Before doing multiple laboratory flotation bench tests in which you need P80 grind controlled or predicted, you need to build a Grind Calibration curve following this short procedure.
You will consistently add sufficient water to the mill to achieve a pulp density of 67% solids (Example: 1 kg solids + 0.5 kg water or 2 kg solids + 1 kg water.) You may need to add more water yet depending on the viscosity of the pulp, by reducing the pulp density.
Add the same weight of grinding rods (or balls) to the mill each time.
You would perform this grind tests over to time range to obtain a P80 size range. Say, for this example, 10 minutes, 20 minutes, 30 minutes.
After you filter the mill discharge product, perform a full screen analysis (wet screen 400 mesh) followed by dry sieve.
Plot the P80 in microns (um) VS Grind Time to obtain your Grind Calibration Curve.
This Grind Calibration graph lets you now estimate that if you want a P80 = 220um you need to grind under the same conditions for 15 minutes.
This other set of Grind Calibrations shows a P80 of 190um takes 12 or so minutes.