Principle of smelting ferromolybdenum by silicon-aluminum thermal method
It is also a common method of smelting iron alloy metal calorimetry. It uses silicon, aluminum (and sometimes magnesium ) as a reducing agent to reduce metal oxides. In smelting, there is usually no need to reheat or supply heat, mainly relying on the heat released by the reaction of the charge itself to produce metal or iron alloy. In addition to iron smelting molybdenum, vanadium may for smelting iron, titanium iron, boron and iron.
The principle of metal thermal method for smelting iron alloy is that at a certain temperature, the affinity of silicon or aluminum for oxygen is greater than the affinity of metal for oxygen in the metal oxide to be replaced. The greater the difference, the easier the metal thermal reaction. The criterion for whether the metal thermal method can be carried out is based on the calculation of thermodynamics: when the reaction free energy Δx 0 <0 (in this case, the reaction is an exothermic process), and the heat released by the reaction is sufficient to cause the reduced metal and the reaction to be produced. The slag is melted enough to compensate for heat loss from heat of the furnace (including feedstock and product), heat of vaporization, heat transfer during heat transfer, heat radiation, and the like. Only in this way can the metal thermal method be self-heating. Shemchuschini proposed a more specific criterion: if the heat released per gram of charge exceeds 2717J, or the reaction enthalpy is greater than 300kJ/mol, the aluminothermic reaction can be self-heated upon ignition.
When the ferromolybdenum is produced by the silicon-aluminum thermal method, the reaction in the furnace is as follows:
Â
2 | MoO 3 +Si = | 2 | Mo+SiO 2 |
3 | 3 |
â–³x 0 =-468745+65.42T
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2 | MoO 3 + | 4 | Al = | 2 | 2 | Al 2 O 3 | |
3 | 3 | 3 | 3 |
â–³x 0 =-631890+51.08T
MoO 2 +Si=Mo+SiO 2
â–³x 0 =-342091+19.48T
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MoO 2 + | 4 | Al = | 2 | Al 2 O 3 | |
3 | 3 |
â–³x 0 =-517902+5.14T
The free energy in several reactions was lower than 0 (â–³x<0). According to the criteria of Shemchuschini, when MoO 3 and MoO 2 were reduced by aluminum, the heat released per gram of charge in the reaction was 4682 J and 3252 J respectively higher than 2717 J; the heat enthalpy per mol was 463.6 kJ. And 400kJ, both higher than 300kJ. Obviously, when the silicon-aluminum thermal method smelting ferromolybdenum, it can be self-heating after the ignition reaction. [next]
During the smelting process, more than 99% of the molybdenum oxide is reduced to metal into the ferromolybdenum alloy. The silicon and aluminum reducing agent reduces the iron oxide (iron ore or scale in the charge) and releases heat (5350 J per 1 kg of Fe 2 O 3 ).
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2 | Fe 2 O 3 +Si = | 3 | Fe+SiO 2 |
3 | 4 |
Â
1 | Fe 2 O 3 +Al = Fe+ | 3 | Al 2 O 3 |
2 | 4 |
In addition to the supply of iron in the ferromolybdenum, approximately 42% of the iron oxide is reduced to metallic iron into the alloy as described above. The remaining iron oxide is only reduced to ferrous oxide and enters the slag:
2Fe 2 O 3 +Si=4FeO+SiO 2
In order to maintain the furnace temperature required for the reaction (1850 ~ 1950 ° C), it is sometimes necessary to add a strong carrier (such as saltpeter), which can release more heat when it is reduced.
Molybdenum can be alloyed with silicon. Common solid compounds are Mo 3 Si, Mo 3 Si 2 , MoSi. The silicon content in ferromolybdenum is often less than 1%. The Mo-Si state diagram under smelting conditions is shown in the figure below.
Figure Molybdenum-silicon state diagram
When smelting ferromolybdenum, the rate of self-heating reaction is very fast. At the end of the angstrom reaction, the furnace temperature drops rapidly. In order to maintain the fluidity of the materials in the furnace, it is necessary to ensure that the ferromolybdenum and the slag are sufficiently separated, and it is necessary to try to reduce the melting point and viscosity of the slag.
Upon reaction, the silicon is oxidized to silica, which together with the silica in the molybdenum calcine forms a highly acidic acidic silicon slag. The alkaline slag such as ferrous oxide and alumina formed by the reaction can neutralize and dilute the silicon slag. But this is not enough. The charge is usually added with fluorite , lime and limestone . They act to dilute the slag and reduce the melting point of the slag.
It should be noted, however, that additives can reduce the melting point and viscosity of the slag, but they also consume a lot of heat when they are melted. Therefore, the amount of additives should be appropriate, and heat loss should be avoided after excessive.
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