Iron-Clad Recovery

In auto shredding systems, the most important magnets for recovery of steel and cleanliness of product are the magnetic drums.

A magnetic drum is defined as a device with an adjustable stationary magnet assembly, made up of either electro- or permanent magnets, around which a non-magnetic shell of stainless steel or manganese is rotated.

Sweeps or bars welded to the shell provide positive transportation of the material over the drum.

DRUMMING UP SUCCESS

Drum magnet units are manufactured with magnetic circuits defined as axial pole (alternating polarity along the drum circumference and the same along the drum width) or radial pole (alternating polarity across drum width and the same along the drum circumference).

Since the feed being presented to the drum is nearly 80 percent ferrous, a pick-up system is required to avoid entrapment of the material.

The axial pole unit provides material agitation as the ferrous product is transported over the drum face, while the radial construction pulls, holds and carries. Today, the axial pole drum is primarily used. Most systems use two drums in tandem, whereby the ferrous recovered by the primary drum is retreated on the secondary unit. Single-drum arrangements can be provided, but the ferrous concentrate may require an air-system cleanup.

The units normally used in shredder operations, depending on volume, are 36 inches, 42 inches, 48 inches, 60 inches and 72 inches in diameter, with drum widths available from 48 inches to 120 inches in increments to suit the feed volume.

The size of the drum is based on the volume of material being fed to it. Normally, the feed is presented to a drum by either a flat belt conveyor or an oscillatory pan feeder. In a 150-ton-per-hour system with a bulk density of 50 pounds per cubic foot, the volume being treated is 100 cubic feet per minute.

If a pan feeder is utilized, the material velocity at most is about 50 feet per minute. If we limit the material depth to one particle (about 4 inches), or 0.33 feet, then 16.5 square feet per minute are moving to the drum.

By dividing the square feet per minute into the total volume of 100 cubic feet per minute, a 6-foot wide feeder or drum would be required. The size is also affected by variations in particle density and velocity.

PARTICLE MATTERS

Now consideration should be given to the maximum particle size, whether non-metallic or nonferrous, that is desired to pass through the gap. This also helps determine the distance of the drum diameter from the stream to provide the strength to produce maximum recovery. The following is a rule of thumb:

Drum Diameter......... Suspension
36 inches.................. 6 to 8 inches
42 inches............... 8 to 10 inches
48 inches............. 10 to 12 inches
60 inches............. 12 to 14 inches
72 inches............. 16 to 18 inches

Normally, permanent magnet circuits can produce depth of field equal to the electro-magnet designs. The electro-magnet, however, typically produces a stronger holding force at the surface of the drum.

Maintenance required on the magnets includes bearing lubrication and observation of the drum shell for excessive wear. Dents in the drum shell can produce interference between the magnet’s outside diameter and the inside of the shell.

Since the electro-magnet is constructed with steel poles exposed to the inside of the drum shell, a rub can wear away the drum shell from the inside. This is not necessarily true with a permanent magnet, which is normally encased in relatively thin-gauge stainless steel. But, the outer diameter of the magnet assembly may wear away and allow the magnet material to jam the inside of the drum.

SUSPENDED ASSISTANCE

Normally, suspension magnet units are suspended at right angles to the residue or fluff belt, while the captured ferrous is discharging to the side of the belt.

In selecting these magnets, it should be recognized that the units must be stronger than the drums at the suspension distance selected to recover the steel missed by the drums. These magnets again can be electro-magnets or permanent in design, but the same cautions apply in jamming situations.

Depending on the volume of ferrous that these separators see, belt wear is a significant factor on these units. This wear can be reduced by going to special belt configurations, such as:

• Heavier outside ply;
• Stainless steel cladding;
• Urethane belts or cladding; or
• Adding a spacer between the belt and the magnet face to reduce holding forces.

The units are normally of a two- or four-pulley construction. Therefore, bearings should be lubricated periodically. The maximum incline on most suspended self-cleaning separators is 15 degrees, otherwise the belt tends to ride to the downside of the incline.

Any steel framework within 1 foot on either side of the magnet should have non-magnetic material in key areas to prevent ferrous material hang-up, since the magnetic field inductively magnetizes these frame parts.

HEAD PULLEYS

After separating the bulk of the ferrous scrap with the cross-belt magnets, a magnetic head pulley is used to pull out the balance of the steel.

Again, two types of construction are available: axial and radial pole. Most magnet manufacturers can build either type. An axial pole unit should be used in these applications because it provides a uniform field across the belt width. Radial pole units (which have alternating poles across the belt width) can lose spherically shaped particles through the air gap, which is normally weak.

Electro-magnet units can be built, but they are not typically as strong as the permanent magnets available through the years. The electro-magnet is normally available only in a radial pole design.

Pulleys should be selected for the maximum size particle to be handled. For instance, if a spur gear is 3 to 4 inches in diameter, the rule of thumb for tramp iron separation can no longer be used. You now require a minimum diameter of 24 inches on this pulley at a slower belt speed than recommended for tramp iron removal.

The splitter used to provide segregation between ferrous and nonferrous product must be made of non-magnetic material for the same reason previously mentioned on suspended equipment: inductive magnetization can cause ferrous hang-up between pulley and splitter.

The magnetic field on a pulley should be about 5/6th the width of the belt to prevent ferrous product from working its way between the belt and pulley face. The total face width of the pulley should be 2 to 3 inches wider than the belt.

Magnetic devices arrayed in this fashion should remove 98 percent of the ferrous material, preparing the rest of the material for its remaining steps, including separation of nonferrous metals using an eddy current.

The author, who has been in the field of industrial magnetics since 1950, is a product manager and magnetic separation specialist with Walker Magnetics, Milwaukee, Wis. He can be contacted at mormag@earthlink.net.