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Calcination of Limestone

Posted by Satyendra [1] on May 2, 2013 in Articles [2] | 0 comments

Calcination of Limestone

Calcination or calcining is a thermal treatment process to bring about a thermal

decomposition. The process takes place below the melting point of the product.

The name calcination is derived from the Latin word ‘Calcinare’ which mean to

burn lime. Limestone is a naturally occurring mineral. It exists nearly all over

the world. The chemical composition of this mineral varies greatly from region

to region as well as between different deposits in the same region. Therefore,

the end product from each natural deposit is different. Typically limestone is

composed of calcium carbonate (CaCO3), magnesium carbonate (MgCO3), silica

(SiO2), alumina (Al2O3), iron (Fe), sulphur (S) and other trace elements.

Limestone is one of the most basic raw materials employed in the steel industry

and is used both in iron making and steel making processes. Lime (CaO) is one

of the oldest chemicals known to man and the process of lime production is one

of the oldest chemical industries. Quicklime was produced in US as early as 1635

in Rhode Island. Technical progress which was non existing in centuries past,

has rapidly advanced the lime industry during the last fifty years in the area of

process methods and design.

Limestone deposits are wide distributed. The limestone from the various

deposits differs in physical chemical properties and can be classified according

to their chemical composition, texture and geological formation. Limestone is

generally classified into the following types:

High calcium – The carbonate content is composed mainly of calcium

carbonate with a magnesium carbonate content not more than 5 % (usually

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less).

Magnesium – This contains magnesium carbonate to about 5 – 20%.

Dolomitic -This is also known as dolomite and contains over 20 % of MgCO3.

However the maximum MgCO3 content does not exceed 45.6%. The balance

amount is calcium carbonate.

Limestones from different sources differ considerably in chemical compositions

and physical structures. The chemical reactivity of various limestones also shows

a large variation due to the difference in crystalline structure and the nature of

impurities such as silica, alumina and iron etc. The varying properties of the

limestone have a big influence on the processing method. Hence it is necessary

to know comprehensive information of the limestone such as physical and

chemical properties, the burning characteristics and kinetic parameters for the

calcination of the limestone. This aids optimal design and operation at lime

kilns.

Calcination process

Calcination reactions usually take place at or above the thermal decomposition

temperature. This temperature is usually defined as the temperature at which the

standard Gibbs free energy is equal to zero. In case of limestone calcination, a

decomposition process, the chemical reaction for decomposition of the

limestone is

CaCO3= CaO + CO2 (g)

The standard Gibbs free energy of reaction is approximated as ?G°r = 177,100 ?

158 T (J/mol). The standard free energy of reaction is zero in this case when the

temperature T is equal to 1121 K or 848 deg C. Hence the chemical

decomposition reaction for pure CaCO3 will start at 850 deg C.

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. This typically is between 850 deg C and 1340 deg C. Once the

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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. The following factors affect the calcination.

Crystalline structure affects the rate of calcination, internal strength of

limestone and resultant crystal size of lime after calcination. The smaller

crystals agglomerate during calcination and forms larger crystals which in turn

cause shrinkage and volume reduction.

Calcination at higher temperature means higher agglomeration and more

shrinkage. Also the density of limestone is related to the crystal structure. The

shape of crystals determines the void space between crystals, and hence the

density of the limestone. Larger voids allow easy passage for CO2 gases during

calcination and it results in a reduction of volume during calcination. Some

limestone, due to its crystalline structure, disintegrates during the calcination

process. This type of limestone is not useful for calcining. There is some other

limestone whose behavior is the opposite. This type of limestone become so

dense during calcination that it prevents the escape of CO2 and become non

porous. This type of limestone is also not suitable for calcination.

The smaller size limestone is more suitable for calcination in rotary kilns and it

allows optimum residence time. The lower calcining temperature also allows less

fuel consumption. In contrast, larger size limestone and low calcining

temperature is needed for vertical kilns. If the temperature rise is too rapid, the

outer layer of the limestone pieces is calcined very fast. As the temperature rises,

the surface of the limestone shrinks and closes the pores created by the escape

of CO2. This causes increased internal pressure within the limestone. Since the

CO2 gas cannot escape, the limestone explodes and disintegrates producing

unwanted “fines” thus reduces the quality of the lime.

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. At relatively low

calcination temperatures, products formed in the kiln contain both unburnt

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carbonate and lime and is called ‘underburnt’ lime. As the temperature

increases, ‘soft burnt’ or ‘high reactive lime’ is produced. At still higher

temperatures, ‘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. A high productive product is relatively

soft, contains small lime crystallites and has open porous structure with an

easily assessable interior. Such lime has the optimum properties of high

reactivity, high surface area and low bulk density. Increasing the degree of

calcination beyond this stage makes formed lime crystallites to grow larger,

agglomerate and sinter. This results in a decrease in surface area, porosity and

reactivity and an increase in bulk density. This product is known as dead burnt or

low reactive lime.

Calcining kilns

Calcining kilns are basically comes in two categories. They are i) rotary kilns and

ii) vertical kilns. Both the types of kilns can be designed with any of the solid,

liquid or gaseous fuels. Rotary kilns can be long kilns with straight rotary 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 disintegration during calcination. Low value of decrepitation

decreases the porosity of the bed thus impeding the flow of the gases and

reducing the kiln efficiency. Rotary kilns also need limestone with good

tumbling index. A rotary kiln with preheater and contact cooler is shown in Fig 1

[3]

[4]

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Fig 1 Rotary kiln with preheater and contact cooler

The schematics of a preheater is shown in Fig 2

[5]

[6]

Fig 2 Schematics of a preheater

The most popular vertical kilns are PFR (Parallel Flow Regenerative) type.

Different types of kilns for calcining limestone are shown in Fig 3. Various

features of these kilns are tabulated in Tab 1.

[7]

[8]

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1. http://ispatguru.com/author/Satyendra/

2. http://ispatguru.com/category/articles/

Fig 3 Different types of kilns for limestone calcination

Tab 1 Comparison of various types of calcining kilns

Type of

calcining kiln

Kiln

capacity

Limestone

size

Specific fuel

consumption

Remarks

Kcal/Kg

Rectangular

PFR kiln

100-

400

30-120 810-870 Highly reactive lime is

produced

Circular PFR

kiln

300-

800

30-160 810-870 Highly reactive lime is

produced

Fine lime kiln 200-

400

15-40 790-850 Highly reactive lime is

produced

Annular shaft

kiln

200-

600

15-200 910-980 High CO2 content in off

gas

Single shaft

kiln

50-300 10-100 980-1100 Medium hard burnt lime is

produced

Rotary kiln

with preheater

300-

1200

10-50 1150-1350 Highly reactive lime is

produced, high production

rate and low sulphur

production

Long rotary

kilns without

preheater

300-

1000

20-50 1600-1700 high production rates,

reactive lime and low

sulphur

Suspension

calcining

300-

1200

0.03-2 1300-1400 Very fine raw material

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3. http://ispatguru.com/calcination-of-limestone/rotary-kiln-with-preheater/

4. http://ispatguru.com/calcination-of-limestone/rotary-kiln-with-preheater/

5. http://ispatguru.com/calcination-of-limestone/preheater/

6. http://ispatguru.com/calcination-of-limestone/preheater/

7. http://ispatguru.com/calcination-of-limestone/different-type-of-kilns-for-

calcination/

8. http://ispatguru.com/calcination-of-limestone/different-type-of-kilns-for-

calcination/