China's Refractories

Chrome oxide free refractories have significant economic benefits for the development of gasification technology. Spinel based and SiC based refractories are corroded by coal slag at 1 500 ℃ under reducing atmosphere. Moreover, interactions between refractory materials and molten slag are also predicted by thermodynamic calculations under the same conditions. The results indicate that spinel based refractories are corroded by dissolution of molten slag, while SiC based refractories are corroded by oxidation of CO and FeO. Both of the spinel and SiC based refractories show good corrosion resistance against coal slag by the present experimental tests. Finally, preliminary developed spinel-SiC composite materials are prepared and corroded by coal slag as well, the research of which shows great potential to be used in slagging gasifiers.

In this study, a kind of microporous sintered alumina was analyzed and applied. The microporous sintered alumina has a lot of nano and sub-micron closed pores in the crystals, the average pore size of 140 nm, and the thermal conductivity of 4.864 at 800 ℃, 38% lower than that of common sintered alumina. The microporous sintered alumina was used as the aggregates in alumina magnesia castables for ladles to replace the common sintered alumina. The properties of the microporous alumina castable were measured, and the temperature field of the ladle outer wall was simulated and calculated. The results show that the refractoriness under load (T_0.6) and the hot modulus of rupture (1 400 ℃, 0.5 h) of the microporous castable are the same as the common castable; the thermal conductivity at 800 ℃ decreases by 15%; the retention ratio of modulus of rupture increases by about 5% after five thermal shock cycles (1 100 ℃, water quenching); and the temperature of the ladle outer wall is reduced by 5-41 ℃.

Aiming at prolonging the service life of refractories for direct reduction of laterite nickel ores in rotary kilns, the slag resistance of ten materials (corundum bricks, chrome corundum bricks, silicon nitride bonded silicon carbide bricks, high alumina silicon carbide bricks, high alumina bricks, magnesia chrome bricks, magnesium aluminate spinel bricks, spinel chrome corundum bricks, chrome corundum castables and magnesia alumina chrome composite spinel bricks) was evaluated by rotary slag tests, which simulate the service conditions in rotary kilns. The corroded residual bricks were analyzed by SEM and EDS. The results show that the magnesia alumina chrome composite spinel brick possesses the advantages of magnesium aluminate spinel bricks and chrome corundum bricks; MgO-rich spinel can absorb the penetrated ferric oxide, and forms a dense zeylanite layer, which prevents the penetration of the molten laterite nickel ores; therefore, it is an ideal lining of rotary kilns for direct reduction of laterite nickel ores.

In order to improve the thermal shock resistance of chrome-corundum bricks, different amounts of andalusite were added to the formulation of chrome-corundum bricks to replace the equivalent white fused corundum with the same particle size. After mixing, shaping, drying and firing, the density, the cold strength, the cold wear resistance, the hot modulus of rupture and the thermal shock resistance were tested. XRD, SEM and elemental surface scanning were used to characterize the specimens. The results show that: (1) the volume expansion of andalusite mullitization reduces the apparent porosity of chrome-corundum bricks; (2) the density and the hardness of mullite are lower than those of corundum so the decrease of the corundum content in brick leads to the decrease of the bulk density, the strength and the cold wear resistance; (3) the cross-distributed columnar mullite in the matrix can effectively improve the hot modulus of rupture and the thermal shock resistance of the specimens; (4) considering comprehensively, the andalusite addition shall not exceed 18%.

Light burned magnesia powders with different particle sizes were obtained by calcining crystalline magnesite. Sintered magnesia was prepared from the light burned magnesia powders by adding different amounts of La₂O₃. The effect of the La₂O₃ addition on the densification of magnesia was studied. The results show that the densification of sintered magnesia can be promoted both by reducing the particle size of the light burned powder and by adding small amounts of La₂O₃. The effect of the La₂O₃ addition is more significant: the maximum bulk density of the sintered magnesia can reach 3.49 g · cm^-3 and the apparent porosity can be reduced to 0.33%. During sintering, Ca²⁺ from the impurities enters La₂O₃ lattices, causing lattice distortion of La₂O₃ thus promoting the formation of La_4.67(SiO₄)₃O phase and CaLa₄(SiO₄)₃O phase, which fill the pores between the MgO grains and promote the densification of sintered magnesia.

The primary impregnation slurry was prepared using active alumina (56.25 mass%), kaolin (15 mass%), zirconia (3.75 mass%), deionized water (25 mass%), and extra adding FS (0.2 mass%) and CMC (0.4 mass%). The effects of the active alumina particle size (d₅₀=5.043, 2.934, and 1.629 μm) on the rheology and the thixotropy of the slurry were researched. It was found that the bimodal active α-Al₂O₃ (AMA-10) with d₅₀=1.629 μm was optimum. The secondary impregnation slurry was prepared using AMA-10, kaolin and zirconia as the main raw materials. Then the alumina-based reticulated porous ceramics were fabricated by the organic foam impregnation method combined with a secondary vacuum impregnation process. The influence of the AMA-10 content on the properties of the ceramics was studied. The residual stress of the specimens was analyzed by finite element analysis. The results show that the smaller alumina particle size and multimodal distribution are beneficial to the thixotropy of the primary impregnation slurry. The secondary vacuum impregnation technique can significantly improve the mechanical properties, the thermal shock resistance and the residual strength of the alumina-based reticulated porous ceramics. With the decrease of alumina content in the secondary impregnation slurry, the residual stress of the external layer of ceramic reinforcement gradually changes from tensile stress to compressive stress, which effectively inhibits the expansion of the surface crack, and remarkably improves the crushing strength retention ratio of alumina reticulated porous ceramics.

Refractory materials have been widely used in high-temperature industries, in which the boron content has a great influence on the performance. To control the detection quality and satisfy the requirement of production and scientific research of refractories, conditional tests involving three methods for the boron determination (alkali titrimetry, inductively coupled plasma atomic emission spectrometry (ICP-AES), and curcumin absorption spectrophotometry) were carried out in this work. The affecting factors on the determination of boron were analyzed and discussed by performing the experiment. The results show that compared with curcumin absorption spectrophotometry, alkali titrimetry and ICP-AES have the advantages of simple operation, quick analysis and high precision (relative standard deviation <5%), which are more appropriate for boron determination in refractories.

Si₂N₂O ceramics were prepared using amorphous Si₃N₄ as the raw material and Li₂CO₃ as the sintering additive through vacuum multi-stage sintering. The influence of the Li₂CO₃ addition (0%, 1%, 2%, 3%, and 5%, by mass) on the phase composition, the microstructure, the porosity, the mechanical properties, the dielectric constant and the tangent of the dielectric loss angle of the porous Si₂N₂O ceramics was investigated. The results reveal that a suitable addition of Li₂CO₃ can promote the generation of Si₂N₂O but excessive or inadequate Li₂CO₃ causes decomposition of Si₂N₂O ceramics. The prepared porous Si₂N₂O ceramics have good mechanical properties, good thermal shock resistance, and low dielectric properties, which have excellent potential for application in microwave sintering furnaces.