Cı ´nthia

T Muranaka
email: cinthia.muranaka@poli.usp.br

Carine Julcour
email: carine.julcour@ensiacet.fr

Anne-Marie Wilhelm

Henri Delmas

Claudio A O Nascimento

Regeneration of Activated Carbon by (Photo)-Fenton Oxidation

This work aims to study the adsorption of phenol on activated carbons (ACs) and the consecutive in situ regeneration of carbon by Fenton oxidation. Two different operations have been carried out: (1) a batch procedure in order to investigate the influence of Fe 2+ and H 2 O 2 concentrations; (2) continuous fixed bed adsorption, followed by a batch circulation of the Fenton's reagent through the saturated AC bed, to examine the efficiency of the real process. Two different activated carbons have been also studied: a both micro-and mesoporous AC (L27) and an only microporous one (S23). In the batch reactor the best conditions found for pollutant mineralization in the homogeneous Fenton system are not the best for AC regeneration: a continuous reduction of adsorption capacity of L27 is observed after 3 oxidations, due to the decrease of both AC weight and surface area. Higher concentration of Fe 2+ and lower concentration of H 2 O 2 (2 times the stoichiometry) lead to a 50% recovery of the initial adsorption capacity during at least four consecutive cycles for L27, while about 20% or less for S23. In the consecutive continuous adsorption/batch Fenton oxidation process, the regeneration efficiency reaches 30-40% for L27 after two cycles whatever the feed concentration and less than 10% for S23. A photo-Fenton test performed on L27 shows almost complete mineralization (contrary to "dark" Fenton) and further improves recovery of AC adsorption capacity although not complete (56% after two cycles).

Introduction

The study of advanced technologies for wastewater treatment has been intensified owing to the raising of public concern about health and related environmental effects.

Phenol and phenolic compounds are among the most prevalent chemical pollutants found in industrial wastewaters, due to their extensive use in petrochemical units, oil refineries, and the polymer and pharmaceutical industries.

Inhibitory effects of phenol on microbial activity have been reported for concentrations larger than 500 ppm. [START_REF] Uygur | Phenol Inhibition of Biological Nutrient Removal in a Four-step Sequencing Batch Reactor[END_REF][START_REF] Amat | Ozonisation Coupled with Biological Degradation for Treatment of Phenolic Pollutants: a Mechanistically Based Study[END_REF] Therefore high loaded effluents requires additional solutions to conventional biological treatment. Among possible techniques, adsorption on activated carbon (AC) is extensively applied, as it is most effective for removing relatively low-molecular-weight organic compounds such as surfactants, pesticides, dyes, and aromatic molecules. [START_REF] Juang | Characterization and Use of Activated Carbons Prepared from Bagasses for Liquid-phase Adsorption[END_REF] However, after adsorption of toxic compounds, activated carbon becomes itself a hazardous waste that must be treated or disposed properly. Incineration of spent activated carbons is often used, but it has some major drawbacks: its operating costs, the destruction of the adsorbent, as well as the emission of possibly toxic compounds, such as chlorodioxines and chlorodibenzofuranes when the activated carbon is saturated with organochloro compounds. [START_REF] Okawa | Regeneration of Granular Activated Carbon with Adsorbed Trichloroethylene Using Wet Peroxide Oxidation[END_REF] The economical viability of processes using adsorption is thus directly related to their efficiency in regenerating and recycling the activated carbon. The most widely used technology is thermal regeneration. However, it presents the disadvantage of high energy consumption in keeping the temperature above 800 °C and carbon loss by burnoff. Another method used to regenerate activated carbons is based on solvent extraction. Studies have shown that activated carbons generally do not recover their full adsorption capacity by this method due to residual solvent. [START_REF] Martin | The Repeated Exhaustion and Chemical Regeneration of Activated Carbon[END_REF] Moreover, it only again displaces pollution.

Advanced oxidation processes (AOPs) have been considered a promising alternative technology for the remediation of toxic organic compounds. AOPs are characterized by the production of free radicals, mainly the hydroxyl radical (•OH) that offers a high oxidation potential (2.8 V) and is able to oxidize a wide range of organic compounds. AOPs can be classified according to the different systems used to generate free radicals, such as the following: O [START_REF] Zhang | TiO 2 -assisted Photodegradation of Dye Pollutants II. Adsorption and Degradation Kinetics of Eosin in TiO 2 Dispersions under Visible Light Irradiation[END_REF][START_REF] Alnaizy | Advanced Oxidation of Phenolic Compounds[END_REF][8] Those last two processes have been largely studied, known as the so-called Fenton and photo-Fenton systems, respectively.

The Fenton process consists in the generation of hydroxyl radicals by the reaction between ferrous ions and hydrogen peroxide (Fenton's reagent), according to eq 1: [START_REF] Walling | Fenton's Reagent Revisited[END_REF] The degradation efficiency of this oxidation process is accelerated by UV light irradiation. UV radiation results not only in the formation of additional hydroxyl radicals but also in an improved regeneration of the ferrous catalyst by reduction of Fe 3+ and destruction of ferric complexes. In this way, the overall reaction is accelerated and mineralization yield increased. [START_REF] Momani | A Comparative Study of the Advanced Oxidation of 2,4-dichlorophenol[END_REF][START_REF] Kavitha | The Role of Ferrous Ion in Fenton and photo-Fenton Processes for the Degradation of Phenol[END_REF] Recently Fenton oxidation has been successfully applied for the recovery of carbons exhausted with chlorinated organic compounds [START_REF] Toledo | Application of Fenton's Reagent to Regenerate Activated Carbon Saturated with Organochloro Compounds[END_REF] and methyl tert-butyl ether. [START_REF] Huling | Fentondriven chemical regeneration of MTBE-spent GAC[END_REF] H 2 O 2 /UV radiation has also been reported for the regeneration of activated carbon saturated with acetone and isopropyl alcohol. [START_REF] Horng | Regeneration of Granular Activated Carbon Saturated with Acetone and Isopropyl Alcohol via a Recirculation Process under H 2 O 2 /UV Oxidation[END_REF] The present work aims to destroy adsorbed phenol in order to in situ regenerate activated carbon by Fenton and photo-

Fe 2+ + H 2 O 2 f Fe 3+ + OH - + •OH (1)
Fenton oxidations in very mild conditions. Compared to usual continuous oxidation treatment, very high conversion is no longer required in this sequential adsorption/oxidation process as water quality is provided by the adsorption step. Moreover, the preconcentration of target pollutants on the AC may minimize radical scavenging and there is no need to remove iron from the purified water (consumption of Fe 2+ is minimized). The two-step process has been conducted using either a batch system (preliminary experiments to study the influence of Fe 2+ and H 2 O 2 concentrations) or a continuous fixed bed adsorption, followed by a batch circulation of the Fenton's reagent through the spent AC bed.

Materials

Ferrous sulfate heptahydrate (FeSO 4 • 7H 2 O) and hydrogen peroxide (H 2 O 2 ; 30%) were used as Fenton's reagents. Phenol was chosen as a model pollutant and the pH of the synthetic wastewater was adjusted by concentrated sulfuric acid (H 2 SO 4 ). During Fenton experiments, samples of the reactant mixture were withdrawn at different reaction times, and a quenching solution containing potassium iodide (KI, 0.1 mol/L), sodium sulfite (Na 2 SO 3 , 0.1 mol/L), and sodium hydroxide (NaOH, 0.1 mol/L) was added to stop the reaction. KI and Na 2 SO 3 decompose H 2 O 2 , and NaOH precipitates Fe 2+ ions. All reagents were analytical grade.

Two types of activated carbons were used, in the form of 1-1.6 mm diameter particles: a both micro-and mesoporous AC (PICA L27 from wood) and an essentially microporous AC (PICA S23 from coconut shell). Their physicochemical properties are given in Table 1.

Apparatus

The experimental setup used for Fenton experiments consisted in a 650 mL stirred Pyrex reactor, equipped with a jacket to control the temperature of the solution.

For preliminary batch experiments, AC was kept inside the reactor in a small basket (Figure 1a), whereas continuous adsorption was carried out in a stainless steel column (18.5 cm high and 1.5 cm internal diameter) packed with AC. A peristaltic pump achieved the continuous feed of the phenol solution through the AC column during the adsorption step. The same pump was used to feed the column with the Fenton solution from the stirred reactor and back to it during the regeneration step (cf. Figure 1b).

For photo-Fenton experiments, an annular photochemical reactor was employed (cf. Figure 1c). A 80 W low pressure mercury UV lamp (Philips HPLN) was used as light source. It was placed in jacketed quartz well, immersed in a borosilicate vessel. The photochemical reactor was connected to the activated carbon column and to a stirred Pyrex tank. The total liquid volume of 2 L was circulated through the reactor and the column by a peristaltic pump. During the first step, 500 mL of 1 g/L phenol solution was contacted at 30 °C with the activated carbon under stirring until adsorption equilibrium was reached (after 13 h). Then, the pH of the solution was adjusted to 3.0 and the Fenton reaction was initiated by simultaneous addition of FeSO 4 • 7H 2 O and hydrogen peroxide. After the reaction step, the solution was withdrawn and another batch adsorption was carried out with a new phenol solution using the same activated carbon. This procedure was repeated three consecutive times with the same operating conditions. Liquid samples were taken just after adsorption and regularly during oxidation (up to 5 h). Different concentrations of Fe 2+ (10-20 mmol/L) and H 2 O 2 (300-1000 mmol/L, corresponding to 2 to 6.5 times the stoichiometric amount for complete mineralization [The stoichiometric amount of H 2 O 2 required for the total oxidation of phenol was calculated based a Calculated from the Horvath-Kawazoe model. [START_REF] Horvath | Method for Calculation of Effective Pore Size Distribution in Molecular Sieve Carbon[END_REF] b Calculated from the Barret-Joyner-Halenda method. [START_REF] Barrett | Determination of Pore Volume and Area Distributions in Porous Substances, I. Computations from Nitrogen Isotherms[END_REF] c C a and C b are the total concentrations of acidic and basic groups per gram of activated carbon, determined from Boehm titration. [START_REF] Boehm | Some aspects of the surface chemistry of carbon blacks and other carbons[END_REF]

Experimental Procedures

Continuous Adsorption System.

In this operation, a 0.1 or 1 g/L phenol solution was continuously fed to the column with a flow rate of 2 L/h until saturation of the AC bed. Then, the stirred reactor was loaded with 500 mL of the same phenol solution which was recycled through the bed at 2-6 L/h. The reactor temperature was set to 30 °C, and the pH was adjusted to 3. The Fenton reaction was initiated by simultaneous addition of FeSO 4 • 7H 2 O and hydrogen peroxide. The oxidative regeneration step was carried out for 5-24 h. Then, the AC bed was drained to operate a new adsorption-oxidation cycle. Samples were regularly withdrawn at the column outlet during the two steps.

Analytical Methods.

During the oxidation step, samples of the reacting solution were withdrawn and treated with the quenching solution (NaOH 0.1 mol/L, KI 0.1 mol/L and Na 2 SO 3 0.1 mol/L) in order to stop the oxidation process. After precipitation of iron and filtering, samples were analyzed for remaining total organic carbon (Shimadzu TOC 5050A analyzer).

The samples were also analyzed by HPLC. The separation was achieved by a C18 reverse phase column (ProntoSIL C18 AQ) using a mobile phase of variable composition (acidified deionized water/acetonitrile). The dual wavelength UV detector (UV2000 Thermo Finnigan) was set at 210 and 254 nm. In this case, the quenching solution was made of NaOH 0.1 mol/L only (so that to precipitate Fe 2+ and remove it from the samples), because KI and Na 2 SO 3 peaks detected by HPLC may interfere with the peaks of phenol and oxidation intermediates. As shown by the results of experiment without Fe 2+ (Figure 2), removal of Fe 2+ from the solution is enough to avoid any further decrease of TOC within a few hours. Moreover, the samples were injected in the chromatograph immediately after the addition of NaOH.

After the last adsorption-oxidation cycle, the spent activated carbons were characterized for BET surface area by measuring nitrogen adsorption isotherms at 77 K with an ASAP 2010 analyzer (Micromeritics). Thermogravimetry analysis (TGA) of the ACs was also performed under nitrogen flow from room temperature to 700 °C at a heating rate of 10 °C/min (Q600 SDT, TA Instrument). To further identify the compounds adsorbed on the activated carbon after the adsorption-oxidation cycles, pyrolysis-GC/MS of used carbons was carried out on a PY-2020iD coupled with a GCMS-QP2010 Plus (Frontier Lab) (590 °C pyrolysis temperature, 320 °C GC injector temperature, and 230 °C MS ion source temperature). Finally, scanning electron microscopy energy dispersive X-ray (SEM/EDX) analysis of the particles (on the surface and in the core) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of acidic leachates after contacting a few days with the ground particles were used to determine the iron content in the activated carbon after Fenton oxidation.

Results and Discussion

Fenton Reaction without AC.

The homogeneous Fenton reaction was first investigated for the treatment of a 1 g/L phenol solution (500 mL), varying the concentration of Fe 2+ (from 0 to 25 mmol/L) and the amount of H 2 O 2 (from 0.15 to 0.50 mol). As shown in Figure 2, the highest mineralization was obtained for 10 mmol/L of Fe 2+ and 0.50 mol of H 2 O 2 , leading to a steady TOC conversion of 71% after about 1 h of reaction. In all experiments with Fe 2+ , phenol was depleted within 10 to 30 min.

A positive effect could be expected on TOC removal by raising the concentrations of Fenton's reagents. However, it has been observed that an excess of Fe 2+ or H 2 O 2 can be detrimental for the process performance, because those species can subsequently react with the most active radical intermediates like •OH, resulting in a competition with the organic compounds to be degraded, [START_REF] Maciel | Phenol Removal from High Salinity Effluents using Fenton's Reagent and Photo-Fenton Reactions[END_REF][START_REF] Torrades | Experimental Design of Fenton and photo-Fenton Reactions for the Treatment of Cellulose Bleaching Effluents[END_REF] as indicated in eqs 2 and 3. Such an effect is noticeable here for Fe 2+ , leading to a better TOC removal with 10 mmol/L than with higher concentrations.

Batch Adsorption System.

The best Fenton conditions without AC were applied for the regeneration of L27 after it had reached adsorption equilibrium with the phenol solution (exp 1). As shown in Figure 3, the TOC reduction obtained in liquid phase by this sequential adsorption-oxidation process was higher than that observed without AC, at least during the first cycle. However in this case, the TOC vs time plot exhibited a minimum, due to desorption of both phenol and oxidation intermediates from AC.

For simplicity the regeneration efficiency was defined as the ratio of phenol amount readsorbed on AC after Fenton treatment to the initial amount adsorbed, even if a more proper definition should consider values at same equilibrium aqueous phase concentration. The calculations were also based on the initial weight of AC. Corresponding results are given in Table 2.

Using the Fenton conditions which led to the highest mineralization of the phenol solution, a continuous decrease of adsorption capacity of L27 from 100% to 23% was observed after three oxidations (exp 1). It could be explained not only by some remaining physisorbed pollutants, but also by a decrease of both surface area (from 1860 to 590 m 2 /g) and AC weight (from 1.7 to 1.45 g after three cycles).

The modification of the chemical surface properties of the ACs by the Fenton's reagent could also be a reason for the lower adsorption observed after regeneration. A blank test (without pollutant) showed that this phenomenon contributed for 25% of the total capacity loss of L27 carbon in the conditions of exp 1.

A lower concentration of H 2 O 2 was used to reduce AC weight loss. Iron was added in two equal parts (at t ) 0 and 60 min) leading to a final concentration of 20 mmol/L (exp 2). Actually, a slight improvement was found when adding it in once at t ) 0 (exp 3). The results of exp 3 showed that the efficiency of phenol adsorption remained then practically constant at 50% for at least four adsorption-oxidation cycles (Table 2). The TOC evolution in the liquid phase exhibited also the same behavior from the second to the fourth adsorption-oxidation cycles (with an overall reduction of about 60% after 10 min), as shown in Figure 4.

No AC weight loss was observed and the surface area only decreased to 1020 m 2 /g in this case. A blank test in the same Fenton conditions resulted in a loss of adsorption capacity of only 2.5%.

These results confirm that the best conditions of Fenton's reagent without AC do not correspond to those leading to the highest regeneration yield of the activated carbon. A probable explanation is the attack of activated carbon surface by an excess of H 2 O 2 resulting in a loss of AC weight.

When later Fenton oxidation was applied to S23, the recovery of its adsorption capacity was found significantly lower and always decreasing. It was probably due to S23 microporous character, access to the micropores being partially blocked by condensation products resulting in a surface area divided by a factor of 6 (from 1230 to 210 m 2 /g).

Deposit of such low-volatile compounds was also indicated by the comparison between TGA spectra of fresh and spent ACs: the weight loss between 200 and 700 °C increased from 16% to 27% for L27 (exp 2) and from 4% to 12% for S23 (exp 4). Corresponding thermograms are shown in Figure 5. This point was later investigated by pyrolysis-GC/MS of the carbons (cf. section 5.3).

Continuous Adsorption System.

To mimic a real process, a continuous adsorption was achieved prior to the in situ regeneration. However the adsorption step was carried out up to complete AC saturation, so that the adsorption capacity of (fresh/regenerated) ACs could be calculated from numerical integration of the breakthrough curves (cf. Figure 8a). According to the previous results, the amount of H 2 O 2 added for the regeneration corresponded to about 2 times the stoichiometry for complete mineralization of phenolsadsorbed and in solution (exps 5-7). Results are shown in Table 3.

In accordance with previous batch observations, the Fenton treatment was much more efficient for L27 than for S23. An interesting result is that the recovery yield appeared to be almost independent of the feed concentration used in adsorption (0.1 or 1 g/L), despite the finding that the decrease of S BET was higher when increasing the adsorbed pollutant amount.

Pyrolysis-GC/MS analysis at 590 °C identified the main compounds adsorbed on L27 AC during continuous adsorptionbatch Fenton oxidation cycles. Phenol mostly but also oxidation intermediates (hydroquinone, resorcinol, and acetic acid) and phenolic dimer were detected on L27 after the last breakthrough (Figure 6). EGA-MS analysis mode indicated that acetic acid is released from 150 to 350 °C and should mainly result from acidic surface functions of the carbon. Phenol is also removed at relatively high temperatures (between 150 and 300 °C). Phenol seems indeed to interact with AC through a chemisorption mechanism based on the bonding between phenol and oxygen-containing surface groups, which can progress to form phenol-derived polymers by an oxidative coupling mechanism. As reported previously, this mechanism could be one of the responsible for the pore blockage. [START_REF] Santiago | Modified Activated Carbons for Catalytic Wet Air Oxidation of Phenol[END_REF] The ICP-AES and SEM/EDX analyses showed very low amount of iron in the Fenton regenerated carbons (about 0.5 wt % in conditions of exp 5), due to some deposit on the surface, but not detected in the core of the particle.

Photo-Fenton Regeneration.

During the Fenton oxidation, mineralization yield reached its maximum after 10 min to 1 h and never decayed afterward (cf. Figures 234). Such reaction inhibition could be explained by the formation of Fe 3+ / carboxylic acids complexes with the degradation products (eq

H 2 O 2 + •OH f •O 2 H + H 2 O (3) (2)
). These complexes are stable in usual Fenton conditions, hindering the Fe 2+ regeneration: [START_REF] Safarzadeh-Amiri | Ferrioxalatemediated Photodegradation of Organic Pollutants in Contaminated Water[END_REF][START_REF] Balanosky | Oxidative Degradation of Textile Wastewater. Modeling Reactor Performance[END_REF] UV radiation is expected to play an important role in the destruction of Fe 3+ /carboxylic acids complexes according to eq 5.

The radical R• can react with oxygen and degrade further.

UV light can also enhance mineralization by the photoreduction of Fe 3+ to Fe 2+ (eq 6), accelerating Fe 2+ regeneration and generating additional hydroxyl radicals.

The photogenerated Fe 2+ from eqs 5 and 6 can participate in Fenton's reaction (eq 1), generating additional hydroxyl radicals.

Another UV contribution to enhanced mineralization, but to a less extent, is the photolysis of hydrogen peroxide, as shown in eq 7.

The effect of UV radiation was investigated for the conditions of exp 8. During the oxidation step, 95% mineralization was achieved in solution after 4 h with photo-Fenton, while only 25% with the usual ("dark") Fenton, as shown in Figure 7.

Figure 8a and b compare the corresponding adsorption curves after Fenton and photo-Fenton treatments. Although the adsorption capacity of L27 decreased upon recycling in both cases, it could be maintained to 56% of its original capacity after two cycles with photo-Fenton oxidation (against 40% with dark Fenton). The change in BET surface area appeared to be the same after both treatments, as well as the nature of adsorbed compounds (from pyrolysis-GC/MS analysis).

Conclusions

The experimental results presented in this work proved Fenton and photo-Fenton processes to be applicable to regenerate spent activated carbons saturated with phenol.

The regeneration efficiency was found to be a complex interplay between the mineralization yield and the modification of both textural and chemical surface properties of the activated carbons.

Concerning Fenton treatment, a lower recovery of phenol adsorption capacity was obtained for the microporous AC (S23) than for the both meso-and microporous AC (L27), whatever the system considered (spent AC immerged in the Fenton solution or circulation of the reagent through the saturated AC bed). It may be explained by a significant decrease of S23 specific area after the oxidation steps, even if in case of L27 similar performances could be obtained for different aging of the AC.

It was also found that UV irradiation improved the mineralization yield and the subsequent readsorption (by more than 15% compared to dark Fenton treatment).

4. 1 .

 1 Batch Adsorption System. The procedure consisted of two steps: (1) batch adsorption of phenol on activated carbon (contained in a basket); (2) Fenton oxidation. The weight of AC (S23 or L27) was chosen in order to adsorb half of the pollutant amount initially in the solution.

  d pH PZC is the pH at the point of zero charge (PZC).

Figure 1 .

 1 Figure 1. Fenton ((a) batch reactor, (b) loop reactor) and photo-Fenton (c) setups used for the regeneration of activated carbon.

Figure 2 .

 2 Figure 2. TOC evolution during Fenton oxidation without AC: H 2 O 2 ) 0.50 mol, no iron (-); H 2 O 2 ) 0.50 mol, [Fe 2+ ] ) 10 mmol/L (0); H 2 O 2 ) 0.15 mol, [Fe 2+ ] ) 1 mmol/L ([); H 2 O 2 ) 0.15 mol, [Fe 2+ ] ) 10 mmol/L (9); H 2 O 2 ) 0.15 mol, [Fe 2+ ] ) 20 mmol/L (b); H 2 O 2 ) 0.15 mol, [Fe 2+ ] ) 25 mmol/L (2).

Figure 3 .

 3 Figure 3. TOC evolution during batch adsorption-Fenton oxidation cycles with L27: exp 1 (H 2 O 2 ) 0.50 mol, [Fe 2+ ] ) 10 mmol/L); (line) Fenton reaction without AC, (dots) successive regeneration cycles [first (+), second (/), and third ( ×) cycles]. Empty circles and squares correspond to the beginning and end of adsorption, respectively.

Figure 4 .

 4 Figure 4. TOC evolution during batch adsorption-Fenton oxidation cycles with L27: exp 3 (H 2 O 2 ) 0.15 mol, [Fe 2+ ] ) 20 mmol/L); (line) Fenton reaction without AC, (dots) successive regeneration cycles [first (+), second (/), third (×), and fourth (4) cycles]. Empty circles and squares correspond to the beginning and end of adsorption, respectively.

Figure 5 .

 5 Figure 5. Thermograms of fresh/spent L27 (a) and fresh/spent S23 (b) (conditions of exps 2 and 4).

Figure 6 .

 6 Figure 6. Pyrogram of spent L27 (conditions of exp 8).

Figure 7 .

 7 Figure 7. TOC evolution in solution during regeneration of L27 using Fenton (a) and photo-Fenton (b) oxidation (conditions of exp 8): first (•) and second (×) oxidation.

  3 /UV radiation, O 3 /H 2 O 2 , O 3 /H 2 O 2 /UV radiation, H 2 O 2 /UV radiation, TiO 2 /UV radiation, Fe 2+ /H 2 O 2 , Fe 2+ /H 2 O 2 / UV radiation.

Table 1 . Physical and Surface Chemical Properties of the Activated Carbons properties

 1

	c C b (mmol/g)	pH PZC d

specific surface area (m 2 /g) microporous volume a (cm 3 /g) mesoporous volume b (cm 3 /g) C a c (mmol/g)

Table 2 . Regeneration Efficiency of L27 and S23 after Fenton Treatment (Batch Adsorption)

 2

	regeneration efficiency (%)

a Added in two equal parts at t ) 0 and 60 min.

Table 3 . Regeneration Efficiency of L27 and S23 after Fenton Treatment (Continuous Adsorption) a

 3 Added in four equal parts and equivalent to 1.1 times the stoichiometry for complete mineralization (Q recirculation ) 2 L/h).

	regeneration efficiency (%)

a t oxidation ) 5 h with Q recirculation ) 6 L/h if not mentioned otherwise. b 1st cycle (t oxidation ) 24 h, Q recirculation ) 2 L/h). c 2nd cycle (Q recirculation ) 2 L/h).

d Added in four equal parts. e Liquid volume ) 2 L. f

Acknowledgment

The authors express their gratitude to the ALFA-Programme, the sixth FP "REMOVALS", INP-ENSIACET, and ANR (Precodd "PHARE") for financial support. They also thank PICA (Veolia group) for supplying S23 and L27 ACs, M. Auriol and C. Rouch (SAP, LGC, Toulouse) for AC characterization, and J. L. Labat for the design and implementation of the Fenton experimental setup.

Note Added after ASAP Publication: Changes have been made to

Table 3 in the version of this paper that was published on the Web November 4, 2009. The corrected version was reposted December 15, 2009. Literature Cited


domain: math scco

text:
Maureen Clerc

Juliette Leblond

Jean-Paul Marmorat

Theo Papadopoulo

Inverse source estimation problem in EEG

Being given pointwise measurements of the electric potential taken by electrodes on part of the scalp, the EEG (electroencephalography) inverse problem consists in estimating current sources within the brain that account for this activity. A model for the behaviour of the potential rests on Maxwell equations in the quasi-static case, under the form of a Poisson-Laplace partial differential equation (PDE). We solve the inverse problem (IP) in spherical geometry, for piecewise constant electric conductivity values, and pointwise dipolar source terms.

The algorithm relies on consecutive steps and is encoded in the software FindSources3D: (i) Singular value decomposition (SVD), in order to separate the time independant activities. (ii) Data transmission from scalp to cortex. (iii) Best quadratic rational approximation on families of 2D slices, compute singularities in circular sections. (iv) Clustering of these singularities in order to localize the sources.

[CLMP] Clerc, Leblond, Marmorat, Papadopoulo. Source localization in EEG using rational approximation on plane sections, Inverse Problems 28, 2012.

Model, assumptions, IP

• Potentials u, subject to pointwise dipolar J supported within the brain

div (σ grad u) = div J = K k=1 p k • grad δ C k • Spherical head geometry, 3 homogeneous isotropic layers Ω 2 , Ω 1 , Ω 0 (scalp, skull, brain) constant conductivity values σ 2 = σ 0 > σ 1 > 0 & ∂ n u = 0 on S 2
• EEG data set: Best constrained quadratic approximation on spheres S i i = 1, 2 by harmonic functions in Ω i , with norm constraint on S i-1

• expansions on spherical harmonics data at electrodes e j

• transmission S 2 ; S 1 then S 1 ; S 0 Boundary Element Methods, Bounded Extremal Problems BEM, BEP min

u ap u -u ap L 2 (S i ) + λ grad u ap L 2 (S i-1 )
u ap on spherical harmonic basis r l Y m l (θ, ϕ), Lagrange parameter λ > 0 3. step (iii) 2d slices of S 0 ; poles singularities

Best quadratic rational approximation on circles

• σ 0 ∆u = div J, convolution by fundamental solution ; u = u s + harmonic, u s (X )

K k=1 p k ,X -C k |X -C k | 3
• from u, ∂ n u on planar sections of S 0

; K poles 2d singularities

• on circle Γ p u, ∂ n u → f = u 2 s polynomials π, q complex variable min π,q f - π q L 2 (Γ p )
iterate on degree q K

• K = 2 poles in P disks D p p = 1, . . . , P, plane Π

; roots of q in D p = poles • Other inverse problems in brain & medical imaging (e.g. estimation of skull conductivity, j.w.w. BESA GmbH)

•
• Other application fields planetary sciences, paleomagnetism, . . . http://team.inria.fr/apics/ juliette.leblond@inria.fr

u

  depends on time t through |p k (t)| pointwise values of potential u = u(t i , e j ) at electrodes locations e j on S 2 • IP: estimate quantity K and locations C k in Ω 0 of sources & moments p k 2. Step (i) SVD ; static ativities Singular value decomposition of EEG matrix time window, EEG = U Σ V , rk R • main asynchronous activities columns u r of U, r = 1, . . . , R • best linear combination, in view of source estimation change basis in span 3. Steps (ii), (iii), (iv) S 2 ; S 0 ; sources in Ω 0 For each activity u = u r = u r (e j ) (P circles in 13 ; 3 directions) (ii) S 2 ; S 0 cortical mapping (iii) S 0 ; Ω 0 K = 2 poles in each 2d slice (iv) Ω 0 ; sources C k 3d clustering 3. step (ii) cortical mapping S 2 ; S 0 Compute harmonic extension of the data on S 2 in Ω 2 , then in Ω 1 ∆ u = 0 in Ω 1 & Ω 2 + transmission conditions on S i : u, σ ∂ n u continuous integral representation, single & double layer potentials

;

  up to K = 3 or 4 pointwise sources + various activities & dipole fit + moments recovery (linear system) wrist, hospital la Timone, Marseille http://www-sop.inria.fr/apics/FindSources3D/ (matlab) 5. Ongoing & further work • Other datasets, MEG, EIT magnetoencephalography, Electrical Impedance Tomography • Realistic geometries, more layers (Calderón)


Laurent Cassayre
email: cassayre@chimie.ups-tlse.fr.

Patrice Palau

Pierre Chamelot

Laurent Massot

Properties of Low-Temperature Melting Electrolytes for the Aluminum Electrolysis Process: A Review

The traditional electrolyte for aluminum electrolysis is composed of molten cryolite (Na 3 AlF 6 ) and alumina (Al 2 O 3 ). One of the objectives of the industry is to lower the operating temperature of the electrolytic process, which is currently around 960 °C. The benefits commonly evoked of a temperature decrease in the aluminum plotlines are multiple: reduction of the energetic consumption and thus of the global environmental footprint, increase of the cell life due to limited corrosion, reduction of production costs, etc. In this work, an overview of some properties of molten fluoride systems (Na-cryolite-based electrolytes (NaF-AlF 3 ) and K-cryolitebased electrolytes (KF-AlF 3 )) is presented. To a lesser extent, the case of all-chloride and mixed chlorofluoride salts is also discussed. Many physicochemical properties of salt mixtures are of major importance to evaluate the potential of a given electrolyte. When available, the following properties are reported and compared: the liquidus temperature, the electrical conductivity, the density, the vapor pressure, the solubility of alumina, and the solubility of liquid aluminum.

Introduction

The Hall-He ´roult process, named after its inventors, is devoted to the production of primary aluminum from alumina in an electrolysis cell, at a temperature of around (950 to 960) °C. It has been running and optimized for more than a century. The electrolyte is mainly composed of cryolite (Na 3 AlF 6 ) and alumina (Al 2 O 3 ), together with some aluminum fluoride (AlF 3 ) and calcium fluoride (CaF 2 ) and in some cases other additives. One of the current trends in R&D, as can be observed from the large amount of recent scientific articles, is to try to lower the operating temperature of the electrolytic process. The benefits commonly evoked of a temperature decrease in the aluminum plotlines are multiple: reduction of the energetic consumption and thus of the global environmental footprint, increase of the cell life due to limited corrosion, reduction of production costs, etc. There has been quite a large amount of work done in recent years concerning the properties of new electrolytes for the alumina reduction process, both in view of using the regular carbon-based anodes and also in the frame of the development of oxygen-evolving anodes. In this latter case, the temperature decrease is also expected to greatly reduce the corrosion rate of the anodes.

The major objective of the present paper is to review the main physicochemical properties of some selected electrolytes supposedly suited for use in the electrolytic process. As hundreds of systems can be envisaged, the study was first refined to salt mixtures containing Al(III) as a main constituent, to avoid masstransfer limitation of the cathodic reaction. The temperature range of the melting point of the considered salt mixtures was chosen to be comprised between 950 °C (current electrolytes) and 660 °C (melting temperature of aluminum), so that aluminum is formed in the liquid phase. Finally, only the systems operating with an alumina feed were considered, which involve an oxygen-containing anodic product (either CO 2 or O 2 ). These criteria lead to an electrolytic process not so different from the Hall-He ´roult process, in the way that dissolved alumina is reduced into liquid aluminum.

An initial overview of the published papers led to a refinement of the present review to two major systems: Na-cryolite-based electrolytes (NaF-AlF 3 ) and K-cryolite-based electrolytes (KF-AlF 3 ), which are the topics of many recently published works. To a lesser extent, the case of all-chloride and mixed chloro-fluoride salts is also discussed. The use of aluminum sulfides, nitrides, or bromides as a feeding material is not discussed: no work has been published on this topic since the mid eighties, and it seems that this kind of process has been discarded. Many physicochemical properties of salt mixtures are of major importance to evaluate the potential of a given electrolyte. The following properties have been considered: the liquidus temperature, the electrical conductivity, the density, the vapor pressure, the solubility of alumina, and finally the solubility of liquid aluminum. The main impact of each of them is described in short in the following (much more details are available in reference books, e.g., Aluminum Electrolysis 1,2 ):

-Alumina solubility: the solubility of alumina in the electrolyte is a key parameter in the running of the process since a sufficient concentration of alumina must be maintained to control the anodic reaction. Indeed, alumina depletion in the vicinity of the anode leads to the so-called anode effect, which consists of the blockage of the anode surface and the production of significant amounts of powerful greenhouse gases, the perfluorocarbons.

-Vapor pressure: the determination of the total vapor pressure above the molten salts is a good indication of their stability, a high vapor pressure indicating that the electrolyte tends to evaporate. Knowledge of the compounds constitutive of the vapor phase and their partial vapor pressure also helps to predict the evolution of composition of the liquid salt phase.

-Electrical conductivity: the electrical conductivity of the electrolyte is of great importance in the energetic balance of the Hall-He ´roult process since the bath resistivity represents more than half of the ohmic drop of industrial cells (∼1.35 V out of ∼2.50 V). This ohmic drop allows the temperature of the cell by the Joule effect to be maintained, but most of the energy is lost and dissipates. The highest electrical conductivity is then required.

-Aluminum solubility: the liquid aluminum solubility in the molten salt influences the faradic efficiency of the process and needs to be minimized. Indeed, dissolved aluminum gets oxidized in the vicinity of the anode (where it reacts with CO 2(g) ) and is responsible for some current loss.

-Salt density: the Hall-He ´roult process operates well because aluminum is heavier than the molten salt, which allows confining the liquid metal at the bottom of the cell. A maximal density gap between aluminum and salt is thus required to ensure optimal phase separation.

While the Na-cryolite mixtures have been widely studied, it is not the case of some of the other systems presented here. A summary of the current knowledge available in the literature is established in this review: first, standard industrial cryolite mixtures are considered, and then the properties of modified cryolite mixtures, K-based cryolites and chloro-fluoride salts, are presented.

Na-Cryolite Systems

Standard Cryolite Melts. The usual industrial electrolyte composition range is, in mass fraction (w): w(AlF 3 ) ) 0.10 to 0.12, w(Al 2 O 3 ) ) 0.02 to 0.04, w(CaF 2 ) ) 0.03 to 0.05, and the balance Na 3 AlF 6 . To compare the properties of the various lower-temperature melting salt mixtures, a summary of the properties of the industrial electrolyte is presented in this first section. The influence of the common additives (CaF 2 , LiF, MgF 2 , and KF) is also discussed.

Liquidus Temperature and Alumina Solubility. The liquidus temperatures of the cryolite melts are highly dependent on their composition. As a general trend, stoichiometric cryolite has a melting point of about 1011 °C, and any modification of composition in the system Na 3 AlF 6 -AlF 3 -Al 2 O 3 -CaF 2 -MgF 2 -LiF-KF leads to a decrease of the liquidus temperature. Many detailed studies on the evolution of the liquidus temperature and alumina solubility in cryolite melts have been carried out. [START_REF] Thonstad | Aluminium Electrolysis[END_REF] Among them, the work of Solheim et al. 3,[START_REF] Solheim | Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolysis[END_REF] is noticeable since it proposes analytical formulas in the (850 to 1050) °C temperature range. A thermochemical modeling of most of these data has also been performed and included in the Hall-He ´roult database (FTHall 5 ) of the FactSage software. [START_REF] Bale | FactSage thermochemical software and databases[END_REF] This database has been used in this article to calculate some phase diagrams.

Industrial mixtures contain an excess of AlF 3 . The molar ratio NaF/AlF 3 , called cryolite ratio (CR), is the usual way to write the cryolite composition: a CR equal to 3 indicates stoichiometric cryolite, while a CR below 3 refers to mixtures containing an excess of AlF 3 . Standard industrial mixtures usually have a CR of about 2.2. As illustrated in Figure 1, the increase of the AlF 3 concentration leads to a sharp decrease of the liquidus temperature. The liquidus temperature of industrial baths is around 955 °C.

The evolution of the liquidus temperature of the industrial bath composition versus the alumina content is plotted in Figure 2. It shows a major property of the cryolite mixtures: alumina is highly soluble, up to w ) 0.07 at 950 °C, which is a key parameter for operating the electrolysis cells. Furthermore, alumina additions decrease the liquidus temperature by about -5 °C per mass percent of Al 2 O 3 . CaF 2 , KF, and LiF reduce the liquidus temperature of cryolite melts. The order of influence (on the mass scale) is AlF 3 < CaF 2 < KF < LiF, LiF leading to the strongest decrease of liquidus temperature. As it will be discussed later, the presence of these compounds also has a drawback since they diminish the solubility of alumina.

Vapor Pressure. The total vapor pressure of the NaF-AlF 3 -Al 2 O 3 mixtures is in the range of (5 to 10) mbar. The gaseous phase is essentially composed of the NaAlF 4 compound. [START_REF] Thonstad | Aluminium Electrolysis[END_REF] For a mixture with a CR equal to 3, an increase of the alumina concentration is linked to a slight decrease of the total vapor pressure. This trend is verified in the CR range 1.8 to 3.0. [START_REF] Robert | Structure and thermodynamics of alkali fluoride-aluminum fluoridealumina melts. Vapor pressure, solubility and Raman spectroscopy studies[END_REF] As shown in Figure 3, additives like CaF 2 and KF have only a slight influence on the vapor pressure. On the other hand, at constant temperature, an excess of AlF 3 noticeably increases the vapor pressure due to an enhanced formation of NaAlF 4(g) . Even if, on first thought, it seems that the most acidic compounds (like MgF 2 and AlF 3 ) implicate an increase of the vapor pressure due to AlF 4

-stabilization (which enhances the formation of NaAlF 4(g) ) on detriment of AlF 5 2-, Gilbert 8 indicates that this way of thinking is not always true, due to the formation of mixed AlF x -MF x compounds.

Electrical ConductiWity. The electrical conductivity of industrial baths is about 2.1 S • cm -1 . This conductivity is essentially driven by Na + cations, which are the most mobile due to their small size compared to larger ionic entities composing the salt. [START_REF] Thonstad | Aluminium Electrolysis[END_REF] The evolution of the electrical conductivity slightly depends on the alumina concentration: 9,10 a decrease is observed with increasing alumina content (about (-0.04 to -0.06) S • cm -1 per mass percent of Al 2 O 3 ). This is likely due to the formation of large Al 2 OF 6 2-and Al 2 OF 8 4-anions which decrease the ions' mobility in the electrolyte.

In a similar way, the increase of AlF 3 content, which enhances the formation of AlF 4

-and AlF 6 2-entities, has a deleterious effect on electrical conductivity, as well as most of the additives (KF, CaF 2 , MgF 2 ). Conversely, LiF and NaCl, which dissociate into small Li + and Na + cations, improve the conductivity of cryolite mixtures. [START_REF] Kan | Electrical conductivity of Na 3 AlF 6 -AlF 3 -Al 2 O 3 -CaF 2 -LiF(NaCl) system electrolyte[END_REF] Aluminum Solubility. Aluminum solubility in industrial melts is about w(Al) ) 0.0005. Even though measurements are quite scattered, it is admitted that the aluminum solubility decreases with higher AlF 3 contents. This is why, as explained by Holmes [START_REF] Holmes | Hall cell ampere efficiency up 12% in three decades[END_REF] in a historical review of the Alcoa potlines evolution, the excess amount of AlF 3 has been increased throughout the optimization of the faradic efficiency of the process. It has also been shown that a temperature decrease reduces the aluminum solubility, which goes in favor of a decrease of the operating temperature of the cells.

Density. The usual density of the industrial baths is about 2.1 g • cm -3 at 960 °C, 1 while aluminum density at the same temperature [START_REF]Handbook of Chemistry and Physics[END_REF]

is 2.3 g • cm -3 .
Common additives do not have a great influence on the density of cryolite melts compared to the density of liquid aluminum: in any case, in the (950 to 1050) °C temperature range, the salt density is comprised between (1.9 and 2.1) g • cm -3 , while aluminum's is always above 2.3 g • cm -3 . A few examples are plotted in Figure 4.

Modified Na-cryolite Systems (T liquidus < 950 °C). To decrease the melting temperature of the cryolite-based mixture below 950 °C, two options (which can be coupled) are possible: either increase the AlF 3 excess or increase the additive concentration. An outlook of the knowledge concerning the physicochemical properties of these modified cryolite mixtures is provided in this section.

Accessible Temperature Range. The phase diagram of the NaF-AlF 3 system indicates a eutectic composition at an AlF 3 molar ratio of 0.46 (corresponding to a CR of 1.7) at a temperature of 698 °C. [START_REF] Thonstad | Aluminium Electrolysis[END_REF] It is then possible, by an increase of the AlF 3 content, to reach a liquidus temperature of about 700 °C.

The addition of additives like KF, LiF, CaF 2 , or MgF 2 also allows decreasing the liquidus temperature. The review of the various analytical formulas presented in ref 1 indicates that the best fit available to estimate the liquidus temperature of such mixtures is the one proposed by Solheim et al. [START_REF] Solheim | Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolysis[END_REF] where t is the liquidus temperature in °C, and all concentrations [] are related to 100 w. The formula is valid for

[AlF 3 ], [CaF 2 ], [LiF] < 20 and [MgF 2 ], [KF] < 5.
The liquidus temperature of various mixtures is plotted in Figure 5, according to the AlF 3 excess (w up to 0.2) for Na 3 AlF 6based salts containing w ) 0.03 of alumina and either w ) 0.05 or 0.15 of additives. At a concentration of AlF 3 equivalent to the industrial one (w ) 0.12), the chart shows that the liquidus temperature can be decreased down to 900 °C, depending on the nature of the additive and its concentration.

As illustrated by the above equation and Figure 5, the mixture combinations allowing a given liquidus temperature to be reached are infinite. However, the properties of the melt (and specifically the alumina solubility and the electrical conductivity) clearly depend on the chemical nature of the additives: the addition of basic compounds like LiF and KF does not have the same influence as CaF 2 or AlF 3 , which have an acid behavior.

Alumina Solubility. The solubility of alumina in the Na 3 AlF 6 -AlF 3 -CaF 2 -MgF 2 -LiF system has been notably studied by Skybakmoen et al. 3 in the (850 to 1050) °C temperature range. The analytical formula proposed in their work indicates the alumina solubility according to the melt composition where t is the temperature in °C and all concentrations [] are related to 100 w.

This equation highlights the coupling between the AlF 3 content and the temperature decrease on the alumina solubility in the NaF-AlF 3 system. Indeed, as illustrated in Figure 6, the solubility of alumina sharply decreases at constant overheating, while the CR decreases: from more than w ) 0.10 in pure cryolite (CR ) 3), the solubility drops to about w ) 0.04 in a mixture of CR ) 1.6 at 850 °C. It is however noticeable, when extending the CR ) 3 curve at lower temperature, that the solubility decrease is mostly due to the temperature effect rather than to the AlF 3 content.

For more acid mixtures exhibiting a liquidus temperature below 850 °C, the published formulas (both for the liquidus [START_REF] Solheim | Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolysis[END_REF] and for the alumina solubility 3 ) are not applicable according to their authors. However, since the FTHall database is claimed to be valid down to CR ) 1.5, some liquidus lines were calculated using the FactSage software in the CR range 2.2 to 1.5, according to the alumina concentration. These calculations, presented in Figure 7, are in excellent agreement with the data of Skybakmoen 3 down to CR ) 1.8 and slightly differ for more acid mixtures and temperatures below 850 °C.

The use of the FTHall database (Figure 7) confirms that the alumina solubility continues to decrease with decreasing CR and temperature. For a cryolite mixture of CR ) 1.5, this solubility does not exceed w ) 0.025. Below 750 °C (CR ) 1.3), the solubility is expected to be lower than w ) 0.02.

The addition of LiF, CaF 2 , and MgF 2 also leads to an alumina solubility decrease. Indeed, the A term of the equation proposed by Skybakmoen et al., which is directly proportional to the alumina solubility, linearly decreases (at constant temperature) with the increase of CaF 2 and MgF 2 . Some selected examples are presented in Figure 8, in which the alumina solubility is plotted versus the additive concentration at various CR and temperatures.

As taken into account in the cross terms of the Skybakmoen equation, a specific interaction between LiF and AlF 3 is driving the alumina solubility. The chemical reason for the interaction still has to be explained, but the practical consequence is that, converse to MgF 2 or CaF 2 , the LiF influence on the alumina solubility is related to the AlF 3 content of the melt. For a given mass, the LiF influence is much more detrimental to the alumina solubility than AlF 3 .

Another way of understanding the influence of additives on the alumina solubility consists of fixing a target liquidus temperature, choosing a set of mixture compositions corresponding to this liquidus temperature, and calculating the alumina solubility. The solubility of alumina for various mixtures having a liquidus temperature of 850 °C is presented in Table 1. The data show that the best way of decreasing the temperature, as long as the highest alumina solubility is the only criterion, is to stay in the Na 3 AlF 6 -AlF 3 system since a melt with CR ) 1.68 exhibits an alumina solubility of w ) 0.048, while all other combinations have a lower solubility. A high LiF content leads to the lowest alumina solubility (below w ) 0.02 for a LiF content of w ) 0.15).

Electrical ConductiWity.

Several studies propose analytical formulas for the calculation of the electrical conductivity of cryolite melts in the (850 to 1020) °C temperature range. The fits from Chrenkova et al. [START_REF] Chrenkova | Density, electrical conductivity and viscosity of low melting baths for aluminium electrolysis[END_REF] and Hives et al. [START_REF] Hives | Electrical conductivity of molten cryolite-based mixtures obtained with a tube-type made of pyrolytic boron nitride[END_REF] are among the most recent and take into account older data. One of the equations proposed by Hives 10 is where σ is in S • cm -1 ; T is in K; and all concentrations [] are related to 100 w.

Figure 9 indicates the electrical conductivity according to the CR at constant overheating. This figure includes additional data from Hives et al. [START_REF] Hives | Electrical conductivity of low-melting electrolytes for aluminium smelting[END_REF] concerning a mixture of CR ) 1.22. It is shown that the electrical conductivity decreases due to the common decrease of the CR and the temperature: from 2.8 S • cm -1 in pure cryolite, the conductivity decreases to 2.0 S • cm -1 for a mixture with a CR of 1.8 at 900 °C and to 1.2 S • cm -1 for a mixture of CR ) 1.22 at 715 °C.

The formula proposed by Hives shows that with the exception of LiF all additives have a detrimental effect on the electrical conductivity of cryolite melts. To illustrate the influence of each of them, the electrical conductivity of some mixtures is evaluated in Table 1: at 850 °C and without LiF, the conductivity of the mixtures is about (1.6 to 1.7) S • cm -1 , which represents about 75 % of the conductivity of the industrial mixtures. However, the addition of LiF allows this conductivity decrease to be fully balanced.

Density. An analytical formula proposed by Chrenkova 14 provides the density of cryolite mixtures in the (850 to 1050) °C temperature range, for an AlF 3 excess between w ) 0 and 0.30 (CR ) 3.0 to 1. 45) where F is in g • cm -3 ; t in °C; and all concentrations [] are related to 100 w.

This formula has been applied to calculate the density of the mixtures at constant overheating (up to 50 °C), as illustrated in Figure 10. It shows that the decrease of the CR induces a decrease of the density of the mixtures and, as a consequence, an increase of the difference of density with liquid aluminum. This trend is confirmed by the measurements of Silny et al. [START_REF] Silny | Density, viscosity, surface tension and interfacial tension in the systems NaF(KF) + AlF 3[END_REF] for a mixture of CR ) 1.22 at lower temperatures [(700 to 850) °C].

This trend is in contradiction with earlier results published by Utigard et al., [START_REF] Utigard | Density of the sodium hexafluoroaluminate-aluminum fluoride-alumina-calcium fluoride system: a key to the performance of Hall-He ´roult cells[END_REF] which indicate that the melt density at constant overheating exhibits a minimum at CR ) 2.1. However, as mentioned in ref 1, these measurements are in disagreement with all other published data.

Vapor Pressure. At constant temperature, the increase of the AlF 3 content leads to a noticeable increase of the total vapor pressure of the cryolite melts. However, this phenomenon is essentially due to the fact that when the AlF 3 concentration increases the liquidus temperature of the mixture sharply decreases: the isothermal representation of the vapor pressure is then not really adapted to evaluate the evolution of the volatility of the cryolite mixtures.

An analytical formula derived by Haupin [START_REF] Haupin | Chemical and physical properties of the Hall-He ´roult electrolyte[END_REF] is where P is the vapor pressure in kPa; T is in K; R is the bath ratio (R ) CR/2); and all concentrations [] are related to 100 w. Indeed, as illustrated in Figure 11, the vapor pressure at constant overheating is almost not sensitive to the influence of CR, with a maximal volatility at around CR ) 2.4. AlF 3 -rich mixtures are finally not more volatile than industrial melts.

This behavior was confirmed by Qiu et al., [START_REF] Qiu | Vapour losses of molten low temperature electrolytes in aluminium electrolysis[END_REF] which showed that for a constant overheating of 10 °C the mass loss of acid mixtures (CR ) 2.35 to 1.6) decreases with the AlF 3 content. The addition of Al 2 O 3 and LiF also tends to decrease the volatility of the salt.

Solubility of Aluminum. The data concerning the solubility of liquid aluminum in the molten salt are quite scarce at low temperatures. The data which were compiled in ref 1 only show the effect of the CR at constant temperature and not at constant overheating. Nevertheless, Qiu et al. [START_REF] Qiu | Metal solubility in low temperature electrolytes in aluminium electrolysis[END_REF] have shown that, on top of the CR effect, the temperature decrease leads to a decrease of the aluminum solubility. In a mixture of CR ) 1.5 at 820 °C, this solubility is reported to be seven times lower than industrial melts at 975 °C.

Conclusions. The use of additives (LiF, CaF 2 , KF) as well as an increase of the AlF 3 excess allows decreasing the liquidus temperature of the cryolite melts down to about 700 °C. The evolution of the main physicochemical properties of these mixtures, which have been widely studied, is due to two main parameters, namely, the chemical composition and the operating temperature.

The temperature decrease has some beneficial effects (decrease of the aluminum solubility in the salt, decrease of the vapor pressure, increase of the density of the aluminum-molten salt difference) as well as negative ones (decrease of the alumina solubility and of the electrical conductivity).

B ) 7.0184 + 0.6844R -0.08464R 3 + 0.010805[LiF] -0.005489R[LiF] + 0.047124[Al 2 O 3 ]/(1 + 0.1893[Al 2 O 3 ]) + 0.0068[CaF 2 ]
The chemical composition of the melts, at an equivalent liquidus temperature, mostly influences the electrical conductivity and the alumina solubility. LiF addition is almost an obligation in view of maintaining the electrical conductivity at a value equivalent to the industrial melts conductivity, mixtures like Na 3 AlF 6 -Li 3 AlF 6 -AlF 3 having for instance an electrical conductivity of about 2.0 S • cm -1 at 850 °C. [START_REF] Fellner | Electrical conductivity of low melting baths for aluminium electrolysis: the system Na 3 AlF 6 -Li 3 AlF 6 -AlF 3 and the influence of additions of Al 2 O 3 , CaF 2 and MgF 2[END_REF] Regarding the alumina solubility, it seems almost impossible to keep it high in such systems, the temperature effect dominating the chemical effect. The increase of the AlF 3 excess, preferably to the use of additives, seems the best solution to enhance alumina solubility.

An illustration of the LiF influence on the liquidus temperature, the alumina solubility, and the electrical conductivity of two mixtures is proposed in Figure 12. It finally seems unlikely to find a composition in the Na-cryolite system that allows an operating temperature below 900 °C without reducing the alumina solubility below w ) 0.05, all other parameters (electrical conductivity, density, vapor pressure) being constant or improved.

K-Cryolite Systems

The KF-AlF 3 -based salts have been widely studied recently. These mixtures are attractive since the system has a eutectic point at 560 °C and an improved alumina solubility compared to NaF-AlF 3 melts. The available data (liquidus temperatures, alumina solubility, vapor pressure, and electrical conductivity) are presented in this section.

KF-AlF 3 -Al 2 O 3 System. Liquidus Temperature. As indicated in Figure 13, the phase diagram of the system KF-AlF 3 is comparable to the NaF-AlF 3 system. Similar to the Na-cryolite system, the cryolite ratio (CR) is defined here as Several recent studies [START_REF] Danielik | Phase diagram of the system NaF-KF-AlF 3[END_REF][START_REF] Fellner | Physicochemical properties of the molten system Na 3 AlF 6 -K 3 AlF 6 -Al 2 O 3[END_REF][START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF][START_REF] Wang | Temperature of primary crystallization in party of system Na 3 AlF 6 -K 3 AlF 6 -AlF 3[END_REF][START_REF] Danielik | Low-melting electrolyte for aluminium smelting[END_REF] have been devoted to the establishment of the liquidus line in function of the CR and the alumina concentration. The main results, summarized in Figure 14, show that in the 1.2 to 3.0 CR range the increase of the alumina concentration slightly decreases the melting point of the KF-AlF 3 mixtures, similar to the Na-cryolite system.

Alumina Solubility. Alumina is highly soluble in KF-AlF 3 mixtures: at 1000 °C, the solubility exceeds w ) 0.20 for CR ) 3.0. In any case, as illustrated in Figure 15, alumina is more soluble in K-cryolite mixtures than in Na-cryolite ones, for equivalent CR and temperature.

As shown in Figure 16, from data established by Robert et al. [START_REF] Robert | Structure and thermodynamics of alkali fluoride-aluminum fluoridealumina melts. Vapor pressure, solubility and Raman spectroscopy studies[END_REF] and Yang et al., [START_REF] Yang | Alumina solubility in KF-AlF 3 based low temperature electrolyte system[END_REF] alumina solubility decreases with CR and temperature. It is noteworthy that alumina solubility is higher than w ) 0.05 at 750 °C for CR above 1.3. Such a solubility is not reached in Na-cryolite melts at temperature below 850 °C (see Table 1).

Vapor Pressure. Similar to Na-cryolite mixtures, when the AlF 3 content increases in the KF-AlF 3 system, the vapor pressure at constant temperature greatly increases due to the increase of the KAlF 4 (g) activity. [START_REF] Danielik | Phase equilibria in the system KF-AlF 3 -Al 2 O 3[END_REF] At constant overheating, the only available data seem to be those from Robert et al. [START_REF] Robert | Structure and thermodynamics of alkali fluoride-aluminum fluoridealumina melts. Vapor pressure, solubility and Raman spectroscopy studies[END_REF] They do not allow the vapor pressure at very low CR to be evaluated but indicate that the presence of alumina slightly decreases the vapor pressure. As illustrated in Figure 17, the K-cryolite mixtures are slightly more volatile than the Na-cryolite ones: for instance, at CR ) 2.0, the vapor pressure of the KF-AlF 3 system is 14.2 mbar, while it is 8.5 mbar for NaF-AlF 3 .

Electrical ConductiWity. Conductivity measurements in the KF-AlF 3 system have been performed only for very low CR (i.e., CR ) 1.22 [START_REF] Hives | Electrical conductivity of low-melting electrolytes for aluminium smelting[END_REF] and CR ) 1.30 [START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF] ). At an equivalent temperature, as indicated in Figure 18, the electrical conductivity of K-cryolite melts is smaller than Na-cryolite melts. For instance, at 750 °C and for a comparable CR of 1.22, the electrical conductivity of the K-cryolite melt is 1.04 S • cm -1 , while it is 1.27 S • cm -1 in the Na-cryolite mixture. These data show the influence of the substitution of Na + by K + ions: the ionic radius of the latter is indeed slightly bigger, and thus K + ions are less mobile.

The effect of alumina additions, illustrated in Figure 19, is similar to the NaF-AlF 3 system: the electrical conductivity decreases with alumina additions.

Density. The density of KF-AlF 3 melts has not been evaluated very much. Nevertheless, Silny et al. [START_REF] Silny | Density, viscosity, surface tension and interfacial tension in the systems NaF(KF) + AlF 3[END_REF] have published data obtained at 750 °C for CR ) 1.22: the density is 1.77 g • cm -3 , while it is 1.92 g • cm -3 in the same conditions for Na-cryolite. K-cryolite mixtures are then slightly less dense than Na-cryolite mixtures.

Influence of AdditiWes. The influence of three main additives is evaluated in this section: LiF, which was shown to improve the electrical conductivity of Na-cryolite mixtures; CaF 2 , which is an inevitable additive since CaO is contained in the alumina feed; NaF, in the case of mixed Na 3 AlF 6 -K 3 AlF 6 systems. For these systems, the available data are mainly restricted to the alumina solubility and the electrical conductivity.

Influence of LiF. The work from Kryukovsky, Apisarov, and Dedyukhin [START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF][START_REF] Apisarov | Conductivity of low temperature KF-AlF 3 electrolytes containing lithium fluoride and alumina[END_REF][START_REF] Dedyukhin | Electrical conductivity of the (KF-AlF 3 )-NaF-LiF molten system with Al 2 O 3 additions at low cryolite ratio[END_REF] shows that, similar to the Na-cryolite system, the electrical conductivity of K-cryolite mixtures increases with increasing LiF content. The case of CR ) 1.3 is illustrated in Figure 20. Nevertheless, the conductivity of LiF-containing systems remains quite low (below 1.3 S • cm -1 for CR ) 1.3). The measurements performed by Kryukovsky et al. [START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF] also show that the addition of w ) 0.05 of LiF allows compensation for the decrease of conductivity induced by w ) 0.035 of alumina.

It is also reported that LiF additions decrease the alumina solubility of K-cryolite mixtures, but very few data are available. Yang et al. [START_REF] Yang | Alumina solubility in KF-AlF 3 based low temperature electrolyte system[END_REF] indicate that a concentration of w ) 0.015 of LiF decreases the alumina solubility by about w ) 0.001 at 700 °C.

Since LiF additions allow partial compensation of the decrease of electrical conductivity due to the temperature decrease, additional experimental data on the alumina solubility in the system KF-AlF 3 -LiF would be valuable.

Influence of CaF 2 . Dedyukhin et al. [START_REF] Dedyukhin | Influence of CaF 2 on the properties of the lowtemperature electrolyte based on the KF-AlF 3 (CR ) 1,3) system[END_REF] reported that the CaF 2 solubility in a K-cryolite mixture of CR ) 1.3 at 700 °C is almost negligible. It reaches w ) 0.023 in a system containing w ) 0.05 of LiF and w ) 0.031 for a LiF content of w ) 0.10. Their measurements regarding electrical conductivity indicate a noteworthy phenomenon: at constant temperature, CaF 2 enhances slightly the electrical conductivity. Nevertheless, this effect is quite small (amplitude of about 0.1 S • cm -1 ).

The presence of CaF 2 in K-cryolite mixtures also decreases the alumina solubility: at 700 °C, Yang et al. [START_REF] Yang | Alumina solubility in KF-AlF 3 based low temperature electrolyte system[END_REF] have measured that the alumina solubility is around w ) 0.038 in a mixture of CR ) 1.22 containing w ) 0.044 of CaF 2 , while this solubility is w ) 0.048 in the same melt without CaF 2 .

KF-AlF 3 -NaF-Based Systems. The expected benefit of using NaF-KF-AlF 3 mixtures is the combination of the good electrical conductivity of Na-cryolite with the enhanced alumina solubility in K-cryolite melts. The liquidus lines of the ternary system are reported in some recent studies, [START_REF] Yang | Alumina solubility in KF-AlF 3 based low temperature electrolyte system[END_REF][START_REF] Danielik | Phase equilibria in the system KF-AlF 3 -Al 2 O 3[END_REF] and the system was integrated in a thermodynamic model by Heyrman and Chartrand. [START_REF] Heyrman | A thermodynamic model for the NaF-KF-AlF 3 -NaCl-KCl-AlCl 3 system[END_REF] As illustrated in Figure 21 from the latest measurements from Apisarov et al. [START_REF] Apisarov | Physical-chemical properties of the KF-NaF-AlF 3 molten system with low cryolite ratio[END_REF] and older data from Grjotheim et al., [START_REF] Grjotheim | Equilibrium studies in the system K 3 AlF 6 -Na 3 AlF 6 and K 3 AlF 6 -Rb 3 AlF 6[END_REF] the whole range of liquidus temperature between (650 and 1000) °C is attainable in the system NaF-KF-AlF 3 (CR is defined here as the molar ratio (NaF + KF)/AlF 3 ). Up to now, there are quite few data concerning the properties of such ternary mixtures, but as indicated by the available measurements of alumina solubility and electrical conductivity, the properties of the melts are intermediate between those of Na-cryolite and K-cryolite.

Indeed, some alumina solubility measurements have been published by Yang et al. [START_REF] Yang | Alumina solubility in KF-AlF 3 based low temperature electrolyte system[END_REF] in KF-AlF 3 systems with small NaF content. As indicated in Table 2, NaF additions decrease the alumina solubility, similar to LiF and CaF 2 .

As for the electrical conductivity, data published by Huang et al. [START_REF] Huang | Electrical conductivity of Na 3 AlF 6 -40 wt %K 3 AlF 6 -AlF 3 melts[END_REF] indicate that NaF-KF-AlF 3 mixtures with a NaF/(NaF + KF) molar ratio equal to 0.65 exhibit an intermediate conductivity between K and Na-cryolite systems in a (1.4 to 3.0) CR range. This trend, illustrated in Figure 22, was confirmed by Dedyukhin et al., [START_REF] Dedyukhin | Electrical conductivity of the (KF-AlF 3 )-NaF-LiF molten system with Al 2 O 3 additions at low cryolite ratio[END_REF] who measured the electrical conductivity of KF-AlF 3 -NaF systems at constant CR ) 1.3 in the whole NaF/(NaF + KF) composition range. It is noteworthy that this work also provides conductivity data for the complex system KF-AlF 3 -NaF-LiF-Al 2 O 3 .

Conclusions. The most obvious advantages of the K-cryolite systems are that they allow low-melting temperatures to be reached and exhibit, at an equivalent temperature, a much higher alumina solubility than Na-cryolite systems. However, on top of a slight increase of their vapor pressure, the major drawback is that the electrical conductivity of K-cryolite systems is noticeably lowered: a conductivity of about 1.0 S • cm -1 is reported at 700 °C, which has an expected consequence of a doubling of the ohmic drop in the salt compared to industrial mixtures (2.2 S • cm -1 at 960 °C).

The majority of the studies on the properties of K-cryolite mixtures has been performed at very low CR, in view of operating temperatures of about 700 °C. However, it would be interesting to characterize some melts having a liquidus temperature of about 800 °C since they should have a high alumina solubility (w ) (0.07 to 0.08) % by mass) and an intermediate electrical conductivity that could be improved by LiF additions.

More data on the NaF-KF-AlF 3 -Al 2 O 3 system would also allow a better evaluation of the systems involving KF as a major component. However, with increasing number of components in the cryolite systems (for which LiF and CaF 2 are also of interest), experimental investigation becomes almost endless. An interesting approach based on an empirical evaluation involving the molar volume of the bath components is being carried out by Redkin and co-workers. [START_REF] Dedyukhin | Influence of CaF 2 on the properties of the lowtemperature electrolyte based on the KF-AlF 3 (CR ) 1,3) system[END_REF][START_REF] Redkin | Empirical evaluation of the cryolite melts properties[END_REF][START_REF] Redkin | Modeling of cryolite-alumina melts properties and experimental investigation of low melting electrolytes[END_REF] This modeling may [START_REF] Hives | Electrical conductivity of low-melting electrolytes for aluminium smelting[END_REF] and Kryukovsky [START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF] ). help time to be spared in view of the evaluation of the electrical conductivity and alumina solubility of such complex cryolite mixtures.

Chloro-Fluoride Systems

Due to the very low solubility of alumina in pure chloride salts, the production of aluminum in such electrolytes requires the use of AlCl 3 instead of Al 2 O 3 as a feeding compound. A process operating with NaCl-KCl-AlCl 3 salts was developed by Alcoa in the seventies, of which the main features were summarized by Layne and Huml. [START_REF] Layne | Mixed chloride-fluoride bath for the electrolysis of aluminium chloride[END_REF] Its advantages are the absence of carbon consumption (Cl 2 is formed on an inert carbon anode) and the high solubility of AlCl 3 which allows continuous feeding, while its main drawbacks are the high AlCl 3 volatility above 700 °C, the sensitivity to moisture, and the low electrical conductivity of the salts.

This process, which produces chlorine gas at the anode, was abandoned, noticeably because of high maintenance costs of the plants and the difficulty of producing oxide-free AlCl 3 . Considering that carbon dioxide production becomes an environmental problem in the current Hall-He ´roult process, chlorine gas handling seems quite a challenge, and all-chloride baths have not been studied further since that time.

Some work was also performed on the properties of mixed chloro-fluoride baths, which may allow the use of alumina feed and consumable carbon anodes. Many phase diagrams obtained from thermodynamic modeling in the system NaF-KF-AlF 3 -NaCl-KCl-AlCl 3 are compiled in a paper by Heyrman and Chartrand: 33 several combinations allow for a decrease in the melting temperature, reaching the (700 to 800) °C temperature range.

However, a critical drawback of the chloro-fluoride mixtures is that the presence of chlorides induces a decrease of the alumina solubility. As a consequence of the small oxide concentration, a mixed anodic reaction is observed, and as mentioned for example by Fellner et al., [START_REF] Fellner | The content of sodium in aluminium during electrolysis of the molten systems Na 3 AlF 6 -NaCl-Al 2 O 3 and NaF-NaCl[END_REF] chlorine evolution may occur at the anode. Indeed, alumina solubility is reported to be almost negligible (about w ) 0.0001) in KCl-NaCl-NaF mixtures containing about one-third NaF, [START_REF] Cherginets | Solubility of Al 2 O 3 in some chloridefluoride melts[END_REF] as well as in LiCl-KCl-NaF mixtures. [START_REF] Picard | Effect of the addition of fluoride on the conditional conductivity of alumina in LiCl-KCl eutectic melt[END_REF] Some properties of the NaF-KCl-Al 2 O 3 mixtures have been studied by Balaraju et al., [START_REF] Balaraju | Studies on low temperature Al electrolysis using composite anodes in NaF-KCl bath electrolyte[END_REF] and the alumina solubility seems to be enhanced in this system. Nevertheless, as indicated in Table 3, the alumina solubility does not exceed w ) 0.012, and the production of chlorine gas can be expected at industrial current densities. The electrical conductivity of the NaF-KCl (0.5-0.5) mixture, determined by Balaraju et al., is quite high, i.e., about 2.2 S • cm -1 in the (700 to 800) °C temperature range.

The properties (melting point, electrical conductivity, density) of the Na 3 AlF 6 -AlF 3 -BaCl 2 -NaCl-Al 2 O 3 system were also recently published by Lu et al., [START_REF] Lu | Low temperature aluminum floating electrolysis in heavy electrolyte Na 3 AlF 6 -AlF 3 -BaCl 2 -NaCl bath system[END_REF] to evaluate a heavy electrolyte with floating aluminum. The solubility of alumina is however not mentioned in this work, nor is the nature of the anodic reaction.

Conclusions

The present review shows that the physicochemical properties of Na-cryolite and K-cryolite mixtures are well established, notably thanks to recent studies which filled some gaps. The influence of additives in K-cryolite mixtures is however not completely covered, as well as the aluminum solubility in lowtemperature electrolytes.

It is noteworthy that K-cryolite mixtures have been mostly studied at a low-temperature range [(700 to 750) °C] and much less in the (800 to 900) °C temperature range, where they exhibit a large alumina solubility. The interest of a temperature decrease of about (100 to 150) °C should be evaluated, to determine if the target temperature is necessarily 700 °C or if intermediate operating temperatures may bring improvements compared to the current process.

On another hand, data concerning chloro-fluoride mixtures are quite scarce. It is probable that this kind of system has not been largely studied because of the concern of the anodic reaction: chlorine gas management will necessarily raise heavy operational troubles.

Finally, apart from the salt mixture properties presented in this article, it is obvious that many other parameters have to be taken into account to operate electrolysis cells at lower temperature than the current Hall-He ´roult process. As discussed for instance by Thonstad and Rolseth 45 in the case of AlF 3 -rich Na-cryolite mixtures, the purity of aluminum, the cell voltage, the handling of the thermally insulating frozen ledge, and the corrosion resistance of lining materials are some of the many problems to be studied with alternative electrolyte compositions for aluminum electrolysis.

Literature Cited

Figure 1 .

 1 Figure 1. Influence of the AlF 3 content on the liquidus temperature of a cryolite mixture containing w(Al 2 O 3 ) ) 0.03 and w(CaF 2 ) ) 0.05 (calculated with the FTHall database).

Figure 2 .

 2 Figure 2. Part of the cryolite-alumina system for CR ) 2.2, w(CaF 2 ) ) 0.05 (calculated with the FTHall database).

Figure 3 .

 3 Figure 3. Total vapor pressure above Na 3 AlF 6 -M mixtures at 1027 °C (plotted from refs 1 and 7).

Figure 4 .

 4 Figure 4.Influence of temperature and composition on the density of cryolite melts.[START_REF] Thonstad | Aluminium Electrolysis[END_REF][START_REF]Handbook of Chemistry and Physics[END_REF][START_REF] Chrenkova | Density, electrical conductivity and viscosity of low melting baths for aluminium electrolysis[END_REF]

Figure 5 .Figure 6 .

 56 Figure 5. Liquidus temperature versus AlF 3 excess (up to CR ) 1.85) for mixtures containing w ) 0.05 (left) and w ) 0.15 (right) of additives (CaF 2 -KF-LiF) and w ) 0.03 of alumina (calculated from ref 4). [Al 2 O 3 ] sat ) A(t/1000) B with A ) 11.9 -0.062[AlF 3 ] -0.0031[AlF 3 ] 2 -0.50[LiF] -0.20[CaF 2 ] -0.30[MgF 2 ] + (42[LiF][AlF 3 ])/ (2000 + [LiF][AlF 3 ]) B ) 4.8 -0.048[AlF 3 ] + 2.2[LiF] 1.5 /(10 + [LiF] + 0.001[AlF 3 ] 3 )

Figure 7 .

 7 Figure 7. Solubility of alumina in the NaF-AlF 3 -Al 2 O 3 system, in the range CR ) 1.3 to 2.2 (calculated with the FTHall database).

Figure 8 .

 8 Figure 8. Alumina solubility in mixtures of various CR versus the LiF, CaF 2 , and MgF 2 content, at the liquidus temperature of the Na 3 AlF 6 -xAlF 3 system (calculated from ref 3).

Table 1 .Figure 9 .

 19 Figure 9. Electrical conductivity in the NaF-AlF 3 system at various CR, in the temperature range T liq -T liq + 50 °C (calculated from refs 10 and 15).

Figure 10 .

 10 Figure10. Aluminum density compared to cryolite mixture density in the temperature range T liq -T liq + 50 °C (from Chrenkova,[START_REF] Chrenkova | Density, electrical conductivity and viscosity of low melting baths for aluminium electrolysis[END_REF] Silny,[START_REF] Silny | Density, viscosity, surface tension and interfacial tension in the systems NaF(KF) + AlF 3[END_REF] and Utigard[START_REF] Utigard | Density of the sodium hexafluoroaluminate-aluminum fluoride-alumina-calcium fluoride system: a key to the performance of Hall-He ´roult cells[END_REF] ).

Figure 11 .

 11 Figure 11. Evolution of the total vapor pressure above NaF-AlF 3 mixtures versus the CR isotherms at 1020 °C and constant overheating (from Haupin 18 and Robert 7 ).

Figure 12 .

 12 Figure 12. Evolution of the liquidus temperature, the alumina solubility, and the electrical conductivity in the NaF-AlF 3 -LiF-Al 2 O 3 system versus the LiF content, at constant overheating (+10 °C).

Figure 13 .

 13 Figure 13. Comparison of the KF-AlF 3 and NaF-AlF 3 phase diagrams.[START_REF] Danielik | Phase diagram of the system NaF-KF-AlF 3[END_REF]

Figure 14 .

 14 Figure 14. Liquidus temperatures in the KF-AlF 3 -Al 2 O 3 system versus the CR and the alumina concentration (plotted from refs 23, 24, and 26).

Figure 15 .

 15 Figure 15. Alumina solubility in K-cryolite mixtures versus CR at 700 °C (Yang 27 ) and 1000 °C (Robert 7 ). Comparison to alumina solubility in Na-cryolite mixtures.

Figure 16 .

 16 Figure 16. Evolution of alumina solubility versus temperature and CR in KF-AlF 3 mixtures (data from Robert 7 and Yang 27 ).

Figure 17 .

 17 Figure 17. Comparison of the vapor pressures of NaF-AlF 3 and KF-AlF 3 mixtures versus CR, at constant overheating, T liq + 10 °C (from refs 7 and 18).

Figure 18 .

 18 Figure 18. Comparison of the electrical conductivity of Na and K cryolite melts versus temperature at CR ) 3.0 and CR ) 1.22 (data from refs 15, 24, 26, and 29).

Figure 19 .

 19 Figure 19. Electrical conductivity of KF-AlF 3 mixture versus temperature at low CR: influence of the alumina content (from Hives[START_REF] Hives | Electrical conductivity of low-melting electrolytes for aluminium smelting[END_REF] and Kryukovsky[START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF] ).

Figure 20 .

 20 Figure 20. Influence of the LiF content on the electrical conductivity of K-cryolite melts (CR ) 1.3).[START_REF] Kryukovsky | Electrical conductivity of low melting cryolite melts[END_REF][START_REF] Apisarov | Conductivity of low temperature KF-AlF 3 electrolytes containing lithium fluoride and alumina[END_REF]

Figure 21 .

 21 Figure 21. Liquidus temperature in the NaF-KF-AlF 3 system with different CR (from Apisarov 34 and Grjotheim 35 ).

Table 2 .

 2 Alumina Solubility (Mass Percent, 100 w) in Mixed Na-K-cryolite Melts at 700 °C27 composition (molar) alumina solubility (100 w) KF-AlF 3 -NaF/CR 48.0-48.0-4.0/CR ) 1.08 2.51 50.0-46.0-4.0/CR )

Figure 22 .

 22 Figure 22. Electrical conductivity versus temperature for NaF-KF-AlF 3 mixtures at CR ) 3.0 (from Huang 36 ) and CR ) 1.3 (from Dedyukhin 31 ).Comparison to the NaF-AlF 3 and KF-AlF 3 systems.[START_REF] Hives | Electrical conductivity of low-melting electrolytes for aluminium smelting[END_REF]


Paul-Louis Fabre

Olivier Reynes
email: reynes@chimie.ups-tlse.fr

Determination of Copper and Zinc in Brass: Two Basic Methods

Alloys constitute many objects of our daily life and are studied for the development of new materials and the improvement of their properties (1,2). Because of their particular characteristics (i.e., conductivity, mechanic or corrosion resistances), alloys find applications in many fields such as aeronautics, building, electronics devices, and jewelry. Brass is an alloy of copper and zinc and is familiar to most students. It is an appropriate material for an educational laboratory experiment because of its (i) low cost, (ii) simple composition, mainly copper and zinc (5-45% by mass) and minor elements (<5%), and (iii) ease of dissolution in nitric acid to give Cu 2þ and Zn 2þ ions. Surprisingly, few pedagogical articles are devoted to the study of brass (3)(4)(5), even though in 1927, Stout published a method for the determination of copper in brass (6).

We describe two basic methods for determining the quantity of copper and zinc in brass. This experiment can easily be carried out during a typical 3-h laboratory period in upper-level undergraduate laboratory and does not require advanced instrumentation. The pedagogical interest lies mainly in the basic concepts of analytical chemistry in solutions: oxidative processes according to the standard potentials; selective or successive precipitations according to the solubility products, K sp ; potentiometric titrations yielding precipitates; and complexometric titrations with the competition between complexing agents to mask an element.

A brass piece is dissolved in concentrated nitric acid to give Cu 2þ and Zn 2þ ions; the metals are oxidized by the oxidizing agents (H þ and NO 3 -). Students are required to write and justify the redox reactions according to the standard potentials (7). The resulting solution is diluted in a volumetric flask and used in the following titrations as the test solution. The first method is based on acid-base titration of the test solution by sodium hydroxide. The titration shows the three equivalent points corresponding to (i) the neutralization of the excess of nitric acid of the redox reaction; (ii) the complexation of Cu 2þ and the precipitation of Cu(OH) 2 , the less insoluble hydroxide, and (iii) the complexation of Zn 2þ and the precipitation of Zn(OH) 2 . Students again write and justify the reactions with the help of the solubility products. Using these reactions and the measured equivalence volumes, they calculate Cu 2þ and Zn 2þ concentrations and then the copper and zinc ratio in the brass piece.

The second method consists of two complexometric titrations by EDTA with and without the use of complexing agents for masking the Cu 2þ species. In the first titration, copper and zinc together are titrated by EDTA. In the second titration, sodium thiosulfate is added to complex the Cu 2þ ions and then the titration of Zn 2þ by EDTA is performed. The titration reactions are written and justified by the formation constants of the complexes . The second titration gives [Zn 2þ ], whereas the first titration gives the sum of [Cu 2þ ] and [Zn 2þ ]. Once again, the copper and zinc ratios are calculated in the brass piece.

Experimental Procedure

Brass Dissolution

Brass pieces, ∼770 mg, are reacted in ∼5 mL of concentrated nitric acid (70%) in a hood. After complete dissolution of brass, the resulting acid solution is diluted in a 100 mL volumetric flask and used in the following experiments as the test solution.

Acid-Base Titrations

The test solution (25 mL) is titrated by a sodium hydroxide solution, ∼0.1 M. The pH is measured during the addition of NaOH. The pH curve as a function of the volume of NaOH is graphed and shows three jumps. The potentiometric titration is repeated twice.

Complexometric Titrations

Complexometric titrations are carried out on two buffered solutions prepared by the addition of 10 mL of the test solution to an equal volume of an acetic buffer so that the pH is between 5 and 6. The buffered solutions are titrated with ∼0.1 M EDTA solution in presence of xylenol orange as a colored indicator. With the titration of the first buffered solution, the equivalence volume for the titration of Cu 2þ and Zn 2þ is indicated by the color change from red to green. Before the titration of the second buffered solution, sodium thiosulfate is added to the solution, which complexes selectively the Cu 2þ ions. When the titration is carried, out the color change from red to yellow gives the equivalence volume for the titration Zn 2þ . Each titration is repeated twice.

Hazards

Sodium hydroxide is caustic and may cause irritation of the eyes and skin. Nitric acid is a strong oxidizer and contact with other material may cause fire. It is a corrosive liquid and may cause severe burns to all body tissue. Dissolution of brass in concentrated nitric acid liberates toxic vapors; the reaction must be carried out in a hood. EDTA and sodium thiosulfate can cause irritation to skin, eyes, and respiratory tract. Students must wear safety glasses, lab coats, and gloves when performing these experiments.

Results and Discussion

The dissolution of brass in concentrated nitric acid to Cu 2þ and Zn 2þ is observed: the solution turns blue and red-brown vapors escape from the vessel. The red-brown vapors are related to NO x gases evolving from the NO/NO 3 -redox couple; the blue color is attributed to the formation of Cu 2þ cations.

The determination of the Cu 2þ and Zn 2þ concentrations is achieved by acid-base titration through the successive precipitations of the hydroxides Cu(OH) 2 and Zn(OH) 2 . Titration of the test solution by sodium hydroxide NaOH is represented by a curve, pH versus NaOH volume, that gives the evidence of three neutralization reactions by three pH jumps. The first jump is related to the remaining nitric acid (strong acid) and the second jump is attributed to the precipitation of copper hydroxide Cu(OH) 2 whose solubility product (K sp1 = 2 Â 10 -20 ) is lower than the solubility product (K sp2 = 3 Â 10 -17 ) of zinc hydroxide Zn(OH) 2 . The blue precipitate is characteristic of the Cu(OH) 2 formation. The third jump is attributed to the precipitation of zinc hydroxide Zn(OH) 2 and a white precipitate appears. Taking into account the different solubility products, K sp , the student analyzes the neutralization curve and calculates the beginning pH of the precipitations. The equivalence volumes for the second and third pH jumps allow the calculation of the concentrations of copper and zinc cations. Then, the relative mass of copper and zinc in the analyzed brass pieces are obtained by considering the initial volume of the test solution and the mass of brass dissolved.

The second method for brass analysis consists in complexometric titrations by EDTA, which is an efficient complexing agent of metallic cations (8). Xylenol orange is used as a colored indicator for the detection of equivalence point (volume). The analysis is carried out via two titrations. In the first titration, the total concentration of cations Cu 2þ and Zn 2þ is ascertained as the complexation constants of Cu-EDTA (log β 1 = 18.8) and Zn-EDTA (log β 2 = 16.5) are similar (8) and the complexes are formed simultaneously. In the second titration, the addition of sodium thiosulfate masks the copper cation. According to the complexation constants, β 1 0 = 10 10.3 for copper and β 2 0 = 10 2.3 for zinc [START_REF]Determination and Use of Stability Constants[END_REF], the complexation of Cu 2þ is selective and Cu 2þ is masked. Thus, titration of this solution by EDTA is restricted to Zn 2þ cations. Taking into account the two titrations, concentrations of copper and zinc are determined.

Comparing the results of the two methods, it appears that the complexometric titrations give the best results when compared to fluorescence X-ray analysis (see the supporting information). The potentiometric titration results were significantly different from those obtained by complexometric and fluorescence X-ray analysis. The difference observed between complexometric and potentiometric titrations can be explained by the approximations used in the mass balance equation. During addition of sodium hydroxide, a basic cupric complex [START_REF] Hidimi | [END_REF] and the soluble [Zn(OH) 2 ] 4-can be formed and were not considered here.

Conclusion

This educative laboratory experiment is easily carried out in a 3-h period, without advanced instrumentation. The experiment uses a protocol similar to those of standard laboratory: sample preparation, dilution, method, analysis, and results. Each step must be rigorously performed. From an academic point of view, this experiment illustrates the basic concepts of solution chemistry. The oxidative dissolution of brass pieces relates to the standard potentials of the redox couples. The potentiometric titrations yield precipitates that imply selective or successive precipitations according to the solubility products K sp . Considering the apparent constants according to Ringbom (11), the complexometric titrations with the competition between complexing agents to mask an element could be the object of a further study.

Acknowledgment

The authors would like to thank Francine Bazer-Bachi for her technical assistance.