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.
没有评论:
发表评论