Calcination

What Is Calcination?

Calcination, in simple words, can be described as a process of heating some solid material or a substance in a controlled environment. Usually, in the process, the temperature is also regulated. Calcination is done to bring about some change in the substance’s physical or chemical constitution.

During calcination, solids are heated at high temperatures. This is done to mainly remove volatile substances and water or oxidise the substance. This process is also known as the purification process. The word Calcination has been derived from the Latin word Calcinare, which translates as “to burn lime”.

Calcination Examples

A typical calcination process involves the conversion of calcium carbonate to calcium oxide.

Some other common examples of calcination include,

• Calcination of limestone involving decomposition of carbonate ores and removal of carbon dioxide.
• Calcination of bauxite and gypsum involving the removal of water of crystallisation in the form of water vapour.
• Decomposition of volatile components from raw petroleum coke.
• Obtaining rutile from anatase or devitrification of glass materials.
• Synthesis of zeolites, where ammonium ions are removed.
• Anhydrous alumina is left behind when bauxite is calcined, and water is removed.

Al₂O3.2H₂O → Al₂O3 + 2H₂O

• Anhydrous iron is formed during iron calcination.

2Fe₂O₃.3H₂O → 2Fe₂O₃ + 3H₂O

In metallurgy, metals are usually obtained through the reduction or electrolysis of their respective oxides. However, the ores of many metals are not available as oxides of the metal. Many ores are obtained as carbonates or sulfates of the metal. In such scenarios, the only way to extract metal from the ores is to process it first and then apply the reduction. The process that is applied is calcination. This process is followed to thermally decompose the sulfate or carbonate ores into the oxides of their metal.

Alternatively, there is another process that is used to convert the ores to their respective oxides, and it is called roasting. Unlike calcination, in roasting, the ores are converted into its oxides by heating them above their melting point in the presence of excess oxygen. We will be discussing the major differences between calcination and roasting in the following paragraphs.

Calcination Process Description

Calcination is usually carried out in furnaces, retorts or kilns, and often, materials are racked over or stirred to make sure the product is uniform. One of the common arrangements that is used for calcination is the reverberatory furnace. The typical construction and process are explained below.

There are different forms of construction for reverberatory furnaces, but in all cases, the flames and hot gases from the fire come in direct contact with the material to be calcined; however, the fuel is separated from it.

In the above figure, the fire burns on the grate at G. Now, the flames passing over the bridge at E are deflected downward by the low-sloping roof of the furnace and pass directly over the surface of the charge or the material under calcination, which is laid on platform B. The fumes and hot gases then escape through throat F into the chimney. The charge is spread out evenly on the bed as a thin layer.

Difference between Calcination and Roasting

The below-given table highlights some major differences between calcination and roasting.

Calcination Reactions

Limestone Calcination

During the calcination of limestone, it follows a decomposition chemical reaction.

CaCO₃ → CaO + CO₂ (g)

We can take the standard Gibbs free energy of the reaction as ΔG°r = 177,100 − 158 T (J/mol). Meanwhile, the standard free energy of the reaction = 0 when the temperature is set at 1121 K, or 848 °C.

Calcination of Gypsum

Gypsum or calcium sulfate dihydrate (CaSo₄.2H₂O) is a mineral of calcium that is mined in various parts of the world. It is also formed as a by-product of flue gas desulphurization in some coal-fired electric power plants. There are different kinds of Gypsum, and they are listed below:

• Alabaster – pure white, fine-grained and translucent gypsum.
• Satin spar, gypsum with fibrous needle-like crystals with a silky lustre.
• Selenite, colourless and transparent crystals.

Gypsum is used in many fields. Some major domains are listed below:

• Building construction
• Soil conditioning
• Food additives
• Pharmaceuticals
• Medical devices

Production of Gypsum Products

Now, let us see what will happen if we let gypsum undergo calcination. Gypsum, when subjected to calcination, creates different products that are industrially important. Some of these include the Plaster of Paris, dental stone, etc.

Initially, the gypsum is subjected to temperatures of 110 degrees Celsius to 130 degrees Celsius (230 degrees Fahrenheit to 266 degrees Fahrenheit) in open containers to drive off part of the water for crystallisation.

The reaction is represented as follows:

CaSo_4.2H₂O → CaSo₄.½ H₂O

The resulting substance is called the plaster of Paris or dental plaster. The compound is a fibrous aggregate of fine crystals with capillary pores. Plaster of Paris is a quick-setting cement constituting fine white powder. It hardens when moistened and dried.

It acquires its distinctive name because of the abundance of gypsum near Paris. Plaster of Paris is a more reliable plaster in the sense that it will not shrink or crack when solidified. It is widely used in sculpture industries and medical institutions.

Now, when the temperature is further increased, the rest of the water of crystallisation will vanish, and the compound becomes anhydrous. This process is called calcination.

CaSo₄.½ H₂O → CaSo_4.

When gypsum is heated in a vat, kettle, or rotary kiln that sustains a wet environment, a crystalline hemihydrate is formed. This substance is called the dental stone, which has a rod or prism crystalline structure.

Frequently Asked Questions

1. Explain the purpose of calcination.

Answer:

Metallurgy