Due to the special nature of China's rare earth raw materials, silicon thermal reduction method before the 1990s has been a major process of preparing rare earth iron alloy of silicon.
  Reaction Thermodynamics of Preparation of Rare Earth Ferrosilicon Alloy by Silicon Thermal Reduction
In the process of preparing rare earth ferrosilicon alloy by silicon thermal reduction, the main chemical reaction is silicon reduction rare earth oxide, accompanied by chemical reactions such as reduction of other oxides, slag formation and alloying. Because the reaction of rare earth ferrosilicon alloy smelting process is very complicated, especially the research on the thermodynamic data of some rare earth element leasing management compounds is not enough. At present, it is difficult to carry out accurate thermodynamic calculation of chemical reaction in smelting process. Thermodynamic data can only be discussed in general terms.
  Thermodynamics of silicon reduction rare earth oxides
Table 1 lists several heats of formation and free formation of oxides [1,2] . It can be seen from the table that the rare earth element has a large affinity for oxygen, and the stability of the most important rare earth oxides (Ce 2 O 3 , CeO 2 , La 2 O 3 ) is close to that of MgO and Al 2 O 3 . More stable than SiO 2 . Therefore, under general smelting conditions, it is extremely difficult to directly reduce rare earth oxides with silicon. Taking silicon reduction of Ce 2 O 3 as an example:
2
Ce 2 O 3
+
Si
====
4
Ce+SiO 2
(1)
3
3
â–³G 0 =289600-13.40T
Table 1 Standard heating and free enthalpy of oxides
Oxide
Oxidation reaction
â–³H 0 298 (J/molO 2 )
â–³G 0 /(J/molO 2 )
Al 2 O 3
4
Al+O 2
====
2
Al 2 O 3
-1116290
-115500+209.20T
3
3
Ca0
MgO
2Ca+O2 ==== 2CaO
2Mg+O2 ==== 2MgO
-1268580
-1202480
-1237800+201.30T
-1196600+208.40T
Ce 2 O 3
4
Ce+O 2
====
2
Ce 2 O 3
-1202000
-1195400+189.10T
3
3
CeO 2
Ce+O 2 ==== CeO 2
-1025080
-1085700+211.30T
La 2 O 3
4
La+O 2
====
2
La 2 O 3
-1195510
-1192000+277.40T
3
3
Si0 2
Si+O 2 ==== SiO 2
-910860
-905800+175.70T
However, the preparation of the rare earth ferrosilicon by the silicon thermal work equivalent method is actually a molten state reduction process. The reaction system consists of a liquid slag phase and an alloy phase, and the reduction reaction is carried out between the two phases, and the substances participating in the reaction are not pure substances. The reaction of silicon to reduce rare earth oxygen can be illustrated as:
2
(RE 2 O 3 )+[Si]====
4
[RE]+(SiO 2 )
(2)
3
3
                   
[1]
Where ΔG 0 is the standard free enthalpy change of the reaction;
aRE - the activity of rare earth silica in the alloy;
asiO 2 - the activity of silica in the slag;
aRE 2 O 3 - the activity of rare earth oxides in the slag;
Asi - the activity of silicon in the alloy.
The standard free enthalpy change ΔG 0 of the reaction has the following relationship with the equilibrium constant K of the reaction:
                    
â–³G 0 =-RTlnK [2]

Thus equation (9-3) can be rewritten as:
                   
â–³G=-RTlnK+RTlnJ=RTlnJ/K [3]
In the middle
According to the principle of the minimum free enthalpy of the chemical reaction, if the reaction formula (2) can be carried out in the direction in which the rare earth metal is formed by the thermal reduction of the rare earth oxide by silicon, it is necessary to make ΔG < 0. It can be seen from the formula [3] that if ΔG < 0, J < K is required. The equilibrium constant K is a function of temperature. K is a constant at a certain temperature. Therefore, in order to make the reaction formula (22) proceed to the right, decreasing the J value, that is, increasing the activity of the reactant or reducing the activity of the reaction product, is the most effective route. A qualitative study of the activity of the reactants is given below. [next]
(1) Activity of rare earth oxides The activity of rare earth oxides in slag has the following relationship with the molar concentration of rare earth oxides:
aRE 2 O 3 =αRE 2 O 3 ·NRE 2 O 3 [4]
Where aRE 2 O 3 - the activity of the rare earth oxide in the slag;
aRE 2 O 3 - activity coefficient;
NRE 2 O 3 - the molar concentration of the rare earth oxide.
It can be seen from the formula [44] that increasing the activity coefficient and concentration of the rare earth oxide in the slag can increase the activity of the rare earth oxide.
When the literature [3, 4, 5] studied the activity of rare earth oxides in slag of different systems, the same law was obtained:
The activity of the rare earth oxide in the slag increases as the concentration of the rare earth oxide increases.
2 When the calcium oxide content (or alkalinity) in the slag increases, the activity coefficient of the rare earth oxide increases, and the activity also increases, as shown in FIG.
Fig.1 Activity change of La 2 O 3 in La 2 O 3 -CaF 2 -CaO-SiO 2 quaternary system
According to the ion theory, the activity of RE2O3 can be expressed by the following formula:
      
[5]
- the activity of oxygen ions.
It can be seen from the formula [5] that the activity of the rare earth oxide is related to the activity of the rare earth ion and the oxygen ion, and the activity of the rare earth ion and the activity of the oxygen ion are related to the rare earth oxide and calcium oxide content in the slag. In the slag, the rare earth oxide and calcium oxide will be ionized into metal cations and oxygen anions, so increasing the content of rare earth oxides and calcium oxide in the slag is beneficial to increase the activity of the rare earth oxide. However, in the process of preparing rare earth ferrosilicon alloy by silicon thermal reduction method, the rare earth oxide is used as the main reactant, and the concentration in the slag is restricted by the rare earth grade of the alloy, and cannot be increased arbitrarily, and the oxygen anion provided by it is limited. Further, amphoteric and acidic oxides such as Al 2 O 3 , SiO 2 , P 2 O 5 and the like contained in the slag will absorb the oxygen anions.
3 When the content of calcium oxide in the slag is too high, the melting point of the slag increases, the viscosity increases, and the activity of the rare earth oxide decreases.
(2) Activity of silicon The rare earth ferrosilicon alloy is prepared by a silicon thermal reduction method. The silicon as a reducing agent is supplied by ferrosilicon, and the content of ferrosilicon has a significant influence on the activity of silicon. The results show that [6] , as Nsi increases, asi also increases accordingly, the relationship is shown in Figure 2.
Fig. 2 Relationship between activity and concentration of silicon in Si-Fe system
It can be seen from Fig. 2 that when Nsi = 0.3 (i.e., 17.64%), asi is almost zero, indicating that ferrosilicon having Nsi < 0.3 cannot be used as a reducing agent. From the technical and economic point of view, the preparation of rare earth ferrosilicon by silicon thermal reduction method is more reasonable with 75 ferrosilicon as a reducing agent.
In the process of producing silicon-calcium alloy by silicon thermal reduction method [7] , it was found that the content of free silicon in ferrosilicon has a great influence on the activity of silicon. 75 ferrosilicon is an alloy composed of free silicon and germanium. During the reduction process, silicon in the germanium phase does not participate in the reaction, and its molecular formula is Fe 2 Si 5 . Therefore, the high content of silicon in the reducing agent is beneficial to increase the activity of silicon.
(3) Activity of SiO 2 in the slag In the process of preparing the rare earth ferrosilicon alloy by the silicon thermal reduction method, the method of increasing the content of calcium oxide in the slag is used to reduce the activity of SiO 2 . In the CaO-SiO 2 binary system, the activity of SiO 2 decreases sharply with the increase of CaO concentration in the melt (Fig. 9-3) [8] .
Calcium oxide can form various compounds with SiO 2 formed by silicon reduction rare earth oxide, such as CaO·SiO 2 , 2CaO·SiO 2 , 3CaO·SiO 2 , etc. [9] , and the activity of SiO 2 is reduced. In actual production, the lime contained in the slag can be combined with almost all of the SiO 2 to make the activity less, and the reaction of reducing the rare earth oxide by silicon can be sufficiently performed. [next]
(4) Activity of rare earth in the alloy melt Reducing the activity of the rare earth metal of the reaction product in the alloy melt is also one of the main contents of reducing the activity. The phase analysis of the rare earth ferrosilicon alloy shows that [10] , the rare earth in the alloy exists in the form of silicide, that is to say, after the rare earth oxide in the slag is reduced to the rare earth metal by silicon, the rare earth metal and the silicon in the alloy melt The alloying reaction occurs to form a rare earth silicide and dissolve it in the alloy melt, taking hydrazine as an example [11] :
                                     

1

[Ce]+[Si]

=

1

CeSi 2

2

2


(3)

CeSi 2 =[CeSi 2 ] (4)
The rare earth silicide is in a stable state in the alloy, thereby reducing the activity of the rare earth in the alloy. In order to allow the alloying reaction of the rare earth to proceed sufficiently, it is required to have a sufficient silicon content in the alloy.
Fig.3 Relationship between activity and concentration in CaO-SiO 2 binary system
In summary, it can be considered that in the process of preparing the rare earth ferrosilicon alloy by the silicon thermal reduction method, in order to sufficiently reduce the rare earth and obtain a high rare earth recovery rate, the high-grade, low-contamination rare earth raw materials, reducing agents and fluxes are selected. It is a condition for making the thermodynamic conditions of the silicon thermal reduction method sufficient.
  Thermodynamics of silicon reduction of other oxides
In addition to REO, CaO and SiO2, the rare earth charge contains a small amount of FeO, MnO, MgO, TiO 2 , ThO 2 , etc., and the flux also carries a large amount of CaO, which are reduced to various degrees by silicon during the smelting process. In addition to the reduction of CaO to promote the reduction of rare earth compounds, the reduction of other oxides consumes a certain amount of silicon, which reduces the silicon content in the alloy melt and reduces the activity of silicon, which is not conducive to the reduction of rare earth oxides.
(1) Reduction of FeO and MnO Since the affinity of iron and manganese for oxygen is much smaller than the affinity of silicon for oxygen, it is easy to reduce FeO and MnO with silicon:
2FeO+Si====2Fe+SiO2 (5)
â–³G 0 ====-368190+38087T
                   
2MnO+Si====2Mn+SiO2 (6)
â–³G 0 =-123090+18.79T
The reaction formula (5) and the reaction formula (6) are exothermic reactions and can be carried out spontaneously, and the produced iron and manganese can react with silicon to form a silicide. The silicide of iron and manganese is stably present in the alloy, which reduces the content of free silicon in the alloy and is not conducive to the reduction of rare earth oxides.
(2) Reduction of CaO and MgO CaO is an extremely stable oxide, and the reaction of reducing CaO with silicon is:
             
2CaO+Si (1) ====2Ca (1) + SiO 2 (7)
â–³G 0 =324430-17.15T
When both the reactants and the product are in a pure state, the reaction formula (7) is extremely difficult to carry out under general smelting conditions. However, in the process of preparing rare earth ferrosilicon by silicon thermal method, silicon can reduce calcium oxide has been confirmed by practice [12] . It has been reported that in the reaction system of ferroalloy production, the reduction of calcium oxide by silicon may have the following reactions:
2CaO+3Si (1) ====2CaSi (1) + SiO 2 (8)
â–³G 0 =465190-240.54T
                
3CaO+5Si (1) ====2CaSi 2(1) +CaO·SiO 2(1) (9)
△G 0 =23220-111.84T-0.75×10 -3 T 2 +2.68TlgT
                
3CaO+3Si (1) ====2CaSi (1) +CaO·SiO 2(1) (10)
△G 0 =37450-67.99T-0.75×10 -3 T 2 +2.68TlgT
Magnesium oxide may be silicon reduction reaction is as follows:
                 
3MgO+2Si (1) ====Mg2Si (1) +MgO·SiO 2(1) (11)
△G 0 =245600+48.50T-1.00×10 -3 T 2 -38.74TlgT
In the presence of CaO, the reaction proceeds as follows:
              
2MgO+CaO+2Si (1) ====Mg2Si (1) +CaO·SiO2 (1) (12)
△G 0 =186010+62.22T+2.0×10 -3 T 2 -43.22TlgT
However, since the actual smelting temperature is higher than the boiling point of magnesium (1105 ° C), most of the reduced magnesium is volatilized in a gaseous state, and only a small part of the alloy is present.
(. 3) reduction of TiO 2 TiO 2 and thermodynamic stability similar 2 SiO, silicon reduction reaction of TiO 2 is:
                      
TiO 2 +Si (1) ====Ti+SiO 2 (13)
â–³G 0 =-8386+21.63T
The TiO 2 introduced in the charge is often regarded as a harmful substance because the reduced titanium enters the alloy and affects the use effect of the rare earth ferrosilicon alloy in iron. Therefore, the reduction of TiO 2 should be minimized during the smelting process.
references
1. Liang Yingjiao, et al., Physical Chemistry, Beijing: Metallurgical Industry Press, 1986, 336~368
2, Huang Xiwei, etc., Principles of Iron and Steel Metallurgy, Beijing: Metallurgical Industry Press, 1986
3, Wangchang Zhen, Yang behalf silver, metal Sinica, 1979,15: 433
4. Wang Changzhen, Zou Yuanzhang, Journal of Metals, 1980, 16:190
5. Wang Changzhen et al., Journal of Rare Earths, 1985, 3:27
6. Cai Tigang, Proceedings of the Second Annual Conference of China Rare Earth Society, Second Volume, China Rare Earth Society, 1990, 156
7, M, A Luis, Ferroalloy Smelting, Beijing: Metallurgical Industry Press, 1989
8. German Engineers Association, Slag Atlas, Beijing: Metallurgical Industry Press, 1989, 148-149
9. Liu Xingshan, Ferroalloy, 1986, 3:15
10. Rare Earth Compilation Group, Rare Earth (Upper and Lower), Beijing: Metallurgical Industry Press, 1978
11, Yan Bingyi, rare earth, 1989, 10 (4): 50
12. Cui Jigang, Fang Xingbo, Rare Earth, 1988, 9(2): 42

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