Effect of mechanical activation on aluminothermic reduction of molybdenum trioxide

Abstract In the present work, two different methods were used to study the effect of mechanical activation on aluminothermic reduction of MoO3. In the first method, a stoichiometric amount of Al + MoO3 and 20% weight percent of aluminum oxide (as the heat absorber) was mixed and mechanically activated by milling process in a planetary ball mill under argon atmosphere. XRD patterns and SEM micrographs of the activated samples indicated that aluminothermic reduction of MoO3 was done in the milling media after 5 h and increasing the milling time, decreased the remaining non reacted Al and MoO3. In the second method, as received MoO3, was mechanically activated in different times in the range of 3 to 24 h. The effect of mechanical activation on molybdenum trioxide grain size was analyzed by Williamson-Hall method, and it was shown that, by increasing the activation time, the grain size and internal strain was reduced and increased, respectively. In the next step, the activated MoO3 powders were mixed with stoichiometric amount of Al and 20 wt% Al2O3 (as the heat absorber) and shaped in the form of cylindrical samples of 10 mm in diameter and height of 15–20 mm by applying 70 MPa uniaxial pressure. Aluminothermic reaction was initiated in the prepared pellets by using conventional microwave oven. The XRD patterns of the combustion products showed that, by mechanical activation of MoO3, the undesired phases of molybdenum dioxide (MoO2) and aluminum molybdate Al2(MoO4)3 was decreased. The aluminothermic reduction of activated MoO3 also was studied by non-isothermal Differential Scanning Calorimetry (DSC) with heating rate of 10 °C min−1 under argon atmosphere. The results showed that by mechanically activating MoO3 for 24 h, the ignition temperature of reduction reaction decreased from 910 to 780 °C and the practical efficiency of the reaction increased from 82.9% (for the nonactivated MoO3) to 92.5%. The comparing results between the milling mixture and the milling molybdenum trioxide indicated that the reaction in mixture cannot be completed during milling and a small amount of Al and MoO3 remained in the activated powder. But for the activated MoO3, and after igniting in a conventional microwave oven the desired reaction went to completion.