Article pubs.acs.org/IECR
Leaching of Spent Pot-Lining with Aluminum Anodizing Wastewaters: Fluoride Extraction and Thermodynamic Modeling of Aqueous Speciation Diego F. Lisbona,†,‡ Christopher Somerfield,† and Karen M. Steel†,§,* †
School of Chemical and Environmental Engineering, The University of Nottingham, Nottingham, NG7 2RD, United Kingdom S Supporting Information *
ABSTRACT: The cotreatment of spent pot-lining (SPL), a high fluoride (20 wt %), and cyanide (up to 1 wt %) waste generated in aluminum smelting, and Al3+ wastewaters generated by the aluminum anodizing industry was identified in this work as a treatment alternative that holds the potential to virtually remove the need for purchasing chemical reagents while keeping carbon dioxide emissions to a minimum. The process proposed involves fluoride leaching with anodizing wastewaters and precipitation of AlF2OH by treatment with caustic waste from the aluminum anodizing industry. AlF2OH can be directly injected into fluidized beds that are used to convert Al(OH)3 to AlF3. Since AlF3 is used in primary aluminum smelting, aluminum and fluoride from aluminum industry wastes such as SPL and anodizing wastewaters are recovered in a form that can be readily used as a feedstock in the aluminum industry. In this paper, the results of SPL leaching studies using aluminum anodizing waste are presented together with a model of the solution equilibria, which has been used to interpret experimental observations. Mild leaching temperatures were used and no chemical reagents other than anodizing waste were necessary to extract the soluble fluoride.
1. INTRODUCTION Spent pot-lining (SPL) is a hazardous waste generated at the end-of-life of carbon cathodes in aluminum smelting electrolysis cells or pots. After 3−10 years,1 degradation of the cathode lining material starts affecting cell performance and must be replaced. Although the composition of the SPL thus generated varies, fluoride contents up to 20 wt % and cyanide contents up to 1 wt % are the main environmental concern.2 An estimated one million tonnes of SPL is produced worldwide every year.3 The uncontrolled dumping of SPL into rivers and the sea, or the open air storage of SPL in unlined landfill sites, was practiced until well into the 1990s,4 which has led to cases of severe pollution of groundwater, water courses, and land in the vicinity of aluminum smelters.5,6 Research into SPL treatment technologies started in the late 1970s, and gained pace after the USA EPA ruling on its hazardous waste status in the late 1980s, when carbon dioxide emissions and their effect on the Earth’s climate were not high on the environmental agenda. Most SPL treatment processes require the purchasing of chemical reagents for leaching, have high energy demands and/or do not fully exploit the potential of SPL as a source of fluoride and graphitic carbon. Only two treatment technologies have made it to industrial scale development, a thermal treatment7 and a low caustic leaching and liming process (LCL&L).8 Plans for tackling the SPL problem have in times leaned toward coprocessing SPL in the cement industry.9 However, there is no treatment technology widely accepted across the industry and controlled storage in secure landfill sites is being practiced by many aluminum smelters. Chemical leaching using acid, caustic, and Al3+ solutions has long been proposed as a valid option for treatment of SPL.10 © 2012 American Chemical Society
However, a truly environmentally sustainable approach for the treatment of SPL based on Al3+ and acid leaching followed by neutralization11−13 would require a sustainable source of Al3+, acid, and caustic solutions or the mechanism for the recovery of caustic and acid values having a low energy demand. Current industrial scale production of water-soluble Al3+ salts of mineral acids involves high temperature leaching of Al(OH)3 with concentrated acids followed by extensive evaporation to crystallize the salt. Low grade bauxite, nepheline syenite, alunite, or laterite could be leached with acid to directly produce Al3+ solutions. This approach has been used in the past, e.g. with nepheline syenite as the source of Al3+ by leaching with HCl or H2SO412. This source of Al3+ represented an advance in terms of process economy: the diluted acid treatment of a low grade aluminum ore to directly generate an Al3+ solution avoided the use of water-soluble Al3+ salts. However, it was still necessary to treat an ore with acid to extract aluminum values and then neutralize or add an extra chemical reagent for fluoride precipitation. Further solid waste was also generated in the acid treatment of nepheline syenite. Al3+, acid, and caustic solutions are nevertheless currently being generated within the wider aluminum industry and could be used for the treatment of SPL. Caustic and acid aluminum anodizing wastewaters have traditionally been neutralized with each other to form a water-rich (80−85%) amorphous sludge and an effluent which is disposed. The residue’s main component is aluminum hydroxide, with Na, Ca, and aluminum sulfates as minor components.14 There are 100 000 tonnes of Received: Revised: Accepted: Published: 8366
March 9, 2012 May 22, 2012 June 10, 2012 June 11, 2012 dx.doi.org/10.1021/ie3006353 | Ind. Eng. Chem. Res. 2012, 51, 8366−8377
Industrial & Engineering Chemistry Research
Article
anodizing sludge produced per year in the EU.14,15 This amount constitutes a continuous loss of sulfuric acid, sodium hydroxide, and aluminum values that has led to the development of numerous recovery processes.16−18 However, the neutralization of wastewaters to form anodizing mud that is taken to a landfill and a solution that is discharged to sewers is the most widespread practice.19 This may be due to the small size of most anodizing companies, which often makes the cost of implementing recovery technologies prohibitive.20 Co-treatment of SPL with Al3+, acid, and caustic values in anodizing waste for aluminum and fluoride recovery as an aluminum hydroxyfluoride product for SG AlF3 manufacture and reinjection in aluminum smelting may offer promise for the development of a viable technology. Co-treatment of SPL with aluminum anodizing wastewaters could overcome the environmental sustainability issues linked to the chemical leaching of SPL. In this work, a study into the leaching of SPL using first water to remove water-soluble fractions and then acid leaching with aluminum anodizing acid wastewaters is presented. The experimental observations have been interpreted using a thermodynamic model of the solution equilibria. In the development of SPL treatment processes based on chemical leaching, quantitatively linking theoretical thermodynamic equilibrium information for the relevant solution species to the observed trends in solution speciation, precipitate composition, or yields has not been carried out to date.
Filtrates obtained by leaching SPL in anodizing wastewater media (ca. 100 mL) together with the washing waters obtained when flushing any residual leachate solution from the residual solid sample were taken to a final volume of 250 mL using Milli-Q water. A 4 mL portion of the sample thus generated was placed in a polypropylene volumetric flask together with 2.00 g of orthoboric acid (ARISTAR grade) and 10 mL of concentrated HNO3 (70 wt/v %). Milli-Q water was added to dilute to a final volume of 100 mL. Sample flasks were equipped with a polypropylene lid and sonicated in an ultrasonic bath at 50 °C for 15 min to allow orthoboric acid dissolution prior to analysis. Blank solutions were prepared following the same procedure with the sample substituted by Milli-Q water. To avoid silicon contamination, reagents and samples were sourced from and kept in plastic bottles at all times. The ICP−AES instrument was periodically calibrated during sample analysis using one blank and two standard solutions for each element. The two reference solutions containing known concentrations of the elements were prepared by dissolving appropriate amounts of element standard solution in equivalent nitric and orthoboric acid media to the samples. For accuracy, each element’s concentration in the reference solutions was chosen so that the expected concentrations in SPL leachates were within the linear zone defined by these lower and upper limits. Reference standard concentrations and the elementspecific emission wavelengths were adjusted according to ICP− AES Optima-3300DV equipment software by Perkin-Elmer. Solid samples were characterized by powder X-ray diffraction (XRD) using a Hiltonbrooks X-ray powder diffraction set consisting of a Hiltonbrooks 3 kW generator model DG3 (40 kV, 20 mA), detector control module and step motor drive module. It was equipped with a Philips PW 1050 goniometer and a proportional detector. The X-ray generator consists of a Seifert copper long fine focus X-ray tube for Cu Kα radiation at 1.5406 Å, a Sietronics curved graphite monochromator, and a Ni filter to absorb Cu Kβ radiation. The solid samples were finely ground using a pestle and mortar and mounted in the sample holders. Data were collected by the Sietronics siehilt automation software and XRD diffraction results were analyzed using the traces v.3 scan processing software. Scanning electron microscopy (SEM) images were taken using a FEI Quanta 600. Energy-dispersive X-ray spectra (EDX) were collected with an EDS−EDAX Genesis instrument. Prior to analysis all samples were dried at 110 °C for 4 h and kept in a desiccator.
2. MATERIALS AND METHODS The f irst cut SPL sample supplied by Anglesey Aluminum Ltd. was crushed using a jaw crusher and dry sieved to recover a −1.18 mm (below 1.18 mm) size fraction. The −1.18 mm size fraction was characterized as a whole by multielemental ICP− AES analysis of fused samples, fluoride, and cyanide determination using fluoride and cyanide ion selective electrodes and a loss-on-ignition procedure for SPL samples (at 800 °C for 5 h in platinum ashing trays). All the chemical reagents used were of analytical reagent grade. Loss-on-ignition data were used as an indication of the samples’ carbon content. Mineralogical information on the sample’s major components was determined by X-ray diffraction analysis (XRD) on raw SPL samples and ashing residues. Elemental analysis was also performed on water-washed SPL samples, for which the watersoluble NaF and Na2CO3 fractions had been removed, as waterwashed SPL was used for acid Al3+ leaching tests. The particle size distribution within the −1.18 mm sample was determined by dry sieving using 850, 500, 250, 106, 75, 53, 38 μm sieve sizes and weighing the collected fractions. Elemental analysis of the size fractions was performed together with an assessment of the degree of mineral liberation by scanning electron microscopy−energy dispersive spectroscopy (SEM-EDS). The anodizing bath sample was supplied by Heywood Metal Finishers Ltd. and characterized by multielemental ICP−AES analysis and potentiometer titration, according to the procedure described in the literature.21 In a typical leaching test, 12 g of water-washed SPL (particle size below 1.18 mm) was placed in a PTFE beaker equipped with a PTFE-coated stirring bar and a polyethylene lid. A 100 mL sample of aluminum anodizing solution was added and stirred at 300 rpm and the selected temperature for 4 h. Leachates were filtered and reserved for characterization using a fluoride determination procedure for SPL samples developed by Besida12 and multielemental ICP−AES analysis.
3. MATHEMATICAL MODELING Solution speciation models have been constructed for a broad range of mixed electrolyte aqueous solutions in numerous chemical and geochemical systems of environmental and industrial importance to predict the effect of influencing factors such as temperature, pH, or the concentration of ionic species.22 With these same goals, a model of the solution equilibria established when SPL is treated with aluminum anodizing wastewaters has been developed and is provided in the Supporting Information that accompanies this publication. The model has been constructed from thermodynamic stability constant data available in the open literature and uses the Davies activity coefficient equation to account for the system’s nonideality. Although the Davies equation is only generally accepted for I < 0.1−0.2 M, it has been suggested that it is valid for I values up to 0.7−1 M.23,24 The validity of this approach 8367
dx.doi.org/10.1021/ie3006353 | Ind. Eng. Chem. Res. 2012, 51, 8366−8377
Industrial & Engineering Chemistry Research
Article
Table 1. Elemental Composition of Raw and Water-Washed SPL Samples (25° C, 4 h, particle size < 1.18 mm) component
C (wt %)
F− (wt %)
Na (wt %)
Al (wt %)
Ca (wt %)
Si (wt %)
Fe (mg/g)
K (mg/g)
Mg (ppm)
Ti (ppm)
Total CN− (ppm)
raw SPL water-washed SPL
62.8 68.5
16.3 13.6
15.2 7.4
4.3 5
2.1 2.5
1.2 1.1
1.93 2.01
1.55 1.48
367 402
120 153
105 84.2
Figure 1. Particle size analysis, carbon and fluoride contents of raw SPL (particle size
