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 :
- (1)Both (A) and (B)
- (2)Only (C)
- (3)Only (D)
- (4)Both (C) and (D)
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.
- (1)Both (A) and (B) — 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.
- (3)Only (D) — 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.
- (4)Both (C) and (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.
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.
MPSC's chemistry section asks metallurgy as method-to-metal recall — which metal is obtained by electrolysis, which by carbon reduction, which by leaching, and what the named processes are called. The efficient preparation is a single table with the metal, its principal ore and its extraction route, because that table answers almost every question the Commission has asked on the subject. This item adds the statement-list packaging that runs through the whole 2024 paper, and it is worth noticing that its labels are printed in upper case, (A) to (D), while neighbouring questions use lower case — the paper is not consistent, so each question's own labelling must be read as printed. The distractor design is also typical: the correct pair is offered alone in one option and again in combination with a false pair in another, so the mark turns on the discipline of judging each statement separately.
- Gold and silver are extracted by cyanide leaching, in which the metal dissolves as a soluble cyanide complex in dilute sodium or potassium cyanide solution in the presence of air.
- The dissolved metal is recovered from the leach liquor by displacement with zinc dust, zinc being the more electropositive metal.
- The cyanide process is known as the MacArthur-Forrest process and made the working of low-grade gold ores economic.
- Sodium is obtained by the electrolysis of fused sodium chloride in the Down's process, not from any aqueous solution, because water would be reduced in preference to the sodium ion.
- Mercury is obtained by roasting its sulphide ore, cinnabar, in air and condensing the metal vapour.
Two things confine cyanide leaching to the noble metals: they occur largely in the native state, finely disseminated through rock, so what is needed is a solvent that picks them selectively out of a huge mass of gangue — and both form unusually stable complexes with the cyanide ion. Oxygen from the air is essential to the dissolution, which is why the leach liquor is aerated.
- Accepting any option that contains gold without checking the metal paired with it
- Assuming a reactive metal such as sodium can be obtained from an aqueous solution; water is reduced first, so a fused-salt electrolysis is required
- Adding a false statement to a true one in a statement-list question, which loses the mark as surely as missing the true one
- Confusing the roasting of a sulphide ore with leaching; mercury needs only roasting, gold needs a solvent
Metallurgy questions in MPSC papers name a metal and ask for its ore or its extraction method, or name a process and ask which metal it belongs to. The named processes worth holding are the cyanide or MacArthur-Forrest process for gold and silver, the Bayer process and the Hall-Heroult cell for aluminium, the Down's process for sodium and the blast furnace for iron. Statement-list packaging of the kind used here changes nothing about the chemistry required but doubles the number of facts that must be settled, so a table learnt in advance is worth far more than reasoning at the desk.
No directly related past PYQ was found.
- practice — not a real PYQ
In the cyanide process for the extraction of gold, the metal is recovered from the leach solution by adding :
- (a)Zinc dust
- (b)Concentrated sulphuric acid
- (c)Sodium hydroxide
- (d)Coke
Answer(a) Zinc dust — zinc is more electropositive than gold and displaces it from the soluble dicyanoaurate complex, so the gold precipitates and can be filtered off. Coke is the reducing agent of furnace metallurgy, not of a leach liquor, and neither the acid nor the alkali would recover the metal.
- practice — not a real PYQ
Sodium cannot be obtained by the electrolysis of an aqueous solution of sodium chloride because :
- (a)Sodium chloride does not dissolve in water
- (b)Water is reduced at the cathode in preference to the sodium ion
- (c)Chlorine is not liberated at the anode
- (d)Sodium reacts with the electrode material
Answer(b) Water is reduced at the cathode in preference to the sodium ion — hydrogen is discharged instead of sodium, so the metal must be won from the fused salt rather than from its solution. That is why sodium is produced by the electrolysis of molten sodium chloride in the Down's process.