Ok, this looks like it could answer to an extent the question:...

Ok, this looks like it could answer to an extent the question:

[0023] The chemical composition of different fly ashes can be substantially different. Fly ash with an excessively high iron oxide content can present economically insurmountable problems because of the degree of stickiness which results. Best results were achieved with fly ash having an iron oxide content of about 4%. Using a fly ash containing in excess of 12% iron oxide can caused insurmountable problems of sticking in the kiln. Iron content in the fly ash should be less than 12%, preferably about 7% or less, and most preferably about 4% or less.

[0024] The invention is suitable for making proppant materials from proppants using fly ash from the combustion of various types of coal (e.g., lignite, bituminous, and anthracite coals). Lignite fly ash typically contains a much higher proportion of CaO (and therefore smaller proportions of Si02 and A1203) than bituminous or anthracite fly ash. Using lignite fly ash, the proportion of bauxite in the core granule can be reduced; in some embodiments, very strong proppant materials could be made without including any bauxite in the core particle - i.e., making the core particles of fly ash and clay. In such embodiments, bauxite is used only as a coating material. Moreover, a single, relatively light coating of bauxite may be used with core granules made using lignite fly ash (e.g., an amount of bauxite equal to about 5% or less of the total weight of the dry ingredients.

Source: patent application, Ralph Enderle
https://www.google.com/patents/WO2015026585A1?cl=en