Carbonitridation of Fly Ash. 4. Hydrolysis of Nitrided Fly Ash

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

Hydrolysis experiments of the nitrided fly ash were carried out in dilute aqueous suspensions.An ammonium electrode and a pH electrode were used to measure the extent of the hydrolysis.For comparison purposes, hydrolysis studies were also performed for the commercially availablenitrides. At the initial stage of the hydrolysis, a linear relationship exists between the hydroxideconcentration and the hydrolysis time. Accordingly, a slope can be extracted from this linearrelationship. A comparison of the slopes could be an effective method to evaluate the extent ofthe hydrolysis of nitride particles. A novel application of the nitrided fly ash is revealed in thispaper. The ammonia released slowly from the hydrolysis of the nitrided fly ash has the potentialto be a very good nitrogen fertilizer. Certain salts can slow the hydrolysis of the nitrided flyash, as for example calcium carbonate, and a slow ammonia release can be achieved.

1. Introduction

The water sensitivity of nitrides such as aluminumnitride (AlN) has been known for a long time. AlN hasthe highest sensitivity to moisture in the nitride group.Through the hydrolysis reaction, AlN decomposes intoaluminum hydroxide and ammonia, and heat is gener-ated simultaneously. The hydrolysis of AlN is similarto the oxidation of metals or alloys. A significantimprovement in water resistance was observed forpowders treated above 800 °C. The reason was theformation of a Al2O3 film on the surface of AlN particles.

Bowen et al.1 (1990) hydrolyzed 20 g of AlN in 100 gof deionized water. The mixture was treated with anultrasonic horn for 2 min. This mixture was then stirredfor 2-24 h (at 25 °C) before being dried. The kinetics ofthe AlN hydrolysis was claimed to be first order withrespect to AlN. Their kinetic data fitted an unreactedcore model with a porous product layer where thesurface chemical reaction controlled the overall kinetics.A Korean paper2 (1994) studied the hydrolysis of a AlNpowder. The heat of the hydrolysis was measured as 172cal/g (29.6 kJ/mol). It was observed that AlN hardlyhydrolyzed in a solution of pH ) 2.0.

Current publications on the hydrolysis of SiAlON aremainly of qualitative character. There is no reportedinformation on the hydrolysis of nitrided fly ash. In thispaper, we studied the degradation of AlN in an excessof H2O at a series of temperatures to estimate theactivation of the process. Studies on the rate andmechanism of the reaction between nitrides and H2Owere carried out to assess the possible use of nitride inaqueous suspensions. Furthermore, the effect of thenitridation conditions on the hydrolysis was also dis-cussed. We have also discovered that the addition of aninorganic additive can change the hydrolysis rate andthus provide an effective way to control the ammoniarelease rate. Unlike previous research on the hydrolysis,the ability to hydrolyze and release ammonia is themain concern of this work. We have also discovered thatthe addition of an inorganic additive can change the

hydrolysis rate and thus provide an effective way tocontrol the ammonia release rate.

Nitrided fly ash has the ability to release ammoniaduring the hydrolysis. Therefore, the nitrided fly ashcan be used as a nitrogen fertilizer. If a reasonableamount of ammonia can be released during the hydroly-sis, nitrided fly ash can be a good candidate for anitrogen fertilizer. Most commercial fertilizers dissolvein water. Therefore, a substantial fraction may bewashed away by rain, irrigation streams, and ground-water. A slow release nitrogen fertilizer is expected tosubstantially increase the fertilizer’s lifetime and reduceits losses. Development of slow release fertilizers isessential for saving the energy and protecting theenvironment. Nitrides, such as AlN, Si3N4, and SiAlON,can hardly dissolve in water. This feature makes thesematerials resistant to be washed away by water. Insteadthese materials slowly hydrolyze in water and releaseammonia, which can be absorbed and utilized by plants.For application as a fertilizer, it is necessary to provethat the product has the ability to slowly releasenitrogen in the form of ammonia.

2. Experimental Procedure

The goal of the hydrolysis experiment is to measurethe kinetic data describing the hydrolysis process ofnitride powders. The basic hydrolysis mechanism of thenitrides is shown below:

The hydrolysis gives rise to a continuous formation ofammonia.3 The hydroxyl ions (OH-) tend to raise thepH of the suspension. Individual pure nitrides werehydrolyzed at temperatures of 35 °C, 50 °C, 60 °C, 70°C, and 80 °C. SiAlON powders prepared from fly ashwere hydrolyzed only at temperature 80 °C due to alimited amount of sample.

* To whom correspondence should be addressed. Tel.: (716)645-3106. Fax: (716) 645-3106. E-mail: qqiu@buffalo.edu.

2AlN+ (3 + m)H2O f Al2O3‚mH2O + 2NH3 (1)

Si3N4 + (6 + 3n)H2O f 3SiO2‚nH2O + 4NH3 (2)

NH3 + H2O f NH4+ + OH- (3)

7359Ind. Eng. Chem. Res. 2005, 44, 7359-7365

10.1021/ie050393g CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 08/17/2005

Figure 1 shows the assembly of the hydrolysis experi-ment. Condensers were connected to the round-bottombottle to collect the reflux of the cooled ammoniasolution. The cooling water of the condenser can reachas low as 4 °C. A water bath was preheated to thedesignated hydrolysis temperature.4 The nitride sus-pension was placed in a three-necked round-bottomglass bottle. Afterward, the bottle holding the suspen-sion was placed in the water bath for hydrolysis.Therefore, the suspension can reach fast the designedtemperature. A temperature controller ((0.1 °C) and animmersion heater were used together to control thewater bath temperature. A magnetic stirrer was em-ployed to disperse the particles in the suspension. Thesuspension is assumed to be homogeneous throughoutthe experiment.

2.1. Sample Preparation and Characterization.First, some commercially available nitride powders wereexamined. These nitrides include the following: R-Si3N4(Alfa Aesar, 95%, 85% (min R phase), SA ) 5.0 m2/g),â-Si3N4 (Aldrich, SA ) 2.56 m2/g), AlN (Aldrich, 98%,<10 micron, SA ) 2.85 m2/g), SiAlON Grade 101 (PredMaterials Int., Inc., SiAlON polytypes 40-100%, AlN(24304-00-5) 0-10%, Al2O3 3.0-5.0%, yttrium oxide YO27.0-9.0%, ferric oxide Fe2O3 < 0.10%, SA ) 12.39m2/g), and AlN (Grade F powder, SA ) 4.3 m2/g; GradeH powder, SA ) 2.7 m2/g, Tokuyama America Inc.,Burlingame, CA). Later, the hydrolysis of nitrided flyash materials was studied. The main composition of thenitrided fly ash is SiAlON. For preparation of nitridedfly ash see our previous papers.5,6

Two grams of nitride was dispersed in 200 mL ofdistilled water in a three-necked round-bottom flask.The solubility7 of ammonia in water at 80 °C is 6.5 g/100g water, which is much higher than the concentrationof the suspension we studied. As for the ammonia inthe vapor phase, the cooling water in the condenser isvery low, 4 °C. Therefore, we can assume that releasedammonia can almost completely dissolve in water underthese experimental conditions. Before the hydrolysis, forbetter dispersion, the ultrasonic mixing was applied tothe suspension for 1-2 min. The hydrolysis reactiontook place in a three-necked flask equipped with a reflux

and a magnetic stirrer. Hydrolyzed samples in the formof a pseudohomogeneous slurry in the amount of 20-30 mL were taken out by using a syringe with a needle.Afterward the solution was filtered. To remove anyexcess water, 2-propanol was applied at the end of thefiltration. Then, the filtered powders were dried under80 °C in an oven for 1 day and stored in desiccators forlater measurements. The filtered clear solution wasused for measuring the pH value and the ammoniumconcentration.

2.2. Chemical Analysis. The supernatants werecollected by filtration. The pH values were measuredby a pH electrode (pH model 320, Corning Inc., Corning,NY). The ammonium concentrations in the supernatantwere measured by an ammonium ion selective electrode(ISE). The ammonium electrode used in this paperworks in the pH range from 4 to 10. For samples witha pH value higher than 10, a small amount of sulfuricacid is added to the sample to adjust the pH value tothe working range of the ISE. A 1 mL ion strengthadjuster (1 M NaCl) is used for every 100 mL of filteredsample solution. Both electrodes were calibrated beforeeach measurement.

The Fourier transform infrared (FTIR) spectroscopy(Galaxy series FTIR 5000 spectrometer, Mattson In-struments, WI) technique was used to characterize thehydrolyzed powder. For the infrared analysis, thesamples were ground with KBr in an agate mortar.Afterward, for analysis, the refined powder was com-pressed to a pellet of 10 mm in diameter.

3. Hydrolysis Experiment Results andDiscussion

The hydrolysis of the commercial nitrides was inves-tigated. The results were compared with the hydrolysisof the SiAlON materials prepared from fly ash. Thehydrolysis slurry was filtered; the supernatant was keptfor analysis. The rules observed in the hydrolysis areexpected to be generalized for the hydrolysis of thenitrided fly ash particles.

3.1. Hydrolysis of Pure Nitrides. The hydrolysisin distilled water was studied for several commerciallyavailable pure nitrides. These nitrides were AlN (GradeF), AlN (Grade H), R-Si3N4, â-Si3N4, and SiAlON (poly-type). The hydrolysis of each nitride was performed attemperatures of 35 °C, 50 °C, 60 °C, 70 °C, and 80 °C.Nitrogen analysis of the hydrolyzed powder revealedthat there was almost no hydrolysis of â-Si3N4 andSiAlON (polytype) even at 80 °C. Other nitrides studiedshowed a different extent of the hydrolysis reaction.Only the pH electrode has been used to measure thehydrolysis of pure nitrides in this study.

The results indicate that the crystal type could be animportant factor affecting the hydrolysis process. De-pending on the type of nitride under hydrolysis, an odorof ammonia can be smelled from the suspension for acertain reaction time. The SiAlON (SA ) 12.4 m2/g)powders from Pred. Materials are a polytype of AlN. Thehydrolysis of this SiAlON is similar to â-Si3N4. Theresults of the nitrogen measurement of the dried powderare represented in Figure 2. The figure reveals thatthere was no hydrolysis of SiAlON at 80 °C.

3.1.1. Activation Energy. Figure 3 shows the hy-drolysis of AlN (F). The pH value of the hydrolyzedsolution is plotted against the hydrolysis time at dif-ferent temperatures. The increasing rate of the pH valueis dependent on the hydrolysis temperature. The hy-

Figure 1. Hydrolysis experiment setup.

7360 Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005

drolysis is faster at higher temperatures. When thehydrolysis reaches pH ≈ 11.44, the hydrolysis ratedecreases to near zero. Oliveira3 reported a maximumpH value of 10 for a suspension of 5 wt % AlN. Zhanget al.8 prepared suspensions of AlN in deionized waterby stirrer mixing. The concentration of the suspensionwas not mentioned in the paper. The highest pH valuethey recorded was approximately 10 after 250 h ofhydrolysis of AlN.

A plot of [OH-] vs the hydrolysis time is displayed inFigure 3b. There is a linear relationship of [OH-] vs

time at the beginning of the hydrolysis. Afterward, theconcentration [OH-] levels off with the residence time.By recording the slopes at the corresponding tempera-tures in Figure 3b, we can calculate the apparentactivation energy for the hydrolysis of AlN(F) powders.Another phenomenon observed is that the concentrationof the hydrolytic slurry can affect the apparent activa-tion energy of the hydrolysis reaction. For more con-centrated slurries, the apparent activation energy islower. The hydrolysis of AlN (H) is presented in Figure4 and that of R-Si3N4 is shown in Figure 5. Comparisonof Figures 3-5 indicates that the increase of the pHvalue is much faster for AlN than for Si3N4. There isan obvious incubation period at lower temperature suchas 35 °C and 50 °C for R-Si3N4, see Figure 5a. Duringthe incubation period, there is not much change in thepH value. The pH value is close to that of the distilledwater, pH ≈ 6-7. The highest pH value Si3N4 suspen-sion can reach is about pH ≈ 9.4; this pH value is muchlower than that of AlN, pH ≈ 11.44.

The apparent reaction rate considered in this paperis a zero order reaction as suggested by part b in Figures3-5. Information on the activation energy of nitrideshydrolysis can be extrapolated from Figure 6 and Table

Figure 2. Hydrolysis of SiAlON powders, 9 80 °C, b 70 °C,2 60 °C. (Solid pointssnitrogen content, hollow pointssoxygencontent).

Figure 3. Hydrolysis of AlN(F) powders. (a) pH value and (b) [OH-] concentration as a function of time. 9 35 °C, b 50 °C, 2 60 °C,1 70 °C, [ 80 °C.

Figure 4. Hydrolysis of AlN(H). (a) pH value and (b) [OH-] concentration as a function of time. 9 35 °C, b 50 °C, 2 60 °C, 1 70 °C,[ 80 °C.

Figure 5. Hydrolysis of R-Si3N4. (a) pH value and (b) [OH-] concentration as a function of time. 9 35 °C, b 50 °C, 2 60 °C, 1 70 °C,[ 80 °C.

Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7361

1. Aluminum nitride powder is more sensitive to hy-drolysis than silicon nitride.

Figure 7 compares the FTIR spectra of AlN (F)hydrolyzed at different time periods. The peak9 centeredat 710 cm-1 is assigned to the vibration of the AlN bond.The spectra display the disappearance of the peak at710 cm-1 and appearance of the peak around 3400 cm-1.The broad peaks around 3400 cm-1 are attributed tothe O-H stretching vibration of AlOH species thatundergo hydrogen bonding with neighboring hydroxylgroups. In addition, the O-H stretching vibration of themolecular H2O is also likely to contribute to this band.After hydrolysis, some vibrations assigned to Al(OH)3were observed near 3500, 1000, and 500 cm-1. Theintensity of the Al(OH)3 increases for longer hydrolysistime.

3.2. Hydrolysis of Nitrided Fly Ash. 3.2.1. BlankExperiment. A blank hydrolysis experiment of theoriginal Huntley fly ash is displayed in Figure 8. Thefigure shows the decrease of the OH- ion concentrationin the fly ash slurry. The initial pH value of the slurryis the same as that of the distilled water pH ≈ 5.7. Theammonium concentration in the fly ash suspension isfar below the detection limit of the ammonium electrode,5 × 10-6 M. Instead, the pH value decreases gradually.The drop in the pH value might be due to the dissolvingof sulfur dioxide chemisorbed by the fly ash materials.In water, SO2 can be converted to H2SO4 under thecatalytic action of the metallic impurities contained in

the fly ash sample. Obviously, the sulfuric acid contrib-utes to the drop in the pH value.

3.2.2. Relationship between pH and AmmoniumConcentration. An analysis of the concentration of theammonia or nitrogen is the principle part of ourresearch of hydrolysis of SiAlON. Depending on the pHvalue, there are two forms of ammonium. These twoforms include the following: the ammonium ion (NH4

+)and dissolved ammonia gas (NH3). Ammonium predomi-nates when pH is < 8.75; ammonia predominates whenpH is > 9.75. There is a transition between NH4

+ andNH3 during a change in the pH values. Because therelationship between [NH4

+] and [NH3] is controlled bythe pH value, the total ammonia can be calculated usingeqs 4 and 5 if the ammonium ion concentration and thepH of the water are known. Equation 5 is the equilib-rium equation of reaction 3

where

[NH4+] is the ammonium concentration, [NH3] is the

ammonia concentration, and Kb is the equilibriumconstant.

To prove that the hydrolysis of nitrided fly ash cancontinuously release ammonia, the concentration ofeither N or ammonia needs to be measured. For SiAlONmaterials prepared from fly ash in the lab, the FTIRtechnique does not show any obvious change afterhydrolysis at 80 °C. Also, the change in the nitrogencontent in the hydrolyzed SiAlON is within the error ofthe N/O analyzer. A change in the FTIR spectra andthe nitrogen content is not obvious and thus cannot beused to measure the ammonia release rate for nitridedfly ash. Consequently, other methods must be used tomeasure the ammonia release rate from SiAlON.

In this paper, [NH4+] was measured by an ammonium

electrode. This ion electrode measures ammonium ionsin the aqueous solution. A calibration curve was pre-pared by measuring the millivolt reading of a series ofammonium standards. The concentration of these stan-dards is near the expected sample concentration. Theconcentration range of the electrode is 5 × 10-6-1 M.

The tendency of the pH change and the ammoniaconcentration change are shown in Figure 9. There is alinear relation between the hydroxide ion concentrationand hydrolysis time as well as between the totalnitrogen concentration and the hydrolysis time. Thusthe change in the pH value, that is the slope ofhydroxide ion concentration, can be used to compare the

Figure 6. Arrhenius plot for hydrolysis of pure nitrides.

Figure 7. FTIR spectra of AlN(F) hydrolyzed at 50 °C.

Table 1. Activation Energies for Hydrolyzed Nitrides

substancesurface area,

m2/g regression lineaEa,

kJ/mol

AlN(H) 2.7 ln k ) 15.27283-7340.442/T 61.03AlN (F) 4.3 ln k ) 18.89904-8323.525/T 69.20R-Si3N4 5.4 ln k ) 15.34646-11020.970/T 91.63

a k ) Ae-(Ea/RT), lnk ) lnA - (Ea/RT).

Figure 8. Blank hydrolysis experiment for Huntley fly ash withcarbon C ) 28 wt %.

total ammonia ) [NH4+] + [NH3] (4)

[NH3] ) ([NH4+]*[OH-])/Kb (5)

7362 Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005

ammonia release rate of the nitrided fly ash. Some otherhydroxides, such as Al(OH)3, may also contribute to theconcentration of the hydroxide ions. The hydrolysisreaction was illustrated in the hydrolysis of AlN.

Let

and Co is the concentration of AlN. Then we can get

divided on both sides of dC/dt ) k(Co - C) by Co, let x )C/Co, we can get

For very small x:ln(1 - x) ≈ -x

thus C has a linear relationship with time.

For a diluted suspension, water is in excess. In this case,the equilibrium is moving toward the right side of the

reaction. Therefore, it is rational to assume that mostof the ammonia is transformed to ammonium ion.

3.2.3. Effect of Nitridation Conditions on Hy-drolysis. 3.2.3.1. Temperature on Hydrolysis. Fig-ure 10 shows the dependence of [OH-] on the residencetime for nitrided fly ash prepared at different temper-atures. The nitrogen content in these samples is N ≈12 wt %. The results show that SiAlON prepared athigher temperatures tends to hydrolyze more easily. Thesequence of the hydrolysis rate is as follows: 1500 °C> 1400 °C > 1200-1300 °C. One possible reason is thatdifferent final products were produced at differentnitridation temperatures; see the XRD results in ourprevious paper.5

To further prove the effect of the nitridation temper-ature on the hydrolysis, several other nitrided fly ashsamples were hydrolyzed at 80 °C for 2 h. These sampleswere prepared with different inorganic additives, suchas KCl, BaF2, K2HPO4, and â-Si3N4. The nitridedproducts have a high nitrogen content, N ≈ 28 wt %.The total amount of nitrogen dissolved in the distilledwater is shown in Figure 11. Under the same hydrolysisconditions, the nitrided fly ash prepared at 1500 °Creleases more ammonia than that prepared at 1400 °C.The results also indicate that the inorganic additivescould also affect the hydrolysis rate of the nitridedproduct. Therefore, the adjustment of the nitridationtemperature and the use of additives are possible waysto control the ammonia release rate.

3.2.3.2. Effect of Nitridation Flow Rate on Hy-drolysis. The nitrogen flow rate in the nitridation hasan impact on the hydrolysis rate. Figure 12 shows theeffect of nitrogen velocity on the hydrolysis rate. Thefigure indicates that when the flow rate is in the regionof 70-169 cm/min, the slopes of the hydrolysis curvesare similar. The SiAlON sample prepared at 14 cm/min

Figure 9. Hydrolysis of nitrided fly ash samples at 80 °C.9 - total N, b - [OH-]. Samples were prepared at 1400 °C, fromHuntley fly ash (C ) 27.6 wt %) with 4 wt % NaCl additive, linearvelocity U[N2] ) 51 cm/min, and residence time t ) 1 h.

AlN + (32 + m)H2O 98k1

NH3 +12

Al2O3‚mH2O

NH3 + H2O 798k2

NH4+ + OH-

d[NH3]dt

) k1[AlN] - (k2,f[NH3] - k2,r[NH4+][OH-])

d[NH4+]

dt) k2,f[NH3] - k2,r[NH4

+][OH-]

C ) [NH4+] + [NH3]

dCdt

)d([NH3] + [NH4

+])dt

) k1[AlN] ) k(Co - C)

dxdt

) k(1 - x)

dx1 - x

) kdt

ln(1 - x) ) -kt

dx ) kdt

d[NH3]dt

) k1[AlN] - (k2,f[NH3] - k2,r[NH4+][OH-])

d[OH-]dt

)d[NH4

+]dt

) k2,f[NH3] - k2,r[NH4+][OH-]

Figure 10. Hydrolysis of nitrided fly ash samples at 80 °C,0 1500 °C, 9 1400 °C, 2 1300 °C, b 1200 °C. Samples wereprepared from original Huntley fly ash at temperatures 1200 °C-1500 °C, carbon lean condition, carbon C ) 17.5 wt %, residencetime ) 4 h, and linear velocity U[N2] ) 42 cm/min.

Figure 11. Nitrided fly ash samples hydrolyzed at 80 °C for 2 h.0 1400 °C, 9 1500 °C. Samples were prepared from Huntley flyash unless specified otherwise; linear velocity U[N2] ) 51 cm/min,residence time t ) 1 h.

Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7363

has a much lower hydrolysis rate. This low hydrolysismay be due to the lower nitrogen content in the SiAlONsamples. As we recall from our previous results,5 thenitrogen content in the final product tends to increasewith the increasing of the nitrogen flow rate until itlevels off at around 49 cm/min.

3.2.3.3. Effect of Nitridation Time on Hydrolysis.Figure 13 shows the effect of the nitridation time onthe hydrolysis reaction. We can see that SiAlON pow-ders prepared at longer nitridation times tend tohydrolyze faster. After hydrolyzing for a certain time,the nitrogen content and hydroxide concentration tendsto level off. A simple calculation was performed for a 2g sample with nitrogen content ≈28 wt % in 200 mL ofdistilled water. If all the nitrogen can be released duringhydrolysis, the maximum theoretical nitrogen concen-tration would be 0.2 mol/L. For samples nitrided for 4h, the extent of hydrolysis is obviously higher asrepresented by Figure 13. The total nitrogen contenttends to slow and finally reaches its highest level at≈0.14 mol/L, see part a of Figure 13. Approximately 75

wt % of the nitrogen could be released from this nitridedfly ash (nitridation time 4 h). Consequently, underlonger nitridation time or by adjusting the nitridationreaction conditions, it would be possible to release ahigher amount of nitrogen from the nitrided fly ash.

3.2.3.4. Effect of Additives on Hydrolysis. Figure14 shows the effect of different additives on the hydroly-sis. These samples were prepared at the same nitrida-tion reaction conditions.10 Comparing Figure 14 withFigure 13, we can see that the salt addition may affectthe hydrolysis of the nitrided fly ash. The total nitrogenconcentration is mostly lower for SiAlON samplesprepared with additives. The additives can decrease thehydrolysis rate of the nitrided fly ash samples. Thehydrolysis of the samples depends on the additives usedduring the sample preparation. The figure suggests thefollowing order of the hydrolysis rate: â-Si3N4 (1400 °C)> R-Si3N4 (1500 °C) > R-Si3N4 (1400 °C) > NaCl, CaCO3.Similar hydrolysis experiments were performed on thenitrided fly ash samples prepared with NH4Cl additive.The results also showed slower hydrolysis with the NH4-Cl nitridation additive than that of the nitrided fly ashwithout additives. As reported in our previous paper,5SiAlON crystals were synthesized when adding NH4Clas additive.

Figure 14 shows that SiAlON can be modified byadditives and vastly different values of the solubility/hydrolysis in water results. In particular, CaCO3 is apromising candidate. The addition of CaCO3 to the flyash mixture does not improve the nitrogen concentra-tion in the SiAlON product; however, the solubility/hydrolysis in water of the resulting product is very low.

4. Conclusions

The hydrolysis of nitride depends on the type ofcrystal of the material. AlN is more prone to hydrolysisthan Si3N4. The change of the pH value can be a useful

Figure 12. Effect of nitrogen linear velocity in nitridation reactoron the hydrolysis. Linear velocity U[N2] ) 9 14 cm/min, b 70 cm/min, 2 113 cm/min, 1 141 cm/min, [ 169 cm/min. Samples wereprepared from Huntley fly ash, residence time t ) 1 h, temperature) 1400 °C, and carbon C ) 27.6 wt %.

Figure 13. Effect of nitridation time on hydrolysis. Concentrations of (a) total nitrogen and (b) hydroxide as a function of the hydrolysistime. 9 1 h, b 2 h, 2 3 h, 1 4 h. Samples prepared from Huntley fly ash (dry milled with carbon black for 4 h, carbon C ) 27.6 wt %),temperature T ) 1500 °C, and linear velocity U[N2] ) 56 cm/min.

Figure 14. Hydrolysis of nitrided Huntley fly ash samples with O - no additive, 1500 °C, 9 - NaCl, 1400 °C, b - R-Si3N4, 1500 °C,2 - R-Si3N4, 1400 °C, 1 - CaCO3, 1500 °C, [ - â-Si3N4 as additive. Concentrations of (a) total nitrogen and (b) OH- ion as a functionof the hydrolysis time. Samples were prepared at temperature T ) 1400 °C, reaction time t ) 1 h, and linear velocity U[N2] ) 51 cm/min.

7364 Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005

indication of the ammonia release rate from pure nitridematerials. There exists a linear relationship betweenthe total ammonia concentration or the hydroxide ionconcentration and the hydrolysis time. The change inthe pH value can be a useful indication of the ammoniarelease rate from the nitrided fly ash materials.

The SiAlON powder prepared at higher temperaturestends to release ammonia faster than that prepared atlower temperatures. Additives used during the nitrida-tion also affect the ammonia release rate of SiAlON. Anexample of the additive is CaCO3. This additive can helpslow the hydrolysis of the nitrided fly ash materials. Thenitrided fly ash has the potential to be used as anitrogen fertilizer.

Acknowledgment

This research was partially supported by the MarkDiamond Research Foundation and NYS/GSEU Profes-sional Development Awards. We also thank NRG Hunt-ley Operations Inc., Mineral Solutions Inc. for providingthe fly ash samples, Pred. Materials International, Inc.for providing SiAlON powders, and Tokuyama AmericaInc. for providing AlN powder samples.

Literature Cited

(1) Bowen, P.; Highfield, J. G.; Mocellin, A.; Ring, A. Degrada-tion of aluminum nitride powder in an aqueous environment. J.Am. Ceram. Soc. 1990, 73 (3), 724-728.

(2) Choi, S.-W.; Jung, H.-S.; Whang, C.-M. Hydrolysis ofaluminum nitride powder. J. Korean Ceram. Soc. 1994, 31 (1), 79-87.

(3) Oliveria, M.; Olhero, S.; Rocha, J.; Ferreira, J. M. F.Controlling hydrolysis and dispersion of AlN powders in aqueousmedia. J. Colloid Interface Sci. 2003, 261, 456-463.

(4) Krnel, K.; Kosmac, T. Protection of AlN powder againsthydrolysis using aluminum dihydrogen phosphate. J. Eur. Ceram.Soc. 2001, 21, 2075-2079.

(5) Qiu, Q.; Hlavacek, V.; Prochazka, S. Carbonitridation of flyash. I. Synthesis of Sialon-based materials. Ind. Eng. Chem. Res.2005, 44 (8), 2469-2476.

(6) Qiu, Q.; Hlavacek, V. Carbonitridation of fly ash. II. Effectof decomposable additives and whisker formation. Ind. Eng. Chem.Res. 2005, 44 (8), 2477-2483.

(7) Lange, N. A. Lang’s Handbook of Chemistry, 15th ed.;McGraw-Hill: New York, 1999; Vol. 1.

(8) Zhang, Y. Effect of surfactant on depressing the hydrolysisprocess for aluminum nitride powder. Mater. Res. Bull. 2002, 37,2393-2400.

(9) Zhang, Y. Hydrolysis process of a surface treated aluminumnitride powder-a FTIR study. J. Mater. Sci. Lett. 2002, 21, 803-805.

(10) Qiu, Q.; Hlavacek, V. Carbonitridation of fly ash. 3. Effectof indecomposable additive. Ind. Eng. Chem. Res. 2005, 44, 7352-7358.

Received for review March 29, 2005Revised manuscript received June 21, 2005

Accepted June 24, 2005

IE050393G

Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7365