Potassium Cryolite Research Articles

LOW-TEMPERATURE ELECTROLYTIC PRODUCTION OF Al-REM ALLOYS IN CRYOLITE MELTS

The process of electrolytic production of Al–Y and Al–Sc alloys in an electrolyte based on potassium cryolite KF–NaF(10 wt %)–AlF3 with a cryolite ratio (CR) of 1.5, containing Al2O3, Sc2O3, or Y2O3 oxides, in a cell with vertical electrodes has been studied. The Fe–Ni–Cu alloy served as an inert anode. The wetted cathode was a graphite plate coated with the aluminum diboride. The electrolysis was carried out at a cathode current density of 0.2 A/cm2 and a temperature of 830°C. The Al2O3 mass was calculated based on the value of the current efficiency of 60%. The Sc2O3 additive was introduced into the melt in an amount of 1 wt %. The mass of the Y2O3 additive was chosen based on its solubility in the melt under study. For this, the influence of Y2O3 additives on the liquidus temperature of the quasi-binary mixture [KF–NaF(10 wt %)–AlF3 (KO = 1.5)]–Y2O3 was determined and it was found that, in contrast to Sc2O3 additives, which lower the liquidus temperature of the cryolite melt, small additions of Y2O3 lead to its sharp increase. It has been found that the efficiency of the electrolytic reduction of Y2O3 is 10 times higher than that of the aluminothermic reduction. Other things being equal, the efficiency of the electrolytic reduction of Y2O3 is higher than that of Sc2O3. Alloys Al–Y and Al–Sc with a REM content of 0.6 wt % have been obtained. However, the time to reach the maximum recovery of yttrium significantly exceeds the time to recover scandium. Metallographic studies of the obtained alloys indicated the presence of Al3Sc and Al2Y intermetallic compounds. A conclusion is made about the fundamental possibility of low-temperature electrolytic production of Al-REM alloys in cryolite melts based on potassium cryolite in vertical cells with an inert metal anode and a wettable cathode.

Borated graphite cathodes for low-temperature aluminum electrolysis

Electrochemical boriding of the graphite plates in the potassium cryolite based electrolytes was studied. The boriding were carried out in a cell with vertical electrodes. The procedure included 2 stages: 1) electrolysis in the KF–AlF3–KBF4 melt (CR=1.3) at low current density (0.01–0.02 A/cm2), required for the boron reduction, at 700 and 750 °C; 2) electrolysis in the KF–AlF3–Al2O3 melt at higher current density (0.2 A/cm2), required for the aluminum reduction. The optimal conditions of electrodeposition for obtaining the borated wettable cathodes were determined. According to the SEM data, a continuous AlB2 layer with a thickness of 7–10 μm was formed on the graphite surface. The borated graphite was tested as a wetted cathode during the low-temperature aluminum electrolysis. Prolonged electrolysis in a vertical cell with the graphite anode and the borated graphite cathode was carried out in the KF–NaF(10 wt.%)–AlF3–Al2O3 electrolyte (CR=1.5) at 830 °C. After 100 h of electrolysis, the thickness of the AlB2 layer on the graphite surface was 5 μm, while the cathode surface was completely wetted with aluminum. Thus, we demonstrated the feasibility of using the borated graphite cathode as a wettable dripping cathode in the low-temperature aluminum electrolysis in the vertical cell.

Thermodynamics, kinetics and mechanism analysis of aluminothermic reduction for preparing Al-Zr alloy

In this study, we investigated the thermodynamics and kinetics of the aluminothermic reduction of ZrO2. The results of thermal analysis and isothermal heat treatment showed that the aluminothermic reduction of ZrO2 was an exothermic reaction that occurred at . 958.8°C. The aluminothermic reduction process does not produce metal Zr, but when the stoichiometric number of Al:ZrO2 was 13:3, Al3Zr and Al2O3 can be obtained by the reaction. The activation energy of the reaction was 1589.89 kJ/mol, and the reaction order was 1.90. The aluminothermic reduction process and mechanism of ZrO2 in the presence of potassium cryolite as the molten salt medium was also studied. The results showed that ZrO2 dissolved in potassium cryolite molten salt and participated in the reduction reaction in the form of Zr–O–F complex ions, while the by–product Al2O3 also dissolved in potassium cryolite to further promote the formation of the Al–Zr alloy.

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study.

The crystal structures of three polymorphs of Rb3ScF6 have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; α) and medium-temperature (β) structures are tetragonal, with space groups I41/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) Å, c = 36.5160(0) Å and a = 14.4093(2) Å, c = 9.2015(1) Å at RT and 187 °C, respectively. The high-temperature (γ) structure is cubic space group Fm3̅m (Z = 4) with a = 9.1944(1) Å at 250 °C. The temperatures of the phase transitions were measured at 141 and 201 °C. The three α, β, and γ Rb3ScF6 phases are isostructural with the α, β, and δ forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the β polymorph, the dynamic rotations of the ScF6 octahedra of both Sc crystallographic sites have been detailed.

KF-NaF-AlF<sub>3</sub>体系结构性质的分子动力学模拟

近年来低温铝电解质的研究已成为铝电解工业上节能减排的热点问题，对其基本物理化学性质的探索是一个必然的过程。本文运用密度泛函理论与经典分子动力学方法相结合的方法系统研究了1100 K下xKF-2.2NaF-AlF3熔盐体系的结构性质和输运性质。研究结果表明在平衡的高温钾冰晶石熔盐体系中六配位[AlF6]3−、五配位[AlF5]2−、四配位[AlF4]−络合离子共同存在，且六配位铝氟离子集团占主导地位(70%以上)，并会发生部分的离解，生成五配位或者四配位离子团；当KF浓度增大时，电解质体系中游氟原子含量会增加，桥氟含量逐渐降低，体系的聚合程度也会随之下降。此外，KF的加入使得桥氟离子解离，Na、Al的自扩散系数会先增大后降低，体系显示出低粘度高电导率的特性，但当KF浓度达到2%后，电导率会逐渐降低。 The study of low temperature aluminium electrolytes has become a hot issue for energy saving and emission reduction in the aluminium electrolysis industry in recent years, and the exploration of their basic physicochemical properties is an inevitable process. In this paper, the structural and transport properties of the nKF-2.2NaF-AlF3 molten salt system at 1100 K have been systematically investigated using a combination of density flooding theory and classical molecular dynamics methods. The results show that in the equilibrium high-temperature potassium cryolite molten salt system, hexacoordinated [AlF6]3-, pentacoordinated [AlF5]2- and tetracoordinated [AlF4]- complex ions co-exist, and the hexacoordinated aluminium-fluorine ion group dominates (more than 70%) and undergoes partial dissociation to produce pentacoordinated or tetracoordinated ion groups; As the KF concentration increases, the amount of free fluorine atoms in the electrolyte system increases, the bridge fluorine content gradually decreases and the degree of polymerisation of the system decreases. In addition, the addition of KF causes the dissociation of the bridging fluorine ions, the self-diffusion coefficients of Na and Al will first increase and then decrease, and the system shows the characteristics of low viscosity and high conductivity, but when the KF concentration reaches 2%, the conductivity will gradually decrease.

Al–Zr alloys synthesis: characterization of suitable multicomponent low-temperature melts

Al–Zr alloys as an advanced metal matrix composite material start to be attractive for various special applications. The electrochemical preparation of aluminium–zirconium alloy from cryolite melts seemed to be very perspective. Characterization of low-temperature sodium and potassium cryolite-based systems with the different additions of ZrO2 and Al2O3 (both as an electrochemical active compounds) was realized. The electrical conductivity of NaF–AlF3–Al2O3–ZrO2 and KF–AlF3–Al2O3–ZrO2 systems as a function of the temperature was studied. The addition of AlF3 varied from 33mol% to 42mol% in the sodium cryolite system and from 40.0mol% to 45.5mol% in the potassium cryolite system. Alumina and/or zirconia were added in amounts up to 1.2mol% and 2mol%, respectively (sodium system), and up to 2.8mol% and 3mol%, respectively (potassium system). The behavior of the electrical conductivity in the studied systems was described by the regression equations valid in a large temperature and concentration ranges.

Study of Cu-Ni-Fe Alloys as Inert Anodes for Al Production in Low-Temperature KF-AlF3 Electrolyte

Cu-Ni-Fe-based alloys are considered as promising O2-evolving anode materials for CO2-free Al production. In the present study, biphased (as-cast) and monophased (postcasting homogenized) Cu65Ni20Fe15, alloys, and monophased Ni65Fe25Cu10 alloy (in wt pct) are evaluated as O2-evolving anodes for Al production in potassium cryolite at 700 °C. The produced Al purity is 99.6 wt pct, and the erosion rate is estimated at 0.4 cm year−1 for both Cu65Ni20Fe15 anodes compared to 95.2 wt pct and 3.2 cm year−1 for the Ni65Fe25Cu10 anode. The compositions, and morphologies of the surface oxide layer and the metal fluoride layer present at the oxide/alloy interface are compared for the three anodes. The deleterious impact of electrolyte infiltration on the surface oxide building is highlighted.

Kinetics of leaching lithium from α-spodumene in enhanced acid treatment using HF/H2SO4 as medium

Abstract An enhanced leaching of Li from α-spodumene was carried out using a mixture of hydrofluoric and sulfuric acid (HF/H2SO4) as the medium. Based on the optimized leaching conditions, the leaching kinetics of Li was investigated in an ore/HF/H2SO4 ratio of 1:3:2 g:mL:mL with leaching temperature ranging from 50 to 100 °C. The results indicate that the leaching kinetics of Li fitted well with a model based on the shrinking core model. In addition, the leaching rate of Li was controlled by chemical reactions and diffusion through the product layers. The apparent activation energy Ea was calculated to be 32.68 kJ/mol. Solid films were formed because of the generation of insoluble products such as cryolithionite (Na3Li2Al2F12), cryolite (Na3AlF6), calcium fluoride (CaF2), potassium cryolite (K2AlF5), aluminum fluoride (AlF3), and fluorosilicates (Na2SiF6 or KNaSiF6). Furthermore, the effects of the ore/HF ratio and leaching temperature on the leaching behavior of Li, Al and Si were investigated. The results indicate that the ore/HF ratio and leaching temperature could clearly affect the distribution of HF molecules on the leaching of Li, Al and Si, which are important for the selective leaching of Li over Al and Si with this fluorine-based chemical method.

Ionic micro-structure and transport properties of low-temperature aluminium electrolytes containing potassium cryolite and sodium cryolite.

Nowadays, low-temperature aluminium electrolytes are reported to have good prospects for application in the industrial process of aluminium production. In this paper, low-temperature electrolytes containing potassium cryolite and sodium cryolite with a cryolite ratio of 1.3 were investigated by using first-principles molecular dynamics simulation. This calculation reproduces the ionic structure of low-temperature 1.3(KF + NaF)-AlF3 electrolytes, indicating that [AlF4]-, [AlF5]2- and [AlF6]3- are the fundamental aluminum-fluoro clusters and [AlF5]2- is the predominant species. The calculated results for the ionic structure indicate that molten 1.3(KF + NaF)-AlF3 electrolytes have a high ionic polymerization degree, which is unfavorable for the transport properties of low-temperature 1.3(KF + NaF)-AlF3 electrolytes. Fortunately, increasing the mass fraction of NaF can effectively reduce the polymerization degree of ionic structure and thus improve the ionic conductivity of low-temperature 1.3(KF + NaF)-AlF3 electrolytes, which is an important guiding factor for the component selection of low-temperature electrolytes in future. Also, DFT calculations were adopted to further analyse the small aluminum-fluoro complexes. The calculated Raman spectrum of the aluminum-fluoro complexes is excellently consistent with literature results. Our calculated ionic conductivity falls in between the estimated value of the empirical equation of different literature studies, demonstrating that our results may be more precise than the literature results. This further proves the practicability of our modified N-E equation.

Efficient Assessment of Physico-Chemical Properties of the Cryolite Melts for Research on the Improvement of Low-Temperature Aluminum Electrolysis

Modern aluminum electrolysis in cryolite-alumina melts is energy-intensive, inefficient and environmentally hazardous production. Addressing these significant shortcomings, the technology of low-temperature electrolyte is directed. The basis of low-temperature electrolysis is potassium cryolite, which results in high magnitude and rate of dissolution of alumina. Additive of sodium and lithium fluorides provide the necessary conductivity. Experimental investigation of these properties is extremely time consuming. In this work, as a parameter, which will allow to characterize effectively and rapidly the complexing ability of cryolite melts, the ratio of cationic ion power of Al3+ to the total power of the other cations of the melt is proposed. Regression analyses of the known experimental data establish the existence with a high level of reliability (R2=0.966-0.995) of a directly proportional dependence of this parameter on solubility of alumina and electrical conductivity of cryolite melts.

Sedimentation behavior of high-temperature concentrated colloidal suspension based on potassium cryolite

ABSTRACTThe paper describes the sedimentation behavior of concentrated high-temperature oxide-fluoride slurries with alumina particles volume fraction range 0.24 ≤ φ ≤ 0.32 at ∼700 °C. The behavior is of interest due to perspectives of the non-Newtonian fluids usage in the future aluminium reduction technology. To characterize sedimentation behavior several techniques were used: density analysis, X-Ray diffraction, microphotography and electron scanning microscopy. Sample with −63 µm dispersed phase as well as smelter-grade alumina which contains particles in a range of 10–150 µm has been examined. It has been found that particle settling occurs with the initial velocity 0.97·10−5 m·s−1 at φ = 0.24 and gradually reduces reaching zero at φ close to 0.32 which was considered as a maximum packing fraction. MS−1 sedimentation mode has been identified, the type of sedimentation was found to be changed from type 1 to type 3 in the sample with φ = 0.24. The alumina partial density distribution in suspension as a function of time was determined. Alumina average motion velocity during sedimentation was calculated to be 154·10−3 kg·m−2·s−1 at φ = 0.24 in the top layer of the sample within the first 150 s of sedimentation. It had been reducing drastically before it reached zero after 300 s. An alumina flow was higher in the top layers. Some of the rheological properties (such as relaxation time, which increased from 7.2·10−3 to 17.7·10−3 s after the increase of φ from 0.24 to 0.28) has been determined.

Electrical Conductivity of Low-Temperature Potassium Cryolite Electrolytes Suitable for Innovation of Aluminum Preparation

Innovations of aluminum electrolysis process are focused on the inert electrodes and the new types of electrolytes with lower temperature of primary crystallization. The main component in new types of molten systems can be a potassium cryolite. Advantages of these electrolytes are low possible working temperature (above 660°C) and sufficient solubility of an electrochemical active compound (alumina) at these temperatures. The electrical conductivity is one of the most important properties of the electrolytes. Electrochemical impedance spectroscopy was used as a suitable measurement method for determination of the electrical conductivity. Potassium cryolite-based mixtures with the different molar ratios (molar ratio of KF and AlF3) and with various additives were studied as a function of the temperature. The molar ratio of binary melt KF-AlF3 varied from 1.2 to 1.5 with the step of 0.1. The temperature dependence (from 655°C to 850°C) and the influence of the addition of Al2O3, CaF2, MgF2 and LiF on the electrical conductivity were obtained. The electrical conductivity was measured by tube-type cell with stationary electrodes; AC-techniques with a sine wave signal in the high frequency range were applied. Temperature and concentration dependencies of conductivity in the studied systems were described by non-linear regression equation.

Advanced technology for Al-Zr alloy synthesis: Electrochemical investigation of suitable low-melting electrolytes

Aluminium-zirconium alloys start to be attractive in various fields such as military, aerospace industry, nuclear reactions or atomic energy. The investigation of low-temperature sodium and potassium cryolite based melts with the different additions of ZrO2 was realized in order to develop the scientific basis of the technology for aluminium-zirconium alloy synthesis. The electrical conductivity of NaF-AlF3-ZrO2 and KF-AlF3-ZrO2 systems as a function of temperature was studied by a tube-type cell with fixed distance between the electrodes, applying ac-techniques with a sine wave signal with small amplitude in the high frequency range. The addition of the aluminium fluoride varied in the range (33–38) mol % in sodium cryolite melts (molar ratio MR = n(Na(K)F)/n(AlF3) = 2.0–1.6) and from 40 mol % to 45.5 mol % (MR = 1.5–1.2) in the potassium cryolite system. Zirconia was added into the basic two-component melts in amounts of (1–4) wt % for sodium system and (1.7–5.2) wt % for potassium system. All measured data were evaluated by the regression analysis (the least square method) for calculation of the electrical conductivities in both investigated melts.

Novel Molten Salts Media For Production of Functional Materials

Physical-chemical properties of molten salt media based on potassium cryolite with additions of boron or scandium oxides have been considered from the point of view of their feasibility in production of functional materials, such as aluminum alloys. Liquidus temperature in the quasi-binary systems: [KF–AlF 3 ]-B 2 O 3 , [KF–AlF 3 ]-Sc 2 O 3 , [KF–NaF–AlF 3 ]-B 2 O 3 , and [KF–NaF–AlF 3 ]-Sc 2 O 3 has been measured by thermal analysis. Solubility of Al 2 O 3 , B 2 O 3 , and Sc 2 O 3 in potassium and potassium-sodium cryolites has been determined. The potassium-cryolite-based melts were found to have an enhance protective function due to a low melting point, and an effective refining ability due to a good alumina solubility. It has been assumed that for aluminum alloys production the potassium-cryolite-based melts can be used as fluxes with improved properties as well as electrolytes for low-temperature electrolysis.

Electrical Conductivity of Low-Melting AlF<sub>3</sub>-NaF and KF-AlF<sub>3</sub> Solutions

In this work the electrical conductivity of molten sodium cryolite solutions (Na3AlF6) and potassium cryolite solutions (K3AlF6) containing different contents of aluminum fluoride were investigated. Experiments were carried out in a temperature range including the liquid phase, using electrochemical impedance spectroscopy (EIS). Based on the conducted studies, it was found that the electrical conductivity of the studied electrolytes decreases with decreasing temperature and increasing content of aluminum fluoride. It was also found that in the temperature range in which the studied salts are in the liquid state, the solutions of potassium cryolite have lower electrical conductivity than the solutions of sodium cryolite.

INTERACTION OF SINTERED ALUMINIUM NITRIDE WITH MELT KF–ALF3 AT TEMPERATURE 700–800 °C

By means of continuous weighing method was investigated the kinetics of the interaction of sintered composite on the base of aluminium nitride with the molten potassium cryolite with cryolite ration (CR) 1,3 at temperatures of 700–800 °C. There was specified that at temperature 700 °C composite practically did not exposed to melt impact. Materials interact with molten salt through the stage of its oxidation in the melt of oxygen. At this, takes place besides merging of originally presented in material of oxygen-containing impurities. In the absence of an oxidizer and at concentration of dissolved in cryolite alumina, close to saturation, interaction of sintered aluminium nitride with examined melt did not observed.

Interaction of sintered aluminum nitride with the KF-AlF3 melt at 700–800°C

Interaction kinetics of the aluminum nitride-based sintered composite material with the potassium cryolite melt with the cryolite ratio KO = 1.3 at 700–800°C is investigated by the continuous weighing method. It is established that the composite is almost unaffected by the melt at t = 700°C. The corrosion rate increases as the temperature increases. The material interacts with the molten salt through its oxidation stage by oxygen dissolved in the melt. This is also accompanied by the dissolution of oxygen-containing impurities that are initially present in the material. No interaction of sintered aluminum nitride with the melt under study was observed in the absence of the oxidant and with the concentration of alumina dissolved in cryolite close to saturation.

A spectroscopic and computational study of Al(III) complexes in cryolite melts: Effect of cation nature

Lithium, sodium and potassium cryolite melts are probed by Raman spectroscopy in a wide range of the melt composition. The experimental data demonstrate a slight red shift of main peaks and a decrease of their half-widths in the row Li+, Na+, K+. Quantum chemical modelling of the systems is performed at the density functional theory level. The ionic environment is found to play a crucial role in the energy of fluoroaluminates. Potential energy surfaces describing the formation/dissociation of certain complex species, as well as model Raman spectra are constructed and compared with those obtained recently for sodium containing cryolite melts (R.R. Nazmutdinov, et al., Spectrochim, Acta A 75 (2010) 1244.). The calculations show that the cation nature affects the geometry of the ionic associates as well as the equilibrium and kinetics of the complexation processes. This enables to interpret both original experimental data and those reported in literature.

Growth of platelet-structured CuAlO2 thin film on CuAl alloy by molten salt electrolysis method

Herein, we report a novel synthesis procedure of CuAlO2 thin film by applying molten salt electrolysis method on a CuAl alloy electrode. The CuAlO2 thin film was directly prepared using CuAl alloy as Cu and Al sources by electrolysis in molten potassium cryolite system (CR=1.3) at 700°C for 10h with a current density of 0.5A/cm2. The as-prepared CuAlO2 film was characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The results indicate that the rhombohedra structural CuAlO2 film has platelet-like structures with particle size of 2–10μm and the membrane thickness is ~75μm. Moreover, the reaction mechanism for the as-synthesized CuAlO2 thin film is proposed.

Liquidus Temperatures of Cryolite Melts With Low Cryolite Ratio

The effect of calcium fluoride on liquidus temperatures of the cryolite melts with a low cryolite ratio (CR) was studied. The systems KF-NaF-AlF3 and KF-LiF-AlF3 with CRs of 1.3, 1.5, and 1.7 have been investigated. The liquidus curves of systems containing CaF2 are different and depend on the K/(K + Na) and K/(K + Li) ratios. In potassium cryolite with CRs of 1.3 and 1.5, the calcium fluoride solubility is low and increases with NaF (LiF) concentration.