Which of the following pair/pairs of metal/metals can be extracted from their ores by using cyanide complex solutions ?
(A) Cu and Au
(B) Co and Ag
(C) Au and Ag
(D) Na and Hg
Answer options :

Question

UnlockIAS · Accepted Answer

Correct — option (2), 'Only (C)', the pair gold and silver. Cyanide leaching is the classic hydrometallurgical route for the two noble metals and for no others in this list. The ore, crushed to a fine powder, is treated with a dilute solution of sodium or potassium cyanide in the presence of air; the metal dissolves as a soluble cyanide complex — the dicyanoaurate ion for gold and the dicyanoargentate ion for silver — and the barren rock is filtered off. The dissolved metal is then recovered by adding zinc dust, which is more electropositive and displaces gold or silver from the complex in a simple displacement reaction, leaving a tetracyanozincate in solution. Oxygen from the air is essential to the dissolution step, which is why the leach liquor is aerated. The process is named after MacArthur and the Forrest brothers, who patented it in the late nineteenth century, and it transformed gold mining by making low-grade ores workable. Two features explain why it is confined to these metals: gold and silver occur largely in the native state, finely disseminated in rock, so a chemical route that dissolves them selectively out of a huge mass of gangue is exactly what is needed; and both form unusually stable complexes with the cyanide ion, which is what makes the selective dissolution possible in the first place. Statement (C) names precisely that pair, so option (2) is the answer.

Why the other options are wrong — (a) This option would require copper and cobalt to be cyanide-leached metals, and neither is. Copper is obtained mainly from sulphide ores by concentration, roasting and smelting to a matte followed by self-reduction in a converter, and where a solution route is used for low-grade material it is an acid leach, not a cyanide one. Cobalt is recovered chiefly as a by-product of copper and nickel processing, again by routes that have nothing to do with cyanide. The option is built to catch a candidate who remembers only that gold appears in the correct answer and therefore accepts any pair containing it — statement (A) does contain Au, but it drags copper along with it.  (c) Sodium and mercury are extracted by routes as far from cyanide leaching as it is possible to get. Sodium is so reactive that it cannot be obtained from an aqueous solution at all; it is produced by the electrolysis of fused sodium chloride in the Down's process, since any water present would be reduced in preference to the sodium ion. Mercury is obtained from its sulphide ore, cinnabar, simply by roasting in air, the metal distilling off and being condensed. A candidate choosing this option has usually noticed that both are unusual metals and assumed that an unusual method must apply to them.  (d) This option identifies the right pair and then adds a wrong one, which is the standard near-miss construction in a statement-list question. Statement (C), gold and silver, is correct, but statement (D), sodium and mercury, is not: sodium comes from the electrolysis of its fused chloride and mercury from the roasting of cinnabar. Adding a false statement to a true one is as fatal as missing the true one, and the discipline that prevents it is to settle every printed statement independently before reading any option.

Concept: Extraction metallurgy chooses its method by the reactivity of the metal and the chemistry of its ore, and the whole subject can be held as three broad routes. The most reactive metals — sodium, potassium, calcium, magnesium, aluminium — are won by the electrolysis of a fused salt or oxide, because nothing chemical is a strong enough reducing agent and because water would be reduced first in any aqueous route; sodium from fused sodium chloride and aluminium from alumina dissolved in molten cryolite are the standard examples. Metals of middling reactivity, whose ores are oxides or are first converted to oxides by roasting or calcination, are obtained by reduction with carbon or carbon monoxide in a furnace, iron and zinc being the classic cases. The least reactive metals need the least work: mercury is obtained by simply roasting cinnabar, and gold and silver, which occur largely uncombined, are recovered by dissolving them selectively in cyanide solution and then displacing them with zinc. That last route, leaching, belongs to hydrometallurgy, in which the metal is taken into solution rather than melted, and it is used wherever the valuable metal is thinly scattered through a great bulk of worthless rock. Its price is environmental: cyanide is acutely toxic, and the safe management of leach liquor and tailings is a standing concern wherever gold is mined.

State PCS 2024 Q67 — Science & Technology PYQ with Answer

State PCS 2024 Q67 — Science & Technology PYQ with Answer