chamber 4 and heated
by the upper carbon heater 6 to a temperature between
1100-1300.degree. C., preferably between 1200 and 1280.degree. C.,
in a vacuum atmosphere; the indium feed in the feed crucible 8
evaporates, condenses principally on the inner surfaces of the
inner tube 3 and drips through the funnel-shaped inlet 10 into the
liquid reservoir 9 in the lower part of the tubular member 11 in
the second heating chamber 5 communicating with the lower part of
the first heating chamber 4. If the pressure in the first heating
chamber 4 is higher than 1.times.10.sup.-3 Torr
(1.3.times.10.sup.-1 Pa) or if the heating temperature is less than
1100.degree. C., the evaporation of indium slows down to lower the
rate of its purification. If the heating temperature exceeds
1300.degree. C., the impurity elements having lower vapor pressure
than indium evaporate in increasing amounts and get into the liquid
reservoir 9 together with indium, making indium purification
difficult to continue.
[0025] Among the various impurity elements contained in the indium
feed, aluminum, silicon, iron, nickel, copper and gallium having
lower vapor pressure than indium stay within the feed crucible 8.
On the other hand, phosphorus, sulfur, chlorine, potassium,
calcium, zinc, arsenic, cadmium and lead having higher vapor
pressure than indium evaporate from within the feed crucible,
condense within the first heating chamber 4 together with indium
and drip through the inlet 10 to get into the liquid reservoir 9.
Further purification of indium has been substantially impossible by
the prior art. To overcome this difficulty, the present invention
applies a special treatment to the indium recovered condensed in
the liquid reservoir 9. In the second heating chamber 5, the liquid
reservoir 9 is maintained at a temperature in the range of
900-1200.degree. C., preferably 1050-1150.degree. C., by means of
the lower carbon heater 7, whereby the vapors of the impurity
elements having higher vapor pressure than indium that have been
generated to convect in the liquid reservoir 9 pass through the
diffuser plates 12 to be discharged from the system whereas the
indium vapor condenses upon contact with the diffuser plates 12 and
drips again into the liquid reservoir 9. If the heating temperature
in the second heating chamber is less than 900.degree. C., the
evaporation of the impurities to be removed slows down; if the
heating temperature exceeds 1200.degree. C., the evaporation of
indium increases abruptly. As will be described later in
Comparative Example 1, even if the diffuser plates 12 are absent
from the interior of the tubular member 11, the impurity elements
having higher vapor pressure than indium evaporate from the
recovered indium mass in the liquid reservoir 9 and can be removed
to some extent. However, by installing the diffuser plates 12
across the upper part of the tubular member 11, evaporation,
convection and condensation of indium are effectively performed so
that not only the surface layer of the recovered indium mass in the
liquid reservoir 9 but also its entire bulk is circulated, whereby
the impurity elements having higher vapor pressure are evaporated
from the entire part of the recovered indium mass to achieve higher
yield in purification. Above all, the indium which evaporates
accompanying the impurity elements having higher vapor pressure can
be recondensed on the diffuser plates 12 so that the loss of the
recovered indium mass from the liquid reservoir 9 that can occur
during the purification process is suppressed to a minimum
industrially feasible level.
[0026] In the present invention, the shape of the inner surface of
the liquid reservoir 9 is designed to be the same as that of a
recovery mold which is to be used in the step subsequent to the
first and second thermal purification steps (herein referred to as
"after enhanced purification"). This eliminates the need of the
prior art technology for remelting the purified indium to be cast
into an ingot and recontamination by the casting operation is
effectively prevented to yield satisfactorily purified indium.
Conventionally, quartz is often used as the refractory material of
the inner tube 3; in the present invention, the inner tube 3 and
the diffuser plates 12 are preferably made of graphite and, more
preferably, substantially all surfaces that are to be contacted by
indium in a gaseous and a liquid form in a vacuum atmosphere,
particularly at least the inner surfaces of the inner tube 3, the
upper heater 6, the lower heater 7, the diffuser plates 12 and the
like are made of high-purity graphite in order to prevent indium
contamination. The shift from quartz to graphite as the constituent
material of the inner tube 3 has the added advantage that the
temperature the inner tube 3 can withstand and, hence, the heating
temperature in it can be elevated to increase the rate of indium
purification. What is more, the thermal conductivity of the inner
tube 3 is also increased. Thus, as will be described later in
Example 2, the rate of condensation and, hence, the rate of indium
purification can be increased given the same heating temperature. A
comparative test was performed to determine the rate of indium
purification at 1150.degree. C., 1250.degree. C. and 1300.degree.
C. using two types of inner tube 3, one being made of graphite and
the other made of quartz. As shown in Table 2 (see Example 2 and
Comparative Example 2), the rate of indium purification was 2.95
g/min (graphite) and 0.8 g/min (quartz) at 1150.degree. C., 10.4
g/min (graphite) and 8.7 g/min (quartz) at 1250.degree. C., and
15.2 g/min (graphite) and 13.3 g/min (quartz).
[0027] The indium thus obtained by enhanced purification was
analyzed with a glow discharge mass spectrometer and the total of
the impurities present was no more than 1 ppm. To determine the
purity of indium, the impurity elements to be measured are
subjected to quantitative analysis with a glow discharge mass
spectrometer and the total sum of the impurity contents is
subtracted from 100%.
[0028] It should be noted that the method and apparatus for
enhanced purification of the invention are applicable not only to
indium but also to all other metals that can be purified by the
difference in vapor pressure, as exemplified by antimony, zinc,
tellurium, magnesium, cadmium, bismuth and silver.
[0029] The present invention is further illustrated by reference to
the following examples which are by no means intended to limit the
scope of the invention.
EXAMPLE 1
[0030] FIG. 1 is a schematic vertical section of the apparatus used
in the examples to perform enhanced purification of indium. It had
a graphite inner tube 3 containing a graphite feed crucible 8 in
its upper part and a graphite tubular member 11 in the lower part.
The tubular member 11 had in its open top a funnel-shaped inlet 10
through which indium would drip into the second heating chamber 5
in the tubular member 11 after condensing in the first heating
chamber 4. The lower part of the tubular member 11 was the liquid
reservoir 9 and the upper periphery of the tubular member 11 was
open to serve as an outlet through which to discharge the vapors of
the impurity elements having higher vapor pressure which evaporated
from the recovered indium mass in the liquid reservoir 9. In the
upper part of the tubular member 11, graphite diffuser plates 12
were installed between the inner surface of the tubular member 11
and the outer surface of the funnel-shaped inlet 10. The diffuser
plates 12 were detachable to facilitate the removal of deposits and
replacement after use. An outer tube 1 generally concentric with
the inner tube 3 was slipped over it and carbon heaters 6 and 7
were installed in the upper and lower parts, respectively, of the
space between the inner and outer tubes.
[0031] Seven kilograms of a metallic indium feed having the assay
shown in Table 1 was charged into the feed crucible 8 and the
interior of the crucible was evacuated through the outer tube 1 and
the inner tube 3 by means of a vacuum pump 2 so that the degree of
vacuum in the crucible was at a pressure of 1.times.10.sup.-4 Torr
(1.3.times.10.sup.-2 Pa). At the same time, the metallic indium
feed was heated to 1250.degree. C. with the upper carbon heater 6
so as to evaporate indium and the impurity elements having higher
vapor pressure. As a result of this first thermal purification
step, the evaporating indium condensed upon contact with the inner
surfaces of the inner tube 3 and dripped through the funnel-shaped
inlet 10 to be recovered in the liquid reservoir 9 in the lower
part of the tubular member 11.
[0032] Part of the impurity elements having higher vapor pressure
than indium did not condense but remained in a vapor phase and were
aspirated by the vacuum pump 2 so that it passed through an intake
port 14 to solidify in a cooling trap 13 provided below the inner
tube 3 in the neighborhood of the suction port of the vacuum pump
2. The solidified product was mainly composed of indium, with the
remainder consisting of phosphorus, sulfur, chlorine, lead and
other impurity elements having higher vapor pressure than indium.
The residue in the feed crucible 8 was chiefly composed of indium,
with the remainder consisting of highly concentrated silicon, iron,
nickel, copper, gallium and other impurity elements having lower
vapor pressure than indium.
[0033] Since the recovered indium mass in the tubular member 1
contained part of the impurity elements having higher vapor
pressure than indium, the second thermal purification step was
performed to remove such impurity elements. To this end, the
recovered indium mass in the liquid reservoir 9 was heated to
1100.degree. C. by the lower carbon heater 7 and the generated
convecting vapors of the impurity elements having higher vapor
pressure than indium were passed through the graphite diffuser
plates 12 to be discharged from the system whereas the indium vapor
was recondensed by contact with the graphite diffuser plates 12 so
that it was recovered as purified indium. By 7-hour purification
procedure, purified indium was obtained in an amount of 6 kg and
analyzed to give the result shown in Table 1 (see the data for
Example 1). The result of analysis for Comparative Example 1 is
also shown in Table 1.
1TABLE 1 Analyses of impurities in the indium feed and the purified
indium (by glow discharge mass spectrometer; unit, ppm) F P Si S Cl
Fe Feed 0.24 0.01 0.14 0.02 0.45 0.15 Examp1e 1 <0.01 <0.01
0.03 <0.01 0.01 <0.01 Comparative <0.01 <0.01 0.12
<0.01 0.01 <0.01 Examp1e 1 Ni Cu Ga Sb Pb Feed 2.3 0.28 0.03
0.02 0.2 Example 1 <0.01 <0.01 <0.01 <0.01 0.01
Comparative <0.01 <0.01 0.03 0.01 0.13 Example 1
Comparative Example 1
[0034] For comparison with Example 1, indium was purified by
repeating the procedure of Example 1 except that the diffuser
plates 12 were omitted and the result of analysis of the purified
product is shown in Table 1 (see the data for Comparative Example
1). Without diffuser plates, indium purification could at least be
accomplished; however, it was only the surface layer of the
recovered indium mass in the liquid reservoir 9 that was
principally purified and compared to Example 1 in which all of the
recovered indium mass in the liquid reservoir 9 was purified, the
performance in removing the impurities was limited and the
difference was particularly noticeable for lead and other impurity
elements having close enough vapor pressures to indium. What is
more, in Comparative Example 1, the indium vapor coming from the
recovered indium mass in the liquid reservoir 9 in the lower part
of the tubular member 11 could not be condensed again for recovery
and indium loss was so great that industrially applicable indium
purification was difficult to perform.
EXAMPLE 2
[0035] Twenty kilograms of 99.99% pure metallic indium feed was
charged into the feed crucible 8 and subjected to the same
purification procedure as in Example 1, except that the heating
temperature in the first thermal purification step was varied at
1150.degree. C., 1250.degree. C. and 1300.degree. C. and that the
duration of the second thermal purification step was 15 hours. In
each of the three test runs, indium could be purified to a purity
of at least 99.9999%. The respective rates of indium purification
are shown in Table 2 below together with the result of Comparative
Example 2.
2TABLE 2 Rates of indium purification Temperature Example 2
Comparative Example 2 1150.degree. C. 2.95 g/min 0.8 g/min
1250.degree. C. 10.4 g/min 8.7 g/min 1300.degree. C. 15.2 g/min
13.3 g/min
Comparative Example 2
[0036] For comparison with Example 2, purification tests were
conducted under the same conditions as in Example 2 by the method
described in Example 1 of Japanese Patent Application No. 8-294430.
The rates of indium purification that could be achieved are shown
in Table 2 (see the data for Comparative Example 2). In Comparative
Example 2, the contents of impurities, particularly those having
higher vapor pressure than indium, were higher than in Example 1
but it was at least possible to produce indium having a purity of
99.9999% and more. However, the use of the quartz inner tube in
Comparative Example 2 caused contamination by silicon and, in
addition, due to the poor thermal conductivity of quartz, the
indium vapor condensed so slowly that this was a rate-limiting
factor in the purification process to realize only a slow rate of
indium purification.
[0037] According to the present invention, not only impurity
elements having lower vapor pressure than indium but also those
having higher vapor pressure can be positively separated from
indium, so high-purity indium with a purity of about 99.9999% or
more can consistently be obtained with the additional advantage of
preventing the loss of indium that may occur during its
purification.
[0038] If desired, at least part, preferably all, of the inner
surfaces of the purifying apparatus which are to be contacted by
indium may in effect be formed of high-purity graphite and this
contributes to preventing contamination by the constituent material
of the apparatus. If necessary, the liquid reservoir 9 may be an
indium recovery mold and this is effective in preventing
recontamination that may occur during the steps of purifying and
casting indium. Conventionally, the inner tube has been made of
quartz but quartz has low softening point and reacts with indium at
elevated temperatures. By forming the inner tube of graphite, the
problem of contamination is resolved and, what is more, the heat
resistance and thermal conductivity of the inner tube are so much
increased that the indium purification temperature and rate are
sufficiently increased to achieve a remarkable improvement in
productivity.
[0039] In addition to indium, the similar metals such as antimony,
zinc, tellurium, magnesium, cadmium, bismuth and silver can be
purified by the method of the invention relying upon the difference
in vapor pressure and equally good results are obtained with these
metals.
[0040] If a vacuum atmosphere is created in the outer tube as well
as in the inner tube, the following advantages are obtained: (1)
sufficient heat insulation is provided to save the cost of energy;
(2) the problem of the heat capacity and convection around the
heaters is resolved to permit easy control of the temperature in
the heating chambers; and (3) the oxidative corrosion of the
heaters is significantly reduced.
* * * * *