7352
Ind. Eng. Chem. Res. 2005, 44, 7352-7358
Carbonitridation of Fly Ash. 3. Effect of Indecomposable Additives Qi Qiu* and Vladimir Hlavacek Department of Chemical and Biological Engineering, State University of New York at Buffalo, 218 Furnas Hall, Buffalo, New York 14260
A series of indecomposable additives was tested for its effect on the nitridation of fly ash. The carbonitridation reaction was carried out in a tubular high-temperature furnace. SiAlON (silicon aluminum oxynitride)-based powders were prepared from fly ash/carbon mixture in a nitrogen atmosphere. Fly ash from the Huntley and Hatfield power plant was examined in this paper. The additives, in the amount of 4 wt %, are represented by several inorganic salts. Systematic analysis made it possible to select proper elements or groups having a strong effect on the nitridation reaction. The results revealed the possibility of manufacturing of expensive SiAlON whiskers at a low cost. This group of additives was never tested in the literature. A nitrogen/ oxygen analyzer measured nitrogen content in the final product. Weight losses were also measured to indicate the extent of the evaporation of the solid materials. Scanning electron microscopy and an energy-dispersive X-ray spectrometer were used to characterize the powder of the final product. 1. Introduction The preparation of SiAlON ceramics typically involves the use of some inorganic additives. These additives include high-purity silicon nitride, alumina, and various other indecomposable inorganic salts. In 1986, a Japanese patent1 used Ca3(PO4)2, CaHPO4, KH2PO4, and Na3PO4 as additives for the nitridation reactions. Five to eight g of the additive was mixed with 100 g of the starting material and treated at 1800-1900 °C. In 1998, Zhang et al.2 utilized a mixture of CaF2 and Na2CO3 as the catalyst to prepare SiAlON from clay in the temperature range of 1350-1450 °C. The residence time in their experiments was 4 h. Their studies showed that CaF2 as well as Na2CO3 could promote the break of the Si-O bond. Simultaneously, there was an acceleration of volatilization of the oxides in the clay. Those volatized oxides were MgO, K2O, Na2O, and CaO. In 1999, Komeya et al.3 used 3 wt % CaF2 as a catalyst. An Al2O3/C mixture was utilized to prepare AlN at 1450 °C. In 1999, Sheppard et al.4 studied a silicothermal nitridation using 1-3 wt % of oxides as additives. The above additives function mainly as a sintering aid to reach the acquired density. Sometimes, the additives work as seeds to accelerate the crystal formation and ultimately accelerate the nitridation reactions. Among the inorganic additives, nitrides function mainly as diluents5 or seeds6 to promote the formation of other nitride crystals, such as the SiAlON product. These diluents were of different particle sizes and compositions, and the diluents were chemically compatible with the system and very effective. The dilutions or seeds can range from 3 to 55 wt %.5,6 The optimum dilution depends on the combustion temperature. The crystal structure of Si3N4 applied as seeds can determine the crystal structure of the SiAlON formed.6 Despite of all the research, the mechanism of the indecomposable additives is still under investigation. Oxides have long been reported to contribute to the nitridation reaction. Most of these oxide additives have * To whom correspondence should be addressed. Tel.: (716) 645-3106. Fax: (716) 645-3106. E-mail:
[email protected].
been applied as sintering aids.5,7 Some oxides/carbonates8,9 have been used to achieve the phase transformation from β-SiAlON to R-SiAlON powders. An addition of yttria,4,10-13 calcia,4 magnesia,4,6 or lithia can lead to a solid solution with R-silicon nitride structure. Raju et al.14 reported on the addition of CaO, MgO, and Y2O3. These additives work as dopants to promote the conversion process and thereby decrease the temperatures of carbothermal reduction and subsequent nitridation reaction. Our previous paper15 indicates that decomposable additives can promote the whisker formation and increase the nitridation extent in the nitridation of fly ash. The additives made it possible to reach a higher extent of the nitridation even under carbon lean conditions. It is expected that indecomposable inorganic additives could have a similar effect on the nitridation. The major constituents of fly ash are in the form of inorganic salts, such as R-quartz (SiO2), mullite (3Al2O3‚2SiO2), hematite (Fe2O3), magnetite (Fe3O4), lime (CaO), and gypsum (CaSO4‚2H2O).16 There are also some trace amounts of heavy metals in fly ash. Since fly ash contains various elements of both metallic and nonmetallic type, some of the elements in fly ash might already have a catalytic effect on carbonitridation reactions. It is also evident that the presence of impurities in the liquid form helps the growth of whiskers via the VLS mechanisms. Thus all these elements presented in fly ash make the system more complicated. Currently, there is still no information about the effect of indecomposable additives on the carbonitridation of fly ash. One challenge of the experimental analysis present in this study is that the compositions of fly ash produced by different power plants differ greatly. This difference is related to the source of the parent coal, the combustion conditions in the furnace, and even the way of collecting fly ash from the flue gases. The consequence of this difference is that additives effective for one fly ash may not behave the same as for the other fly ash. Fly ash is a natural mixture of various oxides. These oxides could already have a certain additive effect on the nitridation reactions. Therefore, oxide additives are
10.1021/ie050392o CCC: $30.25 © 2005 American Chemical Society Published on Web 08/17/2005
Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7353
Figure 1. Effect of indecomposable additives on the extent of the nitridation reaction. b - 1500 °C, O - 1400 °C. Samples were prepared at a linear velocity of N2 U[N2] ) 51 cm/min, and a residence time of t ) 60 min. (a) Huntley fly ash and (b) Hatfield fly ash. Table 1. Chemical Analysis of the Huntley and Hatfield Fly Ash Samples by EDS
a
fly ash
Huntley
Hatfield
SiO2 [wt %] Al2O3 [wt %] Fe2O3 [wt %] CaO [wt %] Na2O [wt %] LOIa [wt %] fraction [wt %] (pass 38 µm sieve) SA [m2/g]
47.0 17.8 10.8 2.1 0.02 17.2 68.5
50.9 20.4 14.9 4.8 0.1 3.5 69.6
8.7
1.0
LOI ) loss of ignition after 5 h at 750 °C in air.
not evaluated in this paper. Besides, these oxides may interact with each other. Such interactions are hard to predict. The goal of this paper is to find certain elements or groups that have a positive effect on the carbonitridation reactions. Eventually, lower temperature/shorter nitridation time is expected. 2. Experimental Procedure 2.1. Sample Preparation and Characterization. For comparison purposes, both Huntley and Hatfield fly ash were utilized in this study. The compositions of the fly ash samples were reported in our previous paper,15 and some major differences of the composition are listed in Table 1. The carbon content in the fly ash materials was adjusted to the same level of carbon concentrations [C] ) 27.7 wt %. Carbon black (Regal 660R, 24 nm, Cabot Co.) was added for the carbon adjustment. The tail gas was released to the air through a bubbler. If not specified in the experiment, fly ash and carbon black were ultrasonic mixed in propanol for 4 h. Then the mixture was dried overnight at 85 °C. Finally, the dried powders passed through a 30 micron sieve. Before the nitridation, additives were mixed with the fly ash/ carbon mixture by hand shaking for 1-2 min. The nitridation conditions were described in our previous paper.17 A nitrogen/oxygen analyzer (TC436/ EF400, LECO Co., MI) was used to measure the nitrogen content. A field emission scanning electron microscope (SEM, Hitachi S4000, Hitachi Instruments Inc., CA) equipped with an energy-dispersive X-ray spectrometer (EDS or EDAX) was used to characterize the image and the composition of the samples in the experiments. 2.2. Reagents. The indecomposable additives studied in this paper were listed in Table 2. Classified by anions, these additives can be oxides/carbonates, phosphates/ hydrophosphates,1 halides,2 and nitrides;5 classified by cations, these additives can be alkali salts18 or alkaline earth salts. In this study, 4 wt % additives were used.
Table 2. List of Indecomposable Additives
a
no.
additive
no.
additive
1 2 3 4 5 6 7
N/Aa CaCO3 CaF2 BaF2 BaCl2 CaCl2 NaCl
8 9 10 11 12 13
KCl K2HPO4 Ca3(PO4)2 R-Si3N4 β-Si3N4 SiAlONb
No additives used. b SiAlON prepared from fly ash in our lab.
These additives are as follows: CaCO3 (A.C.S. Reagent; Fisher Scientific Company, Fair Lawn, NJ), CaF2 (-325 mesh; Var-Lac-Oid Chem Co., Inc., Bergenfield, NJ), BaF2 (99%, -40 mesh; Alfa, Morton Thiokol, Inc., Danvers, MA), BaCl2 (made in house, using Ba(OH)2 and excess HCl, then dried in oven), CaCl2 (dihydrate, approximately 99%; Sigma Chemical Co., St. Louis, MO), NaCl (A.C.S. Reagent; Sigma Chemical Co., St. Louis, MO), KCl (approximately 99%; Sigma Chemical Co., St. Louis, MO), K2HPO4, Ca3(PO4)2 (ortho; Johnson Matthey Catalog Co., Ward Hill, MA), R-Si3N4 (95%, 85% min. alpha form,
