Feed changes, equipment condition, and operating settings all deserve attention. So does the material circulating through the dense medium circuit. Magnetite can look like a straightforward consumable on a purchasing schedule, but what happens to it inside the plant affects the separation you are trying to achieve.

Magnetite improves dense media separation in coal washing by creating a water-based medium with a controlled density. This lets you separate lower-density coal from denser rock and other unwanted material. Its magnetic properties also allow you to recover the magnetite and return it to the process.

The benefit depends on keeping that medium suitable for the job. Buying magnetite is the beginning of that conversation.

What Does Magnetite Actually Do in a Coal Washing Plant?

Raw coal arrives with material the customer does not want. Coal washing, also called coal preparation or coal beneficiation, separates useful coal from some of that unwanted material before the product moves downstream.

In dense media separation, finely ground magnetite is suspended in water to create a medium denser than water alone. Operators control the medium to suit the separation required by the plant.

For a simple picture, consider a dense medium bath. Material less dense than the medium tends to float, while denser material tends to sink. That gives the plant a way to separate coal from heavier material such as shale.

A dense medium cyclone uses the same density principle in a flowing, rotating system. Its behaviour is more complex than a bath, so the medium density measured at one point should not be treated as an exact prediction of the separation achieved. Feed conditions and cyclone operation also matter.

In both cases, magnetite helps create the environment in which separation happens. It does not attract the coal out with a magnet.

Why Use Magnetite Instead of Water Alone?

Water alone cannot provide the adjustable dense medium required by this process. Adding suitable magnetite changes the density of the suspension so the plant can target the density difference between the materials it wants to separate.

Magnetite also has a useful second characteristic: it is magnetic. After separation, the plant can recover it from the dilute medium and reuse it, reducing the amount of fresh material needed to keep the circuit supplied.

That does not make magnetite consumption disappear. Some material is lost from the process and has to be replaced. Good recovery keeps those losses under control; poor recovery can turn a useful circulating material into a surprisingly busy delivery schedule.

Magnetite Density and Medium Density Are Different Measurements

This distinction can cause confusion when product specifications and plant readings are discussed in the same meeting.

Magnetite density describes the solid material. Medium density describes the mixture of magnetite, water, and any other solids present in the circuit. Bulk density, which relates to how a powder occupies a storage volume, is another measurement.

A supplier’s magnetite density specification is therefore not the density at which a coal washing plant should run its medium. The operating target comes from the plant’s separation requirements.

If someone refers to a density figure without identifying what was measured, ask. A perfectly accurate number can still be the wrong number for the decision.

How Does Magnetite Quality Affect Separation?

The medium needs to behave predictably while the plant is running. Magnetite particle size, magnetic content, and density all help determine whether the supplied material is suitable for that circuit.

Particle size affects how the magnetite stays suspended and how the medium flows. Magnetic content matters when the plant recovers the material for reuse. Consistency between deliveries helps operators understand what has changed when performance moves away from the expected result.

An acceptable average specification is useful, but it does not tell the whole story. A plant also needs to know how much variation it can expect and whether each delivery remains within the agreed limits.

Martin & Robson’s magnetite products include several particle size grades for different processing requirements. Selecting the grade should involve the people responsible for the circuit, because the product has to work with the equipment already on site.

Is Finer Magnetite Always Better?

No. Finer material can help keep a medium stable, but the plant also needs a suspension that flows and separates effectively. Changing the particle size can affect both those requirements.

The right balance depends on the separator, the required operating density, and the feed being processed. A grade that performs well in one circuit may create different results in another.

This is why a magnetite grade change needs a clear purpose. If the concern is rising consumption, first establish where the magnetite is being lost. If the concern is separation quality, look at product and reject performance alongside medium conditions. Otherwise, the trial may answer a question the plant was not actually asking.

What Happens to the Magnetite After Separation?

Coal and reject material leave the separation stage carrying some medium with them. Drain-and-rinse arrangements recover medium from those streams, and magnetic separation helps recover magnetite from the diluted material for return to the circuit.

The exact arrangement varies between plants, but the operating objective is familiar: keep usable magnetite circulating and limit losses.

Recovery performance matters because the purchase price is only one part of magnetite cost. A plant that loses more material has to buy more replacement magnetite, even if the delivered price per tonne has stayed the same.

The recovery circuit also needs attention when the incoming product meets specification. A suitable magnetite grade cannot compensate for every screen, spray, separator, or handling problem downstream.

What Should Operators Check When Performance Changes?

Start with what has changed and when it changed. A sudden shift after a delivery is worth investigating, but timing alone does not prove that the material caused it.

Useful magnetite checks include:

  • Whether feed size or feed quality has changed.
  • Whether the medium density measurements are reliable and comparable.
  • Whether the supplied magnetite meets the agreed specification.
  • Whether the medium contains an unusual amount of unwanted fine material.
  • Whether the drain-and-rinse and magnetic recovery stages are working as intended.
  • Whether spillage, handling losses, or inventory records explain higher consumption.

Compare observations over the same operating period. Looking at consumption from one week and product quality from another can hide the relationship you are trying to understand.

Where possible, sample the fresh magnetite and the relevant process streams. That gives the plant and supplier something firmer to work with than “it seems to be using more”.

How Do You Know Whether the Magnetite Is Performing Well?

A stable medium density is encouraging, but the commercial result still has to be checked. The plant needs to recover saleable coal at the required quality while controlling losses and operating costs.

Review coal quality, yield, and magnetite consumption together. A lower consumption figure would be little comfort if the change also sent more recoverable coal to the reject stream.

Keep the basis consistent. Magnetite consumption expressed per tonne of feed is useful only when everyone agrees on the period, the feed tonnage and how additions and stock changes have been recorded.

There is no single consumption target that suits every coal washing plant. The useful comparison is against a sound baseline for the same operation, with enough context to explain why performance has changed.

Getting the Product and the Process to Work Together

Martin & Robson provides magnetite quality control and technical support, including plant audits, training, and research. That creates an opportunity to discuss the circuit as well as the next order.

Bring the current grade, operating density, separator details, and recent performance records into that discussion. If the plant is considering a change, agree on what a successful trial would look like before introducing the new material.

Magnetite earns its place in coal washing because it helps create a controllable, recoverable separation medium. Keeping that benefit depends on suitable material, consistent operation, and effective recovery.

If your plant is reviewing its DMS magnetite, speak to Martin & Robson about the product specification and the process conditions behind it. The most useful starting point is what the circuit needs to do better.