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How Trunnion Magnets are solving

problems and improving profitability in ball and SAG mills

by Jose Marin

                  There are now more than 100 installations of             The Trunnion Magnet is mounted at the
                      Eriez Trunnion Magnets worldwide. This           SAG mill discharge point and used in place of a
                  unique system for separating and removing balls,     trommel screen. It consists of a barrel or blind
                  chips or scats in a typical ball/SAG mill operation  trommel that is mechanically attached to the
                  replaces the dead weight of ball magnets             trunnion or discharge of a ball mill. The barrel
                  with fresh ore. By effectively removing 80           rotates around a fixed assembly of ferrite and
                  percent or more of the worn/broken media, the        rare earth magnets positioned on the outside of
                  Trunnion Magnet reduces power consumption            the barrel. The stationary magnetic assembly
                  from the mill drive and prevents expensive           attracts chips and scats to the inside diameter
                                                                       of the barrel. As the ball mill slurry discharges
    Figure 1                                                           through the barrel, eight strategically placed
                                                                       lifters inside the barrel carry the ball fragments
Trunnion Magnets                                                       to the top, where they fall onto a sloping
                                                                       discharge chute.

                                                                           The practice of using magnets to collect
                                                                       ferrous metal from process streams has a long
                                                                       history in the mining industry. This is especially
                                                                       true in the collection and disposal of grinding
                                                                       ball fragments in ball/SAG mill operations.

                                                                           Often small in size, worn and broken
                                                                       metallic grinding balls can cause big problems,
                                                                       if they are not detected and removed from
                                                                       the milling operation. These metal remnants
                                                                       can cause extensive damage to sumps, pumps,
                                                                       hydrocyclones and interconnecting piping.

                  damage to other equipment such as pumps and          Trunnion magnet performance
                                                                           Trunnion magnets are available in several
                  hydrocyclones.
                                                                       design variations to accommodate specific ball/
                  In a typical grinding mill application, the          SAG mill operations. They effectively remove
                                                                       80 percent or more of the worn/broken media
                  grinding media eventually fractures and wears        and — at the same time — reduce power
                                                                       consumption from the mill drive.
                  into a fine metallic powder because of the
                                                                           Besides removing worn grinding media
                  heavy recirculating load in the mill. Energy is      from the ball mill discharge and saving energy,
                                                                       Trunnion Magnets eliminate the need for the
                  unnecessarily being wasted to mill the milling       higher cost of a trommel screen and can extend
                                                                       pump and hydrocyclone service life by as much
                                          media. Fortunately, through  as 250 percent. Mill throughput is increased up to
                                                                       5 percent, while operators can reduce mill power
Jose Marin, member SME, is                advancements in magnetic     consumption by up to 7 percent. In addition,
director, mining and minerals             separation technology,       many installations increase their mill work index
processing, Eriez, email jmarin@          Eriez offers mill operators  through more efficient grinding.
eriez.com.                                a proven approach with the
                                          Trunnion Magnet.             How the Trunnion Magnet works
                                                                           The Trunnion Magnet system includes four

                                                                       basic components: the blind trommel (barrel),
                                                                       magnet sector, support structure and the
                                                                       discharge hopper (Fig. 1).

                                                                           The barrel is a short extension that bolts
                                                                       directly to the discharge flange of the ball mill.
                                                                       The function of the blind trommel is to transport
                                                                       the mill discharge material through the magnetic
                                                                       field. The blind trommel is fabricated from

2 september 2017     Mınıng engıneerıng	                               www.miningengineeringmagazine.com