Fluoride Melts Research Articles

Development and electrochemistry of a novel Ag/AgCl reference electrode suitable for mixed chloride–fluoride melts

Accurate values of electrode potentials are useful in understanding the electrodeposition of boron from a melt containing fluoride, chloride and fluoroborate, as well as in the determination of the thermodynamic properties of this system. A suitable reference electrode for use with this molten mixture is essential for the determination of these potentials. An Ag/AgCl reference electrode was fabricated for this purpose and its stability, reversibility and polarizability in a melt containing chloride and fluoride were studied in the temperature range 1073–1123 K. Cyclic-voltammograms (CVs) of this melt on a platinum working electrode were recorded at regular intervals over a period of 12 h. These CVs did not show any appreciable variation with time, indicating the stability of this reference electrode. The reversibility of this reference electrode was established by carrying out a micropolarization test. Cathodic and anodic polarization tests were carried out by passing a current of 1 mA and 5 mA through the cell for a duration of 300 s. After these polarization tests, the electrode swung back to its equilibrium potential within about 250 s. Reduction potentials of Fe 2+/Fe and Ni 2+/Ni couples in the KF–KCl melt were measured for the first time.

Solubility of UF4, ThF4, and CeF3 in the NaF-ZrF4 melt

The solubility of UF4, ThF4, and CeF3 in a fluoride melt 52 mol % NaF-48 mol % ZrF4 in the temperature range 550–750°C was studied. The solubility of the fluorides in the melt increases with temperature. At 600°C and higher temperatures, the solubility decreases in the order UF4 > ThF4 > CeF3. The temperature dependences of the solubility of UF4, ThF4, and CeF3 in the NaF-ZrF4 system are satisfactorily described by a linear equation.

In Situ Experimental Approach of the Speciation in Molten Lanthanide and Actinide Fluorides Combining NMR, EXAFS and Molecular Dynamics

We study the nature and the distribution of the different anionic species existing in fluoride melts of nuclear interest. Our approach is based on the combination of NMR and EXAFS spectroscopy, both sensitive to the microstructure of the melt, with Molecular Dynamics calculations. Because of its analogy with actinide fluorides, we have studied zirconium fluoride and its mixtures with alkali fluorides. In MF-ZrF4 systems, MD simulations, in agreement with 91Zr and 19F NMR and EXAFS experiments, put in evidence the coexistence of 3 different Zr-based complexes [ZrF6]2-, [ZrF7]3-, and [ZrF8]4- in the melt. Their proportions depend on the amount of ZrF4 and on the nature of the alkali (Li+, Na+ and K+). In the case of lanthanide fluorides, a similar approach performed in the system MF-LaF3 evidences some structural similarity, mainly with the existence of different complexes and upon free fluorine content evolution.

Fluid and magmatic processes in the formation of the Ary-Bulak ongonite massif ( eastern Transbaikalia)

The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous–saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. An ongonite melt crystallized with the participation of P–Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonite melt, the duration of its crystallization has been estimated for a magma chamber of the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and Rayleigh number of the ongonite melt have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonite melt in a closed magma chamber. Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and then these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P–Q magmatic fluid which coexisted with the melt in the residual chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; the silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid–magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonite melt, intense melt convection in the residual chamber, and rising fluid pressure during its degassing.

Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.

ChemInform Abstract: Vibrational Spectra of Niobium(V) Fluoro and Oxo Fluoro Complexes Formed in Alkali-Metal Fluoride Melts.

Raman spectra of the molten system LiF-NaF-KF-K 2 NbF 7 at 650°C indicate the presence of NbF 7 2− ions. With the addition of small amounts of oxide, vibrational frequences due to a NbOF n (n-3)− species are observed at 921 (p), 583 (p) and 307 cm −1 (dp) for oxide to niobium(V) mole ratios up to approximately 2. Infrared spectra of solidified melts support the existence of a niobium-oxygen double bond in this complex. The vibrational spectrum is in accord with the band pattern of monomeric NbOF 5 2− with a C 4v symmetry

Fluoride and chloride melts included in phenocrysts of agpaitic acid volcanic rocks from Pantelleria Island

978 Many deposits of rare elements (Be, Zr, Nb, REE, Y, and others) are genetically related to acid agpaitic (with the mole ratio (Na2O + K2O)/Al2O3 > 1) mag� mas [1, 2]. The concentration of these elements most likely results from crystallization differentiation lead� ing to their accumulation in the residual melts at the expense of low coefficients of minor element parti� tioning between silicate minerals and melts [3, 4]. Gradual accumulation of incoherent minor elements in melts may be violated by separation of saline melts from magma. According to the experimental data [5], minor elements are most effectively extracted by fluo� ride melts. Because of this, silicate–saline liquid immiscibility is an important factor of geochemical and metallogenic evolution of magmas. The formation of chloride melts during the evolu� tion of magmatic systems was established for some objects [6 and others]. The presence of fluoride melts in acid magmatic systems was described in individual cases [7, 8], and each new finding is of intense interest. In this paper we discuss the results of the study of melt inclusions in phenocrysts from magmatic rocks of Pantelleria Island (Central Mediterranean, Italy) [9], in which liquid immiscibility of alkaline acid magmas with the formation of saline melt was observed. Chlo� ride melts in inclusions and glasses from rocks of Pan� telleria Island were first discovered in 1987 [10] and hereafter were described in glasses of the rock ground� mass [11]. The melt and fluid inclusions were studied in phe� nocrysts from ignimbrite and pantellerite collected in different parts of Pantelleria Island and related to the precaldera and postcaldera stages of volcanism evolu� tion.

Electrochemical intercalation of potassium into graphite in KF melt

Electrochemical intercalation of potassium into graphite in molten potassium fluoride at 1163 K was investigated by means of cyclic voltammetry, galvanostatic electrolysis and open-circuit potential measurements. It was found that potassium intercalated into graphite solely between graphite layers. In addition, the intercalation compound formed in graphite bulk in molten KF was quite unstable and decomposed very fast. X-ray diffraction measurements indicate that a very dilute potassium–graphite intercalation compound was formed in graphite matrix in the fluoride melt. Analysis with scanning electron microscope and transmission electron microscope shows that graphite was exfoliated to sheets and tubes due to lattice expansion caused by intercalation of potassium in molten KF.

Unusual effect of the second coordination sphere on the standard charge transfer rate constants for the Nb(V)/Nb(IV) redox couple in chloride—Fluoride melts

Unusual effect of the second coordination sphere on the standard charge transfer rate constants for the Nb(V)/Nb(IV) redox couple in chloride—Fluoride melts

Crystallization in heat-treated fluorochlorozirconate glasses

Crystallization phenomena of fluorochlorozirconate glasses were investigated bymeans of differential scanning calorimetry and inelastic neutron scattering. Theprecipitation of barium chloride nanoparticles from the glass matrix upon heattreatment was found to be suppressed when re-melting the glass with a reducingagent but not if the agent was present in the initial synthesis. Addition of smallamounts of oxide to the predominantly fluoride melt was found to maintain thepresence of nanoparticles but not to induce the predicted phase transition ofthe barium chloride nanoparticles from hexagonal to orthorhombic structure.Inelastic neutron scattering performed on an ‘as-made’ glass and a heat-treatedglass showed an increase in ‘hardness’, consistent with a more ordered structure.

Distribution of groups I and II elements between liquidus phases of fluorine-saturated Si-Al-Na-K-Li-H-O system

Experimental data allow modeling the behavior of the named elements during formation of fluorine- saturated leucocratic rocks of silicic and alkaline compositions. The distribution of alkaline and alkaliearth elements is discussed at equilibrium between the silica-alumina melt with fluoride phases (crystalline and liquid) and with feldspar. Cryolite crystals form during saturation of silica-alumina melt of normal alkalinity with fluorine. Continuous solid solution of sodium-potassium cryolite is stable at 800°C. The equilibrium between melt and crystals continues up to the maximum molar fraction of 0.1 lithium end member in cryolite, at which two fluoride phases (crystalline and liquid) coexist with the silica-alumina melt of fixed composition. Separation of salt melts during late differentiation stages of granite and alkaline rocks is a regular process continuing the natural evolution of ore-magmatic systems. At equilibrium of two liquid phases, the silica phase is relatively enriched in potassium, and the fluoride phase is substantially enriched in sodium. This detected effect is the only currently possible mechanism for the occurrence of the potassium differentiation trends of granite melts. All effects related to crystallization cause enrichment in sodium. In other cases (with Ca, Sr, Mg, Rb, and Cs), separation of the second liquid phase acts in the same direction and enhances the action of crystallization. Comparison between partition coefficients allows derivation of the following affinity rows of alkaline elements for fluoride melt: Li > Na > K > Rb≈Cs and Mg > Ca > Sr > Ba. Hence, the known rule for joining strong bases with strong acids and weak bases with weak acids is fulfilled.

Study of neodymium extraction in molten fluorides by electrochemical co-reduction with aluminium

This work describes the co-deposition process of Al–Nd alloys in LiF–CaF 2 medium (79–21% mol) on tungsten electrode at 860 °C using electrochemical techniques: cyclic and square wave voltammetries and potentiostatic electrolyses. Specific peaks of Al–Nd alloys formation were observed in cyclic voltammograms between the reduction waves of Nd(III) and Al(III), in a fluoride melt containing neodymium and aluminium ions. The potential difference measured between the solvent reduction and the alloys formation should allow expecting an extraction efficiency of 99.99% by the use of the co-reduction process. The intermetallic compounds (Al 11Nd 3, Al 3Nd, AlNd 2 and AlNd 3) were obtained and characterised by Scanning Electron Microscopy with EDS probe. The validity of the process was tested by carrying neodymium extraction in the form of Al–Nd alloy; the extraction efficiency was more than 95%.

Dynamic reference electrode for investigation of fluoride melts containing beryllium difluoride

Existing designs of reference electrodes for potentiometric measurements in fluoride melts do not meet basic requirements of the long exposure corrosion tests to be performed. A new diaphragm-free three-electrode meter with a nonstationary (dynamic) beryllium reference electrode for the redox potential measurements was developed. Optimum conditions of forming dynamic beryllium reference electrode were determined in the laboratory tests in isothermal cell. The device for redox potential measurement was designed and it was shown that it is highly sensitive to changes redox potential within ±5 mV. Reliability and efficiency of diaphragm-free device with dynamic reference electrode was confirmed in the thermal convection corrosion loop operating more than 1200 h with molten 15LiF–58NaF–27BeF 2 mixture (mol%) at a maximum temperature of about 700 °C.

Potentialities of fluoride-based salts for specific nuclear reprocessing: Overview of the R&D program at CEA

Initially studied in the frame of the first French act on radioactive waste management (December 1991), the pyrotechnology is currently assessed by the Nuclear Energy Direction of the Commissariat à l’Energie Atomique (CEA) within the succeeding act (June 2006) as a potential alternative to hydrometallurgy for the reprocessing of targets or dedicated fuels (coming from accelerator-driven systems or ADS) considered for the minor actinides transmutation. The R&D program is mainly focused on the evaluation of the fluoride melts as interesting media for operating separation between the actinides and the fission products. Two separation techniques are currently evaluated; the first one uses the liquid–liquid extraction technique between molten fluoride and liquid metal at high temperature, the second one is based on an electrolytic separation in a molten fluoride melt. Both are promising in terms of separation efficiency. This paper gives an overview of the current studies and presents the last main experimental results.

Geochemistry of Sc in the magmatic process: Experimental evidence

Data on Sc distribution between aluminosilicate melt and fluoride phases (cryolite in the Na-K parts of the system and fluoride melt in its Li-bearing parts) were experimentally obtained for the Si-Al-Na-K-O-H-F system at 800, 750, and 700°C and \( P_{H_2 O} \) = 1000 bar, at the saturation of the aluminosilicate melt with respect to H2O and F. The Sc partition coefficients between aluminosilicate melt and fluoride phases vary from 0.005 to 1.6 depending on phase relations in the system, which are, in turn, controlled by its bulk composition and experimental conditions. At 800°C, the Sc partition coefficients between aluminosilicate melt and fluoride phases are lower than 1 in all of the examined parts of the system; i.e., Sc is preferably distributed into the fluoride phase regardless of whether it is crystals of (Na, K-Na, or K)-cryolite or alkali-aluminofluoride melt. This is the fundamental difference of Sc behavior from those of REE, and most other elements, under the same conditions. A temperature decrease to 700°C and the transition from quartz-normative to nepheline-normative melts, as well as an increase in their agpaitic coefficient, leads to an increase in the Sc partition coefficients to values greater than 1. This tendency is related to an increase in the degree of depolymerization of the aluminosilicate melt.

Waterlike Structural and Excess Entropy Anomalies in Liquid Beryllium Fluoride

The relationship between structural order metrics and the excess entropy is studied using the transferable rigid ion model (TRIM) of beryllium fluoride melt, which is known to display waterlike thermodynamic anomalies. The order map for liquid BeF2, plotted between translational and tetrahedral order metrics, shows a structurally anomalous regime, similar to that seen in water and silica melt, corresponding to a band of state points for which average tetrahedral (q(tet)) and translational (tau) order are strongly correlated. The tetrahedral order parameter distributions further substantiate the analogous structural properties of BeF2, SiO2, and H2O. A region of excess entropy anomaly can be defined within which the pair correlation contribution to the excess entropy (S2) shows an anomalous rise with isothermal compression. Within this region of anomalous entropy behavior, q(tet) and S2 display a strong negative correlation, indicating the connection between the thermodynamic and the structural anomalies. The existence of this region of excess entropy anomaly must play an important role in determining the existence of diffusional and mobility anomalies, given the excess entropy scaling of transport properties observed in many liquids.

Corrosion Behavior of the Electrochemical Couple: Hastelloy/Carbon Material in Zirconium Fluoride - Sodium Fluoride Melts

Time dependencies for the parameters (current, voltage, inner resistance) of short-circuited galvanic cell: Hastelloy-carbon material have been studied in NaF-ZrF4 eutectic melt at 600- 650oC. Analysis of these data, together with the data on polarization of separate electrodes at open circuit conditions, showed that short-circuit current is related to the rate of corrosion of the alloy contacted with carbon. In the presence of zirconium, the short-circuit current corresponds to the rate of zirconium interaction with the fluoride melt and can be used for studies of its dissolution kinetics. It was found that contact of Hastelloy with carbon material results in an increase of corrosion rate both in pure NaF-ZrF4 eutectic melt and with the additives of LaF3. The presence of zirconium in fluoride melt suppresses the corrosion process.

Peculiarities of electroplating tantalum coatings from fluoride melts

Electroplating tantalum coatings on molybdenum, copper, and steel is studied. The effects of various factors (the composition of a fluoride bath, its temperature, current density, and the electric charge spent) on the electrocrystallization of the deposit, its roughness, the degree of dendrite formation, current efficiency, and a possibility to plate multilayer coatings and produce tantalum wares by electrotyping are investigated. To compare the characteristics of the coatings, the studies in a standard chloride-fluoride salt system containing potassium fluorotantalate (K2TaF7) were also carried out under similar conditions.

On the use of electrochemical techniques to monitor free oxide content in molten fluoride media

The electrochemical behaviour of oxide ions has been studied in fluoride melts(LiF/NaF eutectic) by cyclic voltammetry, square wave voltammetry and chronopotentiometry. The purpose is to determine whether these techniques can be used for titration of free oxide ions (O2-) in molten fluorides released by lithium oxide additions. Cyclic voltammetry is shown to be unsuitable for this purpose due to oxygen bubbling disturbing the oxidation peak, while square wave voltammetry is far more appropriate because the observed signal is a well defined oxidation peak with a height proportional to the oxide content. Thus, the present work is focused on a strategy of oxide ions titration by square wave voltammetry. In addition, this work allows assessing that the electrochemical reduction of oxide ions proceeds by diffusion of these species, and the O2- diffusion coefficient is estimated by chronopotentiometry.

A first finding of anomalously Cs-rich aluminosilicate melts in ongonite: Evidence from melt inclusion study

The chemical properties of cesium allow accumulation of this element in the late silicic derivatives of igneous complexes, in particular, in rare-metal granites, pegmatites, and related metasomatic rocks. However, the Cs content can be high enough only in pegmatites to form its own mineral (pollucite), which occasionally occurs in considerable amounts. In other rocks, Cs concentrates largely in micas and feldspars. The possible maximal level of Cs accumulation in the melt remains poorly studied. Important new information has been obtained from the study of volcanic glass and melt inclusions (MI) in minerals. The rhyolitic glass concentrates 220‐870 ppm Cs, occasionally up to 3770 ppm [1]. To date, the highest Cs 2 O content (1.2‐5.5 wt %) has been detected in MIs captured by quartz from miarolitic pegmatite of the Malkhan field of the central Transbaikal region [2, 3] and the southwestern Pamirs (Leskhozovskaya and Vezdarinskaya veins). These inclusions contain products of crystallization of the late pegmatitic melt [3] and the inferred high-temperature meltlike gels [4]. While studying ongonite of the AryBulak Massif, we detected MIs filled with a residual glass that contains up to 17 wt % Cs. This is reliable evidence in favor of the existence of natural melts extremely enriched in Cs. In this communication, we describe these unusual inclusions and discuss Cs distribution in ongonites. The Ary-Bulak Massif is a dome-shaped stock exposed over ~0.8 km 2 among the Devonian volcanosedimentary rocks [5]. The massif is composed largely of porphyritic ongonite. A zone of aphyric rock 50‐100 m wide occurs only near the southwestern contact zone. Beyond this zone, an ongonite variety with anomalously high contents of CaO (3.3‐21.8 wt %) and F (2.7‐16 wt %) has been found near the same locality. The high CaO (7.8‐18 wt %) and F (7.1‐15.5 wt %) contents are inherent to aphyric rocks as well. Prosopite CaAl 2 F 4 (OH) 4 has been identified in this rock for the first time as an abundant mineral (6‐26 wt %). It was established that interstices between minerals of the groundmass of the Ca- and F-rich rocks are filled with submicrometric intergrowths of “fluoritic” and “K-feldspathic” phases. The “fluoritic” phase is a partly devitrified calcium fluoride melt with the following admixtures (wt %): O (3‐12), Al (0.5‐3.3), Si (0.2‐1.5), Sr (0.3‐ 0.5, occasionally up to 1.0‐1.3), Na (up to 0.5), and S (up to 0.3). The “K-feldspathic” phase is commonly close in composition to sanidine (rims around tabular albite crystals) but characterized by local enrichment in Ca (1.5‐4.0 wt %). We provided evidence for the joint crystallization of immiscible aluminosilicate and calcium fluoride melts in the presence of HF-bearing aqueous fluid during formation of Ca- and F-rich rocks [6, 7]. The Cs content in rocks from the Ary-Bylak Massif