Slag Formation Research Articles

From Henry Bessemer’s invention to comprehensive converter process optimization

The Bessemer process enabling to produce high-quality steels was implemented at steel mills of the Urals region more than 200 years ago. At the beginning of the 20th century, the process was modified by scientists from the Mining Institute and was then adopted by copper and nickel alloy producers. The converter process became one of the key processing stages with Russian copper and nickel smelters. This paper examines certain ways to improve the tuyere air flow in a horizontal converter. The authors came up with a generic equation for calculating specific blast air pressure and discuss the use of hydrothermal and aerodynamic techniques for tuyere purging. The blast air limit was determined for a horizontal converter that is characterized with massive melt ejections. The authors demonstrate that, in spite of all the process and design improvements, the modern converter process still has three major drawbacks. They include process cycling, the gas ductwork not being completely tight, unbalanced thermal and chemical status of the vessel leading to excessive thermal and chemical loads on the tuyere zone and incomplete slag formation in zones that are farther from the tuyeres. Research work is ongoing on a novel continuous converter design which involves the use of special-design unsubmerged tuyeres. Such tuyeres produce spatially oriented high-pressure blasts which enable consistent rotation of the molten matte in a tight cylindrical vessel.

Categorization of small-scale biomass combustion appliances by characteristic numbers

The market offers a broad range of different combustion appliances dedicated to residential heating with biomass. The effect of fuel properties on the formation of slag and emissions varies and the technology influences the impact to a certain extent. The applicability of biomass fuels is not only determined by operational settings but also by the design of boiler components as grate area and combustion chamber. Aspects as the fuel load on the grate, residence time, geometry of grate and combustion chamber design, as well as feeding and de-ashing influence the extent of slag formation and emission release. The determination of characteristic numbers by means of constructional measures allows a systematic comparison and - in a further step - an assessment/categorization of combustion technologies. After conducting a boiler survey relevant parameters regarding grate, combustion chamber, feeding, and ash removal were gathered. Characteristic numbers were specified in order to compare technological aspects. The results of this study allow the investigation of the influence of the combustion technology on the performance. They will assist the systematic and targeted design of small-scale boilers and the optimization of combustion appliances in future, especially when it comes to fuel-flexibility.

Sustainable Biomass Pellets Production Using Vineyard Wastes

Vineyards waste has a great importance as biomass, a renewable source of energy. In this paper eight vine shoot varieties were used for the production of pellets by densification of feedstock materials with four ranges of moisture contents (6–8%, 8–10%, 10–12% and 12–15%). A moisture content of 10% gave durability higher than 97.5% and a calorific value greater than 17 MJ kg−1 and the small durability was obtained for 6–8% moisture. The study shows the significant influence of water during densification. The physicochemical and energetic properties of pellets were evaluated in accordance with ISO 17225-6 (2014). The obtained pellets were also structurally characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). SEM analysis showed the formation of carbon microsphere after pelletization, due to the increase of bulk density and durability of pellets. Also, XRD analysis revealed the crystallinity of cellulose, while TGA analysis showed a total decomposition of pellets. The obtained pellets were burned in a domestic boiler and the flue gases were measured. The preliminary results showed that the vineyard residues had higher emissions, but below the admitted limits, with the exception of carbon monoxide content. The obtained results suggested that the biomass wastes can be used for the production of pellets, aiming to enhance the research for the manufacturing of these sustainable biofuels with some remarks regarding risk of corrosion and slag formation during prolonged use.

Effect of limestone and dolomite flux on the quality of pellets using high LOI iron ore

Owing to the depletion of high-grade iron ore quality, many steel plants over the world are now using pellets after beneficiation of low grade ores as blast furnace feed. For the effective utilization of low-grade iron ore resources (59–62% Fe) with high loss of ignition (LOI), mineralogical characteristics play the vital role to improve the process efficiency. The present work illustrates the effect of limestone and dolomite as flux material in pelletization of an Indian goethetic-hematite iron ore with 59.75% Fe, 4.52% SiO2, 3.84% Al2O3, and 4.85% LOI. As per mineralogical analysis, this ore contains 30.11% goethite and 9.71% kaolinite, which contribute major LOI of the sample. The self-binding properties of the ore ensured the complete elimination of an external binder due to the presence of kaolinite. LOI of the ore indeed affected the quality of indurated pellets by cracking, which might be the reason for the degradation of pellet quality. Concurrently, the same LOI enhanced the pellet quality when suitable fluxes were added. Limestone, dolomite, and their combinations were used as fluxing agent. The effects of CaO and MgO for the improvement of inherent pellet properties were evaluated through the physical, mineralogical, thermal, chemical, and metallurgical properties of the fluxed pellet using different analytical techniques, i.e., Thermogravimetric Analysis (TGA), X-ray diffraction, Stereomicroscopy, Optical Microscopy, Field Emission Scanning Electron Microscopy, and Wavelength Dispersive Microscopy with Electron Microprobe. TGA results indicated the thermal decomposition of goethite, kaolinite, and fluxes at different temperatures. With the addition of dolomite and limestone in the pellet feed material, CCS (Cold Compressive Strength/kg.pellet−1), RI (Reducibility Index/%), RDI (Reduction Degradation Index/%), SI (Swelling Index/%) and porosity could be improved. Due to the loss of mass in flux materials, by increasing the MgO content, it is possible to reduce swelling index and increase the pellet porosity. Simultaneously, the strength of the pellet increases because of slag formation in the form of calcium/magnesium aluminum silicate. The physical and metallurgical properties, i.e., CCS, porosity, SI, RDI, and RI of the fluxed pellets were optimized. At particular ratio of limestone and dolomite, synergistic effect was observed with respect to pellet characteristics.

Smelting of steel semi-product in BOF and EAF under magnesian slags

Basics for the development of a rational composition of the main magnesian slags and technological methods for their formation according to the periods of BOF bath blowing and the periods of smelting in EAF are the results of fundamental studies of the slags physicochemical properties. Implementation in the converter shop of OJSC “EVRAZ NTMK” of a developed set of technological methods of magnesian slags formation in the main period of blowing and a wearresistant skull on the basis of magnesian final slags ensured record durability of converter linings. These slags have low aggressive effect on the lining of converters, while maintaining high refining properties. The durability of the converters’ lining exceeds 7000 heats, while maintaining high technological and technical-economic indicators of the process. During heats of low manganese cast irons in 350-metric ton heavy converters of JSC “EVRAZ ZSMK” under magnesian slags, the features of slag formation and changes in chemical composition of the slag were studied by the periods of the BOF bath blowing. During the process stage of phosphorous cast irons in 300-metric ton BOF at JSC “ArcelorMittal Temirtau”, a set of technological methods was developed for the formation of magnesian slags of the recommended chemical composition by the periods of phosphorus cast irons blowing and wear-resistant skulls based on final magnesian slags of moderate basicity. The implementation of the developed technological methods ensured the lining stability of BOF of more than 5 000 heats while maintaining high technological, technical and economic indicators of phosphorous process stage. In the EAF shop of PJSC “Seversky Pipe Plant” a technology for the formation of magnesian slags of rational composition was developed over the smelting periods in EAF-135. Introduction of the technology ensured the record resistance of the refractory lining of the furnace up to 1900 heats per campaign and high level of technological and technical-economic indicators of the process.

Spectroscopic Characterization of Iron Slags from the Archaeological Sites of Brâncoveneşti, Călugăreni and Vătava Located on the Mureş County (Romania) Sector of the Roman Limes

Iron slag samples unearthed from archaeological sites lying on the Eastern limes sector of Roman Dacia (the Brâncoveneşti and Călugăreni auxiliary forts and the Vătava watchtower) were studied in order to assess the probability of local iron working (smelting and smithing) during the 2nd–3rd centuries CE. Structural-mineralogic aspects revealed by PXRD analysis and FTIR spectroscopy indicate different slag types corresponding to different iron production and processing stages allowing the supposition that refining of the bloom and processing of the refined iron took place on the sites. The FTIR absorption bands obtained in the spectral domain 2000–400 cm−1 show that mineralogically the samples are constituted mainly of silicates associated with minor quantities of aluminates and carbonates. The fayalite, haematite, and magnetite phases appearing on both the X-ray diffractograms and the FTIR spectra agree with the redox conditions of the slag formation process which result from the Fe3+/Fe2+ ratio determined using the EPR-method. The bulk macro-elemental PXRF and ICP-MS spectroscopy data support the slag typization proposed on the basis of the probable working conditions; trace-elemental bulk composition suggests that the provenance of the raw materials may be different.

Monitoring of Waste Management in the Central Ecological Zone of the Baikal Natural Territory

Quantitative assessments are made of the formation of municipal solid waste, ash and slag from heat power plants (boilers), and logging and timber processing activities in the central ecological zone of the Baikal natural territory. The material and raw material balance method and the normative method were used as the basis, and calculations used potential volumes of waste produced (thematic industrial reference books) and specific indicators adopted for particular federal subjects of Russia. These authors are executors of several thematic research projects of federal and regional status, aimed at the environmental and socioeconomic development of the central ecological zone of the Baikal natural territory (seven administrative districts and one urban district of Irkutsk oblast and of the Republic of Buryatia), which has allowed us, over the last two decades, to characterize, evaluate and predict the development of various economic processes in the environment and society of this territory, including issues related to production and consumption waste management. Variants of a comprehensive solution to the problems of waste management are proposed: alternative sources of energy and heat supply in the central ecological zone (logging and timber processing waste and consumer waste) as well as applications in the construction sector and, in particular, in road construction (ash and slag). The findings of this research can serve as the substantiation of particular managerial decisions on the promotion of the ideology of the green economy by developing further the aforementioned individual types of reuse of waste produced in order to safeguard the World Heritage site, Lake Baikal, and sustainable development of society on its shores.

Influence of Sodium Oxide on the Fusion of Solid Municipal Waste Ash

The model composition of municipal solid waste (MSW) and its ash chemical composition are characterized. Mineral compounds and melting points are studied using a variety of analytical methods, including XRD, FTIR, and AFT. The main parameters of slag formation are calculated: base-to-acid ratio, slag viscosity index, and fouling coefficient. Solid municipal waste ash contains significant amounts of CaO, SiO2 and Al2O3. Using the Terra program, thermodynamic calculations of the chemical equilibria of the CaO–SiO2–Al2O3–Na2O systems containing from 5 to 25% sodium oxide are performed. It is experimentally shown that each 5 wt % increase in the Na2O content reduces the characteristic melting temperatures of MSW ash by 30–50°С.

Theoretical Basics and Technology of Smelting Steel Semiproduct in Basic Oxygen Furnaces and Electric Arc Furnaces under Magnesian Slags

The results of basic studies of the physicochemical properties of slags are used as the basis for developing the rational composition of main magnesian slags and the process techniques for their formation by the periods of blowing a basic oxygen furnace (BOF) bath and the periods of smelting in electric arc furnaces (EAFs). The record-breaking durability of furnace linings is ensured by implementing, in the furnace shop of OJSC EVRAZ NTMK, the developed set of process techniques of forming magnesian slags in the main period of blowing and a wear-resistant skull based on the final magnesian slags. These slags produce a low aggressive effect on the lining of furnaces, while maintaining high refining properties. The furnaces’ lining durability exceeds 7000 heats; that said, the high process and key performance indicators of the process are maintained. The treatment of low manganese cast irons in the 350-ton heavy furnaces of OJSC EVRAZ ZSMK under magnesian slags allows studying the features of slag formation and changes in the chemical composition of the slag by the BOF bath blowing periods. In the course of treating phosphorous cast irons in the 300-ton BOFs at JSC ArcelorMittal Temirtau, a set of process techniques is developed for forming the magnesian slags of the recommended chemical composition by the periods of phosphorus cast iron blowing and wear-resistant skulls based on moderate-basicity final magnesian slags. The implementation of the developed process techniques ensures the lining stability of BOFs for more than 5000 heatings while maintaining the high process parameters and key performance indicators of the phosphorous treatment phase. The technology of forming rationally composed magnesian slags by the periods of smelting in EAF-135 is developed in the EAF shop of PJSC Seversky Pipe Plant. The adoption of the technology ensures the record-breaking resistance of the refractory lining of the furnace for up to 1900 heats per campaign and the high process parameters and key performance indicators of the process.

Application of Fly Ash from Pulp and Paper Industries as Slag Formers in Electric Arc Furnace Stainless Steel Production

During electric arc furnace (EAF) steelmaking process, burnt lime is charged together with other slag forming materials to attain a specific basicity of the slag and to achieve purification by removing unwanted elements. Herein, fly ash (FA) containing ≈60% CaO generated from pulp/paper mills is tested to partially (15–50%) replace primary lime (PL) in pilot scale EAF trials. The obtained results show good possibilities. It is found that an increased amount of FA instead of PL reduces the required amount of FeSi (up to 3 kg t−1 of scrap) and increases the sulfur content in the final slag. As a result, the amount of required slag/ton of steel can be decreased. However, the phosphorus content in the metal is slightly increased. The replacement ratio of FA will be limited by the acceptable phosphorus level in the final steel, due to higher phosphorus content in FA from pulp and paper mills compared with that in PL. Applications of FA as slag formers can reduce the consumption of natural resources in the metallurgical processes. In addition, it can reduce the amount of wastes from pulp/paper industries sent to landfill, which is important from an environmental point of view.

Thermochemical equilibrium study of ash transformation during combustion and gasification of sewage sludge mixtures with agricultural residues with focus on the phosphorus speciation

The necessity of recycling anthropogenically used phosphorus to prevent aquatic eutrophication and decrease the economic dependency on mined phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The aim of this work was to identify the P recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these fuel blends in comparison with pure sewage sludge. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could increase the degree of slag formation and enhance the volatilization of K in comparison with combustion conditions. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is emphasized that the results of this theoretical study represent an idealized system since in practice, non-equilibrium influences such as kinetic limitations and formation of amorphous structures may be significant. However, applicability of thermodynamic calculations in the prediction of molten and solid phases may still guide experimental research to investigate the actual phosphate formation in the future.

The influence of intense preliminary mechanical processing on the slag formation in coal-fired utility boilers

Burdukov et al. [1] showed experimentally that enhancement of coal reactivity when micronized in a high-impact disintegrator mill makes it possible to attain self-igniting and self-sustaining (autothermal) compact-flame combustion in a cold environment. But with intensive grinding of coal, the mineral part of the fuel is also being converted, which can lead to both a decrease and an increase in the intensity of the formation of deposits. The effect of grinding on the mineral matter of coal and the formation of deposits in a 5 MW pilot-scale combustor are presented. The experiments showed a decrease in the intensity of formation of deposits after grinding of coal by disintegrating mill as compared to grinding in the ball-tube mill.

Improvement of Corrosion Resistance of 13CrMo4-5 Steel by Ni-Based Laser Cladding Coatings

The 13CrMo4-5 ferritic steel is commonly used in power plants, due to its favorable mechanical properties. According to EN10028-2, this steel can be used at temperatures up to 570 °C because of its creep behavior. The inefficient corrosion resistance limits the application of this steel to lower temperatures depending on the gas temperature and slag formation. Therefore, the application of a highly resistant Ni-based coating is proposed to extend the corrosion resistance of elements made of ferritic steel. The corrosion test was performed in an environment containing a mixture of gases, like O2, COx, and SOx, and deposited ashes with elements, e.g., Na, Cl, Ca, Si, C, Fe, and Al. The exposure time was, respectively, 240 h, 1000 h and 4500 h at a temperature of 600 °C. The oxide scale formed on the 13CrMo4-5 steel was significantly thicker than on the IN686 coating. The microstructure and chemical and phase compositions of the oxide scale were investigated using light optical microscopy together with scanning and transmission electron microscopy techniques. Energy dispersive x-ray analyses were preformed when appropriate.

Behavior of the Sewage Sludge Ash under the Conditions of High-Temperature Processing

The sewage sludge and the chemical composition of its ash were characterized experimentally. Mineral compounds of the ash were studied by X-ray microanalysis, X-ray diffraction, and IR spectroscopy. The fusion temperatures of the sewage sludge ash were determined by the cone method. The main parameters of the slag formation (base to acid ratio, slag viscosity index, and fouling index) were calculated. The ash contains large amounts of P2O5, CaO, SiO2, and Fe2O3. Thermodynamic calculations show that large amounts of Ca3(PO4)2 and CaSiO3, significant amount of Fe2O3, and small amounts of MgSiO3, Al2O3, and SiO2 are observed in the temperature interval 600–1400°С. The sewage sludge ash tends to slagging and fouling on the reactor walls.

Slag Formation during Entrained Flow Gasification: Calcium-Rich Bark Fuel with KHCO3 Additive

Slag property management is of utmost importance for successful operation of entrained flow gasifiers. The present study investigates the influence of potassium introduced as KHCO3 on the ash and slag formation of softwood bark, a calcium-rich fuel, during entrained flow gasification. The bark contained only minor mineral inclusions causing the ash composition to be dominated by calcium and potassium. Wood bark with and without KHCO3 additive was gasified between 850 and 1400 °C at O2 stoichiometric ratio (λ) 0.6. The ash particles collided with a flat impact probe inside the hot reactor at particle impact angles set to 90°, 60°, and 30°. The reactor and probe allowed long-distance microscope data collection close to the probe surface. Particle deposition was optically monitored and resulting deposits were analyzed by SEM-EDS and XRD. Thermodynamic equilibrium and viscosity calculations were used to assist interpretation of experimental results. The predicted temperature window for liquid carbonate formation was experimentally verified, but the melt fraction of the deposit was too low to cause efficient flow and removal of ash from the probe under the prevailing experimental conditions. At higher temperatures, spherical particles indicated lower ash melting temperatures than expected from the bulk ash composition, and a detailed mechanism was proposed.

Technology for the reduction of iron oxides in the processing of metal slag scrap in the oxygen converter

The characteristic features of slag formation and oxidative metal refining in 160-tonne converters of EVRAZ ZSMK JSC using metal slag scrap in the solid metal charge are considered. Technological advantages of the process are established for the reduction of iron oxides from the slag component of the iron-containing product in the processing of waste slags.

Prediction of slag related problems during fixed bed combustion of biomass by application of a multivariate statistical approach on fuel properties and burner technology

Slag is related to the melting properties of ash and is affected by both the chemical composition of the fuel ash and the combustion parameters. Chemical analysis of slag from fixed bed combustion of phosphorus-poor biomass show that the main constituents are Si, Ca, K, O (and some Mg, Al, and Na), which indicates that the slag consists of different silicates. Earlier research also points out viscosity and fraction of the ash that melts, as crucial parameters for slag formation. To the authors’ knowledge, very few of the papers published to this day discuss slagging problems of different pelletized fuels combusted in multiple combustion appliances. Furthermore, no comprehensive classification of both burner technology and fuel ash parameters has been presented in the literature so far. The objective of the present paper was therefore to give a first description of a qualitative model where ash content, concentrations of main ash forming elements in the fuel and type of combustion appliance are related to slagging behaviour and potential operational problems of a biomass fuel in different small- and medium scale fixed bed appliances.Based on the results from the combustion of a wide range of pelletized biomass fuels in nine different burners, a model is presented for amount of slag formed and expected severity of operational problems. The model was validated by data collected from extensive combustion experiments and it can be concluded that the model predicts qualitative results.

Dynamic Modeling and Simulation of Basic Oxygen Furnace (BOF) Operation

Basic oxygen furnaces (BOFs) are widely used to produce steel from hot metal. The process typically has limited automation which leads to sub-optimal operation. Economically optimal operation can be potentially achieved by using a dynamic optimization framework to provide operators the best combination of input trajectories. In this paper, a first-principles based dynamic model for the BOF that can be used within the dynamic optimization routine is described. The model extends a previous work by incorporating a model for slag formation and energy balances. In this new version of the mathematical model, the submodel for the decarburization in the emulsion zone is also modified to account for recent findings, and an algebraic equation for the calculation of the calcium oxide saturation in slag is developed. The dynamic model is then used to simulate the operation of two distinct furnaces. It was found that the prediction accuracy of the developed model is significantly superior to its predecessor and the number of process variables that it is able to predict is also higher.

Effects of clay and temperature on the slag formation of two biomass fuels: Wood from Acacia mangium and rhizome residual from Manihot esculenta

Thermal energy generation from biomass fuels usually leads to dense slag accumulation, causing combustion system problems. This study tested a type of local clay as a slag soothing additive on two biomass fuels: Acacia mangium (kratin-tree) and Manihot esculenta (cassava). Physical characteristics of the slags were studied. In addition, a laboratory method for determination of ash sintering was modified to assay the dense and loose slag accumulation. Compositions of the clay and biomasses were determined. While the clay contains high relative contents of Si (42.3%) and Al (24.5%), the cassava rhizome and kratin-wood contain high relative contents of K (46.8% and 47.2%) and Cl (10.8% and 7.54%), respectively. Gas phase transition of residual ashes was revealed as a result of slag inducing combustion treatments. Addition of the clay (0–5%) decreased the total slag accumulation, and the maximum effect was revealed at 3–5% clay. The modified method in this study requires a small amount of ash sample which is practical and convenient for laboratory testing of slag behavior.

In-situ synthesis of oxides by reactive process atmospheres during L-PBF of stainless steel

Traditionally, reactive gases such as oxygen (O2) and carbon dioxide (CO2) have been avoided during laser powder bed fusion (L-PBF) of metals and alloys based on the notion that it may lead to defect formation and poor properties. Here we show that instead, these gases can be used to form sub-μm-sized oxide particles in-situ during the L-PBF process in an Fe-Cr-Al-Ti stainless steel and lead to improved room temperature and high-temperature mechanical properties. We manufactured cube samples using pure Ar and various reactive gas atmospheres, namely an O2/Argon (Ar) mixture containing 0.2 % O2 and CO2/Ar mixtures containing up to 100 % CO2. Co-axial measurements of infrared radiation emitted from the melt pool showed correlation to the presence of O2 or CO2 in the gas mixture. Builds produced under CO2-containing atmosphere contained complex oxides with an average diameter of ∼40 nm, an Al-rich core and a Ti-rich shell. Due to the high cooling rates typical to L-PBF, agglomeration of oxides and slag formation on the surface of the samples could almost be entirely avoided. Compression tests at temperatures up to 800 °C showed that the samples produced in 100 % CO2 have about 20 % higher yield stress compared to samples produced in Ar. The paper concludes with a discussion of the formation mechanism of the observed oxides. Our results show that in-situ reactions during additive manufacturing processes are a promising pathway to the synthesis of particle-reinforced alloys.