What is Calcination and Roasting?
Metals are usually obtained from oxide ores after going through the electrolysis or reduction process. While oxide ores are easy to reduce, it is not the same with carbonates and sulphides. These ores are turned to metals only after converting sulphides and carbonates to an oxide ore. So how is it done? Well, mostly, the conversion is done through the processes of roasting or calcination. So, here, we will learn about roasting and calcination and try to understand various topics related to them.
Define Calcination and Roasting?
What is Calcination?
As per the traditional definition, calcination can be defined as the process of converting ore into an oxide by heating it strongly. The ore is heated below the melting point either in limited supply or absence of air. We use this method in common for converting hydroxides and carbonates to their respective oxides. During calcination, the volatile and moisture impurities are also removed. We can also describe calcination as a thermal process that can be used to convert ores and other solid materials by bringing about a thermal decomposition. In calcination, most of the time, the reaction occurs at the thermal decomposition tempurature or above it.
Calcination is derived from the Latin word "calculate," which translates "to burn lime." Thus, calcination is mostly used in the decomposition of limestone (calcium carbonate) to carbon dioxide and lime (calcium oxide).
CaCO3 --> CaO + CO2
Whereas, the products derived from calcination are referred to as calcining, and it is regardless of the actual compounds undergoing thermal treatment.
What is Roasting?
When we discuss roasting, basically, it is a process of metallurgy where the ore is converted into its oxide by heating it in the presence of excess air above its melting point. While calcination is the process mostly used in the oxidation of carbonates, roasting is a method that can be used for converting the sulphide ores.
Whereas, during roasting, the non-metallic and moisture impurities in the form of volatile gas are released. The roasting process contains the solid-gas thermal reaction that includes reduction, oxidation, chlorination, sulfation, and also pyro hydrolysis.
However, roasting that involves the sulphides is a major source of air pollution, and the major drawback of this process is, it releases a large amount of metallic, toxic, and acidic compounds as well, which causes harm to the environment.
An example of roasting can be given as when Zinc sulphide is converted into zinc oxide.
2ZnS+3O2 --> 2ZnO + CO2
Differentiate between Roasting and Calcination
A few of the major differences between calcination and roasting are tabulated below..
Explaining the Differences Between Calcination and Roasting using an Example
Calcination typically uses the temperatures roughly twice compared to typical roasting. The idea is to use high temperatures (in excess of 1000℉) and decompose the material that is being heated fairly quickly. Simply we can put, calcination setups are not used for the food products.
The roasting process tries to change the food product by starting and supporting numerous chemical reactions within the food that result in pleasant flavors, greater availability of particular nutrients (or conversely, the toxins or nasty tasting flavor element destruction), and drying of the product. Usually, roasting is done so that the desired chemical reaction, such as the Maillard reaction series (say), starts gently, continues long enough to convert enough of the chemicals of the product into the good flavored stuff, and then stops.
For instance, in roasting coffee, it is not simply placed into a drum at a temperature x for y number of minutes and then dumped out. Each variety of coffee requires different temperature profiles, usually changing the temperature of the drum many times during the length of the roast.
If we used calcination for coffee, we'd end up with ashes.
Why Does Calcination Occur in the Absence of Oxygen?
Calcination is done for the Carbonate Ores. In the calcination process, the ores are heated strongly in the absence of oxygen (air). This is done so as to convert the Metal Carbonates into Carbon Dioxide and Metal Oxides. If these ores are heated in the presence of oxygen or air, no reaction will take place, and the entire process will become vain.
ZnCO3 → ZnO + CO2
ZnCO3 + O2 → No reaction can take place.
These Oxides can be further reduced to oxides by smelting.