What is critical speed?

Critical speed is the rotational speed at which centrifugal force equals gravity, causing the grinding charge to cling to the shell rather than tumble. It is calculated as 42.3 / √D (rpm) where D is the mill diameter in meters. Most mills operate at 65–80% of critical speed.

What is Bond Work Index?

Bond Work Index Wi is the laboratory-measured ore resistance to grinding, expressed in kWh per metric ton. Use a test value representative of the ore and mill duty rather than a generic density-based estimate.

How accurate is this estimate?

Bond’s equation is a first-pass comminution estimate. Use net power delivered to the charge and representative F80, P80, and Wi values. Circuit classification, recycle load, mill type, operating condition, and scale-up corrections can materially change actual throughput.

What are F80 and P80?

F80 is the feed size and P80 is the product size at which 80% of material passes, both entered in micrometres. F80 must be larger than P80 for a grinding duty.

Does this tool account for slurry or wet grinding?

The model uses a dry-equivalent volumetric approach. For wet grinding circuits, the effective throughput depends on pulp density and water addition — apply a 0.85–0.95 correction factor as a rough adjustment.

Mill Throughput Calculator

Enter net mill power, Bond Work Index, feed F80, and product P80 to estimate throughput in metric tons per hour.

Mill Parameters

Mill Internal Diameter (m)

Typical range: 1–12 m

Effective Grinding Length (m)

Typical range: 1–16 m

Rotational Speed (% of Critical Speed)

Typical: 65–80 % Nc

Net Grinding Power (kW)

Net power delivered to grinding, excluding drive losses

Bond Work Index, W_i (kWh/t)

Use a representative laboratory Bond test value

Feed F80 (µm)

Product P80 (µm)

Results

Enter parameters and click Calculate

Typical Reference Values

Parameter	Low	Typical	High
Speed (% Nc)	65%	72–76%	80%
Bond Wi (kWh/t)	7	12–18	25+
Product P80 (µm)	75	150–250	500

Summary

Enter net mill power, Bond Work Index, feed F80, and product P80 to estimate throughput in metric tons per hour.

How it works

Enter the mill's internal diameter and effective grinding length in meters.
Input the rotational speed as a percentage of critical speed (typically 65–80%).
Enter the measured or design net grinding power in kilowatts.
Enter Bond Work Index in kWh/t and the 80%-passing feed and product sizes F80 and P80 in micrometres.
The calculator evaluates Bond specific energy and divides net power by it to estimate throughput.
Review throughput, specific grinding energy, mill volume, and critical-speed reference.

Use cases

Preliminary feasibility studies for new grinding circuits.
Comparing throughput of different mill sizes during plant design.
Estimating production rates when scaling up from pilot to full-scale mills.
Quick sanity-check of vendor-quoted mill capacities.
Educational tool for metallurgical and mining engineering students.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu