Silicon metal smelting is a high-energy-consuming production. my country’s metal silicon production has been around for a long time. With the tightening of the national energy policy, the development of energy saving and emission reduction, and the promotion of new energy, the metal silicon smelting has become a primary product and Technology, many domestic emerging energy companies have built a series of circular industrial chains such as metallic silicon, polycrystalline silicon, monocrystalline silicon, and solar cells, which will inevitably affect the development of my country's entire energy sector and the application of new energy in the next few years.
The following describes the smelting process and flow of metal silicon.
1. The necessity of producing silicon for the chemical industry
The metal silicon produced in my country (the silicon content is mainly 98.5%) was originally mainly metallurgical silicon, and the production of chemical metal silicon (the silicon content is mainly 99.85%) has mainly developed since the mid-1990s. my country's chemical silicon The output and export volume of China have increased rapidly. From 1999 to 2001, my country’s chemical silicon exports to Japan reached 22,000 tons, 30,000 tons, and 40,000 tons. In 2001, my country’s chemical silicon exports to Japan accounted for more than 40% of Japan’s chemical silicon imports. . China has begun to join the ranks of chemical silicon producers and suppliers, and the number of companies producing chemical silicon continues to increase. Since Shanghai Guangji Silicon Materials Co., Ltd. fully disclosed the carbothermic reduction smelting process in 2002, the production capacity of silicon metal in China rapidly increased from 100,000 tons per year to 1.2 million tons between 2002 and 2004. As a result, the National Development and Reform Commission imposed comprehensive sanctions. In 2006, the actual output of silicon metal fell back to 700,000 tons. In total, in 2006, only the Dawu Silicon Plant of Shanghai Guangji Silicon Materials Co., Ltd. successfully constructed a 10,000-ton-level metal silicon plant Jingxin Plant in the border area of Xinjiang. Others have built new silicon plants. Chemical silicon refers to metal silicon used in the production of organic silicon and polysilicon. From a global perspective, the consumption of metallurgical silicon is more than that of chemical silicon. However, with the continuous development of science and technology, the use of chemical silicon in the production of organic silicon and semiconductors has been expanding, and it is widely used in the production of organic silicon. Silicon monomer and polymer silicone oil, silicone rubber, silicone resin building anti-corrosion, waterproofing agent, etc. They have unique properties such as high temperature resistance, electrical insulation, radiation resistance, and waterproofing. Used in electrical, aviation, machinery, chemical, pharmaceutical, defense, construction and other sectors. As the core of integrated circuits, more than 95% of the electronic components are made of semiconductor silicon. Semiconductors are the backbone of the contemporary information industry. The optical fiber in the optical fiber cable, which is widely used in the "information highway", is also produced with metal silicon as a raw material. The consumption of chemical silicon in the United States and the European Union has accounted for more than half of the total consumption of metal silicon. Chemical silicon has been widely used as a high-tech field and an important basic industry, and its consumption tends to increase steadily. Under normal circumstances in the international market, the price per ton of chemical silicon is US$300-400 higher than that of metallurgical silicon. Therefore, it is necessary to vigorously develop the production of chemical silicon, whether from meeting the export and domestic demand, or from improving the economic efficiency of metal silicon enterprises, improving product quality.
2. Raw materials for chemical silicon production In the production of chemical silicon, raw materials are a prerequisite for good operation.
Quartz rocks are used as raw materials for the production of chemical metal silicon, and low-ash carbonaceous materials are used as reducing agents. The raw materials for the production of chemical silicon by the electric furnace method are mainly silica and carbon raw materials. The carbon raw material is mainly petroleum coke, with high-quality anthracite or charcoal, and part of it can also be mixed to increase the specific resistance of the charge. The raw materials are required to have the necessary purity and good reaction ability to meet the product specifications; the reducing agent has different reaction capabilities so that it can fully react with the quartz stone; the charge has different components and different particle sizes, In order to make a good influence on the charge electric furnace through proper coordination.
2.1. The process of smelting metallic silicon from silicon oxide minerals is a slag-free process. Chemical silicon smelting requires strict selection of silica. Not only does it contain less impurities, it also requires high mechanical strength, sufficient thermal stability, and suitable particle size composition. It is best to use silica for chemical silicon smelting. The natural form of silica exists either as an independent quartz mineral, or as a rock that is almost entirely composed of silica—silica, or as a sandstone in the form of silica. In the production of chemical silicon, the impurities and adherents in the silicon oxide minerals are completely reduced during the smelting process, and some are partially reduced, and some enter the product silicon as a compound or generate slag. This not only increases energy consumption, reduces product quality, but also causes difficulties in the smelting process. Therefore, strict requirements are imposed on the chemical composition of silicon oxide-containing minerals used in chemical silicon smelting. It is required that SiO2 is greater than 99%, Fe2O3 is less than 0.15%, Al2O3 is not greater than 0.2%, CaO is not greater than 0.1%, and the sum of impurities is not greater than 0.6%. The silica used must be washed with water and the surface clean before smelting.
The silica entering the furnace requires a certain particle size. The particle size of silica is an important process factor in smelting. The appropriate particle size of silica is affected by various factors such as the type of silica, electric furnace, capacity, operating conditions, and the type and particle size of the reducing agent, and should be determined according to specific smelting conditions. Under normal circumstances, the 6300KVA three-phase electric furnace (built in Dawu Silicon Plant in 1983) requires the silica particle size to be 8-100mm, and the 3200KVA three-phase electric furnace requires the silica particle size to be 8-80mm, and the proportion of the intermediate particle size composition is larger. When the particle size is too large, because it cannot be compatible with the viscous material and reaction speed of the furnace, the unreacted silica will easily enter the liquid silicon, which will increase the amount of slag, difficult to discharge, reduce the recovery rate of silicon, increase energy consumption, and even cause furnace The bottom rises, affecting normal production. If the particle size is too small, although the contact surface of the reducing agent can be enlarged, which is conducive to the progress of the reduction reaction, the gas produced during the reaction cannot be discharged smoothly, and the reaction speed will be slowed down. The particle size is too small. The impurities brought in will increase again, affecting product quality. Generally speaking, silica smaller than 5mm should not be used in production.
2.2 Carbonaceous reducing agent The main reducing agents used in chemical silicon smelting are petroleum coke, bituminous coal, and charcoal. In order to increase the resistivity of the charge and increase the chemical activity, it is also matched with gas coal coke, silica coke, blue charcoal, semi coke, low temperature coke, and wood block. In the chemical composition of carbonaceous reducing agent, fixed carbon, ash, volatile matter and moisture should be mainly considered. Generally, the fixed carbon is required to be higher, and the total amount of reducing agent required is reduced, so that less impurities are brought into the ash, the amount of slag is correspondingly reduced, the power consumption is reduced, and the impurity content in chemical silicon is reduced. The resistivity of the carbonaceous reducing agent is higher and the porosity is higher. The resistivity of the charge mainly depends on the carbonaceous reducing agent. The carbonaceous reducing agent has high resistivity, good chemical activity and high silicon recovery rate.
Petroleum coke has the lowest ash content among the reducing agents used in the production of metallic silicon, with ash content of 0.17-0.6%, fixed carbon of 90-95%, and volatile content of no more than 3.5%-13%. Chemical silicon smelting uses petroleum coke as a reducing agent because of its low ash content, which helps improve product quality. However, petroleum coke has low electrical resistivity and poor reactivity, and is easy to graphitize at high temperatures. When the amount is too large, the furnace condition is not well controlled, resulting in unsintered charge, severe stabbing, high power consumption, and difficulty in furnace discharge.
Charcoal has high specific resistance and reactivity, and has low impurity content. It is an ideal reducing agent for smelting silicon used in industrial chemistry. However, charcoal made with different woods and different methods has very different properties. The ash content of barked charcoal is usually one-half to one-third lower than that of bark charcoal. Bark has a great influence on the ash content of charcoal. The main component of charcoal is carbon, and the ash content is low, generally less than 10%. The resistivity is large and the chemical activity is good. Years of production practice has proved that charcoal is an important carbonaceous raw material that meets the needs of smelting chemical silicon, but the source of charcoal is limited, and charcoal reducing agent can no longer be used.
Judging from the situation abroad, most countries no longer use charcoal. Many domestic manufacturers have also done a lot of work in seeking and using charcoal substitutes. Practice has proved that among various carbonaceous reducing agents, bituminous coal is another ideal reducing agent besides charcoal in terms of reaction capacity and specific resistance.
The characteristics of bituminous coal are large resistance and strong reaction ability. Low-ash bituminous coal is obtained through washing. The ash content can reach about 3%, the Fe2O3 content is 0.2-0.3%, and the Al2O3 content is less than 1%. The ash content of the reducing agent bituminous coal in my country is more than 3%, while the ash content of the foreign reducing agent bituminous coal is more than 1%. The chemical method adopted by the Soviet Union can be used to select bituminous coal to obtain clean coal with an iron oxide content of less than 0.1%. The patent for the smelting process of replacing charcoal with bituminous coal is jointly owned by Shanghai Guangji Silicon Materials Co., Ltd. and Ordos Electric Power Smelting Co., Ltd. The function of the wood block is to increase the resistance of the material layer, and the size of the amount has an impact on the furnace condition. The amount of wood block is too large, the material layer is loose, the condition of the furnace deteriorates, and the power consumption increases. The wood block has a low ignition point and low carbon content, so the actual use of the reducing agent is very small.
The impurities in carbon raw materials are mainly ash, which is entirely composed of oxides. In chemical production, the oxides in the ash are also reduced, which consumes electricity and carbon, and the reduced impurities are still mixed in the silicon liquid, which reduces the strength of silicon. In production practice, every increase of 1% of the ash in the furnace charge will consume 100 degrees -120 degrees more electricity. Therefore, the ash content in the carbon raw materials is required to be as small as possible.
2.3 Electrodes Electrodes are one of the main consumable materials in the production of chemical silicon. Electrodes for chemical silicon smelting generally use graphite electrodes and carbon electrodes. At present, graphite electrodes are mainly used in China.
In the silicon smelting furnace, the electrode is the heart and an important part of the conductive system. Electric current is input into the furnace through electrodes to generate an electric arc, which is used for chemical silicon smelting. Requirements for electrode materials: (1) Good conductivity and low resistivity to reduce power loss. (2) The melting point should be high, the thermal expansion coefficient should be small, and it is not easy to be deformed; (3) It has sufficient mechanical strength at high temperature and low impurity content. Graphite electrode has low ash content, good electrical conductivity, heat resistance and corrosion resistance. It is the best choice for chemical silicon smelting.
3. The smelting process of silicon used in the chemical industry
The process flow of chemical silicon includes charge preparation, electric furnace smelting, silicon refining and casting, and crushing by removing slag inclusions. Before the charge is prepared, all raw materials must be processed as necessary. The silica is crushed in a jaw crusher to a size not greater than 100mm, and the fragments less than 5mm are screened out and rinsed with water. Because the fragments in the furnace are melted in the upper part of the furnace, the air permeability of the charge is reduced and the production process is difficult to carry out. Petroleum coke has a high conductivity coefficient, it must be crushed to a lumpiness not greater than 10mm, and the amount of petroleum coke powder must be controlled. Because it burns directly on the mouth of the furnace, it will cause insufficient reducing agent.
In the production of chemical silicon, bituminous coal can completely replace charcoal, such as Hunan Zhuzhou fine washed bituminous coal, fixed carbon reaches 77.19%, volatile content is 19.4%, ash content is 3.41%, Fe2O3 content is 0.22%, Al2O3 content is 0.99%, and CaO content is 0.17% . Through production practice, it is feasible to use this bituminous coal to smelt chemical silicon.
Wood blocks and chips for the production of chemical silicon are processed by cutting machines and chip chippers. The carbonaceous reducing agent in the charge is mainly petroleum coke and bituminous coal, and the amount of wood block and wood chip depends on the furnace condition. Wood is not used in production, but the product quality is more stable. The proportion of the charge is determined according to the required product level. The ratio of petroleum coke and bituminous coal is determined by the amount of carbon required for each batch of mineral silicon. The ratio of petroleum coke and bituminous coal has a greater influence on the working resistance of the charge.
After each component of the charge is weighed, the charge is uniformly mixed, and after the furnace is pounded, the uniformly mixed charge is concentrated into the furnace. Keep a certain height of the material surface and add evenly.
The production of chemical silicon is continuous. The conditions in the furnace are not permanent. 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 of the furnace are very important. The smelting implements closed arc operation, maintains high temperature furnaces, improves thermal efficiency, and increases the utilization rate of electric furnaces. In the study, one metal silicon furnace with a capacity of 3200KVA and 6300KVA is used. The smelting adopts a certain period of simmering and regular concentrated feeding operation methods. Under normal circumstances, it is difficult for the charge to sink automatically, and generally requires forced sinking. The furnace condition is easy to fluctuate and is difficult to control. Therefore, it must be correctly judged and handled in time during production. The furnace is discharged every 4 hours for scouring casting, crushing and picking slag, sorting and storing.
4. Electric furnace operation Chemical silicon smelting is carried out under submerged arc state.
In order to produce a constant and homogeneous metal silicon, it is necessary to implement the best furnace operation during the smelting process. The main source of heat is electricity. Therefore, the current flow path in the furnace and the current distribution of each route have an important impact on the temperature distribution of each zone in the furnace and the progress of the entire smelting process. Pay attention to maintaining the load balance of the three-phase electric energy in order to increase the output. Ensure quality and reduce power consumption.
4.1 Feeding and stoking furnace To make the silicon smelting furnace achieve high-quality and high-yield purposes, in addition to the requirements of good electric furnace parameters, excellent raw materials, and reasonable proportioning, the quality of the operation method is a very important factor. A reasonable feeding method plays a leading role in the material layer structure, the stability of the electrode in the furnace and the full utilization of heat energy. In production, feeding and stoking furnace are combined. According to the different conditions and characteristics of the smelting process, the feeding and stoking operations should be completed in a timely manner. In order to maintain good air permeability in the furnace, it is necessary to dazzle and pound the furnace. The small stoking furnace is carried out according to the furnace conditions, and the large stoking furnace is usually carried out once every hour or so. The stoking should be completed quickly, and the blocks smashed out of the stoking furnace are pushed to the core. During the stoking process, it is not possible to add new materials while smashing the furnace. It is not advisable to operate the tamping furnace with one electrode zone and one electrode zone. The tamping furnace must be centralized and uniformly charged, so as to maintain a higher melting temperature. The most important factor for stable furnace operation is to maintain a constant temperature distribution in the material bed. If the furnace temperature distribution is destroyed, the furnace operation will be seriously disturbed. The particle size and uniformity of the charge in the production, charging and discharging, and the treatment of the furnace material surface will all affect the electrode movement. Excessive electrode movement and strong stoking furnace will make the furnace operation unstable.
4.2 Closed arc operation
The closed-arc operation is to properly bury the electrode in the charge, and use the semi-melted charge as the resistance body to generate an arc between the electrode and the molten charge. In order to achieve closed arc operation, we must first consider the feeding method. Feeding methods include one-time feeding method, divided feeding method, and multiple feeding method. Except the one-time feeding method is open arc operation, other methods can achieve closed arc operation. In the production of chemical silicon, we adopt the batch feeding method, which has stable material layer structure, low power consumption and long furnace life. There are several problems to be dealt with during operation: one is to choose suitable electrical parameters so that the electrode can be inserted into the material layer with a proper depth; the other is to find a way to control the specific resistance of the charge; the third is that the particle size of the charge has an effect on silicon smelting. Important influence, too large or too small particle size is detrimental to furnace conditions. The advantages of closed arc operation are: ①The material layer structure in the furnace can form a complete system, and the charge will sink in sequence; ②The arc light is not exposed, and the radiant heat loss on the material surface is greatly reduced. The temperature of the blast furnace is maintained and the thermal efficiency is improved, thereby increasing the output, Improve product quality and reduce power consumption. ③The electrode consumption can be balanced and stable, avoiding the accident of breaking. ④The temperature of the material surface is relatively low, so that the equipment on the material surface is less thermally corroded, which extends the life of the equipment and improves the utilization rate of the electric furnace equipment. ⑤With less dust, the furnace surface operation can have a better operation surroundings. Regardless of the size of the furnace, as long as appropriate measures are taken, closed arc operation can be achieved, and ideal production results can be obtained.
4.3 Power Distribution Technology Electric arc furnace is a device that uses the heat generated by the arc to heat. In the process of chemical silicon smelting, physical and chemical changes are closely related to the electrical system. The quality of the power distribution operation has a very important impact on the smelting efficiency. The arc mainly has an electric terminal, and the cavity is affected by the impact of the arc to open the material legs to form a bulb shape. During the smelting process, the electrical parameters of the electric furnace are controlled by power distribution. Generally, it is to control the embedding depth of the electrode. Shallow buried electrodes generally indicate that there is excess reducing agent, fire holes are formed near the electrode, the arc sound is loud, the silicon temperature is low, the quantity is small, and the power consumption is high. For deeply buried electrodes, if there is too little reducing agent in the charge, the electrode will be in a lower position. Because the charge resistance increases as the carbon in the charge decreases, the increase in resistance reduces the current load, the electrode consumption increases, and the productivity decreases. During production, the depth of electrode embedding is determined according to on-site operations. Adjusting the electrode embedding depth is to change the resistance value of the charge, which is the best way to adjust the furnace condition. When the secondary voltage of the electric furnace exceeds a certain value, the electrode will be damaged, the evaporation loss of silicon will increase, the upper part of the furnace will overheat, and the heat loss will increase. The secondary current is limited by the allowable current density of the electrode and cannot be increased at will.
The ratio of current to voltage is an important factor in the operation of the furnace. If the current-voltage ratio is too small, the electrode will not go down, and it is difficult to operate the open arc production. The current-to-voltage ratio is too large, the electrode is inserted too deeply into the charge, and the production is not ideal enough. In production, only when the proper current-voltage ratio is found, the working current is stable, the material is balanced, and the electrode is raised and lowered, can the best production results be achieved. Adjusting the working voltage is an important means to adjust the productivity of the furnace. The working voltage of the furnace depends on two aspects: on the one hand, it is a short network structure, which requires high electrical efficiency and proper power factor. On the other hand, furnace conditions, including furnace body structure and production operation conditions, the resistance value of working resistance in smelting is very important, it is easy to change, and efforts should be made to stabilize and approach the optimal value. Generally, in order to ensure the normal material surface temperature, increase the voltage. The normal surface temperature of the bag is around 600°C. Using raw materials that meet the specifications, the charge size is large, the resistance is small, the branch current is large, and the electrode is not easy to penetrate.
