Effect of fluxing agents on reduction degradation behaviour of hematite pellets

During induration at a high temperature, a considerable amount of slag/melt phase forms inside the iron ore pellets, comprising SiO₂, Al₂O₃, CaO, MgO and FeO. After cooling, the slag phase solidifies and acts as an important bonding phase in the finished pellets and influences their room temperature as well as high temperature properties, especially reduction degradation. Fluxing agents play an important role in forming these bonding phases depending on the type and amount of flux. In the present study, the effect of different fluxing agents, namely, limestone, dolomite, magnesite and pyroxenite, on melt formation and microstructure during induration and on reduction degradation behaviour during reduction was examined. From the results, it was understood that to reduce the disintegration during reduction it is essential to increase the amount and distribution of bonding phases like silicates, which are more stable as compared to oxide phases like hematite. Acid pellets exhibited highest reduction degradation due to the presence of more hematite bonds and less silicate bonds. In limestone fluxed pellets, reduction degradation index dropped considerably with increasing CaO content due to the formation of more amount of bonding phase. Dolomite–pyroxenite pellets, on the other hand, showed lower reduction degradation index up to 0.4 basicity, and beyond that, higher degradation was observed due to the increased pore size, which resulted in poor strength of the reduced pellet matrix and hence more degradation. Low reduction degradation observed in pyroxenite and magnesite fluxed pellets could be due to the formation of magnesioferrite and silicate melt, which are more stable phases compared to hematite.     

Influence of refining slag composition on cleanness and fatigue life of 60Si2MnA spring steel

The influence of basicity and Al₂O₃ content of LF refining slag on T.[O] (total oxygen) as well as type, number and size of non-metallic inclusions in Al killed 60Si2MnA spring steel was investigated. The results showed that with the increase of slag basicity R(CaO/SiO₂) or the decrease of Al₂O₃ content in slag, the T.[O], number and size of non-metallic inclusions decreased significantly. On the one hand, as the slag basicity increased, inclusions in steel were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, which made the formation of voids between inclusions and steel matrix to decrease. Furthermore, thermodynamic calculations showed that CaS could only form in steel (R?≥?3.4). Al₂O₃–SiO₂–CaO–MgO came close to the compositions of the low melting point area, while Al₂O₃–SiO₂–CaO–MgO–CaS deviated from this. On the other hand, as the Al₂O₃ content in slag increased, Al₂O₃–SiO₂–CaO–MgO–CaS came close to the compositions of the low melting point area. In conclusion, the cleanness and fatigue life of 60Si2MnA spring steel had been improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag.     

Study on refining slags targeting high cleanliness and lower melting temperature inclusions in Al killed steel

Abstract

Three high basicity slags (A, B and C) were used in laboratory to refine Al killed steel to target high oxide cleanliness and low melting temperature inclusions. Inclusions were of CaO–MgO–Al₂O₃–SiO₂ system after 90 min reaction, parts of which were MgO based. Total oxygen were in the range of 0·0007–0·0010 and 0·0005–0·0010% respectively when slag A (CaO/SiO₂, 6–8; Al₂O₃, ～40%) and slag B (CaO/SiO₂, 6–8; Al₂O₃, ～30%) were applied, with inclusions all in spherical shape and mainly <5 μm. Inclusion composition concentrated in or around the lower melting point region (<1500°C) under slag A, while it became more scattered under slag B. Total oxygen varied between 0·0008 and 0·0011% under slag C (CaO/SiO₂, 3–4; Al₂O₃, about 20–25%). Many of the inclusions were in larger size, irregular morphology and located far away from the lower melting point region. Formation of MgO based inclusions closely related to solubility behaviour of MgO in the slag.     

Rationale on composition of slags in oxygen steelmaking processes

Abstract

A study of plant analytical data on slags for the BOS and OBM processes revealed consistent interrelations between the concentrations of FeO, CaO, and SiO₂ in the slag at the end of the oxygen blow. Over a wide composition range, the amounts of CaO and SiO₂ decrease with increasing FeO, slag basicity increases with increasing FeO, and the amount of MgO decreases with increasing slag basicity. At all levels of lime and silica contents, the amount of FeO in the slag varies by 5–6% between the low and high contents. Consequently, the slag/ metal distribution ratios (%P)/[%P] and (%S)/[%S] as functions of basic and/or acidic oxide contents are within a composition range bordered by the equilibrium curves for the low and high FeO contents.     

Effects of oxygen atmosphere,FeOx and basicity on mineralogical phases of CaO–SiO2–MgO–Al2O3–FetO–P2O5 steelmaking slag

ABSTRACT

The mineralogical phase of slag after crystallisation is essential to utilisation of steelmaking slag. The mineralogical phases of cooled multicomponent CaO–SiO₂–MgO–Al₂O₃–Fe_tO–P₂O₅ slag with different iron oxide contents and basicities (defined as the ratio of mass percentage of CaO to mass percentage of SiO₂ (w(CaO)/w(SiO₂))) in different atmospheres were investigated in the present work by scanning electronic microscopy and energy dispersed spectroscopy analysis and X-ray diffraction. The mineralogical phases in steelmaking slag cooled in argon are mainly nCa₂SiO₄-Ca₃(PO₄)₂ (thereafter nC₂S-C₃P) solid solution, (Fe, Mn, Mg)O (RO) phase. Some CaMgSiO₄ phases could be found in slag with lower basicity. The mineralogical phases in steelmaking slag cooled in air are mainly nC₂S-C₃P solid solution, spinel phase. The overall crystallisation of slag cooled in both argon and air was enhanced with increasing basicity. However, the crystal sizes become smaller in sample with high basicity. The Fe-enriched phases were transformed from non-faceted RO phase in sample cooled in argon to faceted spinel phases in sample cooled in air. The crystallisation of slag cooled in both argon and air was promoted with increasing FeO_x content. The phosphorus content in solid solution was elevated with decreasing basicity and increasing FeO_x content. It was implied by the present work that appropriate basicity and air oxidation would be beneficial to magnetic separation and phosphorus utilisation.     

Inclusion composition control in tyre cord steel by top slag refining

Abstract

In order to improve the inclusion type and composition in tyre cord steel, ladle furnace refining has been simulated by laboratory experiments and thermodynamic calculation. It was found that slag metal reaction time and top slag composition have an important influence on the inclusion compositions in the final steel. To produce the desired low melting point ductile inclusions the optimum conditions were: reaction time 60 min, basicity (CaO/SiO₂) of top slag in the range of 1·0–1·2 and Al₂O₃ content of slag in the range of 3–9 mass-%. These were then confirmed in industrial trials.     

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%.     

含铁尘泥金属化球团的合理渣系

基于转底炉工艺,结合FeO-SiO_2-CaO三元相图,对金属化球团的渣系进行理论分析,同时开展模拟实验,研究了含铁尘泥金属化球团合理渣系结构。结果表明,对于含铁尘泥球团,当二元碱度为0.37~0.67时,渣系熔点小于1 150℃,球团在较低的还原温度下即可形成液相;随着渣系碱度的逐渐降低,含铁尘泥金属化球团的抗压强度呈现先增大后降低的趋势,当球团碱度为0.61时,抗压强度达到最大;金属化球团的强度与反应温度呈正相关性,反应温度的提高可大幅提高球团的强度。当球团二元碱度为0.85时,反应温度由1190℃提高至1220℃,球团的抗压强度可提高近100%。但随着球团碱度逐渐降低,不同温度条件下球团抗压强度的差异逐渐减小。     

Thermodynamics of chromium oxides in CaO-SiO2-CaF2 slag

The distribution ratio of chromium between a CaO-SiO₂-CaF₂ slag and liquid silver under the oxygen partial pressure used in practical hot-metal dephosphorization treatment was measured at 1623 K. The distribution ratio was minimal when the basicity index of a slag, wt pct CaO/wt pct SiO₂, was about 2. The redox equilibrium between CrO (Cr²⁺) and CrO_1.5 (Cr³⁺) in the slag was also measured as a function of slag composition. The calculated activity coefficient of CrO had a maximum value at wt pct CaO/wt pct SiO₂=2, whereas that of CrO_1.5 decreased monotonously with the increase in slag basicity.     

New indices of sinter basicity

Conclusions The Chusovoi Metallurgical Plant has now accumulated a significant amount of experience in making its own sinter within a broad range of basicities—from 1.03 through 3.81. In order to evaluate sinter and other materials as substitutions for limestone, it would be expedient to express fluxing ability as the percentage concentration of CaO or the sum CaO+MgO in the material minus the percentage of these oxides used to flux SiO₂. Given sinters of equal basicity, their fluxing ability depends on their content of SiO₂ and the basicity of the blast-furnace slag. To prepare for the possibility of having to melt unfluxed Kachkanar pellets in the blast furnaces, it would be best to organize the local production of high-basicity sinter. Chusovoi Metallurgical Plant. Translated from Metallurg, No. 7, pp. 13–14, July, 1999.     

Phosphorous Enrichment in Molten Adjusted Converter Slag： Part I Effect of Adjusting Technological Conditions

 The effects of adjusting technological conditions on the phosphorous enrichment of adjusted converter slag were investigated. The results showed that the phosphorus could be effectively enriched to 2CaO·SiO2 and 2CaO·SiO2-3CaO·P2O5 solid solution (SS, namely phosphorus-rich phase) to be formed with the decreasing of basicity and cooling rate. Moreover, the morphology of the phosphorus-rich phase changed from granular for the original converter slag with higher basicity to the coexistence of granular shape and rod-like for the adjusted slag with lower basicity. P2O5 content in phosphorus-rich phase exceeded 30% while the basicity was 13 at the cooling rate of 10 ℃/min.     

Analisys of Electric Arc Furnace Slag

The black slag produced during the melting process in electric arc furnace can be used as adjunct in the operation of land filling, building operation of road grounds, and production of concrete. Their use limitation is due to the presence of polluting chemical elements, including Cr, Ba, V, Mo, etc, that can be dangerous for human and environment, resulting by using of polluted scraps (i.e., painted, lubricated, or polymeric compound scraps). The release extent of polluting elements appears to behave as a function of the constituent phases, i.e., CaO, SiO₂, Al₂O₃, MgO, etc. The main tools employed in the characterization of the black slag consists in optical basicity calculation, SEM‐EBS and SEM‐EDS analysis. The combination of the data coming from chemical analysis, micro‐structural examination and releasing tests allows to identify the correct chemical range avoiding dangerous chemical release and to develop a working disposal procedure for the investigated slag.     

Effect of the firing temperature and the added MgO on the reduction swelling index of the pellet with low SiO2 content

The use of pellet with low SiO₂ content in blast furnace (BF) will reduce the slag amount as well as fuel rate and increase the productivity. In this paper, the effect of the firing temperature and the added MgO on the reduction swelling index (RSI) and the compressive strength of the reduced pellet with low SiO₂ content was investigated, and the microstructure of the fired and reduced pellets was analysed by means of the electron microscopy. It was found that the decrease of SiO₂ content will raise the RSI and reduce the compressive strength of reduced pellet. When the SiO₂ content of pellet is 4.8%, the RSI is 16.5% and compressive strength of reduced pellet is 423 N/P. When the SiO₂ content is 2.8 and 1.8% fired at 1280°C, the RSI of the reduced pellet will increase to 34.5 and 55.8% and the compressive strength is reduced to less than 200 and 80 N/P. However, if some MgO was added, the RSI and the compressive strength of the reduced pellet could be improved significantly. When pellet’s MgO content was over 1.7% and SiO₂ content was 2.8% or MgO content was 2.5% and SiO₂ content was 1.8% fired at 1280°C, the RSI of the two pellets could drop to less than 20% and the compressive strength could increase to more than 300 N/P. Then, the technical index of pellet will meet the requirement of the charging in large BF. The added MgO pellet with low SiO₂ content have been used in 5500?m³ BF in Shougang Jingtang Corporation and delivered sustainable improvement of cost reduction.     

The Effect of MgO on the Viscosity of the CaO‐SiO2‐20 wt%Al2O3‐MgO Slag System

The viscosities of CaO‐SiO₂‐20 wt%Al₂O₃‐MgO slags (CaO/SiO₂ = 1.0–1.2, wt%MgO = 5–13) were measured to estimate the effect of MgO on the viscous behaviour at elevated temperatures. The slag viscosity at 1773 K decreased with increasing MgO contents, which was typical of a basic oxide component at relatively low basicity (CaO/SiO₂) of 1.0. The FT‐IR spectroscopic analysis of the slag structure seems to verify this behaviour. However, an unexpected contradiction with the temperature dependence was observed above 10 wt%MgO and above CaO/SiO₂ of 1.2. Although the apparent activation energy was expected to decrease with additions of the basic oxide component MgO, the apparent activation energy increased. This unexpected behaviour seems to be related to the change in the primary phase field correlating to the phase diagram corresponding to the slag composition. Therefore, in order to understand the viscosity at both high Al₂O₃ and MgO, not only should the typical depolymerization of the slag structure with high MgO content be considered but also the primary phases of which the molten slag originates.     

Replacement of bentonite in hematite ore pelletisation using a combination of sodium lignosulphonate and copper smelting slag

Bentonite is the most common binder used in iron ore pelletisation owing to its good bonding properties in green and dry pellets at both ambient and elevated temperatures. However, due to its high alumina and silica content, it increases the slag volume and energy consumption in downstream processes. Organic binders may be used to replace bentonite; however, they fail to provide strength at a high temperature (700–900°C) due to poor thermal stability during pellet induration. In the present study, an organic binder Na lignosulphonate (NLS) has been used along with copper smelting slag (Cu-SS). FeO in Cu-SS provides diffusion bonding at high temperature and maintains the strength of pellets even after evaporation/burning of NLS. It also enhances recrystallisation bonding at relatively lower temperature to provide good strength. The study has been carried out with hematite ore and varying amounts of NLS and Cu-SS. Copper smelting slag (1.0%) addition with 0.5%NLS has been found to be optimum to provide very good green properties and ～300?kg/pellet cold crushing strength (CCS) at 1250°C induration temperature. However, hematite pellets of similar basicity with 0.5% bentonite requires higher induration temperature (1300°C) to achieve a similar CCS. The developed pellet also shows better reducibility (80%), similar reduction degradation index (18%) and swelling index (10%) to the usual bentonite pellet. Thus, the induration temperature of hematite pellet has been lowered by 50°C using a combination of NLS and Cu-SS eliminating bentonite completely, which can provide a considerable energy and cost saving.     

Effects of silica addition on chromium distribution in stainless-steel slag

The present study was aimed at highlighting the effect of silica on chromium distribution and enrichment in certain stainless-steel slag when basicity ranged from 0.96 to 1.96. Industrial slag samples of the CaO–Al₂O₃–MgO–SiO₂–CrO_X–Fe_XO system were investigated with different silica addition contents and soaked at the targeted regime. X-ray diffraction, electron probe micro-analyzer with wavelength-dispersive X-ray spectrometer, as well as Fourier-transform infrared spectroscopy and NIH ImageJ software were used for sample characterisation. FactSage version 6.4 was employed to calculate phase equilibria at 1200°C for the slag system to understand the crystallisation procedure. Results indicated that chromium was enriched in CaCr₂O₄ at high basicity, but in spinel at lower basicity. The spinel phase increased in size and chromium distribution with Fe(II) participated in growth with the decrease in depolymerisation of the slag structure.     

Effect of Slag Basicity on Phase Equilibria and Selenium and Tellurium Distribution in Magnesia-Saturated Calcium Iron Silicate Slags

New measurements have been made on the phase equilibria of magnesia-saturated CaO-FeO_x-SiO₂ slags at 1573 K (1300 °C) and an oxygen partial pressure of 10⁻⁹ atm. The thermodynamic behavior of selenium (Se) and tellurium (Te) in the slag and the stability of oxide mineral phases within the slag were examined as a function of slag composition. The measured equilibrium distribution of Se and Te between the slag and the copper showed nonlinear dependence on the slag basicity, reaching maxima at CaO/(CaO + SiO₂) ratios of about 0.2 and 1 and a minimum at a ratio of about 0.5. The solubility of the copper oxide in the bulk slag also passed through a minimum value at a ratio of about 0.5. Results from drop-quench experiments confirmed the stability of various oxide solid solution phases at 1573 K (1300 °C) that had virtually no solubility for Se and Te. The deduced capacity of the liquid slag for Se was found to be independent of basicity in relatively basic slags, and decreased sharply as SiO₂ replaced CaO in relatively acidic slags.     

Chemistry,microstructure, and reduction characteristics of dolomite-fluxed magnetite pellets

The addition of dolomite to iron ore pellets improves high temperature reduction properties, primarily by increasing their softening point and reducibility. This study was made to ascertain the compositional ranges within which dolomite fluxed magnetite pellets show such an improvement over good acid pellets. A variety of compositions with MgO levels from 1 to 2 wt pct and CaO/SiO₂ ratios of 0.6 to 1.8 were prepared using Hibbing magnetic concentrate. These compositions were balled and then pot-grate fired to various temperatures. Petrographic examination and electron microprobe analyses were used to characterize the phase assemblages. Reduction characteristics were also evaluated. As evidenced by the following petrographically observed relationships, the CaO/SiO₂ ratio (c/S) was found to be the controlling factor for pellet microstructure: a) The low c/S (< 0.8) pellets were predominantly oxide-bonded, b) the intermediate c/S (0.8 to 1.3) pellets were slag- and oxide-bonded, and c) the high C/S (> 1.3) pellets were mostly calcium ferrite-bonded. Intermediate c/S compositions contain more magnesioferrite than do either low or high basicity compositions. Pellets must be fired to a high enough temperature to generate sufficient liquid for ion transport but not so high as to cause pellets to stick together. Well-fired pellets with a c/S of 1.3 or less performed adequately in low temperature degradation tests. Pellets with a c/S of 0.8 or greater displayed satisfactory swelling behavior. Therefore, we determined that pellets within a c/S range of 0.8 to 1.3 are technically acceptable. Pellets containing 1.5 wt pct MgO had the best reduction characteristics, but all fluxed compositions reduced faster and more completely than did acid pellets. Increasing MgO content up to 2.0 pct increased the softening temperature by 165‡C over acid pellets. On the basis of pellet test results and taking into consideration the auxiliary influence of economic and operating considerations, we determined the optimum composition of dolomite fluxed Hibbing pellets to be 1.5 pct MgO and a C/S of 0.8.     

Influence of MgO addition on mineralogy of iron ore sinter

The influence of MgO addition on sinter mineralogy was studied on sinters produced in a laboratory installation, with a wide range of MgO/CaO ratios at several basicity indices [B = (CaO + MgO)/ (SiO₂ + A1₂O₃)] between 0.7 to 1.9. The most striking influence of MgO is the suppression of hematite and Ca-ferrite phases and the increase in magnetite phase. In general, MgO favors the formation of glass and suppresses the precipitation of dicalcium silicates in favor of Ca-Fe-Mg olivines and pyroxenes. Microprobe studies revealed that most of the Mg was picked up by the magnetite phase to form mixed spinels of type (Fe, Mg)O · Fe₂O₃. At a constant basicity index, increased replacement of CaO by MgO also led to increased participation of FeO in the slag formation process, thus increasing the overall FeO content of sinter. A mechanism for the formation of mixed spinels has been proposed. The effect on various sinter properties resulting due to change in sinter mineralogy has been outlined. S. C. PANIGRAHY, with Department of Metallurgy and Metallography, State University of Ghent, Ghent, Belgium at the time the experiments were carried out     

Vanadium oxide solubility in CaO–SiO2–VOX system

The solubility of vanadium oxide in the SiO₂–CaO–VO_X system was investigated as a function of basicity (CaO/SiO₂) at a fixed temperature of 1600°C and oxygen partial pressure of 10^?10?atm. Formed phases and microstructures of saturated samples were identified with SEM–EDS analysis and XRD. Maximum solubility of vanadium oxide was between 15 and 20% independent of basicity. Pure karelianite (V₂O₃) was formed in all samples at saturation of vanadium oxide. The morphology of karelianite changed with the change in basicity in the slag, where needles or threads were formed for slags with basicity B2?=?0.54 and B2?=?0.67 and stars or dendritic patterns were formed with basicity B2?=?1.0 and B2?=?1.22. Wollastonite (CaSiO₃) was also formed in the slags with star or dendritic patterns.