SEPARATION OF METAL CHLORIDES BY D lSTl LLATIO N SIDNEY G. PARKER AND ORAN W. WILSON Texas Instruments, Inc., Dallas, Tex.
A batch process for producing pure tantalum metal from tantalite ore was developed. Carbochlorination of the ore converted the metal oxides and/or silicates to their respective metal chlorides. A pure tantalum pentachloride fraction was separated from these metal chlorides by fractional distillation. Sixty to 70% of the tantalirm in the initial ore was recovered as tantalum pentachloride containing less than 10 p.p.m. of metallic impurities.
pure metals, such as tantalum or niobium, from or solvent extraction methods now in use is a long, involved process (4-6, 8, 77, 72). A simplified process for separating tantalum from its ore was experimentally investigated and developed. T h e metal oxides of tantalite ore were reduced with carbon to the corresponding crude metals or metal carbides and then converted to volatile metal halides. These halides were separated by distillation, and the pure TaCls was reduced to the metal in a stream of hydrogen at (elevated temperature. Because tantalum in tantalite ore is readily converted to tantalum pentachloride (TaClS), which we found to boil a t 240' C. without decomposition, it was believed that TaCl5 could be separated froin other metal chlorides by distillation. Lnfamiliarity with the use of distillation of inorganic materials which are solids at 25'' C. probably has discouraged study of distillation as a purification tool. I t is believed that several parties have been using chlorination methods to extract and purify tantalum and niobium from ores and slags, but no detailed information on their proprietary processes has been reported in the open literature. Therefore, it was thought that there would be interest in the experimental results which we obtained. EPARATING
S their ores by the fractional precipitation
Formation of Metal Hailides
A carbochlorination method was used to convert tantalite ore to anhydrous metal chlorides. Although the composition and amount of each element vary with different ore lots, a typical composition is listed in Table I. Tantalites were converted to halides by first reducing them with carbon to crude metals and/or metal carbides, and then making these metals a.nd metal carbides react with chlorine
Table 1. Metal
Composition of Tantalite Ore Sample wt. % Equivalent Oxide
Taa 43.49 TazO6 NbzO6 Xba 18.99 Fen 5.04 FeO Mna 0.52 MnO Sib 1.6 Si02 Tia 1.03 Ti02 Alc 0.1-5 &Os Sna 0.13 SnO:! All other metals less than O.1y0 each. Method of analysis. a Wet. Spectrographic. mated.
gas to give metal halides. T h e following equations illustrate the over-all reactions which occur during carbochlorination : heat
Fe(Ta0s)z
+ 6 C f 6'/2 Clz -+ 2 TaC15 f FeC13 f 6 C O
(1)
heat
Fe(Nb03)zf 6 C f 6'/2 Clz -+ 2 NbCls f FeC13 f 6 C O heat
M n ( T a 0 3 ) ~f 6 C f 6 Clz + 2 TaCIS f MnClz
+ 6 CO
(2)
(3)
Silicates of metals and other materials also react during carbochlorination to give metal chlorides and/or oxychlorides. Some separation of metal halides was obtained during carbochlorination because of differences in boiling points. For example, the percentage of iron and manganese chlorides in the TaClS condensate varied with the carbochlorination temperature. A sketch of the chlorination apparatus is shown in Figure 1. The tantalite ore was ground to a 200-mesh powder, mixed with carbon black, and made into a slurry; then the mixture was pressed into pellets. While the reaction chamber was being heated to about 600' to 800' C. a stream of Nz removed the excess moisture and oxygen. When the desired reaction temperature was reached, a gas stream composed of 507, Clz and 507, He was introduced. The helium swept the volatile halides formed from the reactor and did not affect the reaction kinetics. The product outlet line was maintained a t 300' C. to prevent condensation. An analysis of volatile halides produced from the carbochlorination expressed as mole fraction was: TaC16, 0.422; NtrC15, 0.311 ; MnC12,0.116; FeC13, 0.082; SiC14, 0.069; plus traces of Ti, W, Al, and Cu. Approximately 96% of the tantalum was removed. Separation by Distillation
T h e entire still, constructed from Pyrex brand glass No. 7740, was placed in an oven as illustrated in Figure 2. A temperature of 225' C. was maintained inside the oven in order to keep the metal chlorides molten. The fractionating column made of Pyrex glass pipe was l ' / z inches in diameter and 6 feet in length and was packed with '/16-inch-diameter quartz Fenske rings. A 5-liter flask with an electric heating mantle to supply the required heat was used to contain the charge. Silicone oil was circulated through the condenser
53.1 27.2 6.48 0.67 3.43 1.72 0.19-9.45 0.17
Spectrographic esti-
REACTOR TUBE
Figure 1.
FURNACE HEATER
Carbochlorination apparatus VOL. 4
NO. 4
OCTOBER 1 9 6 5
365
with an Aurora pump capable of pumping fluids at temperatures u p to 390' C. Though the entire still was in an oven, the rectification column was insulated to prevent excessive condensation in this region. Products and samples were removed by solenoid-operated glass ball and socket joint valves. A similarly operated valve was placed in a calibrated section of the return reflux line to measure the throughput rate. Electromagnets used on the solenoid valves were made from ceramic-coated copper wire. The heated product and sample lines kept the material molten until it flowed into flasks outside the oven and quickly solidified. To prevent hydrolysis of metal chlorides, the system was kept under N P atmosphere. Charge and products were transferred in a glove box in an atmosphere of dry N2 or He.
Figure 2. I
1
I
I
I
Because the effectiveness of separation by distillation depends upon differences in boiling points of the individual constituents in a mixture, the first task was to determine the boiling points of the various metal chlorides resulting from carbochlorination of tantalite ore. Table I1 lists melting and boiling points of various chlorides present in the initial still charge. The boiling points of TaC15 and NbC15, the major constituents of tantalite ore, differ by about 15' C., and the relative volatility of TaCl5-SbCIE was given as 1.36. (The value of the relative volatility gives a measure of the ease of separation; a relative volatility of 1.0 means that separation by distillation would be impossible.) British \vorkers (70) proved the basic feasibility of distillation for separating TaClS and NbCl5, but pointed out possible problems. First, in their process tungsten oxychloride (WOCld) was probably formed. As this compound boils 12' C. lower than TaC15, it would be about as difficult to separate TaCle from lVOC14 as from KbC15. They used a ferroniobium alloy as their raw material, and the tungsten may have been introduced with the iron rather than with the niobium. No precautions were taken to exclude air from the system, so oxidation may have occurred during the chlorination or distillation. Second, ferric chloride (FeC13) when heated decomposes to ferrous chloride (FeC12) and chlorine. Ferrous chloride with a melting point of 670' C. may solidify in the distillation system, thus precluding the use of a continuous system for the separation of iron chloride from the other chlorides. Ferric chloride may be removed by scrubbing through XaC1 at 500' C. ( 3 ) . Under these conditions FeC13 forms a nonvolatile, double salt with NaC1, while the other metallic chlorides pass through the NaCl scrubber unchanged. T o produce pure TaClj and NbCl5 by distillation, three separations are necessary: (1) The light impurities must be removed, (2) the heavy impurities must be removed, and (3) the TaClj and XbC15 must be separated from each other. The first two separations are relatively simple, and since the decomposition of FeC13 may be a problem, the present equipment design is based on a batch column to separate a TaC15KbC15 fraction from both the light and heavy impurities.
Distillation unit I
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60
E40
I-
3
t
n I
I W
3
0
a
I I-
40
20
60
I
80
PRESSURE DROP, Crn.H20 Figure 3.
Pressure drop with throughput rate
Melting and Boiling Points of Metal Chlorides in Initial Still Charge Melting Point, Boiling Point, Chloride c. c.
Table II.
L 210'
:.I Figure 4.
366
212
213 214 DENSITY, g,/c c.
215
2l6
2J7
Densities of T a c h and NbClj
I&EC PROCESS D E S I G N A N D DEVELOPMENT
Tin, SnC14" Titanium, TiC1d4 Aluminum, .%lClP Tungsten, \t70C14a Tantalum, TaClja)b Xiobiurn. NbClsagb
-30.2 - 30 Sublimes at 178 21 1 220 209.5
Iron
FeC13" FeC1Za Tungsten, \VC16" a Literature calues ( 7 , 70).
282
114.1 136.4
22;
:5
239.3 254 315
Sublimes 670 346,7 275 Determined experimentally.
2
5
0
1I
.
I
I \c \
I
I
*--TaCIS
1
I
w
i
L
I
\ I
Bn 0 . 8 t
'Io
L13.7
0.8
0.9
A1.1
1.0
V I S C O S I T Y , CENT1 POISES
Figure
5.
3
Viscosities of TaCIS and NbClj
P E R C E N T DISTILLED, M O L E S Figure 6.
The height equivalent to a theoretical plate (HETP) was determined with test mixtures of n-heptane and methylcyclohexane. The HETP with the above mixture was 1.32/inch, which was used as an approximate value in designing the column since H E T P is dependent upon chemical composition. Successful distillation and throughput rate depend upon pressure drop across the column and the possibility of flooding. The pressure drop at different throughput rates for a 50-50 mole % mixture of TaClb-NbCls was determined (Figure 3). h-o flooding occurred up to 60 ml. per minute, the maximum rate attainable with ihe heating mantle used on the charge flask At the maximum throughput rate, no halides passed through the condenser as a gas; very efficient liquefaction was obtained. Holdup in the still at a throughput rate of 40 ml. per minute was about 10% of an initial 4500-gram charge. The Smoker ( 9 ) and Rayleigh (7) equations can be used to calculate distillation curves if enough information is available. Two necessary factor'; for calculation of possible distillation behavior are densities and viscosities of the liquids. T h e densities and viscosities of TaC15 and NbCls as illustrated in Figures 4 and 5 were the values actually determined experimentally. Densities were measured with calibrated volume density bottles which were suspended in an oil bath at the desired temperatures. The bottles were then weighed to find the weight per unit volume. Viscosities were determined in a modified Oswald viscometer suspended in a heated oil bath; in this apparatus the viscosity is calculated from the time required for the liquids to flow through a capillary. Distillation curves obtained from experimental data are shown in Figures 6, 7 , and 8, which also illustrate the similarities between experimental curves and those obtained by calculation from the Rayleigh and Smoker equations for batch distillation. O u r distillation follows the theoretical calculations closely enough so these calculated values could be used as a good approximation for distillation behavior. Deviations between the experimental and calculated distillation curves may be caused by differences in relative volatility, holdup, and/or the actual number of rectification plates present. The calculations based on the Smoker and Rayleigh equations assume negligible holdup, adiabatic conditions, no heat of mixing, and similar thermodynamic properties of all the metal chlorides; these are rather broad assumptions and do not necessarily apply to actual experimental conditions. As shown in Figure 9, TaCl5 of rather high purity was obtained with the metal chlorides from tantalite ore. Most of the metal chlorides with boiling points less than 225' C. were driven through the condenser while the system was brought u p
Distillation curve
Reflux ratio. 1O:l Throughput rate. 1800 ml./hr. Initial charge. 0.45 mole fraction TaCls
C
I
I
PERCENT DISTILLED, MOLES Figure 7.
Distillation curve
Reflux rotio. 20:l Throughput rate. 1800 ml./hr. Initial charge. 0.45 mole fraction TaC15
I
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w
IU
1.c
1 t
2
n 0.8
z
1
1
i
1
I
----TI
Ln
2 0.6 I-
A
z 0
p 0.4
-
CALCULATED EXPERIMENTAL
V
= l
a LL
0.2
-I
0
ZE
0
I
IO
I I I 20 30 40 50 60 PERCENT DISTILLED,MOLES
Figure 8.
70
80
Distillation curve
Reflux ratio. 2 9 : 1 Throughput rate. 1800 ml./hr. Initial charge. 0.47 mole fraction TaClb
VOL. 4
NO. 4
OCTOBER 1965
367
q
handling steps combined with less required floor space. The over-all yield of pure TaClj extracted from the ore is 60 to 70% for the batch distillation approach.
5 1 W
J
-
I-!
1000
L
0 0
40
20
60
80
100
I20
P E R C E N T DlSTl LLED Figure
9.
Conclusions
A mixture of metal chlorides obtained by chlorination of tantalite ore can be distilled to give a pure TaC16 fraction. The molten chlorides behave like other liquids during distillation; the only unusual requirement necessary for distillation is that the entire still be heated to prevent solidification of the halides of moderately high melting point. Theoretical curves based on the Smoker and Rayleigh equations for batch distillation give good approximations for actual experimental values. Actual distillation results were better than predicted ; however, the calculated values were based on several assumptions which probably were not entirely accurate. If FeCI3 decomposition is not a problem, our calculations indicate the feasibility of performing a continuous distillation. However, no experimental work was done to confirm this. literature Cited
Purity of TaClj distillate
to total reflux. Chlorides with boiling points higher than that of TaC16, such as niobium and iron, tended to remain in the charge flask until most of the TaC15 had been removed. Niobium began to appear in the distillate only after a large portion of the initial charge had been distilled. The emission spectrographic analysis showed less than 10 p.p.m. of metallic impurities in some samples of TaC16. The pure TaClj fraction was readily reduced to metallic tantalum by heating TaClj vapors in a stream of Hz at 1200’ C. No further metallic contamination was encountered during this reduction step, as witnessed by
