Calcination and Roasting

Dhristi JEE 2022-24

What are 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 sulfides. These ores are turned to metals only after converting sulfides 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?

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 temperature 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).


\[CaCO_3 \rightarrow  CaO + CO_2\]


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.


\[2ZnS + 3O_2 \rightarrow 2ZnO + CO_2 \]


Purpose of Calcination

Calculation is the process used for the thermal decomposition of any material. It is done by heating a substance to a higher temperature, below its melting point. This heating at high temperatures helps to remove volatile substances and also oxidizes any mass of the substance. Therefore, calcination can be considered as a purification process of metals. However, calcination is mostly used in oxidizing carbonate compounds. 


Purpose of Roasting

Roasting is the process where the substance is heated below its melting point, in presence of excess air or oxygen. Roasting is used in metallurgical processes where ores get converted into oxides, and then metals are extracted. It is usually used for converting sulphide ores. During this process, volatile substances and moisture along with impurities get removed. 

 

Differentiate between Roasting and Calcination

A few of the major differences between calcination and roasting are tabulated below.

 

Calcination

Roasting

Calcination is a process where the air might be supplied in limited quantities, or the ore is heated in the absence of air.

Roasting includes heating of ore lower than its melting point in the presence of oxygen or air.

Calcination involves the thermal decomposition of carbonate ores.

Roasting is mostly carried out for sulfide minerals.

Moisture is driven out from ore during calcination.

Roasting does not involve dehydration of ore.

Carbon-dioxide is given out during calcination.

During roasting, a large amount of toxic, acidic, and metallic compounds are released.

 

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 put, calcination setups are not used for 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 a 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.


Quick Summary

About Calcination

  • Calcination is derived from the Latin word calculate which means to burn lime.

  • In calcination, the ores heated in the absence of air or a limited supply of air

  • Calcination process thermal decomposition of carbonate old and compounds takes place

  • During this process, moisture and volatile substances in form of impurities are removed

  •  are removed 

  • Carbon dioxide gas is released during the calcination process

  • For the calcination process, usually, the decomposition occurs near or above thermal decomposition temperature. 

  • The products derived from the calcination process are called calcining. 


About Roasting

  • Roasting is the process of heating ore to extract the metal, in presence of excess air

  • This process is mostly carried out for ores containing sulphide

  • The roasting process does not involve any dehydrating procedure. 

  • During this process, impurities in the form of moisture, volatile compounds are released. 


Sample Questions for Exam

  1. What is roasting explain with an example

  2. What is calculation?  explain with an example

  3. Differentiate between roasting and calcination

  4. How does the calcination process work?  state and chemical reaction for the same

  5. What is the purpose of roasting?

  6. What are the advantages and disadvantages of calcination and roasting

  7. How is the roasting of FeS done?

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FAQs (Frequently Asked Questions)

1. How is Light Roasting Different From Darker and Medium Roasting?

The difference between darker and medium roasting is during the roasting time of coffee beans. The longer time equals darker roast. Darker roast equals less caffeine, whereas different flavors and aromas. In general, these flavors and aromas of darker roasts are considered as 'richer' and are thus preferred by many.


Each roasting level presents its own flavor profile. At the same time, some people prefer lighter roasting. Robusta coffees (or Robusta coffee) tend to be more lightly roasted, whereas the Arabicas (or Coffea arabica) tend towards the darker roasts. However, this is not a hard-and-fast rule. Also, the Italian espresso tends to use dark roasts of the Robusta blends.

2. What is Meant by Burning in the Absence of Oxygen?

The term that we look for is probably called pyrolysis. Where, pyrolysis is a chemical decomposition reaction involving from little to no oxygen, or at least not enough oxygen required for combustion.


In general, we can use two types of elemental analysis instruments in the laboratory that burn samples. We use combustion to generate H2O and CO2 quantitatively from the organic compound samples in the presence of abundant oxygen gas. Also, the other instrument works purely by the pyrolysis process instead of combustion and forms CO and H2 quantitatively gasses from similar organic sample compounds in purely reducing conditions.

3. What is Calcination?

It is the method of heating an ore strongly in the presence of a very limited quantity of air.

4. List Some Roasting Types?

  • Volatilizing roasting,

  • Oxidizing roasting

  • Magnetic roasting,

  • Sulphur roasting.

5. What is the difference between calcination and smelting?

Calcination is the process where the substance is heated to a high temperature below its boiling point, bringing in thermal decomposition. Smelting is the process of melting substances to extract metal from its ore. Calcination is carried out in absence of air or a limited supply of air. The smelting process is used to extract metals like silver, copper, iron, etc. Calcination is majorly used in the process to obtain gypsum and limestone. 

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