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Патент USA US3071533

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Jan. 1, 1963
J. B. ZADRA ETAL
3,071,523
METHOD FOR ELECTROWINNING MOLYBDENUM FROM MOLTEN ELECTROLYTES
Filed Feb. 6, 1961
2/
INVENTORS
JOHN B. ZADRA
HÂROLD J HE?NE?V
JOHN M. GOMES
BY
(EM Ö. ÇM
W WORNEYS
”iee
&071523
Fatentecl Jan. i, 1953
2
such as molybdenum oxide, with a mixture of alkali or
alkaline earth metal pyrophosphates, borates and halides
34371523
METHOD FOR ELECTROWINNIN G MOLYBDE
NUM FRGM MOLTEN ELECTROLYTES
in 'a crucible. Passage of a direct current of proper our-
rent density therethrough results in the deposition of high
John B. Zadra, Harold J. Heinen, and John M. Gomes, CT ?purity crystalline molybdenum on the cathode.
Reno, Nev., assignors to the United States of America
While the exact theoretical explanation for the success
as represented by the Secretary of the Interior
ful production of molybdenum is not known at present,
Filed Feb. 6, 1961, Ser. No. 87,502
it `Would appear to be due to the particular electrolyte
6 Claims. (Ci. 204-64)
employed. The essential Components of the electrolyte
(Granted under Title 35, U.S. Code (1952), sec. 266)
li) are a molybdenum compound, a pyrophosphate 'and .a
The invention herein described and claimed may be
manufactured and used ?by or for the Government of
the United States of America for government-al purposes
without the payment of royalties vthereon or therefor.
The present invention relates to the production of pure
molybdenum in massive form by the electrolysis of ia
fused electrolyte.
Currently, the most important commercial procedure
for producing molybdenum metal is by the hydrogen re
duction of high purity molybdic oxide or ammonium
molybdate in an electric furnace. Molybdenum powder
produced thereby contains approximately 0.25 to 0.30
percent oxygen. It is then converted to massive metal
and reduced in oxygen content vby sintering in a hydro
gen atmosphere, or by are melting. These methods are
costly, and in addition, -are accompanied by excessive
fume losses and other technical dif?cultíes.
As shown in Bureau of Mínes Information Circular
7723 by Campbell and Jones, issued July 1955, entitled
“A Survey of the Literature on the Electrodeposition of
Molybdenum,” it has been proposed to electrodeposit
molybdenum from many aqueous or fused salt electro
lytic systems. Substantially all of the methods disclosed
sutfer from the serious disadvantage that the metal de
posits are extremely thin and are usually contaminated :
with the lower oxides and hydroxides of the molybdenum.
The Senderoif and Brenner Patent 2,715,093 describes
halide. Molybdic oxide MoOa is usually the most con
venient molybdenum compound to use. However, other
molylbdenum compounds may be employed, such as the
molybdenum chloride for example. The purity of the
compound is not signi?cant in this process, and in fact
one of the advantages claimed is that impure molyb
denum compounds may be employed.
Sodium pyrophosphaite is the most readily obta?'nable
pyrophosphate, and is therefore .the one ordinarily used.
Similarly, sodiurn chloride is the chloride usually em
pioyed. However, the sodiurn in these compounds may
be replaced by lithium, potassium, rubidium, cesiurn,
calcium, -barium, or strontium, and the halogen in the
halide may be ñuorine, bromine, or iodine, as well as
chlorine. The addition of sodium tetraboraite to the elec
trolyte makes'the ratio of sodium chloride to sodium py
rophosphate less critical for the production of molyb
denum essentially free of phosphorus. The sodíurn tet
raborate also facilitates the ñuxing of silicious impurities
that may be present in the molybdenum compounds ern
ployed. Sodium metaborate may be substituted for the
tetraborate, and the sodium may be replaced by other
-alkalí metals.
In percent, the molybdenum content of the electrolyte
may range from 2.0 to 15.0; pyrophosphate from 15 to
However, this
85; tetraborate from 0 to 60; and halide from 15 to 85.
The preferred ranges are: Molybdenum 4 to 8; pyro
phosphate 30 to 50; tetraborate O to 20; and halide 30
to SO. All the foregoing and subsequent percentages re
process requires rigid reaction conditions and control,
such as special high purity alkali metal hexahalomolyb
fer to percent by weight.
The following table lists four compositions, having
date, inert atmosphere and the exclusion of all traces of
oxygen and moisture, resulting necessaríly in a relatively
high cost process.
Components within the -above ranges, found satisfactory
in the process:
TABLE 1
the formation of massive molybdenum 'by electrolysis of
a fused bath of a salt such as K3MoCls.
Patent 2,960,4S 1, issued November 15, 1960 to Slatin,
recites the production of molybdenum by the electrolysis
of a CaC12-CaO-Mo03 electrolyte. The presence of -al
kali metal compounds is stated by the patentee to be
harmful. Also, MoO3 is stated as not being as compat
ible with his CaCIZ-CaO bath as WO3 (col. 6, line 61). .
It is the main object of this invention to provide -a
simple, practical method for electrowinning high purity
molybdenum from its compounds.
A further object of this invention is to produce high
purity molybdenum from its impure compounds by fused
salt electrolysis in an open cell under atmospheric pres
MoOs
Na4P2O7 NagBioç NaBOz
Eutectic of
NaCl-NaF
H
NFPW menus›
The temperature of the electrowinning reaction may
range from 800° to l200° C. A preferred range is from
950° C. to 1050° C., at which the bath is very ?uid.
Lower temperatures result in electrodeposits consisting
largely of molybdenum dioxide and a phosphorus con
A further object of this invention is to produce high
taining compound, presumably a phosphide, with little,
purity molybdenum metal substantially free from oxygen
if any, crystals of metallic molybdenurn.
and phosphorus.
60
Current density on the cathode may range from 20
A 'still further object of the invention is to produce
to 280 amperes per square decirneter (a./dm.2), based on
massive deposits of pure crystalline molybdenum metal.
the original dimensions of the electrode. Since the cath
A further object of this invention is to provide a meth
sures.
od for the electrolytic recovery of molybdenum from a
ode 'surface area changes during the electrolysis, the ac
molybdenum in a fused salt electrolytic process.
density on the `anode is not critical, and is determined by
denum electrolyte by fusing a molybdenum compound
over the whole range.
tual current density during operation is diñicult to de
bath containing sodium.
65
termine. However, that based on the original dimen
A further object of this invention is to provide for ef
sions is adequate for the present purposes. The current
?cient spacíng of the electrodes to produce highpurity
its surface area once the current density of the cathodc
Further objects and advantages will become appa?'ent
is selected.
from the rest of the speci?cation.
70
Current e?iciencies of around 90 percent are obtained
In brief, our method consists in making a molyb
In general, this factor appears to
a,a71,523
'Ë
have less in?uence on the results achieved than either
bath temperature or composition of the electrolyte.
4.
from. This shows the high purity of the product ob
tained.
FEG. l illustrates a cross-sectional View of the electro
TABLE 2
lytic cell showing crucible l, furnace 2, graphite block 3
and cathode 4.
The electrolytic cell or crucible may be made of graphite
MoOa feed,
percent
Element
silicon nitride or silieon carbide. However, the preferred
material of Construction is graphite. Usually the graphite
cell is made the anode.
The cathode may be made of any suitable material, such
as iron, molybdenum or carbon. Graphite is very Satis
factory for the purpose. Any material that is not attacked
may be employed as the anode in electro-Winning. In the
following example the graphite electrolysis cell was em~
ployed as the anode.
Although an all-graphite cell is used, carbon contamina
tion of the electrodeposited molybdenum may be prac
tically eliminated by a proper spacing of the electrodes,
use of optimurn coneentration of molybdenum for the
particular electrolyte employed, and by the hydraulic
0.08
(1)
.07
. 00075-0. 0025
.05
.90
404
.00075- .0025
(1)
Electrowon
molybdenum,
perserit
<0. 001
. 00075-0. 0025
.00075- . 0025
<. 001
<.001
. 00075- .0025
<.001
0025 - .0075
- .05
.OS
<.001
1. 73
<.05
(I)
(1)
034
.066
i Not determined.
A comparison of molybdenum produced by the instant
process with commercially available hydrogen reduced
classiñcation or washing of the processed crystalline metal.
molybdenum powder is given in Table 3.
For example, an electrolyte of the second composition
in Table l, contains molybdenum within the optin?um
TABLE 3
range for excellent over-all electrolytic performance. A
Analysis
of
Feed
Material
and Electrolyt?'c Molybdenum
25
l-inch diameter graphite cathode was centrally positioned
in a 3-inch diameter graphite cell and lowered in the bath
to within two inches from the bottom of the cell. The
P?'od??ced Therefrom and Compamz?'ve Analysis of
Hydi'ogen Redz?ced Molybdenum Powder
resulting electrowon molybdenum metal averaged 0.01
percent carbon. in other runs carbon contamination was
30
lowered to 20 parts per million.
Elements
The following example illustrates one speci?c embodi
ment of the method, and the invention is not to be con
strued as being limited thereto.
Feed
Material
percent
Hydrogen
Eleetrolytic Reduced, C.P.
Molybdenum Molybdenum
percent
Powder,
percent
A graphite crucible 7 inches deep and 3 inches inside
diameter preheated to a temperature of about 750° C.,
about the melting point of the mixture and then charged
with 900 grams of a salt-molybdic oXide electrolyte mix~
ture consisting of 41.6% sodium pyrophosphate, 333%
sodium chloride, 16.7% sodium tetraborate, and &4%
molybdic trioxide. The crucible, open to the atmosphere,
was placed in a vfurnace under atmospheric pressure and
connected to the positive terminal or' a DC. power source
to serve as the anode. After heating for about 30 min
utes at a selected temperature of between 950° to l050°
C. to stabilize the conditions, a graphite cathode 1 inch in
diameter, connected to the negative terminal of the D.C.
power source, was centrally positioned and lowered into
the molten bath to within 1% to 2 inches from the bottom
of the cell and electrolysis was started. Proper spacing of 50
the cathode in the cell is important in order to eliminate
codeposition of crystalline MoO2 and to prevent short
circuiting. It has been found that the spacíng between
1 Not determined.
From the table, it can be seen that electrolytic molyb
denum compares favorably with commercial hydrogen
reduced molybdenum. A portion of each of the two
the immersed tip of the cathode and the bottom of the
cell must be greater than 1% inches because of the down 55 metals are melted into ingots and tested for hardness. The.
ward crystal growth of the molybdenum up to one inch
Vickers hardness number (VHN) for electrolytic mo~
beyond the cathode tip. A shorter spacing than 1%
lybdenum was 186 as Compared to 170-172 for the
inches usually .results in some codeposition of crystalliue
commercial molybdenum metal. This is believed at
MoOz. Current density on the cathode was maintained
tributable to the phosphorus content in the metal. Sub-‹
at between 80 and 90 a./clm.2 and the bath temperatura 60 sequent experiments have produced phosphorus free,
was about 1000° C. Two hours of current passage Wás
metals by a more careful control of conditions.
required to complete the electrolysis.
Various modi?cations of the process may be made'
The cathode was then removed from the bath, allowed
within the scope of the invention, as de?ned by the fol
to cool in the air, and then immersed in water. Adhering
lowing claims.
solidi?ed electrolyte dissolved in the water and was re 65
We claim:
moved from the deposited metal. The molybdenum metal
1. A process for obtaining massive molybdenum which
was then removed from the cathode and further treated
comprises: fusing a mixture of about 41.6% sodium
with boiling dilute hydrochloric acid to dissolve any adher_
pyrophosphate, about 333% sodium chloride, about
ing calcium or phosphate compounds. After water wash
16.7% sodium tetraborate, and about &4% molybdic tri
ing to remove the acid, ultra ?ne particles of carbon de 70 oxide to form an electrolyte bath, heating the electrolyte
rived from the electrodes were removed by hydraulic
to a temperature of from about 950° C. to about 1050°
classi?cation, leaving high purity dendritic crystals of mo
C., electrolyzing the said electrolyte at a current density
lybdenum metal.
on the cathode of from 20 a./dm.2 to about 280 a./dm.2.
The following table gives a comparative analysis of the
2. ,The method of claim 1, whereín the temperature of
Mo03 feed and the molybdenum metal produced there 75 the bath is about 1000° C. and the current density on
3,o71,523
6
Ll
the cathode is between about 80 a./dm.2 to about 90
a./dm.2.
3. A process for obtaining massive molybdenum which
comprises: fusing a mixture of a molybdenum compound,
a metal pyrophosphate, a metal tetraborate, and a metal
halide, said metal being a member of the class consisting
of lithium, sodium, potassium, rubidium, cesium, calcium,
6. A process for obtaining massive molybdenum which
comprises: fusing a mixture of molybdenum oxide, sodi~
um pyrophosphate, sodium tetraborate and sodium chlo
ride to form an electrolyte bath, heating the said elec
trolyte to a temperature of from about 950° C. to about
1050° C. and electrolyzíng the electrolyte at a current
density on the cathode of from about 20 a./dm.2 to about
280 a./dm.2, the amount of the molybdenum oxide being
barium, and strontium, in a vessel serving as an anode to
su?âcient to provide a bath containing about 2 to 15%
form an electrolyte bath, heating the said electrolyte to a
temperature of from about 950° C. to about 1050° C., 10 of molybdenum.
inserting a cathode rod in the said bath, and electrolyzing
References Cšted in the ?le of this patent
the electrolyte while maintaining the lower end of the said
cathode rod a minimum distance of about 1% inches from
UNITED STATES PATENTS
the bottom of the anode vessel, the amount of said mo
2,554,527
Fink et al. __________ __ May 29, 1951
lybdenum compound being su?icient to provide a bath con 15
taining about 2 to 15% of molybdenum.
FOREIGN PATENTS
4. An electrolyte for producing molybdenum metal by
electrolysis consisting of the product obtained by fusing
203,614
together a mixture of a molybdenum compound, a metal
pyrophosphate, a metal tetraborate, and a metal halide, 20
`said metal being a member of the class consisting of
Leo et al.: Transactíons of the Electro-Chemical So
Australia ____________ __ Sept. 13, 1956
OTHER REFERENCES
ciety, Volume 66, pages 461, 469 (1934).
lithium, sodium, potassium, rubidium, cesium, calcium,
Fink et al.: “The Electro Chemical Society,” Preprint
barium, and strontium, the amount of said molybdenum
84-20, October 14, 1943, pages 197-226.
compound being su?'icient to provide a bath containing
25
Pink et al.: “Chem. Trade Journal and Chemical En
about 2 to 15% of molybdenum.
gineer,” January 14, 1944, pages 33-34.
5. An electrolyte for producing molybdenum metal by
Bureau of Mines, Report No. 5554, “Electrowinning
electrolysis consisting of the product obtained by fusing
Tungsten and Associated Molybdenum From Scheelite,"
together a mixture of about 416% sodium py?rophosphate,
Zadra et al. (1959).
about 333% sodium chloride, about 16.7% sodium tetra
30
borate, and about &4% molybdic trioxide.
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