Angle of repose: coal vs iron ore stockpiles

Anyone who has stood at the edge of a stockyard knows a coal pile and an iron ore pile don't look alike, even at the same tonnage. Part of that is bulk density. Part of it is the angle the material settles at once it comes off the stacker. Get that angle wrong in your volume math and the tonnage you back out can be off by a meaningful margin, especially on a tall pile where small angle errors compound fast.

What angle of repose actually means for a stockpile

Angle of repose is the steepest slope a loose bulk material holds without sliding further, measured from horizontal. It's a function of particle size, shape, moisture, and how much the material has been compacted by reclaimer traffic or its own weight. Drop a handful of dry sand and it spreads to a shallow cone. Drop a handful of damp clay and it holds a steeper, almost vertical face for a while before it gives.

Coal and iron ore sit at different points on that spectrum, and the gap matters because so much stockyard tonnage math still works backward from pile shape: measure height and footprint, assume a cone or wedge, apply an angle, get a volume.

Why coal and iron ore don't stack the same way

Thermal and metallurgical coal, run of mine and washed, typically settles in the 35° to 45° range. Moisture content swings it hard. A pile that's been rained on at an open stockyard flattens out noticeably compared to the same coal dry, because water bridges between particles and changes how they shear against each other. Fines content does the same thing in the other direction. A coal blend with more fine material tends to hold a steeper angle than coarse run of mine, up to a point, then collapses faster once it does slide.

Iron ore behaves differently depending on what form it's in. Lump ore, with its coarse, uniform particle size, often sits closer to 30° to 35°. Sinter feed and fines, especially washed fines coming straight off a beneficiation line still carrying surface moisture, can flatten out well below that or, once they dry and cake, hold a steeper crust than the bulk underneath suggests. Concentrate piles at a smelter yard add another wrinkle: they're smaller, more frequently turned over, and the angle at the working face rarely matches the angle at the undisturbed back of the pile.

None of this is academic if you're trying to translate a pile's dimensions into a tonnage figure for a trading desk. A 5° error in assumed angle on a 15-meter pile translates into a volume error that's easy to underestimate until you've actually done the arithmetic. And real piles aren't clean cones or wedges anyway. Reclaimers leave benched faces, stackers build elongated windrows at port, and segregation during stacking puts coarser material at the base and finer material riding higher on the slope. The textbook angle is a starting assumption, not a measurement.

Measuring the pile instead of modeling it

The more reliable path, if you're tracking a named pile over time rather than estimating it once, is to stop re-deriving the angle every reporting period and instead measure the actual pile surface on a repeat basis. Track the change in volume week over week against the same geometry, rather than recalculating tonnage from height and an assumed slope each time. That sidesteps the coal-versus-ore angle argument entirely, because you're reading the pile's actual shape rather than modeling it.

That's the gap Bulk Stockpile Index is built around: a weekly volumetric series per named site, pulled from stereo satellite passes, so a desk analyst gets pile-level trend without leaning on an assumed angle of repose or waiting on the next quarterly disclosure to find out the yard drew down.

If you're tired of reconciling angle assumptions against shipment data that lags the actual drawdown, it's worth seeing what a weekly pile series looks like for your sites.