What is the process for purifying KOH?
According to Armarego, et al. (Purification of Laboratory Chemicals, 6th edition), they only provide a procedure to purify a solution of $\ce{KOH}$, likely owing to its very high solubility.  Further, the only impurity of note they cite is carbonate (from standing in contact with air), so if this is not your impurity then this procedure will not help.

If this is your impurity, they recommend adding a saturated solution of a barium salt, either $\ce{BaCl}$ or $\ce{Ba(OH)2}$, shaking, and then setting aside to allow the insoluble $\ce{BaCO3}$ to settle.  It sounds like you are working on large scale, so it is important to recall that the procedure just described should be done in several small batches to work best.

Not certain this is what you are looking for, but I hope it's a start.