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 currents of air, 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.
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 oxidize; 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, CaCO3, to convert it into lime, CaO, necessary for concrete.
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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.
When the calcination process is analytical, it is generally one of the last essential 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 purification techniques, the precipitate still has water that must be removed. If these 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 defined.
Also, 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 nHtwoO => A + nHtwoO (steam)
A + Q (heat) => B
The undefined solids would be mixtures A / A nHtwoO and A / B, when ideally they should be pure A and B, respectively.
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 yield 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:
Thief3 => CaO + COtwo
Calcium oxide, CaO, is the lime necessary for making cement. If the first reaction is complemented by these two:
CaO + HtwoO => Ca (OH)two
Ca (OH)two + COtwo => CaCO3
CaCO crystal size can be prepared and controlled3 resulting from robust masses of the same compound. Thus, not only is CaO produced, but also microcrystals of CaCO are obtained.3, required 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.
In itself there is no way to classify calcination, unless we base ourselves on the process and the changes that the solid undergoes with increasing temperature. From this last perspective, it can be said that there are two types of calcination: one chemical, and the other physical..
Chemical calcination is one where the sample, solid or precipitate undergoes thermal decomposition. This was explained for the case of CaCO3. The compound is not the same after the high temperatures have been applied.
Physical calcination is one where the nature of the sample does not change at 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 up” or crack as a result.
This last aspect of calcination: controlling the size of the crystals, has not been addressed in detail, but it is worth mentioning it..
Finally, a series of general and specific calcination applications will be listed:
-Decomposition of metallic carbonates into their respective oxides. The same goes for oxalates.
-Dehydration of minerals, gelatinous oxides or some other sample for gravimetric analysis.
-It subjects 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 Mn0.5Zn0.5FaithtwoOR4; that is, they undergo a physical calcination, where the heat influences the size or shapes of the crystals.
-The same previous effect can be observed in simpler solids such as SnO nanoparticles.two, which increase in size when 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 compounds.
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