DEVELOPMENT OF AN IN SITU PREBAKlNG ELECTRODE SYSTEM FOR SILICON METAL PRODUCTION

Transcription

1 NFACON 7, Trndheirn, Nrway, June 1995 Eds.: Tuset, Tveit and Page Publishers: FFF, Trndheirn, Nrway 431 DEVELOPMENT OF AN N STU PREBAKlNG ELECTRODE SYSTEM FOR SLCON METAL PRODUCTON Amfinn Vatland 1 and William Bruff2 lelkem afs Research, Carbn, P.D.Bx 40 Vagsbygd, N-4602 Kristiansand, Nrway 2Lerkeveien 7, N-4622 Kristiansand, Nrway ABSTRACT A prebaked carbn electrde system develped by Elkem a/s in Nrway is nw in regular peratin n silicn metal furnaces. The incrpratin fthe baking prcess int the silicn prductin furnace has reduced the electrde csts cmpared t the cnventinal ff-furnace manufactured prebaked electrdes. The changever with the intrductin f an additinal prcess and new equipment n the electrdes has been successful. The develpment effrts and results are included. NTRODUCTON The S~derberg prcess fr frmatin f carbn electrdes in situ in electric furnaces is very well knwn. n general, the S~derberg prcess vercmes the need fr manufacturing electrdes in a separate manufacturing facility by enabling the prductin f the electrde in situ frm an unbaked electrde paste cmprising a particulate carbn material and a carbnaceus binder. The S~derberg electrdes n electrthermic furnaces are typically equipped with a cnsumable casing which fllws the dwnward mvement f the electrde int the charge. The electric current is supplied t the casing f the electrde bth fr baking the electrde pas te as it prgresses dwnward tward the charge and fr supplying perating current t the electrde.

2 432 Unfrtunately, the cntinuus Sl2Sderberg electrde is nt permitted fr silicn metal prductin fr the fllwing reasns: The prduct will be cntaminated by the high irn cntent f the casing material. Depending n the energy utilized fr the smelting prcess the baking rate f carbn electrdes is slw cmpared with the cnsumptin rate in silicn metal furnaces. Fr silicn metal prductin it is mst cmmn t use the cnventinal ff-furnace manufactured prebaked electrdes. Their level f chemical purity is high t avid plluting the end prduct. The electrde clumn cntains several large pieces each jined tgether. n this way the electrde can be cnsumed at a rapid rate, i.e. twice that f an electrde in 75% ferrsilicn peratin. The jints are the weak pints in the clumn during peratin, and therefre cstly accurate machining, special precautins during transprtatin, special munting prcedure and gd electrde material characteristics are required t avid electrde breakage. The prebaked electrde cst is tw t three times that f the Sl2Sderberg electrde cst. The present paper deals mainly with the R&D-activities by Elkem a/s in Nrway in cnnectin with an imprved in situ prebaking electrde system fr silicn metal prductin. Elkem a/s as a prducer f silicn metal, electrde paste and a supplier f electric reductin furnace equipment has put a lt f effrts int The Bruff Electrde. Nw the system is in regular peratin n silicn metal furnaces representing in ur case an alternative t the ther carbn electrde systems fr the prductin f lw irn silicn. A few years ag C. F. Fulgenzi presented a status f the mst interesting new carbn electrde systems [1]. He tld f the hundreds f reprted effrts t develp better carbn electrdes f which nly three have shwn utstanding prgress and prmise fr cmmercial use: The Cavigli System, The Perssn Electrde, and The Bruff Cncept. DESCRPTON The methd relates t an imprved prcess fr the manufacture f electrdes in situ, and mre particularly t an imprved prcess f frming a carbn electrde in the upper parts f el~ctr-metallurgical furnaces [2]. FG. 1 shws a schematic view f a lngitudinal sectin f an electrde f the cnventinal Sl2Sderberg prcess, and f an electrde in accrdance with the imprved prcess.

3 433 v.s. Patent Jul. 9, ,527,329 /6 ~:~--- 0 ~=A=_~ /0 /4 / '.., 0.' c.. c r; :0 ~ :, v - c c c.( ELECTRC CURRENT /2 " '.,. c c' '... c.. C " " L' 0. c.;.t..,',. ~.. a. : C " C.' '.'.: 0. PROR ART FG. 1. Schematic View f the S~derberg Prcess (Prir Art) and f the mprved Prcess fr the Manufacture f Carbn Electrdes.

4 434 As shwn in FG. 1, the cntinuus in situ prebaked electrde is btained by the prcess f sftening and baking an electrde paste in a relatively small first zne which prvides fr cntrllable heating and baking f the electrde paste. The baked electrde advances frm the first heating zne t a secnd ne with the cntact clamps which supply electric current t the electrde. n this way the heat fr the baking prcess is independent f the energy utilized fr the smelting prcess in the electric furnace. This separatin f the tw prcesses makes it pssible t achieve a fast baking electrde t exceed the cnsumptin rate f the carbn electrde in silicn metal peratin. The heat in the first zne is adjusted until the paste is baked sufficiently slid and strng s that the suspensin f the electrde and slipping can be effected by a separate hlder dispsed between the primary and the secndary zne. n rder t btain cntrllable heating in the first zne a baking furnace is installed [3]. FG. 2 shws schematically the baking furnace t which the energy utilized fr baking f the electrde has t be supplied frm an external surce. The methd fr baking f the carbn electrde is as fllws: 1. The electrde paste is frmed int a simplified steel casing withut internal fins in rder t reduce manufacturing csts f such casings. 2. The casing which is filled with unbaked electrde paste is slwly and cntinuusly r intermittently lwered dwn thrugh the baking furnace. 3. The gases which evlve when heating the electrde paste will flw int the baking furnace thrugh the perfrated casing. By supplying air, the gases burn in the baking furnace. The ttal amunt f supplied external energy and energy generated by burning f the baking gases is sufficient t maintain a baking furnace temperature between and C. 4. The casing is remved frm the baked carbn bdy belw the baking furnace. The baking furnace including equipment fr cutting ff the used casing is arranged abve the suspensin system f the electrde in the electric furnace. n this way the prduct which is prduced in the smelting furnace is nt cntaminated by the irn f the steel casing.

5 v.s. Patent Sep. 16, ,612, : ".. "..-." "., 3 " / FG. 2. Schematic View f a Baking Furnace fr the Prductin f Carbn Electrdes.

6 436 RESEARCH AND DEVELOPMENT General The idea f the imprved in situ prebaking electrde cncept was presented inside the cmpany in Hwever, fr varius reasns research activities did nt start befre ne decade later. As the cncept was new and unprven ne decided t tryut the system step-by-step. The bjective was t incrprate the baking prcess n an existing silicn prductin furnace equipped with the cnventinal fffurnace manufactured prebaked electrdes. Hence, the changever had t include the intrductin f an additinal prcess and new equipment n the electrdes. Baking Furnace n 1977 calculatins with E1kem's electrde temperature mdel indicated gd pssibilities fr the baking f an electrde in a relatively small baking furnace. n 1979 the experiments started in the pilt plant where a self-cnstructed baking furnace was tested ut n a 355 mm (14") diameter electrde. The baking prcess was prmising, and the electrde was able t cnduct electric current in a single-phase pilt smelting furnace. Encuraged by the results the testing in the pilt plant cntinued, and the baking furnace and the electrde were increased step-by-step up t 1400 mm (55") in diameter. During peratin f the baking furnace, fr sme weeks a hard carbnized material was built up in the upper part f it. Because f this strng material it was nt pssible t mve the electrde relative t the baking furnace, and the prcess had t be stpped in rder t remve the layer f carbnized material. This serius prblem was vercme by arranging a cling chamber, fr circulatin f a cling medium, between the upper part f the refractry lining and the carbn bdy which is being prduced [4]. Mvement and Suspensin f Baking Furnace The first versin f the in situ prebaking system was installed in 1983 n ne f the electrdes f a three-phase silicn furnace at E1kem Fiskaa Silicn plant in Kristiansand. The electrde diameter was 890 mm (35").

7 437 The principle f prducing an electrde fast and independently f the energy utilized fr the smelting prcess, was t be tried ut by a suspensin arrangement in which the baking furnace can be mved, n t the electrde. T be prepared fr an electrde breakage, the equipment was designed t keep a reserve f abut 2 meters f extra baked electrde in the clumn. The result f this first cnstructin was prmising, but during lng-term peratin serius prblems ccurred due t deviatin frm a straightlined electrde. After a few adjustments, cnvincing results were achieved fr the actual phase, i.e. 270 days withut any electrde breakage. Based n all experience gathered frm the first installatin, a secnd versin f the in situ prebaking system was intrduced, and has been in regular peratin n all three electrdes fr many years n anther silicn furnace with 900 mm (35.43") clumns [5]. FG. 3 shws a schematic view f an electrde in an electric furnace with the arrangement fr mvement and suspensin f the baking furnace. FG. 3 illustrates a baking furnace with a frame which is fixed t the electrde suspensin system. Frm the tp f the baking furnace frame three rails, in the frm f spindles with threads, are suspended t enable the baking furnace t mve up r dwn. n rder t prduce a hmgeneus electrde the fllwing is imprtant regarding baking furnace mvement: The baking furnace is mving cntinuusly r intermittently, n t the electrde, with a velcity which crrespnds t a pre-set baking speed fr the electrde. The mvement f the baking furnace is intercnnected with the electrde slipping equipment in such a way that when the electrde is slipped a certain increment thrugh the hlding and slipping rings, the baking furnace will autmatically be mved an equal distance dwnwards. When an electrde breakage, r high rate f cnsumptin ccurs, the baking furnace will be mved dwnwards t a lw psitin n t the electrde, which means less reserve f baked electrde. A maximum permitted baking speed is then used t gain the nrmal psitin.

8 438 v.s. Patent Feb. 2, 1988 Sh~t 1 f 4 4,722,684,/////////// ~ /3-..,,- 11/9,, i : Z4~ 23 Z~Z7 7 /7"! d, 1--/7 /6-, -.~ Z/ ",.1.1 i!, 1--/4 ~ /0 11",1,'''''1111'1 1-/5 i i i j! -_. ~i' '.=- : /2 '-FV.: 011' ; :~, 11 i /'7 15- /0\ ~=-=~=:--=rt~-=-~ ~~--_ -= FG. 3. Schematic View f an Electrde in an Electric Furnace with the Arrangement fr Mvement and Suspensin f a Baking Furnace.

9 439 As the electrde has t pass thrugh three fixed pints in the clumn, namely the baking furnace, the electrde hlding and slipping equipment, and the current clamps, any deviatin frm straight-lined electrde will result in clamping r sticking f the electrde. The electrde will thus either break at a pint between the three fixed pints which it runs thrugh r it will be impssible t slip the electrde thrugh the electrde hlder. This serius prblem was vercme by the new arrangement fr suspensin f the baking furnace. Cutting ff Electrde Casing n rder t cut and remve the used steel casing belw the baking furnace, the peratrs frm the beginning used a cutting trch r angular milling cutter. Due t limited free space in the area f the electrdes abve the furnace rf and the danger f shrt-circuiting, the furnace peratrs usually are nt allwed t wrk in this area when the furnace is running. t is therefre necessary t shut dwn the furnace when peratrs are cutting ff the electrde casing manually. A device fr cutting the electrde casing has been develped [6]. By using this device the casing is autmatically cut bth vertically and hrizntally; then the casing can be remved in frm f relatively small pieces. CONCLUSONS Fr many years a lt f effrts have been put int the develpment f an in situ prebaking electrde system fr silicn metal prductin, based n the idea f installing a baking furnace in the upper part f the electrde clumn and using a simplified steel casing which is remved frm the carbn bdy belw the baking furnace. This has led t the fllwing cnclusins: A practical and wrking electrde system is in regular peratin n silicn metal furnaces, representing in ur case an alternative t the cnventinal ff-furnace manufactured prebaked electrdes. The incrpratin f the baking prcess int the smelting furnace has reduced the electrde csts t less than half the csts f the prebaked electrde. The baking rate is sufficient t exceed the cnsumptin rate f carbn electrde in silicn metal peratin.

10 440 When an electrde breakage, r high rate f cnsumptin ccurs, the reserve f abut 2 meters f extra baked electrde kept in the clumn, is utilized. Therefre, even in such cases, cmpared t prebaked there are n extra lsses f metal prductin in the smelting furnace. The prduct which is prduced in the smelting furnace is nt cntaminated by the irn f the steel casing. t is nt necessary t shut dwn the smelting furnace peratin when cutting ff and remving the used electrde casing. The changever with the intrductin f an additinal prcess and new equipment n the electrdes has been successful. Hwever, t avid setbacks it is als imprtant t fcus cnsistently n the baking prcess and the electrde equipment in the same prper way as it is dne in the case f the smelting prcess and the electric furnace. REFERENCES 1. C. F. Fulgenzi, "New Electrdes fr Arc Furnaces", The ABM Cnference, Vitria, Brazil, December W. Bruff, G. S. Santana, and D. G. Oliveira, Prcess fr the Manufacture "n Situ" f Carbn Electrdes. U. S. Patent 4,527,329. Appl. Jul. 12, 1982: Acc. Jul. 9, W. Bruff and L. Olsen, Methd fr Cntinuus Prductin f Elngated Carbn Bdies. U. S. Patent 4,612,151. Appl. Nv. 5, 1984: Acc. Sep. 16, E. Q. Dabl, A. Vatland, and O. T. Vegge, Baking Furnace fr Electrdes. U. S. Patent 4,678,434. Appl. Aug. 7, 1986: Acc. Jul. 7, H. M. Kvivik, Arrangement fr Suspensin f a Baking Furnace fr Electrdes. U. S. Patent 4,722,684. Appl. Jul. 14, 1986: Acc. Feb. 2, O. T. Vegge, Device fr Cutting an Electrde Casing. U. S. Patent 4,663,844. Appl. Apr. 28, 1986: Acc. May 12, 1987.