China's Refractories

Since China’s Reform and Opening-up, the innovation and development of refractories for ironmaking industry in Sinosteel Luonai were narrated, including silica materials, Al₂O₃-SiO₂ refractories and non-oxide composites; and the development direction of refractories for ironmaking industry was prospected.

In order to realize the large-scale and high-value utilization of waste architectural ceramics, high-temperature resistant materials based on waste architectural ceramics were prepared with sodium silicate as the binder, clay/bauxite and metakaolin/bauxite as coating materials, and the cold strength obtaining mechanism was explored. The phase composition, the microstructure and the mechanical properties of the high temperature resistant materials based on waste architectural ceramics were tested and analyzed. The results showed that when the heat treatment temperature was between 110-1 000 ℃, the strength of the samples mainly came from the physical adhesion of sodium silicate and fine powder. When the temperature rose to 1 100 ℃, the strength of the sample was improved since the internal low-melting-point components melted and promoted sintering. The addition of clay and bauxite can effectively enhance the flexural strength of the samples when the heat treatment temperature is 1 000 ℃. When the heat treatment temperature rises from 900 to 1 000 ℃, the flexural strength of the samples will be enhanced owing to the formation of silica alumina spinel and mullite from metakaolin.

Magnesia-chrome refractories are appropriate to work under cyclic temperatures or atmospheric conditions because some oxides of chrome ore readily release oxygen (are reduced) upon heating and pick up oxygen (are oxidized) upon cooling or upon changing the atmosphere. They have been ideal lining materials for RH degassers for many years, despite challenging of Cr⁶⁺ disposal issue after using. The varieties of magnesia-chrome refractories depend on their raw materials and burning temperatures, which could be well demonstrated by their microstructures. But Indian chrome concentrate cannot be directly used in the production of magnesia-chrome bricks because of reducing sintering density. Silicate bonded magnesia-chrome bricks are produced with low purity magnesia after burning at relatively lower temperatures, resulting in liquid film forming as silicate bond around chrome ore particles. Direct bonded and rebonded fused-grains magnesia-chrome refractories are made of high purity magnesia or fused magnesia-chrome grains, forming euhedral and intergranular secondary chromite spinels as the main feature. The amount of secondary spinel increases with the rising burning temperature, leading to increasing hot modulus of rupture and overall hot properties as well. It is important to burn magnesia-chrome bricks under weakly reducing atmosphere. The oxygen content of burning atmosphere from 800 °C to 1 650 °C would better controlled below 0.5% to increase the burnt strength and to avoid inner cracks and loose bonding of magnesia-chrome bricks, while the atmosphere contains 3%-6% O₂ under most economical firing condition.

Thermal-mechanical coupling effect causes a large stress within porous ceramics at high temperatures, resulting in strength attenuation and product reliability decline. Thermal shock resistance is one of the key factors to characterize the reliability of ceramic materials under thermal-mechanical coupling effect. It is important to study the thermal shock resistance of the porous ceramics to evaluate their service performance and improve their service life. To better evaluate the effect of the thermal shock resistance on properties of the porous ceramics, the evaluation theories, experimental characterization methods and influencing factors about the thermal shock performance of the porous ceramics were reviewed in this paper, and some future research directions were prospected.

High thermal conductivity dense silica bricks have the higher thermal conductivity than ordinary silica bricks, which is conducive to the realization of energy saving and emission reduction in the iron and steel industry. The performance of ordinary silica bricks and high thermal conductivity dense silica bricks was compared, and the high thermal conductivity mechanism was analyzed. The results show that (1) compared with ordinary silica bricks, high thermal conductivity dense silica bricks have the characteristics of higher thermal conductivity, lower apparent porosity, higher tridymite content, higher compressive strength, and higher thermal expansion; (2) by increasing the tridymite content and reducing the porosity, the close packing of honeycomb α-tridymite improves the density and continuity of the SiO₂ frame structure of the silica bricks, and the larger area perpendicular to the heat transfer direction improves the thermal conductivity of the bricks; (3) the densification of the silica bricks also increases the thermal expansion of the bricks, but they still meet the standard requirements.

This work intends to provide a comprehensive review on the development of Al₂O₃-C refractories with low/ultra-low carbon content. It covers three parts: carbon materials, microstructure optimization of the refractory matrix by ceramic phases, and application of metal Al as raw material. Carbon black, expanded graphite, and ultrafine microcrystalline graphite, as price-competitive carbon materials, can be chosen to prepare the low-carbon Al₂O₃-C refractories after some special treatment. Ni/Co/Fe-catalyzed phenolic resin contributes to improving the properties of the low-carbon Al₂O₃-C refractories. The performance deterioration of the low-carbon Al₂O₃-C refractories can also be improved by in-situ formed or pre-synthesized ceramic phases. Metal Al, characterized by plasticity forming, acceleration of sintering, oxidation resistance, and high reactivity, can be used as raw materials to completely replace graphite, and the prepared resin bonded Al-Al₂O₃ based refractories are one novel development direction of the ultra-low carbon Al₂O₃-C refractories.

The importance of basic refractories during the development of steel, non-ferrous and cement industries was introduced. The process of magnesia-chrome bricks was expounded. The development history of basic refractories of Sinosteel Luonai as well as the research and application of magnesia-chrome bricks and chrome free bricks for RH vacuum refining furnaces, heavy non-ferrous smelting furnaces, and cement kilns was emphasized.