Kinetics of the Thermal Decomposition of Alum Sourced from Kankara Kaolin

Kinetics of the Thermal Decomposition of Alum Sourced from Kankara Kaolin

1Olubajo O. O., 2S. M. Waziri, 2B. O. Aderemi

1Department of Chemical Engineering, Abubakar Tafawa Balewa University Bauchi, Nigeria

2Department of Chemical Engineering, Ahmadu Bello University, Zaria, Nigeria

ABSTRACT- This study investigated the kinetics of the thermal decomposition of alum produced from Kankara kaolin. The work involved preparation of alum as well as calcination of the prepared alum. The Coats-Redfern kinetic model was adopted in estimating the kinetic parameters for the decomposition reactions. The experiment was conducted using a muffle furnace and the rate data were obtained using thermogravimetric (TG) and Xray fluorescence (XRF) analyses. From the TGA experimental data, an average order of reaction, n of 1.3 with corresponding activation energy value of 321.03 kJ/molK and frequency factor 2.57E+14min-1 gave the smallest residual deviation values and best fitting curves compared to other reaction orders tested within the range of 0 to 2, while equivalent values from XRF residual sulphate data are n = 1, activation energy = 337.22 kJ/molK and frequency factor of 5.27E+14 min-1. The discripances in the corresponding values from the two modes of data generation is attributed to the differences in the reaction mechanisms captured by the different methods of analysis. While, the XRF solely followed the sulfate decomposition phenomenon, TGA was more encompassing including dehydroxylation and impurity decomposition reactions.

(KEYWORDS: Kankara Kaolin, decomposition, Coats-Redfern model, TGA, XRF, kinetics)

1. INTRODUCTION

   The great interest in alumina is mostly due to its ever increasing applications as adsorbents, catalysts support, refractory, filler and as constituents of various household products. Invariably, bauxite has been the dominant source alumina, through the popular Bayers process [1]. However, notwithstanding the acclaimed bauxites relative economic advantages over all other alternative sources, its rapid depletion in quantity and quality globally is an impetus promoting intensive research activities on alumina extraction from clay materials worldwide [2]. In fact, this is a welcome development to many countries such as Nigeria that are not naturaly endowed with bauxite but having kaolin in abundance.

   It must be admitted that in the last decade, a lot of research efforts have been expended in producing alums, alumina and silica for various purposes from Kankara kaolin clay [3-8]. However, it is of note that while much of the efforts were geared towards establishing favourable process conditions,

   apparently, no such attention has been paid to the prevailing kinetics.

   The work of Moselhy et al. (1994) [9] , on thermal treatment of aluminium sulfate hydrate made a fundamental contribution in addressing the peculiarity of alum decomposition including the testfitting of the Coat-Redfern kinetic model. However, they limited their consideration to only the ideal solid-state reaction cases of order n= ½, 2/3 and 1, thus ignoring the obvious peculiarity of alum. At the onset, it is obvious that a well crystalline alum behaves as a non porous material, in which the shrinking core reaction model ought to describe, howbeit, as reaction progresses and it is evacuated of the occluded free water and sulfate ions, it becomes a porous material to mimic a diffusion-progressive phenomenon. Even at that, decomposition of alum to release the bonded waters (water of crystallization) requires different energy level (activation energy) to that of sulfate decomposition, with no clear cut demarcation in time or space of the occurence of these duo phenomena.

   The strenght of the Coat-Redfern model resides in its development, which is free from the majority of the idealized models assumptions. It equally merged the kinetic constants with the thermodynamic parameters in a single equation, thus affording the evaluation of reaction order, activation energy and reaction frequency factor from same set of data. Its major shortcoming being that the order of reaction determination, inherently depends on the trial and error principle, whether by numerical or graphical approach.

   The use of thermogravimetric data to evaluate kinetic parameters for solid state reactions involving weight loss has been investigated by a number of workers as noted by Coats and Redfern (1964) [10] , but sad enough, the single sample, rapid, and continous kinetics calculation over the entire temperature range offered by dynamic thermogravimetry is still not executable in Nigeria, five decades after, due to lack of requisite thermo-balance facility in the neighborhood.

   Hence, this paper attempts to estimate the kinetic parameters such as activation energy, pre-exponential factor and the order of reaction using Coats-Redfern kinetic model. Therefore, the present work attempts to explore the traditional cumbersome isothermal gravimetry under varying holding time and

   temperature. The study also involves the preparation of single alum from local clay and the calcination of the produced alum at various temperatures and holding times. Gravimetric determination was employed in monitoring the overall rate data, while X-ray Fluorescence analyses of the residual sulfate

   dX = dX . dT = dX . 9

   dt dT dt dT

   Substituting Equation 9 into 1 and /, the subject

   of the formula

   concentration in the alumina were obtained to model sulphate decomposition specifically.

   dX = dT

   dX . 1

   =

   X 10

   dt

   Derivation of Coat-Redfern Model

   In general, for the reaction

   dX = X dT 11

   0

   + the disappearance of component A can be described by the formal kinetic expression:

   g X = () = X dT X

   12

   dX g X = =

   13

   dt = kf X 1a

   dX = f X 1b

   dt

   From Arrhenius,

   From calculus:

   Therefore, if

   dX = 1 a

   =

   2

   =

   = 1

   Where X is the fractional conversion; t is the time; A is the pre-exponential factor; E is the activation energy; R is the gas constant; T is the temperature in Kelvin and f (X) is the kinetic function which takes different forms depending on the particular reaction rate equation. In isothermal kinetic studies, the rate equation used to calculate the rate constant has the

   form:

   Differentiating the above with respect to temperature T gives

   dx dt = T 2 dT = T2dx

   Substituting dT = T 2dx into Equation13:

   = 2 =

   14

   g X = t 3

   0

   0 2

   Differentiating with respect to time t

   d g X = 4

   dt

   Substituting the LHS term of Equation 4 into Equation 1a to

   Since dx = 1 a

   Then using integration by part

   = 15

   give

   d g X f X =

   5

   Let = 1 2

   = , then from Equation 14

   dt

   d g X X = 6

   Making dg( X ) subject of the formula

   0

   2

   = 1 2

   0

   + 2 1 3 16a

   g X =

   7

   0 0

   = + 2 16b

   Integrating with respect to X

   0 2

   2

   0

   3

   g X = ()

   0

   8

   Taken the integration by part a step furtherand rewrite Equation 16

   However, non-isothermal methods are becoming more widely

   used because they are more realistic than the classical isothermal methods. In non-isothermal kinetics the time

   2

   + 2 .

   2 3

   2 +

   3 =

   3

   17

   0

   dependence on the left hand side of Equation 1 is eliminated

   dt

   using constant heating rate = dT so that T = To + , where To

   0 +

   4

   is the starting temperature and t is the time of heating. Using integral methods of analysis, from Equation 1:

   Its obvious that the integration continues endlessly, however, from the generic definition of x = 1/T, x4 and above goes to

   zero, rendering the equation undefined. Therefore, it becomes expedient to stop at the second term on the right hand bracket.

   The equation has been written in the form:

   Hence,

   =

   2

   1 1 X 1n

   T2 1 n

   2

   2

   = In R 1 2RT

   E RT (27)

   2

   +

   a 0

   3

   +

   2

   3 .

   18

   E E +

   Substituting a = and x = 1 T

   gives Equation 19

   into Equation 18

   Equation 27 satisfies for n< 1or n > 1 but for n=1, Equation 27 becomes

   = 1 X

   E

   RT2 T

   = e RT

   2R2T3

   +

   T

   eE RT

   19

   2

   T2

   = In R 1 2RT E

   (28)

   E

   E2

   E E +

   RT

   And then factorize Equation 19 to give Equation 20

   E

   = RT2 eE RT 1 2RT 20

   For Equations 27 and 28 to be amenable to graphical solution, the quantity 1 2RT is assumed to be close to 1, hence the need to verify the reasonability of this assumption [10].

   E E

   E E

   = RT2eE RT 1 2RT 21

   Incorporating the power law Equation 21 as follows:

   X

   n dX

2. MATERIALS

   Raw kaolin used in this investigation was obtained from Kankara village, Katsina State, Nigeria, while beneficiated and metakaolin were obtained from processing of the raw clay

   X =

   1 X , then g X =

   0

   X

   and calcination of the beneficiated clay respectively. Fresh alum produced in this work as described in the following

   0

   dX 1 X n

   X

   = 1

   n

   0

   X

   dX (22)

   subsection meets general requirement of 6-9% wt of Al2O3/ wt of fresh alum. All other chemicals used were laboratory grade.

   If Equation 22 was integrated to give:

   EXPERIMENTAL PROCEDURE

   1 1 X 1n

   Preparation of Metakaolin from raw Kankara kaolin clay

   =

   0

   1 n (23)

   Substituting Equation 23 into Equation 21

   1 1 X 1n

   The raw clay was crushed using a mortar and pestle. The resultant product was then beneficiated by soaking in water and intermitent vigorious stirring for 3 days, after each day the spent water was replaced while sand particles were removed

   =

   1 n

   RT2 1 2RT E E

   = E e

   RT

   (24)

   and discarded. The significance of removal of spent water is to faciliate the removal of soluble impurties. The kaolin suspension was then centrifuged and dried overnight at 120oC to remove free water [11]. The dried lump was crushed and screened with 315 microns sieve. The sieved clay powder was

   Dividing Equation 24 through by T2

   then weighed into crucibles and calcined in a muffle furnace at a temperature of 750oC for 2 hrs [2].

   =

   2

   1 1 X 1n

   T2 1 n

   = R E 1 2RT E eE RT

   (25)

   Dealumination of metakaolin using sulphuric acid

   The dealumination of metakaolin (Al2Si2O7) was performed by reacting 50 grammes of metakaolin with 168.03cm3 of

   Taking the negative natural logarithm of both sides

   sulphuric acid of 96wt% (H2SO4) to give a 60 wt% acid solution [12]. A simplified chemical reaction for dealumination process is presented as Equation 29:

   =

   2

   1 1 X 1n

   T2 1 n

   + 3 + 24

   R

   2RT

   2 2 7 ()

   2 4 ( ) 2

   = In E 1

   E + E RT (26)

   2 4 3 .272() + 22 (29)

   Distilled water (184.27cm3) was added to quench the reaction and to enhance the seperability of the alum ladden aliquot

   2

   1 1 X 1n

   T2 1 n

   1 T

   (31)

   from the residual silica. The crystal yield obtained from 50ml of filtrate was observed to increase with decrease in temperature. This is in agreement with the established fact that the solubility of alum decreases at reduced temperatures [13,14]. The crystallization stage was achieved by cooling at – 10oC for 3 hours to initate the growth of alum crystals and resultant content is then filtered [2]. The hydrated alum crystals (Al2(SO4)3.27H2O) were gradually dried in an oven between 40oC and 160oC for 12 hrs. to enhance size reduction and seperation, after which the resultant sample is heated at 350oC for 5 hours to remove the occluded acid and the chemically bonded water from the alum [9]. The dried alum was then ground and sieved with 315 microns [2]. Table 2 presents the compositional analysis for fresh alum and pretreated alums at 160oC and 350oC.

   Calcination of the Pretreated Alum

   40 grammes of ground alum in a crucible was calcined in a muffle furnace at 750oC, 800oC, 810o C, 850o C and 900oC and for holding time intervals of 20,60, 120,150 and 180 minutes respectively. The residues were weighed and also subjected to X-ray flurorescence analysis after cooling to room temperature in a desiccator. A simplified decomposition reaction for aluminum sulphate is as shown in Equation 30.

   2 4 3 () 23() + 3() (30)

   KINETIC PARAMETER ESTIMATION USING COATS-REDFERN MODEL

   Kinetic parameter estimation using thermogravimetric data

   The weight losses obtained were converted to conversion at different temperatures and time interval as shown in Table 5. The calculated conversion and temperature values were inputted into the Coats-Redfern model. Plot of

   T2 1n T

   1 1X 1n vs 1 were made. Regressing the left side of the

   above equation using the least square criteria for different values of n, the slope is E/R and the intercept is equal to

   R E; from these various plots, activation energy, E and the pre-exponential term, A were evaluated [9].

   Discriminatory test for X ray Fluorescence results

   The samples were then characterize using a Energy Dispersive

   -Xray Fluorescence machine to obtain the elemental composition by monitoring the extent of conversion of sulphate decomposition.

   RESULTS AND DISCUSSION

   The raw clay, beneficiated clay and metakaolin

   component analysis

   From the results shown in Table 1, it was observed that the alumina content of the beneficiated clay increased significantly over the raw clay from 41.74 wt % to 43.01 wt.

   % while the silica content slightly decreased from 54.56 wt % to 53.95 wt %.

   TABLE 1: CHEMICAL COMPOSITION OF RAW CLAY, BENEFICIATED CLAY AND METAKAOLIN

   Component	Raw Clay %	Beneficiated clay %	Metakaolin %
   Al2O3	41.74	43.01	44.87
   SiO2	54.56	53.95	52.41
   CaO	0.44	0.34	0.32
   Fe2O3	0.53	0.58	0.53
   MgO	1.74	1.31	1.22
   K2O	0.71	0.56	0.47
   Na2O	0.04	0.05	0.04
   TiO2	0.10	0.07	0.07
   Cr2O3	0.01	0.01	0.01
   Mn2O3	0.02	0.02	0.01
   NiO	0.01	0.03	0.02
   CuO	0.08	0.06	0.03
   ZnO	0.00	0.01	0.00

   This could be attributed to the loss of organic material and free silicia respectively during the process of beneficiation. The color of the calcined kaolin changed from white to brick red indicating the presence of Fe3+ [15].

   1 1 X 1n

   R

   2RT E

   2 =

   T2 1 n = In E 1

   E +

   RT (26)

   Coats-Redfern model was adopted to estimate the kinetic parameters such as activation energy, pre-exponential factor and the order of the reaction were obtained by plotting a graph [9]

   Effect of drying on alum

   Drying at 160oC evidently was able to drive off the adsorbed free water on the alum and possibly part of the occluded water, while heating to 350oC was believed to be potent enough not only to eliminate the remaining occluded water but also the occluded excess acid as well. Table 2 showed these effects resulting in increase in concentration of alumina in the two heating steps; while SO2 increased slightly in the first step, evidence of loss of free water and dropped partially in the second step, indicating loss due to occluded acid evacuation.

   TABLE 2: ELEMENTAL COMPOSITION OF FRESH ALUM, THERMAL PRETREATED ALUMS AT 160OC AND 350OC

   Components	Alum
   	Fresh Alum	Dried at 160oC	Dried at 350oC
   Al2O3	8.41	8.7	16.02
   SO3	60.4	61.4	60.81
   CaO	0.46	0.38	0.2
   Fe2O3	0.18	0.24	0.14
   MgO	0.17	0.02	0.31
   K2O	–	0.15	0.01
   Na2O	0.03	0.09	0.11
   TiO2	0.08	–	0.06
   NiO	–	–	0.01

   This observation is consistent with Moselhy et al. (1994) [9] differential thermal analysis (DTA) on a hydrated aluminium sulfate. The pretreated alum at 160oC comprised of about 8.41 wt/wt % of alumina which agrees with Alan et al, (2000) [16] that liquid alum contains about 8 wt/wt%.

   Calcination of the partially dried alum (at 350oC)

   The residual weight of the alum samples (in percentage of the initial quantity) obtained after calcination at different

   temperatures and time intervals are shown in Figure 1. Figure 2 shows the percentage of sulphate decomposed at various temperatures from thermogravimetric analysis. From Figures

   1 and 2, it could be observed that the minimum residual weight of alum samples stood at 12.67g, while the maximum sulfate decomposition stood at 97.37%. The two figures clearly illustrated that as the temperature increased from 750 900oC, the sensitivity of the decomposition reaction increased, this is evident by steeper initial rates which flattens out as the reaction progressed from 20 mins. to 180 mins.

   KINETIC PARAMETER ESTIMATION USING COATS-REDFERN MODEL

   Kinetic parameter estimation from thermogravimetric data

   The TGA data obtained from the decomposition of the pretreated alum were used to generate values to plot -In g(X) against 1/T, by varying the order n, between 0 and 2. Figure 3 is representative of such Coats-Redfern plots for estimating the kinetic and thermodynamic parameters.

   The summary of the regression analysis and the obtained kinetic parameters are shown in Table 3, indicating that reaction order of 1.3 for TGA gave the best average regression value of 0.9446 as compared with other orders within the range of 0 to 2 tested. The estimated average activation energy and pre-exponential factor corresponding to the reaction order

   1.3 were 321.03 kJ/mol K and 2.57E+14 min-1 respectively.

   TABLE 3: AVERAGE VALUES COMPARISION OF REGRESSION ANALYSIS AS OBTAINED FROM COATS-REDFERN METHOD OF ESTIMATING KINETIC PARAMETERS FROM TGA DATA

   Order n	Equation y	Kinetic parameters
   		Regression value R2	Activation energy kJ/molK	Pre- exponential factor / mins
   0.0	y = 2.386x 6.241	0.884	198.33	1.20E+08
   0.5	y = 2.811x 10.412	0.9178	233.69	8.28E+09
   0.67	y = 2.992x 12.168	0.9274	248.77	5.00E+10
   1.0	y = 3.408x 16.163	0.9407	283.13	3.01E+12
   1.2	y = 3.702x 18.983	0.9442	307.81	5.41E+13
   1.3	y = 3.861x 20.504	0.9446	321.03	2.57E+14
   1.4	y = 3.863x 22.097	0.94416	334.90	1.31E+15
   1.5	y= 4.202x-23.7587	0.9272	349.37	7.18E+15
   1.6	y= 4.383x-24.321	0.9409	364.43	4.2E+16
   1.7	y=4.5709x-27.274	0.9384	380.03	2.61E+17
   1.8	y=4.765x-29.118	0.9353	396.13	1.71E+18
   1.9	y= 4.964x-31.02	0.9317	412.71	1.19E+19
   2.0	y= 5.169x-32.97	0.9278	429.71	8.68E+19

   Effect of reaction time on thermodynamic parameters values

   Table 4 revealed that as decomposition reaction progresses as a function of time while the order of the reaction was held constant at n= 1.3, the thermodynamic parameters: activation energy and pre-exponential factor experiences decrease in their values. This may be attributed to decrease in the barrier to heat and mass transfer due to increase in voidage in the sample matrix as the reaction progresses. The higher activation energy at the early stage of calcinations could also be attributed to the energy required for hydroxylation process coupled with sulfate decomposition, while at later times, sulfate decomposition being the sole reaction, which requires lesser energy for its transformation.

   TABLE 4: TYPICAL VARIATION OF THE THERMODYNAMIC PARAMETERS ESTIMATED WITH REACTION HOLDING TIME FROM TGA DATA AT CONSTANT REACTION ORDER OF 1.3

   Time/ mins	Equation y	Regressio n value R2	Activation energy kJ/molK	Pre- exponential factor / mins
   20		0.9410	333.50	5.79E+14
   60	y = 3.7744x 19.406	0.9432	313.80	8.09E+13
   120	y = 3.6326x 18.535	0.9461	302.01	3.26E+13
   150	y = 3.5632x 18.047	0.9513	296.24	1.96E+13
   180	y = 3.5296x 17.817	0.9415	293.45	1.54E+13

   Kinetic parameter estimation from X-ray Fluorescence data Figure 4 is a representative of the Coats-Redfern plots for estimating kinetic parameters for reaction orders of 0, 0.5, 1.0,

   1.3 and 1.5 respectively from XRF conversion data on sulfate decomposition.

   The summary of the regression analysis and the obtained reaction parameters are shown in Table 5, where order of 1.0 for XRF gave the best average regression value of 0.9818.

   TABLE 5: AVERAGE VALUES COMPARISION OF REGRESSION ANALYSIS AS OBTAINED FROM COATS-REDFERN METHOD OF ESTIMATING KINETIC PARAMETERS FROM XRF DATA

   Order N		Kinetic parameters
   	Equation y	Regression value R2	Activation energy kJ/molK	Pre- exponential factor / mins
   0.0	y = 2.431x 6.400	0.8619	202.14	1.38E+08
   0.5	y = 3.246x 13.468	0.9698	269.85	2.48E+11
   0.67	y = 3.494x 15.744	0.9758	290.50	2.61E+12
   1.0	y = 4.056x 20.834	0.9818	337.22	5.27E+14
   1.3	y = 4.509x 25.266	0.9238	374.86	5.47E+16
   1.4	y = 4.739x 27.322	0.9244	394.01	4.79E+17
   1.5	y = 5.114x 30.281	0.9780	425.19	1.08E+19
   1.6	y= 5.229x- 31.680	0.9241	434.70	4.78E+19
   1.7	y=5.487x-33.976	0.9235	456.17	5.41E+20
   1.8	y=5.753x-36.342	0.9225	478.34	6.59E+21
   1.9	y=6.028x-38.776	0.9213	501.16	8.64E+22
   2.0	y=6.592x-43.492	0.9731	548.04	1.28E+25

   The estimated activation energy and pre-exponential factor for the best average regression analysis of order 1.0 were 337.22 kJ/mol K and 5.27E+14 min-1 respectively. Table 6 clearly shows that as the calcinations progressed, the activation energy and pre-exponential factor decreased as the reaction order was held at n=1. This reduction in the activation energy and frequency factor could be attributed to decrease in the diffusion barrier to heat and mass transfer resulting from increase in voidage as the reaction progresses.

   TABLE 6: TYPICAL KINETIC PARAMETERS VARIATION WITH REACTION TIME FROM XRF DATA

   Time / mins	Equation y	Kinetic parameters
   		Regression value R2	Activation energy kJ/molK	Pre- exponential factor / mins
   60	y = 4.270x 16.42	0.9726	355.01	3.76E+15
   120	y = 4.144x 22.344	0.9977	344.52	1.68E+15
   180	y = 3.754x 19.192	0.9752	312.12	6.50E+13

   From Table 7, the kinetic and thermodynamic parameters obtained from XRF data were slightly higher than those from TGA data. This can be attributed to the fact that X ray fluorescence analysis monitors the sulfate decomposition process only, while that of TGA is more or less average of several reactions including the effect of other metallic sulfates acting as impurities (Table 2) present in the prepared alum which could decrease the overall observed activation energy. It worth mentioning that the kinetic parameters obtained from the XRF data were in good agreement with that of Moselhy et al. from derivatograph data using the standard equipment, dynamic thermogravimetric method for data generation (n=1, E

   = 348 kJmol-1 and A = 1.3E+16 s-1)

   Validation of Coats-Redfern model

   To validate the Coats-Redfern model, the assumption of the term 1 2 1. It was observed that after substituing

   typical activation energy , gas constant R, and temperature T.

   The fraction 2RT/E 0.05, was slightly insignificant resulting in a value of the term in the bracket approximately equal to 1. Thus validating the assumption.

3. CONCLUSION

From this study, it can be concluded that reaction orders of 1.0 and 1.3 were most acceptable for XRF data and TGA data respectively; having activation energy and pre-exponential factor of 337.22 kJ/molK (5.27E+14 min-1) and 321.03 kJ/molK (2.57E+14 min-1) respectively. The large activation energy indicates that the reaction is temperature sensitive. It was observed that as the calcination progressed while the reaction order was held constant there was a reduction in the activation energy and pre-exponential factor for both TGA and XRF results. This can be attributed to decrease in the diffusion barrier to heat and mass transfer most likely due to increase invoidage as the reaction progresses. Based on the assumption considered for Coats-Redfern model, the model was found to fit the rate data obtained satisfactorily.

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