J O U R N A L O F M A T E R I A L S S C I E N C E 4 0 (2 0 0 5 ) 4749 – 4751

Effect of compaction on the kinetics of thermal

decomposition of dolomite under non-isothermal

condition

S. MAITRA∗, A. CHOUDHURY, H. S. DAS, MS. J. PRAMANIKCollege of Ceramic Technology, 73, A.C. Banerjeee Lane, Kolkata-700010, India

Published online: 08 July 2005

Many workers [1–9] studied the kinetics of dolomite decomposition to study the effects ofdifferent parameters like, gas (CO2, N2 etc.) pressures, water vapor, presence of otherimpurities, particle size and grain size of the dolomite samples, crystallinity etc. on thedecomposition kinetics of dolomite using different tools like, thermal analysis,thermo-gravimetric analyses, XRD technique etc. and different values of the activationenergies for the decomposition reaction, order of reactions have been reported. It has beenobserved that pure dolomite decomposed in only two steps. The first stage of the thermaldecomposition of dolomite resulted in the formation of Mg-calcite [(CaMg)CO3] andpericlase (MgO), with the liberation of CO2. It was further observed that under CO2,dolomite decomposed directly to CaCO3, accompanied by the formation of MgO between550 and 765◦C. Calcite decomposed to CaO between 900 and 960◦C and under air,simultaneous formation of CaCO3, CaO and MgO accompanied dolomite decompositionbetween 700 and 740–750◦C. At the latter temperature, the calcite began to decomposeeven though a significant amount of dolomite was still present and simultaneousdecomposition of the two carbonates was terminated at 780◦C. Also, changes indecomposition rates of the various phases correlated with changes in the rate of weightloss determined by derivative thermo-gravimetric analysis.C© 2005 Springer Science + Business Media, Inc.

Again, It has been observed that in inert atmospherethe decomposition of natural dolomites take place viaa single stage process. Study of the thermal behaviourand the kinetics of the decomposition process usingthe Arrhenious equation applied to solid state reactionrevealed that dolomite decomposed via a zero ordermechanism in inert atmosphere with an energy of acti-vation of 175 kJ/mol. The kinetic parameters and mech-anisms were observed to remain unaffected by a changein process variables like, flow rate, heating rate and sam-ple size. It was further observed that besides differencesin the chemical composition of samples, gaseous atmo-sphere and the method of calculation influenced the ki-netic results obtained. Some authors proposed that themechanism of the decomposition of dolomite in con-trolled atmosphere was via the formation of the two sep-arate carbonates of magnesium and calcium. The parti-cle size was found to affect the shape but not necessarilythe position of the DTA curve for dolomite decompo-sition. Detailed results generated on the decomposi-tion of dolomite confirmed that CO2 pressure affectedboth the initial decomposition temperature into MgOand CaCO3 and the final decomposition temperature of

∗Author to whom all correspondence should be addressed.

CaCO3, but in opposite directions. This demonstratedthe existence of different decomposition mechanisms.

In the present investigation kinetics of thermaldecomposition under non-isothermal conditions ofIndian dolomite was studied in relation to the com-paction pressure from thermo-gravimetric data. The ki-netic parameters, like order of the reaction, activationenergy etc., were calculated and were correlated to thecompaction pressure. The kinetic approach to the studyof its thermal decomposition is important not only be-cause of its economic utilization depend on the rate ofthese reactions, but also, kinetic studies give basic in-formation about the inside of the mineral. Most of thestudies on the kinetics of dolomite decomposition weremade using isothermal models and not much work havebeen done on the use of dynamic, non-isothermal ki-netic models to study the decomposition process, whichis likely to be more practical approach.

The dolomite sample used in the present investiga-tion was received in powder form with average particlesize (d50) of 12 µm. It was first chemically analyzed byconventional method and the analysis result is given inTable I. From the chemical analysis of dolomite it was

0022–2461 C© 2005 Springer Science + Business Media, Inc.DOI: 10.1007/s10853-005-0843-0 4749

TABL E I Chemical composition of the dolomite

Constituent Wt (%)

SiO2 0.96Al2O3 0.24Fe2O3 0.34K2O 0.32CaO 31.25MgO 21.55LOI 45.04

observed that the level of major impurity, i.e., SiO2 andFe2O3 is quite low here. The powder sample was com-pacted separately with pressures of 20, 30, 40 and 50MPa respectively, with a hydraulic press in the form ofcircular disc (2.5 cm dia.) using starch as a binder. Thethermo-gravimetric studies on the samples were madeusing a TG furnace. The heating rate of the furnace wasmaintained at 5◦C/min up to a temperature of 1000◦C.

From the TG values of the samples it was observedthat the major weight loss of the material takes placein the temperature range 710–910◦C and this might besubdivided in two different temperature ranges 710–810◦C and 810–910◦C (Fig. 1).

The decomposition of dolomite can be presented bythe following equations.

The first temperature range can be assigned to theformation of MgO and calcite and the second rangewas assigned to the decomposition of calcite [9].

The rate equation of thermal decomposition ofdolomite is given by,

−dW

dt= KW,

W being the concentration of dolomite

Figure 1 DTGA diagram of Dolomite powder compact under differentcompaction pressure.

By incorporating the Arrhenius equation:

K = Ae−E/RT and the heating rate, h = dT/dt , andrearranging, we get:

[− 1

W

dW

dT

]= A

he−E/RT

The rate equation may also be expressed in terms offractional conversion X and the reaction order n as fol-lows:

X = W0 − W

W0 − W∞

(W0 and Wα are taken as the initial weight and weight af-ter equilibrium decomposition, i.e, when further weightchange does not take place)

−dX

dt= K (1 − X )n

By incorporating the expressions of heating rate h andthe rate constant K in the above equation, and rear-ranging and integrating, the following equation can bewritten as:

∫ X

0

dX

(1 − X )= A

h

∫ T

0e−E/RT dT

Different workers gave approximate solutions of theabove equations [10–13]. The solution given byAgarwal and Sivasubramanium [12], which was usedby the authors in studying the dehydration kinetics ofcalcium aluminate cement hydrate [14], was used in thepresent investigation.

Ln

[− Ln(1 − X)

T 2

]

= Ln

[AR

hE

(1 − 2RT/E

1 − 5(RT/E)2

)]− E

RT, for n = 1

Ln

[−Ln{1 − (1 − X )1−n

(1 − n)T 2

]

= Ln

[AR

hE

(1 − 2RT/E

1 − 5(RT/E)2

)]− E

RT, for n �= 1

Therefore, a plot of Ln [−Ln(1−x)T 2 ] vs. 1/T for n = 1 and

Ln [−Ln{1−(1−x)1−n

(1−n)T 2 ] vs. 1/T for n �= 1 give the value of E .The values of A can be calculated from the intercept.

To determine the exact order of the decompositionreaction, the regression coefficient and standard errorvalues of the linear plots for different n values (as de-scribed above) were analyzed by trial and error method.The n value for which correlation coefficient value wasthe maximum, was taken as the order of the reactionand the values are listed in Table II.

The kinetic study was carried out for the two dif-ferent zones where the weight losses were the maxi-mum, i.e., 710–810◦C and 810–910◦C. These are thezones for the formation of MgO along with CaCO3 and

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TABL E I I Kinetic parameters for dolomite decomposition

Compaction pressure E A(MPa) n R2 (KJ/Mol) (min−1)

A. (Temperature range 710–810◦C)

0 0.85 0.9995 103.29 4.13E0520 0.74 0.9992 118.52 3.54E0530 0.55 0.9987 120.41 2.21E0440 0.50 0.9979 123.39 3.21E0350 0.50 0.9982 124.73 1.53E02

B. (Temperature range 810–910◦C)

0 0.75 0.9991 121.17 4.32E0520 0.83 0.9977 118.52 6.04E0430 0.92 0.9967 115.32 5.29E0440 1.0 0.9999 113.43 1.21E0450 1.0 0.9996 109.73 2.23E03

C. (Temperature range 710–910◦C)

0 0.33 0.9998 133.17 1.83E0620 0.75 0.9997 128.52 2.64E0430 1.0 0.9997 127.35 4.27E0440 1.0 0.9999 126.63 5.21E0350 1.0 0.9996 126.33 8.53E01

the formation of CaO. The kinetic study for the entirerange of decomposition, i.e., for the formation of com-plete doloma from the dolomite was also carried out.From the calculated kinetic parameters it was observedthat for the first temperature zone, with the increase incompaction pressure, the order of the decompositionreaction decreased and in all the cases the orders wereof fractional values. The activation energy increasedsteadily with the increase in compaction pressure indi-cating the influence of diffusion-controlled process onthe decomposition reaction at this stage. For the sec-ond temperature range, with the increase in compactionpressure, the order of the reaction increased steadily,and after a critical compaction pressure, the decompo-sition reaction for this stage only, became first order.But the activation energy decreased significantly withthe increase in compaction pressure. This could be dueto the influence of interface at this stage of decomposi-tion, which increased with the increase in compactionpressure. The decomposition reaction of the powdersample for the overall range followed fractional orderkinetics and so also the sample compacted at low pres-sure (20 MPa) although with a higher value. With fur-ther compaction, the decomposition reaction became of1st order and with the increase in compaction pressurefurther increase in the order of the reaction rate wasnot observed. But activation energy of the decomposi-

tion process decreased slightly with the increase in thecompaction pressure.

In the case of dolomite, rhombohedral lattice is con-verted to the cubic lattice after decomposition. As thereaction is interface controlled, it may be assumed thatin between the two stable states of reactant and product,i.e., dolomite crystal and calcined dolomite or doloma,incipiently formed doloma (CaO∗ + MgO∗) remainedin active state. As the reaction approaches an equilib-rium value, the concentration of the active state is ex-pected to rise from zero to a constant value. It is apparentthat the fraction surface available for the reaction willbe dependent upon the removal of oxide away from thereactant dolomite site and to the product site, i.e., latticeof the oxide crystals of the decomposed dolomite [15].With the increase in compaction pressure the possibil-ity of the conversion of the freshly decomposed dolomaintermediate into fully stable form probably increasedand the activation energy for the decomposition processis therefore slightly decreased.

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Received 12 November 2004and accepted 7 March 2005

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