CA1204806A - Electric furnace construction - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus for melting thermoplastic material is disclosed utilizing, in a preferred embodiment, a vessel having a bottom wall and upstanding sidewalls. A protective liner or structure for the vessel is provided, being formed of corrosion resistant material. The furnace may include both through-the-batch and floor electrodes, and also hot spot fining to a conductive outlet channel. A method is set forth for operating the furnace described herein utilizing symmetrical, below the batch heat dissipation for accelerated melting. In order to protect the liner, which in some instances may be fabricated from an oxidizable refractory metal, start-up burners are operated under a reduced oxygen supply.

Description

` Palmquist 1 ` ~Z~)4~1)6 ELECT~IC ~UR~.CE CONSTRUC~TON

Back~round of the Invention This invention rel tPS to a relatively high temperature furnace for ~elting thermo-plas~ic materia7s. More spec~fi-cally, the fu~nace is adapted for ~elting glass whexein the furnace enclosure is a ~essel particularly suited to resist 'he corrosive effects Oc the glass at temperaturas in excess of 1800C~ ~eans are de3cribed for enhancing thermal effi-clen~y and for protecting the furnace during startup. Other 10 details are also her_inafter described in the speciricat~.on.
~e~_ac~L~ l~`ne~ ~ur~aces ha~-2 b2en use~ for many years to mel. glass. ~any standard rerxa_~o~ies, however, nave 2 tendency to become 510wly dissolved or corrcded by th~ glass until the furnace begins to lea~. The rate of th~s corrosion increases .rapidlt~ wi~h incrPasing temperature and ~lass ~luidity. ~hile repairs ~ay be made, ~hev are usually dif~icult to e~ect, e~pensive, and usually short lived.
The re~rac~ory mAy be cooled on the outside surface to 91cw dow~ tnls _orrGsion, but a_ a cost of highe~ energy losse~
~he most severe corros~on usually occurs ~ the sidewalls near the .op of the g~ass ba~h. In conventional furnaces the glass i~ hottest near the ~op, a.~d ~eltlng ana refinin~
tempexatures are limited by ~he re~ractor~ capa~llitie~ to : less tha~ 1600C. As t:^.e rPrractory dissolves ir.to the glass many o~ the cor~osion ~rod~cts ar~ s~ept into rhe molten bath. The dissolved reractcry ~aterials beccme par~
o~ the glass composition and ir. ^ome cases ~.a~ have a del~
t~rious eri3ct on glass cualit~. The hea~ier c~rroslo~
products ~e~d ~o s~r~ to the bo~om or- the ..urnace and ror~

~1 ~V48~6 ~ somewhat loosely arranged protective layer for the bottom wall.
Another area where the reractory corrosion ~ends to ~e high is at the throat o~ exit Portion o~ tne fur~ace. ~lany times such exit areas are clad with metals fcr protection~ In the disclosure oE Spremulli, U.S. Patent No. 4,02~,887 a molybdenum (moly) pipe was used to provide a hishly corrosion resistant conduit from a furnzce to a ~orehearth channel.
Platinum too, has ~een used for exit li~ers. ~n fac~ entire urnaces may be platinum lined, but at extremely high cost.
In vertical electxical melting units, an e~mp`Le of which ~s disclosed in U.S. Patent No. 3,52~,206, the to~ of the molten bath is covere~ with a cold batch blanket.
Corrosion in this type o furnace is typically most sPv~re in the upstanains sidewall near the so-called usion line and around the electrodes entering through the sidawall.
The present invention prov~des means for ~ubstantially reducing such corrosion and~or minimizin~ its e~fect.
In conventional vertical electric melters, havinq a cold ba~ch blankst, ~here ~s a 'endency to retain seeds in the glass since there is no free surf~ce to allow Lor the ~apid escape ~f ~ubbles t~apped i~ the glass~ Therefcre r2siaenco time or ~13ss in the Iurnace must be regulated to assu~e sufficient fining~ Sir.ce freshly melted glass ter.ds to mo~e ~uickly toward thQ exit before i~ can be refined, the fast movlng glass 5et5 Up unwan~ed conve~ion curren~s which contribute to furnace det~rioration~ ~hus, steps mus~
be taken to con~rol co~vection currents and inc~ease the reside~ce tLme of ~he glass in ~he îurnace. One such me~hcd i~ descri~ed in U.S. Pat. No. 4,143,232, wherein contxollQd con~ection currents arP produced by deeply L~mersed electrodes ;r2_ activated in a selected firing arrangement. Another advant-age of the la~ter arrangement is that the heat produced is concentrated away from the walls, thus reducing corrosion around the electrode openlngs thereir.. In the present invention a~ improved arransement of e ectrodes is adapted to provide concentrated central heatlng of the glass and hot spot fining.
~olybdenum, platinum, platin~n alloys, a~d to some extent steel alloys and iron have long b~en recognized as materials having a higher resistance to wear than conventional refractory and are considered useful in the construction of glass melting f~rnaces. Moly~denum, for e~ample, has been used as ~n electrode m~terial and as a lining for stirrer wells where high glass velocities produce rath~r severe corrosion. As mentioned above, furnace outlets are or~en lined with plati~um and sometimes molybdenum.
Platinum ls extremely eY~pensive and its use is o~ten limited to the melting of special glasses such as o~hthalmic or cptical glasses. Tron may ~ used, as disclosed ln British Patent ~o. 601,851, ~ut it has a -elatively low melting point and it can contaminate most glasses with colorants. For certain purposes, however, it may be an acceptabl~ furnace 'iner material.
Moly is r~cogni~ed as a me~al that has high tempera_ure strength, is relatively inexpensive, and i~ chemically compatible with many glasses. ~ distinct disad~antage of this mat~rial is that it will oxidize ab~ve 550C. In the past it has been ~if~icult to fabricate. Now tha~ moly carO
be formed i~to ~lat or curved pla~- and pipe and ~elded lnto structu~es, it is a more a~trac~i~e material. One of th~
most extraordinary advantage~ of moly, which melts at 2O00OC, ~3~

~'~048(~6 is its high temperature strength which allows it to be used up to about 2200 C. Note for example that platinum, which has heretofore been used almos~ e~^lusively in high tempera'ure work, melts at 1730C and can be used up to only about 1600C. Thus, moly is a~ extremely use~ul material since it is substantially less expensive than platinum and has a much higher melting point.
The U.S. patent to Silverman, N~. 3,109,045, suggests the use of molybdenum as a vessel ~terial in a glass melti~g . furnace. ~ ~olybdenum crucible p~rt~on is submerged in an external bath or thermoplastic material to protect the exterior portion ~hereo~ from oxida~ion. The interior portion of the crucible is rilled with molten thermoplasti.c material, thus the moly is protected from the ambient atmospher~
and will not oxidize~ Further, although the exterior of ~he moly crucible is pxotacted by glass, a re~r~ctory tank or contain~ent ~essel fo~ the exterior bath into which the moly crucible is located is lare in comparison ~o the latter.
Thus, the m~lten glass surrounding th~ v2ssel wil} ha~e freedom to con~-ect zr~d ult~ataly ~es.~oy th~ refractory contai~ment vessel.
The Silverman unit is af a siza and confiquration adapted or spec al.y melts an~ would he impractical to sc~e up. In addition it requires a ourge gas arrangemen~
to remove air from the batch materials during operation for ~he purpo~e of ~rotecting the upper portion o the moly ~essel from oxidati~nO Also, since the ~atch materi~l~ 'or mos~ glasses will contain oxidi?ing agent~ such as CO2, SO2 ar.d ~2~ the ba~ch canno~ ~e allowed to contact ~he moly~
On the other h~nd, iI the glass level is maintained above i the noly, it will contact the reractory ring which sits on i ~21~ 6 to~ of the moly, thus causinS the refractory to quickly corrode.
Gas firing o batch materlals would be dif icult to imple~en~ ln a moly lurnace without deleterious effects because the he~t and ~xygen in the flame is h ghest a~ the glass surface, precisely whera protection aga nst corrosion and oxidation is needed. Thus, without the precautions hereinafter suggested by the present invention, a moly liner wou1d ~xidize since it would ke e~posed to the combustion gases.
Joule heatin~ is a preferred method o~ melting glass i~
a furnace of the tvpe dQscribed herein, especially a moly lined furnace. Y.owever, since molybdenum is a conduc~i~e metal, one must place ~Ihe electrodes in selec~ed locations a~d provide appropria~e circuitry in order to optimize current flow in the glass. While it is normally des~rable to avoid a short circuit to ~he liner, it may be desirable to place the elec.~odes and provide circuit~y so that some c~rrent ~lows to the li~er for prov_ding uni or~ power dissipation. ~oreover, t is possible to ~ire directly to the liner if deslredr Bat~h ele~-odes may be suitable for thi~ purpose and various arrang ment3 are illustr2~ed in U.S. patent~ 2~2~,982, 2,978~526 an~ 4,159,392. In a pre~erre~ arra~ge~ent or the prese~t i~rention it is con-templated to use movabl~ batch electrodes. r.~hile the '5~6 pate~t discloses such a concept, the arrangement i5 l mited in flexi~ility a~d ~cu'd drastically inter~er2 wi~h the proper fiiling of ~he furnace.

~2(~

In two related U.S. patents Nos. 4,351,664 and 4,332,687 assigned to the assignee herein, various arrangements of glass transport and conditioning systems useful with the present inven-tion are disclosed in detail .

SUMMARY OF THE INVENTION
A method and apparatus for melting thermoplastic material is disclosed utilizing, in a preferred embodiment, a vessel having a bottom wall and upstanding side walls. A-protective liner or structure for the vessel ls provided, being formed of corrosion resistant material. The furnace may include through-the-batch and floor electrodes and hot spot fining to a condictive outlet channel.
A method is proposed for operating the furnace described herein utilizing symmetrical below the batch heat dissipation, for accelerated melting. In order to protect the rotective structure, which in some instances may be fabricated from an oxidizable refractory metal, start up ~urners are operated with reducing fires.
The present divisional specification provides a method of protecting a refractory furnace vessel for melting thermo-plastic materials from corrosion which comprises, lining an interior portion of said vessel susceptible to relatively high heat with a liner material, closely spacing the liner or protective structure with the vessel, providing a relatively cool zone within said furnace, and extending the protective structure into the relatively cool zone thexeof.
In another aspect the invention provides a method of producing molten glass which comprises, providing a refractory vessel having upwardly extending sidewall po-rtions, positioning a protective structure formed predominantly of molybdenum within said refractory vessel with wall portions of said protective 3~Z~ 6 structure in closely spaced apart relationship to said upwardly extending sidewall portions substantially along the extent of said sidewall portions with the interior and exterior of such protective structure in open communication with each other~
providing molten glass in-teriorly of said protective structure and at least semi-molten glass exteriorly of said protective structure within the space between said protective structure and said sidewall portions with the interior and exterior glasses in physical communication with one another, maintaining said protective structure submerged within said molten and semi-molten glass and completely surrounding said protective structure with said glass, maintaining an upper surface of said molten glass interiorly of said protective structure above the upPer extent of said protective structure and supplying batch material to such surface, and supplying electrical energy centrally of said protective structure ad~acent said batch material to melt said batch material and produce molten glass.
In yet another aspect, the invention provid~s a method of protecting the inside refractory w`alls of a refractory furnace vessel which oridinarily would come into contact with the molten material or the material to be melted and to be refined within the furnace vesselO
Description of the_Drawings Figure 1 is a somewhat schematic elevational view in section of a lined furnace, with cross section lines eliminated for clarity of presentation, illustrating significant features of the present invention.
Figure 2 is a schematic view of a preferred electrode arrangement with superimposed phasors~
Figure 3 is a schematic sectional view in elevation of a lined furnace in a start-up mode.

6a . "

31 2~ 16 Figures 4 and 5 ~re res~ective side and top plan schematic views of an electrode arrangement.
Figure 6 is a bottom view of the urnace of Figure 3 illustrating the electrical isolation of bottom plates and electrodes.
Figure 7 is a scnematic side sectional view or an alternative ~mbodimer.t of the in~ention.
Figure 8 is a t~p plan schematic showing an electrode arrangement ~uitable for th~ furnace shown in Figure 7.
Fisure 9 is a fragmented illustration o a large furnace featuring multiple batch electrodes, a lined bottom and s~lpports ~here~or.
~ igure 10 is a diagr~m or the electrode arr~ngement or the furn~ce or Figure 9.

~esrription of_the ~referred E~bodi~ents The furnace oI th~ present irvention 10 includes an outer shell 1~ having upstanding generally cylindrical t round, poiyhedral, square, or rectangular side~alls 1~ and a bottom wall 16. The bottom wall 16 may be in segmented sections to accommodate thermal ex~a~sion, which sec~ions may be elec~rically isolated one from t~e o~her. The shell 12, orming a main suppor, s~ructure for the fu,nace 10, ~ho~ld be relatively air tigh~, electrically isola~ed rcm bot~om wall 16 by an insulating shim 13, and may be labri-cated from s~eel plat~. ~him 13 als~ allows for th~mal expanslonO ~he furnace 10 is supported ~rom ~he bottom ~v "I" bea~s 18, ~hlch ma~ be electrically isolated from ground by means o~ insulatirlg sh~-ns 75.
~ A layer o~ compressi~le insulation 20A ~uch as FI3E~
FRAX~ manuIac~ured by Carhorundum may be located m~edialely ~7--~L2~48~

interior of ~he shell 12 and extends from the bottom wall 16 to an upper lip 22 thereof. The compxessible insulation 20A
allows ~or t;~e relative mo~men~ of the structural matQrials . during thermal cycling of the furnace. An an~ular formation of rigid insulation 20B is located adjacent the compressible insulation layer 20~.
A re~ractory vessel 24 including upst~nding sidewalls

2~ a~d refrac~ory hottom wall 28 i5 located within the s~ell i2 in spaced relation with the rigid insulat.on 20B. R~ing mix 21 is placed between the rigid insulation 20B and .he refractory vessel 24 to form a glass tight sezl the~ebetween.
Ra~ming ~i~, so~etimes hereinafter referred to as tam~ 21, is a granular refractory material which may be p2cked or tamped in position and fired or sintered on furnace star~up.
The vessel 24 is pre~erably manufactured from knawn refrac~or~
materials resistant to glass corrosion. Coaxially locat-d within the refractory vessel 24 there is pr~vided a protective structure 30 preferably ~o-~ed o~ a highly corrosion resistant _erractory metals~
Molybdenum appe~rs to ~e a use ul and pre~er~ed material for the ;iner 30 altllough tantalum~ rheni~m, niobium, and tu~gste~ may also be suitable. ~oble me~al~ (e.g. platinum, rhodium, et~.) m~y also be suitable fo~ the liner in some situations, especially ~here the glAss is highly oxiAi~ed.
The latter materials are relatively weak a~ high tem~eratures a.~d may the~e ore requir~ bracing or integral ribbing, not shown, to lend additional su~port to the protective struc*ure 30. In such a situation where ~he glass is highly oxiaized cathodic or ~c bi~s may be imposed on the liner in conjun~tion with an anionic sacrificial elec~rode. Such an arranGement would '~e less cos~ly than using no~le me~-als~ It should also be mentioned that., ~or relatively low temperature melting of ~rits and the like below about 1100C steel and nic~el alloys may be useful. Electrodes, hereinafter described for electrically firing the furnace 10, may be fabricated from the above materials bu~ with the same pre~erQnce ~o~ moly.
The other matexials which are noted as being useful are not emphasized because, unless there is some special reason to use them, they are considerably more expensive.
In a preferred ~xd~t the prote~ive structure 30 is fabricated from formed moly plates ri~eted together along lapped seams, no other reinrorcem~nt bein~ deemed necessary. Also plates of ~he zbove ~t~ }s ~ou'd be used as shields for the vessel i tichtly spaced with each other.
upstan~n~ walls 26 of the vessel 24 and the protective structure 30 are in close pxoximit , le2ving a relati~ely narrow annular space 32 therebetween, ~h~oh may e~tend from essentially intLmat~ contact to some wider preferred s?2cins or about 1 - inch or so. For reasons hereinafter set for~h the space 32 may be fitled with ground no~corrosive high viscosity glzss 20 cull~t~ batch or 3.~ er laye~ of refrac~ry tamp (see re erence numeral 23).
It is imnortant that the function of ~rotective s~cture 30be understood. me ~rotective structure 30 shields refracto~ vessel 24 from corrosion caused bv th~ co~ecting ~henmoplastic mater~al (ylass) 43 within the îurnace 10~ Further and ~ery Lmportantly prot~tive stru~ure 30makes it possible to i~rease meltinq ra~s an~
improvP glass quali~y. The rormer is accomplishe~ as a result of the highsr tYmperatures at wh~ch ~he ~urnace 10 may be opera~ed. The higher glass ~uality results 'rom the fact that mcly produces less con~amination of ~he glass than refractory. No matt~r wha~ reIractory is u~ed for c~n~ac.

t ~

with glasses in conventional melting tanks, it will eventually corrode and contaminate the glass. ~dditionally, refractory blocks~ especially the larger ones will crack because of thermal shock. Spaces between courses of refractory blocks and cracks will allow i~spiration of air into the refractory w:nich will react causin~ introduction o~ reactants into the glass. Outgas~ing of the refractory aue to the intense heat o glass furnaces also introduces contaminants into the glass. Glass quality, which is a measure of the absence of drfect~ and contaminants (e.g. cord, s~eds, stones, e~c.), is affe~t~d by all of the above factors pre~alent in conven-tional melting tanks. Thus the protective structure 30 not only protects the re~ractory vessel from corrosion as noted above, i~
allows the melting of glass against an impermeable vessel wall thereby blocking the co~munication o~ con~aminants to th glass.
Since it appears that moly~denum is the most highly corrosion reslstant liner mate~ial prese~tly available for economical high temperature opera.ion, it is the preferred ma erial choice. Molybdenum, being rapidly oxidizable at temp~ratures above 550~C, must be shielded from oxygen. ~n the preferred embodlment of the invention the ~rote~tiv~ structure 30 is ~ositioned ~elow the glas~ le~el by maintaini~g ~he glass le~e~ above the upper marg~n 31 thereof, thus orotecting the in~ide surface of ~he liner~ The materials clo~ely pacXed in s~ac~.32 and trap 2~ shield the outside s~race of protective structure 30 from oxygen contamination.
The bo~tom wall 28 o~ ~he ~ur~ace 10 may be ccmprised prlmarily of refractory, or i~ may be lined with various materials including ~he same ma~ rial forming ~he walls o the protective structure 30. However, f~r purpo~es oE e~planation herein~

~1~

8Q~

a fux~ace with a refractory bottom will be descxi~ed and the other variation~ will ~e described hereinafter with refer~
ence to ~igu-es 7-10.
It is also contemplated herein that a mix~ure o, tightly packed sand, ramming m~x and high viscosity (i.e. hard) glass cullet could be used in place of the las~ers of refractor~
24, insulation ~OA and 20B and tamp 21-23 descri~ed abo~e.
mus, the protective structure 30 with or without a bottom wall oould be located within a ranu~ ar formation surrounded by olter shell 12 a~d of~s~t block 27 (hereaf~er described). In such an arrangement the sand, cullet and tamp would not greatly insulate but w~uld prov.ide protection for the protective structure 30, because upon application of heat such materials would form a high viscosity c~mposite of glass and molten or semi-molten materials shielding the protective structure 30 from oxygen. Heat transfer 10SSQS by co~vection currents externally o~ the lin~r would be greatly reduced because molten or semimolten sand would be hi~y viscous. An ~Yem~lary but none~haustive 11st o useful ma'erials includes sand, silica or zirconia; ramming ~0 mix - C~r~ar~ ~A393 G- 251420.
If kottom or floor electrodes ~0 J hereina~ter described, æ e incorporated into th2 fur~ace, .he bottom wall 28 should be an unlined h~Sh resistivi~y refracto_v such as Corhart ~13~0 Zixcon hav~nq a~ ele~ical resistivity a~ove 100 ~hm-lnch~s at 1800~C. Although not sho~n in Fi~uxe 1, protective structure 30 may be pr~vided so as to extend across botbom wall 28, especially if the melting applica~ior. is or a ha_d glass at high temperatu~e. Such an arrang~ment would preclude the practical use of bo~om el2ct-odes because of t~e low re~istiYity of the protective structure 30 material.

~11--....

~2'f~ 6 In Figure 1, only one bot~om elec ~rode is sho~n in order to simplify the drawing. It may be angula.-ly disposed in an opening 42 in the kGttom wall 28 and extend into ~he interior space 4~ of the furnace. While electrical connec-tions are not shown, each bottom elec~rode ~0 may be e~rg_zed by a connection at its distal end 47. Firing occurs primar-ly from the tip 4g into molten th.ermoplastic material 43 within the space 46. The bottom electrod~ 40 may be movable so the ti~ 44 may b~ axially adjusted in the direction of the arrows al, adjacent same.
A plurality of batch elec~rodes sa may also be used to melt the ~hermopl~stic material 43. ~gain, in order to simplify the drawing, only one is shown. Each is adapted to be positioned about the periphery of the furnace 10 and have ~axious degrees of freedom of orientation~ The batch elec-trode 5~ may include an outer metal or ceramic ~lee~e portion Sl and inner concentric r~Eractory metal elec~rode rod 52.
It may be energized by an electrical connection (not shownl at its distal end 53 while its tip 54 is located i~ the space 45. The electrode rod 52 is preerablv adjustable along axes Al and A2. The batch ele~trode 50 i5 ~.orizGntally supported by arm 56 journaled in sleeve 58. Suppor. struc~ure 60 attached to the snell 12 carries th~ sleeve 58 via a shat 62 s'eeved i~ suppor~ o4.
In operation, the sup ort StrUCtUI:3 60 remains- fixed wit~ respect to t~e shell 12 ,. ho.we~er t.he shaSt 62, sleeved within ~he support 64, is movable axially in the di-ection of the doubls .~eaded arrow a2. The horizcnt~l sleeve 58 is rotatably ~nounted to u~per end 66 of shaf~ 62, see arrow a3.
The suppor~ arm 56 carried in sl eeve 58 may thus be rotated a~out a vertical ~is Al of shaft ~,2 as shown by arrow a ~12~

~Z~4~6 The hoxizontal support arm 56 may be ro~ated about the horiæontal axis A2 as shown by arrow a4, and also may be mo~ed therealong in the directions shown by arro~ aS. The batch el~ctrodes 50 need not ~e vertical as shown. They ma~
be made tiltable by modifying bracket 57 that holds electrode ~0 to arm 56.
The batcn ele~trode 50 carried by the above-~escribed supporti~g structure 60 etc. is mo~able up and down, radially with respect to centexline C o~ the furnace 10, an~ularly of the vertLcal and arcuately in the horizon~al. Thus, each elec-trode 50 has degrees o freedom whereby it may be adjustably orierted durirg o~eratlon to any one of a selected set of coordinates within the space 46.
A center batch electrode 8 a is v~rtically oriented along centerline C of the furnace L0 and is similarly energized at its distal end 82 by an electrical connection (not shown~, It is couplea to a hori~ontal support 8O which in turn is mounted in horizontal sleeve 88, vertical sha-t 90, and suppor~ing structure 92 fLxed to shell 12. ~he 2~ cen~er ~atch electrode 80 is adapted to ~ave ~ertically in the direction o ~he dou~le headed arrow a6 to regulate the loca~ion of i~s tip 96. It may be arransed to be supported from ~he same arm (e.g. 56) as one of the batch elect~odes 50 to sa~e space above the furnace 10. For example, in Flguxes ~ 4 and 5 the center electrode 80 and batch electrode 50 could be supported by the same support 601, one above the other along axis Al. Such an arrangement, with other batch electrodes ~0 loczted 120 degrees apart, e~poses a good deal o~ the to~ surface of tQe ~urnace 10 for ease of filling.
In ano~her embodL~ent of th~ invention flcor elect odes are shut dswn and additional batch electrodes ~not shown~

~13 ~2~48V~

are subs~ituted there~or in approximately the same circumfer-en~ially staggered location xelative to tne k~atch electrodes 50. Such an arrangement of six batch elec~rodes spaced approximately 60 a~art about cen~erline C results in a highly sy~metrical eiectrode arrangement with ~eat apolied near the upper portion of the furnace 10 where most efricient melting occurs2, It has also been found that if the tips 54 and 44 of r~specti~ electrodes 50 and 40 are located approx~mately midway between the centerlinP C of the furnace and the walls of protective structure 30, the melt m g process a~pears bo be enhanced.
For a relati~-ely small version of the furnace 10, i.e.
appro~ ately 4-6 feet in diameter, the electrodes 50 should be akout one half the distance from centerline C to the protective structure 30 and symmetrically locat2~d th2~reabout. Other arrange-men~s are of course possible ard ~ill be described hereinafter.
It appears that both batch electrodes 50 and floor electrodes 40 are most useful for somewhat di~'erant urctionC.
Floor electrodes 40 are particularly suited for start u~
2~ ~sfcre insertion of batch electrodas 5~. ~lso the floor electrodes 40 may be used ~uring full operation for tri~ming and fine control. Batch electrodes 5Q are pri~arily for full time melt~ng at high rate~ and can be useful alone.
The center batch electrode 8Q is pr~marily useful or finin~
'` hard or dif~icult to melt glasses. Fuxther, the elec.ro~es 50, ~Q and 80 may be operate~ either alone or i~ combination so tha~ th~ furn~ce is rendered ex-~remely versa.ile. It should be appr?ciated tha~ altnough no~ 2et~iied herein electrodes 4~, 50 and 80 may ~e wa~er cooled by providins an external ~acket or ~h~ Q ror carrying coolins ~wa~r~
5uch an arrangement prolonss lectro~e li~e.

- ~2~ 6 An ou ~et pipe 100 havinq a central through opening 101 is dispo3ed in an opening 102 in re~xactor~ bottom 28 and is preferably fabricated frcm the sa~e material as the protective structure 30, namely molybdenum. An electrical connection, not illus-trated, is coupled to the outlet pipe 102 at or near distal end 104. The ~utlet pipe 100 may thus be energized with i_s tip 103 firing to the tip ~6 of center elec.rode 80 through the molten thermoplastic materi~l 4~. Since pipe 100 may su~fer corrosio~ by electrical f~ g, cente~ elec~ode 80 which is more easily replaced may be electrically biased with a DC potential so ~hat it becomes a sacrificial electrode.
Upon energi3atlon of ~he center batch ele~trode 80 and outlet p'pe 100 a hot spot 106 is created in the bath of thermoplas~ic material 43 by the passase of la-ge cur-ents therebet~-ee~. The tip 96 of center elec~ode 80 and corres-ponding tip 10~ of outlet pipe lQ0 may be large surface area disks, capable of ~arrying high current. ~e e~ergy-dissi-pated in hDt spot 106 fines the ma~a_ial ~3 just ~erore it lea~e~ the furnace 10 through the opening 101 in outle~ pipe 100. The ining tQmperature being highly elevated and co~centrate~ near the ~-enter of the ~usnace helps to reduce furnace wall deterioration.
In order ~o further reduce furnace wall temperatures, the refrac~ory ~ressel 24 has i~ ve~tical upstar.ding w211 2 stepped near u~er m3rgin 31 of pro~ective structur~ 30. Refractorv block 27 may be offset as shown in Fisure 1 or ~igure 9 to pro~ide ~he step in wall 26, or th~ wall per -~ may be recesse~ as ~h~wn i~ Figure 3. Upstanding r-~rac~ory wall 26 LS th re~r radially recessed at its upper extent away frcm the protective s-~ructure 30, so that the temperature of the ma~erial near block 27 i5 r~duced to a poi~t where eorrosion ~y mol~en th~r~.oplastic ma~erial 43 (e.g. glass) and ~a~cn 110 is insi~nificant.

~15-~4~

A channel or trap 29 is formed between the protective structure 30 and th~ recessed or o~fsQt block 27. In one embodiment, the trap 29 may be f~lled during start-up with noncorros~'ve glass which will melt and slowl~f flow into space 32. A horizontal flange 33 of the protective structure 30 extends radially outwardly to cover an upper face 35 o~ refractory wall 26. After startup, the flange 33 inhlbits the ~low of molten material 43 located in ~he trap 2~ from rapidl~.seeping into ~he space 32, since the outward margi~ 37 of flange 33 is coolest ~ear bloc~ 27 and ~he glass in this location is most viscous. Also, corrosion of bloc~ 27 is ra~uced because it too is remote from ~he hish heat _n ~e c~ ~x po ticr. ol.the.furnace 10.
~ t a kottom end 36 of the protective structure 30, a slot 39 is formed between the bottom wall 23 of the ref.ractory vessel 24 and the protective structure 30. Ihe slot 39 traps and collects a mixture 41' of molten thermoplasti~ material and corrosion products of ~he refractory. mhe molten material at the bottom of ~lot 39 is signiicantly cooler than ~ther portions ~f the furnace 10, and thus, there is a tendency for the mixture 41~ tra?.ed ~herein to ha~e a high ~iscosity and/or to de~rit-ify~ and `' thereby act as a seal ~etween molten ma~erial 43 wit~in ; chamber space ~6 and the material in li~er s?ace 32.
Similarly, the space 32 betw~en the ~rotective structure 30 and the j ups~anding ~efractory wall 26 is ~arxow, and thus convection '. csrrrent-q caused ~y the ~e~t in the fur~ace 10 are elimtnated or su~stantially reduced within su~h space t~her~y materially r~ducing con~ection corrosion or .he refra~tory. The space 32 ac~5 as a tra~ .or a mi~t ~ e 45 of corrosicn prod~cts and therlnoplastic nLaterial. Si~c~ corrosion produc~s con i~ed i~ the space. 32 are nQt conti~ually ~wept away, corro~ion o~

the refractory i5 inhibited.

~16 ~2~4~6 Although passiv~ coolin~ should be eff~ctive to seal space 32 and prevent the circulation o the material retained therein, if desired one or more cooling pipes 112 may be prcvided to carry c~oling gas near end po~tions or margins 36 and 37 of protective structure 30. The resultant extra cooling w~uld ~horoughly assure a seal by virtue of frozen glass adjacent the ends of the liner space 32.
Variations o the above mentioned met~ods and apparatus for pro.tecting refractory from coxrosion and estabiishing cool a~d/or narrow zones to lmpede convectlon cur_ents in sensitive areas of the furnace are available. Exa~ples .includQ ~ er.sio~ o$ the flangQ 33 into the of.set bloc~ 27, flanging the ~ottam end 36 of protective ~tructure 30 into sidewall 26 and tha liXe. All of the above are designed to loca~e interfaces of the protective structure 30 and refractory vessel 24 to positions which are relatively cool an~/~r restricted in volume so ~ha.
glass motion is impeded. Further, if a nonoxidizable ~rotective structure were usea, th~ upper margin 3Q thexeof could be exter.de~
above th~ end of ~he thermoplastic material 43 and the pro-vision for offset block. 27 dispensed with.
It waslearlier mentioned that the bottom wall 28 of therefractory vessel 24 might also be pro~e~t~vely li~ed w~th moly. This feature is describe~ hereira~er with reference to FLgur2s 9 a~d 10. In a~dition, ta~p or chrome o~ide re~ractory could he used aq a b~ttom wall material. Wh~le the latter al~ernatives ar~ not nece~sarily preferable ! ~ecause ~hey ~ay b~ ~wept along and mix with th~ glass, they are poss-ible and useul al~Qrnatives for some types o~
glasses~ Chrome oxid~ being electrically c~nductl~Je would render impractic?l the use of ~ottom elec~rodes.

: -17 .~

:~Z~ 6 me absence of a moly kottom for p~rotect.ive structure 30 is qenerallv preerred except when melting very corrosive or viscous glasses which require high temoeratures, because its absence allows for the flexibility of operation with. bot~om electrodes.
It also reduces the cost of construction of the ~urnace 10.
Further, sin~e for the most par~ the bottom wall 28 is protected hy settling corrosion products, the provision or a bottom liner might not be necessary unless extremely high temperat~res are required. On the o~her hand, since, the presen~ in~ention is pract~cal with or wit~out the pro~ection a~corded by a bottom for protective structure 30, a bottQm lined furna~
will be descr~e~ furth2r in ~è specification.
Referring now to another feature of the present inven-tion, it is i~ended that ~atch materials forming a batch ~lanket 110 may be added to the furnace 10 over the thermo-plastic material 43 in a continuous fashion. Bearing this in mind~ it is important that ~e furnace is operated i~
such a manner that the fusion line lll, separating the batch blar~et 110 ar.d molten material 4~, eYtends acrcss the ~0 furnace lO above the level of flange 33 of protective structure 30. By always kee~ing a layer o molten material over fl~nge 33, the protective structure 30 is ~rotected from oxygen and gaseous roducts contained within the batch blan~e~ llQ. U~per edge 31 of protective structure 30 may protrude i~to batch mate~ial llO and will become oxidized since it will not be co~ere~ ~y mol~en glassO ~owQver, ~he trap 2g for~ned by upper edge 31 and block 27 is useful nLainly ~or startup pu-poses and oxi~ation of upper edg2 31 ~herealter creates no ~ele~erlous effec:ts.
I~ order ~0 fur~er explain t~he impor~an~ fea~ures o~
the present inver.tion it i5 n~cessa-y ~o u~der~and ~he s~art ;up proce~ure. Con~n~ional furnaces ~ ving liners -1&- -8~6 which are susceptible to.oxidation are purged with an inert gas during ~tart-up and ~hereafter as the furnace is operated.
Small scale units for specialty glasses ma~ be operated under vacuum. Such arrangements are difficult to upsiz~, especially where thermal efficiency an~ econcmic factors make vacuum and purge gas arrangements unattracti~e.
I~ ~he present i~vention, star~-up o~ the ~urnace 10 is similar to the start up of a conventional vertical meltex.
Reerring to Figure.3, a cover 11 may ~e placed over the furnace lQ of a type s~ilar to that normally used in conve~ional ver.ical melters. Burners 9 are locat~a through ~3 ~O-~-c- il a-.~ a_e fe~ fuel b~ gas l~es.15 and com~ustion air thxough air lines 17 from some source not shown. The furnace 10 at start-up is normally partial~y filled with cxushed glass, or cullet, up to the dotted line B, rep_e-sen~inS the ang e of repose of the cullet ttypically 45~.
P~eferably, the upper edge. 31 of protective stnlcture 30 is covered. As ~he materlal melts it settles a~d more cullet is ad~e~
through openings 19. If a~ailable, flo~r electroaes 40 are 2Q ener~i ed aSte- the t~a~ch is at 'eas~ par_ially melted.
Once the urnace 10 is full of mol.ten thermoplastic matexial (see glass line G), the protective structure 30 is ~rotected from oxidation.
~ uring start-up, purge sas P m~y be forced i~to fur~.ace 10 under pressure ~hrough an inlet pipe 63 which is sealed or welded in opening 65 in the bottom wall 16 of shell 14 ~he purge gas P may be use~ to. fill the space wi~n ~hQ
shell 14 to prot2c~ the equipmen~ from oxidation. Si~c~ tne shell ~s su~ficiently air tight, the purge gas ~ will ~e reasonably confi~ed in the furnace 1~ and ~uarantee, a~
nominal cos~, ~he sa! 9 and erfeoti~e s~ar~-up of ~rnace 10.
Once the fur~ace is 'ill~d wlth mol~en glass, ~he purge gas ~ay be turned of-In tne present invention, in order to fur~her protect ~e p~t~tive s~ct~e 30 from oxidation, ~e bw~ers 9 ~e o~rated such a way that the ccmbustion products contain no e~cess ox~en and the melt o~ initial cullet B is accomplishe~
under reducad or neutral a~ospheric conditlons. H~ghly reduced fires are not believed desirable since it is thousht tha~, for certain glasses, carbon contamination o~ the protective structure 30 may be harmful to glass quality.
Once.suf~.icient mat~riai is mel.ted the burners 9 may be - shut down, and the b~ttom electro~es 40 may be. energized to maintain ~he temperature of the furnace.. Thereafter the cover ll is removed and the batch electrodes 50 and.80 are inserted at their res~ective positions (see Figures 1, 2, 4 and 5). 3atch materials llQ may then ~e continuous~y added to the furnace la as molten material 43 is removed from the bottom through outlet yipe 100.
~ ill control means, not shown, may be provi~ed to maintaln th~ batch 110 at a desired oxientatian especially in tAe. zone of the trap ~9. .The preerred method o~ operation ~s t~ ~.~;G ~e~aDilit~r t~ control fill and the hydrosta~ic head of gl~ss. Then the fusion line 111 can be controlled by th~ combination of fill, head, and vertical adjus~ment of electrodes sa and 80.
In Figure 2 there is illustrated a schema~ic of one possible elec~ro~e arrangement o~ the present inve~f ion<, The b~t~om ele~trodes 40 are illustrated by th~ ci-cles, the batch electrodes.50 axe illustr.ated by the crosses and t^ne ; cent2r batch electrode 8Q and outlet pipe 10g electrode ar~
respecti~ely illustra.~d ~v a circlP ahou~ a cro~s~ ~he bo~om elec~r.odes 40 may be fired in a closed del~a co~figu rati~n so that each fires to its next adjaeent electrode, as .. ... .. .. .

~204~

shown by phasor arrows 40' between each of ~he electrodes 40. SLmilarly, the batch electrodes 50 may ~ire one to the other in a closed delta configuration su~erim~osed over the first mentioned ~iring pattern, ei~her in the same sense or the opposite sense of ~he bottom electrodes 40, as illustr~ted by the phascr arrows 50'. The electrodes 40 ar.d 50 axe preferably arranged away from the walls of the protective structure 30 to concentrate heat near the. c.enter of the furnace. In t;ne arrang~ent shown t~ere is electrical symmetry due to ~he superimposea or dou~le delta firing ~nd physical symme.ry because the electrodes 4~ and 5Q are circumferent~ally stagyered. The`s~me firing patt rn and symmetry would occur i~ three additional batch electrodes were substitutcd ~or floor electrodes 40.
~ he symmetry referred to above is important for mel.ing efficiency and uniformity. In addition by placing and firing the elect~odes 4Q and 50 symmetrically, tke liner will operate essentially at neutral or ground potential.
mus, the risk of destructive current flow from protective structure 30 to shell ~O, also op~rating at ground potenti~l, is minimized and glass seepage through insulation ~0~-B ~o the shell can be tolerated.
~ n conventional vertical mel~ers, -exter~or walls of th~' refractor~r vessel are cooled to slow do~nn ~ e corrosion of tihe vessel its?l~. Thf~ present in~e~tion, how~er, utilizes th~ insula~ion 2~ ~o retain '~h~ he~t wi~hi~ the furnace 10 while the protecti~e structure 30 resists high t2mperature corrosion. The i~sul2~ed na~tLre o ~h~ ~-urnac~ 10 al~ows for gieat2r energy ~ficiency and hi~her ~ ~ perat ~ e operatio~. thereby significantly i~proving melt ra~s. Since ~h~ re~ractory of furnace 10 is protected by protective s~ructure 30t it is capablP of ~;21~
,r~ .'.

8~6 withstanding hlgher oper.at~ng.t~mperatures allowing the use of insula~ion 20. Even if the ~lass contact refractory in wall 26 become~ softened by the extreme heat of the furnace, the inter~diate la~er of tamp 21 will retard le~X2se of glass into the 'nsulation 20. The various protective layers of materials su~-cessively l~mit the destructiv~ impact of ~he corrosive material on the furnace 10. In addition, wh~re c~nvection currents are lik~ly ~o cause deterioration of the fux~ace 10, .the convection is restricted or confined.
For example, general con~ection ~lows of material in chamber $pace 46 are restricted fxom the walls 26 of refractory vessel 24 by ~rotective structure 30 and the s~ace 32 bet~en the ~rotective structure 30 and the shell 12 is limited so any m wement of materials therein is inhibited. This is especially ~rue i~ a h-~h viscosity material is located in space.32, The same result woula occur iî as previously mentioned the refractory 26, tamp 21, and insulation ~OA and 2aB were replaced by sar.d, cullet,. t~m.p or gro.und up rer~ctory, or m.;~tu~s ~hereof. ~1t~ough heat losses would incre~se, ~0 electrical los.ses wou'd be akeu~ th~ same ~ecause ~he liner operating at ground potential would not be a curre~t sourc~e.
~ no~her feature o~ the present i~nvention as shown in F~sure 1 i~ that the s~dewall 14 o~ ~hell 12 may be directly coup.led to a ground or neutral poten~.al and ~onitoredO
This pr.ovide~ lmportant sa~ety bene$its~ A ground strap 49 csuples sidewall 14 to g:~ound G as a precaution~ ~ curr2nt de~e~or 4~ moni~oEs the curren~ flow rom ~he sidewall 14 ~ to ground G. If current flow occuxs in strap ~ it is a i sign ~hat the sidewalls ar~ no longer isolated. ~he~ this . occurs the o.p2ra~0r should cut ground s~rap 49 ~o preven~
further lea~ase to grour.d and er c~ 3 cage or barrier not :~0~8(~6 shown about the furnace to protect personnel. The additional precauticn of operating and placing the ~lectrodes so that protective structure 30 operates at ground is a feature n~t ava~lable in con~entional furnacesO The bottom 16 of shell 12 is not grounded and normally floats at some voltage V . Insulating shLm 13 i501ztes t~e bot~om 16 ~rom sidewall 14 and similar insulating shLms 25 isolate "I" beams and associated support s~ructures from th~ floating voltage carried on }:ottom 16.
As shown i~ Figure 6, the ~ottom wall 16 of shell 12 may b~ ~ivided into a num~ r of sections 16A.. 16n ~three shown~. If bottom electrodes 4~ are used, t~ey may be sIeeved through the bottom ~all lo through openings 42. The elec~rodes 40 are each isolate~ from the ~ottom wall 16 ~y means of a nonconducting sleeve 5~. Each sec~on 16~-16n of bottom 16 is isolatea ~rom anothex by an insula~ion shim 69.
Thus, if a shor~ develops between elec~rode 4 2 in section 1~ of bottom 1~, current will not be conduc.ed to ad~cent electrodes. ~uxther i more th~n on~ electrode shorts to a respective bottom wall section 16A-lon, ~here will not be a catast~ophic short ~rom one electrode to another. Normally a short ~Q bottom wall 16 should occur, it occurs near openings 42 which may ~av~ ~eco~e filled with hot glass.
Thus a sectional bottom wall 16 i5 a prudent measure. I
g}ass should penetrate into shell 12, oper2t-or. can still continue although with i~cre~sed ~at and elec~rical losses.
~ Because ~arious shell portions are separated, a des~ructive failure may be a~oidedO 5~milarly, glass ~low to hot~om wall 16 of shell 18 ~ill not cause a destruc~i~e cur-en~
flow to ~round be~ause 'lI'/ beams 18 are lso7ated .hererGmO
Although not shown, segm~ntation of ~ottom ~all 16 may ir.clude sec~ions not having an electrode plaoemen~ ~h~rethrough~

~23 It is presen~ly contemplated that the furnace of the present in~ention will operate with maximum tempexatures of 1700-2000C~ As a general rule, ~he melting rate of a glass ~urnace doubles ~or every 100C increase in tempera.ure.
~hus, the urnac2 deccribed herein woull have the same capacity as a conventional electrically fired unit two to four ti~es larger~ Conversely a conventional electrically fired furnace the same size as that of t~e prese~t in~ention would only produce a~out half the glass output thereo~. For example, a 12 ~oot diameter con~entional electric furnace could be replaced by a ~urnace o~ the type descri~ed herein that is onl~ a~out 6 to 9 feet in diame~er. Furthermore the height of ~his ~urnace would be significantly less ~han that o a ty~ical ~ertical melter. A shallow furnace is orererred since it is easier to build and requires less structural material for the lower head o~ glass confir.ed therein.
In addition, because or the ~igher temperatures prac-tically attainable, very hard glasses may ~e economically melted in large quantities. Furt~er, ~ntirely new and only 2Q _heoretical com~ositio~s may ~e attemp~ed.
;Tha emkodiment of the furnace 10 described herein is a rela~iveIy small polyhedral melting unit having a diameter of approximately 4 faet and depth of a~out 3 ~eet. Presently ~he fur~ac~ has operated at and i5 cz~a~la of mel~ing a Corning Code 7~73 borosilicate glass a~ a rate ~f 1.5 sa~
fto per ton. ~hes~ ~igures a_e sisni~ica~t when com~ared with m~lt~ng rates o~ conventi~nal furraces ~hich ranse rom 6-lZ sq~ ft. per to~ or gas fired regenera~i~e ty~es to ; abou 3.0 sq. rt. per to~ ~or a vertical electrically fired 3a glass melti~g unit9 ~24 ~Z~ 6 Xt is conceiva~le tha~ thi~ unit could economically melt soda lime glass at a melting rate of 0. 75 sq. t. per ton and possibly 0.50 sa. ft, per ton. With such ~esults, it has been theorizPd .hat a relatively large capacit~
~urnace of the tvpe described herein would be useful in a so-~alled floa~ glass operation th~ereby eliminating t.~e necessity for t~e large con~entional float furnace.
A fully lined furnace of ~he type descri~ed herein hould ~roduce no refractory cor2, and would reouire only about 2.25 x 106 BTU/ton, ~hile a conventional gas reger ~rative float furnace may require a~out 5-7 x la ~TU/ton and c~n ~ro~u~e cord`and co~se~uent quali~y d~minution. In addition, as the conventional rur~ace wears due ~o usage, the eficiency deteriorates further, whereas in the system conte~.plated herein, the rurnace remains at its hig~ e~r-i-ciency level for virtually it~ enti~e useful life hecause of i~s superior wear characteris~ics.
It should be understood that th~ shape of ~he ~ur?.ace 10 and interior protective structure 30 can be any one of many conceivable arrang-~e~ts ~rom ci~^~alar to ~oly~ral and ~qu~re or r~ctangular in plan view. Further the side walls may be slanted to form a conical structu~e to ~:cntrol co~vect~on currents and~or to move ~he upper margins of the fur~ace away from ~he cent~r, while maintainin~ the hot central zone wi~h a smaller co~centrated body o glass. The ~eat~ures o~
the i~ention for protec~ing the liner an~ xe~ac~ory vessel, however, remæin ess~r.tially ~e same. ~ urnace lOA as ~hown in Fi~ure .' could conc~iva~ly include a conical liner 30A coupled directly to ex~erior outlet pipe lQ0 without an upstandin~ outlet electrode partion 103 above the loor 23 (see ~igur~ o~ spo~ 1~6~ could be achie~ed by losely ~25 ~20~8~1!6 arranged and deeply immer5ed hatch electrodes 50A. Batch electrodes 50 as previously de~cribed in Figure 1 could be located above and staggered be~ween the electrodes 50A (see also Figure g). T~e electrodes 50 may also be less deepl~
immersed and ra~ially spaced fur~her from the cente.r C of the.furnace lOA.
In another.embodiment o the present invention illus-~ra~ed in. Figures 9 and la there is providea a furnace 10~
in ~uding an exterior shell 12, a layer of insulation.2Q, a refractory vessel 24, and protective structure 30B seParated from vessel 24 ~y a narro~ space 32. The vessel 24 incl.udes upstanding sidewalls 26 and floor 28- Protective structure 30B includes sidewal]~
and floor 74 as.well as upper outstanding exterior flange 33 described here~na~ove.
Rerac~ory v~ssel floox 28 may be sli~htl~ inclined downwardly toward ou~tet 100 as shown, and sesarated ~rom lLner floor 74 ~y a relatively narrow space 76. In order to maintain ~he soace 76, varicus sup~or~s mav ke provided.
The ou~er peripheral edge of ~loor 74 i~ supported bv an inwardly extendirlg flanse portion 84 pro.jecting rom side walls 7~. It should be noted ~hat floor 74 could }~e welded - or joined to sidewalls 72 of protective structure 30B or sImply rest on the flange 84 as ~ho~nnO Inte~nediate supports or sh_ms 94 are located i~ recesses 95 in vessel floor 2~. A centr~l suppor.
~8 i~.th~ shape of a~ annular ~ub~ ~as a radially ~xtending .lang~ 99 which supp~r~s an inb~ard por~ion ~7 of line~
.rloo 74. Central sup~Qr~ ~8 rest~ in an inboard recess ~4 of fl~ox 28. ~'lanse 84,. shims 94.and central sup~r~ ~8 mav - be fabricat~d from a~y suita~le material in~l~ding refractor~
and moly as d~sired~ The space 76 is ~orma}lv illed with ~hermoplastic material to pro.~ect t~e liner. Th~ various ~2~-~Z048~6 supports for floor 74 maintain the space 76 and prevent the protective material from being forced out..
A connector 93 extends through the ~,ottom wall 16 or shell 12 and the floor 28 of ves.sel 24 and is coupled to liner floor 74. A distal end o connector 93 i5 coupled to a source of power to thereby electrify the liner floor 74.
Other portions of pro~ective structure 30B may be electrified with addi-tional co~nectors 93 (not shown) to assure good elec~rical sy~metry. Electr.ical h~atexs 104: may be loca~ed at va-ious locations ~ncluding the s~aces 32 and 76 between the ~trotective structure 30B and xefractory vessel 24, ~ slot.36 in 100r 2~ of v~ssel:~4 and ar.nular space a5 ahout oùtlet 100 for startup and ~urnace control.
A pair of batch electr.odes 50 supported by arm 87 are located abo~e furnacta 10B and extend through bat h blanket 110 into. fusion zone below line-111. In a preferred ~mbodi-ment multiple pairs of ~atch electrodes 50B are locate~ at 30 or 6~c i~ter~als about t.~e furnace (see Figure la).
t~hile batch electrodes 50B are shown in pairs other ar_ange-ments and combinations of singlets, triplets, etc. may beused. The preferred arrangeme~t is to. locate one or more elet~r.od,2s i~ sets SO-SOB along radial lines to cover the ~urnace projection s~metrically, Ir. ~igure 10 notice the ~ rela~ively close electrical spacing r~su}tinq f~om the i multiple se~s o~ electrode~ SOB~ The arrang~ment allows ~he electroaes.50-50B to ~ir~ to each o~er a~d create cl~sely coupled electrical cu_renk~ t~erehe~een~ If ~8 ~lec~r.o~e spacing is symmetrical and the ele~trica1 firi~s bal~ced to produre relati~e7y uniform ~emperature across ~0 the furnace, relatively small con~ec~ion rolls will occur.

-~7 In Fig. 9 notice the symme~xy that arises around the radial placement of electrodes. Th~ electrodes sa at different -adii are powered so as to maintain more or less the same temperature at each respecti~e radiai portions thereof. Since ~he glass around each electrode sa will be hotter than the s-~rrounding glass, the glass movement beneath each electrode 50 will be ver~ically upward (see circular arrows 0 & I representing in~er and outer con~ec~ive rolls~.
~etween electrodes 50, ~hsre will he descending glass i 10 current~. If the temperature is maintained about the same :Erc~n the c~ter of the f~rnace lOB ~o the Pr~tective structure 30B, therl each electroda 50 will produce i-5 o~ convective roll which will not he overpowered by a larger or dominant convec~i~e roll~ The many small con~ective rolls move wlth lower ~elocity ~h~n larger con~ecti~e rolls because o the greater shear s~ress therebetween~ Hence the residence time of glass in ~he melter is inc-eased. The depth cf oene~ra~iGn of rolls is a func~ion of glass ~iscosity at t~e me~t temp~ra~
~ure~ Normally it is desirable to keep rolls sm~11 and maintain quie~cer.c2 o~ fining in a lower portion of the furnace. ~owe~er, it appears as ~hough deep pene~ra~io~ or co~te tion currents do not afect glass quali y, especially i~ ~riew o~ the fact that a center batch elect:rode ~shown in F ~gure 1) may b~ used fo~ y., .1 In Figures 1, 7 and !?t ik should be rloted that reqpect:i~re batch el~ctrodes 50 ana 5QB are relatively clos~- ~o ~he top of ~he ~urnace i~us~on li~Le 111. Ry collceIltr2~ing the heat.
high there, in the ~nace~ 10, lOA, a~d lOB, the mol~e~
glass therein doe~s not t~nd to ~riolerltly con~-~c~ and heat is concen~rated whexe r~eeded i. e. near ~he ~atch 110 <, This i~
especiall~ help ul with glass~s which have a resis~i~a~y ;

~28~

that varies rapidly with tempexature ~e.`g. alumino-silicate glasses). More energy can ~e dissipated just below the batch blanket 110 to do a more efficient job of melting.
The arrangement of electrodes- 50, 50~ and 50s should be symmetrical. This is especially true in larger furnaces as for example the furnace 10B shown in ~igs. 9 and 10. The electrodes 50-50B t~erein may ~e located every 30 or 60 with alternate sets exhibiting different firinq pat~erns, e.g. cross iring, peripheral firing, etc. Other combina-tions and arrangements ~re possible up to as close as 15staggered spacing, but the arrangemen~ showm are presently ~referred.
Symmetrical ~iring o~ electrodes positior~ed close to ~he batch blanket has the advantage or producing vertical a~d ~orizon~al temperature stability wh~c~ has been round to r^sult in more ef~icient use or ener~y and produce bet~er qualitv product~ ~',ormally, in a glass melting furnace, freshly hea~ed glass tends to rise because of a reductian in its density wi~h incrsasing temperature. SLmilarly, cooler glass being more dense tends to move dcwnwardly. Con~ecti~e r~lls or rolling movement of th~ melted gla~s are ~U5 prod~ced and maintained within the melt since the di~fa~ential in glass density producas a ~riving force crea~irg such i rolli~g movement. In t~e present in~entlon since ~h~ heat is place~ high in the furnace, ~ust below fusio~ line 111, the heated glass tends to remai~ n ar ~he to~ o f '~he furnace~
O course some ~ooling will ~cur and glasi will -flow do~n-wardly causing con~ection, but ~ince the gl~ss i~ h~ated near the ~op o~ th~ ~urnace lt~ initial motion is restricted, 30 t~ereby reducing displac~ment ~r ~t~er gla~s nearb~. Th~
tendency of the newly heat2~ glass to r~main near the ~op o~

4~

the furnace i5 reinforced because it is at a lccati~n where it is closest to e~uili~rium and is not being rapidly dis-placed and cooled.
Uniformity of horizontal temperature distribution results in suppression of one major convective roll from the hot side ~o the cold side of the r-urnace. ~y balancing heat input hDrizontally there is less of a tendency for any portion of the ~urnace to produce excessive heating or cooli~g of glass which.encourag~s convection.
I'he present invention may fur.ther inciude a system without a liner bottom having floor electrodes as in Fig. 1 bu~ disposed in pairs, triplets, etc. alor.g rad~al lines, such as shown by the arrangement in Figure la.
~ any furnace slzes are passible from the relatively small arrangement of Fig. 1 ~4' diameterl to tne large unit shown in Fig. q ~10-30 ~eet in diameter~. Larser versions are pos.~ible too, howe~er, the number of electrodes might be increased as th~ ~iameter increases. Fux~her it might be necessary to introduce a staggered set of multiple electrodes 0 interme~iate the sets hereir~above descri~ed.
tended that ~eatuxas of each of the em~adiment set forth h~rein may be incorporated in a~d be interchangeable with each other. For example, the layer of refractory ramm~ng mix 21 shown in ~igures 1 and 3 ~ay also be incor-pora~ed in the other emh~dimen~sO As there are many such imp~r~ant interchangea~le features the above is merely exe~plary and illustrati~e and is ~ot in~ended as a limita-tion her~
~hexe ha~ thererore ~een described what a~ present are con~idered to ~e the preferred embodlmentq of the ~re~ent in~en~ion, and it will be ob~ious to ~hose ~killed i~ ~he ~30 3LZ6~48~1~

art that various changes and modiications may be made kherein without departing rom the invention, and it is intended in the appended claims to cover all such changes and modif:ications as ~all within the true spirit and scope of the invention.

Claims (27)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. A method of protecting a refractory furnace vessel for melting thermoplastic materials from corrosion which comprises, lining an interior portion of said vessel susceptible to relatively high heat with a protective material, closely space the protective structure from the inside walls of the melting vessel, providing a relatively cool zone within said furnace, and extending the protective structure into the relatively cool zone thereof.

2. The method of claim 1 further including the step of disposing a thermoplastic material relatively inert with respect to the protective structure between the protective structure and said vessel.

3. The method of claim 1 further including the step of insulating a major protion of the furnace vessel for reducing heat loss therefrom.

4. The method of claim 1 further including the step of providing at least one layer of refractory material in contact with an outer surface of the vessel for limiting seepage of thermoplastic material beyond said layer.

5. The method of claim 4 including the step of sintering said one layer by heat generated with the furnace.

6. The method of claim 1 further including the step of inhibiting low of materials including contaminants between inside and outside portions of the protective structure.

7. The method of claim 6 wherein the step of inhibiting the flow of material includes the step of cooling the material in selected cool zone areas of the furnace to thereby increase its resistance to flow at the relatively cool zone thereof.

8. The method of claim 1 wherein the step of providing the relatively cool zone includes the step of imposing a zone of electrically produced intense heating in an upper portion of the vessel and spaced from said protective structure.

9. The method of claim 1 further including the step of providing at least one zone of electrically produced heating through at least one of an open upper end of the vessel and a sealable through opening in a lower portion of the vessel.

10. The method of claim 9 further including the step of orienting the position of the electrically produced heating in selected symmetrically spaced locations within the vessel.

11. The method of claim 1 including the step of electrically energizing the protective structure with at least one of cathodic direct current bias and an alternating current neutral.

12. The method of claim 1 including the step of forming the protective structure from materials selected from the group consisting of ferrous, refractory and noble metals and alloys thereof.

13. The method of claim 1 including the step of maintaining the protective structure submerged in thermoplastic material when the temperature of the material exceeds an oxidation temperature of the protective structure.

14. The method of claim 1 including the step of locating an outlet of the vessel in a position above a lowermost portion thereof for trapping thermoplastic material below the outlet within the vessel.

15. The method of claim 14 further including the steps of electrically energizing the outlet, and producing relatively high electrical heat in the vicinity thereof.

16. The method of claim 15 further including the step of direct current biasing the outlet to an electrical polarity sufficient to prevent electrolytic corrosion thereof.

17. The method of claim 15 including the step of providing a sacrificial electrically energizable heating means, and direct current biasing said heating means relative to the outlet and sacrificing said heating means by electrolytic corrosion in preference to the outlet.

18. The method of claim 1 including the step of inducing electrical current within the thermoplastic material and segmenting the vessel into electrically isolated portions for inhibiting flow of electrical current therebetween.

19. The method of claim 18 further including the step of monitoring current flow in at least one of said isolated portions.

20. The method of claim 18 further including the steps of grounding at least one of said isolated portions, monitoring current flow therein, and severing said ground in the event the current flow therein exceeds a selected level.

21. The method of claim 1 wherein the furnace has a batch blanket of fusible thermoplastic material overlying molten thermoplastic material, a method for melting said fusible material further including the steps of dissipating electric power in the molten layer of the material immediately under-neath the batch blanket, and concentrating electrical heating energy in said layer.

22. The method of claim 21 further including the step of placing sources of electrical heating energy close to each other for providing electrical currents in the material in a symmetrical pattern.

23. The method of claim 21 further including the step of providing a zone of relatively high temperature in the molten layer immediately below the batch blanket and producing convective rolls within the molten material in the layer.

24. The method of claim 23 further including the step of containing the convective rolls near the layer by concen-trating the dissipation heat energy therein.

25. The method of claim 23 further including the step of optimizing the size of convective rolls by symmetrical dissipation of the heat energy in said layer.

26. The method of claim 23 further including the step of providing relatively small driving force for said convective rolls by relatively uniform dissipation of heat energy into said layer.

27. A method of producing molten glass which comprises, providing a refractory vessel having upwardly extending sidewall portions, positioning a liner formed predominantly of molybdenum within said refractory vessel with wall portions of said liner in closely spaced apart relationship to said upwardly extending sidewall portions substantially along the extent of said sidewall portions with the interior and exterior of such liner in open communication with each other, providing molten glass interiorly of said liner and at least semi-molten glass exteriorly of said liner within the space between said liner and said sidewall portions with the interior and exterior glasses in physical communication with one another, maintaining said liner submerged within said molten and semi-molten glass and completely surrounding said liner with said glass, maintaining an upper surface of said molten glass interiorly of said liner above the upper extent of said liner and supplying batch material to such surface, and supplying electrical energy centrally of said liner adjacent said batch material to melt said batch material and produce molten glass.