A ball mill is a machine that grinds material to a fine size through the impact and attrition of steel balls inside a rotating cylindrical drum. It is used in many areas, such as liberating valuable minerals in ore processing, grinding raw meal and cement in the cement industry, and producing micronised products from industrial minerals (calcite, feldspar, quartz). Because the largest share of energy consumption in mineral processing plants is spent at the grinding stage, correct sizing and operation of the ball mill is one of the determining factors in plant economics.
In this guide we cover comprehensively the working principle of the ball mill, the concept of critical speed, the selection of ball charge and ball size, the logic of the power calculation, closed-circuit grinding, liner selection and maintenance practices.
How Does a Ball Mill Work?
The mill drum rotates around a horizontal axis. Liners and lifters attached to the inner surface of the drum carry the balls and material up to a certain height. From there the balls move in two different ways:
- Cascading: The balls roll down the surface of the charge; attrition and abrasion dominate, giving fine grinding.
- Cataracting: The balls are thrown into the air and fall onto the toe of the charge; impact dominates, which is effective for breaking coarse particles.
Mill speed and liner profile determine the balance between these two motions. If the speed is too high, the balls stick to the drum wall under centrifugal force and grinding stops; this speed is called the critical speed.
Critical Speed and Operating Speed
Critical speed depends on the inner diameter of the mill and decreases as the diameter increases. Ball mills are usually run at about 65–78% of critical speed. A lower speed favours cascading and fine grinding; a higher speed favours impact and breakage of coarse particles. Variable speed drives allow the speed to be adjusted to suit ore properties.
Types of Ball Mill
| Type | Characteristics | Use |
|---|---|---|
| Overflow | Product leaves by overflowing at the trunnion level on the discharge side | Fine grinding, common in ore processing |
| Grate discharge | A grate at the discharge; lower pulp level | Coarser product, less overgrinding |
| Wet grinding | Material is ground as a pulp with water | Ore processing, before flotation |
| Dry grinding | Air classification, separator | Cement, industrial minerals |
| Multi-compartment | Compartments with different ball sizes | Cement and clinker grinding |
Ball Charge and Ball Size
The ball charge (filling) ratio is the percentage of the mill’s internal volume occupied by balls (including the voids between them). In ball mills, this is usually about 30–40% of the volume. As the charge increases, the power drawn by the mill increases up to a point; however, overcharging lowers efficiency and accelerates liner and ball wear.
- Ball size: Chosen according to feed size, ore hardness and target product fineness. Coarse feed and hard ore need larger balls; fine grinding needs smaller balls.
- Ball make-up: Balls shrink as they wear. To maintain the charge, balls of the largest size are added regularly, creating a balanced size distribution in the mill.
- Ball quality: Forged, cast and high chrome balls show different wear and breakage behaviour. Broken balls can damage the liners and increase consumption.
- Power monitoring: The power drawn by the mill is a practical indicator of the charge level. A fall in power may indicate that the charge has decreased or that the pulp density has changed.
The Logic of the Power Calculation: Bond Work Index
The energy required for grinding depends on the ore’s grindability. This property is expressed by the Bond work index (kWh/t), determined in the laboratory. In the Bond approach, the specific energy required is calculated from the work index, the feed size and the product size (usually the sizes through which 80% passes). As the product becomes finer, the energy required increases rapidly.
Reducing the size as far as possible in the crushing stage before grinding can therefore significantly reduce total energy consumption. The “crush small, finish by grinding” approach aims to make good use of the ranges in which crushers work more efficiently than grinding. For energy savings in the crushing and screening stage, see our energy efficiency guide, and for ore crushability and grindability tests, our rock crushability guide.
Closed-Circuit Grinding and Classification
Ball mills usually work in closed circuit with a classifier. In wet grinding, this classifier is usually a hydrocyclone: the mill discharge is pumped to the cyclone, fine particles pass from the overflow to the next stage and coarse particles return from the underflow to the mill. The ratio of material returning to the mill to new feed is called the circulating load.
- A suitable circulating load reduces overgrinding (unnecessary production of fines) and increases mill capacity.
- The cyclone’s cut size is adjusted through feed pressure, solids content and the vortex finder and apex diameters. For details, see our hydrocyclone guide.
- In dry grinding, air separators do the same job.
In large mines, the ball mill is often used as the second grinding stage after a SAG mill. For this circuit configuration, see our SAG and AG mills guide.
Choosing Mill Liners
Mill liners protect the drum against wear and guide the motion of the balls. The liner profile directly affects the mill’s grinding efficiency.
| Liner material | Advantage | Point to watch |
|---|---|---|
| Rubber | Light, quiet, quick to install, long life in fine grinding | Limited with large balls and high impact |
| Alloy steel (Cr-Mo) | High impact resistance | Weight, installation time |
| High chrome white iron | High wear resistance | Brittleness under impact |
| Composite (rubber + metal) | Balance of impact and wear resistance, lighter | Design according to the application |
| Magnetic liner | A protective layer of balls and magnetic particles | Secondary grinding and vertical mills |
For the general properties of wear part materials, see our wear materials guide.
Maintenance and Monitoring
Measuring Liner Wear
Liner thicknesses and lifter heights are measured periodically. As lifter height decreases, ball motion changes and grinding efficiency falls. Measurements are recorded by tonnage to predict replacement time; laser scanning methods can be used on large mills.
Bearings and Lubrication
The mill drum usually turns on trunnion bearings at both ends. These bearings work with hydrostatic or hydrodynamic lubrication; oil pressure, temperature and cleanliness should be monitored continuously. For general principles of lubrication unit maintenance, see our lubrication unit guide.
Girth Gear and Pinion
The girth gear and pinion are the most expensive drive components of the mill. Tooth contact, backlash, the open gear lubrication system and the gear guard should be checked regularly. Vibration and thermography measurements reveal gear problems early.
Bolts and Sealing
Loose or broken liner bolts lead to pulp leakage and drum damage. Bolts and seals should be renewed at liner changes.
Common Mistakes
- Irregular ball make-up: The charge falls, and power and capacity decrease.
- Not controlling pulp density: Very dilute pulp makes balls strike the liner directly; very dense pulp increases viscosity and weakens grinding.
- Leaving the crushed product coarse: Feed coarser than necessary to the mill increases energy consumption.
- Neglecting cyclone adjustment: Overgrinding or coarse bypass occurs.
- Not choosing the liner profile to suit mill speed: Balls striking the wall directly cause liner and ball breakage.
A Field Example
Suppose mill capacity at an ore processing plant is below target and the product is coarse. The investigation shows that the mill feed is coarser than in previous periods, because the cone crusher setting has opened up through wear. When the crusher setting is corrected and screening efficiency increased, the mill feed becomes finer and more tonnage is ground with the same energy. This example is a reminder that grinding performance is often determined at the preceding crushing stage. For crusher setting and liner maintenance, see our cone crusher guide.
Frequently Asked Questions
What is the difference between a ball mill and a rod mill?
In a rod mill the grinding media are long steel rods; it gives a coarser product with a narrower size distribution. A ball mill is used for finer grinding.
How is the ball charge checked?
With the mill stopped, the distance from the charge surface to the top of the drum is measured and the filling ratio calculated; during operation, power draw is a practical indicator.
Rubber liners or steel liners?
Rubber liners are advantageous in fine grinding with small balls; steel or composite liners are preferred in primary grinding with large balls and high impact.
Why is mill speed important?
Speed determines how the balls move. Running too close to critical speed stops grinding; too low a speed reduces impact.
What is the most effective way to reduce grinding costs?
Reducing the feed size in the crushing stage, increasing classification efficiency and choosing balls and liners to suit the ore are the most effective methods.
Ball Mill Spare Parts and Consumables List
| Part / consumable | Type | Check or replacement |
|---|---|---|
| Shell liners and lifters | Wear part | Measurement of thickness and lifter height |
| Head (end) liners | Wear part | Thickness measurement |
| Discharge grates | Wear part | Aperture growth, blockage |
| Liner bolts, nuts and seals | Fasteners | At every liner change |
| Grinding balls | Consumable | Make-up according to power and charge measurement |
| Trunnion bushings / bearings | Mechanical part | Oil temperature, pressure, analysis |
| Girth gear and pinion | Mechanical part | Tooth contact, vibration, thermography |
| Pinion bearings | Mechanical part | Temperature, vibration |
| Clutch and coupling elements | Drive | Alignment, wear |
| Feed chute and trunnion liners | Wear part | Wear and leakage |
You can find parts for the crushers, screens and conveyors that feed the grinding circuit on our spare parts page. See our belt conveyor spare parts page for mill feed conveyors and our cone crusher spare parts page for the cone crushers used for pre-reduction.
Operating Indicators
| Indicator | What it tells you |
|---|---|
| Mill power (kW) | Charge level and load condition |
| Specific energy (kWh/t) | Grinding efficiency |
| Product fineness (P80) | Compliance with the target |
| Circulating load (%) | Classification and mill balance |
| Ball consumption (g/t) | Grinding media cost |
| Liner life (tonnes or hours) | Wear cost and planning |
| Pulp density | Grinding conditions |
Glossary
- Critical speed: The theoretical speed at which the balls stick to the drum wall under centrifugal force.
- Filling ratio: The percentage of the mill’s internal volume filled by the ball charge.
- P80: The sieve size through which 80% of the product passes.
- Bond work index: An energy value expressing the grindability of a material (kWh/t).
- Circulating load: The ratio of material returning from the classifier to the mill to new feed.
- Trunnion: The journal bearing area at the ends of the mill drum.
More Questions
When should mill liners be replaced?
When liner thickness approaches a level at which it can no longer protect the drum, or when lifter height reduces grinding efficiency markedly. Measurement records allow this decision to be planned by tonnage.
What affects ball consumption?
Ore abrasiveness, ball quality, pulp chemistry (corrosion), mill speed and charge level affect ball consumption.
Why does mill sound matter?
Changes in sound give an idea of the charge level and whether balls are striking the wall; some plants use acoustic monitoring.
Support from CSP Mühendislik
CSP Mühendislik supports the design and optimisation of the crushing and screening stages before grinding, the supply of wear parts and maintenance planning. You can find spare parts for the crushers and screens that feed your grinding circuit on our spare parts page and learn about our service, maintenance and repair. If you are planning a plant investment, our crushing and screening plant set-up guide is a good starting point.
