Mineral Composition and Phase Transformations in the Refractory Lining of the Bottom of Aluminum Electrolysis Cells. Part 1.

Results for specimens of the refractory lining sampled from the bottom of an aluminum electrolysis cell with a service life of 3.5 years examined by methods of petrography, chemical analysis, and electron probe x-ray microanalysis are reported. The main products of conversion are sodium aluminosilicates, sodium silicates, a glassy phase of variable composition, oxyfluoride glasses, and eutectics. Some of the specimens analyzed are high in β-alumina (20 – 50%). Fluorides are represented by NaF (about 7%), cryolite Na₃AlF₆ (2 – 5%), malladrite Na₂SiF₆ (2%), and NaF ⋅ MgF₂ (1 – 2%), and the metallic phase, by aluminum (2 – 7%) and ferrosilicon (3 – 10%). The apparent density of the used refractory material is 2.5 – 2.62 g/cm³. __________ Translated from Novye Ogneupory, No. 3, pp. 13 – 17, March, 2005.     

Crystallisation of impurities from black phosphoric acid at high temperature

Technical phosphoric acid when heated at 180°C in a pressure reactor deposits fluoro-aluminate crystals (Na, Mg) A1 (F, OH)₆.H₂O known as ralstonite, together with minor amounts of sodium fluorosilicate, Na₂SiF₆, and aluminum phosphate, AlH₃ (PO₄)₂^.3 H₂O. Uranium and vanadium in solution do not co-precipitate.     

Na和铝电解质对振动成型TiB2惰性阴极的渗透

采用振动成型的TiB2和石墨分别作为Na和铝电解质渗透实验的阴极材料,在工业铝电解条件下电解5 h,考察了Na和铝电解质对其腐蚀渗透情况. XRD分析结果表明,振动成型TiB2阴极的基体中未发现Na或F的单质及化合物,而在石墨阴极中则有NaF和Na3AlF6等物质存在. SEM及EDS分析结果表明,Na, F, Al等元素渗透到TiB2阴极基体中的量极少,渗透到石墨中的量则较多. 可见振动成型TiB2阴极能够有效减缓Na和铝电解质的渗透,但并不能完全阻止.     

Part 3. Phase Transformations within the Layer of a Dry Barrier Mixture for Aluminum Electrolysis Cells

It has been established that transformations within the layer of a dry barrier mixture (DBM) are selective in character and are mainly confined to the upper zones of the innermost part (core) of DBM. The DBM layer undergoes a solid-liquid sintering in these zones. The glass phase thus formed prevents the electrolysis products from penetration into the DBM layer. Modifier admixtures — titanium and iron oxides, products of electrolysis and degradation of the refractory layer AlF, SiF₄, and sodium oxide — decrease the viscosity of aluminosilicate melt. The grains of a disthene-sillimanite concentrate undergo cracking along the cleavage plane. Imperfections of the crystal lattice promote transformations in the grains of normal electrofused corundum. Oxyfluoride AlOF and an oxyfluoride of variable composition AlOF_{1 − x} are formed in the cathode lining of the electrolysis cell. For Parts 1 and 2, see Novye Ogneupory, Nos. 3 and 5, 2005. __________ Translated from Novye Ogneupory, No. 9, pp. 7 – 13, September, 2005.     

Gas permeability of alumina–spinel refractory castables bonded with hydratable magnesium carboxylate

The high level of gas permeability can effectively reduce the explosive spalling risk of refractory castables. The hydratable magnesium carboxylate (HMC) is expected to improve the permeability of castables owing to the thermal decomposition of the HMC hydrates. This study compared the gas permeability and explosive spalling resistance of HMC bonded refractory castables (HMCC) with calcium aluminate cement bonded refractory castables (CACC). Thermal decomposition of (Mg₃(C₆H₅O₇)₂∙11H₂O) (hydrates of HMC), drying behavior, and the pores size distribution of castables were investigated. The level of gas permeability of HMCC is higher than that of CACC, which was confirmed by the higher values of Darcian k₁ and non-Darcian k₂. The degas temperatures of HMC hydrates (156°C) and HMCC (432°C) are lower than those of CAC hydrates (289°C) and CACC (536°C) at a heating rate of 20°C/min, respectively. The large-size and more permeable pores in HMCC were obtained according to the mercury intrusion porosimeter (MIP) results, which formed the connected paths for gases (H₂O, CO₂, C₂H₄, CO, CH₄) released from the castables.     

Ionic Conductivity in Sodium–Alkaline Earth–Aluminosilicate Glasses

The effect of alkaline‐earth ions on Na transport in aluminosilicate glasses was studied by measuring ionic conductivity for a systematic compositional series of Na₂O–RO–Al₂O₃–SiO₂ glasses (R=Mg, Ca, Sr, Ba). The Na transport in aluminosilicate glass could be affected by compositional changes in aluminum coordination and nonbridging oxygen as well as physical properties such as dielectric constant, shear modulus, and ionic packing factor. Through careful experimental designs and measurements, the main determinants among these parameters were identified. ²⁷Al MAS‐NMR indicated that all aluminum species contained in these glasses are four‐coordinated. The activation energy for ion conductivity decreased with increasing aluminum content and decreasing ionic radii of the alkaline‐earth ion in the region where [Al] < [Na]. When the aluminum content exceeded the sodium content ([Al] > [Na]), the composition dependence of the activation energy depended on the specific alkaline earth. These results are explained based on variations in free volume and dielectric constant caused by structural changes around the AlO₄ charge compensation sites. These structure changes occur in response to the smaller size and higher field strength of the alkaline‐earth ions, and are most prevalent in the compositions which require bridging of two AlO₄ sites by the alkaline‐earth ion for charge compensation.     

Structure and crystallization behavior of complex mold flux glasses in the system CaO-Na2O-Li2O-CaF2-B2O3-SiO2: A multi-nuclear NMR spectroscopic study

The structure of mold flux glasses in the system CaO-(Na,Li)₂O-SiO₂-CaF₂ with unusually high modifier contents, stabilized by the addition of ∼4 mol% B₂O₃, is studied using ⁷Li, ²³Na, ¹⁹F, ¹¹B, and ²⁹Si magic-angle-spinning (MAS), and ⁷Li{¹⁹F} and ²³Na{¹⁹F} rotational echo double-resonance (REDOR) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate that the structure of these glasses consists primarily of dimeric [Si₂O₇]⁻⁶ units that are linked to the (Ca,Na,Li)-O coordination polyhedra, and are interspersed with chains of corner-shared BO₃ units. The F atoms in the structure are exclusively bonded to Ca atoms, forming Ca(O,F)_n coordination polyhedra. This structural scenario is shown to be consistent with the crystallization of cuspidine (3CaO·2SiO₂·CaF₂) from the parent melts on slow supercooling. The progressive addition of Li to a Na-containing base composition results in a corresponding increase in the undercooling required for the nucleation of cuspidine in the melt, which is attributed to the frustrated local structure caused by the mixing of alkali ions.     

Coating electroaccretion on galvanized iron and aluminum in seawater

Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawater was investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential <−1.2 V versus Ag/AgCl, coating is composed of aragonite and brucite as calcareous deposits on pure iron, aragonite forming before the growth of brucite. Even when coupled to a magnesium anode, the zinc layer can corrode and large aggregates of brucite and aragonite form on the bare steel. Coatings are composed of zinc hydroxychloride Zn₅(OH)₈Cl₂·H₂O and aragonite without brucite if electroaccretion is performed in natural seawater at potential >−1.2 V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg₄Al₂(OH)₁₄·2H₂O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg₆Al₂(OH)₁₈·4H₂O islands.     

铝合金表面氟硅酸盐转化新工艺

以6063铝合金为基体,在由氟硅酸盐和氟化铵组成的转化液中制得无铬转化膜。以168h盐雾试验后试样未腐蚀面积分数为指标,研究了转化液组成和工艺条件对氟硅酸盐转化膜耐蚀性的影响。优化后氟硅酸盐转化的工艺参数为:Na2SiF63～5g/L,NH4F5～7g/L,pH5.5～6.5,温度25～35°C,转化时间12～16min。经氟硅酸盐处理后,铝合金表面得到由F、Al、Na、O和Si组成的、致密的无铬转化膜,铝合金的自腐蚀电位显著正移,耐蚀性提高。     

Characterization of fluorine-, aluminum-, silicon-, and phosphorus-containing complexes in wet-process phosphoric acid using nuclear magnetic resonance spectroscopy

Wet-process phosphoric acid is one product of the reaction between phosphate rock and sulfuric acid. The limiting step in this process occurs when the acid is filtered from the reaction slurry, which also contains calcium sulfate (gypsum). The acid-soluble impurities present in the phosphate rock (e.g., fluorine, silicon, and aluminum) form complexes in wet-process acid which can alter the optimum size and habit of gypsum crystals, thereby reducing the filtration rates. Fluorine-containing complexes are strongly suspected of being potent modifiers of the crystal habit of gypsum. However, the identities of the complexes responsible for the habit modification have not been established. The identities of the complexes formed in phosphoric acid (28% P₂O₅) containing additions of fluorine (HF and H₂SiF₆) and aluminum [Al(NO₃)₃ · 9H₂O or AlF₃ · 9H₂O] were established in this study by using fluorine-19 (¹⁹F) and phosphorus-31 (³¹P) nuclear magnetic resonance (NMR) spectroscopies. Peaks due to aluminum fluoride, fluorosilicate, and fluoroaluminum phosphate complexes were observed in the NMR spectra recorded from these solutions. In addition, the¹⁹F and³¹P NMR spectra of wet-process acids were recorded. These spectra contained peaks assigned to the hexafluorosilicate ion (major species), along with aluminum fluoride and fluoroaluminum phosphate complexes (minor species).     

Analysis of the performance of the electrodes in a natural gas assisted steam electrolysis cell

The performance of solid oxide electrolysis (SOE) cells while operating in the natural gas assisted steam electrolysis (NGASE) mode was evaluated. The SOE cells used yttria-stabilized-zirconia (YSZ) as the oxygen ion conducting electrolyte, Co-CeO₂-YSZ as the H₂-H₂O electrode, and Pd-doped as the CH₄-oxidation electrode. The cell electrochemical performance was evaluated as a function of the H₂O/H₂ ratio and the extent of conversion of CH₄. The results of this study provide insight into the factors that control electrode performance and further demonstrate the viability of an NGASE cell for the production of H₂.     

Water electrolysis in the presence of an ultrasonic field

The energy efficiency of water electrolysis has been considerably improved in the presence of an ultrasonic field. This was demonstrated by measuring the cell voltage, efficiency and energy consumption of the generated gas from the electrolysis. These measurements were carried out in alkaline solution using linear sweep voltammetry (LSV) and galvanostatic polarization techniques. A large reduction of the cell voltage was achieved under the ultrasonic field, especially at high current density and low electrolyte concentration. With the same current density, the cell voltage difference with and without the ultrasonic field fell as the concentration of the electrolyte was increased. The efficiency of H₂ generation was improved at a range of 5-18% at high current density in the ultrasonic field but the efficiency of O₂ generation fell a little due to the difference in the behavior of the gas bubbles. The energy saving for H₂ production by using the ultrasonic field was about 10-25% for a certain concentration of the electrolyte when a high current density was used. On the other hand, the energy consumption for O₂ production with and without the ultrasonic field was almost the same.     

Mechanistic Study of Carbon Oxidation with NO2 and O2 in the Presence of a Ru/Na‐Y Catalyst

The oxidation of model soot by NO₂ and O₂ in the presence of a Ru/Na‐Y catalyst under conditions close to automotive exhaust gas after‐treatment systems is investigated. Isothermal oxidation experiments of a physical mixture of carbon black and catalyst were performed in a temperature range of 300–400 °C. A remarkable increase of the oxidation rate by NO₂ and O₂ in the presence of the Ru/Na‐Y catalyst was observed. An overall mechanism involving oxygen transfer from the Ru catalyst to the carbon surface leading to an increase of C(O) complexes is proposed. These C(O) complexes are destabilized in the presence of NO₂ increasing the carbon oxidation rate.     

Effect of NiO/YSZ cathode support pore structure on CO2 electrolysis via solid oxide electrolysis cells

Gas diffusion within supporting cathodes of solid oxide electrolysis cells (SOECs) plays an important role in CO₂ electrolysis process. This study has investigated the effect of cathode pore structure on gas diffusion during CO₂ electrolysis. The cathode pore structure was adjusted by applying the different amounts of pore former during cathode preparation. The more pore former added produced the higher porosity of cathode and the higher limiting current density. High limiting current densities are beneficial to diminish or even eliminate gas diffusion limitation in practical applications, where the electrolysis is expected to be operated at low CO₂ concentrations to increase CO₂ conversion. An advanced impedance spectroscopy study is performed to confirm the limiting current density measured according to current-voltage curves. It was revealed that CO₂ electrolysis performance is greatly affected by gas diffusion, which is determined by the employed cathode pore structure.     

First-principles simulation of the growth of in situ synthesised β-Sialon and its effects on the thermo-mechanical properties of Al2O3-C refractory composites

Columnar β-Sialon bonding phases were in situ synthesised in Al₂O₃-C refractory composites and their growth mechanism was simulated based on first-principles calculations. The experimental results indicated that the addition of Fe₂O₃ as a catalyst accelerated the transformation of Si₃N₄ to β-Sialon, resulting in a well-developed columnar structure. The (100) facet was the primary surface for crystal growth during the transformation process of Si₃N₄ into β-Sialon. According to first-principles calculations, the surface energy of the (100) facet decreased greatly due to the substitution of (Si, N) pairs with (Al, O). The catalyst could promote the adsorption of gaseous phases on the (100) facet of Si₃N₄ and decreased the gas adsorption energy of both SiO and Al₂O. Owing to the presence of in situ synthesised columnar β-Sialon bonding phases, the residual crushing strength of Al₂O₃-C refractory composites after 5 thermal shock cycles increased by 25.1%.     

Prospects of the use of aluminosilicate refractories for aluminum electrolyzers. Part 3. The role of the glass phase formed in the operation of aluminum electrolyzers

The compositions of glasses formed in the process of operation of an aluminum electrolyzer, the lowest content of aluminum at which the glass is separated into aluminosilicate and oxyfluoride components, and the temperatures of softening and of formation of drops of titanium-bearing glasses composition close to the glass in ShPD-45 refractory (with an additive of mullite-corundum chamotte) are determined. It is shown that the oxyfluoride glasses consist of both a silica skeleton and a corundum skeleton. The titanium-bearing glass phase can hinder penetration of aggressive gaseous byproducts of electrolysis. __________ Translated from Novye Ogneupory, No. 12, pp. 10–12, December, 2007. Parts 1 and 2 of the paper appear in “Novye Ogneupory” Nos. 9 and 10 of 2007.     

Slag of the production of aluminum alloys from secondary resources as a raw material for the refractory industry

Results of chemical, x-ray phase, and petrographic investigations of slag are presented. It is established that slag from the production of aluminum alloys from secondary raw materials can be classified as a high-alumina refractory material (85–95% Al₂O₃ + SiO₂, refractoriness 1670–1770°C). The refractory properties of the slag can be retained by magnetic separation. There is a practical possibility of using this slag in the production of refractories. Semidry pressing of a charge based on the slag can give a refractory of grade ShA in accordance with GOST 390-83.Translated from Ogneupory, No. 2, pp. 24 – 26, February, 1996.     

Copper extraction from copper ore by electro-reduction in molten CaCl2-NaCl

Sintered solid porous pellets of copper sulfide (Cu₂S) and Cu₂S/FeS were electrolysed at a cell voltage of 2.2-2.8 V to elemental Cu, S and Cu, Fe, S, respectively in molten CaCl₂-NaCl at 800 °C under the protection of argon gas. The process parameters for optimal electrolysis are presented. The electrolysis products are characterized by microscopic techniques and XRD. The product characteristics are linked to the process parameters. The direct electrolysis of the sulfide to copper with the emission of elemental sulphur offers an attractive green process route for the treatment of copper ore.     

铝电解质体系初晶温度、密度和电导率

采用先进的实验技术测定了Na3AlF6-AlF3-Al2O3-CaF2-LiF-NaCl系电解质的初晶温度、密度和电导率. 首先提供了测量的实验数据,接着基于实验数据给出了该体系的初晶温度、密度和电导率的计算公式,并与以前的经验公式作对比,最后从理论上分析了AlF3, Al2O3, LiF和NaCl对电解质物理化学性质的影响. 实验表明,LiF和NaCl能够有效地降低铝电解质的初晶温度并能提高电导率,是十分优良的添加剂,两者配合使用效果良好. 本工作旨在探索新型低温铝电解质组成,为铝电解工业选择适宜的电解质成分提供科学的理论依据.     

Erosion mechanism of refractories in a pyro-processing furnace for recycling lithium-ion secondary batteries

The refractory lining in a furnace is always damaged and peels off when spent lithium-ion secondary batteries (LIB) are pyro-processed in a rotary kiln. To develop highly durable refractories and to elucidate the erosion behavior, various analyses such as scanning electron microscopy/energy dispersive X-ray spectroscopy, laser-induced breakdown spectroscopy, inductively coupled plasma atomic emission spectroscopy, ion chromatography, and X-ray diffraction were performed on the linings sampled from different sections of a refractory. Our results suggested the following mechanisms in Al₂O₃–SiO₂–CaO refractory damage during pyro-processing of spent LIB packs. First, Li₂O, P₂O₅, LiF, and HF were formed by thermal decomposition of electrolyte constituents of the lithium-ion secondary batteries. When HF reacts with SiO₂, Al₂O₃, and CaO on the surface of the refractory, each fluoride that forms vaporizes and melts. When Li₂O and P₂O₅ (as well as LiF) react with the Al₂O₃–SiO₂ refractory, an Li₂O–Al₂O₃–SiO₂–P₂O₅(-LiF) phase with a low melting point forms and penetrates into the refractory through pores, grain boundaries, and cracks, resulting in peeling off.