AIR AGITATION
If air is forced to the bottom of a liquid-filled tank, and allowed to exit through a
perforated pipe, the resulting bubbles will rapidly expand and race toward the liquid surface.
Figure 3 conveys the general idea.
Air agitation has its undesirable facets. It will stir up bottom sludges and anode sludges,
which, if not properly filtered, will cause roughness in the plating. It can cause excess
foaming. It is poorly suited as a substitute for a mixer because powdered and low-density
chemicals will tend to float on the surface rather than being drawn down into the mixing zone.
It is rarely applied in pollution control operations because of its tendency to cause flotation
of the precipitates. Thoughtless design or improper maintenance can mean that the air conveys
inordinate volumes of oil and atmospheric dust into the process tank. Further, by its nature,
the air will provide an oxidizing potential; while acid zinc batbs might profit from this through
oxidation and precipitation of iron, air agitation is never used in acid tin baths because the tin
itself would oxidize, and rarely in cyanide solutions where it promotes formation of
troublesome carbonates.
These limitations notwithstanding, air agitation is sufficiently simple, inexpensive,
efficient, and generally applicable that it is probably the most common method of producing
agitation in the metal-finishing shop today.