- Email: [email protected]
Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor
Journal of
ALLOYS A~D COM~OU~NDS ELSEVIER
Journal of Alloys and Compounds 238 (1996) 18-22
Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor S.P. Chakraborty, P.K. Tripathy, I.G. Sharma, D.K. Bose High Temperature Materials Section, Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay, 400085, India Received 2 September 1995; in final form 2 December 1995
Abstract
Studies on thermal decomposition of ammonium polymolybdate were carried out in a fluidized bed reactor using air as a fluidizing medium. The ammonia vapour released during the decomposition process was passed directly into hydrochloric acid solution. The change in pH of the solution was monitored with respect to time and temperature and the degree a of decomposition was calculated at intermediate stages of the process. The kinetics of the overall decomposition process were found to fit well into a 'power law' model. The overall activation energy E a and the rate constant k were determined to be 16.3 kJ mol-1 and 1.24 × 10-3 s-1 respectively. The value of the activation energy suggests the process to be surface controlled. The preparation of MoO 3 from ammonium polymolybdate in a fluidized bed reactor was found to be accomplished at a lower temperature and in a lesser time than that needed in a static bed reactor. Keywords: Thermal decomposition; Ammonium polymolybdate; Activation energy; Fluidized bed; Surface controlled
1. Introduction
Molybdenum is a strategically important metal in the industrial sector owing to its widespread applications as a steel additive to impart specific properties such as improved strength, resistance to pitting corrosion and increased hardness, etc. Some of the other vital applications of this metal include: a mandrel for making filaments in the lamp industry, a heating element for furnaces, and above all a catalyst in the chemical industry. Pure molybdenum trioxide (MOO3) is by far the most important intermediate compound of molybdenum, from which most of the known compounds containing molybdenum, as well as metal Mo, are prepared either directly or indirectly. Pure M o O 3 (99.98 mass%) can be prepared from technical grade oxide (93 mass%) either by chemical dissolution followed by crystallisation or by a sublimation process. The sublimation process is generally preferred over the chemical process for preparing high purity oxide. The conventional technique for the production of M o O > particularly from molybdenite concentrate (MoS2), involves (i) roasting of the sulphide to oxide, (ii) dissolution of the oxide in ammoniacal solution, (iii) precipitation of molybdenum value in the solution 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 5 ) 0 2 1 8 0 - 9
as ammonium polymolybdate and (iv) decomposition of ammonium polymolybdate (APM) to molybdenum trioxide. The experimental parameters at various stages of dissolution, precipitation and decomposition can critically affect the quality of the oxide powder. Calcination of ammonium polymolybdate to molybdenum trioxide is normally carried out in an open static bed reactor at 500 °C [1] for a duration up to 12 h depending upon the quantity of the charge. The decomposition of APM, if carried out in a closed environment, may lead to the formation of MoO2 along with M o O 3 [2]. Formation of MOO2, however, can be prevented if the decomposition is carried out in a fluidized bed reactor using oxygen/air as the fluidizing medium. The present investigation has been aimed at studying the decomposition of ammonium polymolybdate in the presence of air in a fluidized bed reactor for the preparation of M o O 3. Owing to the excellent heat as well as mass transfer characteristics which can be achieved in a fluidized bed reactor, it is expected that both the time and temperature required to achieve complete decomposition would be reduced substantially. The experimental data, collected during the de-
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
19
composition process, were analyzed using standard non-isothermal kinetic models for the evaluation of overall activation energy as well as rate constant.
2. Experimental 2.1. Materials 2.1.1. Molybdenum trioxide Technical grade MoO 3 (purity 93 mass%) was used as a starting material for the preparation of ammonium polymolybdate.
2.1.2. Ammoniacal solution 25 rnass% NH 3 solution (GR) was used for the dissolution of MoO 3.
2.1.3. Air Compressed air was used as a fluidizing medium to determine both the minimum fluidizing velocity and the subsequent decomposition of ammonium polymolybdate.
1. RUBBER BUNG 2. REACTOR COLUMN 3. DISTRIBUTOR PLATE
10. AIR COMPRESSOR 11. FLANGES 12. TEFLON 'G RING
4. ZIRCONIA PIEQES
13. HEATING COIL
5. FUJIDIZED PARTICLES
14. CALMING :~t;/ION
6. AIR FLOW LINE
15. GLASS BEAKER
7. ROW METER
2.1.4. llydrochloric acid solution Hydrochloric acid (GR) solution of known initial pH was taken in a glass beaker. The ammonia vapour evolved as a result of decomposition was allowed to pass through the solution. The change in pH was continuously monitored.
2.2. Apparatus The fluidized bed reactor assembly consisted of a 0.05 m inner diameter, 0.6 m long Inconel reactor tube, held vertically over a conical calming section from the bottom. A distributor plate, made up of nickel (0.001 m hole, 0.003 m pitch), was held between the reactor column and the calming section by a flange system. The conical calming section at the bottom had an inle~: for gas introduction to the reaction zone. The reactor column was wound up externally with a cord heater, up to a height of 0.12 m from the top of the inert bed material. ZrO 2 pieces of 0.002-0.003 m size formed the material for the inert bed, spread evenly over the distributor plate up to a height of 0.05 m. The air, used for fluidization, was taken from a compressor supplying air at an input pressure in the range 0.1960.245MPa. The temperature at the reaction zone during the course of decomposition was monitored by an iron-constantant thermocouple, placed inside the reactor Fig. 1 gives a schematic diagram of the complete experimental set-up.
CONTAINING HO. SOI..N.
8. PRESSURE REGULATOR
16. GLASS ELECTRODE
9. PRESSURE SENSOR
17. MANOMETER
Fig. 1. Fluidized bed reactor set-up.
2.3. Procedure ZZ1. Preparation of ammonium polymolybdate 500 ml of 25 mass% ammoniacal solution (GR) was mixed with 500 ml of water to prepare 12.5 mass% ammoniacal solution. This solution was then heated to 80-90 °C and 200 g of technical grade MoO 3 (Table 1) was slowly dissolved in the solution with continuous stirring and kept for digestion for 2 h to prepare 10 g pl 1 Mo laden solution. The pH of molybdenum laden solution was maintained at 9 by adding 12.5 mass% NH 3 solution. Then the pH of the solution was adjusted to the range 1.1-1.25 by adding HC1 for the precipitation of molybdenum in the form of ammonium polymolybdate. The greenish-white precipitate Table 1 Chemical analysis of technical grade MoO~ Composition
Amount (mass%)
MoO 3 Fe Cu Ni SiO 2 S
93.17 0.75 0.52 0.36 1.7 3.5
20
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
thus formed was filtered, washed with 12.5 mass% NH 3 solution and then dried at 85-90 °C. Its X-ray diffraction pattern has confirmed the precipitate to be ammonium polymolybdate ((NHn)20-4MoO3). Subsequently, the precipitate was kept for further drying overnight at 90 °C in order to render it free flowing for fluidization. The particle size of the completely dried powder was found to be in the range 50-150 ~m.
on a 100 g scale by heating the reactor externally up to a temperature of 310 °C under air flow. The ammonia vapour released during the reaction was passed directly into HC1 solution and the change in pH of the solution was measured continuously.
2.3.2. Determination of the minimum fluidization
The decomposition of APM was studied with respect to the following two parameters.
velocity Uml Initially a few experiments were conducted to determine the minimum fluidization velocity at room temperature. For this, about 100 g of experimentally prepared APM was kept over the inert bed up to a height of 0.07 m. Compressed air was then passed via the flow meter to the fluidizing zone of the reactor through the calming section. A glass manometer tube (filled with C2H5OH ), one end of which was immersed inside the expanded bed at the reaction zone, was used to measure the pressure drop across the expanded bed against the flow rate of air. The inert bed remained stationary during the course of the fluidization. It was observed that the pressure drop started rising with the increase in air flow rate, attained a constant value with further increase in the air flow rate, and then started to fall (Fig. 2). The flow rate at the beginning of the plateau region, called the minimum fluidization gas flow, was determined to be 7.16 × 10 -5 m 3 s -1, and the corresponding minimum fluidizing velocity Umf 3.65 × 10 : m s 1. The plateau region extended up to a flow rate of 1.66× 10-4 m3s 1, during which the APM remained in the free flowing state. An operating flow rate of 8.33 × 10 -5 m 3 s -1, corresponding to 1.1 × Umf, was maintained in the present studies. The decomposition of APM was carried out
3. Results and discussion
3.1. Temperature Fig. 3 shows the degree of decomposition with temperature in the range 150-310 °C. A sharp rise in the extent of decomposition was observed in the temperature range 190-270 °C. More than 95 mass% decomposition could be achieved by the time the temperature reached 270°C. However, in order to ensure complete decomposition, the temperature was finally raised up to 310°C. About 99.5 mass% decomposition could be accomplished within a duration of 20 min. The product formed at the final temperature was analyzed both by X-ray diffraction (XRD) and chemical analysis. While XRD patterns revealed the presence of only MOO3, chemical analysis (Table 2) confirmed the quantitative conversion of APM into MoO 3 at 310 °C.
3.2. Time Fig. 4 shows the change in pH as a function of time and temperature. The figure indicates that up to about 15 min there was no change in pH when the tempera-
TEMPERATURE E
t#
I.o
g
x v ,~. id m .m
(eC)
I ~ 0 170 190 210 23C) 25C) 2TO ; ~ 0 310 I I I I I I I I I
~ 0.8
40
u)
~ 3o
~ 0.6 IL 0 hi ~ 0.4 0.2
0 0
I 1
I 2
I 3
I 4
I 5
I 6
I T
I 8
1 9
I 10
I 11
AIR FLOW RATE ( X '1~)3 m 3 / M I N )
Fig. 2. Determination of minimum fluidization velocity.
0
/
~'-
~ T E D
"( 4
I 8
I 12 TIME
| 16
•
I 20
24
(MIN)
Fig. 3. Degree of decomposition with time and temperature.
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22 Table 2 Chemical analysis of decomposed ammonium polymolybdate
Composition
Amount (mass%)
MoO, Fe Cu Ni SiO 2 S
99.7 0.05 0.02 0.01 0.09 0.14
of the nuclei and the evolution of the product due to the dissociation of APM can be described by the equation W = C exp(-AZ/kT)
"L 310 270
10 ,
I 98
250 -
2~ wn.. D
~6J5
p5 '170
2 I
I C
10
20
I
~0 TIME
I
40 (MIN)
50
150 60
-
Fig. 4. Measured values of pH with time and temperature.
ture was around 150 °C. A sharp rise in pH took place in a small interval of about 5 rain, after which a plateau region appeared at 270 °C.
4. Kinetic considerations
The degree a of decomposition (amount decomposed at time t to the total amount decomposed) was defined as pH, - pH i a - pHf - pH i
(1)
where pH, is the pH at time t, 0 < t i
21
(2)
where r is a constant having a value of 1/3. The transformation of APM to MoO 3 is governed by the laws of topochemical reactions. These reactions are concentrated in a certain way in space and develop most frequently on interfacial surfaces. A substantial obstacle in the beginning stage of the process is the origination of the oxide phase and the appearance of an interfacial boundary [4]. The kinetics of the growth
exp(-AE/kT)
(3)
where e x p ( - A Z / k T ) reflects the rate of formation of the nuclei and e x p ( - A E / k T ) reflects their rate of growth. The shape of the curve a vs. t (Fig. 3) reflects all phases of the process. The lower section of the curve relates to the inductive period of the decomposition process when nuclei are formed and a boundary appears between the old and new phases. The middle section, which shows a rapid increase in a and the rate of decomposition of the APM, reflects the growth of the interfacial surface and the autocatalytic acceleration of the process concentrated on it. The upper section of the curve exhibits the reaction zone within the lump of the reacting substance and the subsequent reduction in its span. Although in these types of decomposition process the shape of o~ vs. t curve varies greatly with the pressure of the released gas [5], the activation energy, being a physical quantity, does not vary significantly. The overall activation energy and the rate constant for the reaction were determined to be 16.3 kJ mo1-1 and 1.24 × 10 -3 s 1 respectively. If a comparison is drawn between the two possible approaches (both static and dynamic modes of decomposition) it can be said that although the static method gives more accurate information on the reaction mechanism, the dynamic method provides a better indication of the state of the sample at any instant of time with respect to temperature and extent of decomposition [6]. Therefore, the dynamic method is more suitable for obtaining data on the kinetics of the reaction from a single curve for the entire range of temperature. From a process efficiency point of view, the present investigation offers three distinct advantages over the static mode of decomposition. These are: (i) avoidance of formation of lower oxide (MOO2) which is normally found in a static bed reactor under closed conditions, (ii) considerable reduction in the decomposition temperature (310 °C as compared with 500 °C in a static bed) and (iii) substantial decrease in time (from 5-6 h to 20 min for a charge of 100 g).
5. Conclusions
The decomposition of ammonium polymolybdate in a fluidized bed reactor was found to be the most effective method for obtaining MoO3 using air as a fluidizing medium. A substantial reduction was accom-
22
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
plished in both time and temperature required to achieve complete decomposition of APM. The overall activation energy of the process was determined to be 16.3kJ mo1-1, which suggests the reaction to be surface controlled. The rate constant was determined to be 1.24 x 10 -3 s -1. The present investigation has shown that the dynamic mode of decomposition (in a fluidized bed reactor) enjoys an edge over the conventional static bed decomposition.
Acknowledgements The authors wish to express their sincere thanks to Dr. C.K. Gupta, Director of the Materials Group, and Dr. S. Banerjee, Head of the Metallurgy Division, B.A.R.C for their valuable suggestions and keen interest in the present investigation. Dr. P.V. Ravindran
is also acknowledged for his assistance in anlayzing the kinetic data.
References [1] T.K. Mukherjee, Studies on processing of sulphide resources of nickel and molybdenum, Ph.D. Thesis, University of Bombay, India, 1985. [2] Z.L. Yin, X.H. Li, W.S. Zhao, S.Y. Chen, G.R. Lin and S.Q. Wang, Thermochim. Acta, 244 (1994) 283-289. [3] P.V. Ravindran and A.K. Sundaram (eds.), Thermal Analysis Manual, Indian Chemical Society, Bombay, 1974 p. 40. [4] S. Filippov, The Theory of Metallurgical Processes, Mir Publishers, Moscow, 1975, pp. 134-140 (translated from the Russian by G. Kittel). [5] P.D, Garn, Thermoanalytical methods of investigation, Academic Press, New York, 1965. [6] A. Blazek, Thermal Analysis, Van Nostrand Reinhold, London, 1973, p. 61.
ALLOYS A~D COM~OU~NDS ELSEVIER
Journal of Alloys and Compounds 238 (1996) 18-22
Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor S.P. Chakraborty, P.K. Tripathy, I.G. Sharma, D.K. Bose High Temperature Materials Section, Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay, 400085, India Received 2 September 1995; in final form 2 December 1995
Abstract
Studies on thermal decomposition of ammonium polymolybdate were carried out in a fluidized bed reactor using air as a fluidizing medium. The ammonia vapour released during the decomposition process was passed directly into hydrochloric acid solution. The change in pH of the solution was monitored with respect to time and temperature and the degree a of decomposition was calculated at intermediate stages of the process. The kinetics of the overall decomposition process were found to fit well into a 'power law' model. The overall activation energy E a and the rate constant k were determined to be 16.3 kJ mol-1 and 1.24 × 10-3 s-1 respectively. The value of the activation energy suggests the process to be surface controlled. The preparation of MoO 3 from ammonium polymolybdate in a fluidized bed reactor was found to be accomplished at a lower temperature and in a lesser time than that needed in a static bed reactor. Keywords: Thermal decomposition; Ammonium polymolybdate; Activation energy; Fluidized bed; Surface controlled
1. Introduction
Molybdenum is a strategically important metal in the industrial sector owing to its widespread applications as a steel additive to impart specific properties such as improved strength, resistance to pitting corrosion and increased hardness, etc. Some of the other vital applications of this metal include: a mandrel for making filaments in the lamp industry, a heating element for furnaces, and above all a catalyst in the chemical industry. Pure molybdenum trioxide (MOO3) is by far the most important intermediate compound of molybdenum, from which most of the known compounds containing molybdenum, as well as metal Mo, are prepared either directly or indirectly. Pure M o O 3 (99.98 mass%) can be prepared from technical grade oxide (93 mass%) either by chemical dissolution followed by crystallisation or by a sublimation process. The sublimation process is generally preferred over the chemical process for preparing high purity oxide. The conventional technique for the production of M o O > particularly from molybdenite concentrate (MoS2), involves (i) roasting of the sulphide to oxide, (ii) dissolution of the oxide in ammoniacal solution, (iii) precipitation of molybdenum value in the solution 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 5 ) 0 2 1 8 0 - 9
as ammonium polymolybdate and (iv) decomposition of ammonium polymolybdate (APM) to molybdenum trioxide. The experimental parameters at various stages of dissolution, precipitation and decomposition can critically affect the quality of the oxide powder. Calcination of ammonium polymolybdate to molybdenum trioxide is normally carried out in an open static bed reactor at 500 °C [1] for a duration up to 12 h depending upon the quantity of the charge. The decomposition of APM, if carried out in a closed environment, may lead to the formation of MoO2 along with M o O 3 [2]. Formation of MOO2, however, can be prevented if the decomposition is carried out in a fluidized bed reactor using oxygen/air as the fluidizing medium. The present investigation has been aimed at studying the decomposition of ammonium polymolybdate in the presence of air in a fluidized bed reactor for the preparation of M o O 3. Owing to the excellent heat as well as mass transfer characteristics which can be achieved in a fluidized bed reactor, it is expected that both the time and temperature required to achieve complete decomposition would be reduced substantially. The experimental data, collected during the de-
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
19
composition process, were analyzed using standard non-isothermal kinetic models for the evaluation of overall activation energy as well as rate constant.
2. Experimental 2.1. Materials 2.1.1. Molybdenum trioxide Technical grade MoO 3 (purity 93 mass%) was used as a starting material for the preparation of ammonium polymolybdate.
2.1.2. Ammoniacal solution 25 rnass% NH 3 solution (GR) was used for the dissolution of MoO 3.
2.1.3. Air Compressed air was used as a fluidizing medium to determine both the minimum fluidizing velocity and the subsequent decomposition of ammonium polymolybdate.
1. RUBBER BUNG 2. REACTOR COLUMN 3. DISTRIBUTOR PLATE
10. AIR COMPRESSOR 11. FLANGES 12. TEFLON 'G RING
4. ZIRCONIA PIEQES
13. HEATING COIL
5. FUJIDIZED PARTICLES
14. CALMING :~t;/ION
6. AIR FLOW LINE
15. GLASS BEAKER
7. ROW METER
2.1.4. llydrochloric acid solution Hydrochloric acid (GR) solution of known initial pH was taken in a glass beaker. The ammonia vapour evolved as a result of decomposition was allowed to pass through the solution. The change in pH was continuously monitored.
2.2. Apparatus The fluidized bed reactor assembly consisted of a 0.05 m inner diameter, 0.6 m long Inconel reactor tube, held vertically over a conical calming section from the bottom. A distributor plate, made up of nickel (0.001 m hole, 0.003 m pitch), was held between the reactor column and the calming section by a flange system. The conical calming section at the bottom had an inle~: for gas introduction to the reaction zone. The reactor column was wound up externally with a cord heater, up to a height of 0.12 m from the top of the inert bed material. ZrO 2 pieces of 0.002-0.003 m size formed the material for the inert bed, spread evenly over the distributor plate up to a height of 0.05 m. The air, used for fluidization, was taken from a compressor supplying air at an input pressure in the range 0.1960.245MPa. The temperature at the reaction zone during the course of decomposition was monitored by an iron-constantant thermocouple, placed inside the reactor Fig. 1 gives a schematic diagram of the complete experimental set-up.
CONTAINING HO. SOI..N.
8. PRESSURE REGULATOR
16. GLASS ELECTRODE
9. PRESSURE SENSOR
17. MANOMETER
Fig. 1. Fluidized bed reactor set-up.
2.3. Procedure ZZ1. Preparation of ammonium polymolybdate 500 ml of 25 mass% ammoniacal solution (GR) was mixed with 500 ml of water to prepare 12.5 mass% ammoniacal solution. This solution was then heated to 80-90 °C and 200 g of technical grade MoO 3 (Table 1) was slowly dissolved in the solution with continuous stirring and kept for digestion for 2 h to prepare 10 g pl 1 Mo laden solution. The pH of molybdenum laden solution was maintained at 9 by adding 12.5 mass% NH 3 solution. Then the pH of the solution was adjusted to the range 1.1-1.25 by adding HC1 for the precipitation of molybdenum in the form of ammonium polymolybdate. The greenish-white precipitate Table 1 Chemical analysis of technical grade MoO~ Composition
Amount (mass%)
MoO 3 Fe Cu Ni SiO 2 S
93.17 0.75 0.52 0.36 1.7 3.5
20
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
thus formed was filtered, washed with 12.5 mass% NH 3 solution and then dried at 85-90 °C. Its X-ray diffraction pattern has confirmed the precipitate to be ammonium polymolybdate ((NHn)20-4MoO3). Subsequently, the precipitate was kept for further drying overnight at 90 °C in order to render it free flowing for fluidization. The particle size of the completely dried powder was found to be in the range 50-150 ~m.
on a 100 g scale by heating the reactor externally up to a temperature of 310 °C under air flow. The ammonia vapour released during the reaction was passed directly into HC1 solution and the change in pH of the solution was measured continuously.
2.3.2. Determination of the minimum fluidization
The decomposition of APM was studied with respect to the following two parameters.
velocity Uml Initially a few experiments were conducted to determine the minimum fluidization velocity at room temperature. For this, about 100 g of experimentally prepared APM was kept over the inert bed up to a height of 0.07 m. Compressed air was then passed via the flow meter to the fluidizing zone of the reactor through the calming section. A glass manometer tube (filled with C2H5OH ), one end of which was immersed inside the expanded bed at the reaction zone, was used to measure the pressure drop across the expanded bed against the flow rate of air. The inert bed remained stationary during the course of the fluidization. It was observed that the pressure drop started rising with the increase in air flow rate, attained a constant value with further increase in the air flow rate, and then started to fall (Fig. 2). The flow rate at the beginning of the plateau region, called the minimum fluidization gas flow, was determined to be 7.16 × 10 -5 m 3 s -1, and the corresponding minimum fluidizing velocity Umf 3.65 × 10 : m s 1. The plateau region extended up to a flow rate of 1.66× 10-4 m3s 1, during which the APM remained in the free flowing state. An operating flow rate of 8.33 × 10 -5 m 3 s -1, corresponding to 1.1 × Umf, was maintained in the present studies. The decomposition of APM was carried out
3. Results and discussion
3.1. Temperature Fig. 3 shows the degree of decomposition with temperature in the range 150-310 °C. A sharp rise in the extent of decomposition was observed in the temperature range 190-270 °C. More than 95 mass% decomposition could be achieved by the time the temperature reached 270°C. However, in order to ensure complete decomposition, the temperature was finally raised up to 310°C. About 99.5 mass% decomposition could be accomplished within a duration of 20 min. The product formed at the final temperature was analyzed both by X-ray diffraction (XRD) and chemical analysis. While XRD patterns revealed the presence of only MOO3, chemical analysis (Table 2) confirmed the quantitative conversion of APM into MoO 3 at 310 °C.
3.2. Time Fig. 4 shows the change in pH as a function of time and temperature. The figure indicates that up to about 15 min there was no change in pH when the tempera-
TEMPERATURE E
t#
I.o
g
x v ,~. id m .m
(eC)
I ~ 0 170 190 210 23C) 25C) 2TO ; ~ 0 310 I I I I I I I I I
~ 0.8
40
u)
~ 3o
~ 0.6 IL 0 hi ~ 0.4 0.2
0 0
I 1
I 2
I 3
I 4
I 5
I 6
I T
I 8
1 9
I 10
I 11
AIR FLOW RATE ( X '1~)3 m 3 / M I N )
Fig. 2. Determination of minimum fluidization velocity.
0
/
~'-
~ T E D
"( 4
I 8
I 12 TIME
| 16
•
I 20
24
(MIN)
Fig. 3. Degree of decomposition with time and temperature.
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22 Table 2 Chemical analysis of decomposed ammonium polymolybdate
Composition
Amount (mass%)
MoO, Fe Cu Ni SiO 2 S
99.7 0.05 0.02 0.01 0.09 0.14
of the nuclei and the evolution of the product due to the dissociation of APM can be described by the equation W = C exp(-AZ/kT)
"L 310 270
10 ,
I 98
250 -
2~ wn.. D
~6J5
p5 '170
2 I
I C
10
20
I
~0 TIME
I
40 (MIN)
50
150 60
-
Fig. 4. Measured values of pH with time and temperature.
ture was around 150 °C. A sharp rise in pH took place in a small interval of about 5 rain, after which a plateau region appeared at 270 °C.
4. Kinetic considerations
The degree a of decomposition (amount decomposed at time t to the total amount decomposed) was defined as pH, - pH i a - pHf - pH i
(1)
where pH, is the pH at time t, 0 < t i
21
(2)
where r is a constant having a value of 1/3. The transformation of APM to MoO 3 is governed by the laws of topochemical reactions. These reactions are concentrated in a certain way in space and develop most frequently on interfacial surfaces. A substantial obstacle in the beginning stage of the process is the origination of the oxide phase and the appearance of an interfacial boundary [4]. The kinetics of the growth
exp(-AE/kT)
(3)
where e x p ( - A Z / k T ) reflects the rate of formation of the nuclei and e x p ( - A E / k T ) reflects their rate of growth. The shape of the curve a vs. t (Fig. 3) reflects all phases of the process. The lower section of the curve relates to the inductive period of the decomposition process when nuclei are formed and a boundary appears between the old and new phases. The middle section, which shows a rapid increase in a and the rate of decomposition of the APM, reflects the growth of the interfacial surface and the autocatalytic acceleration of the process concentrated on it. The upper section of the curve exhibits the reaction zone within the lump of the reacting substance and the subsequent reduction in its span. Although in these types of decomposition process the shape of o~ vs. t curve varies greatly with the pressure of the released gas [5], the activation energy, being a physical quantity, does not vary significantly. The overall activation energy and the rate constant for the reaction were determined to be 16.3 kJ mo1-1 and 1.24 × 10 -3 s 1 respectively. If a comparison is drawn between the two possible approaches (both static and dynamic modes of decomposition) it can be said that although the static method gives more accurate information on the reaction mechanism, the dynamic method provides a better indication of the state of the sample at any instant of time with respect to temperature and extent of decomposition [6]. Therefore, the dynamic method is more suitable for obtaining data on the kinetics of the reaction from a single curve for the entire range of temperature. From a process efficiency point of view, the present investigation offers three distinct advantages over the static mode of decomposition. These are: (i) avoidance of formation of lower oxide (MOO2) which is normally found in a static bed reactor under closed conditions, (ii) considerable reduction in the decomposition temperature (310 °C as compared with 500 °C in a static bed) and (iii) substantial decrease in time (from 5-6 h to 20 min for a charge of 100 g).
5. Conclusions
The decomposition of ammonium polymolybdate in a fluidized bed reactor was found to be the most effective method for obtaining MoO3 using air as a fluidizing medium. A substantial reduction was accom-
22
S.P. Chakraborty et al. / Journal of Alloys and Compounds 238 (1996) 18-22
plished in both time and temperature required to achieve complete decomposition of APM. The overall activation energy of the process was determined to be 16.3kJ mo1-1, which suggests the reaction to be surface controlled. The rate constant was determined to be 1.24 x 10 -3 s -1. The present investigation has shown that the dynamic mode of decomposition (in a fluidized bed reactor) enjoys an edge over the conventional static bed decomposition.
Acknowledgements The authors wish to express their sincere thanks to Dr. C.K. Gupta, Director of the Materials Group, and Dr. S. Banerjee, Head of the Metallurgy Division, B.A.R.C for their valuable suggestions and keen interest in the present investigation. Dr. P.V. Ravindran
is also acknowledged for his assistance in anlayzing the kinetic data.
References [1] T.K. Mukherjee, Studies on processing of sulphide resources of nickel and molybdenum, Ph.D. Thesis, University of Bombay, India, 1985. [2] Z.L. Yin, X.H. Li, W.S. Zhao, S.Y. Chen, G.R. Lin and S.Q. Wang, Thermochim. Acta, 244 (1994) 283-289. [3] P.V. Ravindran and A.K. Sundaram (eds.), Thermal Analysis Manual, Indian Chemical Society, Bombay, 1974 p. 40. [4] S. Filippov, The Theory of Metallurgical Processes, Mir Publishers, Moscow, 1975, pp. 134-140 (translated from the Russian by G. Kittel). [5] P.D, Garn, Thermoanalytical methods of investigation, Academic Press, New York, 1965. [6] A. Blazek, Thermal Analysis, Van Nostrand Reinhold, London, 1973, p. 61.