Twiflex Limited/Hilliard Corp. has recently supplied their largest grinding mill braking system to Metso Minerals for installation on the Boliden Mineral AB, Autogenous (AG) gearless driven mills.
Two of the largest grinding mills commercially available and supplied by Metso have replaced the existing
five mills.
These 22.5 MW primary mills with wrap around motors measure 11.6m in diameter and 13.7m long, meaning the two milling lines now have a capacity of 2025 t/h.
The grinding technique used in the Boliden concentrator is two stage fully autogenous grinding where the
crushed ore constitutes the only grinding media in the primary mills.
Pebbles (25-70mm) are extracted from the primary mills to make up the grinding media in the secondary
‘pebble’ mills.
The Twiflex braking system is designed specifically for mine grinding mill installations giving both static
and dynamic braking functions.
In static operation the braking system is used to hold the mill during liner replacement and general mill
maintenance.
For dynamic operation the system can operate in two modes, stopping the mill from full speed in an emergency or providing inching/creeping functions in the event of bearing lubrication problems or power failures.
For the first, a controlled application of the brakes is required and for the second, the brakes are operated quickly to give accurate stops needed by the mill operator.
The scope of supply for this project was eight Twiflex VMS-DP brake calipers with pedestals plus hydraulic power pack for each primary mill.
The Twiflex VMS-DP caliper has an adjustable braking force from 590 to 737 kN and is a floating spring-applied, hydraulically-retracted brake suitable for disc/flange thicknesses from 117mm to 130mm.
The brake can be used on installations with a braking path of at least 7.6m outside diameter (there is no
upper limit) and 7m inside diameter.
The hydraulic powerpack offers an advanced and versatile brake control as it allows both local and remote operation for inching and creeping duties through a control panel.
The braking system at Boliden can generate up to 38 MNm braking torque for each mill acting on a 12.97m mill flange diameter.
The VMS-DP calipers weigh 1.67 tonnes each and are able to deliver 940 kN clamping force.
With a full process charge of 1375 Mg the braking system is able to stop a mill in less than 2 seconds.