July 11, 1997
Keeping tramp material out of a screw press is a common difficulty. While stones and glass can be a problem, the most common tramp material is ferrous steel. Because of their magnetic properties, carbon steel and 400 Series stainless steels are relatively easy to deal with. The non- magnetic 300 Series stainless steels present special challenges.
The magnetic separators commonly used in industry consist of a vertical diverter ductwork. This ductwork is mounted so that, when a magnetic field senses metal falling through, the contaminated flow is diverted out of the flow. The flow with tramp material is usually diverted to a portable dumpster. This works exceptionally well on dry material, such as the material leaving a dryer and going to a pellet mill.
The weakness of this magnetic separator in protecting a screw press is that the inbound flow will frequently have varying moisture content. Since the resonance of the magnetic field varies with moisture, the metal detector sensitivity must be set for a wide range of moistures. That way it is effective during normal operation conditions as well as when sloppy material comes to the press as a result of CIP (Clean In Place) procedures upstream in the system. Unfortunately, the required range is so broad that the metal detector loses its usefulness. For this reason magnetic resonance metal detectors are rarely employed on wet peel in the citrus industry.
Permanent magnets are useful on wet materials. These are generally installed in sloped transitions between screw conveyors. Clean-out is effected by picking the material off the magnet during shut-down periods.
With permanent magnets, high levels of magnetism are frequently employed. A class of magnets referred to as ceramic magnets are most commonly specified. Rare earth magnets are also popular.
Another effective way of separating tramp materials from a wet flow is to pass the material through a separation chamber. The heavy tramp material is allowed to accumulate in the bottom of the chamber. In the citrus industry, the addition of molasses to citrus peel can be achieved by injecting the molasses into the bottom of a trap compartment. The peel is dragged through the top of the compartment by a screw conveyor. The molasses coming from below fluidizes the mass in the compartment. The agitation and movement that occurs is very effective in allowing tramp materials to accumulate in the bottom of the separator chamber.
In this system the molasses are effectively added to the peel ahead of the reaction and pressing operations, while tramp material inclusion is minimized.
These traps are effective in removing significant amounts of sand along with a wide range of larger tramp materials.