Hello NCL,
In my experience, as long as it is well controlled, raw meal quality alone is rarely a major cause of ID fan blade buildups. Buildups of this sort are likely to be due to a change in the moisture content of the dust or gas stream (dew point), the quantity/mineralogy of the clay component of the dust, or even an increase in the dust load impacting the fan blades... or a combination of one or more of these factors.
Sometimes, simply an increase in the speed of the fan (over its nominal capacity) can increase the frequency of fan blade buildups, due to the increased impact speed between dust particles and blade surface.
If buildups on the fan blades are not the issue then it could be a problem with the impeller itself.
This paper may be helpful;-
http://www.thomasnet.com/pdf.php?prid=100562
Also there is another article on the subject given in this thread;-
http://www.cemnet.com/cs/forums/thread/2512.aspx
Regards,
Ted.