The siliceous raw material is directly mixed with the carbonaceous reducing agent to prepare the furnace charge, and is sent into the furnace for smelting as required. The energy consumption of the reduction process is very high, about 14kw.h/kg.
Chemical reaction equation:
General metallurgical process requirements for the chemical composition of siliceous raw material quartz sand: SiO2>98.5%, Al2O3<0.8%, Fe2O3<0.4%, CaO<0.2%, MgO<0.15%.
The process flow of industrial silicon includes charge preparation, electric furnace smelting, silicon refining and casting, and crushing by removing slag inclusions.
Taking silica as an example, the silica and carbonaceous reducing agent are mixed, and all raw materials must be treated as necessary before the charge is prepared. The silica is crushed in the jaw crusher to the degree of lumpiness not greater than 100mm, and the fragments less than 5mm are screened out and rinsed with water. Qualified particle size silica, stone tar, and electrodes are proportioned on the ground and added evenly into the furnace. Petroleum coke has a higher electrical conductivity. It must be crushed to the extent that the lumpiness is not greater than 10mm, and the amount of petroleum coke powder must be controlled. It burns directly on the mouth of the furnace, which will cause insufficient reducing agent. 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. Maintain a certain height of the material surface when feeding, and the feeding is even.
Carbonaceous reducing agents include petroleum coke, etc. The total carbon content is the total weight of each batch of silica multiplied by the SiO2 content in the silica multiplied by 24/60. The reducing agent must meet the following requirements: necessary purity, sufficient reactivity, and high electrical resistance , Suitable particle size, suitable ash content, sufficient supply and cheap price.
The amount of silica used in each batch is set to 200kg, and the theoretical carbon requirement of silica is
Silica dosage×theoretical ingredient ratio=200kg×0.4=80kg
If it is used in the production of solar cell polysilicon, attention must be paid to the control of harmful impurities such as B, P and other elements that affect the conversion efficiency of solar cells. Because the carbonaceous reducing agent generally contains high P, so the petroleum coke reducing agent is used in the production, so the amount of petroleum coke needed to fix the carbon is
Therefore, the theoretical ingredient ratio is
Silica: Petroleum Coke=200:120
The above is an estimate. Depending on the specific situation, the ratio can be adjusted, the purity of the raw materials is as high as possible, and the impurities are as little as possible.
In principle, smelting industrial silicon is a slag-free process, because the ash content of natural silica and reducing agent contains impurities, and the slag formed during the smelting process accounts for 2% to 3% of the silicon. The generation of slag disturbs the smelting process of the furnace. , The impurities in the raw materials will also reduce the quality of silicon. Therefore, in the raw materials, there are strict requirements on the content of harmful impurities (aluminum, calcium, iron, phosphorus, arsenic and other oxides).
The oxides of transition metals (such as Ni, Co, Fe) are much less stable than SiO2, and transition metal oxides tend to be reduced to simple metals, which is higher than the requirement for reducing SiO2 to Si. Na2O is also quite stable at room temperature, but its stability decreases rapidly as the temperature rises. This is due to the lower melting and boiling points of alkali metals. Pure Na2O can be reduced to a gaseous metal element at about 1000°C. The same is true for the more volatile K, which is more unstable than Na2O.
Alkaline earth metals and aluminum oxides are the most impurity in the raw materials of electric furnaces, and they can be reduced to a metallic state with lower carbon requirements than SiO2. TiO2 is more stable than SiO2. Its monooxide TiO has a melting point of 1750℃, and its stability is similar to that of Al2O3. The stability of the middle oxides Ti3O4 and Ti2O3 lies between TiO2 and TiO. When the clay is heated, the hydroxyl group is released quickly and becomes a concentrated state, and it is under the reducing action of free carbon and SiC at 1600°C. Of course, the alkali metal oxide has been reduced and evaporated before SiC is formed. Aluminosilicate and CaO still remain in the dispersed small particles, and the silicate therein is selectively reduced to SiC and separated from Al2O3 during further heating. In the presence of liquid Si, almost all Ca and Al are dissolved in it. If pure Cr and Mn oxides are added to the mixture of SiO2 and C, before any reaction of SiO2 occurs, Cr and Mn have been completely reduced. Because the temperature required for complete reduction of Mn is just enough to produce liquid slag, SiO2 is still not reduced at a melting point of about 1700°C, but Mn has been dissolved in the slag. Iron can be reduced well during smelting, and almost all of it enters the silicon. Almost half of aluminum, magnesium and calcium are reduced into the silicon. The rest is evaporated and the rest is not reduced, forming slag.
Mixing of materials has the following effects: ①Good air permeability; ②Improve thermal conductivity and heat exchange rate; ③Decompose and increase combustion speed. The material is dried at 200~300℃ for 1~2h, especially the new furnace must be baked for 2 days in order to completely remove the moisture on the refractory material. Newly repaired or mid-repaired furnaces and furnaces that have been discontinued must be oven-baked before being put into use. The purpose of the oven is to dry and preheat the furnace body. In order to remove moisture and prevent the furnace body from cracking due to too fast receipt, the temperature of the furnace should be slowly raised.
Submerged arc furnace smelting uses conventional single-phase and three-phase electric furnaces to make the furnace temperature above 1800°C, and the prepared charge is added in batches for smelting. The smelting implements closed arc operation to maintain high temperature furnace, improve thermal efficiency, and increase utilization rate of electric furnace. The furnace is discharged every 4h for refining casting, crushing and picking slag, sorting and storing.
Large current and low voltage are beneficial for the electrode to be buried in the charge stably and deeply. During production, the secondary voltage is well controlled to make the furnace condition stable and without fluctuations, and the electrodes are buried deep and stable; and when the secondary voltage is too low, the charge surface will die and large sticky blocks will be formed.
Secondary voltage: 120-165V (adjust according to the actual situation, such as 132V);
Secondary current: 24kA;
Average load: 3800kw;
Embedding depth of electrode: 1200~1400mm;
Material surface height: 400~500mm;
Refining time: 30~40min.
This is the common industrial silicon smelting process. The conditions of different manufacturers are different, and the process parameters may also be different.