Throughput Optimisation in Milling Circuits
All mineral processing plants have some form of comminution, with SAG and Ball Mills drawing the highest power. 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. Millions of dollars are spent breaking down these large rocks into smaller more refined product for further processing in your plant. Any improvement to stabilising milling circuit with advanced online measurements, will help optimise this process to provide a high rate of return.
Implementing advanced process control with advanced online instrumentation, results in higher throughput and recovery rates in the mill. Process IQ offers MillStar Advanced Process Control to stabilise mill feed, discharge and optimise the circuit. MillSlicer and Ball MillScan mill vibration technology help reduce liner wear, Ultrasonic density probes provide robust high frequency measurements and CyLas hydrocycle underflow flare provides optimised classification and detect any roping. In a comprehensive implemented advanced control strategy, this will improve process control in your comminution circuit.
Mill Vibration Technology - SAG Mills
MillSlicer – detects liner strike position and help control the mil fill levels
Mill Vibration Technology - Ball Mills
Ball MillScan – detects liner strike position and help maintain consistent power draw in Ball Mills
SAG Mill Power
SAG milling can be optimised to achieve maximum breakage and throughput (or any other number of objectives). The relationships between the key variables are highly non-linear and shift as the ore hardness, steel load and liner conditions change. Traditional and basic control and modelling techniques are therefore not veryfective.
A Model Predictive Control System such as MillStar Advanced Process Control (APC) can continuously seek 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 Process Control (APC) should address the following areas:
- Mill feed stabilisation
- Mill discharge stabilisation
- Mill throughput optimisation
Advanced Control Process
A good advanced control system such as MillStar APC can significantly increase throughput and grinding efficiency. Additionally, the liner and lifter life can be extended and the percentage grinding media per ton of ore processed can be reduced. 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
Controlling the feed and discharge of a mill is mostly a stabilisation exercise – you need to be able to quickly and accurately control 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 control is quite involved due to 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 is installed it can be included in the control strategy to 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.
Each milling circuit is unique, and as such the goals and most optimal control strategy might differ for individual cases. Process IQ is an expert in implementing control systems for SAG and Ball milling circuits, making use of advanced stabilisation and optimisation strategies developed by Mintek.
And last, but not least, one should estimate the power load curve and optimum load operating point for maximum productivity in real time.