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Material World
Process

Smelting

Heating an ore with a reducing agent to separate metal from the elements it is chemically bound to.

Smelting extracts a metal from its ore by chemical reduction — supplying something, historically carbon in the form of charcoal or coke, that oxygen or sulfur prefers to bond with more than the metal does.

It is distinct from melting, and the distinction is frequently blurred. Melting changes a substance's state; smelting changes its chemistry. Melting copper gives liquid copper. Smelting copper ore gives copper that was not there before, because it was chemically combined.

History

Copper smelting appears in the archaeological record from around 5000 BCE. The technique required both an understanding that heating certain rocks produced metal and the furnace technology to reach the necessary temperatures, and its independent appearance in several regions is one of the more striking cases of parallel technological development.

Uses

Smelting is how most metals are separated from their ores, and the variations are named for the metal. Iron smelting in a blast furnace, using coke as both fuel and reducing agent, accounts for the greatest tonnage by a wide margin.

Copper, lead, zinc, nickel and tin are all won by smelting, usually after concentration and, for sulfide ores, roasting to convert them to oxides first. Where the metal binds oxygen too strongly for carbon to strip it — aluminium and magnesium — smelting in this sense does not work at all, and electrolysis is used instead. That distinction is why aluminium arrived so late.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • Chalcopyrite — mineral · roasted to drive off sulfur, then reduced to copper
  • Cassiterite — mineral · reduced with carbon to metallic tin
  • Hematite — mineral · reduced with coke in a blast furnace
  • Oxygen — element · as the element removed, not added
  • Chromite — mineral · reduced directly to ferrochrome; the iron in the ore is wanted rather than removed
  • Pyrolusite — mineral · reduced to ferromanganese, which is how nearly all manganese reaches steel
  • Scheelite — mineral · after conversion to the oxide
  • Apatite — mineral · the phosphate rock charged to the electric furnace, with coke to reduce it and silica to take up the calcium
  • Coal — rock · as coke: heating coal without air drives off the volatiles and leaves the carbon that reduces iron oxide in a blast furnace, which is what replaced charcoal and lifted the ceiling on iron production
  • Charcoal — material · as both the fuel and the reducing agent, and it smelted every metal humanity had until coke — which is why ironworks were built in woods rather than at the ore

produces

  • Copper — element · as blister copper, refined electrolytically afterwards
  • Tin — element · reduced from cassiterite at relatively low temperature
  • Iron — element · as pig iron, high in carbon and brittle until refined
  • Cast iron — alloy · tapped directly from the blast furnace, before the carbon is removed to make steel
  • Thallium — element · captured from the flue dusts, before it disperses
  • Chromium — element · as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal
  • Manganese — element · as ferromanganese, by the same route and for the same reason — steel wants the alloy, not the element
  • Nickel — element · reduced from roasted pentlandite concentrate, with the platinum-group metals following into the refinery
  • Cobalt — element · generally alongside copper or nickel rather than from a cobalt-first operation
  • Antimony — element · reduced from the oxide left by roasting stibnite
  • Tungsten — element · reduced from the oxide, at a temperature few other metals demand
  • Molybdenum — element · reduced from the oxide that roasting molybdenite produces
  • Silicon — element · carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved
  • Vanadium — element · as ferrovanadium, reduced from vanadium slag and going straight into steel without being separated as the metal
  • Niobium — element · as ferroniobium, reduced aluminothermically from pyrochlore concentrate — the form nearly all niobium is used in
  • Phosphorus — element · white phosphorus, reduced from phosphate rock with coke and silica in an electric furnace and condensed under water

was succeeded by

  • Basic oxygen steelmaking — process · smelting produces the iron; steelmaking refines it, and the two are consecutive stages rather than alternatives
  • Electrorefining — process · smelting produces metal at about 99%; electrorefining takes it the rest of the way and harvests what is left behind
  • Alloying and melting — process · smelting produces the metal; alloying combines metals into something neither was

succeeded

  • Roasting — process · roasting converts the sulfide to an oxide; smelting then reduces the oxide to metal
  • Froth flotation — process · or straight to the smelter, where the concentrate is reduced without a separate roasting stage — the porphyry route, which delivers a concentrate at around twenty-five per cent copper from ore at less than one

is associated with

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Smelting back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Smelting → takes as input (as both the fuel and the reducing agent, and it smelted every metal humanity had until coke — which is why ironworks were built in woods rather than at the ore) → Charcoal → is produced by (the kiln burns a small fraction of the wood to supply the heat that pyrolyses the rest) → Pyrolysis → takes as input (to make coke, and the coal gas that came off it lit European cities for a century as the by-product) → Coal → is sourced from (every coal seam in the world was a peat bog. Bury it, compress it, heat it, and it passes through lignite and bituminous coal to anthracite, losing water and volatiles at each step) → Peat → is sourced from (waterlogging is the whole mechanism: a bog is anoxic below a few centimetres, so the organisms that would decompose the plant matter cannot work, and it accumulates instead) → Water
  • Smelting → takes as input (as coke: heating coal without air drives off the volatiles and leaves the carbon that reduces iron oxide in a blast furnace, which is what replaced charcoal and lifted the ceiling on iron production) → Coal → is sourced from (every coal seam in the world was a peat bog. Bury it, compress it, heat it, and it passes through lignite and bituminous coal to anthracite, losing water and volatiles at each step) → Peat → is sourced from (waterlogging is the whole mechanism: a bog is anoxic below a few centimetres, so the organisms that would decompose the plant matter cannot work, and it accumulates instead) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
  • Smelting → takes as input (reduced with coke in a blast furnace) → Hematite

These are the most distinct paths back. Smelting can be traced through others besides.

Downstream — what it becomes

  • Smelting → produces (reduced from cassiterite at relatively low temperature) → Tin → is an input to (the addition that hardens copper into bronze) → Alloying and melting → produces (tin into copper — the first alloy anybody made deliberately) → Bronze → is used to make (cast in a two-piece mould with a core, which is what bronze does better than the copper it replaced) → Bronze socketed axe → is associated with → Bronze Age complete chain
  • Smelting → produces (reduced from the oxide that roasting molybdenite produces) → Molybdenum → is a source for (recovered from the flue dusts of molybdenum roasting; rhenium forms no ore of its own) → Rhenium → is a component of (two or three per cent, and essentially the only use rhenium has — most of world consumption goes into turbine blades) → Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is associated with (the jet engine created the superalloy industry, because no existing material survived the turbine inlet — and turbine temperature has risen about 500 °C since, almost entirely on the blade) → The wartime materials programmes complete chain
  • Smelting → produces (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Silicon → is an input to (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Czochralski process → produces (grown as a boule and sliced, though it is harder to keep stoichiometric than silicon because the arsenic evaporates) → Gallium arsenide → is used in (radio-frequency and optoelectronic devices, where silicon's indirect band gap and lower carrier mobility are the limits) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Smelting → produces (as blister copper, refined electrolytically afterwards) → Copper → is a component of → Bronze → is used to make (cast in a two-piece mould with a core, which is what bronze does better than the copper it replaced) → Bronze socketed axe → is associated with → Bronze Age complete chain
  • Smelting → produces (tapped directly from the blast furnace, before the carbon is removed to make steel) → Cast iron → is an input to (as pig iron tapped from the blast furnace, carrying the carbon the process removes) → Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain
  • Smelting → produces (as pig iron, high in carbon and brittle until refined) → Iron → is a component of → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain

These are the most distinct paths onward. Smelting ends up in others besides.