China's Refractories

The use of a combined additive consisting of silica-containing additives “F” (thinning) and “S” (accelerating castable hardening) instead of the previously used silica-containing additive “A” and a mixture of dispersing additives based on polyethylene glycol in low-cement silicon carbide castable allows to improve the castable workability by increasing (in 5.4 times) the flowability of a freshly prepared castable, and increase by 1.2 and 1.1 times the cold crushing strength of heat-treated at temperatures of 110 and 1 580 °C, respectively, castable samples from low-cement silicon carbide castable with practically equal values of open porosity (15.8% and 16.0%). The petrographic studies of the castable sample microstructure after firing at a temperature of 1 580 °C showed that the investigated silica-containing additives “F” and “S” contribute to the denser and more uniform structure formation. As a result of the carried out research, the manufacturing technology of a low-cement silicon carbide castable was improved, of which the production was mastered at JSC “URIR named after A. S. Berezhnoy”.

This paper analyzes the extreme application conditions of high temperature, high thermal shock and strong erosion of Al₂O₃-SiC-C refractories for molten iron ladles. It is considered that the main damage mechanism of the refractories is chemical corrosion of oxides (high contents of CaO, FeO and MnO) in the materials, reaction melting erosion, oxidation or decomposition of C or SiC in the materials, melting erosion at high temperatures, and damage under high thermal shock. The analysis results show that the low carbon and low silica Al₂O₃-SiC-C materials with high oxidation resistance should be used. The ratio of Al₂O₃ to SiO₂ in the materials should be greater than 2.5, and the andalusite or pyrophyllite with microcracks, Al₂O₃ or Si₃N₄ with in-situ particles or fibers, and the clay or metal with ductility at high temperatures should be added to improve the high temperature corrosion resistance and thermal shock resistance.

According to the dual needs of heat transfer and thermal insulation in incinerators, different parts of the incinerators are designed with refractories of different SiC contents. Water vapor is mainly concentrated in the zones of feeding, drying and combustion, and the main damage factors of SiC refractories include high temperature and hydration. The results of hydration resistance of typical SiC-containing refractories show that the higher the SiC content, the worse the hydration resistance. Si₃N₄-SiC refractories after oxidation treatment have good hydration corrosion resistance, which are appropriate for water-cooled wall of waste incinerators.

Al₂O₃-SiC castables, for the relining of BF hearths in mid-campaign repair, were sampled and investigated after two years’ service. The chemical and mineralogical characteristics of the residual castables were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry to study the erosion mechanism. The results show that as gaseous K diffuses in the castables, leucite (KAlSi₂O₈) and multiple cracks are formed. Molten iron and slag penetrate through the cracks to form anorthite ([Ca,Na][Al,Si]₄O₈) and iron, which will not form an embrittlement layer similar to carbon bricks. The entire repair of the hearth with Al₂O₃-SiC castables combined with the design of thin-wall lining hearth eliminates “elephant foot shaped” erosion, greatly prolonging the service life of the hearth lining.

The mechanical force borne by the lintel and “brackets” in the inclined channel area of CDQ and the thermal stress generated by the top-down temperature gradient were simulated by ANSYS software. The following conclusions are drawn. (1) The mechanical force has very mild effect on the damage of the “brackets”. (2) The temperature gradient caused by the uneven temperature distribution of CDQ oven is the key factor affecting the “bracket” damage. The different expansion of refractory materials in various parts results in the thermal stress concentration which tears the “brackets”. (3) The structure adjustment of the “brackets” has very little effect on the thermal stress distribution. (4) The multi-phase nitride bonded silicon carbide for CDQ can effectively alleviate the stress concentration at the “brackets”, realizing the long-life and stable operation.

This paper summarizes the current situation of China’s industrial hazardous waste treatment, the disposal technology and the characteristics of rotary kiln incineration system, analyzes the refractory lining design of rotary kiln incineration system in China, and puts forward optimization suggestions according to the process characteristics.

Carbon-free Al₂O₃-MgO dense bricks were produced by the pressing method, using tabular alumina, white fused alumina, alumina micro-powder as main raw materials, and inorganic powder as the binder. The comprehensive properties and performance in steel ladle side wall were made a comparison between Al₂O₃-MgO dense bricks and precast blocks. The results show that Al₂O₃-MgO dense bricks exhibit high dense structure and strength, as well as superior thermal shock resistance and better penetration and corrosion resistance to slag than precast blocks. While replacing precast blocks with dense bricks in 250 t steel ladle side wall in some domestic steel mills, the thickness of the metamorphic layer from slag penetration and the corrosion rate decrease evidently. The damage of dense bricks during service is mainly caused by the corrosion from molten steel and slag, and the structure spalling of the metamorphic layer also plays an important role.

This scientific paper discusses the information on the structure of the three-component system FeO-TiO₂-Al₂O₃, which is necessary for the creation of heat-resistant oxide materials. The structure of binary systems: FeO-Al₂O₃, FeO-TiO₂, and Al₂O₃-TiO₂ is described, and the data on the FeO-TiO₂-Al₂O₃ system are presented. The thermodynamic data of all compounds of the system are given, on the basis of which the change in the Gibbs free energy in the temperature range of 8 00-1 900 K for twenty-one exchange reactions was calculated. It has been established that the triangulation of the FeO-TiO₂-Al₂O₃ system changes in five temperature ranges: up to 1 408 K (TiO₂ exists in the polymorphic modification-anatase), 1 408-1 537 K (TiO₂ exists in the polymorphic modification-rutile and pseudobrookite is stable), 1 537-1 630 K (thialite is stable), 1 630-2 076 K (rearrangement cannot occur) and above 2 076 K (the presence of the stoichiometric compound Al₄TiO₈ is allowed). Two-phase equilibria up to 1 408 K are stable: Al₂O₃-FeTiO₃, FeTiO₃-FeAl₂O₄, and FeAl₂O₄-Fe₂TiO₄; in the temperature range of 1 408-1 537 K: FeAl₂O₄-TiO₂, FeAl₂O₄-FeTi₂O₅, FeAl₂O₄-FeTiO₃, and FeAl₂O₄-Fe₂TiO₄; in the temperature range of 1 537-1 630 K: FeAl₂O₄-TiO₂, FeAl₂O₄-FeTi₂O₅, FeAl₂O₄-FeTiO₃, FeAl₂O₄-Fe₂TiO₄, and FeAl₂O₄-Al₂TiO₅; in the temperature range of 1 630-2 076 K: FeTi₂O₅-Al₂TiO₅, Al₂TiO₅-FeTiO₃, FeTiO₃-Al₂O₃, FeTiO₃-FeAl₂O₄, and FeAl₂O₄-Fe₂TiO₄; over 2 076 K: FeTi₂O₅-Al₂TiO₅, FeTi₂O₅-Al₄TiO₈, Al₄TiO₈-FeTiO₃, Al₄TiO₈-Fe₂TiO₄, Al₄TiO₈-FeO, and Al₄TiO₈-FeAl₂O₄.

To satisfy the requirements of alumina castables in specific environment and enhance their properties, alumina castables were prepared using tabular alumina, various alumina powders (07RAL, 15RA, 22RABL, 40CA, 99-20um, and 99-325mesh), and calcium aluminate cement as main raw materials, and heat treating at 110, 1 000, 1 600 ℃, respectively. Their flowability and physical properties were tested, and the morphology was researched. The results show that compared with calcined alumina and tabular alumina powders, reactive alumina has higher specific surface area and better filling performance leading to lower water demand and higher viscosity, and the castables prepared with reactive alumina have higher CCS and CMOR after heat treating at 110, 1 000 and 1 600 ℃ as well as lower PLC than those with calcined alumina and tabular alumina powders.