Search

To improve the properties of high alumina castables containing calcium aluminate cement (CAC) after firing at elevated temperatures, micro-sized basic zinc carbonate (BZC) was introduced as ZnO-based nano fragments into castables. To account for the influence of BZC on the evolution of castable properties, the phase composition and microstructure of castable matrices were examined with X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectroscopy, respectively. Properties of castables having BZC were compared with those with Zn(OH)2. Results show that strength of castables added with BZC after firing at both medium temperatures and 1550 °C was improved. The volume stability and hot modulus of rupture at 1550 °C of fired castables containing BZC were also enhanced.

This study presents a novel strategy for fabricating high-strength mullite using Al3+/Ce4+-doped SiO2 and γ-Al2O3 powders. Doping of Al3+/Ce4+ ions in SiO2 results in the formation of a large amount of low-viscosity liquid phase in a narrow temperature range, which benefited the particle arrangement (closer) and reduced pores formation, thereby accelerating the mullitization rate and promoting the mullite grains growth. At elevated temperatures, almost all-SiO2 liquid phase were consumed due to a relatively high mullitization rate, and cerianite (CeO2) with high melting point gradually precipitated. Thus, the obtained ceramics with highly dense structure, and fully developed elongated columnar slab-shaped mullite grains form an interlocking structure, thereby leading to a high compressive strength (308.0 MPa) and excellent refractoriness under load (1672 °C).

To assess the impact of changes in slag type on the degradation of refractory linings of secondary copper smelters, the degradation behavior of direct-bonded and fused grain magnesia–chromite refractories in ZnO-containing ferrous calcium silicate slags was studied via dynamic refractory finger and static refractory crucible tests at 1200 °C under Ar atmosphere. The effect of ZnO level in the slag and refractory type on the degradation behavior was determined. A combination of microstructural and compositional characterizations of tested refractories, and thermodynamic calculations leads to comprehensive understanding of the degradation mechanism of the refractories. Based on the degradation mechanism, conclusions can be given about the prediction of lifetime of refractory lining when the slag type changes from fayalite-based to ferrous calcium silicate-based slags for secondary copper smelting.

Hydratable alumina (HA) is a superior Ca-free refractory binder, but the quick hydration rate restricts the working time of castables bonded with HA. In this work, HA was grounded for 1 h and 6 h by a rotational ball mill to study the effect of grinding on the hydration of HA and properties of HA-bonded castables. HA samples with and without grinding were cured at 30 °C and then terminated by freeze-vacuum drying. The phase composition and microstructure of the dried HA samples were then examined. Moreover, flow ability and mechanical strength of castables containing ungrounded and grounded HA were also investigated. The results indicate that the specific area of HA particles were decreased by grinding as the micro-pores and micro-cracks on the surface of HA particles were blocked by smaller HA particles, thereby decreasing the hydration rate of HA and increasing the flow ability of castables.

In this work, the effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)2 after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)2 can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)2 also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)2 decomposition in castables.

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA6 resulted from the more uniformly distributed hydromagnesite in castables.

The formation of CA2 and CA6 in calcium aluminate cement bonded castables lead to a large volume expansion, which probably results in the generation of cracks, thereby increasing slag penetration and decreasing mechanical strength. In this study, the influence of curing time on the dimensional stability of calcium aluminate cement bonded castables is considered to improve the volumetric stability. Phase compositions and morphologies evolution of matrix samples and castables are characterized by XRD and SEM, respectively. The results indicate that the volume stability of castables is greatly improved by increasing curing time from 0.5 days to 6 days after firing at 1600 °C. This can be attributed to two main reasons. First, the improvement in migration of CaO through extending curing time favors more uniform distribution of in situ CA6. Second, more hydrates after dehydration creates more micro-pores, thereby providing space for the generation of in situ CA6.

Calcium aluminate cement (CAC) was ground for 1 and 2 h to investigate the impact of mechanical grinding on CAC hydration at 30 °C and CAC-bonded castable strength. Phase composition and microstructure of unground and ground cements after hydration for predetermined times and terminated by the freeze-vacuum drying were compared. The results indicate that the particle size and particle size distribution of CAC were reduced and narrowed, respectively by grinding, thereby favoring the hydration rate and the conversation of C2AH8 to C3AH6. Then enhanced cement hydration also increases the strengths of castables bonded with milled CAC after drying and firing.

The change in demoulding strength of colloidal silica-bonded castables with and without magnesia is investigated with emphasis on the relationship between the demoulding strength and chemical bond changes. It was confirmed that the demoulding strength was raised with the presence of magnesia in colloidal silica-bonded castables because of the increased chemical bonding between the sol particles. The X-ray photoelectron spectroscopy (XPS) and the Fourier transformation infrared spectroscopy (FTIR) results indicate the formation of new Si–O–Mg chemical bond from the decreased O 1s and Si 2p binding energy, and the appearance of weak vibration peaks at 668 and 419 cm−1 in the spectrum of colloidal silica with the addition of MgO after curing at 30°C for 24 hours. The reaction between colloidal silica and magnesia could promote the formation of –Si–O–Mg–O–Si–bonds, which is the primary reason for the demoulding strength improvement.

To assess the feasibility of potassium aluminosilicates (KAlSiO4, KAS2 and KAlSi2O6, KAS4) as the refractory sagger material for the calcination of Li-ion battery cathode materials, the interaction behavior between KAS2 (KAS4) and the Li(NixCoyMnz)O2 (LNCM) material was investigated at temperatures ranging from 800 to 1100 °C. The phase composition and microstructure of calcined KAS2 (KAS4)-LNCM precursor mixtures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicate that potassium aluminosilicates exhibit superior resistance to corrosion by LNCM materials compared with mullite and MgO-based refractories, and rarely react with LNCM materials during calcination.

We found in our research that andalusite aggregate fired in a reducing atmosphere exhibits a lower mullitization rate than that fired in an air atmosphere. For investigating the effect of atmosphere on the transformation of andalusite and the properties of andalusite-containing refractories, andalusite powder (≤0.074 mm) and refractories containing andalusite aggregate (3–1 mm) were fired in air and carbon embedding, respectively. The phases and microstructure of the andalusite fired in both atmospheres were characterized by X-ray diffraction and scanning electron microscopy, respectively. The correlations of the properties of the andalusite-bearing refractories with the firing atmospheres were investigated in terms of volume stability, mechanical strength, and thermal shock resistance. The difference in the properties of the refractories was discussed with respect to the varied transformation rates of andalusite, and in terms of the different viscosities of the silica-rich glass caused by the different atmospheres.

Micro-sized MgCO3 and Al(OH)3 were used as inorganic porogenic agents for increasing porosity in alumina bubble lightweight castables. The influence of 0–4 wt% micro-sized MgCO3 and 0–8 wt% micro-sized Al(OH)3 additions on the porosity, thermal conductivity and strength of castables after firing was investigated. The results indicate that porosities in castables containing micro-sized Al(OH)3 or MgCO3 after heat-treatment were raised by augmenting micro-sized Al(OH)3 or MgCO3 contents. It is found that MgCO3 favors the porosity formation more than Al(OH)3 in castables because of the different formation behavior of calcium aluminates and MgAl2O4 spinel. The dependence of properties of castables on the micro-sized Al(OH) or MgCO3 addition was discussed with respect to the pore evolution.

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO2-rich liquid phase during the sintering process, and the liquid phase is transient in the Al2O3-SiO2 system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105 MPa) have been obtained after fried at 1700 °C.

In this study, the degradation behavior of Al2O3-Ti3AlC2-Si-C refractories and a complete degradation process of Ti3AlC2 under a temperature gradient along the penetration direction were investigated by a dynamic corrosion at approximately 1630 °C. The degradation process of Ti3AlC2 affected the local liquid components by assisting to form CaAl12O19 and restrained CaO penetration via trapping into high-melting phase. On refractories-steel/slag interfaces affected by Marangoni convection, the molten steel penetration was suppressed in the samples with Ti3AlC2. A refined texture and increased viscosity, accompanying Ti3AlC2 oxidation and dissolution in molten steel, blocked the pathways and restrained the penetration of molten steel.

To assess the viability of micro-sized hydromagnesite as a precursor in refractory castables bonded with calcium aluminate cement, the volumetric stability and thermo-mechanical properties of high-alumina castables containing different micro-sized hydromagnesite amounts (0–1.6 wt%) after firing at 1550 °C were investigated. Phase composition and microstructure evolution in fired castable matrices with different micro-sized hydromagnesite contents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dimensional stability, hot modulus of rupture and thermal shock resistance of castables were improved by adding micro-sized hydromagnesite. The microstructure evolution in castable matrix added with micro-sized hydromagnesite was discussed to understand the mechanism of enhanced volume stability and thermo-mechanical properties.

In comparison with the typically worm-like α-Al2O3 powders formed from an unground Al(OH)3 precursor calcined at 1450°C, spherical α-Al2O3 powders with ~1 μm in size were prepared by the calcination of a ground Al(OH)3 precursor with 5 wt.% [NH4]+BF4 − under the same calcination conditions. The influence of a high-energy ball milling pretreatment as well as of the additives on the morphological evolution of α-Al2O3 powders was studied using the commercial precursor Al(OH)3 as raw material. The results indicate that the morphology of α-Al2O3 powders is closely related to the morphology of the Al(OH)3 precursor and to the introduction of different additives. The refinement of the Al(OH)3 precursor in aggregate size and of the primary crystal size by high-energy ball milling has effectively suppressed the neck growth of α-Al2O3 grains. In contrast to the findings made previously with the introduction of 5 wt.% [NH4]+Cl−, the morphology of the α-Al2O3 particles could be significantly improved from a ground Al(OH)3 precursor with the addition of 5 wt.% [NH4]+BF4 −, which resulted in the formation of spherical α-Al2O3 powders with 1 μm size at 1450°C.

To assess the viability of an alumina-chromia refractory alternative for secondary copper smelters, the degradation behavior of alumina-chromia refractories in a secondary copper smelting slag and a Cu-CuxO mixture was studied via lab scale refractory finger testing. Microstructural characterization of the worn refractory samples resulted in a comprehensive understanding of the corrosion and penetration resistance and a comparison with the degradation of magnesia-chromite refractories. Alumina grains in the refractory are preferentially corroded by the slag. The alumina-chromia refractory exhibits superior infiltration resistance to the slag and the Cu-CuxO mixture compared to magnesia-chromite refractories.

Used mullite-based saggars from a Li(NixCoyMnz)O2 (LNCM) materials-making plant were collected to characterize the degradation occurring during application. Based on the post-mortem analysis, laboratory scale experiments were designed using mixtures of mullite powders and commercial Li-ion battery cathode precursors. The mixtures were calcined at temperatures ranging from 800 °C to 1100 °C, and characterized with X-ray diffraction and scanning electron microscopy. The influence of temperature on the interactions between mullite and LNCM materials was determined. The combination of the post-mortem study and the laboratory scale tests allowed a more comprehensive understanding of the evolution of the saggar microstructure during degradation.

Dense NiTi alloy was prepared by liquid phase spark plasma sintering from a TiH2 and Ni powder mixture at 1050 °C for 4 min. Dehydrogenation of TiH2 in the powder mixture was rapidly completed after heating to 850 °C. The sintered alloy showed no obvious transformation peaks, whereas the homogenized alloy, thermally treated in Ar at 1150 °C for 2 h, displayed clear transformation peaks with a high transformation enthalpy of 10.6 J/g. The hardness of the sintered and homogenized alloys were statistically the same.

Used direct-bonded magnesia–chromite refractory bricks from a secondary Cu smelter were collected to characterize the degradation occurring during application. Based on this post-mortem analysis, lab scale experiments were designed using the direct-bonded refractory type and a fused grain magnesia–chromite refractory. Firstly, the infiltration behavior of a Cu–CuxO and a Cu–CuxO–PbO mixture was investigated. Secondly, the influence of temperature and the prior infiltration of a Cu–CuxO mixture on the infiltration of the ZnO containing fayalite slag was determined. The combination of the post-mortem study and the lab scale tests allowed a more comprehensive understanding of the evolution of the refractory microstructure during degradation. Likewise, it allowed to evaluate how certain parameters, such as temperature and refractory type, affect the infiltration and degradation behavior. As a consequence, conclusions can be drawn about measures to minimize refractory wear in copper smelter linings.

The corrosion behavior of MgO in iron-saturated ZnO-rich fayalite (ZFS) slags having various FeO/SiO2 ratio and CaO/SiO2 ratio was investigated using MgO crucible tests for 12 h at 1200°C. The FeO/SiO2 and CaO/SiO2 ratios in the ZFS slags were varied from 1.0 to 2.2, and from 0.04 to 0.32, respectively. In all of the tests, it was observed that MgO dissolves into ZFS slags and that (Zn,Fe,Mg)2SiO4 olivine and (Zn,Fe,Mg)O solid solution are formed at the crucible/slag interface. The MgO dissolution decreased with the FeO/SiO2 ratio up to a value of 1.7 and then slightly increased, whereas it continuously increased with the CaO/SiO2 ratio. There is no obvious relationship between the amount of olivine and the FeO/SiO2 ratio or CaO/SiO2 ratio. In comparison, the formation of (Zn,Fe,Mg)O solid solution is enhanced by increasing the FeO/SiO2 ratio or CaO/SiO2 ratio in ZFS slags. The results suggest that MgO corrosion is the lowest for FeO/SiO2 and CaO/SiO2 ratios around 1.7 and 0, respectively.

To determine the effect of ZnO in ZnO-containing fayalite (ZFS) slags on the degradation of magnesia-chromite refractories, the corrosion behaviour of a direct-bonded magnesia-chromite refractory was studied by rotating refractory finger tests at 1200 °C under reducing atmosphere. In addition, MgO crucible tests at these conditions were performed to investigate in particular the effect of ZnO on the slag/MgO interaction. The dissolution of MgO from periclase into the ZFS slags is not enhanced by increasing the ZnO content in the slags. (Fe,Mg,Zn)2SiO4 olivine and (Fe,Mg,Zn)O solid solution are the only two products in the slag/MgO interfacial reactions, even when the ZnO content in the ZFS slag reaches 16.7 wt%. The diffusion of ZnO into the chromite grains to form the (Fe,Mg,Zn)(Fe,Al,Cr)2O4 spinel was favoured with increasing the ZnO content in ZFS slags.

The corrosion behavior of alumina-silica (Al 2 O 3 –SiO 2 ) refractories by liquid carbon steel was studied. This was performed by a refractory crucible test in a vacuum induction furnace at 1520–1545 °C under reducing atmosphere. The results indicate that Mn, Si and a minor amount of Fe in the steel were oxidized at steel/refractory interface, combing with refractory components to form a MnO–SiO 2 –Al 2 O 3 –based slag. This slag was liquid at the testing temperature, and infiltrated along the open-pore network and the grain boundaries into the Al 2 O 3 –SiO 2 refractories. Both Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the refractories dissolved into the infiltrating slag. As a result, the refractories were structurally weakened and residual Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the hot face of the refractories were washed away into the liquid steel, affecting steel quality.

This study evaluates commercially available magnesia–chromite, magnesia–carbon and magnesia–doloma bricks for their use in a Vacuum Oxygen Decarburisation ladle. The corrosion behaviour of these bricks by stainless steelmaking slags is, therefore, investigated through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (5.3 and 3.0×10 −11 atm). The results reveal that magnesia–carbon bricks are severely corroded due to the high dissolution of MgO, while magnesia–chromite and magnesia–doloma refractories exhibit an excellent corrosion resistance. The MgO enrichment in the slag is believed to be the reason of the low wear rate of the MgO–doloma refractories. Rebonded and direct-bonded MgO–chromite refractories show similar corrosion resistance against the slags because of the ‘secondary chromite inactivation’. Decreasing the slag basicity enhances the dissolution of MgO into the slag, thereby increasing the corrosion of the magnesia-based refractories.

To investigate the densification of porous alumina plugs, hydrogen measurements, using the Hydris probe system, were performed in molten steel with different oxygen levels at around 1600 °C. The oxygen contents in molten steel were controlled by Al and Fe-Si alloy additions to 664, 296, 92, 51 and 2.5 ppm, respectively. High oxygen levels (> = 92 ppm) in molten steel were found to favor the infiltration of steel into porous alumina plug. The infiltrating steel interacts with SiO2-containing phases and Al2O3 inside the plug, forming liquid FeOnAl2O3-SiO2 slag and FeAl2O4 (hercynite). These newly formed phases, along with the infiltrating steel fill the porous structure of the alumina plugs. As a result, severe densification was observed inside the alumina plugs in contact with molten steel containing high oxygen levels. In comparison, no densification occurred in the plugs contacting with the steel having low oxygen contents of 51 and 2.5 ppm. ispartof: Journal of the European Ceramic Society vol:38 issue:6 pages:2662-2670 status: published