The
1. Introduction
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 4CaO Al2O3 Fe2O3.
2. Binary compounds
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:
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
CaCO3+Al2O3=CaO·Al2O3+CO2 | 161088.3 | -244.1 | 298~1200 |
CaCO3+Fe2O3=CaO·Fe2O3+CO2 | 151677.8 | -220.9 | 298~1200 |
The relationships between Gibbs free energy (
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
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.
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
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 |
|
12764.7 | -16.7 | 298~1696 |
The relationships of
Figure 2 shows that, the
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
Figure 3 shows that, the
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
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
FeO+SiO2 =FeO·SiO2 | 26524.6 | 18.8 | 847~1413 |
2FeO+SiO2 =2FeO·SiO2 | -13457.3 | 30.3 | 847~1493 |
Figure 4 shows that, the
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
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
3CaO+ Al2O3=3CaO·Al2O3 | -9.9 | -28.4 | 298~1808 |
|
318.3 | -44.5 | 298~1800 |
CaO+ Al2O3=CaO·Al2O3 | -15871.5 | -18.1 | 298~1878 |
|
-6667.2 | -13.8 | 298~2023 |
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
|
13939.7 | -65.8 | 298~1800 |
|
-18843.8 | -13.0 | 298~1878 |
|
-6011.2 | -10.9 | 298~2023 |
|
-38544.8 | 18.8 | 298~1878 |
|
-9541.1 | -1.2 | 298~2023 |
CaO•Al2O3+ Al2O3=CaO•2Al2O3 | 2543.8 | -9.5 | 298~2023 |
When CaO is insufficient, redundant Al2O3 may promote the newly generated high calcium-to-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
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
2.6. SiO2- CaO system
SiO2 can react with CaO to form CaO SiO2 (CS), 3CaO 2SiO2 (C3S2), 2CaO SiO2 (C2S) and 3CaO SiO2(C3S) in roasting process. The reactions are shown in table 6, and the relationships between △G0 of the reactions of SiO2 with CaO and temperature are shown in figure 7.
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
CaO+SiO2 = CaO·SiO2(pseud-wollastonite) | -83453.0 | -3.4 | 298~1817 |
CaO+SiO2 = CaO·SiO2(wollastonite) | -89822.9 | -0.3 | 298~1817 |
|
-108146.6 | -3.1 | 298~1700 |
3CaO+SiO2 = 3CaO·SiO2 | -111011.9 | -11.3 | 298~1800 |
2CaO+SiO2 = 2CaO·SiO2(β) | -125875.1 | -6.7 | 298~2403 |
2CaO+SiO2 = 2CaO·SiO2(γ) | -137890.1 | 3.7 | 298~1100 |
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, the thermodynamic order of forming calcium silicate is C2S, C3S, C3S2, CS.
Figure 5 ~ figure 7 show that, CaO reacts with SiO2 and Al2O3 firstly to form C2S, and then C12A7. Therefore, it is less likely to form aluminium silicates in roasting process.
2.7. SiO2- calcium aluminates system
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 between
Figure 8 shows that, the
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
(3)CaO·2Al2O3 +SiO2=3CaO·SiO2+6Al2O3 | -69807.8 | 70.8 | 298~1800 |
(3)CaO·Al2O3 +SiO2=3CaO·SiO2+3Al2O3 | -62678.8 | 42.6 | 298~1800 |
|
-111820.6 | 66.7 | 298~1800 |
(2)CaO·2Al2O3+SiO2 =2CaO·SiO2+4Al2O3 | -98418.8 | 48.1 | 298~1710 |
|
-87585.9 | 38.0 | 298~1700 |
CaO·2Al2O3+SiO2= CaO·SiO2+2Al2O3 | -76146.6 | 27.1 | 298~1817 |
CaO·Al2O3+SiO2 =CaO·SiO2+Al2O3 | -73770.2 | 17.7 | 298~1817 |
|
-84021.4 | 23.8 | 298~1700 |
(2)CaO·Al2O3+SiO2 =2CaO·SiO2+2Al2O3 | -93666.1 | 29.2 | 298~1710 |
|
-90150.8 | 25.7 | 298~1800 |
|
-108592.3 | 35.9 | 298~1700 |
|
-126427.4 | 45.3 | 298~1710 |
|
-86654.2 | 9.4 | 298~1808 |
3CaO·Al2O3+SiO2= 3CaO·SiO2+Al2O3 | -100774.6 | 16.9 | 298~1808 |
|
-103069.3 | 11.0 | 298~1700 |
|
-119063.3 | 12.1 | 298~1710 |
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.
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
CaO+Fe2O3=CaO·Fe2O3 | -19179.9 | -11.1 | 298~1489 |
2CaO+Fe2O3=2CaO·Fe2O3 | -40866.7 | -9.3 | 298~1723 |
2CaO·Fe2O3+Fe2O3=(2)CaO·Fe2O3 | 2340.8 | -12.6 | 298~1489 |
2.9. Al2O3- calcium ferrites system
Figure 1 shows that, the
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
(3)CaO•Fe2O3+ Al2O3 = 3CaO•Al2O3+3Fe2O3 | 47922.7 | 4.5 | 298~1489 |
|
49.6 | -1.2×10-2 | 298~1723 |
|
32685.1 | -24.5 | 298~1489 |
|
34514.4 | -35.0 | 298~1723 |
CaO•Fe2O3+ Al2O3 =CaO•Al2O3+Fe2O3 | 3626.6 | -7.5 | 298~1489 |
|
3215.1 | -8.8 | 298~1489 |
|
3168.6 | -11.0 | 298~1723 |
|
4009.5 | -12.8 | 298~1723 |
The relationships between
3. Ternary compounds in Al2O3-CaO-SiO2-Fe2O3 system
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 formed form CaO, Al2O3 and Fe2O3. The equations are shown in table 10:
Reactions | A, J/mol | B, J/K.mol | Temperature, K |
CaO·SiO2+ CaO·Al2O3=2CaO·Al2O3·SiO2 | -30809.41 | 0.60 | 298~1600 |
|
-47997.55 | -7.34 | 298~1826 |
Al2O3 + 2CaO + SiO2=2CaO·Al2O3·SiO2 | -50305.83 | -9.33 | 298~1600 |
Al2O3 + CaO + SiO2=CaO·Al2O3·SiO2 | -72975.54 | -9.49 | 298~1700 |
|
-112354.51 | 20.86 | 298~1700 |
4CaO +Al2O3 + Fe2O3=4CaO·Al2O3·Fe2O3 | -66826.92 | -62.5 | 298~2000 |
Al2O3 + 2CaO + SiO2=2CaO·Al2O3·SiO2 (cacoclasite) | -136733.59 | -17.59 | 298~1863 |
The relationships between
C2AS may also be formed by the reaction of CA and CS, the curve is presented in figure 11. Figure 11 shows that, the
Figure 12 shows that, in thermodynamics, C2AS and C4AF are firstly formed when Al2O3, Fe2O3, SiO2 and CaO coexist, and then calcium silicates, calcium aluminates and calcium ferrites are generated.
4. Summary
1) When Al2O3 and Fe2O3 simultaneously react with CaO, calcium silicates are firstly formed, and then calcium ferrites. In thermodynamics, when one mole Al2O3 reacts with CaO, the sequence of generating calcium aluminates are 12CaO 7Al2O3, 3CaO Al2O3, CaO Al2O3, CaO 2Al2O3. When CaO is insufficient, redundant Al2O3 may promote the newly generated high calcium-to-aluminum ratio calcium aluminates to transform to lower calcium-to-aluminum ratio calcium aluminates. Fe2O3 reacts with CaO easily to form 2CaO Fe2O3, and CaO Fe2O3 is not from the reaction of 2CaO Fe2O3 and Fe2O3 but form the directly combination of Fe2O3 with CaO. Al2O3 cannot replace the Fe2O3 in calcium ferrites to generate 3CaO Al2O3, and also cannot replace the Fe2O3 in CaO•Fe2O3 to generate 12CaO 7Al2O3, but can replace the Fe2O3 in 2CaO•Fe2O3 to generate 12CaO 7Al2O3 when the temperature is above 1000K; Al2O3 can react with calcium ferrites to form CaO Al2O3 or CaO 2Al2O3.
2) One mole SiO2 reacts with Al2O3 much easily to generate 3Al2O3 2SiO2, Fe2O3 can not react with SiO2 in the roasting process in the air. Al2O3 can not directly react with Fe2O3, but can react with wustite (FeO) to form FeO Al2O3.
3) In thermodynamics, the sequence of one mole SiO2 reacts with CaO to form calcium silicates is 2CaO SiO2, 3CaO SiO2, 3CaO 2SiO2 and CaO SiO2. Calcium aluminates can react with SiO2 to transform to calcium silicates and Al2O3. CaO 2Al2O3 can not transform to 3CaO SiO2 when the roasting temperature is above 900K; when the temperature is above 1500K, 3CaO Al2O3 can not transform to 3CaO SiO2; but the other calcium aluminates all can all react with SiO2 to generate calcium silicates at 800~1700K.
4) Reactions among Al2O3, Fe2O3, SiO2 and CaO easily form 2CaO Al2O3 SiO2 and 4CaO Al2O3 Fe2O3. 2CaO Al2O3 SiO2 does not form from the reaction of CaO Al2O3 and CaO SiO2, but from the direct reaction among Al2O3, CaO, SiO2. And 4CaO Al2O3 Fe2O3 is also not formed via mutual reaction of calcium ferrites and sodium aluminates, but from the direct reaction of CaO, Al2O3 and Fe2O3. In thermodynamics, when Al2O3, Fe2O3, SiO2 and CaO coexist, 2CaO Al2O3 SiO2 and 4CaO Al2O3 Fe2O3 are firstly formed, and then calcium silicates, calcium aluminates and calcium ferrites.
5. Symbols used
Thermodynamic temperature: T, K
Thermal unit: J
Amount of substance: mole
Standard Gibbs free energy:
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