Differences between the Process of Calcination and Roasting.

We get metals from ores of oxides by electrolysis, also called reduction. The ores of oxides can be reduced, but that of carbonates and sulphides is more difficult. These ores can be changed to a metallic state only when the carbonates and sulphides are converted to the ore of an oxide. Either calcination or roasting carries out the process of conversion. 

What is Calcination

Calcination is converting ore to the state of oxide by solid heating. The particular ore needs to be warmed below the melting point range in either a controlled supply or no presence of air. This method is used to get oxides after converting hydroxides and carbonates into them. The evaporative and moist kinds of impurities are also removed in the process. Calcination can also be defined as a thermal process used to change ores and similar solid materials by thermal decomposition. It has been derived from the Latin word ‘calculate,’ which means ‘to burn lime.’ So we can deduce the fact here that calcination is generally used to decompose limestone(calcium carbonate)into carbon dioxide and lime(calcium oxide).

Or symbolically, it is—

CaCO3 → CaO + CO2

The end products obtained from the process are called Calcining.

Roasting

Roasting is the method of metallurgy in which the ore gets changed into its corresponding oxide by warming it up in a large amount of air above its melting point. The calcination process is mainly seen in the oxidising of carbonates, while that of roasting can change the ores of sulphides.

In the method of roasting, the moist impurities of non-metals are released in vaporous gas. The process of roasting has the thermal reaction of solid to gas which includes the processes of oxidation, reduction, sulfation, chlorination, and pyro hydrolysis. However, roasting sulphides causes much pollution of the environment and air and releases a massive amount of acidic, toxic, and metallic compounds. 

Roasting can be seen in the form of an equation as—

2ZnS + 3O2 → 2ZnO + CO2

Importance and use of Calcination

The process of calcination is mainly used to decompose any substance thermally. A substance is heated below its melting point to a higher temperature. The heating at higher temperatures assists in removing volatile substances while oxidising any substance. So calcination is a process of purification of metals. Calcination finds its use in oxidising carbonate compounds.

Process of Calcination

The process of calcination is carried out in furnaces, retorts, and kilns. One of the most famous and frequently used arrangements for calcination is the reverberatory furnace. These reverberatory furnaces are constructed in many ways. The flames and hot gases from the fire come in close contact with the calcining material, but the fuel is kept unaffected by it.

Use of Roasting

Roasting is heating any substance below its melting point in an excessive amount of air or oxygen. Roasting is mainly used to convert sulphide ores.

Types of roasting

The types of roasting are—

1. Volatilizing roasting

2. Oxidising roasting

3. Magnetic roasting 

4. Sulphur roasting

Difference between the process of calcination and roasting

Calcination

1. Calcination is the method where ore is heated in the absence of air.

2. Calcination is the thermal decomposition of ores of carbon.

3. All the moisture is removed from the ore in calcination.

4. Carbon dioxide is given out in the process of calcination.

Roasting

1. The roasting method involves heating the ore at a temperature lower than its melting point in the air.

2. Roasting is generally done for sulphide minerals.

3. The roasting process does not lead the ore to get dehydrated.

4. The process of roasting leads to the release of acidic, metallic, and toxic compounds.

Differentiating between the processes of calcination and roasting with the help of an example:

The process of Calcination uses twice the temperatures used for roasting. In this process, high temperatures beyond 1000℉ are used to break down the substance being heated quite quickly. Calcination is not used for edible items. The roasting method tries to modulate the food item resulting in the coming of fresh and pleasant flavours, more availability of specific nutrients, destruction of toxins, and drying. The roasting process leads to the start of wanted chemical reactions like the Maillard Reaction Series to continue till some time to convert the chemicals of the substance into some flavorful thing and finally stop. For example, when coffee is roasted, it is not put in a vessel at x degree for some time and then taken out. The different varieties of coffee need different temperature profiles generally by changing the vessel’s temperature quite some times during the process of roasting. Moreover, if calcination is used for coffee, we will get ashes instead of coffee. 

Reason for the occurrence of calcination in oxygen’s absence 

Calcination is carried out to get the ores of carbon. During calcination, ores are strongly heated in the oxygen’s absence. It is done to convert the carbonates of metals into oxides of metals and carbon dioxide. There will be no reaction to heating these ores in oxygen’s presence, and the whole method will be useless. 

ZnCO3 → ZnO+CO2

ZnCO3 + O2 → No reaction will be seen.

The resulting oxides can be further reduced to the same state by smelting.

Conclusion

The result of both the thermal processes of roasting and calcination is the same, but they are two different methods used to get metals from their respective ores. After the end of the two processes of calcination and roasting, an ore is changed into an oxide ore. In roasting, the ore is heated in the presence of air which leads to reactions from the gas to the solid-state taking place at high temperatures. On the other hand, calcination is heating the ores in a limited supply or availability of air or oxygen.