Prospector Trails
Equipment/Processing · Separation
After the mills have ground the ore to powder, the valuable grains are still mixed with many times their weight of worthless rock. Froth flotation separates them with air. Bubbles rise through a tank of ground ore and water, the copper, zinc or lead minerals cling to them, and a froth loaded with metal spills over the top. What is left, the tailings, has to be stored for ever.
01 · The problem
A typical copper ore holds less than one per cent copper, in grains a fraction of a millimetre across. Grinding sets them free, but they are still mixed with a hundred times their weight of crushed rock. No sieve or magnet can sort a powder like that.
Flotation does it with air. In this tank at Anaconda in 1942, bubbles loaded with copper sulphide rise to the top as a heavy, grey froth.
Sources: 911 Metallurgist; Library of Congress.
02 · 1905
By 1900, Broken Hill in New South Wales had dumped millions of tonnes of zinc-rich tailings that no one could treat. Charles Potter and Guillaume Delprat patented ways of floating the zinc minerals out in 1901–1903, and in 1905 the Minerals Separation company's version, with a little oil and violent stirring, went to work on the dumps.
In 1912 the Zinc Corporation ran the world's first selective flotation plant, splitting lead from zinc. These cells were built in Melbourne for the same company in 1945.
Sources: Australian Dictionary of Biography; University of Melbourne; 911 Metallurgist.
03 · How it works
The trick is chemistry. A small dose of a collector coats the grains of copper or zinc mineral and makes them water-repellent, while the rock grains stay wet. A frother keeps the bubbles small and stable.
An impeller at the bottom of each cell churns the slurry and breaks the air into bubbles. The water-repellent grains stick to them and ride up into the froth, which paddles or gravity spill over the lip. These are two machines at Utah Copper's Arthur plant in the 1940s.
Sources: Fuerstenau, SME; Library of Congress.
04 · At scale
Flotation made low-grade copper deposits worth mining. By the 1940s Utah Copper's mills at Magna and Arthur, which treated ore from the Bingham Canyon pit, ran long halls of flotation machines, each cell passing its froth to the next for cleaning.
Fuerstenau called the 20th century "the century of froth flotation". By one 2003 estimate, nearly 2 billion tonnes of ore a year go through flotation cells.
Sources: Yianatos, IMPC 2003; Fuerstenau; Library of Congress.
05 · Today
Modern plants use fewer, much bigger tank cells. In 2014 Outotec, now part of Metso, launched the TankCell e630 at 630 m³, and FLSmidth's 600 Series SuperCell, with 660 m³ of active volume, went into service at KGHM's Robinson copper mine in Nevada. Both makers called theirs the world's largest; they measure volume in different ways.
Sources: Mining Weekly; Metso.
06 · New ideas
In 1985 Mount Isa Mines asked Professor Graeme Jameson of the University of Newcastle to improve flotation in its zinc circuit. His answer dropped the impeller: slurry is jetted down a pipe, the downcomer, pulling air in with it, so the grains meet bubbles in an intense mixing zone.
The first full-scale units went in at the Newlands coal plant and at Mount Isa in 1988–89. Glencore Technology, which sells the cell, says it is now in hundreds of plants.
Source: Glencore Technology.
07 · What's left
Because ore is mostly rock, almost everything that goes into a concentrator comes out as tailings: finely ground rock in water. At big mines that can be tens of thousands of tonnes a day or more, pumped by pipeline to a storage facility.
The water is recycled where possible, and the solids settle behind a dam built to hold them for ever. This is the tailings pond at Antamina, a copper and zinc mine high in the Peruvian Andes.
Source: ICMM.
08 · 2019
On 25 January 2019 a tailings dam at Vale's Córrego do Feijão iron ore mine, near Brumadinho in Brazil, collapsed. A wave of mud ran down the valley towards the Paraopeba River and killed 270 people. The satellite views show the valley in August 2018 and August 2019.
In August 2020 ICMM, the UN Environment Programme and the Principles for Responsible Investment published the Global Industry Standard on Tailings Management, which covers a facility's whole life, from siting to closure.
Sources: ICMM; ICMM, GISTM.
09 · The product
The froth is thickened and filtered into concentrate: a damp powder that is often around a quarter copper, or about half zinc, many times richer than the ore. Here Oliver filters at Anaconda in 1942 squeeze the water out of flotation concentrate before it goes to the smelter.
Some mines now sort the ore before it even reaches the mill. Next: ore sorting.
Source: Library of Congress.
The full story
Froth flotation is a way of separating fine mineral grains by how well they are wetted by water. Ore that has been crushed and ground in mills is mixed with water into a slurry and fed to a series of tanks called flotation cells. Chemicals make the wanted mineral water-repellent, air bubbles carry it up into a froth, and the froth overflows as a concentrate. It is the main way copper, zinc, lead, nickel, molybdenum and many gold ores are concentrated, and it is used for coal, phosphate and potash too.
Collectors are reagents that attach to the surface of one mineral and make it hydrophobic. Frothers help form small, stable bubbles. Other reagents, depressants and activators, stop or help particular minerals float, which is how a plant can float lead first and zinc second from the same ore. Plants run cells in banks: roughers take the first froth, scavengers catch what was missed, and cleaners refloat the concentrate to raise its grade.
The classic machine has an impeller turning in a box, drawing in air and mixing the slurry. Modern plants use round tank cells of hundreds of cubic metres, column cells that let bubbles rise through a tall water-washed froth, and designs like the Jameson Cell, which mixes slurry and air in a downcomer pipe. Makers include Metso, FLSmidth, Glencore Technology, Eriez and BGRIMM. Names here are facts about who makes what, not a ranking or advice.
Tailings are what is left after flotation: finely ground rock, water and traces of reagents. They are usually stored in a tailings storage facility behind an embankment. Some embankments are raised over time using the tailings themselves, and how they are built matters for their stability. Alternatives include thickening tailings to a paste, filtering them and stacking them dry, or pumping them back underground as backfill. After the Brumadinho failure in 2019, the Global Industry Standard on Tailings Management set requirements for independent review, public information on the consequences of failure, and accountability at board level.
More machines: all mining equipment, crushers and ore sorting. The metals: copper, zinc and lead.
Ore is ground to a fine powder and mixed with water to make a slurry. Chemicals called collectors coat the grains of the wanted mineral, such as copper sulphide, and make them water-repellent. Air is blown or drawn into the tank as fine bubbles. The water-repellent grains stick to the bubbles and rise with them into a froth on top, which overflows into a launder. The rest of the rock stays in the water and leaves at the bottom as tailings.
Flotation was developed at Broken Hill in Australia in the early 1900s, where millions of tonnes of zinc-rich tailings had piled up. Charles Potter and Guillaume Delprat patented early methods in 1901–1903, and the Minerals Separation company's version, with a small dose of oil and hard stirring, was adopted there in 1905. The Zinc Corporation ran the first selective flotation plant, separating lead from zinc, in 1912.
Tailings are the finely ground rock and water left after the valuable minerals have been removed. Because most ores contain only a few per cent of metal or less, almost all of the ore milled ends up as tailings. They are usually pumped to a storage facility held back by a dam, though some mines now thicken or filter them and stack them dry.
On 25 January 2019 a tailings dam at Vale's Córrego do Feijão iron ore mine near Brumadinho, Brazil, collapsed. The mud flowed down the valley and killed 270 people. In response, ICMM, the UN Environment Programme and the Principles for Responsible Investment convened the Global Tailings Review, which published the Global Industry Standard on Tailings Management in August 2020.
Makers have claimed several records. FLSmidth's 600 Series SuperCell has an active volume of 660 m³ and the first was installed at KGHM's Robinson copper mine in Nevada. Outotec, now part of Metso, launched the TankCell e630 in 2014 with 630 m³ as standard. The volumes are measured in different ways, so the claims are not directly comparable.
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Pictures in this story: Nina111955, Wikimedia Commons (CC BY-SA 4.0); Office of War Information, Library of Congress, no known restrictions (Public domain); Chas Ruwolt Pty Ltd, Museums Victoria (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); Xtremizta, Wikimedia Commons (CC BY-SA 2.0); Copernicus Sentinel-2 (Aug 2018, Aug 2019), true colour, processed by Prospector Trails (Public domain); Office of War Information, Library of Congress, no known restrictions (Public domain). 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; Ball mills and SAG mills by photosmith2011, Flickr, CC BY-SA 2.0; 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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