Karn, F. S., Schultz, J. F., Anderson, R. B., Ind. Eng. Chem. Prod. Res. Develop. 4, 265 (1965). Kemball, C., Taylor, H. S., J . A m . Chem. SOC. 70, 345 (1948). Morikawa, K., Benedict, W. S., Taylor, H. S., J . A m . Chem. SOC. 58, 1795 (1936). Morikawa, K., Trenner, N., Taylor, H . S., J . Phys. Chem. 59, 1103 (1937). Simons, J. B., M. S. thesis, University of Notre Dame, Notre Dame, Ind., 1963. Sinfelt, J. H., Taylor, W. F., Yates, D. J. C., J . Phys. Chem. 69, 95 (1965). Sinfelt, J. H., Yates, D. J. C., J . Catal. 8, 82-90 (1967).
Sinfelt, J. H., Yates, D. J. C., J . Catal. 10, 362-7 (1968). Tajbl, D. G., Feldkirchner, H. L., Lee, A. L., Aduan. Chem. Ser., No. 69, 774 (1967). Tajbl, D. G., Simons, J. B., Carberry, J. J., Ind. Eng. Chem. Fundamentals 5 , 171 (1966). Yates, D. J. C., Taylor, W. F., Sinfelt, J. H., J . A m . Chem. SOC.86, 2996 (1964). RECEIVED for review September 9, 1968 ACCEPTED April 3, 1969 Part of the basic research program of the Institute of Gas Technology.
REACTIONS OF TITANIUM TETRACHLORIDE IN A RADIO-FREQUENCY PLASMA TORCH REID
C .
MILLER'
A N D
R I C H A R D
J.
A Y E N '
Department o f Chemical Engineering, University of California, Berkeley, Calif. 94720 Thermodynamic equilibrium yields, experimental coupling efficiencies, and experimental conversions for reduction of titanium tetrachloride in a radio-frequency plasma torch operated with argon at 1-atm. pressure were studied. Calculated equilibrium results are presented for systems Ar-TiCl4, At-TiC14-Mg, and Ar-TiCl4HZ with temperatures to 14,000° K. at 1-atm. pressure. Experimental coupling efficiencies obtained for a 4-MHz. radio-frequency torch operated with argon indicated that about 60% of the generator plate power was transferred to the plasma. Efficiency appeared to be nearly independent of the power level and whether the stabilizing flow was vortex or coaxial. Hydrogen addition showed little effect on coupling efficiency, while Tic14 produced a decrease. Tic14 was reduced to Tic13 by HZ with 61 to 87% yields. Power input and Tic14 feed rate had little effect on the per cent conversion. Without HZno appreciable reduction of Tic14 could be obtained with the quenching methods employed.
THEinduction-coupled plasma torch, described by Reed (1961), consists of a gas flowing through a confining tube, with electrical coupling effected by means of an induction coil. The coil carries high-frequency current and surrounds the tube. Argon is the most common working gas for the torch, as it is readily available, does not have too high an ionization potential, and does not require energy for molecular decomposition. The plasma torch may be employed as a chemical reactor in a wide variety of arrangements. Solid, liquid, or gaseous reactants may either be passed through the plasma or mixed with the hot tail flame, and many quenching techniques have been studied. A number of good reviews of plasma chemical synthesis studies have been written. Specifically, Jolly (1960), Beguin et al. (1964), Dryden (1963-64), Hulburt and Freeman (1963), and a recent book edited by Baddour and Timmins (1967) cover most of the relevant topics. A good technological survey of
' Present address, Department of Chemical Engineering, University of Wyoming, Laramie, Wyoming 82070 Present address, Stauffer Chemical Co., Richmond, Calif. 94804 370
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plasma jet reactions appeared in a book by NASA (Dennis et al., 1965). Rogers (1966), a previous investigator a t this university, also compiled a comprehensive survey of reported investigations, with the radio-frequency plasma torch of primary concern. A number of simple endothermic reaction products have been prepared in plasma torches-e.g., acetylene, cyanogen, hydrogen cyanide, and various metallic nitrides. The economics of some of the processes appear favorable and should improve with time, as electrical costs are remaining relatively steady and may even decrease in the future. Another area of potential application is the reduction of refractory ores. Both MnSi03+ MnO + Si02 and MgO + C Mg + CO have been carried out in arc torches (Harris et al., 1959). Using the radio-frequency torch described in this paper, Rogers (1966) decomposed powdered MnSi03 with good conversion (-30%). The solid silicate was fed directly into the hot plasma region through a water-cooled tube, and the products were collected in a cooled separator. In another recent investigation Huska and Clump (1967) decomposed molybdenum disulfide powder in a radio-frequency torch, and found --f
that the conversion to the metal was proportional to the plasma power input and inversely proportional to the sulfide feed rate. Reduction of titanium-containing compounds has attracted much attention, as the metal is still relatively expensive, and its excellent properties are producing a greater demand. The most common commercial methods of production employ either sodium or magnesium reduction of T i c & . The tetrachloride is a liquid a t room temperature and easily purified by distillation; thus, it is a common intermediate in the production of various titanium compounds from the naturally occurring oxide ores. The trichloride finds use a t present as a polymerization catalyst and in melt electrolysis for production of titanium metal. I t may be either brown or violet, and is highly reactive, pyrophoric in air, and very rapidly hydrolyzed in water. Titanium dichloride is reported to be a black solid, unstable in air. Schumb and Sundstrom (1933) produced TiCL by hydrogen reduction of TiCL at 650°C. in an electrically heated tubular reactor, collecting the product on a cold finger. I n an early work TiC14 was reduced by HZ to TiCL in the silent electric discharge (Bock and Moser, 1912). More recently Ingraham, Downes, and Marier (1957) produced pure Tic13 by hydrogen reduction in a low-pressure a.c. arc (pncl, 10 to 13 mm. of Hg, pH9 4 to 5 mm. of Hg). They found that the type of discharge strongly affected yields; a 60-cycle per second arc had no effect, while a Tesla coil discharge was effective. A 200-kc. per second high-voltage spark discharge was found the most efficient. The blue glow observed in the reaction zone was attributed to excitation of molecular hydrogen, while the red glow of hydrogen atoms could not be detected. Titanium trichloride as produced was described as a red powder on the reaction zone walls and appeared upon transfer to a receiver flask. Without hydrogen both Gutman et al. (1955), in a low-pressure electrodeless discharge, and Ingraham et al. (1957) found no visible reaction of TiCL. Harnisch, Heymer, and Schallus (1963) reported over 6070 conversion of TiCL to TiCL in the presence of H P in a high temperature arc plasma torch. The product was collected on a water-cooled rotating metal drum and contained small amounts of T i c & if the molar ratio of H 2 to Tic14 was greater than 6 to 1. They stated that the power required was only about 5 kw.-hr. per kg. of Tic13 produced, production from their torch being as high as 6 kg. per hour. No work relating to TiCla reduction in a thermal radio-frequency plasma was found in the literature. It was decided to study TiCL reduction in the radio-frequency torch to gain knowledge about this type of chemical reactor and about the reduction process.
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High-Temperature Equilibria for Plasmas Containing Tic14
I t is essential to obtain thermodynamic information, if plasma reactors are to be designed for chemical processing. At high temperatures chemical reactions generally proceed very rapidly, and the time required to approach equilibrium may be very short, even compared to the time of motion in a flow system. Thus equilibrium compositions may be obtained in the highest temperature regions of a plasma reactor. Direct applications of the knowledge of these compositions include prediction of processing temperatures required and maximum possible
yields, determination of physical or transport properties required in an analysis, choice of the construction material for confinement purposes, and prediction of species available for recombination reactions in the quenching process. Equilibrium concentrations are determined by the relative free energies of the possible system components. These free energies must be either determined by integration of heat capacity expressions or calculated from statistical mechanical relations. The latter method is almost exclusively used for high-temperature gases, and thus energy levels must be available from spectroscopic data for each of the various species. Accurate data of this type are available for most molecular and atomic species commonly encountered, but estimates may have to be made for certain unusual molecules. Equilibrium constants, K,, calculated for formation of a large number of gas-phase atoms and molecules from their reference species are to be found in the JANAF tables (1960, 1964) for temperatures up to 6000°K. This source was used for all un-ionized species in this work. For interpolation and extrapolation, the data were fitted by least squares to the equation
log K , = A
B ++ CT T
For ionization equilibria the Saha-Eggert equation may be used to obtain the required equilibrium constants. Drawin and Felenbok (1965) have evaluated and tabulated the terms involved in this equation for the first few ionic states of most of the common elements. Their tabulated data were interpolated a t 100" K. intervals for computational purposes in a manner similar to the treatment of the molecular constants discussed above. The general method of obtaining the equilibrium composition of a complex reacting system a t a given temperature and pressure is straightforward. For a system containing S species the problem is reduced to the solution of S equations in the S unknown equilibrium concentrations of the species (Miller, 1968). Equilibrium calculations are described below for three plasma systems: Ar-Ti-C1 (72:1:4), Ar-Ti-C1-Mg (72: 1:4:2), and Ar-Ti-C1-H (72:1:4:4). The over-all system atom ratios are the numbers in parentheses. Argon appears in large proportion, as it was the working gas or primary component of the plasma. A complete list of the species considered for each system may be found in Table I. The basic assumptions necessary to carry out plasma thermodynamic calculations such as those of this work are local thermodynamic equilibrium, charge neutrality, and ideal gases. The S nonlinear equations for each system were solved on a digital computer a t each desired temperature (Miller, 1968). The method of solution involved local linearization of the equations in an iterative scheme using a program of the University of California Computer Center, Berkeley (Baer and Martin, 1964). The thermodynamic equilibrium gas-phase compositions for the three systems studied are presented in Figures 1, 2, and 3. I n the Ar-TiC14 system Ar and Tic14 are the lowtemperature equilibrium species (Figure 1). The initial partial pressure of TiC14 (0.0137 atm.) was chosen to correspond to the room temperature vapor pressure of the liquid. T o obtain any reduction of this compound by VOL. 8 N O . 3 J U L Y 1969
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Table 1. Species List
Species for Ar- TiC1, Ar
Ar
+
e Ti Ti Ti+i
Additional Species for Ar-TiC14-Mg Mg Mg' Mg2+ MgCl
Additional Species for Ar-TiCl,-H H H'
HZ HCl
+
Y LL
y I
c1 C1' c1-
Cln TiCl TiCL Tic12 (s) TiCb TiCb(s) TiC1, Ti(1) Ti(s)
004
0
002
00
2030
4aM
6000 8000 TEMPERATURE
p,000
12,000
14,000
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I
Figure 2. High temperature equilibrium diagram for system Ar-Ti-CI-Mg (72:1 :4:2)
0.08
0.06 0
4 U LT
y
004
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0
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4000
6000 8000 TEMPERATURE
10,000 ,
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14.000
O K
Figure 3. High temperature equilibrium diagram for system Ar-Ti-CI-H (72:1:4:4) Figure 1. High temperature gas-phase equilibrium diagram for system Ar-Ti-CI (72:1:4)
thermal action alone some means of freezing out high temperature species (quenching) is necessary. TiC13 reaches a maximum concentration a t about 2100" K., corresponding to a little over one third of the total titanium present. From 2600" to 3200"K. almost all of the titanium exists as TiCL. Above 4000°K. there are no polyatomic species which exist in appreciable amounts. TiCl, Clz, and C1- never attain significant concentrations, but are present in trace amounts. At higher titanium tetrachloride pressures the dissociations would be pushed to higher temperatures and the opposite effect would result from lower pressures. The argon pressure does not affect the equilibrium of the other species a t temperatures below which significant argon ionization takes place (ca. 8500" K.). An interesting aspect of Figure 1 is that the electron concentration remains appreciable down to much lower temperatures than for a pure argon plasma, because of the low ionization potential of titanium. This would modify the inductive coupling process. I n the second system (Figure 2) magnesium metal was considered as a reducing agent in sufficient quantities 372
l & E C PROCESS DESIGN A N D DEVELOPMENT
to allow the reaction to proceed to completion. The ratio of Ar to Tic14 was the same as in the first system. The room-temperature equilibrium species for this system are MgClz(s) and Ti(s). Going from higher to lower temperatures, titanium starts precipitating first at about 1500"K., and then liquid magnesium dichloride starts falling out just above 1200"K. Thus, as in most industrial production of titanium metal, the process required to reduce TiC14 is to heat it in the presence of Mg to a temperature a t which the reduction proceeds a t the desired rate. There would be no benefit in rapid quenching for this system. An increase in Mg pressure would cause titanium to start condensing a t higher temperatures. In addition to the species mentioned as not significant in the Ar-TiClc system, TiCL does not appear in Figure 2 for magnesium reduction. Since atomic magnesium is an easily ionized species, the electron concentration between 5000° and 12,000"K. is much larger than in the first system. As a result, under inductive coupling this system would exhibit behavior even further removed than Ar-TiCla from that of a pure argon plasma. The third system studied contains hydrogen in stoichiometric proportions as a reducing agent for titanium tetrachloride: TiCL + 2H2 -+ T i + 4HC1. HC1 and HZ are the gas-phase equilibrium species a t low temperatures
(Figure 3), because the titanium precipitates in the form of TiCla(s) below about 750”K. The TiC13(g) and TiC12(g) peaks extend to much lower temperatures than in the case of Ar-TiClr. If the H 2 pressure were increased, these peaks would move to even lower temperatures, resulting in TiCh(s) forming a t higher temperatures and eventually some TiClr(s) or even Ti(s) forming, in agreement with the experimental results of Harnisch et al. (1963), noted above. Hydrogen has a fairly high ionization potential, so that the ionization equilibrium is not greatly different from that for Ar-TiClr over the temperature range studied.
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Inner Tube Gas Inlet
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Annular Gas Chamber for Six Inlet Holes
Annular Gas Inlet
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Torch Head
Characteristics of R-F Plasma Torch Reactor
Torch Description. A 4-MHz. Lepel generator of 10-kw. maximum output was used to supply power to a fourturn, 39-mm.-diameter work coil made of %E-inch copper tubing. A diagram of the torch (Figure 4) shows that the coil enclosed three concentric tubes. Cooling water (1 to 4.5 liters per minute) was passed through the annulus between the outer borosilicate glass tube and the 28-mm. i.d. quartz confinement tube. The 22-mm. 0.d. inner tube, which terminated about 2 cm. above the work coil, was used for the coaxial jet experiments and was removed when vortex flow was required. I n the latter case a torch head with a tangential gas inlet was used instead of the head pictured. The torch was started by use of a Tesla coil discharge, using the technique described by Marynowski and Monroe (1964). The stability of the thermal plasma depended strongly on the lengths of the confinement tube both upstream and downstream of the plasma (Miller, 1968). The only previous references to such an effect were by Huska and Clump (1967), who found that an unstable plasma was stabilized simply by attaching a quench chamber to the open confinement tube end, and by Beguin et al. (1964), who found by moving a rod up and down inside the confinement tube below the plasma that instabilities were set up a t certain rod positions. From our results (Miller, 1968) it was postulated that the instability was associated with nodes and antinodes for the longest-wave-length natural sound wave that could be driven in the tube by the plasma. The plasma and open tube ends are located a t antinodes and the torch inlet a t a node. Placing an exit head on the tube bottom stabilized the plasma completely. Whether or not the 360-cycle power supply ripple had anything to do with the instability was not investigated; operation for all reported runs was restricted to a stable configuration. There was very little visual difference between the vortex and coaxial flow plasmas, both exhibiting maximum light intensity a t the center of the tube with a visual cutoff a t about 22-mm. diameter when viewed perpendicular to the tube axis through a blackened protective window in the torch enclosure. When viewed from the open tube bottom through a similar window, the plasmas exhibited maximum intensity in a ring of approximately 15-mm. diameter; there was a definite darkening toward the tube axis. This observation is in accord with an off-axis maximum temperature. An increase in power generally increased both the brightness and visual cutoff diameter on the plasma. Coupling Efficiency. A definition of radio-frequency torch coupling efficiency that has been widely used is the ratio
ner Tube (Quartz) nfinement Tube
Work Coil-
Water Jacket ( P y r e x )
Water Distribution Ring for Six I n l e t Holea
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Figure 4. Diagram of plasma torch
2a T.ocrh
Exit
water-cooled radio-frequency
of power absorbed by the plasma to the d.c. input power for the oscillator tube (plate power). This coupling efficiency should be a satisfactory measure of the overall power transfer efficiency for the induction plasma. Losses in the oscillator tube, tank circuit, transmission lines, and work coil, as well as dielectric heating of the torch walls, may all contribute to less than perfect coupling efficiency. As stated by Marynowski and Monroe (1964), the oscillator is operated as “class C,” meaning that the tube is “on” for only a small portion of the r-f cycle, and thus the plate efficiency can be as high as 75 to 80%; however, additional losses will normally reduce the coupling efficiency to the 50 to 60% range. These figures assume that the generator is operated under matched impedance conditions. The highest efficiency claimed by a manufacturer, even with carefully tuned equipment, is about 70%. Plate power can be measured directly by d.c. voltmeter and ammeter, while the plasma power input is commonly determined by calorimetry. Comparison of a number of previous determinations from the literature indicated that coupling efficiency is dependent on the particular generator employed (Miller, 1968). However, values a t least as high as 50 to 70% can be obtained with proper equipment operating in the megacycle frequency ranges with pure argon. Coupling efficiency measurements were made for the present system by employing the water-cooling jacket for the torch walls and attaching a heat exchanger to the bottom of the confinement tube to return the argon to room temperature. The cooling water was circulated VOL. 8 N O . 3 J U L Y 1 9 6 9
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through the entire system, its temperature being monitored a t the inlet and outlet with thermometers. By operating the generator without a plasma load, power losses due to dielectric heating in the torch were determined to be on the order of 1% of the plate power. This small correction was applied to all reported efficiencies, since this heat ends up in the torch cooling water. If it was desired to trap the radiation losses as well, carbon ink was added to the cooling water and a recirculation system used instead of the house water. The quantity of ink required was determined by adding an amount sufficient so that further additions did not increase the cooling water temperature rise. More than twice the required ink was used in all reported determinations. The blackened cooling water allowed almost no visible light to pass. I t was determined that there was no significant power coupling to the solid carbon particles in the ink by the magnetic field. The first investigation was concerned with how to tune the generator for optimal power transfer. Maximum efficiency was always achieved if the grid drive was adjusted so that the grid current registered 10 to 12% of the plate current, in agreement with the recommendations of Lepel and the operating procedure of Marynowski and Monroe (1964). All efficiencies reported were determined with the generator tuned in this manner. Coupling efficiencies determined for the coaxial flow system handling 22 liters per minute of argon are shown in Figure 5, both with and without radiation. The estimated accuracy of each experiment was &5%. No correlation could be found between efficiency and argon volumetric flow split between the annulus and inner tube, the split ranging from 1.4 to 03. The figure shows that there is also very little change of coupling efficiency with power input; all total efficiency points are between 56 and 61 for plate powers roughly from 3 to 7 kw. The radiation contribution increases with plate power, ranging from about 17 to 22% of the plasma power input over the above range of plate power. This reaction level is in agreement with the results of the theoretical work
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Figure 5. Effect of plate power on radio-frequency torch coupling efficiency for coaxial and vortex flow of argon 374
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Figure 6. Effect of argon flow rate on radio-frequency plasma torch coupling efficiency for coaxial and vortex flow
of Miller and Ayen (1968), which may also be used to estimate convection losses in r-f torches. A correlation of efficiency with total gas flow rate was found (Figure 6). I n the series of runs shown, the coupling efficiency decreased from 64 to 5770, as the argon flow was increased from 9 to 28 liters per minute a t 4.5-kw. plate power. Similar determinations, although not in such great numbers, were made for the vortex flow arrangement. The distance between the inlet head was adjusted to two positions, 4 and 7 inches above the coil, respectively. In either case the efficiencies agreed well with the results presented for the coaxial scheme. No important difference in coupling efficiency could be established between the two flow arrangements for this generator. This is in disagreement with the observations of Huska and Clump (1967), who found much higher efficiencies for the vortex arrangement, using a Lepel generator similar to that used in this work. Effect of Foreign Gases. If an induction argon plasma is to be used as a chemical reactor, it is necessary to know the coupling efficiency with the actual reactants being fed to the torch. I n connection with the study of titanium tetrachloride reduction it was desired to determine the effect of small concentrations of both TiCL and HP in the torch Argon feed. Stable plasma operation with TiCL was restricted to concentrations less than 1 mole %. Upon addition of this species a very intense sky-blue radiation was emitted from the coil region, with an orange tail flame visible for a considerable distance down the torch (8 to 12 inches). The blue region appeared to fill the entire confinement tube within the coil. The higher the TiC14 concentration the more intense the blue radiation became, until upon a further increase the plasma extinguished itself. At higher power inputs TiCL could be fed in greater quantities, the maximum being about 1 mole % for 8- to 10-kw. plate power. Hydrogen had much less effect on plasma operation and appearance. No noticeable change could be detected for less than 2 mole %, while a t 5 mole 5% the plasma radiation had a noticeable red coloration. No operational problems were encountered for H P concentrations as high as 5 to 10 mole %. Plots of coupling efficiency us. mole per cent TiC1, in Ar and mole per cent Hz in Ar are shown in Figure
2
$401 0
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< 4 p.p.m. 0 2 , and < 7 p.p.m. N2, all on a molar basis. The argon was dried by a silica gel trap a t dry iceacetone temperature, but the oxygen and nitrogen were not removed. Sufficient TiCL was always fed so that complete reaction with these materials constituted less than 1.5% conversion of the inlet titanium. Titanium tetrachloride was introduced by bubbling a portion of the dried argon stream through two glass sparger bottles containing liquid Tic14 that had been distilled into them under vacuum. The sparger bottles were maintained in constant temperature baths to control the tetrachloride vapor pressure. I t was determined that the pressure of the TiC14 in argon was equal to 1.3 mole % TiC1, a t 23.5'C., in agreement with the vapor pressures measured by Schafer et al. (1954), and concentrations measured in a similar sparger setup by Srinivasan and Bhatnagar (1964). Both total argon flow rate and the argon flow rate to the bubblers were monitored with rotameters. The bubblers and torch functioned a t very close to atmospheric pressure. The coaxial jet torch head provided separate gas inlets for flow through the annular region and through the central tube. The gas from the bubblers could be supplied to either of these inlets or injected into the plasma tail flame through a quench ring. Downstream of the coil region the apparatus consisted of a cooling section and a vent to the hood. The quench ring could be inserted through the bottom of the confinement tube and raised to within about 2 inches of the bottom of the work coil. N o destruction or coupling of the quench ring could be detected. Either pure argon or the argon and tetrachloride feed could be introduced. As an alternative, a stainless steel, water-cooled finger of 19-mm. diameter could be inserted into the tail flame of the plasma, providing a cold surface for solid deposition. Operation with Ar and TiCL Concentrations of TiC14 in Ar on the order of 0.2 mole 5% could be handled a t low to medium power levels, and thus this amount was chosen for the preliminary runs. It was necessary to purge all system lines with argon before runs to eliminate air contamination and thus prevent reaction of TiC14 with 0 2 and N2 in the torch.
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Figure 7. Effect of feed composition on radiofrequency torch coupling efficiency
7. HShas very little influence a t low concentrations, while TiC14 additions decrease the efficiency. These observations may be explained in terms of changes in the electrical characteristic of the plasma load. The ionization potentials for Ar, H, C1, and T i are, respectively, 15.8, 13.6, 13.0, and 6.8 e.v. Thus if titanium is present, it will be significantly ionized a t much lower temperatures than pure argon, and the plasma electrical conductivities, and consequently the power input and temperature profiles, will be changed. When 0.37 mole cio H2 and 0.15 mole % TiCL were fed simultaneously to the torch, the efficiencies amounted to 56 to 58%, in good agreement with the result for the same quantity of TiC14 alone, in spite of the fact that the TiCL was being almost quantitatively reduced to TiCL. This is a reasonable result in view of the fact that the plasma temperatures are so high that only atomic species should exist in regions of significant power input. Reactions of Titanium Tetrachloride
Experimental Equipment. The 10-kw. Lepel r-f generator described above was employed for the studies of TiC14 decomposition. A schematic for the entire plant is shown in Figure 8. Argon was used in all studies as supplied by Matheson Co., with specifications of < 6 p.p.m. H 2 0 ,
Control
-n
Torch Head
U
Argon
Control Valve
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Figure 8. Schematic diagram of apparatus used to study reaction of titanium tetrachloride
Meter Dryer
Hydrogen
-Cooling
Water Inlet Exhaust
Trap
VOL. 8 N O . 3 JULY 1 9 6 9
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I n a typical run 10 liters per minute of argon containing 0.2 mole % TiC14 was fed to the torch (operating a t 5-kw. plate power) for 10 minutes. N o quenching other than natural heat losses to the cooled tube walls was used. An inspection of the apparatus after the run indicated that no solid reduction products-i.e., TiCL, TiClZ, or Ti-had formed from gases passing through the plasma and on down the tube. A very shiny metallic mirror of silver-gray color was deposited on a portion of the cooled confinement tube adjacent to the work coil. I t covered -6 sq. cm. of area and was surrounded by black, then blue, and then yellow deposits. All deposits except the metallic mirror were loosely attached and easily removed. Upon sitting in air for a short time, the blue color of some of these deposits disappeared, only black remaining. This latter material was determined by x-ray scattering to contain large quantities of Ti203.The metallic tube deposit was too thin to scrape off and analyze, but it was definitely a titanium compound, as a plasma containing some O2 was found to convert the mirror instantly to the white Ti02. A reasonable explanation would appear to be that the mirror was Ti, surrounded by TiC12 (black) and then TiCL (violet); nevertheless, the total deposit represented an insignificant percentage of the titanium fed to the torch. The above result was typical of all attempts to obtain reduction by quenching the high-temperature gas-phase equilibria for TiC14 in the argon plasma (cf. Figure 1). Impingement on the stainless steel cold finger, injection of cold argon through the quench ring, feeding the TiCl, through the quench ring, feeding the TiCla through the small central tube directly into the plasma, changing the power levels, changing the TiCL concentration in the feed, and converting to the vortex-stabilized plasma torch were all tried without appreciable success. Reduction of TiCl, with Hydrogen. Based on the previous investigations using hydrogen reduction in discharges, it was felt that reduction to TiCL should be feasible in the r-f torch. Other candidates might be reducing gases, such as CO, or certain of the metals, such as Mg, Ca, Na, or K, but most of the latter would be difficult to feed to the r-f torch. High purity hydrogen (
