The Alum Molecules, AlSO4, are held together by an inert binder. The binder creates an almost hardened plaster of paris consistency. I have broken high quality boat oars trying to break the stuff up in the mixing tank. If you mix the hydrated lime and alum together, it becomes gelatinous and impossible to pump/spray.
My setup for spraying is 2-fold....On my pond water intake, I want to strain out plants and fish fry. On the chemical slurry side, you are correct, I only want dissolved alum being sprayed, but mostly because I use numerous boomless spray nozzles that have been drilled out for higher flow....2GPM (at 40psi) to around 6 GPM at 30 psi. Even drilled out for higher flow, and strained to very small particle sizes, the Alum and hydrated lime can still harden on the brass nozzles and will need cleaning at times. Spraying this stuff utterly destroys equipment, fast! My 3 current trash pumps are less than 6 months old....One motor is totally locked up, and the metal throttle controls on the other 2 are almost rusted through. None of the pumps got hit with the alum or lime, yet just the dust will destroy the metal
