F80 P80 Reduction Ratio Calculation

What F80 and P80 actually describe

The steadiest way to express how much work a crusher performs is not the size of the largest lump. Sieve analysis is run separately on feed and product, and the aperture matching the 80 percent point of the cumulative passing curve is read off each curve. The feed value is F80, the product value is P80. An F80 P80 reduction ratio calculation is simply one number divided by the other, carried out stage by stage rather than once for the whole plant.

The reason for avoiding top size is statistical. A single oversize block, or a piece of tramp material caught on the grizzly, drags top size upwards and misrepresents the pile. The 80 percent point tracks the mass distribution instead, so two samples taken on different shifts from the same rock land close together. That repeatability is what makes the ratio usable for stage selection and for power estimates.

Workable ratio band by machine type

Every crusher family works efficiently inside a limited ratio band. Compression machines are bounded by chamber geometry and nip angle. Impact machines deliver a far larger single pass ratio but are sensitive to how abrasive the rock is. The table below summarises the band and the usual position in the circuit.

These bands are geometric. They say nothing about hardness or abrasiveness, so the same 6:1 can be achieved in limestone and in granite while wear rate and drawn power differ sharply between the two. The ratio is therefore the first filter in machine selection, never the only criterion.

F80 P80 reduction ratio calculation: splitting the total across stages

The circuit total is the product of the stage ratios. A quarry with a 600 mm feed F80 that must deliver a 20 mm product P80 needs 30:1 overall. No single machine carries that. Split across two stages as 6:1 and 5:1, the product is 30 and both machines stay inside their band. In a three stage circuit the same total breaks into gentler pieces such as 5:1, 3:1 and 2:1, which lowers both wear and energy per tonne.

The order of work is fixed: fix P80 from the product specification, measure F80 on the blasted muckpile, obtain the total, then distribute it so each stage lands inside its band. When choosing the machine for a given stage, the behavioural differences set out in the jaw impact cone crusher comparison decide the outcome.

Sampling rules that protect the number

The measurement is only as good as the sample. A feed sample is taken by stopping the belt and lifting everything within a marked length, not by skimming the surface with a shovel, because skimming under represents fines. The product sample is drawn at the crusher discharge ahead of the screen; a sample taken after screening measures the screen, not the crusher. Required sample mass rises with top size, and coarse feed can demand several hundred kilograms.

When the sieve series is chosen, the apertures corresponding to the saleable fractions must be included. Otherwise the 80 percent point falls between two sieves and has to be interpolated, which shifts the ratio by a few points.

How closed circuit changes the arithmetic

Where oversize is screened off and returned to the crusher, two different ratios exist. The single pass ratio seen by the machine is smaller than the ratio between fresh feed and final product, because returned rock enters the chamber a second and third time. The circuit ratio is computed from fresh feed F80 and screen undersize P80, not from the mixed feed at the crusher mouth. Confusing the two makes a machine look as if it is running above its band when the load actually comes from circulating material.

As circulating load grows, conveyors, screen and motor carry the same tonnes more than once. Keeping the stage ratio slightly conservative in closed circuit can therefore reduce total energy per saleable tonne. That call cannot be made without measuring the return percentage, and a belt scale reading is enough for the purpose.

The cost of leaving the band

Running a machine above its band saves a stage on paper and costs three things on site. Throughput falls, because breakage inside the chamber turns increasingly into grain against grain. Wear part life shortens, because more work is done on the surface of the part for the same tonnage. Product shape deteriorates, the proportion of flaky and elongated particles rises, and a rejection risk appears against concrete and asphalt specifications.

The opposite direction wastes just as much. A machine well below its band cannot fill its chamber, spends power at idle and passes needlessly coarse material to the next stage. A distribution is correct when every stage works near the middle of its band.

A three minute check at shift start

Three figures suffice: the crusher setting, the F80 of a sample from the feed pile, and the P80 of screen undersize. If the measured ratio drifts from target, check the setting first, then whether the chamber is choke fed, and only then the wear part profile. That sequence prevents parts being changed for a problem that was never mechanical.

Operations that want circuit design on a numerical footing will find the rest of the stage planning arithmetic under mining engineering. With your muckpile gradation and product specification at hand, the stage split can be worked through together.