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Jan. 4, 1949.
n. s. cHlsHoLM Er-_AL
Filed nay so, 1945
2 sheets-sheet 1
Coo/ing lAir `
Patented Jan. 4, 1949v
«2,458,253 .
Douglas S. Chisholm and Thomas Griswold, Jr.,
Midland, Mich., assignors to The Dow _Chemical
Company, Midland, Mich., a lcorporation of
Application May 30, 1945, Serial No. 59è,710
2 claims.
î .
This invention relates to apparatus for the
distillation of metals. It particularly concerns
improved means for disposing the feed meta1 in
the vaporizing zone in the form of a continuously
rates can be realized without overheating` the feed
to the point that boiling and its resulting entrain- Y
ment of alloy in the vapor occur. Another object
is to provide a still in which no part of the feed
Ialloy can become stagnant, and in which molten
ñowing thin film.A
' A number of metallurgical processes involve`- .- metals containing solid matter in suspension can
distilling a desired metal from an alloy thereof
with less Volatile metals.
In certain .of these '
processes, the purity of the product is affected
very materially by the manner in which the alloy
is disposed on the heated vaporizing surfaces in
the still. This eifect is most pronounced in the
vacuum distillation of alloys of high density. «l
Thus, the volatilization of metals from dense
alloys takes place largely by surface evaporation.
Boiling, meaning the formation of vapor bubbles
in the body of the metal, is restrained by the highv
hydrostatic pressures which exist at all points
below the surface. Boiling may occur, however,
be handled without diñ‘lculty.
In metallurgical stills according to the inven
tion, the feed alloy is introduced into a shallow
spiral trough which slopes continuously down the ,
vaporizing section of the still. During its traverse
. of the trough, the alloy is heated to„a distilling
temperature and evolves vaporsof the volatiliza.
ble component rapidly and effectively without
undergoing ebullition,l Because of the slope of
Àthe trough, the' alloy is in continuousv now at all
times while it is being heated. >lis a result,4all
portions ofthe heated metal are repeatedly
_brought to the vapor-disengaging surface of -the
' when the alloy is heated drastically. In such a 20 _flowing stream to release their volatilizable com
case, the action is usually so violent that there is ‘ ponent; stagnation does not occur. In addition,
a serious entrainment of the boiling alloy in the
evolving vapors, with a -corresponding decrease in
solid particles entering with the feed are washed
through to the outlet by the moving metal; the
the purityof the distilled product. In designing
still is thus to a large extent self-cleaning.
a still for Vaporizlng metals, then, one of the ma 25
The invention may be further explained'with
reference to the accompanying drawings, in
`ior problems is to provide means for disposing the
feed alloy in a form such that it will Apresent a
maximum surface area for the disengagement of
' vapor and yet will not be subject to such over
Fig. 1 is a vertical assembly, largely in cross
section, of one form of metallurgicalistiil em
30 bodying the invention;
heating as produces violent boiling.
Fig. 2 is a vertical cross-section of a portion of
One solution to `the problem'is to arrange in
the vaporizing section of the still a number of
the vaporizing zoneof a still embodying another
superposed trays which hold the metal as a series;
form of the invention; and
of pools through which it flows While it is being '
Fig. 3 is a fragmentary perspective view of the
heated. Unfortunately, since in this design struc 35 construction shown in Fig. 2.“ a ,
tural limitations usually require that the trays be
The apparatus may be described with reference
'of substantial depth, stagnation of apart of the
to the distillation of magnesium from a lead
magnesium alloy in which lead preponderates,
metalA invariably occurs. In addition, since the
ratio of vapor-disengaging surface of the metal
although it is to be understood that the invention
to total metal volume is small in the case of deep 40 is not thus limited. _
trays, adequate vaporizing rates can only be ob
In the equipment of Fig. 1, the liquid lead-mag
l tained either by going to a still which has a con
nesium alloy to be distilled is stored in an alloy
siderable number'of trays, and hence is of exces
steel tank 4 surrounded byinsulating brick i.
-The alloy is withdrawn by a sump pump 6 driven
sive size', or by overheating the metal to induce
violent boiling, in which case the distilled product 45 by' a variable-speed motorr 1 and is forced through a feed line 8A into the top of the vaporizing sec
is contaminated with entrained feed alloy. In
tion of the still 9.
addition, if the feed contains solid matter in sus
This still is constructed of a large-diameter
pension, the trays tend to fill up and lose their
alloy steel pipe set vertically and provided at the
effectiveness. For these reasons, tray boilers,
although perhaps the most usual form of metal 50 lower end with a conical bottom l0 from which
a drain pipe Il extends into the reservoir 4. At
vaporizer, leave muchl to be desired.
. the upper end, the still is closed' by, a ñanged
It is, therefore, an object of the present'inven
dome l2 from which a vacuum line I3 leads to an
‘tion to provide an improved metallurgical stillin
exhaust pump I4, the. line being provided with a
which the ratio of vapor-disengaging area to
metal volume is large, sothat high vaporlz'ing 55 clean-out flange i5. The still is divided into a
The motor ‘l is adjusted so that the pump 6
lower vaporizing section I6 and an upper con
forces lead-magnesium alloyat a desired ratev
into the still, where it falls into the Icup 21.
The alloy then flows continuously down the con~
densing section I1, the two zones being separated
by radiation shielding baiïles I8. The vaporizing
section I6 ls set in a brick furnace I9 heated by
gas burners 20, and the condenser -I1 is cooled ex
volutions of _the spiral trough 28 and finally
ternally by an air-circulating jacket 2 I.
Mounted within the vaporizing section- I 6 of
the still is atruncated graphite'conical pillar 22,
through the openings 25 and out the drain Il,
being' at the same time heated by radiation from
the hot wall I6 and by contact with the pillar
22 which is lself _heated by radiation. As the
alloypasses ldown the trough 28, part of the
magnesium in it is vaporized without appre
ciable occurrence of ebullition and with little,
. formed, for convenience, of three pieces held to
gether by dowels 23. The pillar rests on >a
graphite plate 24, which ñts snugly into vthe
bottom piece IU and is provided at its edge with
one or more drain holes 25, and is secured at \\
if any, entrainment of the alloy.
the top by alloy steel hold-down lugs 26 welded
tothe wan of the stm. A cup 2l is formed in
the top of the pillar v22 to- receive alloy issuing
into the condenser I1 where they are condensed
to liquid magnesium, which leaves as product
from the feed pipe 9.
In the side of th’e pillar 22, there is cut a spiral
alloy-carrying trough 28, starting' at the cup 21
and continuing at a smooth slope until it runs
out at the bottom just above the support plate
24. The precise cross-sectional shape of the
i trough is not critical, but it is highly desirable
that its rdepth be a minor fraction of its width,
i. e. that the trough be as shallow as possible.
It is likewise preferable that the downward slope
The vapors rise .
through the line 30. .
The alloy in thev tank 4, which would other
wi?se become depleted in magnesium as opera.
tion- proceeds, is enriched continuously .with a
material rich inmaenesium from a source not shown.
The purpose of the conical shape of thepillar
22 illustrated is to provide an increasing area
~for vapor flow from bottom to topof the vapor
izing section I6. To this end the pillar might
equally well be conoidal. Less advantageously,
of the trough be gentle, which is achieved by ` . it may be cylindrical, in which case its diameter
cutting thespiral with a pitch which is a minor
would have to be considerably less than that of
the still.
fraction of the diameter vof the pillar 22, as
shown in Fig. 1. The base diameter of the pillar :in
The use of graphite or other carbon asa ma- '
should ordinarily be nearly equal to the inside
terial of construction for the pillar 22 is pre
diameter of the still, while thetop diameter is
ferred because of its inertness to thermal shock,preferably a major fraction of the still diameter.
its high emissivity, which is importantv where
Magnesium vapors generated in the Vaporiz
the pillar is to be heated by radiation, and the
ing section I6 escape upwardly into the con
fact that it does not corrode with introduction
densing section I'l, where they come into con-4
of undesired substances into the íiowing alloy.
tact with the cooled walls and are condensed to
However, the pillar may, in some instances, be
liquid magnesium. The condensateA trickles
made of other materials, such as alloy steel or
ceramic brick.
down into an vannular collecting gutter 29 welded
to the wall of the >condenser at the bottom, and
t In an alternative construction according to- the
from there'runs through a product line 30 into
invention, the'spiral metal-retaining trough, in- ‘
a holding-pot 3l set in brickwork 32 and fired
stead of being formed on a central pillar within
by a gas burner 33.
the still, is located adjacent rto the inside wall of
The alloy tank 4 and condenser l‘l are pro
the still and receivesy heat by conduction. As
vided wìth heaters as shown, and the associated
illustrated in Figs. 2 and 3, the trough 34 is formed
pipes 8, Il and 30 with heaters, not illustrated,
of a continuous strip 35 ofV alloy steel which is
to raise them to the appropriate temperatures
set at a slight angle to the horizontal to provide
when beginning operation. These parts are also
a shallow gutter and which winds helically down
jacket-ed with thermal insulation, not shown, to
the still. For convenience in construction, the
prevent freezing of the lead-magnesium alloy
trough assembly is preferably made up of acy- ,i
and the magnesium product once circulation is
lindrical sleeve 3_6 which slides snugly into the
still and to theinside of which the spiral strip
established. ‘
In operation of the apparatus shown in Fig. 1,
the lead-magnesium alloy in the tank 4 is kept
under a protective saline flux or inert gas at
mosphere. The tank'4 and feed line 8 are main
tained at a temperature above the freezing point
of the alloy, preferably at 550° to 600° C. The
burners 28 are adjusted to maintain the lower
section I6 of the still at a vaporizing tempera
Y ture, usually 775° to 875° C., and the cooling air
to the condenser I1 is controlled so that the tem
perature 'of the magnesium condensate is above
its freezing point (651° C.), preferably 655° to
665° C. The lexhaust pump I4 is operated to
hold the still at a pressure which is suñiciently
reduced to permit boiling to take'place at a
high rate, but is above the vapor pressure of the
magnesium condensate; absolute pressures be
tween 0.i and about 0.5 inch of mercury are most
satisfactory. At this pressure, the alloy in the
feed line 8 and the return drain II, and the
magnesium in' the product line-30 rise to levels
well abovel those of the tank 4 and pot 3l, form
ing barometric seals.
35 is welded. The sleeve and associated trough
are preferably made up in a number of identical -
sections which are slipped into Athe still body
through the top and are superposed within the
vvaporizing section of the still to form a continu
ous spiral trough of any desired height. (One
such section is shown in Fig. 2.)
In operation of the still of Figs. 2 and 3, the
alloy feed is introduced into the' upper end of the
' trough 34 and circles downwardly through the
still, being heated from the hot wall I6 and va
porizing its volatile component, The vapors rise
through the open central core and escape into the
.condenser I '1.
It is to be understood that the foregoing de
scription is illustrative rather than strictly limi
tative, and that the invention is co-extensive in
scope with the following claims.
What is claimed is:
1. In a still 4for the vacuum distillation of a
volatile metal fromV a molten alloy thereof: a ver
tical elongated thermally-conductive closed still
body having therein a vaporizing zone and a con
densin-g zone in communication with each other.
The following references are of record in the
means for evaouatíng- the still body, means for
cooling the condensing zone, a furnace surround
fue of this patent:
. lng the still body at the vaporizing zone for raising
- '
the latter to a distilling temperature by heat 5
applied externally to the still body, a conical pil
lar mounted axially in the still body Within the
Divine et al ________ __ May 20, 1366
vaporizing zone and having formed in the outer
surface thereof a shallow open spiral trough ex
tending in a continuous downward slope from the 10
upper to the lower part of the zone, means for in
troducing the alloy to be distilled into the upper
Behrens __________ _.. Jan'. 17, 1905 l
_ Donk ___--' _______ __ Mar. 13, 1917
Payne ____________ _.. Feb. 28, 1933
part of the trough and for collecting residual
, 2,309,644
alloy flowing from the bottom of the trough, and
means for withdrawing condensate from the 15 2,337,042
2. A still according to claim 1 wherein the
pillar is constructed of graphite. v‘
> 20
DuPont __________ __ Aug. 22, 1893
Osborn _____-_-___,_~_ Apr. 22, 1941
Hansgirg __________ __‘Feb. 2,
Gloss ____________ __ Dec. 21,
Pidgeon __________ _.- Nov. 14,
McConica _______ _- Dec. 25,
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