Where Magnetic Separators Belong in a Grain Line and When to Clean Them

The number of magnetic separators a grain line needs is decided by where iron can enter the material and where the plant itself produces it. It is not decided by how strict the specification sounds. Two questions asked at every step of the flow give the answer: can tramp iron get in here, and does anything here wear metal into the product?

There are two sources, and both matter:

• Iron that arrives with the grain. Nails, wire, tool fragments, staples from bags, machine parts from the field. It enters at the intake and at any point where material is handled by hand.
• Iron the plant generates. Worn bolt heads, weld spatter, fragments from screw flights and troughs, screen frames and tensioning parts, elevator chain and buckets, and any dehulling or cracking component. It appears downstream of the machine that made it.

Three positions carry the job

• At the intake, before any other machine. This protects everything downstream and it pays for itself, since a piece of wire in a cleaner can damage a screen deck or jam a bucket elevator.
• Downstream of any operation that wears metal. This is the position that gets missed when a line is specified from a catalogue sheet instead of from the process.
• Before the final outlet or the bagging point. This is the last safeguard, and it is the position buyers and their customers ask about as a matter of course.

Two further positions are added when the process calls for them: after a dryer, because heated and moving internal parts can shed material, and after dehulling or cracking, where the working parts wear comparatively fast. These are not optional in any sense that matters. If the process includes such a step, the magnet belongs there.

A line protected at the intake is protected against iron that arrives from outside. A good share of the iron found in a finished bag came from inside the plant. The test is straightforward: walk the flow and ask which machines have metal parts that rub, scrape or strike. Screw conveyor flights and troughs, screen frames and tensioning hardware, elevator chain, sprockets and buckets, dehulling and cracking parts, fan impellers. Downstream of every one of them, iron can appear in the product, and the longer the line runs and the more wear parts it has, the more this matters.

Match the type to the position

A magnet works when iron has a chance to come close to the magnetic face. The type has to suit the place it is fitted, or it will be installed and ineffective.

• Grate and grid magnets, fitted inside spouts, hoppers and discharge chutes so the material passes between the bars. Contact is thorough. With poor-flowing or damp material a grid can bridge, so provide access for cleaning and watch the flow.
• Plate magnets, fitted in a spout wall or under a discharge lip. They do not obstruct the flow, which suits material that bridges. The face must be presented to the stream, and the material has to sweep across it.
• Suspended magnets, hung over a belt. These act on the surface of the bed, so their reach falls away as the bed gets deeper. Use them where the bed is thin and the iron sits near the top of it.
• Magnetic pulleys or drums at the head of a belt. The iron is carried around and discharged continuously, which suits a steady throughput with a steady iron load and avoids frequent manual cleaning.

Cleaning frequency, and checking that the magnet still works

A magnet face covered in iron holds nothing more. At that point the separator is fitted and not working, and nothing in the plant will announce it.

• Set the cleaning interval from what you find, not from a calendar. Where tramp iron is heavy that can mean every shift; where the material arrives clean it can be far less often. Record how much you remove and from which position, and let that record set the interval.
• Clean onto a tray rather than the floor, and look at what comes off. Bolts and nuts point to a machine shaking itself apart, screen wire points to a deck, spatter points to fabrication work upstream. The shape of the iron tells you where it came from.
• Verify the pull on a schedule with a test piece. Permanent magnets weaken over time under heat and vibration, and the change is invisible. A test piece that no longer holds is your warning; there is no other signal before iron reaches the product.
• Clean before a sample is taken rather than after, so no sample is ever drawn through a saturated magnet.

Placement mistakes that leave a magnet installed but useless

• The face is not in the material path. Fitted on the wrong wall, behind a liner, in a pocket where material does not flow, or on the side of a chute while the stream runs along the other side.
• The face is buried under a thick bed. A plate magnet under a full spout, or a suspended magnet over a deep bed on a belt, acts on a fraction of the flow.
• The magnet sits downstream of clean material and upstream of nothing. It guards a product that was already clean while the wear point generating the iron sits further downstream.
• The grid is too small for the flow, so it bridges, blocks, or is bypassed by an operator who has grown tired of clearing it.
• There is no access for cleaning. A magnet that needs tools and a ladder to reach will be cleaned less often than it should be, whatever the instruction says.
• The return or sweep line is left unprotected. Material recirculated back into the process carries iron around with it, and that path is easy to forget.
• Magnetic separation is treated as a metal detector. Magnets hold ferromagnetic material; stainless steel and non-ferrous metal pass through. Where a customer requires all metal foreign matter to be excluded, a metal detector and reject device is a separate requirement, and it belongs in the technical agreement rather than in an assumption.

Actions: map the flow, then decide the positions

• Draw the process as a single line, in order, with every machine on it.
• Mark the points where material enters from outside, and the points where metal parts rub, scrape or strike.
• Place a magnet at the intake, at the outlet of each wear point whose product goes forward, and before the final outlet or bagging point.
• Choose the type to suit each position and the flow there, and confirm that the face will actually meet the stream.
• Set the cleaning interval from the iron you collect, and put the magnet check on the maintenance schedule with a test piece.
• Write the magnet scope into the technical agreement: which metals it covers, and whether a metal detector is required as well.

If iron keeps appearing in the finished product and you cannot find where it comes from, list the process order and the magnet positions you already have. The gap is usually visible in that list.