Synthesis of β-Sialon from Coal Gangue

J. Mater. Sci. Technol., vol 19 No. 1, 2003Synthesis of B-Sialon from Coal GangueXingyuan LUOt, Jialin SUChengji DENG and Yanruo HONGT114AState Laboratory ou Solid Electrolytes and Metallurgical Testing Techniques, University of Science and Technology beijing.Beijing 100083, ChinaManuseript received July 18, 2001, in revised form November 21,2001]It is worth to study the synthesis of B-Sialon from coal gangue, because coal gangue is a waste of coal production anda high quality kaolin contained carbon which is a perfect raw material of contained reducer itself for synthesis of B-Sialon. The study showed that a high conversion rate of 95% from coal gangue to B-Sialon could be obtained by usingSialon were discussed detailedly and the phase diagrams of oxygen pressure vs composition for Sig N AIN-Al2O3-Siosystem at 1350, 1500, and 1600 C were constructed in the papeKEY WORDS: B-Sialon, Coal gangue, Refractory, Thermodynamic1. IntroductionB-Sialon is a new developing advanced ceramics. It hasbeen used in a very wide area, so how to fabricate it fromheaper or natural raw materials is a studying focus now.Especially, it is worth to study the fabrication process of B-Sialon applied in refractories from natural resource, becausit is unnecessary to use highly pure B-Sialon for refractoryindustry. Many works have been done on synthesis ofB-Sialon from natural resources such as kaolin and revealehat it was an effective method to synthesize cheap B-sialon.Except for kaolin, coal gangue which is a waste of coal pro-duction, composed of silica, alumina and some carbon will bbest and cheap raw material for synthesizing B-Sialon, butless works have been performed on it so far. China is a countrywith rich coal resource and there are much more coal ganguecontained only very few impurities of iron and basic metals. Fig.1 Relation between log (PN /p)and temperature forIt is meaningful to synthesize B-Sialon with coal gangue asreaction (3)raw materialsBut most research works have showed that the conversion Gibbs energy for Eq (3)israte from kaolinite to B-Sialon was not more than 90% bycarbothermal reduction-nitridation, because Sic and AiN beC=1491,485-08397+0.0191T{75kg(peo/p°)-gan to appear when the temperature was higher than 1450CThey suggested that the best synthesis temperature must be25g(s,/p°)between1350°Cand1450°C, and not more than1500°C.Theim of this work is to increase the conversion rate from coaccording to ee thermodynamic condition for syrgangue to B-Sialon and study the mechanism of synthesisthesis of Sialon is shown in Fig. 1. Four lines represent fourequilibrium(4G=o)conditiong at COssure oPco=0.1 MPa, 0.035 MPa, 0.01 MPa and 0.001 MPa respe2. Principle for Synthesis of B-sialontively. The upper area of the line is the stable area of B-sialonwhile the lower area of the line is the quartz and mullite stableLee and Cutler pointed out that the reaction for syn- area. It reveals that the fabrication temperature of 6-sialonthesis of B-Sialon from kaolin is as follows:decrease rapidly with the partial pressure of carbon monoxide decreasing at the same nitrogen pressure. For example,3Al2(SiyO5)(OH)4+15C+5N2+ 2Si3AlyO3Ns+6H20+15C0 p.=0. 1 MPa, Pco=0. 1 MPa, the fabrication temperature ofFor the simplicity we divide it into only e licated pro(1) B-sialon is higher than 1499 C, but if Pco reduce to 0.01 MPa,Actually, the synthesis of B-Sialon is a comthe fabrication temperature reduces to 1244"C. If the processo steps firstly: of reduction-nitridation is under the bed of carbon powder inclay-like mineral dehydrates and forms mullite and quartof oxygen in the product is removed by carbonair,Pco=0.035 MPa, PNa=0.065 MPa, the fabrication tem-perature is13929°csubstituted by nitrogen to form B-Sialon simultaneously. Theelative reactions are as:3Al2Si2Os(OH)4-3Al2O3 2SiO2+4SiO2+6H20(2)Raw materialsl/2(3Al2O32siO2)+4SO2+75C+25N2→province), coal with high quality (fromrovince), carbon black, industrial nitrogerSi3 AI3O3N5+7.5CO中国煤化工 DTA analysis suggestst Ph. D to whom correspondence should be addressed,CNMHGwith 3% calcium ligmosulwugu carbon black as redetion agent and another added coal, shown in Table 2. All thePresent address: Liaoning Jiayi Metals minerals co. LTD, mixtures were mixed by vibration-ball-miller and then madeTable 1 Chemical compositions(mass fraction)of raw materialsSiO2 Al2O3 TiO2 C N2 Ash Volatile L L/%Coal gangue4.2040080.592013.3668562025.27NitrogeTable 2 Compositions( mass fraction)forsynthesis of 3-SialonCoal gangueCoal1835/0.6853Carbon black18.35Fig2XRDo「2#at1400°C×8.0h~1600°C×20hFig4 SEM image of2#at1400°C×80h~600°C×2.0h5. Thermodynamic Analysis of Synthesis B-Sialon5.1 8-Sialon is not a final productB-sialon is not aroduct. it will be reduced contin-uously by C and N2strong reduction atmosphere atFig3 XRD of1#at1400°C×80h~1600°C×2.0h4/3SiAO3N5+2C+2/3N2=SiAl4O2N4(15R)+it into balls with different diameters by adding 27% waterSi3N4+2Co(g)After dried in oven at 110C for 24 h, the balls were△G=377.878-0.7607T(5into corundum cruciblein molybdenum-wire furnunder N2 atmosphere of 100 ml/min fow rate. The sampleSi3 Al3O3Ns +3C+N2= 3AIN+ Si3N4 +3Co(g)was heated at the heating rate of 280C/h until 1600 C, at△G=807.021-0.3878T(6)1400 C and 1600.C holding 8.0 h and 2.0 h respectively, thenSisAl3O3Ns+6C= 3AIN + 3SiC +3CO(g)+ Nzcooled at the same rate.△G°=1354917-0.6948T(7)That is why the moree were unable to get a high con-version rate of kaolin to 3-Sialon and showed that the temper-Figures 2 and 3 are the Xrd patterns of sample 2# andure of synthesis could not over 1450.C. So it is importa1# after heating to 1400 Cx80 ha1600 Cx20 h, from which to know the thermodynamic condition of synthesis a-Sialonit can be seen that(1) the main phase in the products is p-Sialon with a only little a O3 and( 2)both coal and car- 5.2 Mechanism of p-Sialon synthesis from coal ganguebon black can be used as reducer to synthesize A-Sialon. TheThe process of p-Sialon synthesis from coal gangue can bevalue of Z of B-Sialon is 2.99 for this product, measured by divided into 4 stepsXRD and calculated from the relationships between lattice(1)Kaolinite decomposed to mullite and cristobalite asparameter and value of Z. And the content of nitrogen in follows:the product is 23.0% measured by instrumental analysis. Therate of conversion from SiO2 and Al2 O3 in coal gangue to B中国煤化工22+4SiO2+6H2O(8)Sialon is approximately 95%(95.56% Si3.01Al299 O2.99N5o13.81% Al203, 0.63% TiN)calculated from the contents ofwill be produced in theSiO2, Al2O3 and TiO 2 in coal gangue with the value of ZCNMHGitridationand content of nitrogen in B-Sialon. At this time, the pris-SiO2+C=Sio(g)+co(g)matic crystals of B-Sialon have grown very well after heatingto1450°×8h~l600°C×2h, as shown in Fig4△G=688520-03439T(9)J Mater. Sci. TechnoL., VoL 19 No. 1, 20033ALOs. 2SiO2 +2C=3Al2O3+ 2Sio(g)+2CO(g)△G°=1286.96-0.6T(10)△G°=1287:751-0.5477T(133R(SiAl1oO2N10)are formed from Sio and Al2oSio(g)+C=p-SiC Co(g)AG°=-83262+0.00431T(12)al2 O3+ Sio(g)+5N2+14C= SiAl1oO2Nlo(33R)+14co(gGo=3408. 172-1.5861 T (13) Fig. 5 XRD of 2# at 1600 2 h and different atmospher5Al20(g)+SiO?+5N2+5C= SiAl1oO2N1o(33R)+5Co(s△G°=-2299.809+07803T(14)(4)B-Sialon will be formed from SiC and 33R as follows:SiC+ 3Al20(g)+3SiO2+5N2=2Si3 Ns 3Co(g)△G°=-2677.422+0.9779T(15)3SiAl10O2 N10+27Sio(g)+ 10N2 3C10Si3Al3O3Ns(B)+3Co(g)yson.Cerium.39+63436T(16)The standard Gibbs energies for reactions above are obtainedt020A s1 bar(5)SiC, 15R, Si3N4 and AlN will be formed by the reduction of B-Sialon under strong reduction atmosphere at hightemperature as above reactions 547)For the sake of convenience of discussion, the reac-tions above usually change into only relative with partial pressure of oxygen. The relations above can be ob-tained by combining above reactions and C+1 /202=Co10(△G°=-114391+0.08577T), for exampleSiO2= Sio(g)+ 5O2(g)△G°=802.91-0258T3Al2O3·2S02=3Al2O3+2Si0(g)+O2(g)△G°=1151.192-0.425TA,=lbarG°=1511.983-0.3727T105.3 Phase diagram of oxygen partial pressure vs compositionThe experimental results showed that the products wereH-SsBon+Candodifferent in different atmosphere although synthesis temper-ature was the same(1600%C). For example, 15R(reaction 5)was the product while the process of reduction-nitridation10was in strong reduction atmosphere (in the bed of graphiteder), and B-Sialonreduction-nitridation was in weak reduction atmosphere(inthe bed of B-Sialon bonded Sic powder), see Fig. 5. The re-sults have also been found by others (2). In order to studp2.1 barhe relationship between atmosphere and product quantifica-tionally a doXygwas constructed. Figure 6 is the diaof oxygen partial中国煤化工tion AlAL oessure us composition for Si-Al-O-Nat 0.1 MPa niogen pressure and temperature from 1350 to 1600C, which Fig. 6CNMH Gare va compositionare calculated by Thermo-Calc and thermodynamic data op-Si-AL-O-Nat 0.1 MPa nitrogen prtimized by Lucia. The following conclusions can be obtainedfrom the diagramJ, Mater. Sci. Technol., Vol 19 No 1,2003in log(Po,/p)=-175~-18.5(3×10-1~3×1020MPa)at1450C. It means that a high synthesis rate of B-Sialon cabe achieved by well controlling oxygen partial pressure in equi-7. Conclusions(1)A high conversion rate of 95% from coalSalon. ANnitridation at 1600 Cx2h when the conditions ofB Sialonto SiC, 15R, Si N4 and AIN under over-nitridation coneFig7 Diagram of temperature vs partial pressure of O2of strong reduction atmosphere and high temperature(3)DiagraIns of oxygen pressure vs composition for Si3.AIN-Al2O3-SiO2 system at 1350, 1500, and 1600C are there and temontrolling the condition of B-Sialon synthesis.ature are constant, the existence of phase depends on ox(4)The best conditions of B-Sialon synthesis from coalgen partial pressure, such as mullite and tridymite phase are gangue are the existent area of single phase p-Sialon in abovestable at higher oxygen pressure; -Sialon and B-Sialon-AINwill be stable at lower oxygen pressure. As mentioned abovewhen the experiment is performed in weak reduction atmo-sphere and pN=0. 1 MPa which partial pressure of oxygenis just in equilibrium with B-Sialon, the product must be gThis work was supported by the National Natural ScienceSialon. When the experimental condition is strong reduction Foundation of China, No. 59772024atmosphere which is over reduction-nitridation condition, theoxygen partial is in equilibrium with AIN polymorphs, reac-tions(5n7)will be occurred, the products are Si3 N4, Sic,AIN and its polymorphsThe stable range of oxygen partial pressure of B-Sialon [1]Gvaries with temperatures, and the higher the temperature theL Higging and A Hendry: Elsevier Applied Science,higher the oxygen partial pressure for the stable range of B-C.J. Space: Elsevier Applied Science, 1986, 133.J. Mukerji and S Bandyopadhyay: Advanced Ceramic Materi-6. Effect of Temperature on B-sialon Synthesis[5] EKokmeijer, CScholte, F Blomer and R Metselaar:J.Materay reports[l-7, 16-18) on the synthesis tem- [6] Alfredo D Mazzoni, Esteban F. Aglietti and Enrique Pereira: J.perature of B-Sialon from kaolin-lIike minerals. It is gener- [7|K.D. Mackenzie,ly thought that the synthesis temperature must be between1350 and 1450.C, and not more than 1500 C. When the temC M.Sheppard: J Mater. Sci., 1994, 29, 2611i M.E. Bowden, K.J. D. Mackenzie and J H.Johnston: Mater. Sci.erature is higher than 1450 C, Sic and aiN begin to ap-Forum,1988,34-86,599pear. And it is suggested the best synthesis temperature it [91ramics Systems, The Mineral, Metals& Materials SoclaJha:is about 1420C, and not more than 1500.C. But our resultshowed that there is no evident SiC and AIN existence, evenwhen synthesis temperature is up to 1600", which means no iii kH.Jack: Silicon Nitride, Sialons, and Related Cerartemperature limitation of synthesis B-Sialon from kaolin-likeminerals by carbothermal reduction-nitridation. The reasonCeramic and Civilisation, VoL Ill. High Technology Ceof temperature limitation is that thermodynamic conditionof synthesis did not be well controlled. In order to know [12] E.T. Turkdogan: Physical Chemistry of High Temperaturethe effect of temperature on B-Sialon synthesis, a diagram (13) JANAF Thermochemical Table(3rd Ed), American ChemicalD.A. Gunn: J. Eur. Ceram. Soc., 1993, 11,35Wen Hongjie: Ph. D Thesis, University of Science and Technol-ogy Beijing, 1997.raw material may give diferent products at different oxy- [16] Hongquan ZHANG, Ying DAI and Ningfang LI: Taoci Yanjiugean pressare changes grad taml ramu high wh low x yge p pase(a7Long'wLiU, Runzhou YU, Deyi LI and Shao-combinations will change along the following order: quartz+mullite, quartz +X1, O+X1, A-Sialon +Xl, p-Sialon, B- [18) L.K. Naik, L.J. Gauckler and T Y Tien: J. Am. Ceram. Soc.,Sialon AIN (polymorph). B-Sialon will be formed only1978,61(7-8),3中国煤化工CNMHGi