Alumina Solubility in KF-NaF-AlF<Subscript>3</Subscript>-Based Low-Temperature Electrolyte

KF-NaF-AlF₃-based electrolyte is a promising low-temperature electrolyte for aluminum reduction. Alumina solubility in molten KF-NaF-AlF₃-based electrolyte was determined as a function of the melt composition and temperature by measuring the weight loss of a rotating corundum disk and by using a LECO RO500 oxygen analyzer (LECO Corporation, St. Joseph, MI). The investigated temperature range is 1023 K to 1073 K (750 °C to 800 °C), and the total cryolite molar ratio (CRt = ([KF] + [NaF])/[AlF₃]) is 1.3 to 1.5; the content of NaF ranges from 0 mol pct to 50 mol pct. The effect of temperature, CaF₂, and LiF on alumina solubility is discussed as well.     

The apparent solubility of aluminum in cryolite melts

The apparent solubility of aluminum in cryolite melts saturated with A1₂O₃ has been determined by titration with electrolytically generated O₂. The results may be expressed by wt pct Al = − 0.2877 + 0.0268 (NaF/AlF₃ wt ratio) + 2.992 × 10⁻⁴ (temp °C) − 0.00192 (% CaF₂) −0.00174 (% Li₃AlF₆) −0.00288 (% NaCl) with a standard deviation of ±0.017. Ranges covered were ratio 0.8 to 2.3, temperatures 969° to 1054°C, CaF₂ ≤ 14 pct, Li₃AlF₆ ≤ 20 pct, and NaCl ≤ 10 pct. There was no significant effect of adding 0 to 38. pct K₃A1F₆ or 0 to 10 pct MgF₂. It was found that solubility was approximately proportional to activity of aluminum when Al-Cu alloys were used. Possible mechanisms of solution are discussed. Monovalent aluminum is ruled out on the basis of the variation of solubility with NaF/AlF₃ ratio and a_Al. The favored, but not proven, mechanism involves formation of both sodium atoms and a colloidal dispersion of aluminum.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part I. Solubility of FeO,FeAl2O4, NiO,and NiAl2O4

The solubilities of FeO, FeAl₂O₄, NiO, and NiAl₂O₄ were measured in cryolite-alumina melts at 1020 °C. FeO was found to be the stable solid phase at alumina concentrations below 5.0 wt pct, while FeAl₂O₄ was stable above that. The corresponding figure for the nickel system was 3.0 wt pct Al₂O₃. These values correspond to Gibbs energies of formation of the aluminates (from the constituent oxides) of −17.6 and −29 kJ/mol, respectively. In alumina-saturated melts in the range 980 to 1050 °C, the solubilities of both aluminates increased with increasing temperature, the apparent enthalpies of solution being 65 kJ/mol for FeAl₂O₄, and 249 kJ/mol for NiAl₂O₄. Investigation of the solubilities of the aluminates as a function of the NaF/AlF₃ ratio in alumina-saturated melts at 1020 °C showed maxima at a molar ratio of around 5. The results are discussed in terms of the species apparently existing in the solution, and are consistent with the solute species being fluorides, not oxyfluorides. The activity coefficients of FeF₂ (liquid) and NiF₂ (solid) in dilute solution in cryolite are found to be 0.22 and 1.2, respectively.     

Thermodynamics of the system NaF-AlF3. Part III: Activities in liquid mixtures

Measurements have been made of a) the sodium content of aluminum in contact with NaF-AlF₃ melts at 1020° and 1080°C, and in contact with liquid-solid mixtures along the cryolite liquidus, b) the position of the A1F₃ liquidus line, and c) the electromotive force of solid-electrolyte cells with one side in the NaF-Na₃AlF₆ two-phase region and the other on the cryolite liquidus. With previously determined data for the activity coefficient of sodium in aluminum and for ΔH°_298f for cryolite, activities of NaF and A1F₃ are calculated. As a limiting condition the melts conform to a regular solution model withRTlnγ’₁ = ∔12,200(1 − n’₁)² cal where γ’₁=a₁/n’₁ and n’₁ the molar fraction of either NaF or NaAlF₄ calculated with them as species. This model breaks down progressively as the NaAlF₄ composition is approached, the deviations starting earlier at higher temperatures. The most plausible explanation is the disproportionation equilibrium 2A1F-₄ ⇌ A1₂F-₇ + F^-, stoichiometric NaAlF₄ containing about 70 pct AlF^- ₄ at 1020° to 1080°C. The hypothetical undisproportionated NaAlF₄ has a free energy of formation from NaF(I) and AlF₃(s) of ΔG° = •101,235 + 32.085T + 5.929 × 10⁷/T. This equation, together with that above for γ’NaAlF₄, defines α AlF₃ in all regions where the regular solution model holds. Solutions of sodium in aluminum are found to conform to Henry’s law in the range 100 to 2 ppm, contrary to recent suggestions.     

The interfacial tension between aluminum and cryolite melts saturated with alumina

A Method was developed for measuring the capillary depression of Met.allic aluminum in an alumina tube in a cryolite melt. As the tube was progressively lowered through the melt into the Met.al the volume of gas expelled was measured by the movement of a meniscus of liquid in a horizontal glass tube. No movement occurred after the tube entered the aluminum until it was far enough down that the Met.al could enter it. A correction was applied for the finite radius of the crucible holding the melt, the necessary theory being derived. The contact angle between Met.al, alumina, and cryolite was determined from the shape of the Met.al frozen in the crucible. It was found that the contact angle of the Metal on alumina is very close to 180 deg, and that the interfacial tension at 1000°C is 460 ± 27 mN · m^-1 (standard deviation). Interfacial tension decreases with increasing NaF/AlF₃ ratio, and increases with addition of MgF₂ or Li₃AlF₆. CaF₂ has no significant effect. The hypothesis is advanced that the effect of ratio is due to adsorption of Na atoms, generated by the reaction 3 NaF + Al ⇌ A1F₃ + 3Na, at the interface. Application of the Gibbs adsorption isotherm suggests that at molar ratios NaF/AlF₃ above 2.8 the interface is covered with a monolayer of sodium atoms.     

Solubility of Chromium(III) Oxide in the Molten Cryolite System

The solubility of Cr(III) species originating from dissolution of Cr₂O₃ in cryolite-based melts was studied in the temperature range 1173 K to 1293 K (900 °C to 1020 °C). The molar ratio n(NaF)/n(AlF₃) was in the range of 1.4 to 2.6. It was found that the solubility depends markedly on the molar ratio n(NaF)/n(AlF₃), high ratios resulting in higher solubility. A semi-empirical model describing the solubility of Cr₂O₃ was developed. The standard deviation between calculated and experimental data is 10 pm (ca 2.4 pct).     

The transported entropy of Na+ in solid state cryolite

The transported entropy of Na⁺ in mixtures of NaF (s) and Na₃AlF₆ (s) is determined from thermocell experiments. The experiments were favorably described by the electric work method. The variation observed in the thermocell electromotive force (emf) with composition can be explained from the probable path of charge transfer in the electrolyte. The transported entropies are S*^cry _Na+ = 140 ± 7 J K^?1 mol^?1 for cryolite and S*^NaFNa+ = 81 ± 8 J K^?1 mol^?1 for sodium fluoride between 380 °C and 500 °C. The value obtained for sodium in the solid cryolite makes us predict that the transported entropy for Na⁺ in themolten electrolyte mixture for aluminum production is substantial and that the reversible heat effects in the aluminum electrolysis cell are the same.     

The interfacial tension between aluminum and cryolite melts saturated with alumina

A method was developed for measuring the capillary depression of metallic aluminum in an alumina tube in a cryolite melt. As the tube was progressively lowered through the melt into the metal the volume of gas expelled was measured by the movement of a meniscus of liquid in a horizontal glass tube. No movement occurred after the tube entered the aluminum until it was far enough down that the metal could enter it. A correction was applied for the finite radius of the crucible holding the melt, the necessary theory being derived. The contact angle between metal, alumina, and cryolite was determined from the shape of the metal frozen in the crucible. It was found that the contact angle of the metal on alumina is very close to 180 deg, and that the interfacial tension at 1000°C is 460 ± 27 mN ⋅ m⁻¹ (standard deviation). Interfacial tension decreases with increasing NaFJAlF₃ ratio, and increases with addition of MgF₂ or Li₃AlF₆. CaF₂ has no significant effect. The hypothesis is advanced that the effect of ratio is due to adsorption of Na atoms, generated by the reaction 3 NaF + Al ⇌ AlF₃+ 3Na, at the interface. Application of the Gibbs adsorption isotherm suggests that at molar ratios NaF/AlF₃ above 2.8 the interface is covered with a monolayer of sodium atoms.     

Interfacial tension of aluminum in cryolite melts

The interfacial tension between aluminum and cryolite melts containing different salt additions has been measured based on a combination of the sessile drop and X-ray radiographie technique. A computer program was used to calculate the interfacial tension from approximately twenty randomly measured coordinate points of the drop profile. Aluminum and salt mixtures containing different amounts of Na₃AlF₆, A1F₃, NaF, A1₂O₃, CaF₂, KF, LiF, and NaCl were melted in a graphite or alumina crucible in a graphite resistor furnace under an argon atmosphere. The interfacial tension was found to be strongly dependent on the NaF/AlF₃ ratio. At the cryolite composition the interfacial tension was 481 mN/m at 1304 K, while it was 650 mN/m when the NaF/AlF₃ ratio was equal to 1.5. The change in interfacial tension with composition is explained by sodium enrichment of the Al/melt interface. Additions of A1₂O₃ increased the interfacial tension for a given NaF/AlF₃ ratio. KF was found to be surface active, while CaF₂, LiF, and NaCl slightly increased the interfacial tension by decreasing the sodium activity.     

The measurement of the thermodynamic properties of NiO−MnO solid solutions by a solid electrolyte cell technique

The activity of NiO in NiO?MnO solid solutions has been measured using the cell Ni, NiO |Zr_0.85 Ca_0.15 O_1.85|Ni, [NiO]_s.s. in the temperature range 900° to 1200°C for compositions between 0 and 80 mol pct NiO. The activity-composition relationships for MnO have been determined by integration of the Gibbs-Duhem equation, and the partial and integral thermodynamic quantities for the system have been calculated. The system exhibits a positive deviation from ideal behavior with a maximum heat of mixing of 850±150 cals per mole, and a positive excess entropy of mixing of 0.24±0.1 cal per mole °C was detected at the equimolar composition.     

The V-VO phase system

A phase diagram is proposed for the V-VO system based on melting point determinations, differential thermal analyses, metallographic observations, and X-ray parametric measurements. A eutectic reaction occurs at 1640°C and 29 at. pct O. The intermediate phases V₉O and V₂O form peritectoidally at 510° and 1185°C, respectively, while V₄O forms by a peritectic reaction at 1665°C. The VO phase melts congruently at 1790°C. The terminal solubility of oxygen in vanadium increases from 3.2 at. pct at room temperature to a maximum of 17.0 at. pct at the peritectic temperature. There is also extensive solid solubility associated with each of the intermediate phases. Two martensite-like phases form in alloys in the composition range 6 to 9 at. pct O upon quenching from above the 510°C peritectoid horizontal.     

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between A_cm and A₁. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.     

V-Ni system

The vanadium-rich portion of the V-Ni system was investigated by thermal analysis, microscopic examination, and X-ray diffraction studies and a phase diagram is proposed. The vanadium solvus was found to exhibit a marked temperature dependence increasing from 6.8 at. pct Ni at 800°C to 24.0 at. pct Ni at 1280°C. A compound having a substituted V₃Ni (A15) type structure was shown to occur at the substoichiometric composition of 22 to 23 at. pct Ni. It forms peritectoidally at 900°C with an enthalpy of formation of 1595 cal per mole.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

On the solubility of aluminum carbide in cryolitic melts

The solubility of aluminum carbide in cryolitic melts was determined as a function of NaF/A1F₃ molar ratio (CR), temperature, and the concentrations of A1₂O₃, CaF₂, MgF₂, and LiF. At 1020 °C a maximum concentration of 2.1 wt pct aluminum carbide was found at CR = 1.80. The following model for the aluminum carbide dissolution reaction based on activity data for NaF and A1F₃ was found to fit the experimental solubility data: Al₄C₃(s) + 5AlF₃(diss) + 9NaF(l) = 3Na₃Al₃CF₈(diss). From the solubility data for aluminum carbide an empirical equation giving the equilibrium carbide concentration was derived for CR > 1.80.     

Thermodynamics of the system NaF-Alf3. part ii: The free energies of formation of cryolite (Na3AlF6) and chiolite (Na5Al3F14)

The theory of the solid-electrolyte cells is given, and it is shown that cryolite itself with Ca²⁺ in solid solution is a suitable Na⁺-ion conductor. Experimental electromotive forces for the ranges 570° to 725°C and 570° to 670°C, r − 18,960 cal with a standard deviation of ±36 cal (based on a third-law calculation). For 5NaF(s) + 3AlF₃(s) = Na₅Al₃F₁₄(s), ΔG° = −38,560 − 7.081T with a standard deviation of ±130 cal. Combination of these results with recent values for Al + 3/2 F₂ = A1F₃ and for 6NaF + Al = Na₃AlF₆ + 3Na gives ΔH°_f298(Na₃AlF₆) = −792,400 cal and ΔH°_f298(NaF) = −137,530 cal. The latter is in excellent agreement with the most recent critical assessment.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part II. Solubility of TiO2

The solubility of TiO₂ in cryolite-alumina melts at 1020 °C was measured; it decreased with increasing alumina concentration up to ∼3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an alumina-saturated melt. Modeling indicated that the most probable titanium species are TiOF₂ and Na₂TiO₃, which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 °C to 2.8 wt pct Ti at 1035 °C, the apparent partial molar enthalpy of dissolution of TiO₂ being 88±4 kJ mol⁻¹.     

On the solubility of aluminum carbide in cryolitic melts

The solubility of aluminum carbide in cryolitic melts was determined as a function of NaF/A1F₃ molar ratio (CR), temperature, and the concentrations of A1₂O₃, CaF₂, MgF₂, and LiF. At 1020 °C a maximum concentration of 2.1 wt pct aluminum carbide was found at CR = 1.80. The following model for the aluminum carbide dissolution reaction based on activity data for NaF and A1F₃ was found to fit the experimental solubility data: Al₄C₃(s) + 5AlF₃(diss) + 9NaF(l) = 3Na₃Al₃CF₈(diss). From the solubility data for aluminum carbide an empirical equation giving the equilibrium carbide concentration was derived for CR > 1.80.     

Effects of austenitizing temperature on the kinetics of the proeutectoid ferrite reaction at constant austenite grain size in an Fe-C alloy

Austenitizing an Fe-0.23 pct C alloy at 1300°C and further at 900°C prior to isothermal transformation was found to increase the growth kinetics of grain boundary ferrite allotriomorphs while decreasing their rate of nucleation. A scanning Auger microprobe was used to establish that sulfur segregates to the austenite grain boundaries and does so increasingly with decreasing austenitizing temperature. A binding free energy of sulfur to these boundaries of approximately 13 kcal/mole (54.4 kj/mole) was calculated from theMcLean adsorption isotherm. The kinetic results were explained in terms of preferential reduction of the austenite grain boundary energy decreasig nucleation kinetics, and adsorption of sulfur at α:γ boundaries increasing the carbon concentration gradient in austenite driving growth.     

Electrocapillarity in the aluminum reduction cell

The effect of the current density on the interfacial tension between aluminum and cryolite containing melts was measured based on the sessile drop method and an X-ray radiographic technique. The experiments were carried out under constant current densities in graphite crucibles with BN lining. When the aluminum drop was the cathode, the interfacial tension was almost independent of the current density. During electrolysis, the interfacial tension increased with decreasing NaF/AlF₃ ratio in a similar manner to that observed when no electrolysis was performed. The interfacial tension between aluminum and an electrolyte containing between 5 to 10 wt pct A1F₃, 5 wt pct CaF₂, and 5 wt pct A1₂O₃ is 690 ± 60 mN/m for cathodic current densities between 0.1 and 0.6 A/cm₂. Interruption of electrolysis caused an instantaneous decrease in the interfacial tension followed by a slow increase with time. This sudden drop together with a decrease in interfacial tension with reversal of cell polarity indicate that the metallic side of the interface has an excess positive charge. The interface was enriched with NaF during electrolysis as indicated by the slow recovery of the interfacial tension after current interruption. T. UTIGARD, formerly Graduate Student, is now Research Engineer, Alusuisse, CH-3965 Chippis, Switzerland