It sounds like you have the wrong type of mixer for this application.  I can't be specific without knowing the chemicals involved, but hydroscopicity may be a factor.  Also, one material may be of a much finer size than the other one or have a highly conductive surface (i.e., metal).  The October 2009 issue of Chemical Processing has an article entitled "Clamp Down on Clumping."  The factors that lead to clumping are outlined and techniques to avoid clumping are described. The leading cause is moisture.  The article doesn't go into detail about equipment types and how they may be prone to clumping, but most mixers that result in clumps should be modified to have a high shear section mounted at right angles to the mixer shaft to break up the lumps.  Mixing time and intensity needs to be determined experimentally.  There are no theoretical models.