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Thermodynamics of Reactions Among Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes

Written By

Zhongping Zhu, Tao Jiang, Guanghui Li, Yufeng Guo and Yongbin Yang

Submitted: 15 November 2010 Published: 02 November 2011

DOI: 10.5772/21545

From the Edited Volume

Thermodynamics - Interaction Studies - Solids, Liquids and Gases

Edited by Juan Carlos Moreno-Pirajan

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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.

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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

Table 1.

The Δ G T θ of Fe2O3-Al2O3-CaCO3 system( Δ G T θ = A + B T , J/mol; P C O 2 =30Pa, i.e., the partial pressure of CO2 in the air)

The relationships between Gibbs free energy ( Δ G T θ ) and temperature (T) are as shown in figure 1.

Figure 1.

Relationships between Δ G T θ 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 Δ G T θ of reaction with Al2O3 is more negative than the Δ G T θ 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.

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
3 2 Al 2 O 3 + SiO 2 = ( 1 2 )3Al 2 O 3 2SiO 2 12764.7 -16.7 298~1696

Table 2.

The Δ G T θ of Al2O3-SiO2 system( Δ G T θ = A + B T , J/mol)

The relationships of Δ G T θ and temperature in Al2O3-SiO2 system is shown in figure 2.

Figure 2.

Relationships of Δ G T θ and temperature in Al2O3-SiO2 system

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

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

Al 2 O 3 + FeO = FeO · Al 2 O 3 = 3 0 172 . 2 + 9 . 3T , J . mol 1   843 ~ 165 0 K     E1

Figure 3.

Relationship of Δ G T θ and temperature in Fe2O3-Al2O3 system

Figure 3 shows that, the Δ G T θ is negative at 843~1650K, reaction can happen and generate FeO Al2O3; the Δ G T θ 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 Δ G T θ and temperature is shown in figure 4, and the chemical reactions of the equations are shown in table 3.

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

Table 3.

The Δ G T θ of SiO2- Al2O3 system( Δ G T θ = A + B T , J/mol)

Figure 4 shows that, the Δ G T θ 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).

Figure 4.

Relationships of Δ G T θ 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 Δ G T θ 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 roasting temperature, the thermodynamic order that one mole Al2O3 reacts with CaO to generate calcium aluminates such as C12A7, C3A, CA, CA2.

Reactions A, J/mol B, J/K.mol Temperature, K
3CaO+ Al2O3=3CaO·Al2O3 -9.9 -28.4 298~1808
12 7 CaO+Al 2 O 3 =( 1 7 )12CaO 7Al 2 O 3 318.3 -44.5 298~1800
CaO+ Al2O3=CaO·Al2O3 -15871.5 -18.1 298~1878
1 2 CaO+Al 2 O 3 =( 1 2 )CaO 2Al 2 O 3 -6667.2 -13.8 298~2023

Table 4.

The Δ G T θ of Al2O3-CaO system( Δ G T θ = A + B T , J/mol)The relationships between Δ G T θ and temperature (T) are shown in figure 5.

Figure 5.

Relationships between Δ G T θ and temperature in Al2O3-CaO system

Reactions A, J/mol B, J/K.mol Temperature, K
( 4 3 )3CaO Al 2 O 3 +Al 2 O 3 =( 1 3 )12CaO 7Al 2 O 3 13939.7 -65.8 298~1800
( 1 2 )3CaO Al 2 O 3 +Al 2 O 3 =( 3 2 )CaO Al 2 O 3 -18843.8 -13.0 298~1878
( 1 5 )3CaO Al 2 O 3 +Al 2 O 3 =( 3 5 )CaO 2Al 2 O 3 -6011.2 -10.9 298~2023
( 1 5 )12CaO 7Al 2 O 3 +Al 2 O 3 =( 12 5 )CaO Al 2 O 3 -38544.8 18.8 298~1878
( 1 17 )12CaO 7Al 2 O 3 +Al 2 O 3 =( 12 17 )CaO 2Al 2 O 3 -9541.1 -1.2 298~2023
CaO•Al2O3+ Al2O3=CaO•2Al2O3 2543.8 -9.5 298~2023

Table 5.

. The Δ G T θ of Al2O3-calcium aluminates system( Δ G T θ = A + B T , J/mol)

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 Δ G T θ of reactions of Al2O3-calcium aluminates system and temperature (T) are shown in figure 6.

Figure 6.

Relationships between Δ G T θ 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 Δ G T θ 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.

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
3 2 CaO+SiO 2 =( 1 2 )3CaO 2SiO 2 -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

Table 6.

The Δ G T θ of SiO2-CaO system( Δ G T θ = A + B T , J/mol)

Figure 7.

Relationships between Δ G T θ and temperature

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 Δ G T θ and temperature are shown in figure 8.

Figure 8 shows that, the Δ G T θ of all the reactions increases with the temperature increases; 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 silicate; when the temperature is above 1500K, the Δ G T θ of reaction(3CA+ SiO2=C3S+3Al2O3) 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 with SiO2 is firstly C3A, and then C12A7, CA, CA2.

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
( 1 4 )12CaO 7Al 2 O 3 + SiO 2 = 3CaO SiO 2 + 7 4 Al 2 O 3 -111820.6 66.7 298~1800
(2)CaO·2Al2O3+SiO2 =2CaO·SiO2+4Al2O3 -98418.8 48.1 298~1710
( 3 2 )CaO 2 Al 2 O 3 + SiO 2 = ( 1 2 ) 3CaO 2 SiO 2 + 3 Al 2 O 3 -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
( 3 2 )CaO Al 2 O 3 + SiO 2 = ( 1 2 ) 3CaO 2 SiO 2 + 3 2 Al 2 O 3 -84021.4 23.8 298~1700
(2)CaO·Al2O3+SiO2 =2CaO·SiO2+2Al2O3 -93666.1 29.2 298~1710
( 1 12 )12CaO 7 Al 2 O 3 + SiO 2 = CaO SiO 2 + 7 12 Al 2 O 3 -90150.8 25.7 298~1800
( 1 8 )12CaO 7 Al 2 O 3 + SiO 2 = ( 1 2 ) 3CaO 2 SiO 2 + 7 8 Al 2 O 3 -108592.3 35.9 298~1700
( 1 6 )12CaO 7Al 2 O 3 + SiO 2 = 2CaO SiO 2 + 7 6 Al 2 O 3 -126427.4 45.3 298~1710
( 1 3 )3CaO Al 2 O 3 + SiO 2 = CaO SiO 2 + 1 3 Al 2 O 3 -86654.2 9.4 298~1808
3CaO·Al2O3+SiO2= 3CaO·SiO2+Al2O3 -100774.6 16.9 298~1808
( 1 2 )3CaO Al 2 O 3 + SiO 2 = ( 1 2 ) 3CaO 2 SiO 2 + 1 2 Al 2 O 3 -103069.3 11.0 298~1700
( 2 3 )3CaO Al 2 O 3 + SiO 2 = 2CaO SiO 2 + 2 3 Al 2 O 3 -119063.3 12.1 298~1710

Table 7.

Table 7. The Δ G T θ of the reactions SiO2 with calcium aluminates( Δ G T θ = A + B T , J/mol)

Figure 8.

Relationships between Δ G T θ and temperature in SiO2-calcium aluminates system

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

Table 8.

The Δ G T θ of Fe2O3-CaO system( Δ G T θ = A + B T , J/mol)

Figure 9.

Relationships between Δ G T θ and temperature in Fe2O3-CaO system

2.9. Al2O3- calcium ferrites system

Figure 1 shows that, the Δ G T θ 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:

Reactions A, J/mol B, J/K.mol Temperature, K
(3)CaO•Fe2O3+ Al2O3 = 3CaO•Al2O3+3Fe2O3 47922.7 4.5 298~1489
( 3 2 ) 2 CaO Fe 2 O 3 + Al 2 O 3 = 3CaO Al 2 O 3 + 3 2 Fe 2 O 3 49.6 -1.2×10-2 298~1723
( 12 7 ) CaO Fe 2 O 3 + Al 2 O 3 = ( 1 7 )12CaO 7 Al 2 O 3 + 12 7 Fe 2 O 3 32685.1 -24.5 298~1489
( 6 7 ) 2 CaO Fe 2 O 3 + Al 2 O 3 = ( 1 7 )12CaO 7 Al 2 O 3 + 6 7 Fe 2 O 3 34514.4 -35.0 298~1723
CaO•Fe2O3+ Al2O3 =CaO•Al2O3+Fe2O3 3626.6 -7.5 298~1489
( 1 2 ) CaO Fe 2 O 3 + Al 2 O 3 = ( 1 2 )CaO 2 Al 2 O 3 + 1 2 Fe 2 O 3 3215.1 -8.8 298~1489
( 1 4 ) 2 CaO Fe 2 O 3 + Al 2 O 3 = ( 1 2 )CaO 2 Al 2 O 3 + 1 4 Fe 2 O 3 3168.6 -11.0 298~1723
( 1 2 ) 2 CaO Fe 2 O 3 + Al 2 O 3 = CaO Al 2 O 3 + 1 2 Fe 2 O 3 4009.5 -12.8 298~1723

Table 9.

The Δ G T θ of the reaction Al2O3 with calcium ferrites( Δ G T θ = A + B T , J/mol)

The relationships between Δ G T θ 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 form CA or CA2, the higher temperature, more negative Δ G T θ . 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.

Figure 10.

Relationship between Δ G T θ and temperature in Al2O3- calcium ferrites system

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

Table 10.

The Δ G T θ of forming ternary compounds ( Δ G T θ = A + B T , J/mol)

The relationships between Δ G T θ and temperature (T) are shown in figure 11. Figure 11 shows that, except for C3AS3(Hessonite), all the Δ G T θ of the reactions get more negative with the temperature increasing; the thermodynamic order of generating ternary compounds at sintering temperature of 1473K is: C2AS(cacoclasite), C4AF, CAS, C3AS3, C2AS, CAS2.

C2AS may also be formed by the reaction of CA and CS, the curve is presented in figure 11. Figure 11 shows that, the Δ G T θ of reaction (Al2O3+CaO+SiO2) is lower than that of reaction of CA and CS to generate C2AS. So C2AS does not form from the binary compounds CA and CS, but from the direct combination among Al2O3, CaO, SiO2. Qiusheng Zhou thinks that, C4AF is not formed by mutual reaction of calcium ferrites and sodium aluminates, but from the direct reaction of CaO, Al2O3 and Fe2O3. Thermodynamic analysis of figure 1~figure11 shows that, reactions of Al2O3, Fe2O3, SiO2 and CaO are much easier to form C2AS and C4AF, as shown in figure 12.

Figure 11.

Relationships between Δ G T θ of ternary compounds and temperature

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.

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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.

Figure 12.

Relationships between Δ G T θ and temperature in Al2O3-CaO-SiO2-Fe2O3 system

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.

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5. Symbols used

Thermodynamic temperature: T, K

Thermal unit: J

Amount of substance: mole

Standard Gibbs free energy: Δ G T θ ,J

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Written By

Zhongping Zhu, Tao Jiang, Guanghui Li, Yufeng Guo and Yongbin Yang

Submitted: 15 November 2010 Published: 02 November 2011