Prospector Trails
Equipment/Processing · Sorting
Not every rock that comes out of a mine is worth grinding. For centuries people sorted ore from waste by eye and by hand, often children. Today sensor-based sorters do it at conveyor speed: a camera, laser or X-ray looks at each rock, a computer decides in milliseconds, and a blast of compressed air knocks it into the right bin.
01 · By hand
For most of mining history, ore was sorted by people. Rock from the mine was spread on tables or belts, and workers picked out the pieces that glinted with metal, or threw away the plain rock, by eye and by hand.
Here women sort zinc and lead ore at the Biały Szarlej mine in Upper Silesia, Poland, in 1927.
Source: Henryk Poddębski, 1927.
02 · 1911
In the anthracite coal breakers of Pennsylvania, the picking was done by children. Boys, some of them nine or ten years old, sat on boards over chutes of moving coal, pulling out slate with their bare hands.
Lewis Hine photographed them for the National Child Labor Committee in January 1911. Of this view of the Pennsylvania Coal Company's Ewen Breaker, he wrote that the dust was so dense at times as to obscure the view.
Source: Lewis Hine, Library of Congress.
03 · 1913
Machines began to take the boys' place. Hine photographed this one in 1913 with its inventor, Mr A. Langerfeld, noting that it "does away completely with the use of breaker boys". Child labour laws, and machines like this, ended the job.
But sorting by what a rock is made of, not just its size or weight, needed sensors.
Source: Lewis Hine, Library of Congress.
04 · How it works
A sensor-based sorter spreads crushed rock in a single layer on a fast belt. As each piece passes a sensor, a computer decides whether it is ore or waste. A few milliseconds later, a row of air valves fires a puff of compressed air that knocks the chosen pieces off their path into a separate chute.
The first photometric and radiometric sorters were built in the 1970s for gold, uranium and industrial minerals. These two belt sorters work at the Mittersill tungsten mine in Austria.
Sources: Minerals (2019); Metso.
05 · Sensors
Different sensors see different things. Colour cameras and lasers read the surface. Near-infrared and X-ray fluorescence read minerals and elements. X-ray transmission, XRT, shines X-rays through each piece and measures how much gets through, which reveals its atomic density, whatever it looks like on the outside.
Sorters now also come in shipping containers, like the one at the top of this page, so they can be set up beside a crusher.
Sources: E&MJ; Minerals (2019).
06 · Karowe
Diamonds are carbon, far lighter in atomic terms than the kimberlite rock around them, so they show up clearly under X-rays. That lets a plant find big stones without crushing them first. Karowe, in central Botswana, began mining in 2012, and Lucara commissioned an XRT circuit there in 2015.
The satellite views show the site in 1991, before the mine, and in 2025.
Sources: Lucara (2019); E&MJ.
07 · The giants
Within months of the XRT circuit starting, Karowe produced the 1,109-carat Lesedi La Rona. The 1,758-carat Sewelô followed in 2019, and in 2024 Lucara reported a 2,492-carat diamond, the second largest ever mined after the Cullinan of 1905.
Lucara says its X-ray sorters found all three; TOMRA, which made them, says the same. Those are company reports.
Sources: Canadian Mining Journal; TOMRA; Lucara.
08 · Why it matters
For metal ores, the gain is in what never reaches the mill. Every tonne of waste rejected after crushing is a tonne that does not have to be ground, floated and stored as tailings, which saves energy and water.
What gets through goes on to the mills and flotation. Next: the machines that work underground.
Sources: Metso; Minerals (2019).
The full story
Ore sorting is the separation of individual pieces of rock into ore and waste, usually after crushing and before grinding. It is one of the oldest steps in mining: miners have always picked out the richest pieces by hand. Modern sensor-based sorting does the same thing with machines, one rock at a time, at hundreds of tonnes an hour.
Crushed rock is screened to a size range, spread into a single layer on a belt or chute, and passed through a sensor. Software classifies each piece from the sensor data, and a bank of compressed-air valves fires at the right moment to deflect the selected pieces. Because each piece is judged on its own, sorting works best when ore and waste are in separate lumps rather than mixed in every grain.
Traditional diamond plants crush kimberlite and recover diamonds by density separation and X-ray luminescence, which risks breaking large stones. XRT sorting can pick out large diamonds from coarser rock. Lucara reports that its XRT circuit at Karowe in Botswana, commissioned in 2015, recovered the 1,109-carat Lesedi La Rona (2015), the 1,758-carat Sewelô (2019) and a 2,492-carat stone in 2024. These are company-reported facts.
Makers include TOMRA, Steinert, Bühler and MineSense, among others. Names here are facts about who makes what, not a ranking or advice.
More machines: all mining equipment, froth flotation and crushers. Places: mining in Botswana, Austria and Poland; the metal: tungsten.
Ore sorting separates pieces of rock into ore and waste before they are ground. In sensor-based sorting, crushed rock is spread out on a belt or chute, a sensor measures each piece, and a computer fires a jet of compressed air to knock the selected pieces into a separate stream. Rejecting waste early means less rock goes through the energy-hungry mills.
An X-ray source shines through each rock and a detector measures how much passes through at two energy levels. That reveals the rock's atomic density rather than its colour, so a dense metal-bearing piece, or a diamond inside a lump of kimberlite, shows up differently from the waste around it.
Breaker boys were children who sat over chutes in the coal breakers of the Pennsylvania anthracite fields, picking slate and other waste out of the coal by hand. The photographer Lewis Hine documented them for the National Child Labor Committee around 1911, and his pictures helped the campaign against child labour.
In 2024 Lucara Diamond reported a 2,492-carat diamond from its Karowe mine in Botswana, the second largest ever mined after the 3,106-carat Cullinan. Lucara said it was detected and recovered by its X-ray transmission sorting. Karowe's 1,758-carat Sewelô (2019) and 1,109-carat Lesedi La Rona (2015) were also recovered by its XRT circuit, according to the company.
Makers of sensor-based ore sorters include TOMRA, Steinert, Bühler and MineSense, among others. TOMRA, a Norwegian company better known for bottle-return machines, entered mining sorting by buying Commodas in 2006. This is a list of facts, not a ranking or advice.
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Pictures in this story: Tomra Sorting GmbH, Wikimedia Commons (CC BY-SA 4.0); Henryk Poddębski, via Wikimedia Commons (Public domain); Lewis Hine, National Child Labor Committee, Library of Congress (Public domain); Lewis Hine, National Child Labor Committee, Library of Congress (Public domain); Tomra Sorting GmbH, Wikimedia Commons (CC BY-SA 4.0); USGS/NASA Landsat (Aug 1991) and Copernicus Sentinel-2 (Aug 2025), true colour, processed by Prospector Trails (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; Froth flotation and tailings by Nina111955, 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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