New concepts help mill operators improve profitability and eliminate downstream damage from broken grinding media
By Jose Marin
The presence of grinding ball fragments in the milling circuit can impact two critical areas in minerals processing. One is the crushing circuit where companies have observed damage to crushers, unscheduled downtime and loss of production. Grinding ball fragments, for example, can cause damage to a cone crusher mantle.
The second is the grinding circuit where ball fragments have resulted in wear to pumps and damage to pump impellers, as well as sumps, piping, hydrocyclones and liners. Fragments can also lead to inefficient grinding, increased power consumption and less-than-optimal throughput.
There are several proven magnetic separation techniques available today to reduce or eliminate grinding ball fragments, depending upon the application and circuit location. Each has its own set of challenges, and each is approached in a different manner.
SUSPENDED ELECTROMAGNET IN PEBBLE CRUSHER CIRCUITS
Suspended electromagnets (SE) are designed to capture pieces of tramp metal with a large mass (e.g., steel scats) rather than smaller metal pieces. The larger the mass, the more likely the SE will pick up the object in spite of maximum burden depth.
However, in a pebble crusher application, the majority of the metal is typically minus 4–5 in. To effectively separate 100% of +1/2-in. chips, at least two magnets and a metal detector or three magnets in a series are recommended. The first magnet can be crossbelt, but the second and third are recommended to be positioned in-line.
SE’s are typically mounted in the crossbelt or in-line position over a pebble crusher conveyor belt, mostly depending upon belt speed. If belt speed is fairly slow (300 ft/min or less), then pulling tramp metal out of burden depth should not be an issue. When the belt is traveling faster, it’s better to have the SE mounted just over the stream of material leaving the head pulley; material coming off faster-moving belts results in a higher trajectory free-fall off the head pulley, clear of any burden. The SE can also capture the tramp metal as it flies off the head pulley; however, a non-magnetized head pulley is recommended in this case.
Be aware that properly sized magnets for pebble crusher circuits do not follow conventional magnet selection norms. Through experience, companies like Eriez have adopted new selection norms to ensure the pebble crusher circuits operate continuously and trouble-free.
SUSPENDED MAGNETIC DRUM FOR OVERSIZE SCREEN DISCHARGE FROM SAG MILLS
By placing a magnetic drum over the discharge of the vibrating screen, a milling circuit will realize a number of benefits:
- Provides a relatively slow-moving ore;
- Eliminates the dead burden of a conveyor belt, since this is essentially a mono-layer approach;
- Agitates material to aid the physical release of entrapped ore;
- Allows placing a magnetic drum close (150 to 250 mm) to the oversize discharge screen; and
- 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 separate discharge. The nonmagnetic material falls freely from the screen onto a conveyor 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 magnetic 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 relatively 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 discharge 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 discharge chute.
In a typical grinding mill application, the grinding media eventually fractures and wears into a fine metallic powder because of the heavy recirculating load in the mill. Fragments of grinding 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 magnetic 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 trunnion magnet is installed:
- Eliminates the higher capital cost of a trommel screen and associated maintenance.
- Pump and hydrocyclone service life is extended by as much as 250% in documented 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 consumption 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 benefits are weighed against the cost. There are more than 100 installations of trunnion 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 [email protected].