China's Refractories

Three groups of cylinder specimens (φ10 mm×10 mm) with 20%, 30% and 40% (by mass, the same hereinafter) of ZrO₂, were prepared using analytically pure MgO, Al₂O₃ and ZrO₂ powders as the starting materials. The specimens were fired at 1 600 ℃ for 5 h, 1 700 ℃ for 0 h and 1 800 ℃ for 7 h, respectively. The microstructure of the MgO-MgAl₂O₄-ZrO₂ subsystem was studied by FESEM. The results show that (1) in the temperature range of 1 600-1 700 ℃, solid phase reactions happen in the MgO-MgAl₂O₄-ZrO₂ subsystem, leaving three phases: periclase, spinel and ZrO₂ (solid dissolving with Mg²⁺ ) co-existing; spinellization affects the densification; (2) the specimen with 40% ZrO₂ fired at 1 800 ℃ has heterogeneous melting microstructure; some areas form typical ternary eutectic MgO-MgAl₂O₄-ZrO₂ with composition of MgO 35.2%-37.2%, Al₂O₃ 24.2%-26.6%, and ZrO₂ 37.2%-39.8%.

The development status of the downstream industries of refractory industry, such as iron & steel, cement, glass, and nonferrous metals was analyzed, and it was pointed out that the market demand of refractory industry entered a platform period. Effect of supply-side reform on the development of refractory industry was deeply discussed, the viewpoints of multi-dimensional innovation of refractory industry were put forward, and the development trend of automation and intelligence in refractory industry was emphasized. It was clarified that the “Belt and Road Initiative” brought new opportunities and pointed out the development direction of China’s refractory industry.

In this paper, calcium hexaaluminate (CA₆) porous ceramics were prepared by a foaming method combined with cement solidification molding. The effects of the foaming agent addition on the microstructure and the properties of the ceramics were studied. The results show that the microstructure and the porosity of the ceramics can be controlled by adjusting the addition of the foaming agent. As the addition of the foaming agent increased, the bulk density reduced from 1 g · cm^-3 to 0.35 g · cm^-3, the porosity increased from 70.4% to 89.6%, the compressive strength reduced from 18.46 MPa to 1.66 MPa, and the thermal conductivity at 1 000 °C decreased from 0.382 W · m^-1 · K^-1 to 0.144 W · m^-1 · K^-1. It is indicated that calcium hexaaluminate porous ceramics with low density, high strength and low thermal conductivity can be obtained by the foaming method. As the lining material of high temperature electric furnaces, the ceramic has excellent energy saving effect.

During calcining the aluminum hydroxide according to the Bayer process in a rotary kiln, the process of incomplete conversion to alpha alumina by soft calcination leads to a special intermediate alumina product containing microcrystalline of primary crystal alpha alumina particles less than 0.4 μm, and other alumina phases with finer microcrystalline of less than 0.2 μm particles. This special alumina powder has similar properties to silica fume or clay, such as high surface area and reactivity, fast wetting, low water demand and high stick-ability. It can partially or completely replace silica fume and clay in dry gunning mixes. It works towards improving the high temperature resistance of the gunning material matrix as well as the corrosion resistance and service life of the gunning materials. The application results show that: the ladle cement-free alumina-magnesia dry gunning mix with this special alumina sub-micro powder, compared with traditional dry gunning materials, has lower rebound loss (<10%), better adhesion rate, higher corrosion resistance and peeling resistance; and it can replace the veneering casting repair to achieve the similar performance. A further improvement goal is to replace the initial casting linings, thus this concept will give a new revolution to the monolithic steel ladles.

The structure of SEN and upper nozzles was optimized and simulated. Relevant trials were conducted on site during the casting of Al-killed steel. These optimizations effectively solve the nozzle clogging and reduce the defects of the casting slab.

In order to improve the flow ability and early-stage strength of colloidal alumina bonded corundum castables, the colloidal alumina with 20% of solid content was used as the binder, and tabular corundum aggregates and fines, and active alumina micropowder CL370 as the main starting materials. Influences of setting accelerating agents (magnesia fines and sodium meta-aluminate) on the linear change rate, the modulus of rupture and the microstructure of corundum castables after different heat-treatments were researched. The results show (1) cement bonded corundum castables have the highest modulus of rupture after 24 h of demolding, and colloidal alumina bonded corundum castables have the lowest one; both magnesia fines and sodium meta-aluminate can improve the strength after 24 h of demolding; (2) after firing at 1 000 ℃ for 3 h, the modulus of rupture of all specimens decreases, even lower than that after 24 h of demolding; (3) after firing at 1 600 ℃ for 3 h, the castable specimen added with 0.1 mass% magnesia fines has the highest modulus of rupture; the tabular phases appear interwovenly in the specimen; (4) taking the comprehensive properties into account, 0.1 mass% of magnesia fines is appropriate, and the addition of sodium meta-aluminate shall be lower than 0.3%.

In this paper, an ideal structural model and a scientific and practical mathematical model of high temperature permeability testing technology were established based on the formula of gas permeability at room temperature according to Darcy’s equation and Forchheimer equation and combined with the basic law of gas mechanics and the resistance loss of gas movement process at high temperatures. Through a gas heater, the safe heating of gas from room temperature to the test temperature was realized; the pressure sealing of high temperature permeability test technology was studied using high temperature resistant flexible material instead of silica gel material and combining preloading with system expansion and continuous loading. Based on the above research, a high temperature gas permeability tester for refractories has been developed and the test temperature can be up to 1 000 ℃. The equipment was applied to research refractories, showing well using effects.

The factors restricting the life of the refining furnace cover were introduced, including the airflow erosion of the refining dust removal system, the melting loss caused by the arc radiation of the electrode, the chemical erosion and penetration of slag and gas, and the condition of refining slag. The improvement measures are adjusting the material of the small furnace cover from corundum to chrome corundum, using a large shaking table to vibrate, optimizing the size design of the small furnace cover, and appropriately thickening the weak areas in the triangular area. The average service life of the refining furnace cover has been increased from one week to two months, reaching 4 maintenance cycles, which meets the needs of the refining production.

The steam oxidation of Si₃N₄-bonded SiC was determined at 1 000 ℃ for 50, 100, 150, 200, 250 and 300 h, respectively, according to ASTM C863-2000. The evolution of the phase composition and the microstructure as well as their relationship was investigated by XRD and SEM. The results show that the oxidation rate of Si₃N₄-bonded SiC is periodic. The presence of nitrogen element can impede the crystallization of SiO₂ glass; the local enrichment of CaO impurities is unfavorable for the existence of fibrous SiO₂. SiO₂ mainly exists as cristobalite when the CaO/SiO₂ ratio reaches a suitable level, but gradually transforms to quartz along with the oxidation time when the SiO₂ content increases, or the CaO/SiO₂ ratio decreases, due to the insufficient mineralization of CaO. The crystallization of SiO₂ glass, especially the formation of quartz is the key factor leading to the volume expansion and structural stress. When the cracks extend and reach the surface, the degradation of the material accelerates.