Carbonitridation of Fly Ash. II. Effect of Decomposable Additives and

Department of Chemical & Biological Engineering, State University of New York at Buffalo, 218 Furnas Hall, Buffalo, New York 14260. Ind. Eng. Chem...
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Ind. Eng. Chem. Res. 2005, 44, 2477-2483

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Carbonitridation of Fly Ash. II. Effect of Decomposable Additives and Whisker Formation Qi Qiu* and Vladimir Hlavacek Department of Chemical & Biological Engineering, State University of New York at Buffalo, 218 Furnas Hall, Buffalo, New York 14260

A series of additives releasing ammonia after thermal decomposition were examined to evaluate their effect on the carbothermal reduction and nitridation of fly ash in the preparation of SiAlON (silicon aluminum oxynitride) powders. The reactions were studied at 1400 °C and times of 1-4 h. The addition of certain decomposable additives can accelerate the nitridation. Therefore, a reduction of the nitridation temperature and/or a shorter nitridation time can be achieved. CaSiAlON whiskers were observed in the reaction products by scanning electron microscope, X-ray diffractometer, and energy-dispersive X-ray spectrometer measurements. The nitrogen content in the final product was measured by nitrogen/oxygen analyzer as an index of the extent of the nitridation reaction. Fly ash powders from the Huntley and Hatfield power plants were investigated in this work. 1. Introduction SiAlON (silicon aluminum oxynitride) based materials have received considerable attention since their discovery in the 1970s1 because of their desirable engineering properties. Typically, the preparation of SiAlON ceramics involves the use of high-purity silicon nitride, alumina, and aluminum nitride, and sometimes other substances are employed. Although excellent ceramics can be obtained, the use of pure starting materials and high-temperature processing in controlled atmospheres results in high costs of the product and inhibits widespread use in such fields as metallurgy. Recent research2-7 has shown that it is possible to synthesize SiAlON-based materials from fly ash. If this process can be scaled up and carried out economically on an industrial scale, SiAlONs could become very useful refractory materials and waste fly ash could become a valuable resource. The current procedures for making nitride ceramics have often been plagued by an extremely long reaction time (several hours to even days) and high reaction temperatures required due to slow diffusion rates in solids. The same problem is encountered in SiAlON powder preparation when crystalline silicates are used in the nitridation process. We have tested the effect of several additives to determine whether a significant increase of reaction rate is possible. The characteristic feature of the selected decomposable additives was the fact that upon heating these additives could completely decompose to release NH3 (ammonia) and other gases such as H2O, HCl, and CO2. Ammonia gas has been reported8,9 to be utilized in the nitridation reactions instead of N2; its benefit has been that the evolved hydrogen participated in the process. Ammonia from the decomposition of the additives can have the same effect as nitrogen. SiAlON whisker formation from pure reactants has been observed by several other authors9-16 in the past. In 2000, Vaidhyanathan et al.14 used NH4Cl as an * To whom correspondence should be addressed. Tel.: (716) 645-3106. Fax: (716) 645-3106. E-mail: [email protected].

additive to improve the synthesis of metal nitride powders from high-purity powders of Al, V, Ga, and Ti in a nitrogen atmosphere. At temperatures below 1600 °C, nitrides with good crystallinity, structural uniformity, and phase purity could be prepared. In 2002, Chen, K. et al.15 used NH4F (3.85 wt %) as an additive in the nitridation reaction with starting materials such as AlN, Si3N4, Al, Si, SiO2, and Y2O3. As a result, novel rodlike yttrium R-SiAlON crystalline powders were fabricated by a combustion synthesis process. With the addition of NH4F, more β-SiAlON crystals were formed. According to their observations, when both NH4F and R-SiAlON (23.08 wt %) were added as the additives, more R-SiAlON was formed. The R-SiAlON represents both a diluent and a seed. They stated that NH4F could provide an easier route for the nitridation of silicon and forming β-SiAlON. Qiu et al.9 used a mixture of NH4Cl and KCl as the additive to prepare AlN from Al powder in a flowing ammonia atmosphere. The ratio of Al, NH4Cl, and KCl in their experiments was 1:1:1. Chen et al.16 observed generation of SiAlON whiskers from the fumed silica and R-alumina in a nitrogen atmosphere. He concluded that the growth of whiskers was governed by the vapor-solid-liquid crystal growth involving SiO, FeCl3, and other vapor species. Additives have long been shown to have a positive effect on the extent of carbothermal reduction and nitridation (CRN) reactions. However, all these above listed investigations were done with high-purity starting materials. We have not found studies in the literature on the effect of additives on the nitridation of fly ash. Besides NH4Cl as suggested by previous research, a series of chemicals that can completely decompose into gases were proposed as additives for the first time in this work, such as urea, ammonia carbamate, and ammonium bicarbonate. We believe that fly ash has good potential as a starting material for large-scale preparation of SiAlONs, and investigating possible enhancement of the CRN reaction seemed a useful undertaking. Therefore, the goal of this work is to find the best additives to decrease the reaction temperature and/or shorten the reaction time. In addition, as our experiments revealed that certain additives promote

10.1021/ie049006d CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005

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Table 1. Chemical Analysis of the Huntley and Hatfield Fly Ash Samples by EDS fly ash

Huntley

Hatfield

SiO2 [wt %] Al2O3 [wt %] Fe2O3 [wt %] CaO [wt %] K2O [wt %] TiO2 [wt %] SO3 [wt %] MgO [wt %] Na2O [wt %] LOIa [wt %] fraction [wt %] (pass 38 µm sieve) SA [m2/g]

47.0 17.8 10.8 2.1 1.5 1.6 1.5 0.8 0.02 17.2 68.5 8.7

50.9 20.4 14.9 4.8 1.9 1.4 1.6 0.4 0.1 3.5 69.6 1.0

a

LOI ) loss of ignition after 5 h at 750 °C in air.

formation of SiAlON whiskers, and it might also be possible to obtain some degree of crystal size control of the product, which is extremely difficult in these substances. 2. Experimental Procedure 2.1. Additives. The additives studied in this work all decompose on moderate heating. We used the following additives: ammonium chloride (NH4Cl, 99.5%; Alfa Aesar, Ward Hill, MA), urea (NH2CONH2; A.C.S. Reagent; Fisher Scientific Co., Chemical Manufacturing Division, Fair Lawn, NJ), ammonia carbamate (NH2CO2NH4, 99%; Aldrich Chemical Co., Inc., Milwaukee, WI), ammonium bicarbonate (NH4HCO3; J. T. Baker, Phillipsburg, NJ), and ammonium nitrate, (NH4NO3; Alfa, Chemicals, Morton Thiokol, Inc, Danvers, MA). In our experiments, the amount of the decomposable additives ranges from 20 to 67 wt %. These additives evaporated or sublimated during heating and left no residue in the final product. 2.2. Sample Preparation and Characterization. Huntley fly ash samples were used in this work, and Hatfield fly ash was also investigated at the end of this paper for comparison purposes. The compositions of the two fly ash samples are given in Table 1. The description of the high-temperature furnace and nitrogen/oxygen analyzer was published in our previous paper.6 If not specified in the experiment, fly ash and carbon black materials (Regal 660R, 24 nm, Cabot Co.) were ultrasonically mixed in propanol for 4 h. Afterward, the mixed powder was dried overnight at 85 °C and then passed through the 30-micron sieve. Before the nitridation, the selected additives were mixed with the reactants by simple hand shaking for 1-3 min. The nitridation conditions were as follows: the temperature T was 1400 °C, and the linear velocity of the nitrogen gas was U ) 38 cm/min. A nitrogen/oxygen analyzer (TC436/EF400, LECO Co., Michigan) was used to measure the nitrogen content in the tested samples. A field emission scanning electron microscope (SEM, Hitachi S4000, Hitachi Instruments Inc., CA) equipped with an energy-dispersive X-ray spectrometer (EDS or EDXS) was used to characterize the image and the composition of the samples produced in the experiments. X-ray diffraction (XRD, Siemens X-ray diffractometer, D-500, graphite monochromator) was also employed to determine the crystal phase in the final product. For the detailed setting of the XRD, see our previous paper.6

Figure 1. Effect of type I additives on the nitridation reaction. Huntley fly ash (original) at temperature T ) 1400 °C, linear velocity U (N2) ) 38 cm/min, and additive concentration ) 50 wt % (-O-, no additive; -∆-, NH4Cl; ‚‚0‚‚, NH4HCO3, -b-, NH2CO2NH4; - - + - -, NH2CONH2).

3. Results and Discussion of the Effect of Decomposable Additives 3.1. Effect of Additives. A blank test of each pure additive was performed in the furnace in a nitrogen atmosphere; the results showed that all the additives evaluated were completely decomposed into gases and there was nothing left behind in the boat. The ammonium nitrate, NH4NO3, was our first choice as an additive because of its high nitrogen content. We anticipated an acceleration of the nitridation reaction. However, the blank experiment with NH4NO3 showed that the decomposition of NH4NO3 is so violent that it is not practical to use it as an additive for this nitridation process. Therefore, the results below discuss only four additives: NH4Cl, NH2CONH2, NH2CO2NH4, and NH4HCO3. Based on the results of some preliminary experiments, the amount of decomposable additives in the starting material was set to 50 wt %; the Huntley fly ash was studied first. The effect of the four additives on the nitridation of Huntley fly ash is shown in Figure 1. Here the degree of nitridation is plotted vs the reaction time at a constant temperature. All four tested additives improved the nitridation degree. Therefore, it may be concluded that the additives promoted the nitridation reaction. Ammonium chloride, NH4Cl, proved to be the most effective in this case of nitridation of fly ash materials. All the additives considered in this paper release ammonia upon heating. We assume that the ammonia released takes part in the nitridation reaction as a source of active nitrogen. NH3 dissociates9,17 into reactive nitrogen (N) and hydrogen (H) radicals at temperatures above 900 °C (see eq 1); H further promotes the decomposition of NH3 (see eq 2). The mixture of N2 and NH3 has been recommended to decrease the reaction temperature and to achieve a higher conversion.9 The N-H bond energies are lower compared with the N-N bond energies in N2; therefore, less energy is required to break the bonds. Furthermore, the nitrogen and hydrogen gas mixture might have a similar effect as the ammonia released from decomposable additives investigated by us.

NH3(g) ) N(g) + 3H(g)

(1)

NH3(g) + 3H(g) ) 1/2N2(g) + 3H2(g)

(2)

There have been several patents18,19 that proposed some pore-inducing agents. The expanded pores result

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in an increased contact area. After the sublimation of NH4Cl or the decomposition of the other additives, the pores of the reaction compact became filled with the NH3 gas or other gases such as CO2, H2O, and HCl. The expanded pores result in an increased contact area, which has the same effect as the raw materials with a high specific area. It has also been speculated that the presence of the decomposable additives in the reaction mixture prevents the coalescence and sintering of the oxide particles during the nitridation process. Therefore, these additives may work as a pore-inducing agent for the reactant mixtures. Ammonium bicarbonate20 decomposes into NH3, CO2, and H2O at temperatures above 35 °C; the decomposition of ammonium carbamate21 starts at 35 °C; the products of the decomposition are ammonia and carbon dioxide. The decomposition of urea is more complicated; when the temperature exceeds its melting point (132.7 °C), this material starts to decompose. Nineteen reactions have been suggested by Schaber et al.22 for the urea thermal decomposition reaction. The primary species from the decomposition are HNCO, NH3, and CO2. The major thermal decomposition reaction of the dry urea is represented by eq 3:

NH2CONH2 f NH3 + HNCO

(3)

As seen from Figure 1, NH4Cl has a strong effect on the nitridation process of the fly ash and carbon mixtures. Ammonium chloride decomposes at a much higher temperature than the other three additives studied in this paper. At a temperature of 350 °C,20 NH4Cl sublimates and decomposes completely into ammonia and hydrogen chloride. NH4Cl is more effective than other additives; one reason might be that this additive decomposes at a higher temperature than the other additives. However, the major difference between NH4Cl and the other additives is that there is hydrochloride gas generation without H2O and CO2 being liberated. Water generated by ammonium bicarbonate may have an adverse effect on the nitridation process. Carbon dioxide may deplete some of the carbon employed for the reduction purpose. Furthermore, the HCl gas released by NH4Cl might react with some of the SiO2, Al2O3, and Fe2O3 in the fly ash and decrease the activation energy of the nitridation reaction. Hence, the results suggest that NH4Cl probably has a unique contribution to the CRN reactions. The sublimation of NH4Cl can be monitored by the appearance and disappearance of the white smoke leaving the furnace. This white smoke is the NH4Cl vapor. In our experiments the sublimation of NH4Cl started around 400 °C14 and was completed in several minutes after reaching this temperature by observing the appearance and the disappearance of the NH4Cl vapor. The mechanism of the effects of these additives is still under debate and is not well understood. These additives are essentially all decomposed at ∼400 °C. However, they have a significant effect on the CRN reactions, which generally take place well above 1000 °C. Our speculation is that ammonia is adsorbed on the carbon matrix and is completely released at higher temperature. The carbothermal nitridation is represented by the reaction of two solids with gases; obviously the reaction occurs on the solid surface and the adsorption of the nitrogen is much weaker than the adsorption of ammonia and therefore more active nitrogen is available for the reaction. The feature of NH4Cl here might also

Figure 2. Effect of NH4Cl concentration on the nitridation reaction. Huntley fly ash (original) at temperature T ) 1400 °C, linear velocity U (N2) ) 38 cm/min. R ) ratio of fly ash to NH4Cl additive (-O-, no additive; -0-, R ) 0.5; ‚‚4‚‚, R ) 1; - -/- -, R ) 2; -1-, R ) 4).

indicate that there could be some chemical interactions between the fly ash/carbon mixture and the additives. Therefore, some components released by the additives might be incorporated into the reactants through some chemical fixation and form some intermediates, which take part in the CRN reactions at higher temperatures. Eventually, these intermediates decompose and the flowing nitrogen gas carries the resulting gas products away. These additives might have the same effect as a catalyst, which can lower the activation energy of the nitridation reactions. 3.2. Effect of NH4Cl. In a silicon-containing system, the effect of NH4Cl was discussed in a paper by Lee et al.23 They used Si and NaN3 as the reactants, and NH4Cl was added as a catalytic agent. They found that the addition of NH4Cl made the reaction possible and also enabled a high product conversion. They considered that SiClx formed by the chlorination process could react with nitrogen in the gas phase to generate silicon nitride. A possible reaction mechanism is shown in eqs 4 and 5.

SiClx(g) + N2(g) + NH3(g) f Si3N4(s) + HCl(g) HCl(g) + Si(s)(l) f SiClx(g) + H2(g)

(4) (5)

The amount of NH4Cl they employed was in the range of 9-45 wt %. In the range they examined, 45 wt % of ammonium chloride resulted in the highest conversion. We studied the effect of NH4Cl in more detail since the preliminary experiments indicated its strong effect. Figure 2 shows the effect of the amount of NH4Cl on the nitridation reaction. We investigated a series of NH4Cl concentrations from 20 to 67 wt %. When the concentration of ammonium chloride is g50 wt %, the curves in Figure 2 almost overlap. This suggests that there is no obvious difference in the extent of the nitridation. When [NH4Cl] < 50%, Figure 2 shows that the extent of the nitridation reaction decreased. The SEM analysis was adopted to characterize the product with and without the addition of NH4Cl. There is no extra carbon added to the Huntley fly ash in the above study. Obviously the study for Huntley fly ash was under a carbon lean condition, carbon content C ≈ 17.2 wt %. Whisker formation was observed under this carbon lean condition: see Figure 3b. This suggests that by adjusting the carbon content in the reactant, we might be able to adjust the size of whiskers. A whisker24,25 is defined as a short, discontinuous, rod- or needle-shaped single crystal in the size range of 0.1-5 µm in diameter and greater than 5 µm in length. These

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Figure 3. SEM picture of SiAlON-based product (a) without and (b) with addition of NH4Cl (50 wt %). Samples were prepared from Huntley fly ash as received, carbon content [C] ) 17.7 wt %. Flow linear velocity U [N2] ) 38 cm/min, temperature T ) 1400 °C, and time t ) 60 min.

Figure 4. Whiskers produced by additives (a) NH4HCO3, (b) urea, (c) NH4Cl, and (d) H2NCONH4. Huntley samples were prepared from Huntley fly ash as received with 50 wt % additives added, carbon content [C] ) 17.7 wt %. Flow linear velocity U [N2] ) 38 cm/min, temperature T ) 1400 °C, and time t ) 120 min.

single crystal filament type materials can be produced by the solid, liquid, or gaseous reactants. The advantage of a whisker is that it could have extremely high strengths. 3.3. Observation of Whisker Formation. What could be the reason for the formation of whiskers? There could be two explanations for the whisker formation. It has been reported24 that the existence of impurities such as Fe, which possibly acts as a catalyst (or solutionforming agent), could contribute to the growth of whiskers. The presence of HCl may promote the cata-

lytic effect of the iron in the fly ash reactants. Another explanation could be the liberation of the gases produced from the additives and the interaction between the evolved gases and the reactants. Consequently, more surface area is generated in the reaction system. As a result, the diffusion resistance for the nitrogen and the resultant carbon monoxide gases is lowered significantly. It seems instructive to measure the SEM image for the SiAlON products prepared from different additives. The results are shown in Figure 4. For all additives, we can notice well-developed whiskers. There-

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Figure 5. EDS spectrum for the whiskers illustrated in Figure 4a.

Figure 6. EDS spectrum for the trunks illustrated in Figure 4a.

Figure 7. EDS spectrum for the balls illustrated in Figure 4a.

fore, HCl is not the only factor that boosts the whisker formation; all the gases generated and released from the decomposition of these additives contribute to the whisker formation. Because NH4Cl addition can increase the nitridation reaction better than other decomposable additives, this suggests that element chloride contributes more to increase the extent of the nitridation reactions. The mechanism of these additives is still not fully understood, and further study in this area is necessary. As shown in Figure 4, two main parts of all these products are the long needle-shaped whiskers and some

trunks. EDS analysis was performed on both parts, and similar elements were represented by the EDS spectrum for the same part in different samples. Illustration of the EDS spectrum for the whiskers is given in Figure 5. EDS illustration of the trunks is shown in Figure 6. As the elements N, O, and C cannot be detected by EDS, the main elements shown by the EDS spectrum are Si, Al, and in some cases, very small amounts of Ca and Fe. Furthermore, by carefully observing the images in Figure 4, we can see that there are some sphere-shaped balls in the product. These balls are mainly scattered around in the products. EDS analysis was also performed on those small balls, see Figure 7. The spectrum shows that the main elements are Fe and Si, which should correspond to Fe3Si. To prove that for the production of whiskers, a different source of fly ash material can be used, the carbonitridation reaction of Hatfield fly ash was also examined. Because of the low carbon content in the original Hatfield fly ash, the carbon content was adjusted to 28 wt %, which is the optimum amount for CRN reaction from our previous paper.6 In accordance with our expectation, the generation of whiskers was observed as well: see Figure 8b. The XRD pattern of the SiAlON product in Figure 8b is represented in Figure 9. The pattern of the crystal shows that the major components from the carbonitridation of Hatfield fly ash are Ca-SiAlON, which are in the form of Ca0.8Si9.2Al2.8O1.2N14.8 and Ca0.68Si9.96Al2.04O0.68N14.8. The EDS spectrum of the whiskers in Figure 8b is measured to confirm that the whiskers formed are Ca-SiAlON, see Figure 10. There are also small amounts of SiC, AlN, and Si3Al3O3N5 present in the system. The whiskers shown in Figure 8b are thicker than those whiskers prepared under carbon lean conditions. This may suggests that it is possible to control the thickness of the whiskers by adjusting the amount of carbon. Certainly, other reaction conditions can also affect the whisker formation, such as the type of additives, the amount of additive, and all the reaction conditions that might affect the CRN reactions. Another interesting phenomenon is that the Fe3Si balls stay on the tip of those whiskers in this case for samples prepared from Hatfield with optimum carbon added. One enlarged picture of the whiskers from Figure 8b is given in Figure 11 to illustrate the ball-shaped tip on the whiskers.

Figure 8. SEM picture of SiAlON (a) without and (b) with the addition of NH4Cl (50 wt %). Samples were prepared from Hatfield fly ash, carbon content [C] ) 27.7 wt %, milled 4 days. Nitrogen linear velocity U[N2] ) 51 cm/min, temperature T ) 1400 °C, and time t ) 60 min.

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Figure 9. X-ray diffraction patterns for SiAlON powders shown in Figure 8b (O, Ca0.8Si9.2Al2.8O1.2N14.8 (International Committee on Diffraction Data (ICDD) 33-0261)/Ca0.68Si9.96Al2.04O0.68N14.8 (ICDD 42-0252); #, Fe3Si (ICDD 35-0519); 9, AlN (ICDD 25-1133); /, Si3Al3O3N5 (ICDD 36-1333); b, SiC (ICDD 29-1129).

4. Conclusion

Figure 10. EDS spectrum for the whiskers illustrated in Figure 8b.

The effect of the additives on the nitridation degree of the fly ash sample depends on the chemical compositions as well as the amount of the additives used. The decomposable additives studied in this work all have a positive effect on the carbothermal reduction and nitridation reaction of the fly ash/carbon mixtures. Therefore, a lower temperature or a shorter nitridation time can be achieved with the addition of the decomposable additives. SiAlON whiskers can be prepared with all the decomposable additives studied in this work. NH4Cl (≈50 wt %) proved to be the most effective in nitridation among the additives explored in this study. The release of ammonia and hydrochloride gases from NH4Cl can accelerate the nitridation reaction and promote the formation of whiskers. Hydrochloride might promote the catalytic effect on the nitridation process. Acknowledgment This research was partially supported by the Mark Diamond Research Foundation and NYS/GSEU Professional Development Awards. We also thank Dave Underwood from NRG Huntley Operations Inc. and Brian Borowski from Mineral Solutions Inc. for providing the fly ash samples.

Figure 11. Magnified SEM picture of the whiskers illustrated in Figure 8b showing the ball-shaped tip of the whiskers.

The evident increase in the degree of the nitridation with the decomposable additives leads to one important conclusion of this study. It suggests that either a lower temperature or a shorter reaction time is achievable without sacrificing the degree of the nitridation. We expect that the nitridation temperature can be decreased to ∼1100 and 1250 °C, when the reaction is taken at a reaction time of 1 h and a nitrogen linear velocity of 50 cm/min. To determine whether the addition of NH4Cl leads to chlorine retention in the system, which we do not expect, EDS analysis of the final products was performed. The EDS spectra suggested that the amount of chloride left was below the detection limit, which is ∼1% depending on the element. Ultimately, it should be reasonable to infer that all the similar materials that can totally decompose to release gases such as CO2, HCl, and NH3 could help to decrease the nitridation temperature, shorten the reaction time, and promote the formation of whiskers. One possible candidate for future research could be the amine compounds group.

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Received for review October 13, 2004 Revised manuscript received December 7, 2004 Accepted December 10, 2004 IE049006D