Freda Rebecca SAG milling circuit optimisation


Simulation work was carried out to analyse how feeding a finer size distribution to a single stage SAG mill in closed circuit with hydro cyclones at Freda Rebecca Gold Mine would affect throughput and specific energy consumption using an Excel based milling process simulator developed by Hinde, a comminution specialist who has worked for Mintek for many years. Other initiatives to increase throughput were evaluated which included; the installation of a regrind mill (1000 kW) to treat the cyclone underflow and the installation of a 100µm aperture fine screen to reduce the re-circulating load. ModSim software was used to study the later. It was observed that when the feed size distribution became finer there was an increase in throughput, a decrease in specific energy consumption and cyclone overflow fineness. A 13.1% increase in throughput was observed when F80 was reduced from 240 mm (split ratio=1) to 45 mm (split ratio=0), the specific energy dropped from 20.66 kWh/t to 18.5 kWh/t and the cyclone overflow product size distribution fineness decreased from 78.33% passing 75 microns to 74%. The optimum throughput was obtained at a split ratio of 0.2 which corresponds to an F80 of 70 mm. The throughput at this point is 88.17 t/h (10.2% increment), specific energy consumption of 18.94 kWh/t and the cyclone overflow product size distribution is 75 % passing 75 microns (plant process requirement). The payback period of the project at optimum throughput is 1.2 years.

Installing a 1000 kW regrind ball mill increased plant capacity by 56% through treating 29% of the cyclone underflow and reduced the SAG mill specific energy from 20.66 to 16 kWh/t and the investment has a payback period of 1 year and a month, in 10 years the project would have earned the company more than 15 million dollars of profit.

It was also observed from the plant survey that there was a high recirculating load of fine material to the SAG mill amounting to 51.2 t/h affecting mill capacity as this took space and limited the capacity for new feed. Using ModSim simulator, incorporation of a 100 micron screen on the cyclone underflow stream was simulated. The main purpose of the screen is to reduce the recirculating load by removal of fines that are deemed gold liberated enough and directed to the leaching circuit. The recirculating load was reduced from a base case of 238% to 175% and a 50% saving on energy required.


Prepared by Thamsanqa Ncube (690341)

Submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa

Supervisor(s): DR M. BWALYA

January, 2015