Hydrogen Flow Rate. A higher hydrogen flow enhances the material exchange due to the more rapid removal of water vapor. Therefore, the flow is inversely proportional to the average grain size.
Hydrogen Flow Direction. Concurrent hydrogen flow with respect to the tungsten flow generates a higher dynamic humidity at the later part of the reduction, while counter current flow (which is the standard condition) provides higher humidity during the early reduction stages.
Hydrogen Dew Point. The dew point of the incoming hydrogen influences the overall humidity during reduction. More “wet” hydrogen enhances the tungsten particle growth.
It is important to understand that the final average grain size of the tungsten powder is a consequence of combining all the aforementioned parameters. The general rules are:
Small grain size: low temperature, dry hydrogen, high hydrogen flow rate, low dew point, small powder layer height, high porosity, low oxide feed.
Large grain size: high temperature, wet hydrogen, low hydrogen flow rate, high dew point, large powder layer height, low porosity, high oxide feed.
Empirically based equations for calculating the reduction time and average particle size have been derived for rotary furnaces but have been applied industrially. The main difficulty is that the properties of the raw material play a crucial role in the reduction process, and these characteristics are not well enough represented by corresponding equations. This is particularly true for smaller grain sizes. For larger grain sizes, the influence of the oxide precursor is less pronounced.
In industrial practice, the choice of proper reduction parameters is based exclusively on empirical experience. Besides the average grain size, the reduction parameters also influence the grain size distribution, agglomeration, apparent density, and grain morphology.
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