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Ind. Eng. Chem. Res. 2004, 43, 190-196
Supercritical Fluid Extraction of Heavy Metals from Fly Ash Christof Kersch,*,†,‡ Geert F. Woerlee,‡ and Geert J. Witkamp† Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands, and FeyeCon Development & Implementation B.V., Rijnkade 17a, 1382 GS Weesp, The Netherlands
Currently expensive measures for disposal of fly ash residues from coal-fired power plants (CFP) or from municipal solid waste incinerators (MSWI) are required to prevent the pollution of groundwater with water leachable metals. For the reuse in road pavement as a substitute for sand- or cement-stabilized subbases, only a minimal leachability of metals is allowed by national legislation. Supercritical fluid extraction (SFE) is a promising method to lower the leachable metal content of fly ash to such an extent that the legal demands are obeyed. This paper presents results of SFE with SC-CO2 from fly ash of both CFP and MSWI for Zn, Pb, Mn, Cd, Cu, V, Sb, Ni, Mo, Cr, and Co. Extraction from 2 kg of fly ash was carried out with a 12 L rotating extraction vessel, a constant solvent flow, and various complexing extractants. Depending on the metal, extraction efficiencies from 0 to 98% were obtained. The positive influence of time and initial moisture content was demonstrated for SFE with Cyanex 302. SFE with equimolar mixtures of TBP-D2EHPA resulted in synergistic effects for Pb and Cu, while extraction of Zn was insignificantly low with TBP, D2EHPA, or a mixture of TBP-D2EHPA. Introduction The annual production of fly ash from coal-fired power plants (CFP) in Europe is about 38 million tons.1 A yearly production of about 250 million tons of municipal solid waste only in Europe has a potential to deliver about 12 million tons of fly ash from incinerators (MSWI).2,3 These fly ash residues are contaminated with water leachable metals. Contact with water leads to release of metals and pollution of the groundwater. Therefore, isolated and expensive disposal of the ash is required. A sustainable solution for the growing amount of this heavy metal-contaminated fly ash requires the reduction or entire removal of the toxic metals. For the reuse as a substitute for sand- or cement-stabilized subbases in road pavement, upper limits of metal leachability are set by national legislation. Supercritical fluid extraction (SFE) with CO2 has become a promising method for extraction of such metals from solids, as demonstrated, for example, by Smart et al. with samples of up to 0.5 g.4 Because the water leachable compounds apparently have a high mobility, a successful extraction from fly ash is expected. Together with suitable extractants, application of SC-CO2 is particularly attractive because it is an environmentally benign solvent with a high diffusivity and a low viscosity allowing easy penetration into the smallest pores of particles. With SFE, usage of organic solvents is prevented, while SCCO2 can be recycled easily. The extraction efficiency (EE) and selectivity depend on a number of process parameters and on the choice of extractant molecule. This study is aimed at the development of a largescale process to (i) remove heavy metals from fly ash and (ii) reduce metal leachability. The equipment, a revolving cylinder of 12 L, was designed to work at pressures up to 35 MPa. An earlier study5 showed the * To whom correspondence should be addressed. Fax: +31 15278 6975. E-mail:
[email protected]. † Delft University of Technology. ‡ FeyeCon Development & Implementation B.V..
effect of moisture content and different extractants on SFE from spiked sand. The preliminary study with CFP fly ash demonstrated a similar influence of moisture content and covered four different extractants with SFE. Similarly to conventional solvent extraction, different extractants and process times have an effect on the extraction. SFE experiments were carried out with MSWI fly ash using extractants such as bis-2,4,4trimethylpentyl)monothiophosphinic acid (Cyanex 302) and mixtures of tributyl phosphate (TBP) and di(2ethylhexyl)phosphoric acid (D2EHPA) to study the extraction of Zn, Pb, Cu, V, Mn, Cd, Sb, Ni, Mo, Cr, and Co. Up to 2 kg of fly ash was treated, and variation of extractant concentration and trends with process time were investigated. The metals were analyzed by high resolution inductively coupled plasma mass spectrometry (ICP-MS). The fly ash surface, crystalline phases, and metal concentrations before and after treatment were studied using (i) scanning electron microscopy (SEM), (ii) N2 physisorption and mercury intrusion porosimetry, (iii) X-ray diffraction (XRD), and (iv) X-ray fluorescence (XRF). Experimental Section Reagents. Two different fly ashes were used for the experiments: alkaline CFP fly ash originating from an electric power plant in The Netherlands (Amer, Geertruidenberg) and MSWI fly ash, which was supplied by a municipal waste incinerator (AVR, Rotterdam). SFE was carried out using the extractants Cyanex 302 (Cytec Inc.), Cyanex 923 (tertiary octyl- and hexyl-phosphine oxides, Cytec Inc.), D2EHTPA (bis(2-ethylhexyl)monothiophosphoric acid, Bayer), D2EHPA (Alfa), NaDDC (sodium diethyldithiocarbamate, Acros), and TBP (British Drug Houses). Analytical grade acids for digestion were purchased from Merck (HF, 40 wt %) and J.T Baker (HCl, 38 wt %; HNO3, 65 wt %), and a solution of 4 wt % boric acid (analytical grade H3BO3 from Merck) was prepared with ultrapure water.
10.1021/ie030114u CCC: $27.50 © 2004 American Chemical Society Published on Web 11/21/2003
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Apparatus and Procedures. A detailed process description and flow diagram with the CO2 cycle has been presented earlier.6 For preliminary experiments of both CFP fly ash and MSWI fly ash, a static vessel (0.4 L) was used. In the case of CFP fly ash, the preheated vessel (40 ( 10 °C) was first filled with extractant (5 mL) and second with CFP fly ash (100 g). For experiments with Cyanex 923, the ash was wetted with 9 mL of 1.7 M NaCl solution prior to SFE because the solvating extractant requires the presence of monovalent anions (such as Cl-). Subsequent pressurization of the vessel with SC-CO2 to 16 MPa was followed by 10 min () 0.17 h) of static extraction and 30 min () 0.5 h) of dynamic extraction using a CO2 flow rate of 1.5 kg/h. For the preliminary experiments with MSWI fly ash, the static vessel was filled with 50 g of ash. After pressurization of the vessel to 20 MPa, the SC-CO2 flow was adjusted to 5-10 kg/h at 40 °C. The extractant was mixed with methanol (equal weight %) to reduce the extractant’s viscosity and to facilitate a continuous addition into the CO2 flow. Furthermore, the presence of methanol in SC-CO2 changes the solvent polarity, facilitates dissolution of metal extractant, and thus enhances the extraction.5,7,8 After extraction of the MSWI fly ash for 1 h, rinsing with pure SC-CO2 for 0.5 h enabled the removal of the remaining extractant and its complexes. Extraction of 2 kg of MSWI fly ash was carried out using a tilted revolving cylindrical extraction vessel (12 L). After pressurization to 20 MPa, pure CO2 was passed through the vessel for about 0.5-1.0 h to heat the fly ash and to attain a constant flow. Metal extraction was commenced by continuously adding a mixture of equal amounts of complexing agent and methanol into the warm (40 °C) CO2 flow. The tilted extraction vessel contained a mixing device that maintained both continuous mixing and extraction when the vessel was rotated (50 rpm) throughout the process. After extraction (between 0.5 and 6 h), a rinsing step with pure CO2 removed the remaining complexing agent and metal complexes. Sample Handling and Homogenization. The CFP fly ash originated from a homogeneous 1500 kg batch. Ash samples (0.5 g) show little variation (
