The calcination is a process in which a solid sample is subjected to high temperatures in the presence or absence of oxygen. In analytical chemistry it is one of the last steps of gravimetric analysis. The sample can therefore be of any nature, inorganic or organic; but especially, it is about minerals, clays, or gelatinous oxides.
When calcination is carried out under air currents, it is said to occur in an oxygenated atmosphere; such as simply heating a solid with fire product of combustion in open spaces, or in furnaces to which vacuum cannot be applied.
Rudimentary or alchemical calcination under the open sky. Source: Pixabay.
If oxygen is replaced by nitrogen or a noble gas, then calcination is said to occur under an inert atmosphere. The difference between the atmospheres that interact with the heated solid depends on its sensitivity to oxidation; that is, to react with oxygen to transform into another more oxidized compound.
What is sought with calcination is not to melt the solid, but to modify it chemically or physically to meet the qualities required for its applications. The best known example is the calcination of limestone, CaCO 3, to convert it into the lime, CaO, necessary for concrete.
Process
The relationship between the heat treatment of limestone and the term calcination is so close that in fact it is not uncommon to suppose that this process applies only to calcium compounds; However, this is not true.
All solids, inorganic or organic, can calcine as long as they do not melt. Therefore, the heating process must occur below the melting point of the sample; Unless, it is a mixture where one of its components melt while the others remain solid.
The calcination process varies depending on the sample, the scales, the objective and the quality of the solid after its heat treatment. This can be globally divided into two types: analytical and industrial.
Analytical
When the calcination process is analytical, it is generally one of the last indispensable steps for gravimetric analysis.
For example, after a series of chemical reactions a precipitate has been obtained, which during its formation does not look like a pure solid; obviously assuming that the compound is known in advance.
Regardless of the purification techniques, the precipitate still has water that must be removed. If such water molecules are on the surface, high temperatures will not be required to remove them; but if they are "trapped" inside the crystals, then the oven temperature may have to exceed 700-1000ºC.
This ensures that the precipitate is dry and water vapors are removed; consequently, its composition becomes definite.
Likewise, if the precipitate undergoes thermal decomposition, the temperature at which it must be calcined has to be high enough to ensure that the reaction is complete; otherwise, you would have a solid of undefined composition.
The following equations summarize the two previous points:
A nH 2 O => A + nH 2 O (vapor)
A + Q (heat) => B
The undefined solids would be mixtures A / A · nH 2 O and A / B, when ideally they should be pure A and B, respectively.
Industrial
In an industrial calcination process, the quality of the calcination is just as important as in gravimetric analysis; but the difference is in the assembly, the method and the quantities produced.
In the analytical one seeks to study the performance of a reaction, or the properties of the calcined; while in the industrial sector, it is more important how much is produced and in how long.
The best representation of an industrial calcination process is the heat treatment of limestone so that it undergoes the following reaction:
CaCO 3 => CaO + CO 2
Calcium oxide, CaO, is the lime necessary for making cement. If the first reaction is complemented by these two:
CaO + H 2 O => Ca (OH) 2
Ca (OH) 2 + CO 2 => CaCO 3
The resulting CaCO 3 crystals can be prepared and sized from robust masses of the same compound. Thus, not only CaO is produced, but also CaCO 3 microcrystals are obtained, necessary for filters and other refined chemical processes.
All metallic carbonates decompose in the same way, but at different temperatures; that is, their industrial calcination processes can be very different.
Types of calcination
In itself there is no way to classify calcination, unless we base ourselves on the process and on the changes that the solid undergoes with the increase in temperature. From this last perspective, it can be said that there are two types of calcination: one chemical, and the other physical.
Chemistry
Chemical calcination is one where the sample, solid or precipitate undergoes thermal decomposition. This was explained for the case of CaCO 3. The compound is not the same after the high temperatures have been applied.
Physical
Physical calcination is one where the nature of the sample does not change in the end once it has released water vapor or other gases.
An example is the total dehydration of a precipitate without undergoing a reaction. Also, the size of the crystals can change depending on the temperature; at higher temperatures, the crystals tend to be larger and the structure can "puff" or crack as a result.
This last aspect of calcination: the control of the size of the crystals, has not been addressed in detail, but it is worth mentioning.
Applications
Finally, a series of general and specific calcination applications will be listed:
-Decomposition of metallic carbonates in their respective oxides. The same goes for oxalates.
-Dehydration of minerals, gelatinous oxides or any other sample for gravimetric analysis.
-Submits a solid to a phase transition, which could be metastable at room temperature; that is, even if your new crystals were cooled, they would take time to return to how they were before calcination.
-Activates alumina or carbon to increase the size of its pores and behave as well as absorbent solids.
-Modifies the structural, vibrational or magnetic properties of mineral nanoparticles such as Mn 0.5 Zn 0.5 Fe 2 O 4; that is, they undergo physical calcination, where heat influences the size or shapes of the crystals.
-The same previous effect can be observed in simpler solids such as SnO 2 nanoparticles, which increase in size when they are forced to agglomerate by high temperatures; or in inorganic pigments or organic colorants, where the temperature and the grains influence their colors.
-And desulfurizes coke samples from crude oil, as well as any other volatile compound.
References
- Day, R., & Underwood, A. (1989). Quantitative Analytical Chemistry (fifth ed.). PEARSON Prentice Hall.
- Wikipedia. (2019). Calcination. Recovered from: en.wikipedia.org
- Elsevier. (2019). Calcination. ScienceDirect. Recovered from: sciencedirect.com
- Hubbe Martin. (sf). Mini-Encyclopedia of Papermaking Wet-End Chemistry. Recovered from: projects.ncsu.edu
- Indrayana, IPT, Siregar, N., Suharyadi, E., Kato, T. & Iwata, S. (2016). The calcination temperature dependence of microstructural, vibrational spectra and magnetic properties of nanocrystalline Mn 0.5 Zn 0.5 Fe 2 O 4. Journal of Physics: Conference Series, Volume 776, Issue 1, article id. 012021.
- FEECO International, Inc. (2019). Calcination. Recovered from: feeco.com
- Gaber, MA Abdel-Rahim, AY Abdel-Latief, Mahmoud. N. Abdel-Salam. (2014). Influence of Calcination Temperature on the Structure and Porosity of Nanocrystalline SnO 2 Synthesized by a Conventional Precipitation method. International Journal of Electrochemical Science.