he surging behaviour of the balling-drum circuits used in pelletising is identified as a limit-cycle oscillation by a control-theory analysis. The principles resulting from this analysis are then used to devise and assess a range of possible schemes to control the surging behaviour. Two basic approaches to the problem are identified:

1.
(1) conventional control (in balling-circuit terms), where the moisture content is used to modulate the surge level of the circuit; and
2.
(2) circuit modification to design out the surge which corresponds to the classical control-system technique of feedback compensation.
The various schemes are tested and their effectiveness assessed using a digital simulation of the balling-drum circuit, which is based upon the fundamental kinetics of balling. The circuit-redesign approach is shown to be a robust technique with both good practical properties and a sensible interpretation in terms of feedback control theory. Conventional control strategies, however, require a significant pay-off between surge level and product quality to be effective.