Back to the Basics

In the past decade there has been a concentrated effort to upgrade the nonferrous metals collected at auto shredding plants. This was justifiable and helped to provide new revenue streams for these companies. I believe that the emphasis placed on the nonferrous system has allowed some recyclers to overlook their ferrous system. However, there are some fundamental ways in which recyclers can accomplish the goal of collecting more "clean" ferrous scrap.

There are many proven design philosophies. It is assumed that most shredders strive to produce a dense uniform size scrap (minimum of pokers) to enhance the cleaning process. I will break down the ferrous shredding line into three sections: conveyors, magnet area, and z-box/air system.

(To see a larger version of the above schematic click on this link)

"Wider is better"! The most efficient way to separate the ferrous scrap is to spread it out, and have the separation system 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 would 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 has 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 is also common to see a two-man picking station on each side of the conveyor. Many picking conveyors are enclosed with appropriate air/heat depending on weather conditions. The radial stacker transports the clean ferrous scrap from the picking conveyor to the finished ferrous pile. A tropher belt design is common since it is only moving material from one location to another (no picking, no cleaning or separation.) The radial wheels allow for a larger area to stack the finished ferrous scrap. It is also wise to line all receiving hoppers with bolt in AR liners to extend the life.

I will offer some representative conveyor widths:

Magnet Area

It has been my experience 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 can extend its magnetic field into the feeder pan and lift the ferrous scrap away from the nonferrous/fluff. The width and length of the first magnet feeder should be sufficient to allow for effective separation. If you are feeding an axial pole magnet that is 84 inches wide, the feeder pan should be about 78 inches wide.

I believe that the majority of the magnet manufacturers have made substantial improvements to their designs. The current axial pole magnets offered have numerous enhancements over older radial pole designs. These improvements include:

• Stronger magnetic field, which allows for better ferrous separation

• Wider field – the horizontal field allows the ferrous to spread across the face of the drum, reducing the burden depth, maintenance, and wear on the drum cylinder. (An 84 inch wide drum will use 78 inch of the wear surface.)

• The axial pole design alternates the magnetic field polarity with a north pole followed by a south pole followed by another north pole. Steel reacts to the alternating polarity of the changing fields by flipping each time it leaves one pole and is attracted to the next one. This flipping process helps release non-magnetic material that may have been trapped against the face of the drum cylinder by a radial pole magnet.

• Replaceable wear covers extend the drum shell life.

The axial pole magnets have altered the thinking of some manufacturers who now offer one magnet system versus two. It was not uncommon for an older dual radial pole system to have ferrous metal loss of between 2-4 percent. Newer axial pole magnets have been successful in reducing the metal loss to as low as 1 percent. The keys to successful magnetic separation:

• Spread out the material to single burden depth

• Feed the magnet with a vibrator or oscillator

• Axial pole magnet diameter and width should be based on expected maximum volume surges

(To see a larger version of this schematic click on this link)

The magnet stand would be the structure that supports the magnet feeders and the magnet. A typical design would consist of a catwalk on both sides accessible by stairs on each side. It would be constructed of heavy I-beams and/or tubing. The magnet area would include side guards made of UMHW or equivalent material to restrict flying scrap. Some magnet areas are enclosed to reduce the noise to neighbors close by. Examples of past and current magnet sizes:

Old 80/104 2500 hp motor (2) 42/72 radial pole magnets

New 80/104 2500 hp motor (1) 60/72 or 60/84 axial pole magnet

Old 98/104 4000 hp motor (2) 48/84 radial pole magnets

New 98/104 4000 hp motor (2) 60/84 axial pole magnets or (1) 72/96

Z-Box/Air System

The z-box has been a very successful method for cleaning scrap. The fabricated box is in the shape of a "Z", allowing material to bounce off at least three walls to help separate the ferrous from non metallic items. The z-box will be lined with bolt in AR liners. The traditional z-box arrangement will have a closed loop air system that blows through the box to help with the ferrous scrap separation. About 75 percent of the air used would be recirculated, while 25 percent would be exhausted to the atmosphere. However, there are newer models that can recirculate 100 percent of the air.

The cyclone, fan, and drive would be sized accordingly with the expected shredder volume. Many times the cyclone is undersized for the expected volume, leading to premature cyclone wear. There are numerous cyclone designs, but a typical unit would 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 also should have replaceable bolts in AR panels, and/or rebar to extend the wear life.

The location of the z-box in the ferrous line has changed over the years. Prior to the introduction of eddy currents (late 1980’s and before), it was common to see the z-box in front of the magnets. This allowed the z-box to clean both ferrous and nonferrous metals. The downside to this location was that light nonferrous metals were exhausted into the cyclone and shipped to the landfill. With eddy currents becoming a standard item, the z-box was moved behind the magnets. This allowed most of the fluff/nonferrous to be separated from the stream that fed the z-box, reducing the burden on the z-box to clean the ferrous. Further, the location of the z-box will be determined on how you plan to upgrade your nonferrous metals.

The shredding system allows for many variations to achieve similar goals. There is no right or wrong design, but there are some that have proven successful. I would offer a few key points to help with your ferrous metal separation:

• Wide flat belt conveyors

• Feed your magnet with a vibrator/oscillator

• Use an axial pole drum magnet(s)

• Z-box air system after the magnet(s)

The author is president of Metal Shredding Solutions. He can be reached at mark@metalshreddingsolutions.com, or by telephone at (210) 481-5510.