A study on thermal oxidation of burnt zirconium fines in a fluidized-bed reactor
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a 0.15-m-long disengaging section of 0.1-m i.d. at the top and a multiorifice nickel distributor plate at the bottom, as shown in Figure 1. The disengaging section is provided to minimize the carryover of the fines. A distributor plate (0.001-m hole, 0.003-m pitch) is held between the reactor column and a calming section at the
bottom by a flange system. The conical calming section at the bottom has an inlet for gas introduction to the reaction zone. The reactor column is wound up with cord heater up to a height of 0.12 m from the top level of the inert bed material. An alumina tube of 0.04-m i.d., 0.048-m o.d., and 0.6-m length is used as a liner to the main reactor. The liner rests on the distributor plate and is fixed to it with the help of alumina cement. Over the distributor plate lies a bed of inert material (ZrO2 pieces of 0.002- to 0.003-m size) 0.05 m in height. The air used is taken from a compressor that supplies air at input pressures ranging from 0.196 to 0.245 MPa. To start with, the flash point of as-received zirconium fines has been determined on a 0.01-kg scale in a static bed reactor. It has been found that at a temperature of 473 -+ 10 K, the material begins to be oxidized rapidly, with a red glow. Prior to the thermal oxidation run, the minimum gas flow rate for the fluidization of burnt zirconium powder, using air as the fluidizing medium, was determined experimentally and found to be 6.6 x 10 -4 m3/s, the corresponding minimum fluidization gas velocity being 0.53 m/s. In the subsequent studies with different gases, the gas flow rate has been maintained at 6.6 • 10 -4 m3/s. The influence of different experimental parameters such as the mode of charging (batchwise, semicontinuous), the fluidizing gas (oxygen, mixture of air and argon, and air), and inert bed material (bubble alumina, porcelain beads, and zirconia pieces) has been studied to obtain the optimum parameters for an efficient conversion of zirconium fines to zirconium oxide. Two modes of charging the material into the reactor have been examined. In the first case, i.e., batch charging, the entire charge material (0.01 to 0.1 kg) is kept over the inert bed before triggering the reaction. Then the temperature in the reaction zone is gradually raised, and air flow is simultaneously put on at a flow rate of 6.6 • 10 -4 ma/s. The oxidation reaction gets triggered at around 473 -+ 10 K. It has been found that instantaneous propagation of the oxidation reaction at a rapid rate does not permit efficient heat removal and eventually results in agglomeration. An yield of 80 pct has been achieved in this run. The second mode involves charging zirconium powder from the top of the reactor at a controlled rate of 1.33 to 1.66 • 10- kg/s through a screw feeder. Before commencing charging, a temperature of 523 K at the reaction zone is attained first, and then air flow is initiated. As soon as zirconium fines reach the reaction zone, the oxidation reaction is triggered. External heating is then discontinued, and zirconium powder feeding at the rate o
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