Formation and leachability of hexavalent chromium in the Al2O3-CaO-MgO-Cr2O3 system

The Al₂O₃-CaO-MgO-Cr₂O₃ system has immense potential for refractory castables applications. However, Cr(III) can potentially be oxidized to carcinogenic Cr(VI) during the usage, which can cause subsequent problems with disposal. Equilibrium experiments on the Al₂O₃-CaO-MgO-4wt%Cr₂O₃ system were performed at 1500?°C in air. The effect of MgO addition (0 and 20?wt%) on the formation and leachability of hexavalent chromium was investigated using XPS, XRD, SEM-EDS, the TRGS 613 standard Cr(VI) leaching test and multiple leaching tests. A Cr(VI)-containing phase Ca₄Al₆CrO₁₆ predominantly formed in the samples with 0 and 5?wt% MgO, while the Cr(III)-containing Mg(Al, Cr)₂O₄ spinel phase formed in the samples with 10 and 20?wt% MgO. Addition of MgO suppressed the formation of Cr(VI) while favored the formation of spinel phase. Concentrations of Cr(VI) in the leachates from TRGS 613 tests exceeded the European limit of 0.0002?wt% in all samples, although decreased significantly with 20?wt% MgO addition.     

Inhibited Cr(VI) formation in Cr2O3-containing refractory castable using reactive MgO as hydraulic binder

Reactive MgO was used in the first time as alternative hydraulic binder of calcium aluminate cement (CAC) to prepare Cr₂O₃-bearing refractory castable. The formation of Cr(VI), and physical and mechanical properties of MgO-bonded refractory castables after heat-treating were investigated. Microstructural characterization and phase composition analyses on the heat-treated MgO-bonded refractory castable matrices resulted in a comprehensive understanding of the mechanism for the inhibition of Cr(VI), and of the strength development during firing. The results indicate that compared with CAC, Cr(VI) levels were 6.7–28.1 times lower using reactive MgO after firing at 700–1300 °C. The in situ Mg(Cr,Al)₂O₄ spinel formed from the preferential interactions among MgO and Cr₂O₃ and Al₂O₃ would be the main reason leading to the inhibited Cr(VI) formation and strength development during firing.     

Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

This study investigated the effect of Al₂O₃/SiO₂ mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO₂–MgO–Al₂O₃–Cr₂O₃ stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)₂O₄ spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)₂O₄ spinel phase gradually decreased for slags with higher Al₂O₃/SiO₂ mass ratios. When the Al₂O₃/SiO₂ mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al₂O₃/SiO₂ mass ratio and MgO mole percent comprising less than the combined sum of the Cr₂O₃ and Al₂O₃ mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr³⁺ with Al³⁺ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al₂O₃/SiO₂ mass ratios from 0.125 to 5 and then increased for slags with an Al₂O₃/SiO₂ mass ratio of 1. The results suggest that the addition of appropriate amounts of Al₂O₃ to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Interaction of Al2O3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al₂O₃ rich (～15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr)₂O₄ spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr)₂O₄ spinel formation is proposed. Initially, hot face periclase grains take up Cr₂O₃, and to a much lesser extent, Al₂O₃ from the slag. The further diffusion of Cr₂O₃ and Al₂O₃ from the slag establishes a spinel layer of three distinct compositions of the type MgAl_2(1?x)Cr_2xO₄, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr₂O₃ in the hot face periclase decomposes and forms chromium-rich metal droplets.     

CaAl2Cr2O7: Formation,synthesis, and characterization of a new Cr(III) compound under air atmosphere in the Al2O3–CaO–Cr2O3 system

Despite huge potential, Al₂O₃–CaO–Cr₂O₃ system has been one of the least investigated one due to the generation of carcinogenic and toxic Cr(VI) compounds. Herein, we investigated the system under air atmosphere varying Cr₂O₃ while keeping Al₂O₃:CaO ratio constant in order to identify the Cr(VI) dominant region, eventually to avoid it. The Ca₄Al₆Cr^VIO₁₆ phase predominantly formed in the air atmosphere with Cr₂O₃ content up to ∼12 mol%. However, an unprecedented Cr(III) phase appeared under air at higher Cr₂O₃ content (26.43 mol%). We then synthesized the new polycrystalline ternary Cr(III) compound (CaAl₂Cr₂O₇) at 1500 °C under air atmosphere for the first time. A trigonal symmetry of hexagonal crystal family with space group P3 (143), lattice parameters a = b = 7.7909 Å and c = 7.6506 Å were determined from the X-ray powder diffraction pattern study. Electron microscope studies revealed uniform hexagonal microcrystals with similar lattice parameters. Most significantly, the binding energies of 586.1 and 576.2 eV for Cr2p_1/2 and Cr2p_3/2 respectively implied the +3 oxidation state of Cr in this compound.     

High-temperature stability and leaching kinetics of the hexavalent chromium compound,chrome-hauyne (Ca4Al6CrO16)

Cr-refractories usually contain Cr(III) compounds that can convert into carcinogenic Cr(VI) compounds such as CaCrO₄ and Ca₄Al₆CrO₁₆ under alkaline and oxygen-rich conditions. Literature provides insight into the characteristics, thermal stability, and aqueous solubility of CaCrO₄, while there is hardly any data available on Ca₄Al₆CrO₁₆. The present paper comprehensively studied high-temperature stability and leaching kinetics and other characteristics of Ca₄Al₆CrO₁₆ by using high-temperature XRD, TG-DSC, XPS, Raman, and TRGS 613. XPS confirmed that chromium in Ca₄Al₆CrO₁₆ is present in the +6 oxidation state. Ca₄Al₆CrO₁₆ is thermally stable up to 1500 °C in the air but decomposes in nitrogen above 1258 °C to form the Cr³⁺-containing phases CaCr₂O₄ and Ca₆Al₄Cr₂O₁₅. The Ksp of Ca₄Al₆CrO₁₆ in deionized water is 1.0381 × 10⁻²², 4.5723 × 10⁻²¹ and 2.3489 × 10⁻²⁰ at 12, 25, and 40 °C, respectively. The leaching process of Ca₄Al₆CrO₁₆ is chemical reaction controlled, with an apparent activation energy of 58.78 kJ/mol.     

Interfacial Reactions Between Anorthite (CaAl2Si2O8) and Al 7075 Alloy at 850°C and 1150°C

The present work reports an investigation of the interactions of Al 7075 alloy and anorthite at 850°C (150 h) and 1150°C (24 h). Transmission electron microscopy, electron probe microanalysis, X‐ray diffraction, and scanning electron microscopy coupled with energy‐dispersive spectroscopy were used to identify the mineralogical and microstructural changes at the metal–ceramic interface. At 850°C, the phase formation mechanisms were (a) Si⁴⁺–Al³⁺ interdiffusion between the Al alloy and anorthite to form calcium dialuminate (CA₂) and Ca²⁺–Mg²⁺ interdiffusion between the Al alloy and calcium dialuminate to form spinel. At 1150°C, spinel + Al₂O₃ and calcium hexaluminate (CA₆) + CA₂ were the major and minor phase mixtures, respectively in the corroded area. A thin layer of calcium monoaluminate (CA), gehlenite, and Si was present in the immediate vicinity of anorthite. The early stages of corrosion at 1150°C and 850°C were identical. However, due to thickening of the corroded region (viz., spinel formation) and enhanced evaporation of Mg at the higher temperature, the interdiffusion path evolves from Si⁴⁺–Al³⁺ + Ca²⁺–Mg²⁺ to Si⁴⁺–Al³⁺ + Ca²⁺–Al³⁺, thus establishing the following phase evolution path at the interface:      

Enhancing mechanism of improved slag resistance of Al2O3-spinel castables added with pre-synthesized (Al,Cr)2O3 micro-powder

In order to enhance the slag resistance of Al₂O₃-spinel castables, (Al,Cr)₂O₃ is added into Al₂O₃-spinel system as a pre-synthesized micro-powder. Firstly, (Al,Cr)₂O₃ micro-powder is synthesized by sintering under reduction conditions to prevent the formation of hexavalent chromium. The Al₂O₃-spinel castables are prepared using tabular alumina, fused spinel, α-Al₂O₃ micro-powder, calcium aluminate cement and (Al,Cr)₂O₃ micro-powder as the raw materials. The bulk density, porosity, mechanical properties, and slag resistance of the samples are tested. Afterward, the effects of (Al,Cr)₂O₃ micro-powder (0–3 wt%) on the slag resistance and microstructures of the Al₂O₃-spinel castables are assessed by X-ray diffraction (XRD) and energy-dispersive (SEM-EDS) analysis. The results show that the addition of (Al,Cr)₂O₃ micro-powder can could inhibit the deteriorating effects of Cr³⁺ on the mechanical properties of the samples. The microstructure results also shows that with the addition of the (Al,Cr)₂O₃ micro-powder, a secondary solid solution of Ca(Al,Cr)₁₂O₁₉ formed, causing the unit cell to become larger. In the slag erosion area, CA₆ crystals formed with network-like interwoven structures, high density, and greater thickness. These characteristics significantly reduce the erosion and permeability indices of the castables, and improve the slag erosion resistance of the material.     

The phase conversions and properties of refractories based on the MgO-Al2O3-Cr2O3system

Conclusions An investigation of the phase conversions in the MgO-Al₂O₃-Cr₂O₃ system at a below-unity ratio MgO/R₂O₃ and varied content of sesquioxides showed that the magnesiochromite initially being formed in all mixtures interacts with the Al₂O₃ at temperatures above 1000°C with the result that two kinds of solid solutions are formed, viz., a spinel one Mg(Cr_xAl)_1–x)₂O₄ and (Al_xCr_1–x)₂O₃. The predominance of a given component in the original composition remains preserved in both phases.The presence was established of a solid solution of Al₂O₃ in the spinel phase which contained a signficant amount of MgAl₂O₄. No solid solutions of R₂O₃ are formed in the spinel when the chromite component predominates in both phases.The changes in the properties of the specimens correspond to the phase conversions. After high-temperature firing materials of the type Mg(Al, Cr)₂O₄-(Al, Cr)₂O₃ possess adequate property indices.The analysis is concerned with that part of the system for which MgO/(Al₂O₃ + Cr₂O₃) is less than unity.Translated from Ogneupory, No. 8, pp. 48–53, August, 1976.     

Transport of Chromium(VI) through a Supported Liquid Membrane Containing Tri-n-octylphosphine Oxide

ABSTRACT

In this study the transport of chromium(VI) from aqueous solutions of pH 2–4 through a supported liquid membrane (SLM) with tri-n-octylphosphine oxide (TOPO) dissolved in kerosene as a mobile carrier was investigated. The transport flux of Cr(VI) increased with an increase in the concentrations of Cr(VI) in the feed phase and of TOPO in the membrane phase, but with a decrease in pH of the feed phase. Considering the equilibria of various Cr(VI) species in the aqueous phase and of the Cr(VI)—TOPO complexes formed in the membrane phase, a permeation model including the aqueous film diffusion of HCrO₄ ^? and Cr₂O₇ ^2? toward the membrane, the interfacial chemical reaction between them and TOPO, and the membrane diffusion of the Cr(VI)—TOPO complexes (H₂CrO₄(TOPO) and H₂Cr₂O₇(TOPO)₃) was proposed to describe the transport of Cr(VI) through the SLM. By best fitting the transport flux equations of Cr(VI) with the experimental data using the Rosenbrock method, the apparent mass-transfer coefficients of HCrO₄ ^? and Cr₂O_7^. across the aqueous film, and those of H₂CrO₄(TOPO) and H₂Cr₂O₇(TOPO)₃ across the membrane phase, were obtained. This work helps to clarify the transport mechanism of Cr(VI) through an SLM.     

Effect of Mg doping on magnetic,and dielectric properties of NiCr2O4 nanoparticles

The structural, magnetic, and dielectric properties of spinel Magnesium (Mg) doped Nickel chromite (NiCr₂O₄) nanoparticles (NPs) have been studied in detail. The X-ray powder diffraction exhibited normal spinel phase formation of Mg_xNi_1-xCr₂O₄ (x = 0, 0.2, 0.4, 0.6, and 1) NPs with a maximum average crystallite size of about 44 nm for x = 0.2 composition. The FTIR spectra of these NPs revealed the characteristic Ni–O and Mg–O and Cr–O bands around 639 cm^?1 and 497 cm^?1, respectively which confirmed the spinel structure. Temperature-dependent zero field cooled and field cooled graphs of NiCr₂O₄ NPs showed phase changes from ferrimagnetic to paramagnetic state at 86 K, while MgCr₂O₄ NPs showed antiferromagnetic (AFM) transition at Neel temperature (T_N) at 15 K due to corner-sharing of Cr³⁺ ions at a tetrahedral lattice site resulting in a highly magnetic frustrated structure. The field dependent magnetization (M ? H) loops of Mg_xNi_1-xCr₂O₄ NPs confirmed the competing AFM interactions and ferrimagnetic interactions resulting in a sharp decreased saturation magnetization with Mg doping. Dielectric constant, dielectric loss, and ac conductivity of these NPs showed size-dependent variation and depicted maximum value at x = 0.2 Mg concentration. In summary, the magnetic and dielectric properties of Mg doped NiCr₂O₄ NPs were modified by variations in the average crystallite size and magnetic exchange interactions, which may be suitable for different technological applications.     

Photocatalytic reduction of chromium and oxidation of organics by polyoxometalates

The photocatalytic reduction of Cr(VI) to the less toxic Cr(III) is presented in the presence of the polyoxometalates (POM) PW₁₂O₄₀³⁻ or SiW₁₂O₄₀⁴⁻ as photocatalyst and an organic substrate (salicylic acid or propan-2-ol) as electron donor. Cr(VI), as dichromate, is reduced to Cr(III), according to the 6:1 stoichiometry of PW₁₂O₄₀⁴⁻ versus Cr₂O₇²⁻ indicated from experiments in the dark. Increase of POM or salicylic acid (SA) concentration accelerates, till a saturation value, both the reduction of metal and the oxidation of the organic, suggesting that these two conjugate reactions act synergistically. The photocatalytic action of POM is not so important in the case of highly concentrated solutions of organics that exhibit direct photochemical reduction of Cr(VI), i.e. propan-2-ol (i-prOH), while it becomes important at low concentrations of i-prOH, especially for organics that do not react directly photochemically with Cr(VI), such as SA. Increase of Cr(VI) concentration enhances consumption of SA and Cr(VI) till an optimum value, due to inner filter effect. The method is suitable for a range of chromium concentration from 5–100 ppm achieving complete reduction of Cr(VI) to Cr(III) up to non-detected traces (>98%). The presence of oxygen does not influence the efficiency of SA and Cr(VI) consumption. In contrast to the semiconductor-based heterogeneous photocatalysis, the POM-based homogeneous process seems superior in the frame that: (i) it remains catalytic throughout illumination by providing more active sites and (ii) among the two POM used, the one that is more efficient in the degradation of the organic, that is PW₁₂O₄₀³⁻ compared to SiW₁₂O₄₀⁴⁻, is also more efficient in reducing Cr(VI), due to a kinetic effect, and a compromise is not needed.     

Immobilizing chromium in stainless steel slags with MnO addition

This work investigates the stabilizing impact of MnO on the leaching behavior of hazardous Cr-containing CaO-SiO₂-Al₂O₃-Cr₂O₃-MnO stainless steel slags after equilibrating at various elevated temperatures and evaluates the potential immobilization of Cr into a MnCr₂O₄ spinel phase from the existing Cr₂O₃ phase. The MnCr₂O₄ spinel phase was found to be an excellent Cr-stabilizer in stainless steel slags, where the leaching tendency of potentially hazardous Cr-related ions decreased with higher MnO content and lower equilibration temperatures within the range of 0 to 15 mass pct. and 1500 to 1300°C, respectively. Thermodynamic calculations by conducting the phase stability diagram also showed that the MnCr₂O₄ spinel phase was relatively stable and the Ca₃Si₂O₇ (Ca_3-xMn_xSi₂O₇) phase was relatively unstable compared with other crystal phases in acid extractant with pH value of 3.2. Combined with the scanning electron microscopy and X-ray powder diffraction results along with the thermodynamic calculations, the leached Cr-related ions was predominantly originating from the unstable amorphous glass phase.     

Synthesis of alcohols from carbon oxides and hydrogen: XVIII. Preparation chemistry,phase transformations and catalytic behaviour of unpromoted MnCrO systems in the synthesis ofalcohols from carbon monoxide and hydrogen

Unpromoted MnCrO catalysts with manganese-to-chromium (Mn/Cr) atomic ratios ranging from 0.5 to 5 have been prepared and characterized by XRD, IR, UV-VIS diffuse reflectance, differential thermal analysis-thermogravimetric analysis and surface area measurements. The preparation chemistry is based on the redox reaction 3Mn^II+Cr^VI→3 Mn^III+Cr^III and the phase transformations occurring during activation depend on the Mn/Cr ratio and the annealing atmosphere. Activation in air at 400–600°C results in the formation either of α-Cr₂O₃ and a spinel phase or of α-Mn₂O₃ for low and high Mn/Cr ratios, respectively. Activation in hydrogen at similar temperatures results in the formation of MnCr₂O₄ for Mn/Cr = 0.5 and of an MnO-type phase for high Mn/Cr ratios, which is pyrophoric when exposed to air at room temperature. Activation in nitrogen is most suited for the preparation of MnCrO catalysts because it prevents both pyrophoric effects and the formation of oxides of Mn and Cr, which are known to catalyse the formation of undesirable reaction products during the synthesis of higher alcohols. Activation in air at higher temperature causes the disappearance of α-Cr₂O₃ and α-Mn₂O₃ and the formation of a spinel-like phase; for high manganese contents a tetragonal distortion of the cubic spinel phase is observed owing to the Jahn-Teller stabilization of octahedrally coordinated Mn^III. In the synthesis of higher alcohols, (i) Mn/Cr ratios > 0.5 ensure high activity in methanol synthesis, (ii) high Mn/Cr ratios depress the productivity with respect to ethers and methane because of the lower surface acidity and (iii) irrespective of the Mn/Cr ratio, all MnCrO catalysts are effective systems for the water-gas shift reaction.     

The Structure of Fusion-Cast High-Chrome Oxide Refractories in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - Al<Subscript>2</Subscript>O<Subscript>3</Subscript> System

The phase composition and structure of fusion-cast refractories composed of 57.0 – 84.2% Cr₂O₃, 4.3 – 36.1% MgO, 2.0 – 9.7% Al₂O₃, and 2.4 – 6.9% SiO₂ have been studied by petrographic and x-ray spectral microprobe analysis methods. Refractories high in MgO with modulus M = (Cr₂O₃ +Al₂O₃)/MgO = 1.64 – 3.1 are shown to consist of spinel phase Mg(Cr, Al)₂O₄ and silicate glass. Refractory materials (80.8 – 84.2% Cr₂O₃, 4.3 – 4.7% MgO, 2.0 – 9.7% Al₂O₃, and 2.7 – 6.9% SiO₂ with M = 18.7 – 20.2) are three-phase systems composed of spinel, escolaite, and glass phase. These materials, owing to their high corrosion resistance, have promising potentiality for practical applications.__________Translated from Novye Ogneupory, No. 12, pp. 69 – 74, December, 2004.     

Enhanced Thermoelectric Performance Induced by Cr Doping at Ca‐Sites in Ca3Co4O9 System

Thermoelectric performance of Ca₃Co₄O₉ system has been enhanced through the misplaced substitution with Cr doping at Ca‐sites. The study of Cr doping at Co‐sites has also been performed for comparing. Base on the analysis of the structural data, we conclude that the doped Cr ions should be in the form of Cr⁺⁶, and enter into Ca‐sites of Ca₂CoO₃ layers in Ca_3?xCr_xCo₄O₉ lattice and Co‐sites of CoO₂ layers in Ca₃Co_3.90Cr_0.10O₉ lattice, respectively. As Cr ions are doped into Ca‐sites in Ca_3?xCr_xCo₄O₉ lattice, both resistivity and thermopower increase obviously, while thermal conductivity decreases. Among all samples, Ca_2.95Cr_0.05Co₄O₉ shows the maximum ZT value, which is enhanced by 12% than that of Ca₃Co₄O₉. The results show that the misplaced substitution with a proper Cr doping at Ca‐sites is beneficial to the enhancement of the thermoelectric performance in Ca₃Co₄O₉ system compared to the traditional idea: Cr doping at Co‐sites, which is suggested to originate from the combined action of the carrier concentration, the electronic correlation, and the lattice disharmony.     

Properties of compositions based on the system Al2O3-Mg(Al1−x,Crx)2O4

Conclusions Magnesia-alumina spinel and highly chromic spinel MgAl_0.4Cr_1.6O₄ and MgCr₂O₄ retard the grain growth of corundum during firing of the product. Low-chromic spinel MgAl_1.6Cr_0.4O₄ and MgAlCrO₄ added in small quantities intensify the corundum recrystallization.Small (5–10% by weight) additions of spinel Mg(Al_1–x, Cr_x)₂O₄ increase the strength of the corundum specimens, but only the high-alumina spinel improves their sintering.The sintering of mixtures of Al₂O₃ and Mg(Al_1–x Cr_x)₂O₄ is impaired during the substitution of magnesia-alumina spinel by magnesia chromite, and with an increase in the quantity of spinels from 5 to 30%.A small addition (5–10%) of high-alumina spinel of the composition Mg(Al_1–x, Cr_x)₂O₄ where x 0.5 to the alumina precalcined at 1450°C enables us to obtain dense, strong, and thermally shock-resistant corundum products.Translated from Ogneupory, No. 3, pp. 53–56, March, 1972.     

Assessment of conversion efficiency of Cr4+ ions by aliovalent cation additives in Cr:YAG ceramic for edge cladding

0.25at.% Cr:YAG ceramics were successfully fabricated as the edge cladding of Yb:YAG transparent ceramic slabs through vacuum sintering of co‐precipitated powders, using oxide additives to introduce different cations. The effects of various cation additives (Si⁴⁺, Ca²⁺, and Si⁴⁺ + Ca²⁺) on the conversion efficiency of Cr⁴⁺ ions and optical characteristics of the Cr:YAG edge cladding were investigated. Measurements of the absorption spectra of the Cr:YAG ceramics without any additives revealed 2 absorption bands centered at 430 nm and 600 nm, which imparted the sample with a green color. The introduction of only Si⁴⁺‐bearing additive did not promote the transition of Cr ions from the 3+ to 4+ state. Theoretical analysis and experimentation revealed that the addition of CaO not only enhanced the microstructure and improved the transmittance of the Cr:YAG ceramic, but also introduced vacancies that assisted in the formation of Cr⁴⁺ ions. It was determined that CaO has the same effect on the conversion efficiency of Cr⁴⁺ ions whether it is added as a single additive or in combination with SiO₂. The underlying mechanisms by which these aliovalent cation additives influence the formation of Cr⁴⁺ ions and affect optical properties are discussed in detail. High quality composite ceramics with Yb:YAG transparent ceramic slabs and dark brown‐colored Cr⁴⁺: YAG ceramic edge cladding were achieved through the addition of 0.05 wt.% CaO to the edge cladding, with no interfacial effects between the 2 regions being observed.     

Corrosion of high chromia refractory materials by basic coal slag under simulated coal gasification atmosphere

In this paper, dynamic corrosion experiment of a high chromia refractory interaction with basic coal slag under slagging gasifier conditions was conducted by using rotary drum corrosion test with the FactSage thermodynamic analysis. The microstructures and chemical compositions of the corroded samples were analyzed by scanning electron microscopy (BEI and EDS), and the corrosion mechanism was investigated by combining thermodynamic simulation and SEM analysis. The results show that the simulation results were consistent with the results of corrosion test. Reaction layer and penetration layer are formed from the surface to the interior of the sample after corrosion. The (Mg, Fe) (Al, Cr)₂O₄ spinel solution was formed in the reaction layer, which make the matrix structure become dense and change the overall structure of the particles’ uniformity. Corrosion of Cr₂O₃ aggregate is relatively weak by slag. The Cr₂O₃ dissolves into the slag through the formed spinel solution layer on the surface of aggregates. While, Cr₂O₃ and Al₂O₃ dissolve into molten slag through the spinel solution layer formed in the matrix. ZrO₂ in the matrix directly dissolved into molten slag and penetrates inner the matrix with the penetration of the slag to form a ZrO₂-free region. The liquid sintering of the matrix has happened in the melt, causes the structure of the penetration layer become dense, which is different from that of the original sample.