**2.3 Fe2O3-Al2O3 system**

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

$$\text{Al}\_2\text{O}\_3 + \text{FeO} = \text{FeO} \cdot \text{Al}\_2\text{O}\_3 \cdot \text{Al}\_T^{\theta} = \text{-30172.2} + \text{9.3T} \text{.} \text{J} \cdot \text{mol}^{-1} \cdot \text{843} \cdot \text{1650K} \tag{1}$$

Fig. 3. Relationship of *GT* and temperature in Fe2O3-Al2O3 system

Thermodynamics of Reactions Among

generate calcium aluminates such as C12A7, C3A, CA, CA2.

2 3 2 3

2 3 2 3

**-80**

23 23 2 3

23 23 2 3

23 23 2 3

23 23 2 3

23 23 2 3

4 1 ( )3CaO Al O +Al O =( )12CaO 7Al O

1 3 ( )3CaO Al O +Al O =( )CaO Al O

1 3 ( )3CaO Al O +Al O =( )CaO 2Al O

<sup>1</sup> <sup>12</sup> ( )12CaO 7Al O +Al O =( )CaO Al O

<sup>1</sup> <sup>12</sup> ( )12CaO 7Al O +Al O =( )CaO 2Al O

Reactions A,

**-60**

△

 *G* 

/(KJ·Mol-1) *T*

G,kJ.Mol-1

**-40**

**-20**

**0**

*GT* of Al2O3-CaO system(

<sup>12</sup> <sup>1</sup> CaO+Al O =( )12CaO 7Al O 7 7

1 1 CaO+Al O =( )CaO 2Al O

2 2

The relationships between

Fig. 5. Relationships between

3 3

2 2

5 5

5 5

17 17

Table 5. The

Table 4. The

Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes 829

roasting temperature, the thermodynamic order that one mole Al2O3 reacts with CaO to

3CaO+ Al2O3=3CaO·Al2O3 -9.9 -28.4 298~1808

CaO+ Al2O3=CaO·Al2O3 -15871.5 -18.1 298~1878

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

T,K

CaO•Al2O3+ Al2O3=CaO•2Al2O3 2543.8 -9.5 298~2023

*GT* of Al2O3-calcium aluminates system(

**2**

**CaO+Al**

**O3**

**2**

**2**

**O3**

**O3**

**=(1/7)12CaO2**

*GT* and temperature in Al2O3-CaO system

**=CaO2**

**3CaO+Al**

**·7Al**

**2**

J/mol

**·2Al**

**2**

**O3**

**2**

**O**

**3**

B, J/K.mol

13939.7 -65.8 298~1800





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

Temperature, K

**O3**

**12/7CaO+Al**

**1/2CaO+Al**

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

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

**=(1/2)CaO2**

> **·Al**

**O3**

**2**

**2**

**O3**

**=3CaO·Al**

*GT* and temperature (T) are shown in figure 5.

318.3 -44.5 298~1800


Figure 3 shows that, the *GT* is negative at 843~1650K, reaction can happen and generate FeO·Al2O3; the *GT* rises with the temperature, the higher temperature is, the lower thermodynamic reaction trends.

### **2.4 SiO2-Fe2O3 system**

SiO2 also does not directly react with Fe2O3, but Al2O3 may react with wustite (FeO) to form FeO·SiO2 (FS) and 2FeO·SiO2(F2S). The relationships of *GT* and temperature is shown in figure 4, and the chemical reactions of the equations are shown in table 3.


Table 3. The *GT* of SiO2- Al2O3 system( *G A BT <sup>T</sup>* , J/mol)

Figure 4 shows that, the *GT* of SiO2- Al2O3 system are above zero at 847~1500K, so all of the reactions can not happen to form ferrous silicates (FS and F2S).

Fig. 4. Relationships of *GT* and temperature in SiO2-Fe2O3 system

#### **2.5 CaO-Al2O3 system**

Al2O3 can react with CaO to form calcium aluminates such as 3CaO·Al2O3(C3A), 12CaO·7Al2O3(C12A7), CaO·Al2O3(CA) and CaO·2Al2O3 (CA2). As regard as the calcium aluminates only C12A7 can be totally soluble in soda solution, C3A and CA dissolve with a slow speed, and the other calcium aluminates such as CA2 are completely insoluble. Equations that Al2O3 reacted with CaO to form C3A, C12A7, CA and CA2 are presented in table 4.

Figure 5 shows that, the *GT* of reactions of Al2O3 with CaO decreases with the rise of temperature; all reactions automatically proceed to generate the corresponding calcium aluminates at normal roasting temperature (1473~1673K, same as follows); At the same

SiO2 also does not directly react with Fe2O3, but Al2O3 may react with wustite (FeO) to form

*GT* is negative at 843~1650K, reaction can happen and generate

*GT* of SiO2- Al2O3 system are above zero at 847~1500K, so all of

*GT* of reactions of Al2O3 with CaO decreases with the rise of

*GT* and temperature is shown in

*GT* rises with the temperature, the higher temperature is, the lower

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

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

FeO+SiO2 =FeO·SiO2 26524.6 18.8 847~1413 2FeO+SiO2 =2FeO·SiO2 -13457.3 30.3 847~1493

**800 900 1000 1100 1200 1300 1400 1500 1600**

**T/K**

*GT* and temperature in SiO2-Fe2O3 system

Al2O3 can react with CaO to form calcium aluminates such as 3CaO·Al2O3(C3A), 12CaO·7Al2O3(C12A7), CaO·Al2O3(CA) and CaO·2Al2O3 (CA2). As regard as the calcium aluminates only C12A7 can be totally soluble in soda solution, C3A and CA dissolve with a slow speed, and the other calcium aluminates such as CA2 are completely insoluble. Equations that Al2O3 reacted with CaO to form C3A, C12A7, CA and CA2 are presented in

temperature; all reactions automatically proceed to generate the corresponding calcium aluminates at normal roasting temperature (1473~1673K, same as follows); At the same

Figure 3 shows that, the

**2.4 SiO2-Fe2O3 system** 

thermodynamic reaction trends.

Figure 4 shows that, the

Fig. 4. Relationships of

**2.5 CaO-Al2O3 system** 

Figure 5 shows that, the

table 4.

FeO·Al2O3; the

Table 3. The

FeO·SiO2 (FS) and 2FeO·SiO2(F2S). The relationships of

figure 4, and the chemical reactions of the equations are shown in table 3.

*GT* of SiO2- Al2O3 system(

the reactions can not happen to form ferrous silicates (FS and F2S).

**FeO + SiO2 = FeO·SiO2**

> **2FeO + SiO2 = 2FeO·SiO2**

△G/(KJ**·**Mol-1

/(KJ·Mol-1)

*T G* 

)



Table 4. The *GT* of Al2O3-CaO system( *G A BT <sup>T</sup>* , J/mol)

The relationships between *GT* and temperature (T) are shown in figure 5.

Fig. 5. Relationships between *GT* and temperature in Al2O3-CaO system


Table 5. The *GT* of Al2O3-calcium aluminates system( *G A BT <sup>T</sup>* , J/mol)

Thermodynamics of Reactions Among

**-150 -140 -130 -120 -110 -100 -90 -80 -70**

△

*T G* 

Fig. 7. Relationships between

**2.7 SiO2- calcium aluminates system** 

between

Figure 8 shows that, the

G/(KJ·Mol-1

/(KJ·Mol-1)

)

Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes 831

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

**T/K**

Figure7 shows that, SiO2 reacts with CaO to form γ-C2S when temperature below 1100K, but β-C2S comes into being when the temperature above 1100K. At normal roasting temperature,

Figure 5 ~ figure 7 show that, CaO reacts with SiO2 and Al2O3 firstly to form C2S, and then

In the CaO-Al2O3 system, if there exists some SiO2, the newly formed calcium aluminates are likely to react with SiO2 to transform to calcium silicates and Al2O3 because SiO2 is more acidity than that of Al2O3. The reaction equations are presented in table 7, the relationships

the reaction (3CA2+SiO2=C3S+6Al2O3) can not happen when the roasting temperature is above 900K , i.e., the lowest calcium-to-aluminum ratio calcium aluminates cannot transform to the highest calcium-to-silicon ratio (CaO to SiO2 molecular ratio) calcium

is also more than zero; but the other calcium aluminates all can react with SiO2 to generate calcium silicates at 800~1700K. The thermodynamic sequence of calcium aluminates reaction

·

**2**

**(**γ**)**

**SiO2** **(**

**3CaO+SiO2**

ß

**=(1/2)3CaO**

**)**

**=2CaO**

·

*GT* and temperature

**SiO2**

**=CaO**

**(3/2)CaO+SiO**

**CaO+SiO2**

·

**SiO2**

**=CaO**

(wollastonite)

·

**SiO2** (

pseud-wollastonite

·

**2SiO2**

**=3CaO**

·

**SiO2**

*GT* of all the reactions increases with the temperature increases;

*GT* of reaction(3CA+ SiO2=C3S+3Al2O3)

)

**2CaO+SiO2**

**=2CaO**

**2CaO+SiO2**

*GT* and temperature are shown in figure 8.

silicate; when the temperature is above 1500K, the

with SiO2 is firstly C3A, and then C12A7, CA, CA2.

the thermodynamic order of forming calcium silicate is C2S, C3S, C3S2, CS.

C12A7. Therefore, it is less likely to form aluminium silicates in roasting process.

**CaO+SiO2**

When CaO is insufficient, redundant Al2O3 may promote the newly generated high calciumto-aluminum ratio (CaO to Al2O3 mole ratio) calcium aluminates to transform into lower calcium-to-aluminum ratio calcium aluminates. The reactions of the equations are presented in table 5:

The relationships between *GT* of reactions of Al2O3-calcium aluminates system and temperature (T) are shown in figure 6.

Fig. 6. Relationships between *GT* of reactions Al2O3-calcium aluminates system and temperature

Figure 6 shows that, Gibbs free energy of the reaction of Al2O3-calcium aluminates system are negative at 400~1700K, and all the reactions automatically proceed to generate the corresponding low calcium-to-aluminum ratio calcium aluminates; Except for the reaction of Al2O3-C12A7, the *GT* of the rest reactions decreases with the rise of temperature and becomes more negative. Comparing figure 4 with figure 5, it can be found that Al2O3 reacts with CaO easily to generate C12A7.
