⛏️ Mining: How Ore Leaves the Ground

What you’ll learn

1. Grams Per TonneAdopt the course's through-line — that mining is a grade problem, not a digging problem — and feel what grams per tonne actually means.Mining is not fundamentally about digging; it is about a ratio between valuable metal and worthless rock. Porphyry copper deposits, which supply more than 60% of the world's copper, averaged 0.44% copper in 2008 according to the USGS deposit model, so a tonne of ordinary ore holds roughly 4.4 kg of copper and 995.6 kg of waste; gold is measured in grams per tonne because percentages would be all zeros. Every subsequent decision in mining — open pit versus underground, how fine to grind, which reagents to use, what to do with the leftovers — is an attempt to improve one side of that ratio or to stop the other side from bankrupting the operation.
2. Ore Is a Price, Not a RockUnderstand that 'ore' is an economic verdict rather than a rock type, and that cut-off grade, reserves and resources all follow from that.Ore cannot be identified by inspection, because the standard definition contains a price: rock is ore only if its metal is worth more than the cost of extracting it. Every mine therefore draws a cut-off grade — a line built from metal prices, diesel, power, haulage and recovery rather than from geology — and when that line moves, identical rock crosses between ore and waste without changing at all, which is why mines re-process their own old waste dumps. The same logic splits "how much is left" into resources (geologically present, in a known quantity and grade) and reserves (demonstrably economic under today's prices, costs and permits), so reserve figures and claims about the world's largest reserves are statements about knowledge and prices, and should be attributed rather than asserted.
3. Finding Something You Cannot SeeUnderstand exploration as a cheap-clues-then-expensive-certainty funnel, and grasp that an ore body model is an inference from a scatter of drill cores.An ore body is a buried, irregular, invisible three-dimensional object, and hard-rock exploration cannot image it the way the oil industry images layered basins, because ore bodies are irregular masses sitting in rock that is acoustically similar to them. Instead, exploration runs a funnel: cheap, broad, ambiguous methods first — geochemistry, which hunts parts-per-billion metal haloes leaked into soils and stream sediments, and geophysics, which looks for magnetic, dense or electrically conductive shapes that do not belong — both of which find anomalies rather than ore. Only diamond drilling settles anything, cutting an intact cylinder of core that can be logged, assayed and archived; but because core samples only a few centimetres across, everything between holes is statistical inference, which is why resource estimates are regulated and signed by a named, liable person.
4. The Stripping RatioUse the stripping ratio to explain the open-pit-versus-underground decision and the physical shape of a pit.The choice between an open pit and an underground mine is governed less by depth than by the stripping ratio — the tonnes of barren overburden that must be drilled, blasted, hauled and stored to free one tonne of ore, which in metal mining is a straight waste-to-ore weight ratio. Lower is better, because waste removal is cost while ore extraction is revenue, so grade and stripping ratio together largely decide whether a deposit is a mine at all: at 0.44% copper and a 3:1 strip, more than nine hundred tonnes of rock move for every tonne of contained copper. Because rock cannot stand in vertical walls, a pit must be a cone cut back to a stable slope angle, so each step deeper requires stripping a wider ring from the whole wall; waste therefore grows with the cube of the pit's dimensions while the ore is a fixed prize, the ratio climbs relentlessly, and the pit's final depth is the computed point at which the next ring of waste costs more than the ore it frees.
5. Why a Pit Is a StaircaseRead the physical form of an open pit — benches, slope angles, haul roads and truck size — as consequences of the grade ratio and rock mechanics.An open pit's staircase of benches is functional, not decorative: each bench is a flat working floor wide enough for drill rigs and enormous trucks, a catch bench that intercepts rockfall from the steep face above, and a carrier of the haul road spiralling down at a climbable grade — together building an overall wall angle the rock can actually hold. Ultra-class haul trucks follow from the same economics. The record holder, the BelAZ 75710, carries 450 tonnes and very few were built; the fleet that does the world's hauling sits in the 220–400 tonne class. Because a truck's driver, engine, tyres and fuel costs do not scale with payload, bigger trucks cut cost per tonne moved — so the truck is enormous precisely because the ore grade is low, with bench width and haul-road strength setting the practical ceiling.
6. When the Lid Stays OnUnderstand underground mining as the inversion of open-pit logic, and see block caving as the same grade economics expressed underground.Underground mining inverts the open-pit principle — instead of removing everything that is not ore, you remove only the ore and leave the rest of the mountain standing overhead — which nearly eliminates the stripping ratio but adds two costs the surface never charges: continuous ground support (rock bolts, mesh, shotcrete) against rock stress that wants to close every opening, and ventilation, which must supply air, dilute diesel and blasting fumes and remove the heat of the rock itself, consuming large amounts of power without pause. Underground methods therefore trade selectivity against cost per tonne, and block caving sits at the cheap extreme: tunnels are driven beneath the ore body and an undercut blasted out, removing its support so it collapses under its own weight and flows out through draw points for decades. It suits massive, steeply dipping, highly fractured and typically low-grade ore bodies — meaning caving exists precisely because low grade forbids expensive precision — at the cost of surface subsidence, documented at the Climax and Henderson mines, and the risk of catastrophic air blast if caving stalls while drawing continues.
7. Drill, Blast, Load, HaulSee the drill-blast-load-haul cycle as one designed chain, and understand blasting as a size-reduction manufacturing step rather than mere destruction.Every mine, surface or underground, runs the same four-beat cycle — drill, blast, load, haul — repeated until the ore body is exhausted, with each beat existing to serve the next. Blasting is best understood not as destruction but as the first size-reduction step in manufacturing: its product is a particle size distribution, and engineers design hole diameter, spacing, depth, charge and above all the millisecond delay sequence so that holes fire in order, each row breaking toward space opened microseconds before. Getting fragmentation wrong propagates downstream — too coarse and shovels, trucks and crushers choke on boulders; too fine and explosive is wasted and the carefully engineered pit walls are shaken — and crucially the blast, though the cheapest size reduction available, leaves the ore far too coarse to release its metal.
8. The Energy Goes Into Breaking RockUnderstand comminution as the energy centre of mining, and grasp liberation size as the physical reason it is unavoidable.Comminution — the progressive crushing and grinding of rock — is where a mine's energy actually goes: Marc Allen's 2021 study for CEEC puts it at about 25% of an average mine site's final energy, within a mining industry consuming roughly 12 EJ a year, or about 3.5% of global final energy. The study's global estimates for comminution (up to 1% of final energy, up to 3% of primary energy) are contested across methods, and the report was commissioned by an equipment manufacturer, so they should be read as a range. Comminution is unavoidable because metal exists as discrete microscopic mineral grains locked inside gangue grains, and no separation process can distinguish two things that are physically stuck together — so the ore must be broken down to its liberation size, a threshold set by the deposit's natural grain size. It is expensive because breakage creates new surface area, which by Rittinger's law grows inversely with particle size, making the cascade from near-free blasting through cheap crushing to fine grinding progressively costlier, and turning how-fine-to-grind into an economic optimisation that moves with metal prices.
9. Bubbles That ChooseUnderstand froth flotation as selective surface chemistry, and see how it redefined what counts as ore.Froth flotation solves the problem left by grinding — separating trillions of liberated mineral grains from identical-looking gangue grains of similar size and density — by exploiting surface chemistry rather than density: it separates particles that repel water from particles that do not, and it is the highest-tonnage separation process in industry, treating more than a billion tonnes of ore a year. Collectors (for sulphides, xanthates, dosed at anything from a few tens to a few hundred grams per tonne of ore) bond to the target mineral and present an oily tail to the water, making only that mineral water-repelling so rising bubbles can attach; frothers (pine oil, MIBC, polyglycols, xylenol) stabilise the froth, and depressants (starch, polyphenols, lye, lime) hold unwanted minerals down when two minerals both float. Developed over decades — Bessel 1877, the Elmores' first industrial-scale plant at Glasdir in 1897, Sulman-Picard-Ballot patented 1905 — flotation drove the cut-off grade down so far that US copper ore averaging about 2.5% before 1907 gave way to about 0.6% by 1991, and to a 0.44% world porphyry average by 2008: a record of capability rather than depletion, since ore is defined economically.
10. Concentrate — and Where This Course StopsSee the mine's product as concentrate rather than metal, understand the mine as a machine for raising a ratio, and locate the clean handoff to smelting.Mines do not produce metal: a copper flotation plant's product is concentrate — mostly chalcopyrite, a compound of copper, iron and sulphur, plus unrejected gangue — dewatered and shipped, typically running 25–30% copper against a feed of roughly 0.44%. There is a hard ceiling on that, because chalcopyrite itself is only about 35% copper by weight, so a 30% concentrate is already nearly pure mineral and there is no more rock left to remove. Every step of mining is physical rather than chemical: break, grind, sort, discarding the overwhelming majority of the mass by weight while keeping nearly all the metal, so a mine is best understood as a machine for raising a ratio cheaply. Freeing the metal means breaking the chemical bond between copper and sulphur, which requires smelting — heat, oxygen, slag, sulphur dioxide capture and refining — and belongs to extractive metallurgy; likewise iron concentrate's journey through the blast furnace to steel belongs to steelmaking. The chain runs rock, ore, broken rock, powder, concentrate, metal, with mining owning the middle and grade and cost-per-tonne governing every arrow.
11. What's LeftFace the waste side of the grade ratio honestly: why tailings exist by arithmetic, why tailings dams fail differently from water dams, and why acid mine drainage outlives the mine.Low grade means enormous waste by definition, so the roughly 995 kg left from every tonne of ordinary copper ore becomes tailings — flour-fine flotation residue in a water slurry, stored in some of the largest earth structures on Earth. Tailings dams differ from water dams because they are often raised incrementally from the tailings themselves (in the cheap upstream design, partly founded on previously deposited wet tailings), and saturated fine tailings can liquefy and flow. The two best-documented failures differ sharply: at Mount Polley (4 August 2014) the operator estimated 21–25 million m³ of discharge, and the independent panel found not liquefaction but undrained failure of an unrecognised glaciolacustrine silt and clay layer in the foundation, with no deaths; at Brumadinho (25 January 2019) the expert panel found flow liquefaction released 9.7 million m³, about 75% of the stored tailings, in under five minutes, killing 272 people, with no instrument detecting any warning — driving bans on upstream construction and a shift toward dry-stack tailings. The slower problem is acid mine drainage: grinding sulphides to flour to liberate metal also exposes vast surface area, so pyrite oxidises (2 FeS₂ + 7 O₂ + 2 H₂O → 2 Fe²⁺ + 4 SO₄²⁻ + 4 H⁺), ferric iron attacks more pyrite in a self-feeding cycle accelerated by Acidithiobacillus ferrooxidans, reaching pH −3.6 at Iron Mountain in water up to 47 °C, where the remaining sulphide is estimated to need some 3,000 years to oxidise — treatable, but only by someone still present to pay.

Questions this course answers

Grounded in trusted sources

• John, D.A., and others (2010), Porphyry copper deposit model: USGS Scientific Investigations Report 2010-5070-B — porphyry deposits supply >60% of world copper production and ~65% of resources; grades 0.3–2.0% Cu; average grade 0.44% Cu in 2008, down from 0.49% in 2002. https://pubs.usgs.gov/sir/2010/5070/b/
• Singer, D.A., Berger, V.I., and Moring, B.C. (2008), Porphyry copper deposits of the world: USGS Open-File Report 2008-1155 — grade and tonnage models for 422 deposits; median copper grade 0.44%. https://pubs.usgs.gov/of/2008/1155/
• US Geological Survey, Mineral Commodity Summaries — copper: reserves are the economically extractable portion of a resource, so published reserve figures move with price, cost and how much engineering work has been done. https://www.usgs.gov/centers/national-minerals-information-center/copper-statistics-and-information
• Allen, M. (engeco) (2021), 'Mining Energy Consumption 2021', published by CEEC and commissioned by The Weir Group — comminution ≈25% of an average mine site's final energy; mining ≈12 EJ/yr ≈3.5% of global final energy; mine-site split 46% diesel, 25% comminution, 15% mining and ventilation electricity, 14% other. https://www.ceecthefuture.org/resources/mining-energy-consumption-2021
• Wills, B.A., and Finch, J.A., Wills' Mineral Processing Technology (Elsevier) — comminution and Rittinger's law; liberation size; froth flotation collectors, frothers and depressants; and the standard account that US copper ore averaged about 2.5% before 1907, against about 0.6% by 1991.
• Fuerstenau, M.C., Jameson, G.J., and Yoon, R.-H. (eds.) (2007), Froth Flotation: A Century of Innovation, Society for Mining, Metallurgy and Exploration — the Bessel, Elmore and Sulman-Picard-Ballot developments, and flotation as the technology that made low-grade porphyry ore economic.
• Nordstrom, D.K., and Alpers, C.N. (1999), Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California: PNAS v.96, p.3455–3462 — pH as low as −3.6; water measured to 47 °C underground; ~8 Mt of sulphide remaining and roughly 3,000 years to oxidise at current rates. https://doi.org/10.1073/pnas.96.7.3455
• Nordstrom, D.K., Alpers, C.N., Ptacek, C.J., and Blowes, D.W. (2000), Negative pH and extremely acidic mine waters from Iron Mountain, California: Environmental Science & Technology v.34, p.254–258 — pH to −3.6, dissolved metals to 200 g/L, sulfate to 760 g/L in the Richmond Mine. https://doi.org/10.1021/es990646v

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