Giant mills rely on high-torque hydraulics
Boliden AB, Stockholm, is Europe’s third largest producer of both copper and zinc. Its operations focus on the initial stages of the processing chain: exploration, mining, milling, smelting, refining, and recycling. Its biggest open-pit mine, Aitik 36, in northern Sweden, was discovered in the 1930s, and is the largest open pit copper mine in Europe. Aitik 36 yields ores that are somewhat low grade, but plentiful. This makes high productivity of processing essential to maintain profitability.
Boliden recently invested in new facilities that will eventually double Aitik 36’s annual ore production to 36 million tons and increase copper production by 50% by 2014. The successive ramping up of production began in 2010 and will continue until it reaches full capacity.
A key component of the upgrade called for replacing five grinding mills with two — each being the largest grinding mill commercially available. This reflects a trend over the last five years to use fewer comminution (pulverizing) machines per line, so machines have increased considerably in size and capacity. These mills were supplied by Metso Minerals Industries, Helsinki. Together, the two mill lines can now process 2025 tons of ore per hour.
The technique used by Boliden is two-stage fully autogenous grinding, where the grinding medium is the crushed ore itself. Pebbles 25- to 70- mm in diameter are extracted from the primary mills to make up the grinding medium in the secondary pebble mills.
Each mill is rated 22.5 MW and is driven by a wrap-around motor measuring 11.6 m (38 ft) in diameter and 13.7 m (45 ft) long. The motors are direct drive, with no geared speed reducer. This technique is effective at driving the mill, but it is not able to quickly stop a rotating mill or provide an inching function. This is where a separate braking system comes in.
Twiflex Ltd., Middlesex, England, recently supplied its largest grinding mill braking system to Metso Minerals for installation on Boliden’s mills. The Twiflex braking system is designed specifically for mine grinding mill installations and provides both static and dynamic braking functions. In static operation, the braking system holds the mill during liner replacement and for general maintenance. For dynamic operation, the system can operate in two modes: stopping the mill from full speed in an emergency due to bearing lubrication problems or power failure. Dynamic braking requires a controlled application of the brakes, but for inching, the brakes are operated quickly to give accurate stops needed by the mill operator.
Twiflex supplied eight VMS-DP brake calipers with pedestals for the project, plus a hydraulic power unit for each of the two mills. The Twiflex VMS-DP caliper has an adjustable braking force from 590 to 737 kN and is a floating spring-applied, hydraulically-released brake for disc thicknesses from 117 to 130 mm. The hydraulic power unit offers a versatile brake control by allowing both local and remote operation for inching duties through a control panel.
The braking system at Boliden can generate up to 38 MN-m braking torque for each mill acting on a 12.97-m mill disc diameter. Each VMS-DP caliper weighs 1.85 tons and can deliver 940 kN clamping force. Even with a full process charge of 1375 Mg (3,025,000 lb) in the mill, the braking can stop the mill in less than 2 sec.