Getting Metal Out of Grain and Cereals

Grain arrives at the mill carrying metal. Some of it comes from the field: wire, bolts, and fragments from harvesting equipment. Some comes from the trucks, railcars, and augers that moved it. More is added inside the plant as elevators, conveyors, screens, and rolls wear.

Because metal enters at several points, it has to be removed at several points. One magnet at intake does not protect a roller mill three floors up, and it does nothing about a stainless steel fragment from a broken sifter screen.

This guide follows a typical wheat mill stage by stage and shows where each type of equipment goes, what it catches, and what it protects. Every mill is laid out differently, so use it as a starting map for your own line.

Two ways to remove metal

Magnetic separators and metal detectors do different jobs. Most mills need both.

The usual pattern is magnets early and often, to protect equipment as grain moves through the mill, and metal detectors near the end, as the final inspection before product ships.

The mill, stage by stage

Six points in a wheat mill where metal is most likely to enter or do damage.

1. 1

   Truck and railcar receiving

   Grain intake

   This is the highest risk point in the mill and where the largest tramp metal turns up. Anything that came from the field or the transport arrives here in one load.

   Grate Magnet

   Sits in the intake pit or hopper and catches large tramp iron as grain is dumped.

   Plate Magnet

   Mounted on the intake conveyor or spout ahead of the elevator to catch smaller pieces.

   Magnetic Head Pulley

   Alternative to the plate magnet on belt conveyors.

   Replaces the head pulley on the intake belt and removes ferrous metal continuously as grain discharges.

   Protects: the receiving elevator, conveyors, and every stage downstream.

2. 2

   Between the blending bins and the mill mix bins

   After blending

   Grain from different lots is blended to hit a target protein and moisture. Moving it through the bins frees metal that was buried in the grain on the first pass.

   Plate Magnet

   A second plate magnet on the discharge from the blending bins.

   Hump Magnet

   Alternative for vertical gravity chutes.

   Two plate magnets offset in a dogleg housing. Grain falls onto the first magnet, tumbles, and falls onto the second, which exposes metal the first magnet could not reach.

   Protects: the cleaning equipment in the next stage.

3. 3

   Screens, aspirator, and destoner

   Cleaning

   Before milling, wheat is cleaned. Screens remove oversize and undersize material, the aspirator pulls off dust, chaff, and light impurities, and the destoner removes stones and other heavy debris. The clean wheat is then dampened and held in tempering bins so the bran toughens and separates cleanly in the mill.

   Center Flow Magnet

   An inline gravity magnet ahead of the cleaning equipment. The magnetic element sits in the center of the flow and holds captured metal until the magnet is cleaned.

   Plate Magnet after the destoner

   A safety net for the cleaning equipment itself. If a screen tears or a machine sheds a part, this magnet catches the fragments before the wheat goes to tempering.

   Protects: the tempering system and the mill.

4. 4

   Feed to the first break rolls

   Before the roller mills

   After tempering, wheat enters the most expensive equipment in the plant: roller mills, sifters, and purifiers. A piece of tramp metal here can chip or score a roll, tear sifter screens, and stop the line until repairs are made.

   Rare earth Center Flow Magnet

   One on each feed to the first break rolls, with a neodymium rare earth element to capture the small ferrous pieces that got past earlier magnets.

   Protects: roller mills, sifters, and purifiers.

   

5. 5

   After the sifting system

   Bran and flour streams

   Milling splits the wheat into two main streams: flour, or semolina when milling durum, and bran. Each needs its own protection, because metal worn off the rolls and sifters travels with both.

   Rare earth Plate Magnet, bran stream

   Ahead of the hammer mill or bran finisher. Protects it from damage and reduces wear before the bran goes to storage for shipment.

   HF Drawer Magnet, flour stream

   After the reduction rolls and before bleaching or treatment, where used. A high-intensity drawer magnet for fine iron in dry, gravity-fed flour.

   Center Flow Magnet or Pneumatic In-Line Magnet

   After treatment and ahead of flour storage. Use a center flow magnet on gravity lines and a pneumatic in-line magnet on pneumatic lines.

   Protects: the hammer mill, flour storage, and finished product.

   

6. 6

   Bulk load-out and bagging

   Before product leaves the mill

   This is the last chance to catch metal, and the point where metal detection matters most, because it is the only way to find non-ferrous and stainless steel contamination.

   Bulk load-out

   Just before loading trucks or railcars, flour passes through a Center Flow Magnet, an HF Drawer Magnet, or a Performer Elite Food P pneumatic metal detector, depending on how the line conveys product.

   Performer Elite GF gravity-fall metal detector

   Ahead of the packer. Inspects flour as it falls and rejects metal before it reaches a bag, so less finished product is rejected later.

   Conveyor metal detector for packed product

   Every finished bag passes through a detector on the packing line. Many end customers require this check before they accept product.

   Protects: your customers and your shipments.

The full layout at a glance

Why so many points?

Ten or more separation and detection points can look like overkill. Each one is there for a reason: to protect a specific machine, or to catch metal introduced by the stage before it. Take one out and the risk moves downstream, usually to more expensive equipment or into finished product.