My lack of kn0wledge has disallowed me to give you a prompt...

My lack of kn0wledge has disallowed me to give you a prompt answer in the way I would prefer to word it.

A simple answer is that when classifying for size, the kaolin particles rejected as being too coarse are sent back to the grinding step, rather than being regarded as waste. Hence, for WAK to produce the fine-grained K99F that Stanco wants, incurs a greater milling cost.

The elaboration that I am struggling to verbalise is that in a setting where WAK also sells non-K99F products (K99C for ceramics, for instance), then WAK would not want to grind the particles too much, because to achieve fast drainage, pressure-cast ceramics requires coarse kaolin.

Ignoring real-world kaolin beneficiation capability, an imaginary beneficiation plant could be flexible enough to handle multiple contradictory requirements of different kaolin products. In this setting, a coarse product like K99C would be optimally beneficiated, and the fine-grained K99F likewise via a different stream. Later, a delaminated K99P product for the paper sector could be added to go down its optimal path. Large kaolin particles with excellent crystal formation are ideal for delamination, so one would not want to grind the kaolin to the point required by the continuous0-fibreglass sector.

It is a pity that WAK did not start its commercial life with an offtake agreement for K99C, rather than scrambling to get into the ceramics sector belatedly.