Roasting and Calcination: Turning Ores into Oxides
Most metals are not found pure in nature. They are locked inside compounds — usually sulphides, carbonates, or oxides. To get the metal out, you first need to break that compound down into a metal oxide, because oxides are much easier to reduce to the pure metal. Roasting and calcination are the two main ways to do this conversion.
The Intuition
Think of an ore as a metal atom that is "holding hands" with something else — sulphur or a carbonate group. You want to make it let go and hold hands with oxygen instead, because oxygen is easier to kick out later.
If the ore is a sulphide (metal + sulphur), you need to burn the sulphur away. That requires a lot of oxygen — you roast it in excess air.
If the ore is a carbonate (metal + carbonate group), the carbonate group breaks apart when heated, releasing carbon dioxide. You don't need extra oxygen for this; you just need heat. Too much air would be wasteful or could cause unwanted side reactions, so you use limited air.
That is the core difference: roasting needs oxygen to react with sulphur; calcination just needs heat to decompose the carbonate.
The Precise Definitions
Important
Roasting: Heating a sulphide ore strongly in the presence of excess air (oxygen) to convert it into the metal oxide, while sulphur is driven off as sulphur dioxide gas.
Calcination: Heating a carbonate ore (or hydrated ore) strongly in the limited supply of air (or in the absence of air) to decompose it into the metal oxide and carbon dioxide gas.
Roasting — The Sulphide Route
A typical sulphide ore is zinc blende, ZnS. When roasted in excess air:
2 ZnS+3 O2Δ2 ZnO+2 SO2↑
The sulphur atom leaves as SO2 gas, and the zinc is left behind as zinc oxide. The reaction is exothermic — it gives off heat, which helps sustain the process.
Roasting is not just about oxide formation. It also removes volatile impurities like arsenic and antimony, which burn off as their oxides. Sometimes, if the temperature is not controlled, a small amount of sulphate can form instead of the oxide — that is undesirable and is avoided by careful heating.
Watch out
A common mistake: students think roasting is just "heating strongly." It is specifically heating in excess air. If you heat a sulphide ore without enough oxygen, you get the metal sulphide back, not the oxide — or worse, you may get toxic fumes without proper conversion.
Calcination — The Carbonate Route
Take limestone, CaCO3, which is a carbonate ore of calcium. When calcined:
CaCO3ΔCaO+CO2↑
No oxygen from the air is needed. The carbonate ion (CO32−) is unstable at high temperature and simply falls apart. The calcium oxide (quicklime) is left behind, and carbon dioxide escapes.
Calcination is also used for hydrated ores — like removing water from bauxite (Al2O3⋅2H2O) before electrolysis. In that case, the water is driven off as steam. …
Roasting and calcination are both pretreatment steps that convert an ore into its oxide before reduction, but they differ in the type of ore treated and the role air plays. …
Roasting = heating a sulphide ore in excess air to the oxide (oxidation, gives SO2); Calcination = heating a carbonate/hydroxide ore in little/no air to the oxide (decomposition, gives CO2/H2O).
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2019Set HE1 markMCQ
Q.Carbon dioxide is released on heating lime stone ore, in metallurgy. This is called -
(a) Smelting
(b) Ore dressing
(c) Calcination
(d) Roasting
›Reveal solutionSolution
Calcination is the process of strongly heating a carbonate/hydrated ore, usually in absence or limited supply of air, to remove volatile impurities like CO₂ or H₂O without melting the ore.
Limestone (CaCO₃) on strong heating decomposes as:
CaCO₃ → CaO + CO₂↑
This loss of a volatile substance (CO₂) on heating an ore below its fusion point is specifically called calcination.
This is distinct from:
Roasting — heating an ore (usually a sulphide ore) strongly in excess of air/O₂, e.g. converting sulphides to oxides. …
Calcination converts a carbonate/hydrated ore into its oxide by strong heating in limited air.
Calcination is a metallurgical step used mainly for carbonate and hydrated ores. The powdered ore is strongly heated below its melting point, usually in a limited supply of air (to avoid oxidation reactions typical of roasting), which:
Removes moisture and volatile matter.
Expels water of crystallisation from hydrated ores.
Decomposes carbonates to oxides, releasing CO₂.
Makes the ore porous, which helps in subsequent reduction steps.