The crushing and milling equipment selected for the Anglo Platinum’s Mogalakwena (previously PPRust) mine near Mokopane, is notable for its sheer size. In addition to the primary gyratory crusher, which is believed to be the biggest anywhere in the world, two 8m diameter by 11,7m long gearless ball mills, both rated at 17.5MW have also been supplied. These are the biggest ball mills yet supplied for a platinum mining industry application in South Africa and are also among the biggest anywhere in the world. The new concentrator expands milling capacity by 600,000 tons per month, producing an additional 230,000 ounces of platinum per annum and bringing total platinum production at the mine to 430, 000 ounces per year.
With each ball mill drum having a monumental total inertia of 14,195,400kgm2, controlling the drums (i.e. stopping, holding and positioning them) presented its own unique problems. Periodically they have to be stopped and brought into an equilibrium position while new liners are inserted or essential maintenance is performed. In addition, there is the requirement for emergency stop should a power outage occur, or a problem develop with the hydraulics serving the drum.
The unique nature and demands of the application necessitated a special type of brakeThe robust VMS-DP units are spring applied, hydraulic release safety brakes designed to operate in the most arduous conditions, where reliable operation all year round is expected. The VMS-DP brakes are a compact, modular mine-ready design that, unlike calliper brakes, does not need guarding. Moreover, they are about onethird smaller than calliper units, providing a more compact assembly, and one that reduces cost, both in terms of the size of hydraulic power pack – and the volume of oil required – to release the brakes.
Each of the 8m ball mills has six of the VMSDP brakes, providing a total braking force of 4.4MN or 737KN per brake. What this means in operational terms is that even with a full process charge (1.29Mg) the braking system could stop the mill in a staggering 0.5 seconds. This stopping time assumes that the charge mass assists the braking; it is based on the brakes coming ‘hard’ on and the coefficient of friction (μ) being 0.4. However, in practice the customer wanted the braking to be controlled to avoid damage to his equipment. To facilitate this, the hydraulic powerpack is set to stop the mill between 6 and 10 seconds using a fast approach, soft braking option: (i.e. brake pads come onto the disc quickly with zero force and then the amount of force is controlled to give the desired stopping time).
The Twifex system is designed to provide 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 giving inching/ creeping operations 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 actuated quickly giving the accurate stops required by the mill operator.