The Causes of Sludge
Most dips contain coarse clays that require extensive high shear mixing or steam (hot water) in order to swell the particles or suspend them in solution. Suspending the clay particles is imperative to assure they don't fall out of the solution, dropping to the bottom of the tank as sediment or sludge.
The best way to suspend clay particles without hot water is to use high shear mixing. Using a propeller blade in your mix tank is probably making the problem worse because it does not provide any shear to break up the particles; it just 'sloshes' them around the tank and then they settle somewhere else (like a pipe or pit).
Even slurry products are prone to sludge. Slurry products contain pre-swelled clays that tend to cling to the sides of tanks and in hoses. To properly disperse a pre-swelled clay dip, it is necessary to mix with either hot water or to use a high shear mixer like a Cowles or Rotor-Stator type. Otherwise, the slurry tends to clump and cling.