Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
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• Permanent magnets provide compact design with no power consumption. As material reaches the end of the screen, the magnetic field lifts and holds ferrous chips and scats on the drum shell. As the drum revolves, it carries the material through the stationary magnetic field to the top of the drum onto a sepa- rate discharge. The nonmagnetic materi- al falls freely from the screen onto a con- veyor belt. A heavy-duty magnetic drum separator is commonly used in minerals processing. The drum consists of a stationary, shaft- mounted magnetic circuit completely enclosed by a rotating drum. The magnet- ic circuit has segments of alternating rare-earth magnets and steel pole pieces that span an arc of 120°. The steel poles are induced and project a high-intensity, high-gradient magnetic field. The nonmagnetic material discharges in a natural trajectory from the screen. The magnetic material is attracted to the drum shell by the magnetic circuit and is rotated out of the nonmagnetic particle stream. The drum separator treats rela- tively coarse material in a high-capacity, severe-duty application. Trunnion Magnet With Ball Mill Discharge The trunnion magnet is an enhanced system for separation and removal of balls, chips or scats in a typical ball or SAG mill operation. The trunnion magnet is mounted at the SAG ball mill dis- charge point and is used in place of a trommel screen. The trunnion magnet consists of a barrel or "blind trommel" that is mechanically attached to the trunnion or discharge of a ball mill. The barrel rotates around a fixed assembly of ferrite and rare-earth magnets, positioned on the outside of the barrel. The stationary magnetic assembly, approximately 28 in. long, covers an arc of about 210°, and attracts chips and scats to the inside diameter of the barrel. As the ball mill slurry discharges through the barrel, eight strategically placed lifters inside the barrel carry the ball fragments to the top, where they fall onto a sloping dis- charge chute. In a typical grinding mill applica- tion, the grinding media eventually fractures and wears into a fine metallic powder because of the heavy recirculat- ing load in the mill. Fragments of grind- ing media eventually accumulate in the ore being processed and can cause serious damage to other equipment in the grinding circuit such as pumps and hydrocyclones. The foremost reason to use a trunnion magnet system in a ball mill is to replace the deadweight of ball fragments with fresh ore. The effective removal of chips and scats from a ball mill leads to lower power consumption from the mill drive. Eriez' trunnion magnet can provide the solution—a powerful and efficient mag- netic separator that can effectively remove as much as 80% or more of the worn/broken media. There are significant advantages and measurable improvements when a trun- nion magnet is installed: • Eliminates the higher capital cost of a trommel screen and associated main- tenance. • Pump and hydrocyclone service life is extended by as much as 250% in docu- mented cases. • Can increase mill throughput up to 5%. • Reduces mill power consumption 5%–7%. • Can increase mill work index due to more efficient grinding. One example of the trunnion magnet's effectiveness was recorded at the Kemess mine in British Columbia, an open-pit copper and gold operation. There, the effect of a trunnion magnet system on the ball mill was significant: Total mill feed remained essentially unchanged, averaging approximately 1,300 t/h; however, total mill power con- sumption dropped 8% from an average of 7,600 kW to 7,000 kW. The mill work index dropped 10% from an average of 5.5 kW-hr/T to 5 kW-hr/T. Retrofitting a ball mill with a trunnion magnet is easy, particularly when bene- fits are weighed against the cost. There are more than 100 installations of trun- nion magnets worldwide. Jose Marin is Eriez Magnetics' director of mining and minerals processes. He can be reached at 814-835-6000 or by email at jmarin@eriez.com . SEPTEMBER 2015 • E&MJ; 65 www.e-mj.com M A G N E T S This Eriez trunnion magnet is mounted at the SAG mill discharge point and is used in place of a trommel screen. Placing a magnetic drum at the discharge of a vibrating screen can provide a number of operational benefits.