Papers by Keyword: Magnesia

Abstract: Based on thermodynamic analysis, the reduction and volatilization of magnesium in ludwigite were studied using carbothermal reduction-nitridation method. The experimental result show that the total mass loss rate of samples increase with temperature rising, which the maximum is 52.88 wt% in the range from 1440°C to 1470°C. Magnesia in ludwigite was reduced and volatilized as gaseous magnesium vapour in the process of carbothermal reduction, and its mass loss rate go up to 98.138%. Part of the volatilized matter formed white powder deposited at the opening of furnace tube and adhered to tube wall together with boride/silicon volatilized. It was proved that there is volatilization of MgO from ludwigite in the process of carbothermal reduction-nitridation.

Abstract: The MgO-Al₂O₃-Fe₂O₃ composite was prepared at 1600°C for 3h with magnesia powder and alumina powder as raw materials and with ferric oxide powder as additive. The effect of the molar ratio of MgO/Al₂O₃ on the sintering and thermal shock resistance of the composite materials was studied. The results showed that increasing the molar ratio of MgO/Al₂O₃ could improve the densification of the Al₂O₃-Fe₂O₃ composites, with the molar ratio of MgO/Al₂O₃ increased from 3 to 5, densification of the composites didn't increased but decreased. The optimum molar ratio of MgO/Al₂O₃ of the Al₂O₃-Fe₂O₃ composite is equal to 3, the sample MA-3 exhibits better sintering densification and thermal shock resistance. Its primary crystal phases are spinel and periclase, and less amount of sosoloid 20MgO·19Al₂O₃·Fe₂O_3. So, the MgO-Al₂O₃-Fe₂O₃ composite is acceptable for application in cement rotary kiln linings and high temperature gas filter support.

Abstract: The composites in the MgO-Al₂O₃-Fe₂O₃ system were prepared using high pure magnesia and alumina as raw materials and ferric oxide powder as additive. The effect of sintering temperatures on the sintering performance and thermal shock resistance of the composites was studied. The results showed that both the apparent porosity and linear change ratio of the samples decreased with the increase of sintering temperatures, and their bulk density and bending strength increased accordingly. The sample sintered at 1550°C exhibits excellent thermal shock resistance. The XRD and SEM results indicated that the crystal phase of the samples remained the same, but their microstructure became denser as the sintering temperatures increased from 1500°C to 1600°C. As a result, the composites could be sintered at about 1550°C, which has a potential application in cement rotary kiln linings.

Abstract: The chemical shrinkages of alkali-activated slag cement (AASC), and the effect of fly ash, MgO burnt at 900°C and the curing solutions were studied. The shrinkages were compared with that of ordinary portland cement (OPC). The results show that the chemical shrinkage of AASC is lower than that of OPC. Adding fly ash and light-burnt MgO reduced the early age chemical shrinkage, while the shrinkage-reduction effect decreased with the age. The alkality of the curing solution has significant effect on the hydration and shrinkage of AASC. The chemical shrinkage of AASC increased with the alkali concentration of the curing solution. The mechanisms of fly ash, MgO and curing solution on the shrinkage were discussed.

Abstract: In terms of this work formulations of carbon bonded castables based on new binder approaches and nanoadditions will be demonstrated. The new binder system allows the manufacturing of water based magnesia carbon castables with the same properties and chemistry of pressed magnesia carbon bricks. This binder can be also applied in oxide castables offering them high refractoriness and workability during processing. According to the workability nano-additions improve significantly the spreading diameter of carbon castables and as a result their flowability.

Abstract: Slide gates are used to control the flow rate of molten steel in continuous casting. The properties that are important for materials for slide gate application are corrosion resistance, thermal shock resistance, abrasion resistance, and strength. However, operating conditions and in particular steel chemistries vary by shop and require development of different refractory material qualities. Alumina-carbon refractories are the prevalent family for slide gate application due to their superior thermo-mechanical properties. However, they suffer from extensive corrosion in aggressive applications. On the other hand, magnesia based refractories show promising corrosion resistance, but they are poor in terms of thermal shock and abrasion resistance. A novel approach using material based solution was employed to utilize a combination of properties of both refractory material families. This approach resulted in a material composite that jointly exhibits the thermomechanical properties and abrasion resistance of alumina based refractory and the corrosion resistance of magnesia based refractory. This paper will present the properties, key limiting factors, and results of usage at three steel plants.

Abstract: Magnesium phosphate cement (MPC) paste is prepared by mixing dead burned magnesia powder and potassium di-hydrogen phosphate with a proper proportion of water. The superior properties such as fast setting, high early strength, low shrinkage and good bonding makes MPC suitable for applications in rapid repair of infrastructures. However, the setting time of MPC is sometimes too fast, only taking a few minutes, which causes difficulties in manipulation. How to retard and control the setting time of MPC becomes a challenging topic in both scientific and engineering fields. It was found that the setting of MPC was greatly influenced by the reactivity of magnesia, which could be determined by a quick, accurate and convenient test method. Two kinds of magnesia, namely, high-reactivity magnesia and low-reactivity magnesia were blended in order to control the setting time of MPC. Effects of magnesia to phosphate molar ratio, blending ratio and borax addition on setting time, workability and mechanical properties of MPC cement paste were investigated. Finally, the suitable ranges of the different constitutes of MPC based repair materials with satisfied workability and strength development were proposed for the practical engineering applications.

Abstract: The citrate gel method, similar to the polymerized complex method, was used to synthesize homogenous tetragonal zirconia at 800oC and 1000oC. Nanocrystalline tetragonal single phase has been fully stabilized with 3, 7, 10 mol% CaO and 10, 15 mol% MgO at 800oC, respectively. In addition, the XRD analysis showed the absence of monoclinic phase after addition of 7 and 10 mol% CaO into zirconia-based solid solutions, which have been fully stabilized both 800oC and 1000oC. The crystallite sizes of the t-ZrO2 with 3, 7 and 10 mol% CaO at 1000oC were 32, 28 and 29nm, respectively. For ZrO2- x mol% MgO (x=3, 10, 15) solid solution, the crystallite sizes of samples at 800oC were less than 29nm, however it was increased up to 69nm at 1000oC. The prepared gel and subsequent heat-treated powders were characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) to get detail information regarding to differentiation of polymorphs of zirconia as well as formation of powders.

Abstract: Porous HAp scaffolds have been prepared by using the slurry including HAp and magnesia based on the replication of polymer sponge substrate. The influence of HAp and MgO content in slurry on the pore morphology and size, and density, porosity, and mechanical strength of porous HAp scaffolds was investigated. The obtained scaffolds with average pore sizes ranging 150 to 300 μm had open, relatively uniform, and interconnected porous structure regardless of HAp and MgO content. As the MgO content increased, the pore network frame of scaffolds became to be relatively stronger, even though the pore size was not much changed. The compressive strength of the scaffolds increased rapidly with the increase of MgO content at a fixed HAp content because of increasing the pore wall thickness and density of the scaffolds. As a result, the porosity, density, and compressive strength of the porous HAp scaffolds scaffolds prepared by the sponge method were significantly affected by the addition of MgO.

Abstract: To prevent the shrinkages by the densification during the application of unfired Al2O3-C refractories or Al2O3 castables in steel making conditions, MgO was added as aggregate or matrix powder and the expansion caused by spinel formation was studied. Because the spinel was formed at the contacting areas between Al2O3 and MgO particles and the volume of in-situ formed spinel increased more abnormally at the side of Al2O3 particles than MgO, the addition of MgO aggregates was not recommendable due to the formation of large voids around the MgO aggregates. Nevertheless, corrosion resistance was increased with the amount of fine MgO added, and the finer MgO powder added, the better residual expansion and minute structure formation was observed. In the contrary, in case of castables volume expansion due to spinel formation was not obvious because the degree of densification was less than high-pressure formed refractories. But CA6 phase would not form around alumina aggregates during corrosion so the corrosion resistance was much more enhanced.