SAG Balls

Need SAG balls that stand up to heavy impact? William Boulton supplies forged steel SAG balls for semi-autogenous grinding (SAG) mills across the mining sector. A SAG mill is one of the toughest places grinding media can work, so the balls you charge have to survive constant heavy impact while still wearing slowly and evenly.

We help mineral processing operations choose the right ball for their mill, ore and grind target, and supply grinding media tailored to each application. Every product is tested and quality checked before dispatch, and our global network means we can deliver to and support customers around the world.

Not sure which size or grade you need? Call our team on 01782 816158 or send us your mill details.

Expert support

Our experienced engineers provide ongoing technical support to help you get the best performance from your grinding circuit.

What are SAG balls?

SAG balls are large steel grinding balls used in semi-autogenous grinding mills. A SAG mill is a large-diameter rotating drum that is usually the first grinding stage after the primary crusher. The ore does much of the grinding itself – hence “semi-autogenous” – with a charge of steel balls added to break the larger, harder rocks and keep throughput up.

As the mill turns, lifters on the liners carry the charge up the shell until it tumbles back down. SAG balls are lifted with the ore and dropped onto it, breaking rock by impact as well as grinding it by abrasion. The ball charge is only a small part of the mill load – often around 8–15% of the mill volume – but it has a big effect on throughput, power draw and grind size.

Because the impacts are so heavy, SAG balls are much larger than the media used in ball mills. They are typically between about 100 mm and 150 mm (4–6 in) in diameter, with some mills running sizes either side of that range.

Why forged steel balls are the standard for SAG mills

Grinding balls have to resist two things: impact, which causes breakage and spalling, and abrasion, which wears them down. In a secondary ball mill, abrasion dominates. In a SAG mill, impact is far more severe, so toughness matters as much as hardness.

Forged low-alloy steel balls combine high hardness with the fracture toughness to survive repeated heavy impacts. High chrome cast balls are extremely wear resistant but more brittle, so they are more often used where impact is lower, such as secondary ball mills and cement grinding. We supply high chrome grinding balls for those duties too.

Using media that isn’t tough enough for a high-impact SAG mill can lead to:

• Higher steel consumption – broken and spalled balls are steel you have paid for but lost.
• Lower grinding efficiency – fragments and undersize balls do little useful work in a SAG mill.
• Damage downstream – ball fragments can leave through the mill grates and end up in pumps and pebble crushers.

What to look for in a SAG ball

• Surface and volumetric hardness – hardness should run deep into the ball, not just sit at the surface, so it keeps wearing slowly as it gets smaller. Forged SAG balls are commonly heat treated to the mid-50s to low-60s on the Rockwell C (HRC) scale.
• Impact toughness – drop ball testing shows how well a ball resists breakage and spalling under repeated impact.
• Consistent chemistry – tightly controlled steel chemistry gives consistent heat treatment results from batch to batch.
• Accurate size and weight – consistent balls keep your charge predictable and make ball consumption easier to track.

Choosing the right SAG ball size

There is no single right size for every SAG mill. The best ball diameter depends on:

• Ore hardness and competency – tougher, more competent ore needs higher-energy impacts to break it.
• Feed size – coarse primary-crushed feed generally calls for larger balls than a finer, secondary-crushed feed.
• Grate and pebble port openings – balls need to stay in the mill long enough to do useful work before they wear small enough to pass through the grates.
• Mill speed, lifters and liners – these control how high the charge is lifted and how hard the balls land.
• Your circuit priorities – maximum throughput or a finer transfer size to the ball mill.

As a rule, a larger ball delivers fewer but more powerful impacts and can lift throughput on hard ore, but it can also coarsen the mill discharge and increase the risk of liner damage. Smaller balls give more impacts and more grinding surface. Changes in ball size are best made step by step, using marked ball tests or ball consumption data to compare performance.

SAG ball size guide

Weights are calculated for a solid steel sphere at 7.85 g/cm³ and are a guide only. Actual weights vary with size tolerance.

Not sure which size is right? Send us your mill diameter, feed size, grate aperture and current ball size and we’ll recommend a starting point.

Why choose William Boulton for SAG balls?

William Boulton is a British engineering company based in Stoke-on-Trent, founded in 1852. Alongside our own process machinery, we supply a wide range of grinding media and the practical advice to get the most from it.

• Tailored grinding media – we recommend the media type, size and material that suits your process, rather than a one-size-fits-all product.
• Technical support – our experienced engineers provide ongoing support to help optimise your grinding performance and costs.
• Quality you can rely on – our grinding media is tested and quality checked to meet industry standards.
• Global delivery – our network supports customers around the world with timely delivery.
• A wide grinding media range – from forged steel and high chrome balls to ceramic and zirconia media for fine grinding. View all grinding media.

SAG ball FAQs

What are SAG balls used for?

SAG balls are used in semi-autogenous grinding mills, mainly at large mining operations processing ores such as copper, gold and iron ore. They add impact energy to the ore charge so the mill can break larger rocks and maintain throughput.

What size SAG balls do I need?

Most SAG mills use balls between about 100 mm and 150 mm (4–6 in). The right size depends on your ore, feed size, grate openings, mill speed and lifter design. Send us your mill details and we’ll recommend a size.

What is the difference between SAG balls and ball mill balls?

SAG balls are larger and need much higher impact resistance, because they are lifted and dropped onto coarse rock. Balls for secondary ball mills are smaller and work mainly by abrasion and attrition, so wear resistance is the priority and high chrome cast balls are often an option.

Why do SAG balls break?

Breakage and spalling usually come from very high-energy impacts, internal defects in the ball, or a hardness and toughness profile that doesn’t suit the mill. Operating conditions matter too: high mill speeds and low charge levels can throw balls directly onto the liners. A tough forged ball, correctly sized for the mill, helps to reduce breakage.

How much of a SAG mill is filled with balls?

A SAG mill typically carries a ball charge of around 8–15% of its internal volume, within a total load of ore and balls that is often around 25–35%. The best ball charge for your mill depends on its available power, your ore and your throughput target.

How hard are SAG balls?

Forged SAG balls are typically heat treated to the mid-50s to low-60s HRC, with good hardness right through the ball rather than just at the surface. Harder isn’t always better: hardness has to be balanced against toughness so the balls resist breakage in a high-impact mill.

Do you supply SAG balls outside the UK?

Yes. Our global network supports customers around the world with timely delivery and technical support. Contact us with your location and requirements.