This work investigates the phase transformations in silica (SiO2) during heating to a target temp... more This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate.
In this paper we investigate flow phenomena related to tapping of silicon melt from a submerged a... more In this paper we investigate flow phenomena related to tapping of silicon melt from a submerged arc furnace. The multiphase flow model predicts how the gas pressure variations inside the furnace crater zone may effect phenomena like the gas flow pattern inside different zones of the charge materials inside the furnace, the metal tapping flow rate, and the gas blow out from the furnace tap hole. The full 3D multiphase flow model of the furnace is based on the real geometry and the most probable physical properties of different zones in the furnace, such as density, charge material particles size, porosity and permeability of the packed beds on the furnace bottom. In addition different spatial arrangements of charge material zones have been investigated. The results of the model show that existence of the mentioned high pressure crater zone in the furnace has a considerable effect on the metal tapping and the gas flow rate from the tap hole. The predicted tapping rate compares well to...
The aim of the work is to carry out a fundamental study on the solidification of 75% ferrosilicon... more The aim of the work is to carry out a fundamental study on the solidification of 75% ferrosilicon cast under different conditions. It is attempted to predict resulting properties of the alloy such as gram size and segregation.This research work is divided in four main topics. Each topic starts with a theoretical introduction. Then the experimental work and the results are presented.The first part presents temperature-time results from eleven experiment. The accuracy of the measurements is evaluated. The results are used to describe solidification of different casting methods. The cooling rate through the mushy (two-phase) zone is varied from 0.2 °C/min to 1170 °C/min. Eutectic solidification and solid state reaction are discussed. An important result is the recognition that the solid state reaction ( consist of two reactions. The first reaction seems to be harmful to the structure, Normally the reaction only reach the equilibrium-state after a prolonged heat treatment.The second part of the thesis report macro-segregation in cast ferrosilicon which have a vertical heat flux. The Silicon-content in one cast have been found to vary from 65.7% Si to 83.2% Si. The difference between segregation in top and bottom cooled solidification is described. A simple model for segregation with solidification from the top is presented.The third part consist of numerical calculations of the temperature-time histories of different positions in the ingots. Three cases have been computed and measured. Heat transfer from the surface of the block controls the cooling rate. Modelling results for one and three dimensional calculation are given. The heat transfer resistance from the metal to the mould is an important factor in the models.The last part of the thesis reports results concerning structure and strength of 75% ferrosiicon, The gram diameter for 75% ferrosilicon are measured and computed with a grain-coarsening model based on the theory of Kattamis and Flemings. The gram diameter (in i m) is given as a function of the local solidification time (in seconds) as:Gram diameter = 31.5 (Solidification time) 1”3Abrasion strength and compression strength are measured. Small gram size and high Silicon-content seems to favour high strength of the solid ferrosilicon.
Kirk-Othmer Encyclopedia of Chemical Technology, 2016
Silicon, a low density chemical element having nonmetallic characteristics, is the second most ab... more Silicon, a low density chemical element having nonmetallic characteristics, is the second most abundant element in the lithosphere (after oxygen). Silicon occurs naturally in the form of oxides and silicates and constitutes over 25% of the earth's crust. The production of silicon from silica and silicon refining technology are discussed in this article as well as the primary uses in the chemical, aluminum, solar electronics, and ceramic industries are also discussed. Silicon is the chemical precursor of silicones, a versatile, fast-growing market, and of advanced ceramics such as silicon nitride. It forms alloys with a large number of metallic elements; many of these alloys are of importance to the production and refining of irons and steels. The most prevalent are the ferrosilicons, production of which is discussed. Silicon is also one of the leading elements used for making photovoltaic cells for conversion of sun light to electricity and for manufacturing semiconductors in the electronics industry. Keywords: ferrosilicon; semiconductors; silicon; silicones; silicon alloys
Abstract The production of silicon requires input of carbon and electric power, both high in exer... more Abstract The production of silicon requires input of carbon and electric power, both high in exergy, in addition to the silicon source (quartz). We report exergy efficiencies of a new furnace for two choices of carbon raw material mixtures and compare this to reported data. From the collected experience for this industry, we propose to evaluate the silicon furnace using a set of two exergy performance indicators: the overall exergy efficiency and the silicon yield exergy indicator, introduced in this work. The new indicator is a measure for the performance of furnace operation. The overall exergy efficiency was 0.30 for both cases of the present furnace. The new silicon yield exergy indicator, together with data from the specific power consumption, favoured the mixture without coal for the present furnace (0.41). Additional exergy introduced as volatiles and through the consumption of electrodes accounted for 8 and 11%, respectively, of the total exergy destruction in the furnace. At 800 °C, the off-gas contained a potential of roughly 25% of the exergy input. This documented the potential to increase the overall exergy efficiency beyond 0.30.
The Norwegian ferroalloy industry is at present responsible for about 3 million tonnes of CO2-emi... more The Norwegian ferroalloy industry is at present responsible for about 3 million tonnes of CO2-emissions a year. These come from fossil carbon, mainly coal and coke that are used as reductants in the smelting process. If biocarbon is used as a substitute for coal and coke it is possible to reduce these emissions considerably. The term biocarbon means carbon from biological and renewable resources, for example charcoal and wood chips. This paper gives a brief review of the reasons for using biocarbon and presents the trends in biocarbon use in the Norwegian ferroalloy industry. Results are presented from the pilot scale smelting experiments that have used biocarbon as reductant. As charcoal strength and reactivity are important for process performance, these features have been measured. Although the charcoal is not produced in Norway, the possibilities for increasing the charcoal yield in the carbonization process have been studied and the results from laboratory experiments are inclu...
The Norwegian ferroalloy industry has high ambitions regarding environmental standards. Basic res... more The Norwegian ferroalloy industry has high ambitions regarding environmental standards. Basic research work has been performed to improve the fundamental understanding of ferroalloy processes and the environ- mental aspects. This paper will present work dealing with the understanding of formation and control of NOx, PAH and dioxins. During this work various techniques and methods have been applied, such as comprehen- sive industrial measurements, laboratory experiments, model development and application of Computational Fluid Dynamics (CFD). The main goal has been to reduce the environmental impact of ferroalloy plants, by means of improving the combustion processes done by introducing new charging routines and optimized fur- nace operations, instead of installing expensive gas treatment systems. The Norwegian obligations in accordance with the Gothenburg protocol are to reduce the NOx emissions by 30%, compared with the 1990 level within 2010. Measures developed in this work have r...
As a part of the FUME research programme that is being done by the Norwegian Ferroalloy Research ... more As a part of the FUME research programme that is being done by the Norwegian Ferroalloy Research Association (FFF), the SINTEF Group and NTNU (Norwegian University of Science and Technology), an energy balance for a 45 MW (ferro-) silicon submerged arc furnace have been made. The energy content and dynamics in and out of the process have been measured. New and improved equipment for flow and temperature measurement of cooling water has been utilized to increase the accuracy of the calculations of energy losses to the furnace equipment. The results from the energy balance visualize the energy potential in the different material flows. In addition, the paper presents how surplus energy from the Elkem Salten plant is being used.
The Norwegian FerroAlloy Association (FFF) has been promoting research work in order to understan... more The Norwegian FerroAlloy Association (FFF) has been promoting research work in order to understand and to improve the silicon and ferrosilicon process. One of the projects has dealt with the waste gas dynamics. The objective of this project has been to achieve a better understanding of the process as well as the environmental aspects. The project includes comprehensive measurements at a 42 MW ferrosilicon furnace as well as theoretical studies and modelling work. The main gas pollutants NOx, CO2 and SO2 has been examined. Also crater pressure dynamics and charge temperatures has been measured. Large variations in waste gas composition as well as off-gas temperature during the normal furnace operations such as stoking and charging has been found. The knowledge from this project will be used in order to meet new and stricter environmental regulations as well as to improve the furnace operations.
Gassing from the taphole of submerged arc furnaces used for silicon production poses a significan... more Gassing from the taphole of submerged arc furnaces used for silicon production poses a significant problem in the view of environment, operation and economy of production. The reason for the gassing is the pressure of the process gasses in the furnace (mainly CO and SiO) which may escape through the taphole. The combustion of the off gasses creates a high temperature gas containing very fine particles of SiO 2 and the result may be internal pollution in the tapping area. This study presents development of a model for combustion and capturing of the off gasses from the furnace taphole as the preliminary design of a new hood system for Elkem Company. The developed model is 3D and is constructed based on CFD method as a powerful tool to reach the desired results. In this paper several case studies consist of different velocities and components for the off gasses and different suction rates for the fans have been considered. The results show that based on the designed geometry and selec...
This work investigates the phase transformations in silica (SiO2) during heating to a target temp... more This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate.
In this paper we investigate flow phenomena related to tapping of silicon melt from a submerged a... more In this paper we investigate flow phenomena related to tapping of silicon melt from a submerged arc furnace. The multiphase flow model predicts how the gas pressure variations inside the furnace crater zone may effect phenomena like the gas flow pattern inside different zones of the charge materials inside the furnace, the metal tapping flow rate, and the gas blow out from the furnace tap hole. The full 3D multiphase flow model of the furnace is based on the real geometry and the most probable physical properties of different zones in the furnace, such as density, charge material particles size, porosity and permeability of the packed beds on the furnace bottom. In addition different spatial arrangements of charge material zones have been investigated. The results of the model show that existence of the mentioned high pressure crater zone in the furnace has a considerable effect on the metal tapping and the gas flow rate from the tap hole. The predicted tapping rate compares well to...
The aim of the work is to carry out a fundamental study on the solidification of 75% ferrosilicon... more The aim of the work is to carry out a fundamental study on the solidification of 75% ferrosilicon cast under different conditions. It is attempted to predict resulting properties of the alloy such as gram size and segregation.This research work is divided in four main topics. Each topic starts with a theoretical introduction. Then the experimental work and the results are presented.The first part presents temperature-time results from eleven experiment. The accuracy of the measurements is evaluated. The results are used to describe solidification of different casting methods. The cooling rate through the mushy (two-phase) zone is varied from 0.2 °C/min to 1170 °C/min. Eutectic solidification and solid state reaction are discussed. An important result is the recognition that the solid state reaction ( consist of two reactions. The first reaction seems to be harmful to the structure, Normally the reaction only reach the equilibrium-state after a prolonged heat treatment.The second part of the thesis report macro-segregation in cast ferrosilicon which have a vertical heat flux. The Silicon-content in one cast have been found to vary from 65.7% Si to 83.2% Si. The difference between segregation in top and bottom cooled solidification is described. A simple model for segregation with solidification from the top is presented.The third part consist of numerical calculations of the temperature-time histories of different positions in the ingots. Three cases have been computed and measured. Heat transfer from the surface of the block controls the cooling rate. Modelling results for one and three dimensional calculation are given. The heat transfer resistance from the metal to the mould is an important factor in the models.The last part of the thesis reports results concerning structure and strength of 75% ferrosiicon, The gram diameter for 75% ferrosilicon are measured and computed with a grain-coarsening model based on the theory of Kattamis and Flemings. The gram diameter (in i m) is given as a function of the local solidification time (in seconds) as:Gram diameter = 31.5 (Solidification time) 1”3Abrasion strength and compression strength are measured. Small gram size and high Silicon-content seems to favour high strength of the solid ferrosilicon.
Kirk-Othmer Encyclopedia of Chemical Technology, 2016
Silicon, a low density chemical element having nonmetallic characteristics, is the second most ab... more Silicon, a low density chemical element having nonmetallic characteristics, is the second most abundant element in the lithosphere (after oxygen). Silicon occurs naturally in the form of oxides and silicates and constitutes over 25% of the earth's crust. The production of silicon from silica and silicon refining technology are discussed in this article as well as the primary uses in the chemical, aluminum, solar electronics, and ceramic industries are also discussed. Silicon is the chemical precursor of silicones, a versatile, fast-growing market, and of advanced ceramics such as silicon nitride. It forms alloys with a large number of metallic elements; many of these alloys are of importance to the production and refining of irons and steels. The most prevalent are the ferrosilicons, production of which is discussed. Silicon is also one of the leading elements used for making photovoltaic cells for conversion of sun light to electricity and for manufacturing semiconductors in the electronics industry. Keywords: ferrosilicon; semiconductors; silicon; silicones; silicon alloys
Abstract The production of silicon requires input of carbon and electric power, both high in exer... more Abstract The production of silicon requires input of carbon and electric power, both high in exergy, in addition to the silicon source (quartz). We report exergy efficiencies of a new furnace for two choices of carbon raw material mixtures and compare this to reported data. From the collected experience for this industry, we propose to evaluate the silicon furnace using a set of two exergy performance indicators: the overall exergy efficiency and the silicon yield exergy indicator, introduced in this work. The new indicator is a measure for the performance of furnace operation. The overall exergy efficiency was 0.30 for both cases of the present furnace. The new silicon yield exergy indicator, together with data from the specific power consumption, favoured the mixture without coal for the present furnace (0.41). Additional exergy introduced as volatiles and through the consumption of electrodes accounted for 8 and 11%, respectively, of the total exergy destruction in the furnace. At 800 °C, the off-gas contained a potential of roughly 25% of the exergy input. This documented the potential to increase the overall exergy efficiency beyond 0.30.
The Norwegian ferroalloy industry is at present responsible for about 3 million tonnes of CO2-emi... more The Norwegian ferroalloy industry is at present responsible for about 3 million tonnes of CO2-emissions a year. These come from fossil carbon, mainly coal and coke that are used as reductants in the smelting process. If biocarbon is used as a substitute for coal and coke it is possible to reduce these emissions considerably. The term biocarbon means carbon from biological and renewable resources, for example charcoal and wood chips. This paper gives a brief review of the reasons for using biocarbon and presents the trends in biocarbon use in the Norwegian ferroalloy industry. Results are presented from the pilot scale smelting experiments that have used biocarbon as reductant. As charcoal strength and reactivity are important for process performance, these features have been measured. Although the charcoal is not produced in Norway, the possibilities for increasing the charcoal yield in the carbonization process have been studied and the results from laboratory experiments are inclu...
The Norwegian ferroalloy industry has high ambitions regarding environmental standards. Basic res... more The Norwegian ferroalloy industry has high ambitions regarding environmental standards. Basic research work has been performed to improve the fundamental understanding of ferroalloy processes and the environ- mental aspects. This paper will present work dealing with the understanding of formation and control of NOx, PAH and dioxins. During this work various techniques and methods have been applied, such as comprehen- sive industrial measurements, laboratory experiments, model development and application of Computational Fluid Dynamics (CFD). The main goal has been to reduce the environmental impact of ferroalloy plants, by means of improving the combustion processes done by introducing new charging routines and optimized fur- nace operations, instead of installing expensive gas treatment systems. The Norwegian obligations in accordance with the Gothenburg protocol are to reduce the NOx emissions by 30%, compared with the 1990 level within 2010. Measures developed in this work have r...
As a part of the FUME research programme that is being done by the Norwegian Ferroalloy Research ... more As a part of the FUME research programme that is being done by the Norwegian Ferroalloy Research Association (FFF), the SINTEF Group and NTNU (Norwegian University of Science and Technology), an energy balance for a 45 MW (ferro-) silicon submerged arc furnace have been made. The energy content and dynamics in and out of the process have been measured. New and improved equipment for flow and temperature measurement of cooling water has been utilized to increase the accuracy of the calculations of energy losses to the furnace equipment. The results from the energy balance visualize the energy potential in the different material flows. In addition, the paper presents how surplus energy from the Elkem Salten plant is being used.
The Norwegian FerroAlloy Association (FFF) has been promoting research work in order to understan... more The Norwegian FerroAlloy Association (FFF) has been promoting research work in order to understand and to improve the silicon and ferrosilicon process. One of the projects has dealt with the waste gas dynamics. The objective of this project has been to achieve a better understanding of the process as well as the environmental aspects. The project includes comprehensive measurements at a 42 MW ferrosilicon furnace as well as theoretical studies and modelling work. The main gas pollutants NOx, CO2 and SO2 has been examined. Also crater pressure dynamics and charge temperatures has been measured. Large variations in waste gas composition as well as off-gas temperature during the normal furnace operations such as stoking and charging has been found. The knowledge from this project will be used in order to meet new and stricter environmental regulations as well as to improve the furnace operations.
Gassing from the taphole of submerged arc furnaces used for silicon production poses a significan... more Gassing from the taphole of submerged arc furnaces used for silicon production poses a significant problem in the view of environment, operation and economy of production. The reason for the gassing is the pressure of the process gasses in the furnace (mainly CO and SiO) which may escape through the taphole. The combustion of the off gasses creates a high temperature gas containing very fine particles of SiO 2 and the result may be internal pollution in the tapping area. This study presents development of a model for combustion and capturing of the off gasses from the furnace taphole as the preliminary design of a new hood system for Elkem Company. The developed model is 3D and is constructed based on CFD method as a powerful tool to reach the desired results. In this paper several case studies consist of different velocities and components for the off gasses and different suction rates for the fans have been considered. The results show that based on the designed geometry and selec...
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