**4. Final remarks**

In this chapter three different approaches to the chlorination equilibrium study of an oxide were presented. The first two are based on the construction of <sup>o</sup> *G*<sup>r</sup> <sup>x</sup>*T* diagrams (topic 2.2.1) and on the calculations, first introduced by Kang Zuo (1989) (topic 2.2.2), respectively. Both of them take into consideration that each chlorinated compound is produced independently. The third one has its fundamental based on the total Gibbs energy

On the Chlorination Thermodynamics 823

apparently contradicting the information contained in the predominance diagrams of Figures (15) and (16), is a mere consequence of the fact that on topic (3.1.3.2) the gaseous chlorides build an ideal gas solution. The chemical potentials of VCl3 and also of VCl2 are lower than their pure molar Gibbs energies. The species become more stable in the gaseous solution in relation to the pure state, and their mol fractions assume higher values for the same temperature imposed. The same idea explains why VCl4 is formed in significant amounts at 1073 K for a *P*(Cl2) value lower than the one observed in the predominance

The effect of adding more Cl2 after all vanadium has been converted to gaseous chlorinated compounds is also consistent with the expectations. At 1073 K the results indicate that the mol fraction of VCl4 grows while all other relevant chlorinated species reduces (topic 3.1.3.2). This can be explained by the reaction of VCl2 and VCl3 with Cl2 resulting in VCl4,

The study of the impact of varying *P*(O2) over the gas phase composition at 1373 K indicated that the mol fractions of CO and CO2 experience significant elevation as *P*(O2) becomes higher, a fact that is also observed in the case of VOCl3 (Table 4). The concentration of all other chlorinated compounds reduces for the same studied range of *P*(O2). The influence of the oxygen chemical activity over the gas phase speciation can be explained by a group of proposed reactions between VCl4, VCl3 and VCl2 with O2 resulting in VOCl3 (topic 3.1.3.2). All these reactions have equilibrium constants much higher than one, indicating an

The conclusions about the exothermic nature of the chlorination process in the temperature range between 1000 K and 1300 K and the observation that it becomes progressively more endothermic as 1700 K is approached (topic 3.1.3.3), are perfectly consistent with the fact that the atmosphere becomes progressively diluted in VCl4 and VOCl3, whose formations are associated with negative molar enthalpies and becomes richer in VCl2 and VCl3, whose

Finally, we can conclude that the study of the equilibrium states achievable through the reaction between a transition metal oxide and gaseous Cl2, can be now approached through the implementation of methods of different complexity levels. The most general one, in which the total Gibbs energy of the reaction system is minimized, enables the construction of a more detailed picture of the equilibrium state. However, as it is evident from the comparisons explained above, the most general method must be consistent with the

Allain E., Djona M., Gaballah I. Kinetics of Chlorination and Carbochlorination of Pure

Brewer L., Ebinghaus, B. B. The thermodynamics of the solid oxides of vanadium.

Brocchi, E. A.; Moura, F. J. Chlorination methods applied to recover refractory metals from

Cecchi, E. et al. A feasibility study of carbochlorination of chrysotile tailings. International

Esquivel, M. R., Bohé, A. E., Pasquevich, D. M. Carbochlorination of samarium sesquioxide.

Tantalum and Niobium Pentoxides. Metallurgical and materials transactions B, v.

diagram of Figure (14) for the equilibrium between VCl2(l) and VCl4(g).

which have a significant negative driving force at 1073 K (Table 3).

expressive thermodynamic driving force at 1373 K (Table 6).

molar enthalpy of formation are considerably positive (Figure 31).

Thermochimica Acta, v. 129, p. 49 – 55, 1988.

Thermoquimica Acta, v. 403, p. 207 – 218, 2003.

tin slags. Minerals Engineering, v. 21, n. 2, p. 150-156, 2008.

Journal of Mineral Processing, v. 93, n. 3 - 4, p. 278-283, 2009.

tendencies predicted by simpler calculations.

28, p. 223 - 232, 1997.

**5. References** 

minimization of the reaction system and the gas phase equilibrium composition is calculated considering that the formed species are produced simultaneously (topic 2.2.3).

The method based on the construction of <sup>o</sup> *G*<sup>r</sup> <sup>x</sup>*T* was applied on topic (3.1.2) for studying the thermodynamic viability of the reaction between gaseous Cl2 and V2O5. The discussion evidenced that the chlorination is thermodynamically feasible only in the presence of a reducing agent (graphite in the case of the present work) and was initially focused on the production of VCl4 and VOCl3. The same approach was employed for studying the possible mechanisms associated with the formation of VCl4 and VOCl3. According to the results (topic 3.1.2.1), the synthesis of these two compounds is subdivided in different stages, which can vary in nature, depending on the temperature range considered. In global terms though, both VOCl3 and VCl4 have molar reaction Gibbs energies of the same magnitude order. So, it was not possible to clearly identify, which one of them should be produced in greater quantities. The problem of the relative stability between VOCl3 and VCl4 was then addressed by the implementation of the method of Kang Zuo (1989) (topic 3.1.3.1). The results indicated that VCl4 should have a higher concentration in comparison with VOCl3 in the temperature range between 1073 K and 1373 K.

It can de said that both, the method based on the <sup>o</sup> *G*<sup>r</sup> <sup>x</sup>*T* diagrams construction as well as the Kang Zuo method (1989), incorporate some simplifications and are very easy to implement. However, they lead to only a superficial knowledge of the true nature of the equilibrium state achievable. Thanks to the development of computational thermodynamic software, of which *Thermocalc* is a good example, more complex computations can be realized. For example, by allowing the chlorides and oxychlorides to build a gaseous solution, the minimization of the total Gibbs energy of the system results in the direct computation of the mol fraction of each chlorinated species present in the gas phase (topic 3.1.3.2). This method can be seen as an improvement of the idea put forward by Kang Zuo (1989), in that all equilibrium equations are solved simultaneously, with the further advantage that one does not need to formulate a group of independent reactions that cover all possible chemical interactions among the components, a task that can become very complex for metals, as in case of vanadium, which can produce a family of chlorides and oxychlorides.

The conclusion that graphite strongly promotes the thermodynamic driving force necessary to chlorination and that VCl4 should be formed preferentially in relation to VOCl3 are perfectly consistent with the results based on the total Gibbs energy minimization. However, by the application of this last method it was possible to go a little further, through investigation of the effect of *P*(Cl2) over the chlorination enthalpy and by studying the effect of temperature, Cl2 and O2 partial pressures over the concentrations of vanadium chlorides and oxychlorides in the gas phase.

The predictions associated with the effect of temperature over the gas phase speciation (Table 4) indicate that the mol fractions of VCl2 and VCl3 grow significantly in the range between 1073 K and 1473 K and, as a result, the concentrations of VOCl3, VCl4, CO and CO2 exhibit a significant reduction (topic 3.1.3.2). This finding agrees with the tendency depicted by the predominance diagrams constructed for the system V – O – Cl, where the VCl4(g) and VOCl3(g) fields shrink and that of VCl3(g) grows (Figures 14, 15 and 16). Also, the calculated mol fraction of CO is at all temperatures much higher than the mol fraction of CO2, a fact that is consistent with the establishment of the Bourdouard equilibrium for temperatures higher than 973 K, where the concentrations of the two mentioned carbon oxides have the same magnitude.

The fact that the speciation computation indicates appreciable amounts of VCl3 for temperatures higher than 1100 K and of VCl2 for temperatures higher than 1473 K,

minimization of the reaction system and the gas phase equilibrium composition is calculated considering that the formed species are produced simultaneously (topic 2.2.3). The method based on the construction of <sup>o</sup> *G*<sup>r</sup> <sup>x</sup>*T* was applied on topic (3.1.2) for studying the thermodynamic viability of the reaction between gaseous Cl2 and V2O5. The discussion evidenced that the chlorination is thermodynamically feasible only in the presence of a reducing agent (graphite in the case of the present work) and was initially focused on the production of VCl4 and VOCl3. The same approach was employed for studying the possible mechanisms associated with the formation of VCl4 and VOCl3. According to the results (topic 3.1.2.1), the synthesis of these two compounds is subdivided in different stages, which can vary in nature, depending on the temperature range considered. In global terms though, both VOCl3 and VCl4 have molar reaction Gibbs energies of the same magnitude order. So, it was not possible to clearly identify, which one of them should be produced in greater quantities. The problem of the relative stability between VOCl3 and VCl4 was then addressed by the implementation of the method of Kang Zuo (1989) (topic 3.1.3.1). The results indicated that VCl4 should have a higher concentration in comparison with VOCl3 in

It can de said that both, the method based on the <sup>o</sup> *G*<sup>r</sup> <sup>x</sup>*T* diagrams construction as well as the Kang Zuo method (1989), incorporate some simplifications and are very easy to implement. However, they lead to only a superficial knowledge of the true nature of the equilibrium state achievable. Thanks to the development of computational thermodynamic software, of which *Thermocalc* is a good example, more complex computations can be realized. For example, by allowing the chlorides and oxychlorides to build a gaseous solution, the minimization of the total Gibbs energy of the system results in the direct computation of the mol fraction of each chlorinated species present in the gas phase (topic 3.1.3.2). This method can be seen as an improvement of the idea put forward by Kang Zuo (1989), in that all equilibrium equations are solved simultaneously, with the further advantage that one does not need to formulate a group of independent reactions that cover all possible chemical interactions among the components, a task that can become very complex for metals, as in case of vanadium, which

The conclusion that graphite strongly promotes the thermodynamic driving force necessary to chlorination and that VCl4 should be formed preferentially in relation to VOCl3 are perfectly consistent with the results based on the total Gibbs energy minimization. However, by the application of this last method it was possible to go a little further, through investigation of the effect of *P*(Cl2) over the chlorination enthalpy and by studying the effect of temperature, Cl2 and O2 partial pressures over the concentrations of vanadium chlorides

The predictions associated with the effect of temperature over the gas phase speciation (Table 4) indicate that the mol fractions of VCl2 and VCl3 grow significantly in the range between 1073 K and 1473 K and, as a result, the concentrations of VOCl3, VCl4, CO and CO2 exhibit a significant reduction (topic 3.1.3.2). This finding agrees with the tendency depicted by the predominance diagrams constructed for the system V – O – Cl, where the VCl4(g) and VOCl3(g) fields shrink and that of VCl3(g) grows (Figures 14, 15 and 16). Also, the calculated mol fraction of CO is at all temperatures much higher than the mol fraction of CO2, a fact that is consistent with the establishment of the Bourdouard equilibrium for temperatures higher than 973 K, where the

The fact that the speciation computation indicates appreciable amounts of VCl3 for temperatures higher than 1100 K and of VCl2 for temperatures higher than 1473 K,

concentrations of the two mentioned carbon oxides have the same magnitude.

the temperature range between 1073 K and 1373 K.

can produce a family of chlorides and oxychlorides.

and oxychlorides in the gas phase.

apparently contradicting the information contained in the predominance diagrams of Figures (15) and (16), is a mere consequence of the fact that on topic (3.1.3.2) the gaseous chlorides build an ideal gas solution. The chemical potentials of VCl3 and also of VCl2 are lower than their pure molar Gibbs energies. The species become more stable in the gaseous solution in relation to the pure state, and their mol fractions assume higher values for the same temperature imposed. The same idea explains why VCl4 is formed in significant amounts at 1073 K for a *P*(Cl2) value lower than the one observed in the predominance diagram of Figure (14) for the equilibrium between VCl2(l) and VCl4(g).

The effect of adding more Cl2 after all vanadium has been converted to gaseous chlorinated compounds is also consistent with the expectations. At 1073 K the results indicate that the mol fraction of VCl4 grows while all other relevant chlorinated species reduces (topic 3.1.3.2). This can be explained by the reaction of VCl2 and VCl3 with Cl2 resulting in VCl4, which have a significant negative driving force at 1073 K (Table 3).

The study of the impact of varying *P*(O2) over the gas phase composition at 1373 K indicated that the mol fractions of CO and CO2 experience significant elevation as *P*(O2) becomes higher, a fact that is also observed in the case of VOCl3 (Table 4). The concentration of all other chlorinated compounds reduces for the same studied range of *P*(O2). The influence of the oxygen chemical activity over the gas phase speciation can be explained by a group of proposed reactions between VCl4, VCl3 and VCl2 with O2 resulting in VOCl3 (topic 3.1.3.2). All these reactions have equilibrium constants much higher than one, indicating an expressive thermodynamic driving force at 1373 K (Table 6).

The conclusions about the exothermic nature of the chlorination process in the temperature range between 1000 K and 1300 K and the observation that it becomes progressively more endothermic as 1700 K is approached (topic 3.1.3.3), are perfectly consistent with the fact that the atmosphere becomes progressively diluted in VCl4 and VOCl3, whose formations are associated with negative molar enthalpies and becomes richer in VCl2 and VCl3, whose molar enthalpy of formation are considerably positive (Figure 31).

Finally, we can conclude that the study of the equilibrium states achievable through the reaction between a transition metal oxide and gaseous Cl2, can be now approached through the implementation of methods of different complexity levels. The most general one, in which the total Gibbs energy of the reaction system is minimized, enables the construction of a more detailed picture of the equilibrium state. However, as it is evident from the comparisons explained above, the most general method must be consistent with the tendencies predicted by simpler calculations.

## **5. References**


**30** 

*China* 

K

*G A BT <sup>T</sup>* , J/mol; *CO*<sup>2</sup> *P* =30Pa, i.e., the

**Thermodynamics of Reactions** 

**During Roasting Processes** 

*School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan 410083,* 

**Among Al2O3, CaO, SiO2 and Fe2O3** 

Zhongping Zhu, Tao Jiang, Guanghui Li, Yufeng Guo and Yongbin Yang

The thermodynamic of the chemical reactions among Al2O3, CaO, SiO2 and Fe2O3 in the roasting processes was investigated in this chapter. The chemical reactions are classified into SiO2-Al2O3 system, Fe2O3-Al2O3 system, SiO2-Fe2O3 system, CaO-Al2O3 system, SiO2-CaO system, SiO2-calcium aluminates system, CaO-Fe2O3 system, Al2O3-calcium ferrites system and Al2O3-CaO-SiO2-Fe2O3 system. When the roasting temperature is over 1100K, 3Al2O3·2SiO2 is preferentially formed in SiO2-Al2O3 system; FeO·Al2O3 can be formed in Fe2O3-Al2O3 system; ferric oxide and SiO2 could not generate iron silicate; 12CaO·7Al2O3 is preferentially formed in CaO-Al2O3 system when one mole Al2O3 reacts with CaO; 2CaO·SiO2 is preferentially formed in SiO2-CaO system; except for CaO·2Al2O3 and CaO·Al2O3, the other calcium aluminates can transform into calcium silicate by reacting with SiO2 in SiO2-calcium aluminates system; 2CaO·Fe2O3 is preferentially formed in CaO-Fe2O3 system; alumina is unable to form 3CaO·Al2O3 with calcium ferrites(2CaO·Fe2O3 and CaO·Fe2O3), but able to form 12CaO·7Al2O3 with 2CaO·Fe2O3; when CaO, Fe2O3, Al2O3,SiO2 coexist, they are more likely to form ternary compound 2CaO·Al2O3·SiO2 and

Fe2O3 and Al2O3 can all react with limestone during roasting to generate corresponding aluminates and ferrites. In Fe2O3-Al2O3-CaO system, the reaction Fe2O3 and Al2O3 with

CaCO3+Al2O3=CaO·Al2O3+CO2 161088.3 -244.1 298~1200 CaCO3+Fe2O3=CaO·Fe2O3+CO2 151677.8 -220.9 298~1200

*GT* of Fe2O3-Al2O3-CaCO3 system(

Reactions A, J/mol B, J/K.mol Temperature,

**1. Introduction** 

4CaO·Al2O3·Fe2O3.

Table 1. The

**2. Binary compounds** 

**2.1 Fe2O3-Al2O3-CaCO3 system** 

partial pressure of CO2 in the air)

CaCO3 coexist, and the reactions equations are as followed:

