Usually, metals are obtained from oxide ores after going through reduction or electrolysis process. While oxide ores are easy to reduce it is not the same with sulphides and carbonates. These ores can be turned to metals only after converting carbonates and sulphides to an oxide ore. So how is it done? Well, the conversion is mostly done through the processes of calcination or roasting. So, here in this page we will learn about calcination and roasting and try to understand the various topics related to them.
What is Calcination?
As per the popular definition, calcination is defined as the process of converting ore into an oxide by heating it strongly. The ore is heated below its melting point either in absence of air or in limited supply. This method is commonly used for converting carbonates and hydroxides to their respective oxides. During calcination, moisture and volatile impurities are also removed. Calcination can also be described as a thermal process that is used to convert ores and other solid materials by bringing about a thermal decomposition. In calcination, the reaction most of the time occurs at or above the thermal decomposition temperature.
Calcination is derived from a Latin word calculate which translates as “to burn lime”. So, calcination is mostly used in the decomposition of limestone (calcium carbonate) to lime (calcium oxide) and carbon dioxide.
CaCO3 —> CaO + CO2
Meanwhile, the products that are derived from calcination are known as calcining, and it is regardless of actual compounds undergoing thermal treatment.
What is Roasting?
When we talk about roasting, it is basically a process of metallurgy where ore is converted into its oxide by heating it below its melting point in the presence of excess air. While calcination is mostly used in the oxidation of carbonates, roasting is a method that is used for converting sulphide ores. During roasting, moisture and non-metallic impurities in the form of volatile gases are released. Process of roasting consists of solid-gas thermal reaction which includes oxidation, reduction, sulfation, chlorination and pyro hydrolysis.
However, roasting which involves sulphides are a major source of air pollution and the main drawback of this process is that it releases a large amount of metallic as well as toxic and acidic compounds which causes harm to the environment.
An example of roasting is when Zinc sulphide is converted into zinc oxide.
2ZnS+3O2 —> 2ZnO + 2SO2
Important Differences Between Calcination and Roasting
Here are some of the major differences between calcination and roasting.
|Calcination is a process in which ore is heated in the absence of air or air might be supplied in limited quantity||Roasting involves heating of ore lower than its melting point in the presence of air or oxygen.|
|Calcination involves thermal decomposition of carbonate ores.||Roasting is carried out mostly for sulfide minerals.|
|During calcination, moisture is driven out from an ore.||Roasting does not involve dehydrating an ore.|
|Carbon dioxide is given out during calcination||During roasting large amount of toxic, metallic and acidic compounds are released.|
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