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Separator Tromp Curve

Take three samples at the same moment — what goes into the separator, what it sends back to the mill, and what leaves as product — sieve all three the same way, and this tells you your circulating load, how well the separator is really working, and where it is actually cutting, which is rarely where the setpoint says. Press SAMPLES on the board to type them in.

Why this matters now

Clinker reduction, high electricity prices and tighter cement specifications put the grinding circuit under pressure. The fastest gains usually come from finding the real constraint before changing equipment or operation.

What this tool helps decide

Use three routine samples to identify separator cut size, bypass and circulating load without stopping the mill or commissioning a separate study.

What subscription adds

Build a connected grinding audit: save every mill separately, reload previous measurements, compare changes over time and issue an editable report for the plant team.

Connected to the industry conversation around clinker-factor reduction, grinding energy, blended cement and quality control reported in World Cement, CemNet and Global Cement.See process-tool plans

Your separator, as sampled

The circuit

Fresh feed to the mill Tonnes per hour of new material, not counting the return.

Quick check at one sieve

The circulating load and the efficiency come from these three numbers alone. Use the sieve your lab reports every shift.

Sieve sizeMicrometres. Usually 45 for cement.

Mill discharge — % retained What the separator is being fed.

Coarse return — % retained What it sends back to the mill. Always the coarsest of the three.

Fine product — % retained What leaves as cement. Always the finest of the three.

Full sieve analysis

All three streams, % retained on each sieve. This is what draws the curve and finds the real cut size. Leave a row at 0 to skip it.

Sieve µm	Discharge	Return	Product

THE CIRCUIT

THE CUT

Circulation factor

—

Circulating load

—

Coarse return

—

Mill throughput

—

Separator efficiency

—

Real cut size d50

—

Bypass to return

—

Sharpness of cut

—

What this means for your separator

The other half of the picture

Circulating load tells you how much material the mill is really handling. The power tool tells you whether the drive can carry it.

Check the drive

How the numbers are worked out

Circulating load. C = (Rg − Rf) / (Rg − Rm), workbook formula #9b, then CL = (C − 1) × 100. It is a mass balance on one sieve — nothing more, nothing fitted. It only holds while the three residues are correctly ordered: product finest, return coarsest.

Efficiency. Rf (C − 1) × 100 / (C × Rg), formula #11.

The curve. For each size class, the share of the feed that ends up in the return: T = (C−1)/C × g/f, where g and f are the mass fractions in that class, taken as the difference between residues on neighbouring sieves. Plotted at the geometric mean of the two sieve sizes.

d50 is where the curve crosses 50 % — the size the separator is really splitting at, which is often nothing like the speed setpoint would suggest. Bypass is the flat tail at the fine end: fine material dragged into the return without being classified at all, usually poor air distribution or a leaking rotor seal. Sharpness is (d75 − d25) / (2 d50) — lower is sharper; a good high-efficiency separator sits near 0.3 to 0.5.