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Sources and Properties of Impurities in Silicon Metal

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Sources and Properties of Impurities in Silicon Slag

Impurities in silicon metal exist in the form of simple substances and compounds. Thermodynamic calculations show that when Fe2O3, SiO2, MgO, Al2O3, CaO, etc. are reduced under normal pressure, the reduction temperature of Fe2O3 is the lowest, followed by SiO2, and then Al2O3, MgO and CaO. Because the reduction temperature is different, most of Fe2O3 and SiO2 are reduced, while Al2O3, MgO and CaO can only be partially reduced. Unreduced Al2O3, MgO and CaO form slag together with SiO2. Some of this slag accumulates together to form an obvious light-colored slag block, which is confined between the grain boundaries and has a clear boundary. It can be removed by hand when it is broken; other slag becomes dark and can only be seen under a microscope. Particles, mixed with silicon, become impurities in silicon.

To extract silicon metal from silicon slag, the first step is to separate the obvious light-colored slag and dark-colored slag in the silicon slag with a manual hammer, and then use the principle of middle selection to further separate to increase the silicon content in the silicon slag; the second part is to carry out Smelting separation is mainly to separate impurities in molten silicon.

Impurities in silicon slag exist in two forms: reduced and unreduced. Add flocculants composed of Na2O5~50% and SiO250~95% to industrial silicon or Na2O5~50%, SiO250~95% and low MgO and CaO. With a flux composed of 35%, the melting point of the slag can be reduced from the original 1450~1480 to about 1000. It can also reduce the density of the slag, improve its viscosity and surface tension, and facilitate the separation of slag and silicon. Aluminum and calcium are removed by oxidation reaction with synthetic flux. The oxidation rate constant of aluminum and calcium has a linear relationship with temperature, and the oxidation process of aluminum and calcium is carried out in the area close to diffusion. The slag can be completely dissolved in the synthetic flux after 40 minutes, and 83% of aluminum and 92% of calcium can be removed. This flux is made of soda ash, dolomite, silica, quicklime, serpentine or cullet, etc. It is inert to silicon and can oxidize aluminum and calcium well and make non-metallic inclusions gather, float and separate, but No iron removal effect. The amount of flux added is generally 10-20% (mass) of refined industrial silicon. The batching work requires accuracy, uniformity, and timeliness. The quality of the batching work has an important impact on the output of the product, the unit consumption of raw materials and the cost, and even determines whether the smelting and purification process can be carried out.

The silicon slag is purified and smelted with a power frequency furnace for heating and heat preservation.

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    Requirements of the Raw Materials for High Carbon Ferro Chrome Smelting

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    Requirements of the Raw Materials for High Carbon Ferro Chrome Smelting

    1. The grade and purity of raw materials: ferro chrome smelting requires the highest possible grade of raw materials Chrome Ore, Coke and Silica. Raw materials with high purity can achieve high yield, high quality and low consumption

    2. Harmful impurities: sulfur and phosphorus are the most harmful impurities in raw materials, because after they enter the ferrochrome alloy, they will eventually affect the quality of steel and steel products.

    3. The particle size of raw materials: Whether the particle size of raw materials is suitable has a great influence on the smelting process. If the particle size of the raw material is too large, it is not easy to melt, and the reason is difficult, the conductivity increases, the amount of slag increases, the furnace condition deteriorates, and various technical and economic indicators of smelting deteriorate. However, if the particle size of the raw material is too small and there are many powders, the gas permeability of the furnace material will be poor, and the pressure around the electrode will be high, resulting in stinging. Moreover, the powder material is easy to melt, which will cause the upper layer of the furnace to sinter and suspend the material, resulting in material collapse. As a result, the electrode is unstable, the ignition is frequent and the material collapse is frequent, and the unreturned raw material directly enters the crucible, which will also deteriorate the technical and economic indicators. Therefore, there should be strict requirements on the particle size of raw materials for submerged arc furnaces.

    ∙ Cr2o3 > 40% in chromium ore, Cr2o3/ΣFeO > 2.5, s < 0.05%, P < 0.07%, MgO and Al2o3 content should not be too high; particle size 10-70mm, if it is refractory ore, the particle size should be appropriately smaller.

    ∙ Coke should contain no less than 84% fixed carbon, less than 15% ash, S<0.6%, particle size 3-20mm.

    ∙ Silica is required to contain SiO97%, Al2o3 <1. o%, good thermal stability, no soil, particle size 20-80mm.

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      Effect of Carbon Additive on Microstructure and Properties of Cast Iron

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      Effect of Carbon Additive on Microstructure and Properties of Cast Iron

      The influence of carburizing process on the structure of cast iron after carburizing treatment with carbon additive generates a large number of dispersed heterogeneous crystallization cores in molten iron, reduces the supercooling degree of molten iron, and promotes the formation of graphite-based At the same time, due to the small amount of pig iron used, its genetic effect is greatly weakened, so that the branches of the ink sheet are developed and difficult to grow, making the graphite short and uniform.

      1. Furnace material requirements: no oil and no rust, scrap steel requires no excessive oxidation on the surface.
      2. Generally, it is prepared by adding 4 kg of recarburizer for every 100 kg of scrap steel.
      3. The temperature of the furnace is controlled at 1550°C. It is expected that the nodularization temperature will drop by 100°C, and the temperature of the bag will drop by 50°C.
      4. During the production of the first furnace, 10-20 kg of high-quality iron chips are added to the bottom of the electric furnace. When the second furnace starts production, 20-40 kg of the remaining molten iron from the previous furnace is used.
      5. After the iron chip is added, put the prescribed recarburizer into the plastic bag and put it on the iron chip. From the second furnace, 20-40 kg of the remaining molten iron is put into a plastic bag and the recarburizer is put into the molten iron surface.
      6. Add 50 kg of scrap steel with small carbon particles (less than 50×50 area), and cover the entire furnace pond tightly.


      7. Start melting, add remaining steel scrap → add pig iron → add returned iron (note that there should not be too much sand on the surface of the returned iron (to prevent the carburant from bonding with sand to affect the absorption).
      8. Cover the molten iron with a covering agent after melting, and clean the slag repeatedly 2-3 times when the temperature reaches 1400°C.
      9. Spheroidizing treatment Put the spheroidizing agent and ferrosilicon into the hanging package and cover the surface with high-quality iron chips.
      10. After smelting, use a high-quality slag remover to clean the slag on the liquid surface in the furnace for 2-3 times, and check the temperature of the molten iron at 1550°C-1600°C.

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        Effect of Carburizer Particle Size on Absorption Rate

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        Effect of Carburizer Particle Size on Absorption Rate

        The carburizing process using a carburant includes a dissolution diffusion process and an oxidation loss process. The particle size of the carburant is different, the dissolution diffusion rate and oxidation loss rate are also different, and the absorption rate of the carburant depends on the combined effect of the carburant dissolution diffusion rate and oxidation loss rate.

        coal carbon additives
        graphite petroleum coke
        petroleum coke carbon raiser

        In general, the carburant particles are small, the dissolution rate is fast, and the loss rate is large; the carburant particle size is large, the dissolution rate is slow, and the loss rate is small.

        For example, in a 110kg high-frequency induction furnace, the carbon raiser with a particle size of 15-18mm dissolves very quickly, and most of it is dissolved in the molten iron before it is oxidized and lost, and only a small part is lost, so the absorption rate is high.

        In a 60kg induction furnace, the diameter and capacity of the hearth are relatively large, and the particle size of the recarburizer is 15-18mm, which is too small compared to the diameter and capacity of the hearth, so the loss rate is fast and the absorption rate is low; while the particle size of 116-312mm is relatively small compared to the diameter and In terms of capacity, the recarburizer has a faster dissolution rate and a slower loss rate, and the dissolution plays a dominant role, with a high absorption rate.

        Therefore, the choice of the particle size of the carburizer is related to the diameter and capacity of the furnace. In general, the larger the diameter and capacity of the furnace, the larger the particle size of the carburant; otherwise, the smaller the particle size of the carburant.

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          Why Adding Carnon Additive in Steelmaking Process?

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          Why Adding Carnon Additive in Steelmaking Process?

          It is generally believed that the higher the temperature of molten iron, the longer the action time, and the higher the carbon absorption rate. But the reality is just the opposite. In the induction furnace, carbon is increased at low temperature and silicon is increased at high temperature. That is, at high temperature, instead of increasing carbon, it is reducing carbon.

          This is because graphite carbon is mainly lost in the gas phase diffusion to the atmosphere outside the furnace; the oxidation of molten iron is related to the balance of C-Si-O, CO in molten iron is continuously oxidized to CO2, and CO2 will be reduced by C, and the reaction produces CO, CO2 gas floats up and overflows the surface of the molten iron, reducing the carbon content in the molten iron. The reaction rate is related to the equilibrium temperature, and the equilibrium temperature varies with the content of carbon and silicon. For ductile iron, the equilibrium temperature is about 1450°C±20°C, and for gray iron, it is about 1400°C±20°C. The oxidation of carbon in molten iron becomes violent above the equilibrium temperature. As a result of the reaction, the carbon in the molten iron is continuously oxidized and burnt, and the burnt loss of silicon is reduced. At this time, the carburizer added on the surface of the molten iron makes the carburization and carbon reduction of the molten iron reach a balance.

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            Influence and Application of Carbon Raiser in Smelting Industry

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            Influence and Application of Carbon Raiser in Smelting Industry

            With the same chemical composition, different smelting processes, different ingredients and ingredient ratios, the metallurgical quality of molten iron is completely different. To obtain a good carburizing effect, the electric furnace adopts the carburizing technology, and the cupola adopts the high-temperature smelting technology. The influence of carburant on smelting mainly has three aspects.

            1. The molten iron carburization technology can increase the graphite crystal nucleus during the smelting process, especially the electric furnace smelting. Adding silicon carbide to cupola smelting can also increase the long-term graphite nuclei of molten iron and reduce the oxidation of molten iron.

            2. Carbon increase is the best measure to prevent or reduce shrinkage tendency. Due to the effect of graphitization expansion in the solidification process of molten iron, good graphitization will reduce the shrinkage tendency of molten iron.

            3. Under the condition of high carbon content, in order to obtain high-strength gray iron castings, the smelting process adopts the process of adding carburant to all scrap steel, so that the molten iron is more pure, and the castings produced have high performance materials.

            For smelting, clean materials without oil pollution should be used to avoid electric leakage or excessive scum. The previous furnaces of a certain factory used oil-soaked scrap iron chips, which caused electric sparks in the coils. It was thought that the furnace lining material contained too much iron and caused leakage. In fact, it is because the smelted iron filings contain oil and are prone to carbon deposition. The carbon deposition part is on the cold surface of the furnace lining, even deposited in the heat insulation layer. Since the furnace lining has not been fully sintered, CO infiltrates into the back of the furnace lining, and a CO→C+O2↑ reaction occurs, and C is deposited on the cold surface of the furnace lining or the heat insulation material in the stomata. When carbon deposition occurs, it will cause the furnace body to ground and leak electricity, causing the coil to spark. It can be avoided by switching to pure materials. In another factory, because the sources of steel scrap purchased were chaotic, paint, lime, coal and other substances were even coated on the surface, resulting in a lot of scum, and a lot of manpower and material resources were consumed in the later work of slag removal.

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              Advantages of Adding Carburant During Smelting

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              Advantages of Adding Carburant During Smelting

              There are many coarse hypereutectic graphites in pig iron. This kind of coarse graphite is hereditary, the melting temperature is low, and the coarse graphite is not easy to be eliminated. The coarse graphite is inherited from the liquid state to the solid cast iron structure. On the one hand, it reduces the cast iron. The strength reduces the performance of the material, on the other hand, it weakens the expansion of the graphitized precipitation that should have occurred during the solidification process, and increases the shrinkage tendency during the solidification process of the molten iron.

              In cupola smelting, try to reduce the amount of pig iron charge, use carburizer to ensure high carbon equivalent, and relatively increase the amount of scrap steel. In this way, under the condition of high-temperature smelting, carbon with good activity and more significant graphitization can be obtained by carburizing. It is reflected in the casting that the morphology of graphite is better, which is beneficial to improve the mechanical properties, reduce the shrinkage tendency and improve the processing performance.

              During electric furnace smelting, high-quality molten iron can be obtained by carburizing with low pig iron consumption or even zero consumption.

              In terms of material performance, the mechanical properties of the large proportion of pig iron used in the past are half a grade lower than that of high scrap steel with the same composition. Therefore, the new process of adding carburant smelting is superior to the traditional large proportion of pig iron in terms of cost and product performance.

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                The Choice of Carbon Raiser and its Character

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                The Choice of Carbon Raiser and its Character

                During the smelting process, due to improper ingredients or charging and excessive decarburization, sometimes the carbon content in steel or iron does not meet the expected requirements. At this time, carbon should be added to steel or molten iron. The main substances usually used to increase carbon are anthracite coal powder, carbon-increasing pig iron, electrode powder, petroleum coke powder, pitch coke, charcoal powder and coke powder. The requirement for the carburant is that the higher the fixed carbon content, the better, and the lower the harmful impurities such as ash, volatile matter and sulfur, the better, so as not to contaminate the steel.

                The smelting of castings uses high-quality recarburizer after high-temperature firing of petroleum coke with few impurities, which is the most important link in the carburization process. The quality of the recarburizer determines the quality of the molten iron and also determines whether a good graphitization effect can be obtained. In short, the carburizer plays a pivotal role in reducing the shrinkage of molten iron.

                When all scrap steel is smelted in an electric furnace, the recarburizer that has undergone graphitization treatment is preferred. Only after high-temperature graphitization treatment, the carbon atoms can change from the original disordered arrangement to the flake arrangement, and the flake graphite can become graphite-shaped. The best core of the nucleus to facilitate graphitization. Therefore, we should choose a recarburizer that has been graphitized at a high temperature. Because during the high-temperature graphitization process, the sulfur content is reduced by the escape of SO2 gas. Therefore, high-quality recarburizers contain very low sulfur content. At the same time, this is also an indirect indicator to judge whether it has been subjected to high-temperature graphitization treatment and whether the graphitization is good. If the selected recarburizer has not undergone high-temperature graphitization treatment, the nucleation ability of graphite will be greatly reduced, and the graphitization ability will be weakened. Even if the same amount of carbon can be achieved, the result is completely different.

                The so-called carburizer is to effectively increase the carbon content in the molten iron after adding it, so the fixed carbon content of the carburizer must not be too low, otherwise, to achieve a certain carbon content, it is necessary to add a higher carbon More samples of recarburizer will undoubtedly increase the amount of other unfavorable elements in the recarburizer, so that the molten iron cannot obtain better returns.

                Low sulfur, nitrogen, and hydrogen elements are the key to preventing nitrogen pores in castings, so the lower the nitrogen content of the carburizer, the better. Other indicators of the carburant, such as moisture, ash, and volatile matter, the lower the amount of fixed carbon, the higher the amount of fixed carbon, so the higher the amount of fixed carbon, the content of these harmful components must not be high.

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                  Influence of Carburizing Process on Structure and Properties of Cast Iron

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                  Influence of Carburizer on Structure and Properties of Cast Iron

                  The influence of carburizing process on cast iron structure After carburizing with carbon additive, a large number of dispersed heterogeneous crystallization cores are formed in the molten iron, which reduces the supercooling degree of the molten iron and promotes the formation of A-type Graphite-based graphite structure; at the same time, due to the small amount of pig iron, its genetic effect is greatly weakened, so that the A-type graphite flakes are well-branched and difficult to grow, making the graphite short and uniform.

                  1. Furnace material requirements: no oil and no rust, scrap steel requires no excessive oxidation on the surface.

                  2. Generally, it is prepared by adding 4 kg of recarburizer for every 100 kg of scrap steel.

                  3. The temperature of the furnace is controlled at 1550°C. It is expected that the nodularization temperature will drop by 100°C, and the temperature of the bag will drop by 50°C.

                  4. During the production of the first furnace, 10-20 kg of high-quality iron chips are added to the bottom of the electric furnace. When the second furnace starts production, 20-40 kg of the remaining molten iron from the previous furnace is used.

                  5. After the iron chip is added, put the prescribed recarburizer into the plastic bag and put it on the iron chip. From the second furnace, 20-40 kg of the remaining molten iron is put into a plastic bag and the recarburizer is put into the molten iron surface.

                  6. Add 50 kg of scrap steel with small carbon particles (less than 50×50 area), and cover the entire furnace pond tightly.

                  7. Start melting, add remaining steel scrap → add pig iron → add returned iron (note that there should not be too much sand on the surface of the returned iron (to prevent the carburant from bonding with sand to affect the absorption).

                  8. Cover the molten iron with a covering agent after melting, and clean the slag repeatedly 2-3 times when the temperature reaches 1400°C.

                  9. Spheroidizing treatment Put the spheroidizing agent and ferrosilicon into the hanging package and cover the surface with high-quality iron chips.

                  10. After smelting, use a high-quality slag remover to clean the slag on the liquid surface in the furnace for 2-3 times, and check the temperature of the molten iron at 1550°C-1600°C.

                  11. When the molten iron is taken out of the furnace and two-thirds of the molten iron is used, immediately add the secondary silicon barium inoculant downstream of the furnace mouth.

                  12. Use a high-quality slag remover to clean up the slag.

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                    Smelting Process of Silicon Metal for Chemical Industry

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                    Smelting Process of Silicon Metal for Chemical Industry

                    The technological process of silicon for chemical use includes furnace charge preparation, electric furnace smelting, silicon refining and casting, and crushing for removing slag inclusions. All raw materials must undergo necessary treatment before the charge is prepared. The silica is crushed in the jaw crusher until the size of the pieces is not more than 100mm, and the pieces smaller than 5mm are screened out and washed with water. Because the fragments in the furnace are melted in the upper part of the furnace, the gas permeability of the charge is reduced, making the production process difficult. Petroleum coke has a relatively high electrical conductivity, so it must be broken to a size of no more than 10mm, and the amount of petroleum coke powder must be controlled. Because it burns directly on the furnace mouth, it will cause insufficient reducing agent.

                    In chemical silicon production, bituminous coal can completely replace charcoal, such as Hunan Zhuzhou refined bituminous coal,

                    The fixed carbon is 77.19%, the volatile matter is 19.4%, the ash content is 3.41%, the Fe2O3 content is 0.22%, the Al2O3 content is 0.99%, and the CaO content is 0.17%. According to production practice, it is feasible to use this kind of bituminous coal to smelt chemical silicon.

                    Blocks and chips for the production of silicon for chemical use are processed with timber cutters and wood chippers. The carbonaceous reducing agent in the furnace charge is mainly petroleum coke and bituminous coal, and the amount of wood blocks and wood chips depends on the furnace conditions. Wood is not used in production, but the product quality is more stable. The ratio of the charge is determined according to the grade of the product required to be produced. The proportion of petroleum coke and bituminous coal is determined according to the amount of carbon required for each batch of ore silicon. The ratio of petroleum coke and bituminous coal has a great influence on the working resistance of the charge.

                    After each component of the furnace charge is weighed, the charge is mixed evenly, and after the furnace is pounded, the uniformly mixed charge is added to the furnace. Keep a certain height of the material surface and feed evenly. Chemical silicon production is continuous. The conditions inside the furnace are not eternal either. Chemical silicon production is a process in which electric energy is converted into heat energy in an electric furnace, and then the heat energy is used to directly heat the material to produce a chemical reaction. Therefore, the electrical characteristics in the furnace are very important. The smelting operation is closed-arc to maintain the high temperature furnace, improve the thermal efficiency, and improve the utilization rate of the electric furnace. In the research, a metal silicon furnace with a capacity of 3200KVA and 6300KVA is used. The smelting is carried out by simmering for a certain period of time and regular concentrated feeding. Under normal circumstances, it is difficult for the charge to sink automatically, and it is generally necessary to force the charge to sink. The furnace condition is easy to fluctuate and difficult to control. Therefore, it is necessary to judge correctly and deal with it in time in production. The furnace is fired every 4 hours for scouring and casting, and the slag is broken and sorted into storage.

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