Throughput Optimisation in Milling Circuits
Mill Vibration Technology
Particle Size Distribution
Hydrocyclone Flare Angle Measurement
Optimisation of milling needs a systematic approach. One has to ensure the stable operation of the milling circuit before you can aim to optimise power input. Bear in mind that SAG and Ball mills are the highest power consuming equipment in the plant. All Mineral Processing plants have some form of comminution, but SAG and Ball milling is the most common. Comminution is required to liberate valuable minerals from gangue (waste), and then upgrade to a more concentrated form from which the ultimate product is made. Every day millions of dollars are spent breaking big rocks into smaller rocks. It is not a trivial problem and any improvement in these processes can provide a very high rate of return.
SAG Mill Power
Finally, the SAG mill power can be optimised to get maximum breakage and throughput. The power-load relationship is highly non-linear and shifts around as the ore hardness and steel load and liner conditions change. Traditional control and modelling techniques are therefore not very effective. An intelligent control system will be able to continuously seek for the optimum mill operation by changing the solids feed or load setpoint, e.g. by continuously determining whether the mill is overloaded or under-loaded. Safety controllers can be used to change the solids feed rate and feed water to prevent mill overloads.
Advanced Control Process
A good advanced control system (APC) can make a big difference in efficiency of operation and throughput optimisation.
The APC system will improve power control, throughput and efficiency of grinding mills. If this is done successfully, then internal liners and lifters in the mill be protected, less grinding media will be used per ton, power consumption per ton of material grounded will be reduced while throughput is increased.
Advanced Process Control (APC) should address the following areas:
- mill feed stabilisation
- mill discharge stabilisation and lastly
- mill throughput optimisation
Controlling the feed and discharge of a mill is mostly a stabilisation exercise. For example, you need a good fast responding control of the mill feed rate. If the mill feed setpoint is changed, then the actual feed rate needs to stabilise at the new target in a short period of time. The feed rate setpoint can then be tied into a SAG mill power optimisation strategy.
Sag Mill Discharge
SAG mill discharge control is somewhat more involved, because there exist a few interactive variables, such as mill discharge sump level, classification hydro-cyclone variables (feed density, flow and pressure) and water feed rate to the sump. Model predictive control (MPC), which is a multi-input multi-output (MIMO) controller, is very suitable for this purpose. If a particle size analyser exists it can also be included into the control strategy. This will ensure an optimal particle size for flotation or other downstream processes.
This strategy will ensure significant increases in throughput, typically anywhere from 5% to 16% has been observed.
And last, but not least, one should estimate the power load curve and optimum load operating point for maximum productivity in real time. Please watch this video to get an idea of how the power-load relationship in a SAG mill can change over a period of 24 hours.