Tungsten Carbide by Pyrolysis of Tungsten Hexacarbonyl DALLAS T. HURD, H. R. RICENTEE', AND P. H. BRISBIN Research Laboratory, General Electric Co., Schenectady, N . Y .
HERE have been several indications that the pyrolysis of tungsten carbonyl a t high temperatures might lead directly t o tungsten carbide. Various sources in the scientific lit,erature describe the formation of tungsten carbides from tungsten metal by reaction with carbon monoxide or methane a t temperatures as low as 800' t o 1000" C., whereas the direct reaction between tungst,en and elemental carbon proceeds rapidly only a t temperatures upwards of 1400" C. In their work on vapor-phase plating with molybdenum and tungsten by the pyrolysis of carbonyls a t intermediate temperatures, Lander and Germer ( 5 ) found that contamination of the metal with carbon always occurred unless special means were taken to prevent it. They observed the formation of hexagonal molybdenum carbide (Mo2C) a t relatively low temperatures, but under their reaction conditions (below 800" C.) they did not obtain either tungsten carbide or the normal hexagonal form of W2C. Korton (6) of this laboratory analyzed the vapor of tungsten carbonyl with the mass spectrometer and found that the material decomposes stepwise under the electron bombardment in the instrument into the fragments WC(C0)s +,W(CO)5+, WC(CO), *, W(CO)r+,WC( C0)s +,W(CO)Q+, WC(CO)2+, W(CO)g+, WC(C0)+, SVC +,and W+. Although this type of stepwise decomposition under electron bombardment may not be representative of the decomposition of tungsten carbonyl at elevated temperatures, it is interesting to note the retention of carbon atoms in the intermediates, including t,he molecule WC +. Although the direct conversion of tungsten carbonyl t o tungsten carbide would be of considerable interest, it also was considered that under certain reaction conditions carbides might be formed by a decomposition of the carbonyl into free tungsten and carbon monoxide, and a subsequent carburization of the tungsten by the carbon monoxide. EXPERIMENTAL
The tungsten hexacarbonyl used in the investigations was prepared by reducing tungsten hexachloride xvith finely divided aluminum metal in a reaction medium of anhydrous ethyl ether in the presence of carbon monoxide a t pressures of 2000 t o 3000 pounds per square inch. The best yields of W(CO)e-about 85 t o 90'%-were obtained a t reaction temperatures in the neighborhood of 100" C. ( 4 ) . The tungsten carbonyl was purified by steam distillation from a 5% solution of sodium hydroxide, a process which removes the metallic impurities usually associated with tungsten, yet has no appreciable effect on the almost inert tungsten carbonyl. 1he carbonyl vapor condenses and separates in the condenser in the form of fine, snow-white crystals. These are separated from the distillate water and dried on a suction filter. The larger crystals used in the second phase of the investigation described below were prepared by a vacuum sublimation of the fine crystalline material a t about 60" C. PRELIMINARY IKVESTIGATIOSS. Prior to investigating the actual pyrolysis of tungsten carbonyl, a number of experiments were performed to determine the reactions of tungsten metal and tungsten carbide (WC) with various gases under the antici1
Present address, Chemical Department, General Electric Co., Waterford,
N. Y.
pated reaction conditions. The tungsten nietal and the tungsten carbide both were General Electric products; the carbide was verified both by x-ray analysis and by chemical analysis before it was used. The materials, bothin the form of very fhepowders (2 to 20 microns), were heated in Alundum boats in a horizontal quartz tube furnace while different gases were passed through the tube, usually for periods of several hours. A check was maintained on the loss or gain in weight of the samples, and the reacted materials were analyzed variously by x-ray, by combustion analyses for carbon, and by chemical analysis t o determine the presence of tungsten oxide. The results of these experiments verified, in general, the information found in the literature concerning the chemical behavior of tungsten and tungsten carbide under the particular reaction conditions. Qualitatively, such results are: Tungsten heated in carbon monoxide a t 1000" C. is carburized, but at a very slow rate. Tungsten heated in methane a t 1000° C. is carburized very rapidly to form tungsten carbide, but excess carbon also is deposited. Tungsten carbide is decarburized slowly by hydrogen a t 1nnno r! -. Tungsten carbide heated in carbon dioxide is oxidized and loses carbon a t 1000" C. It also will lose carbon a t $50' C., but a t a much slower rate. Tungsten trioxide is reduced by carbon monoxide a t 1000" C. and is carburized a t a slow rate Tungsten carbide remains unchanged in carbon monoxide at 1000" C., a t least for several hours.
-"-"
The conditions used for these tests were not equilibrium conditions. Hotwver, it was questionable whether equilibrium conditions would be reached in the experimental apparatus for pyrolyzing tungsten carbonyl, so the foregoing information wa6 considered pertinent. It appeared from the literature that of thetv, o tungsten carbides, only WC should exist as the stable form in a temperature range of approximately 800" t o 1850" C., provided that equilibrium was reached. At temperatures below about 800" C., TV2C apparently is the thermodynamically stable carbide form ( 2 , 3 ,7 ) . Therefore, it was concluded that the pyrolysis experiments should be done a t a temperature well above the 800" C. limit. VAPORIKJECTION APPARATUS.The apparatus used in the initial experiments in carbonyl pyrolysis is illustrated in Figure 1. It comprised a boiler, or heated chamber, from which tungsten carbonyl could be vaporized into a stream of inert or reactant carrier gas and injected through a jet system into a hot tube where the pyrolysis occurred, The jet system was maintained a t a constant temperature by a surrounding bath of boiling liquid; methyl Cellosolve acetate, boiling point 144" C., was used in most of the experiments. The pyrolysis furnace v a s a tube of chemical porcelain wrapped with Nichrome ribbon and insulated with asbestos tape and magnesia steam-pipe lagging; it could be operated a t temperatures up t o 1100" C. and in a few instances was held at 1200" C. for several hours. At the bottom of the pyrolysis tube was a cylin: drical aluminum tube with a removable system of baffles t o collect the fine dust of the product. The carbonyl boiler was heated with a separate resistance-&ire n-inding and always was
2432
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
October 1952
operated at a temperature below the temperature of the jet system t o avoid depositing crystals of carbonyl within t h e jet. No observable decomposition of carbonyl occurred within the jet system in any of the runs. Plugging of the jet orifice with tungsten owing t o a decomposition of the carbonyl by radiant heating from the furnace was encountered only in a few runs made at very low rates of gas flow. Temperatures of the carbonyl boiler, jet tube, and pyrolysis tube were measured with thermocouples. The flow of gas through the system was measured on rotameters, including a rotameter on the outlet of the apparatus t o determine any volume changes during the reaction and t o indicate any major leaks in the system.
CONDENSER
-4E
BOILING LIQUID
,SWEEP GAS
TABLE I. PYROLYSES IN VAPORINJECTION APPARATUS Temp., O
a
c.
ZbYka
Feed Gas Na
94 38
Product W
w wsc w;c
97 WZC, wc 15 w, w2c in feed gas flow.
sis apparatus, since the size range desired for the production of cemented carbides is from about 1 t o 20 microns. A more serious difficulty was that the desired carbide, WC, waa not produced. Table I presents data from a few experiments selected from a considerable number of such runs. It is apparent from the experimental data that in the temperature range of these investigations, the pyrolysis of tungsten carbonyl does not yield WC as a decomposition product. It also is evident that the desired carburization reactions did not go t o completion;
t
PLUNGER
-VALVE --GAS
/-FJYROLYSIS
Contact Time, Sec. 4
1000 3.55 1000 co 2.75 1000 co 0.63 1100 co 1.08 900 co 0.156 Calculated concentration of carbonyl vapor
/GAR
I
2433
BONYL FOR CARBONYL FROM ROTAMETER
TUBE
TRAP
-TO Figure 1.
HOOD
Apparatus for Tungsten Carbide Preparation
In making an experimental run with this apparatus, the system was continuously flushed with nitrogen while the pyrolysis tube was brought up to temperature, then the gas flow was changed t o the desired rate of flow of the particular sweep gas being used, and finally the carbonyl boiler was brought t o the temperature calculated to give the desired concentration of carbonyl vapor in the gas flow through the jet. At the completion of the run, the boiler was cooled, the feed gas was replaced with nitrogen, and the furnace was turned off. The products from the various runs were subjected to analysis by x-ray diffraction, to analysis for carbon content, and, in some instances, to a particle size determination by electron microscopy. Practically all of the runs yielded the product in the form of an extremely h e black powder which, however, was retained remarkably well by the baffle system used as a collector. All of the material in the samples examined by electron microscopy appeared to be under 1 / ~ micron in diameter, with particles ranging down to the limit of resolution of the instrument in size. This appeared t o be a major disadvantage of this type of pyroly-
' L T O HOOD Figure 2.
System for Tungsten Carbide Preparation
the general formation of W2C rather than WC even in the presence of excess carbon monoxide is evidence for this conclusion. The calculated contact times, however, were based only on the velocity of gas through the apparatus and neglected the effects of convection and settling. Since the heavy powder does tend to settle rapidly in slow-moving gas, it is possible that the actual contact times were somewhat shorter than the figures indicate, particularly in the runs with very long contact times-Le., slow gas flow. I n a few runs (not shown in Table I)in which the contact times were very short and the concentrations of carbonyl in the feed gas were very high, some tungsten oxide (WZOS)was found in the products.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2434
SOLIDINJECTION APPARATUS.Since it was obvious that the desired particle size range was not achieved with the vapor injection apparatus, and that WC was not formed directly under the available conditions of operating temperature, the apparatus illustrated in Figure 2 was designed. The pyrolysis furnacre in this apparatus was similar to that in the vapor injection apparatus, but the vapor injection jet system was replaced with a carbonyl reservoir having a plunger valve a t the bottom through which solid crystals of tungsten carbonyl could be dropped into the pyrolysis zone. Situated within the bottom part of the pyrolysis tube, well up into the heated zone, was a long narrow bucket of stainless steel which was packed with granular material to collect and hold the pyrolysis product. Various materials \+ere tried as packing in this bucket, such as quartz, iron, and porcelain; of these, small porcelain chips appeared to be the most satisfactory. Carbon monoxide, alone or mixed with other gases, was introduced through the injection system during the operation.
TABLE 11. PYROLYSES IN SOLIDINJECTION APPARATUS TEmp.,
c.
1000
1000 1000 1500 1000
1000
Feed Gas
co co co
CO CO-Nz 1-6 CO-Xa: 3-1
Soak Period, Hr. 4
3 2 1 4 4
X-Ray Analysis
wc: wc wc, wzc
WC, trace WIC W. some WC W C , trace WzC
Carbon,
Yo
9 2 6 9 5 18 6 2 1 01 5 40
Packing Quartz Quartz Quartz Quartz Porcelain Porcelain
l h e general procedure for operation was to bring the pyrolysis furnace up to temperature with a slow flow of carbon monoxide through the apparatus, then periodically throttle the flow of gas and drop a small amount of carbonyl into the system by deprcssing the plunger on the reservoir. The interruption of the gas flow was necessary since the decomposition of the carbonyl generated large volumes of gas. The furnace end of the injection system was uwally maintained a t 100" C. by a surrounding water bath, which was cooled in turn by a water jacket. Following the complet,ion of the carbonyl injection, the furnace temperature was adjusted to the desired soak temperature, and the pyrolysis product then was soaked in carbon monoxide ,or a mixture of reactant gases for an extended period of time. The furnace then was cooled rapidly with the gas flow maintained through it, Finally, the bucket was .v;ithdrawn and the product powder was washed from the packing material and dried. The pyrolysis step generally was performed in the temperature range from 650" to 700" C., although in some of t'he mns the pyrolysis was niade at 1000' C. Surprisingly, no significant differences in particle size were observed between the materinls produced by pyrolyses a t the two different temperature9. Mlien sublimed tungsten carbonyl crystals with an average lengt,h of about 1 mni. were used, the particle size range of the product from the reaction was 1 to 20 microns, Kith the prinripal fraction in the range 6 to 10 microns. This is in a desirable particle size range for the production of cemented t,ungsten carbide. Later, the fine crystalline carbonyl obtained from the steam distillation purificat'ion was used directly. With this material the particle size range was generally about I/* to 8 microns, with the principal fraction being 2 to 4 microns; this also is in the range of particle sizes useful for cementation. The carburization step was performed a t temperatures from 1000" to 1150" C. It became apparent early in the investigations that although the pyrolysis product from the carbonyl could be converted completely t o WC by heating it in carbon monoxide a t 1000" C., the process was uneconomically slow and some excess carbon was deposited. It also was observed that the packing in the bucket markedly influenced the deposition of free carbon, Table I1 presents data from a few selected experiments made with the solid injection apparatus. It will be seen
Vol. 44, No. 10
that relatively long times are required for the pyrolysis product to be converted completely to WC at 1OOO" C. At 1150" C. the process is considerably more rapid; temperatures even higher than 1150" C. probably would be much more useful than the range of temperatures covered in these investigations. THERMOUYKARIIC CONSIDERATIOKS. It was considered a t this point that the primary objectives of the investigation had been realized: Tungsten carbonyl had been pyrolyzed and converted to tungsten carbide of the required particle size distribution. A problem that still remained -cas the control of the carbon content in the product,. One possible answer (perhaps the easiest) would be to mix the raw pyrolysis product, with the required amount of powdered carbon and fire the mixture according to tire technique now in use for converting tungsten t o tungsten carbide. For a gas-phase carburization process, the problem of controlling the carbon content would mainly be one of preventing the formation of excess carbon by the equilibration of carbon monoxide into carbon dioxide and free carbon a t the operating temperature; in view of the thermodynamic data available on the carbonoxygen system, it is considered that this phase of the problem vould not be difficult t o solve. For example, to repress the formation of free carbon in a continuous flow of carbon monoxide over tungsten at, say, 1000° C., it w u l d be necessary merely to add an amount of carbon dioxide slightly in excess of the equilibrium partial pressure a t this temperature for the reaction 2CO C COZ, provided, however, that the partial pressure of carbon monoxide in resulting gas mixture would be sufficient to carburize the tungsten to tungsten carbide. Unfortunately, no thermodynamic data were available on tungsten carbide. To this end, therefore, the equilibrium constant for the reaction
-
+
\v
+ 2co
n-c + coz
e-,
;I 1
was estimated to be In K = 24557/T
- 0.9687 In 2"
+ 0.8857 X lOW3T- 14.444
(2)
In determining this relationship, it was necessary to approximate the heat capacity and the standard molal entropy of tungsten carbide. Knowing the specific heat of one grade of cemented tungsten carbide (95% U-C, 5% Co) and some thermodynamic constants for certain other heavy metal carbides, the values for Cypand SM were assumed to be 9.4 and 9.0 calories per gram mole per K., respectively. On the basis of Equation 2 and known therniodynamic data on the carbon-oxygen system (I),it then n - a e ralculated that a t 1000° C. the reaction
W(C0)s
+ WC 4COS + 4CO
(3)
should proceed to the formation of pure TVC in a closed system, or, similarly, tungsten should be carburized t o pure WC with no excess carbon a t 1000° C. in a continuous stream of a 4 t o I mixture of carbon monoxide and carbon dioxide. The observation that a small sample of tungsten carbonyl sealed in vacuo in a quartz tube and heated at 1000" C. for 16 hours did ,yield relatively pure R C (by x-ray analysis, and by analysis for carbon: carbon = S.l%, theoretical = 6.12%) indicates that these approximations are a t least reasonable estimates. Similar calculations can be made for other carburizing gas systems, such a3 methane and hydrogen. It is apparent t'hat to be economically practical a gas-phase carburization process will require rathrr high temperatures so that the residence time in the furnace will be at a minimum. ACKNOWLEDGTIENT
The authors would like to ackno~vledyethe assistance of L. B. Bronk, Mrs. B. F. Decker, and E. F. Fullam, all of the General Electric Research Laboratory, for carbon analyses, x-ray diffraction analyses, and particle size determinations, respectively.
October 1952
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY LITERATURE CITED
(1) Austin, J. B., and Day, M. J., IND.ENG.CEIEM., 33, 23 (1941). (2) Beoker, K.‘, 2. Metallkunde, 20, 437 (1928). (3) Gregg, J. L.,“Alloys of Iron and Tungsten,” Chapt. IV, New York, McGraw-Hill Book Co., Inc., 1934. (4) Hurd, D.T., U. S. Patent 2,554,194(June 1950). ( 5 ) Lander, J. J., and Germer, L. H., Metals Technot., 14 (September 1947).
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(6) Norton, F. J . , private communication to D. T. Hurd. (7) Schenk, R., Kursen, F., and Wesselkoch, H., 2. anorg. u. allgevn. Chem., 203, 159 (1931). RECEIVED for review April 14, 1952. ACCEPTEDM a y 20, 1952 Presented at the X I I t h International Congress of Pure a n d Applied Chemistry, New York, September 1951.
Reaction between Ferrous Iron
and Dissolved Oxygen in Brine D. C. BOND AND G. G. BERNARD The Pure Oil Co., Research and Development Laboratories, Crystal Lake, Ill.
I
APPARATUS
N OIL field technology the reaction between ferrous iron
Figure 1 is a diagram of the apparatus used. All rertotions and oxygen in brine is of considerable importance. In an were carried out in a 1-liter Florence flask. A magnet, sealed in oil- or gas-producing well, water from two sands, one containing glass, was placed in the flask and rotatbedby means of a rotating oxygen and the other containing ferrous iron, can mix and react. In a water aeration tower ferrous iron is oxidized to form insoluble magnet under the flask. Preliminary tests showed that if the flask, partially filled with ferric hydroxide, which can be settled and filtered out of the water. brine, was stoppered and allowed to stand, no measurable change In a “closed” system for disposal of oil-well brine, the accidental in oxygen content of the brine in the flask would occur in 3 hours. entrance of air into the system can lead to the oxidation of ferrous In all experiments lasting an hour or less, samples were merely iron in the brine. Brine or fresh water containing oxygen can withdrawn from the flask by means of a pipet, and the flask was be injected into a sand, where it may mix withwater that contains then stoppered. ferrous iron: or in areas where air has been injected into oil In experiments lasting more than an hour, samples of brine sands for secondary recovery of oil, oxygen can be dissolved, were displaced through the capillary tube in the rubber stopper reacting with ferrous iron in the connate water in the sand. in the neck of the flask. This was done by inflating a rubber In all of these cases it ie important to know something about balloon inside the flask with water from a separatory funnel, the rate of reaction between ferrous iron and dissolved oxygen. Tests showed that if the balloon was filled with air-saturated This information is needed in order to predict the retention time water, no measurable amount of oxygen diffused through the required after aeration of water, to tell whether plugging of balloon into the brine in the flask in 3 days. Thus, it was possible filters or injection sands by ferric hydroxide can occur, t o predict to stir the reaction mixture and withdraw samples from time to what will happen when various fluids mix in underground rocks time without having air come in contact with the brine in the or in well bores, or to predict whether a given mixture of brines flask. The reaction flask was immersed in a water bath mainwill be corrosive. The reaction between ferrous iron and dissolved oxygen has tained at constant temperature (=k0.Zo F.). Dissolved oxygen determinations were made with a polarobeen studied by many investigators. In most cases a procedure has been used which involves the air blowing of solutions ( 4 , 6, graphic dissolved oxygen meter ( 1 7 ) 7, 1.2, 14, 99). In these cases determination of the rate of PROCEDURE Water reaction between ferrous iron The reaction between ferand oxygen has been obscured rous iron and dissolved oxygen by possible changes in rate of Produces hydrogen ion. Since solution of oxygen and changes it was desired that the experiin solubility of oxygen. With ments be carried out a t constant pII, the brines used were few exceptions quite concenRemoving Samples Slirrer buffered by the addition of trated (6, 13) or strongly boric acid, potassium acid Florence acidified solutions have been phthalate, and potassium hy“--h Nater Healer droxide. Table I shows the used (10,13, 18). J composition of the solutions It appears that no work has used in the experiments. been done on systems comBrine of the desired concenmonly encountered in the oil tration was pre ared with disfield, that is, mixtures containtilled water a n f reagent-grade sodium chloride. The soluing about 1 to 10 p.p.m. distion was buffered a t the desolved oxygen and 1 to 100 sired pH and then air blown p.p.m. ferrous iron with high Regulator for several hours to produce concentrations of salts. The a brine that was nearly satuMagnetic rated with air. The pH valuee present work was undertaken were determined with the glass in order t o obtain infsrmation electrode, corrected for sodium about the behavior of such content of solutions tested. Figure 1. Apparatus for Studying Reaction between systems. Dissolved Oxygen and Ferrous Ion The buffered brine solution,
Y
!I Illya
Yh
