Iranian Journal of Materials Science & Engineering V ol. 8, Number 1, W inter 2011

KINETIC STUDY OF SYNTHESIS OF BY METHANOTHERMALREDUCTION OFTITANIUM DIOXIDE

R. Alizadeh 1,* and O. Ostrovski 2 * [email protected] Received: August 2010 Accepted: January 2011

1 Department of Chemical Engineering, Sahand University of T echnology, T abriz 51335-1996, Iran. 2 School of Material Science and Engineering, The University of New South W ales, UNSW Sydney, NSW 2052,Australia

Abstract: Reduction of the T itanium dioxide, T iO2, by methane was investigated in this work. The thermodynamic of reaction was examined and found favorable. The r eaction of with methane was carried out in the temperature range 1 150°C to 1450°C at atmospheric pressure with industrial high porosity pellets prepared from titanium dioxide powder. The evolved gas analyzing method was used for determination of the extent of r eduction rate. The gas products of the r eaction are mostly CO and trace amount of CO2 and H2O. The synthesized product powder was characterized by X-ray diffraction (XRD) for elucidating solid phase compositions. The effect of varying temperature was studied during the r eduction. The conversion-time data have been interpreted by using the grain model. For first order r eaction with r espect to methane concentration, the activation energy of titanium dioxide reduction by methane is found to be 51.4 kcal/gmole. No detailed investigation of kinetic and mechanism of the reaction was r eported in literatures.

Keywords: Titanium Dioxide, Reduction, Methane, T itanium Carbide, Kinetic S tudy.

1. INTRODUCTION synthetic rutile or T iO2-rich slag [3]. Titanium oxycarbide can be chlorinated at low Titanium metal has become known as a space- temperatures. Mostert et al.[4-5] reported that age metal because of its high strength-to-density carbonitride produced by reduction/nitridation of ratio and inertness to many corrosive from and titanium slag environments. T itanium is mainly utilized in the was chlorinated at 200-500 °C. In the low form of titanium dioxide (TiO2) as paint filler, temperature chlorination, impurity-oxides do not paper, rubber and plastic industries [1], and the chlorinate or chlorinate very slowly [6]. This demand for titanium pigment is growing steadily permits selective chlorination of titanium [2]. T itanium minerals are also processed to oxycarbide, decreases the chlorine consumption produce metallic titanium and titanium and waste generation, and makes the whole compounds, particularly titanium carbide (TiC) technology of ilmenite processing more efficient Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 and nitride (TiN), which are used in the and environmentally friendly. manufacture of composite materials and Kinetic study of the T itanium dioxide or as catalyst. reduction by methane was not reported in Production of T itania white pigments and literature. The purpose of this work is to obtain metal titanium include processing of titanium kinetic parameters of reduction of T itanium minerals to . Chlorination dioxide, T iO2, with methane, toward assessing of titanium dioxide is carried out almost the feasibility of using methane to produce exclusively by fluidised-bed process, which metallic titanium and titanium compounds. requires high temperature of 800-1100 °C, and involves the use of petroleum coke (250-400 kg/t 2.THERMODYNAMIC CONSIDERATIONS TiO2) as the reducing agent. Existing technology for titanium tetrachloride production requires The reduction of titanium dioxide with minerals of high quality, with low impurities carries out in the temperature range 1200 K to level, which are processed prior to chlorination to 1500 K. The Free enthalpy change and the overall reaction are as follows [7]:

1 R. Alizadeh and O. Ostrovski

Table 1. Equilibrium constants for carbothermal (1) and methanothermal reduction of (2). TiO3 C TiC 2 CO ' H0 124 kcal / gmo l 2 rxn (1) T (K) K K 10 4 K 10 15 CH 4 C H2 1300 7 16 3 4 Methane also has a strong reducing capability, and can react at lower temperature simply: 1400 1 4064 100 80

TiO3 CH TiC 2 CO 6 H ' H0 190.7 kcal / gmo l 1500 1 3645 8831 2000 1000 2 4 2 rxn (2)

Consider a general equation, as follows:

¦XRR ¦ X P P (3) The experimental setup is shown in Figure 1. The vertical type electric high temperature furnace where R and P represent reactants and (Model HT 08/17, Engineering, X X products, and R and P are the stoichimetric Australia) has been used for heating the reactor to coefficients of reactants and products, the reduction temperatures. The pellets were respectively. The equilibrium constant of this made using titanium dioxide powder. The system reaction can be calculated from the following was heated to the desired temperature under an equation [7]: inert gas stream such as argon. Then the isothermal period begins and after temperature X  X logK¦P log K fP, ¦ R log K fR , (4) stabilization, the reducing gas (a mixture of CH4/H2/Ar) is introduced from the top of the where K is the equilibrium constant of the reaction tube. The gas streams passed through the reaction and Kf,R and Kf,P are equilibrium Hydropurge gas purifiers filled with 4A constants for the formation of reactants and molecular sieve to remove moisture. The products, respectively. From equation (4), the composition of reducing gas was achieved by equilibrium constants of Reactions (1) precisely controlling the flow rate of each gas (carbothermal reduction) and (2) with Brooks mass flow controllers. Direct (methanothermal reduction) are computed and measurement of temperature in the vicinity of the K tabulated in T able 1 in which Kc and CH 4 are the solid pellet was difficult to implement. A equilibrium constants for the Reactions (1) and thermocouple was shielded by a sheath and the (2) respectively. T able 1 shows that methane temperature of gas mixture at the inlet of reaction reagent has greater reducing capability than chamber was measured. The reducing gas was the carbon, which creates a potential to decrease the mixture of methane with high purity (99.95%), operating temperature of reduction of T itanium hydrogen with super high purity (99.999) and Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 dioxide. argon with high purity. The use of argon as an Table 1 also shows the equilibrium constant inert gas facilitated adjusting of gas composition K for hydrogen reduction ( H2 ) of titanium and provided a reference for calculation of off- dioxide. From this we can discover that hydrogen gas flow rate. The gases were provided by BOC reaction with T iO2 is thermodynamically Gases, Australia, in gas cylinders. impossible. Hydrogen reduction of titanium The scheme of the reactor used in the dioxide can be represented as follows: experiments is depicted in Figure 2. A recrystallised alumina tube of 8.6 mm inner   TiO22 H 2 Ti 2 HO 2 (5) diameter was used as a sample holder. A porous magnesia plug was fixed at the tube bottom by an alumina pin. The temperature at the inlet of the 3. EXPERIMENTAL SETUP tube was measured by a type B thermocouple and is referred as the reduction temperature. The The evolved gas analyzing method was used inner tube was inserted into an outside for determination of the extent of reduction rate. recrystallised alumina sheath of 19.0 mm inner

2 Iranian Journal of Materials Science & Engineering V ol. 8, Number 1, W inter 2011

particles were calculated by using the average specific surface area of pellet that measured by BET analysis. Titanium dioxide pellets were made by mechanical pressing of the powder in a pressing mould (1500 kg/cm2 ). The pellets were assumed semi-spherical in kinetic modeling. The reduction experiments were carried out in the temperature range of 1 150-1450°C under atmospheric pressure. Experiments were conducted with excessive reducing gas. Preliminary experiments showed that the reaction rate was not affected by the gas flow rate if the Fig. 1. Flow diagram of experimental system for kinetic flow rate is greater than 1 L/min, therefore a total study of reduction of titanium dioxide. gas flow rate was maintained at 1 L/min which means that external mass transfer resistance was negligible during the reaction. Phase composition of products was measured by powder X-ray diffraction (XRD, Simens D5000). A sample was ground to fine powder and closely packed in the cave of a plastic sample holder. The diameter of the cave was 10mm. The X-ray Diffractometer has a monochromator and a copper K D X-ray source. The voltage and current exerted on to the X-ray emission tube were set at 30 kV and 30 mA, respectively. The scanning was performed from 20 to 80°. The scanning rate of samples of reduced titanium dioxide was set at 0.75° /min with a step of 0.05 °.

4. FORMULATION OF KINETIC MODEL Fig. 2. Schematic of experimental reactor. The methanothermal reduction of titanium dioxide is expressed by Equation (2). The Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 diameter. The inner tube, outside sheath and “conversion” is defined as the oxygen loss of a thermocouple sheath were fixed by special metal sample at a given time divided by the sample fittings and sealed with O-rings. total oxygen when it is unreacted, i.e., total The outlet gas stream also leaves the system conversion is defined when all oxygen atoms are from the top of reaction tube, firstly passed liberated from reactant sample. Thus, the through a dew point sensor for evaluating conversion-time curves can be obtained from the content of the outlet stream then passed from the gas analyzer data using the following relation: online infrared CO/CO2/CH4 gas analyzer. ªt º X (2 nCO  n  CO  ndt  H O )2 WM0 TiO The mole fraction of methane in the input gas ¬«³0 2 2 ¼» 2 (6) mixture was fixed at 5 %. The titanium dioxide  nCO n nH O (TiO2) powder employed in the study is rutile where , CO2 and 2 are molar rate of CO with a T iO2 content of 99.9+ % and with a mean , CO2 and H2O production respectively and W0 is MTiO particle of about 2 microns that was supplied by the initial pellet weight, 2 is the molecular Aldrich Chemical Company, Inc. (Milwaukee weight of titanium dioxide. WI, USA, Catalog number 22422-7). The sizes of The grain model introduced by Szekely et al.

3 R. Alizadeh and O. Ostrovski

was used to analyze experimental data. This wr* = -a model is well described in the literature [8] and wTg (9) only a brief development is offered here. The titanium dioxide-methane reaction may be With the initial and boundary conditions: represented by the following equation:

Tg 0,r 1 (10) A()gas bB () soid o cC () gas  dD () solid (7) wa y 0, 0 where b and d are stoichiometric coefficients wK (11) of the solid reactant B and solid product D, respectively and c is the stoichiometric coefficient of the product gas. The modeling is y 1, a 1 (12) based upon the following assumptions: a C C where A Ab is dimensionless gas 1. The pellet retains its initial size throughout concentration, CA and CAb are the concentrations of the reaction reactant A in position K with in a pellet and in bulk 2. The reaction system is isothermal gas, K is the dimensionless radius of the pellet, r r/ r 3. The external mass transfer resistance is gc g0 is dimensionless unreacted radius in the negligible grain, rg0 and rgc are initial and unreacted core T 4. The reaction is irreversible and first order radius of grains, respectively, g is dimensionless with respect to methane concentration time for grain model defined as follows: 5. The pseudo-steady approximation is valid. bkC t T Ab g U r The solid reactant is visualized as being B g 0 (13) composed of a large number of highly dense, spherical grains. Each of these grains reacts where k is reaction rate constant; b is the time U individually according to unreacted shrinking and B is the true molar density of solid reactant core model. In the overall pellet, however, the B; V is the gas-solid reaction modulus. By reaction occurs in a zone rather than at a sharply ignoring structural changes of the pellet, for flat defined boundary. Reactant gas undergoes mass pellet consisting of spherical grains this modulus transfer from the bulk gas stream to the pellet is defined as: surface. From the surface the gas must diffuse to arrive at a sharp interface between the grain 3.(1H ) k V R 0 particle and the product layer for reaction to take D r (14) eA g 0 Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 place. H where 0 is the pellet porosity which was 4. 1. Calculation of Rate Constant estimated from the volume and weight of a pellet, and DeA is effective diffusion coefficient of In the starting of reaction, for calculating of gaseous reactant A in the pellet. DeA was rate constant we use simple grain model and we estimated using the random pore model [10]: neglect the ash layer resistance in the vicinity of 2 zero time for first step. The dimensionless DeA D A H0 (15) governing equations of the simple grain model for spherical pellet with spherical grains are as where DA was determined by combining follows [9]: molecular and Knudsen diffusion coefficients as w2a2 w a follows:  =V 2 .a.r 2 2 * (8) 1 1 1 wK K w K  (16) DA DAM D AK

4 Iranian Journal of Materials Science & Engineering V ol. 8, Number 1, W inter 2011

DAM and DAK are molecular and Knudsen Fg diffusivity of gaseous reactant A in the pellet, x 1  ( r ) (21) respectively. DAM was evaluated using the Chapman-Enskog formula [11] and the DAK was where x is the local conversion of the grains calculated from the following equation [8]: and Fg is the grain shape factor, which equals as 1, 2 and 3 for slablike, cylindrical and spherical 1 4 §8R T · 2 grains, respectively. However, conversion of D g k AK ¨ ¸ 0 (17) pellet (X) is defined as overall conversion and is 3 ©S M A ¹ calculated from local conversion integration over where Rg is the gas constant, T is the absolute the pellet as follows:

temperature, MA is the molecular weight of 1 X 1  3K 2 .(r ) 3 . dy gaseous reactant and K0 was calculated from the ³0 (22) ‘dusty gas model’ of Mason et al. [12] as follows: 1§ 128 · §S · If the Eq. (20) was combined with Eq. (22) and n r 2 1  ¨ ¸d g ¨ ¸ (18) then integrated, the conversion of pellet could be k0 ©9 ¹ © 8 ¹ found at the time near zero. Then the slope of nd , the number of solid grains per unit volume conversion time curve in the vicinity of the zero of porous solid, was calculated as follows: can be shown to be as follows: 3(1H ) dX9b . k . C [1V  e 2V ( V  1)] n 0 Ab . d 4S r3 (19) dtU. r [( e 2V  1) V 2 ] (23) g t 0 B g 0

By using above equation, the governing All of the above nonlinear equation parameters

equations can be solved and r was found in the are known except k, which is unknown. By time near zero as follows: solving the above equation, rate constant for the Sinh(V . y ) reduction of titanium dioxide by methane was r 1  .T Sinh (V ) g (20) calculated and listed in T able 2. Extent of reaction or conversion for non- 5. RESULTS AND DISCUSSION porous grains is defined as local conversion and is the ratio of mass of solid product produced at Reduction of T itanium Dioxide by methane each time to mass of solid product if the grain was examined at different temperatures using a converts to solid product completely; so for methane-hydrogen-argon gas mixture containing grains we have: only 5 vol% methane. Figure 3 presents the

Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 100

90

80

70 ) 60 on i s er 50

onv 1150 C % ( 40 1200 X 1250 30 1300

1350 20 1400

10 1450

0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 Time(min)

Fig. 3. Isothermal reduction curves of titanium dioxide (5 vol% CH4,75 vol% H2 and 20 vol% Ar) at different temperatures.

5 R. Alizadeh and O. Ostrovski

Table 2. Rate constants for the reduction of T itanium Dioxide by methane at different temperatures T (°C) 1150 1200 1250 1300 1350 1400 1450

k( cm / s ) 10 3 0.8 2.0 3.7 6.3 13.8 15.2 18.3

conversion-time curve of T itanium Dioxide reduction at different temperature. The XRD patterns of solid product for different temperatures are presented in figure 4. Their analysis clearly indicates that the majority the solid phase is T iC. In the metal oxide reduction reactions with methane, the reduction process starts with adsorption of methane on the active sites of the oxide surface and its decomposition described by the following reactions [13]:

CH4() gas CH 4 () ad (24)

 CH4() ad CH 3 () ad Had ( ) (25)

CH3() ad CH 2 () ad  H () ad (26)

 CH2 () ad CHad () Had () (27)

Fig. 4. XRD patterns of T iC formed at different  temperatures. CH() ad C () ad H () ad (28)

where k’ is the frequency factor and Ea is the

2()H ad H2 ( gas ) (29) apparent activation energy. Arrehenius plot for this reaction is shown in Figure 5. From the Downloaded from ijmse.iust.ac.ir at 2:40 IRST on Sunday October 3rd 2021 The overall reaction of methane adsorption resulting straight line, the temperature and cracking may be presented as: dependency of the reaction rate constant is as follows:

CHo o C  2 H 25876 4ad 2 (30)  k 8.1 u 104 e T (32) In which Cad represents active carbon species adsorbed on solid surface, and is substantially From equation (32) the activation energy of different from deposited solid carbon. The the T itanium Dioxide reduction is found to be reaction rate constant calculated using equation 51.4 kcal/gmole. (23) is presented in T able 2. One of the most important parameters of the The effect of temperature on the reaction rate reduction reaction is the apparent activation can be evaluated by Arrhenius equation: energy as it defines the reactor dimensions and

E the energy consumption. Making comparison for  a k kc e Rg T (31) activation energy of methanothermal reduction reaction is more interesting. The activation

6 Iranian Journal of Materials Science & Engineering V ol. 8, Number 1, W inter 2011

Table 3. Comparison of activation energy of methane reduction reactions. Me tal oxid e T (K) Reference Ea (kcal/gmole)

Fe2O3 1148-1223 52.70 [14] ZnO 1113-1203 67.09 [15] PbO 9 73-1123 51.62 [16] CoO 1073-1223 37.18 [17]

Cr 2O3 1143-1248 47.09 [18] NiO 873-998 63.89 [19]

BaSO 4 1173-1248 96.00 [20]

-3.5 51.4 kcal/gmole, which is within the same order -4 of magnitude of other metal oxide

-4.5 methanothermal reduction.

-5 r = 0.9 8 ACKNOWLEDGMENT -5.5 Ln(k)

-6 The financial support for this research project

-6.5 has been provided by the Australian Research Council that is gratefully acknowledged. The -7 authors wish to acknowledge to Professor G . -7.5 5 5.5 6 6.5 7 7.5 Zhang who participated in this project and was so 10000/T generous in sharing his knowledge and expertise. Fig. 5. Arrhenius plot of the rate constant of the reaction of Titanium Dioxide with methane REFERENCES

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