Investigation into the behavior of solid and liquid aluminum during the dissociation of lithium carbonate in vacuum

The oxidability of aluminum powder with carbon dioxide is studied theoretically and experimentally in the scope of the developed technology of the aluminothermic acquisition of metal lithium as a dissociation product of lithium carbonate. Our evaluation of the interaction of solid and liquid aluminum with CO₂, as well as (for comparison) with oxygen, shows that aluminum should be oxidized with two gases both in solid and in liquid states. It is established experimentally that oxide film protects the aluminum of oxidation with carbon dioxide during the dissociation of lithium carbonate. It is proven experimentally that during dissociation the protective layer of Al₂O₃ on the surface of aluminum powder interacts with Li₂CO₃ with the formation of a new denser layer of lithium monoaluminate, which protects the aluminum of oxidation with CO₂ better than Al₂O₃.     

Thermal and carbothermic decomposition of Na2CO3 and Li2CO3

In order to elucidate the decomposition mechanism of Na₂CO₃ and Li₂CO₃ in mold-powder systems employed in the continuous casting of steel, decompositions of Na₂CO₃ and Li₂CO₃ were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na₂CO₃, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li₂CO₃, however, the decomposition occurred at much faster rates than that for Na₂CO₃. When carbon black was added to the carbonate particles, the decomposition rates of both Na₂CO₃ and Li₂CO₃ were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na₂CO₃ and Li₂CO₃ with carbon black take place according to the respective reactions of Na₂CO₃ (1) + 2C (s) = 2Na (g) + 3CO (g) and Li₂CO₃ (l) + C (s) = Li₂O (s) + 2CO (g). It was found that liquid droplets of Na₂CO₃ were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li₂CO₃, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na₂CO₃ and Li₂CO₃ with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole⁻¹.     

Procedure of calculation of lithium in intermediate products and aluminum

The procedure for calculating the amount of lithium in froth cryolite is suggested for monitoring the Li content in aluminum in those electrolysis shops where no introduction of a purpose additive of its carbonate into the electrolyte is anticipated. A content of 0.0001% Li in aluminum of shops B and C will be reached during the operation of one shop A for 2–3 months. It is calculated that an additional increase of the lithium content in aluminum in the shop with the use of Li₂CO₃ is possible due to Li ingress with flotation and regeneration cryolites, the bath removed from crucibles, and recycled cryolite. For example, an additional increase by 0.0001% in shop A for the operation of 720 baths with the use of lithium carbonate will be attained in 1 month.     

Thermal oxidation behavior of an Al-Li-Cu-Mg-Zr alloy

The chemical composition of oxide films formed during thermal treatments of an Al-Li-Cu-Mg-Zr alloy has been studied by means of Auger electron spectroscopy and X-ray photoelectron spectroscopy. The oxide layers formed after oxidation of 2.5 minutes to 30 minutes at 530 °C in lab air have been characterized. In the early stages of oxidation the surface is composed of both the lithium rich oxides and magnesium rich oxides. However, after longer oxidation times the oxidation of lithium becomes predominant and the air/oxide interface is completely covered by lithium compounds. Oxidation products formed on the alloy surface have been studied by X-ray diffraction analysis. The following three phases, namely, Li₂CO₃, α-Li₅AlO₄, and γ-LiAlO₂, were identified. During heat treatment in lab air at 530 °C and at atmospheric pressure the dominating reaction product is Li₂CO₃. Due to the selective oxidation of lithium a soft surface layer is developed. The width of the soft layer formed during solution heat treatments carried out in lab air and in salt bath environments has been determined by microhardness measurements. The lithium concentration profiles were calculated from a diffusion equation. The depletion of alloying elements from the near surface region during heat treatments has been investigated using energy dispersive X-ray analysis. The oxide morphology was examined using scanning electron microscopy and optical microscopy.     

Electrodeposition of coatings of double carbides of tungsten and molybdenum from tungstate–molybdate–carbonate solutions

The electrochemical deposition of coatings of double tungsten and molybdenum carbides from tungstate–molybdate–carbonate melts is investigated. The composition of formed coatings is investigated by X-ray fluorescent and X-ray phase analyses. The crystal size and coating thickness are determined using scanning electron microscopy. Optimal deposition parameters of W₂C · Mo₂C coatings are as follows: the melt composition is Na₂WO₄–(1.0–4.0 mol %) Li₂WO₄–(1.0–4.0 mol %) Li₂MO₄–(1.0–5.0 mol %) Li₂CO₃, the cathode current density is 750–1500 A/m², the process temperature is 1123–1173 K, and the electrolysis duration is 4 h.     

Corrosion behavior of a Kh30N45YuT alloy and 20Kh23N18 steel in a lithium and potassium carbonate melt during anode polarization

The effect of temperature, the gas phase composition, and the addition of sodium peroxide on the corrosion behavior of a Kh30N45YuT alloy and 20Kh23N18 steel in a eutectic Li₂CO₃-K₂CO₃ melt is studied by measuring the corrosion potential and during steplike anode polarization.     

Study of the process of dissociation of lithium carbonate in the presence of aluminum powder

The results of a study of the dissociation process of lithium carbonate in vacuum in the presence of aluminum powder are presented. It is revealed that at a process temperature of 700°C, the degree of dissociation achieves ∼60% and the thermal decomposition of lithium carbonate retards. It follows from the phase diagram of the Li₂CO₃-Li₂O system that an increase in the temperature leads to the appearance of a liquid phase. On the basis of the experimental data with the use of the half-division method, a maximum rate of heating from 700 to 740°C of 0.33 ± 0.02°C/min is determined at which there is no formation of the low-melting eutectic between the carbonate and lithium oxide. The partial micromelting does not affect the mechanical properties of the briquette. The selected temperature mode provides the uniform thermal decomposition of lithium carbonate over the whole mass of the briquette, which minimizes the possibility of contacts between the carbonate and lithium oxide. Under these conditions, the time of the total degree of dissociation is 2 h. A decrease in the heating rate leads to an increase in the duration of the dissociation process, while its increase leads to the partial melting of the briquette. Aluminum plays the role of an inert additive accelerating the dissociation process at the stage of dissociation.     

Influence of Li2O on the carbonate capacity of CaO- CaFv2- AI2O3 Melts

The influence of Li₂O on the carbonate capacity of CaO-CaF₂-Al₂O₃-based fluxes is examined by a thermogravimetric technique over the temperature range 1250 °C to 1350 °C. The values of the carbonate capacities (Cc = wt pct CO₂/P_CO2) were calculated by the solubility and the partial pressure of carbon dioxide. The replacement of CaO by Li₂O resulted in a decrease of the carbonate capacity. The addition of Li₂O, from 0.4 to 2.0 pct, to the CaO-CaF₂-Al₂O₃ increases the carbonate capacity at 1300 °C by 50 pct. At 0.4 pct Li₂O,Cc is 1.2, and at 2.1 pct Li₂O,Cc is 2.1. The replacement of CaF₂ by Al₂O₃ was found to have no significant influence on the carbonate capacity of the investigated ternary system. SIMEON SIMEONOV, formerly Visiting Research Fellow, Institute of Industrial Science, University of Tokyo. KOJI FUKTTA, formerly with the Institute of Industrial Science, University of Tokyo.     

An experimental investigation of the gasification of graphite by carbon dioxide

The gasification of graphite by carbon dioxide was studied under atmospheric pressure in a fixed bed reactor in the temperature range of 1173–1773?K, CO₂ partial pressures 2–10?kPa and gas flow rate 0·5–2·0?L?min^?1. Iron presented in a small amount in graphite ash had a catalytic effect on the gasification reaction at 1373?K; this effect was weaker at 1473?K due to the melting of iron saturated with carbon. The gasification rate increased with increasing CO₂ partial pressure and total gas flow rate.     

Magnesium-thermic synthesis of molybdenum carbide in ion melts

The physicochemical aspects of the synthesis of the powders of molybdenum carbide by the magnesium-thermic reduction of its oxide in melts of lithium, sodium, and potassium carbonates are considered. The thermodynamic evaluation of reactions based on the synthesis is given. The influence of the melt temperature on the granulometric characteristics of the carbides is revealed. It is shown that the powders with the largest specific surface are formed in the melt of lithium carbonate (Mo₂C of 7.96 m²/g) at 750°C.     

Interaction of exogenous refractory nanophases with antimony dissolved in liquid iron

The heterophase interaction of Al₂O₃ refractory nanoparticles with a surfactant impurity (antimony) in the Fe–Sb (0.095 wt %)–O (0.008 wt %) system is studied. It is shown that the introduction of 0.06–0.18 wt % Al₂O₃ nanoparticles (25–83 nm) into a melt during isothermal holding for up to 1200 s leads to a decrease in the antimony content: the maximum degree of antimony removal is 26 rel %. The sessile drop method is used to investigate the surface tension and the density of Fe, Fe–Sb, and Fe–Sb–Al₂O₃ melts. The polytherms of the surface tension of these melts have a linear character, the removal of antimony from the Fe–Sb–Al₂O₃ melts depends on the time of melting in a vacuum induction furnace, and the experimental results obtained reveal the kinetic laws of the structure formation in the surface layers of the melts. The determined melt densities demonstrate that the introduction of antimony into the Fe–O melt causes an increase in its compression by 47 rel %. The structure of the Fe–Sb–O melt after the introduction of Al₂O₃ nanoparticles depends on the time of melting in a vacuum induction furnace.     

Electroreduction of chromium(III) chloride in a thermal battery

The discharge characteristics and the reduction products of the CrCl₃ cathodic material in a thermal battery are studied. The maximum discharge capacity of the cathodic material is found to be 0.45 A h g^–1 (550°C). A mechanism for the reduction of CrCl₃ to metallic chromium via an intermediate formation of the melt of Li₃CrCl_6x(Br_6(1–x)) complexes is proposed.     

Electroreduction of Chromium(III) Chloride and Molybdenum(VI) Oxide Mixtures in a Thermally Activated Battery

The discharge characteristics of a thermally activated battery element with cathode materials based on CrCl₃–MoO₃ mixtures are studied. The composition and the morphology of the products of reduction of the cathode materials are determined. A mechanism is proposed for their reduction to metallic chromium and molybdenum through an intermediate stage of formation of the melts of the complex Li₃CrCl₆ compound and lithium molybdates with lithium chloride.     

Corrosion-electrochemical behavior of nickel in an alkali metal carbonate melt under a chlorine-containing atmosphere

The corrosion-electrochemical behavior of a nickel electrode is studied in the melt of lithium, sodium, and potassium (40: 30: 30 mol %) carbonates in the temperature range 500–600°C under an oxidizing atmosphere CO₂ + 0.5O₂ (2: 1), which is partly replaced by gaseous chlorine (30, 50, 70%) in some experiments. In other experiments, up to 5 wt % chloride of sodium peroxide is introduced in a salt melt. A change in the gas-phase composition is shown to affect the mechanism of nickel corrosion.     

Effect of Lithium Carbonate on the Structure Formation of Ceramics Based on Si3N4

Structure formation in the system Li₂CO₃ Si₃N₄ both during heating in the powder state (500-1450°C) and also during specimen sintering (1450-1750°C) is studied. The most active formation of binary Li Si nitrides (LiSi₂N₃, Li₂SiN₄, Li₈SiN₄) is observed at 1450-1550°C. With a controlled sintering temperature and the amount of added Li₂CO₃ it is possible to prepare materials based on silicon nitride with a prescribed phase composition and corresponding properties.     

Formation of Metallic Phase on Reducing-Gas Injection in Multicomponent Oxide Melt. Part 3. Separation of Ferronickel and Oxide Melt

Using the equations of physicochemical hydrodynamics and experimental results regarding the surface and interphase properties of metallic and oxide melts, the conditions in which metallic phase is formed in the bubbling of carbon monoxide through molten oxidized nickel ore are described. The critical dimensions of the gas bubble (R_b.cr) and the metal droplet (r_d.cr) moving in oxide melt without change in size are determined in the range 1550–1750°C. It is found that R_b.cr increases slightly from 6.35 × 10^–2 m at 1550°C to 6.58 × 10^–2 m at 1750°C. With change in the droplet composition and the temperature, r_d.cr varies from 2.1 × 10^–3 to 2.9 × 10^–3 m. The dimensions of the metal droplet formed at a single bubble during the reduction of nickel and iron from oxide melt are determined. As the content of nickel and iron oxides in the melt decreases with increase in the overall CO consumption, the nickel content in the ferronickel droplets falls from 89 to 18%, while the droplet diameter decreases from 1.4 × 10^–3 to 8.0 × 10^–4 m. The droplet mass falls correspondingly from 9.4 × 10^–5 to 1.6 × 10^–5 kg. The conditions in which the bubble–droplet system rises through the melt are determined. Over the whole range of temperature and Ni content, the bubble–droplet system begins to rise through the oxide melt when r_d/R_b is less than 0.68–0.78. To assess the stability of the bubble–droplet system, with the given bubble and droplet dimensions, the parameters determining their joint motion are calculated. It is found that breakaway of the metal droplet from the bubble is not possible in pyrometallurgical systems. The formation of metal phase as a result of the bubbling of carbon monoxide through the oxide melt is described. In this process, the interaction of the oxide melt with the gas is accompanied by the formation of metal droplets, which become attached to the surface of gas bubbles and move to the surface of the oxide melt. Metal with 80–90% Ni is formed at first. With decrease in the nickel content in the oxide melt, its content in the metal declines to 20%. At the surface of the oxide melt, the metal droplets coalesce. When their diameter is greater than 5 × 10^–3 m, they break away from the surface and fall to the bottom. If the falling drop collides with ascending bubble–droplet systems, they may coalesce with it or flow around it. On coalescence, the small droplets will be assimilated and rise to the surface. The breakaway force of the droplet from the bubble significantly exceeds the gravitational force on the droplet. Therefore, the bubble–droplet system is stable for all the size ratios considered.     

Influence of alloying titanium carbonitride by transition metals of groups IV–VI on the interaction with the nickel melt

The influence of alloying titanium carbonitride TiC_0.5N_0.5 by transition metals of Groups IV–VI on the mechanism of contact interaction with the nickel melt is studied. It is established that alloying metals exert a strongly destabilizing influence on titanium carbonitride TiC_0.5N_0.5, simultaneously increasing both its dissolution rate in nickel and the degree of process incongruence (the preferential transition of alloying metal and carbon into the melt). The influence of alloying on the phase stability of titanium carbonitride TiC_0.5N_0.5 in contact with a nickel melt manifests itself in its dehomogenization or phase separation. The destabilizing effect of alloying additives enhances in a series Me^IV–Me^V–Me^VI parallel with a decrease in their nitrogen affinity.     

Hot corrosion of Fe-Cr-Ni-AI-Si alloys in molten carbonate

The change in weight of five newly developed steel alloys containing Fe, 10–11.3 % Cr, 15–18.27 % Ni, 4.98–5.76 % Mn, 4.18–5.52 % Al and 0.36–1.5 % Si together with Inconel 690 and stainless steel 304 has been investigated in Na₂CO₃ + K₂CO₃ eutectic mixture at 800°C under atmospheric pressure and under isothermal and cyclic conditions. The scales formed were examined with optical and scanning electron microscopy and were analysed by energy dispersive X-ray diffraction techniques. It has been shown that the corrosion resistance of the alloys is far better than that of stainless steel 304, but slightly less than that of Inconel 690. The relatively high contents of Si and Al gave alloy 5 its highest corrosion resistance. About 0.2 % N were found to refine the grains without the danger of depleting the grain boundaries from Al and Cr. The results have been explained in the light of the alkaline conditions prevailing in the carbonate melt.     

Kinetics of oxidation of carbonaceous materials by CO2 and H2O between 1300 °C and 1500 °C

The kinetics of the oxidation of graphite, metallurgical coke, and glassy carbon by CO₂ and H₂O were investigated at temperatures between 1300 °C and 1500 °C. The experimental technique employed a lance-crucible geometry with continuous gas analysis to measure the reaction rate. The experiments were designed to ensure that the carbon reaction behavior was in the limited mixed regime, where only a small volume of material close to the surface is reacting, and external gas phase mass transfer was fast. The results demonstrated the importance of internal pore structure, particularly as it develops in the reacted layer during the course of the reaction. This was believed to be responsible for the higher rates measured in graphite than in coke and the time-dependent rate increase that was observed in nonporous glassy carbon during experiments. For a commercial grade graphite and metallurgical coke, the rate constants depended strongly upon the carrier gas species, indicating that molecular diffusion was the primary transport mechanism in the pores of these materials. In contrast, for a specially purified graphite, the rate constant was found to be independent of the carrier gas species, which suggested Knudsen diffusion control dominates in this carbon. The results are in good agreement with extrapolations of previous work carried out at lower temperatures.     

Laboratory studies on smelting reduction – a contribution to reaction mechanisms in the production of liquid iron from iron oxide melts

Iron oxide slags are subjected to reduction at 1550 °C with the use of various reducing agents. The experiments with carbon monoxide are subdivided into series of top-blowing, injection or bottom-blowing experiments. Moreover, solid carbon is employed for further experiments. As a result of the necessary addition of Al₂O₃ – because of the corrosivity of FeO_n towards the crucible material during experiments – reaction-inhibiting barrier layers precipitate in the form of spinels during reduction. Foams containing CO_n exert the same effect. From the experimental results, it can be concluded that the reduction of liquid oxides by carbon monoxide or by solid carbon does not consistently obey any law of reaction during the entire process sequence. The essential, decisive parameter for the carbon monoxide blowing experiments is the blowing rate. The experiments performed with the use of solid carbon exhibit the highest reduction rates, on the whole. For the purpose of elucidating the reaction mechanisms, only the experiments conducted by blowing carbon monoxide onto the melt yield clearcut results. After a possible initial steep rise, a reaction of first order is then established, as a first approximation. The further course of the experiment is characterized by covering of the blowing trough. Mass transfer can then proceed only through the spinel layer near the surface, or through the channels between the spinels.