Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

EAF Stainless Steel Refining ‐ Part II: Microstructural Slag Evolution and its Implications for Slag Foaming and Chromium Recovery

The microstructural evolution of the slag during the electric arc furnace (EAF) austenitic stainless steelmaking was investigated with respect to its effect on chromium recovery and slag foaming. Two distinct EAF types were followed up: (1) an eccentric bottom tapping furnace (EBTF) and (2) a spout tapping furnace (STF). Slag samples were collected from 36 industrial heats at three (EBTF) or five (STF) distinct moments in the process. The microstructure of the slag samples was characterised with electron probe microanalysis using energy dispersive spectroscopy (EPMA‐EDS) and X‐ray diffraction (XRD). From the microstructural analysis it is concluded that at the processing temperature the slag consists of a liquid slag matrix, two different types of metallic particles and MgO·(Al,Cr)₂O₃‐based spinel particles. The evolution of the microstructure is highlighted, the interactions between the phases are discussed and the effects of changing conditions on the slag microstructure are illustrated by thermodynamic calculations. Special attention is given to the effect of slag basicity on the activity of chromium oxide in the slag. It is shown that a thorough study of the microstructural evolution of the slag is instrumental to understanding and improving slag foaming and chromium recovery.     

An Experimental and Computational Study on the Melting Behaviour of AOD and Chromium Converter Slags

The campaigns of the AOD and chromium converters are often limited by the strong wear of the refractory materials in certain areas (usually tuyere zone). Slag protection (in which the slag is left to solidify at the converter walls after the tapping of the metal) has been considered to be an option in prolonging the campaigns of the converters. In order to study the possibilities of the slag protection, the purpose of this work was to determine the melting behaviour of the AOD and chromium converter slags; i. e. the solidus and liquidus temperatures and the composition of solid phases as a function of temperature in different slag compositions. Examination was executed using both computational (FactSage software) and experimental (optical dilatometer/SEM) methods. It was noticed that when burned dolomite was added into the slags, the increased MgO‐contents had a strong influence on the melting behaviour. Even small changes (a few percent) in the MgO‐content had a very strong influence (a few hundred degrees centigrade) on the solidus and liquidus temperatures. Solid phases that are formed during the solidification are also different in slags with and without burned dolomite addition.     

铬盐清洁工艺中含铬铁渣回收钾及资源化处理

研究铬盐清洁新工艺产生的含铬铁渣在酸性条件下浆化回收钾的过程,最终得到富铁渣及含钾滤液,富铁渣可用作炼铁原料,含钾滤液进一步处理后可制备相应产品。考察pH值、温度、液固比、反应时间对脱钾率的影响,优化工艺条件。结果表明,在pH值3.5～4.5,反应温度100℃,液固比为7∶1,反应时间90 min条件下,可脱除铁渣中80%以上的钾,反应条件较温和,所选择的酸性脱钾剂为硫酸亚铁,其中Fe2+对铁渣中的微量Cr6+具有还原解毒作用,有利于工业实施。     

Enrichment of Fe‐Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from BOF Slags

Steel slag normally contains a large amount of iron and its oxides. Therefore, it is a potential renewable resource in case of inadequate iron ore supply. To recover the metals from steel slag, two types of BOF slags were remelted at 1873 K. The liquid slags were cooled using four types of cooling conditions, namely, water granulation, splashing, air cooling, and furnace cooling, to investigate the influence of cooling rate on mineral components, especially the enrichment behavior of Fe‐containing minerals. Subsequently, wet magnetic separation was conducted to examine the relations between iron recovery ratio and cooling conditions. The results show that the slags under the four cooling conditions mainly contained dicalcium silicate, RO phase, Fe_tO, 2CaO(Fe,Al)₂O₃, and calcium ferrite. However, tricalcium silicate, 12CaO·7Al₂O₃, M‐A spinel, and free CaO and MgO were occasionally observed. The amount of glass matrix decreased, the Fe‐containing minerals increased, and the minerals more fully crystallized when the cooling rate of the liquid slag was decreased. From granulation to furnace cooling of the slags, the iron content in the recovered concentrate and the iron recovery ratio both increased. This result is in agreement with the findings on phase transformation through SEM analysis.     

EAF Stainless Steel Refining ‐ Part I: Observational Study on Chromium Recovery in an Eccentric Bottom Tapping Furnace and a Spout Tapping Furnace

An observational study of the electric arc furnace (EAF) process for austenitic stainless steel was performed. A comparison was made between two distinct EAF types: (1) an eccentric bottom tapping furnace (EBTF) and (2) a spout tapping furnace (STF). In order to study the slag evolution during the EAF process, per heat several slag samples were collected at consecutive process stages. They were subjected to electron probe microanalysis using energy dispersive spectroscopy (EPMA‐EDS). Compositional and mineralogical data from 36 heats corroborate that both thermodynamic and kinetic conditions exert a strong influence on the final chromium oxide content of the slag. In the STF, chromium oxide reduction predominantly occurs during tapping, owing to the intimate mixing of steel and slag. A multivariate linear regression analysis reveals that the main parameters determining the overall chromium recovery are the slag basicity and the content of dissolved silicon of the steel. These parameters explain 70% of the observed variance in final chromium oxide levels and can be used as control parameters to improve the chromium recovery. For the EBTF, lower chromium recoveries are recorded due to the absence of sufficient mixing during tapping.     

Application of an Off‐Gas Analysing System to Control Oxidation during Stainless Steelmaking in an EAF

During stainless steelmaking in the electric arc furnace at Deutsche Edelstahlwerke GmbH, oxygen is injected to oxidize unwanted tramp elements mainly carbon and silicon. Unfortunately the injected oxygen also oxidizes valuable elements such as chromium and iron, which causes an economical loss and has a negative environmental effect. Since the temperature and dilution techniques to minimise chromium oxidation are seldom applied in the electric arc furnace, a new strategy to minimise chromium oxidation has to be developed. This paper proposes a new strategy which involves the use of a continuous off‐gas analysing system to minimise chromium oxidation by monitoring the oxidation products in the off‐gas, i.e. CO and CO₂. During stainless steelmaking in the electric arc furnace, for which initial carbon and silicon input cannot be precisely known due to imprecise scrap analysis, the installed off‐gas analysing system should provide precise information concerning an efficient oxygen injection. This would then directly prevents excess chromium and iron oxidation. A continuous off‐gas analysing system installed at Deutsche Edelstahlwerke GmbH delivers a promising result for future applications. During the plant trial, the efficiency of oxygen injection as well as the chromium and iron yields were increased.     

高炉冲渣水余热回收应用

高炉冲渣水作为一种低温废热源,具有温度稳定、流量大、热容量大的特点,充分回收利用高炉冲渣水余热可以为企业带来可观的经济效益,其环境效益和社会效益显著。     

The Activities of FexO in {CaO‐SiO2‐Al2O3‐MgO‐FexO} Slags at 1723 K

In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO‐SiO₂‐Al₂O₃‐MgO} slags, and this research revealed that those slags at 35 wt%‐Al₂O₃ would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of Fe_xO were determined by using solid‐state electrochemical cells incorporating MgO‐stabilized zirconia and Mo + MoO₂ reference electrode.     

废钨回收中钨酸钠结晶料的除铬处理研究与实践

针对碳酸钠熔炼法回收废钨过程中因钨酸钠结晶料溶解液中铬含量高而造成后续工艺分相困难这一问题,通过工艺原理分析及对比试验,进行了在硫化除钼时同时除铬的研究,并确定了相关工艺的优化条件。结果表明,当硫化钠添加量为理论量的4倍时,控制pH=8～9,并选择合适的温度和陈化时间,除铬效果良好,可满足后续萃取工艺的生产,且产品APT质量达到GB/T 10116—2007中APT-0级的要求。     

Effect of Titanium Addition on As Cast Structure and Macrosegregation of High‐Carbon High‐Chromium Steel

In casting heavy ingots of high‐chromium high‐carbon cold work steels, macrosegregation develops in the center of the ingot, causing difficulties during subsequent hot working. Heat transfer and solidification of an industrial scale high‐carbon high‐chromium steel ingot was simulated and thereafter a laboratory scale representative ingot was designed to model the solidification of the industrial scale ingot. Titanium in the range of 0.3–1% was added to the high‐chromium high‐carbon (12%Cr–2%C) steel during melting process. Microstructures, macrosegregation and phase formations were studied using optical microscopy, scanning electron microscopy, energy dispersive X‐ray spectrometry, wave dispersive X‐ray spectrometry, optical emission spectroscopy, and X‐ray diffraction. Addition of 0.3% titanium was sufficient to diminish the macrosegregation; however it did not have a significant effect on the grain size. Addition of 0.7 and 1% titanium had a substantial effect on grain size in the longitudinal direction and refined the primary carbides structure. The formation of small TiC carbides that precipitated before solidification of liquid iron acted as nuclei for primary pro‐eutectic austenite grains.     

Galvannealing of (High‐)Manganese‐Alloyed TRIP‐ and X‐IP®‐Steel

In this study the influence of Mn on galvannealed coatings of 1.7% Mn‐1.5% Al TRIP‐ and 23% Mn X‐IP®‐steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of ‐25 °C (5% H₂) hamper the Fe/Zn‐reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface‐reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X‐IP®‐steel which was bright annealed at 1100 °C for 60 s at DP ‐50 °C (5% H₂) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ‐phase at lower temperatures by participating in the Fe‐Zn‐phase reactions, it was expected that the metallic Mn of the X‐IP®‐steel increases the Fe/Zn‐reactivity in the same manner. The approximation of the effective diffusion coefficient (D_eff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn‐reaction was found by microscopic investigations. Residual η‐ and ζ‐phase fractions prove a hampered Fe/Zn‐reaction. As explanation for the observed hampered Fe/Zn‐reaction the lower Fe‐content of the high‐Mn‐alloyed X‐IP®‐steel was suggested as the dominating factor for galvannealing.     

水淬铜渣还原熔分回收粒铁和铜

采用煤基还原熔分技术研究了熔分温度、熔分时间、氟化钙配比,煤粉配比对铜渣熔分后铁、铜回收率的影响,并运用 XRD及 SEM等分析手段对熔分产物进行了详细的研究。结果表明,在1380~1450℃之间进行熔分,通过对铁、铜的综合收得率计算,熔分温度应该控制在1430℃,在此基础上确定最佳熔分时间为20 min,此时金属铜的回收率达到最高,为90％左右。配入一定比例的煤粉和氟化钙均能提高铁的回收率,尤其以添加氟化钙的效果最好,适宜的添加比例不应超过2％(质量分数)。对分离后的渣铁分别进行表征分析,表明粒铁中的主要组织为铁素体和浮氏体,炉渣主要呈非晶态,其中夹杂有尚未凝聚的铁晶粒。最后,在最优条件下进行了转底炉半工业试验。     

Viscosities of Slags ‐ an Overview

Viscosities of slags constitute an important physical property needed for an understanding of the mass transfer phenomena in metallurgical processes. Viscosity is also the key that leads to a better understanding of the structure of slags. It is well‐known that the viscosities of silicate slags decrease with the addition of basic oxides due to the breaking of the silicate network. The measurements of slag viscosities often pose experimental challenges, especially with respect to the choice of materials. This paper takes up the experimental problems and the various techniques adopted. Dynamic viscosity measurement, as a powerful tool towards an understanding of the kinetics of some high temperature reactions is brought out. A number of semi‐empirical models have been developed to estimate the slag viscosities in the case of multicomponent slags. A critical survey of these models is presented in this paper. Earlier models by Riboud et al., Urbain et al. and Mills et al. have been developed further into a new generation of models. Some of the current important models are, (1) Model by lida et al., (2) CSIRO model, (3) Pyroresearch Model, (4) Model by Tanaka et al., (5) Model approach by Reddy et al., and (6) KTH‐model. The predictions and capabilities of the various models are compared. Estimations of viscosities from thermodynamic data and prediction of liquidus temperatures from viscosities are presented. The concept of surface viscosities with reference to slags and viscosities of two‐phase mixtures are also taken up in the presentation.