Improvement of reaction efficiency in hot metal dephosphorisation

Abstract

In Japan, the hot metal pretreatment process has been developed to refine hot metal under conditions where each impurity can be removed most efficiently. At Nippon Steel, three types of hot metal pretreatment process, using torpedo car, hot metal ladle, or LD converter, are employed, that make a great contribution to the reduction of slag volume. Recently, Nippon Steel has developed a new hot metal pretreatment called the multirefining converter (MURC) process, in which dephosphorisation and decarburisation are carried out continuously in the same converter. Nevertheless, since the dephosphorisation efficiency of CaO is less than 30%, CaO is now being used in far greater quantities than that stoichiometrically required to make 3CaO.P₂O₅. As hot metal dephosphorisation is a non-equilibrium reaction, in which hot metal is in contact with slag whose oxygen activity differs greatly from that of hot metal, it is important to increase the interfacial oxygen activity. From the results of a fundamental experiment, the 2CaO.SiO solid phase, in which P₂O₅ can be dissolved, has a great effect on the dephosphorisation reaction.     

高磷铁矿处理及高磷铁水脱磷研究进展

介绍了高磷铁矿选矿脱磷和炼钢脱磷反应机理研究的最新进展,对生产低磷钢的各个环节如铁水预处理,转炉冶炼,钢水炉外脱磷尤其是炉外脱磷的渣系组成及脱磷工艺进行了详细的论述.探讨了高磷铁水炼钢的可行性.     

Recovery of metallic iron from high phosphorus oolitic hematite by carbothermic reduction and magnetic separation

Abstract

This study aims to provide theoretical and technical basis for economical and rational use of high phosphorus oolitic hematite. Following physical, chemical and microscopic characterisation of high phosphorus oolitic hematite ore the feasibility of separation of phosphorus and metallic iron by reduction roasting and magnetic separation process were investigated. The results indicate that such a process is a feasible and efficient method for iron and phosphorus separation of high phosphorus oolitic hematite. The recovery of metallic iron and dephosphorisation rate is relatively low without additives but is significantly improved by appropriate CaO and Na₂CO₃ addition. With 8%CaO and 3%Na₂CO₃ the recovery of metallic iron and dephosphorisation rate reach 95.1 and 94.0% respectively.     

260t转炉高磷铁水冶炼低磷钢生产实践

针对鞍钢铁水中ω([P])≥0.085%的高磷铁水冶炼低磷钢的难题,鞍钢炼钢总厂四工区通过调整铁水硅含量、优化供氧制度及温度控制度,采用高碱度单渣法、双渣法、复吹、出钢后炉外脱磷等冶炼工艺进行了低磷钢生产,实践结果表明,不同冶炼工艺对高磷铁水生产低磷钢均起到了有益的作用。     

Effect of MgO content in slag on dephosphorisation in converter steelmaking

Abstract

In this study, the effect of MgO content in slag on the phosphorus distribution ratios, and the equilibrium solubility of MgO in slag during converter steelmaking were analysed using the modified Kozheurov regular solution model and the thermodynamic package FactSage. Thermodynamic analysis shows that the MgO content in the final slag should be greater than 5% to saturate the slag with MgO and minimise the chemical wear of the refractory lining. However, the phosphorus distribution ratio decreases dramatically with increasing MgO. Industrial tests were performed using an 80-t top-and-bottom combined blown converter showed that the average phosphorus distribution ratio reduces significantly from 120.7 to 75.7 when the average MgO content in the final slag increases from 7.0 to 9.8%, due to thermodynamics and kinetics reasons. By optimizing the MgO content between 6 and 8% dephosphorisation and refractory wear are optimised.     

Production of Low-P Steel

Production of low phosphorus steel using the basic oxygen steelmaking process is conventionally achieved by maintaining a highly basic slag. This leads to high flux consumption, generates a large volume of slag and restricts recycling of the slag due to presence of free lime. The environmental impact goes further, since the calcined lime involves depletion of a mineral resource, consumption of fossil fuels and release of CO₂. However, recent studies indicate that adequate dephosphorisation is possible in the BOF even if the slag basicity is reduced from the current practice (3.3–3.6) to a level of 2.7–2.8. Phosphorus partitioning deviates considerably from equilibrium and post stirring of bath helps in lowering bath phosphorus. Induction furnace is widely used to produce structural grade steel. Since lining is usually acidic, effective phosphorus removal is not achieved. But using basic lining reasonably low phosphorus steel could be produced.     

Simulation of Steel Refining Process in Converter

Steel refining is a complex phenomenon which depends on numerous variables, so, a kinetic approach is necessary for precise understanding of the refining process. In this study, based on a previously proposed model for hot metal dephosphorization, a new simulation model for the steel refining process in BOF is presented. In most cases, steelmaking slag is saturated with dicalcium‐silicate (C₂S) and it is well known that C₂S forms solid solution with tricalcium‐phosphate (C₃P) in a wide composition range and the partition ratio of phosphorus between C₂S and liquid slag is large. On the other hand, C₂S formed around the lime surface is known as a barrier to lime dissolution into liquid slag. In this simulation model not only the effect of solid phase in slag is considered but also the effects of temperature dependence of variables as well as top and bottom blowing and scrap melting are taken into account. The calculation results are compared with industrial data and the good agreement between experimental and simulation results evidence the validity of this kinetic approach to steel refining process in BOF. Moreover, by using this model the influence of various parameters on the reaction efficiency is discussed.     

钒钛铁水冶炼提升废钢比的探索与实践

西昌钢钒有限公司高炉使用钒钛矿,铁水含钒钛较高,炼钢采用提钒-炼钢的双联工艺,提钒过程的冷却工艺削弱了热源优势,半钢碳低、硅低、锰低和温度低的特点又造成炼钢转炉热源不足,成渣难等问题,限制了提钒炉和炼钢炉废钢比的提升。通过对提钒炉、炼钢炉提高废钢比的工艺研究和探索,西昌钢钒成功将全流程的废钢比从2%～6%的水平提升至7%～10%水平,并保证了转炉的炉体状况和钢水洁净度,在钒钛铁水冶炼的企业中处于先进水平。     

Dephosphorisation of high phosphorus oolitic hematite by carbon composite pre-reduction and fast melting separation

High phosphorus oolitic hematite deposit is a kind of refractory iron ore resource of huge amount. At present, it is difficult to be utilised by traditional physical and chemical technology efficiently and economically. A novel process for utilisation of the high phosphorus oolitic hematite based on carbon composite pre-reduction and fast melting separation has been put forward in the paper. High grade pig iron nugget of low phosphorus could be obtained in the present research. The influence of experimental conditions, such as pre-reduction temperature, C/O (molar ratio) and basicity, on the dephosphorisation behaviours was studied in detail. The thermodynamic basis and reduction and melting separation process were also analysed. The phosphorus content in the iron nugget decreased with the increasing of basicity and increased with the increasing of C/O. The optimum parameters were pre-reduction temperature of 1200°C for 30?min, C/O of 0.95 and basicity of 1.7. After melting separation of molten iron and slag at 1400°C for 10?min, the iron nugget containing 0.02?wt-% [P] would be obtained. The dephosphorisation degree and iron yield in the form of iron nugget were 97.5% and 96.9% respectively. The iron nugget may be directly used as the raw materials of steelmaking from the view point of its high grade.     

基于投掷式探头的转炉终点自动控制技术的应用

利用投掷式探头模拟副枪进行钢水测量,同时改进静、动态模型,实现了100 t转炉无副枪、无烟气分析自动炼钢技术。应用结果表明,终点双命中率提高到90.82%,补吹率由投运前的平均28.3%减少到6.76%,金属消耗降低,吹炼过程平稳,喷溅减少。同时还建立了终点低磷的吹炼模型,转炉的脱磷率达90%以上,平均出钢磷质量分数控制在0.010%~0.015%左右。     

A review of phosphorus partition relations for use in basic oxygen steelmaking

Phosphorus removal in basic oxygen steelmaking is a significant problem for integrated steelmakers. Phosphorus removal is required due to its deleterious effect on the mechanical properties of steel. However, this is progressively becoming more difficult due to the increasing phosphorus content of many iron ores. Many studies have investigated dephosphorisation and published empirical phosphorus partition (L_P) equations for a range of conditions. The structure of these equations has been used to develop a new partition relation that allows the effect of minor slag constituents such as TiO₂, Al₂O₃ and V₂O₅ on steel dephosphorisation to be tested. Al₂O₃ was found to have a weak negative effect on the measured L_P, except at the lower oxygen potential range tested, where a positive correlation was observed. Increasing TiO₂ and V₂O₅ contents were found to decrease the measured L_P; however, these correlations became less prevalent at the higher oxygen potential ranges tested.     

A Thermodynamic Model of Phosphorus Distribution Ratio between CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5 Slags and Molten Steel during a Top–Bottom Combined Blown Converter Steelmaking Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the phosphorus distribution ratio between top–bottom combined blown converter steelmaking slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating mass action concentrations of structural units in the slags, i.e., CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ slags, based on the ion and molecule coexistence theory (IMCT). Not only the total phosphorus distribution ratio but also the respective phosphorus distribution ratio among four basic oxides as components, i.e., CaO, MgO, FeO, and MnO, in the slags and molten steel can be predicted theoretically by the developed IMCT phosphorus distribution ratio prediction model after knowing the oxygen activity of molten steel at the slag–metal interface or the Fe _t O activity in the slags and the related mass action concentrations of structural units or ion couples in the slags. The calculated mass action concentrations of structural units or ion couples in the slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than the mass percentage of components, can present the reaction ability of the components in the slags. The predicted total phosphorus distribution ratio by the developed IMCT model shows a reliable agreement with the measured phosphorus distribution ratio by using the calculated mass action concentrations of iron oxides as presentation of slag oxidation ability. Meanwhile, the developed thermodynamic model for calculating the phosphorus distribution ratio can determine quantitatively the respective dephosphorization contribution ratio of Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O in the slags. A significant difference of dephosphorization ability among Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O has been found as approximately 0.0 pct, 99.996 pct, 0.0 pct, and 0.0 pct during a combined blown converter steelmaking process, respectively. There is a great gradient of oxygen activity of molten steel at the slag–metal interface and in a metal bath when carbon content in a metal bath is larger than 0.036 pct. The phosphorus in molten steel beneath the slag–metal interface can be extracted effectively by the comprehensive effect of CaO and Fe _t O in slags to form 3CaO·P₂O₅ and 4CaO·P₂O₅ until the carbon content is less than 0.036 pct during a top–bottom combined blown steelmaking process.     

氧气转炉高磷铁水炼钢及鄂西高磷铁矿资源的利用

氧气转炉高磷铁水炼钢技术包括顶吹法（LD-AC、OLP）和底吹法（OBM、LWS）,这些方法都可以利用高磷  铁水生产出优质钢并得到磷肥（P₂O₅ 18%～20%）。中国鄂西高磷矿储量达16.9亿t,可以采用上述技术用于钢铁生产,对解决中国铁矿石不足,减少进口有重要作用。建议国家尽快开发利用鄂西高磷矿资源。     

利用数学模型分析复吹转炉吹炼工艺

在冶金热力学和动力学及反应工程学原理的基础上，建立了模拟复吹转炉冶炼过程的数学模型．在用梅钢150t复吹转炉工业试验的结果对模型加以验证之后，利用模型对具有中高磷铁水特点的梅钢转炉吹炼工艺进行了分析讨论．     

K-OBM-S转炉冶炼不锈钢过程的数学模拟和应用

K-OBM-S转炉是以铁水和电弧炉预熔钢液为原料冶炼不锈钢的精炼设备。以转炉冶炼普碳钢的顶吹模型和AOD法冶炼不锈钢模型为基础,建立了适用于80 t K-OBM-S转炉冶炼不锈钢的数学模型。对二步法冶炼2Cr13型不锈钢和三步法冶炼0Cr18Ni9型不锈钢的过程验证结果表明,大部分终点碳含量的误差≤±0.03%,终点铬含量误差≤±0.3%,110炉0Cr18Ni9钢目标碳(0.10%～0.25%)命中率为95.6%,终点目标铬(17.1%17.6%)的命中率为85.2%。     

Effects of BOF top blowing and bottom stirring conditions on suppressing excessive oxidation

Abstract

The effects of top blowing and bottom stirring conditions during steelmaking in a 6 t basic oxygen furnace (BOF) were investigated in studies with the aim of suppressing excessive oxidation. With low oxygen feed rate and high stirring energy, the apparent partial pressure of CO P′_CO was calculated from the equilibrium of carbon and oxygen in molten steel as being <1 atm. The relationship between top blowing/bottom stirring conditions and mass transfer at the slag/metal interface was analysed. It is proposed that mass transfer at the hot spot is significantly affected by the reaching rate of oxygen to the steel bath and bottom stirring. Mass transfer at the slag/metal interface, outside the hot spot, is sufficient to allow equilibrium to be attained in combined blowing BOF processes. Thus, the oxygen that is not consumed for decarburisation is distributed between steel and slag, i.e. deoxidation from steel to slag takes place, which makes it possible to obtain P′_CO <1 atm under atmospheric conditions. The decarburisation model developed based on the analysis reproduces the suppression of excessive oxidation under a decreased, top blown oxygen feed rate and is in good agreement with results from both 6 t BOF experiments and 350 t commercial BOF operation.     

Concept of dynamic foaming index and its application to control of slag foaming in electric arc furnace steelmaking

Abstract

To sustain a foam in steelmaking processes, two basic requirements should be fulfilled, i.e. appropriate physical properties of the slag such as high viscosity, low density, and low superficial tension, and the generation of sufficient reaction gas. To date, foaming indexes have been focused on the physical properties of refining slags. In the present paper a dynamic foaming index (DFI) that involves both above requirements is proposed, using a kinetic model of the electric arc furnace process to calculate the generation rate of reaction gas, mainly CO. When the arc distortion, as affected by electrode submergence in the foam, is compared with the DFI, calculated via the kinetic model, it is observed that both parameters follow very similar trends. This finding indicates the feasibility of knowing the foaming conditions of a heat in advance, or of using the kinetic model online to control the foaming phenomena. Furthermore, experimental results relating to dynamic behaviour of the slag chemistry are well simulated using the kinetic model. To take into account the effect of size distribution of carbon particles injected into the slag to reduce FeO, a Monte Carlo simulation has been integrated into the process simulator, allowing a more realistic prediction of the current steelmaking process.     

Thermal physical properties and dephosphorisation kinetics of rare earth oxides containing slags

ABSTRACT

The melting properties and viscosity of CaO–SiO₂–MnO–La₂O₃–CeO₂ slags, with mass ratios of 0.97 and 1.05 between CaO and SiO₂, were measured using the hot stage microscopy method and rotating cylinder method, respectively. In addition, the dephosphorisation kinetics of rare earth (RE) oxides containing slags was studied. For increasing mass contents of the RE oxides, La₂O₃ and CeO₂ (0, 3, 6, 9, and 12?wt.%), in the slag, the melting temperature initially decreases and then increases. Minimum values appeared for mass contents of 6?wt.%. RE oxide concentrations below 9?wt.% are beneficial to the decrease in viscosity, which increases significantly with the mass fraction of RE oxides in molten slags with a basicity of 1.05. In slags with a basicity of 0.97, the RE oxides favour the decrease in viscosity, which does not change noticeably as their mass fraction is increased above 6?wt.%. Finally, it was found that the rate-limiting step of the dephosphorisation process is the diffusion of phosphorus.     

转炉铁水预处理脱磷的影响因素

 基于炉外铁水深度预脱硫+转炉铁水预脱磷的铁水预处理工艺是当今低磷或超低磷钢冶炼的重要工艺平台,其中转炉铁水预处理脱磷是关键的技术环节。以国内“双联转炉炼钢法”预脱磷炉实践为出发点,在实验室高温炉上通过顶加脱磷剂、浸入吹氧进行了铁水模拟转炉预脱磷影响因素的试验研究,比较了铁水温度、铁水初始硅质量分数w(Si)_i、脱磷渣碱度、供氧制度、搅拌强度、萤石加入量对脱磷效率的影响。结果表明,各因素对脱磷率影响的顺序为铁水温度＞w(Si)_i＞供氧制度＞脱磷渣碱度、搅拌强度＞萤石加入量;适宜的工艺参数为铁水温度为1 300 ℃,w(Si)_i 为0.10%～0.26%或低于0.30%,脱磷渣碱度为2.9～3.0,供氧制度中气氧与固氧各占50%或固氧稍偏多,维持较高的搅拌强度;转炉内铁水预脱磷处理可不加萤石。