Investigation of the Kinetics of Reduction of Nickel Tungstate by Hydrogen

S. SRIDHAR, DU SICHEN, and S. SEETHARAMAN

In the present work, the kinetics of reduction of nickel tungstate, NiWO4, by hydrogen was investigated by a thermogravimetric method in the temperature range 891 to 1141 K. The ex- periments were conducted under both isothermal and nonisothermal conditions. The products were examined by X-ray diffraction analysis. The results indicate that the reduction reaction proceeds in two steps; first, reduction of NiWO4 to nickel as well as WO2 and then WO2 to tungsten. From the isothermal experiments, the activation energies of the two reaction steps were calculated to be 95.3 + 4.9 and 80.8 -+ 6.4 kJ- mol -l, respectively. The activation energy value obtained from nonisothermal experiments for the first step is in agreement with the iso- thermal experiments. The values are compared with the activation energies reported in other literature for the individual oxides.

I. INTRODUCTION

THE application of tungsten-reinforced nickel as a structural material for application under extreme condi- tions, as in the case of turbo-jet engines, LIJ necessitates an examination of the various methods for producing these alloys. In synthesizing nickel alloys with tungsten fibers by the reduction of oxides, it is important to have a knowledge of the reduction kinetics. A thermodynamic investigation of the ternary system Ni-W-O has recently been carried out in the present laboratory.~2~ The results of this study indicate that the only ternary oxide that ex- ists in this system is NiWO4. Woo and Roy t3~ have iden- tified this compound as the major oxide formed in the oxidation of nickel-tungsten composites. A similar ob- servation has also been made by Kvernes and Kofstad.t4] A possible route to the production of the alloy coatings is to reduce NiWO4 oxide layers over suitable substrate using a reductant like hydrogen or carbon monoxide. Studies of the kinetics of reduction of NiWO4 are also quite relevant in the production of complex carbides which are of importance in the hard metal industry. In this con- nection, an earlier study on the influence of nickel on the hydrogen reduction of WO3 tSj shows that nickel metal is formed in the first stage of reduction and tungsten whiskers are formed in the second stage.

The study of the reduction of nickel tungstate is of interest even from a theoretical viewpoint. The Department of Theoretical Metallurgy, Royal Institute of Technology, t2~ is seriously involved in a project on the study of the reduction mechanisms of complex oxides. Toward this aim, experimental and theoretical studies have been carried out earlier on the kinetics of hydrogen re- duction of the oxides of molybdenum t61 and tungsten.J7] The reduction kinetics of nickel oxide by H 2 is currently being investigated. It would be interesting to compare

S. SRIDHAR, formerly with Royal Institute of Technology, Stockholm, Sweden, is Special Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. DU SICHEN, Research Associate, and S. SEETHARAMAN, Professor, are with the Division of Theoretical Metallurgy, Department of Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden.

Manuscript submitted July 29, 1993.

the reduction kinetics of the complex oxide NiO-WO3 with that of the corresponding simple oxides under iden- tical experimental conditions. The present investigation seeks to develop an understanding of the reduction ki- netics of nickel tungstate by hydrogen in the temperature range 891 to 1141 K. The studies have been carried out using a thermogravimetric method.

II. EXPERIMENTAL

A. Materials

NiWO4 powder (99 pct purity) was supplied by Johnson and Matthey Ltd., United Kingdom, and was calcined at 1023 K for 24 hours. The samples were preserved in a desiccator before being used in the experiments. The argon gas used during the prereduction period of the thermogravimetric experiments contained a maximum of 2 ppm impurity and was supplied by AGA Special Gas, Stockholm. The hydrogen gas used for reduction was of SR grade supplied by AGA Gas.

B. Apparatus and Procedure

The thermogravimetric studies were carried out in a SETARAM TGA 92 (made in France) with a detection limit of 1 /zg. The experimental assembly is illustrated in Figure 1. The instrument was controlled by an IBM PC* through a CS 92 controller.

*IBM PC is a trademark of International Business Machines Corporation, Armonk, NY.

A detailed description of this apparatus is given in an earlier publication.t6] The NiWO~ sample of known weight was laid out in the form of a bed at the bottom of an alumina crucible with an 8-mm i.d. and 14-ram height. The crucible was hung from the balance using a platinum suspension wire inside an alumina reaction tube that, in turn, was positioned inside a graphite furnace. It was ascertained during assembly of the experimental setup that the crucible was in the even temperature zone of the furnace and was not touching the walls of the reaction tube.

During the isothermal experiments, the reaction tube

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 25B, JUNE 1994--391

was evacuated before the start of the experiment to a vacuum of 10 Pa for 15 minutes and was filled with argon gas through the auxiliary gas inlet. The sample was heated to the experimental temperature under a steady argon flow of 0.05 L . min -j . No weight change was observed dur- ing the heating period. When the temperature was stable, the argon atmosphere was replaced by hydrogen gas in- troduced through the carrier gas inlet at the top of the reaction tube. Even in the case of the nonisothermal ex- periments, the reaction tube was filled with hydrogen gas through the cartier gas inlet at the top of the reaction tube. A constant flow rate of hydrogen (0.6 L- min -t) was maintained during the heating period.

In the case of both isothermal and nonisothermal ex- periments, the bed heights were generally maintained at about 1.0 mm corresponding to 146 mg of the sample. In all the e x~eriments, a constant hydrogen flow rate of 0.6 L- min- was maintained during the reduction period. Preliminary experiments with different flow rates of the reductant gas showed that this flow rate was above the starvation rate for the reaction. The weight losses were registered by the data collection system at intervals of 2 seconds. The reproducibility of the measurements was confirmed in the case of some selected experiments. The lower temperature limit of the present measurements was kept at 891 K, as the reaction rate was found to be ex- tremely slow below this temperature. For nonisothermal experiments, three different scanning rates, 15, 20, and 22 K - m i n -~, were employed.

In the case of some of the thermogravimetric experi- ments, the reaction was arrested at different stages of the experiments and the reductant gas was replaced by argon as the sample was cooled at the maximum possible rate, viz. 99 K . min-L These samples were subsequently ex- amined by X-ray diffraction method in order to identify the phases present. For this purpose, a PHILIPS* X-ray

*PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.

diffraction instrument, x-pert system, available in the department was used.

I I I . R E S U L T S

A. Isothermal Reduction

The reduction curves obtained in the case of the iso- thermal experiments are presented in Figure 2. The mass loss is given by AW which represents the dimensionless mass change; namely, the ratio of the mass loss to the initial mass. In view of the large number of data points, the raw data are not presented in this article but could be provided by the authors on request. Figure 2 shows that the reaction is extremely slow at 791 K. This seems to be in accordance with the observations made in the case of the reduction of molybdenum oxide.J6] In the case of the reduction of pure tungsten oxide under similar conditions, 171 the reduction was found to be slow even at 891 K. This difference could be attributed to the pres- ence of NiO in the compound in the present experiments, which is more easily reduced. A careful examination of the raw data revealed that the reaction proceeds with a sharp decrease in the reaction rate around AW = 0.104.

,•1 THERMOBALANCE

CARPJER-GAS AUXILIARY-GAS INLET INLET liii I GRAPHITE FURNACE

' I l l

Fig. 1 - - T h e experimental assembly.

0

-5

OE) -10

-15

-20

I ' I

791 K

1191

- - 1 1 4 1 K I I = I

0 500 1000

TIME(s) Fig. 2 - - I so thermal reduction curves.

This value would correspond to the theoretical comple- tion of the reduction of NiWO4 to Ni and WO2. It should be noted that this decrease in rate is not evident in Figure 2. On the other hand, this is shown clearly in Figure 3, where the first derivative of dimensionless mass change with respect to time, d(AW)/dt, has been plotted as a function of time. The "break" points in these curves occur around AW = 0.104. It can also be seen in Figure 3 that the discontinuity gets less pronounced.as the temperature of reduction is lowered.

In Figure 4, the effect of the change of the bed height on the reduction rate has been illustrated. The mass loss is expressed in terms of Am, in grams. The curves for the different bed heights coincide with each other during the first part of the reduction (corresponding approxi- mately to AW = 0.104). In this part, the reduction seems

392--VOLUME 25B, JUNE 1994 METALLURGICAL AND MATERIALS TRANSACTIONS B

to proceed independent of the height of the bed. On the other hand, the latter portion of the curves in Figure 4 shows a strong dependency on the bed height indicating the prevalence of a diffusion mechanism for the reduc- tion process. This seems to be somewhat contradictory to the observations made in the case of the reduction of WO2 to W carried out earlier in this laboratory. Iv]

B. Nonisothermal Reduction

The results of the nonisothermal reduction studies at heating rates of 15, 20, and 22 K are presented in

' U ) v A

o x ' - I

0 . 0 0

- 0 . 0 1 -

-0.02-. [ ~ - .

-0.03 -

-0.0,.

-0.05.

"0"061

-0.07

-0,os I I I

0 2 o o 4 o o

TIME (s)

I

600

Fig. 3 - -Dif fe ren t ia l isothermal reduction curves. The arrows marked in the figure correspond to AW = 0.104,

Figure 5. Even in the case of the nonisothermal reduc- tion, the differential reduction curves d (AW) /d t vs time for the heating rates 15, 20, and 22 K- min - ' have been given in Figure 6. It is seen that the reaction rate shows a decrease around AW = 0.104 marked by arrows in the figure for the heating rates 20 and 22 K" min- ' . The curve corresponding to a heating rate of 15 K does not show this discontinuity. This could possibly be due to the fact that the reaction time for the first step is much longer, as can be seen in Figure 5, and hence, the second step could get merged with the first.

-5

O -10 (3

-20

I I I

2 2 K / m i ~

, I , I i I ,

0 200 400 600 800

TIME (s)

Fig. 5 - -Non i so the rma l reduction curves.

-10

O) -20 E

v

E <~ -30

-40

-50 0

I I I I

1 2: 1.3mm \ ~

4:: 2.1mm. " ~

= I = I I I = I =

200 400 600 800 1000

TIME (s)

Fig. 4 - - I so the rma l curves for different bed heights at 1041 K. The arrows marked in the figure correspond to AW = 0.104.

0.00,

~. -0.02

~:~ -0.04 Q I

-0.06 20K/min

-0.08 - ~ 200 400 6O0 800

TIME (s)

Fig. 6 - -Di f fe ren t ia l nonisothermal reduction curves. The arrows marked in the figure correspond to AW = 0.104.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 25B, JUNE 1994--393

C. X-Ray Diffraction Analysis

In the present work, an X-ray diffraction method was employed in order to identify the phases present in par- tially reduced and quenched specimens. The percentages of weight loss for the various samples studied are 5.94, 10.41, 17.43, and 20.4 reduced at 1041 K, the last one corresponding to complete reduction. Cu Ks radiation was used for the diffraction studies. The phases present in the various samples, identified by X-ray diffraction, are presented in Table I. The peaks for the nickel phase were found to be slightly shifted from those for the pure metal. This is possibly due to the slight solubility of tungsten in nickel, which is in conformity with the phase diagram for the Ni-W-O system arrived at by thermodynamic cal- culations, t2~ On the other hand, the tungsten metal peaks coincided with the literature data, I8J indicating that the solubility of nickel in tungsten is negligible. This is in complete agreement with the phase diagram of the Ni-W binary system, rg]

IV. DISCUSSION

The break in the reduction rate in the case of both isothermal and nonisothermal reductions at AW = 0.104 would correspond to the completion of the reduction of NiWO4 to Ni and WO2. The reaction could be repre- sented as

NiWO4(s) + 2H2(g) = Ni(s) + WO2<s) + 2H20(g) [1]

This suggests that for the system NiO-WO3, the reduc- tions of NiO to nickel and WO3 to WO2 occur simul- taneously. This seems to be in agreement with the earlier observations of Haubner et al. TM The foregoing reaction sequence does not take into consideration any reduction of the oxide of tungsten to the metal and its subsequent alloying with nickel. The shifting of the X-ray diffrac- tion lines for nickel in the case of the samples reduced to AW = 0.104, as mentioned in Section III, indicates the occurrence of this reaction to some extent.

The second step in the reaction would correspond to the reduction of WO2 to tungsten, viz.

WO2(s ) '~ 2Hz(g) = W(s) + 2H20(g ) [2]

The steps corresponding to the reduction of WO29 to WO272 and another for W02.72 to W O 2 could not be ob- served in the present work. In their study of the reduc- tion kinetics of the oxides of tungsten, Bustnes et al. F~ could observe only two steps, one corresponding to WO3-WO2 and the other to WO2-W. The present results are in agreement with this observation.

The isothermal sections of the Ni-W-O phase diagram

Table I. Phases Identified by X-Ray Diffraction at Different Stages of the Reduction of NiWO4

AW • 100 at the Phases Identified by Time of Quenching X-Ray Diffraction

5.94 NiWO4, WO2, Ni 10.41 NiWO4(trace), WO> Ni, 17.43 Ni, WO2, W 20.44 Ni, W

at 1273 and 1073 K recently computed in the Department of Theoretical Metallurgy 12] are presented in Figures 7 and 8. At 1273 K, the reduction path would go through the three-phase equilibrium NiWO4-WO2.72-Ni , then the two-phase region WO2.72-Ni, and subsequently the three- phase area WO2.Tz-WO2-Ni before entering the WO2-Ni area (Ni stands for nickel-rich alloy with tungsten). It is also seen that the nickel metal could alloy with tungsten metal forming a nickel-rich alloy. The phase relation- ships at 1073 K are somewhat different and indicate the occurrence of a four-phase equilibrium involving NiWOa- WO2.9-WO2 72-Ni between these two temperatures. This

1/202

1.0

0.9 /wo, o8/ I

, o.,/ 0 . 6 Z / / / / / I "wo,

~ / I \ 0.4

0.3

0.2

Ni 0 0.2 0.4 0.6 0.8 1.0 W

X(W) -.

Fig. 7--Isothermal section from the Ni-W-O phase diagram at 1273 K. m

1/202

1.0

0.gf /wo, , 0.8/I' ]

o.7I

0.5

0.4

0.3

0.2

0.1

0 Ni 0 0.2 0.4 0.6 0.8 1.0 W

X(W) -* Fig. 8--1sothermal section from the Ni-W-O phase diagram at 1073 KY I

394--VOLUME 25B, JUNE 1994 METALLURGICAL AND MATERIALS TRANSACTIONS B

would mean that there may be a change in the mecha- nism of reduction at temperatures below the transition point. This is supported by the present observation that the isothermal reduction curves for the temperatures 1141 and 1191 K show a lower rate of reduction than at 1091 K below AW = 0.12.

From the rates of the reaction obtained by isothermal reduction experiments at different temperatures, the ac- tivation energies for the two reaction steps could be cal- culated using Arrhenius plots. Such a plot for the first step involving Reaction [1] is given in Figure 9. In order to avoid the experimental errors owing to the transient hydrogen partial pressure when argon is replaced by H2, the rate for each reduction was taken at the point when the rate was maximum. The activation energy calculated is 95.3 kJ. mo1-1. From the scatter in the experiments, the error associated with this value is found to be ---4.9 kJ. mol- ' . It is interesting to compare this value with the activation energies of the reduction reactions for the individual oxides, viz.

NiO(s) + H2(g ) = Ni<s) + H20(g ) [3]

and

WO3(s) + Hz(g) = W O2(s ) q- H20(g ) [4]

The reduction of NiO by hydrogen was studied earlier by Szekeley and Evans tl~ and Deb Roy and Abraham. t1,1 While the former workers suggest an activation energy value of 54.4 kJ, rnol -~, they admit that the reaction mechanism is somewhat complicated and large uncer- tainties could be associated with their value. On the other hand, Deb Roy and Abraham I~~ have estimated the ac- tivation energies for Reaction [3] from the initial rates obtained by thermogravimetric studies as 21.8 kJ. mol- which would correspond to the chemical reaction con- trolling the rate. In order to resolve this, the present au- thors conducted some preliminary studies in the Department of Theoretical Metallurgy on the reduction

' I ' I ' I '

-7.5 .

-8.0

"o

-8.5 "o

-9.0

-9.5

0.00090 0.00095 0.00100 0.00105 0.00110

lfr (K 1)

Fig. 9 - -Arrhen ius plot for Reaction [1] during the reduction of NiWO4.

of pure NiO under similar conditions, as in the case of NiWO4 with shallow powder beds where the chemical reaction would be the rate-controlling step. The activa- tion energy value obtained in this work was of the order of I8 kJ-mo1-1 in support of the result of Deb Roy and Abraham. Bustnes et al. [71 have studied the reduction of WO3 in the present laboratory under similar conditions; but, unfortunately, these authors have not reported the activation energy for Reaction [4]. From their raw data, the value for the activation energy was estimated to be 75.7 -+ 3.6 k J" mol ~. It is evident that the dominating part of Reaction [ 1 ] would be the reduction of WO3 to WO2.

The Arrhenius plot for the second step in the isother- mal reduction experiments is presented in Figure 10. The rate values for this plot were taken at the early stages of the second step where chemical reaction would be rate controlling. The corresponding activation energy is 80.8 + 6.4 kJ. mol -~. This value is in good agreement with a value of 83.17 kJ. mol -~ for the activation energy of Reaction [2] reported by Bustnes et al. lTl This would suggest that in the case of the reduction of the complex oxide, NiWO4, the presence of nickel metal in the sec- ond step does not affect the reduction mechanism of WOz to tungsten. The correlations between the activation en- ergy values for the reduction of complex oxides and those for the simple ones expose the possibility of predicting the kinetics of reactions involving complex compounds from a knowledge of those of the simple systems under similar experimental conditions.

In the treatment of the data for the nonisothermal ex- periments, Bustnes et al. 171 have developed a theoretical model on the basis that the reaction proceeds by the movement of the reaction front, with the velocity of the movement being constant. According to this treatment, the function In ( d A W / d t ) would be a linear function of (1 /T) as represented by the equation

In (dAW/dt) = In [AW~ (2nbMsCako/hp)J - (Q/RT) [5]

where n is the average number of horizontal rows of small particles building up the reaction front, b is a stoichio- metric constant, M8 is the molecular weight of the reac- tant solid, Ca is the bulk concentration of the reactant gas, k0 is the rate constant for the gas-solid reaction, h is Planck's constant, p is the density of the reacting par- ticle, Q is the activation energy for the reaction, R is the universal gas constant, and T is the temperature of the reaction in kelvin. From Eq. [5], it can be seen that the slope of a plot of In (dAW/d t ) with respect to (1 I T ) should represent - ( Q / R ) . This provides a means of es- timating the activation energy of a reaction by noniso- thermal kinetic studies. This model was applied to the first part of the reduction data presented in Figure 6, and a corresponding plot is given in Figure 11 for the three heating rates used in the present work. The activation energy obtained is 88 + 4.4 kJ- mol -~, which is in good agreement with the value obtained by isothermal exper- iments. Attempts to apply the model to the second step in the isothermal reduction were not successful. This could be due to the inadequacy of the model to deal with sit- uations where the mass transfer plays a significant role

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 25B, JUNE 1994--395

" I ' ! , ! * | ,

-7.5 �9

-8.0

-9.5

I = I , I , I ,

0.00085 0.00090 0.00095 0.00100 0.00105 0.00110

1/T (K 1) Fig. 10 - - Arrhenius plot for Reaction [2] during the reduction of NiWO4.

"10 -8.5

<1

t'-

-9.0

-11.1

-11.2.

-11.3-

-11.4-

1 "o -11.5 ~

-11.7-

-11.8-

�9 15 K/min

~ f l ~ K f l B x 20 K/min 2 22 K/min

o o o x o

o �9 o

Q = 8 8 k J/mole

-11.9

1 I ' I ' I ' I ' I l I '

0.00093 0.00094 0.00095 0.00096 0.00097 0.00098 0.00099 0.00100

1F1" K -1

Fig. 1 1 - - A plot of In (dAW/dt) vs I / T for Reaction [1] by noniso- thermal reductions of NiWO4.

in the reaction mechanism. The importance of mass transfer in this step is well illustrated in the case of iso- thermal experiments with different bed heights presented in Figure 4, where the rate is affected by the height of the bed. A modification of the model incorporating the effects of mass transfer is deemed necessary, and efforts are under way for the improvement of the model in this direction.

IV. S U M M A R Y AND C O N C L U S I O N S

The reduction kinetics of NiWO4 by hydrogen gas was studied in the present work by means of the thermo- gravimetric method in the temperature range 891 to 1141 K. Both isothermal and nonisothermal techniques were employed. The results indicate that the reaction proceeds in two steps: one involving the reduction of NiWO4 to nickel and WO2, and a second step in which the WO2 is reduced to metallic tungsten. From the rates of the reaction obtained by isothermal studies, the acti- vation energies for the two steps were calculated to be 95.3 and 80.8 kJ. mo1-1, respectively. The results of the nonisothermal studies for the first step are in agreement with those of the isothermal experiments. For the second step, the mass transfer seems to play an important role. The activation energy values are compared with those obtained for the reduction of NiO and WO3 by hydrogen under similar experimental conditions.

A C K N O W L E D G M E N T

The present work has been carried out in conjunction with a consortium on "Computer Assisted Materials and Process Development."

REFERENCES

1. D.W. Petrasek and R.A. Signorelli: Composite Materials: Testing and Design, ASTM, Philadelphia, PA, 1969, pp. 405-16.

2. R.E. Aune, S. Sridhar, and Du Sichen: Report No. TRITA-MEL 124, Royal Institute of Technology, Stockholm; accepted for Presentation at the 1994 TMS Annual Meeting, San Francisco, CA, Feb. 1994.

3. O.T. Woo and C. Roy: 26th Canadian Metal Physics Conf., Kingston, ON, Canada, 1976.

4. I. Kvernes and P. Kofstad: Scand. J. Metall., 1973, vol. 2, pp. 291-97.

5. R. Haubner, W.D. Schubert, E. Lassner, and B. Lux: Int. J. Refract, Hard Met., 1988, vol. 7 (1), pp. 47-56.

6. Du Sichen and S. Seetharaman: Metatl. Trans. B, 1992, vol. 23B, pp. 317-24.

7. J.A. Bustnes, Du Sichen, and S. Seetharaman: Metall. Trans. B, 1993, vol. 24B, pp. 475-80.

8. U.S. National Bureau of Standards Circ., no. 539, 28/1, Swanson, Tatge, 1953.

9. H. Okamoto: J. Phase Equilibria, 1991, vol. 12 (6), p. 706. 10. J. Szekely and J.W. Evans: Metall. Trans., 1971, vol. 2,

pp. 1699-710. 11. T. DebRoy and K.P. Abraham: Physical Chemistry of Process

Metallurgy: the Richardson Conf., The Institute of Mining and Metallurgy, London, 1973, pp. 85-93.

396--VOLUME 25B, JUNE 1994 METALLURGICAL AND MATERIALS TRANSACTIONS B