Skip to content
Material World
Process

Roasting

Heating a sulfide ore in air to convert it to an oxide — the step that makes zinc and lead smeltable, and historically the largest source of industrial sulfur dioxide.

Most base metals occur as sulfides, and a sulfide cannot be reduced with carbon directly. Roasting fixes that by heating the ore in air until the sulfur burns off as sulfur dioxide and the metal is left as an oxide, which carbon will then reduce.

It is a preparatory step rather than a production step — nothing comes out of a roaster that anybody wants as a final product — but it is the hinge on which sulfide metallurgy turns.

Environmental impact

Roasting is where the sulfur in an ore body becomes somebody's problem. Released untreated, the sulfur dioxide produces acid rain, and the damage around historical smelters was severe enough to be visible from the air decades later.

The modern answer is to capture it rather than disperse it, feeding the gas to a contact-process plant and selling the sulfuric acid. That turns the principal pollutant of the industry into one of its products, and is the reason a smelter and an acid plant are so often found together.

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 · a copper sulfide; roasting drives off the sulfur that would otherwise prevent reduction
  • Pyrite — mineral · roasted for its sulfur as much as for its iron
  • Galena — mineral · roasted to the oxide, the sulfur going to an acid plant rather than the air
  • Sphalerite — mineral · roasted to zinc oxide before reduction or electrolytic winning
  • Cinnabar — mineral · roasted in air; the mercury leaves as vapour rather than staying behind as an oxide
  • Molybdenite — mineral · roasted to the oxide; the flue dust carries the rhenium
  • Pentlandite — mineral · a sulfide, roasted before reduction like the others
  • Cobaltite — mineral · roasted with care — the arsenic is what gave cobalt its reputation and its name
  • Stibnite — mineral · roasted to the oxide before reduction
  • Arsenopyrite — mineral · roasted to destroy the mineral and free the gold locked inside it, which is what releases the arsenic

produces

  • Zinc — element · as the oxide, which is then reduced — roasting is the step that makes zinc sulfide smeltable
  • Lead — element · as the oxide, from galena, by the same route
  • Rhenium — element · recovered from the flue dusts of molybdenite roasting; it forms no ore of its own
  • Mercury — element · roasting cinnabar drives the mercury off as a vapour, which is condensed — one of the few metals won directly by heating its ore in air
  • Arsenic — element · recovered as the trioxide from roasting arsenical ores, and reduced to the metal from that; the furnace yields the compound rather than the element
  • Cadmium — element · collected from the flue dust of zinc roasting and refined from it — a by-product of zinc throughout, never mined for itself

was succeeded by

  • Smelting — process · roasting converts the sulfide to an oxide; smelting then reduces the oxide to metal

succeeded

  • Froth flotation — process · the concentrate goes to the furnace; flotation moved the mineral, and roasting is where its chemistry is finally changed

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Roasting 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

  • Roasting → takes as input (a copper sulfide; roasting drives off the sulfur that would otherwise prevent reduction) → Chalcopyrite
  • Roasting → takes as input (roasted for its sulfur as much as for its iron) → Pyrite
  • Roasting → takes as input (roasted to the oxide, the sulfur going to an acid plant rather than the air) → Galena
  • Roasting → takes as input (roasted to zinc oxide before reduction or electrolytic winning) → Sphalerite
  • Roasting → takes as input (roasted in air; the mercury leaves as vapour rather than staying behind as an oxide) → Cinnabar
  • Roasting → takes as input (roasted with care — the arsenic is what gave cobalt its reputation and its name) → Cobaltite

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

Downstream — what it becomes

  • Roasting → produces (as the oxide, which is then reduced — roasting is the step that makes zinc sulfide smeltable) → Zinc → is an input to (the addition that makes brass; it boils below copper's melting point, which is the difficulty) → 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
  • Roasting → produces (as the oxide, from galena, by the same route) → Lead → is a source for (most silver reaches the market as a by-product of lead mining rather than from silver mines, recovered from the refinery's residues) → Silver → is a component of (a few per cent in the lead-free alloys, which measurably raised world silver demand when the transition happened) → Solder → is used in (and the move away from tin-lead under RoHS from 2006 is the most visible material change the industry has made) → 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
  • Roasting → produces (recovered from the flue dusts of molybdenite roasting; it 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
  • Roasting → produces (roasting cinnabar drives the mercury off as a vapour, which is condensed — one of the few metals won directly by heating its ore in air) → Mercury → is used as (the vapour discharge in fluorescent tubes, now being phased out) → Lighting
  • Roasting → produces (recovered as the trioxide from roasting arsenical ores, and reduced to the metal from that; the furnace yields the compound rather than the element) → Arsenic → is used as (as gallium arsenide, in infrared and laser diodes) → Lighting
  • Roasting → produces (collected from the flue dust of zinc roasting and refined from it — a by-product of zinc throughout, never mined for itself) → Cadmium → is used as (cadmium yellows and reds, now largely withdrawn on toxicity grounds) → Pigment

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