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Ind. Eng. Chem. Res. 1997, 36, 3602-3606
A Comparative Study on the Thermal Decomposition of Ammonium p-Tungstate in Batch and Fluidized-Bed Reactors P. K. Tripathy,* S. P. Chakraborty, I. G. Sharma, and D. K. Bose High Temperature Materials Section, Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Thermal decomposition of ammonium p-tungstate (APT) to tungsten trioxide (WO3) was carried out in both batch and fluidized-bed reactors. In a batch reactor, the temperature and holding time for achieving complete decomposition were found to be much higher as compared to the respective values obtained in a fluidized-bed reactor using air as the fluidizing medium. The present studies have shown that fluidized-bed decomposition of APT offers a number of distinct advantages such as lower decomposition temperature as well as time, faster kinetics, and better product morphology. The decomposition kinetics, in the fluidized-bed reactor, was found to fit well into the “Power Law” model, dR/dt ) k(1/r)R1-r, where R ) fraction decomposed, k ) rate constant, and r (a constant) is 1/2. The activation energy of the decomposition reaction suggests the overall process to be surface controlled. 1. Introduction Tungsten has received a unique position in many important areas of application such as a filament in electrical lamps and parts in electronic tubes, an alloy additive to high-speed and other tool steels, and as tungsten carbide in tool materials and wear-resistant parts. Because of the unusual sizes and surface characteristics, ultrafine tungsten powder may find application as catalyst, filler, and nucleation agent in alloy production and other diverse applications as well as in the more conventional field of the hard metals industry (Basu and Sale, 1975). High-purity tungsten is prepared from its oxide (WO3) through ammonium ptungstate (APT) intermediate, which is further reduced to tungsten powder by hydrogen at a temperature of ∼800-1000 °C. The preparation and subsequent thermal decomposition of APT to produce high-purity WO3 is a fairly well-known process, widely used in the tungsten industry. In fact, in the extractive metallurgy of tungsten, conversion of solid APT to tungsten oxides is an essential first process prior to hydrogen reduction. The particle size and morphology of the metal powder are considerably influenced by the size and morphology of the oxides, which, in turn, depend on the characteristic features of APT (Basu and Sale, 1977). Thermal decomposition of APT in a batch reactor results in the formation of nonstoichiometric tungsten oxides, WO3-x (0 < x < 0.2) (Pfeifer et al., 1993), depending on the reaction parameters. This is particularly so if the decomposition is carried out in a closed reactor. Both in closed type and in a static bed reactor, the decomposition of APT often leads to the formation of WO2.9 along with WO3 as the principal product (Klar, 1984). The formation of nonstoichiometric oxides takes place because of two reasons, viz. (i) the ammonia, evolved during the heating process, provides a reducing atmosphere and (ii) there is insufficient air/O2 in the bulk of the APT. The presence of lower oxides (other than WO3), after the final stage of decomposition, would adversely affect further processing of WO3, leading to the production of metal with undesired ductility. That is why the decomposition of APT is normally carried out in the presence of a continuous flow of a stream of filtered air/O2 in order to (i) facilitate the faster removal of NH3 and water vapor from the product zone and (ii) make effective contact with APT so that the formation S0888-5885(96)00667-7 CCC: $14.00
of nonstoichiometric oxides could be avoided. However, this difficulty can be surmounted fully if the decomposition is carried out in a fluidized-bed reactor where a dynamic flow of air during decomposition does not allow the formation of nonstoichiometric oxides. Moreover, due to better heat- and mass-transfer characteristics, it is expected that the temperature and holding time for achieving complete decomposition would substantially decrease as compared to that required in a static/ batch reactor. In the present investigation, decomposition studies of APT have been carried out in batch as well as fluidized-bed reactors, using air as the fluidizing medium in the latter case. A comparative account with respect to the temperature and time of decomposition as well as the product quality has been presented. The overall kinetics of the decomposition process in the fluidized-bed reactor has also been discussed using various nonisothermal models for solid-state reactions. 2. Experimental Section 2.1. Apparatus. 2.1.1. Batch Reactor. A horizontal silica tube, 80 mm in diameter (internal) and 600 mm long, with both ends open was used as the reactor. The reactor was externally wound with a nichrome wire, embedded in insulating refractory material. The entire setup was encased in a mild steel shell. 2.1.2. Fluidized-Bed Reactor. A 50 mm diameter (internal) and 700 mm long inconel tube, vertically placed, was used for carrying out the decomposition studies. The bottom end of the reactor was fitted with a nickel distributor (0.001 m long hole, 0.003 m pitch) for the upward movement of air from a compressor. A combination of flow meter and valve was used to control the air supply to the reactor. The reactor was heated externally by a cord heater, wound on the reactor tube, from outside. Bubble alumina of 0.002-0.003 m size, distributed over the nickel distributor, was used as an inert-bed material. The temperature was measured by attaching an iron-constantan thermocouple to the outer wall of the heating zone. The pressure drop across the bed was measured by connecting a manometer through a 2 mm (internal diameter) silica tube. Figure 1 represents the schematic diagram of the experimental setup. 2.2. Procedure. 2.2.1. Preparation and Chemical Analyses of APT. Low-grade wolframite contain© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3603 Table 1. Chemical Analysis of Experimentally Prepared APT composition
amount (wt %)
composition
amount (wt %)
APT Fe P
99.44 00.25 00.03
Si Mn Na
00.18 00.05 00.05
Table 2. Chemical Analysis of WO3 (Prepared in the Fluidized-Bed Reactor) composition
amount (wt %)
composition
amount (wt %)
WO3 Fe P
99.790 00.016 00.010
Si Mn Na
00.080
