Powder Calculations

How to Calculate Grinding Energy, Moisture, and Size Yield in Powder Processing

The four mass and energy balances every mineral powder plant runs on, worked out with real units and numbers.

The core math in mineral powder processing reduces to four balances: energy per ton through the mill, moisture on a dry versus wet basis, bulk density between mass and volume, and size yield across a screen deck. Get these right and the rest of the plant model follows. Start with grinding energy. Specific energy E equals gross mill power P in kW divided by fresh dry feed rate Q in metric tons per hour. A mill drawing 1,250 kW at 45 t/h runs 27.8 kWh per ton. The Grinding Mill Throughput calculator inverts this to size feed for a target power draw.

To predict throughput from ore hardness, use Bond's equation. Work input W in kWh per ton equals 10 times Wi times the quantity 1 over the square root of P80 minus 1 over the square root of F80, where Wi is the Bond work index and F80 and P80 are feed and product size at 80 percent passing, in microns. For limestone at Wi 11.6, grinding from F80 12,000 microns to P80 150 microns gives W equal to 10 times 11.6 times the quantity 0.0816 minus 0.00913, which is 8.4 kWh per ton. Multiply by feed rate for gross power.

Moisture trips more balances than any other input because two conventions exist. Wet basis moisture equals water mass divided by total wet mass. Dry basis equals water mass divided by bone dry solids. A stream at 8 percent wet basis carries 0.08 times 1,000, or 80 kg of water per wet ton, leaving 920 kg of solids, so dry basis is 80 over 920, which is 8.7 percent. The Moisture Content Adjustment calculator converts between the two and restates tonnage. Never blend a wet basis spec into a dry basis mass balance, because the error compounds through every downstream rate.

Powders sell by mass but store and ship by volume, so bulk density ties the two together. Volume V in cubic meters equals mass M in kg divided by bulk density rho in kg per cubic meter. Ground calcium carbonate at 1,400 kg per cubic meter packs 1,000 kg into 0.714 cubic meters, while a fluffy precipitated grade at 400 kg per cubic meter needs 2.5 cubic meters for the same ton. Use poured density for silo fill and tapped density for settled bags. The Bulk Density Conversion and Silo Inventory Days calculators run these conversions directly from a metered mass.

Product size yield is a two stream split. If a screen feeds 30 t/h and oversize returns 8 t/h, on size yield is the quantity 30 minus 8 over 30, or 73.3 percent. Screening efficiency compares actual undersize passing to the undersize present in the feed: if feed holds 70 percent undersize at 21 t/h and 19 t/h actually passes, efficiency is 90.5 percent. The Particle Size Yield and Screening Loss calculators separate saleable fines from recirculating load, which sets both mill duty and the true first pass yield you can promise a customer.

Drying energy is a water evaporation balance. Heat Q in kJ equals water removed times the sum of sensible heat to boiling plus latent heat, roughly 2,600 kJ per kg of water for a hot air dryer at typical inlet conditions. To pull a 12 percent wet basis feed to 0.5 percent on 10 t/h of dry solids, water removed is about 1,310 kg per hour. At 2,600 kJ per kg and 55 percent thermal efficiency, fuel demand is near 6.2 GJ per hour. The Dryer Energy Cost calculator carries this to a per ton figure once fuel price and efficiency are entered.

Tie it together with a mass balance that must close within 1 percent. Dry solids in equals product out plus screening loss plus dust captured plus moisture evaporated. If 45 t/h of feed yields 40.5 t/h of product, 1.5 t/h returns as oversize and 0.4 t/h reports to the Dust Collection Airflow Load system, the remainder is moisture and unaccounted loss. Convert product tonnage to bags with the Bagging Line Capacity calculator: 40.5 t/h into 25 kg bags is 1,620 bags per hour, or 27 bags per minute, the pace the packer must hold to keep up with the mill.

Published 2026-07-02.