Smelting, Flux and Slag Formation
Smelting is the high-temperature stage that actually reduces a roasted or calcined ore to the crude metal, and flux is the reagent added specifically to remove the remaining gangue as a separate, low-melting liquid layer called slag — the details of exactly what that flux and slag are made of are recurring exam facts.
A phase change alters the arrangement and motion of particles without creating a new substance.
Particles change their freedom of movement
Heat → particles move more freely
Heating increases particle motion: solid → liquid → gas. During a phase-change plateau, energy separates particles instead of raising temperature.
Flux removes gangue by forming slag
Flux is a substance added during smelting specifically to react with the gangue (the silica-rich, unwanted rocky material still present after roasting/calcination) and convert it into a low-melting, fusible compound called slag, which then separates as a distinct liquid layer (usually floating above the denser molten metal) and can be physically drained off. Flux is not there to make the ore porous, to directly facilitate reduction, or to facilitate oxidation — its one job is removing gangue by converting it to slag.
Copper extraction: silica as the flux, iron silicate as the slag
In copper extraction from copper pyrites (CuFeS₂), silica (SiO₂) is added as the flux specifically to react with the iron oxide gangue (formed during roasting) rather than with the copper compounds: FeO + SiO₂ → FeSiO₃ (iron silicate), which is the slag that separates and is removed, leaving the copper-containing matte behind. When copper ore is heated with silica in a reverberatory furnace, the remaining copper matte that forms (the molten mixture left after the iron silicate slag separates) is specifically a mixture of copper(I) sulfide and iron(II) sulfide, Cu₂S and FeS — not copper(II) or iron(III) species, since the sulfide environment favours these lower, more sulfide-stable oxidation states.
Iron purity grades from the blast furnace onward
Iron extracted from the blast furnace (pig iron, roughly 4% carbon plus other impurities) is progressively purified in later stages: cast iron (re-melted pig iron, still around 2-4% carbon, brittle) is one further-processed form, but WROUGHT IRON is the purest commercially available form of iron, produced by further oxidising away almost all the remaining carbon and other impurities from cast/pig iron (down to well under 1% carbon), giving a much more malleable, tough material historically used for items like chains, horseshoes and decorative ironwork, precisely because of this very low impurity content.
OlympIQ exam lens
- ✓Flux is added during smelting specifically to convert gangue into slag, a low-melting liquid that separates from the metal
- ✓In copper extraction, silica is the flux (reacting with FeO gangue), and iron silicate (FeSiO₃) is the resulting slag
- ✓Copper matte from a reverberatory furnace is a mixture of Cu₂S and FeS (copper(I) and iron(II) sulfides)
- ✓Wrought iron, not cast iron or pig iron, is the purest commercially available form of iron, with the least residual carbon and impurities
Do not assume pig iron or cast iron is the purest form of iron just because pig iron is the first product out of the blast furnace. Both still carry substantial carbon and other impurities — wrought iron, produced by further oxidising those impurities away, is the purest commercially available form.
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