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Purification of molybdenum : volatilisation processes using MoO3
Polyhedmn Vol. l,No.5,pp.479-482,1982 Printed in Great Britain
PURIFICATION OF MOLYBDENUM : VOLATILISATION PROCESSES USING MoO3 D.A. JOHNSON#, J.H. LEVY#, J.C. TAYLOR# and A.B. WAUGH*#
Chemical Technology Division and J. BROUGH Isotope Division Australian Atomic Energy Commission Lucas Heights Research Laboratories Private Mail Bag, Sutherland, 2232, NSW, Australia
ABSTRACT
Various volatilisation processes for the purification of Moo3 were investigated. Of these, fusion of MoO3 with NaCl at 600°C gave the best results. Thermal analysis of the MoO3/NaCl and W03/NaCl systems illustrates details of the processes involved. INTRODUCTION This study arose from an investigation into the purification of
MoOg
for production
of radiopharmaceuticaltechnetium. Rhenium and tungsten are the impurities of major concern since their high neutron absorption cross sections produce excessive activity. Rhenium is a particularly undesirable contaminant as it exhibits similar chemistry to the final radionuclide required, its group VII partner, Tc.
Tungsten is difficult to remove
because of its chemical similarity to molybdenum. We felt that a volatilisationprocess using simple molybdenum compounds could result in a suitable purification procedure. RESULTS AND DISCUSSION Four likely processes were tested and their results are summarised in the table which also shows the results of analysis for rhenium and tungsten by spark source mass spectrometry. The first two processes involve sublimation of MoO 3 from a tube furnace at 800°C to room temperature, in silica apparatus. Both processes result in significant reductions in rhenium content because volatility of rhenium oxides is higher than that of molybdenum and tungsten oxides. The high rhenium result observed in the MoO 3 fraction furthest from the furnace illustrates this convincingly. Sublimation under vacuum does not lower the tungsten content but this cannot be explained by volatilisation. The crystal structures of MOO3 and WO3 are quite different. 1 MOO3 is layered' and volatilises as polynuclear species, whereas WO3 exhibits an infinite lattice structure and hence is non-volatile. It is possible that WO3 is physically carried over in the process. Sublimation in oxygen flow rates at 100-200 mL/min would be less likely to carry particulate matter and may explain the lower result for tungsten in process 2. # Current address :
Division of Energy Chemistry, CSIRO, Lucas Heights Research Laboratories, Private Mail Bag 7, Sutherland, NSW, 2232, Australia.
POLY1:s -D.
479
480
Notes
L
0 .r
6
*
\
.r *
t.r
~4-0
E
.r
~~
:..
2
c?
d p
0” r” -.. .-..._
. .. ... ...-.
a..
.
,...*
0.
.
. . . . . ...*
_....*,...-
. ...* _....-. .
.
-**
?
L)
1./
9
2!
”
z
z”
.r
d
u-
. .
e
f:
0
cu
03
N
co
d
d
d
d
d
481
TABLE PURIFICATION PROCESSES AND RESULTS FOR MOLYBDENUM TRIOXIDE
PURIFICATION PROCESS
ANALYSIS
SAMPLE Ah'ALYSED
0.
Re (ppm) t
Starting Material Ampere Bulk MOO3
1.
w(Ppm)t
5.5
83
Sublimation of MoO3 at 800°C (in vacuum) Residue co.2 170 ...................................... ................... MoOa closest to furnace exit co.4 130 ...................................... ................... MOO3 furthest from furnace exit 42 310 ...................................... ...................
2.
Sublimation of MOO3 at 800°C in flowing 02 (sl50 cm3 min-I) co.9
M&3
3.
MoO3 refluxed in SCC12 to produce
MoWlt,
MoOCl4 sublimed at 100°C* 4.
26
2.3
75
~0.6
~0.6
Fusion of MoO3 with NaCl at 600°C MoO2Clr sublimate*
*Molydenum oxide chloride samples were hydrolysed, washed and dried then analysed as MoO3 tAnalysis by spark source mass spectrometry, estimated accuracy : f25%
Notes
482
Unlike the processes 1 and 2 which involve high temperatures,process 3 uses low temperatures. ~003 is refluxed in thionyl chloride, producing MoOCls2 which is subsequently sublimed at 1OO'C. However, no significant purification occurs because of formation of rhenium and tungsten oxide halides which have similar volatilities to MoOCls at this temperature. The fourth process utilised the fusion of
MoOa
with NaCl to form MoO2C12 which
volatilises immediately. This product has greatly reduced concentrationsof both rhenium and tungsten. A more thorough investigation of the process by thermochemical and X-ray diffraction techniques was carried out to determine the reactions involved. Thermogravimetry was carried out on a Cahn RH Electrobalancewith hanging platinum sample cup in quartz tubing inside a vertical furnace. In an atmosphere of flowing nitrogen, samples were heated at 5 Co per minute. Amplified mass and temperature signals were processed and stored by an LSI-11 computer. Differential thermograms were calculated from TG data by computing the smoothed first derivative as described by Whittem, Stuart and Levy3. X-ray diffraction apparatus consisted of a 114 mm dia. Debye-Scherrer powder camera, attached to a Philips X-ray generator producing Ni-filtered copper radiation. In the preparation of MoO2C12 by the fusion process4, the reaction has been claimed to be 2 MOO3 + 2 NaCl B
NanMoOs + MoOzC12
However thermogravimetricstudies on this system, using a range of McO3 to NaCl ratios from 1:2/3 to 1:3, gave the stoichiometry 3 MOO3 + 2 NaCl -
NaZMopO7 + MoOzC12
A thermogram for this reaction is shown in Figure la. Powder X-ray diffraction studies did not show the presence of NazMoOs. Sodium dimolybdate was confirmed as the sole non-volatile product. Thermogravimetry on the WOs/NaCl fusion system indicated the same stoichiometry as the MoO3/NaCl system. A comparison of derivative thermograms (Figure lb) shows that the reaction in the tungsten system occurs 300°C higher. We believe that it is this temperature difference of the solid state fusion reactions which results in the excellent separation observed for these two metals in impure Moos. If increased yields of purified molybdenum are desired, the residue may be dissolved in hydrochloric acid solution, dried, and recycled through the fusion process, Na2Mo207 + 2 HCl -
2 NaCl + 2 MoO3 + Hz0
The MoOs/NaCl fusion system at 600°C probably represents the simplest, most effective method of purifying MoO3 for technetium production. REFERENCES 1.
Advanced Inorganic Chemistry. 2nd Edition, F.A. Cotton and G. Wilkinson, Interscience,
2.
R. Colton, I.B. Tomkins and P.W. Wilson, Aust. J. Chem. 18, 447 (1965).
3.
R.N. Whittem, W-1. Stuart and J.H. Levy, Thermochimica Acta, in press (1982).
4.
A.N. Zelikman and N.N. Gorovits, Zh. Obshsh. Khim. 24, 1916 (1954).
Great Britain, 1966.
PURIFICATION OF MOLYBDENUM : VOLATILISATION PROCESSES USING MoO3 D.A. JOHNSON#, J.H. LEVY#, J.C. TAYLOR# and A.B. WAUGH*#
Chemical Technology Division and J. BROUGH Isotope Division Australian Atomic Energy Commission Lucas Heights Research Laboratories Private Mail Bag, Sutherland, 2232, NSW, Australia
ABSTRACT
Various volatilisation processes for the purification of Moo3 were investigated. Of these, fusion of MoO3 with NaCl at 600°C gave the best results. Thermal analysis of the MoO3/NaCl and W03/NaCl systems illustrates details of the processes involved. INTRODUCTION This study arose from an investigation into the purification of
MoOg
for production
of radiopharmaceuticaltechnetium. Rhenium and tungsten are the impurities of major concern since their high neutron absorption cross sections produce excessive activity. Rhenium is a particularly undesirable contaminant as it exhibits similar chemistry to the final radionuclide required, its group VII partner, Tc.
Tungsten is difficult to remove
because of its chemical similarity to molybdenum. We felt that a volatilisationprocess using simple molybdenum compounds could result in a suitable purification procedure. RESULTS AND DISCUSSION Four likely processes were tested and their results are summarised in the table which also shows the results of analysis for rhenium and tungsten by spark source mass spectrometry. The first two processes involve sublimation of MoO 3 from a tube furnace at 800°C to room temperature, in silica apparatus. Both processes result in significant reductions in rhenium content because volatility of rhenium oxides is higher than that of molybdenum and tungsten oxides. The high rhenium result observed in the MoO 3 fraction furthest from the furnace illustrates this convincingly. Sublimation under vacuum does not lower the tungsten content but this cannot be explained by volatilisation. The crystal structures of MOO3 and WO3 are quite different. 1 MOO3 is layered' and volatilises as polynuclear species, whereas WO3 exhibits an infinite lattice structure and hence is non-volatile. It is possible that WO3 is physically carried over in the process. Sublimation in oxygen flow rates at 100-200 mL/min would be less likely to carry particulate matter and may explain the lower result for tungsten in process 2. # Current address :
Division of Energy Chemistry, CSIRO, Lucas Heights Research Laboratories, Private Mail Bag 7, Sutherland, NSW, 2232, Australia.
POLY1:s -D.
479
480
Notes
L
0 .r
6
*
\
.r *
t.r
~4-0
E
.r
~~
:..
2
c?
d p
0” r” -.. .-..._
. .. ... ...-.
a..
.
,...*
0.
.
. . . . . ...*
_....*,...-
. ...* _....-. .
.
-**
?
L)
1./
9
2!
”
z
z”
.r
d
u-
. .
e
f:
0
cu
03
N
co
d
d
d
d
d
481
TABLE PURIFICATION PROCESSES AND RESULTS FOR MOLYBDENUM TRIOXIDE
PURIFICATION PROCESS
ANALYSIS
SAMPLE Ah'ALYSED
0.
Re (ppm) t
Starting Material Ampere Bulk MOO3
1.
w(Ppm)t
5.5
83
Sublimation of MoO3 at 800°C (in vacuum) Residue co.2 170 ...................................... ................... MoOa closest to furnace exit co.4 130 ...................................... ................... MOO3 furthest from furnace exit 42 310 ...................................... ...................
2.
Sublimation of MOO3 at 800°C in flowing 02 (sl50 cm3 min-I) co.9
M&3
3.
MoO3 refluxed in SCC12 to produce
MoWlt,
MoOCl4 sublimed at 100°C* 4.
26
2.3
75
~0.6
~0.6
Fusion of MoO3 with NaCl at 600°C MoO2Clr sublimate*
*Molydenum oxide chloride samples were hydrolysed, washed and dried then analysed as MoO3 tAnalysis by spark source mass spectrometry, estimated accuracy : f25%
Notes
482
Unlike the processes 1 and 2 which involve high temperatures,process 3 uses low temperatures. ~003 is refluxed in thionyl chloride, producing MoOCls2 which is subsequently sublimed at 1OO'C. However, no significant purification occurs because of formation of rhenium and tungsten oxide halides which have similar volatilities to MoOCls at this temperature. The fourth process utilised the fusion of
MoOa
with NaCl to form MoO2C12 which
volatilises immediately. This product has greatly reduced concentrationsof both rhenium and tungsten. A more thorough investigation of the process by thermochemical and X-ray diffraction techniques was carried out to determine the reactions involved. Thermogravimetry was carried out on a Cahn RH Electrobalancewith hanging platinum sample cup in quartz tubing inside a vertical furnace. In an atmosphere of flowing nitrogen, samples were heated at 5 Co per minute. Amplified mass and temperature signals were processed and stored by an LSI-11 computer. Differential thermograms were calculated from TG data by computing the smoothed first derivative as described by Whittem, Stuart and Levy3. X-ray diffraction apparatus consisted of a 114 mm dia. Debye-Scherrer powder camera, attached to a Philips X-ray generator producing Ni-filtered copper radiation. In the preparation of MoO2C12 by the fusion process4, the reaction has been claimed to be 2 MOO3 + 2 NaCl B
NanMoOs + MoOzC12
However thermogravimetricstudies on this system, using a range of McO3 to NaCl ratios from 1:2/3 to 1:3, gave the stoichiometry 3 MOO3 + 2 NaCl -
NaZMopO7 + MoOzC12
A thermogram for this reaction is shown in Figure la. Powder X-ray diffraction studies did not show the presence of NazMoOs. Sodium dimolybdate was confirmed as the sole non-volatile product. Thermogravimetry on the WOs/NaCl fusion system indicated the same stoichiometry as the MoO3/NaCl system. A comparison of derivative thermograms (Figure lb) shows that the reaction in the tungsten system occurs 300°C higher. We believe that it is this temperature difference of the solid state fusion reactions which results in the excellent separation observed for these two metals in impure Moos. If increased yields of purified molybdenum are desired, the residue may be dissolved in hydrochloric acid solution, dried, and recycled through the fusion process, Na2Mo207 + 2 HCl -
2 NaCl + 2 MoO3 + Hz0
The MoOs/NaCl fusion system at 600°C probably represents the simplest, most effective method of purifying MoO3 for technetium production. REFERENCES 1.
Advanced Inorganic Chemistry. 2nd Edition, F.A. Cotton and G. Wilkinson, Interscience,
2.
R. Colton, I.B. Tomkins and P.W. Wilson, Aust. J. Chem. 18, 447 (1965).
3.
R.N. Whittem, W-1. Stuart and J.H. Levy, Thermochimica Acta, in press (1982).
4.
A.N. Zelikman and N.N. Gorovits, Zh. Obshsh. Khim. 24, 1916 (1954).
Great Britain, 1966.