Which of the following corundum and cryolite are important ores ?

Question

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Correct — B, Aluminium. Read the stem as "of which of the following are corundum and cryolite important ores?" — the English is inverted, but the chemistry is not in doubt. Corundum is crystalline aluminium oxide, Al2O3: the same compound as alumina, hardness 9 on the Mohs scale (second only to diamond), and the mineral whose coloured varieties are ruby, tinted red by traces of chromium, and sapphire, tinted blue by iron and titanium. Cryolite is sodium hexafluoroaluminate, Na3AlF6 — again an aluminium compound, historically mined at Ivigtut in Greenland until that deposit was worked out, and now made synthetically because industry cannot do without it. Both minerals therefore contain aluminium and nothing else in the option list. Cryolite's role is the more interesting one: in the Hall-Héroult process, discovered independently in 1886 by Charles Martin Hall in the United States and Paul Héroult in France, purified alumina is dissolved in a bath of molten cryolite at roughly 950-980 degrees Celsius. Alumina on its own melts only above 2,000 degrees Celsius, which would make electrolysis impossible; dissolving it in cryolite drops the working temperature below 1,000 degrees and makes the melt electrically conducting, and molten aluminium collects at the carbon-lined cathode. Standard Indian textbook lists of aluminium ores name bauxite, cryolite, corundum, alunite and kaolinite together, which is the list this question is drawn from — though in commercial practice bauxite is the ore actually smelted, and cryolite is the electrolyte rather than a feedstock.

Why the other options are wrong — (a) Silver's ores are sulphides and halides — argentite/acanthite (Ag2S) and horn silver or chlorargyrite (AgCl) — and most of the world's silver is actually recovered as a by-product of lead, zinc and copper smelting. Neither an oxide like corundum nor a fluoride like cryolite has any silver in it.  (c) Iron's ores are haematite (Fe2O3), magnetite (Fe3O4), limonite and siderite (FeCO3). Haematite is an oxide, which is the resemblance that traps candidates who half-remember that corundum is 'an oxide' — but corundum's oxide is of aluminium, and iron in corundum appears only as a colouring impurity in sapphire.  (d) Tin has effectively one commercial ore, cassiterite (SnO2), also called tinstone; in India it is worked mainly in the Bastar region of Chhattisgarh. Neither corundum nor cryolite is associated with tin, and tin metallurgy is a carbon-reduction smelt, not an electrolytic process requiring a fluoride bath.

Concept: An ore is a mineral from which a metal can be extracted profitably — every ore is a mineral, but not every mineral of a metal is its ore. Aluminium illustrates the distinction better than any other element. It is the most abundant metal in the Earth's crust at about 8 per cent, and the third most abundant element after oxygen and silicon, yet it was a precious curiosity until the 1880s because its oxide bond is so strong that carbon reduction, the route used for iron, will not break it. Two discoveries changed that: the Bayer process (Karl Josef Bayer, 1888), which digests bauxite in hot caustic soda to yield pure alumina, and the Hall-Héroult electrolytic process of 1886, which reduces that alumina in a cryolite bath. Bauxite itself is not a single mineral but a weathered lateritic mixture of gibbsite, boehmite and diaspore.

State PCS 2024 Q62 — Science & Technology PYQ with Answer

State PCS 2024 Q62 — Science & Technology PYQ with Answer