US2238793A - High-temperature electric furnace - Google Patents

Description

April 15, 1941. D. HANAWALT ET AL 3 7 HIGH-TEMPERATURE ELECTRIC FURNACE Filed July 28, 1939 Patenta& Apr, 35 3.941

HIGH-TEDMEMT URE ELECTRIC FURNACE 'Joseph lD; Hanawalt, (Charles E. Nelson, and John S. Peake,.Midlanl, Micic., aesignors to The Dow Chemical Compare Midlandl, mich., a co-poration ol Michigan Application July 28, 3939, entel Ne.. %87,134

a claim& WL lui- 3313 This invention relates to an improved construction tor electric furnaces operated at temperatures above 1000 C. More particularly it relates to .the thermal insulation of such urnaces.

In various metallurgical processes, such as the liberaton and vaporization of magnesium and the alkaline earth metals from their ores, it is customary to operate in a graphite vessel at temperatures well in excess of 1000 C., say at 1300- 1800 C., and preferably under vacuum or in the presence of an inert gas. Since it is a virtual impossibility to construct a completely vacuumtight vessel which will withstand such temperatures, it has been found necessary to enclose the heated vessel within a larger container or chamber which is at a much lower temperature and hence can be made vacuum-tight, and to interpose a large body of heat insulating material between the heated metallurgical vessel and the surrounding cool walls of the vacuum chamber.

OI the various possible heat-insulating materials which may be employed i'or this purpose, finely-divided carbon, e. g. lampblack or graphite dust, would seem highly preferable because it does not attack a graphite metallurgical vessel even at temperaturesewell above 1000 C., and because at these temperatures it still has an outstandingly low thermal oonductvity. Unfortunately, however, finely-divided carbon, after exposure to air, always contains large amounts of adsorbed gases, especially water, which, upon heating of the carbon, are eventually evolved or outgassed. Thus, when ordinary lampblack is used as the sole heat insulator in an electric vacuum furnace of the type described, the insulation temperatures 'are such that water vapor and other gases are slowly desorbed and evolved within the vacuum chamber, gradually attacking the metallurgical vessel and other graphite parts, and requiring continuous removal. Thus, in a vacuum electric urnace of moderate commercial size, after each shut-down for cleaning or repairs during which time the lampblack insulation is exposed to the air, this outgassing sometimes continues for a considerable period after operation of the fumace is resumed, during which time anumber of the graphite parts may become so badly attacked as to decrease greatly the life of* the iurnace before another shut-down is necessai-y. In addition, for certain metallurgical processes which must be operated at very low pressure, this evolution of gas may be sufiicient to make it difiicult to attain the Operating pressure. For this reason the use of finely-divided carbonas the sole heat insulator for vacuum furrraces has "ri-een. considered impractical, and resort has seen had to other materials whichare poorer heat insulators and are often seriously afiected by contact with 'the inner graphite vessel at the high temperatures.

We have now' "found, however, that it is not only possible but highly desirable to use lamphlacl; or other nely-dvded carbon as the principal thermal insulator in electric Vacuum furnaces, provided the umace be so constructed that during operation substantially all the mass of lampblack is maintained above its desorption temperature, i. e. above the temperature at which the lampblacis will retain adsorbed gases or more than a short period, usually above about 500 C. 'When this temperature condition is met, outgassing of the nely-divided carhon insulation is completed within a relatively short period after operation of the furnace is begun, the gases being entirely removed before there is time for any significant attack on the iurnace parts to occur. It is thus possible to utilize the excellent thermal properties and Chemical inertness of the finely-divided carbon insulation without encountering the outgassing diiculties of prior art prec tice. Maintenance costs are lowered and the sizes of the Iurnaces may he reduced below that of i'urnaces using poorer insulating material. Any form of finely-divided carbon, i. e. carbon having a particle size be1ow 0.1 mm., may he employed.

A variety of means may be used in an eiectric y furnace according to the invention to maintain the flnely-divided carbon insulation above its desorption temperature; for instance, heaters may be placed in the. carbon. However, the simplest construction in practice is to dispose the nely divided carbon insulation immediately around the graphite furnace chamber where it serves as the primar-y insulation, being exposed to the highest temperatures and most severe conditions, and then to surround this carbon with another or secondary heat-insulating material. This secondary insulation is employed in such thickness that the thermal gradient therein is sufiicient' to insure that the finely-divided carbon insulation is at a temperature at least as high as the desorptlon temperature of the carbon. The secondary insulation should not, however, be so thick that the temperature at the carbon insulation-secondary insulation interface is so high that the Chemical interaction between the two materials might occur. In this Construction the secondary thermal insulation serves chiefiy as a lining or blanket to keep the carbon insulation g aas-lar e at the desired temperature. It is itself not ex- `posed to the extreme temperatures of the furnace interior and hence may be made of any reiractory material not subject to outgassing, e. g. rebrick, magnesite brick, etc. or combinations thereof.

One form of apparatus utilizing the principle of the invention is illustrated in the accompanying drawing which shows an electric-vacuum furnace in diagrammatic vertical section. such a furnace finds use, :for example in metallurgcal processes in which a solid ore charge is reduced, liberating metal in vapor form, and leaving a solid resiclual slag.

Referring to the drawing, the entire furnace .structure is 'shown housed in an air-tight shell i made of steel or other suitable material and open at the top through a manhole 2 having an air tight cover 3 With a valved outlet d. vounted within the chamb-er i on gran-hite piers 5 is a graphite metallurgical vessel ti having a sloping hearth ll and a slag pit and provided with a charge inlet a vapor outlet w, and a slag outlet ll. solid material entel-s the inlet 9 through a charging lock [12, and gas may enter through a valvecl inlet 03. Slag leaves the iurnace outlet ll through a discharge lock l l. The inetallurgical charnber G is heated by electric resistor bars iii which receive current through suitable gast'ght leads not illustrated. The principal furhace insulation is a bed of lampblack i@ in contact with and entirely surrounding the vessel Between the lampblack and the walls or the chamber i is a lining consisting of layers of secondary insulation, i. e. magnesite brick Il'l, chrome brick iS, and ordinary firebrick 09. 'The lampblaclg is covered on top by a mat of rock wool which also serves as seconclary heat insulation.

in operation, current'is passed through the resistors 15 so that the temperature in the graphite vessel 6 is well above 1000 C., say at 1300-i800 C., the exact temperature depending upon the particular metallurgical process being carried out. A charge of ore is introducecl through the .inlet il, and falls onto the hearth ll, where under the temperature prevailing it reacts to liberate metal vapor, which escapes through the outlet i@ to a condenser not shown; any slag formed falls into the pit from which it may be withdrawn through the outlet M. "When it is desired to operate under reduced pres'sure, the v alve t! is closed. and vacuum is appliecl through the outlet lt When Operating in an inert atmosphere, the inert gas may be introduced through the inlet 03 and allowed to escape through theoutlet W. in either case, the furnace is being operated in the absence In working under vacuum, when the iurnace is first started up, the lampblacl i@ evolves adsorbed gases. These gases may be withdrawn from the chanber l by applying a vacuurn to the outlet l, which is then closed when outgassing is complete. In an alternative construction, the outlet 6 may be omitted, the evolved gas es seening through the graphite walls of the vessel G and being withdrawn through the outlet lil. It will be appreciated that because of the presence of the secondary insulation il, 063, se, and ao, the lampblack (i is maintained well above its desorption temperature, and the initial outgassing just rnentionecl` is complete within a compare,- tively short period. The structure illustrated thus utilizesto the iullest extent the ideal thermal properties and Chemical inertness of lamplolack,

and yet avoids the outgassing difficulties of previous iurneces.

'The relative thiclrnesses of the finely-divided carbon insulation and the secondary insulation necessary to insure that the ormer remains above its desorption temperature, i. e. above about 500 C., when the vessel G-is at temperatures above lll0 C., may be oomputeol by familiar thermal principles from the known heat conductivities of the various refractory materials used. In the most economical Construction, the secondary in` sulation is just thick enough to keep the outer portion of the lampblach just above the desorio tion temperature, hut greater thicknesses, such that the lampblacl; is at much higher temperatures throughout, may he employed.

lt is to he understood that the invention is not limited to the specific structure shown but is applicahle to any high temperature furnace utilizing electric heating means and operating at above i C. in the absence of air, i. e. under reduced pressure or with an inert atmosphere.

Vie claim:

i. An electrical iurnaoe for operation at temperatures above NOW C. comprising: an air-tight shell; a graphite furnace chamber supported within the shell; electric heating means within the chamber; a bed of lampblaci: immediately surrounding the chamber and acting as a priinary therinal insul-ation between the chamber and the shell; and means ior maintaining the said lampblaci substantially all at a temperature above eow 0.; and .means for removing gases ironi within the shell.

2. .in an electric urnace, the oombination of: an eir-tight shell; a graphite urnace chainber supported within the shell; electric heating means within the chamber; a bed of fnely-clivided carhon immediately surrounding the chamber and acting as primary thermal insulation between the chainloer and the shell; and a. lining ofheat reiractory secondary insulating material between the finely divided carhon and the shell, said material being of such thickness that when the furnace is operating at temperatures above 1000 C. the therrnal gredient in such reiractory is sufrlcient to maintain the finely-divcled carbon substantially all at a temperature above its desorption temperature, and means :for removing gases from within the shell.

3. in an electric furnace, the combination of: an air-tight shell; a graphite iurnace chamber supported within the shell; electric heating means within the chamber; a bed of nely-divlcled carbon immediately surrounding the ohamber and acting as primary thermal insulati'on between the chamber and the shell; and a lining of heat refractory seconclary insulating material between the nely-divided carbon and the shell, said material being of such thickness that when the iurnace is Operating at temperatures above iooo C. the therrnal gradient in such refractory is sufcient to maintain the firely-divioled carhon suhstantially at a temperature above 0 C.: and means for removing gases froni within the shell.

4. in an electric furnace, the combinaton of: an air-tight shell; a graphite furnace chamber supported within the shell; electricheating means within the chamber; e charge inlet extending :from outside the shell into the said. chamber; a bed of lampblacl ;immediately surrounding the chamber and actng as primary insulation between the chamber and the shell; and a lining o refractory secondary heat insulating material between the lampblack and the shell, said material being of such thickness that when the furnace is Operating at temperatures above 1000 C. the thermal gradient in said refractory is such as to maintain the lampblack substantially all at a temperature above 500 C.; and means for removing gases from within the shell.

5. In an electric vacuum fumace, the combination of: an air-tight shell; a graphite furnace chamber supported within the shell; electric heating means within the chamber; a vapor outlet leading from said chamber to outside the shell; a bed of finely-divided carbon immediately surrounding the chamber and acting as primary thermal insulation between the chamber and the shell; and a lning of refractory secondary heatinsulating material between thecarbon and the shell, said material being of such thickness that when the furnace is Operating at a temperature above 1000 C. the therma gradient in said refractory is such as to maintain the carbon substantially all at a temperature, above 500 C.

6. In an electric vacuum furnace, the combination of: an air-tight shell; a graphite furnace chamber supported within the shell; electric heating means within the chamber; a charge inlet extending from outside the shell into the said chamber; a vacuum charge lock in the said charge inlet; a slag outlet leading from said chamber to outside the shell; a vacuum discharge lock in the said outlet; a vapor outlet leading from said chamber to outside the shell; a bed of lampblack immediately surrounding the chamber and, acting as primary insulation between the chamber and the shell; and a lining of. refractory secondary heat insulating material between the lampblack and the shell, said material being of such thickness that when the urnace is Operating at temperatures above 1000 C. the thermal gradient in said refractory is such as to maintain the lampblack substantially all at a temperature above 500 C.

7. In an electric vacuum furnace, the combi- I nation of: an air-tight shell; a valved gas outlet in said shell; a graphite furnace chamber mounted on refractory piers within the shell; electric resistance heating elements within the chamber; a charge inlet extending from outside the shell into the .chamber; a vacuum charge lock and a valved gas inletin the saidtcharge inlet; a slag outlet leading from said chamber to outside the shell; a vacuum discharge lock in said outlet; a vapor outlet leading from said chamber to outside the shell; a bed of lampblack immediately surrounding the chamber and acting as primary insulation between the chamber and the shell; and a lining of refractory secondary heat insulating material between the lampblack and the shell, said material being of such thickness that when the fumace is Operating at temperatures above 1000 C. the thermal gradient in said refractory is such as to maintain the lampblack substantially all at a. temperature above 500 C.

8. In an electric furnace, the combination of: -an air-tight shell; a graphite fu'nace chamber supported within the shell; electric heating means within the chamber; a bed of finely-divided carbon immediately surrounding the chamber and acting as a primary thermal lnsulation between the chamber and the shell; and a linin! of secondary thermal insulation between said bed of carbon and the shell consisting of successive layers of magnesite brick, chrome brick, and firebrick, the said brick layers being of suflicient thickness that when the furnace is Operating at temperatures above 1000 C. the thermal gradient in the brick is such as to maintain the primary insulation all at a temperature above 500 C., and means for removing gases from within the shell.

JOSEPH D. HANAWALT. CHARLES E. NELSON. JOHN S. PEAKE.

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