**1. Introduction**

784 Thermodynamics – Interaction Studies – Solids, Liquids and Gases

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1022.

Chlorination roasting has proven to be a very important industrial route and can be applied for different purposes. Firstly, the chlorination of some important minerals is a possible industrial process for producing and refining metals of considerable technological importance, such as titanium and zirconium. Also, the same principle is mentioned as a possible way of recovering rare earth from concentrates (Zang et al., 2004) and metals, of considerable economic value, from different industrial wastes, such as, tailings (Cechi et al., 2009), spent catalysts (Gabalah Djona, 1995), slags (Brocchi Moura, 2008) and fly ash (Murase et al., 1998). The chlorination processes are also presented as environmentally acceptable (Neff, 1995, Mackay, 1992).

In general terms the chlorination can be described as reaction between a starting material (mineral concentrate or industrial waste) with chlorine in order to produce some volatile chlorides, which can then be separated by, for example, selective condensation. The most desired chloride is purified and then used as a precursor in the production of either the pure metal (by reacting the chloride with magnesium) or its oxide (by oxidation of the chloride).

The chlorination reaction has been studied on respect of many metal oxides (Micco et. al., 2011; Gaviria Bohe, 2010; Esquivel et al., 2003; Oheda et al., 2002) as this type of compound is the most common in the mentioned starting materials. Although some basic thermodynamic data is enclosed in these works, most of them are related to kinetics aspects of the gas – solid reactions. However, it is clear that the understanding of the equilibrium conditions, as predicted by classical thermodynamics, of a particular oxide reaction with chlorine can give strong support for both the control and optimization of the process. In this context, the impact of industrial operational variables over the chlorination efficiency, such as the reaction temperature and the reactors atmosphere composition, can be theoretically appreciated and then quantitatively predicted. On that sense, some important works have been totally devoted to the thermodynamics of the chlorination and became classical references on the subject (Kellog, 1950; Patel Jere, 1960; Pilgrim Ingraham, 1967; Sano Belton, 1980).

Originally, the approach applied for the study of chemical equilibrium studies was based exclusively on <sup>o</sup> *G*r x *T* and predominance diagrams. Nowadays, however, advances in computational thermodynamics enabled the development of softwares that can perform more complex calculations. This approach, together with the one accomplished by simpler techniques, converge to a better understanding of the intimate nature of the equilibrium states for the reaction system of interest. Therefore, it is understood that the time has come

On the Chlorination Thermodynamics 787

O Cl M

25 25 22 5 2 2 5

The development of the chlorination reaction can be followed through introduction of a

ratio of its molar content variation of each specie participating in the reaction and the

 22515 Cl2 OM <sup>52</sup> O2 MCl5 *dn dn dn dn*

The numbers inside the parenthesis in the denominators of the fractions contained in equation (5) are the stoichiometric coefficient of each specie multiplied by "-1" if it is represented as a reactant, or "+1" if it is a product. The equilibrium condition (Eq. 4) can

2 5 22 5

*gd d d d*

2

At the desired equilibrium state the condition defined by Eq. (6) must be valid for all

parenthesis is equal to zero. This last condition is the simplest mathematical representation

2 5 22 5 s g gg M O Cl O MCl

The chemical potentials can be computed through knowledge of the molar Gibbs energy of each pure specie in the gas phase, and its chemical activity. For the chloride MCl5, for

MCl5 <sup>5</sup> <sup>5</sup>

introducing equations analogous to Eq. (8) for all components of the gas phase, Eq. (7) can be rewritten according to Eq. (9). There, the activity of M2O5 is not present in the term located at the left hand side because, as this oxide is assumed to be pure, its activity must be

<sup>522</sup> 2 5 <sup>5</sup> <sup>2</sup>

<sup>5</sup> 2 5 <sup>2</sup> ln *<sup>r</sup> g g gg a a <sup>G</sup> a RT RT* 

g gg <sup>s</sup> 2 5/2 MCl O Cl M O MCl <sup>O</sup>

MCl5 *a* represents the chemical activity of the component MCl5 in the gas phase. By

 

<sup>2</sup> *<sup>g</sup>*

g MCl

<sup>5</sup> 5 20

g MCl

 

<sup>5</sup> 5 20

2 5 22 5

*g d*

 

2

s g gg M O Cl O MCl

 

g

s g gg M O Cl O MCl

**/**

<sup>5</sup> 5 20

 

> 

 

. This can only be accomplished if the term inside the

ln*aRTg* (8)

s g gg MO MO Cl Cl O O MCl MCl

*g dn dn dn dn*

*dG*

*s g*

reaction coordinate called *degree of reaction* (

*d*

now be rewritten in the following mathematical form:

for the chemical equilibrium associated with reaction (1).

example, the following function can be used (Robert, 1993):

2

5 Cl  

stoichiometric coefficient(Eq. 1).

possible values of the differential *d*

Where <sup>g</sup>

equal to one (Robert, 1993).

,, , ,

*TPn n n*

*dG g dn dn dn dn*

25 25 22 5 2 2 5

MO MO Cl Cl O O MCl MCl

g g

0

),whose first differential is computed by the

(4)

(5)

(6)

(7)

(9)

0

for a review on chlorination thermodynamics which can combine its basic aspects with a now available new kind of approach.

The present chapter will first focus on the thermodynamic basis necessary for understanding the nature of the equilibrium states achievable through chlorination reactions of metallic oxides. Possible ways of graphically representing the equilibrium conditions are discussed and compared. Moreover, the chlorination of V2O5, both in the absence as with the presence of graphite will be considered. The need of such reducing agent is clearly explained and discussed. Finally, the equilibrium conditions are appreciated through the construction of graphics with different levels of complexity, beginning with the well known

<sup>o</sup> *G*r x *T* diagrams, and ending with gas phase speciation diagrams, rigorously calculated through the minimization of the total Gibbs energy of the system.
