Middlings Regrinding in Magnetic Separation Circuits

MINERAL PROCESSING KNOWLEDGE · IRON ORE BENEFICIATION

Middlings are the fraction of an iron ore that is neither clean concentrate nor clean tailings, and regrinding them is how a magnetic circuit recovers that value without diluting its product.

Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.

A magnetic separator sorts by magnetic susceptibility, not by liberation, and that distinction is the whole reason middlings exist. A particle made of magnetite still attached to quartz responds to the magnet as strongly as the separation requires, so it enters the concentrate carrying gangue with it. Send it forward and the concentrate grade falls. Discard it and recovery falls. This article explains how middlings form in an iron ore magnetic circuit, why regrinding is the standard answer, and how the screening and regrinding steps are arranged.

Technical note: The following is a principle-level explanation of middlings regrinding in magnetic separation circuits. It is not an engineering design, a mass balance or a guarantee of concentrate grade or recovery. Circuit configuration must be confirmed through mineralogy, testwork and detailed design for each orebody.

1. How middlings form in a magnetic circuit

In wet magnetic separation of iron ore, the separator responds to magnetite content. A composite particle consisting of magnetite still attached to quartz or silicate gangue is pulled into the magnetic product, because the magnetite in it is enough to deflect the particle. These iron-silicon intergrowths are the middlings.

They are not a fault in the separator. They are a consequence of the grind size, and the quantity produced is a direct signal that the liberation size of the ore has not yet been reached at that stage.

2. Why neither destination works

Middlings create a decision with no good answer inside the separator itself. Both obvious routes cost the circuit something it was built to deliver.

This is why middlings handling is treated as a separate duty rather than as a setting on the separator. The material is not waste and it is not product. It is ore that has not yet been liberated, and it needs grinding rather than a different magnetic field.

3. Screen first, then regrind

Implementation starts with classification rather than with the mill. A high-frequency vibrating fine screen classifies the magnetic intermediate product by size. The oversize, which carries the coarse intergrowths, is the fraction that needs more work. The undersize is already fine enough and does not benefit from further grinding; sending it to the regrind mill would only add cost and generate more slimes.

Screening also protects the mill. A mill fed the whole separator product spends much of its energy on material that is already liberated, and it grinds fines it should never have received.

4. Choosing the regrind mill

The oversize returns to a middlings regrind mill, usually a vertical mill or a fine-grinding ball mill. The duty differs from primary grinding. The target is not a new top size for the whole ore but complete liberation of magnetite inside particles that are already fine, which calls for a mill that grinds by attrition rather than relying on the impact of a falling charge.

A vertical mill suits the duty because it works material in a compact, high-intensity zone and grinds efficiently at fine sizes, while a fine-grinding ball mill is the conventional alternative where the circuit arrangement or the existing equipment favours it. In either case the loop grinds to liberation, not to a nominal screen size.

5. Returning the reground product to cleaning

Screen undersize and reground product are combined and sent to a cleaner magnetic separator for a second pass. After regrinding, magnetite in the composite particles is free of gangue, so the cleaner can do what the rougher could not: hold the iron and reject the silicate.

The loop also has to be seen in the context of the wider magnetite flowsheet. A typical route applies multistage grinding with low-intensity wet magnetic separation at field strengths of 0.1-0.15 T, grinds the magnetic semi-concentrate to 85%-95% passing 200 mesh in the second stage, and aims for a final concentrate of 66%-68% Fe. That second-stage target is only reachable if the particles the first stage could not liberate are handled separately, which is exactly what a middlings regrind loop does.

6. Checks before committing to a regrind loop

• Establish the liberation size from mineralogy, not from the primary grind set point.
• Test the middlings as a separate stream to measure what regrinding actually releases.
• Size the screen cut so that the regrind mill receives coarse particles only.
• Define the return path for new middlings from the cleaner, so the circulating load stays controlled.

Conclusion

Middlings are a grind-size problem wearing the appearance of a magnetic separation problem. Trying to solve them at the separator forces a choice between diluted concentrate and lost iron. Screening the intermediate product, regrinding only the coarse intergrowths and returning the product to a cleaner separator resolves both sides of that trade-off, and it spends grinding energy where liberation is actually limited. The wider circuit this loop sits within is described under iron ore beneficiation.

How LiCheng Tech can help

LiCheng Tech matches equipment to your material, feed size and target capacity from our product catalogue, and connects you with suitable Chinese suppliers where a specification falls outside our standard range. For a middlings regrind loop that means the vibrating screen that classifies the intermediate product, the ball mill used for fine regrinding, the classifier that closes it and the magnetic separator that cleans the combined product. Where coarse waste can be rejected ahead of the mill, a dry magnetic separator is the alternative to consider.

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