Preparation of Powder Grades

A generalized flow sheet, outlining the different modes of preparation, is presented in by Fig. 9.23. 

FIGURE 9.23. Schematic flow chart for hardmetal production, showing the variety of production routes. 

Any particular mill and mill charge combination has a limiting particle size, beyond which no further comminution will occur. For most industrial aggregates, this is in the range of 0.5-1 pm [9.25]. Only in optimized mills is further comminution possible, finally reaching a size limit of about 100-150 nm [9.24]. This limit is inherent to the WC (i.e., to its mechanical properties). This kind of milling is used only by specialists. 

The choice of the correct mill and milling conditions depends on the hardmetal grade produced, mainly on the WC grain size. Attritors are used today for most powder qualities (1-12 pm). They have become increasingly popular because of the significantly shorter milling times. Optimized ball mills are still preferred for submicron qualities, which are very sensitive to all types of heterogeneities. 

Grade powder milling is always conducted in an organic liquid to prevent local overheating and oxidation. The enormous milling energy, which is converted to heat, must be removed from the mill by cooling (mills equipped with water jackets). 

---