Circuit Control Improvement Project
A leading Australian nickel mine has a grinding circuit comprising a primary single stage SAG mill in closed circuit with cyclones. The SAG cyclone underflow passes through a flash flotation before returning to the SAG mill.
A portion of the flash flotation tails reports to a separate ball mill which helps to reduce load on the SAG mill. Feed to the circuit is variable and comes from a range of different ore sources (underground, open pit, toll treatment) with varying characteristics in terms of hardness, grade and blast fragmentation.
In early 2020, Orway Mineral Consultants were called to site to assist with SAG mill operation. Control of the grinding circuit has historically been reliant on operator knowledge, historical experience and rules of thumb. Orway IQ developed a live mass balance and detailed understanding of the circuit constraints and assisted in implementation of a new control philosophy around the grinding circuit.
- Key constraints to circuit control were identified and solutions were implemented and trialed. These include:
- The interdependence of SAG mill and flash flotation operation does not allow for independent control of SAG mill and flash flotation density. This forces operation of dilute SAG mill in order to ensure flash flotation is operable.
- Continual feed stoppages due to fixed tonnage control philosophy (resulting in mill weight excursions and requiring grind out to rectify)
- Control of circulating load (cyclones online) based on mill discharge hopper level is being replaced with implementation of constant pressure control on both SAG and ball mill cyclones to stabilize circulating loads.
- SAG mill grate design causing flow restrictions and slurry pooling. Investigation of new grate design is underway.
- Diagnosis of throughput limiting issues like grate blockages
- Control of the SAG mill weight using feed rate has been implemented and controller is being used more frequently.
- SAG Cyclone pressure control implementation ongoing
- BM cyclone pressure control ongoing
Image: Results of controlled SAG weight