Prospector Trails
Equipment/Processing · Crushing
Every tonne of hard-rock ore that leaves a pit goes first to a crusher. Truck-loads of blasted rock, some of it in pieces the size of a car, drop into a machine that squeezes them down to fist-sized gravel in seconds, then hands them on for finer crushing and grinding. Here is how jaw, gyratory and cone crushers work.
01 · First stop
At a big mine, every loaded haul truck backs up to the primary crusher and tips its 200 or 300 tonnes into a pocket above it. This is the primary crusher at Brockman 2, an iron ore mine in Western Australia's Pilbara.
Blasted rock arrives in every size, from dust to boulders a metre or more across. The crusher's job is to bring all of it down to pieces small enough to ride a conveyor to the next stage.
Sources: 911 Metallurgist; SME.
02 · 1858
The first successful rock-breaking machine was Eli Whitney Blake's jaw crusher, patented in the US in 1858. Two heavy plates stand in a V. One is fixed; the other is driven back and forth by an eccentric shaft and a toggle, squeezing the rock on every stroke until it cracks and falls lower.
Blake's toggle design was so sound that jaw crushers are still built on it. This "Samson" jaw stood in the Sound Democrat mill in Colorado's San Juan Mountains.
Sources: 911 Metallurgist; HAER, Library of Congress.
03 · 1881
In 1881 Philetus W. Gates of Chicago patented a crusher with all the essentials of the modern gyratory. A cone-shaped mantle on a long shaft gyrates inside a steel bowl, so rock is always being squeezed somewhere around the ring, with no wasted return stroke.
In a contest in 1883, a Gates gyratory crushed its load about three times faster than a Blake jaw of the same opening. This gyratory was crushing copper ore for Anaconda in Montana in the 1940s.
Sources: SME, Primary crushing history; Library of Congress.
04 · Inside
From above, a gyratory is a ring-shaped gap around the top of the mantle. Rock falls into it and is crushed as it works its way down to the narrow gap at the bottom, the setting, which decides how big the pieces are when they leave.
Anaconda's crusher reduced mine-run ore to a maximum of four inches, about 10 cm, in one pass. When a piece jammed, a worker freed it by hand.
Source: Library of Congress, Anaconda Copper Mining Company.
05 · The biggest
Gyratories are sized by two numbers in inches: the feed opening and the diameter of the mantle. Metso's largest, the Superior MKIII 60-110, has a 1,525 mm (60-inch) opening and a 1,500 kW motor, and its maker's table rates it at up to about 17,000 tonnes an hour.
Big crushers need big motors: this one drove a gyratory at Utah Copper's mills in the 1940s.
Sources: Metso; Metso brochure.
06 · Finer
After the primary crusher, ore usually goes through one or two more stages. Cone crushers, like smaller, flatter gyratories, break it finer, and vibrating screens send anything still too big back round. This is a Symons cone crusher in a Colorado mill, an early version of a design still used today.
Some modern plants use high pressure grinding rolls instead, which squeeze a bed of ore between two rolls and crack it from the inside.
Sources: HAER, Library of Congress; 911 Metallurgist.
07 · The plant
A crushing plant is a tower of steel: crushers, screens, chutes and conveyors, with stockpiles or bins to keep the mill fed when the trucks stop.
At some mines the crusher moves into the pit itself, and conveyors carry the crushed ore out, replacing a long uphill truck haul. Smaller crushers on tracks, like this mobile plant, go to the rock instead, and are common in quarries and small mines.
Sources: 911 Metallurgist; SME.
08 · Next
Crushing takes rock from boulders to gravel. To free the tiny grains of copper, gold or zinc minerals inside, the ore must be ground much finer, to powder. That is the job of the mills, and it takes far more energy: in one Australian study, grinding and crushing together used about half of the mines' electricity.
Next: ball mills and SAG mills.
Source: AusIMM (2012).
The full story
A crusher is a machine that breaks rock into smaller pieces by squeezing it between hard steel surfaces. In a mine it is the first step of processing, called comminution: crushers take blasted rock down from boulders to gravel, and mills then grind it to powder so the valuable minerals can be separated.
A jaw crusher has a fixed jaw and a moving jaw set in a V. An eccentric shaft drives the moving jaw through a toggle, so it closes on the rock with great force on each stroke. Rock is crushed as it falls through the narrowing gap and leaves at the bottom. Eli Whitney Blake's design, patented in 1858, is still the reference. Jaw crushers are compact and robust, so they are common as primary crushers at smaller mines, in quarries, underground and on mobile tracked units.
A gyratory has a cone-shaped head, the mantle, on a long vertical shaft that gyrates inside a bowl lined with concaves. Because the gap is closing somewhere all the time, a gyratory crushes continuously, and it can take ore straight from trucks without a feeder. That makes it the usual primary crusher at large open pits. Sizes are given as feed opening by mantle diameter in inches. Metso's Superior MKIII 60-110, the largest in its range, has a 1,525 mm opening and 1,500 kW motor and a top rated capacity of about 17,000 tonnes an hour.
Secondary and tertiary crushing uses cone crushers, which work like short gyratories with a wider, flatter head, in circuit with vibrating screens. High pressure grinding rolls (HPGR) squeeze a bed of ore between two counter-rotating rolls, creating micro-cracks that make the ore easier to grind, and are spreading in hard copper, gold and iron ores to save energy.
The main makers of large mining crushers include Metso, FLSmidth (which took over thyssenkrupp's mining business), Sandvik, Weir and TAKRAF. Liners and wear parts are a large business of their own, because the steel surfaces wear out and are replaced regularly. Names here are facts about who makes what, not a ranking or advice.
More machines: all mining equipment, drilling and blasting and haul trucks. The ores: copper, iron ore and gold.
Rock falls between two heavy steel plates, the jaws, set in a V. One is fixed; the other is pushed back and forth by an eccentric shaft and a toggle. Each stroke squeezes the rock until it cracks, and the pieces fall lower until they are small enough to drop out of the gap at the bottom. Eli Whitney Blake's 1858 design set the pattern still used today.
A jaw crusher squeezes rock in strokes between two jaws. A gyratory crusher has a cone-shaped mantle that gyrates inside a bowl, so it crushes continuously all the way round. Gyratories handle far more rock per hour and are used as the first crusher at big mines; jaws are cheaper, smaller and common at smaller mines, quarries and underground.
A cone crusher works like a smaller, flatter gyratory. It takes rock that has already been through the primary crusher and breaks it finer, often in two stages with screens in between, before the ore goes to the mill. Some plants now use high pressure grinding rolls (HPGR) for this step.
Metso's Superior MKIII 60-110 is the largest in its range, with a 1,525 mm (60-inch) feed opening and a 1,500 kW motor, and a top rated capacity of about 17,000 tonnes an hour in the maker's table. Gyratories are sized by feed opening and mantle diameter in inches, so a 60-110 has a 60-inch opening and a 110-inch mantle.
The main makers of large mining crushers include Metso, FLSmidth (which took over thyssenkrupp's mining business), Sandvik, Weir and TAKRAF, with many makers of smaller and mobile units. This is a list of facts, not a ranking or advice.
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Pictures in this story: photosmith2011, Flickr (CC BY-SA 2.0); Calistemon, Wikimedia Commons (CC BY-SA 4.0); Historic American Engineering Record, Library of Congress (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); Office of War Information, Library of Congress, no known restrictions (Public domain); Historic American Engineering Record, Library of Congress (Public domain); Antti Leppänen, Wikimedia Commons (CC BY 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; Ball mills and SAG mills 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.
