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Asian Journal of ; Vol. 26, No. 11 (2014), 3227-3232 ASIAN JOURNAL OF CHEMISTRY

http://dx.doi.org/10.14233/ajchem.2014.15965

Carbo-Chlorination of Silica: Thermodynamic Simulation and Experimental Study

* MEILONG HU , MEI SONG, XUEWEI LV, LU LIU and CHENGUANG BAI

College of and Engineering, Chongqing University, Chongqing 400030, P.R. China

*Corresponding author: E-mail: [email protected]

Received: 27 June 2013; Accepted: 6 January 2014; Published online: 25 May 2014; AJC-15219

Carbo-chlorination of silica represents an interesting route to prepare SiCl4,which is an important precursor used in the and microelectronic industry. Chlorination of several silica-containing materials in the presence of was studied at atmospheric pressure in an isothermal reactor at laboratory scale, with continuous and stable gas flow. The influencing factors such as temperature,

C/SiO2 mole ratio and binder content in the mixture were studied. The experimental results showed that diatomite is the optimal material

for carbo-chlorination to prepare SiCl4. The conversion ratio of dioxide increases with C/SiO2 mole ratio, decreases with binder

content increased in the mixture. SiO2 + 2Cl2 + 2C = SiCl4 + 2CO to be the dominant reaction above 800 °C according to the thermodynamic calculation. Based on the results, a probable procedures of the carbo-chlorination is proposed.

Keywords: Carbo-chlorination, Silica, .

INTRODUCTION This article describes a chlorination way to prepare SiCl4 using different silicas. In the process, the silicon dioxide Silicon tetrachloride (SiCl4) is an important inorganic reacts with in the presence of C to form chlorides. compound used as silicon source materials for the production The generated products in gaseous phase are carried away by of organo , silicon esters, organo silicon halides, silicone the chlorine gas stream and cooled outside the reactor, then 1-3 polymers , etc. Meanwhile, SiCl4 is one of the most important SiCl4 can be further processed after distillation. Based on the 4,5 precursors for the manufacturing of silica nano-particle , thermodynamic simulation, some experiments were conducted 6 7 8 spherical silica particle , fumed silica , poly-silicon which to judge which material is the optimal one for carbo-chlori- 9 includes metallurgical-grade silicon, solar-grade silicon , nation. After the optimal material determined, the influence electronic-grade silicon and optical fiber10. of parameters, such as temperature, C/SiO2 ratio, binder content Several routes for silicon tetrachloride production have in the mixture was studied. been reported in literatures, such as (i) preparing SiCl by 4 EXPERIMENTAL reaction of silicon with chlorine and under irradiation with 11 ultra-violet light and (ii) producing SiCl4 by reaction of silicon Thermodynamic simulation: The main chlorination 12 13 carbide with chloride . Iwei et al. conducted a reactions of SiO2 in the presence of Cl2 and Cl2 with C/CO as process to produce silicon tetrachloride by the reaction between reductant are given by eqn. 1 to 4. The standard free energy a silicon dioxide containing substance and trichloride. changes of these reactions are given in Fig. 1. All of the thermo- The generated di- was reduced and chlorinated dynamic data mentioned in this article were obtained from to boron trichloride. Rice husk (a by-product of rice processing Factsage 6.2. From Fig.1 it can be deduced that the chlorination constituted mainly of silica and carbon) have also been used to of SiO2 in the presence of C/CO (according to reaction [1]- 14-16 prepare silicon compounds . In the industry, SiCl4 is produced [3]) is thermodynamically feasible in random cases while reaction as a by-product along with other chlorides17, specially the poly- [4] is not thermodynamically favorable in the investigated crystalline silicon through the chemical vapor deposition process18, temperature range at standard atmosphere. As the vertical dotted also, almost 50 % silicide in bearing line shows, reaction 1 has greater negative free energy values high titania converted into SiCl4 by chlorination process, the than reaction 2 and 3 at temperature above 700 °C. This thermo- generated SiCl4 can be separated from TiCl4 by fractional dynamic calculation proves that CO and SiCl4 are expected to distillation after condensed, or SiCl4 was produced by the direct be the main gas phases when the temperature exceeds 700 °C 19 chlorination of ferrosiliconor SiO2/C mixture with chlorine . when sufficient carbon is used as the reducing agent. r equilibr are plottedinFigs.2and3. reaction temperaturesandnormalpressurewereobtained SiO performed forcalculatingequilibrium. Cl Fig. 2. Fig. 1. 3228 range of800-1000°C. c lations ofSiO eaction systemsar Equilibrium mole fraction (%) hosen toallo ∆G°/KJ mol –1 2 = 6mol,C4mol.Excesschlorineandreductantwere –800 –700 –600 –500 –400 –300 –200 –100 2 10 20 30 40 50 60 70 SiO As forthereductants,otherpossiblereactionsin SiO SiO C The initialamountsofthereactantswerechosentobe(i) SiO Based onthea 100 200 = 1mol,Cl Hu 0

(s) +CO 20 0 60 0 10 10 10 1600 1400 1200 1000 800 600 400 200 0 0 reactions Cl Chemical equilibriumcompositions forreactants:SiO Standar ium compositionsoftheser 2 2 2 2 etal. 2 (s) +2Cl (s) +2Cl (s) +2Cl (s) +2Cl = 6mol,CO10mol 50 00 50 00 2500 2000 1500 1000 500 0 d fr w ther 2 /Cl 2 2 ee ener (g) = 6mol,CO10moland(ii)SiO 2 C (g)+2Cl CO 2 2 e insignif 2 2 /CO andSiO (g) +2CO(s) (g) +2C(s) (g) +C(s) (g) → bo eactions tog

(g) +Cl g 2CO v → y c e discussion, SiCl hang Temperature (°C) Cl Temperature(°C) icant,

(g) 2 2 e withdif 2 (g) 4 → (g) (g) (g) +O → 2 → especiall o tocompletion /Cl SiCl → → SiCl eaction systemsa → SiCl 2 2Cl CCl /C reactionsystemsare COCl ther f er 4 CiCl Cl CO 2 (g) +CO 4 ent temper 4 (g) 2 (g) +2CO[1] (g) +2CO 4 y inthetemper (g) 4 mod 2 (g) CCl CO ynamic sim 2 a 20 tur 2 (g) 4 . Chemical 2 t dif e ofse 2 (g) [3] 2 = 1mol, = 1mol, [2] [5] [4] [7] [8] [6] Cl f COCl er a v tur ent er u- al e Fig. 3. f r with CO/Casr COCl mole fractionisdecreasingwithtemperatureincreased.(4) w g por pr imental set-upemplo w Chlor conc reg chlorination processinthetemperaturerangeconsidered, thermodynamics. (2)SiCl reaction ofSiCl at alltemperatureconsidered(200-1500°C)astheformation to measur becomes dominant. formation ofCO OD 51mm, tube andtheco the v in ag g w 112 mm)inw a sealedglasspr and theotherw with theg tw w used toa condenser in theairtoCO or SiO adical w ases w as tightness. as usedaniner hic hic er Equilibrium mole fraction (%) oduced onl o-sta ar ts inthesteelco e tw –5 10 15 20 25 30 35 40 45 50 55 60 65 Fr F The reactorconsistedofanalundumtube(length840mm, T lusions canbedr h w h w acuum pumpandthethir dless ofw 0 5 ra ine andar 2 ig he SiCl 20 0 60 0 10 10 10 1600 1400 1200 1000 800 600 400 200 0 concentr om ther phite cr Cl Chemical equilibriumcompositionsforreactants:SiO g 2 as r . 4.sho o por as hea /Cl e scr er bsorb thec 2 as obser e thetemper , = 6mol,C4mol as, w e madeofsteel. e 2 /C r w gula distr her y w ts inthesteelbase ub hic 4 T ted b ucib r eductant: v all thic esults r as f he tubew 2 g ws asc hether ther ber usingsa a er andbasew e theg eaction systems, eaction pr e . h ther ted b on w hen thetemper tion isal v par 2 4 ib , attemperatureshigherthan800°CCO ed a or theg le (OD44mm, can proceedspontaneouslyintermsof y MoSi v hlor t diluentandcar uted equa er ed f y meter y er a kness 6mm), e w , t temper a ener e onew ed inthecarbo-c wn f tur por hema ine andtheg e suppliedfr (1)SiO or thecondensedpr as placedv er wa 2 as inlet. e inther 4 oduct w Temperature (°C) a ted inF hea isthemoststableproductof e someholestoallo eductant isCorCO or thecarbo-c ted g tur T ys lo SiCl CCl CO Cl tic r as f er ing v he jointsbetw bly. d w ting elements. a a e f tur , 2 ted causticsodasolutionw a 4 4 onew aseous phasesar or thether can becon lo tur e w w a ix as f T pr alv es higherthan1000°C.(5) igs. 2and3, as collectedinar eaction z all thic w temper ed b e isbelo withaco he solidpelletsw Cl er r om c esenta ener CO t alltemper ier g es andf or theg COCl 2 ticall as f y steelr 2 hlor a ylinder hlor as. ted COw kness 2mm, 2 or ather mocouple inor tion ofthee v y insideafur oducts. F een thealundum w 600°Candits er one T ina a T lo as outlet. ina her v w g tur ted completel he f thef w meter Asian J ings toensur er andabase tion ofSiO , a . (3)CCl s. onew e cooledand tion ofSiO e f e w tur ood contact lo T mocouple as b av ollo he ar e 2 w ofthe er er inall ing lik .T = 1mol, . Chem. or length e thr e held T ur as f s. he Cl xper wing s the nace her g ned y, 4 der is on ee or as a 2 y e e e - 2 . Vol. 26, No. 11 (2014) Carbo-Chlorination of Silica: Thermodynamic Simulation and Experimental Study 3229

Pellets

Thermoelectric couple

Stainless steel capping Furnance

Pressure Pressure reducing reducing valve valve Condenser

Collector Exit gas treatment unit

Silicon Cylinder Cylinder of argon of chlrine rods Thermoelectric couple

Fig. 4. Schematic representation of the experimental setup

Supporting equipments: An analytical balance was used XRD patterns show that the diatomite was poorly crystallized. for the measurement of the samples mass before and after Diatomite, mainly consists of amorphous silica (SiO2·nH2O) carbo-chlorination. The balance has a maximum sensitivity derived from opalescent of , is a fine-grained, of 0.0001 g. Chemical analysis was used to determine the mass low-, soft, light weight biogenic sediment. Owing to its per cent of silicon containing materials. X-Ray Diffraction high surface area and amorphous structure, an improvement 21-23 (Rigaku CuKα radiation (λ = 0.154 nm)) is used to observe the of the chlorination ratio was obtained by previous workers . phases present before and after carbo-chlorination. A thermo- Experimental procedure: The silicas, carbon and binder gravimetric analyzer (Model NETZSCHSTA449F3-tempera- (Na2SiO3·9H2O) were weighed accurately and mixed together, ture ranging from 33 to 1400 °C) was used to determine the then pressed pellets at 20 Mpa. content of unreacted carbon in the carbo-chlorination sample. The graphite crucible containing the samples was placedin Characterization: Dinas , , silicon dioxide the middle of the reactor and argon was introduced to purge and diatomite are chosen as the silica-bearing raw materials the system. The reaction temperature was set and the furnace in this study. Dinas rock and carbon was crushed and milled was switched on. The heating rate was controlled at 10 °C/min through a 200-mesh sieve and the rest materials were analytic to prevent the samples from cracking as a result of thermal reagent. The content of SiO2 in the four raw materials is shown shock. The system was evacuated after the required temperature in Table-1. The industrial analysis of the carbon used as reached. Chlorine was then fed into the reactor to make the reducing agent is shown in Table-2. system to be filled and the flow rate of supplying chlorine was kept at 300 mL/min. After the specified chlorination time TABLE-1 elapsed, chlorine is replaced by argon to purge the system. CONTENT OF SIO 2 IN FOUR RAW MATERIALS Then, the furnace gets switched off and cooled. When the Silicon Dinas rock Quartz sand Diatomite reactor is cooled to room temperature, argon is interrupted 99.20 97.90 92.90 89.66 and the samples are taken out from the furnace and weighed.

TABLE-2 To obtain the mass per cent of SiO2 in the unreacted INDUSTRIAL ANALYSIS OF CARBON samples exactly, the samples after carbo-chlorination were S Moisture C ash volatiles roasted in a muffle furnace at 1000 °C for 3 h. Then, the powder Content (%) 0.44 6.71 65.41 16.86 11.01 sample is identified by chemical analysis. RESULTS AND DISCUSSION X-ray patterns of the four raw materials are presented in Fig. 5. It is noticed that crystal phase such as quartz, silicon According to the calculation by Factsage 6.2, it was shown oxide, formed in silicon dioxide, dinas rock, quartz sand, while earlier that there will be little or no SiCl4 generated by reaction was found in diatomite and the intensity of between SiO2 and Cl2. Since the formation of heat of SiCl4 the peaks was lower than the other materials. The results of from Cl2, SiO2 and C is -161 kJ/mol, so it is clear that SiCl4 can 3230 Hu et al. Asian J. Chem.

1400 (a) (b) 1000 1200 quartz quartz 1000 800 silicon oxide silicon oxide

800 600 y y t t i i s s

n 600 n e e t t 400 n n I I 400 200 200 0 0

20 40 60 80 20 40 60 80

2θ (°) 2θ (°) 250 1200 (c) (d)

quartz 200 1000 cristobalite silicon oxide 800 150 y t i s

n 600 e

t 100 n I Intensity 400 50 200

0 0

20 40 60 80 20 40 60 80 2θ (°) 2θ (°) Fig. 5. X-ray patterns of the four raw materials, (a. dinas rock, b. quartz sand, c. silicon dioxide, d. diatomite) be generated easily when carbon exists in the system. Factsage 55

6.2 was used to calculate the optimal generation of SiCl4 with 50

different temperature and carbon addition. The result is that ) 45 % (

40 the optimal C/SiO2 mole ratios are 1.2, 1.4, 1.6, 1.8 at 900, 2 O 1000, 1100 and 1200 °C, respectively. i 35 S

f o

The degree of conversion of SiO2 is defined as: 30 o i t

0 a 25 r MSiO − MSiO 2 2 n

o 20 η = 0 [9] i s MSiO r 2 e 15 v

0 n where MSiO2 is the initial mass of SiO2 in the starting material o 10 silicon oxide C dinas rock powder and MSiO2 is the mass of SiO2 in the powder after a 5 specific reaction. diatomite 0 quartz sand Several experiments were made with the four silicas –5 mentioned above according to the calculations with a chlorine 900 950 1000 1050 1100 1150 1200 flow rate of 500 mL/min. The experiment result shown in Temperature (°C) Fig. 6. Fig. 6. Conversion ratio of different silicas at different temperature From Fig. 6, it can be obtained that the conversion ratio of several silica materials increased with temperature raised. of diatomite, small quantity of by-product FeCl3 and rich storage

At the same temperature, the conversion ratio of diatomite make it a suitable raw material for the preparation of SiCl4 by was the highest among all the materials. Actually, silica can carbo-chlorination. be found in more than one state-crystalline as in a quartz Subsequently, a series of experiments were made through crystal, which is quite common in and amorphous as in the change of mass carbon to study the effect of C/SiO2 mole 24 the remains from a . ratio on the conversion of SiO2. The result showed that the Because of its amorphous structure and , so it is conversion ratio increased with carbon increases in Fig. 7. feasible for diatomite to react with Cl2 at a lower temperature At 1100 °C, the optimal C/SiO2 mole ratio of the chlori- than any other silicon-bearing ores. Moreover, the lower price nation was 1.6 according to thermodynamics calculation. Ho V the highestwhenC/SiO should bead the conditionsf for thattheequilibriummoduleofthermodynamicscalculated dia and carbonof and con changing themassofbindtostudyrelationbetweenporosity tw ding toeqn.[1], inlet intother enough carbonispresentinthefeed.Besides,aschlorine constr pr oxide suchascarbo-thermicreduction SiO a par ther molten saltf inter temperature. the surf (c dominant ing As itw rides r no necessar in thequantityofcoal.Somereportersconsideredthatthere is the c c noted tha is unf so her p and nic with aacti eaction, hlor F yr ol. 26, hlor

oposed tha Conversion ratio of SiO (%) ig 2 o solidr tes.T wev ol 30 40 50 60 70 80 . 7.Con 2 mod Pr Se hlor Amor media decreasedwithbinderincreasing. 29,30 t ofcarbonw ides, ytic carbonsurf av aints anditr ine) ar e c v er No.11(2014) ocedur as ther k o mak ace ofthepar or v er . ynamic anal ina w el f t c , er hlor Ilic F ab al r v e va . 15 . 25 3.0 2.5 2.0 1.5 1.0 tes suc hic hlor sion r er eactants (SiO tion de bieta ig y f le er e in outes w t carbo-c tion ener sion ofSiO or someacti ine a ded e SiO . 7indica et al aseous phasea f 1100 °C, 120 min °C, 1100 120 . h inaccor mod r eactor withoutinter or carbonando er r es ofthecarbo-c ine w T ites withorwithoutad v 31 thisisag her or thec a . Besides, h asCOCl olv eg . studiedthedecompositionof c gr eaction ar tio, ould tur 28 toms ar ynamicall 2 con ef discussedthec er ar ee ofNi, as easytodissocia ed ysis pr aces andf tic ther hlor or g e usedto ded allthecarbonasr 2 india ted tha . y a v e, les aslongasitg dance withsomer 2 T er moleratioisequalto3.Itaccounts C:SiO mole ratio mole C:SiO hlor 2 w v ina he pr andC)oneg bout ahalfofitsbondener n intosomec e sug t intoSiCl e metals as-solid r esult indica esented ear somee 2 e supposeeqn.[1]tobetheglobal eas f tomite withdif , F t hightemper y f tion occur Cla ina t thecon e, 2 xides tocontact, oducts ar ound tha xtr av g Crincr tion underalar or thef ested tobetheacti tom andr or hlor act metalfr r hlor upt, 27 xper ab 4 eaction systeminw inv and carbo-c te onthecarbonsurf le f v ina ina lier sho diti s b eased withtheincr tes tha COisr er hlor t c or e g f iments ar 26 ener er ast, sion r tion ofnic or thef a y meansofg , electrolysisfrom tion ofSiO v esear ma hlor ent C/SiO especti tur eductant. aseous a es andf ides suc e om somemetallic a tion ofinter aseous r e, t con xcessi Carbo-Chlor ted w ine dissocia a c dir pr emo ed tha tio ofSiO hes g hlor , or v hlor soeqn.[3] o e madeb e r 2 e o v ectl moler vided tha k ma ound tha t r v 25 h asCCl v er Actuall el o v e carbon ed fr ine o . ang xyc ina gy e inter eaction sion of eactant t COis Accor tion of aseous y 2 : . . Itis xides tion. hic And ease e of a hlo- T me- ace ina tio om v ted 2 he er is y, y h tion ofSilica: - - 4 t t , ra reductant. isthoughttobetheglobalreactionduringprepa- chlorination process.Here,carbonisthecatalystand Conclusion Funds f of por 1. SiO 5. ma 2. and equa The proceduresdiagram(Fig.8)ofcarbo-chlorinationSiO 7. 3. 6. 8. 4. (a) theca r in thepr r dioxide. chlorine atom.(b)thereductant,capturingoxygenfromsilicon carbon incr to beastha out mor surf mediates formedbyCl Chlor

eaction betw upted b

tion ofSiCl

F

i

f t h

s ter t e Cl Cl p ace isin 2 It ispossib Due toitsstructureofamorphoussilicaandproperties B T Cur Y 187 J Adhesion J H.-B X.L. Zhangand S (1995). Z. Luo, Inor The proceduresofcarbo-chlorinationofSiO increasedwithC/SiO 2 • . Song .R. Hele . P .Z. Qiao, ialto pr ine a . his w osity andhighsurf T Ar , 357(2012). or theCentr e full r Fig. 8.ProceduresdiagramofcarbochlorinationSiO eng g esence ofC, . Cl y acti . Chen, her . Chem.Ind iw tions e tal A • , ppl. Ph Cl X.Cai, toms ar t c , Z.-B ahjoedi, easing N or mod ytic , y, • y v R.P hlor oduce SiCl 33 . F D k w een acti 4 . Inthissystem, olv v Y , w . Cao, , 1(2011). ed c eng , le f ynamic Sim xpr . J .-L. Liand . Ma, acti ACKNOWLEDGEMENTS ys ine decomposedintoacti R.Y ed inthef Kh.FuadandM. as suppor hen suf Cl , ac Y CO ., Cl 2CO +SiO theacti , e f . D , or SiO essed asf .L. F

• kson andP hlor al Uni 12 H.-P subsequentl 44 Z.K.Zhang va . Hong 2 . Hu, or +C Cl va

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