Mineral Composition and Phase Transformations in the Refractory Lining of the Bottom of Aluminum Electrolysis Cells. Part 2.

Specimens of post-service ShA-5-grade chamotte refractory are analyzed for total elemental aluminum and silicon. Products of the electrolysis of cryolitic melt Na (gas), AlF, Al₂O, and, possibly, Na₂F are shown to diffuse through pores of the carbon refractory bottom lining blocks of an aluminum electrolysis cell. The lower compounds and Na(gas) as they reach the refractory layer enter into a reaction with the refractory material. The Gibbs free energy of hypothetic reactions in the Al - Si - O - F - Na system involving a gas phase has been calculated. It is inferred that the performance characteristics of the ShA-5 refractory fail to meet standard operational requirements. __________ Translated from Novye Ogneupory, No. 5, pp. 24 – 26, May, 2005.     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.     

Micellization and Clouding Behavior of EO–PO Block Copolymer in Aqueous Salt Solutions

Aqueous solution properties of polyethylene oxide–block-polypropylene oxide–block-polyethylene oxide TBP [(PEO)₁₀₃(PPO)₃₉(PEO)₁₀₃] were studied in the presence of sodium salts with different anions (NaI, NaBr, NaCl, NaF, Na₂SO₄, Na₃PO₄) to investigate unimer-to-micelle transition [critical micelle concentration (CMC), critical micellization temperature (CMT)], micelle size and the phase separation (cloud point). This TBP, due to its very hydrophilic (80% PEO) nature does not form micelles at ambient temperatures. Micellization can be induced much below its CMT in water on addition of sodium salts having different anions. Analytical methods viz. fluorescence, FTIR and dynamic light scattering (DLS) were used to monitor the salt-induced micellization. The hydration of respective anion and resultant contribution to its salting-out effect was found to be the governing factor in promoting micellization. The presence of salt decreases the CMC, CMT and phase separation temperature. The salts affect the aggregation process in agreement with an order mentioned in Hofmeister series.     

Study of the mineral and phase composition of aluminosilicate refractory ShPD<Subscript>M</Subscript>-45 after service in an aluminum electrolyzer of OAO Zaporozh’e aluminum combine

Comprehensive studies have established that ShPD_M-45 refractory produced by OAO Zaporozhogneupor exhibits good anticorrosion properties towards the action of electrolysis bath corrosive extraneous components. The degree of degradation of ShPD_M-45 refractory after two years of service is 10%. The average concentration of entry of Na_g and F_tot into ShPD_M-45 refractory through hearth blocks with 50% graphite is correspondingly 4.08 × 10^–4 and 3.32 × 10^–4 g-mole/day. It is established that ShPD_M-45 refractory is a more reliable barrier material than unmolded aluminosilicate mixes. The penetration rate of corrosive components of the electrolysis bath into unmolded aluminosilicate materials is greater by a factor of 10 – 30 than for ShPD_M-45 refractory. It is shown that the protective function of titanium-containing components and glass phase in an electrolyzer lining develops as a result of forming fluoride, subfluoride and oxyfluoride titanium compounds. Industrial approval of ShPD_M-45 refractory under OAO Zaporozhe Aluminum Combine conditions has shown that it exhibits good operating properties in electrolyzers with hearth blocks with 50% graphite The authors thank D. V. Pruttskov and D. Yu. Boguslavskii for help with this work. Translated from Novye Ogneupory, No. 3, pp. 45 – 50, March 2009.     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.     

Friedel–Crafts catalysis using supported reagents. Synthesis, characterization and catalytic application of ZnCl2 supported on fluoride‐modified sol–gel‐derived aluminosilicates

ZnCl₂–aluminosilicate catalysts were prepared via a sol–gel route involving fluoride‐catalyzed hydrolysis of aluminum and silicon alkoxides in the presence of NaF, KF, NH₄F and ZnF₂. The catalysts were characterized by employing ²⁹Si, ²⁷Al and ¹⁹F solid‐state MAS NMR. The dependence of the activities of the catalysts on the nature and amount of fluoride present in the catalysts were investigated using Friedel–Crafts alkylation reaction of benzene with benzyl chloride. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of adding liquid glass on the compaction and reactive sintering of cermet Al2O3-Al

The salient features of the process of compaction and reactive sintering of Al₂O₃-Al cermet compacts with additions of liquid glass are studied. The introduction of 3–28 wt.% glass into the initial aluminum powder, consisting of plate-shaped particles with aluminum-oxide surface films, yields granular material with bulk mass 0.4–0.5 g/cm³. The salient features of the compaction of granular powder are studied by analyzing logarithmic diagrams. It is shown that reactive sintering of samples in air occurs in a filtration combustion regime, and the dry residue of liquid glass activates sintering, resulting in the formation of γ-Al₂O₃, θ-Al₂O₃, α-Na₂Si₂O₃, and Si nanodispersed inclusions in an aluminum matrix with a layered structure. The flexural strength of 2.40–2.45 g/cm³ cermet is 300–320 MPa. __________ Translated from Steklo i Keramika, No. 7, pp. 28–31, July, 2007.     

Interfacial tension between aluminum and cryolite melts during electrolysis of the systems Na3AlF6–AlF3 (NaF)–Al2O3

The interfacial tension between aluminum and cryolite melts containing different salt additions has been measured by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when the capillary, which is moved vertically down through the molten salt layer, passes through the salt/metal interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension was found to be strongly dependent on the n(NaF)/n(AlF₃) ratio (cryolite ratio, CR). At the cryolite ratio 2.28 (80 wt.% Na₃AlF₆ + 10 wt.% AlF₃ + 10 wt.% Al₂O₃ // Al, t = 1000 °C) the interfacial tension was 546 mN m⁻¹, while it was 450 mN m⁻¹ at the cryolite ratio 4.43 (80 wt.% Na₃AlF₆ + 10 wt.% NaF + 10 wt.% Al₂O₃ // Al, t = 1000 °C). Experiments under current flow conditions were also performed. During the electrolysis the interfacial tension at n(NaF)/n(AlF₃) ratio 2.28 decreased from 546 mN m⁻¹ at zero current to 518 mN m⁻¹ at 0.112 A cm⁻². The same trend was observed in the system with a cryolite ratio 4.43. The interfacial tension decreased from 450 mN m⁻¹ at zero current to 400 mN m⁻¹ at 0.112 A cm⁻². The consequent increase in interfacial tension of these systems caused by interruption of electrolysis was observed. Electrolysis of the system 25 wt.% NaF + 75 wt.% NaCl (eutectic mixture)/Al indicated no influence of applied current on the interfacial tension at 850 °C.     

Wear-resistant refractories in aluminum pyrometallurgy

Service conditions for refractories and physicochemical processes at their contact with molten aluminum are provided. Thermodynamic calculations establish the relative wear resistance of a number of oxides, silicates, and oxygen-free compounds towards aluminum in the range 700–1200°C (937–1473 K). The role is revealed of contact physicochemical processes at the boundary of a refractory with molten aluminum. The high resistance towards aluminum is demonstrated for high-alumina calcium aluminates, particularly bonite CaO · 6Al₂O₃ and spinel MgO · Al₂O₃. The most practicable wear-resistant fuzed materials for preparing lining of melting furnaces are lime-aluminate slags of OAO Klyuchevsk Ferroalloy Plant. __________ Translated from Novye Ogneupory, No. 9, pp. 15–19, September 2007.     

On some distinctive features of the structure of composite ceramic materials obtained by the method of directed reaction impregnation

The method of directed reaction impregnation (DRI), known as a Lanxide process in foreign countries, provides a wide spectrum of ceramic composite materials with a variable content of metallic phase, which possess high physicomechanical properties. An important advantage of the method is the absence of shrinkage, which diminishes substantially the amount of heat-treatment rejects and the cost of the finishing mechanical treatment. The structural characteristics of composites obtained by impregnation of the pore space of \-SiC and A1₂O₃ ceramic matrices with an aluminum alloy with 5 wt.% Si and 2 wt.% Mg are presented. The former matrix is obtained by free pouring of a granular powder of silicon carbide, and the latter matrix is represented by a densely sintered alumina preform with through cylindrical channels. The process is conducted at 1150 – 1200‡C for 24 h in air. The structure of the composites bears Al₂O₃ crystals and a mixed Al/Al₂O₃ phase formed from the metallic melt. The microhardness of the phases that compose the structure of the composite with a silicon carbide matrix is determined (HV = 28.4 – 73.2 GPa for Al₂O₃,HV = 10.3 – 19.7 GPa for SiC, andHV = 0.5 – 0.8 GPa for A1/A1₂O₃). Directed reaction impregnation through cylindrical channels of the aluminum oxide matrix yields highly porous fibers with a typical dimple structure. The impregnation process in this composite is considered from the standpoint of equilibrium (the Le Chatelier principle). Its occurrence in different stages is considered from the standpoint of thermodynamics. Transleted from Ogneupory i Technicheskaya Keramika, No. 8, pp. 14 – 20, August, 2000.     

Fusion-Cast Refractories in the High-Zirconia Region of the ZrO<Subscript>2</Subscript> - SiO<Subscript>2</Subscript> - Cao System

The synthesis and properties of fused materials in the ZrO₂ - SiO₂ - CaO system (83.1 – 91.7% ZrO₂, 0.3 – 9.2% SiO₂, and 0.6 – 7.8% CaO) are reported. CaO-assisted stabilization of ZrO₂ to yield a cubic modification is reached at a SiO₂ concentration of ≤ 4.3%. The arc furnace technology developed can be used to fabricate crucibles, tubes, rings (of diameter 320 mm and height 310 mm), and other shaped components from high-zirconia refractory materials with more than 90% ZrO₂. __________ Translated from Novye Ogneupory, No. 3, pp. 37 – 40, March, 2005.     

Aluminosilicate Refractories Based on High-Alumina HCBS.

Ceramic concrete specimens are prepared based on HCBS of mullite-silica composition (58% Al₂O₃) and fillers of similar and chamotte compositions, having markedly better properties than for traditional refractories with the same Al₂O₃ content. An ultimate strength in compression of 150 – 250 MPa with porosity of 6 – 11%, and a temperature for deformation under load T _g of 1540 and 1460 MPa are obtained correspondingly for ceramic concrete specimens with an Al₂O₃ content of 58 and 42%. With respect to T _g, mullite-silica ceramic concretes (58% Al₂O₃) correspond to traditional mullite (62 – 72% Al₂O₃), chamotte (42% Al₂O₃) and mullite-silica (45 – 62% Al₂O₃) refractories.     

Vibrational spectra and dissociation of aqueous Na2SiO3 solutions

To facilitate molecular spectroscopic observation of the mysterious transition of dissolved sodium silicate molecules into nanoparticles of desired silica gel and zeolite structures, the IR and Raman spectra of Na₂H₂SiO₄ monomers are studied here in details. It is demonstrated that the 3–0.2 mol/L aqueous solutions of Na₂SiO₃ and Na₂SiO₃ × 9H₂O contain mostly Na₂H₂SiO₄ monomers dissociated about 30%–80%, respectively. In contrast to the common belief the Si–O vibrations of these monomers depend on their dissociation level generating FTIR and Raman bands which are frequently associated with polymer silica structures in the current literature. To stay consistent with the molweight and dissociation measurements, these vibrational assignments are revised in this paper. Some unique and unexpected effects of D₂O used instead of H₂O as solvent are also reported.     

Synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient pressure using sodium silicate precursor

The present paper deals with the synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient pressure using low cost sodium silicate precursor. The hydrogels were prepared by sol–gel processing of sodium silicate precursor and acetic acid catalyzed water followed by vapour passing and solvent exchange with methanol. The mixture of MeOH:trimethylchlorosilane (TMCS):hexane was used for the end capping of the silanols present on the silica surface. The process was optimized by varying vapour passing time, gel ageing time, molar ratios of H₂O/Na₂SiO₃, CH₃COOH/Na₂SiO₃ and TMCS/Na₂SiO₃ and silylation period. The aerogels have been characterized by bulk density, % of volume shrinkage, porosity, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis and contact angle measurements. The best quality silica aerogels in terms of low density (0.073 g/cc), higher porosity (96%) and better hydrophobicity (θ = 146°) have been obtained with the molar ratio of Na₂SiO₃:H₂O:CH₃COOH:TMCS at 1:166.6:2.25:11.9.     

Composition of beryllium oxide ceramics

The physicochemical and electrophysical properties of ceramics based on BeO without impregnation and after impregnation with an aqueous solution of sodium carbonate Na₂CO₃ are studied. It is established that impregnation leads to preparation of ceramic specimens with a white color, which develop increased porosity, lower amount of impurities and smaller average microcrystal size, although it has little effect on their electrophysical properties. In order to improve these properties considerably it is necessary to increase ceramic density by increasing its sintering temperature by 20 – 40 K. Values of electrical permittivity δ and dielectric loss tangent tg δ are determined for a series of BeO ceramic specimens (732 pieces), having identical geometric dimensions, but prepared from original BeO powder of different batches. Studies show considerable scatter in the distribution of these parameters, which points to a requirement for further improvement of BeO-ceramic preparation technology.     

Evaluation of the rheological properties of sulfonic acids and sodium sulfonates

Sulfonic acids of linear alkylbenzene (LAB) are converted into the corresponding sodium salts to produce the most widely used anionic surfactant worldwide, linear alkylbenzene sulfonate (LAS). Used in many industrial applications and consumer products, the physical and mechanical properties of the sulfonates are strongly dependent on the LAB manufacturing process. Until recently, commercial alkylation of benzene has employed aluminum chloride or hydrogen fluoride catalysts, but a new fixed-bed alkylation process (DETAL) has been developed with improved 2-phenyl isomer selectivity and low tetralin concentration. In order to better understand the rheology of LAS in aqueous media, a comprehensive comparative evaluation of sulfonic acids and sodium sulfonates of the three LAB process derivatives has been done using dynamic mechanical rheometry, steady shear viscometry, and X-ray diffraction for phase identification. LAB sulfonic acids are Newtonian fluids in the temperature range of 20–60°C. The neat AlCl₃, HF, and DETAL sulfonic acids are Newtonian fluids within the temperature range of 20–60°C. At 30 wt%, all three sulfonates are Newtonian at 20–60°C, and the 40 wt% AlCl₃ sodium sulfonate remains in the Newtonian regime within this temperature range. Lamellar liquid crystalline phases have been identified for the sulfonates in the concentration range of 40–60 wt% in water at 20–60°C, and a hexagonal lattice phase also has been identified for DETAL sodium alkylbenzene sulfonate at 40 wt%, 60°C. The presence of anisotropic phases results in non-Newtonian pseudoplastic behavior with time-dependent viscosity functions.     

Synthesis and characterization of mesoporous molecular sieve nanoparticles

Mesoporous molecular sieve was prepared hydrothermally by a two-step method with materials of cetyltrimethyl ammonium bromide (CTAB), as a template, and sodium metaaluminate (NaAlO₂) and sodium silicate (Na₂SiO₃·9H₂O), as aluminum and silicon sources, respectively. The mesoporous molecular sieves are well ordered and have high thermal and hydrothermal stabilities. The as-prepared samples were characterized by powder X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), thermogravimetry-differential scanning calorimetry (TG/DSC), Fourier transform infrared spectroscopy (FT-IR) and nitrogen adsorption experiments. Particle size distribution was in the 30–50 nm range, BET surface area was more than 800 m²/g, thermal stability was higher than 1023 K, the mesoporous structure was not entirely damaged at a calcination temperature of 1123 K and there was no clear change in ordering degree, pore size, and surface area of the mesoporous molecular sieve after hydrothermal treatment at 373 K for 10 days. The activity and selectivity of benzene hydrogenation to cyclohexane by mesoporous molecular sieve-supported Pt was up to 100%. The catalytic activity didn’t decline in a reaction period of 30 h.     

Process and quasi-static compressive properties of highly porous palladium bulks

Highly porous palladium bulks with open porosities from 77.8% to 82.0% are prepared by powder metallurgical process with Na₂CO₃ as the filler material. Compressive properties of the prepared porous Pd bulks have been investigated at strain rates of 10⁻³–10⁻¹ s⁻¹. It has been found that the porous Pd bulks first show a short elastic region, then a long and oblique stress yield region, and finally, a densification region where the stress increase rapidly in the nominal stress–nominal strain curves. The effect of strain rate on the compressive properties of the porous Pd bulks is also discussed.     

Development of technology for the production of microspherical alumina support for the alkane dehydrogenation catalyst: II. The influence of hydrothermal treatment conditions on the operational characteristics of microspherical alumina support and chromium oxide/alumina catalyst for the dehydrogenation of iso-butane

For the purpose of improving the mechanical strength and reducing the abrasive activity of micro-spherical chromium oxide/alumina catalysts for the processes of alkane dehydrogenation in fluidized bed, we have developed technology for the production of new highly durable boehmite support with the use of technical aluminum trihydrate as an initial raw material. The technology includes two consecutive stages: the dehydration of aluminum trihydrate and the subsequent hydrothermal treatment of dehydration product into boehmite. Part II covers the results on the influence of conditions for the hydrothermal treatment of aluminum trihydrate dehydration products in an industrial autoclave of special construction on the phase composition, physicomechanical and structural characteristics of boehmite support, the acidic properties of its respective oxide form and chromium oxide/alumina catalyst based on this support and also on the catalytic properties of this catalyst in the reaction of iso-butane dehydrogenation into iso-butylene. It has been shown that differently sized boehmite crystallites and also gibbsite are formed in the volume of microgranule under hydrothermal conditions of χ-Al₂O₃ dehydration depending on a chosen regime (temperature, time). For the production of iso-butane dehydrogenation selective chromium oxide/alumina catalyst with the minimal content of strong acidic sites, catalyzing the cracking reactions, it is necessary to provide such hydration conditions, under which large (43–47 nm) boehmite crystals are formed. The appearance of strong acidic sites is caused by small boehmite crystals and the presence of nonhydrated χ-Al₂O₃ phase. In the absence of the impurity gibbsite phase, the highly durable microgranules of boehmite support and catalyst are formed. The conditions, providing the complete χ-Al₂O₃ hydration into macrocrystalline boehmite, have been defined. The application of the developed two-stage technology gives the iso-butylene yield of 45–49%, the selectivity of 88–90%, and the abrasivity of 0.10 g/(m² h). The given technology for the production of catalyst is realized in an industrial unit with the capacity of 100 t per month at OAO Karpov Mendeleevsk Chemical Plant (Mendeleevsk). Industrial catalyst lots are currently under operation at the Synthetic Isoprene Rubber Plant of OAO Nizhnekamskneftekhim.     

Structure of NaF–TeO2 glasses and glass-ceramics

The structure of NaF–TeO₂ glasses and glass-ceramics has been studied by XRD, TEM, SEM, Raman and FTIR techniques. The results suggest that, for NaF ≤10 mol%, the entire NaF content enters the structure to convert TeO₄ units into TeO_3/2F and Na⁺[TeO₃₊₁]^– units. It has also been shown that NaF partially forms amorphous and/or crystalline phases for higher NaF content, where the relative concentration of each phase depends on the NaF content. SEM images show agglomerates of different sizes, which are discrete and spread within the structure. XRD revealed formation of crystalline Te₂O₃F₂ for NaF ≤50 mol%, and a dominant phase of crystalline NaF for NaF >50 mol%. Raman and FTIR spectra have been analyzed to calculate the concentrations of the various structural units in glasses and glass-ceramics.