**2.1 Fe2O3-Al2O3-CaCO3 system**

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 coexist, and the reactions equations are as followed:


Table 1. The *GT* of Fe2O3-Al2O3-CaCO3 system( *G A BT <sup>T</sup>* , J/mol; *CO*<sup>2</sup> *P* =30Pa, i.e., the partial pressure of CO2 in the air)

Thermodynamics of Reactions Among

Fig. 2. Relationships of

Figure 2 shows that, the

**2.3 Fe2O3-Al2O3 system** 

Fig. 3. Relationship of

cannot happen; the

  **-20**

Al2O3+FeO=FeO·Al2O3

**-40**

**-30**

△**G/**(**KJ**·**Mol-1**

 /(KJ·Mol-1) *T G* 

)

**-20**

**-10**

**0**

**-10**

△G/(KJ·Mol-1

 /(KJ·Mol-1) *T G*

)

**0**

**10**

**3/2Al2 O3 + SiO2**

**Al2 O3 + SiO2 = Al2 O3 ·SiO2**

aluminium silicates is A3S2, AS(andalusite), AS(fibrolite) at 1100~1700K.

**20**

Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes 827

**= (1/2)3Al2 O3 ·2SiO2**

**Al**

**Al2 O3 + SiO2 = Al2 O3 ·SiO2 (**fibrolite)

**O3**

**+ SiO2**

**= Al**

**(**andalusite)

**2**

**O3**

**·SiO2**

**(**kyanite)

*GT* of kyanite is greater than zero at 1000~1700K, so the reaction

*GT* =-30172.2+9.3T, J.mol-1 843~1650K (1)

*GT* and

*GT* of andalusite and fibrolite alter little with temperature changes; the

**2**

**200 400 600 800 1000 1200 1400 1600 1800**

**T/K**

*GT* and temperature in Al2O3-SiO2 system

*GT* of A3S2 decreases with the rise of temperature. The thermodynamic order of forming

Al2O3 does not directly react with Fe2O3, but Al2O3 may react with wustite (FeO) produced during roasting process to form FeO·Al2O3. No pure ferrous oxide (FeO) exists in the actual process. The ratio of oxygen atoms to iron atoms is more than one in wustite, which is generally expressed as FexO(x=0.83~0.95), whose crystal structure is absence type crystallology. For convenience, FeO is expressed as wustite in this thesis. Al2O3 may react

with wustite(FeO) to form FeO· Al2O3 in the roasting process. The relationship of

**Al**

**2**

**O3**

temperature is shown in figure 2, and the chemical reaction of the equation is as followed:

**800 900 1000 1100 1200 1300 1400 1500 1600 1700 -50**

**T/K**

*GT* and temperature in Fe2O3-Al2O3 system

**+FeO=FeO**·**Al**

**2**

**O3**

The relationships between Gibbs free energy ( *GT* ) and temperature (T) are as shown in figure 1.

Fig. 1. Relationships between *GT* and temperature in Fe2O3-Al2O3-CaCO3 system

Figure 1 shows that, the Gibbs free energy of reactions on Fe2O3 and Al2O3 with CaCO3 decreased with the rise of temperature in normal roasting process (due to decomposition of CaCO3 over 1200K, so the curve has no drawing above 1200K), and the reactions all can automatically react to generate the corresponding calcium aluminate and calcium ferrite. The *GT* of reaction with Al2O3 is more negative than the *GT* of reaction with Fe2O3 at the same temperature. CaCO3 has actually decomposed at 1473~1673K industrial roasting temperature, therefore, only CaO is taken into account on the following analysis.

## **2.2 SiO2-Al2O3 system**

SiO2 mainly comes from the ore and coke ash in the roasting process. SiO2 reacts with Al2O3 to form aluminium silicates. The aluminium silicates mainly include Al2O3·2SiO2(AS2), Al2O3·SiO2(AS,andalusite), AS(kyanite), AS(fibrolite), 3Al2O3·2SiO2(A3S2). Thermodynamic calculation indicates that, AS2 can not be formed from the reaction of Al2O3 and SiO2 under the roasting condition. The others equations are shown in table 2.


Table 2. The *GT* of Al2O3-SiO2 system( *G A BT <sup>T</sup>* , J/mol)

The relationships of *GT* and temperature in Al2O3-SiO2 system is shown in figure 2.

200 400 600 800 1000 1200

Figure 1 shows that, the Gibbs free energy of reactions on Fe2O3 and Al2O3 with CaCO3 decreased with the rise of temperature in normal roasting process (due to decomposition of CaCO3 over 1200K, so the curve has no drawing above 1200K), and the reactions all can automatically react to generate the corresponding calcium aluminate and calcium ferrite.

same temperature. CaCO3 has actually decomposed at 1473~1673K industrial roasting

SiO2 mainly comes from the ore and coke ash in the roasting process. SiO2 reacts with Al2O3 to form aluminium silicates. The aluminium silicates mainly include Al2O3·2SiO2(AS2), Al2O3·SiO2(AS,andalusite), AS(kyanite), AS(fibrolite), 3Al2O3·2SiO2(A3S2). Thermodynamic calculation indicates that, AS2 can not be formed from the reaction of Al2O3 and SiO2 under

Al2O3+SiO2=Al2O3·SiO2(kyanite) -8469.3 9.0 298~1696 Al2O3+SiO2=Al2O3·SiO2(fibrolite) -4463.8 -0.9 298~1696 Al2O3+SiO2=Al2O3·SiO2(andalusite) -6786.1 0.6 298~1696

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

*G A BT <sup>T</sup>* , J/mol)

*GT* and temperature in Al2O3-SiO2 system is shown in figure 2.

temperature, therefore, only CaO is taken into account on the following analysis.

**CaCO3**+**Fe2O3=CaO**·**Fe2O3+CO2**

**2O3=CaO**·**Al**

**CaCO3**<sup>+</sup>**Al**

**T/K**

**2O3+CO2**

*GT* and temperature in Fe2O3-Al2O3-CaCO3 system

12764.7 -16.7 298~1696

*GT* of reaction with Fe2O3 at the

*GT* ) and temperature (T) are as shown in

The relationships between Gibbs free energy (


*GT* of reaction with Al2O3 is more negative than the

the roasting condition. The others equations are shown in table 2.

*GT* of Al2O3-SiO2 system(

 23 2 23 2 3 1 Al O SiO ( )3Al O 2SiO



△G/(KJ·Mol-1

/(KJ·Mol-1)

*T G*

Fig. 1. Relationships between

)

0

50

100

figure 1.

The

**2.2 SiO2-Al2O3 system** 

2 2

The relationships of

Table 2. The

Fig. 2. Relationships of *GT* and temperature in Al2O3-SiO2 system

Figure 2 shows that, the *GT* of kyanite is greater than zero at 1000~1700K, so the reaction cannot happen; the *GT* of andalusite and fibrolite alter little with temperature changes; the *GT* of A3S2 decreases with the rise of temperature. The thermodynamic order of forming aluminium silicates is A3S2, AS(andalusite), AS(fibrolite) at 1100~1700K.
