Screen Blinding and Deck Media Selection: Fixing Capacity You Already Paid For
A dense medium circuit is only as consistent as the medium itself. You can specify the best cyclone geometry available and still get a wandering cut point if the magnetite feeding that cyclone doesn’t meet spec because the medium’s density and its stability under shear are properties of the magnetite, not the cyclone.
Our own 2024 technical work on optimizing clean coal yield for coking coal publishes the specifications we hold ground magnetite to for heavy media cyclone circuits: 98.5% Fe₃O₄ content, specific gravity of 4.90 to 5.15, 90% passing 45 microns, and 68 to 70% Fe content. Each of those four numbers is doing a specific job, and it’s worth understanding what happens when any one of them drifts.
None of the four is redundant with the others, which is why a spec sheet that only quotes one or two of them isn’t enough to qualify a supplier. A magnetite can pass on Fe content and still be wrong on granulometry, or it can pass on particle size and still be wrong on SG. Qualifying a source properly means checking all four together, not treating any single number as a stand-in for the rest.
98.5%
Fe₃O₄ content
4.90–5.15
Specific gravity
90%
Passing 45 microns
68–70%
Fe content
Fe₃O₄ Content: What You're Actually Buying
Fe₃O₄, an impurity: magnetite proper is the fraction of the material that’s both magnetic and dense enough to do the job. Everything that isn’t Fe₃O₄ is an impurity: typically silica, other iron oxides, or gangue minerals that add bulk without adding either the density or the magnetic recoverability you’re paying for. A magnetite that tests below the 98.5% benchmark isn’t just slightly weaker; the impurity fraction dilutes the achievable medium density at any given solids loading, and because that same impurity fraction is largely non-magnetic, it’s also harder for your magnetic separator to recover cleanly, showing up later as higher makeup consumption from an entirely different cause than the mechanical loss points that drive consumption in an otherwise correctly specified circuit.
Specific Gravity: The Ceiling on Your Cut Point
The SG of the magnetite particles themselves, 4.90 to 5.15 in spec, sets the practical ceiling on how dense a medium you can run at a workable solids concentration. Pushing the cut point density higher than a given magnetite’s SG comfortably supports, the circuit has to compensate with higher solids loading to hit the same medium SG, which raises viscosity and starts working against separation efficiency rather than for it. Magnetite running below spec SG effectively narrows the density window you can operate in before medium rheology becomes the limiting factor, which is most important for coking coal circuits running higher cut points to hold ash targets.
Particle Size: The Balance Between Stability and Recovery
Ninety percent passing 45 microns is a specific, deliberate balance point, not an arbitrary fineness target. Finer magnetite stays in suspension longer and settles more slowly, which gives a more stable, more homogeneous medium with less density segregation across the cyclone directly relevant to holding a tight Ep value. But finer particles are also harder for a magnetic separator to recover efficiently; recovery efficiency generally falls as particle size drops, because finer magnetite has less mass per particle for the same magnetic force pulling it toward the drum. Grind magnetite finer than spec, and you buy medium stability at the cost of separator recovery. Leave it coarser than spec and recovery gets easier while medium stability and cut point sharpness suffer. The 90% passing 45-micron point is where those two costs roughly balance for typical coal washery duty.
Fe Content: The Cross-Check
The 68 to 70% Fe content figure tracks closely with Fe₃O₄ purity and functions mostly as a supplier quality cross-check. A magnetite testing below 68% Fe is very unlikely to meet the Fe₃O₄ and SG numbers either, since Fe content is what those other properties are built from. It’s the number worth putting on a certificate of analysis requirement precisely because it’s hard to fake without also fixing the underlying purity. A supplier can’t blend their way to a high Fe reading without the Fe₃O₄ content coming along with it.
Magnetite Consumption Benchmarks
Off-spec magnetite also drives up makeup consumption; see our benchmark and diagnostic guide for the mechanical side of that number.
Why This Matters More on High Near-Gravity Indian Coal
Every one of these four parameters matters more as near-gravity material content in the feed increases, which is precisely the situation on much of Indian coking coal. High NGM feed means a larger fraction of particles sit close to the actual separating density, so the cut has to be sharp, with a low Ep value, to avoid misplacing material on either side of the cut. Ep is a function of medium stability, and medium stability is a direct function of the granulometry and SG spec covered above. A magnetite that would be entirely adequate for an easy-washing thermal coal with low NGM can produce a visibly worse cut on a difficult coking coal simply because the circuit has less room for error before misplaced material shows up in yield and ash figures. Magnetite spec is only one input to Ep, though the cyclone’s own mechanical condition and operating pressure matter just as much, which we cover in our post on cut point control in heavy media cyclone circuits.
This is also why magnetite specification and magnetite consumption are related but separate conversations. A circuit can be losing magnetite through the mechanical points covered elsewhere: the drum, the screens, and housekeeping while running magnetite that’s fully in spec. Equally, a circuit can hold its consumption number close to the benchmark while running off-spec magnetite and simply pay the cost in a wider Ep and lower yield instead. Checking consumption and checking specification are both worth doing, and neither one substitutes for the other.
Checking Your Own Supply Against Spec
Request a current certificate of analysis with every delivery, not just at the start of a supply contract, since magnetite quality from a given source can drift over time even when the paperwork doesn’t change. Cross-check Fe₃O₄ and Fe content against each other rather than trusting either number alone. And if the cut point has been drifting on a circuit where cyclone geometry and density control haven’t changed, checking the magnetite specification is worth doing before assuming the problem is mechanical.
The spec exists because each number was chosen to solve a specific, real trade-off in the circuit, not as a generic purchasing requirement. Holding to it is usually the cheapest lever for cut point consistency, because it costs nothing beyond specifying it correctly and checking that it stays that way.
How John Finlay Helps
Holding magnetite to this specification is the cheapest lever available for cut point consistency, and it’s one John Finlay’s engineers can help you check directly. We can review your current supplier’s certificate of analysis against these four benchmarks, and if the cut point has been drifting on a circuit where nothing else has changed, we can help confirm whether magnetite quality is the actual cause. This sits alongside our Magnetic Separators, Dense Media Cyclones, and Slurry Density Meters as part of one dense medium circuit offering.
Not sure your magnetite supply is holding spec?
Our engineers can review your certificate of analysis against these benchmarks.
Talk to an ExpertRelated Equipment
Dense Media Cyclone
Two Product & Three Product HMC series engineered for optimal EP values.
Intelligent Slurry Density Meter
Real-time underflow/overflow density measurement to hold cut point.