Are the particles actually single crystals or agglomerates? Breaking up crystals takes a lot more energy than breaking up agglomerates.
What do you mean by laminar? Do you mean gentle?
What type of impeller is installed in the vessel?
What is the brake horsepower (not the motor rated...
Jet mixing is not going to work. The viscosity is too high and Reynolds number too low.
The question is a common one for producers of viscous fluids (paint is a very common example). How long to wait before you can take a sample that is truly representative of the contents of the whole...