A non-linear observer model of a semi-autogenous grinding mill is developed. The observer model distinguishes
between the volumetric hold-up of water, solids, and the grinding media in the mill. Solids
refer to all ore small enough to discharge through the end-discharge grate, and grinding media refers to
the rocks and steel balls. The rocks are all ore too large to discharge from the mill. The observer model
uses the accumulation rate of solids and the mill’s discharge rate as parameters. It is shown that with
mill discharge flow-rate, discharge density, and volumetric hold-up measurements, the model states and
parameters are linearly observable. Although instrumentation at the mill discharge is not yet included
in industrial circuits because of space restrictions, this study motivates the benefits to be gained from
including such instrumentation. An extended Kalman filter is applied in simulation to estimate the model
states and parameters from data generated by a semi-autogenous mill simulation model from literature.
Results indicate that if sufficiently accurate measurements are available, especially at the discharge of
the mill, it is possible to reliably estimate grinding media, solids and water hold-ups within the mill. Such
an observer can be used as part of an advanced process control strategy.