Investigation of the chlorination mechanism of metal oxides by chlorine

Journal of Materials Processing Technology 142 (2003) 145–151

Investigation of the chlorination mechanism of metal oxides by chlorine N.V. Manukyan, V.H. Martirosyan∗ Department of Chemical Technologies and Ecology, State Engineering University of Armenia, Teryan Str. 105, Yerevan, Armenia Received 3 May 2002; received in revised form 4 February 2003; accepted 26 February 2003

Abstract The mechanisms of chlorination of metal oxides by chlorine in various media are reviewed. The contacting-diffusion mechanism was proposed based on the investigation of chlorination processes and the data analysis. Chlorination of the oxides in carbon-thermal media is carried out via two-step mechanism, e.g. through the formation of oxy-carbochlorides and then, chlorides. The proposed mechanism is applicable for all types of oxide compounds. © 2003 Elsevier B.V. All rights reserved. Keywords: Oxides and chlorides; Carbon-thermal and gaseous media; Reduction and chlorination processes; Thermodynamic characteristics and data; Reaction mechanism

Chlorination of metal oxide by chlorine can be described: Mex Oy + 21 xzCl2 → xMeCl2 + 21 yO2 where x, y are the stoichiometric coefficients, and z the valence of the metal. Analysis of the data, presented in Table 1, shows that carbon enhances chlorination process, i.e. when oxygen is removed from the reaction zone:

Equilibrium of reaction (2) shifts to the right along with the increase of temperature, e.g. towards higher CO content range. Then the partial pressure PCO2 drops, while PCO increases. Equilibrium condition responds to chemical potentials of carbon phases, when πC cond. = πC gas . Carbon moves into the condensed state when πC cond. < πC gas , and carbon gasifies, if πC cond. > πC gas . The mechanism of chlorination of oxides is complicated and much remains here to be revealed. Particularly, the chlorination initiation temperature is still unknown. Nevertheless, as it was shown in [1], the higher is the chlorination ∗ Corresponding author. E-mail address: [email protected] (V.H. Martirosyan).

0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00595-8

initiation temperature the larger is the periodic number of the element forming oxide. According to [2], carbon presence lowers the chlorination initiation temperature during chlorination of oxides. Also, carbon presence reduces the overall reaction temperature. However, the chlorination temperature of oxides coincides for both the oxidizers and reductants’ presence as was shown in [3]. Thus, the chlorination process can be described as follows [2]:

As per the given scheme, the oxide (Mex Oy ) is reduced up to its corresponding metal (Me), which interacts with chlorine (Cl2 ) to produce chloride (MeClz ). This mechanism could prove valid for the oxides prone to easy reduction, because chlorination occurs at early stages of the process [4]: for NiO at 523 K, MoO3 → 673 K, Fe2 O3 → 723 K. However, the proposed stepwise reaction pathway (4)–(6) cannot be applied to hard-to-reduced oxides, such as Al2 O3 , Cr2 O3 , TiO2 , etc. Solid reductants (graphite, soot, coke, etc.) are used for industrially produced metal chlorides obtained from oxides’ chlorination. However, the role played by the reductants are not understood fully, particularly that of the carbon. As it was already mentioned, the oxygen—displaced by

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N.V. Manukyan, V.H. Martirosyan / Journal of Materials Processing Technology 142 (2003) 145–151

from the point that phosgene dissociates (COCl2 CO + Cl2 ) at over 923 K, with follow-up loss of its capacity [10]:

Table 1 G◦T values for the chlorination reactions of oxides N

1 2 3 4 5 6 7 8 9 10 11 12 13

Reactions

NiO + Cl2 ↔ NiCl2 + 0.5 O2 FeO + Cl2 ↔ FeCl2 + 0.5 O2 MgO + Cl2 ↔ MgCl2 + 0.5 O2 Al2 O3 + 3Cl2 ↔ 2AlCl3 + 23 O2 Cr2 O3 + 3Cl2 ↔ 2CrCl3 + 23 O2 TiO2 + 2Cl2 ↔ TiCl4 + O2 TiO2 + 2Cl2 + C ↔ TiCl4 + CO2 TiO2 + 2Cl2 + 2C ↔ TiCl4 +2CO TiO2 + 2Cl2 + 2CO ↔ TiCl4 +2CO2 C + 21 O2 ↔ CO C + O2 ↔ CO2 CO + 21 O2 ↔ CO2 C + CO2 ↔ 2CO

G◦T (kcal/mol) 1273 K

1237 K

−9.20 −7.60 4.00 19.50 14.30 19.00 −57.70 −51.20 −64.30 −43.10 −47.20 −51.58 −8.56

−6.40 −5.20 6.20 31.95 18.60 37.60 −64.10 −79.40 −50.90 −57.70 −47.30 −41.20 −12.56

chlorine—moves towards carbon and interacts with it thus producing CO or CO2 , as per reactions (1)–(3). Nevertheless, the carbon would have not necessarily contact with the oxide directly. While investigating chrome oxides’ chlorination, Magidson et al. [5] proposed another mechanism. The reactions sustain thanks to CO, which is obtained from oxidation of coal by oxygen adsorbed on its surface (C+(1/2)O2 = CO), i.e.:

As it can be seen from (7), chlorination takes place with the participation of CO, rather than with solid carbon (C), which serves only for CO generation (8). Adsorptive-catalytic action of carbon during chlorination of the oxides is elucidated in [6,7]. The first stage includes the formation of chlorcarbonic radicals: C + Cl2 → CClads. → C+ · · · Cl2 − , Cm+ · · · Cl− 2

Temperature, T (K)

Degree of dissociation of COCl2 , α (%)

573

673

773

1073

3.9

19.8

55.0

100.0

Chemosorption of chlorine on the carbon’s surface is accompanied by its dissociation: Cl2 → 2Cl∗ . The energy of rupture of Cl–Cl bond is 57.9 kcal/mol, and the dissociation constant is 1.56 × 10−4 , at 1300 K. The calculated values of G◦T for the chlorination of SiO2 and Al2 O3 by the chlorine molecule and chlorine atom are presented in Fig. 1. As it was anticipated, oxides’ chlorination by chlorine atom proceeds much easier than with the molecular chlorine. Thus, for many instances, proposed mechanisms of chlorination of oxides are contradictory and do not describe fully the physico-chemical processes occurring in the Mex Oy –Cl2 –C system. In this regard, let us examine some properties of the components of this system from the standpoint of their reduction and chlorination. There are two basic types of inter-atomic bonds in metal oxides: covalent and ionic. For readily reducing oxides (CuO, V2 O5 , MoO3 , etc.), the share of ionic bond is 30–40%, whereas for hard-to-reducing ones is 60–70%. The lesser is the r (the distance between the ions) and the larger are the z1 and z2 the stronger is the ionic bond. The ionic mobility increases considerably upon the elevation of temperature, while the electron mobility is decreasing. Bivalent metal oxides are crystallized in NaCl lattice-structure type, where the lattice consists of four oxygen anions and metal cation; for the trivalent ones—in the corundum type (␣-Al2 O3 ) having rhombohedral lattice, and tetravalent metals—that of a tetragonal cubic lattice. The so-called degree of ionicity (the difference of electronegativity as between the oxygen and metal) corresponds to their melting temperature (Tmel. ) and to the commencement of interaction with carbon (Tc.i. ), in other

(10)

while at the second stage these radicals interact with oxides. According to Stefaniuk and Morozov [8], at first, the carbon catalyses the formation of phosgene, being the active chlorinating agent: CO + Cl2 → COCl2

(11)

MeO2 + 2COCl2 → MeCl4 + 2CO2

(12)

C + CO2 → 2CO

(13)

Preference of COCl2 against the Cl2 are not supported in several works. Thus, Voronin and Galinker [9] found no difference with these two compounds while chlorinating Al2 O3 . Phosgene (COCl2 ) enacts below 923 K, which is explicable

Fig. 1. Free energy change for the following reactions: (1) SiO2 + 2Cl2 → SiCl4 + O2 ; (2) Al2 O3 + 3Cl2 → 2AlCl3 + (3/2)O2 ; (3) SiO2 + 4Cl → SiCl4 + O2 ; (4) Al2 O3 + 6Cl → 2AlCl3 + (3/2)O2 .

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Table 2 Dependence Tmel. and Tc.i. of metal oxides on the degree of ionicity Metal oxides

Electronegativitya of metals (eV)

Difference of electronegativities (eV)

Melting temperature of oxides (K)

Reaction beginning with carbon (K)

Radius of cation (metal) (Å)

ZrO2 Al2 O3 TiO2 WO3 MoO3

1.5 1.5 1.6 2.0 2.1

2.0 2.0 1.9 1.5 1.4

2973 2323 2130 1748 1068

1823 1800 1103 993 690

0.82 0.57 0.64 0.65 0.65

a

Electronegativity of oxygen = 3.5 eV.

words—with the chemical activity of an oxide (Table 2). As it follows from Table 2, Tmel. and Tc.i. temperatures are elevating when the ionicity of an oxide increases, therefore, the chemical activity of oxides subside. This trend follows also from G◦T data for the chlorination reactions of Al2 O3 and TiO2 at 773 and 1273 K (Table 1, reactions (4) and (6)). For the majority of oxides, the stoichiometric composition is not upheld (Table 3), i.e. they possess so-called subtraction-vacancy structure in the oxygen and metallic sub-lattices impurities and imperfections of a crystalline structure result in non-stoichiometric composition of oxides. Deviation of oxide compounds from stoichiometric composition affects strongly on their thermodynamic characteristics ( G◦T , S), and consequently, on the processes of reduction and chlorination. In oxides, metal cations have greater mobility, than that of anions of oxygen (according to Holdshmidth, rO2− = 1.3 Å). Wagner [11] presumes that the anions are practically immobile. Thus, the rate of diffusion of Zn2+ is higher to the order of 3, than the rate of diffusion of O2− in ZnO at 1373 K. Nevertheless, in some works, the immobility of oxygen anion is disclaimed, because it is impossible to explain redox processes without taking into account oxygen diffusion. Lebedev [12] calculated that rO2− = 0.46 Å, which considerably is lesser than for the ions of many metals. Earlier, a hypothesis was proposed by Yesin and Geld [13], according to which displacement of oxygen occurs in an atomic manner (rO = 0.6 Å). Based on this consideration, Vorontsov has shown that the diffusion rates of cations and anions are somehow comparable [14]. Unfortunately, these and other similarly performed works are not supported experimentally. As the metals possess various chemical affinity towards oxygen, the thermodynamic strength of their oxide com-

pounds is determined by G◦T = f(T) relationship. The thermodynamic characteristics of some oxides are presented in Table 4. Ca, Mg, Al, Zr, Ti, Si, Cr produce durable compounds, while Mo, Cu, Fe, Ni, Co—less durable ones, etc. The thermodynamic strength of carbon monoxide (CO) increases along with the temperature elevation ( G◦T = −96.0 to −136.6 kcal/mol O2 ), and this is considered as an exception. This means that practically any metal oxide can be reduced by carbon at high temperatures. From Table 4 it follows, that the thermodynamic strength of higher oxides decreases with the elevation of temperature (for the metals having various valences), while the thermodynamic strength of lower oxides increase. Therefore, the higher oxides (Fe2 O3 , MoO3 , V2 O5 , TiO2 , etc.) are reduced more easily, than the lower ones (FeO, VO, TiO, etc.). Chlorination of higher oxides proceeds with difficulties compared to lower ones. Let us examine interaction of metal oxides with carbon, the mechanism of which is still considered unclear. As it was said, carbon monoxide (CO) is the reductant for (7)–(9) reactions, while the role played by carbon (C) is reduced to its regeneration. According to adsorptive-autocatalytic theory of reduction [13,15], at first, the gas-reductant (CO) adsorbs on the surface of an oxide, then the oxygen dislodges from an oxide with further formation of gas phase (CO2 ), and thus removes out of the lattice-structure of metal [Me]: MeO + CO → MeO · (CO)ads. → Me(CO2 )ads. → Me + CO2 ↑

(14)

As it can be seen, the mechanism of reduction of oxides involves chemical interaction of substances in an adsorptive (surface) layers with follow-up desorption of

Table 3 Non-stoichiometricity of metal oxides The stoichiometric formula of oxides TiO2 Ti2 O3 TiO FeO V2 O5 WO3

Form of compound With a deficiency of cations

With a deficiency of anions

– Ti(0.93–1.0) O Ti(0.7–1.0) O Fe(0.95–1.0) O – W(0.98–1.0) O3

TiO(1.9–2.0) TiO(1.42–1.5) TiO(0.6–1.0) – VO(2.47–2.5) WO(2.95–3.0)

Limits of a homogeneity of an oxide on oxygen 1.90–2.00 1.42–1.57 0.60–1.30 1.00–1.05 2.47–2.50 2.95–3.00

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Table 4 Thermodynamic characteristics of formation of oxides Oxides

Temperature of formation, T (K)

− H ◦T (kcal/mol O2 )

− S ◦T (kcal/mol K)

− G◦T (kcal/mol O2 )

−log Po2

Al2 O3

1000 2000

254.2 229.6

−0.0373 −0.0313

216.73 166.60

51.36 18.20

TiO2

1000 2000

214.5 195.6

−0.0324 −0.0281

182.10 139.40

39.80 15.20

Ti3 O5

1000 2000

220.9 199.4

−0.0285 −0.0233

192.40 152.76

42.08 16.68

Ti2 O3

1000 2000

227.1 203.0

−0.0289 −0.0224

198.27 158.27

43.33 17.27

TiO

1000 2000

246.1 246.6

−0.0404 −0.0397

205.70 166.70

44.90 18.10

V 2 O3

1000 2000

181.0 156.8

−0.0263 −0.0193

154.73 118.07

33.80 12.87

WO3

1000 2000

123.4 94.3

−0.0299 −0.0183

93.47 57.67

20.40 6.27

CO

1000 2000

42.4 25.8

0.0536 0.0554

96.00 136.60

21.00 14.80

gaseous reaction products. Higher oxide lattice is restructured into that of a lower oxide lattice, and then into metal lattice-structure, along with the oxygen depletion. Therefore, reduction1 of oxides is carried out by stepwise mechanism of topochemical reactions: Fe2 O3 → Fe3 O4 → FeO → Fe

(15)

TiO2 → Ti3 O5 → Ti2 O3 → TiO → Ti

(16)

The adsorptive capacity of an oxide (MeO) and graphite (C) have an immense impact on reaction activity, and consequently, on reduction and chlorination processes. Thus, hydrogen (H2 ) reduces Fe2 O3 many-fold faster, than carbon monoxide does (CO). However, H2 reduces CuO much slower than CO. This is explicable from the differences of mutual adsorption of oxides and reductants. By the way, the adsorbance of H2 O, H2 , Cl2 , CO and CO2 on graphite is rather good. To be stated also, that usually the adsorption is an exothermic process, while the desorption is an endothermic one. It is interesting to clarify the role of carbon being both in the solid state and gaseous state, and elucidate the true mechanism of reaction (14). For this end, it has to be revealed as whether CO and CO2 are the main products of the reactions of this type: MeO + C → Me + CO(CO2 )

(17)

when metal oxide directly interacts with carbon, or they simply are the products of indirect reduction: MeO + CO → Me + CO2

(18)

1 Similar mechanism holds for the oxidation reactions, e.g. from metal up to higher oxide (Fe–Fe2 O3 , Ti–TiO2 ).

CO2 + C → CO

(19)

In [16], the gaseous media was formed through the interaction of metal oxides with carbon at up to 973 K. As it turned out, no gasification of carbon (C + CO2 → 2CO) occurs. The experiments were conducted on Fe2 O3 , V2 O5 , MoO3 , WO3 and soot (C). For the elimination of side reactions, neutral helium gas served as a carrier aiming to remove the reaction products (C and CO2 ). Gas media characteristics were monitored by chromatographic analysis tool method running in non-stop (continuous) mode. Experiments unequivocally showed that C and CO are the primary products of reaction (17). Consequently, the carbon-thermal reduction process is performed by the contact-diffusive mechanism, i.e. via the direct reduction. Evacuation of carbon monoxide (CO) from the reaction zone by a stream of helium gas would have to retard the reduction processes if the reduction was carried out by the carbon monoxide. According to authors, boosting the helium stream velocity speeds up the reduction process. Analysis of the data [16] shows that reduction reactions are carried out via oxides’ removal into the gaseous state with follow-up transportation onto the carbon particles’ surface. High vapor pressure elasticity of oxide supports this mechanism at elevated reduction temperatures. As it is apparent from the above said, even for reduction of oxides, the role and mechanism of an action of carbon remains disputable, not speaking of its participation in chlorination processes. Magidson et al. [5] assumes that the carbon—in the form of CO—participates in chlorination reactions (7)–(9). Essentially, it is similar to (1)–(3), except for the participation of CO, instead of C. Formal description of a reaction (7) is of no help as how the CO reduces the

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Mex Oy . In a molecular form, the CO—taking into account that rCO = 1.6 Å—is incapable to diffuse into the oxide (Mex Oy ). The oxygen potential of CO (πCO ) is higher, than of C (πC ), so reduction capability of CO is much lower. Besides, the thermodynamic strength of CO considerably increases along with temperature increase (Table 4), especially in between the 1273 and 1473 K range, where practically the processes of chlorination are carried out. The formation of chlorine–carbon radicals (CCl, CCl2 , . . . ) and phosgene (COCl2 ) being referred, and so their impact on reduction and chlorination processes, is of catalytic in nature, and could not serve as an argument and proof of the mechanism’s path. In Mex Oy –Cl2  system, the chlorine is not capable to displace the oxygen completely, even during its dissociation (Cl2 → 2Cl∗ ), as the ionic radii is 1.81 Å. Besides, the oxygen (3.5 eV) is more electronegative, than chlorine (3.0 eV). Nevertheless, the thermodynamic calculations show the possibility of some reactions of the type (Table 1, − G◦T values for (1) and (2) reactions); however, there is no valid mechanism described for these reactions to proceed. Thus, reactions (1) and (2) lose their sense, and so does the whole process of chlorination represented by (1)–(3) and (4)–(6) reactions. Our investigations permit to propose a new interpretation of contact-diffusion mechanism of oxides’ chlorination by the solid carbon. The basic prepositions come down to the following. The carbon-thermal reduction of oxides run by two-step mechanism:

For example, via the formation of Mex (Oy , Cz ) oxy-carbides. It is not to be excluded, that reaction (20) can be accompanied by the partial reduction of an oxide, with the increase of a carbon potential:

149

The reduction is carried out by the direct contact of an oxide both with the carbon (C), and with the gas phase (CO) via disproportion reaction: 2CO → CO2 + C∗ and thermal dissociation: CO → C∗ + O. Carbon atom, C∗ (rC4+ = 0.2 Å) is more active and easily introduced into the crystal lattice-structure of an oxide, thus disarranging it. The carbon-thermal reduction of refractory metals’ oxides differs in the sense that the reaction path predisposes towards enhanced carbonization and carbide-formation: ZrO2 + 2C ↔ Zr + 2CO,

G◦1 = 198 600 − 81.4T (26)

ZrO2 + 3C ↔ Zr + 2CO,

G◦2 = 158 600 − 89.1T (27)

G◦1

G◦2

= 37.9 kcal and = −17.2 kcal Gibbs energy, at T = 1973 K. Consequently, reaction (26) is less probable (+ G◦1 ) thermodynamically, than reaction (27), for which the G◦2 value has a negative sign. In Table 5, the thermodynamic characteristics of oxides, oxy-carbides and carbides of vanadium of various compositions are presented. As we can see, the enthalpy of formation of oxy-carbide in the V–O–C system decreases with the increase of carbon content. Content of oxygen also decreases, and thermodynamic strength of oxy-carbide V(O, C). Oxy-carbides occupy an intermediate position as between the oxides and carbides as per their heat of formation values ( H ◦298 ). The same peculiarity holds for all oxy-carbide compounds. Thus, the thermodynamic strength of oxy-carbides in Ti–O–C system (Table 6) decreases during titan monoxide’s (TiO) transformation into carbide (TiC). The enthalpy of formation of oxy-carbides could be defined via additivity rule with adequate accuracy. In the MeO–MeC system, the possibility of formation of oxy-carbide compounds of Mex Oy Cz type could not be overlooked. The degree of interaction is determined by the thermodynamic stability of the components, which considerably diminishes upon transferring from group IV metals to group V metals of the periodic table. So, in the Ti–O–C system, TiOy Cz oxy-carbides has a wide variety of homogeneity, whereas in Zr–O–C and Hf–O–C systems, the limiting compositions satisfy to the formulas: ZrO0.37 C0.63

Table 5 Thermodynamic characteristics of V–O–C system compounds [17] Compounds

Specific thermal-capacity, CP (cal/(g atom K))

Specific entropy, S0 (cal/(g atom K))

Enthalpy, H ◦298 (cal/(g atom))

VO0.86 VO1.24 VO1.30 VO0.24 C0.69 VO0.12 C0.75 VC0.71 VC0.83 VC0.86

4.60 4.95 5.30 4.50 4.10 4.30 4.20 4.10

4.15 4.20 4.40 3.60 3.45 3.60 3.25 3.20

727 753 785 657 622 641 601 593

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Table 6 Thermodynamic characteristics of titanium oxy-carbides Oxy-carbides of titanium

Specific thermal-capacity, CP (cal/(g atom K))

Specific entropy, S0 (cal/(g atom K))

Enthalpy, H ◦298 (cal/(g atom))

TiO1.01 TiO1.00 C0.10 TiO0.81 C0.28 TiO0.60 C0.41 TiO0.35 C0.60 TiO0.23 C0.73 TiO0.11 C0.84 TiO0.04 C1.00

5.05 4.90 4.80 4.65 4.50 4.40 4.30 4.10

4.30 4.15 4.02 3.80 3.55 3.40 3.20 2.98

761 739 720 691 668 622 608 562

and HfO0.25 C0.75 accordingly. Similar limitations beheld in

Me–O–C systems for the transition metals of group V elements (V, Nb, Ta, etc.). Nevertheless, these limitations have but a small impact on their activity, because chlorination processes are taking place at the earlier stages of carbonization of oxide. It is known [7,18], that out of other chemical compounds, the oxides are harder to chlorinate, while the carbides— much more easily. As the oxy-carbides are intermediate products, so the role of carbon in chlorination processes becomes apparent: TiO2 + 2Cl2 ↔ TiCl4 + O2 ,

G◦1273 = 21.4 kcal

(28)

of these calculations are presented, from which it follows, that in the gas phase TiCl4 occupies 45.5–33.5 vol.% within the 873–1273 K temperature range. In actual conditions, the phase composition differs from being at equilibrium. Let us consider chlorination of TiO2 by chlorine, free of carbon: TiO2 + 2Cl2 ↔ TiCl4 + O2 ,

G◦1273 = 21.4 kcal

TiO2 + 2Cl2 ↔ TiOCl2 + 21 O2 ,

G◦1273 = 48.3 kcal (33)

TiOCl2 + 2Cl2 ↔ TiCl4 + 21 O2 ,

G◦1273 = −26.9 kcal

TiO0.7 C0.3 + 2Cl2 ↔ TiCl4 + 0.3CO + 0.2O2 , G◦1273

= −54.3 kcal

TiC + 2Cl2 ↔ TiCl4 + C,

G◦1273

= −103.8 kcal

(34) (29)

TiOCl2 + 2Cl2 + C ↔ TiCl4 + CO,

(30)

G◦1273 = −81.2 kcal

Juxtaposition of reactions (28)2 and (29) shows, that chlorination of TiO0.7 C0.3 is possible ( G◦1273 = −54.3 kcal), while TiO2 cannot be practically chlorinated as G◦1273 = 37.6 kcal. As to TiC, reaction (30), the titanium carbide can be chlorinated completely, i.e. without residue. The processes of chlorination of an oxide (Table 1, 8) and titanium oxy-carbide—with the participation of carbon— proceed almost with identical heat effects: TiO2 + 2Cl2 + 2C ↔ TiCl4 + 2CO, G◦1273 = −79.4 kcal

(31)

TiO0.7 C0.3 + 2Cl2 + 0.4C ↔ TiCl4 + 0.7CO, G◦1273 = −83.6 kcal

(32)

This result was predictable as their chlorination mechanism is similar. Thus, during carbon-thermal reduction, the reaction proceeds through energy-favorable mechanism, i.e. chlorination of oxides is carried out via the formation of oxy-carbides. During chlorination of TiO2 in carbon-containing environment, composition of a gas phase could be calculated by equilibrium constants of reactions. In Table 7 the results 2

The thermodynamic calculations are based on [19–21].

(35)

As it was stated before (Table 1, 6), in fact, reaction (28) would not occur. However, chlorination of TiO2 would happen through the formation of an oxy-chloride, TiOCl2 , presented by reactions (33) and (34). Thus, oxygen is displaced by chlorine stepwise: TiO2 → TiOCl2 → TiCl4 . Without doubt, the chlorination processes carried out as per reactions (33) and (34), could not be overlooked, although their energetic level is lower as compared to reaction (29). The validity of thermodynamic approach via oxides’ chlorination through the chlorine is confirmed by reactions (31), (32) and (35), which almost have identical values of G◦T −79, −83.6 kcal. Also, due to mobility of chlorides, one could not reject the possibility of formation of oxy-chlorides in the system

MeO–MeCl: Fe2 O3 + FeCl3 ↔ 3FeOCl,

G◦1273 = −9.03 kcal

(36)

Table 7 Partial pressures of the components of a gas phase for reaction (31),

TiO2 –Cl2 –C system Temperature (K)

PTiCl4

PCO

PCO2

PCl2

PCOCl2

873 1073 1273

0.455 0.353 0.335

0.175 0.600 0.661

0.370 0.047 0.003

1.47 × 10−5 7.40 × 10−5 1.12 × 10−4

5.7 × 10−7 4.9 × 10−7 9.9 × 10−6

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Nb2 O5 + 3NbCl5 ↔ 5NbOCl3 ,

G◦1273 = −31.01 kcal (37)

MoO3 + MoCl ↔ MoO2 Cl2 + MoOCl3 , G◦1273 = −73.45 kcal

(38)

In the system Mex Oy –C–Cl2 , the processes of chlorination could be carried out by the concurrently occurring mechanism, i.e. via the formation of oxy-carbo-chlorides being the end-products of interaction: oxy-carbide–chlorine and

oxy-carbide–oxy-chloride. It is to be demonstrated on the following reaction pathway, as an example:

As it can be seen from (39) to (42), during chlorination, the principle of a sequential redox reactions [13,15] is observed, when from lower up to higher chlorides were obtained with stage by stage displacement of oxygen during chlorination of the oxide: WO3 → WO2 Cl∗2 → WOl∗4 → WCl6

(43)

In above equations, the asterisk denotes oxide-carbonchloride, i.e. oxy-carbides, doped by carbon. In conclusion, it is to be said that two-stage mechanism is observed during chlorination of oxides by chlorine: 1. In the system Mex Oy –C–Cl2 , by the mechanism when oxy-carbides, then chlorides, are formed under the impact of carbon-thermal reduction as per reactions (20) and (32). 2. In the system Mex Oy –Cl2 , by the mechanism when oxy-chlorides, then chlorides, are formed, without carbon participation as per reactions (33) and (34). Thermodynamically, realization of this mechanism is more likely, than via the direct chlorination of an oxide as per reaction (28). 3. Direct chlorination of oxides without carbon participation as per reactions (1) and (28), and also as per reactions (4)–(6) where carbon-reduced metals form from oxides, there is no mechanism for these reactions to succeed. Chromatographic, X-ray spectroscopic analysis tool method investigations reinstated the validity of the proposed mechanisms of chlorination of metal oxides by chlorine [22,23].

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