The effect of argon flow intensity on mass loss rate in the carbothermic reduction of SiO2 to SiC was examined. The graphite and natural quartzite in a molar ratio of C/SiO2=3 were used. Samples were reduced for 6 hours at 1 500 °C. In the range of argon flow intensity from 0,1 to 3,4 dcm3/min, the high level of rate of mass loss of the sample and the 70 – 80 % of SiC formed in the residue of the sample were obtained

J. Węgrzyn

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The influence of intensity of argon flow on the rate of mass loss in carbothermal reduction of quartzite to silicon carbide

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185METALURGIJA 55 (2016) 2, 185-188J. WĘGRZYNTHE INFLUENCE OF INTENSITY OF ARGON FLOW ON THE RATE OF MASS LOSS IN CARBOTHERMAL REDUCTION OF QUARTZITE TO SILICON CARBIDE Received – Primljeno: 2015-03-18Accepted – Prihvaćeno: 2015-10-15Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 55(2) 185-188 (2016)UDC – UDK 669.782:669.784:669.052:66.094.1=111J. Węgrzyn - Silesian University of Technology, Faculty of Materials Engineering and Metallurgy, Katowice, PolandThe effect of argon flow intensity on mass loss rate in the carbothermic reduction of SiO2 to SiC was examined. The graphite and natural quartzite in a molar ratio of C/SiO2=3 were used. Samples were reduced for 6 hours at 1 500 °C. In the range of argon flow intensity from 0,1 to 3,4 dcm3/min, the high level of rate of mass loss of the sample and the 70 – 80 % of SiC formed in the residue of the sample were obtained.Key words: silica, carbon, carbothermal, reduction, intensity of argon flow INTRODUCTIONVery few articles concern the influence of flow rate of inert gas on mass loss of the sample in carbo-thermic reduction of SiO2 to SiO, among them for example [1-3]. In the paper by F. K. Van Dijen, R. Metselaar [1] the increase in the flow of He from 0,05 to 0,1 dcm3/min resulted in a modest increase in the speed reduction. The increase from 0,1 to 0,2 dcm3/min results in a mini-mal increase in the rate of weight loss. Whereas, in the paper by C.Y. Chen et al. [2] with an increase in the flow of He from 0,6 to 3 dcm3/min decreases the yield of SiC and also decreases conversion of SiO and C. The authors of work [2] claim, that the authors of work [1] reported opposite results and gave no explanation of this.V. Sahajwalla et al. [3] show the influence of inten-sity of Ar flow on the rate of mass loss such that there is optimum value. Maximum of mass loss is at inten-sity of Ar flow 1 dcm3/min. Lower (0,5 dcm3/min) and higher flows (2 dcm3/min) make mass loss smaller. This work, however, applies to a relatively low SiO2 content in the sample (10 % of mass) with very high molar ratio C/SiO2 = 45 and in some ways differs from other studies [4].Therefore, first of all it was decided to investigate the effect of VAr on the rate sample weight loss in carbothermic reduction of SiO2 to SiC. The main con-cern here was not the highest rate of weight loss, rather the rate of argon flow to obtain the biggest possible yield of SiC. Thus, the aim of the research is to find a VAr intensity to get the greatest amount of SiC in the shortest possible time. PROCEDURE OF EXPERIMENTSInvestigation of the effect of argon flow rate on mass loss in carbothermic reduction of SiO2 to SiC was per-formed in thermobalance “Thermogravimat” type Sar-torius 4304. Thermobalance was equipped with vertical tubular reactor of fused Al2O3 - “Alsint 99,7” – of inner diameter 27 mm. The reactor has a gas delivery and gas removal to enable directing the gas flow both from top to bottom and reverse. During carbothermic reduction of SiO2 to SiC (CTR) argon was blown from top to bot-tom of the reactor with a quartz tube of internal diame-ter 2,7 mm. The end of the blowing tube was positioned 32,5 cm from the edge of the crucible with the sample. Thermocouple PtRh10-PtRh6 was used to measure the temperature in the reactor. The analysis of oxygen content in samples was per-formed on a LECO analyzer ONH836. The study was conducted with the use of natural quartz-ite marked BG and graphite from SGL Carbon Group Po-land SA Raciborz. Sieve analysis resulted in obtaining quartzite and graphite particles smaller than 63 μm. Sintered Al2O3 crucibles produced by Refractory Materials Branch in Gliwice were used. The dimensions of crucibles: diameter outer/internal 15,2/12,5 mm, height external/internal 18,5/16,94 mm. Separate components of SiO2 and C samples were weighed and then subjected to drying at approx. 200 °C for about 2 h. After that time, the components showed no weight loss. Then the components were carefully weighed and divided into samples of molar ratio C/SiO2 = 3. Dried and continuously heated ingredients were thoroughly mixed. The portions of samples of 500 mg weighed from the mixture, were then compressed with a force of 161,41 MPa, forming cylindrical pellets hav-ing a diameter of 12 mm and a height of about 2,5 mm. Pressed pellets were ejected from the die directly to a previously dried and weighed crucibles. Each sample 186J. WĘGRZYN: THE INFLUENCE OF INTENSITY OF ARGON FLOW ON THE RATE OF MASS LOSS IN CARBOTHERMAL... METALURGIJA 55 (2016) 2, 185-188with the crucible was dried prior to reduction of about 2 hours, i.e. not appearing until the weight loss. The cru-cibles with samples were then suspended on a platinum wire rod thermobalance. Warming the furnace reactor assembly to 1 500 °C, and moving it upwards to close the reactor of thermo-balance launch the reduction. Samples were reduced for about 6 h in vertical stream of argon flowing from top to bottom. Turning off the heating and retraction of the furnace to such a height that the sample was in the upper part of the reactor tube being in a stream of argon (this time flowing from bottom to top), stops the reduction. When the temperature dropped to about 200 °C, the sample was removed from the thermobalance suspen-sion. The crucible with the sample was weighed. Crucibles weight with the samples minus the weight of crucibles themselves are the mass of the samples be-fore reduction, referred to as W0. The differences be-tween the mass of crucibles with residue of samples af-ter reduction and the mass of empty crucibles are deter-mined as Wf. Theoretically, Wf is the weight of the resi-due of the sample after reduction. The loss of mass during the reduction of the sample is W 0 – W f. Due to tumbling, pressing and pushing from the die, initial masses of samples were not exactly the same. In order to compare the weight loss curves of the samples at dif-ferent flow rates of argon relative mass loss of samples (fractional weight loss) was introduced (W 0 – W f) / W 0. RESULTSThe resulting weight loss curves of a relative weight loss of the sample (W 0 – W f) / W 0 as a function of re-duction time at different values of flow rate of argon at 1 500 °C are shown in Figure 1. Generally, the higher the intensity of argon flow rate, the higher go the curves (Figure 1). However, there are some exceptions. The curve obtained at the rate of 1,6 dcm3/min runs lower than the curve of 1,2 dcm3/min. Similarly, curves 2,6 and 3,4 dcm3/min go below the curve 2 dcm3/min. While the curve obtained at a flow of 0,1 dcm3/min passes above the curves 0,2 and 0,4 dcm3/min in the range of time reduction. These dif-ferences can be regarded as a characteristic of disper-sion of results of empirical research. Basically, they do not change the general tendency of increase in weight loss with increasing VAr. Due to short distances between the curves on the graph in Figure 1, relative mass loss after 360 minutes is shown in Figure 2. On the surface of some samples there appeared a layer of crust in light grey with a hint of pink or brown. Crust thickness decreased with the increase of argon flow rate. At a flow rate of 2, 2,6, 3,4 dcm3/min the crust is not present or is so thin that it can’t be identified. The crust layers were separated from the rest of samples and separately weighed and analyzed for oxygen content.  Weighing the content of crucibles revealed a smaller mass of residue samples W f2 than W f resulting from the calculation   W f = W 0 – m (1)At the same time, it turned out that the weight of crucibles after the reduction is greater than the weight of crucibles before the reduction. The mass of this is determined mx.0,300,350,400,450,500,550 0,8 1,6 2,4 3,2 4(W0-Wf ) / W0at t = 360VAr / dcm3/minFigure 1  Curves of relative mass loss (W0 – Wf) / W0 vs time at different intensity of argon flow, VAr, at 1 500 °CFigure 2  Relative mass loss received at 360 min reduction (W0 - Wf) / W0 vs intensity of argon flow, VAr187J. WĘGRZYN: THE INFLUENCE OF INTENSITY OF ARGON FLOW ON THE RATE OF MASS LOSS IN CARBOTHERMAL...METALURGIJA 55 (2016) 2, 185-188Between the components of samples (SiO2, C, SiC, SiO and CO) and the material of the crucible made of sintered Al2O3 a series of reactions is possible, for ex-ample [5]. Their detailed analysis is beyond the scope of this article. However, for purposes of the calculation of the mass balance of the sample before and after the CTR, it was formally assumed that SiO escaping from the sample reacts with Al2O3 crucible. Hence, the mass of the crust mcrust and the difference in weight between the observed loss, W f, and weighed remnant of the sam-ple W f2, and also weight gain crucible, Δmx, are related as follows  W f = W f2 + mcrust – mx (2)It is shown in a formal expression of modified mass balance equations, which, briefly are:    (3)   (4)Equations (3) and (4) supplemented with balance equations of the number of moles of Si, O, C, and the values of oxygen content in samples, enable to calculate unknown sample mass components. Mass balance sys-tem of equations will be presented in detail in the next publication.Relative proportion of components of the residue mass m fi referred to the mass of the sample residue W f2, after stopping the reduction after 6 hours is shown in Figure 3. This graph reveals gradual changes of con-stituents in the residue of the sample with an increase in the flow of argon. Up to the value of 2 dcm3/min we can observe a clear decrease in SiO2, CFree and CTot content and increase of SiC content. After exceeding 2 dcm3/min, there are visible fluctuations which do not show a clear tendency. One can discern slight reverse trends, i.e. increase in the content of SiO2 and CFree and decrease of SiC. DISCUSSION OF RESULTSThe values of the weight loss achieved referred to the mass of initial sample after 360 min (W 0 – W f) / W 0 (Figure 2), substantially increase with increasing argon flow rate to 2 dcm3/min. The data points in Figure 2 suggest that an increase in the rate of weight loss in the reduction of SiO2 with increasing argon flow rate approaches a certain value. However, it is not known whether it is a global maxi-mum dependence (W 0 – W f) / W 0 = f(VAr). This would require further research with even greater VAr. However, the scope of VAr used in the present study suggests that, starting with 2 dcm3/min does not form a crust layer and we obtain a fairly high content of 70 – 80 % SiC of sample residue. This can be considered a rather large value for CTR 6 h at 1 500 °C. The colour of formed crust layers on the surface of samples in the range of argon flow rate of 0,1 to 1,6 dcm3/min suggests that it may be a phase comprising SiO2. Under the conditions of the experiment the reac-mfSiO2mfCFreemfSiCCTot01020304050607080900 0,8 1,6 2,4 3,2 4mf/ Wf 2/ %VAr / dcm3/minFigure 3  Relative ma ss content of sample ingredients, mf / Wf2, and total carbon content, CTot vs intensity of argon flow, VArtions SiO + 1/2O2 = SiO2 and 2SiO = Si + SiO2 are pos-sible [6]. Just as in [7] it was observed, that SiO was condensed and deposited as silicon suboxide, SiOx, on the surfaces of cooler parts. In the present study SiOx was condensed on the ends of the wire slings Pt crucible and on the weld of PtRh10 – PtRh6 thermocouple. The oxygen content in mixture of Si + SiO2 is 36,29 %, and in SiO2 it is 53,26 %. Under the conditions of this ex-periment the measured oxygen content in crust layers is maintained at the average level of 52,2 % with a stan-dard deviation of 0,86 %. Such oxygen content suggests that the main component of crust layers is most likely SiO2. The observed mass loss rate and the calculated amount of SiC are a complex function of many param-eters of the study. These include properties of mixtures of C + SiO2, such as grain size, molar ratio of C/SiO2 sample, compression force, and the like, but also geo-metric conditions of the reactor. The latter refers to the distance from the end of the tube blowing to crucible, height of crucible walls, the shape of the sample and others, which results from, for example test [2] and ob-servations done in this study. Mass transfer conditions to the sample surface are crucial here. They affect the amount of SiO discharged, as they decide whether or not a crust layer is formed on the sample surface. Crust layer affects the rate of weight loss and the amount of SiC formed in the core sample. With the increase of VAr the thickness of crust layer de-188J. WĘGRZYN: THE INFLUENCE OF INTENSITY OF ARGON FLOW ON THE RATE OF MASS LOSS IN CARBOTHERMAL... METALURGIJA 55 (2016) 2, 185-188creases in such a way that, starting with 2 dcm3/min and above the crust does not form. It can be assumed that with further increase of VAr the amount of SiC formed may be reduced. However, within the range of VAr used in this study such a phenomenon did not occur.CONCLUSIONSThe experiment of carbothermic reduction of SiO2 to SiC for the intensity of argon flow rate: 0,1, 0,2, 0,4, 0,8, 1,2, 1,6, 2, 2,6, 3,4 dcm3/min was carried out here. Reactions were carried out for 6 hours. After that time the reduction was stopped (though it could be contin-ued). The study revealed the effect of argon flow intensity on the rate of mass loss of the sample. With the increase of flow rate increases the speed of weight loss of sam-ples. Achieved relative mass loss after 360 min is the largest for argon flow rate 2 dcm3/min. Similar behav-iour revealed SiC contents, calculated from mass bal-ance equations on the basis of oxygen content in residue samples. The calculated SiC content requires verifica-tion. It can be performed by analyzing the total carbon content, CTot, which will be the subject matter of the next publication. Determining the range or a single value of argon flow rate at which maximum created SiC occurs, is a parameter for further carbothermic reaction kinetic studies of SiO2 to SiC. LIST OF SYMBOLS CTR – carbothermic reduction of SiO2 to SiC – mass of crucible before CTR / g – mass of crucible after CTR / g – mass of the crust layer / gm fi  –  the final mass of the i-th component after stop-ping the reduction reaction / gVAr – intensity flow of argon / dcm3/min W 0 – initial mass of the sample / g(W 0 – W f) / W 0– mass loss achieved relative to the mass of the initial sample / - W f –  mass of the sample residue after stopp ing the reduction reaction calculated from the weight loss W f = W 0 – m / g, where  / g – mass of residue sample after stopping the reduction reaction, determined from a weighing sample without  and mx/ g – mass gain crucible / gREFERENCES[1] F. K. Van Dijen, R. Mets elaar: The Chemistry of Carbo-thermal Synthesis of β-SiC: Reaction Mechanism, Reac-tion Rate and Grain Growth; Journal of the European Ce-ramic Society 7 (1991), 177-184.[2] C.Y. Chen, C.I. Lin, and S.H. Chen: Kinetics of synthesis of silicon carbide by carbothermal reduction of silicon dio-xide; British Ceramic Transactions 99 (2000) 2, 57-62. [3] V. Sahajwalla, Ch. Wu, R . Khanna, N. Saha Chaudhury, J. Spink: Kinetic Study of Factors Affecting in Situ Reduc-tion of Silica in Carbon-Silica Mixtures for Refractories; ISIJ International 43 (2003) 9, 1309-1315. [4] J. Węgrzyn: comparison o f Mass Loss Rate in Reaction of Silica with Carbon from Different Investigation Results; Metalurgija 54 (2015) 3, 579-582. [5] Z. Zhu, T. Jiang, G. Li, Y. Guo and Y. Yang: Thermodyna-mics of Reactions Among Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes; Thermodynamics - Interaction Studies - Solids, Liquids and Gases, (J.C. Moreno PirajÃ¡n, ed.), InTech, (2011), pp. 825-838. Available from: http://www.intechopen.com/books/thermodynamics-interaction-studies-solids-liquidsand-gases/thermodynamics-of-reac-tions-among-al2o3-cao-sio2-and-fe2o3-during-roasting-processes[6] B. Machulec: Teoretyczne podstawy procesu elektroter-micznego wytopu żelazokrzemu oraz krzemu technicznie czystego, Zeszyty Naukowe Politechniki Śląskiej, nr 1589, Gliwice 2003, pp. 49, 50. [7] S. V. Komarov, D. V. Kuz netsov, V. V. Levina and M. Hi-rasawa: Formation of SiO and Related Si-Based Materials Through Carbothermic Reduction of Silica-Containing Slag; Materials Transactions 46 (2005) 4, 827-834. Note:  The responsible translator for English language is Jadwiga Węgrzyn, MA, Senior Lecturer, the Silesian University, Kato-wice, Poland

The influence of intensity of argon flow on the rate of mass loss in carbothermal reduction of quartzite to silicon carbide

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