Material builds up in slurry tanks and slurry piping for one of two reasons, settling or scaling. If you have build-up in your slurry system it is likely from one or both of these reasons. It is important to determine which is the cause because the method(s) for addressing them are different.
Look into your tank and/or pipe and it should be relatively easy to determine which is causing your issues. If you have build-up predominantly at the bottom of your pipe or tank you are likely looking at a settling problem. Settling is normally a result of material falling out of suspension due to low flow velocity or dead legs in you piping.
If the build-up is on the walls of the tank, or around the full perimeter of the pipe you are dealing with scale. Scale is a result of a chemical reaction between the chemical being handled and the impurities in the water making up the slurry.