Calcination and Roasting

Introduction

Calcination and Roasting are two processes that convert ores into their oxide form. Ores are a natural substance in the earth's crust. These ores concentrate on various minerals and valuable metals. Metals extracted from ore are required by applying lots of heat either in the presence or absence of oxygen. The conversion of ores to oxides comes under processes like calcination and roasting. Here is the step-by-step diagram for converting ores into oxide and obtaining pure metals.

Raw ore is needed to convert into oxide. During this process of conversion, it expels volatile substances and gas. This removal of volatile impurities in the solids and gas forms results in the extraction of metals from its oxides obtained from ores. Furthermore, metal purifies by electrolytic refining.

What is Calcination?

It is a process of converting carbonate ore into oxide below its melting point by the action of heat in the absence of air or a limited supply of oxygen. It is also known as the thermal decomposition process because it decomposes the ore or solid substance without altering the chemical properties and only removes volatile and organic impurities.

For example −

$$\mathrm{ZnCO_3 →ZnO +CO_2}$$

Define Calcination

Calcination is a thermal or heat process that occurs when a solid substance like carbonate ore is subjected to high temperatures but strictly under its melting point in an absence or limited supply of oxygen. Calcination word is obtained from Calcinare which means 'to burn lime'. Therefore, the most commonly used ore limestone (calcium carbonates), in the absence of air or oxygen at a temperature of 900 to 1050 °C, produces quicklime (calcium oxide). The following reaction is −

$$\mathrm{CaCO_3 (s)→ CaO(s) + CO_2 (g)}$$

Purpose of Calcination

Calcination is the process of purifying ores. Heating the ores or solids at a high temperature below their melting point leads to the decomposition or removal of volatile impurities, moisture and water, and organic matter. For example −

• Removal of carbon dioxide from carbonated ores.

• Removal of hydrated molecules from bauxite and gypsum. The reaction is as follows −

  $$\mathrm{Al_2 SO_3.2H_2 O →Al_2 O_3 +2H_2 O }$$

• Removal of volatile liquids from petroleum and coke.

• Removal of ammonium ions in the preparation of zeolites.

What is Roasting?

It is a process of converting mainly sulphide ores into their respective metal oxides when subjected to heat in the presence of air or oxygen. It is one of the processes used in metallurgy.

Define Roasting?

Roasting is the heat process of converting sulphide ores into metal oxides below their melting point over the presence of air or oxygen. The conversion of ore into oxides changes the chemical properties of the solid ores and forms a new product with the removal of impurities. The roasting of zinc sulphide into zinc oxide and the reaction is as follows −

$$\mathrm{2ZnS + 3O_2→2ZnO +2SO_2}$$

Purpose of Roasting

Roasting process − It is conducted at an elevated temperature in the presence of oxygen and converting sulphide ores into oxides. It is used in the metallurgy process to obtain metals or their oxides by removing metallic, non-metallic, toxic, and moisture impurities in the form of volatile substances in the ores. During conversion the following impurities are removed −

• Removal of sulphur from sulphide ores.

• Removal of phosphorus and silicon present in flux. Flux is added to remove impurities as slag.

Differentiate between Roasting and Calcination

Roasting	Calcination
Ores are heated in the presence of an excess of oxygen or air.	Ores are heated in the absence or limited supply of oxygen or air.
This method is employed for sulphide ores.	This method is employed for carbonate ores.
In this process, oxygen is reacted with sulphide ores.	In this process, decomposition takes place only and oxygen does not take part in the reaction.
Toxic impurities are expelled out.	Organic matter and water are expelled out as impurities.
Sulphur dioxide is produced along with metal oxide.	Carbon dioxide is produced along with metal oxide.
It is done in a reverberatory furnace. The holes in the furnace were kept open for the entry of oxygen or air.	It is also done in a reverberatory furnace. The holes in the furnace were kept closed.

Why Does Calcination Occur in the Absence of Oxygen?

Process of Calcination − The naturally occurring ores (carbonate ores) undergo this process to form into their oxide and carbon dioxide. It needs to be done in the absence of oxygen under high temperatures as it only decomposes to form metal oxides and carbon dioxide as the presence of oxygen leads to the formation of new products. For example, zinc carbonate (Calamine) is subject to heat and decomposes to form zinc oxide and carbon dioxide.

$$\mathrm{ZnCO_3 (s)→ ZnO(s) + CO_2 (g)}$$

Therefore, calcination is always done in the absence of oxygen.

Conclusion

Roasting and calcination methods are applicable for the extraction of metals. The ores or minerals containing metals are not always present in the oxide form, in that case, they are subjected to undergoing roasting and calcination processes. Both processes change the ore into their oxide form. This makes the extraction process easier. These undergo changes only when high heat is applied. However, roasting takes place in the presence of oxygen and calcination takes place in the absence of oxygen. Roasting gives new products during thermal decomposition while calcination decomposes the solid substance. Both give metal oxides which are further reduced to metals.

FAQs

1. What type of reaction is calcination?

Calcination is a thermal decomposition process. In this process, solid substances are subjected to high heat in the absence of air to bring the phase transition. This helps solid substance changes to liquids and gas impurities which are easily removed by this process.

2. Is calcination endothermic or exothermic?

It is a process of heat that applies to the ores to convert them into their respective metal oxides. Hence it is an endothermic process.

3. Is calcination a physical process?

It is a process of decomposition where solids substances are broken down when high heat is applied. During this process, no new products are formed. Therefore, it is only a physical change or a physical process.

4. How are the calcination reactions done?

Reactions to calcination take place in retorts and furnaces. In this, the ores or solid substances are stirred to make a uniform product.

5. Why is roasting and calcination done below the melting point?

If ores heat above their melting point, they will melt and mix with carbonate and sulphur impurities that are difficult to separate.

6. What types of minerals are subjected to roasting and calcination processes?

For roasting sulphide ores and for calcination carbonate ores.