Back to Basics

In the past decade there has been a concentrated effort to upgrade the nonferrous metals that are generated by the shredding of automobiles and white goods. This was justifiable because the nonferrous content in these products has been increasing and because it helped to provide new revenue streams for auto shredder operators.

However, I believe the emphasis that shredder operators have placed on the nonferrous portion of their downstream systems has allowed some to overlook their ferrous systems.

There are numerous ways to accomplish the intended goal of "clean" ferrous scrap and many proven design philosophies, and I will review some of the successful methods in the field.

I will break down the ferrous shredding line into three sections: conveyors, magnets and z-box/air systems. I’ll also start with the assumption that most shredder operators strive to produce a dense, uniformly sized shred (with a minimum of pokers) to help enhance the cleaning process.

When it comes to conveyors, wider is better. The most efficient way to separate the ferrous scrap generated by an automobile shredder is to spread it out so your separation system can attack a single particle size depth burden. A flat belt design will allow for the best potential in providing maximum width across the belt. A typical system would have flat belt designs for the first transfer, the z-box feed and the picking conveyor. The first transfer conveyor will convey the shredded scrap away from the shredder toward the magnet area. The width of the flat belt will be determined by the anticipated volume and size of the magnet.

The z-box feed conveyor will transport the material from the magnet area to the z-box. This conveyor should be the same width as the z-box and should have a hood with rubber seal flaps at the top to reduce air loss from the z-box.

The picking conveyor often will have a variable speed drive to allow for volume fluctuations. The flat belt horizontal design, along with the slower speed, allows for the most efficient picking. It also is common to see a two-person picking station, with pickers stationed on each side of the conveyor. Many picking conveyors are enclosed with air or heat, depending on the weather conditions in the area.

The radial stacker transports the clean ferrous scrap from the picking conveyor to the finished ferrous pile. A trough belt design is common for this portion of the conveying system because the conveyor is only transporting material from one location to another. (There is no picking, no cleaning and no separation occurring at this stage.) The radial wheels allow for a larger area to stack the finished ferrous scrap. It is also a good idea to line all receiving hoppers with bolt-in AR (abrasion-resistant) liners to extend their lives.

For an 80/104 shredder with a 2,500-horsepower motor, the first transfer conveyor should measure 48 or 60 inches wide. The same conveyor widths are recommended for feeding the z-box. For the picking conveyor, a width of 36 inches or 48 inches and a length of 30 feet are typical. A 36-inch wide stacking conveyor with a length of 60 to 70 feet is recommended.

When it comes to a 98/104 shredder with a 4,000-horsepower motor, the first transfer conveyor should measure 60 inches or 72 inches in width. Again, the same conveyor widths are recommended for feeding the z-box. A 48-inch wide picking conveyor with a length of 30 feet is common, as are dual picking lines. A stacking conveyor measuring 48 inches long and 70 to 80 feet long is typical.

It has been my experience that the most efficient method for feeding a magnet is with a vibratory or oscillating feeder. This allows the scrap to spread out and "dance" on the feeder. The magnet is then able to extend its magnetic field out into the feeder pan and lift the ferrous scrap away from the nonferrous and fluff.

The width and length of the first magnet feeder should be sufficient enough to allow for effective separation. If a system’s axial pole magnet is 84 inches wide, then the feeder pan should be approximately 78 inches wide.

The majority of the magnet manufacturers have made substantial improvements to their designs. The current axial pole magnets on the market have numerous enhancements relative to older radial pole designs. These include:

The axial pole magnets have altered the thinking of some manufacturers, who now offer a one-magnet system vs. a two-magnet system.

It was not uncommon for older dual-radial pole systems to lose 2 percent to 4 percent of the ferrous metal. The newer axial pole magnets have been successful in reducing ferrous metal loss to as low as 1 percent.

The keys to successful magnetic separation are:

In older 80/104 shredder systems, two 42-inch-by-72-inch radial pole magnets were often used, while in newer installations, one 60-inch-by-72-inch or 60-inch-by-84-inch axial pole magnet can be used. Two 48-inch-by-84-inch radial pole magnets were common in older 98/104 shredder installations.

In newer systems, two 60-inch-by-84-inch axial pole magnets or one such magnet measuring 72 inches by 96 inches can be used.

The magnet stand will be the structure that supports the magnet feeders and the magnet. A typical design in today’s market consists of a catwalk on both sides of the magnet accessible by stairs on each side. It would be constructed of heavy I-beams and/or tubing. The magnet area will have side guards made of UHMW (ultra-high molecular weight) or equivalent material to restrict flying scrap. Some magnet areas are enclosed to reduce noise.

The z-box has been a successful method for cleaning the scrap. The fabricated box, lined with bolt-in AR liners, is in the shape of a "Z." This allows the material to bounce off at least three walls, helping to liberate ferrous metals from nonmetallic items. The traditional z-box arrangement will have a closed loop air system that blows through the box to help with the ferrous scrap separation. Approximately 75 percent of the air is re-circulated, and 25 percent is exhausted to the atmosphere. Newer models are available that re-circulate 100 percent of the system’s air.

The cyclone, fan and drive of the z-box are sized in accordance with the expected shredder volume. When the cyclone is undersized for the expected volume, it can lead to premature wear in the cyclone.

Numerous cyclone designs are available, but a typical unit will have bolt-in AR liners in the high-wear areas, and the cones would be lined with rebar. The system design should consider short duct runs to avoid loss of air efficiency and unnecessary maintenance. The high-wear areas of the ductwork should have replaceable bolt-in AR panels and/or rebar for extended life.

The location of the z-box in the ferrous line has changed throughout the years. Before eddy currents, it was common to see the z-box in front of the magnets. This allowed the z-box to clean the ferrous and the nonferrous metals. The downside to this location was that light nonferrous metals were exhausted into the cyclone and ultimately shipped to the landfill. Starting in the late 1980s, with the eddy currents becoming a standard feature downstream of the shredder, the z-box was moved behind the magnets. This allowed for most of the fluff/nonferrous material to be separated from the stream that fed the z-box. This reduced burden allowed the z-box to aggressively clean the ferrous metals. It is safe to say that the location of a system’s z-box is determined by how an operator plans to upgrade the nonferrous metals in the material stream.

Shredding systems allow for many variations to achieve similar goals. There is no right or wrong design, but there are some that have proven successful. I suggest using wide flat belt conveyors, feeding magnets with a vibratory or oscillating conveyor, using an axial pole drum magnet(s) and placing the z-box air system after the magnet(s).