What Makes the Ball Mill Liner So Important for You
New grids can slow grinding speeds: some mills switch grids by way of liner changes, while other users often alternate high or low grid lift bars to maintain grinding performance. Backflow from unloaded rocks can limit pulp lift performance and add to wear at the base of pulp lifters. Adequate depth from the front to the rear of the pulp lifters is necessary to ensure good pulp lifting, with some SAG mill operators having adopted curved pulp lifters, especially in the ‘hockey stick’ way. These help discharge the pulp, especially from larger pebbles, and substantially reduce any backflow. The use of the ball mill liner is also there now.
- The problem of whether to use grids or a pulp discharge overflow system is not limited to SAG grinding. According to van den Bosch, Magotteaux tests have shown that contrary to popular belief there is a small practical difference in performance between the overflow and grinding designs of the secondary and re-grinding mills in this application, although this is not the case for SAG grinding, where the main criterion is the total yield instead of the fine grinding.
- In an effort to demonstrate to South African mill operators that the practice of overflow grinding can save costs by eliminating the need for grids and their replacements, Magotteaux has now designed an interchangeable discharge system. Just by loosening a few bolts, van den Bosch said, the mill discharge can be switched from one mode to the other, back to the previous mode, if it proves to be more effective, at any time. The company has also been working on the design of the lifters at the discharge end of the mill, with a new design that can operate under overflow, semi-overflow or grid conditions now under test in a pilot mill.
Coating Management Logistics
Re-lining a mill is an arduous task, not only because of the sheer weight of each segment of a liner, but also because of the precision with which the liner has to be installed if the mill is to be operated efficiently. And while the locally made liner sections used at Waihi 100 years ago were probably manageable due to their relative lack of sophistication, re-lining a modern mill presents a whole different set of challenges.
Take the 18-MW SAG mill now operating at copper mine in Zambia as an example. Commissioned in 2008, the 38- × 20-foot mill is the largest in Africa, with a total output of 20 million mt / year. Back then, the company reported that it contained 600 mt of coatings, with the heaviest individual section weighing 2.2 mt, while re-coating requires around four at a cost of $ 1.5 million.
Clearly, handling a liner section weighing more than 2 mt goes beyond manual handling capabilities within the difficult but confined mill interior. Consequently, Equinox has turned to Russell Mineral Equipment (RME), the Australian company that claims to have an 80% market share of mill lining machines, with units sold to more than 160 mines in more than 40 countries.
Striking the Right Balance
As with most things in life, selecting the most appropriate coating for a specific grinding operation can be a balancing act between duration, cost, and several other factors. As indicated by several people interviewed here, the goals of the maintenance manager and the mill superintendent may not always be the same, especially where there are separate cost centers. Lower costs for coatings can keep the maintenance budget low, but they will certainly have an impact on the downtime of a mill and therefore on its availability.