China's Refractories

Si₃N₄ porous ceramics were fabricated by a combined foam-gelcasting and catalytic nitridation method at 1 473-1 623 K using silicon powder as the starting material, hexadecyl trimethyl ammonium bromide (CTAB) as the foaming agent, and different amounts of micron Fe powder as the catalyst. The effects of the nitridation temperature and the Fe powder addition on the phase composition, the mechanical properties, and the microstructure of the samples were researched. The results show that when nitriding at 1 573 K for 5 h and adding 1 mass% Fe powder (with respect to the Si powder), the sample has a high porosity and suitable mechanical properties: the porosity of 76.5%, the compressive strength of 16.2 MPa, and the specific strength of 22.7 MPa ? cm3 ? g-1.

It is hard to manufacture high purity, dense sintered-magnesia from natural magnesite in the prevailing process of calcining and sintering, for two reasons: a) impurity, and b) low sintering activity of coarse-grained magnesite. In this paper, 95% MgO magnesite was used as the starting material and finally magnesia of high density (3.47 g ? cm-3) and high purity (98% MgO) was obtained after the flotation purification and a three-step sintering process. An industrial scale of the beneficiation system of natural magnesite was introduced for magnesite purification. In lab experimental, the three-step sintering process was implemented by calcining, hydrating, re-calcining and sintering, which has proven the possibility of densified magnesia from natural magnesite.

The article analyzed the main groups of thermal insulation (TIM) and slag-forming (SFM) mixtures produced by leading foreign companies: acidic ones based on the SiO₂-Al₂O₃ system, basic ones based on the CaO-MgO-Al₂O₃ system and medium basic ones based on the CaO (MgO) system-Al₂O₃-SiO₂. It is shown that achieving the necessary properties of the SFM and TIM requires strict observance of the mass content of the components of mixtures produced on the basis of synthetic materials in granular form with a given dispersion. Compliance with a given composition and dispersion is associated with the need to provide a given melting temperature (for intermediate ladles 1 250-1 450 °C, for molds 950- 1 150 °C), neutrality to the cast metal, protection of the melt from secondary oxidation, minimization of heat loss from the melt mirror, absorption of non-metallic inclusions of various natures (Al₂O₃, CaO, SiO₂, TiO2, less often MgO-containing) and sizes; as well as a number of specific requirements: compatibility with the working layer of the lining of the bucket, maintaining the work of the stoppers, minimizing harmful gaseous emissions, etc. It is known that Al₂O₃, TixOy, and CaO refractory inclusions in steel cause the majority of defects on the surface of a cold-rolled sheet, therefore, the aim of the work was to analyze the physicochemical regularities and melting paths of the wollastonite-containing compositions during the assimilation of nonmetallic inclusions by them. To analyze the melting paths of the wollastonite-containing compositions in the CaO-Al₂O₃-SiO₂ system, a diagram of the primary crystallization field of anorthite and the adjacent boundary curves is shown, indicating the directions of temperature drop and designating invariant points. The presented scheme adequately reflects the topological regularities of the state diagram of the CaO-Al₂O₃-SiO₂ system. To analyze the melting paths of the wollastonite-containing SFM compositions during the assimilation of TiOx impurities from steel melts, a diagram of the primary crystallization field of CaTiSiO5 sphene with adjacent boundary curves, the direction of temperature drop, and the designation of invariant points is presented. It is shown that steric and kinetic factors have a predominant effect on the formation of the three-layer layer in the SFM (molten, partially molten and unmelted layers). The influence will be the lesser, the more uniform the thickness of the entire SFM layer (when it is heated from the melt mirror), and the competing processes of solid-phase interaction will start, which provide a gradual accumulation of more refractory compounds and increase the melting temperature of the SFM.

As the essential materials for high temperature industrial production and technological development such as metallurgy, refractories have an important influence on the safety and efficiency of production for high-quality steels. Slag corrosion is one of the main reasons causing the wear of refractories. The slag resistance of refractories can be enhanced by regulation and control of the composition and structure, while applying the external electromagnetic field can achieve the good performance. Electromagnetic field can not only change the thermo-physical properties of slags, but also have a significant effect on the slag resistance of refractories. Suitable electric field can slow down the slag corrosion while the influence of conductivity of refractories is obvious. The magnetic field with millitesla level has a significant impact on the high temperature properties of the slags, and the alternating magnetic field can accelerate the slag corrosion of the refractories, while the static magnetic field has a promising potential to improve the slag resistance of the refractories. Furthermore, the interaction mechanism between refractories and slags under magnetic field needs to be clarified in order to develop the magnetic resistance of slag corrosion of refractories.

Ti(C,N) is a raw material for refractories with excellent properties. The preparation of Ti(C,N) by reducing TiO₂ has the characteristics of low cost and simple process. In this paper, the research status of resin-bonded Al-Al₂O₃ refractories containing TiO₂ under high temperatures and low oxygen partial pressure was introduced.

The application of CaO refractories was limited due to their poor hydration resistance. Introducing appropriate additive is an effective way to improve their hydration resistance. Additive WO₃ was introduced in the preparation of CaO aggregates using a calcination method in this study. The introduction of WO₃ decreased the apparent porosity of CaO aggregates and promoted the grain growth of CaO by the formation of liquid at high temperatures. The average grain size of the specimens with 0.4 mass% WO₃ and treated at 1 550 °C was increased to 51.6 μm comparing to 33.5 μm of those without WO₃ additive. As a result, the hydration resistance of CaO aggregates was significantly improved. Besides, WO₃ sublimated easily, therefore, less WO₃ remained after the heat treatment. The further increase of WO₃ addition had little effect on the hydration resistance of CaO specimens.

Due to the good thermal stability and excellent resistance to slag erosion, used refractories can be recycled as the main raw materials for some refractories. In this article the latest development about used refractories in metallurgical industry has been reviewed, focusing on the results reported in the past decade. The research and reutilization methods of used refractories were discussed. For the research of used refractories, two aspects, the performance and the surface erosion characteristics, were summarized. Then, the advances in research on recycling technology of several main kinds of used refractories, such as MgO-C, Al₂O₃-SiC-C, Al₂O₃-MgO-C, magnesia-chrome, and corundum-spinel refractories were summarized and discussed in detail. Some results of the author’s group were reported accompanied by these comments. The microstructure and the chemical composition were studied by scanning electron microscopy and energy dispersion spectra, and the properties of the refractories were analyzed. Afterwards, the application scope of the materials was determined according to the classification and the analysis of refractories. Finally, the large-scale application of used refractories and an outlook were given to future developments of the entire recycling industry.