Equipment/Processing · Grinding
Most of the metal in a mine is locked in grains smaller than a grain of sand. To set it free, the ore is ground to powder in giant turning drums filled with steel balls and rock. The mills are among the biggest machines in mining and the hungriest for power. Here is how ball mills and SAG mills work.
01 · The mill hall
Walk into a concentrator and the first thing you hear is the mills: steel drums several metres across, turning day and night, full of ore, water and tumbling steel. This long row of ball mills ground copper ore at one of Utah Copper's concentrators during the Second World War.
Crushers take the ore to gravel. The mills take it to powder, finer than beach sand, so the tiny grains of copper minerals come free from the rock around them.
02 · Inside
Look inside a ball mill and you see what does the grinding: thousands of steel balls. As the drum turns, they are carried up the side by ribbed liners and fall back, crushing and rubbing the ore between them. The balls and liners wear away and are topped up and replaced.
The mill turns at a set fraction of the speed at which the load would stick to the shell, so the balls cascade rather than spin round with it.
03 · SAG
A SAG mill, short for semi-autogenous grinding, is larger and squatter than a ball mill. It takes coarse ore straight from the primary crusher and grinds it mainly with the ore itself, big lumps breaking smaller ones, with some steel balls added. An AG mill uses the ore alone.
Most big copper and gold plants now run a SAG mill first, then ball mills to finish the grind. Before SAG mills, plants used rod mills and long rows of ball mills, like these at Phelps Dodge's Morenci concentrator in Arizona in the 1940s.
Sources: OneMine; Library of Congress.
04 · 1998
In June 1998 the Cadia gold and copper mine in New South Wales started a SAG mill 40 feet (12.2 m) in diameter, driven by a 20 MW gearless motor. It was the largest grinding mill in the world, over 40% bigger than any SAG mill then running, and its owners chose it because one huge mill line cost less to build than two.
From space you can see the mine and its plant grow out of the farmland around it.
05 · The motor
Turning a drum that size takes a special motor. In a gearless mill drive, the magnetic poles are bolted round the mill shell, which becomes the motor's rotor, and the stator wraps around it. There are no gears to wear out.
The first, a 6.4 MW drive, turned a cement mill at Le Havre in France in 1969. The first in a mine followed at Bougainville, Papua New Guinea, in 1985. Metso was later chosen to supply a 42 ft SAG mill with a 28 MW gearless drive for the Conga project in Peru.
Sources: ABB; ABB, Conga.
06 · Power
Breaking rock finer and finer takes more and more energy. A 2021 report for the Weir Group found crushing and grinding typically use about a quarter of mining's energy for copper, gold, iron ore, nickel and lithium, and about 1% of all the energy used in the world.
Measured as electricity alone, the share is higher: an Australian study found comminution used about 52% of the mines' reported electricity.
Sources: MINING.COM, Weir report; AusIMM (2012).
07 · Scale
Even an old, modest ball mill is big. The end of this one, with the ring gear that turned it, came from a former cement works and now stands in a Brisbane park. The largest SAG mills today are 12 to 13 m across, about as tall as a four-storey building.
They are built from shell sections, end walls and trunnion bearings shipped in pieces and assembled on site.
08 · Next
The ground ore leaves the mill as slurry, mixed with water. Next it goes to separation, most often froth flotation, where air bubbles carry the copper, zinc or lead minerals to the surface. What is skimmed off and dried is concentrate, like this copper concentrate, ready for the smelter.
Source: ABB, Mills and GMDs.
The full story
A grinding mill is a large rotating steel cylinder, lined with replaceable wear plates, in which ore is ground to a fine powder with water. Grinding follows crushing and is the last step of comminution. Its aim is liberation: grinding the ore fine enough that each grain of the valuable mineral is free of the waste rock, so it can be separated.
A ball mill is partly filled with steel balls. As the shell turns, liners lift the balls, which cascade and fall onto the ore, grinding it by impact and attrition. Ball mills are longer than they are wide and usually do the finer grinding, taking feed from a SAG mill or a crushing circuit. Hydrocyclones classify the slurry leaving the mill and send coarse particles back for more grinding.
Semi-autogenous (SAG) mills are wider and shorter, and take coarse ore straight from the primary crusher. The ore itself does most of the grinding, helped by a smaller load of steel balls; autogenous (AG) mills use the ore alone. Pebbles that resist grinding are often taken out, crushed and returned. The largest SAG mills are 40 to 42 feet (12.2 to 12.8 m) in diameter. Cadia in New South Wales commissioned the first 40 ft SAG mill in 1998, driven by a 20 MW gearless motor; Metso was later chosen to supply a 42 ft, 28 MW SAG mill for the Conga project in Peru.
Comminution is the biggest single use of energy at most hard-rock mines. A 2021 report for the Weir Group put it at about 25% of mining's final energy for five major commodities and about 1% of world energy use; an Australian study found about 52% of reported mine electricity. The largest mills are turned by gearless mill drives, where the motor is built around the mill shell. The first, 6.4 MW, drove a cement mill at Le Havre in 1969, and the first in mining was at Bougainville in 1985. Makers of mills include FLSmidth, Metso and CITIC Heavy Industries; drives come from ABB and Siemens; liners and grinding media from companies such as Weir, Metso, Magotteaux and ME Elecmetal. These are facts about the supply chain, not a ranking or advice.
More machines: all mining equipment, crushers and drilling and blasting. The metals: copper, gold and zinc.
Both are turning steel drums. A ball mill is filled with steel balls that tumble and grind the ore to a fine powder. A SAG (semi-autogenous grinding) mill is larger and takes coarser ore straight from the crusher; it grinds mainly with the ore itself, with some steel balls added. Many plants run a SAG mill first and ball mills after it, a layout called SABC when it includes a pebble crusher.
The biggest are 40 to 42 feet (12.2 to 12.8 m) in diameter. Cadia in Australia commissioned a 40 ft, 20 MW SAG mill in June 1998, then the world's largest grinding mill. Metso was later chosen to supply a 42 ft, 28 MW gearless SAG mill for the Conga project in Peru.
A 2021 report for the Weir Group, by the director of the Coalition for Eco-Efficient Comminution, found crushing and grinding typically use about 25% of mining's final energy for copper, gold, iron ore, nickel and lithium, and about 1% of world energy use across hard-rock mining. An Australian study found they used about 52% of reported mine electricity.
A motor built around the mill itself: the magnetic poles are fixed to the mill shell, which becomes the motor's rotor, and the stator wraps around it. There are no gears to wear. The first, 6.4 MW, drove a cement mill at Le Havre in France in 1969; the first in a mine was at Bougainville in 1985. Today the largest SAG mills run on gearless drives of 20 MW and more.
The ground ore, mixed with water as a slurry, goes to separation, most often froth flotation, which floats off the valuable minerals as a concentrate. Gold ores may go to leaching instead. What is left becomes tailings.
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Pictures in this story: photosmith2011, Flickr (CC BY-SA 2.0); Office of War Information, Library of Congress, no known restrictions (Public domain); Office of War Information, Library of Congress, no known restrictions (Public domain); Office of War Information, Library of Congress, no known restrictions (Public domain); USGS/NASA Landsat (Aug 1995) and Copernicus Sentinel-2 (Aug 2025), true colour, processed by Prospector Trails (Public domain); Kgbo, Wikimedia Commons (CC BY-SA 4.0); Nuno Madeira Alves, Wikimedia Commons (CC BY-SA 4.0). Cover photos from Wikimedia Commons: Bucket wheel excavators by bluecloud9, CC BY-SA 2.0; Draglines by Roy Luck, CC BY 2.0; Haul trucks by Lechhabmed, CC BY-SA 4.0; Autonomous trucks by RobSimmons223311, CC BY-SA 4.0; Drilling and blasting by Siim Roov, CC BY-SA 4.0; Crushers by photosmith2011, Flickr, CC BY-SA 2.0; Froth flotation and tailings by Nina111955, CC BY-SA 4.0; Ore sorting by Tomra Sorting GmbH, CC BY-SA 4.0; Underground mining equipment by Kaupo Kikkas, CC BY-SA 4.0; Longwall mining by Bochum1805, Flickr, CC BY-SA 2.0; Conveyors, rail and ports by Kabelleger / David Gubler, CC BY-SA 4.0; Who supplies the mines by Alexandre Prevot, CC BY-SA 2.0.
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Cover photos from Wikimedia Commons: Super Pit, Kalgoorlie by Kgbo, CC BY-SA 4.0.
