In 2012, Kevitsa, a First Quantum (FQM) copper-nickel-platinum mine in Finland, began production with throughputs of about 5 Mt/y. The original scope included a grinding circuit consisting of two AG mills and a secondary pebble mill, with successive copper and nickel flotation to produce two separate concentrates. However, according to Kevitsa’s Chief Metallurgist, Ishmael Muzinda, some 12% of the copper could not be recovered in the copper flotation stage due to poor liberation and grade-recovery limitations. “If the liberation is poor, then you don’t get a good copper-nickel separation,” said Muzinda. “And that copper that is reporting to the nickel concentrate is not paid for at the same rate as copper in copper.”
After completing a detailed mineral liberation analysis, Muzinda was able to identify the grain size was too small for proper liberation, sometimes as low as 15 microns. “Because of the issue of copper-nickel separation, we identified the need to regrind to 20 microns,” Muzinda added.
In February 2015, FQM achieved improved copper recovery and concentrate grade quality at its Kevitsa mine by installing a vertical stirred mill, or HIG mill, provided by Outotec. FQM considered other options but settled on Outotec HIGmills “because of its energy efficiency and for delivering fine and ultra-fine grinding solutions.”
HIGmill technology has been developed and utilised for more than 30 years. Today there are more than 200 installed units around the world in the limestone industry, with now 10 new HIGmill orders for the introduction of this technology into the crossover field of hard rock metallic mining. HIGmills are proven in full-scale operation up, with installed mill power up to 5,000kW. Outotec HIGmills are so far the largest units in the market place.
“We could safely say that we can save our customers between 30-40% in energy savings with Outotec´s HIGmill compared to traditional ball mills, and 15-25% compared to other stirred mills,” says Steve Schmidt, Outotec’s Commercial Product Manager for HIGmills.
The HIGmill comprises a mill body, shaft with grinding discs, shell mounted counter rings, gearbox and drive. The grinding chamber is filled up to 70% with grinding beads. Rotating discs stir the charge and grinding takes place between beads by attrition. The number of discs (grinding stages) depend on the application and can be up to 30. Feed slurry is pumped into the mill via bottom connection. When the flow transfers upwards, it passes all consecutive grinding stages. Final product discharges at open atmosphere at the top of the machine. Because of the tall, narrow, vertical mill body arrangement, grinding media is evenly distributed and mineral particles remain in constant contact, significantly increasing grinding efficiency. The most obvious difference between the HIGmill and its competitors is its tall, narrow aspect ratio, unique agitator design and the existence of the stationary counter-discs.
In 2016, FQM accomplished its goal of improving recovery, when the amount of copper reporting to nickel dropped to 8%. Overall copper recovery also increased by 1%. Muzinda said with the benefits of increased throughput, he estimates the payback time at under two years.