AGITATION VERSUS MIXING
Another complicating factor is the troublesome word agitation itself, because it implies
a need for violent solution movement, whereas the usual actual requirement is for proper
mixing--and the two are not necessarily congruent.
To clarify this, consider a cylindrical tank with a centrally mounted paddle-type mixer as
shown in Fig. 1. Such a system might be employed in diluting a reagent, or in a pollution
control reactor, or for batch carbon treatment of a nickel solution. The mixer will cause the
contents to swirl rapidly, but actual mixing will be minimal (to appreciate this assertion, think
about the fact that each drop!et of solution is following an essentially fixed orbit around the
mixer shaft). Actions taken to disrupt this orbiting, including installing baffles or moving the
mixer off center, will cause the mixer's power to be absorbed in the desired shearing action
rather than being available to rotate the contents as a whole, with the result that the solution
may appear to the casual observer as less agitated.
Another example of movement, but little mixing, can occur with the typical rotating
plating barrel illustrated in Fig. 2. There may be much agitation both within and outside of the
barrel envelope, but little mixing through the envelope. Again, whether or not the mixing is
sufficient depends on the particular requirements. Typically, the barrel rotation provides
sufficient solution interchange to allow the plating process to proceed satisfactorily; but,
conversely, rinsing will be improved if the barrel is momentarily lifted out of the rinse tank
(allowing it to drain and refill) instead of being left immersed.
In deciding how to agitate a particular solution, the principal factors to consider include
how much agitation is actually required, and what side effects are foreseeable.