**2.8 CaO- Fe2O3 system**

Fe2O3 can react with CaO to form CaO·Fe2O3(CF) and 2CaO·Fe2O3(C2F). When Fe2O3 is used up, the newly formed C2F can react with Fe2O3 to form CF. The reaction equations are shown in table 8, and the relationships between △G0 and temperature are shown in figure 9.

Figure 9 shows that, Fe2O3 reacts with CaO much easily to form C2F; CF is not from the reaction of C2F and Fe2O3, but from the directly reaction of Fe2O3 with CaO. When Fe2O3 is excess, C2F can react with Fe2O3 to form CF.


Table 8. The *GT* of Fe2O3-CaO system( *G A BT <sup>T</sup>* , J/mol)

Thermodynamics of Reactions Among

form CA or CA2, the higher temperature, more negative

**(12/7)CaO**·**Fe2**

> **O3**

formed form CaO, Al2O3 and Fe2O3. The equations are shown in table 10:

*GT* of forming ternary compounds (

**+ Al**

> **O3**

> > **O3**

**2**

**+ Al**

**+ Al**

**2**

**(1/2)2CaO**·**Fe2**

**2**

**O3 = (1/2)CaO**·**2Al**

**O3 = (1/2)CaO**·**2Al**

**O3 = CaO**·**Al**

**(3)CaO**·**Fe2 O3 + Al2 O3**

**(3/2)2CaO**·**Fe2**

**(6/7)2CaO**·**Fe2**

> **O3**

**2**

**O3**

**O3**

**+ Al**

**+ Al**

> **+ Al**

**O3 = (1/7)12CaO**·**7Al**

**2**

**2**

**O3 = 3CaO**·**Al**

**O3 = (1/7)12CaO**·**7Al**

and the ternary compounds are not considered.

**-20**

Fig. 10. Relationship between

22 2

3 3

Table 10. The

**0**

**CaO**·**Fe2**

> **/2)CaO**·**Fe2**

**(1/4)2CaO**·**Fe2**

**3. Ternary compounds in Al2O3-CaO-SiO2-Fe2O3 system** 

 2 3 <sup>2</sup> 23 2 11 1 Al O CaO SiO ( )CaO Al O 2SiO

 2 3 <sup>2</sup> 23 2 1 1 Al O CaO SiO ( )3CaO Al O 3SiO

Al2O3 + 2CaO + SiO2=2CaO·Al2O3·SiO2 (cacoclasite)

**(1**

**20**

△G/(KJ·Mol-1

 *G* 

/(KJ·Mol-1) *T*

)

**40**

**60**

Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes 835

CaO more easily to generate C2F (Fig.9), therefore, C12A7 is the reaction product at normal roasting temperature(1073~1673K) under the conditions that CaO is sufficent in batching

 **= 3CaO**·**Al2**

**O3 +3Fe2 O3**

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

**O3 + Al2**

**2**

**O3** **+Fe2**

**O**

**2**

**O3**

**2**

**O3**

**3**

**2**

**O3 = CaO**·**Al2**

**2**

**O3**

> **O3**

**T/K**

The ternary compounds formed by CaO, Al2O3 and SiO2 in roasting process are mainly 2CaO·Al2O3·SiO2(C2AS), CaO·Al2O3·2SiO2(CAS2), CaO·Al2O3·SiO2(CAS) and 3CaO·Al2O3·3SiO2(C3AS3). In addition, ternary compound 4CaO·Al2O3·Fe2O3(C4AF) is

CaO·SiO2+ CaO·Al2O3=2CaO·Al2O3·SiO2 -30809.41 0.60 298~1600

Al2O3 + 2CaO + SiO2=2CaO·Al2O3·SiO2 -50305.83 -9.33 298~1600 Al2O3 + CaO + SiO2=CaO·Al2O3·SiO2 -72975.54 -9.49 298~1700

4CaO +Al2O3 + Fe2O3=4CaO·Al2O3·Fe2O3 -66826.92 -62.5 298~2000

**2**

**+3/2Fe2**

> **O3**

**+6/7Fe2**

**+(1/2)Fe2**

> **O3 +(1/2)Fe2 O3**

*GT* and temperature in Al2O3- calcium ferrites system

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

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

**+(1/4)Fe2**

**+12/7Fe2**

**O3**

**O3**

> **O3**

**O3** **O3**

*GT* . Because Fe2O3 reacts with

K




Fig. 9. Relationships between *GT* and temperature in Fe2O3-CaO system

### **2.9 Al2O3- calcium ferrites system**

Figure 1 shows that, the *GT* of the reaction of Al2O3 with CaCO3 is more negative than that of Fe 2O3 with CaCO3, therefore, the reaction of Fe2O3 with CaCO3 occurs after the reaction of Al2O3 with CaCO3 under the conditions of excess CaCO3. The new generated calcium ferrites are likely to transform into calcium aluminates when CaCO3 is insufficient, the reactions are as followed:


Table 9. The *GT* of the reaction Al2O3 with calcium ferrites( *G A BT <sup>T</sup>* , J/mol)

The relationships between *GT* and temperature (T) are shown in figure 10. Figure 10 shows that, Al2O3 cannot replace the Fe2O3 in calcium ferrites to generate C3A, and also cannot replace the Fe2O3 in CaO•Fe2O3(CF) to generate C12A7, but it can replace the Fe2O3 in 2CaO•Fe2O3(C2F) to generate C12A7 when the temperature is above 1000K, the higher temperature is, the more negative Gibbs free energy is; Al2O3 can react with CF and C2F to

**O3**

**O3** **+Fe2**

**=CaO·Fe2**

**O3**

=**CaO·Fe2**

> **O3**

**O3**

**O3**

*GT* of the reaction of Al2O3 with CaCO3 is more negative than that

J/mol

B, J/K.mol Temperature,

49.6 -1.2×10-2 298~1723

32685.1 -24.5 298~1489

34514.4 -35.0 298~1723

3215.1 -8.8 298~1489

3168.6 -11.0 298~1723

4009.5 -12.8 298~1723

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

*GT* and temperature (T) are shown in figure 10. Figure 10

K

**2CaO·Fe2**

**CaO+Fe2**

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

**O3** **=2CaO·Fe2**

*GT* and temperature in Fe2O3-CaO system

**2CaO+Fe2**

T,K

of Fe 2O3 with CaCO3, therefore, the reaction of Fe2O3 with CaCO3 occurs after the reaction of Al2O3 with CaCO3 under the conditions of excess CaCO3. The new generated calcium ferrites are likely to transform into calcium aluminates when CaCO3 is insufficient, the

(3)CaO•Fe2O3+ Al2O3 = 3CaO•Al2O3+3Fe2O3 47922.7 4.5 298~1489

CaO•Fe2O3+ Al2O3 =CaO•Al2O3+Fe2O3 3626.6 -7.5 298~1489

Reactions A,

**-60 -50 -40 -30 -20 -10 0**

23 23 23 23

23 23 23 23

23 23 23 23

23 23 23 23

23 23 23 23

23 23 23 23

*GT* of the reaction Al2O3 with calcium ferrites(

shows that, Al2O3 cannot replace the Fe2O3 in calcium ferrites to generate C3A, and also cannot replace the Fe2O3 in CaO•Fe2O3(CF) to generate C12A7, but it can replace the Fe2O3 in 2CaO•Fe2O3(C2F) to generate C12A7 when the temperature is above 1000K, the higher temperature is, the more negative Gibbs free energy is; Al2O3 can react with CF and C2F to

3 3 ( )2CaO Fe O Al O 3CaO Al O Fe O 2 2

6 16 ( )2CaO Fe O Al O ( )12CaO 7Al O Fe O 7 77

1 11 ( )CaO Fe O Al O ( ) CaO 2Al O Fe O 2 22

1 11 ( )2CaO Fe O Al O ( ) CaO 2Al O Fe O 4 24

1 1 ( )2CaO Fe O Al O CaO Al O Fe O 2 2

<sup>12</sup> <sup>1</sup> <sup>12</sup> ( )CaO Fe O Al O ( )12CaO 7Al O Fe O 7 77

△G,kJ.Mol-1

/(KJ·Mol-1)

*T G* 

Fig. 9. Relationships between

Figure 1 shows that, the

reactions are as followed:

Table 9. The

The relationships between

**2.9 Al2O3- calcium ferrites system** 

form CA or CA2, the higher temperature, more negative *GT* . Because Fe2O3 reacts with CaO more easily to generate C2F (Fig.9), therefore, C12A7 is the reaction product at normal roasting temperature(1073~1673K) under the conditions that CaO is sufficent in batching and the ternary compounds are not considered.

Fig. 10. Relationship between *GT* and temperature in Al2O3- calcium ferrites system
