calcination Flashcards
definiton
Calcination is the process in which the inorganic substances are strongly heated so as to remove their
volatile contents and a fixed residue are obtained.
The IUPAC defines it as: “Heating to high temperatures in air or oxygen”.
The calcination process normally takes place at temperatures below the melting point of the product
materials.
origin
The process of “calcination” derives its name from the Latin “calcinare” (to burn lime).
The name is given due to its most common application, the decomposition of calcium carbonate
(limestone) to calcium oxide (lime) and carbon dioxide, in order to produce cement.
examples
Decomposition of carbonate minerals, as in Calcination of Limestone to drive off Carbon dioxide to
produce cement.
Decomposition of hydrated minerals, as in Calcination of Bauxite and Gypsum to remove crystalline
water as water vapor.
Removal of Ammonium ions in the synthesis of Zeolites.
actual calcination
Actual Calcination
o Actual calcination is that brought about by actual fire, from wood, coals, or other fuel, raised
to a certain temperature.
potential calcination
Potential Calcination
o Potential calcination is that brought about by potential fire, such as corrosive chemicals; for
example, gold was calcined in a reverberatory furnace with mercury and sal ammoniac; silver
with common salt and alkali salt; copper with salt and sulfur.
Calcination reactions usually take place at the thermal decomposition temperature (for decomposition).
For example: In limestone calcination, a decomposition process, the chemical reaction is
CaCO3 → CaO + CO2 (g)
apparatus
Shaft furnaces
Calcining kilns
Multiple hearth furnaces
Fluidized bed reactors
shaft furnances
A vertical, refractory-lined cylinder in which a fixed bed (or descending column) of solids is
maintained, and through which an ascending stream of hot gas is forced.
calcining kilns
This process is done in kilns.
Calcining kilns basically comes in two categories.
o Rotary kilns
o Vertical kilns
rotatory kilns
Rotary kilns can be long kilns with rotatory coolers while verticals kilns can be several types. Calcining
kilns need lime stone with decrepitation index.
Decrepitation index of limestone is a measure of its susceptibility to disintegrate during calcination.
Low value of decrepitation decreases the porosity of the lime thus impeding the flow of the gases and
reducing the kiln efficiency.
The smaller size limestone is more suitable for calcination in rotary kilns.
vertical kilns
A kiln consisting of a steel shell with a vertical axis and a lining of firebrick.
The most popular vertical kilns are PFR (Parallel Flow Regenerative) type.
multiple hearth furnance
For the decomposition of limestone, a large number of heat is requiring as we know calcination is an
endothermic process and it is done on industrial scale. This is done by multiple hearth furnaces.
fluidized bed reactions
A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of
multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a
granular solid material at high enough velocities to suspend the solid and cause it to behave as though
it were a fluid. This process, known as fluidization.
process
Calcination (also referred to as Calcining) is a thermal treatment process in presence of air or oxygen
applied to ores and other solid materials to bring about a thermal decomposition.
Calcination of calcium carbonate is a highly endothermic reaction, requiring 755 M Cal of heat input
to produce a ton of lime. The reaction begins when the temperature is above the dissociation
temperature of the carbonates in the limestone.
Once the reaction starts the temperature must be maintained above the dissociation temperature, and
carbon dioxide evolved in the reaction must be removed. Dissociation of the calcium carbonate
proceeds gradually from the outer surface of the particle inward, and a porous layer of calcium oxide,
the desired product, remains.
high temp
This calcination means higher agglomeration and more shrinkage. At this limestone becomes so dense
during calcination that it prevents the escape of CO2 and becomes non porous. The internal pressure
of limestone increases and it explodes, producing unwanted materials and reduces the quality of lime.
Increasing the degree of calcination beyond the limited temperature makes formed lime crystallites to
grow larger, agglomerate. This results in a decrease in porosity and reactivity and an increase in bulk
density. This product is known as Dead burnt or Low reactive lime.
low temp
This calcination allows less fuel consumption.
At relatively low calcination temperatures, products formed in the kiln contain both unburnt carbonate
and lime and is called ‘under burnt’ lime. As the temperature increases ‘Soft burnt’ or ‘high reactive
lime’ is produced. At still higher temperature, dead burnt or low reactive lime is produced.
Soft burnt lime is produced when the reaction front reaches the core of the charged limestone and
converts all carbonate present to lime. Such lime has the optimum properties of high reactivity, high
surface area and low bulk density.
The production of good quality lime depends upon the type of kiln, conditions of calcination and the
nature of the raw material i.e. limestone.
