Article pubs.acs.org/ac
Direct Determination of the NaF/AlF3 Molar Ratio by Raman Spectroscopy in NaF−AlF3−CaF2 Melts at 1000 °C Cedric Malherbe and Bernard Gilbert* Laboratory of Analytical Chemistry and Electrochemistry, Department of Chemistry, University of Liege, Liege, Belgium S Supporting Information *
ABSTRACT: For the last 40 years, Raman spectroscopy has been very useful in investigating the structure of corrosive molten salts, such as the cryolite-based melts widely used as electrolyte in the Hall−Heroult process. Even if this process remains the most economically efficient for metallic aluminum electro-production, it suffers from a high energy loss, which is dependent on the melt composition. Therefore, controlling the chemical composition of the electrolyte is essential. The present paper proposes to apply Raman spectroscopy for the direct determination of the NaF−AlF3 molar ratio in NaF− AlF3−CaF2-based melts. Despite the experimental difficulties, a calibration curve based on equilibria taking place in the melt has been developed and the procedure has been successfully compared to industrial samples of known compositions. The possible exportation of the laboratory scale procedure to an industrial environment application for the control of the Hall−Heroult process is finally discussed.
A
the cryolitic ratio (CR), which is defined as the molar ratio of NaF and AlF3 within the melt.4,5 Currently, the Hall−Heroult process remains the most economically efficient process, even if it suffers from major drawbacks, such as relatively low bath conductivity, digestion of the carbon lining, and consumption of the carbon anode, leading to carbon dioxide emission and high energy loss.5 Consequently, much research has tried to minimize these problems.6 Some have been dedicated to the development of inert electrode materials7 such as Cu−Ni−Fe alloys, which has attracted attention in recent years.8 Other research has focused on optimization of the bath composition, by attempting to increase the cell conductivity and reduce the working temperature. Make note of one very important point: the overvoltage and the working temperature required for the electrolysis turns out to be heavily dependent on the melt composition.9 Therefore, controlling the industrial bath composition and understanding the structure of the melt are critical to reduce the energy loss. Dating back to the 1970s, the first model generally accepted for the structure of molten cryolite proposed that (i) cryolite dissociates into its constituents ions, Na+ and AlF63− (eq 2), and (ii) AlF63− subsequently dissociates into AlF4− and F− (eq 3). Raman spectroscopy was very useful for that matter, providing direct molecular evidence of the existence of AlF4− in molten cryolite melts.10
luminum represents a key material for future industrial applications, because of its good physical properties and rather low density, compared to steel. However, its utilization is still limited, for economical reasons mainly related to the high cost of production.1 Since the end of the 19th century, worldwide metallic aluminum production has involved the Hall−Heroult process. The metal is electro-deposed from aluminum oxide dissolved in a molten salts electrolyte. Aluminum is reduced at a carbon cathode, forming a pool of liquid metal at the bottom of the industrial cell while the carbon anodes are oxidized in carbon dioxide. The global reaction can be summarized by the following equation: 2Al 2O3 + 3C ⇌ 4Al + 3CO2
(1)
The industrial electrolyte, also known as the cryolitic melt, consists essentially of molten cryolite (Na3AlF6) to which several additives (such as aluminum fluoride (AlF3) and calcium fluoride (CaF 2 )) are added to improve the physicochemical properties of the melt. With the additives, the working temperature decreases from 1011 °C to 940−970 °C, and the viscosity of the molten electrolyte also decreases while the conductivity increases.2 Furthermore, aluminum oxide, whose concentration decreases during the electrolysis process, must be added recurrently to maintain the cell activity under its optimal conditions, or as close as possible.3 A typical industrial electrolyte contains 6−13 wt % AlF3, 4−6 wt % CaF2, and 2−4 wt % Al2O3. Finally, it is common, in the industrial world, to characterize the cryolitic melt composition by using © 2013 American Chemical Society
Received: May 17, 2013 Accepted: August 10, 2013 Published: August 10, 2013 8669
dx.doi.org/10.1021/ac401490j | Anal. Chem. 2013, 85, 8669−8675
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Article
Na3AlF6 ⇌ 3Na + + AlF6 3 −
(2)
AlF6 3 − ⇌ AlF4 − + 2F−
(3)
located at 180 cm−1, which is amid the range of Rayleigh decay. Access to very low frequencies (well below 100 cm−1) is then necessary in order to evaluate the baseline properly. Recent instrumental developments on CCD-based spectrometers and the advent of new edge filters, which allow one to record spectra closer to the exciting laser line, made the method more feasible and justified the reinvestigation of this proposal. The present paper demonstrates a laboratory-scale analytical procedure leading to the determination of the cryolitic ratio from the Raman spectrum of a melt, without referring to an internal standard of intensity. The procedure was developed on reference melts prepared in the laboratory and was tested on industrial samples that we have received from a factory (Hydroaluminium, Ǻ rdal, Norway). An extension to the industrial field is also proposed. The main purpose of this paper is then to describe the development of a procedure which allows a fast and as accurate as possible determination of the melt composition, based on in situ Raman measurement. To our knowledge, this is the first time such a method is proposed successfully in the literature. The measurement of the oxide content, which involves a more complex procedure, will be proposed in a subsequent publication.
Incoherencies of this model regarding thermodynamic data emerged later, and Dewing11 suggested a new model that explains the thermodynamic data more accurately. This model postulates that AlF63− dissociates, forming not only AlF4− but also an intermediate species, AlF52−, as shown in eqs 4 and 5. AlF6 3 − ⇌ AlF52 − + F−
(4)
AlF52 − ⇌ AlF4 − + F−
(5) 12
New Raman measurements were then performed, using much-improved instrumentation, and the results obtained on a large range of melt compositions could indeed not be understood without taking into account AlF52−, together with AlF4−. Actually, it turned out that AlF52− was the major species in molten NaF−AlF3-based systems. Employing this model, thermodynamic and reinvestigated Raman spectroscopic data were now in agreement. However, since the spectra are extremely difficult to obtain, considering the high temperatures and the corrosive character of the melts, the Raman data providing molecular proof of the existence of AlF52− in molten cryolitic melts have been criticized by several authors and the model has been subject to controversies for almost 20 years.13 Nonetheless, more Raman measurements14 made with a new furnace design from room temperature to 1050 °C, together with results obtained from NMR data15 confirmed the two-step dissociation model. Currently, this model seems generally accepted.16−19 Understanding the molten fluoride salts structure could, in principle, help to control the industrial process, but a direct measurement of the composition would be much more valuable. Unfortunately, since the electrolyte is highly corrosive, there has been, until now, no in situ analytical method able to perform a direct chemical control of the bath composition. Most of the process control relies only on the measurement of the cell voltage and temperature. However, this control is quite complex, since a variation in the voltage can be caused by numerous factors such as electrodes spacing, local bath temperature, the amount of undissolved material or sludge in the cell, and electrolyte composition.4,20 This is the reason why, in practice, the control is reinforced by time-to-time verification of bath composition by analyses made offline on frozen samples using mostly X-ray fluorescence (XRF).3 Raman spectroscopy is a highly valuable method for quantification, although publications are scarce in that field, mostly for experimental reasons. Considering our experience in the study of the highly corrosive cryolitic media, we proposed in the past to apply that technique to an offline determination of the melt composition,21 since the Raman spectra, if their quality is good enough, are clearly a function of the melt composition. A setup able to perform the measurements directly on the electrolysis cell was also proposed. Unfortunately, the latter could not be implemented at the time, since the recorded signal was too weak and too much time was needed for a good quantification. Indeed, experiments made in the factory showed that the crust covering the electrolysis cell, which must be broken to perform a measurement, starts to be renewed after 30 s. This means that a maximum of only 20 s are available for the laser to reach the liquid. In addition, the most likely usable Raman band for measuring the oxides content is
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MATERIALS AND METHODS Chemicals. Na3AlF6 (hand-picked cryolite from Greenland) was dried under vacuum at 600 °C overnight. NaF (Prolabo) and CaF2 (Sigma−Aldrich, optical grade) were dried overnight under vacuum at 500 and 600 °C, respectively. AlF3 (BDH Fluortran grade) was sublimed22 twice at 930 °C under vacuum, in a graphite container that was designed for this purpose. All chemicals were stored within a nitrogen-filled glovebox, where the water content never exceeded 3 ppm. Preparation of Melts. Inside the glovebox, batch powder mixtures of reference were prepared by weighing, mixing, and crushing the accurate masses of chemicals (Na3AlF6, AlF3, and CaF2) in an agate mortar to ensure mechanical homogeneity. For comparison purposes, other reference mixtures were prepared by again weighing, mixing, and crushing the required amounts of two batch mixtures together in an agate mortar. Twenty-five (25) compositions with a molar ratio NaF/AlF3 between 1.99 and 3.00 were prepared. The reference mixtures were melted in vitreous carbon crucibles inserted in an airtight quartz cell. All vitreous carbon crucibles were cleaned with great care before use. They were first immersed within an aqueous solution of sodium citrate (0.1 mol L−1) and sodium hydroxide (0.4 mol L−1) for several days in order to dissolve any trace of possibly remaining fluoroaluminates. They were then cleaned three times in an ultrasonic manner in water and finally subjected to an ethanol bath. Finally, they were dried under vacuum at 600 °C overnight, and then at 1000 °C for 2 h. Raman Spectroscopy and Furnace Apparatus. Two different cell-furnace assemblies have been used, both allowing collection of the scattered Raman light from melted samples in the same direction as the incident light. The spectrometer (a Jobin−Yvon Labram 300 confocal spectrometer) is provided with a microscope where the first cell-furnace assembly is set whereas the optical fiber carrying the Raman signal from the second assembly is connected to the spectrometer optical fiber input (see below). The first homemade cell-furnace assembly was previously described.14 It mainly consists of a regulated temperature furnace fixed in place of the moving table of the microscope. 8670
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RESULTS AND DISCUSSION Qualitative Description of the Spectra. In this study, we have recorded spectra from reference melts (25) characterized by cryolitic ratio values (CR) mainly in the range of 2−3, which are typical values met in the industrial field. Selected spectra of different melts recorded at the same temperature with the developed instrument are depicted in Figure 1. In this figure,
The melt is held in a 0.5 mL glassy carbon crucible inserted itself in a quartz cell. The latter is provided with a water-cooled copper lid with a quartz window in its center. The objective of the microscope is simply replaced by a 80-mm focal distance lens to focus, through the quartz window, the laser beam right into the melt. This setup allows measurements from 25 °C to 1100 °C and is specifically designed for the study of hightemperature molten fluoride salts with the purpose of avoiding condensation of fluoride vapors on the quartz window. In the present study, the melts were kept under inert argon atmosphere at 946 ± 5 °C for the recording. This is the assembly that has been used for the development of the analytical method. In order to simulate measurements made on top of an industrial electrolysis cell, we have also used a remote probe that we have developed in collaboration with Jobin−Yvon. This probe serves to illuminate the sample and collect the Raman light, and it has been described in more detail in ref 23. Now called Power-Head, the probe can be set far away from the spectrometer (then protecting the electronics from the high magnetic fields generated by the electrolysis cells), since it is linked to the exciting laser and the spectrometer through optical fibers. The cell and furnace are similar in principle to those previously described, except that they are much larger. The reason is that, to protect the probe lens from the strong heat that would be emitted by the melt on an industrial cell (measuring ∼4 m × 8 m), the lens focus must be longer here. Consequently, the analyzed depth in the sample is longer and to avoid emission by the bottom of the cell, the cell must be higher. The glassy carbon cell now contains 20 mL of melt and is fitted into a 15 cm height quartz tube. The quartz tube is closed by a water-cooled copper lid provided on its top with a quartz window. Possible clouding of the quartz window from condensation of the melt vapors is avoided by tangentially blowing argon on the window inside the lid. During the recording of the Raman spectrum, only the quartz tube is inserted into the furnace while the copper lid remained outside. The probe head is set vertically on the same axis as the cell and quartz tubing and the measurement is made from the top, then simulating an online measurement. All measurements are made with the cell under an argon atmosphere, at 1010 °C. For the Raman measurements, the spectrometer is provided with new edge optical filters cutting off the signal from below 40 cm−1 (Iridian Spectral Technologies). The 488-nm line of an argon-ion laser (Spectra Physics, Model 164) was used with a power of 100 mW at the sample. Every spectrum was accumulated twice for 20 s. If needed, the signal/noise ratio could be improved by averaging successive spectra recorded on the same melt, under the condition that the spectra do not change with time. For that reason, no more than eight spectra were averaged for one melt. In order to maintain the quality of the data, no smoothing was ever applied on the spectra. Despite the confocal nature of the spectrometer and the furnace design, some background originating from the blackbody emission of the hot vitreous carbon crucible still enters the spectrometer. To take this problem into account, the procedure is as follows: for each sample, the spectrum is first recorded at the requested temperature (946 °C); the laser beam is then turned off and the background of the blackbody is recorded with the same integration time as the preceding spectrum. This background is then subtracted digitally and quantitatively from the sample spectrum.
Figure 1. Averaged Raman spectra for different compositions of cryolitic melts: cryolitic ratio (CR) = 0.99, [CaF2] = 0.00 wt %, and T = 765 °C (spectrum a), CR = 1.99, [CaF2] = 5.24 wt %, and T = 936 °C (spectrum b), CR = 2.50, [CaF2] = 5.24 wt %, and T = 942 °C (spectrum c), and CR = 3.00, CaF2 = 5.24 wt %, and T = 949 °C (spectrum d). In all spectra, the blackbody emission has been subtracted, except in spectrum a, where the blackbody was not measurable, considering the lower temperature (765 °C).
the spectrum of a mixture with CR = 0.99 has also been included, since it is one of the most difficult to record because of the high vapor pressure above such melt at elevated temperature. The very good quality of this spectrum testifies to the efficiency of the entire instrumental setup. For instance, the blackbody background intensity here is, on average, 10%−20% of the total Raman signal. The spectra of cryolitic melts as a function of composition have already been discussed.12 The most suitable model for the structure of such NaF−AlF3−CaF2 mixtures indicates that the three fluorocomplexes are in equilibrium with each other. To understand the present paper, it is only necessary to focus on the totally symmetrical strong and polarized bands that constitute the spectrum: these peaks are located at 515, 560, and 622 cm−1 and were assigned to AlF63− (octahedral geometry), AlF52− (trigonal bipyramidal geometry), and AlF4− (tetrahedral geometry), respectively. If, in Figure 1, the presence of the AlF63− band (515 cm−1) can barely be seen, its existence was demonstrated by the increase in the CR, the increase of the cation size, and a reduction in the temperature.14 In any case, the relative intensities of the bands presented in Figure 1 change with the melt composition, as a result of the species equilibria, and this change is the basis of our procedure. Indeed, in order to determine the CR value from the recorded spectrum of the melt, we thought that this behavior could be quantified. As far as CaF2 is concerned, it has been added in all our mixtures. It is indeed present in all industrial baths, mainly originating as a contaminant of alumina and its concentration reaches a stable value (∼5 wt %) after some cell electrolysis 8671
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In order to evaluate the homogeneity of the reference mixtures, spectra of different samples from a same powdered mixture batch were recorded and an example of such spectra, recorded at different dates, is presented in Figure 3. Since the
time. The amount of added calcium is actually counterbalanced by the calcium extraction in the produced metallic aluminum. CaF2 is also known to act as a weak fluorobase24 in AlF3-rich melts (low CR). This is the reason why, in all our mixtures, the amount of CaF2 was kept constant at 5.17 ± 0.18 wt %, which is assumed to be representative of the content of a classical industrial bath. Sample Stability and Homogeneity. Before considering the CR determination method itself, it was necessary to investigate the stability of the Raman measurements (because of the unavoidable evaporation of the melts) and the homogeneity of the reference melts (because of the preparation method). Many sets of experiments were conducted for this purpose. As an example, 15 Raman spectra, recorded every minute after melting a powder mixture of CR ≈ 2, are overlapped in Figure 2. These spectra were corrected for the blackbody
Figure 3. Superimposition of three Raman spectra of different samples taken from the same powdered reference mixture of which has been melted (CR = 2.09 and [CaF2] = 4.89 wt %) and recorded at different dates (recording temperature were 942, 945, and 948 °C). The baseline, which includes the Rayleigh decay and blackbody emission, has been evaluated and subtracted from the averaged spectrum.
Rayleigh decay may change from melt to melt for experimental reasons, it was digitally subtracted from the spectrum. To evaluate the Rayleigh decay, a Gaussian-exponential hybrid mathematical function was fitted to each spectrum from a set of seven experimental points chosen every 10 cm−1, from 60 cm−1 to 120 cm−1 (thus, below any Raman bands). The relative intensities of the three spectra superimposed in Figure 3 are similar: the standard error affecting the intensity is
