Top gas recycling blast furnace developments for ‘green’ and sustainable ironmaking

Abstract

The ultralow CO₂ steelmaking blast furnace process (ULCOS-BF) aims at minimising the CO₂ emissions of the BF by at least 50%. This process is based on the replacement of hot blast by oxygen, the recycling of hot decarbonated top gas into the lower shaft and normal hearth tuyeres, and the capture of CO₂ and its storage in a geological trap (full CO₂ capture and storage process). The paper highlights the main technologies of this process and the expected benefits for CO₂ mitigation. The ULCOS-BF has been demonstrated during three campaigns of 7 weeks each by coupling the LKAB experimental BF in Luleå to a pilot vacuum pressure swing absorption unit for CO₂ removal. The concept, preparation and results of the campaigns are described.     

Optimisation study of ironmaking using biomass

Abstract

The economic advantage of using biomass as partial substitute for fossil reductants in the blast furnace (BF) process was studied by simulation. A thermodynamic model of the BF was used in combination with simple models of the coke plant, sinter plant, hot stoves, basic oxygen furnace and power plant. Pretreatment of the biomass before its injection in the BF is considered in a pyrolysis unit where the carbon content and heating value are raised and the oxygen content is lowered, which is beneficial for the BF. The system was optimised with respect to the price of raw steel, considering costs of raw materials, energy and CO₂ emissions of the unit processes. The study demonstrates that biomass in partially pyrolysed form is a potential auxiliary reductant and that the optimal states of operation within certain regions depend strongly on the price structure of the raw materials and emissions.     

Study of the Effect of Hellenic Diasporic Bauxite Addition on Blast Furnace Operations

The blast furnace (BF) operation, as well as the steel‐shop downstream, can derive benefits from adjusting the slag alumina content with the proper addition of bauxite flux to its burden. Additions of 5‐15 kg of bauxite/t hot metal lead to a raise of the alumina content of the resultant BF‐slag from 10% to the level of about 12%. In Hellas extensive deposits of diasporic bauxite exist which, however, are under‐utilized by the alumina refineries due to their rather low solubility during the Bayer‐process. The above BF application, therefore, as an alternative use of the diasporic bauxite, is of great interest to the bauxite producers of Hellas. The influence of the bauxite additions on the metal/slag reactions under BF‐conditions was simulated in the laboratory by smelting BF‐slag with diasporic bauxite additions in a graphite crucible. It was achieved the production of BF‐slags with higher alumina contents under the condition of constant slag basicity. The results show the beneficial effect of the bauxite addition to the quality of the produced hot metal. In accordance to its desulphurization and desiliconization as well as the increase of BF‐slag sulphur capacity and fluidity, a thermodynamic evaluation of the metal/slag reaction in respect of SiO₂ reduction and S‐transfer to the slag in relation to its Al₂O₃ content was carried out. This evaluation led to simple algebraic functions and graphs easily applicable by the BF operators. The attained metallurgical results were verified in comparison to industrial test heats with bauxite additions in the BF‐burden in USA, Canada and Europe.     

Application and analysis of slag holdup model in BF deadman

The operation of blast furnace is considerably influenced by slag performance, particularly in BF hearth. Slag holdup in deadman has negative effects on hearth, such as sharp peripheral flow of hot metal and carbon brick erosion. Therefore, it is urgent to monitor the slag holdup. In this paper, a slag holdup model was proposed to give a transient snapshot of the internal state of BF. Based on the analysis of practical example of Jingtang (Hebei, China) No. 1 blast furnace, three typical models and the slag holdup model were compared. Furthermore, the effects of slag basicity(CaO/SiO₂), MgO/Al₂O₃ ratio, temperature of hot metal, diameter of coke, shape factor of coke and voidage of deadman on slag holdup were investigated. The results show that current slag holdup model can timely predict the general trend of the permeability of the deadman and internal state of hearth. In addition, the influence extent of the six factors which affect the slag holdup is in the following order (from large to small effect): the voidage of deadman, shape factor of coke, diameter of coke, basicity(CaO/SiO₂), MgO/Al₂O₃ ratio, temperature of hot metal. The weighting ratio of these factors is 1:0.92:0.69:0.69:0.38:0.23.     

煤制气直接还原铁两段串联流程估算

为了降低直接还原铁能耗,根据试验数据研究了煤制气直接还原铁两段串联流程。串联流程中第一段竖炉用煤制气粗煤气余热和含碳球团冶炼直接还原铁,含碳球团以焦粉、半焦粉或无烟煤粉为还原剂,铁精矿、无机黏结剂混合后加压制作,电炉熔化直接还原、脱硫和生产水渣。串联流程中第二段竖炉以第一段净化后的炉顶煤气为第二段直接还原铁还原气,以氧化球团为原料。结果表明,煤制气直接还原铁两段串联流程估算能耗为394.8kg/t;与铁水比可比能耗为487.8kg/t,比高炉低41.2kg/t,生产过程中产生的污染物和温室气体排放低于高炉,接近天然气直接还原铁。     

Research on characteristics and modelling estimation of molten BF slag viscosity in the process of slag waste heat recovery

The object of this study was to explore the effects of basicity (CaO/SiO₂ ratio), MgO and Al₂O₃ on viscosity and melting temperature of molten slag in the process of coal gasification with blast furnace (BF) slag as heat carrier to recover the waste heat. The results showed that the viscosity and melting temperature of BF slag decreased first and then increased with CaO/SiO₂ ratio increasing, and the suitable CaO/SiO₂ ratio was 1.10–1.20. Both viscosity and melting temperature increased with the increasing of MgO and Al₂O₃ in the slag, which should be lower than 8.22 and 11.00%, respectively. What is more, the viscosity estimation model for molten BF slag was established according to the Urbain model and experimental data of slag viscosity. In the model, the activation energy was calculated using slag compositions based on the redefine of material types in the slag, and the temperature was described by the Weymann–Frenkel equation. The viscosity of BF slag system estimated by proposed viscosity estimation model (modified Urbain model) fitted well with the experimental data and the mean deviation was about 17.98%.     

Numerical simulation on novel blast furnace operation of combining coke oven gas injection with hot burden charging

A novel blast furnace operation of coke oven gas (COG) injection simultaneously with hot burden charging has been proposed to solve the problem of insufficient heat in the BF shaft zone under the condition of COG injection and make full use of the abundant sensible heat of high temperature burden. In this paper, the novel process has been simulated with a multifluid blast furnace model. The results show that, in comparison with the operation of COG injection only, under the operation of COG injection together with hot burden charging, the temperature in the upper zone of the shaft increases while that in lower zone decreases. Furthermore, the reduction of iron bearing material is improved in the top zone, and the cohesive zone tends to descend and narrow. The coke ratio, fuel ratio and CO₂ emissions of the operation of charging hot pellet and coke with the temperature of 800°C are decreased by 4.0, 4.7 and 5.3% respectively, while the hot metal productivity is increased by 7.14%. Therefore, COG injection combined with hot burden charging operation not only increases temperature in the upper part of the blast furnace but also decreases energy consumption per tonne hot metal.     

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.     

Effect of composition on the crystallisation behaviour of blast furnace slag using single hot thermocouple technique

When the content of glass in blast furnace slag is over 95%, it can be used as a raw material in the manufacture of cement. The critical cooling rate required for the formation of glassy slag is one of the important characteristics for molten BF slag. The crystallisation behaviour of molten BF slag has been studied by in situ observation with the single hot thermocouple technique. The isothermal and non-isothermal experiments were performed to construct time–temperature-transformation and continuous-cooling-transformation diagrams. The effect of MgO, Al₂O₃ and binary basicity (CaO/SiO₂) on the critical cooling rate of the CaO–SiO₂–MgO–Al₂O₃ slags were studied under conditions of CaO/SiO₂?=?1.1–1.4, 6–12?mass% MgO and 10–16?mass% Al₂O₃. The following finding are reported in the present paper: (i) Higher MgO content increased the critical cooling rate; higher Al₂O₃ content decreased the critical cooling rate; higher CaO/SiO₂ increased the critical cooling rate. (ii) The crystallisation temperature of molten BF slag lowers as the cooling rate increases, the slag have larger critical cooling rate, higher crystallisation temperature. The results could be used to design proper cooling path of molten BF slag for the formation of glassy.     

Crystallization behavior of blast furnace slag modified by adding iron ore tailing

Blast furnace(BF)slag is a by-product of the ironmaking process and could be utilized to manufacture slag fiber by adding iron ore tailing.The crystallization behavior of the modified BF slag is significant to the fibrosis process.To investigate the influence of basicity on the crystallization behavior,BF slag was modified by adding iron ore tailing at room temperature and melted at 1 500°C.FactSage simulation,X-ray diffraction,scanning electron microscopy backscattered electron imaging coupled to an energy dispersive spectrometer,and hot thermocouple technique analysis were performed to explore the crystallization behavior of the modified BF slag during the cooling process.It was found that the initial crystallization temperature increased with the increase in basicity.Melilite,anorthite,clinopyroxene,and wollastonite could be generated during the cooling process as basicity ranged from 0.7 to 0.9.Spinel could be found as one of the phases;however,wollastonite disappeared under a basicity of 1.0.The initial crystallization temperature was controlled by the crystallization of melilite during the cooling process when the basicity of the modified BF slag ranged from 0.7 to 1.0.Moreover,the cooling rate could also influence the crystallization of the modified BF slag.     

首钢京唐炼铁余热余能回收及潜力

京唐炼铁余热余能占炼铁工序能耗的60%左右,分布于热风炉、高炉煤气除尘、炉前除尘、渣处理和高炉本体冷却水等系统。重点分析现有工艺技术流程,通过高炉煤气回收、干式TRT和热风炉烟气预热空煤气及制粉三项利用技术,已实现炼铁主要余热余能回收80.8%,指出热风炉烟气和高炉煤气物理显热利用率仅为30%～40%,还有待进一步提高。同时,以末端温度为基础分析了各项低品位余热潜力尚有65.9kgce/t,并提出有效利用放散高炉煤气、热风炉烟气和冲渣水余热等措施和建议,为余热梯级回收和合理高效利用提供依据。     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

Slag-foaming phenomenon originating from reaction of titanium-bearing blast furnace slag: effects of TiO2 content and basicity

ABSTRACT

The foaming behaviour originating from the reduction of iron oxide in molten CaO–SiO₂–MgO–Al₂O₃–TiO₂ slag was studied at 1500°C with the aid of a real-time foaming process monitoring system. The effect of TiO₂ content and binary basicity on the slag foaming were investigated. It was found that the TiO₂ content has a significant influence on the foaming degree, while the basicity of the slag has a smaller influence. The foam-generation time, foam duration, and foam-elimination time greatly increased with increasing TiO₂ content, while they slightly increased with increasing basicity. Furthermore, the source of gas and the effect of physical properties of the slag on the foaming behaviour were also discussed. This study can provide a guideline for the utilisation of high-ratio titanium-bearing magnetite ore in the blast-furnace iron-making process, which is seriously affected by the slag-foaming problem.     

Development of Slag Recycling Process in Hot Metal Desulfurization with Mechanical Stirring

In order to develop an environment‐friendly steelmaking process, a slag recycling process for hot metal desulfurization by mechanical stirring was designed. The process was developed with 70 kg‐scale hot metal experiments and actual plant tests. The recycled slag has a 70% desulfurization ability compared with that of virgin flux (CaO‐5%CaF₂). The lower efficiency of the recycled slag was caused by SiO₂ contamination carried over from the previous process. There is no particular size requirement for the recycled slag, as the effect of the recycled slag size on the desulfurization ability is small. The ratio of CaO in the recycled slag to total CaO should be less than 60% in order to prevent an increase in the amount of slag. Slag recycling operation can be repeated more than twice when the optimum conditions are applied. The slag recycling process was established in an industrial operation, and consumption of desulfurization flux decreased by 40% with the process compared with that without slag recycling. Slag hot recycling was adopted at another plant where consumption of desulfurization flux decreased by 50% compared to operation without slag recycling. The positive effect of hot slag recycling is estimated to be a result of the temperature of the recycled slag.     

Research on slag modifying agents for CaO–Mg based hot metal desulphurisation

Abstract

To improve the slag/iron separation problems in CaO–Mg based hot metal desulphurisation, experiments were carried out to generate a more fluid slag and to reduce the amount of hot iron entrapped within the desulphurising slag. The optimised slag modifying agent effectively decreased the melting temperature and viscosity of CaO–Mg based desulphurising slag. The optimised modifying agent has been successfully applied to industrial production. Industrial tests show that the average desulphurisation exceeded 82% when using the 80CaO–15Mg–5CaF₂ desulphurising agent at 3·5–4·0 kg t^–1, and with an average final S of 0·005%. The skimming time decreased from nearly 12 min to ～6·5 min. The heat loss during desulphurisation decreased to ～19°C which is favourable to the subsequent steelmaking process. Average total Fe content in post-desulphurisation slag is 34% with 0·7–0·8 kg t^–1 modifying agent, a decrease of ～30% in iron loss compared with current status.     

Co2 Accounting and Abatement: An Approach for Iron and Steel Industry

The issue of energy use in iron and steel making operations has been focused by several authors. Since current operations use predominantly fossil fuel based carbon bearing inputs, energy use is related to CO₂ emissions which is a Green House Gas. Abatement of CO₂ emissions requires a proper CO₂ accounting procedure to be established for an existing operation. Varied approaches have been used, but information on mass and energy balance for individual process steps and the plant as a whole have scarcely been correlated to emission patterns for individual process steps as well as the total plant. In the accounting procedure suggested, the first step is to establish a carbon balance encompassing input carbon, fuel gases generated and solid/liquid fuel produced. Once the quantities of by-product fuel gases are established through carbon accounting, consumption route of these gases are examined using a carbon flux diagram, to ascertain which process steps emit maximum CO₂. Predictive models can be used to work out downstream fuel gas streams requirements as well as surplus generated. Use of analytical models for reduction of CO₂ emissions at source can be combined with an objective of overall cost reduction objective for evolving implementation strategies without sequestration. Greater CO₂ abatement can be obtained through use of carbon capture and sequestration, as applied to steel mills. An example case for sequestering the stove flue gas of a conventional blast furnace using an amine system for capture combined with slag sequestration is discussed. Use of a ‘top gas recycle system’ with CO₂ separation using VPSA is shown to lead to greater degree of abatement. The emission allocation issues, related to Life Cycle Analysis are important for arriving at the total emission pattern. Implementing carbon abatement requires additional auxiliary technology development, such as recovery of low grade heat, for supporting energy requirements associated with CO₂ separation.     

Cost and Life Cycle Analysis for Deep CO2 Emissions Reduction for Steel Making: Direct Reduced Iron Technologies

Among heavy industrial sectors worldwide, the steel industry ranks first in carbon dioxide (CO₂) emissions. Technologies that produce direct reduced iron (DRI) enable the industry to reduce emissions or even approach net-zero CO₂ emissions for steel production. Herein, comprehensive cradle-to-gate (CTG) life cycle analysis (LCA) and techno-economic analysis (TEA) are used to evaluate the CO₂ emissions of three DRI technologies. Compared to the baseline of blast furnace and basic oxygen furnace (BF–BOF) technology for steel making, using natural gas (NG) to produce DRI has the potential to reduce CTG CO₂ emissions by 33%. When 83% or 100% renewable H₂ is used for DRI production, DRI technologies can potentially reduce CO₂ emissions by 57% and 67%, respectively, compared to baseline BF–BOF technology. However, the renewable H₂ application for DRI increases the levelized cost of steel (LCOS). When renewable natural gas (RNG) and clean electricity are used for steel production, the CTG CO₂ emissions of all the DRI technologies can potentially be reduced by more than 90% compared to the baseline BF–BOF technology, although the LCOS depends largely on the cost of RNG and clean electricity.     

A new method for evaluating cooling capacity of blast furnace cooling stave

The detailed process of the heat transfer of the cooling stave in blast furnace (BF) has been systematically analyzed and the simplified mathematical model was constructed based on heat transfer theory. Precise definitions of the cooling capacity, stable working slag thickness and safe working slag thickness were put forward so as to evaluate the cooling capacity of cooling stave systematically. The results show that 95% of heat is carried off by cooling water through convection and the heat taken away through convective heat transfer between furnace shell and atmosphere only account for 5%. The entire heat transfer process can be divided into four modules and the cooling system is divided into three parts. The cooling capacity φ is defined and function curve of temperature of cooling stave hot surface T_b with changes of brick thickness is drawn and the safe working area and stable working area are put forward.     

X-ray observation of phosphorus vaporization from steelmaking slag and suppression method of phosphorus reversion in liquid iron

The behavior of phosphorus in CaO-SiO₂-Al₂O₃-MgO-P₂O₅-Fe _t O slag systems during solid Al and carbon deoxidation was investigated at 1873 K. Furthermore, an X-ray apparatus (vertical resistancetube furnace equipped with an X-ray inspector system, which enhances the real-time observation of the generated gas in slag) was introduced to confirm the possibility of the vaporizing phenomena of phosphorus from slag. The X-ray-assisted observations proved that phosphorus gas is produced from P₂O₅-containing slag during the deoxidation process and can be applied to suppress the reversion of phosphorus into the liquid iron by removing it into the air. The results of experiments between slag and metal showed that Al decreased the Fe _t O and P₂O₅ contents in slag simultaneously because of the strong reducing power, but carbon decreased only the Fe _t O content in slag to a certain extent without reducing the P₂O₅. For the prevention of the phosphorus-content increment in liquid iron during the deoxidation process, it was ascertained that the Fe _t O content, the absorption site of phosphorus gas, should be decreased to some extent at the time of phosphorus generation. It could be proposed that it is the two-step deoxidation process that decreases the Fe _t O content by carbon while maintaining the P₂O₅ content in the slag at a nearly constant level and then decreases the remaining P₂O₅ content quickly by vaporizing as a gas phase by Al, without considerable reversion of the phosphorus in hot metal.     

Studies on dissolution kinetics of dolime in electrothermal magnesium slag

The electrothermal process of magnesium metal production is a promising route, where large sized internally heated reactor is used for magnesium production resulting in less energy and labour intensive and high space-time yield process. However, the dissolution behavior of dolime in the electrothermal slag has been found critical for the process optimization. In this paper, the dissolution kinetics of the dolime in the slag was discussed. Quaternary slag (CaO-Al₂O₃-SiO₂-MgO) was prepared having basicity CaO/SiO₂ ≥ 1.8 and Al₂O₃/SiO₂ ≥ 0.26 for dolime dissolution studies by static hot dip method. Prior to the experiments, FactSage calculations were carried out varying temperatures and slag compositions. In the kinetic studies, dolime particles 10–15 mm size was added in slag melted at 1450, 1500 and 1550°C and samples were taken at various time intervals. The chemical analysis of slag sample was carried out to investigate the dissolution kinetics to establish the rate expression. The activation energy for the process was calculated for different models used in study and was found to be in the range of 130–270 kJ/mol. SEM analysis was done for surface analysis of reacted particles. This study would be helpful in optimizing the dolime charging rate during pilot scale trials for electrothermal magnesium production at CSIR-NML, Jamshedpur.