Effect of coal slag on the wear rate and microstructure of the ZrO2-bearing chromia refractories

The typical properties of slag and refractories for slagging coal gasifier were investigated. In the range of 0–25% CaO/ash, the characteristic ash-fusion temperatures (AFTs) and viscosity of a coal-ash slag decreased with an increase of CaO additive. When the CaO/ash ratio was greater than 25% in the mixture of the coal and limestone, AFTs no longer reduced. The slag viscosity for limestone addition with CaO/ash = 25% was very low and in a narrow range (4–13 Pa.s) at coal gasification temperatures between 1300 °C and 1500 °C. However, corrosion resistance of the ZrO₂-bearing chromia refractories reduced with increasing CaO content in coal slag, especially for a slag with more than 30% CaO content. Increased chromia in three kinds of the ZrO₂-bearing chromia refractories resulted in increased corrosion resistance. The higher the Cr₂O₃ content and the lower the SiO₂ content, the less the deterioration of microstructures in the materials. Thermochemical spalling of the ZrO₂-bearing 80% Cr₂O₃ refractory after 807 operating hours of a coal gasifier was considered as the primary attack mechanism.     

Oxidation protection of CaO–ZrO2–C refractories by addition of SiC

The effect of SiC used as antioxidant in carbon-containing CaO–ZrO₂ refractories and the behaviour of SiC in CO gas were studied. SiC was found to react initially with CO to form SiO₂(s) and C(s) at 1200 °C, and then the formed SiO₂ reacted with CaO in the refractories to form belite (2CaO·SiO₂). The refractory microstructure was modified by addition of SiC. Due to the deposition of SiO₂ in the large (2–10 μm) pores of the refractory through the reaction of SiO(g) with CO, the percentage of large pores decreased and a dense layer, mainly consisting of belite, was formed near the surface of the refractory after it was heated at high temperature (1500 °C). The oxidation resistance of CaO–ZrO₂–C refractories was improved by reaction of SiC with CO to deposit C(s) and decrease the size of the large pores. The oxidation resistance of such refractories can be improved significantly when such a dense layer is formed near their surfaces.     

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO2

A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO₂ (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO₂ on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO₂ slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO₂. ZrO₂ containing samples always show better resistance to thermal shock than the ZrO₂ free samples. Scanning electron micrography shows that ZrO₂ occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.     

Densification and properties of ZrO2 nanoparticles added magnesia–doloma refractories

In this work the effect of nano- and microZrO₂ addition on the densification and hydration resistance of MgO–CaO refractories was investigated. 0, 2, 4, 6 and 8 wt% ZrO₂ was added to MgO–CaO refractories that contain 35 wt% CaO. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. Results show that with addition of ZrO₂ the bulk density and hydration resistance of the samples increased while apparent porosity decreased. Also the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also it revealed that the nanoZrO₂ addition was more effective than microZrO₂ due to its higher activity.     

Recycling of spent magnesite and ZAS bricks for the production of new basic refractories

Changing environmental awareness and regulations, the cost of waste disposal, and concerns about future liabilities have caused increased interest in recycling of spent refractories. The densification parameters of spent magnesite containing up to 10.0 wt.% spent ZAS (zirconium aluminium silicate) sintered at 1450–1550 °C were investigated. X-ray, microstructure and microchemistry analysis were used to establish the present phases. The technological parameters in terms of RUL (refractoriness under load) and TSR (thermal shock resistance) of the prepared refractories attaining their maximum densification were evaluated. The results revealed that, the addition of spent ZAS up to 5.0 wt.% to spent magnesite enhanced the physico-mechanical, refractory and thermal properties due to the development of highly refractory phases MA spinel and MgO·ZrO₂ solid solution. Samples containing more than 5.0 wt.% spent ZAS exhibited lower refractory and thermal properties due to the formation of M₂S ferroan beside the high flux CMS phase.     

热处理温度对ZrO2纤维复合ZrO2-C材料性能的影响

浸入式水口是钢铁连铸工序中关键的功能耐火材料,其中以渣线部位的工作环境最为恶劣。目前,最适合的渣线材料是ZrO₂-C材料。为了提高浸入式水口的性能,本文以氧化锆与鳞片石墨为主要原料,添加增强材料氧化锆纤维及金属硅粉等,以酚醛树脂为结合剂制备ZrO₂-C复合材料。比较了1 000 ℃、1 200 ℃和1 500 ℃三种热处理温度对ZrO₂-C材料的性能及显微结构的影响,结果表明,在热处理温度高于1 200 ℃时,ZrO₂-C材料中的硅粉与石墨发生反应生成碳化硅,大量晶须状碳化硅与ZrO₂纤维交错在一起形成网络结构,提高了材料的力学性能和抗热震性。     

Autothermal reforming of methane over Ni catalysts supported over CaO–CeO2–ZrO2 solid solution

Ni catalysts supported on various solid solutions of ZrO₂ with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO₂ solid solution showed a high activity. Incorporation of CaO in the ZrO₂ matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO₂–ZrO₂ solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane.     

BAS, CMAS and CZAS glass coatings deposited by plasma spraying

In this study, three different industrial frits BaO–Al₂O₃–SiO₂ (BAS), CaO–MgO–Al₂O₃–SiO₂ (CMAS), CaO–ZrO₂–Al₂O₃–SiO₂ (CZAS) have been deposited on porcelainized stoneware tiles by plasma spraying. In the as-sprayed conditions, the microstructure of the coatings is defective because of pores, microcracks and low intersplat cohesion. Hot stage microscope and differential thermal analysis measurements made on the glass powders allowed to characterize the frits thermal behaviour. Post process thermal treatments have been arranged, following these indications as well as preliminary tests, in order to achieve the lowest porosity and the highest resistance to abrasion. At the chosen temperatures, a microstructural improvement has been induced, but in the BAS specimens, an optimal sintering has not been accomplished because of the unavoidable full overlapping of the sintering and crystallization processes.     

Enhancement of Densification and Thermal Conductivity in AlN Ceramics by Addition of Nano-Sized Particles

The effect of addition of nano-sized particles on densification and thermal conductivity of AlN ceramics was investigated. The commercially available AlN powder (∼0.9 μm) was mixed with 1.89 mass% nano-sized AlN particles (<0.1 μm), 3.53 mass% Y₂O₃, and 2.0 mass% CaO as sintering aid. The mixture was fired at 1500° and 1600°C in a tungsten resistance furnace under flowing N₂ atmosphere. The results showed that a fully densified specimen was obtained at the lower temperature of 1600°C by addition of nano-sized particles. The thermal conductivity of the resulting product was 133 W/m°C. The value is much higher than the 52 W/m°C for the sample prepared without adding the nano-sized AlN powder. This study indicates a strong potential for the use of nano-sized particles as additives in the densification of AlN ceramics.     

Sintering of tricalcium phosphate–fluorapatite composites with zirconia

Zirconia (ZrO₂) addition effects on densification and microstructure of tricalcium phosphate–26.52 wt% fluorapatite composites were investigated, using X-ray diffraction, scanning electron microscopy and by analysis using ³¹P nuclear magnetic resonance. The tricalcium phosphate–26.52 wt% fluorapatite–zirconia composites densification increases versus temperature. At 1300 °C, the composites apparent porosity reaches 9% with 5 wt% zirconia. XRD analysis of the composites reveals the presence of tricalcium phosphate, fluorapatite and zirconia without any other structures. Above 1300 °C, the densification was hindered by grain growth and the formation of both intragranular porosity and new compounds. The ³¹P MAS-NMR analysis of composites sintered at various temperatures or with different percentages of zirconia reveals the presence of tetrahedral P sites. At 1400 °C, XRD analysis of the tricalcium phosphate–26.52 wt% fluorapatite–20 wt% zirconia composites shows the presence of calcium zirconate and tetracalcium phosphate. This result indicated that partial decomposition of tricalcium phosphate during sintering process of composites when 20 wt% or less ZrO₂ was added. Thus, zirconia reacts with tricalcium phosphate forming calcium zirconate and tetracalcium phosphate.     

CO2 capture by double metal modified CaO-based sorbents from pyrolysis gases

High-temperature pyrolysis technology can effectively solve the problem of municipal solid waste pollution. However, the pyrolysis gas contains a large amount of CO₂, which would adversely affect the subsequent utilization. To address this problem, a novel method of co-precipitation modification with Ca, Mg and Zr metals was proposed to improve the CO₂ capture performance. X-ray diffraction (XRD) patterns and energy dispersive X-ray spectroscopy analysis showed that the two inert supports MgO and CaZrO₃ were uniformly distributed in the modified calcium-based sorbents. In addition, the XRD results indicated that CaZrO₃ was produced by the reaction of ZrO₂ and CaO at high temperatures. The effects of doping ratios, adsorption temperature, calcination temperature, CO₂ concentration and calcination atmosphere on the adsorption capacity and cycle stability of the modified calcium-based sorbent were studied. The modified calcium-based sorbent achieved the best CO₂ capture performance when the doping ratio was 10:1:1 with carbonation at 700 ℃ under 20% CO₂/80% N₂ atmosphere and calcination at 900 ℃ under 100% N₂ atmosphere. After ten cycles, the average carbonation conversion rate of Ca-10 sorbent was 72%. Finally, the modified calcium-based sorbents successfully reduced the CO₂ concentration of the pyrolysis gas from 37% to 5%.     

Floor tile glass-ceramic glaze for improvement of glaze surface properties

Simultaneous improvement of surface hardness and glossiness of floor tile glaze, without changing its firing temperature, was the main purpose of the present paper. Thus, various glazes in the system of CaO–MgO–SiO₂–Al₂O₃–ZrO₂ were prepared and their crystallization behaviors within a fast firing cycle were investigated. With increasing amounts of calcium and magnesium oxides to base glass, the optimum glass-ceramic glaze was obtained. The results showed that with increasing of CaO and MgO part weights in frit, the crystallization peak temperature was gradually decreased and the intensities of diopside and zirconium silicate were increased. The comparison of micro hardness for the optimum glass ceramic glaze derived in this work with a traditional one used in floor tile industries indicates an improvement of 21%. It was found that the glaze hardness not only depend on the amount and type of crystalline phases, but also on the residual glass composition. Furthermore, it was observed that the glaze micro hardness is only slightly affected by thermal expansion mismatch of body and glaze.     

Synthesis of dimethyl carbonate from propylene carbonate and methanol using CaO–ZrO2 solid solutions as highly stable catalysts

CaO–ZrO₂ catalysts were prepared by coprecipitation and their catalytic performances were evaluated in the synthesis of dimethyl carbonate from propylene carbonate and methanol. The characterization by XRD, N₂ adsorption, XPS and CO₂–TPD indicated that Ca²⁺ ion substituted for Zr⁴⁺ ions in the host lattice to form homogeneous CaO–ZrO₂ solid solution when Ca/(Ca + Zr) ratio changed from 0.1 to 0.3, and CaO segregated at grain boundaries with Ca/(Ca + Zr) ratio from 0.4 to 0.5. As a result, the catalysts showed different activity and stability towards the transesterification of propylene carbonate and methanol into dimethyl carbonate. The activity of catalysts was improved with increase in Ca content, whereas high stability was shown with Ca/(Ca + Zr) ratio below 0.3. The formation of homogeneous CaO–ZrO₂ solid solution was responsible for the stability of catalysts.     

利用铝热还原金属铬炉渣制备彩色氧化锆陶瓷

为提升氧化锆陶瓷的使用性能,采用氧化钇稳定的四方氧化锆为基体（yttria stabilized tetragonal zirconia,3Y-TZP）,将铝热法生产金属铬所得炉渣（铝铬渣）按照不同比例（质量分数为5%~15%）加入,利用无压烧结在1 400 ℃保温2 h制备出彩色氧化锆陶瓷。通过X射线衍射（XRD）、扫描电镜（SEM）、显微硬度计及万能材料试验机测试了试样的物相、显微结构及力学性能。结果表明：掺杂铝铬渣可以制备出粉红色系氧化锆复合陶瓷,其物相主要为四方氧化锆、单斜氧化锆和含铬的氧化铝,并且铝铬渣的加入促使更多的四方氧化锆保留到室温。铝铬渣的加入不利于试样的烧结致密性,随着其含量增加复合陶瓷烧结后的体积收缩率降低,基体内出现部分孔隙。但是,铝铬渣的加入提升了试样的力学性能,当其加入量为5%（质量分数）时,氧化锆复合陶瓷的显微硬度和抗折强度均达到最大值,分别为1 755.3 HV和421.3 MPa。     

Reversible sorption–desorption of NOx by mixed oxides under various atmospheres

Characteristics of MnO_y–ZrO₂ and Pt–ZrO₂–Al₂O₃ as reversible sorbents of NO_x were investigated under dynamic changes in atmosphere. These sorbents can be used reversibly with a change of C₃H₈ concentration in the reaction gases. Catalytic reduction of NO occurred in the presence of propane, which was more pronounced on Pt–ZrO₂–Al₂O₃ than on MnO_y-ZrO₂ due to high activity of Pt surface for this reaction on MnO_y in MnO_y–ZrO₂. The sorption was observed as soon as the atmosphere changed from a reducing to an oxidizing one. This implies that a high equilibrium partial pressure of O₂ is necessary for NO uptake since the sorbed NO⁻₃ species becomes stable. The beginning of NO_x desorption atmospheres was somewhat dependent on the amount of stored NO_x. The presence of propane in the gas phase strongly affected the characteristic sorption and desorption properties of MnO_y–ZrO₂ and Pt–ZrO₂–Al₂O₃. The sorption and desorption properties are different for MnO_y–ZrO₂ and Pt–ZrO₂–Al₂O₃, since the noble metal or metal oxide possesses unique activity for the NO reaction with C₃H₈ and the amount of oxygen available for oxidative sorption of NO.     

Toughening alumina with silver and zirconia inclusions

Both silver and zirconia inclusions are added into an alumina matrix, the strength and toughness of the composites are determined. The toughening agents prohibit the grain growth of the matrix, the strength of alumina is, therefore, enhanced. The addition of two toughening agents also enhances the toughness of alumina. The presence of Ag inclusions raises the transformation ability of ZrO₂; however, the toughness increase of the Al₂O₃–ZrO₂–Ag composites is slightly lower than the sum of the toughness increase of Al₂O₃–ZrO₂ and of Al₂O₃–Ag composites. The present study demonstrates that the toughening effects contributed by a transformation toughening agent and a ductile toughening agent can interact with each other; nevertheless, such interaction depends strongly on the microstructure of the composites.     

Ethylene dimerization over NiSO4 supported on Fe2O3-promoted ZrO2 catalyst

The NiSO₄ supported on Fe₂O₃-promoted ZrO₂ catalysts were prepared by the impregnation method. Fe₂O₃-promoted ZrO₂ was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe₂O₃–ZrO₂. The addition of nickel sulfate (or Fe₂O₃) to ZrO₂ shifted the phase transition of ZrO₂ (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe₂O₃) and ZrO₂. 15-NiSO₄/5-Fe₂O₃–ZrO₂ containing 15 wt.% NiSO₄ and 5 mol% Fe₂O₃, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO₄/Fe₂O₃–ZrO₂ catalysts was very effective for ethylene dimerization even at room temperature, but Fe₂O₃–ZrO₂ without NiSO₄ did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe₂O₃ up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe₂O₃ and ZrO₂ and due to consequent formation of Fe–O–Zr bond.     

Zirconia and titania composite membranes for liquid phase separation: preparation and characterization

Crack-free tubular TiO₂/ZrO₂ composite ceramic membranes on ZrO₂ supports have been successfully synthesized from colloidal titania sols by the sol-gel technique. The pore sizes of the prepared membrane were mainly controlled by the sol properties and the calcination conditions. The influence of different parameters such as additives and solvents on the hydrolysis and polycondensation properties of the sols was studied by DTA–TG and viscosity measurements. The TiO₂/ZrO₂ composite ceramic membranes were characterized by X-ray diffraction and scanning electron microscopy. Composite membrane permeability and rejection properties were also investigated. The results proved that the range of composite membrane flux and retention could be controlled by controlling the amount of polyethylene glycol (PEG). PEG in titania sol systems was suitable for use as an organic additive to fabricate crack-free composite membranes. The larger the amount of PEG added to the precursor solution, the larger the size and number of pores produced in the prepared membranes when the PEG was completely combusted during heat-treatment. By controlling the amount of PEG added to the precursor solution, TiO₂/ZrO₂ composite ceramic membranes with different pore sizes were prepared, which can be used in photocatalytic membrane reactors and applied in liquid–liquid or liquid–solid separation in future research.     

Methane steam reforming over Ni/Ce–ZrO2 catalyst: Influences of Ce–ZrO2 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics

Ni/Ce–ZrO₂ showed good methane steam reforming performance in term of stability toward the deactivation by carbon deposition. It was first observed that the catalyst with Ce/Zr ratio of 3/1 showed the best activity among Ni/Ce–ZrO₂ samples with the Ce/Zr ratios of 1/0, 1/1, 1/3, and 3/1. Temperature-programmed oxidation (TPO) experiments indicated the excellent resistance toward carbon formation for this catalyst, compared to conventional Ni/Al₂O₃; the requirement of inlet H₂O/CH₄ to operate without the formation of carbon species is much lower. These benefits are related to the high oxygen storage capacity (OSC) of Ce–ZrO₂. During the steam reforming process, in addition to the reactions on Ni surface (*), the redox reactions between the gaseous components present in the system and the lattice oxygen (O_x) on Ce–ZrO₂ surface also take place. Among these reactions, the redox reactions between the high carbon formation potential compounds (CH₄, CH_x-*n and CO) and the lattice oxygen (O_x) can prevent the formation of carbon species from the methane decomposition and Boudard reactions, even at low inlet H₂O/CH₄ ratio (1.0/1.0).

Regarding the intrinsic kinetic studies in the present work, the reaction order in methane over Ni/Ce–ZrO₂ was observed to be approximately 1.0 in all conditions. The dependence of steam on the rate was non-monotonic, whereas addition of oxygen as an autothermal reforming promoted the rate but reduced CO and H₂ production selectivities. The addition of a small amount of hydrogen increased the conversion of methane, however, this positive effect became less pronounced and the methane conversion was eventually inhibited when high hydrogen concentration was added. Ni/Ce–ZrO₂ showed significantly stronger negative impact of hydrogen than Ni/Al₂O₃. The redox mechanism on ceria proposed by Otsuka et al. [K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517; K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493; K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193] can explain this high inhibition.     

Catalytic combustion of hydrocarbons with Mn and Cu-doped ceria–zirconia solid solutions

The catalytic activity of a series of CeO₂–ZrO₂ mixed oxides in the total oxidation of methane and light hydrocarbons has been investigated. The influence of dopants like Mn and Cu has also been studied. It is shown that both MnO_x and CuO at low loading dissolve within the ceria–zirconia lattice. This strongly influences the redox behaviour of the catalysts by promoting low-temperature reduction of Ce⁴⁺. In addition, the ternary oxides show better stability to repeated redox cycles, which is attributed to the presence of ZrO₂. The catalytic activity of pure CeO₂ is also enhanced in the presence of ZrO₂, reaching a maximum with Ce_0.92Zr_0.08O₂; a further promotion of activity is observed with the introduction of MnO_x and CuO dissolved into CeO₂–ZrO₂ lattice.