Trace Elements in the Si Furnace. Part I: Behavior of Impurities in Quartz During Reduction

Quartz and carbonaceous materials, which are used in the production of silicon as well as electrodes and refractories in the silicon furnace, contain trace elements mostly in the form of oxides. These oxides can be reduced to gaseous compounds and leave the furnace or stay in the reaction products—metal and slag. This article examines the behavior of trace elements in hydrothermal quartz and quartzite in the reaction of SiO₂ with Si or SiC. Mixtures of SiO₂ (quartz or quartzite), SiC, and Si in forms of lumps or pellets were heated to 1923 K and 2123 K (1650°C and 1850°C) in high purity graphite crucibles under Argon gas flow. The gaseous compounds condensed in the inner lining of the tube attached to the crucible. The phases present in the reacted charge and the collected condensates were studied quantitatively by X-ray diffraction (XRD) and qualitatively by Electron Probe Micro Analyzer (EPMA). Contaminants in the charge materials, reacted charge and condensate were analyzed by Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS). Muscovite in the mineral phase of quartz melted and formed two immiscible liquid phases: an Al-rich melt at the core of the mineral, and a SiO₂-rich melt at the mineral boundaries. B, Mn, and Pb in quartz were removed during heating in reducing atmosphere at temperature above 1923 K (1650°C). Mn, Fe, Al and B diffused from quartz into silicon. P concentration was under the detection limit. Quartzite and hydrothermal quartz had different initial impurity levels: quartzite remained more impure after reduction experiment but approached purity of hydrothermal quartz upon silica reduction.     

Trace Elements in the Si Furnace-Part II: Analysis of Condensate in Carbothermal Reduction of Quartz

Silicon feedstock for production of solar-grade silicon should be as pure as possible to decrease the cost of manufacturing of solar cells. Impurities in quartz, carbonaceous materials, electrodes, and refractories are mostly present in the form of oxides. These oxides can be reduced to volatile gaseous compounds in presence of SiO(g) and CO(g) atmosphere and potentially leave the furnace or stay in the condensed reaction products, metal, and slag. This work investigates the conditions under which volatile impurities report to the gas phase in laboratory experiments with lumpy and pelletized mixtures of SiO₂, SiC, and Si at 1923 K and 2123 K (1650 °C and 1850 °C), respectively, were carried out. The volatile compounds were generated by the reduction of quartz and collected in the form of condensate. The effects of the reaction temperature, quartz type, charge composition, pellets, and lumps on the composition of the condensate were studied. The trace elements in the charge input, reacting charge, and condensate were analyzed using inductively coupled plasma (ICP)-mass spectroscopy (MS) and X-ray diffraction (XRD). CO(g) and SiO(g), which are the major components in reduction reactions, formed four types of condensate: white, brown, green, and orange. The condensate constituents were amorphous SiO₂, 3C:SiC, Si, and α-quartz. Each impurity present in the quartz charge entered the gas phase during quartz reduction and was detected in the condensate. Al and Fe show limited volatility. The volatility of Mn, P, and B depends on the charge mix: a higher P_CO enhances the concentration of these elements in the gas phase. Fluid inclusions, common in hydrothermal quartz, enhance the distribution of the contaminants to the gas phase. Industrial campaigns on Si and Fe-Si production confirm the experimental results.     

Carbothermic Reduction of Amorphous Silica Refined from Diatomaceous Earth

Aimed at developing solar-grade Si (SOG-Si) resources, amorphous silica (AS) refined from diatomaceous earth was reduced carbothermically. The reactivity of quartz—typically crystalline silica—also was investigated for comparison. Preliminary experiments confirmed an intermediate phase of SiC during the carbothermic reaction. SiC was produced more easily by heating AS mixed with graphite within 2 hours at 1773 K in a resistance furnace, whereas quartz remained unreacted under the same condition. The AS mixed with SiC then was heated in an electrode impulse furnace. An Si peak was identified in the X-ray diffraction (XRD) pattern of the sample reacted within 30 seconds at 2273 K. Chemical analysis indicated that the mole ratio of reduced Si to initial SiO₂ increased with a heating time of 15–30 seconds. It almost reached a constant depending on the heating temperature. The initial stage may correspond to a significant reduction from SiO₂ to Si in the solid–solid or solid–gas reaction systems. The next stage probably is a slow vaporization of SiO(g). Once the reduced Si melts with SiO₂ at the high temperature, the melt partially covers the surface of SiO₂ to prevent contact with SiC. A better reactivity for refined AS is observed than for quartz.     

Process Modeling and Optimization of a Submerged Arc Furnace for Phosphorus Production

This article presents a process model of a phosphorus-producing, submerged arc furnace. The model successfully incorporates accurate, multifield thermodynamic, kinetic, and industrial data with computational flow dynamic calculations and thus further unifies the sciences of kinetics and equilibrium thermodynamics. The model is structurally three-dimensional and uses boundary conditions, initial values, and material specifications provided by industrial measurements, laboratory experiments, and a combination of empirical and thermodynamic data. It accounts for fully developed gas flows of gaseous product generated from within the packed bed; the energy associated with chemical reactions, heating, and melting, as well as thermal conductivity and the particle–particle radiation within the burden. The model proves the existence of a narrow, gas–solid reduction zone where the bulk of phosphorus is produced. It shows that fast reaction rates in this narrow reaction zone in combination with long residence times diminish the influence changing reaction rates have on the process. It indicates that most heat exchanged between the new pellets entering the furnace and the gaseous product produced in the reduction zone takes place in the top 0.5 m of the furnace bed. The gaseous product and flow information shows low and recirculating gaseous flow velocity areas that cause dust accumulation.     

Heat-transfer model for the acheson process

The Acheson process is used to manufacture silicon carbide (SiC) in a resistor furnace using petroleum coke and silica as raw materials. The process is highly inefficient, as only 10 to 15 pct of the charge gets converted into silicon carbide. No published attempt has been made to optimize this century-old process by applying mathematical modeling. Therefore, a simultaneous heat- and mass-transfer model has been developed for the resistance-heating furnace, considering silicon carbide formation as a typical carbothermal reaction. Coupled transient partial differential equations have been worked out. These equations have been solved numerically, using the implicit finite-difference method in their nondimensional form, to obtain the profiles of solid temperature and volume fraction reacted in the furnace. The trend of the computed results appears to be realistic; comparison of the results with published experimental work validates the applicability of the model’s predictions. The effects of various parameters on the process have been studied. These include void fraction, power inputs, initial concentration of silicon carbide present in the charge, etc.     

The use of siderites in blast furnaces operated on unfluxed pellets

Bakal siderites represent an important source of iron ore that can be used in the production of high-quality pig iron and steel. Their value in this regard stems from the stability of their chemical composition, low content of harmful impurities (sulfur, phosphorus), good reducibility, and good strength in the roasted state. These advantages, combined with the favorable composition of the country rock-which has a basicity (based on CaO + MgO/SiO₂) of up to 2.6-makes it possible to classify these siderites as iron-fluxes that can be used effectively in a blast furnace. Roasted siderite concentrate (RSC) can be used efficiently in the blast-furnace charge to make different grades of manganese-bearing foundry iron as long as unfluxed pellets are also employed. On the average, the addition of 10 kg of RSC to the charge per ton of pig iron reduces the consumption of dry skip coke by 0.6–1.5 kg/ton pig and increases furnace productivity by 0.9–2.0 tons/day by decreasing the amount of raw flux needed and increasing the iron content of the fluxed part of the charge. __________ Translated from Metallurg, No. 8, pp. 51–55, August, 2006.     

中关铁矿制备熔剂性球团生球性能试验

 通过增加熔剂性球团矿的入炉比例,能够改善炉料结构,降低炼铁系统能耗,并且通过“源头减量”的途径可以降低炼铁过程中污染物的排放。实现高球比冶炼的核心环节是制备熔剂性球团,而熔剂性球团质量取决于生球的性能,因此,保证生球质量是探究熔剂性球团制备工艺较为重要的环节。由于中关铁矿硅含量较低、镁含量适宜,适合作为低硅熔剂性球团的原料。以中关铁矿为原料探究熔剂性球团的制备工艺,并在此基础上分析了影响熔剂性球团生球质量的因素(粒度、时间、水分、膨润土、SiO₂含量、碱度和MgO含量)。试验结果表明,生球的抗压强度、落下强度及爆裂温度受碱度、SiO₂和MgO含量变化的影响不大;生球的抗压强度、落下强度及爆裂温度主要受造球时间、水分、黏结剂用量、铁矿粉及熔剂的理化性能影响,并在造球时间维持为12 min、水分维持为8%～9%、膨润土用量为2%时,生球抗压强度、落下强度及爆裂温度较优且满足运输与入炉要求。     

Study of steel production with the use of hot-briquetted iron in the cold charge

The use of hot-briquetted iron in large furnaces makes their operation more efficient. The time of furnace operation under current is shortened by 4 min, while unit electric-power consumption is cut by 15 kWh/ton for the melting of the charge and by 8 kWh/ton for the completion of the heat as a whole. These results are obtained when the amount of hot-briquetted iron in the cold charge corresponds to 0.3–0.4 of the weight of the scrap and the ratio of the mass of the cold charge to the mass of the metallized pellets in the charge is within the range 1.0–1.1. __________ Translated from Metallurg, No. 3, pp. 50–51, March, 2007.     

Macrosegregation of Impurities in Directionally Solidified Silicon

Directional solidification of molten metallurgical-grade Si was carried out in a vertical Bridgman furnace. The effects of changing the mold velocity from 5 to 110 μm seconds^–1 on the macrosegregation of impurities during solidification were investigated. The macrostructures of the cylindrical Si ingots obtained in the experiments consist mostly of columnar grains parallel to the ingot axis. Because neither cells nor dendrites can be observed on ingot samples, the absence of precipitated particles and the fulfillment of the constitutional supercooling criterion suggest a planar solid–liquid interface for mold velocities ≤10 μm seconds^–1. Concentration profiles of several impurities were measured along the ingots, showing that their bottom and middle are purer than the metallurgical Si from which they solidified. At the ingot top, however, impurities accumulated, indicating the typical normal macrosegregation. When the mold velocity decreases, the macrosegregation and ingot purity increase, changing abruptly for a velocity variation from 20 to 10 μm seconds^–1. A mathematical model of solute transport during solidification shows that, for mold velocities ≥20 μm seconds^–1, macrosegregation is caused mainly by diffusion in a stagnant liquid layer assumed at the solid–liquid interface, whereas for lower velocities, macrosegregation increases as a result of more intense convective solute transport.     

Kinetic Studies of Iron Ore–Coal Composite Pellet Reduction by TG–DTA

An attempt has been made to study the effect of coal quality on the reduction kinetics of iron ore–coal composite pellets under non-isothermal condition in inert atmosphere. During non-isothermal reduction of composite pellets, it is observed that (i) reduction rate of iron oxide increases with increasing temperature, (ii) reduction rate increases with increase in porosity of pellets and (iii) the computed values of activation energy (E) are lower during the initial stage of reduction (0.86–8.82 kJ mol⁻¹) than those in the later stages of reduction (12.37–38.32 kJ mol⁻¹). These values indicate that the initial stage reduction is controlled by gaseous diffusion mechanism and at final stage, mixed control reaction mechanism (i.e., both gaseous-diffusion and chemical reaction) is the rate controlling step. The present investigation aims at to assess the effect of Fe_tot/C_fix ratio in pellet, volatile matter in coal, and temperature on the reduction kinetics of iron ore–coal composite pellets using simultaneous thermogravimetric and differential thermal analyser (TG–DTA).     

首钢高炉新炉料结构下球团技术的探索与应用

以首钢京唐钢铁厂高炉大比例球团矿炉料结构为背景,以京唐二期球团带式焙烧机生产线为研究对象,在试验研究和工艺设备等方面阐述了首钢京唐高炉大比例球团矿炉料结构下的带式焙烧机工艺和设备的创新点和优势.生产实践结果表明,生产出了超低硅高碱度球团矿,2条线均日产量为12500 t,碱度为1.1～1.2,SiO2质量分数为2.0％...     

济钢生产镁质球团矿的试验研究与实践

济钢选择配加高氧化镁铁矿经济原料,分别在实验室和球团竖炉进行了氧化镁含量2%镁质球团矿试验研究和生产试验应用。研究和生产实践表明:在原料配比碱性精粉35%～40%、酸精粉50%～55%,球团矿氧化镁含量2.0%～2.3%、硅含量5%～6%,焙烧温度控制在1160～1180℃情况下,能够生产出抗压强度2350N/个的合格镁质球团矿,吨矿降低原料成本72.68元,具有显著的经济效益。     

Charging technology for modern electric-arc furnaces

Conclusion  Electric steelmaking is assuming an increasingly larger role in steel production, with the use of larger quantities of scrap substitutes and scrap of different levels of quality. Thus, the goal is to improve the economic and technical indices of the furnaces, despite the difficulties and limitations connected with the use of this new type of charge. In improving charging and melting operations and furnace equipment, certain basic requirements must be met:

Aspects of the smelting of high-vanadium pig iron

Conclusions The use of CVS in amounts up to 44 kg/ton made it possible to increase the vanadium content of the pig from 0.497 to 0.638% while keeping L_V at the usual level (80–82%). An increase in the percentage of CVS in the charge leads to a moderate increase in the manganese content of the pig. There is almost no change in the concentrations of the other elements, except for phosphorus. Converter slag introduces additional phosphorus into the blast furnace, resulting in an increase in the phosphorus content of the pig iron from 0.038 to 0.043%. The fact that the phosphorus content of the pig in isolated taps was greater than 0.05% can again be attributed to an increase in the amount of phosphorus introduced by the coke and pellets. The use of CVS makes it possible to moderately reduce the consumption of raw limestone in the blast-furnace charge. Chusovoi Metallurgical Plant, AMI Company, and the Institute of Metallurgy (in the Ural Division of the Russian Academy of Sciences) (Ekaterinburg). Translated from Metallurg, No. 7, pp. 18–20, July, 1999.     

Sintering Characteristics of Indian Chrome Ore Fines

Chrome ore concentrate consists of high-temperature melting oxides such as Cr₂O₃, MgO, and Al₂O₃. The presence of these refractory constituents makes the ore a very high melting mineral. Hence, it is difficult to produce sinter from chrome ore by a pyrometallurgical route. Currently, chrome ore is ground to below 75 μm, pelletized, heat hardened through carbothermic reaction at 1300 °C to 1400 °C, and then charged into a submerged electric arc furnace (EAF), along with lumpy ore for ferrochrome/charge-chrome production. Electricity is a major cost element in this extraction process. This work explores the sinterability of chrome ore. The objective of this study was to: (1) determine whether chrome ore is sinterable and, if so, (2) ascertain ways of achieving satisfactory properties at a low temperature of sintering. Sintering of the raw material feed could be a way to reduce electricity consumption, because during sintering a partial reduction of minerals is expected along with agglomeration. Studies carried out by the authors show that it is possible to agglomerate chrome ore fines through sintering. The chrome ore sinter thus produced was found to be inferior in strength, comparable to that of an iron ore sinter, but strength requirements may not be the same for both. Because the heat generation during chrome ore sintering is high owing to some exothermic reactions, compared with iron ore, and because chrome ore contains a high amount of fines, shallow-bed-depth sinter cake production was attempted in the laboratory-scale pot-sintering machine. The sintered product was found to be a good conductor of electricity because of the presence of phases such as magnetite and maghemite. This characteristic of the chrome ore sinter will subsequently have a favorable impact in terms of power consumption during the production of ferrochrome in a submerged EAF. The sinter made was melted in the arc furnace and it was found that the specific melting energy is comparable to that of heat-hardened chrome ore pellets but lower than briquettes and lump ore.     

Three-Layer Electrorefining of Silicon

To make electrical energy from photovoltaic (PV) silicon (Si) solar cells competitive, the cost in each of the PV manufacturing process steps has to be diminished. Today, high-purity Si is produced by an energy-intensive process exhibiting high irreversible thermodynamic losses. The purity of the product from this process (99,9999999 pct [9 N]) far exceeds what generally is accepted to be the requirements for PV purposes (4 to 6 N). Here we show a novel method for the purification of Si based on the principle of electrochemical refining in a molten high-melting-point fluoride electrolyte at temperatures above the melting point of silicon 1685 K (1412 °C). The method comprised a vertical stack of three molten layers with a metal alloy at the bottom, an intermediate electrolyte layer, and purified metal at the top. The integrity of the layers being secured was through the immiscibility of the liquids and the careful tailoring of the individual densities. Boron (B), exhibiting similar thermodynamic properties to Si, effectively was not removed. A suitable low-B feedstock may be identified in kerf from the sawing of mono- or multicrystalline Si blocks into wafers. To produce purified metal in the 6 N range, practice from electrorefining of aluminum shows that long-term, stable operation in large-scale industrial reactors is needed. The trends and mechanisms observed in the laboratory scale indicate that high purity also can be achieved for Si provided that these criteria can be met.     

Blast-furnace operation with coke fines

Conclusion Depending on the smelting conditions, the use of coke fines (36-25 mm) for up to 22% of the coke charged into a blast furnace having a standard bell-and-hopper charging appratus, led to a decrease in pig-iron output by 1.0–1.6% (or 0.2–0.4% for each 1% of coke fines), an increase in unit coke consumption by 0.8–1.0% (or by 1.0%-0.19-0.17%), and a deterioration in the drainage capacity of the hearth. The performance indices of the furnace equipped with a revolving distributor show that it is possible to force smelting while using up to 25% coke of the 36-25-mm fraction in the charge. If the coke in the charge consists of more than 30% fines, smelting rate decreases and hearth performance declines. These developments are accompanied by a 3.3–3.6% reduction in furnace productivity (0.12–0.13% for each 1.0% of the 36-25-mm fraction of coke) and a 0.85–0.90% increase in unit coke consumption (0.03–0.04% for each 1% of coke fines). West Siberian Metallurgical Combine. Translated from Metallurg, No. 1, pp. 38–39, January, 1999.     

Secondary Silicon Carbide Formed in the Interaction of Nanosized Silicon Carbide with Iron Oxide

The characteristics of superfine powder composites formed in the interaction of nanosized silicon carbide with iron oxide in vacuum and argon at 1200 and 1400°C, respectively, are analyzed. Silicon carbide (β-SiC), iron silicide and carbide, silicon oxide, and silicon oxynitride are main components of the powder composites. The lattice parameter of SiC in the powder composites synthesized in the SiC–Fe2O3 system is determined. In the interaction in the SiC–Fe2O3 powder mixture in vacuum, secondary SiC is synthesized with a lattice parameter that corresponds to the standard parameter for cubic β-SiC. The interaction in an argon atmosphere is accompanied by the synthesis of secondary SiC with a decreased lattice parameter. The minimum lattice parameter (0.4336 nm) is 0.6% smaller than the standard parameter for cubic β-SiC. The morphology of the powder composite synthesized in the SiC–Fe2O3 system is studied. The average particle size of the powder composite decreases with increasing weight content of secondary SiC.     

Study of a blast-furnace smelting technology which involves the injection of pulverized-coal fuel, natural gas, and an oxygen-enriched blast into the hearth

Studies were made of features of a blast-furnace smelting technology that involves the injection of natural gas (NG), oxygen (O₂) and pulverized-coal fuel (PCF) into the hearth. The technology has been implemented in the compensation and overcompensation regimes, which has made it possible to maintain or improve the gasdynamics of the furnace, the conditions for the reduction of iron oxides, the heating of the charge, and PCF combustion in the tuyere zone as PCF consumption is increased and coke use is decreased. Under the given conditions, with the blast having an oxygen content of 25.64–25.7%, the hearth injection of 131–138 kg PCF and 65–69 m³ NG for each ton of pig iron has made it possible to reduce coke consumption by 171–185 kg/ton pig (30.2–32.7%), reduce the consumption of comparison fuel by 36–37 kg/ton (5.2–5.3%), and lower the production cost of the pig iron by 43–49 hryvnas/ton (3.7–6.4%). Here, furnace productivity has increased 3.8–6.5%, while the quality of the conversion pig iron remains the same as before. Measures are being implemented to further increase the level and efficiency of PCF use. __________ Translated from Metallurg, No. 5, pp. 41–44, May, 2006.