A composite carbonaceous reducing agent pellet for industrial silicon and its preparation method

Abstract

This invention relates to a composite carbonaceous reducing agent pellet for industrial silicon and its preparation method, belonging to the field of resource-efficient and clean utilization technology. The composite carbonaceous reducing agent pellet for industrial silicon of this invention is prepared by microwave roasting, comprising an inner layer pellet and a non-low-caking coal coating layer covering the outside of the inner layer pellet. The inner layer pellet is formed by pressing charcoal powder, petroleum coke, and low-rank non-caking coal as carbonaceous materials, combined with water, a binder, and alkali metal additives. The non-low-caking coal coating layer is formed by layering charcoal powder and caking coal as carbonaceous materials, combined with water, a binder, waste biomass, and alkali metal additives onto the surface of the inner layer pellet, wherein the number of non-low-caking coal coating layers (n≥2) is greater than or equal to 2. The particle size of the charcoal powder, petroleum coke, and low-rank non-caking coal in the inner layer pellet is greater than the particle size of the caking coal in the coating layer, and the caking index of the low-rank non-caking coal in the inner layer pellet is less than the caking index of the caking coal in the coating layer. The first coating layer covers the surface of the inner layer pellet.

Description

Technical Field

[0001] This invention relates to a composite carbonaceous reducing agent pellet for industrial silicon and its preparation method, belonging to the field of efficient and clean resource utilization technology. Background Technology

[0002] When using whole coal as the carbonaceous reducing agent in industrial silicon smelting, the reaction rate of the furnace charge can be accelerated under high current and low voltage conditions, resulting in a more complete and thorough carbothermic reaction, thus increasing silicon yield while reducing energy consumption. However, untreated caking coal has a low fixed carbon content, a high volatile matter content, and poor permeability, which may cause severe sparking of the furnace charge, leading to raw material waste. Low-rank non-caking coal, on the other hand, has extremely low utilization rates and is inexpensive. Summary of the Invention

[0003] To address the problems of existing carbonaceous reducing agents for industrial silicon, this invention proposes a method for preparing composite carbonaceous reducing agent pellets for industrial silicon. The method uses charcoal powder and non-low-caking coal with different caking indices as carbonaceous raw materials for the non-low-caking coal coating layer. Microwave roasting of the caking coal reduces the dielectric loss of the carbon material, heating the entire carbon material to the sintering temperature to achieve densification and improve pellet strength. Charcoal powder, petroleum coke, and low-rank non-caking coal are used as carbonaceous materials for the inner layer of pellets. The non-low-caking coal coating layer covers the carbonaceous materials of the inner layer of pellets, reducing their burn-off rate. The addition of alkali metal additives promotes structural rearrangement and crystallization of biomass and caking coal in the later stages of industrial silicon smelting, increasing the porosity of the pellets. This allows the alkali metals in the biomass ash and the added additives to penetrate into the pores of the carbon material, increasing the contact area between reactants and improving reaction efficiency. This invention not only ensures high fixed carbon content, good reactivity, low ash content, and suitable bonding index of the product, but also recycles waste biomass and wasted charcoal powder. The microwave roasting and the use of waste biomass and inorganic binders do not affect the porosity of the outer layer and can improve the overall strength of the pellets. Furthermore, the addition of alkali metal additives can increase the porosity and play a prominent role in the later production process. It can be fully used in the smelting and production of industrial silicon.

[0004] A composite carbonaceous reducing agent pellet for industrial silicon: The composite carbonaceous reducing agent pellet for industrial silicon is prepared by microwave roasting, comprising an inner layer pellet and a non-low-caking coal coating layer covering the outside of the inner layer pellet. The inner layer pellet is formed by pressing charcoal powder, petroleum coke, and low-rank non-caking coal as carbonaceous materials, combined with water, binder, and alkali metal additives. The non-low-caking coal coating layer is formed by layering charcoal powder and caking coal as carbonaceous materials, combined with water, binder, waste biomass, and alkali metal additives onto the surface of the inner layer pellet, wherein the non-low-caking coal coating layer n≥2.

[0005] The particle size of charcoal powder, petroleum coke and low-rank non-caking coal in the inner layer pellets is larger than that of caking coal in the coating layer. The caking index of low-rank non-caking coal in the inner layer pellets is less than that of caking coal in the coating layer. The first coating layer in the coating layer coats the surface of the inner layer pellets.

[0006] The alkali metal additive is K2CO3, Na2CO3, NaOH, KOH, or NaHCO3.

[0007] The particle size of the charcoal powder, petroleum coke, and low-rank non-caking coal is 0.20~0.40mm, and the caking index of the charcoal powder, petroleum coke, and low-rank non-caking coal is 0. Based on the mass fraction of carbonaceous materials in the inner layer pellets as 100%, the inner layer pellets contain 20~35% charcoal powder, 20~35% petroleum coke, 30~50% low-rank non-caking coal, 1~1.5% alkali metal additives, 0.9~1.2% binders, and 9~12% water.

[0008] Taking the mass fraction of carbonaceous materials in a single-layer non-low-caking coal coating as 100%, the charcoal powder accounts for 20-35% of the single-layer non-low-caking coal coating, the remainder is caking coal, the amount of alkali metal additives is 1-1.5%, the amount of binder is 0.9-1.2%, the amount of waste biomass is 3-6%, and the amount of water is 9-12%.

[0009] The nth non-low caking coal coating layer covers the surface of the (n-1)th non-low caking coal coating layer, wherein the particle size of the caking coal in the (n-1)th non-low caking coal coating layer is greater than the particle size of the caking coal in the nth non-low caking coal coating layer; and the caking index of the caking coal in the (n-1)th non-low caking coal coating layer is less than the caking index of the caking coal in the nth non-low caking coal coating layer.

[0010] Preferably, the particle size of the caking coal in the first non-low caking coal coating layer is 0.15~0.18mm, and the caking index is 65~85; the particle size of the caking coal in the second non-low caking coal coating layer is <0.1mm, and the caking index is >85.

[0011] The charcoal powder has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%; the petroleum coke has a fixed carbon content of 85-91 wt.%, volatile matter of 8-11 wt.%, ash content of 0.2-0.3 wt.%, and moisture content of 1.0-2.0 wt.%; the low-rank non-caking coal has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%; and the caking coal has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%.

[0012] The industrial silicon composite carbonaceous reducing agent pellets have a bonding index of 62~82 and a cold strength of 9~13MPa.

[0013] The specific steps for preparing the composite carbonaceous reducing agent pellets for industrial silicon are as follows:

[0014] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and caking coal with different caking indices are crushed into preset particle sizes respectively;

[0015] (2) Mix charcoal powder, petroleum coke and low-rank non-sticky coal with a preset particle size evenly as carbonaceous material for inner layer pellets, then add water, binder and alkali metal additives and stir to mix evenly, then press under pressure of 5~20MPa to obtain inner layer cylindrical pellet embryos.

[0016] (3) Mix the caking coal and charcoal powder corresponding to the first non-low caking coal coating layer evenly as the carbonaceous material of the first non-low caking coal coating layer, then add water, binder and alkali metal additive and stir to form a mixture viscous body, and coat the inner cylindrical pellet surface with the mixture viscous body to form the first pellet embryo.

[0017] (4) Mix the caking coal and charcoal powder corresponding to the second non-low caking coal coating layer evenly as the carbonaceous material of the second non-low caking coal coating layer, then add water, binder and alkali metal additive and stir to form a mixture viscous body. Coat the surface of the first pellet embryo with the mixture viscous body to form the second pellet embryo; and so on, prepare the nth pellet embryo in the same way;

[0018] (5) Place the nth pellet in a microwave tube furnace, introduce argon gas, microwave roast at 500~600℃ for 1~2h, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0019] The adhesive may be an inorganic adhesive or an organic adhesive, preferably water glass or self-modifying starch.

[0020] Preferably, in step (5), the argon gas introduction rate is 1~1.5L / min and the microwave power is 1~5kw.

[0021] The beneficial effects of this invention are:

[0022] (1) This invention mainly uses charcoal powder and caking coal as carbonaceous raw materials, and uses different non-low caking coals to coat charcoal powder and low-rank non-caking coal. The smaller the particle size and the finer the particles of the caking coal coating the inner layer pellets containing charcoal powder, petroleum coke and low-rank non-caking coal, the better the coating effect of the caking coal, which forms a porous structure. This results in a large number of voids inside the pellets, which is beneficial to improve its utilization rate in smelting, reduce the burning rate of charcoal, make full use of it, ensure the efficient utilization of carbonaceous materials, and at the same time appropriately improve the stability of combustion.

[0023] (2) This invention uses microwave roasting of caking coal to reduce the dielectric loss of carbon materials, thereby heating the carbon materials to the sintering temperature and achieving densification, thus improving the strength of the pellets. The charcoal powder and low-rank non-caking coal are coated to reduce their burn-off rate. At the same time, during the roasting process, the caking coal burns at a certain temperature to produce a gel, which makes the internal structure of the pellets more tightly bound, thereby improving the strength of the composite carbonaceous reducing agent pellets. Meanwhile, the increase in temperature also increases the bonding index of the composite carbonaceous reducing agent pellets, making its bonding index suitable and meeting the requirements of industrial silicon smelting.

[0024] (3) The addition of alkali metal additives in this invention will promote the structural rearrangement and crystallization effect of biomass and caking coal in the later industrial silicon smelting, increase the porosity of the pellets, and allow the alkali metals in the biomass ash and the added additives to penetrate into the pores of the carbon materials, increase the contact area between the reactants, and improve the reaction efficiency. The addition of alkali metal additives in this invention can increase the porosity and play a prominent role in the later production, and can be fully used in the smelting and production of industrial silicon.

[0025] (4) After the composite carbonaceous reducing agent pellets for industrial silicon of the present invention are microwave roasted at a certain temperature, the volatile matter of the raw materials will be released in large quantities and the volatile matter content will be reduced sharply. The fixed carbon content of the roasted pellets can reach more than 75 wt.%, the volatile matter content is 15~20 wt.%, the ash content is less than 4 wt.%, the cold strength can reach 9~13 MPa, and the bonding index can reach 62~82. It ensures that the fixed carbon content is high, the reaction activity is good, the ash content is low, the bonding index is suitable, and the waste biomass and the wasted charcoal powder can be recycled and utilized. It can also produce high-quality industrial silicon, which can be used for the smelting and production of industrial silicon.

[0026] (5) The modified starch binder used in the inner and middle layers of the composite carbonaceous reducing agent pellets for industrial silicon of the present invention is released as volatiles during the industrial silicon smelting process, which makes the pellets have a large number of pores and forms a porous structure, thereby improving the chemical reactivity of the pellets. Furthermore, the use of waste biomass and inorganic binders in the outer layer does not affect the porosity of the outer layer and can improve the overall strength of the pellets, thus greatly improving the industrial silicon smelting process. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0028] Example 1: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 78.2 wt.%, a volatile content of 18.7 wt.%, an ash content of 3.1 wt.%, a bonding index G of 80, and a cold strength of 13 MPa.

[0029] The charcoal has a fixed carbon content of 66 wt.%, a volatile matter content of 22.6 wt.%, and an ash content of 1.5 wt.%; the low-rank non-caking coal has a fixed carbon content of 57 wt.%, a volatile matter content of 28.6 wt.%, and an ash content of 2.3 wt.%; the petroleum coke has a fixed carbon content of 88 wt.%, a volatile matter content of 10.2 wt.%, and an ash content of 0.22 wt.%; caking coal A has a fixed carbon content of 60 wt.%, a volatile matter content of 36.5 wt.%, and an ash content of 1.4 wt.%; and caking coal B has a fixed carbon content of 61 wt.%, a volatile matter content of 33.2 wt.%, and an ash content of 0.6 wt.%.

[0030] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0031] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.30 mm and their caking indices were all 0. The particle size of caking coal A was 0.16 mm and its caking index was 85. The particle size of caking coal B was 0.065 mm and its caking index was 93.

[0032] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (K2CO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 20MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 40mm and a height of 40mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 20% charcoal powder, 35% petroleum coke, 45% low-rank non-caking coal, 1.3% alkali metal additive K2CO3, 1% binder, and 10% water.

[0033] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 60 mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 20% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (modified starch) added is 1%, the amount of waste biomass added is 3%, and the amount of water added is 10%.

[0034] (4) The caking coal B corresponding to the second non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 80 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 20% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (water glass) added is 1%, the amount of waste biomass added is 3%, and the amount of water added is 10%.

[0035] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.5L / min), microwave calcinate at 550℃ for 1.5h, microwave power is 4kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0036] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 30:35:35;

[0037] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 102 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 87.2 kg of industrial silicon product was obtained, consuming 12190 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.6 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, and Ca ≤ 0.05 wt.%.

[0038] Example 2: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 79.8 wt.%, a volatile content of 17 wt.%, an ash content of 3.2 wt.%, a knot index G of 75, and a cold strength of 11 MPa.

[0039] The charcoal has a fixed carbon content of 67 wt.%, a volatile matter content of 26.6 wt.%, and an ash content of 1.5 wt.%; the low-rank non-caking coal has a fixed carbon content of 61 wt.%, a volatile matter content of 26.6 wt.%, and an ash content of 1.8 wt.%; the petroleum coke has a fixed carbon content of 89 wt.%, a volatile matter content of 9.2 wt.%, and an ash content of 0.2 wt.%; and the caking coal A has a fixed carbon content of 58 wt.%, a volatile matter content of 34.5 wt.%, and an ash content of...

[0040] The amount was 1.2 wt.%; the fixed carbon content of caking coal B was 56 wt.%, the volatile matter content was 36.2 wt.%, and the ash content was 1.6 wt.%.

[0041] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0042] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.30 mm and their caking indices were all 0. The particle size of caking coal A was 0.15 mm and its caking index was 85. The particle size of caking coal B was 0.068 mm and its caking index was 95.

[0043] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (Na2CO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 13MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 40mm and a height of 40mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 30% charcoal powder, 30% petroleum coke, 40% low-rank non-caking coal, 1.5% alkali metal additive Na2CO3, 1.2% binder (modified starch) and 12% water.

[0044] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (Na2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 65mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (Na2CO3) added is 1.5%, the amount of binder (modified starch) added is 1.2%, the amount of waste biomass added is 4%, and the amount of water added is 12%.

[0045] (4) The caking coal B corresponding to the second non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (Na2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 85 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (Na2CO3) added is 1.2%, the amount of binder (water glass) added is 1.2%, the amount of waste biomass added is 4%, and the amount of water added is 12%.

[0046] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.5L / min), microwave calcine at 600℃ for 1.5h, microwave power is 5kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0047] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 40:30:30;

[0048] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 101 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 91.9 kg of industrial silicon product was obtained, consuming 12090 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.6 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, Ca ≤ 0.05 wt.%.

[0049] Example 3: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 77.3 wt.%, a volatile content of 19 wt.%, an ash content of 3.7 wt.%, a knot index G of 73, and a cold strength of 10 MPa.

[0050] The charcoal has a fixed carbon content of 63 wt.%, a volatile matter content of 23.6 wt.%, and an ash content of 2.0 wt.%; the low-rank non-caking coal has a fixed carbon content of 64 wt.%, a volatile matter content of 24.4 wt.%, and an ash content of 1.6 wt.%; the petroleum coke has a fixed carbon content of 87 wt.%, a volatile matter content of 10.5 wt.%, and an ash content of 0.3 wt.%; caking coal A has a fixed carbon content of 59 wt.%, a volatile matter content of 36.7 wt.%, and an ash content of 1.2 wt.%; and caking coal B has a fixed carbon content of 55 wt.%, a volatile matter content of 38.2 wt.%, and an ash content of 0.4 wt.%.

[0051] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0052] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.25 mm and the caking index was 0. The particle size of caking coal A was 0.18 mm and the caking index was 76. The particle size of caking coal B was 0.075 mm and the caking index was 90.

[0053] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (NaHCO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 10MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 45mm and a height of 45mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 35% charcoal powder, 20% petroleum coke, 45% low-rank non-caking coal, 1.2% alkali metal additive NaHCO3, 0.9% binder and 9% water.

[0054] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (NaHCO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 60 mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 35% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (NaHCO3) added is 1%, the amount of binder (modified starch) added is 0.9%, the amount of waste biomass added is 5%, and the amount of water added is 9%.

[0055] (4) The caking coal B corresponding to the second non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (NaHCO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 90 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 35% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (NaHCO3) added is 1%, the amount of binder (water glass) added is 0.9%, the amount of waste biomass added is 5%, and the amount of water added is 9%.

[0056] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1L / min), microwave calcine at 550℃ for 1h, microwave power is 4kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0057] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 45:20:35;

[0058] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 103 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 85.4 kg of industrial silicon product was obtained, consuming 12690 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.6 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, Ca ≤ 0.05 wt.%.

[0059] Example 4: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 79.6 wt.%, a volatile content of 17 wt.%, an ash content of 3.4 wt.%, a knot index G of 78, and a cold strength of 12 MPa.

[0060] The charcoal has a fixed carbon content of 66 wt.%, a volatile matter content of 29.4 wt.%, and an ash content of 0.8 wt.%; the low-rank non-caking coal has a fixed carbon content of 62 wt.%, a volatile matter content of 23.4 wt.%, and an ash content of 1.9 wt.%; the petroleum coke has a fixed carbon content of 89 wt.%, a volatile matter content of 8.5 wt.%, and an ash content of 0.3 wt.%; caking coal A has a fixed carbon content of 59 wt.%, a volatile matter content of 33.5 wt.%, and an ash content of 1.2 wt.%; and caking coal B has a fixed carbon content of 57 wt.%, a volatile matter content of 35.2 wt.%, and an ash content of 1.6 wt.%.

[0061] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0062] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.30 mm and their caking indices were all 0. The particle size of caking coal A was 0.16 mm and its caking index was 83. The particle size of caking coal B was 0.068 mm and its caking index was 94.

[0063] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (KOH) are added and stirred and mixed. Then the mixture is pressed under a pressure of 15MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 30mm and a height of 30mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 25% charcoal powder, 35% petroleum coke, 40% low-rank non-caking coal, 1.4% alkali metal additive KOH, 1.1% binder, and 11% water.

[0064] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (KOH) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 60 mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (KOH) added is 1.1%, the amount of binder (modified starch) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0065] (4) The caking coal B corresponding to the second non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (KOH) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 100 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (KOH) added is 1.1%, the amount of binder (water glass) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0066] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.5L / min), microwave calcinate at 600℃ for 1.5h, microwave power is 4kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0067] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 35:35:30;

[0068] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 100 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 91.4 kg of industrial silicon product was obtained, consuming 12250 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.6 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, and Ca ≤ 0.05 wt.%.

[0069] Example 5: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 80.3 wt.%, a volatile content of 16.2 wt.%, an ash content of 3.5 wt.%, a knot index G of 69, and a cold strength of 11 MPa.

[0070] The charcoal has a fixed carbon content of 69 wt.%, a volatile matter content of 21.6 wt.%, and an ash content of 0.7 wt.%; the low-rank non-caking coal has a fixed carbon content of 60 wt.%, a volatile matter content of 25.2 wt.%, and an ash content of 2.1 wt.%; the petroleum coke has a fixed carbon content of 89 wt.%, a volatile matter content of 8.5 wt.%, and an ash content of 0.3 wt.%; caking coal A has a fixed carbon content of 64 wt.%, a volatile matter content of 34.5 wt.%, and an ash content of 1.2 wt.%; and caking coal B has a fixed carbon content of 56 wt.%, a volatile matter content of 37.2 wt.%, and an ash content of 1.6 wt.%.

[0071] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0072] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.20 mm and their caking indices were all 0. The particle size of caking coal A was 0.18 mm and its caking index was 65. The particle size of caking coal B was 0.068 mm and its caking index was 90.

[0073] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (K2CO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 13MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 50mm and a height of 50mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 30% charcoal powder, 35% petroleum coke, 35% low-rank non-caking coal, 1.1% alkali metal additive K2CO3, 1.2% binder, and 12% water.

[0074] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 70 mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (K2CO3) added is 0.9%, the amount of binder (modified starch) added is 1.2%, the amount of waste biomass added is 5%, and the amount of water added is 12%.

[0075] (4) The caking coal B corresponding to the second non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 90 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (K2CO3) added is 0.9%, the amount of binder (water glass) added is 1.2%, the amount of waste biomass added is 5%, and the amount of water added is 12%.

[0076] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.5L / min), microwave calcine at 500℃ for 1.5h, microwave power is 3kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0077] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 40:35:25;

[0078] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 99 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 82.5 kg of industrial silicon product was obtained, consuming 12370 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.5 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, Ca ≤ 0.05 wt.%.

[0079] Example 6: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 80.7 wt.%, a volatile content of 18 wt.%, an ash content of 1.3 wt.%, a knot index G of 64, and a cold strength of 9 MPa.

[0080] The charcoal has a fixed carbon content of 63 wt.%, a volatile matter content of 31.2 wt.%, and an ash content of 0.8 wt.%; the low-rank non-caking coal has a fixed carbon content of 55 wt.%, a volatile matter content of 28.4 wt.%, and an ash content of 2.3 wt.%; the petroleum coke has a fixed carbon content of 91 wt.%, a volatile matter content of 8.2 wt.%, and an ash content of 0.2 wt.%; caking coal A has a fixed carbon content of 60 wt.%, a volatile matter content of 36.5 wt.%, and an ash content of 0.3 wt.%; and caking coal B has a fixed carbon content of 65 wt.%, a volatile matter content of 33.3 wt.%, and an ash content of 0.2 wt.%.

[0081] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0082] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.30 mm and their caking indices were all 0. The particle size of caking coal A was 0.16 mm and its caking index was 83. The particle size of caking coal B was 0.068 mm and its caking index was 94.

[0083] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 8MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 45mm and a height of 45mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 35% charcoal powder, 25% petroleum coke, 40% low-rank non-caking coal, 1% alkali metal additive K2CO3, 1.1% binder, and 11% water.

[0084] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 65mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 35% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (K2CO3) added is 0.8%, the amount of binder (modified starch) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0085] (4) The caking coal B corresponding to the second non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the second pellet with a diameter of 95 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 35% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (K2CO3) added is 0.8%, the amount of binder (water glass) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0086] (5) Place the second pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1L / min), microwave roast at 500℃ for 1h, microwave power is 3kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0087] In this embodiment, the mass ratio of inner layer pellets, caking coal A, and caking coal B is 45:25:30;

[0088] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 98 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 80.6 kg of industrial silicon product was obtained, consuming 12690 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.5 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, Ca ≤ 0.05 wt.%.

[0089] Example 7: The industrial silicon composite carbonaceous reducing agent pellets in this example have a fixed carbon content of 79.5 wt.%, a volatile content of 17 wt.%, an ash content of 3.4 wt.%, a bonding index G of 81, and a cold strength of 12 MPa.

[0090] The charcoal has a fixed carbon content of 67 wt.%, a volatile matter content of 28.3 wt.%, and an ash content of 0.9 wt.%; the low-rank non-caking coal has a fixed carbon content of 63 wt.%, a volatile matter content of 22.4 wt.%, and an ash content of 1.9 wt.%; the petroleum coke has a fixed carbon content of 88 wt.%, a volatile matter content of 7.4 wt.%, and an ash content of 0.4 wt.%; caking coal A has a fixed carbon content of 59 wt.%, a volatile matter content of 33.5 wt.%, and an ash content of 1.2 wt.%; caking coal B has a fixed carbon content of 58 wt.%, a volatile matter content of 34.3 wt.%, and an ash content of 1.5 wt.%; and caking coal C has a fixed carbon content of 64 wt.%, a volatile matter content of 34.4 wt.%, and an ash content of 1.3 wt.%.

[0091] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0092] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.35 mm and their caking indices were all 0. The particle size of caking coal A was 0.17 mm and its caking index was 80. The particle size of caking coal B was 0.085 mm and its caking index was 90. The particle size of caking coal C was 0.065 mm and its caking index was 94.

[0093] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (K2CO3) are added and stirred and mixed. Then the mixture is pressed under a pressure of 15MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 50mm and a height of 50mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 30% charcoal powder, 35% petroleum coke, 35% low-rank non-caking coal, 1.3% alkali metal additive K2CO3, 1.1% binder, and 11% water.

[0094] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 65mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (K2CO3) added is 1.1%, the amount of binder (modified starch) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0095] (4) The caking coal B corresponding to the second non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet embryo to form the second pellet embryo with a diameter of 75 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (K2CO3) added is 1.1%, the amount of binder (modified starch) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0096] (5) The caking coal C corresponding to the third non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the third non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the second pellet to form the third pellet with a diameter of 90 mm. Assuming that the mass fraction of carbonaceous material in the third non-low caking coal coating layer is 100%, the charcoal powder accounts for 30% of the third non-low caking coal coating layer, the remainder is caking coal C, the amount of alkali metal additive (K2CO3) added is 1.1%, the amount of binder (water glass) added is 1.1%, the amount of waste biomass added is 4%, and the amount of water added is 11%.

[0097] (6) Place the third pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.2L / min), microwave calcinate at 550℃ for 1.5h, microwave power is 4kw, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

[0098] In this embodiment, the mass ratio of inner layer pellets, caking coal A, caking coal B and caking coal C is 35:20:20:25;

[0099] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 100 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 90.4 kg of industrial silicon product was obtained, consuming 12390 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.5 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, Ca ≤ 0.05 wt.%.

[0100] Example 8: The industrial silicon composite carbonaceous reducing agent pellets of this example have a fixed carbon content of 80.5 wt.%, a volatile content of 18 wt.%, an ash content of 1.5 wt.%, a bonding index G of 82, and a cold strength of 13 MPa.

[0101] The charcoal has a fixed carbon content of 69 wt.%, a volatile matter content of 21.5 wt.%, and an ash content of 0.8 wt.%; the low-rank non-caking coal has a fixed carbon content of 60 wt.%, a volatile matter content of 25.1 wt.%, and an ash content of 2.2 wt.%; the petroleum coke has a fixed carbon content of 88 wt.%, a volatile matter content of 9.5 wt.%, and an ash content of 0.3 wt.%; caking coal A has a fixed carbon content of 64 wt.%, a volatile matter content of 34.4 wt.%, and an ash content of 1.3 wt.%; caking coal B has a fixed carbon content of 57 wt.%, a volatile matter content of 36.1 wt.%, and an ash content of 1.7 wt.%; caking coal C has a fixed carbon content of 58 wt.%, a volatile matter content of 37.5 wt.%, and an ash content of 1.4 wt.%; and caking coal D has a fixed carbon content of 59 wt.%, a volatile matter content of 36.5 wt.%, and an ash content of 1.4 wt.%.

[0102] A method for preparing composite carbonaceous reducing agent pellets for industrial silicon, the specific steps of which are as follows:

[0103] (1) The charcoal powder, petroleum coke, low-rank non-caking coal and two caking coals with different caking indices were crushed into preset particle sizes. The particle size of the charcoal powder, petroleum coke and low-rank non-caking coal was 0.40 mm and the caking index was 0. The particle size of caking coal A was 0.18 mm and the caking index was 65. The particle size of caking coal B was 0.15 mm and the caking index was 81. The particle size of caking coal C was 0.075 mm and the caking index was 90. The particle size of caking coal D was 0.055 mm and the caking index was 95.

[0104] (2) The carbonaceous material of the inner layer pellets is uniformly mixed with charcoal powder of the preset particle size, petroleum coke and low-rank non-caking coal. Then water, organic binder (modified starch) and alkali metal additive (K2CO3) are added and stirred and mixed. Then the mixture is pressed under pressure MPa to obtain an inner layer cylindrical pellet embryo with a diameter of 30 mm and a height of 30 mm. Based on the mass fraction of carbonaceous material in the inner layer pellets as 100%, the inner layer pellets contain 25% charcoal powder, 40% petroleum coke, 35% low-rank non-caking coal, 1.2% alkali metal additive K2CO3, 1.2% binder, and 10% water.

[0105] (3) The caking coal A corresponding to the first non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the first non-low caking coal coating layer. Then, water, binder (modified starch) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the inner cylindrical pellet to form the first pellet embryo with a diameter of 45 mm. Assuming that the mass fraction of carbonaceous material in the first non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the first non-low caking coal coating layer, the remainder is caking coal A, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (modified starch) added is 1.2%, the amount of waste biomass added is 6%, and the amount of water added is 10%.

[0106] (4) The caking coal B corresponding to the second non-low caking coal coating layer and charcoal powder are mixed evenly as the carbonaceous material of the second non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet embryo to form the second pellet embryo with a diameter of 60 mm. Assuming that the mass fraction of carbonaceous material in the second non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the second non-low caking coal coating layer, the remainder is caking coal B, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (modified starch) added is 1.2%, the amount of waste biomass added is 6%, and the amount of water added is 10%.

[0107] (5) The caking coal C corresponding to the third non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the third non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet embryo to form the third pellet embryo with a diameter of 75 mm. Assuming that the mass fraction of carbonaceous material in the third non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the third non-low caking coal coating layer, the remainder is caking coal C, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (modified starch) added is 1.2%, the amount of waste biomass added is 6%, and the amount of water added is 10%.

[0108] (6) The caking coal D corresponding to the fourth non-low caking coal coating layer and the charcoal powder are mixed evenly as the carbonaceous material of the third non-low caking coal coating layer. Then, water, binder (water glass) and alkali metal additive (K2CO3) are added and stirred to form a mixture. The mixture is coated on the surface of the first pellet to form the fourth pellet with a diameter of 90 mm. Assuming that the mass fraction of carbonaceous material in the fourth non-low caking coal coating layer is 100%, the charcoal powder accounts for 25% of the fourth non-low caking coal coating layer, the remainder is caking coal D, the amount of alkali metal additive (K2CO3) added is 1%, the amount of binder (water glass) added is 1.2%, the amount of waste biomass added is 6%, and the amount of water added is 10%.

[0109] (7) Place the fourth pellet embryo in a graphite boat, put the graphite boat into a microwave tube furnace, introduce argon gas (argon gas flow rate is 1.2L / min), microwave calcinate at 600℃ for 2h, microwave power is 5kw, and cool to room temperature with the furnace to obtain composite carbonaceous reducing agent pellets.

[0110] In this embodiment, the mass ratio of inner layer pellets, caking coal A, caking coal B, caking coal C and caking coal D is 35:15:15:15:20;

[0111] 200 kg of silica (99.5% SiO2, 0.05% Al2O3, 0.03% Fe2O3, 0.05% CaO) was crushed into blocks with a particle size of 25-100 mm. After washing and sieving, it was mixed evenly with 101 kg of composite carbonaceous reducing agent pellets as described in this embodiment and added to a submerged arc furnace for smelting. After smelting, 91.3 kg of industrial silicon product was obtained, consuming 12270 kWh of electricity. The composition of the industrial silicon product obtained after smelting was Si > 99.6 wt.%, Fe ≤ 0.19 wt.%, Al ≤ 0.09 wt.%, and Ca ≤ 0.05 wt.%.

[0112] The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A composite carbonaceous reducing agent pellet for industrial silicon, characterized in that: The industrial silicon composite carbonaceous reducing agent pellets are prepared by microwave roasting, comprising an inner layer of pellets and a non-low-caking coal coating layer covering the outer surface of the inner layer of pellets. The inner layer of pellets is formed by pressing charcoal powder, petroleum coke, and low-rank non-caking coal as carbonaceous materials, combined with water, binder, and alkali metal additives. The non-low-caking coal coating layer is formed by layering charcoal powder and caking coal as carbonaceous materials, combined with water, binder, waste biomass, and alkali metal additives onto the surface of the inner layer of pellets, wherein the non-low-caking coal coating layer n≥2; the alkali metal additives are K2CO3, Na2CO3, NaOH, KOH, or NaHCO3. The particle size of charcoal powder, petroleum coke and low-rank non-caking coal in the inner layer pellets is larger than that of caking coal in the coating layer. The caking index of low-rank non-caking coal in the inner layer pellets is less than that of caking coal in the coating layer. The first coating layer in the coating layer coats the surface of the inner layer pellets. The particle size of the charcoal powder, petroleum coke, and low-rank non-caking coal is 0.20~0.40mm, and the caking index of the charcoal powder, petroleum coke, and low-rank non-caking coal is 0. Based on the mass fraction of carbonaceous materials in the inner layer pellets as 100%, the inner layer pellets contain 20~35% charcoal powder, 20~35% petroleum coke, 30~50% low-rank non-caking coal, 1~1.5% alkali metal additives, 0.9~1.2% binders, and 9~12% water. Based on a mass fraction of 100% for carbonaceous materials in a single-layer non-low-caking coal coating, the single-layer non-low-caking coal coating contains 20-35% charcoal powder, with the remainder being caking coal. The amount of alkali metal additives added is 1-1.5%, the amount of binder added is 0.9-1.2%, the amount of waste biomass added is 3-6%, and the amount of water added is 9-12%. The nth non-low caking coal coating layer covers the surface of the (n-1)th non-low caking coal coating layer, wherein the particle size of the caking coal in the (n-1)th non-low caking coal coating layer is greater than the particle size of the caking coal in the nth non-low caking coal coating layer; and the caking index of the caking coal in the (n-1)th non-low caking coal coating layer is less than the caking index of the caking coal in the nth non-low caking coal coating layer. The particle size of the caking coal in the first non-low caking coal coating layer is 0.15~0.18mm, and the caking index is 65~85; the particle size of the caking coal in the second non-low caking coal coating layer is <0.1mm, and the caking index is >85.

2. The composite carbonaceous reducing agent pellets for industrial silicon according to claim 1, characterized in that: Charcoal powder has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%; petroleum coke has a fixed carbon content of 85-91 wt.%, volatile matter of 8-11 wt.%, ash content of 0.2-0.3 wt.%, and moisture content of 1.0-2.0 wt.%; low-rank non-caking coal has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%; caking coal has a fixed carbon content of 55-70 wt.%, volatile matter of 22-40 wt.%, ash content of 3.0-4.0 wt.%, and moisture content of 1.0-4.0 wt.%.

3. The method for preparing composite carbonaceous reducing agent pellets for industrial silicon according to any one of claims 1 to 2, characterized in that, The specific steps are as follows: (1) The charcoal powder, petroleum coke, low-rank non-caking coal and caking coal with different caking indices are crushed into preset particle sizes respectively; (2) Mix charcoal powder, petroleum coke and low-rank non-sticky coal with a preset particle size evenly as carbonaceous material for inner layer pellets, then add water, binder and alkali metal additives and stir to mix evenly, then press under pressure of 5~20MPa to obtain inner layer cylindrical pellet embryos. (3) Mix the caking coal and charcoal powder corresponding to the first non-low caking coal coating layer evenly as the carbonaceous material of the first non-low caking coal coating layer, then add water, binder and alkali metal additive and stir to form a mixture viscous body, and coat the inner cylindrical pellet surface with the mixture viscous body to form the first pellet embryo. (4) Mix the caking coal and charcoal powder corresponding to the second non-low caking coal coating layer evenly as the carbonaceous material of the second non-low caking coal coating layer, then add water, binder and alkali metal additive and stir to form a mixture viscous body. Coat the surface of the first pellet embryo with the mixture viscous body to form the second pellet embryo; and so on, prepare the nth pellet embryo in the same way; (5) Place the nth pellet in a microwave tube furnace, introduce argon gas, microwave roast at 500~600℃ for 1~2h, and cool with the furnace to room temperature to obtain composite carbonaceous reducing agent pellets.

4. The method for preparing composite carbonaceous reducing agent pellets for industrial silicon according to claim 3, characterized in that: The binder is water glass or self-modifying starch.

5. The method for preparing composite carbonaceous reducing agent pellets for industrial silicon according to claim 3, characterized in that: In step (5), the argon gas introduction rate is 1~1.5L / min, and the microwave power is 1~5kw.