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Hydrothermal conversion of chrysotile asbestos using near supercritical conditions
Journal of Hazardous Materials 179 (2010) 926–932
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Hydrothermal conversion of chrysotile asbestos using near supercritical conditions Kalliopi Anastasiadou, Dimosthenis Axiotis, Evangelos Gidarakos ∗ Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, P.C. 73100, Greece
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 22 March 2010 Accepted 22 March 2010 Available online 27 March 2010 Keywords: Chrysotile asbestos Mineralogical conversion Hydrothermal treatment Supercritical conditions
a b s t r a c t The present research investigates, develops and evaluates the transformation of chrysotile asbestos into a non-hazardous material, such as forsterite, using an economically viable and safe method. The aim of this study is to convert fibrous chrysotile asbestos into an anhydrous magnesium silicate with a nonhazardous lamellar morphology using supercritical steam. The treatment method is characterized as hydrothermal in a temperature and pressure range of 300–700 ◦ C and 1.75–5.80 MPa, respectively. Small amounts of asbestos (2.5 g) were treated in each experiment. Deionised water was used as the treatment solution. The treatment duration varied from approximately 1–5 h. Additional experiments took place using solutions of distilled water and small amounts of acetic acid, with the aim of attaining optimal treatment conditions. Crystal phases of the samples were determined by X-ray diffraction (XRD). The main phases present in the treated samples were forsterite, enstatite, and chrysotile asbestos. Lizardite and periclase were also found. The morphology of the treated chrysotile asbestos fibers was identified by scanning electron microscope (SEM). The fibrous form of chrysotile asbestos was converted into nonfibrous form of forsterite. In fact, none of the fibrous-needle-like morphology, with length equal to or greater than 5 m and diameter less than 3 m, which was responsible for the toxicity of the original material, was visible in the solid phase. The dissolution of magnesium from chrysotile asbestos was measured using volumetric determination by titration with EDTA. Leaching of magnesium into the liquid phase was observed. Clearly, the highest concentrations of dissolved magnesium are observed after hydrothermal treatment of chrysotile asbestos using acetic acid 1% (8.4–14.6%). Lowest concentrations of dissolved magnesium are obtained after hydrothermal treatment of chrysotile asbestos without using additives. Observing the results of the hydrothermal treatment using additives, the mineralogical conversion does not depend on the presence of a small quantity of weak organic acid (<1%). The addition of acetic acid 1% during hydrothermal treatment did not involve changes in the conditions of the chrysotile asbestos’ mineral conversion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Asbestos has been characterized universally, and by Greek legislation in particular, as a harmful and hazardous material for human health (i.e. a toxic and carcinogenic substance). A key hazard is the release of fibers from products that contain asbestos [1]. The fibers penetrate respiratory systems and cause asbestosis and carcinogenesis. However, due to its resilient characteristics, asbestos has been widely utilized in industry and currently has over 3000 uses. Thus, today it can be found in a vast array of products (construction materials, fire retardants, binders, etc.) in varying proportions, ranging from 5% to 100%. Chrysotile asbestos, or white asbestos, is the form of asbestos most commonly used (95%) [2].
∗ Corresponding author. Tel.: +30 2821037789; fax: +30 2821037850. E-mail address: [email protected] (E. Gidarakos). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.094
Chrysotile asbestos is a 1:1 trioctahedral phyllosilicate with the chemical formula Mg6 Si4 O10 (OH)8 with some Mg substituted by Fe2+ . This 1:1 phyllosilicate comprises one Mg-octahedral sheet bonded to a Si-tetrahedral sheet. An ideal Mg-octahedral sheet has a lateral dimension of b ≈ 9.43 Å and an ideal Si-tetrahedral sheet has a lateral dimension of b ≈ 9.1 Å [3]. These dimensional differences seem to cause a lateral misfit between the octahedral and tetrahedral sheets along the X and Y axes [3,4]. To compensate partially for this misfit, the larger octahedral Mg sheet curls over the smaller tetrahedral silica sheet, thus generating chrysotile’s tubular morphology. The outer surface of the octahedral magnesian sheet exposes hydroxyl ions (OH− ). Chrysotile’s tubular structure produces four reactive sites: (1) an outer hydroxyl sheet; (2) the ends of the fiber; (3) the exposed edges of the curled sheet; and (4) the interior of the hollow central channel [5]. Fig. 1 presents chrysotile’s tubular structure, curled sheets developed from lateral misfits and cross-section showing Mgoctahedral sheet overlying Si-tetrahedral sheet.
K. Anastasiadou et al. / Journal of Hazardous Materials 179 (2010) 926–932
Fig. 1. Chrysotile’s curled sheets developed from lateral misfits and cross-section showing Mg-octahedral sheet overlying Si-tetrahedral sheet [6].
Considering the nature of asbestos products as time passes, it is certain that, at some point, asbestos fibers will be released. Once an asbestos product loses its characteristics, or has been abandoned, or is due to be abandoned, it is deemed Asbestos Containing Material—ACW. According to current legislation, ACW’s must be removed and properly managed in accordance with safety regulations. The most common way is the transportation and rejection of ACW’s in suitable landfills and disposal of this kind is forbidden without pre-treatment. Few treatment technologies of ACW’s have been developed, such as chemical, thermal, thermo-chemical, mechanical and others. The scope of most chemical processes is to cover the surface of the asbestos fibers with organic or inorganic substances, aiming to hinder those elements that are believed to react with human cells
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and cause fibroses and cancer [7,8]. In addition, strong acids can be used to destroy the fiber structure, but unfortunately such acids are considered to be more hazardous than chrysotile asbestos. According to literature the most popular natural treatment is the thermal conversion of chrysotile asbestos into non-hazardous substances at temperatures ranging between 900 and 1200 ◦ C [9]. Some efforts have been made to reduce the operational temperature and to lower the cost of this technique [10]. Other physical treatments worth mentioning are the conversion of chrysotile asbestos using the radiation of microwaves and mechanical methods [11–14]. The basic parameters for the material’s physical treatment are (i) temperature and (ii) duration. Decreasing the operational temperature would be a significant step towards the economic viability of these methods, considering that the mineralogical conversion of chrysotile asbestos occurs at temperatures >700 ◦ C [15]. Water in supercritical conditions is used for waste treatment, but mainly for the treatment of organic compounds [16]. Although the dehydration of chrysotile asbestos under hydrothermal conditions is described in the literature [17], Sigon et al. [18] for the first time converted asbestos fiber into non-hazardous material using as the only “reagent”, water in supercritical conditions. The present research investigates, develops and evaluates the conversion of chrysotile asbestos into a non-hazardous material, such as forsterite under economically viable and safe conditions using supercritical steam. The aim of this research was to decrease the operational temperature as much as possible using this hydrothermal procedure. For this reason the experimental temperature was kept below 700 ◦ C. The treatment method is characterized as hydrothermal in a temperature and pressure range of <700 ◦ C and <8.00 MPa, respectively. During the hydrothermal treatment the pressures and the temperatures applied were in the supercritical steam region, according to the diagram present in Fig. 2. Small amounts of asbestos (2.5 g) were treated in each experiment. Deionised water was used as the treatment solution. The treatment duration varied from approximately 1 to 5 h. Some additional experiments took place using solutions of deionised water and small amounts of chemicals, aiming to attain optimal treatment conditions. The crystal phases of the samples were determined by X-ray diffraction (XRD). The presence of chrysotile asbestos fibers was identified by scanning electron microscope (SEM). The dissolution of magnesium from chrysotile asbestos was measured using volumetric determination by titration with EDTA.
Fig. 2. Supercritical conditions and experimental area [16].
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Fig. 3. Experimental equipment used for the hydrothermal treatment of chrysotile asbestos [16].
2. Methods and materials Chrysotile asbestos was supplied by the Asbestos Mines of Northern Greece [19]. The mineral particle size was approximately 1 mm after grinding. Wet grinding was carried out for 15 min on chrysotile asbestos samples using a planetary grinding mill. The mill had a rotating speed of 180 rpm. The chrysotile asbestos samples were pulverized using acetone to prevent the diffusion of the fibers and to ensure the health protection of the researchers during the placing of these samples in the sample holder for XRD measurement. Acetone was used as dispersing liquid because it reduces grinding times and consequently the risks of amorphization [20,14]. The entire procedure of sampling preparation took place in a closed environment in a glove box under negative pressure. The mineralogical phase analysis of the asbestos samples was conducted before and after treatment. The test sample was placed in a holder which was then placed in a Rigaku XRD machine with a copper target ( = 15.406 nm). A diffraction angle of between 4◦ and 70◦ (2), and a scanning rate of 2◦ /min was utilized to analyze the crystal phases of the mineral samples.
X-ray diffraction showed that the chrysotile asbestos samples before treatment also contained a small amount of dolomite and calcium sulfate. A SEM was also used and the suspect fibres were examinated with an energy dispersive X-ray for their composition [19]. EDS analysis of chrysotile asbestos samples before treatment revealed that the major elements were SiO2 (57.1%), MgO (29%) and CaO (2.8%). An S2 Ranger EDS (Bruker Ltd.) was then used to qualitatively analyze the chemical composition of the asbestos samples. The ignition crystallic water content was estimated to be 14% [16]. Hydrothermal treatment is a simple process. An amount of chrysotile asbestos with deionised water is imported into a 4740 Haynes Alloy 230 high pressure reactor (manufactured by Parr Instruments Ltd.). The reactor is heated by a controller until the desired operational temperature is achieved. The temperature is monitored and adjusted by the controller while the measured pressure depends on the water volume inserted into the reactor. The volume of the autoclave reactor is 70 ml. The deionised water is transformed into supercritical steam. The reaction between the supercritical steam and chrysotile asbestos occurs in a suitable environment by means of hydrolysis.
K. Anastasiadou et al. / Journal of Hazardous Materials 179 (2010) 926–932 Table 1 Operational parameters of performed experiments during hydrothermal treatment. Temperature (◦ C)
Time (h)
Pressure (MPa)
DW (mL)
Additives
500 580 600 650 670 690 700 700 700 700 700 700 700 700 300 500 600 650
3 3 3 3 3 3 1 2 3 4 3 3 3 5 3 3 3 3
1.75 1.80 1.85 1.90 2.00 2.10 2.40 2.60 2.25 3.00 4.70 5.80 7.40 2.15 1.80 2.20 2.90 2.50
20 20 20 20 20 20 20 20 20 20 23 24 25 20 20 20 20 20
– – – – – – – – – – – – – – Ac. Acid 1% Ac. Acid 1% Ac. Acid 1% Ac. Acid 1%
Fig. 3 presents the experimental equipment used for the hydrothermal treatment of chrysotile asbestos. The supercritical steam penetrates the braided bands of the asbestos, determines a hydrolysis process that separates the fibres from each other, degrades the silicon oxide tetrahedron (SiO2 ), which becomes a solution, and modifies the structure with fibrous morphology into crystals of forsterite. The reaction that comes about can be represented by the following stoichiometry [18]: 2Mg3 (Si2 O5 )(OH)4 → 3Mg2 SiO4 + 2SiO2(aq) + 4H2 O
(1)
Temperature and pressure in the reactor are monitored by the controller. The time of operation depends mainly on the amount of the sample and the operational conditions (temperature and pressure). After completion of the process, the mixture (sample + deionised water) is cooled to room temperature, centrifuged and filtered, using a MCE filter, for the separation of the solid (treated chrysotile asbestos) and liquid phases (deionised water enriched with Si4+ and Mg2+ . Table 1 presents the operational parameters of the experiments performed. It is well known that acids destroy the fiber structure of chrysotile asbestos. According to the literature various acids such as HCl, oxalic acid, phosphoric acid are used for the destruction of the crystal body of chrysotile asbestos [21–23]. To make the process more effective and attractive, additives such as acetic acid in different concentrations of less than 1% were used. The concentration of 1% acetic acid was chosen mainly to avoid the corrosion of the reactor and to maintain acidity levels as low as possible. Indeed, while HCl is considered to be the best solution for chemical treatment, nevertheless it is a very strong acid. A weak organic acid such as acetic acid was chosen because it appears to effectively corrode the layer of brucite of the chrysotile asbestos fiber. The concentration was decreased in order to maintain the environmentally friendly character of the hydrothermal treatment. The morphology of the fibrous bodies was characterized using the SEM technique, which, because of its high resolutive power, allows the detection of even the smallest fibers. The analysis of the solid residue was carried out by means of Xray diffractometry. The total amount of Mg2+ removed during the hydrothermal treatment was determined by titration with disodium-ethylenediaminetetraacetic acid (EDTA) as described by Harris [24]. Each sample was repeated three times and averaged. The results are presented in the next section.
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3. Results The thermal decomposition of chrysotile asbestos follows a two-stage sequence of dehydroxylation and breakdown, and the mechanism has been researched in several studies [17]. The dehydroxylation of chrysotile asbestos takes place in the temperature range 600–780 ◦ C and at 800–850 ◦ C the dehydroxylated noncrystalline residue is said to recrystallize to give anhydrous silicate, such as forsterite and silica in amorphous form [20] or forsterite and enstatite [25]. Gualtieri and Tartaglia observed during thermal decomposition of asbestos that a complete re-crystallisation to forsterite and enstatite is obtained at 1100 ◦ C after 1 h [25]. Fig. 4 presents the spectrums produced using the XRD technique on the chrysotile asbestos samples before and after hydrothermal treatment at several operational temperatures and durations. The main phases present in the treated samples were forsterite, enstatite, and chrysotile asbestos. Lizardite and periclase were also found. During hydrothermal treatment, conversion of chrysotile asbestos into forsterite is observed at 650 ◦ C and the eventual absence of chrysotile asbestos is achieved at 690 ◦ C in 3 h. Total mineralogical conversion of chrysotile asbestos into forsterite is also achieved at a temperature of 700 ◦ C in 1 h. As can be observed in Fig. 4, in the sample hydrolyzed with supercritical steam at 700 ◦ C for 1 h and analyzed using the XRD technique, the presence of chrysotile asbestos was not detected. If present, it was in concentrations lower than the instrument’s lowest detectable limit. Time duration of up to 5 h does not appear to change the final result [19]. The total absence of fibers, that demonstrates the effectiveness of the innovative treatment proposed, was demonstrated by the SEM which, even at greater resolutions, did not detect the presence of fibrous solid. Fig. 5(a) shows an image of the sample of asbestos before the hydrothermal hydrolysis treatment while (b) shows an image of the sample after hydrothermal hydrolysis treatment with supercritical steam. The SEM photos of the treated sample (Fig. 5) taken at higher magnification (1000×) show that they had been completely transformed as a result of the treatment. In fact, none of the fibrousneedle-like morphology, with length equal to or greater than 5 m and diameter less than 3 m, which was responsible for the toxicity of the original material, is visible in the solid. Observing the results of the hydrothermal treatment using additives, the mineralogical conversion does not depend on the presence of a small quantity of weak organic acid (<1%). The addition of acetic acid 1% during hydrothermal treatment did not involve changes in the conditions of the chrysotile asbestos’ mineral conversion. However, it did cause the release of important quantities of magnesium ions, more than those released by the dissolution of chrysotile asbestos under basic conditions [5]. These magnesium ions are released from the brucite layer of the chrysotile asbestos fibers. Previous research has demonstrated that dissolution in 5% acetic acid can considerably decrease the chrysotile asbestos crystal structure (a reduction of 30–90%) [26]. Unfortunately, as shown in Fig. 6, it is not sufficient to completely eliminate the chrysotile phase at lower temperatures and so improve the hydrothermal treatment process by converting chrysotile asbestos into forsterite. However, the dissolution of chrysotile asbestos during the hydrothermal treatment is improved by the use of acids, and the amount of Mg released into the solution is increased. The dissolution of chrysotile asbestos in water can be expressed as the reaction [5]: Mg3 Si2 O5 (OH)4 + 5H2 O → 3Mg3+ + 6OH− + 2H4 SiO4
(2)
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Fig. 4. Spectrum produced using the XRD technique on the sample of asbestos before and after hydrothermal treatment at several temperatures and durations.
According to Choi and Smith [5] concentrations of Mg2+ and OH− ions are increased at the initial stage of the reaction. The concentration of OH− increases logarithmically with time at room temperature. However, after several hours (>5 h) the pH value tends to decrease. This reduction is probably created as a result of the formation of MgOH+ or re-absorption of Mg(H2 O)6 2+ and MgOH+ ions on the negatively charged silicon layer that has been revealed due to the dissolution of the brucite layer. Table 2 shows the amounts of Mg2+ leached from chrysotile asbestos into the solution during the hydrothermal treatment with and without acetic acid, determined by titration with EDTA. Clearly, the highest concentrations of dissolved Mg are observed after hydrothermal treatment of chrysotile asbestos using acetic acid 1% (8.4–14.6%). Lowest concentrations of dissolved Mg are obtained after hydrothermal treatment of chrysotile asbestos without using additives (<1%). This is in total agreement with previous research that the dissolution of chrysotile asbestos and the release
of Mg2+ increase with temperature and acid concentration [20]. The proportion and rate of chrysotile asbestos dissolution depends on the operational temperature, the concentration of the acid used, and on the type and origin of the mineral. The present innovative method provides for operating at relatively low temperatures (690–700 ◦ C) compared to 900 ◦ C and above for the traditional thermal treatments, while maintaining relatively low pressures (1.8–5.8 MPa). The hydrolysis time period, and thus the duration of the transformation process for obtaining a final product without any toxic/noxious residuals, is less than 3 h, according to the operative conditions chosen. It must be pointed out that, in contrast to other chemical treatments, the hydrothermal process of hydrolysis in near supercritical steam does not require the use of a strong chemical reagent or a substance with a high impact on the environment. The energy savings gained by using subcritical steam are significant due to lower temperature needed for the conversion
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Fig. 5. Pictures from a scanning electronic microscope of a sample of asbestos containing fibrous chrysotile asbestos before (a) and after (b) the hydrothermal treatment, enlarged 1000 times.
Fig. 6. Spectrum produced using the XRD technique on the sample of asbestos before and after hydrothermal treatment using acetic acid 1% at several temperatures.
Table 2 Percent of Mg leached into several solutions of hydrothermal treatment. Temperature (◦ C)
Time (h)
Pressure (MPa)
DW (mL)
Additives
Percent Mg2+ removed
500 580 600 650 670 690 700 300 500 600 650
3 3 3 3 3 3 3 3 3 3 3
1.75 1.80 1.85 1.90 2.00 2.10 2.25 1.80 2.20 2.90 2.50
20 20 20 20 20 20 20 20 20 20 20
– – – – – – – Ac. Acid 1% Ac. Acid 1% Ac. Acid 1% Ac. Acid 1%
<1% <1% <1% <1% 1% 1.4% 1.6% 8.4% 9.5% 12.4% 14.6%
of chrysotile asbestos into forsterite. The temperature needed for hydrothermal process (700 ◦ C) is lower than the temperature for conventional thermal treatment (1200 ◦ C). In other words, energy consumption of conventional thermal treatment (based on artificial
reproduction of the temperature conditions necessary for the mineralogical transformation of asbestos minerals) is much higher than the energy consumption of the hydrothermal process for the same treatment time (1 h). Concerning laboratory scale observations,
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a common furnace, which can be used for conventional thermal treatment (1200 ◦ C) of chrysotile asbestos into forsterite, requires from 1200 to 6000 W, while the controller of the hydrothermal process only needs 400 W [16]. This means that for the same treatment time (heating time and operating time) the furnace consumes more energy than the controller used for the hydrothermal process. Of course, these results apply to laboratory scale treatment and not industrial application. 4. Conclusions The environmental, energy and productive advantages that would be obtained with the new hydrothermal treatment of chrysotile asbestos in supercritical steam are multiple and unquestionable. The advantages and benefits can thus be summed up as follows: • the treatment procedure allows work to be carried out in a confined space, keeping the risk of environmental contamination to a minimum, • the better solvent properties of the water in near supercritical conditions improve the solubility characteristics of the solid materials of asbestos, accelerating the penetration processes of the “reagent” fluid, • no hazardous additives are used in comparison to other processes based on etching, • the process is characterized by low energy consumption in comparison to other treatment for asbestos such as vitrification, • chrysotile asbestos is completely neutralized and converted into forsterite (a raw mineral for production of several materials). The use of acetic acid in the experiments, in low concentrations, did not improve the hydrothermal treatment process by converting chrysotile asbestos into forsterite but there was an increase in the amount of Mg released from the chrysotile asbestos structure into the solution. References [1] E. Gidarakos, K. Anastasiadou, E. Koumantakis, N. Stappas, Investigative studies for the use of an inactive asbestos mine as a disposal site for asbestos wastes, J. Hazard. Mater. A 153 (2008) 955–965. [2] Directive 1999/77/EC, Adapting to technical progress for the sixth time—Annex I to Council Directive 76/769/EEC on the approximation of the laws, regulations and administrative provisions of the Member States relating to restrictions on the marketing and use of certain dangerous substances and preparations (asbestos) (1999). [3] F.J. Wicks, D.S. O’Hanley, Serpentine minerals: structure and petrology, Rev. Miner. Geochem. 19 (1988) 91–159.
[4] D.R. Veblen, A.G. Wylie, Mineralogy of amphiboles and 1:1 layer silicates, health effects of mineral dusts, Rev. Mineral Geochem. 28 (1993) 61–137. [5] I. Choi, R. Smith, Kinetic study of dissolution of asbestos fibers in water, J. Colloid Interface Sci. 40 (2) (1972) 253–262. [6] T. Sugama, R. Sabatini, L. Petrakis, Decomposition of chrysotile asbestos by fluorosulfonic acid, Ind. Eng. Chem. Res. 37 (1998) 79–88. [7] M. Gulumian, An update on the detoxification processes for silica particles and asbestos fibers: successes and limitations, J. Toxic. Environ. Health. Part B 8 (2005) 453–483. [8] M. Klockars, M. Hedenborg, E. Vanhala, Effect of two particle surface-modifying agents, polyvinylpyridine-N-oxide and carboxymethylcellulose, on the quartz and asbestos mineral fiber-induced production of reactive oxygen metabolites by human polymorphonuclear leukocytes, Arch. Environ. Health. 45 (1990) 8–14. [9] C.J. Martin, The thermal decomposition of chrysotile, Miner. Mag. 41 (1977) 453–459. [10] M. Fujishige, R. Sato, A. Kuribara, I. Karasawa, A. Kojima, CaCl2 addition effect and melt formation in low-temperature decomposition of chrysotile with CaCO3 , J. Ceram. Soc. Jpn. 114 (10) (2006) 844–848. [11] T. Zaremba, M. Peszko, Investigation of the thermal modification of asbestos wastes for potential use in ceramic formulation, J. Therm. Anal. Calorim. 92 (2008) 873–877. [12] C. Leonelli, P. Veronesi, D.N. Boccaccini, M.R. Rivasi, L. Barbieri, F. Andreola, I. Lancellotti, D. Rabitti, G.C. Pellacani, Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics, J. Hazard. Mater. B135 (2006) 149–155. [13] D.N. Boccaccini, C. Leonelli, M.R. Rivasi, M. Romagnoli, P. Veronesi, G.C. Pellacani, A.R. Boccaccini, Recycling of microwave inertised asbestos containing waste in refractory materials, J. Eur. Soc. 27 (2007) 1855–1858. [14] P. Plescia, D. Gizzi, S. Benedetti, L. Camilucci, C. Fanizza, P. Simone, F. Paglietti, Mechanochemical treatment to recycling asbestos containing waste, Waste Manage. 23 (2003) 209–218. [15] M. Cheshire, N. Guven, Conversion of chrysotile to a magnesian smectite, Clay Miner. 53 (2) (2005) 155–161. [16] D. Axiotis, Hydrothermal treatment of asbestos, Diploma thesis. Technical University of Crete, Greece, 2009. [17] M.C. Ball, H.F.W. Taylor, The dehydration of chrysotile in air and under hydrothermal conditions, Min. Mag. 33 (1963) 467–482. [18] F. Sigon, A. Servida, S. Grassi, Process for the hydrothermal treatment of asbestos and/or asbestos containing materials in supercritical water and relative production plant, US Patent, 2006/0149118 A1. [19] K. Anastasiadou, E. Gidarakos, Toxicity evaluation for the broad area of the asbestos mine of Northern Greece, J. Hazard. Mater. A 139 (2006) 9–18. [20] A. Cattaneo, Z.A.F. Gualtieri, Z.G. Artioli, Kinetic study of the dehydroxylation of chrysotile asbestos with temperature by in situ XRPD, Phys. Chem. Miner. 30 (2003) 177–183. [21] F. Turci, M. Tomatis, S. Mantegna, G. Cravotto, B. Fubini, A new approach to the decontamination of asbestos-polluted waters by treatment with oxalic acid under power ultrasound, Ultrason. Sonochem. 15 (4) (2007) 420–427. [22] S. Habaue, T. Hirasa, Y. Akagi, K. Yamashita, M. Kajiwara, Synthesis and property of silicone polymer from chrysotile asbestos by acid-leaching and silylation, J. Inorg. Organomet. P 16 (2) (2006) 155–160. [23] L. Heasman, G. Baldwin, The destruction of chrysotile asbestos using waste acids, Waste Manage. Res. 4 (1986) 215–223. [24] D.C. Harris, Quantitative Chemical Analysis, third ed., WH Freeman and Co, New York, 1991, pp. 782. [25] A.F. Gualtieri, A. Tartaglia, Thermal decomposition of asbestos and recycling in traditional ceramics, J. Eur. Ceram. Soc. 20 (2000) 1409–1418. [26] W. Mirick, Method for treating asbestos, Assignee: Austen Chase Industries, US Patent 5.041.277, 1991.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Hydrothermal conversion of chrysotile asbestos using near supercritical conditions Kalliopi Anastasiadou, Dimosthenis Axiotis, Evangelos Gidarakos ∗ Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, P.C. 73100, Greece
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 22 March 2010 Accepted 22 March 2010 Available online 27 March 2010 Keywords: Chrysotile asbestos Mineralogical conversion Hydrothermal treatment Supercritical conditions
a b s t r a c t The present research investigates, develops and evaluates the transformation of chrysotile asbestos into a non-hazardous material, such as forsterite, using an economically viable and safe method. The aim of this study is to convert fibrous chrysotile asbestos into an anhydrous magnesium silicate with a nonhazardous lamellar morphology using supercritical steam. The treatment method is characterized as hydrothermal in a temperature and pressure range of 300–700 ◦ C and 1.75–5.80 MPa, respectively. Small amounts of asbestos (2.5 g) were treated in each experiment. Deionised water was used as the treatment solution. The treatment duration varied from approximately 1–5 h. Additional experiments took place using solutions of distilled water and small amounts of acetic acid, with the aim of attaining optimal treatment conditions. Crystal phases of the samples were determined by X-ray diffraction (XRD). The main phases present in the treated samples were forsterite, enstatite, and chrysotile asbestos. Lizardite and periclase were also found. The morphology of the treated chrysotile asbestos fibers was identified by scanning electron microscope (SEM). The fibrous form of chrysotile asbestos was converted into nonfibrous form of forsterite. In fact, none of the fibrous-needle-like morphology, with length equal to or greater than 5 m and diameter less than 3 m, which was responsible for the toxicity of the original material, was visible in the solid phase. The dissolution of magnesium from chrysotile asbestos was measured using volumetric determination by titration with EDTA. Leaching of magnesium into the liquid phase was observed. Clearly, the highest concentrations of dissolved magnesium are observed after hydrothermal treatment of chrysotile asbestos using acetic acid 1% (8.4–14.6%). Lowest concentrations of dissolved magnesium are obtained after hydrothermal treatment of chrysotile asbestos without using additives. Observing the results of the hydrothermal treatment using additives, the mineralogical conversion does not depend on the presence of a small quantity of weak organic acid (<1%). The addition of acetic acid 1% during hydrothermal treatment did not involve changes in the conditions of the chrysotile asbestos’ mineral conversion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Asbestos has been characterized universally, and by Greek legislation in particular, as a harmful and hazardous material for human health (i.e. a toxic and carcinogenic substance). A key hazard is the release of fibers from products that contain asbestos [1]. The fibers penetrate respiratory systems and cause asbestosis and carcinogenesis. However, due to its resilient characteristics, asbestos has been widely utilized in industry and currently has over 3000 uses. Thus, today it can be found in a vast array of products (construction materials, fire retardants, binders, etc.) in varying proportions, ranging from 5% to 100%. Chrysotile asbestos, or white asbestos, is the form of asbestos most commonly used (95%) [2].
∗ Corresponding author. Tel.: +30 2821037789; fax: +30 2821037850. E-mail address: [email protected] (E. Gidarakos). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.094
Chrysotile asbestos is a 1:1 trioctahedral phyllosilicate with the chemical formula Mg6 Si4 O10 (OH)8 with some Mg substituted by Fe2+ . This 1:1 phyllosilicate comprises one Mg-octahedral sheet bonded to a Si-tetrahedral sheet. An ideal Mg-octahedral sheet has a lateral dimension of b ≈ 9.43 Å and an ideal Si-tetrahedral sheet has a lateral dimension of b ≈ 9.1 Å [3]. These dimensional differences seem to cause a lateral misfit between the octahedral and tetrahedral sheets along the X and Y axes [3,4]. To compensate partially for this misfit, the larger octahedral Mg sheet curls over the smaller tetrahedral silica sheet, thus generating chrysotile’s tubular morphology. The outer surface of the octahedral magnesian sheet exposes hydroxyl ions (OH− ). Chrysotile’s tubular structure produces four reactive sites: (1) an outer hydroxyl sheet; (2) the ends of the fiber; (3) the exposed edges of the curled sheet; and (4) the interior of the hollow central channel [5]. Fig. 1 presents chrysotile’s tubular structure, curled sheets developed from lateral misfits and cross-section showing Mgoctahedral sheet overlying Si-tetrahedral sheet.
K. Anastasiadou et al. / Journal of Hazardous Materials 179 (2010) 926–932
Fig. 1. Chrysotile’s curled sheets developed from lateral misfits and cross-section showing Mg-octahedral sheet overlying Si-tetrahedral sheet [6].
Considering the nature of asbestos products as time passes, it is certain that, at some point, asbestos fibers will be released. Once an asbestos product loses its characteristics, or has been abandoned, or is due to be abandoned, it is deemed Asbestos Containing Material—ACW. According to current legislation, ACW’s must be removed and properly managed in accordance with safety regulations. The most common way is the transportation and rejection of ACW’s in suitable landfills and disposal of this kind is forbidden without pre-treatment. Few treatment technologies of ACW’s have been developed, such as chemical, thermal, thermo-chemical, mechanical and others. The scope of most chemical processes is to cover the surface of the asbestos fibers with organic or inorganic substances, aiming to hinder those elements that are believed to react with human cells
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and cause fibroses and cancer [7,8]. In addition, strong acids can be used to destroy the fiber structure, but unfortunately such acids are considered to be more hazardous than chrysotile asbestos. According to literature the most popular natural treatment is the thermal conversion of chrysotile asbestos into non-hazardous substances at temperatures ranging between 900 and 1200 ◦ C [9]. Some efforts have been made to reduce the operational temperature and to lower the cost of this technique [10]. Other physical treatments worth mentioning are the conversion of chrysotile asbestos using the radiation of microwaves and mechanical methods [11–14]. The basic parameters for the material’s physical treatment are (i) temperature and (ii) duration. Decreasing the operational temperature would be a significant step towards the economic viability of these methods, considering that the mineralogical conversion of chrysotile asbestos occurs at temperatures >700 ◦ C [15]. Water in supercritical conditions is used for waste treatment, but mainly for the treatment of organic compounds [16]. Although the dehydration of chrysotile asbestos under hydrothermal conditions is described in the literature [17], Sigon et al. [18] for the first time converted asbestos fiber into non-hazardous material using as the only “reagent”, water in supercritical conditions. The present research investigates, develops and evaluates the conversion of chrysotile asbestos into a non-hazardous material, such as forsterite under economically viable and safe conditions using supercritical steam. The aim of this research was to decrease the operational temperature as much as possible using this hydrothermal procedure. For this reason the experimental temperature was kept below 700 ◦ C. The treatment method is characterized as hydrothermal in a temperature and pressure range of <700 ◦ C and <8.00 MPa, respectively. During the hydrothermal treatment the pressures and the temperatures applied were in the supercritical steam region, according to the diagram present in Fig. 2. Small amounts of asbestos (2.5 g) were treated in each experiment. Deionised water was used as the treatment solution. The treatment duration varied from approximately 1 to 5 h. Some additional experiments took place using solutions of deionised water and small amounts of chemicals, aiming to attain optimal treatment conditions. The crystal phases of the samples were determined by X-ray diffraction (XRD). The presence of chrysotile asbestos fibers was identified by scanning electron microscope (SEM). The dissolution of magnesium from chrysotile asbestos was measured using volumetric determination by titration with EDTA.
Fig. 2. Supercritical conditions and experimental area [16].
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Fig. 3. Experimental equipment used for the hydrothermal treatment of chrysotile asbestos [16].
2. Methods and materials Chrysotile asbestos was supplied by the Asbestos Mines of Northern Greece [19]. The mineral particle size was approximately 1 mm after grinding. Wet grinding was carried out for 15 min on chrysotile asbestos samples using a planetary grinding mill. The mill had a rotating speed of 180 rpm. The chrysotile asbestos samples were pulverized using acetone to prevent the diffusion of the fibers and to ensure the health protection of the researchers during the placing of these samples in the sample holder for XRD measurement. Acetone was used as dispersing liquid because it reduces grinding times and consequently the risks of amorphization [20,14]. The entire procedure of sampling preparation took place in a closed environment in a glove box under negative pressure. The mineralogical phase analysis of the asbestos samples was conducted before and after treatment. The test sample was placed in a holder which was then placed in a Rigaku XRD machine with a copper target ( = 15.406 nm). A diffraction angle of between 4◦ and 70◦ (2), and a scanning rate of 2◦ /min was utilized to analyze the crystal phases of the mineral samples.
X-ray diffraction showed that the chrysotile asbestos samples before treatment also contained a small amount of dolomite and calcium sulfate. A SEM was also used and the suspect fibres were examinated with an energy dispersive X-ray for their composition [19]. EDS analysis of chrysotile asbestos samples before treatment revealed that the major elements were SiO2 (57.1%), MgO (29%) and CaO (2.8%). An S2 Ranger EDS (Bruker Ltd.) was then used to qualitatively analyze the chemical composition of the asbestos samples. The ignition crystallic water content was estimated to be 14% [16]. Hydrothermal treatment is a simple process. An amount of chrysotile asbestos with deionised water is imported into a 4740 Haynes Alloy 230 high pressure reactor (manufactured by Parr Instruments Ltd.). The reactor is heated by a controller until the desired operational temperature is achieved. The temperature is monitored and adjusted by the controller while the measured pressure depends on the water volume inserted into the reactor. The volume of the autoclave reactor is 70 ml. The deionised water is transformed into supercritical steam. The reaction between the supercritical steam and chrysotile asbestos occurs in a suitable environment by means of hydrolysis.
K. Anastasiadou et al. / Journal of Hazardous Materials 179 (2010) 926–932 Table 1 Operational parameters of performed experiments during hydrothermal treatment. Temperature (◦ C)
Time (h)
Pressure (MPa)
DW (mL)
Additives
500 580 600 650 670 690 700 700 700 700 700 700 700 700 300 500 600 650
3 3 3 3 3 3 1 2 3 4 3 3 3 5 3 3 3 3
1.75 1.80 1.85 1.90 2.00 2.10 2.40 2.60 2.25 3.00 4.70 5.80 7.40 2.15 1.80 2.20 2.90 2.50
20 20 20 20 20 20 20 20 20 20 23 24 25 20 20 20 20 20
– – – – – – – – – – – – – – Ac. Acid 1% Ac. Acid 1% Ac. Acid 1% Ac. Acid 1%
Fig. 3 presents the experimental equipment used for the hydrothermal treatment of chrysotile asbestos. The supercritical steam penetrates the braided bands of the asbestos, determines a hydrolysis process that separates the fibres from each other, degrades the silicon oxide tetrahedron (SiO2 ), which becomes a solution, and modifies the structure with fibrous morphology into crystals of forsterite. The reaction that comes about can be represented by the following stoichiometry [18]: 2Mg3 (Si2 O5 )(OH)4 → 3Mg2 SiO4 + 2SiO2(aq) + 4H2 O
(1)
Temperature and pressure in the reactor are monitored by the controller. The time of operation depends mainly on the amount of the sample and the operational conditions (temperature and pressure). After completion of the process, the mixture (sample + deionised water) is cooled to room temperature, centrifuged and filtered, using a MCE filter, for the separation of the solid (treated chrysotile asbestos) and liquid phases (deionised water enriched with Si4+ and Mg2+ . Table 1 presents the operational parameters of the experiments performed. It is well known that acids destroy the fiber structure of chrysotile asbestos. According to the literature various acids such as HCl, oxalic acid, phosphoric acid are used for the destruction of the crystal body of chrysotile asbestos [21–23]. To make the process more effective and attractive, additives such as acetic acid in different concentrations of less than 1% were used. The concentration of 1% acetic acid was chosen mainly to avoid the corrosion of the reactor and to maintain acidity levels as low as possible. Indeed, while HCl is considered to be the best solution for chemical treatment, nevertheless it is a very strong acid. A weak organic acid such as acetic acid was chosen because it appears to effectively corrode the layer of brucite of the chrysotile asbestos fiber. The concentration was decreased in order to maintain the environmentally friendly character of the hydrothermal treatment. The morphology of the fibrous bodies was characterized using the SEM technique, which, because of its high resolutive power, allows the detection of even the smallest fibers. The analysis of the solid residue was carried out by means of Xray diffractometry. The total amount of Mg2+ removed during the hydrothermal treatment was determined by titration with disodium-ethylenediaminetetraacetic acid (EDTA) as described by Harris [24]. Each sample was repeated three times and averaged. The results are presented in the next section.
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3. Results The thermal decomposition of chrysotile asbestos follows a two-stage sequence of dehydroxylation and breakdown, and the mechanism has been researched in several studies [17]. The dehydroxylation of chrysotile asbestos takes place in the temperature range 600–780 ◦ C and at 800–850 ◦ C the dehydroxylated noncrystalline residue is said to recrystallize to give anhydrous silicate, such as forsterite and silica in amorphous form [20] or forsterite and enstatite [25]. Gualtieri and Tartaglia observed during thermal decomposition of asbestos that a complete re-crystallisation to forsterite and enstatite is obtained at 1100 ◦ C after 1 h [25]. Fig. 4 presents the spectrums produced using the XRD technique on the chrysotile asbestos samples before and after hydrothermal treatment at several operational temperatures and durations. The main phases present in the treated samples were forsterite, enstatite, and chrysotile asbestos. Lizardite and periclase were also found. During hydrothermal treatment, conversion of chrysotile asbestos into forsterite is observed at 650 ◦ C and the eventual absence of chrysotile asbestos is achieved at 690 ◦ C in 3 h. Total mineralogical conversion of chrysotile asbestos into forsterite is also achieved at a temperature of 700 ◦ C in 1 h. As can be observed in Fig. 4, in the sample hydrolyzed with supercritical steam at 700 ◦ C for 1 h and analyzed using the XRD technique, the presence of chrysotile asbestos was not detected. If present, it was in concentrations lower than the instrument’s lowest detectable limit. Time duration of up to 5 h does not appear to change the final result [19]. The total absence of fibers, that demonstrates the effectiveness of the innovative treatment proposed, was demonstrated by the SEM which, even at greater resolutions, did not detect the presence of fibrous solid. Fig. 5(a) shows an image of the sample of asbestos before the hydrothermal hydrolysis treatment while (b) shows an image of the sample after hydrothermal hydrolysis treatment with supercritical steam. The SEM photos of the treated sample (Fig. 5) taken at higher magnification (1000×) show that they had been completely transformed as a result of the treatment. In fact, none of the fibrousneedle-like morphology, with length equal to or greater than 5 m and diameter less than 3 m, which was responsible for the toxicity of the original material, is visible in the solid. Observing the results of the hydrothermal treatment using additives, the mineralogical conversion does not depend on the presence of a small quantity of weak organic acid (<1%). The addition of acetic acid 1% during hydrothermal treatment did not involve changes in the conditions of the chrysotile asbestos’ mineral conversion. However, it did cause the release of important quantities of magnesium ions, more than those released by the dissolution of chrysotile asbestos under basic conditions [5]. These magnesium ions are released from the brucite layer of the chrysotile asbestos fibers. Previous research has demonstrated that dissolution in 5% acetic acid can considerably decrease the chrysotile asbestos crystal structure (a reduction of 30–90%) [26]. Unfortunately, as shown in Fig. 6, it is not sufficient to completely eliminate the chrysotile phase at lower temperatures and so improve the hydrothermal treatment process by converting chrysotile asbestos into forsterite. However, the dissolution of chrysotile asbestos during the hydrothermal treatment is improved by the use of acids, and the amount of Mg released into the solution is increased. The dissolution of chrysotile asbestos in water can be expressed as the reaction [5]: Mg3 Si2 O5 (OH)4 + 5H2 O → 3Mg3+ + 6OH− + 2H4 SiO4
(2)
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Fig. 4. Spectrum produced using the XRD technique on the sample of asbestos before and after hydrothermal treatment at several temperatures and durations.
According to Choi and Smith [5] concentrations of Mg2+ and OH− ions are increased at the initial stage of the reaction. The concentration of OH− increases logarithmically with time at room temperature. However, after several hours (>5 h) the pH value tends to decrease. This reduction is probably created as a result of the formation of MgOH+ or re-absorption of Mg(H2 O)6 2+ and MgOH+ ions on the negatively charged silicon layer that has been revealed due to the dissolution of the brucite layer. Table 2 shows the amounts of Mg2+ leached from chrysotile asbestos into the solution during the hydrothermal treatment with and without acetic acid, determined by titration with EDTA. Clearly, the highest concentrations of dissolved Mg are observed after hydrothermal treatment of chrysotile asbestos using acetic acid 1% (8.4–14.6%). Lowest concentrations of dissolved Mg are obtained after hydrothermal treatment of chrysotile asbestos without using additives (<1%). This is in total agreement with previous research that the dissolution of chrysotile asbestos and the release
of Mg2+ increase with temperature and acid concentration [20]. The proportion and rate of chrysotile asbestos dissolution depends on the operational temperature, the concentration of the acid used, and on the type and origin of the mineral. The present innovative method provides for operating at relatively low temperatures (690–700 ◦ C) compared to 900 ◦ C and above for the traditional thermal treatments, while maintaining relatively low pressures (1.8–5.8 MPa). The hydrolysis time period, and thus the duration of the transformation process for obtaining a final product without any toxic/noxious residuals, is less than 3 h, according to the operative conditions chosen. It must be pointed out that, in contrast to other chemical treatments, the hydrothermal process of hydrolysis in near supercritical steam does not require the use of a strong chemical reagent or a substance with a high impact on the environment. The energy savings gained by using subcritical steam are significant due to lower temperature needed for the conversion
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Fig. 5. Pictures from a scanning electronic microscope of a sample of asbestos containing fibrous chrysotile asbestos before (a) and after (b) the hydrothermal treatment, enlarged 1000 times.
Fig. 6. Spectrum produced using the XRD technique on the sample of asbestos before and after hydrothermal treatment using acetic acid 1% at several temperatures.
Table 2 Percent of Mg leached into several solutions of hydrothermal treatment. Temperature (◦ C)
Time (h)
Pressure (MPa)
DW (mL)
Additives
Percent Mg2+ removed
500 580 600 650 670 690 700 300 500 600 650
3 3 3 3 3 3 3 3 3 3 3
1.75 1.80 1.85 1.90 2.00 2.10 2.25 1.80 2.20 2.90 2.50
20 20 20 20 20 20 20 20 20 20 20
– – – – – – – Ac. Acid 1% Ac. Acid 1% Ac. Acid 1% Ac. Acid 1%
<1% <1% <1% <1% 1% 1.4% 1.6% 8.4% 9.5% 12.4% 14.6%
of chrysotile asbestos into forsterite. The temperature needed for hydrothermal process (700 ◦ C) is lower than the temperature for conventional thermal treatment (1200 ◦ C). In other words, energy consumption of conventional thermal treatment (based on artificial
reproduction of the temperature conditions necessary for the mineralogical transformation of asbestos minerals) is much higher than the energy consumption of the hydrothermal process for the same treatment time (1 h). Concerning laboratory scale observations,
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a common furnace, which can be used for conventional thermal treatment (1200 ◦ C) of chrysotile asbestos into forsterite, requires from 1200 to 6000 W, while the controller of the hydrothermal process only needs 400 W [16]. This means that for the same treatment time (heating time and operating time) the furnace consumes more energy than the controller used for the hydrothermal process. Of course, these results apply to laboratory scale treatment and not industrial application. 4. Conclusions The environmental, energy and productive advantages that would be obtained with the new hydrothermal treatment of chrysotile asbestos in supercritical steam are multiple and unquestionable. The advantages and benefits can thus be summed up as follows: • the treatment procedure allows work to be carried out in a confined space, keeping the risk of environmental contamination to a minimum, • the better solvent properties of the water in near supercritical conditions improve the solubility characteristics of the solid materials of asbestos, accelerating the penetration processes of the “reagent” fluid, • no hazardous additives are used in comparison to other processes based on etching, • the process is characterized by low energy consumption in comparison to other treatment for asbestos such as vitrification, • chrysotile asbestos is completely neutralized and converted into forsterite (a raw mineral for production of several materials). The use of acetic acid in the experiments, in low concentrations, did not improve the hydrothermal treatment process by converting chrysotile asbestos into forsterite but there was an increase in the amount of Mg released from the chrysotile asbestos structure into the solution. References [1] E. Gidarakos, K. Anastasiadou, E. Koumantakis, N. Stappas, Investigative studies for the use of an inactive asbestos mine as a disposal site for asbestos wastes, J. Hazard. Mater. A 153 (2008) 955–965. [2] Directive 1999/77/EC, Adapting to technical progress for the sixth time—Annex I to Council Directive 76/769/EEC on the approximation of the laws, regulations and administrative provisions of the Member States relating to restrictions on the marketing and use of certain dangerous substances and preparations (asbestos) (1999). [3] F.J. Wicks, D.S. O’Hanley, Serpentine minerals: structure and petrology, Rev. Miner. Geochem. 19 (1988) 91–159.
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