Reaction Mechanism of Preparation of Rare Earth Ferrosilicon Alloy by Silicon Thermal Reduction

Preparation of a silicon thermal reduction of rare earth silicon iron alloy process, due to insufficient thermodynamic data and a rare earth metal compound, containing data relating to the activity of the lack of rare earth elements and the slag melt-based RE-Si-Fe alloy melt, thus creating a The thermodynamic data is used to calculate the difficulty of the actual smelting process. However, the basic principles of metallurgical thermodynamics can be utilized, combined with production practices, to infer the possible chemical reactions in the smelting process, thereby further deepening the understanding of the reaction mechanism.
  Chemical reaction during melting period of charge
The melting period refers to the smelting stage from the start of the addition of rare earth raw materials and lime to the addition of ferrosilicon, the task of which is to melt the charge to form a slag phase. Rare earth rich slag or rare earth concentrate slag is used as raw material [the mineral composition is strontium calcium silica , gunite, fluorite and calcium sulfide, etc., rare earth elements are present in strontium calcium silica minerals (3CaO·Ce2O3·SiO2)], when smelting When the temperature reaches 1100 to 1200 ° C, the molten slag reacts with the lime and promotes the melting of the lime, and the following reaction occurs.
1 铈 calcium silica decomposition:
3CaO·Ce 2 O 3 ·2CiO 2 +CaO====Ce 2 O 3 +2(2CaO·SiO 2 ) (1)
2 shot spar decomposition:
              
3CaO·CaF 2 ·2SiO 2 +CaO====CaF 2 +2(2CaO·SiO 2 ) (2)
3 Under sufficient CaO conditions:
               
2CaO·SiO 2 +CaO====3CaO·SiO 2 (3)
  Chemical reaction during the reduction period
The reduction period is the smelting stage of adding ferrosilicon to the alloy. As the ferrosilicon melts, two phases, the molten slag phase and the alloy phase, appear in the furnace. The chemical reaction at this time consists of three parts: a reduction reaction carried out at the interface of the two phases, a slagging reaction in the slag phase, and an alloying reaction in the alloy phase.
(1) Silicon Reduction of Rare Earth Oxide Due to the presence of a large amount of free RE 2 O 3 in the slag, a large amount of free silicon exists in the ferrosilicon, and RE 2 O 3 is reduced by silicon at the interface of the two phases [Reaction formula (-1 )].
The phase analysis results show that [13] , the rare earth in the alloy exists in the form of silicide, and SiO 2 in the slag exists in the form of silicate. It is thus proved that the rare earth metal and the silicon are alloyed to form a rare earth silicide in the alloy phase:
              
[RE]+[Si]====[RESi] (4)
              
[RESi]+[Si]====[RESi 2 ] (5)
The SiO 2 produced by the reduction reacts with CaO in the slag to form calcium silicate which is present in the slag:
          
(CaO)+(SiO 2 )====(CaO·SiO 2 ) (6)
         
2(CaO)+(SiO 2 )====(2CaO·SiO 2 ) (7)
           
3(CaO)+(SiO 2 )====(3CaO·SiO 2 ) (8)
The formation of rare earth silicide and calcium silicate greatly reduces the activity of the rare earth in the alloy and the activity of SiO 2 in the slag, so that the reaction formula and the following formula can be smoothly carried out.

2

( RE 2 O 3 ) +[Si]

====

4

[RE]+ ( SiO 2 )

3

3

(2) Reduction of rare earth oxides by silicon-calcium In order to further explore the reduction mechanism of rare earth oxides, the researchers obtained the actual conditions of rare earth ferrosilicon alloys by silicon thermal method and prepared them into synthetic slag containing no rare earth. The composition is shown in Table 1. After the synthesis slag is melted, it is reduced with 75 ferrosilicon. The changes of calcium content and silicon content of the alloy during the smelting process are shown in Table 2.
Table 1 Composition of synthetic slag
composition
CaO
SiO 2
CaF 2
Al 2 O 3
S
content/%
48.97
14.53
28.14
3.20
0.82
Table 2 Changes in the content of calcium and silicon in the alloy
Time/min
0
2.5
5
10
15
30
40
50
75
120
Alloy calcium content /%
0.39
15.93
21.53
21.15
22.33
21.87
21.30
19.05
15.20
Alloy silicon content /%
75.70
67.50
59.10
56.10
56.10
55.70
55.80
57.00
   
It can be seen from Table 1 that the reduction of rare earth-free synthetic slag with ferrosilicon can obtain a calcium-calcium alloy containing 22.33% of calcium, but under the same conditions, the rare earth slag is reduced by ferrosilicon, and finally the calcium content of the rare earth ferrosilicon alloy is not More than 5%. In the process of smelting rare earth ferrosilicon alloy, sampling and analysis of the change of silicon calcium, confirming that the reduced calcium or silicon calcium is involved in the reduction of rare earth oxides, the following reactions exist:
     
(RE 2 O 3 )+[CaSi] === 2[RE]+(CaO·SiO 2 ) (9)
               
[RE]+[Si] === [RESi] (10)
Therefore, CaO in the slag is reduced by silicon, which is advantageous for the reduction of rare earth oxides.
Auxiliary reaction
In the process of smelting rare earth ferrosilicon alloy, the arc furnace has a large amount of flue gas escaping, and as the temperature rises, the boiling of the melt is also generated. This is because the electric arc furnace uses a carbon furnace lining and a graphite electrode, among which carbon Can also participate in the reduction reaction, for example:
                      
(FeO)+C === [Fe]+CO↑ (11)
                      
(MnO+C)=== [Mn]+CO↑ (12)
                        
(SiO 2 )+C === SiO↑+CO↑ (13)
A large amount of CaF 2 is present in the slag and acts with SiO 2 :
              
2(CaF2)+2(SiO2) === (2CaO·SiO2)+SiF↑ (14)
The SiO2 in the slag reacts with Si in the alloy:
(SiO2)+[Si] === 2SiO↑ (15)
The gas generated by the above reaction causes the melt to boil, acts as a stirring, improves the contact conditions of the molten slag phase and the alloy phase, is also favorable for the diffusion of the reactants, and improves the kinetic conditions of the reduction reaction.
In short, based on years of experimentation and production practice, it can be inferred that the reaction of the silicon thermal reduction method to obtain the rare earth ferrosilicon alloy is that under the condition that a large amount of lime participates in the reaction, the silicon first reduces the lime to calcium to form a silicon calcium alloy, and the silicon calcium and then the rare earth The reduction of the oxide to a rare earth metal does not exclude the possibility that silicon directly reduces the rare earth oxide to a rare earth metal. The rare earth metal is further alloyed with silicon and is present in the alloy as a silicide phase. This is a rather complicated redox reaction process, so the alloy composition can be effectively controlled by controlling the smelting process conditions such as charge ratio, reduction temperature and time.
references
13, Dong Yicheng, etc., Steel, 1983, 18 (12): 43

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