The mill sizes were designed taking into account the most efficient process technology by applying state-of-the-art simulation programmes. The necessary installed grinding capacity of 34 MW in total derived from this was implemented as following:
Semiautogenous mill 10.4 x 6.1 m – 2 x 7.5 MW installed motor power
Ball mill 8.2 x 14.0 m – 2 x 9 MW installed motor power
In the machine design, new approaches were found with regard to the drive concept. For example, both mills are driven by four pinions, each with the help of two double pinion gears. The long transport distances in connection with the machine sizes had to be taken into account in the design.
In mid-2012, in addition to the grinding equipment, the remaining circuit equipment, including an online particle size determination in real time using CEOPS (Cemtec Online Particle Sizer), was commissioned. The commissioning finally took place in 2015.
During the first two years of operation, the production rate was raised to a level of 1350 t/h through optimisation measures. This mainly involved working on the transfer size between the SAG and ball mill circuits. The semi-autogenous mill is operated in a closed circuit with two vibrating screens and a 750 kW pebble crusher. The mesh size of the vibrating screens determines the transfer grain size to the ball mill circuit. The ball mill is fed by the underflow of 20 hydrocyclones.
Several sampling campaigns showed that the overall performance of the circuit can be can be optimised by using finer meshed screen decks at the vibrating screens after semi auto-genous grinding mill. For example, the ball mill circuit was operated with circulating loads of up to 500 %, while that of the primary comminution was only 50 %. The level of the circuit loads was also reflected in the electrical power consumed by the two grinding units. While the total grinding capacity was only utilised to 85 %, the ball mill already absorbed 17.4 MW of 18.0 MW. During the last years the process circuit also had to be adapted to changes of ore mineralogy. Coarser gold liberation size the fines of grind changed from originally 80 % < 74µm to 80 % < 130 µm.
Gold enrichment was therefore almost completely switched to density separation by gravity concentrators. In addition to the described changes in the transfer size and an increased installed capacity of 20 MW at the ball mill, the new target production capacity is 2000 t/h. The high availability of the plant, in combination with the optimisation work, not only led to a significant gold production by the end of 2020, but also to the highest customer satisfaction.