The basic principle of electric furnace melting

Electric furnace smelting can be divided into two processes, namely, thermal processes (such as electrical energy conversion, thermal energy distribution, etc.) and smelting processes (such as furnace melting, chemical reaction, slag separation, etc.). For the convenience of description, the basic principle of electric furnace melting, the following three aspects of electrical energy conversion, material melting, smelting reaction and product are discussed.
1) Conversion of electric energy If a current is supplied to a solid or a liquid, the electric energy is converted into thermal energy due to the action of the electric resistance, and the Joule equation can be used to determine:
Q=I ² Rt
Where Q is heat, J;
I-current through the resistor, A;
R-resistance, Ω;
T-time, h.
Since 1J=0.239ca1 or 0.239/1000kca1, and according to Ohm's law IR=U, the above formula can be written as:
Q=0.239/1000IUt (kcal)
If the electrical energy is in kilowatts, it will be consumed within t hours:
Q=IUt/1000(kWh)
Since 1kW = 0.239kcal.s ^-1 , 1kWh = 0.239 × 3600 = 860kcal, then the heat generated in t hours is:
Q=0.86IUt(kcal)×4.184=3.6IU(kJ)
Electric heating is widely used in various technical fields is particularly important for the metallurgical industry. In the metallurgical industry, electric furnaces are an important type of metallurgical furnaces.
Equipped with an electric furnace with a three-phase transformer rated at PkVA, when the electric furnace is overloaded to P rated, the heat generated in the solution pool is calculated as follows:

The electric furnace is equipped with three single-phase transformers with a rated capacity of PkVA. When the electric furnace is overloaded to P rated, the heat of the pool is calculated as follows:
Q=3×860P _rated cosøt/1000=2.58I _phase U _phase cosøt(kcal)=10.79I _phase U _phase cosøt(kJ)
Where Q - heat, kcal or with 1kcal = 4.184kJ;
P _rated - rated capacity of the transformer, kVA or kW; [next]
U _line -line voltage, V;
I _line -line current, A;
U _phase - phase voltage, V;
_Phase I - phase current, A;
Cos ø power factor;
T—time, h.

2) Melting of materials The electric furnace for smelting sulfide ore and concentrate can be regarded as a high-temperature molten pool with two layers of melt (see Figure 1). The upper slag layer is 1700-1900 mm thick and the lower layer is 600-800 mm thick. The solid material loaded into the bath sinks into the slag layer in the form of a pile to form a slope.
The material is melted by heat from the main source of electrical energy, which is fed into the furnace through three or six electrodes. The depth of the electrode inserted into the slag layer is 300-500 mm, and the conversion of electrical energy into heat energy occurs in the slag layer. 40% to 80% of the heat is generated on the contact surface of the electrode-slag, and the rest of the heat is generated in the slag layer in the electrical circuit.
Most of the heat is generated on the contact surface of the electrode-slag because there is a gas layer around the working end of the electrode. This is called a gas bag. The current is in the form of a large number of particle discharges, that is, in micro-forms. The form of micro-arc passes through this air pocket. The air bag is formed by the mechanical pressure of the electron flow, the slag is separated from the electrode, and the formed void is filled by the gas generated by the combustion of the electrode and the gas escaping from the slag. This gas layer has a high resistance. Therefore, a large voltage drop occurs when the current passes through the gas layer, and the corresponding heat is released.
In the electric field of the electric furnace, from the center line of the electrode to the range of the diameters of the two electrodes close to the electrode, it is the conductive portion of the molten pool (it is estimated that 90% of the current is passed through an electrode diameter range from the center line of the electrode). It is in this region that electrical energy is converted into heat, away from the portion of the molten pool where the centerline of the electrode exceeds the diameter of the electrode, not in the electrical circuit, and no heat is generated.
There are two lines of current through the electric furnace: [next]
(1) From the electrode through the slag → nickel锍 → slag → electrode, that is, star load.
(2) One electrode flows through the slag to the other electrode, that is, a triangular load.
When the distance between the electrodes is constant, the magnitude of the star load and the triangle depends on the depth of the electrode inserted into the slag layer, the thickness of the slag layer, and the size of the material slope in the furnace. When the electrode insertion depth is not large, the triangular load can reach 70% of the total load; as the electrode insertion depth increases, the triangular load gradually decreases, and the electrode insertion is deep, which is reduced to 30% to 40%. When the electrodes are inserted and inserted, the star current and the triangular load are gradually decreased, and the electrode insertion depth is increased proportionally, but the star current is increased faster than the triangular current.
In the furnace, where no heat is generated, heat is transferred from the heat to the cold due to the convective movement of the slag inside the molten pool.
The convection flow rate of the slag is caused by the difference in heat between the various parts of the slag pool. It has been pointed out that the greatest heat is generated in the contact zone of the electrode-slag. In this region, the slag layer near the surface of the electrode has been greatly overheated, and its temperature may exceed 1500 to 1700 ° C or higher. As the slag contains a large amount of bubbles, its expansion greatly reduces its density. The slag near the electrode surface and the slag density away from the electrode make a difference. The superheated slag having a small density continuously rises close to the electrode to the surface of the molten pool and spreads to the periphery of the molten pool.
The superheated slag meets the floating charge during its movement, and melts the lower surface of the material slope that sinks into the molten pool. After the moving slag is mixed with the low-temperature melting charge, it settles down the slag pool downstream, and near the lower end of the electrode, a part of the slag continues to drop to the lower layer of the slag pool where the flow is very weak, where the nickel slag and slag are carried out. Separation. The hot slag transfers its excess heat to the cooler portion of the molten pool during the flow away from the electrode, thereby maintaining the thermal balance of this portion of the molten pool. The parts where the hot slag rarely flows, or the part where the temperature is lowered, are insufficient in heat, and the temperature is only 1250 to 1250 °C. The four corners of the furnace, the vicinity of the furnace wall, and the area under the electrodes are prone to generate knots.
The convection movement of the slag is one of the most important working processes in the electric furnace. The convective movement ensures the heat exchange and melting of the material in the molten pool of the electric furnace. The large amount of melting of the material occurs in the molten pool area where the electrode is inserted, that is, the intense occurrence occurs. Convection in the loop area. From the plane of the electric furnace, this area is within the range of 1.5 to 2 electrode diameters from the center line of the electrode.
Since the electric energy is converted into heat energy in the molten pool, the temperature of each part of the molten pool is also inconsistent. The temperature near the upper layer of the melt is higher and the lower layer is lower. The temperature of the slag layer is uniform in the longitudinal and transverse directions, only the vertical direction changes, mainly the temperature below the electrode changes, and is actually isothermal in the electrode insertion depth range, which can be explained by the existence of intense convective heat exchange. .
Since the heat exchange conditions of the molten pool are different, it is obvious that the melting speed of the charge decreases sharply as the distance from the electrode increases. Therefore, most of the charge (80% to 90%) is added in the range of 1.5 to 2 times the electrode diameter from the center line of the electrode.
3) Smelting reaction and product [next]
(1) Melting reaction The physicochemical reaction of the electric furnace smelting mainly occurs on the contact surface of the slag and the charge, and the furnace gas hardly participates in the reaction. Therefore, the phase reaction of the liquid phase and the solid phase of the electric furnace smelting is mainly performed, and the chemical reaction of slag formation and nickel ruthenium can be completed at one time.
The materials added to the electric furnace are mainly concentrates and calcined sand, followed by soot, returning charge and liquid converter slag, flux and carbonaceous reducing agent. The mineral composition of copper- nickel mineral materials is:
Sulfide: Fe ₇ S ₈ , (FeNi)S ₂ , CuFeS ₂ , CoS;
Oxide: Fe ₂ O ₃ , Fe ₃ O ₄ , NiO, CuO, SiO ₂ , MgO, CaO, Al ₂ O _{3 ,} etc.;
Silicate: mMO.nSiO ₂ , the material may also contain a small amount of sulfate (MSO ₄ ), carbonate (MCO ₃ ), hydroxide [M(OH) ₂ ] and precious metals.
When the superheated slag meets the surface of the material during convective movement, it transfers the heat from the excess potential to the material. When the material is heated to 1000 ° C, the thermal decomposition of complex sulfides, certain sulfates, carbonates and hydroxides occurs in the material, resulting in a relatively simple and stable compound. If the incoming material is calcined rather than concentrate, the above reaction is completed at the time of calcination. When the material is heated to 1100-1300 ° C, it is mainly the interaction between sulfide and oxide. The liquid product of Ni ₃ S ₂ , Cu ₂ S, FeS and CoS fused to each other is low nickel ruthenium. A small amount of Fe ₃ O ₄ and Cu, Ni, Fe metal and precious metal are dissolved.
Alkaline oxides (FeO, CaO, MgO, etc.) react with acidic oxides (SiO ₂ ) to form various silicates of the mMO.nSiO ₂ type, which are fused together in a molten state to produce an electric furnace. Another product of smelting - slag. The nickel ruthenium and slag in a molten state are separated in the molten pool due to the difference in density.
When the material is heated and melted, in addition to the liquid product, a gas is generated, such as S ₂ is oxidized to SO ₂ . Carbon reduction, such as CO ₂ produced by MO, most of the gas rises to the surface of the molten pool and enters the furnace space and is discharged along with the flue gas. A small part of the gas is encased in the slag, which is why the slag contains a large amount of gas.
In the melting furnace, since the thermal decomposition of the sulfide produced with high iron sulfide metal oxide reaction, the sulfur can be removed in the charge part. When the electric furnace smelts the uncalcined sulfide concentrate, the desulfurization rate is 15% to 18%; when the concentrate after smelting and roasting and the carbonaceous reducing agent are appropriately added, the desulfurization rate is much smaller.
(2) Smelting products When the electric furnace smelts sulfide copper-nickel concentrate, its products have low nickel sulfur, slag, flue gas and soot. Low nickel niobium is an intermediate product of smelting, which is sent to the converter process for further enrichment. The slag is discarded due to the low valence metal content. The smoke is collected into the atmosphere after the dust is collected, and the smoke is returned to the electric furnace for melting. [next]
1 low nickel bismuth. It is mainly composed of Ni ₃ S ₂ , Cu ₂ S, FeS. In addition, there are some cobalt sulfide and some free metals and alloys in the low nickel niobium. A small amount of magnetic oxidation is also dissolved in the low nickel bismuth.
In the electric furnace smelting process, the completeness of separation of low nickel bismuth and slag depends mainly on their density difference. The greater the density difference, the more thorough the separation, and the lower nickel ruthenium scale depends on the various vulcanizations that make up the low nickel bismuth. The content of the substance. The density of various sulfides is different, for example, FeS is 4.6 g/cm ³ , Ni ₃ S ₂ is 5.3 g/cm ³ , Cu ₂ S is 5.7 g/cm ³ , and the lower the nickel niobium grade, that is, the FeS content. The higher the density, the lower the density of nickel bismuth. The solid low nickel niobium density is generally 4.6 to 5.0 g/cm ³ . The density of the molten low nickel niobium is slightly smaller because a certain amount of sulfur dioxide gas is melted in the molten nickel niobium to increase the volume and decrease the density. The slag density is generally between 3 and 4 g/cm ³ .
In the low-nickel niobium smelted by the electric furnace of Jinchuan Company, the ratio of nickel to copper is about 2:1, and the ratio of nickel to copper smelted by Norilsk electric furnace is about 3:2. The sum of the nickel and copper contents in the low nickel niobium is 15% to 22%, and the sulfur content fluctuates in the range of 22% to 27%. The amount of sulfur in the low nickel bismuth is not sufficient to bring all of the metals contained therein into a state of sulphide because a part of the metal can be melted in the low nickel bismuth. Table 1 lists the low nickel bismuth components smelted in the electric furnace of each plant.

Can be seen from Table 1. The low nickel niobium obtained by smelting copper-nickel deuterated ore and concentrate electric furnace has a nickel content fluctuation ranging from 7% to 18%. The content of various metals in the low outer length of the electric furnace is determined by the content of the metal in the furnace, the higher the yield of low nickel niobium, and the content of the valuable metal in the low niobium (low niobium grade). The lower. Therefore, the higher the desulfurization rate of the electric furnace smelting, the smaller the yield of the low nickel niobium, and the higher the content of the valuable metal in the low nickel niobium. [next]
The low nickel niobium has the same melting point as its density, depending on the content of various metal sulfides constituting the low nickel niobium. The melting point of pure sulfide was 790 ° C for Ni ₃ S ₂ , ₁ 120 ° C for Cu ₂ S, and 1150 ° C for FeS. The melting point of low nickel sulfur, between the melting points of various sulfides.
The industrial production of low nickel bismuth melting point is between 1000 ~ 1050 ° C, but the thermal furnace has overheating characteristics, the temperature of low nickel bismuth released from the electric furnace reaches 1250 ° C due to overheating, at this temperature, low nickel bismuth Very easy to flow, it is easy to see into the gaps of the electric furnace brick body and the construction is not tight. Therefore, in the design of the mine electric furnace and the furnace body masonry, the wet masonry method is adopted for the low nickel niobium area, and the requirements are very strict. In addition, due to the superheated molten low nickel niobium, it is very aggressive and can melt metal iron and cast iron parts well. Therefore, the chutes that emit low nickel niobium are all lined with refractory materials, and the buns containing low nickel niobium must also pass through the converter slag. Use it only after hanging out the protective layer.
Low nickel niobium has good electrical conductivity. In the molten sulfide, the conductivity of Ni ₃ S ₂ is the largest, and the conductivity of Cu ₂ S is the lowest, and the change is compliant with Ni ₃ S ₂ >CoS>FeS>Cu ₂ S. The low nickel ruthenium conductivity produced by the factory is generally (35 ~ 45) × 102 (Ω.cm) -1 at 1100 ~ 1350 ° C, and its value depends on the content of sulfide and the temperature of the melting. Since the conductive property of the melting crucible is close to that of the metal, the turning of the electric furnace occurs, and after the nickel crucible is floated and contacted with the electrode, the current control of the electric furnace is unstable, and an overcurrent trip accident occurs.
(2) Slag. The slag produced by electric furnace smelting is mainly composed of the following five main components: SiO ₂ , FeO, MgO, Al ₂ O ₃ and CaO, and their total accounts for about 97% to 98% of the total. It also contains small amounts of Fe ₃ O ₄ , ferrite and metal oxides and sulfides. Examples of the composition of the electric furnace smelting slag are shown in Table 2.
The metal content of the slag depends on the properties of the slag and the low nickel niobium, the slag temperature and the technical level of operation. Generally, the metal content (%) in the slag is: Ni 0.07 to 0.25, Cu 0.05 to 0.10, and Co 0.025 to 0.1. The slag composition has a great influence on the properties of slag and metal loss. At the same temperature, as the content of SiO ₂ increases, the conductivity of the slag decreases, the viscosity increases, and the heat capacity increases, and the melting of the furnace pellet increases the mechanical inclusion loss. Therefore, in the electric furnace smelting, in order to reduce the metal loss, it is suitable to control the SiO ₂ content in the slag to be 38% to 41%.

FeO can greatly change the properties of slag, especially electrical conductivity. As the FeO content increases, the conductivity of the slag increases, the melting point decreases, the flowability of the high iron slag is good, but the density is high, the surface tension at the interface between the low nickel niobium and the slag is lowered, and the separation conditions of the low nickel niobium and the slag deteriorate, resulting in metal loss. increase. In addition, high iron slag can dissolve sulfides well, which also increases metal loss. In the smelting process, the reasonable content of ferrous oxide in the slag is 25% to 32%.
Fe ₃ O ₄ is a stable compound with a high melting point (1597 ° C) and a high density (5018 g / cm ³ ), so the behavior during the smelting process has a great influence on the smelting process. There is a certain amount of Fe ₃ O ₄ in the slag and low nickel bismuth of the electric furnace, and the source thereof is mainly the product of the physicochemical reaction of the calcine, the converter slag and the charge.
Due to the high temperature of the slag smelting in the electric furnace, the low partial pressure of oxygen in the furnace gas, and the vigorous movement of the slag in the molten pool, the decomposition of Fe ₃ O ₄ is easier than other smelting methods. However, the slag has a high solubility to Fe ₃ O ₄ in the high temperature zone, and the slag temperature decreases after entering the precipitation zone, and the solubility of Fe ₃ O ₄ in the slag is reduced, and it is precipitated from the slag. Practice has proved that in the electric furnace smelting, there is often a layer of separator between the low nickel bismuth and the slag, also known as the "sticky slag layer", which is a deposit with a higher viscosity and a semi-melting state, affecting the slag and low. The precipitation of nickel ruthenium is separated to increase the nickel content. When the low nickel ruthenium temperature is too low, it is easy to form a barrier layer therefrom, which brings a lot of trouble to the production operation. In the production practice, the following measures can be taken to eliminate the Fe ₃ O ₄ slag layer: 1 reduce the working voltage, insert the electrode deep, increase the temperature of the bottom of the molten pool, and increase the Fe ₃ O ₄ in the slag and the low nickel bismuth. Solubility, eliminate the slag layer; 2 Add a certain amount of coke to the slag to reduce Fe ₃ O ₄ to FeO. And properly increase the content of dioxin and silicon in the charge, destroy the Fe ₃ O ₄ ; 3 with a certain amount of high-sulfur concentrate, on the one hand increase the proportion of FeS in the charge, make Fe ₃ O ₄ easy to decompose, another On the one hand, it can reduce the nickel niobium grade and improve the melting ability of low nickel niobium to Fe ₃ O ₄ ; 4 add pig iron or other iron-containing materials to make Fe ₃ O ₄ be reduced by metal iron and slag with SiO ₂ ; The internal power increases the bath temperature, which increases the solubility of Fe ₃ O ₄ and facilitates the decomposition reaction.
The first two of the above measures are often used in production practice, and the effect is good. The use of cast iron to reduce magnetic iron oxide is generally only used to treat local freezing in the furnace. As for the high-sulfur concentrate, it is only used when the low-nickel niobium grade is high. [next]
Treatment of converter slag in an electric furnace is also one of the sources of Fe ₃ O ₄ in the slag. The converter slag is a complex melt composed of iron silicate, free silica, magnetic iron oxide, and a small amount of nickel-copper-iron sulfide, nickel-cobalt oxide, and the like. The purpose of returning the converter slag to the electric furnace is to recover valuable metals. 1 After the liquid converter slag is injected into the electric furnace, it is superheated at 100-200 °C to reduce its viscosity. Therefore, it is beneficial to the sedimentation and separation of low-nickel niobium particles which are mechanically mixed. 2 Due to the high temperature in the love, there is silica and metal sulfide. In the presence of a large amount of material, Fe ₃ O ₄ is reduced and reacted with silica to form slag. The more complete the reduction of Fe ₃ O _{4 , the} more complete the sedimentation and aggregation of the dispersed mechanically mixed low outer long 锍 particles; 3 contained in the converter slag A part of nickel exists in the form of NiO. Most of the cobalt exists in the form of CoO. In the electric furnace, it undergoes a vulcanization reaction with ferrous sulfide to form Ni ₃ S ₂ and CoS, and then enters the nickel crucible.
In order to facilitate the recovery of cobalt from the converter slag, the converter slag generally does not return directly to the electric furnace or only returns the pre-converted slag to the electric furnace.
Although there is not much data on the slag containing Fe ₃ O ₄ in the nickel metallurgical literature, it has been found that the slag containing Fe ₃ O _{4 is} related to the slag containing Ni (or Cu), and the amount of Fe ₃ O ₄ is commonly used as control in the operation. Parameter.
The slag contains a high amount of MgO, which is a characteristic of the smelting of copper sulfide nickel ore electric furnace. When the slag contains MgO less than 10%, the slag containing MgO measured by the slag-removing nickel plant is closely related to the following:
The slag contains MgO/% 10 11 12 13 14 15 16 17 18 19 20
Smelting power consumption / (kWh.t-1) 680 700 715 730 740 750 770 790 820 830 850
However, when the amount of MgO contained in the slag is higher than 22%, the electrical conductivity of the slag increases. As MgO increases and FeO decreases, the valence metal in the slag decreases. Reasonable magnesium oxide content of slag smelting furnace is 10% to 12%.
The amount of calcium oxide contained in the electric furnace slag is not high, generally 3% to 8%, and has little effect on the quality of the slag. As the CaO content increases to 18%, the slag conductivity increases by 1 to 2 times, the slag density and viscosity decrease, and the solubility of the sulfide (the substance plus Co) decreases in the slag.
The slag contains Al ₂ O _{3 in an amount} of 5% to 12%. As calcium oxide, as the presence of a small amount of alumina has little effect on the slag properties with increasing alumina content, the viscosity of the slag and metal loss increases.

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