Slag Formation Research Articles

Variation and optimization of acid-dissolved aluminum content in stainless steel

As a key step in secondary refining, the deoxidation process in clean stainless steel production is widely researched by many scholars. In this study, vacuum oxygen decarburization (VOD) deoxidation refining in a 40-t electric arc furnace + VOD + ingot casting process was analyzed and optimized on the basis of Al deoxidation of stainless steel and thermodynamic equilibrium reactions between the slag and steel. Under good stirring conditions in VOD, the deoxidation reaction reaches equilibrium rapidly, and the oxygen activity in the bulk steel is controlled by the slag composition and Al content. A basicity of 3–5 and an Al content greater than 0.015wt% in the melt resulted in an oxygen content less than 0.0006wt%. In addition, the dissolved oxygen content decreased slightly when the Al content in the steel was greater than 0.02wt%. Because of the equilibrium of the Si–O reaction between the slag and steel, the activity of SiO2 will increase while the Si content increases; thus, the Si content should be lowered to enable the formation of a high-basicity slag. A high-basicity, low-Al2O3 slag and an increased Si content will reduce the Al consumption caused by SiO2 reduction.

Metallurgical Properties and Phase Transformations of Barium-Strontium Modifier

Metallurgical properties and phase transformations of barium-strontium modifier were tested in laboratory conditions resembling steel processing in furnace and ladle. When heating barium-strontium modifier start of melting, kinetics of decomposition, phase and structure transformation were studied. The concentrate under consideration has been revealed to be a complex mineral compound containing barytocalcite, calcite, calciostrontianite, dolomite and siderite. The reaction kinetics of decomposing mineral components of barium-strontium modifier to oxides does not considerably affect slag formation in conditions of out-of-furnace steel processing.

Slag island characteristics and weld penetration in very low sulphur 18 % Cr stabilized ferritic stainless steel

The role of minor elements in the development, intensity and composition of slag islands visible on the weld bead was investigated. Thirty-one very low sulphur heats of ferritic stainless steel EN 1.4509 (“441”, S43940) were autogenously gas tungsten arc welded using multiple welding scenarios. The welds were analysed by using conventional metallography and scanning electron microscopy. Furthermore, the level of penetration was examined as the phenomena are connected to each other. There were two types of slag: (1) slag associated with arc ignition comprising mainly Al2O3-rafts and Al2O3-rich Al2O3-CaO, which was promoted by the presence of aluminium in the steel and high arc voltage, and (2) slag spots comprising TiO2-rich TiO2-CaO-Al2O3 with less than 40 % Al2O3. The extent of slag spots increased with increasing calcium and titanium in the steel but decreased with increasing aluminium. Calcium and titanium intensified the slag island formation and severely impaired penetration. Silicon had a detrimental effect on weld penetration but had no effect on slagging. Aluminium had a decisive role in the formation of ignition slag, but had no clear effect on slag spot formation or penetration. No statistically significant effect of sulphur or oxygen on weld penetration was found for the ranges studied.

Efficient liquid metallurgy synthesis of Fe–TiB2 high modulus steels via in-situ reduction of titanium oxides

We studied the in-situ reduction of Ti oxides by Al as an alternative and cost effective route for the liquid metallurgical synthesis of low density, high stiffness steels (high modulus steels) containing about 10vol.% TiB2. TiO2, TiO1.83 and TiO were inserted via iron tubes into Fe–B melts, with Al either premixed with the oxide powders or liquid in the melt. Depending on Ti oxide type and location of the redox partner Al, greatly differing reaction kinetics, slag formation and corresponding microstructures of the high modulus steels were observed. TiO1.83 and TiO premixed with Al showed the highest TiB2 yield in the cast steel and are thus favourable candidates for the cost effective production of high modulus steels. Based on our findings, a novel synthesis process is proposed, based on filling wire injection into a continuous casting process, allowing the utilisation of the additionally formed oxide particles for the further improvement of the property profile of high modulus steels.

Critical review of predictive coefficients for biomass ash deposition tendency

Energy production by solid biofuel combustion is the most commonly used biomass-to-energy technology. Nevertheless, the ash generated from the combustion process leads to problematic phenomena. Depending on the origin and chemical characteristics of the biomass used, the combustion technology and conditions, and the melting temperature of the ash, there may be considerable sinter and slag formation during biomass combustion. A great number of coefficients developed for coal ash have been applied to approximately predict the slagging and fouling tendencies of biomass ash. However, because biomass composition is significantly different from that of coal, such formulas are not accurate when applied to the prediction of a biomass ash melting point.This paper presents an overview of indices or coefficients commonly used in the literature to predict biomass slagging and fouling tendencies in order to compare them and apply them to several types of biomass having different origins. The results show no relationship or predictive trend among the existing indices when applied to biomass fuels. Indeed, the contradictory findings come to demonstrate that further research and experimental tests under real conditions are needed to create or validate any of these indices. These coefficients should base on results from real combustion experiences, considering the influence of the combustion conditions and technologies, the heterogeneity of solid biomass fuels, and their physicochemical characterization.

Effects of the composition on the structure and viscosity of the CaO–SiO2-based mold flux

In this study, to investigate the effects of composition on the structure and viscosity of CaO–SiO2-based mold flux, the structure of molten slag was investigated by Raman spectroscopy, and the effect of the components on the viscosity was analyzed. The results show that the formation of the molten slag with multicomponent oxides involves the equilibrium and redistribution processes of O2− ions. The network breakers offered O2− ions and the network formers accepted the O2− ions. Al3+ ions can act as both the network formers (O2− acceptor) and the network breakers (O2− donor). The Al3+ ions were absorbed into the silicate structure, mainly acting as the network formers in the mold flux with a high CaO/SiO2 ratio, but mainly as the network breakers in the mold flux with a low CaO/SiO2 ratio. Because the network breakers (Li2O, Na2O, CaO, and MgO) have different O2− donor powers, the effect of equimolar substitutions between the network breakers was different in two aspects: the degree of polymerization of the molten slags and the viscosity properties. CaF2 also can dissolve into Ca2+ and F−, and F− can depolymerize the molten slag and decrease the viscosity and therefore is also used as a network breaker. The equimolar substitutions between the network breakers can be used to adjust the structural and the viscosity properties of the molten slags. Hence, the proportional control of the network breakers is very important to adjust the structure and viscosity properties during the design of mold flux.

Thermodynamic analysis of the carbothermic reduction of electric arc furnace dust in the presence of ferrosilicon

Thermodynamic analysis of zinc recovery from electric arc furnace (EAF) dust by carbon in the presence of ferrosilicon was studied. A preheating process was performed to remove volatile compounds from dust to avoid impurities in the final zinc product. The main process includes reduction of zinc oxide and gasification and condensation of zinc vapor. Equilibrium condition was computed using FactSage program for the processes of reduction and gasification. The effect of ferrosilicon addition on the recovery of zinc was investigated for the temperature range of 950–1050°C at the constant pressure of one atmosphere. It is shown that ferrosilicon provides a better kinetic condition for zinc recovery by facilitating slag formation. The preferred condition for formation of liquid slag was predicted based on thermodynamic calculations and verified by performing some experiments. The experimental results show that good agreement between calculations and experiments can be obtained with higher amounts of silicon contents and at higher temperatures where liquid slag is presented. A mixture of the dust with 10% C and 7% FeSi(75%) can be used to achieve over 97% zinc recovery.

Effect of Basicity, Al2O3and MgO content on the softening and melting properties of the CaO-MgO-SiO2-Al2O3high alumina quaternary slag system

The flow characteristic temperatures of the selected quaternary CaO-MgO-SiO2 -Al2 O3 slags have been experimentally measured by performing softening and melting tests using a high temperature microscope in accordance with German standard 51730. An effective statistical model is developed through constructing empirical equations by regression analysis method to predict Softening temperature (ST), Hemispherical temperature (HT), and Flow temperature (FT) based on Al2 O3 (25%–30%) content, CaO/SiO2 (C/S ratio: 0.9–1.4), and MgO (4%–12%) content. The effect of basicity, MgO and Al2 O3 content on characteristic temperature are also studied. FactSage 6.4 data base is used to calculate liquidus temperature of the above quaternary slags and compared with the experimental and regression results. The experimental results show that, increase in Al2 O3 content increases ST and HT while decreases FT of the slags. The ST, HT and FT increase with increase in C/S ratio. Increase in MgO content decreases both ST and HT while increases FT of the slags. The liquidus temperature obtained from FactSage 6.4 are found to correspond well with the experimental and regression data. Under the range of composition examined, a high Al2 O3 (30%) and high C/S ratio (1.4) coupled with low MgO (4%) content is beneficial for the blast furnaces running with high Alumina slags, as it ensures the formation of a short slag.

In situ study on the formation mechanism of bubbles during the reaction of captured chars on molten slag surface

Bubbles in the liquid slag affect the rheology and thermal conductivity, and formed porous structures after cooling or quench in the industrial gasifier. In this study, we applied a high temperature stage microscope (HTSM) to investigate the formation mechanism of bubbles during char gasification on molten slag surface. The initial bubble diameter and the formation time were measured by ImageJ software. Bubbles formed on the char–slag interface and formation time of the initial bubble increased with increasing initial particle size. Bubble accumulative volume for a single char particle increased with the shrinkage of particle. Results also showed that gas product from fine particles tended to diffuse into liquid slag and form the bubbles, with the comparison of bubble accumulative volume and gas product volume from a single particle. In addition, particle diameter at the initial bubble formation time was proportional to the initial particle size. The formation mechanism of bubbles was analyzed and predicted, which showed approximate results for the experiment data and was available for the bubble formation.

Adoption of Sinter Addition in Steelmaking Converter to Control Spitting

Steel Melting Shop-I, Rourkela Steel Plant, Rourkela, produces 0.5 MT of various special steels through BOF-VAR/VOR-LF-CC route. One of the most serious problems in BOF operations was lance skulling, hood jamming, and build-up of metal in the mouth and cone of the furnace due to spitting and slopping. As spitting occurring during blowing increases, these particles of metal are deposited inside the mouth and cone and affect badly BOF productivity. In the present work, control of spitting has been established by addition of sinter during the period of spitting. It also helped to improve the slag formation and fluidity of slag through increase of FeO content during peak decarburisation period.

Alkali transformation during single pellet combustion of soft wood and wheat straw

Controlling slag and deposit formation during thermochemical fuel conversion requires a fundamental understanding about ash transformation. In this work, a macro-TGA reactor was used to determine the release of ash forming elements during devolatilization and char combustion of single pellets. Soft wood and wheat straw were combusted at two temperatures (700°C and 1000°C) and the residual ashes were collected and analyzed for morphology, elemental and phase composition. The results showed that the single pellet combustion exhibit similar release character as in grate boilers. The temporal release was found to be both temperature and fuel dependent. For wood, the release of potassium occurred mostly during char combustion regardless of furnace temperature. Similar results were found for straw at 700°C, but the temperature increase to 1000°C implied that the release occurred already during devolatilization. The differences are presumably explained by different fuel phase compositions. The residual ash were composed of three different categories of phases; crystalline compounds, molten ash (glass) and char, and the work concludes that K was captured by crystalline K/Ca-carbonates as well as in amorphous glassy silicates for wood, and by almost fully molten ash of glassy silicates for straw. The fuel conversion processes occurring on a grate influence the fuel combustibility in terms of e.g. burnout, slag formation and release of fine particle and deposit forming matter, and the present work has given novel insights into the specific alkali behavior during biomass fuel conversion.

Slag formation path during dephosphorization process in a converter

The slag formation path is important for efficient dephosphorization in steelmaking processes. The phosphorus capacity and the melting properties of the slag are critical parameters for optimizing the slag formation path. Regarding these two factors, the phosphorus partition ratio was calculated using the regular solution model (RSM), whereas the liquidus diagrams of the slag systems were estimated using the FactSage thermodynamic package. A slag formation path that satisfies the different requirements of dephosphorization at different stages of dephosphorization in a converter was thus established through a combination of these two aspects. The composition of the initial slag was considered to be approximately 15wt%CaO–44wt%SiO2–41wt%FeO. During the dephosphorization process, a slag formation path that follows a high-iron route would facilitate efficient dephosphorization. The composition of the final dephosphorization slag should be approximately 53wt%CaO–25.5wt%SiO2–21.5wt%FeO. The composition of the final solid slag after dephosphorization is approximately 63.6wt%CaO–30.3wt%SiO2–6.1wt%FeO.

Increase in the efficiency of electric melting of pellets in an arc furnace with allowance for the energy effect of afterburning of carbon oxide in slag using fuel–oxygen burners

The problems of increasing the efficiency of electric steelmaking using fuel–oxygen burners to supply oxygen for the afterburning of effluent gases in an arc furnace are considered. The application of a new energy-saving regime based on a proposed technology of electric melting is shown to intensify the processes of slag formation, heating, and metal decarburization.

Exploration of Slagging Behaviors during Multistage Conversion Fluidized-Bed (MFB) Gasification of Low-Rank Coals

Four slag samples of Shenmu bituminite and Huolinhe lignite (SSM(A), SSM(B), SHLH(A), and SHLH(B)) selected from two upper parts of a multistage conversion integrated fluidized bed (MFB) were analyzed by X-ray fluorescence spectrometry, scanning electron microscopy, and X-ray diffraction analyses. The results show that the ash sintering temperature and ash fusion temperature of slag samples are below those of the raw coal ashes and increase in the order SSM(A) < SSM(B) < SHLH(A) < SHLH(B) because of differences in acid/base composition. The slag samples are composed of amorphous matter and some crystals, and the crystal content of the two SHLH samples are higher than that of the two SSM samples. The section of the MFB above the site of oxygen injection may be divided into three zones based on particle movement. Fine mineral particles contain much more iron and calcium than other particles. The possible mechanism of slag formation may involve mineral exposure by char gasification, interaction of minerals, ...

Tackling the challenges in modelling entrained-flow gasification of low-grade feedstock

Development of a new technology for conversion of residual biomass into a liquid fuel via pyrolysis–gasification–gas cleaning–synthesis is the overall objective of the on-going bioliq® project. The present paper gives an overview on research activities dedicated to mathematical modelling of entrained-flow gasification for conversion of biomass-based suspension fuels into a medium calorific (LCV around 15 MJ/kg) synthesis gas. The objective is to identify knowledge gaps that currently prohibit a knowledge-based mathematical description of reacting high-pressure multi-phase flows so as to model the bioliq® gasification reactor in particular and biomass conversion in entrained flow gasifiers in general. Substantial knowledge gaps for high pressure process conditions have been identified for atomization of high viscous liquids, gasification chemistry for biomass-based fuels, radiative heat transfer as well as slag formation mechanisms. The paper proposes an interdisciplinary research approach in a holistic manner to close these gaps.

Production technology for low-carbon, low-sulfur boron steel

A technology for slag formation in the ladle–furnace unit is considered; the slag is based on the CaO–SiO2–MgO–Al2O3–B2O3 system. This technology permits both microalloying of the steel with boron (reduced from the oxide phase) and desulfurization of the steel. The resulting boron content in the steel is 0.001–0.008%; the sulfur content in low-alloy steel and pipe steel is low (0.004–0.010%); and the consumption of manganese ferroalloys is reduced to 0.5 kg/t for 08кп steel and 1.4 kg/t for 09Г2C steel. In addition, the proposed technology increases the strength of the rolled steel, without loss in its plasticity; and reduces the environmental impact thanks to the replacement of fluorspar by colemanite.

Controlling the formation of Na2ZrSiO5 in alkali fusion process for zirconium oxychloride production

In the alkali fusion process for the production of zirconium oxychloride, the formation of the water-insoluble by-product, Na2ZrSiO5, largely affects NaOH recovery and results in the formation of silicon slags. Thus, controlling Na2ZrSiO5 formation is necessary. In this paper, we studied the effects of roasting temperature, NaOH/zircon mole ratio, and particle size of zircon sand on Na2ZrSiO5 formation. The optimum reaction parameters, when the roasting time was 1h, were as follows: ⩾95μm average particle size of zircon sand, 6:1 NaOH/zircon mole ratio, and 750°C roasting temperature. To avoid an uneven roasting temperature during alkali fusion, the use of a rotary kiln reactor is recommended. The amount of Na2ZrSiO5 was significantly decreased under these optimum conditions, and the content of Na2O was reduced from 7.049% to only 2.21% after three water-leaching cycles.

Processing of low-silicon hot metal with slag formation by ferruginous lime

Economic aspects of the converter process with decrease in silicon content from 0.6 to 0.1% are considered. Industrial experience with low-silicon hot metal is described.

Thermochemical Equilibrium Study of Slag Formation during Pressurized Entrained-Flow Gasification of Woody Biomass

The potential slag formation behavior during pressurized entrained-flow gasification (PEFG) of woody biomass has been studied from a thermodynamic perspective with respect to compositional, temperature, and pressure variations. An ash transformation scheme was proposed on the basis of the melt formation potential that arises when gaseous K species are present with Si and Ca. Databases and models in FactSage 6.4 were used to carry out thermochemical equilibrium calculations within ChemSheet. It was found that increasing pressure and increasing Si content expanded the range of operating conditions that are conducive of melt formation, while increasing temperature and increasing Ca content diminished the range. The results from the calculations compared qualitatively well to experimental results and provide further information needed in the development of PEFG reactors for woody biomass.

Investigation of rye straw ash sintering characteristics and the effect of additives

The understanding of ash sintering during combustion of agricultural residues is far from complete, because of the high heterogeneity of the content and composition of ash forming matters and the complex transformation of them. In order to make agricultural residues competitive fuels on the energy market, further research efforts are needed to investigate agricultural residues’ ash sintering behavior and propose relevant anti-sintering measures. The aim of this work was to investigate the ash characteristics of rye straw and effects of additives. Three additives were studied regarding their abilities to prevent and abate rye straw ash sintering. Standard ash fusion characterization and laboratory-scale sintering tests were performed on ashes from mixtures of rye straw and additives produced at 550°C. Ash residues from sintering tests at higher temperatures were analyzed using a combination of X-ray diffraction (XRD) and scanning electron microscopy–energy dispersive X-ray spectrometry (SEM–EDX). High sintering and melting tendency of the rye straw ash at elevated temperatures was observed. Severe sintering of the rye straw ash was attributed to the formation and fusion of low temperature K–silicates and K–phosphates with high K/Ca ratios. Among the three additives, calcite served the best one to mitigate sintering of the rye straw ash. Ca from the calcite promoted formation of high temperature silicates and calcium rich K–phosphates. In addition, calcite may hinder aggregating of ash melts and further formation of large ash slag. Therefore, the chemical reactions and physical restraining effects arose by calcite addition contributed to reduction of ash melts and sintering degree. Upon addition of kaolin, compositions of rye straw ash shifted from low temperature melting K–silicates to high temperature melting K–Al–silicates. The changes of ash chemistry were favorable for reducing sintering of the rye straw ash. As the Ca-sludge was added, reduction of sintering of the rye straw ash was less pronounced. Only K4CaSi3O9 and a small amount of KCaPO4 were identified in the rye straw ash as Ca-sludge was added.