In contrast to the pusher furnace, where a static powder layer passes the furnace, a dynamic powder flow exists in the rotary kiln. Furnace rotation and the incline continuously move the powder through the hot zone. Feed rate, rotational speed, incline, and lifters inside the tube determine the depths of the dynamic powder layer. High oxide feed, longer retention time (low rotation rate and low inclination), and higher temperature result in coarser tungsten grains.
The powder layer is constantly disturbed by the rotating motion and powder from inside moves to the surface, and vice versa. Therefore, the water-vapor retaining capability of the layer is less compared to a static layer, and the material exchange rate H2O→H2 is of the layer is enhanced. The humidity in a rotary furnace powder layer is thus lower and reduction proceeds under drier conditions as compared to the pusher. Lifters inside the tube combined with stepwise turning motion make sure that a powder bed is maintained for a certain time giving the chance to build up higher humidity both temporarily and locally.
In order to maintain the humidity for a certain grain growth, only a small excess of hydrogen is necessary. This corresponds to the lowest values in pusher furnaces, applied only for very coarse powder. Moreover, the influence of the H2/WO3 molar ratio on grain growth is less pronounced. The hydrogen is cleaned and dried and recycled to the furnace after preheating. Due to the constant motion of the powder bed, fine particles are swept away by the hydrogen flow and form a dust, which must be removed by filtering devices.
Very coarse blue oxide has improved flow characteristics and helps to prevent powder from sticking to the tube wall.
Due to the "dry" conditions, rotary-furnace-produced tungsten powder shows a high degree of agglomeration, lower apparent density, and closer grain size distribution. The upper size limit is approx 6 µm.
Furnaces in use possess different sizes: inner diameter between 300 and 750 mm and heated drum length from 3000 to 8000 mm. The most common equipment has an inner diameter of 600mm and heated length of 6500mm.
Advantages of the rotary furnace compared to a pusher are less specific energy consumption, while automation is less complicated and consequently maintenance is cheaper. Finally, the product is more homogeneous and less contaminated, but the particle morphology may differ significantly from powders produced in pusher furnaces.
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