Fly ash, extraction of metals

Ligand-Assisted Extraction of Metals from Fly Ash with Supercritical CO2 A Comparison with Extraction in Aqueous and Organic Solutions... 

Janssen-Jurkovicova et al. (1994) presented a conceptual model for CCB weathering, which breaks the process into four phases. Figure 2 illustrates their model for weathering of alkaline fly ash (i.e., fly ash producing alkaline water extracts defined as fly ash having Ca/S > 2.5 by Ainsworth Rai 1987), modified using observations of Warren Dudas (1984). In phase 1, oxides and soluble salts present on ash particle surfaces dissolve in contact with water. Hydrolysis of metal oxides (especially CaO) results in a rapid rise of pH to values of approxi-... 

Carbon-based sorbents are relatively new materials for the analysis of noble metal samples of different origin [78-84]. The separation and enrichment of palladium from water, fly ash, and road dust samples on oxidized carbon nanotubes (preconcentration factor of 165) [83] palladium from road dust samples on dithiocarbamate-coated fullerene Cso (sorption efficiency of 99.2 %) [78], and rhodium on multiwalled carbon nanotubes modified with polyacrylonitrile (preconcentration factor of 120) [80] are examples of the application of various carbon-based sorbents for extraction of noble metals from environmental samples. Sorption of Au(III) and Pd(ll) on hybrid material of multiwalled carbon nanotubes grafted with polypropylene amine dendrimers prior to their determination in food and environmental samples has recently been described [84]. Recent application of ion-imprinted polymers using various chelate complexes for SPE of noble metals such as Pt [85] and Pd [86] from environmental samples can be mentioned. Hydrophobic noble metal complexes undergo separation by extraction under cloud point extraction systems, for example, extraction of Pt, Pd, and Au with N, A-dihexyl-A -benzylthiourea-Triton X-114 from sea water and dust samples [87]. 

The potential of ultrasonic extraction for field-based extractions has been put into use in the industrial hygiene and environmental single-element analysis of, for example, lead from glass fibre filter ambient air samples [13,14] or from lead-based paint, urban dust and river sediment [15] hexavalent chromium from coal fly ash and paint chips [16] and strontium from river sediment [17]. Ultrasonic extraction has also proved effective as a prior step in multi-element determinations of heavy metals. 

The DPHSE technique has also been used for the determination of organic pollutants and metals in fly ash and coal, respectively. The extraction of dioxins [48,179] and PAHs [180] from fly ash was accomplished with toluene [48,180] or a toluene-methanol mixture [179], with results as good as or even better than those provided by Soxhiet extraction for 24 h. On the other hand, the extraction of major ash-forming elements (Fe, AI, Ca, Mg, Na and K) [148] and minor inorganic pollutants (As, Se and Hg) [46] from coal was done with acidified water. In the latter case, a combination of static and dynamic extraction was found to provide quantitative recoveries within a shorter time and with less dilution of the extracts than dynamic extraction alone. Acidified water is more corrosive than pure water, so the high temperatures required for extraction (150-200°C) call for the use of an extractor made of a material more corrosion-resistant than steel hastel-loid. However, in proportions above 4%, nitric acid — the acidulant most frequently added to the water — has been found to result in clogging of the system and the restrictor, so the recommended acid concentration is much lower than that. 

The fly ash formed in coal combustion also represents a disposal problem (see also Chap. 14). Although there are some uses such as in concrete and bricks, soil stabilization, soil conditioner, and landfill cover, more need to be found.24 Additional uses in wallboard, concrete blocks, and other construction materials should be possible. Other ashes include bottom ash and boiler slag. Experiments have been run on the recovery of iron, aluminum, and other metals from the ashes, but the processes may not be economical at this time. This could reduce the need to mine for these other materials. Coal-fired power plants produce over 100 million tons of ash annually in the United States. Coal fly ash is routinely mixed with water and put into settling basins. This process extracts some arsenic, cadmium, mercury, selenium, and strontium into water, which can then cause abnormalities in amphibians.25... 

Sequential Extraction of Heavy Metal Fractions in Sediments (52) Removai/Destruction of Dioxins from Fly Ash (53)... 

Part of the liquor removed by the filters is recycled and part is purged from the system via the effluent treatment plant. This uses lime to precipitate metals (extracted from the gases, the residual fly ash and the limestone), which are removed as a sludge. 

Supercritical (SC) CO2 is advmitageous for extaaction of metals from solid particles such as e.g. fly ash (5), because CO2 is a benign and cheap solvent for vdiich suitable extractants are available. SFE does not require any expensive drying of the final product. Evaporation of solvent CO2 by release of pressure results in both a solvent free matrix and a separate metal-extractant complex. 

The aim of this work was to sturfy the influence of water on metal extraction from the fly ash, for SFE at a scale of 2 kg solids. The final aim is to provide design parameters for a larger scale SFE unit. We concentrate on the main metals of municipal solid waste incinerator (MSWI) fly ash (Zn, Pb, Cu, Cd, and Mn, Figure 1). Since die ZnO content of the studied ash was high, Zn was chosen as model compound. Three types of eiqieriments were performed ... 

A stepwise extraction mechanism is shown in Figure 3. The fly ash contains a number of metals, as presented in Figure 1. Exemplarily the exhaction of zinc(Il) ion Zr ) with the acidic chelating i ent fflT=Cyanex 302 is described. 

The fly ash that was used for all experiments came fixim a single homogenous 200 kg batch. This batch showed variation in metal concentiation of up to 15% (sample size 0.Sg). To compensate for this variation a homogenous sample (0.5 g) of untreated ash was taken prior to each extraction and analysed. The variation of those samples was ftien about 5%. An average of the analyses served as a reference for eveiy set of extraction experiments, allowing a more reliable expression of the extraction efficiency. For the studies of wet ashes, homogeneous humidification was desired. The ash was mixed with neutral water (pH=7) prior to SFE, an excess of water was removed after sedimentation, and the ash was dried to a humidity of 38wt.%. 

Liquid phases were analysed by ICP-MS for the determination of EE after both aqueous leaching and solvent extraction. Based on these analyses, the amount of extracted metals per mass of dry initial fly ash was related to the initial metal concentration of the applied fly ash as,... 

At the liquid-SC CO2 inter ce, a constant, and sufficient high concentration of extractant Cyanex 302 is assumed. Concentration effects of generated metal-complexes are assumed to be negligible, due to diffusion coefficient of solutes in supercritical fluids of about 10" m /s (17), which is approximately 2 orders of magnitude fester than in the aqueous phase. A continuous flow of solvent during extraction even reduces surfece effects, due to both continuous supply of exbactant and continuous removal of metal-extractant complex. Thus, mass transfer is a limiting factor at the liquid-SC CO2 interface, as studied previously by Tai et al. (18). Furdier research is required to study a possible impact on the overall extraction of SFE from humid MSWI fly ash. 

For a good extraction of the studied metals from municipal waste fly ash, SFE might be enhanced by a pre-leaching step. Leaching efficiency by aqueous acidic solutions obeys a logarifemic power rate law model. 

Dominici F, Peng RD, Bell ML et al (2006) Fine particulate air pollution and hospital admission for cardiovascular and respiratory diseases. JAMA 295 1127-1134 Dreher KL, Jaskot RH, Lehmann JR et al (1997) Soluble transition metals mediate residual oil fly ash induced acute lung injury. J Toxicol Environ Health 50 285-305 Dye JA, Lehmann JR, McGee JK et al (2001) Acute pulmonary toxicity of particulate matter filter extracts in rats coherence with epidemiologic studies in Utah Valley residents. Environ Health Perspect 109(Suppl 3) 395-403... 

Information on the historical and spatial distributions of SCP, IAS and associated pollutants (trace metals, sulphur, PAHs, etc.) can be obtained or inferred from interpreting particle concentration data or transforming these data into particle accumulation rate, or inventory form. However, further information is available from fly-ash particles, extracted from lake sediments, when they are allocated to their fuel-type such that the total concentration is sub-divided into fuel-classes. 

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