Densification of fly ash under alkaline hydrothermal hot pressing conditions Z. Matamoros-Veloza 1,*, J.C. Rendón-Angeles 2, K. Yanagisawa 3 and M.M. Cisneros-Guerrero 1 1 Department of Metal-Mechanical, Technological Institute of Saltillo, Coahuila 25280, México *E-mail: zullyma@fenix.its.mx 2 Research Center for Advanced Studies of the NPI, Campus Saltillo, Saltillo, Coahuila 25900, México 3 Research Laboratory of Hydrothermal Chemistry, Kochi University, Kochi 780-8520, Japan Large volumes of fly ash are annually produced at power plant Co. of the Federal Electricity Council (Coahuila, MEXICO), and this solid waste has become an environmental problem because of shortage of storage restrictions and environmental pollution. The fly ash consists of spherical particles (particle size of 20 m) and contains an amorphous silicate phase and two crystalline phases (Mullite and Quartz) as major constituents. Fly ash compacts were prepared with the addition of 15 wt % of an alkaline solvent (NaOH solution 5 and 10 M) by hydrothermal hot pressing at temperatures in the range of 150-250 ºC, for different reaction intervals (30-120 min) with a loading pressure of 17 MPa. Tensile strength of the compacts was measured by the Brazilian test. The structural changes of the crystalline phases during the treatment were determined by X- ray diffraction analyses, and microstructural aspects of the specimens were observed by scanning electron microscopy. Although X-ray diffraction results showed that the crystalline Mullite and Quartz phases did not react even in concentrated NaOH solutions at high temperature, the densification was achieved by the dissolution of the amorphous phase. A low strength (3 MPa) was obtained on the compact prepared with 5 M NaOH solution at 250 C. In contrast, the strength of the fly ash compacts was further increased up to 30 MPa by increasing the NaOH concentration to 10 M. 1. Introduction Nowadays, the annual production of fly ash is high each year. An amount between 10-20 wt% of fly ash is commonly produced by the combustion of bituminous coal in power plants. The combustion of bituminous coal in power plants commonly produces fly ash, 10-20 wt% from the coal. The fly ash waste solid is collected by cyclones, electric precipitators, and/or bag filters from the gas flowing at the burner top, while clinker ash is recovered from the bottom as mass of fused rock. More than 150 million of tons/year is generated in the world, and at least, one-third is used as raw material in the fabrication of cement. In 2002, 1.68 million of tons were produced at the electric plant Jose Lopez Portillo Coahuila-Mexico, and only 4.5% was recycled. The half of the fly ash finds no application and most of this waste is disposed as a solid waste in landfills. This route for solid waste recycling has been expensive increasingly and brings enormous environmental contamination problems. Fortunately, many research works are now evaluating new routes for fly ash processing. However, attempts for recycling of the fly ash have been limited due to the chemical composition of this specific waste. It is well known that fly ash major inorganic constituents are the compounds such as SiO 2 and Al 2 O 3. There are other minor undesirable s elements, such as: As, B, Ba, Cd, Cr, Pb, Hg, Mo, Se, S, which are considering highly contaminants. Nevertheless, the potential use for power plant fly ashes was reported by many researchers [1-5], as well as some applications, in order to increase the amount of the recycled fly ash solid wastes [1-5]. Hitherto, the fly ash had been used as an additive for cements and concrete, because its lightweight characteristic and morphology of agglomerated spheres or cenospheres (1-100 µm in diameter). These aggregates mainly consist of crystalline inorganic phases of quartz (-SiO), mullite (3Al 2 O 3 SiO 2 ), hematite (Fe 2 O 3 ), magnetite (FeOFe 2 O 3 ) and a significant amount of a vitreous amorphous phase [6-7]. A great number of experiments have been carried out under hydrothermal alkaline conditions, in order to transform fly ash into inorganic zeolite 360
materials. Under these conditions, different types of crystalline zeolite phases were obtained at low concentration of alkaline solutions [2.0 M] within the temperature range of 100-200 ºC. The main zeolites obtained are the analcime, Na-A and Na-P1 type zeolite, and hydroxyl-sodalite. The former results suggested that the type of zeolite strongly depended on the nominal Si/Al ratio of the raw fly ash [8-10]. In the present research work, we have aimed to study the influence of the alkaline solution on the preparation of the compacts under hydrothermal hot-pressing conditions. Tensile strength and apparent density of the prepared compacts were measured. 2. Experimental Procedure Fly ash produced at CFE power plant Co. (Coahuila, MEXICO) was employed as a raw material. The fly ash spherical particles with an average particle size of 20 m, consisted of amorphous silicate phase and two crystalline phases (mullite, 3Al 2 O 3 2SiO 2 and quartz, SiO 2 ). The chemical composition determined by wet analyses, showed that the major constituents are SiO 2 (62.37%), Al 2 O 3 (26.14%), and minor Na 2 O (0.34%), CaO (1.99%), MgO (0.85%) and TiO 2 (0.71%). Figure 1 shows the typical XRD pattern of the raw fly ash, which depicts that this material is formed by crystalline quartz, SiO 2 (); and mullite, Al 6 Si 2 O 13 (). The fly ash (5 g) was well kneaded with 15 wt % of the NaOH solution used as a solvent with two different concentrations, 5 and 10 M. The mixture was then placed in the chamber of the hydrothermal hot pressing autoclave, and the experiments were conducted at various temperatures between 150-250 ºC for several reaction intervals (30-120 min) with a constant loading pressure of 17 MPa and heating rate of 5 ºC/min. After the HHP treatment, the autoclave was cooled down to room temperature and the solidified compact was taken out from the autoclave. The tensile strength of the compacts was determined by the Brazilian test. The structural changes of the crystalline phases during the treatment were determined by X-ray powder diffraction, and microstructural aspects of the compacts were observed on polished surfaces by scanning electron microscopy. Intensity (Arbitrary Units ) 10 20 30 40 50 2 ( degree, CuK ) Fig. 1. X-Ray diffraction pattern of raw fly ash powder. () SiO 2, Quartz, () Al 6 Si 2 O 13, Mullite. 2.1. Experimental Apparatus A schematic representation of the cross section for the hydrothermal hot pressing autoclave is shown in Fig. 2. The autoclave is a piston cylinder type with an inner diameter of 20 mm. The pistons in contact with the powder have an open space for solvent release which occurs during the compaction process [12,13]. This apparatus also has two teflon glad packings which are normally fixed between the piston and the push rod. The main function of Teflon packing is to prevent the leakage of high pressure steam. Fig. 2. Schematic representation of the hydrothermal hot-pressing autoclave. 361
3. Results and Discussion 3.1. Reactivity of fly ash in NaOH solution under hydrothermal hot-pressing conditions Initial efforts were directed to investigate the reactivity of fly ash under alkaline hydrothermal conditions, and the experiments were carried out by using two contents (15 and 20 wt%) of NaOH solution with a concentration of 5 and 10 M. The preliminary hydrothermal hot pressing treatments were conducted at 200 C for 120 min with a uniaxial loading pressure of 17 MPa. Fig. 3. SEM Micrographs of polished surfaces of fly ash compacts prepared at 200 C for 120 min with 15 wt% of the NaOH solution with concentration of a) 5 M and b) 10 M. In general, the compaction of the fly ash powder samples was found to be strongly dependent on experimental variables, such as reaction temperature, concentration, and amount of alkali solution, which play an important role in enhancement of densification of the fly ash particles. Figure 3 shows a typical microstructure of compacts prepared under HHP conditions with two different concentrations of the NaOH solution, 5 M (Fig. 3a) and 10 M (Fig. 3b). When the fly ash powder was treated with 5 M NaOH solution, the fly ash particles did not react with the alkaline solution. The compact had the residual porosity and the morphology of the remained particles kept their original spherical shape even after the treatment. A small amount of a continuous phase was also observed in this sample (Fig. 3a, zone 3). On the other hand, the increase in the NaOH concentration up to 10 M resulted in a significant decrease of the residual porosity and formation of a continuous phase that covers the large solid fly ash spheres (Fig. 3b). The formation of this new phase might be associated with the dissolution of the original vitreous amorphous phase, which was more reactive than the crystalline mullite and quartz (zone 2) under strong alkaline hydrothermal conditions. Vitreous amorphous silicate materials have proved to be highly reactive even in pure water under hydrothermal hot pressing conditions. The above mentioned results are in good agreement with those determined by X-ray powder diffraction analyses, which were conducted on the hydrothermal hot pressed compacts treated at 200 C for 120 min. Figure 4 shows typical X-ray diffraction patterns of the compacts prepared with different NaOH concentrations. In general, the diffraction patterns do not show significant differences in the constituent phases. However, we surmise that the dissolution of the amorphous is likely to proceed under HPP conditions, rather than that of the crystalline phases, and this process achieves the densification of the compact by a viscous flow mechanism [14]. These results agree with that reported in conventional hydrothermal treatments, where it was found that the mullite phase exhibited a high chemical stability in comparison with quartz, even employing concentrated (3.5 M) alkaline mineralizers (NaOH) at 200 C for 24 h [10]. Figure 5 shows the tensile strength of the fly ash compacts prepared at 200 C for 120 min. with 17 MPa of confining pressure. The compacts prepared by using 10 M NaOH solution had higher tensile strength (34.7 MPa) when compared with those obtained in 5 M NaOH solution. The tensile strength was further increased as the hydrothermal treatment was conducted for long reaction intervals. The abrupt increase of mechanical strength on the fly ash compacts might be attributed to the dissolution of amorphous vitreous phase, which causes a viscous flow so that the continuous phase covers the crystalline particles and also reduces the 362
porosity in the sample during the hydrothermal reaction [14]. continuous phase which might be homogeneously distributed so that it cover all the non reacted particles and reduce the residual pores inside the compact. Intensity (Arbitrary Units) [10 M] [5 M] Raw 10 20 30 40 50 2 ( degrees, CuK ) Fig. 4. XRD Pattern of the fly ash compacts prepared at 200 C and 17 MPa of load pressure for 120 min with 15% NaOH solutions. () SiO 2, Quartz, () Al 6 Si 2 O 13, Mullite. 3.2 Effect of the amount of NaOH solution on the mechanical strength of fly ash compacts The influence of the NaOH solution content on the densification of fly ash powders was evaluated under standard hydrothermal hot pressing conditions: 200 ºC, 17 MPa, 120 min, and 10 M NaOH solution. The results are shown in Fig. 6. Both tensile strength and apparent density of the compacts exhibited a marked increase when the compacts were prepared with 15wt% of 10 M NaOH solution. The substantial strengthening obtained in these samples might be associated with the reactivity of the amorphous phase, which seems to be limited by the amount and concentration of the alkaline solution in contact with the fly ash powder. The addition of a low amount of 10 M NaOH solution gave the compacts with low tensile strength and apparent density, because of a partial reaction of the amorphous vitreous phase. It is well known that during the densification of powders under hydrothermal hot-pressing conditions, the addition of a large amount of a solvent solution results in filling the interstices between the raw particles. After the interstices are filled, densification does not proceed anymore. We surmise that the addition of 15 wt% of 10 M NaOH solution gave the sufficient dissolution of the amorphous vitreous phase of the fly ash to form the Tensil Strength ( MPa ) 40 35 30 25 20 15 10 5 0 20 40 60 80 100 120 140 Time ( min ) Fig. 5. Tensile strength of the compacts prepared at 200 C and 17 MPa with 15 wt% of () 5 M and () 10 M NaOH solution for various reaction time. The maximum value of tensile strength and apparent density obtained from the fly ash compacts were 34.7 MPa and 2.30 g/cm 3, respectively. These results show that the compacts prepared form the fly ash by hydrothermal hot pressing conditions had a higher tensile strength than those of the compacts prepared at similar HHP standard conditions from ordinary concrete (13 MPa) and fibber reinforced Portland cement (14 MPa) [15,16]. 4. Conclusions Fly ash powder was densified by hydrothermal hot-pressing method at standard conditions: temperature 200 C, loading pressure of 17 MPa, reaction time of 120 min, with 15 wt% of NaOH 10 M. The maximum tensile strength obtained on these compacts was 34.7 MPa, while the apparent density was 2.30 g/cm 3. Among the phases that are incorporated in the fly ash, the amorphous phase reacts with the solvent to enhance densification of the particles. The reactivity of the vitreous amorphous phase was mainly achieved by increasing the alkalinity of the solvent, resulting in the formation of a viscous flow that covers the remaining crystalline phases (SiO 2 and Al 6 Si 2 O 13 ) and reduces the porosity on the compact. 363
Tensile strength ( MPa ) 50 40 30 20 10 0 1 5 10 15 20 25 Solvent Content ( wt% ) 3 2.5 2 1.5 Apparent density ( g/cm 3 ) Fig. 6. Tensile strength () and apparent density () of compacts obtained at 200 ºC for 120 min with different amounts of a 10 M NaOH solution. Acknowledgements The authors would like to acknowledge the authorities at the electric plant Jose Lopez Portillo Coahuila-Mexico for supplying the fly ash used in this investigation. One of the authors ZMV is indebted to CONACYT for the financial support through the research funds (Project 36706-U), additional funds were given by COSNET through the project 699.01-P. Many thanks are expressed to MSc Roberto Suarez of the Research Center for Advanced Studies (CINVESTAV-Saltillo), for his help on the preparation of the samples for helium picnometry and SEM observations. References [1] A. Singer and V. Berkgaut, American Chemical Society Environmental, Science & Technology, 29, 1748 (1995). [2] C.F. Lin and H.C. His, Resource, American Chemical Society Environmental, Science & Technology, 29, 1109 (1995). [3] C. Amrheim, G.H. Hagnia, T.S. Kim, P.A. Mosher, R.C. Gagajena, T. Amanios and L. de la Torre, American Chemical Society Environmental, Science & Technology, 30, 735 (1996). [4] X. Querol, A. Alastuey, A. Lopez-Soler and F. Plana, American Chemical Society Environmental, Science & Technology, 31, 2527 (1997). [5] Bulletin of CFE North Division, José López Portillo 2002. [6] N. Shigemoto, H. Hayashi and K. Miyaura, Journal of Material Science, 28, 4781 (1993). [7] C. Swan, D. Brown, S. Levine, and J. Zabel, Home page http://www.flyash.info/, (2003). [8] H. Maenami, H. Shin, E.H. Ishida and T. Mitsuda, Proceedings of Join 6 th ISHR and 4 th ICSTR, Ed. K. Yanagisawa and Q. Feng, Kochi University, Japan, 175 (2000). [9] T. Kori, S. Suzue and T. Besshi, 5 th Internacional Conference on Solvothermal Reactions Ed. R. E. Riman, Rutgers University, USA, 187 (2002). [10] J. Hojo, M. Inada, Y. Euguchi, M. Uehara, N. Enomoto, and J. Hojo, 5 th Internacional Conference on Solvothermal Reactions, Ed. R. E. Riman, Rutgers University, USA, 281(2002). [11] M. Nishioka, T. Murano, K. Olfa, and Y. Kozuki, Proceedings of Join 6 th ISHR and 4 th ICSTR, Ed. K. Yanagisawa and Q. Feng, Kochi University, Japan, 203(2000). [12] M. Nishioka, S. Hirai, K. Yanagisawa and N. Yamasaki, J. Mater.Sci. Lett, 5, 335 (1986). [13] Y. Matsumoto, T. Kamamura, N. Yamasaki and T. Hashida, Proceedings of Join 6 th ISHR and 4 th ICSTR, Ed. K. Yanagisawa and Q. Feng, Kochi University, Japan, 215(2000). [14] M. Nishioka, K. Yanagisawa, and N. Yamasaki, Journal of ceramic society of Japan, 94,11, 1119-24 (1986). [15] Y. Nakane, T. Hashida, H. Takahashi, and N. Yamasaki, Journal of Ceramic Society of Japan, 103, 511 (1995). [16] A. J. Majumdar and J. F. Ryder, Glass Tech, 9, 60 (1968). 364