CDS (II) 2021 General Knowledge · Question 11 of 120

Correct — B, Roasting. The stem is the textbook definition word for word: sulphide ores are converted into oxides by heating strongly in the presence of excess air, and that process is called roasting. Zinc blende is the standard illustration — zinc sulphide heated in a current of air gives zinc oxide and sulphur dioxide. The reason this step exists at all is that a metal is far easier to recover from its oxide than from its sulphide or its carbonate, so ores of the moderately reactive metals in the middle of the activity series, such as iron, zinc, lead and copper, are turned into oxides first and reduced afterwards with carbon. Two words in the stem do the sorting between roasting and its twin. 'Sulphide' and 'excess air' point to roasting; a carbonate ore heated strongly in limited air is calcination, which drives off carbon dioxide and leaves the oxide behind.

• (a)Calcination — The companion process, applied to carbonate ores and carried out in limited air rather than excess air. Heating zinc carbonate this way gives zinc oxide and carbon dioxide, with no sulphur involved.
• (c)Smelting — Comes later in the sequence. Smelting is the reduction of the oxide to the metal, usually by heating with carbon, so it acts on the product of roasting rather than producing it.
• (d)Incineration — A waste-management term for burning refuse, not a metallurgical operation. It has no place in the standard ore-to-metal sequence.

Turning an ore into a metal runs through three stages — concentrating the ore, converting it to the oxide, and reducing that oxide to the metal. The middle stage has two named routes chosen by the chemistry of the ore. Roasting heats a sulphide ore strongly in excess air, giving the oxide and sulphur dioxide. Calcination heats a carbonate ore strongly in limited air, giving the oxide and carbon dioxide. Both produce the same kind of intermediate for the same reason: an oxide gives up its metal more readily than a sulphide or a carbonate does.

Two of the four options are real metallurgical processes and the item turns on telling them apart, so the safe method is to read the stem for its two clues. The ore type comes first — sulphide points to roasting, carbonate to calcination — and the air supply confirms it, since excess air is what oxidises sulphur to sulphur dioxide while limited air merely supplies heat for decomposition. A useful memory hook is that calcination shares its root with calcium carbonate: limestone heated in a kiln to give quicklime is the oldest example of the process. Smelting sits one step further down the line and is worth keeping separate, since a question that names carbon or coke as the other reagent is asking about reduction, not about oxide formation.

• Sulphide ores are converted into oxides by heating strongly in the presence of excess air; this is roasting.
• Carbonate ores are changed into oxides by heating strongly in limited air; this is calcination.
• Roasting zinc blende gives zinc oxide and sulphur dioxide; calcining zinc carbonate gives zinc oxide and carbon dioxide.
• Metals in the middle of the activity series — iron, zinc, lead and copper — usually occur as sulphides or carbonates and go through this oxide stage.
• The oxide is then reduced to the metal with carbon, or with a highly reactive metal such as aluminium in the thermit reaction.

• Swapping roasting and calcination because both involve strong heating; the ore type and the air supply are what separate them.
• Calling the whole ore-to-metal sequence smelting; smelting is only the reduction step.
• Assuming excess air always means combustion of a fuel — here it is the oxidant that converts sulphide to oxide.

As a define-the-process item like this one, as a match list pairing processes with industries, or as an equation-recognition question showing a sulphide or carbonate being heated.

• practice — not a real PYQ

  Heating a carbonate ore strongly in a limited supply of air to convert it into the oxide is called

  • (a)Roasting
  • (b)Calcination
  • (c)Smelting
  • (d)Refining

  Answer(b) Calcination — the carbonate decomposes to the oxide and carbon dioxide, without the excess air that roasting needs.

• practice — not a real PYQ

  Which gas is released when zinc blende is roasted in air?

  • (a)Carbon dioxide
  • (b)Hydrogen
  • (c)Sulphur dioxide
  • (d)Nitrogen dioxide

  Answer(c) Sulphur dioxide — the sulphur of the sulphide ore is oxidised while the zinc is left behind as the oxide.