Metal is obtained from its ore by the process of electrolysis or reduction. In the process of reduction, the ore that is reduced is an oxide ore. Oxide ore is much easier to reduce as compared to its carbonates and sulphides. If the ore is not an oxide ore then first it is converted to an oxide ore by the process of calcination or roasting. In calcination, the ore is heated below its melting point either in the absence of the air or less amount of air supply while in roasting, the ore is heated strongly in the presence of air below its melting point.
What is Roasting?
It is a metallurgical process which involves gas-solid reactions at very high temperature with the aim of purify the metal from impurities. Roasting is a step of the processing of different ores, but before roasting the ore has already been partially purified from impurities by the process called froth floatation. Process of roasting consist of solid-gas thermal reaction which includes reduction, oxidation, sulfation, chlorination and pyro hydrolysis. In roasting, the ore or ore concentrate is heated in the presence of very hot air. Roasting is mostly done for sulfide minerals where sulfide is converted into oxide and the sulfur is changed into sulfur dioxide (gas) which is then released into the environment. It is the serious source of air pollution. The gaseous content of roasting that is sulfur dioxide is often used in making of sulphuric acid. Roasting is not used for dehydrating an ore. In roasting of sulfide ore, if the sulfur content from the ore is almost completely removed it results in a dead roast. The main disadvantage of this process is that it releases large amount of toxic, metallic and acidic compounds. These compounds causes harm to our environment.
What is Calcination?
In IUPAC, it is defined as “heating to high temperatures in the presence of air or oxygen”. But calcination is also defined as a thermal process in the absence of air or very limited supply of oxygen that is applied to ores and other solid materials to bring about thermal decomposition. Calciner is a cylinder made up of steel that rotates inside a heated furnace. Calciner performs indirect high temperature processing (550-1150 °C) within a controlled atmosphere. Calcination is derived from a Latin word called calcinare which means “to burn lime”. Calcination is most commonly applied in decomposition of calcium carbonate (lime stone) to calcium dioxide (lime) and carbon dioxide in order to make cement. Carbon dioxide is released into the environment. The products obtained from calcination is termed as calcine, regardless of actual compounds undergoing thermal treatment. In calcination, reaction takes place at or above the thermal decomposition temperature. Process of calcination can be used in driving out the moisture from an ore.
- In the process of roasting, ore is heated above its melting point in the presence of air or oxygen while in calcination, ore is heated above its melting point in the absence of air or very limited amount of oxygen supply.
- Roasting is mostly done for sulfide minerals while calcination is mostly applied in decomposition of carbonate ores.
- Roasting is not used for dehydrating an ore while calcination can be used to drive out moisture from an ore.
- In roasting, sulfur dioxide is released while in calcination carbon dioxide is released.