MODELING THE TAPPING OF SILICON MELT FROM THE SUBMERGED ARC FURNACES

Transcription

1 Seventh Intenational Confeence on CFD in the Mineals an Pocess Inusties CSIRO, Melboune, Austalia 9-11 Decembe 009 MODELING THE TAPPING OF SILICON MELT FROM THE SUBMERGED ARC FURNACES Mehi KADKHODABEIGI 1, Halva TVEIT 1 an Stein Toe JOHANSEN 1 Depatment of Mateials Science an Engineeing, Nowegian Univesity of Science an Technology (NTNU), PO Box7491, Tonheim, NORWAY SINTEF Mateial an Chemisty, Flow technology goup, PO Box 7465, Tonheim, NORWAY ABSTRACT In this pape we investigate flow phenomena elate to tapping of silicon melt fom a submege ac funace. The multiphase flow moel peicts how the gas pessue vaiations insie the funace cate zone may effect phenomena lie the gas flow patten insie iffeent zones of the chage mateials insie the funace, the metal tapping flow ate, an the gas blow out fom the funace tap hole. The full D multiphase flow moel of the funace is base on the eal geomety an the most pobable physical popeties of iffeent zones in the funace, such as ensity, chage mateial paticles size, poosity an pemeability of the pace bes on the funace bottom. In aition iffeent spatial aangements of chage mateial zones have been investigate. The esults of the moel show that existence of the mentione high pessue cate zone in the funace has a consieable effect on the metal tapping an the gas flow ate fom the tap hole. The peicte tapping ate compaes well to inustial tapping tials, using cate pessue levels that has been measue peviously on a feosilicon funace. The moel also shows that gas blow out fom the tap hole can change the gas flow patten insie the chage mateials all the way to the funace top. NOMENCLATURE ρ v ε g μ L P p α C T ensity of phase velocity of phase voi faction (poosity) gavity acceleation viscosity of phase epth of the pace be pessue mean paticle iamete pemeability inetial esistance tempeatue INTRODUCTION Silicon commecially is pouce though cabothemic euction of quatz (SiO ) in the submege ac funaces. The funace consists of a boy with the shape of a shallow cucible, mae of a steel shell an a lining that can withstan extemely high tempeatues an attac by chemicals. The funace is fille up to its im with aw mateials chage by gavity though a system of silos an tubes. As they ae consume by eactions (smelting) in the inne zones of the funace the aw mateials ae tanspote ownwas in the funace. The enegy equie fo the chemical eactions is elivee as -phase altenating cuent though thee cabon electoes positione vetically an submege eep into the chage mateials. The supplie enegy is by lage issipate as heat though the electic acs in the gas-fille cavities suouning the tips of the electoes insie the funace (Schei (1998)). Silicon poucing chemical eactions mainly happen in the high tempeatue zone of the funace nea by the electoe tips (the cate zone). The molten silicon, as the main pouct, is pouce though the net chemical eaction below (1) an is accumulate on the funace bottom. The metal is tappe fom the tap hole esigne in the funace bottom. SiO ( s) + C( s) = Si( l) + CO( ) (1) g The tapping is a citical opeation in the silicon pouction pocess. The tapping pocess is simply how to bing molten silicon fom the funace into the lale. The tapping pocess has a iect impact on the total pocess pefomance. If the tapping of pouce silicon fom the funace heat fails, the pouction must be stoppe until silicon can be tappe in a pope way. Goo aining of the molten metal in the funace is essential to obtain an optimum silicon yiel. The tapping pocess an hence the metal flow ate fom the tap hole is influence by the funace intenal conitions. Depening on the iffeent opeating paametes of the funace, such as tempeatue istibution, chemical eactions ate insie the cate zone, flow of chage mateials into the cate zone an flow of eactive gases though chage mateials in the uppe pat of the funace, iffeent zones ae fome insie the funace. Each of these zones theefoe has its own physical popeties such as ensity, paticles size, poosity an pemeability. The existence of these egions insie the funace can affect the tapping pocess in many iffeent ways. The silicon metal accumulate ove the funace bottom is emove fom the funace by opening of the tap hole uing the tapping pocess. Howeve the silicon Copyight 009 CSIRO Austalia 1

2 metal is not the only mateial on the funace bottom. The esults of funace excavations have shown that besie silicon metal thee ae layes of soli an poous mateial mainly compose of silicon cabie (SiC) an to a lesse extent uneacte an high viscosity molten silica (SiO ) ove the entie funace bottom. Existence of this poous egion aoun the tap hole is the main eason fo euce metal flow ate fom the tap hole. In fact the pouce silicon metal must ain though this poous egion an existence of high viscosity molten silica in the poous egion aoun the tap hole causes the phenomenon which is well nown as tap hole clogging. The secon issue elate to the tapping pocess is the existence of high gas pessue conition insie the cate zone of the funace. In fact ue to the existence of chemical eactions insie the chage mateials, thee is always a high gas pessue situation in the funace heat. Because of euce pemeability of chage mateials ue to these eactions an melting of chage mateials in the egion nea by the electoe tips (the cate walls), the gas pessue in the cate zone inceases to highe levels than what coul be peicte by the opeation. Existence of this high pessue gas zone some times leas to blowing out of high velocity hot gasses fom the taphole uing tapping. Investigations an measuements of the gas pessue insie the cate zone of the submege ac funace fo silicon an feosilicon pouction have been one in pevious stuies (Johansen (1998), Tveit (00)). A D insie view of a submege ac funace use fo silicon pouction togethe with the peviously mentione issues is pesente in figue 1. Poous zone of chage mateials Electoes Gas flow though chage mateials High pessue cate zone Pace be at the funace bottom Gas flow fom taphole Tapping Figue 1: Schematic of submege ac funace use fo Silicon pouction. In this pape a D CFD multiphase moel fo the tapping pocess of silicon melt fom the submege ac funace is popose. Ou goal is to investigate how iffeent paametes such as high gas pessue in the cate zone an a low pemeability situation on the funace bottom an nea the tap hole can affect the silicon melt tapping flow ate. Moeove the flow patten of eactive gases though the chage mateials in the iffeent zones insie the funace an the off gases fom the funace top is stuie. The geometical an opeational ata ae fom the eal opeating funace an the insie conition an physical popeties of iffeent zones in the funace has been selecte base on the most pobable conitions insie of the funace base on liteatue an pevious scientific wos (Shei (1998, 1967), Otani (1968), Myhaug (00)). The esults ae in goo ageement with eal ata fom inusty which itself shows the valiity of the evelope moel. MODEL DESCRIPTION In oe to simulate the tapping pocess of silicon melt fom the submege ac funaces, a full D multiphase moel of the funace was evelope. The multiphase system is compose of chage mateials, molten metal an eaction gases insie the funace. The bul of chage mateials insie the funace wee ivie into iffeent zones an each zone has its own physical popeties. The paticles size an the poosity of the pace be of chage mateials in each zone is the cucial paamete which etemines the pemeability an hence esistance of the zone against the flow of pocess gas though chage mateials up to the funace top. The pemeability istibution futhe contols the ainage of pouce molten metal though the pace be on the funace bottom towas the taphole. In oe to moel the high gas pessue insie the funace a gas souce was applie insie the cate zone. The electoes, mae of cabon mateials, wee efine as soli pats in the moel. The pemeability of the pace be of soli paticles in the bottom laye of the funace, whee the molten silicon has been accumulate, was consiee to ecease as the istance fom the funace cente inceases. The taphole an the funace top wee consiee as pessue outlets at atmospheic pessue. MODEL EQUATIONS The physical moel use in this stuy is base on the unsteay tubulent multiphase flow of pocess gasses an molten silicon. The fluis ae teate as immiscible but ae inteacting with poous meia of chage mateials fo the gas phase an pace be of soli SiC an slag fo the liqui an gas phases in the funace. Theefoe the govening equations fo each phase in the system ae continuity equation: ρ + t An the momentum equation: t ( v ) + ( ρ v v ) ( μ v ) S ( ρ v ) = 0 () ρ = ε P + ερ g + () Whee the poous meia viscous esistance to the flow is base on the Egun s equation fo poous meia as follows (Egun 195): ΔP 150μ (1 ε ) 1.75ρ(1 ε ) = v + v ΔL p ε pε μ 1 S = v + C ρ v v α ε α = p 150 (1 ε).5 (1 ε ) = (4) (5) (6) C p ε (7) Copyight 009 CSIRO Austalia

3 The tubulence moel use in this wo is the stana -ε moel using the tanspot equations fo the tubulent inetic enegy () an its issipation ate (ε). NUMERICAL METHOD In oe to pefom the numeical solution of the moel equations, the FLUENT softwae vesion 6..1 was use. To constuct the intephase between liqui an gas phases in the funace, the volume of flui (VOF) metho togethe with the CICSAM scheme, suitable fo fluis with big viscosity iffeence, was chosen. In oe to moel the effect of the pace be on the flui flow the poous meia bounay conitions available in the softwae was applie to the moel. The volume of flui (VOF) metho is itself able to inclue the effects of intefacial tensions an wall ahesion wheneve these effects ae consieable. SIMULATION SETUP Geomety The eal ata fo the geomety of the submege ac funace was use in this stuy. The effective funace iamete an its effective height ae 10.5m an.m espectively. The taphole iamete an the taphole length wee 0.1m an 0.m. Electoes wee epesente as soli meiums an the iamete of each electoe is 1.8m. The eal geomety of the funace which has been use in this stuy is pesente in figue. =1.8m mateials an the pace be laye at the funace bottom wee base on the ata foun in the liteatue an the esults of funace excavations. Depening on the position of the zone in the funace, the poosity anges fom 15% up to 60%. A D view of the pace be of chage mateials, consisting of iffeent zones insie the funace is pesente in figue. top chage cust chage top of gas zone Chage besie Cate cate zone wall Pace metal zone Active top chage Active cust Chage top of gas zone Gas zone Metal zone top chage cust chage top of gas zone Cate Chage besie wall cate zone Pace metal zone Figue : D view of iffeent zones insie the silicon pouction funace. RESULTS AND DISCUSSIONS In oe to show the effect of high pessue in the cate zone on the silicon metal flow ate uing the tapping pocess, iffeent cases stuies wee evelope. In the fist case thee was no excess gas pessue in the cate zone an only the effect of gavity ainage of the silicon melt fom the pace be laye on the funace bottom was investigate. The flow ate of silicon melt fom the funace taphole vs. time is pesente in figue 4. It shoul be note that in all case stuies the initial metal height in the funace was consiee to be the same as the taphole level which is 0.11m Ø=0.1m h=.m Tapping flow ate (g/s) D=10.5m Figue : Pessue pattens insie the submege ac funace. Due to the complexity of the funace geomety an the equie accuacy in oe to epesent the flui flow in the egion nea by the taphole, non-confomal gi meshes wee use. Mateial Popeties The popeties of the silicon melt an eaction gasses insie the funace wee selecte base on the opeating tempeatue an flui compositions. Fo the gas phase we consiee a mixtue of SiO an CO gas. The ensity of the gas mixtue in the opeating tempeatue is 0. g/m. In the silicon melt the ensity an viscosity wee epesente as follows (Lia (1998), Rhim (000)): t (sec) Figue 4: Tapping flow ate of silicon melt fom the funace taphole without excess gas pessue. In the next five cases we investigate the high pessue in the cate zone an its effect on the molten silicon flow ate an the stating time fo the gas blowing out fom the taphole. This phenomenon is nown as taphole gassing. The ange fo the gas pessues in the cate zone of the funace was vaie in the ange between 0-41mba. The consiee ange is base on the measuements epote in []. The pessue patten insie the cate zone of the funace is pesente in figue 5. ρ m = ( T 1687) g / m (8) ( T 1687) g / m s μ (9) 6 m = The woing tempeatue of the funace was constant at C. The popeties of the poous zone of chage Copyight 009 CSIRO Austalia

4 950 Stating time fo gassing taphole phenomenon (s) Cate pessue (mba) Figue 5: Pessue pattens insie the submege ac funace. A compaison between the above mentione case stuies fo iffeent gas pessue insie the cate zone of the funace is shown in figue 6. As it is obseve fom this figue the pessue incease in the cate zone will lea to incease metal flow ate an hence eceases the tapping time befoe gas blowing out fom the funace taphole is initiate. By compaing with the case of zeo excess gas pessue, figue 4, it is clea that in the existence of high pessue cate zone can incease the metal flow ate up to times. The eason fo selecting the consiee ange fo cate gas pessues is the measuements epote in [, ]. Figue 8: Relation between cate pessue an stating time of gassing taphole phenomenon. As it can be seen fom figue 8 the cate pessue also plays an impotant ole in etemining when the gassing taphole phenomenon stats. Futhe moe the esults of the moel show that the cate pessue affects on the total amount of metal tappe fom the funace. As seen fom figue 9, an incease in cate pessue will lea to incease ainage of metal fom the funace. At high cate pessue tapping is faste, but a smalle faction of metal can be aine befoe sevee gas blowing stats. The velocity patten of eaction gasses insie the poous bul of chage mateials insie the funace is shown in figue Tapping flow ate (g/s) Cate pessue=0 mba Cate pessue=5 mba Cate pessue=7 mba Cate pessue=8 mba Cate pessue=41 mba Tappe metal befoe stat of gassing taphole phenomenon (%) t (sec) Figue 6: The effect of cate gas pessue on both the tapping flow ate an stating time of gassing taphole phenomenon Cate Pessue (mba) Figue 9: Pecentage of total metal in the funace befoe gassing taphole phenomenon stats. The esults of the CFD moel show that thee is a iect elation between maximum tapping flow ate an cate pessue. The inceasing maximum metal flow ate with cate pessue is seen clealy fom figue Maximum tapping flow ate (g/s) Cate Pessue (mba) Figue 7: Maximum tapping flow ate inceases with incease of cate pessue. Figue 10: Velocity contous of eaction gasses in the chage mateials uing the tapping pocess Fom the figues 10, 11 we see that the gas flow patten though the funace top epens on the tapping situation. Duing nomal opeation the gas flow is symmetic though the chage suface, while uing tapping togethe with gassing phenomenon, moe gas exits the chage suface close to the taphole aea. Duing tapping with gassing moe eactive funace gasses flow close to the taphole an may incease the Copyight 009 CSIRO Austalia 4

5 local metal poucing eaction in an intemittent manne (gassing-tapping cycle). In oe to valiate the moel ata fom tapping of metal fom an inustial feosilicon funace is use. In the inustial campaign weight of tappe metal vesus tapping time was ecoe. The paametes of the CFD moel wee tunne base on the inustial funace esign. Because the actual feosilicon funace was smalle than in the pevious calculations, the cate pessue was set to 9 mba. With these moifications (funace size an cate pessue) almost pefect ageement with the inustial ata was obtaine. The oveall esults ae shown in table 1. a b The esults of the moel ae in goo ageement with eal ata fom the tapping pocess an the moel is able to geneate the inustial ata with easonable egee of accuacy. REFERENCES SCHEI, A., TUSET, J. an TVEIT, H., (1998), Pouction of High Silicon Alloys, Tapi folag, Tonheim, Noway. JOHANSEN, S. T., TVEIT, H., GRÅDAHL, S., VALDERHAUG, A. an BYBERG, J., (1998), Envionmental aspects of Feo-Silicon funace opeations - an investigation of waste gas ynamics, Infacon 8, Beijing, 7-10 June. TVEIT, H., HALLAND, T., LANDRØ, K. I., JOHANSEN, S. T. an RAVARY, B., (00), The tapping pocess in silicon pouction, Silicon fo the chemical inusty VI, Leon, Noway, 17-1 June. SCHEI, A., (1967), On the chemisty of feosilicon pouction, Tissift Kjemi Begvesen, Metallugi 7, 8-9, OTANI, Y., SAITO, M., USUI, K. an CHINO, N., (1968), The inne stuctue in submege ac funace, N. 1, iscussion N 117D, VIth Intenational congess of electo heat, Bighton, Englan. MYRHAUG, E. H., (00), Non-fossil euction mateials in the silicon pocess-popeties an behaviou, D.-ing thesis, NTNU, Noway. ERGUN, S., (195), Flui Flow though Pace Columns, Chem. Eng. Pog, 48(), LIDA,T. an GUTHRIE, R.I.L., (1998), The physical popeties of liqui metals, Claenon Pess, Oxfo. RHIM, W.K. et al., (000), Jounal of Chystal Gowth, 08, N.1-4, 1-1. Figue 11: Gas velocity patten on the funace top (a)uing nomal tapping an (b) uing the taphole gassing situation Paamete Inustial Data Moel Result Aveage metal flow ate (g/s) Aveage tapping time (s) Total metal tappe (g) Stating time of gassing taphole phenomenon (s) Vaiable 1817 Table 1: Moel esults in compaison with inustial ata CONCLUSION A full D multiphase moel of tapping of metal fom the submege ac funace has been evelope. The effects of high gas pessue insie the cate zone an at the pemeability of the pace be in the funace bottom laye, on the tapping flow ate of metal fom funace into the lale was stuie. Moeove the flow of eaction gasses though the poous meia of the chage mateials insie the funace an also the off-gasses fom the funace top was investigate. Existence of a high gas pessue insie the cate zone inceases the tapping flow ate an it is a majo eason fo the taphole gassing phenomenon. Incease gas pessue in the cate zone ue to euce pemeability of chage mateials above the cate zone can lea to highe metal flow ate an ealie bea though of gas fom taphole. Copyight 009 CSIRO Austalia 5