Dissolution is defined as the solution of solid (particles) in a liquid to form a homogeneous phase. It is generally a sub-category of solids suspension. Note that complete suspension is a sufficient specification for most dissolution operations. When all particles are suspended, mass transfer from the particle surface to the liquid controls the rate, and additional mixing is not cost effective. It is seldom practical to design a mixing system to mechanically break solid particles as an aid to dissolution. Agitated slurries often show evidence of some particle attrition, but this usually has only a small effect on dissolution rate. Specialized attrition devices are available if the application demands it.
Vessel Design Considerations
The optimum vessel for solids dissolution is round and has an aspect ratio of about 1:1. It must either be baffled with the mixer on center or the mixer can be angle-offset mounted. A center mounted mixer in an unbaffled tank will not effectively suspend solids. Offset mixers in unbaffled vessels will leave a significant heal of solids in one sector of the vessel floor. Angle offset mixers are generally restricted to relatively small volumes although we have used them successfully in 25,000 gallon vessels.