Recently, magnetic separation techniques have been developed to effectively collect tramp metal, such as grinding ball fragments, from mill process streams. Worn or broken steel balls are mixed with the ore and these metal remnants can cause extensive and costly damage to pumps and cyclones located downstream from the mills.

Eriez Magnetics says its trunnion magnet separator, available in several design variations to accommodate specific ball mill configurations, effectively removes up to 98% of the steel balls and ferrous chips from milled ore in SAG/ball mill operations. By eliminating these potentially destructive pieces, harm to downstream equipment is significantly reduced. The Pennsylvania, USA-based company says it has installed more than 50 trunnion magnet systems over the past three years.

Positioned at the downstream end of the mill, the trunnion magnet consists of a drum shell that rotates around a fixed assembly of ferrite and rare earth magnets. The separator receives ground ore in a slurry that also may contain residual broken balls. The slurry enters the mill magnet feed box and is spread evenly across the drum surface.

The powerful permanent magnet circuit attracts and holds worn/broken media to the surface of the drum. The ball fragments are then removed from the slurry stream by the rotating drum and discharged into a separate hopper. The feed box, drum surface and slurry guide cover are rubber-lined to minimize wear.

Eriez recently helped a Canadian copper producer solve a common problem in which ball fragments discharged from a grinding mill were causing extreme wear to downstream processing equipment. These ball fragments, circulating in the milling circuit, caused excessive wear to sumps, pumps, hydrocyclones and interconnecting piping. Attempts to remove tramp metal from the discharge stream by using screens to catch the fragments were only moderately effective, and the screens were expensive.

After Eriez engineers studied the application, they recommended installing a magnet, mounted to the mill’s discharge trunnion. During the first several hours of operation, the magnet removed nearly 20 tons of tramp steel. The removal rate eventually stabilized at a few tons per day. The installation was so successful that the company purchased another magnet the following year.

According to Eriez, in applications such as these the trunnion magnet pays for itself quickly by removing the steel chips that circulate in the ball mill/cyclone circuit and reducing the wear on the feed pump, distributor and hydrocyclone parts. This, in turn, lowers maintenance costs and increases production.

Eriez also claims an improvement in efficiency will also occur as cyclone apex wear life is extended. Better cyclone performance provides a finer flotation feed and improved copper recovery. The use of longer-wearing ceramic cyclone linings also becomes a possibility when the steel chips are eliminated, as they could otherwise damage ceramic linings. There may also be a drop in flotation impeller wear as well as a reduction in labor when the cost of cleaning steel chips out of the cells is taken into account.