China's Refractories

The abnormal corrosion of hot iron ladles was investigated. The performance, the composition and the structure of bricks for hot iron ladles were analyzed. The results show that (1) compared with the alumina-silicon carbide-carbon bricks for the ladle bottom, those for the ladle wall have more pyrophyllite and the Al₂O₃ content of 36.32 mass%; their bulk density, apparent porosity and cold compressive strength are lower than the requirement of industry standard; they have poor anti-oxidation performance and are oxidized to form a porous layer during service, which loosens the brick lining structure thus leading to fracture, local wear and structural damage of bricks; (2) without preheating, steel scraps are not completely melted, resulting in slag or steel attachment at the mouth or the bottom of ladles thus increasing damage of ladles; (3) and the residual bricks react with the attached slag to form low melting point phases affecting their hot properties. The refractories for the lining of hot iron ladles must be improved in combination with process changes, not entirely by raw materials replacement to reduce costs.

Three lightweight Al₂O₃-MgO castables were fabricated with tabular alumina or microporous corundum as the aggregates, reactive α-Al₂O₃ micropowder, tabular alumina powder, and fused magnesia powder as the matrix, calcium aluminate cement as the binder, and MgO ultrafine powder (d₅₀=5.4μm) and Al(OH)₃ ultrafine powder (d₅₀=8.2μm) as additives. The influence of aggregates and ultrafine powders on the properties, including pore size distribution, heat conductivity, thermal shock resistance, and slag resistance of lightweight refractory castables was investigated. The results show that the incorporation of microporous corundum reduces the bulk density of Al₂O₃-MgO castables, and MgO and Al(OH)₃ ultrafine powders further increases the proportion of micropores in castables, which is beneficial to reducing the heat conductivity, and improving the thermal shock resistance and slag resistance of castables. Additionally, MgO ultrafine powder and Al(OH)₃ ultrafine powder increase the fluidity and the strength of castables.

In order to improve the anti-explosion performance of ρ-Al₂O₃ bonded corundum castables, H₂O₂ was added (0, 0.025%, 0.050%, 0.075%, 0.100% and 0.125%, by mass) as the anti-explosion agent. After mixing and casting, specimens were prepared. Some specimens were cured at room temperature for 12 h and demoulded for the anti-explosion performance test at different temperatures (450, 500, 550, 600, 650, 700, 750 and 800 ℃); the other specimens were cured, dried and fired, and tested in terms of the apparent porosity, the density, the cold mechanical properties, the air permeability and the pore size distribution. The results show that: (1) with the increase of the H₂O₂ addition, the anti-explosion performance of castables increases gradually, the average pore size increases gradually, and the density and the strength decrease gradually; (2) by comprehensive consideration, the appropriate addition of H₂O₂ shall be within 0.075%.

To extend the service life of the upper checker bricks in the regenerator of glass furnaces with petroleum coke as fuel, the corrosion resistance of magnesia-chrome and alumina-chrome bricks with the similar apparent porosity was systematically researched. The results show that the Cr₂O₃ content and the microstructure present significant effects on the corrosion resistance. The molten corrosion reagent forms silicate and vanadate phases with low melting points between MgO crystals in magnesia-chrome bricks, and the volume strain generated by the melting and solidification process leads to the cracking and spalling of refractories, which intensifies the corrosion process. The encapsulation of alumina particles by alumina-chrome solid solutions in alumina-chrome bricks avoids contact with the low melting point liquid phases and improves the corrosion resistance of refractories.

Using zircon, boric acid and carbon black as starting materials, ZrB₂-ZrO₂-SiC composite powder was synthesized by calcining at 1 500 ℃ in flowing argon atmosphere. The effects of the soaking time (3, 6 and 9 h) and the addition of additive AlF₃ (0, 0.5%, 1.0%, 1.5%, 2.0% and 2.5%, by mass) on the phase composition and the microstructure of the synthesized products were investigated. The results show that: (1) ZrB₂-ZrO₂-SiC composite powder can be synthesized by carbothermal reduction at 1 500 ℃ in flowing argon atmosphere; ZrB₂ and ZrO₂ are granular-like, and SiC crystals are fiberous; (2) with the soaking time increasing, the amount of ZrB₂ increases, the amounts of m-ZrO₂ and SiC decrease, and the total amount of non-oxides ZrB₂, SiC and ZrC gradually increases; the optimal soaking time is 3 h; (3) compared with the sample without AlF₃, the sample with 0.5% AlF₃ has decreased m-ZrO₂ amount, noticeably increased ZrB₂ amount but decreased SiC amount; however, when the addition of AlF₃ increases from 0.5% to 2.5%, the m-ZrO₂ amount increases, the ZrB₂ amount decreases, and the SiC amount changes slightly; the optimum AlF₃ addition is 0.5%.

The dissolution of alumina-based refractory ceramics in CaO-Al₂O₃-SiO₂ slag melts was performed based on the in-situ observation system of an ultra-high-temperature laser confocal microscope, and the effect of the CaO/SiO₂ slag mass ratio (C/S ratio) on the dissolution rate of alumina-based refractory ceramics was investigated. The results indicate that the dissolution rate increases with an increase of the C/S ratio and is mainly controlled by diffusion. During the early stage of dissolution, for all C/S ratios, the dissolution process conforms to the classical invariant interface approximation model. During the later stage of dissolution, when the C/S ratio is ≥6, the dissolution process is significantly different from the model above because of the formation of a thick interfacial layer, which can be explained by dissolution kinetics.

Ceramic slurry of 78 mass% solid loading was prepared using photosensitive acrylic resin and dispersant SP-710 as the liquid phase, Al₂O₃ powder (d₅₀=2.38 μm) and TiO₂ powder additive as the solid phase. Alumina ceramics were prepared by DLP, sintering for 4 h at 1 450, 1 500, 1 550 or 1 600 ℃, respectively. The effects of the TiO₂ addition (0, 1%, 2%, 3% and 5%, by mass) on the properties of the ceramics were studied. The results show that the addition of TiO₂ can improve the sintering of Al₂O₃ ceramics, significantly improve the densification, and reduce the sintering temperature. With the optimum TiO₂ addition of 3% and the optimum sintering temperature of 1 600 ℃, the obtained Al₂O₃ ceramics have shrinkage of 15.7%, 15.8% and 23.8% at the x axis, the y axis, and the z axis, respectively, the porosity of 2.4%, the bulk density of 3.74 g·cm^-3 and the three-point bending strength of 251.1 MPa. Compared with the undoped alumina ceramics, the doped alumina ceramic has increased bulk density by 0.56 g·cm^-3, decreased apparent porosity from 20.2% to 2.4%, and the three-point bending strength increases by 2.5 times. Therefore, the density and the strength of DLP prepared ceramics can be improved effectively by adding an appropriate amount of TiO₂, and the performance of the DLP prepared ceramics is close to that of the pressed samples. Thus, it is hopeful to apply DLP in refractories field.

Aluminous refractory materials with high alumina contents are widely used in the steel industry, and the higher the alumina content, the higher the working temperature. Properties such as high refractoriness and thermal shock resistance lead these refractory materials to be used as channel linings of blast furnaces, where they are exposed to the attack by slag, molten steel, working cycles and sudden temperature changes between 25 ℃ (room temperature) and 1 520 ℃ (the temperature of molten pig iron). In this work, microstructural changes in post-mortem aluminous refractory bricks were investigated by apparent porosity, X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy, and X-ray dispersion energy spectrometry (SEM/EDS). The results showed an increase in the apparent porosity and the bulk density and the presence of the phases mullite, sillimanite, alumina, and quartz in the post-mortem brick. Calcium and magnesium were not detected in the microstructure of the post-mortem brick, indicating that slags did not corrode these refractory materials. Therefore, the microstructural changes that occurred in the post-mortem bricks must be due to thermal cycling. In the X-ray diffraction (XRD) test, mullite, sillimanite, quartz, and α-alumina phases were identified. These results indicate that the aluminous refractory was obtained from sillimanite. In infrared spectroscopy (FTIR) it was possible to identify the vibration bands referring to the Si-O and Al-O bonds. The increase in the porosity is a result of cracks caused by work cycles at high temperatures and the temperature gradient to which the refractory was subjected during use. Through the micrograph it was possible to identify the presence of acicular mullite. The absence of magnesium and calcium in the microanalysis results by energy dispersed X-ray spectrometry (EDS) indicates that there was no infiltration by slag or liquid iron. These results indicate that the microstructural changes that occurred in the post-mortem aluminous refractory were of a thermal nature.

To reduce production costs and make full and reasonable use of raw materials, high alumina bricks were prepared using tabular corundum and mullite as aggregates, sillimanite as intermediate particles, and white fused corundum powder, α-alumina micropowder, and Suzhou soil as the matrix, firing at different temperatures (1 420, 1 440, 1 460, 1 480, 1 500 and 1 520 ℃) for 4 h. The apparent porosity (AP), the bulk density (BD), the cold crushing strength (CCS), the thermal shock resistance (TSR), the refractoriness under load (RUL) and the creep rate of the samples were tested. The effects of the firing temperature on the creep rate (1 450 ℃×50 h, under a load of 0.2 MPa) of the samples were studied. The results show that with the sillimanite addition of 22.5 mass%, the sample fired at 1 460 ℃ for 4 h performs the best comprehensive properties: the AP of 17.5%, the BD of 2.75 g·cm^-3, the CCS of 100.5 MPa, the TSR number of 35 cycles, the RUL of 1 682 ℃, and the creep rate of -0.428%, which can prolong the service life of furnaces.