Oscillations in a laboratory flotation machine

In a laboratory Denver machine, worn impeller/stator assemblies lead to strong, rapid oscillations of the water level in the standpipe at impeller speeds just below self-aeration. Frequency depends linearly on impeller speed, decreasing with impeller depth. Amplitude passes through a maximum with speed, increasing with depth. A mechanism accounting for oscillation is suggested. The possibility that the phenomena lies in the operating range of commercial-size machines is discussed. The described effects are confined to standpipe/self-aeration machines. Oscillation can be prevented by supplying air with an external blower, a growing trend even with traditionally self-aerating designs. Oscillation recommences just prior to flooding.