In this case, the primary grinding stage in a concentrator comprises a rod mill in open circuit and a semi-autogeneous mill in closed, circuit with a hydrocyclone classifier.

All the measurements have computer interfaces and all the control circuits have been implemented by the process control computer.

The crushed ore feed to the rod mill is measured by a belt-weigher and controlled by a belt-feeder. The pulp density in the rod mill is stabilized by proportioning the waterfeed at the mill inlet to the crushed ore feed.

The slurry level in the pump sump is measured by a pressure transducer and it is controlled by charging the speed of the direct-current drive pump.

Design of the Controller

A dynamic process model was developed with the aid of step responses. According to the present control strategy the manipulated variables are the crushed ore feed and the water feed to the cyclone pump sump. The final controlled variables are the cyclone overflow particle size and the cyclone feed density.

When only single loop controllers are applied it is obvious from the model that charges in the setpoint of the cyclone feed density cause strong effects in the particle size. This is due to tho small delay (112 s) and the snail time constant in the transfer function between the water feed and the particle size. The primary aim of the controller design is to make the two control loops as independent as possible.

The system is clearly not diagonally dominant at higher frequencies. In the first design step a lag compensator (T = 600 s) is added to the control loop between the water feed and the cyclone feed density. This is done in order to make the interaction from the control loop between the water feed and the cyclone feed density to the particle size slower.