Tungsten Oxide Reduction
A on reduction rate of pelletized tungsten di- and triPREVIOUS ARTICLE
d
(2)
oxides with hydrogen showed that reactions for the two compounds were similar, and dependent on temperature and pellet size. It was believed that a diffusional process was the controlling step, and work was continued to determine the effect of inert gases and pellet density on reaction rate. Mixtures of 25% hydrogen and 75% nitrogen, helium, and argon were used to reduce pellets of both oxides at temperatures between 700' and 850' C. Also, a series of pellets was used-right cylinders, height equal to diameter, bulk density of 4.5 grams per cc., and available in 3/16-, 5/18-, '/pinch sizes. The original pellets, l/4, l/s, and inch in diameter, had a bulk density of 3.7 grams per cc. Reduction rates for these two types of pellets were compared and identity of the low temperature phase of tungsten metal (beta material) was investigated. The equipment has been described generally ( 2 )and in detail ( 3 , 4 ) . Nitrogen was used as an inert medium. Reduction isotherms were constructed by reducing pellets for various time intervals and determining gravimetrically the per cent of oxygen removed. The reduced pellets were sectioned to observe reaction progress in the interior. Colored photographs of these are available (3). Using hydrogen-water vapor mixtures to produce pellets of tungsten dioxide by partially reducing the trioxide required several modifications in equipment and procedure. Water condensing in the gate valve at the top of the reactor caused the porcelain tube to shatter from thermal stress when the condensate ran down the inside of the tube. To avoid this, a second reactor-a 1-inch
TIME, SECONDS
-
Figure 1. Effect of inert gases (- -) and pellet density (-) on reduction rate Pellet Density 0 4.5 g./cc.
4 3.7 g./cc.
Gas Mixtures A Hz 0 H r N z H r H e
*
Hz-A
nominal schedule 40 Inconel pipe-was installed in parallel to use hydrogen saturated with water vapor. This proved satisfactory for humidifier temperatures of about 45' C. Data in Figure 1 (broken lines), typical for effect of inert gases on reduction rate, were obtained using a flow rate of about 0.4 cubic foot per minute and total pressure of 5 pounds per square inch gage. I n general, the reaction rate for heliumhydrogen is higher than that using nitrogen or argon as a diluent. The isotherms for the latter two gases were often so closely identified that it was impossible to distirguish between them. Effect of the inert can be attributed to change in diffusivity of hydrogen or water caused by presence of a third body. From the Stefan-Maxwell diffusion equation, M'ilke (5)derived the following expression for the effective diffusion coefficient of a gas in a multicomponent mixture of stagnant gases
Here B' is the point value effective diffusion coefficient and y, mole fraction of the various components. The mutual diffusion coefficient of two gases at a given temperature and total pressure is
where M is the molecular weight of the various gases. Although Equation 2 is derived for transport through a stagnant film of gas, it can be used to show effect of the inert gas on diffusivity of water vapor during the reaction process. If B is hydrogen; A, water; and C, various inerts, then Y B , yc, 1 - y A , and DAB in Equation 1 become constant for reduction at the same temperature and pressure. For nitrogen and argon, whose molecular weights are 28 and 44, respectively, the two values of DAC as given in Equation 2 are almost identical. Either of these two inerts give almost the same value of D'A. Since helium has a molecular weight of 4, DAc and D'A give the highest value. Hence, for equivalent points in reducing pellets of tungsten trioxide with hydrogen and one of the inerts, this interpretation supports the observation that the helium isotherm is the highest and the nitrogen and argon reductions are almost identical. I t is impossible by this approach to determine whether the transfer of hydrogen or water vapor is controlling.
No consistency appears in the initial rate of the various inerts. The initial rate must be characteristic of a period when the pellet has been penetrated with gas and a small amount of reduction occurs throughout. Lower density pellets reduce at faster rates (Figure 1, solid lines). Since material comprising both pellets was approximately 200-mesh, the higher density pellets have a smaller pore size which must impede gas flow. Smaller pellets such as l/s inch, reduced at relatively low temperatures (500' C.), show no phase boundary. X-ray analysis showed tungsten on the surface of the sectioned pellet, but gave combined spectrum of the metal and dioxide when the pellet was crushed and analyzed as a fine powder. A particlewise diffusion process seems to take place at lower temperatures. Since the reaction rate increases exponentially with temperature and the diffusion rate with the 3/2 power, increase in temperature increases the reaction rate faster than the diffusion. At a relatively higher temperature, diffusion in the pellet becomes the controlling step and produces the observed phase boundary. Extensive data on effect of water vapor on reduction rate for 3/16-inch pellets of tungsten dioxide at 700' C. were also obtained. Hydrogen was saturated with water by bubbling through humidifiers at 45' C. No reduction occurred beyond 34y0which would correspond to tungsten dioxide. If the hydrogen is assumed saturated when leaving the humidifier, concentration of water would be approximately 7%, below the equilibrium value given in the literature (7). This was noted previously in reducing l/d-inch pellets, but is even more striking for 3/,6-inch sizes.
Acknowledgment National Science Foundation and past support of the Research Corp. and Rensselaer Research Grants Committee made this work possible.
literature Cited (1) Chaudron, G., Ann. chim. 16, 221 (1921). (2) Hougen, J. O., Reeves, R. R., Mannella, G. G., IND.END.CHEM.48, 318 (1956). (3) Mannella, G . G., M.Ch.E. and Ph.D. theses, Rensselaer Polytechnic Institute, Troy, N. Y., 1955-56. (4) Reeves, R. R., Ph.D. thesis, Rensselaer Polytechnic Institute, Troy, N. Y . , 1954. ( 5 ) Wilke, C. R., Chem. Eag.. Progr. 46, 95 (1950).
G.G. MANNELLA and J. 0. HOUGENI Rensselaer Polytechnic Institute, Troy, N. Y.
Present address, Monsanto Chemical Co., St. Louis, Mo. V O L 49, NO. 5
M A Y 1957
893
