High-Temperature Reactions and Microstructure of Slag–Iron in the Hydrogen Blast Furnace Hearth Based on Liquid Nitrogen-Quenching and Dissection

The paper describes a method for converting carbon powder obtained as byproduct while decontaminating pot liner by acid treatment route, into blast furnace tap hole mass. Producing an useful industrial product from waste helps in paying back the cost incurred during decontamination process of pot liner and thus improves the overall economics of the process. Blast furnace tap hole mass is a prepared muddy material used to close the tap hole of iron making blast furnace. At iron melting temperature the material carbonizes and becomes hard in order to hold the metallostatic pressure inside blast furnace hearth. Since iron gets oxidized at high temperature this tap hole compound is prepared predominantly from carbonaceous material. Further these carbonaceous materials used needs to be semicrystalline in nature in order to withstand high temperature of the furnace. With rising cost of such carbonaceous material used in tap hole compound, it will be advantageous if a source of cheap carbon material matching the properties of carbon material already in use in the tap hole recipe , could be found in order to bring down cost of production of commercial taphole recipe. The aim of this research was to find such a carbonaceous material from an industrial waste which is difficult to dispose because of its toxic contaminants. Such a material chosen for this work is called SPENT POT LINER of aluminum smelter plant. At present majority of the aluminum smelter plants round the world use Hall-Heraoult's electrolysis process for extracting aluminum metal from molten cryolite. The electrolysis cell used for this purpose constitute large carbon blocks as cathode laid at the bottom and side wall of the cell. These carbon electrodes are known as pot and basically made of anthracite, graphite and binder/electrode pitch. Various manufactures uses 20-30% (1) graphite in their recipe in order to meet electrical properties needed for the pot liner. While graphite is a superior electrical conductor than anthracite (which has similar structure to graphite), 100% graphite is never used in the manufacture of pot as it is soft and can not withstand turbulent molten cryolite contained in the cell. Anthracite on the other hand provides required mechanical property to the finished pot liner. The prefabricated pot liner produces carbon matrix with short range order. Thus prepared pot ultimately contain both crystalline (graphite) and amorphous carbon. During service molten cryolite slowly gets reduced and the sodium fluoride crystal deposits within the fine crevices of pot liner creating defect spots . As time passes, these crystals grow and exert pressure within these crevices resulting in the propagation of crack. As a consequence, with time the pot liner looses its electrical property and ultimately being discarded. These rejected waste pot are called spent pot liners (in short SPL). Spent pot are not only contaminated by fluoride but also by other toxic elements such as cyanides (formed at high temperature reaction with atmospheric nitrogen), alkalis and aluminum. Table-1 below shows typical range of the contaminants in such discarded pot along with concentration of these toxic elements in SPL carbon powder after decontamination with oxidizing acids. In practice suitability of a specific component in a commercial recipe is tested by evaluating some gross property of the modified recipe against the production recipe of the compound. For example, in development of foundry chemicals (like mould coating, tundish cover , hot tops etc) which basically a combination of various components in a formulation , various substitutes are being tried with above procedure of gross evaluation of certain properties of the compound in order to determine its suitability. Similarly in present case of developing suitable substitute of carbonaceous material in commercial tap hole compound recipe, following properties are important in order to determine its suitability. Apparent porosity. This property determines the ease with which gas generated in the tap hole compound during carbonization can escape easily without breaking or decreasing strength of the carbonized tap hole mass. This value is generally maintained in the range 25-35%. a) Bulk density. This is maintained in the range 1.3-1.6 gm/cc in all commercial recipe in order to match with standard pushing length required by the equipment to fill the tap hole.

Blast furnace hearth thermal state is one of the most important indexes for the evaluation of blast furnace hearth production status. By analyzing the relevant relationships among the Tc (Coke burning temperature in direct reduction zone), Tf (Theoretical combustion temperature), hot metal temperature and silicon content in hot metal, a mathematical model for the predictions of hot metal temperature and hot metal silicon content has been established, based on which the blast furnace physical heat index model was also put forward. The online calculation of the model was realized and practiced in a blast furnace production by the technologies of data acquisition, processing and programming, which built a high speed information channel of blast furnace hearth thermal state. Practice proved that the model could effectively help foremen to better understand the condition of blast furnace hearth, so that the blast furnace stability, hearth activity and hot metal quality were promoted greatly.

The extraction of molten iron and slag in the liquid phase from the lower part of a blast furnace (hearth) is usually accomplished according to operational experience and involves a high degree of uncertainty, mainly because the liquid level cannot be directly measured. This study presents a methodology for obtaining multistep models to forecast the hearth liquid level by measuring a voltage generated on the blast furnace shell, which is strongly correlated with the hearth liquid level. The results show that this electrical signal is a nonstationary and nonlinear time-series that, after appropriate treatment, can be represented by a time-delay neural network (TDNN) model. Some comparisons are made with linear time-series models represented by an autoregressive moving average model and a seasonal autoregressive integrated moving average model, and the results indicate that the TDNN model provides better forecasting performance up to one hour ahead.

The skulling behavior in the blast furnace (BF) hearth has yet to be investigated as few (if any) industrial/experimental studies with particular focus on hot metal are reported in the open literature. As a necessary first step toward a better understanding of the sophisticated behavior, an experimental technique is introduced in the present paper. The experimental apparatus, which mainly consists of a vertical tube furnace, a rotating and moveable pedestal, and a moveable water‐cooled probe covered with a multi‐layer structured refractory sleeve can utilize industrial coke, pig iron, and BF hearth carbon brick as raw materials. The technique is shown to be capable of producing chemical, thermal, and mechanical conditions similar to those in the real process. The feasibility and potential of the technique are demonstrated by a set of experimental runs. The results indicate that the air gap between the cooling device and the refractory lining plays a decisive role in both skull formation and lining erosion. Furthermore, the microstructure of graphite precipitated during solidification is influenced by the cooling rate, which in practice is affected by the BF hearth operating parameters. It is hoped that the current contribution will stimulate the growing research interest in this subject.

To monitor and diagnose the erosion status of the blast furnace hearth lining during operation, this study utilises the high precision and rapid processing capabilities of ANSYS finite element software for secondary development. It integrates temperature data from various thermocouples at the blast furnace hearth bottom with samples for erosion thickness calculation, developing a model based on extensive datasets of erosion status. This model can display isotherms for any user-specified temperature and combines calculated longitudinal profile data to create a three-dimensional representation of the hearth bottom through the secondary development of Solidworks 3D modeling software. This flexibility allows for an arbitrary view of the blast furnace's erosion state. This method significantly enhances both the speed and accuracy of calculations compared to other erosion models. It addresses the challenge of constructing a reliable heat transfer model for the furnace hearth and bottom when numerous thermocouples fail in the later stages of furnace operation. By improving the model's applicability throughout the entire lifecycle of the blast furnace, it ensures real-time feedback to users by continuously calculating the erosion status.

A study on liquid flow in the blast furnace hearth

In order to clarify the causes of the excess accumulation of molten materials in the blast furnace hearth which results in blast furnace troubles, such as hanging and slipping, the behaviour of flowing out and accumulation of slag in the furnace hearth is investigated.The results of model studies of slag flow in the furnace hearth during tapping were analyzed as the scaleup problem based on the theories of fluid dynamics. The dimensionless flow-out coefficient FL in eq.(32) has been found to be closely related to the slag residual ratio, i.e. the ratio of residual amount of slag at the end of tapping to that accumulated at the begining (Fig. 8).Further investigation using this relation shows that eight independent variables, i.e. viscosity, tapping amount, tapping rate and depth of slag, hearth diameter, effective hearth area, number of tapping and permeability of packed coke, determine the behaviour of flowing out and accumulation of slag in the furnace hearth. The results of investigations are as follows;1) The residual amount of slag and the depth of slag layer increase with the increase in slag tapping rate and of slag viscosity.2) The increase of the number of tapping operation is beneficial in maintaining the smooth furnace operation, if the tapping rate and/or slag viscosity increase.

The blast furnace ironmaking is a “black box” operation and the blast furnace hearth plays a vital role in production. In this paper, the blast furnace hearth visualisation system is based on the actual blast furnace production data and the raw data processing is completed by feature engineering. It is worth noting that firstly, based on heat transfer and finite element method, BP neural network is used to predict and simulate the erosion state of the furnace hearth. Secondly, the temperature measurement points and temperature field derived parameters of the furnace chamber area are visualised, and the XGboost algorithm is used to achieve accurate prediction of key parameters; finally, the online operation of the blast furnace hearth visualisation system is realised based on the industrial internet platform, contributing to the intelligence of blast furnace ironmaking. Finally, the online operation of the blast furnace hearth visualisation system is realised based on the industrial internet platform, contributing to the intelligence of blast furnace ironmaking.

Stable blast furnace operation is required to reduce energy consumption in iron and steelmaking industry. For the stable blast furnace operation, precise controlled drainage is one of the important factors. However, the effects of the various in-furnace conditions on the stable operation were not examined well. Therefore, in this work, basic characteristic features of drainage in a blast furnace hearth were examined.Two- and three-dimensional mathematical model were developed based on the finite difference method to simulate molten iron and slag flow in a hearth of a blast furnace. Pressure drop evaluation model in a taphole was developed to reflect pressure variation in a blast furnace hearth on drainage rate of molten iron and slag for the three-dimensional mathematical model.The two-dimensional mathematical model results were validated with measured interfaces shapes obtained using an experimental model. The three-dimensional mathematical model results were validated with measured total, iron and slag drainage rate of Chiba No. 6 blast furnace. The results indicate that the drainage behavior and residual iron and slag volume were affected by the conditions in the hearth. The taphole conditions dominate the total drainage rate under the term of assumed blast furnace conditions. In order to reduce the residual slag volume, the taphole diameter change during the tap should be controlled. The decrease of the coke diameter causes increase of the residual slag volume, decrease of the residual iron volume.

An in-depth study on the characteristics of coke in the hearths of blast furnaces is of great significance for explaining the mechanism of coke deterioration in blast furnaces. In the present work, the changes in macromorphology, degree of graphitization, and microstructure of the coke taken from different hearth locations of a 5,800 m3 superlarge blast furnace during its intermediate repair period were systematically studied. Significant differences were found between cokes obtained from the edge ("edge coke") and from the center ("center coke") of the hearth in terms of properties and degradation mechanisms. Edge coke was severely eroded by liquid metal, and only a small amount of slag was detected in the coke porosity, whereas center coke was basically free from erosion by liquid metal, and a large amount of slag was detected in the coke porosity. The degree of graphitization of edge coke was higher than that of center coke. The carburizing effect of liquid metal was the main cause of the degradation of edge coke and made it smaller or even disappear. Center coke was degraded due to the combination of two factors: slag inserted into micropores on the surface of center coke loosened the surface structure; and graphite-like flakes that appeared on the center coke surface lowered the strength and caused cracks in the surface.

An in-depth study on the characteristics of coke in the hearths of blast furnaces is of great significance for explaining the mechanism of coke deterioration in blast furnaces. In the present work, the changes in macromorphology, degree of graphitization, and microstructure of the coke taken from different hearth locations of a 5,800 m3 superlarge blast furnace during its intermediate repair period were systematically studied. Significant differences were found between cokes obtained from the edge (“edge coke”) and from the center (“center coke”) of the hearth in terms of properties and degradation mechanisms. Edge coke was severely eroded by liquid metal, and only a small amount of slag was detected in the coke porosity, whereas center coke was basically free from erosion by liquid metal, and a large amount of slag was detected in the coke porosity. The degree of graphitization of edge coke was higher than that of center coke. The carburizing effect of liquid metal was the main cause of the degradation of edge coke and made it smaller or even disappear. Center coke was degraded due to the combination of two factors: slag inserted into micropores on the surface of center coke loosened the surface structure; and graphite-like flakes that appeared on the center coke surface lowered the strength and caused cracks in the surface.

To prolong the campaign life of the furnace hearth for high demand in the steel market, the theme of preventing the hearth wall from erosion phenomenon is worthily studied for steel industry. The titanium carbide (TiC) concentration distributions in the blast furnace hearth can be used to suppress the erosion phenomenon of the hearth wall. In this work, we solve the momentum and the thermal-energy-balance equations, as well as the mass transfer equation with chemical reaction effects to investigate the TiC concentration profiles in the hearth of Port Kembla no. 5 blast furnace (PKBF5) by means of a computational fluid dynamics (CFD) package, Fluent (version 6.2). As shown in the results, the elephant foot erosion and pot-like erosion in the hearth may be restrained based on the calculated TiC concentration distributions. Additionally, this work illustrates that deadman type may be inferred based on the calculated TiC concentration profiles when the blast furnace is revamped.

The aim of the work is to study modern ways to increase the operational reliability of the furnace and hearth of blast furnaces, which largely determine the duration of the blast furnace campaign. The article analyzes the ways to increase the stability of the furnace and hearth, presents the results of the analysis of thermal work and ignition of the lining of metal receivers of blast furnaces of different designs. The modern directions of construction of the metal receiver of blast furnaces are determined. It is shown that the modern methodology of construction of blast furnace furnaces develops two main directions: the use of a coordinated combination of refractory materials with a cooling system; use of a combination of wear-resistant materials based on carbon and ceramics. However, even the improvement of the design and cooling system of the metal receiver does not allow to fully increase the duration of the campaign. To assess the service life of the furnace, it is necessary to provide regular automated control of the ignition of the furnace lining and hearth. In Ukraine, during the renovation of blast furnaces, the design of metal receivers with the use of "ceramic glass" was preferred. To date, the system of monitoring the thermal work and ignition of the furnace has been implemented in 10 blast furnaces using the automatic control system "Horn" developed by the HMI NASU. The implementation of continuous control over the ignition of the furnace in blast furnaces allowed us to assess the effect of the use of ceramic cups. The value of heat losses of the furnace and the cost of coke for their compensation are estimated. Methods and models for determining the thermal state and wear of the metal receiver lining based on a combination of calorimetric and thermometric control methods have been developed. Comparison of heat losses of the metal receiver in the cooling system of blast furnaces allows to quantify the thermal performance of controlled areas and the furnace as a whole. It is shown that the specific value of heat loss of the metal receiver per unit volume of the blast furnace can serve as an integral parameter. It is established that the value of specific heat losses per unit volume of the blast furnace with a ceramic cup is ~ 0.4-0.7 kW/m3, which is much less than blast furnaces without it (~ 0.9-1.1 kW/m3). Ceramic glass saves coke about 1 kg/t of cast iron.

The materials’ properties in the hearth of the blast furnace are very crucial for the hearth conditions. In this study, a number of coke, slag, metal, and aggregate samples were collected from the hearth of the LKAB’s experimental blast furnace (EBF). Subsequently, the coke, slag, and metal samples were chemically analyzed by X-ray fluorescence (XRF) or optical emission spectrometer (OES); the aggregate samples were analyzed by scanning electron microscope combined with energy-dispersive X-ray spectroscopy (SEM/EDS). The possible flow field of the liquid in the EBF hearth before quenching is depicted according to Cu tracers in the metal samples. Selected elements in the coke, slag, and metal were mapped for two sampling layers in the hearth, as well as in one cross section of the flow field. The results indicate that there exists an area beneath, and in front of, tuyere 3, where the flow resistance of the liquid was high. The high flow resistance contributed to the formation of a cold zone in the close-to-wall region and at the bottom of the EBF hearth. The temperature distribution in the EBF hearth has significant impacts on the chemical properties of the materials in different positions of the EBF hearth, as well as on the radial and vertical distributions of certain elements/components.

Stable blast furnace operation is required to reduce energy consumption in iron and steelmaking industry. For the stable blast furnace operation, precise controlled drainage is one of the important factors. However, the effects of the various in-furnace conditions on the stable operation were not examined well. Therefore, in this work, basic characteristic features of drainage in a blast furnace hearth were examined.Two- and three-dimensional mathematical model were developed based on the finite difference method to simulate molten iron and slag flow in a hearth of a blast furnace. Pressure drop evaluation model in a taphole was developed to reflect pressure variation in a blast furnace hearth on drainage rate of molten iron and slag for the three-dimensional mathematical model.The two-dimensional mathematical model results were validated with measured interfaces shapes obtained using an experimental model. The three-dimensional mathematical model results were validated with measured total, iron and slag drainage rate of Chiba No. 6 blast furnace. The results indicate that the drainage behavior and residual iron and slag volume were affected by the conditions in the hearth. The taphole conditions dominate the total drainage rate under the term of assumed blast furnace conditions. In order to reduce the residual slag volume, the taphole diameter change during the tap should be controlled. The decrease of the coke diameter causes increase of the residual slag volume, decrease of the residual iron volume.