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Theoretical Methods of Silicon Smelting
Source: | Author:佚名 | Published time: 2022-11-03 | 763 Views | Share:

The reduction of silicon dioxide with carbon is a more complicated physicochemical reaction process than metal smelting. After comprehensive scientific research, the basic laws of the physicochemical reaction in the production of metal silicon are obtained, and the correctness and reference of the laws obtained are demonstrated from the production process of metal silicon.


The following is a brief description of the experience and achievements of predecessors in the industry over the past 54 years, and a deeper scientific summary based on this.


1. Si-O system

In the Si-O system, * * * stable "phase" is SiO2, its theoretical melting point is 1933K (1660 ℃) [crystalline silicon dioxide is 1414 ℃], and its boiling point is 3048K (2775 ℃). The specific resistance at 1973K is 90 Ω· m. The research shows that there are more than ten varieties of quartzite (SiO2) in the "phase" transition during the heating process, that is, its crystal structure also follows, with a large volume change. At the metal temperature of 1143~2001K, when the quartz phase changes to tridymite, its volume increase value can reach 14.7%. Such a large volume change will result in hot cracking of quartz (commonly known as silica) as a component of the furnace charge on the surface of the furnace charge. The higher the water absorption and conductivity of quartz, the more explosive it is.


In the metallurgical theory of metal silicon, the major theoretical breakthrough is the discovery of a new silicon low valence oxide - SiO. This is confirmed by the large amount of SiO2 dust recovered from the metal silicon dust collector. It can be further confirmed that the oxide SiO plays a very important role in reduction during smelting, and it is a necessary intermediate product. Formation of SiO in Si-O system:


Under liquid state: SiO2+Si=2SiO (1)

Relationship between equilibrium constant and temperature: LgPSiO=- (15200/T)+7.38

In liquid state, the evaporation of pure SiO2 will generate SiO according to the following reaction:


SiO2=SiO+1/2O2 (2)

At 1773K~1983K:

△G0=762240-244.06T


Therefore, if we can fully understand the basic theory of the formation of SiO in the furnace and its participation in the reaction, we can understand the importance of preventing the loss of SiO generated from escaping from the furnace mouth. Reducing flue gas, reducing heat energy emission, and eliminating dust (also known as micro silica powder) are important works to improve the efficiency of metallurgical furnace and improve the technical and economic indicators of the metallurgical furnace.


2. Si-C system

When smelting metal silicon, the understanding of SiC is also very important, because it, like SiO, is an indispensable product in the transition. If its formation is completed in solid state during smelting, solid SiO2 reacts with solid C in reducing agent according to the following formula:


SiO2+3C=SiC+2CO (3) SiO2+C=SiC+O2 SiO2+2C=Si+2CO SiO2+4C= SiO+ SiC+3CO

△G0=555615-322.11T

If PCO=100Kpa, △ G=0 at 1725K. Therefore, in the smelting reaction process, a large amount of SiC will be generated in the solid state at a lower temperature, that is, when the reduction temperature is not reached. That is, if there is enough carbon and there is 100% contact surface between SiO2 and C, SiO2 will be completely converted into SiC (the production of silicon carbide is based on this theory).


*Liquid silicon and solid carbon react to form SiC according to the following formula:

Si+C=SiC (4) Si+2C=Si+2CO


At 1683~2000K:

△G =-100600+34.9T,

At 2880K, △ G=0. That is to say, SiC is stable before 2880K, and it will start to dissociate above this temperature.


*Under high temperature, gaseous SiO and excess C generate condensed SiC according to the following formula:

SiO+2C=SiC+CO (5) (normal: SiO+C=Si+CO SiO+SiC=2Si+O)

Under normal conditions, SiC does not melt but changes from solid state to gas state.

SiC will dissociate into liquid silicon and solid carbon according to the following formula:

SiC → Si+C (6) or SiO2+0.5SiC=1.5SiO+0.5CO SiO2+2SiC=3Si+2CO 2SiO2+SiC=3Si+CO 0.5SiO2+SiC=1.5Si+CO


Fully understanding the theory of SiC formation, dissociation and participation in the reaction is very important to reduce the residual amount of SiC in the furnace and the smooth operation of the furnace.


3. Thermochemical reaction of carbon reducing SiO2

When producing metal silicon, the total process of reducing silicon from SiO2 with carbon can be expressed as:

SiO2+2C=Si+2CO (7)

△G0T=697390-359.07T

When the temperature reaches 1942K, it can be regarded as the initial reaction temperature.

Due to the activity aC carbon, both aSi silicon and aSiO2 silicon dioxide are equal to 1. So the equilibrium constant is KP=P2CO.


Logarithm of CO partial pressure: LgPCO=(- 697390/38.308T)+9.37


The carbon distribution in the ingredients produced by submerged arc furnace is carried out according to the above reaction formula (7), which is called the theoretical carbon distribution. However, in fact, at different temperatures, the reduction of SiO2 by C is carried out through the formation of intermediate solid SiC, gaseous SiO and condensed SiO. Therefore, the reaction process cannot be simply carried out according to Formula (7). The thermodynamic scope of the production of metal silicon must grasp the phase equilibrium principle of the cross reaction between the concentration ratio of elements and alloys and the smelting temperature in the Si-O-C system.


A large amount of heat (69~72% of the heat consumption) required for smelting metal silicon reduction reaction mainly comes from the high temperature burning area of the arc at the bottom (working end) of the electrode. In this burning area, a gas hole (also known as the high temperature reaction area) is formed. In this hole with extremely high temperature, a variety of intense and complex reactions such as material melting, decomposition, ionization, vaporization, boiling, sublimation and phase change are carried out. In order to study this complex system, we established a visualized crucible reaction zone model.


When we format this model, the constraints (conservation) formed are:

(1) When the vapor partial pressure of the "condensed phase" substances participating in the reaction is equal to the saturated vapor pressure of these substances, the equilibrium of all evaporation and condensation reactions can be achieved in the system.

(2) Maintain the balance of all possible reactions in the system, including the mass balance of chemical elements in the gas phase and the balance of compound atomization constants, and achieve the balance of dissociation and combination reactions in the system.

(3) Volume equilibrium of gas components in the system.


The restriction conditions of the above three equilibria may be fulfilled in the laboratory, but this restriction is an inevitable law, which reflects the reaction of the state equilibrium of the basic substances in the model. According to the basic theory of thermodynamics, this model reveals the relationship between the initial reaction, intermediate reaction and the gas/liquid/solid phase of the terminal reaction of metal silicon formation and temperature and pressure. Therefore, we believe that there is a reaction zone similar to a "pot" in the submerged arc furnace for smelting metal silicon, which should be used as the basis for establishing the model.


Metallurgists have studied and calculated this system based on the basic theory of thermodynamics:

(1) In this high-temperature Si-O-C system, there are four condensed systems: solid or liquid SiO2, C, SiC and Si. CO, CO2, SiO, SiO2, O, O2, C, Si and SiC in the gas phase. These constitute a complete thermodynamic research system of Si-O-C system.

(2) At the high temperature of arc combustion in the reaction zone of the crucible, the condensed phase (solid liquid mixture) and the gas phase react violently as follows:

①SiO2+C=SiO+CO (8)

LgKP=(-33445/T)+17.19

②2 SiO2+SiC=3SiO+CO (9)

LgKP=(-75290/T)+34.45

③SiO+2C=SiC+CO (10)

LgKP=(4580/T)-0.14

④SiO2+Si=2SiO (11)

LgKP=(-33020/T)+15.05

⑤SiO+SiC=2Si+CO (12)

LgKP=(-9330/T)+4.35

⑥SiO+C=Si+CO (13)

LgKP=(-2420/T)+2.14


For this system, the total pressure of the gas phase is determined by each partial pressure:

P=PCO+Pco2+PSiO+Psio2+PO+Po2+PSi+Pc+PSiC=100KPa。


Using the above relationship between reaction equilibrium constant and temperature, the vapor partial pressure of all participating substances at metallurgical temperature can be calculated.


Under P=0.1 MPa and different temperatures, the component state diagram of phase composition and composition in homogeneous mixture can be concluded as follows:

(1) The initial temperature of the interaction between C and SiO2 is 1754K to form SiC. In the condensed liquid phase, there are C and SiO2 in addition to SiC,

The presence of C and SiO2 indicates the residual amount of non generated SiC.

(2) From 1754K to 2005K, there is liquid SiO2 condensed phase.

(3) The formation of silicon carbide should have started at 1962 K. Before reaching the evaporation point, its amount increases with the continuous increase of temperature.

(4) SiO and CO exist in all gas phases, and the concentration of SiO increases significantly with the increase of temperature.


Although the reaction theory disclosed above is very important, the electrothermal coupling temperature measurement technology and PCL automatic control system have not been used before, so it is difficult to measure the smelting temperature in the furnace, which makes the application of these theories to the actual calculation of the material balance in the metallurgical process have great limitations, or even be strongly rejected. In terms of logical inference, from SiO