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 ball charge fraction should I use?

Typical ball charge fractions are 0.30–0.40 (30–40% of mill volume). Lower charges reduce power draw and throughput; charges above 0.45 can cause mechanical issues and are rarely used.

How accurate is this estimate?

This is a volumetric first-pass estimate suitable for order-of-magnitude sizing. Accurate throughput prediction requires feed size distribution, Bond Work Index, target product size, and a full grinding circuit simulation.

What ore density should I enter?

Use the bulk density of the mill feed (ore + voids), typically 1.6–2.0 t/m³ for most crushed ores. Dense ores (iron, lead) range 2.0–2.8 t/m³. If uncertain, 1.8 t/m³ is a reasonable starting point.

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

Name: Mill Throughput Calculator
Availability: InStock
Author: Nham Vu

Enter mill diameter, length, speed, ball charge, and ore density to estimate grinding throughput in 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

Ball Charge Fraction (J_b)

Fraction of mill volume (0.05–0.50); typical 0.30–0.40

Ore Bulk Density (t/m³)

Typical: 1.6–2.8 t/m³ for most ores

Results

Enter parameters and click Calculate

Typical Reference Values

Parameter	Low	Typical	High
Speed (% Nc)	65%	72–76%	80%
Ball Charge (J_b)	0.25	0.32–0.38	0.45
Ore Density (t/m³)	1.6	1.8–2.2	2.8

Summary

Enter mill diameter, length, speed, ball charge, and ore density to estimate grinding throughput in 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%).
Set the ball charge fraction (volume fraction of mill filled with grinding media, typically 0.25–0.45).
Enter the ore bulk density in t/m³ (typically 1.6–2.8 for most ores).
The calculator computes mill volume, power draw, and estimated throughput using a standard volumetric model.
Review the results panel showing throughput (t/h), mill volume, and power draw.

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: 2026-07-24 · Reviewed by Nham Vu