Magnetite Consumption in Dense Medium Circuits: Benchmarks and How to Diagnose Yours
Every dense medium circuit consumes magnetite. The number that actually affects your operating cost isn't whether it consumes magnetite; it's how much it should consume and whether your circuit's real figure is close to that or has quietly drifted away from it.
For a correctly specified low-intensity wet drum separator, that benchmark sits close to 800 grams of magnetite per tonne of ROM coal washed. That figure comes from our own 2024 technical work on magnetite recovery in Indian coal duty, and it holds for circuits where the drum, the launders and the screens are all doing what they're designed to do. It isn't a theoretical minimum reserved for a perfect plant on a good day. Correctly running circuits sit on it.
Most plants don't. Ask five washery managers what their circuit consumes and you'll usually get five different numbers, most of them well above 800 g/t, and almost none of them broken down by cause. That's the real problem. Not that consumption is so high that nobody can say why.
Magnetite is also one of the few genuinely controllable variable costs in a dense medium circuit. Coal quality is what it is. Throughput is set by the plant's design and the market. Magnetite consumption, on the other hand, is almost entirely a function of how well the circuit is running, which means it's one of the fastest places to find real savings without touching capacity or product spec.
It matters more in Indian coal duty than the number alone suggests. Higher ash content and larger near-gravity fractions put more physical load through the medium circuit per tonne washed than many imported coals do, which makes the discipline around magnetite recovery worth more here, not less. A circuit that's leaking magnetite on an easy coal is expensive. The same leak on a difficult Indian coal is worse because the circuit is already working harder to hold separation efficiency.
Where The Number Actually Goes
The arithmetic is straightforward once you have your number. Take the difference between what your circuit actually consumes and the 800 g/t benchmark, multiply it by tonnes of ROM washed, and multiply it by your delivered magnetite price. That's what the gap is costing, every month, independent of anything else that might be off in the circuit.
It's rarely one cause on its own. Most circuits we've looked at closely turn out to have two or three loss points running at the same time, each one modest by itself, together adding up to a consumption figure that looks unremarkable until it's actually benchmarked against what the circuit is capable of.
A magnetic separator recovering to its rated efficiency is the foundation the rest of this diagnosis depends on. Every other loss point on this list is easier to isolate once you know the drum itself isn't the source. If your separator hasn't been checked against rated recovery performance recently, that's where you should begin.
None of this requires shutting the circuit down to find out. A shift-length mass balance, a tailings sample, and an honest look at your density trend will tell you within a day whether you're close to 800 g/t or carrying a gap worth chasing and roughly which of the five loss points above is the likely cause.
Diagnosing Your Own Number
Before making any changes to the circuit, please establish your current position.
Run a magnetite mass balance over a full shift, comparing the mass of magnetite added to the tonnes of ROM washed in that same shift, rather than relying on monthly purchase records averaged against monthly throughput. Monthly figures smooth out exactly the upsets you're trying to find.
Check the magnetic separator tailings directly. A bucket sample and a hand magnet will tell you, qualitatively and quickly, whether magnetite is riding out with the reject in a way that shouldn't be happening. If it is, the drum is your starting point for investigation, not the makeup rate.
Sample what's leaving on the product and discard screens, not only what's leaving in the tailings stream. Carryover on clean coal is invisible unless someone specifically checks for it, and it's one of the more common blind spots on sites that only monitor the obvious loss points.
Track medium density against a setpoint across a full shift rather than relying on a single spot check. A circuit that chronically runs under density and receives fresh magnetite additions has a dilution problem disguised as a magnetite consumption problem.
Separate consumption from loss. Some magnetite leaves the circuit because it's genuinely degraded past the point of recovery for its consumption, and better magnetite quality addresses it. Some leave because a piece of equipment isn't performing to specification; that's a loss, and no amount of premium magnetite fixes it. Sites that treat both the same way keep buying their way around mechanical problems.
What Closing The Gap Is Worth
Before making any changes to the circuit, please establish your current position.
Run a magnetite mass balance over a full shift, comparing the mass of magnetite added to the tonnes of ROM washed in that same shift, rather than relying on monthly purchase records averaged against monthly throughput. Monthly figures smooth out exactly the upsets you're trying to find.
Check the magnetic separator tailings directly. A bucket sample and a hand magnet will tell you, qualitatively and quickly, whether magnetite is riding out with the reject in a way that shouldn't be happening. If it is, the drum is your starting point for investigation, not the makeup rate.
Sample what's leaving on the product and discard screens, not only what's leaving in the tailings stream. Carryover on clean coal is invisible unless someone specifically checks for it, and it's one of the more common blind spots on sites that only monitor the obvious loss points.
Track medium density against a setpoint across a full shift rather than relying on a single spot check. A circuit that chronically runs under density and receives fresh magnetite additions has a dilution problem disguised as a magnetite consumption problem.
Separate consumption from loss. Some magnetite leaves the circuit because it's genuinely degraded past the point of recovery for its consumption, and better magnetite quality addresses it. Some leave because a piece of equipment isn't performing to specification; that's a loss, and no amount of premium magnetite fixes it. Sites that treat both the same way keep buying their way around mechanical problems.
How John Finlay Helps
Closing a magnetite consumption gap starts with knowing whether your separator is actually recovering to spec. John Finlay's LF-HMDS wet drum separators are built for full counter-flow recovery efficiency, and our engineers can walk through a shift-length mass balance with your team to pinpoint which of these five loss points is driving your number. We support this as part of complete dense medium circuit design and ongoing service, not equipment supplied and left.
Not sure where your circuit's gap is?
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