Recovery of fluoride values from spent pot-lining: Precipitation of an aluminium hydroxyfluoride hydrate product

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1 Available online at Separation and Purification Technology 61 (2008) Recovery of fluoride values from spent pot-lining: Precipitation of an aluminium hydroxyfluoride hydrate product Diego Fernández Lisbona, Karen M. Steel School of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, United Kingdom Received 29 June 2007; received in revised form 8 October 2007; accepted 18 October 2007 Abstract Treatment of spent pot-lining (SPL) from aluminium smelting cells by a two-stage leaching scheme comprising a water wash and an Al 3+ leach and fluoride recovery as an aluminium hydroxyfluoride product has been studied for extraction of fluoride and then recovery as smelter grade AlF 3. The NaF content of a 1.18 mm size fraction was removed by the water wash, while the more refractory Na 3 AlF 6 and CaF 2 were removed by treatment with 0.34 M Al 3+ solution at 25 C for 24 h, which yielded an overall fluoride extraction of mol%. Mathematical modelling using experimental stability constant data was carried out to predict the effect of combining solutions and identify ways to manipulate the solution equilibria to maximise fluoride precipitation yields. The predictions were then tested experimentally. In the ph range, selective precipitation of fluoride as an aluminium hydroxyfluoride hydrate product was achieved by neutralisation of the combined solutions with addition of 2 M NaOH solution. Higher ph values lead to the co-precipitation of hydrolysed sodium fluoroaluminates. Characterisation of precipitates using X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, thermogravimetric analysis, differential thermal analysis, aluminium and fluoride determination have pointed out both the AlF 2 (OH) 1.4H 2 O stoichiometry and possible thermal decomposition pathways to yield a dehydrated aluminium hydroxyfluoride product, AlF 2 (OH), that could be used for smelter grade AlF 3 production. The kinetics of hydrolysis are such that nucleation dominates while particle growth is restricted. Techniques to allow slow hydrolysis are necessary to form smelter grade AlF Elsevier B.V. All rights reserved. Keywords: Spent pot-lining; Aluminium hydroxyfluoride hydrate; Smelter grade aluminium fluoride 1. Introduction Spent pot-lining (SPL) is a hazardous waste generated at the end-of-life of carbon cathodes in aluminium smelting electrolysis cells or pots. After a variable period of time (3 8 years [1]), degradation of the carbonaceous cathode material, with an initial composition anthracitic/graphitic in nature, starts affecting cell performance and must be replaced. For this material, graphitization progressed with time, along with sodium intercalation (especially within anthracitic grains) and side reactions due to bath material penetrating the structure [2,3]. Although the composition of the SPL thus generated varies, fluoride contents up to 20 wt.% and cyanides up to 1 wt.% are the main environmental concern [4]. Fluoride is present as NaF, Na 3 AlF 6 and CaF 2 and cyanides as NaCN, Na 4 Fe(CN) 6 and Na 3 Fe(CN) 6. An estimated 1 million tonnes of SPL is produced worldwide every year [5,6]. Corresponding author. Tel.: ; fax: address: karen.steel@nottingham.ac.uk (K.M. Steel). A number of treatment technologies for SPL have been developed over the years. These can be classified into four categories: co-processing of SPL in third-party industries [6 9] (where either its fluoride or carbon fraction can be used), physical separation methods, thermal treatment and chemical leaching approaches [10]. Physical separation methods usually take advantage of differences in density and surface properties between the materials present in SPL to separate carbon from the inorganic fraction. Thermal treatment alternatives [10 16] reduce the amount of waste to the refractory components: carbon content is burnt, cyanides destroyed and fluoride essentially recovered as a gaseous HF effluent which, ideally, can be reacted with smelter grade Al 2 O 3 in a fluidised bed to yield smelter grade AlF 3 [10,14]. In contrast, all leaching processes available are aimed at dissolving soluble inorganic matter present in the SPL to recover the fluorides as CaF 2 for HF production [10,17], Na 3 AlF 6 for bath material [18] and/or AlF 3 [19 21]. Two main technologies have been considered for development at industrial level, a thermal approach by Ausmelt Alcoa to produce AlF 3 and Alcan s caustic leaching to produce NaF or CaF 2 [15,22] /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.seppur

2 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) However, there is no technology widely accepted and controlled storage in secure landfill sites is largely practiced by aluminium smelters while awaiting a cost-effective recycling process. In order for an SPL process to achieve widespread acceptance, it needs to have a low cost and generate products of high value and/or guaranteed value. Chemical leaching processes offer promise because they can be done at low temperatures and pressures with equipment made of inexpensive materials of construction, enabling separation and recovery of both fluoride and graphite in useful forms. While NaF in SPL is easily removed by a water wash [23,24], Na 3 AlF 6 and CaF 2 can only be removed through chemical leaching. Chemical leaching with mineral acids and bases has been applied to SPL to essentially dissolve Na 3 AlF 6 [17,19 21,25 27]. However, caustic leaching yields incomplete fluoride extraction due to the limited fluoride solubility (e.g. 12 g F /l in g/l NaOH solutions). CaF 2 is insoluble in most caustic and acidic solutions. In addition, sodium aluminium silicates can precipitate on SPL with caustic leaching, which hinders mass transfer [17]. Also, less commercially attractive fluoride products are recovered by conventional precipitation methods (NaF, Na 3 AlF 6 or CaF 2 ). In contrast, Al 3+ solutions are more effective at extracting fluoride from fluoride-bearing materials including CaF 2 [28]. Improved fluoride extraction is due to the formation of highly stable aluminium fluoride complexes with the formula AlF i (3i)+, where i = 1 6 [29]. In particular, formation of highly stable AlF 2 + and AlF 2+ are responsible for Na 3 AlF 6 and CaF 2 leaching [23,24], respectively: Na 3 AlF 6 + 2Al 3+ 3Na + + 3AlF 2 + CaF 2 + 2Al 3+ Ca AlF 2+ (I) (II) Leaching with Al 3+ solutions has been applied to SPL in a number of studies [24,25,30]. Generally, the fluoride products recovered from SPL leachates, such as NaF and CaF 2, have a relatively low value. Na 3 AlF 6 is also in surplus, whereby it is produced in aluminium smelting through reaction of Al 2 O 3 sodium impurities with AlF 3 [31]. AlF 3 is constantly needed in aluminium smelting cells for improving current efficiency [32] and must be added throughout the life of the cell to compensate various aluminium fluorideconsuming processes taking place [31]. This makes AlF 3 the desirable product for fluoride recycling from SPL. Unfortunately, AlF 3 has a relatively high solubility in aqueous solution [33]. Furthermore, Na + and Ca 2+ concentrations in the resulting leachates promotes precipitation of sodium fluoroaluminates upon fluoride recovery [34] as in the system described by Grobelny [35]: cryolite (Na 3 AlF 6 ), chiolite (Na 5 Al 3 F 14 ) and sodium tetrafluoroaluminate (NaAlF 4 ), and/or CaF 2. Gradual neutralisation of Al 3+ leachates with NaOH solutions yields an aluminium hydroxyfluoride product AlF x (OH) 3x nh 2 O, with x = F/Al 2 [25]. It is claimed that this can be converted into a mixture of AlF 3 and Al 2 O 3 (F/Al 2), which can then be added to aluminium smelting cells after calcination at C. It follows that further work along these lines should be carried out to fully explore the potential of producing the aluminium hydroxyfluoride hydrate. In particular, the possibility of combining a water wash and Al 3+ leaching of SPL to fully extract fluoride and raise F/Al ratio in the final product while minimising the amount of Al 3+ should be investigated. No information is available in the open literature to confirm whether the reported aluminium hydroxyfluoride product can be selectively precipitated from Al 3+ and/or acidic leachates, nor the precipitation yields. Perhaps more importantly, whether this material can be converted into smelter grade AlF 3. To minimise dust, anode effects and crust formation, apart from purity requirements, Al 2 O 3 flow properties and granulometry are critical for aluminium smelters [36,37]: coarse particles of Al 2 O 3 (92% above 44 m) and AlF 3 are required. This work includes modelling of the aluminium fluoride complexation equilibria in an attempt to confirm experimental results (effect of ph) and expose ways that the chemistry could be manipulated to achieve a desired effect. Further work will focus on manipulating the precipitate s morphology to minimise nucleation and maximise crystal growth with the ultimate objective of recovering a product convertible into smelter grade AlF Materials and methods 2.1. Reagents and samples All chemical reagents used in this work were analytical reagent grade. The SPL sample was a first cut obtained from Anglesey Aluminium Ltd. (UK) in the form of cm long 5 10 cm wide lumps. Before further processing, SPL was washed with water and dried for 24 h. Particle size was reduced using a jaw crusher. Crushed SPL was sieved and size fraction 1.18 mm collected for leaching tests. Fluoride content was determined by an alkali fusion method Leaching tests SPL leaching consisted of two steps: a 30.0 g sample (fraction 1.18 mm) was first washed with 125 ml of de-ionised water at 25 C for a period of 4 h. Following filtration using Fisherbrand QL100 filter paper, the filtrate was reserved for subsequent fluoride precipitation and the water-washed SPL residue was dried at 110 Cfor4h.AnAl 3+ leaching solution was prepared by dissolving 27.0 g of Al(NO 3 ) 3 9H 2 O in 200 ml of de-ionised water. Water-washed SPL sample was agitated in the Al 3+ leaching solution using a magnetic stirrer at 25 C for 24 h. Leaching time was selected from previous Na 3 AlF 6 and CaF 2 model compound solubility results [38]. The mixture was filtered and dried at 110 C for 4 h. The filtrate was reserved and later used for aluminium hydroxyfluoride precipitation Precipitation of fluoride, effect of temperature and ph Filtrates from SPL water wash and Al 3+ treatment were combined at various temperatures ranging from 50 C to the

3 184 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) solution s boiling temperature (101.4 C) and ph values to increase F/Al ratio in solution from 2 to 3, study fluoride recovery, precipitates morphology and calcination behaviour. ph values were determined with a Thermo Orion combination ph electrode and data acquired with a Thermo Orion ph-ise meter. Solution volume was kept constant under reflux conditions and, where necessary, ph was adjusted with 2 M NaOH solution. Precipitates were recovered by centrifugation at 2500 rpm, washed with de-ionised water to remove unreacted ionic species and dried at 110 C for a period of 4 h. Precipitates recovered were characterised by powder X-ray diffraction (XRD) with a Hiltonbrooks X-ray generator and electronics equipment linked to a Philips goniometer and using Cu K radiation. Carbon content in SPL samples was estimated by ashing 1.50 g of a finely ground sample in a platinum ashing tray at 800 C for a period of 5 h. Fluoride analyses were performed both on SPL samples, to determine fluoride extraction, and precipitates recovered by the alkali fusion and oxine precipitation method developed by Besida [24]. This involved fusing a finely ground solid sample with a mixture 1.00 g of sodium carbonate and 0.50 g of sodium tetraborate in a platinum crucible using a Meker burner. The fusion pellet was dissolved in aqueous HCl and metallic species removed by precipitation with 8-hydroxyquinoline reagent as described in Vogel s textbook of quantitative chemical analysis [39]. Solution was buffered to ph 4 5 with the total ionic strength adjustment buffer (TISAB) III. Fluoride standard solutions for electrode calibration were prepared from NaF 1 M solution. Fluoride concentration was then determined using a Thermo Orion fluoride ion selective electrode. In the fluoride analysis of aluminium hydroxyfluoride hydrate solid phases, the mass of precipitates recovered from the 8-hydroxyquinoline can be taken as a reliable approximation to the aluminium content to calculate F/Al ratios. Thermogravimetric analysis on aluminium hydroxyfluoride samples was performed in a thermogravimetric analyser TA SDT Q600 by heating samples to 800 C (heating rate 10 C/min) under N 2 flow (100 ml/min). Initial sample size was kept constant at 5 mg Calcination of aluminium hydroxyfluoride samples Samples recovered from NaOH neutralisation experiments were calcined in an Elite tube furnace fitted with a corundum tube, PTFE connectors and tubing, and polyethylene bottles for HF absorption. Temperature was kept constant at the desired value for a period of 2 h with an initial heating rate of 10 C/min. Compressed air was passed through the system at a constant rate of 1 l/min. For each calcination experiment, a 0.5 g sample of aluminium hydroxyfluoride product (precipitated at 70 C and ph 5.00) was weighed in corundum boats. The sample was then placed in the tube furnace, calcination temperature selected and off-gases bubbled through 0.1 M NaOH solution. Fluoride determination on the solution recovered was performed with the fluoride ion selective electrode. TISAB III was again used as buffer solution. Textural characterisation of solids was performed by N 2 physisorption in a Micromeritics ASAP 2010 Analyser. Samples were first degassed at 110 C under vacuum and N 2 adsorption data were subsequently taken at 196 C. Apparent surface area was determined by the Brunauer Emmett Teller (BET) method. Scanning electron microscopy (SEM) images were taken using an FEI Quanta 600. Energy-dispersive X-ray spectra (EDS) were collected with an EDS-EDAX Genesis. 3. Mathematical modelling The concentration of Al F and ternary Al F OH species in leachates obtained by Al 3+ treatment of SPL at 25 C, as well as the effect of ph on the complexation equilibria, can be modelled from experimental stability constant data available in the literature. Modelling is important to both confirm experimental observations and to search for possible ways to manipulate the chemistry and achieve desired results. Stability constant values for the ternary Al F OH species at zero ionic strength (I) were taken from Sanjuan and Michard [40]. Stability constants for Al F and Al OH de-protonation equations were taken from Nordstrom and May [41]. Equations for the precipitation of CaF 2 or NaAlF 4 were not included as quantities of these precipitates were minimal. Eqs. (1) (19) were used to predict the concentration of species in solution: [AlF 2+ ] = 1 [Al 3+ ][F ] (1) [AlF 2 + ] = 2 [Al 3+ ][F ] 2 (2) [AlF 3 ] = 3 [Al 3+ ][F ] 3 (3) [AlF 4 ] = 4 [Al 3+ ][F ] 4 (4) [AlF 5 2 ] = 5 [Al 3+ ][F ] 5 (5) [AlF 6 3 ] = 6 [Al 3+ ][F ] 6 (6) [AlF(OH) + ] = 7 [Al 3+ ][F ]/[H + ] (7) [AlF(OH) 2 ] = 8 [Al 3+ ][F ]/[H + ] 2 (8) [AlF(OH) 3 ] = 9 [Al 3+ ][F ]/[H + ] 3 (9) [AlF 2 (OH)] = 10 [Al 3+ ][F ] 2 /[H + ] (10) [AlF 2 (OH) 2 ] = 11 [Al 3+ ][F ] 2 /[H + ] 2 (11) [AlF 3 (OH) ] = 12 [Al 3+ ][F ] 3 /[H + ] (12) [Al(OH) 2+ ] = 13 [Al 3+ ]/[H + ] (13) [Al(OH) 2 + ] = 14 [Al 3+ ]/[H + ] 2 (14) [Al(OH) 3 ] = 15 [Al 3+ ]/[H + ] 3 (15) [Al(OH) 4 ] = 16 [Al 3+ ]/[H + ] 4 (16) [HF] = 17 [H + ][F ] (17) [HF 2 ] = 18 [H + ][F ] 2 (18) [H + ][OH ] = K w (19)

4 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) Balances to aluminium, fluoride and charged species are summarised in Eqs. (20) (22): [Al] initially = [Al 3+ ] + [AlF 2+ ] + [AlF 2 + ] + [AlF 3 ] + [AlF 4 ] + [AlF 2 5 ] + [AlF 3 6 ] + [AlF(OH) + ] + [AlF(OH) 2 ] + [AlF(OH) 3 ] + [AlF 2 (OH)] + [AlF 2 (OH) 2 ] + [AlF 3 (OH) ] + [Al(OH) 2+ ] + [Al(OH) + 2 ] + [Al(OH) 3 ] + [Al(OH) 4 ] (20) [F ] initially = [F ] + [AlF 2+ ] + 2[AlF 2 + ] + 3[AlF 3 ] + 4[AlF 4 ] + 5[AlF 2 5 ] + 6[AlF 3 6 ] + [AlF(OH) + ] + [AlF(OH) 2 ] + [AlF(OH) 3 ] + 2[AlF 2 (OH)] + 2[AlF 2 (OH) 2 ] + 3[AlF 3 (OH) ] + [HF] + 2[HF 2 ] (21) [F ] + [AlF 4 ] + 2[AlF 2 5 ] + 3[AlF 3 6 ] + [AlF(OH) 3 ] + 2[AlF 2 (OH) 2 ] + [AlF 3 (OH) ] + [Al(OH) 4 ] + [HF 2 ] + [OH ] + [NO 3 ] = [Al 3+ ] + 2[AlF 2+ ] + [AlF + 2 ] + [AlF(OH) + ] + 2[Al(OH) 2+ ] + [Al(OH) + 2 ] + [H + ] + [Na + ] + 2[Ca 2+ ] (22) The system of equations consists of 22 unknowns ([Na + ], [Ca 2+ ], [NO 3 ], [F ] initially and [Al] initially were constant) and 22 equations. [Na + ] and [Ca 2+ ] can be calculated from sodium added as NaOH, and sodium and calcium extracted from SPL sample. [NO 3 ] is determined by the quantity of Al(NO 3 ) 3 9H 2 O used for leaching. [F ] initially and [Al] initially can be calculated from the total amount of fluoride extracted from SPL or added to the solution initially, and the amount of aluminium (reduction in aluminium content in parent SPL sample plus Al 3+ from Al(NO 3 ) 3 9H 2 O leaching). The solution to the system of equations can be approximated by injecting stability constant data in MEDUSA software for chemical equilibrium diagrams [44]. Most stability constant data available in the literature for the Al F system have been obtained at a I = 0.5 and data can also be found for I = 0. Stability constant data at I = 0 are presented in Table 1 and were used for the model calculations. Ternary stability constant values for Al F OH were taken from Sanjuan and Michard [40]. Ternary stability constant values can also be calculated from binary Al F and Al OH stability constant data using the statistical approach presented in Martin s work [43]. Experimental data and statistical approximations were within the range of tentative (±0.2) to doubtful values (±1) range, which concur with the estimated reliabilities of binary stability constant data in the Al F system [41]. Table 1 Stability constant data for aluminium fluoride species at I = 0, log β (25 C) Reaction log β (25 C) 0.0 Al 3+ + F AlF 2+ (III) 7.0 Al F AlF 2 + (IV) 12.7 Al F AlF 3 (V) 16.8 Al F AlF 4 Al F AlF 5 2 Al F AlF 6 3 (VI) 19.4 (VII) 20.6 (VIII) 20.6 Al 3+ + F H + + AlF(OH) + (IX) # Al 3+ + F 2H + + AlF(OH) 2 (X) # Al 3+ + F 3H + + AlF(OH) 3 (XI) # Al F H + + AlF 2 (OH) (XII) # Al F 2H + + AlF 2 (OH) 2 (XIII) # Al F H + + AlF 3 (OH) (XIV) # Al 3+ H + + Al(OH) 2+ (XV) 5.0 Al 3+ 2H + + Al(OH) 2 + (XVI) 10.1 Al 3+ 3H + + Al(OH) 3 (XVII) 16.8 Al 3+ 4H + + Al(OH) 4 (XVIII) 23.0 H + + F HF (XIX) 3.17* H + + 2F HF 2 (XX) 3.67* H + + OH H 2 O (XXI) 14.0* Reviewed values by Nordstrom and May [41]; + Sanjuan and Michard [40]; *Martell [42], # statistical approach by Martin [43]. Once the concentration of the different species in solution has been determined, the accuracy of the model can be checked by recalculating I according to Eq. (23): I = 1 m i z 2 i (23) 2 i where m is the species concentration in mole per kilogram of solvent and z represents the species charge. As an example, at ph 5.25, Eq. (23) gave I = 0.89 for equilibria modelled with stability constant data at I = 0 and solution background resulting from water wash and Al 3+ leach with 0.34 M Al(NO 3 ) 3, according to procedure detailed in Section 4.1. Stability constant data can be iterated to match the calculated I value. Nevertheless, most experimental stability constant data in the literature for the Al F system at I > 0.5 are incomplete. Alternatively, calculations could be based on stability constant data at I = 0 and a correction for higher I values could be applied through activity coefficients; this approach is used by the MEDUSA software [44]. It must be pointed out that I did not change equilibria to a great extent and values can be considered reliable: at I = 0.89 AlF 6 3 fraction increased compared to values at I = 0 and AlF 2 (OH) fraction slightly dropped due to higher AlF 3 (OH) and AlF 2 (OH) 2 stability constants. Nevertheless, no definitive

5 186 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) Fig. 1. Modelled equilibrium distribution diagram of Al F species at 25 C, ionic strength (I) = 0.90, [Al 3+ ] initially = 0.29 M and ph 3.5: Al 3+ ( ), AlF 2+ ( ), AlF 2 + ( ), AlF 3 ( ), AlF 4 ( ), AlF 5 2 ( ) and AlF 6 3 ( ). quantitative conclusions can be extracted due to the uncertainty in the stability constant values for the ternary Al F OH complexes. I was recalculated by injecting the new equilibrium concentrations in Eq. (23). A value of 0.90 was obtained, which is in agreement with the 0.90 value assumed. Fig. 1 shows the modelled aluminium fluoride distribution diagram at ph 3.5 for [Al] initially = 0.29 M and varied [F ] initially. [F ] initially represents the total amount of fluoride in solution as shown in Eq. (21), and its value for SPL leachates is determined by a fluoride determination procedure (Section 2.3). At ph below 4 the presence of Al OH or ternary Al F OH complexes in solution was not predicted. This is in agreement with the formation of highly stable AlF 2 + for fluoride extraction by Al 3+ leaching. AlF 2 + is the major species in Al 3+ leachates for the typical [F ] initially value of 0.66 M. Treatment of Al 3+ leachates with water wash solution shifts equilibria towards more fluoridated Al F species, AlF 3, whereby [F ] initially = 0.78 M (Fig. 1). Nevertheless, both the metastability of AlF 3 saturated solutions and the high solubility of AlF 3 in aqueous solutions [33] suggest that no -AlF 3 or AlF 3 3H 2 O are expected to precipitate. The caustic nature of the water wash solution is expected to raise ph values when mixed with Al 3+ leachates. Fig. 2 shows the modelled effect of ph in the reaction between water wash and Al 3+ leachates: at ph between 4.5 and 7.0 AlF 2 OH fraction increases as ph increases and there is a drop in both AlF 3 and AlF 2 +, accordingly. At ph above 6.0, formation of negatively charged hydrolysed species, AlF 3 (OH), AlF 2 (OH) 2 and AlF(OH) 3 and, to a lesser extent highly fluoridated AlF 4, AlF 5 2, AlF 6 3 species, characterise the complexation equilibria. This is in agreement with AlF 3 (OH) and AlF 2 (OH) 2 being the dominant species in the ph 6 8 range reported by Sanjuan and Michard [40]. Mathematical modelling work shows the effect that ph has on the Al F complexation equilibria established as water wash and Al 3+ leach solutions are reacted and suggests ways whereby equilibria can be manipulated to selectively precipitate fluoride. In this work, the effect of ph on aluminium hydroxyfluoride hydrate precipitation has been investigated. Fig. 2. Modelled ph influence on the equilibrium distribution diagram of Al F species at 25 C, I = 0.90, [Al 3+ ] initially = 0.29 M and [F ] initially = 0.78 M: AlF 2+ ( ), AlF + 2 ( ), AlF 3 ( ), AlF 4 ( ), AlF 2 5 ( ), AlF 3 6 ( ), Al(OH) 4 ( ), AlF(OH) + ( ), AlF(OH) 2 ( ), AlF(OH) 3 ( ), AlF 2 (OH) (), AlF 2 (OH) 2 (+) and AlF 3 (OH) (). 4. Results and discussion 4.1. Leaching of SPL samples Fluoride content in the raw SPL sample (size fraction 1.18 mm) was found to be 15.8 wt.% by alkali fusion and carbon content upon ashing at 800 C for 5 h was wt.%. Water-washed SPL samples gave carbon contents between wt.% after one water wash and wt.% after a second water wash; carbon content increased up to wt.% after leaching with 200 ml of 0.34 M Al 3+ solution at 25 C for 24 h. Weight reduction was found to be between 23 and 24 wt.%, overall fluoride extractions between 76 and 86 mol% and residual fluoride in SPL between 3 and 4 wt.% after water wash and Al 3+ leach treatment. Fig. 3 shows the XRD pattern of the raw SPL calcined at 800 C for 5 h. High intensity peaks correspond to NaF and CaF 2 and lower intensity peaks to NaAl 11 O 17 and aluminium silicates. After water wash and Al 3+ leach, the intensities of NaF and CaF 2 peaks relative to Al 2 O 3 and NaAl 11 O 17 were reduced as shown in Fig. 4. Remaining peaks were assigned Fig. 3. XRD pattern of inorganic fraction from raw SPL recovered by ashing at 800 C for 5 h. Marked peaks correspond to NaAl 11 O 17 (*), CaF 2 (#) and NaF ( ).

6 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) Fig. 4. XRD pattern of inorganic fraction from SPL residue after water wash and Al 3+ leach recovered by ashing at 800 C for 5 h. Marked peaks correspond to NaAl 11 O 17 (*), CaF 2 (#), NaF ( ), Na 1.95 Al 1.95 Si 0.05 O 4 (ˆ), Al 2 O 3 (+) and LiAlSi 3 O 8 ( ). to Fe 2 O 3, aluminium metal, and sodium, calcium and lithium aluminosilicates, which are either present in SPL residue or result from SPL ashing. Presence of CaF 2 was in agreement with Besida s work [24]; the more refractory and finely divided CaF 2 partly remained within the SPL sample for leaching conditions and size fraction considered. Higher degrees of extraction are expected for smaller size fractions. The peak at θ corresponds either to aluminium metal or, more probably, to NaF from the decomposition of refractory fluoroaluminates upon ashing Precipitation of fluoride, effect of temperature Water wash solutions and Al 3+ leachates from the SPL sample were combined to shift the aluminium fluoride complexation equilibria towards AlF 3, and the temperature was adjusted between 50 C and the solutions boiling point of C to precipitate fluoride. The effect of temperature on fluoride recovery was followed by XRD, SEM EDS, TGA and F/Al determination. In this work, the term fluoride recovery refers to fluoride precipitated from the aqueous solutions whereas fluoride extraction refers to fluoride removed from the solid SPL sample. XRD results presented in Fig. 5 matched an AlF x (OH) 3x nh 2 O pattern for the whole temperature range studied and no precipitation of the hexagonal tungsten bronze (HTB) -form was observed. Detailed fluoride precipitation yields, fluoride to aluminium ratios (by alkali fusion and SEM EDS) and bound water content are shown in Table 2. Fluoride recovery from SPL leachates increased with temperature to a maximum of 19.1 mol% at 90 C, Fig. 5. XRD patterns on aluminium hydroxyfluoride product recovered at: (1) 50 C, (2) 70 C, (3) 90 C and (4) C. but decreased when solution was heated to boiling under reflux conditions (101.4 C). Surface sodium and calcium contents presented in Table 2 fluctuated between and wt.%, respectively, for temperatures below the boiling point and F/Al ratios remained close to two determined both by alkali fusion and SEM EDS. Bound water contents did not show a significant change in the range of temperature and, overall, the aluminium hydroxyfluoride product agreed well with a 1:2:1:1.4 Al:F:OH:H 2 O stoichiometry. Preliminary model compound work with Al 3+ leachates (from Na 3 AlF 6 and CaF 2 leaching with 0.34 M Al(NO 3 ) 3 9H 2 O) and 1 M NaF solution (mimicking water wash treatment) had yielded a Na 3 AlF 6 precipitate at 45 C and essentially NaAlF 4 at higher temperatures. In the same system, an aluminium hydroxyfluoride product precipitated at ph above 4.5 [34]. When treating actual SPL samples, water wash solution not only increased fluoride content in reaction media as in the model compound work, but also neutralised Al 3+ leachates from ph to This caused the precipitation of the aluminium hydroxyfluoride hydrate product. In this ph range, according to the mathematical model in Section 3, AlF 2 OH concentration increases with ph. The basicity of the water wash solution is largely caused by Na 2 CO 3 content being dissolved [23]. Experimentally, the ph of the water wash varied within the range. Precipitates were dried at 110 C for 4 h. Fig. 6a shows SEM micrographs for the submicron spherical particles for all temperature conditions evaluated, with a typical size of 300 nm at 70 and 90 C. Slightly larger particles were precipitated as temperature increased (375 nm at C, Fig. 6b). Table 2 Influence of temperature on fluoride recovery from Al 3+ leachates and water wash solution T ( C) F recovery (mol%) F/Al ratio (alkali fusion, mol/mol) F/Al ratio (EDS, mol/mol) Water (TGA, mol/formula) Water (EDS, mol/formula) Na (EDS, wt.%) Ca (EDS, wt.%)

7 188 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) Fig. 6. SEM images of (a) aluminium hydroxyfluoride product precipitated at 70 C and (b) C. Fig. 7. ph effect on fluoride recovery (, wt.%) and precipitates fluoride content (, wt.%) at 70 C Effect of ph on the precipitation of aluminium hydroxyfluoride AlF x (OH) 3x nh 2 O The effect of ph was further evaluated by neutralisation of the combined Al 3+ and water wash leachates with 2 M NaOH. Fig. 7 shows the effect of ph on fluoride recovery and the precipitates fluoride content. As ph increased up to 5.50, fluoride recovery from Al 3+ leachates reached a maximum at 79.6 mol%. Mathematical modelling work in Section 3 anticipated the increase in AlF 2 OH concentration with ph up to ph XRD patterns shown in Fig. 8 were consistent with an AlF 2 OH 1.4H 2 O product up to ph 5.5. F/Al ratios, bound Fig. 8. XRD patterns on aluminium hydroxyfluoride product recovered at: (1) ph 4.5, (2) ph 5.0, (3) ph 5.5, (4) ph 6.0, (5) ph 7.0. Na 5 Al 3 F 14 (*) and Na 3 AlF 6 -like precipitates (+) at ph above 5.5. water, surface sodium and calcium contents are detailed in Table 3. Increased sodium content from ph 5.75 resulted from the gradual co-precipitation of aluminium hydroxyfluoride and sodium fluoroaluminates that also showed in XRD results (first asana 5 AlF 14 -like product from ph , and also as Na 3 AlF 6 at ph 7.00). A patent describing an equivalent trend for the recovery of aluminium hydroxyfluoride hydrate from model cryolite Al 3+ leachates was filed by Bush and Gaydoski [45]. Nevertheless, precipitates recovered from actual SPL leachates reported here did not show increased F/Al ratios either by alkali fusion or SEM EDS as would have been expected if chiolite Table 3 Influence of ph on fluoride recovery from Al 3+ leachates and water wash solution ph F recovery (mol.%) F/Al ratio (alkali fusion, mol/mol) F/Al ratio (EDS, mol/mol) Na (EDS, wt.%) Ca (EDS, wt.%) Water (TGA, mol/formula) Water (EDS, mol/formula)

8 Na 5 Al 3 F 14 (4.7), cryolite Na 3 AlF 6 (6), CaF 2 or calcium hydroxyfluoride co-precipitated as is shown by increased sodium and calcium contents in Table 3. As ph is raised from 4.50, the mathematical model in Section 3 predicted an important increase in the concentration of hydrolysed AlF 3 OH and AlF 2 (OH) 2 species with respect to AlF 4 which accounts for sodium fluoroaluminates co-precipitation. However, the hydrolysed species act as the main fluoroaluminates precursors at near neutral ph conditions and explain the high degree of hydrolysis of the sodium fluoroaluminate co-products. Particle morphology is shown in Fig. 9; submicron spherical particles (300 nm typical size by SEM) of the aluminium hydroxyfluoride product are recovered at ph 5.00 (Fig. 9a). The inhomogeneous mixing conditions inherent to neutralisation with bases promoted primary nucleation while particle growth was minimal. Nonetheless, particle size slightly increased to 0.45 m for precipitates recovered at ph 5.50 (Fig. 9b). A strong dependence of growth rate on ph has been previously reported in other fluoride systems such as CaF 2 precipitation under fluidised-bed conditions [46]. The spherical shape was lost as ph was increased above 5.75 due to sodium fluoroaluminate co-precipitation (Fig. 9c). Particle growth should be promoted if the aluminium hydroxyfluoride product is to be converted into smelter grade AlF 3. Section 4.4 summarises the effect of calcination temperature on particle agglomeration and composition of the aluminium hydroxyfluoride product precipitated at ph Despite CaF 2 and sodium fluoroaluminates being absent in XRD patterns at ph (Fig. 8), higher ph samples yielded minor NaCaAl 2 F 9 peaks upon calcination at temperatures above 600 C. Surface sodium and calcium concentrations detailed in Table 3 anticipated these impurities as sodium and calcium contents increased from 2.52 and 0.82 wt.% at ph 5.00, to 7.37 and 1.91 wt.% at ph 5.75, respectively Calcination of the aluminium hydroxyfluoride product D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) The addition of AlF 3 to smelting cells is done in an analogous manner to Al 2 O 3 and, therefore, AlF 3 should have similar flow properties to smelter grade Al 2 O 3. Smelter grade AlF 3 is produced by treatment of gibbsite agglomerates with anhydrous HF at temperatures between 500 and 600 C in a fluidised-bed reactor [33]. The main challenge is to yield coarse AlF 3 particles by precipitation in aqueous solution from SPL leachates. Precipitation of an aluminium hydroxyfluoride hydrate product by neutralisation with NaOH up to ph 5.5 is reported in Sections 4.2 and 4.3. Precipitates showed submicron particle sizes after drying at 110 C for 4 h (Fig. 9). AlF 1.82 (OH) H 2 O obtained by neutralisation to ph 5.00 at 70 C was calcined at temperatures between 250 and 800 C to remove bound water and hydroxide, study particle agglomeration and fluoride losses. Calcination temperatures were chosen according to thermogravimetric analysis presented in Fig. 10. The TG curve showed an initial dehydration stage up to 274 C which corresponds with a wt.% mass loss (1.0 mol of bound water). Further dehydration took place Fig. 9. SEM images of aluminium hydroxyfluoride product recovered at 70 C: (a) ph 5.0, (b) ph 5.5 and with fluoroaluminate impurities at ph 7.0 (c). between 274 and 425 C, which accounted for the remaining bound water. A 0.67 wt.% loss was consistently observed at temperatures between 440 and 450 C when TG analysis was repeated. This is thought to be most probably related to the transition between the hydroxyfluoride crystalline structure to -AlF 3, initial release of hydroxyl groups and HF, and was investigated

9 190 D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) Fig. 10. Thermogravimetric analysis of AlF 1.82 (OH) H 2 O precipitated at 70 C and ph 5.0. by ex situ XRD (Fig. 11). Weight loss above 485 C is attributed to a hydrolytic reaction between hydroxyl groups and fluorine atoms to gradually decompose into Al 2 O 3 and HF. An overall reaction is by reaction (XXII): 3 AlF 2 (OH) AlF 3 + Al 2 O HF (XXII) Calcination at 250 C for 2 h partially removed water (15.9 wt.% loss in parent AlF 1.82 (OH) H 2 O sample) but particles retained both size and spherical shape as shown in Fig. 12a. In contrast with the aluminium hydroxyfluoride hydrate XRD pattern for the parent sample dried at 110 C(Fig. 11), the product calcined at 250 C was characteristic of a dehydrated Fig. 11. XRD patterns on aluminium hydroxyfluoride product calcined at: (1) 110 C, (2) 250 C, (3) 425 C, (4) 485 C, (5) 612 C and (6) 800 C. aluminium hydroxyfluoride. This crystal structure was retained up to 425 C. There was also a slight decrease in sharpness of the diffraction pattern between 110 and 425 C, probably due to a crystallinity loss as water was removed from the crystal structure. Fluoride loss at these low temperatures was minimal ( wt.%) but increased to 2.20 wt.% at 485 C. At this temperature pyrohydrolysis is thought to occur. Fluoride loss from aluminium fluoride and hydroxyfluorides has previously been reported to start at temperatures above 300 C for -AlF 1.95 (OH) H 2 O (from a (NH 4 ) 3 AlF 6 H 2 O precursor) or 377 C for - AlF 2.20 (OH) H 2 O from AlF 3 3H 2 O [47]. The loss of hydroxyl groups is a surface-generating process, as it can be Fig. 12. Calcination of aluminium hydroxyfluoride sample: (a) 250 C, (b) 425 C, (c) 485 C and (d) 612 C.

10 Table 4 Influence of calcination temperature on AlF 3 /Al 2 O 3 product D.F. Lisbona, K.M. Steel / Separation and Purification Technology 61 (2008) T ( C) Weight loss (wt.%) F loss (wt.%) F/Al ratio (pyro, mol/mol) F/Al ratio (EDS, mol/mol) Na (EDS, wt.%) Ca (EDS, wt.%) BET surface area (m 2 /g) seen from BET surface area values presented in Table 4. Surface area went through a maximum of m 2 /g for calcination at 485 C(Fig. 12c) and dropped as calcination temperature was increased and particles agglomerated (Fig. 12d). XRD patterns for calcination products above 485 up to 800 C confirmed an -AlF 3 phase; Al 2 O 3, on the other hand, remained amorphous and did not show on XRD analysis. Overall, samples became depleted in fluorine as pyrohydrolysis of AlF 3 progressed above 485 C and HF was absorbed in NaOH scrubbers. F/Al ratio for the calcined product was calculated by subtracting the amount of HF released from the initial fluoride content (F/Al ratio by pyrohydrolysis) and is presented in Table 4 along with surface F/Al ratio determined by SEM EDS. SEM EDS data, on the other hand, showed F/Al ratios that remained consistently at initial or higher values despite HF being released. High surface concentration of fluorine (2.38 F/Al ratio) compared with the bulk of sample (1.42 F/Al ratio) was observed for the calcination product at 800 C. It is thought that, as pyrohydrolysis occurs, HF is released and may fluoridate surface oxide groups to form AlF 3.Al 2 O 3 /AlF 3 product mixture is shown in Fig. 13. Prismatic particles gave surface F/Al ratios on SEM EDS consistently close to 3 whereas smaller dispersed particles gave lower values (2). These values were, nonetheless, above the overall F/Al ratio determined by measuring HF released. 5. Conclusions Fluoride can be extracted from spent pot-lining and precipitated as an aluminium hydroxyfluoride product, AlF x (OH) 3x nh 2 O(x 2 and n 1.4) using a water wash, Al 3+ wash, combining solutions and then controlling ph through addition of OH. Overall fluoride extractions reached mol% at ambient temperature, and overall fluoride recovery as the hydroxyfluoride product reached a 79.6 mol% optimum at ph 5.5. Higher ph values led to co-precipitation of hydrolysed sodium fluoroaluminates with chiolite and cryolitelike crystal structures. For the aluminium hydroxyfluoride product, ph affects crystal growth but not composition to a great extent. Calcination at temperatures above 485 C yielded a mixture of -AlF 3 and Al 2 O 3, as characterised by XRD analysis. Mathematical modelling of the Al F complexation equilibria highlighted the effect of ph on the equilibria and predicted both the formation of AlF 2 (OH) and hydrolysed fluoroaluminate species. Specifically, the modelling agreed with the precipitation of fluoride values as the aluminium hydroxyfluoride product AlF 2 (OH) 1.4H 2 O up to ph 5.5 and the co-precipitation of sodium fluoroaluminate impurities beyond that point. Inhomogeneities introduced by the addition of base promoted primary nucleation and restricted crystal growth. Homogeneous supersaturation conditions in the reaction media by controlled generation of hydroxyl ions may promote particle growth to yield an aluminium hydroxyfluoride hydrate product suitable for conversion into smelter grade AlF 3. Acknowledgements The authors gratefully acknowledge financial support from the University of Nottingham s Young Lecturer s Fund and thank Samantha Fagan at Anglesey Aluminium Ltd. for supplying the SPL and David Clift for XRD and SEM EDS analyses. References Fig. 13. Calcination of aluminium hydroxyfluoride sample at 800 C (2 h). [1] Y. Courbariaux, J. Chaouki, C. Guy, Update on spent potliners treatments: kinetics of cyanides destruction at high temperature, Industrial & Engineering Chemistry Research 43 (2004) [2] P.Y. Brisson, G. Soucy, M. Fafard, H. Darmstadt, G. Servant, Revisiting sodium and bath penetration in the carbon lining of aluminum electrolysis cell, Light Metals (2005) [3] L.P. Lossius, H.A. Oye, Melt penetration and chemical reactions in 16 industrial aluminum carbon cathodes, Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science 31 (2000)

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