Adsorptive removal of an anionic dye Congo red by flower-like hierarchical magnesium oxide (MgO)-graphene oxide composite microspheres

The effect of TiO2- and Al-additions on the oxidation resistance and the mechanical properties of MgO–C refractories is evaluated in terms of a phase evolution as a function of the coking temperature The formation of TiCN and TiC crystalline phases in the bonding matrix contributes to superior mechanical, thermal and chemical properties of carbon-bonded refractories

Microstructure evaluation of MgO-C refractories with TiO2- and Al-additions

The effect of antioxidants on the oxidation behaviour of magnesia-carbon refractory bricks

Corrosion resistance and corroded microstructures of MgO–C refractories containing various antioxidants in a model EAF slag (CaO/SiO2 weight ratio=1.38) were investigated after 30 h at 1650°C. Antioxidants influenced their corrosion resistance by affecting both carbon (C) oxidation and MgO dissolution in the slag. Al additions improved C oxidation resistance at 1650°C only a little, but accelerated MgO dissolution, resulting in a minor effect on corrosion resistance. Additions of Si or Al+Si improved C oxidation resistance slightly but accelerated MgO dissolution more than Al additions, resulting in worse corrosion resistance than Al addition and no addition. B4C conferred the worst corrosion resistance, since boron-containing liquid formed in the refractory, greatly accelerating MgO dissolution and resulting in C (mainly graphite) in the matrix being eroded easily by the slag. With double addition of Al+B4C, boron-containing liquid formed, which not only inhibited effectively C (mainly graphite) oxidation, but also accelerated formation and growth of MgAl2O4 spinel (MA) crystals between graphite in the matrix at the test temperature. In this case, even though MgO dissolution was accelerated to some extent, graphite was not easily washed away by the slag because it was effectively protected from oxidation and held in place by MA crystals. This maintained the integrity of the refractory texture, giving the Al+B4C containing refractory the best corrosion resistance.

Influence of additives on corrosion resistance and corroded microstructures of MgO–C refractories

The dissolution of magnesia particles in synthetic CaO–Al 2 O 3 –SiO 2 (CAS)-based slags with and without MgO addition was investigated in situ with a confocal scanning laser microscope (CSLM) at 1500 and 1600 °C. The dissolution process was recorded. The effects of slag composition and temperature on the dissolution process and the time dependency of the MgO particle size during dissolution were obtained. Increasing the temperature increases the dissolution rate. However, MgO addition to the slag retards the dissolution rate significantly. The rate limiting steps are discussed. It is shown that boundary layer diffusion is responsible for the dissolution. By combining in situ observations with post mortem analyses, thermodynamic calculations of local and global equilibrium, and kinetic considerations, the conditions under which MgAl 2 O 4 spinel can be formed at the particle–slag interface are clarified.

In situ observation of the direct and indirect dissolution of MgO particles in CaO–Al2O3–SiO2-based slags

Reactions on firing MgO–C refractories with added Al, Si and B 4 C have been predicted by thermodynamic calculations and observations made of microstructures given equivalent treatments. At 1200 and 1500 o C, addition of Al leads to generation of Al 4 C 3 , AlN, Al 2 O 3 and magnesium aluminate spinel, MA. Gaseous species, such as Al(g), participate in their formation processes as predicted and suggested by their morphologies. For example, AlN occurs as whiskers and MA as fine precipitates in the matrix and as surface layers on MgO aggregates. Additionally, Al 2 O 3 and Al 4 C 3 shells occur surrounding porous cores at the location of the original Al particles. At 1200 o C, addition of Si leads to formation of SiC, Si 3 N 4 , SiO 2 and forsterite, M 2 S. These phases are still present at 1500 o C except for Si 3 N 4 which is not thermodynamically stable. Gaseous species such as SiO(g) were also involved in formation of these product phases as predicted and suggested by their morphologies. M 2 S occurred as fine precipitates in the matrix and as a layer on MgO aggregate surfaces. SiO 2 formed either directly as a layer on Si particles or indirectly as a shell around a SiC core. Thermodynamic calculations predict that at 1200 and 1500 o C B 4 C reacts with N 2 from the atmosphere to form BN and/or with CO from the atmosphere to form B 2 O 3 which further reacts with MgO to form low melting 3MgO.B 2 O 3 , M 3 B. M 3 B becomes liquid >1350 o C and takes up impurities from MgO and/or graphite raw materials, forming more liquid, which will be detrimental to the refractories corrosion resistance. M 3 B was detected in the microstructures but there was no direct evidence of BN formation and its reaction with CO. Gaseous species such as B 2 O 3 (g) were also involved in M 3 B formation.

Thermochemistry and microstructures of MgO–C refractories containing various antioxidants

Penetration and corrosion resistance of high purity sintered and fused magnesia grain by model EAF (CaO/SiO2 = 1·38) and BOF ‘late’ slags (CaO/SiO2 = 3·29) at 1600 and 1700°C were investigated by SEM, EDS, and XRD analysis. Thermodynamic calculations were performed to assist interpretation of the reaction processes involved. At the test temperatures, Fe andMn ions from both model slags diffused into themagnesia grain to form a magnesiowustite, (Mg,Fe,Mn)O. The magnesiowustite directly adjacent to the slag had a much larger crystal size than that of the bulk MgO far from the MgO/slag interface. The large magnesiowustite grains limit the potential for grain boundary penetration into the sintered magnesia. The magnesiowustite layer formed with the EAF slag took up more FexO from the slag than that formed with the BOF slag, which was partially responsible for a lower slag penetration into sintered magnesia grain since the remaining silica rich local liquid was rendered more viscous. The EAF slag was n...

Penetration and corrosion of magnesia grain by silicate slags

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Thermochemistry and microstructures of MgO–C refractories containing various antioxidants

Penetration and corrosion of magnesia grain by silicate slags