China's Refractories

A SiC assembled large block for blast furnace tuyeres was prepared using silicon carbide particles (3-1 and 1-0.088 mm) and fine powder (<0.088 mm), silicon powder (<0.088 mm), industrial carbon black (N990), microsilica,ρ-Al₂O₃ powder, etc. as raw materials. The developed block was compared with a silicon nitride bonded silicon carbide brick, a self-bonded silicon carbide brick and an imported self-bonded silicon carbide block to analyze and evaluate their service performance. The results show that: (1) in the 0-100 mm zone, the SiC large block mainly consists of β-SiC and nitrides such as O’-SiAlON, β-SiAlON, α-Si₃N₄, and Si₂N₂O, the bulk density is 2.68-2.70 g·cm^-3, the apparent porosity is 14%-15%, and the material structure is uniform; (2) in the 0-100 mm zone, β-SiC nano-whiskers intercalate with nitrides; with the depth increasing, the number of flocculent β-SiC nano-whiskers increases, while the number of nitrides decreases; when the depth reaches 150 mm or more, the main bonding phases are β-SiC and mullite; (3) compared with the reference products, the developed SiC large block has a good basic performance, and after alkali corrosion, the mass change rate is -0.1%, which is obviously superior to the imported self-bonded silicon carbide and the homemade silicon nitride bonded silicon carbide materials.

Alumina-magnesia dry materials are widely used in induction furnace linings, but they show different kinds of damage when melting different kinds of alloy steel. In this paper, the chemical composition, phase composition, and microstructure of the post-use dry materials for the working liners melting different kinds of steel were evaluated. Furthermore, the corrosion mechanism of the steel on the furnace lining materials was comprehensively analyzed. The findings reveal a significant ability of the Mn element in the molten steel to diffuse and penetrate into the refractories. Mn oxidizes to form MnO at the steel-refractory interface, and then forms a liquid phase with Al₂O₃. The Cr element is dissolved into corundum and spinel of the refractories, resulting in lattice defects and structural damage of the materials. TiO₂ reacts with Al₂O₃ to form Al₂TiO₅, which plays a crucial role in preventing crack formation and propagation. Part of Ti⁴⁺ dissolves into magnesia-alumina (MA), densifying the materials. TiO₂ also slows down the reaction between the Cr element and refractory components, further improving the corrosion resistance of the materials.

Al₂O₃-MgO and Al₂O₃-spinel low cement castables (LCC-AM and LCC-AS) have been extensively used in steel ladles as working linings. Nevertheless, the use of alumina-magnesia gunning mixes has been mainly kept for maintaining these castable linings, because of high rebound loss, poor green strength, high porosity and short life-span. Thanks to a high BET alumina (MC-G), it is now possible to develop a series of high-performance no-cement or low-cement Al₂O₃-MgO gunning mixes (NCG-AM or LCG-AM). The paper focuses on the BOF slag resistance of NCG-AM, LCG-AM, LCC-AM and LCC-AS. The corrosion mechanisms of rotary slag samples were studied by scanning electron microscopy (SEM/EDS). The results reveal different microstructures around MgO particles, depending on the four used compositions. Continuous and thicker spinel phases were formed in NCG-AM, which was proved to have the best corrosion resistance after the dynamic slag test. MC-G can provide a high diffusion flux of Al³⁺ in terms of kinetics and hence inhibits Kirkendall porosity around MgO particles. In addition, a continuous spinel phase acts like a pinning nail to reinforce the matrix and thus decreases erosion by slag. In contrast to NCG-AM, the porous spinel phase was found around unreacted MgO particles and some particles were carried away near the interface of LCC-AM and slag. The NCG-AM containing MC-G had been tested in two steel plants, and it extended the service life of the ladles up to 50%. In addition, this study suggests the potential application of NCG-AM as steel ladle linings.

Boron carbide (B₄C) has excellent high-temperature oxidation resistance, high hardness, low relative density, high melting point and excellent abrasive resistance, which is widely used in fields such as refractories, wear-resistant materials and lightweight protective materials. The research progress and application of B₄C materials in China and overseas in recent years were summarized. The influences of sintering processes (pressureless sintering, hot-pressing sintering, hot isostatic pressing sintering, spark plasma sintering and microwave sintering) and sintering additives (simple substances, oxides and carbides) on the B₄C densification were analyzed. The development of B₄C materials was prospected.

In order to improve the properties of corundum based dispersive purging plugs, dispersive purging plug specimens were prepared using tabular corundum (1-0.15 and ≤0.15 mm) as the aggregates, tabular corundum (≤0.044 mm) andα-Al₂O₃ micropowder (d₅₀=0.6 μm) as the matrix, Secar 71 cement as the binder, introducing different amounts of nano-CaCO₃, casting into shapes, and firing at different temperatures (1 200 or 1 600 °C) for 4 h. The effects of the nano-CaCO₃ extra-addition (0, 0.5%, 1.0%, 1.5% and 2.0%, by mass) on the consistency of the castables as well as the properties and microstructure of the dispersive purging plug specimens were studied. The results show that: (1) with the fixed water addition, the consistency of the corundum castables decreases as the nano-CaCO₃ addition increases; (2) with the increasing nano-CaCO₃ addition, the bulk density of the specimens fired at different temperatures for 4 h decreases, the apparent porosity, the cold strength and the hot modulus of rupture all increase, the gas permeability does not change significantly; (3) the specimens fired at 1 600 °C for 4 h have obviously better cold comprehensive performance than those fired at 1 200 °C for 4 h; (4) when the nano-CaCO₃ addition is 1.5%, the comprehensive performance of the specimen is the optimal.

Low grade magnesite is one of the main research directions in the future as the raw material for the preparation of magnesia based insulating refractories. Periclase-forsterite (MgO-Mg₂SiO₄) lightweight insulating refractories were prepared by the molten salt method with high silica magnesite and tertiary talc ore as raw materials by pretreating them to get light burnt magnesia and talc, and NaCl molten salt as the reaction medium. The effects of the NaCl addition, the sintering temperature, the holding time and the raw material ratio on the sample preparation were studied. The results show that when the NaCl addition is 20% of the mass of light burnt magnesia and talc mixture, the sintering temperature is 1 200 ℃, the holding time is 6 h, and m(light burnt magnesia) : m(talc)=5 : 5, the sample has the optimal comprehensive properties: the bulk density of 1.46 g·cm^-3 and the apparent porosity of 55.0%. In addition, it is found that self-decomposition of talc and the formation of forsterite can form pores inside the sample.

In order to enhance the sintering resistance of lanthanum aluminate based high emissivity coatings, Ca²⁺-Fe³⁺ doped LaAlO₃/ZrO₂ specimens were prepared by solid state sintering using La₂O₃, Al₂O₃, CaCO₃ and Fe₂O₃ as raw materials, extra-adding fused Y₂O₃-stabilized ZrO₂ (0, 5%, 10%, and 15%, by mass), ball milling, pre-calcining, pressing into shapes and sintering at 1 600 ℃ for 2 h. The effect of the ZrO₂ addition on the properties of lanthanum aluminate based high emissivity coatings was explored. It is concluded that ZrO₂ does not react with LaAlO₃, which can inhibit the sintering of the specimens. With the increase of the ZrO₂ addition, the linear shrinkage rate and the thermal conductivity decrease significantly, while the emissivity decreases, but the emissivity of all the specimens is all higher than 0.9. The optimal addition of ZrO₂ is 10%.

With the rapid industrialization and urbanization, the demand for air quality management is more and more urgent. High temperature dust filtration is one important environmental management technology. Porous ceramics are used as filter materials in the field of high-temperature dust filtration because of their unique advantages such as high filtration efficiency, as well as high temperature stability, particle loss resistance, corrosion resistance and durability. This paper mainly introduced several common preparation techniques of porous ceramics, including the traditional organic foam impregnation method, foaming method, in-situ combustion method, pore-forming method and other new methods such as the template method, gel injection molding method, freeze-drying method, multi-component co-precipitation method and hydrogel method. The principle, advantages and disadvantages of these preparation technologies and their research status were described. The application of these technologies in the field of high temperature dust filtration was briefly reviewed. Finally, the application prospect of the porous ceramics in the field of high temperature dust filtration was prospected.