Year 2017. Volume 37

Abstract

The work presented here deals with simulation assisted evaluation of fracture testing of ordinary ceramic  refractory materials. Two tests are applied. One of them, a wedge splitting test, is already established for this purpose. An inverse evaluation procedure was developed to derive more information from the test results: It enables the simultaneous determination of the specific fracture  energy, the tensile strength and the Young’s modulus. Moreover specific fracture energy can also be determined in the case that the test has to be interrupted at some residual load due to relatively low material brittleness. The other test method, a laser irradiation disc test, was developed in order to determine specific fracture energy and tensile strength for fine ceramic refractory materials behaving relatively brittle. From the time elapsed until crack initiation occurs (t₁) and a stable/instable transition of crack propagation takes place (t₂), respectively, the tensile strength and the specific fracture energy are calculated based on a simulation of the mode I fracture behavior which applies the fictitious crack model according to Hillerborg.

Abstract

This paper deals with the characterization of crack networks on the surface of refractory castable slabs, which are induced by stresses arising during the curing and drying (CD) stages. Images of the surface of castable samples placed in a house-made climatic chamber were recorded during the CD stages with a digital camera and processed by Digital Image Correlation (DIC) to measure the displacement field. Post-processing of DIC results were performed to estimate the length, orientation and opening of cracks in networks. The methodology is detailed and first analyses of experimental results are shown.

Abstract

Gasifiers are reaction vessels used to process carbon feedstock such as coal and/or petcoke at elevated temperature, high pressure, and in a reducing atmosphere (low oxygen partial pressure) to form CO and H2, called synthesis gas or syngas. Syngas is used as a fuel in power generation or as a feedstock material in chemical production. By-products of the gasification process include unreacted carbon, gases such as CO₂ and H₂S, and slag formed from mineral impurities or organic metallic compounds in the carbon feedstock that liquefy during gasification. In the gasifier, slags interact with the high chrome oxide refractory liner, causing wear and eventual failure of the refractory lining by two primary means - spalling (structural and chemical) and chemical dissolution. Failure of the refractory lining causes the gasifier to be shut down for repair, with increased service time identified by users as important for greater usage of gasification as an industrial process. Phosphate additions to high chrome oxide refractories have been found to increase service life during commercial service by reducing spalling and lowering chemical dissolution of the refractory liner. The mechanism of how they improve service life is not well understood. The microstructure and physical properties of high chrome oxide refractories with and without phosphate additions removed from a commercial gasifier after approximately eight months of exposure to a coal slag are evaluated in this report, with the emphasis on evaluating slag/refractory interaction in refractory pores. Details of the investigation are presented and possible mechanisms of how phosphate additives improve wear resistance discussed.

Abstract

The present study aims to analyze the corrosion mechanism of Al₂O₃-SiC-C castables for blast furnace main trough. The anti-oxidation and corrosion properties of the castable caused by slag were studied with residual lining investigation and microstructure analysis. A rapid recognition of grain distribution, ingredients, and mineral phases was achieved by using cathodoluminescence (CL) in conjunction with stereo microscopy. Different materials such as brown fused alumina, spinel, and low-melting CaO-containing phases etc. can be detected due to their distinct fluorescence. The result indicated that when carbon in the matrix of castables was oxidized, its non-wetting property to slag was reduced, which causes slag penetrate into the castable and formed low-melting CaO-containing phases. On the contrary, when the anti-oxidation property of castable was improved, the carbon in the matrix was not easily oxidized and corroded by slag. Furthermore, MgO of slag reacted with Al₂O₃ in the matrix to form a continuous spinel layer, which retarded slag to penetrate and corrode the castable.

Abstract

Corrosion of refractories results from reactive transport namely, transport of agents and chemical reactions of these agents with impregnated medium. On one hand, the transport involves either diffusion or impregnation depending on the state of the corrosive agents and the microstructure of the host media. On the other hand, chemical reactions may be very numerous and complex. This study focused on the reactive impregnation of Al₂O₃-CaO slag into porous high alumina refractory.
Transport properties of the porous media have been assessed by performing wicking test.
Chemical reactions between the solid high alumina skeleton and Al₂O₃-CaO slag involve successive dissolution/precipitation mechanisms forming aluminates of lime. Contrary to the thermodynamic properties of the binary system, the kinetics of these solid/liquid reactions is not well known.Corrosion tests associated with quenching method, XRD and high temperature XRD were performed for a better understanding of the kinetics.

Abstract

Damage of SiC oxide bonded refractories in waste-to-energy facilities (WtE) has been characterized. Different phenomena were observed: wear by slag phases, volume expansion of tiles and fracture in different locations. These results are in agreement with laboratory experiments. The role of gas composition and tiles temperature profile on deposit composition, on condensation of gaseous alkali chloride and on formation of liquid phase inside the porosity of the refractories has been emphasized. Gaseous alkali species are involved, not only in the formation of liquid phases, but also as a precursor of cristoballite formation around the SiC grains as well as in the rich alumina-silica matrix. On the hot face of the refractories, oxo-reduction reactions produce the formation of wollastonite. Post-mortem analysis after several thousand hours of operation point to three main corrosion mechanisms:
- molten salt/slag attack (Na, K, Ca sulphate phases)
- salt condensation inducing new phase formation (SiO₂-CaSO₄-K₂SO₄ or K₂O-Al₂O₃-SiO₂) and SiO₂ cristoballite crystallization
- SiC oxidation, cristoballite and wollastonite crystallisations These mechanisms explain the volume expansion of SiC refractories observed in WtE plants and makes it possible to understand cracks propagation related to stress load. The relative contributions of chemical degradation mechanisms are discussed and different axes of improvement are suggested.

Abstract

The MgO-C bricks are extensively used as lining work on different steel containers. Due to the high temperatures of the process, these refractories are subjected to severe wear and corrosion processes, principally in the area of contact with the slag. These slags have variable contents of CaO, SiO₂, Al₂O₃, MgO and FeO, varying in composition according to the process stage.
In this paper, cup tests were performed at 1650 °C during 2 hours in air, using two commercial MgO-C bricks. They were put in contact with two different grades of slags, one with high basicity (HB) and the other rich in FeO (RF). The corrosion degree suffered by the refractory materials were analyzed and compared. Microstructural observations were performed in order to postulate the probable corrosion mechanisms acting on each material. The results establish that, in the case of HB slag, the attack is carried mainly through the filler region (matrix). In the case of RF slag attack, it is observed that Fe is the main specie that diffuses through the matrix area of the bricks, and to a lesser extent through MgO grains. In both cases, the quality of the raw material used in manufacturing each refractory bricks be playing an important role in the corrosion degree.

Abstract

The addition of biomass in co-combustion reactors for energy production is becoming a promising way for reducing the use of non-renewable fossil fuels. However the presence of alkalis in biofuel can degrade more rapidly the refractory lining of the combustion chamber. The alkalis can penetrate in the refractory and dissolve material components, forming new liquid phases that physically degrade the coherence of the material, increasing the risk of refractory failure.
The use of selection criteria based on this degradation mechanism is useful to a proper choice of materials, limiting the traditional trial-and-error approach.
In this work an integrated approach helping the selection of materials is presented.
The proposed approach combines thermodynamics and laboratory experiments.
Thermodynamic evaluation of the chemical reactions between systems representing the refractory and the aggressive environment of the combustion chamber is used to predict the formation of new phases, potentially altering the refractory structure. Laboratory experiments are performed to evaluate the extent of the modification.
The predictions obtained by the integrated approach have been validated on samples tested in real combustion chambers.

Abstract

This study aims to numerically analyze the refractory wear of the blast furnace main trough. The three dimensional transient Navier-Stocks equation associated with the volume of fluid (VOF) was developed to describe the flow fields of air, molten iron and slag in the main trough of the blast furnace during tapping process; and then solved by the finite volume method (FVM) subject to the pressure implicit with split operator (PSIO). Based on the Newton’s law of viscosity, the computed shear stress profile in the impingement region consists with the erosion rate of main trough from the no. 4 blast furnace at China Steel Corporation (CSC BF4). The influence of the tapping angle and the ratio of iron to slag in tapping stream on the wall shear stress of main trough was also examined for the suggestion to minimize the refractory wear of blast furnace main trough in this work.

Abstract

It is a common practice to design refractory linings with the help of thermal computations, thermochemistry analyses and strong workman know-how. Their mechanical design is often limited to simple thermo-elastic computations. Sometimes computations are refined considering non-linear mechanical behaviour, even if corrosion often induces additional chemical strain and strong change in service of the mechanical behaviour of the refractory. The aim of this presentation is to briefly recast the irreversible thermodynamic framework in order to underline the implications of some basic thermodynamic concepts in term of refractory behaviour modelling. Then, the use of these concepts to develop fully 3D finite element simulations accounting simultaneously for thermal, mechanical and chemistry phenomena will be illustrated on the particular case of SiC-based refractory. Comparison between long duration oxidation test at high temperature and model prediction allows the validation of the proposed approach. Then, an extension to the industrial case of refractory lining in Waste to Energy plant will be illustrated. The interest of taking into account the thermo-chemo-mechanical coupling effects is shown.

Abstract

A thermo-mechanical finite element model was developed by using FEA commercial software, in order to estimate thermal stress levels in a refractory lined steelmaking ladle shell during its first preheating stage. The model considers two refractory linings: an inner working lining and a permanent one besides the external ladle shell. In this way, stresses on both refractory lining and metallic shell due to restricted lining thermal expansion were simulated. Stress-strain curves of refractory lining materials at different temperatures were obtained through lab tests and then used for model optimization. Additionally, creep behavior in the steel shell under different tensile stresses and temperatures was also estimated taking into account previously obtained results from laboratory tests. In plant strain and temperature measurements were done by instrumenting the lower part of the ladle sidewall shell with high temperature strain gauges and thermocouples. Predicted stress evolution showed good agreement with results obtained from in plant measurements.