Removing reinforced steel from concrete

Recycled concrete requires the crushing and removal of reinforced steel during processing. Malcolm Davy discusses how magnetic separators can assist in  the process.

Today, the recycling of materials is important to preserve our reserves of raw materials. We see it everyday in liquids, glass, metals, rubber, plastics and even concrete.

Recycling concrete has its hazards, as once it is broken into large lumps, it is crushed to reduce it to a smaller particle size. Eventually, it is recycled as an aggregate or used as a road base. Concrete generally has reinforcing steel rods embedded in it and during the crushing process the steel is broken into short lengths with very sharp points.

The steel has to be removed before the concrete can be reused and the most efficient method of recovering the steel is through magnetic separators. Below is an outline of the magnetic separators available for the recycling of aggregates and concrete.

These magnets are suspended above the conveyor belt at different stages of the crushing cycle with the first unit post-primary crushing. Here, it encounters the larger lumps of concrete with the loose, tangled bundles of steel. The feed from the secondary crusher is smaller in size with more steel being liberated so the clean-up is more intense. Due to the composition of concrete, there is an endless supply of steel that the suspended magnet needs to remove from the stream. The most efficient way of doing this is to use a self-cleaning design which incorporates a rubber cleated belt moving past the magnet face and conveying the steel out of the magnetic field to discharge into a skip or down a chute.

Unfortunately, the jagged points of the steel can damage the self-cleaning belt through cutting and spearing. The standard belt fitted to a magnetic separator is of two- or three-ply with rubber cleats to assist with clearing the steel from the magnetic field. To increase normal belt life in these conditions, it is preferable to have at least a 10mm natural rubber impact section vulcanised into the centre of the belt to absorb the impact with stainless steel angle cleats fastened to the belt via elevator bolts. Additional belt protection can come from fitting heavy duty fabricated stainless steel cleats and slats over the impact padded section of a heavier three-ply belt, reducing wear and impact on the belt.

Drum magnets are of a heavy duty, rugged construction and can be top-fed by a conveyor or vibratory feeder. Alternatively, the drum can be mounted above the discharge of a vibratory feeder or conveyor and then the steel is lifted up and transported over the drum to a discharge point. This method minimises entrapment of concrete between the steel and the magnetic drum and provides a clean steel product.

An advantage of a drum magnet is the heavy duty manganese steel shell and its high wear resistant qualities, plus minimal moving parts and control of the steel transfer.

For final recovery of the smaller pieces of steel within shallower burdens, a permanent magnetic pulley can be utilised in place of a standard head pulley – often referred to as a self-cleaning head pulley magnet. The steel is attracted to the magnetic pulley by inducement and held to the belt as it travels around to the bottom where the steel leaves the magnetic field and drops off into a chute.

Today?s demand on recycling has created a market for portable recycling crushing plants, enabling them to be set up where the particular requirement demands, either in crushing concrete or rocky outcrops within property development sites. This saves on the expense of transporting the material to a fixed crushing plant. Portable crushing plants include primary jaw crushing, a conveyor to carry the material to a secondary crusher, screening and discharge onto various stock piles and specially designed lighter magnets located on the conveyor after the jaw crusher.

These magnets are also self-cleaning and are of permanent (non-electro) design. The self-cleaning belt can be hydraulically driven for convenience, removing the need for electrical power. The magnet can be backed up with a metal detector fitted to the conveyor to protect the secondary crusher from having smaller aperture settings.

As magnets only attract ferrous metals, a metal detector can be installed after a suspended magnet to detect the non-ferrous metals. A metal detector can be supplied with contacts that will activate an alarm system, flashing beacon, a flip-flop gate to reject the tramp metal or even turn the conveyor off. It can also be supplied with a bag dropper system that drops a brightly coloured bag in the vicinity of the tramp metal detected. Manual extraction of the tramp metal can be done once the conveyor is stationary.

Malcolm Davy is the general manager of Eriez Magnetics Pty Ltd.

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