US3721743A - An electric smelting furnace of the closed type - Google Patents

Abstract

A closed electric smelting furnace is provided with a cylindrical partition wall of substantially the same height as the furnace lid secured to the underside of the lid in a position to surround the electrodes and material feeding chutes. The partition together with the lid defines an annular gas collecting chamber. The raw materials are charged into the furnace to a level where they extend into the annular gas collecting chamber. Dust rich furnace gases rise quickly around the electrodes and collect in the upper portion of the chamber after which they pass slowly outwardly through the raw material so that they are filtered before leaving the furnace though a vent in the furnace lid which is positioned radially outwardly of the partition.

Description

Unite States Patent 1 Shiina et al.

1March 20, 1973 AN ELECTRIC SMELTING FURNACE OF THE CLOSED TYPE [75] Inventors: Toshio Shiina, Kokubungi-shi, Tokyo; lltatu Tisaki, Setagaya-ku, Tokyo, both of Japan [73] Assignee: Tanabe Kakoki Co., Ltd., Niigataken, Japan [22] Filed: Sept. 15, 1971 [21] Appl. No.: 180,764

[30] Foreign Application Priority Data Sept. 25, 1970 Japan ..45/8370O Sept. 28, 1970 Japan ..45/84542 [52] 11.8. C1 ..13/9, 13/33 [51] int. Cl ..H05b 7/00, F27d 13/00 [58] Field of Search ..13/9, 33

[5 6] References Cited UNITED STATES PATENTS 1,830,992 11/1931 Frenzel ..l3/33 X 12/1964 Collin et a1. ..l3/33 X Primary Examiner-Roy N. Envall, Jr. Attorney-Solon B. Kemon [5 7] ABSTRACT A closed electric smelting furnace is provided with a cylindrical partition wall of substantially the same height as the furnace lid secured to the underside of the lid in a position to surround the electrodes and material feeding chutes. The partition together with the lid defines an annular gas collecting chamber. The raw materials are charged into the furnace to a level where they extend into the annular gas collecting chamber. Dust rich furnace gases rise quickly around the electrodes and collect in the upper portion of the chamber after which they pass slowly outwardly throughthe raw material so that they are filtered before leaving the furnace though a vent in the furnace lid which is positioned'radially outwardly of the partitlon.

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PATENTEDmeoxm SHEET 3 [IF 3 RATE OF AIR FLOW (Nm /hr) ELECTRIC SMELTING FURNACE OF THE CLOSED TYPE BACKGROUND OF THE INVENTION This invention relates to a closed electric smelting furnace, and more particularly to a type with a built-in dust collecting device for the generating furnace gas.

In an electric smelting furnace, most of the furnace gas mainly consisting of carbon monoxide evolving from the reaction zone at the lower part of the furnace spurts to the furnace top through a narrow columnar pile of mixed raw materials, whereas only small volumes of the furnace gas rise through the raw material disposed apart from the electrodes. Since the furnace gas released from the neighborhood of the electrodes goes up at a considerable speed, the gas ejected from the furnace top is accompanied with large amounts of dust consisting of finely divided raw materials. On the other hand, the gas rising through the raw materials distant from the electrodes passes very slowly through a broad area and contains minor amounts of dust. Since this dust is filtered away by the heaped raw materials, the furnace gas coming out of the periphery top is substantially free therefrom.

In an electric smeling furnace of open type, the furnace gas discharged from the raw materials surface immediately burns of itself. This burnt gas is scattered into the air with much amounts of dust, often giving rise to public nuisance. To resolve this problem, there should be additionally provided a dust collector. Since, however, the volume of air to be mixed with the burnt gas amounts to 50 to 100 times that of the latter, there would be required a tremendously bulky dust collecting apparatus, the realization of which would be practically feasible. With respect to a large capacity electric furnace, therefore, there has been developed a closed type, wherein the furnace gas is drawn outside of the furnace without being burned at the furnace top to remove most of the dust entrained with the gas by an externally constructed dust collector. Since, however, the gas contains 50 to 100 g/m of dust, its substantially complete elimination would still require very expensive dust collecting equipment which would be moreover accompanied with the considerable danger of handling explosive gases and demand a great deal of time and work in its inspection and maintenance.

On the other hand, if raw materials are crushed to a proper uniform particle size before they are charged into an electric furnace, then the furnace gas will be prevented from being concentrated around the electrodes, and in consequence slowly and uniformly rise from various sections of the charged raw materials, offering a certain dustproof effect. In this case, however, there would also have to be expended considerable amounts of capital in providing a device for pretreating raw materials. Therefore, the last mentioned method is in no way a satisfactory solution of the problem.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to remove dust from furnace gas in an electric smelting furnace itself without constructing the aforementioned external apparatus for dust collection or pretreatment of raw materials, and draw out the dust-stripped pure gas mainly consisting of carbon monoxide for utilization in any application.

Another object of this invention is to burn the furnace gas so as to preheat the charged raw materials with said combustion heat, remove dust from the burnt gas in the furnace and discharge the dust-stripped gas to the outside.

These objects may be attained in accordance with the present invention by fixing a cylindrical partition wall, whose height is substantially the same as that of the furnace lid, to the ceiling thereof in a manner that it surrounds three electrodes and several chutes for feeding raw materials into the furnace so as to heap up the raw materials inside of said partition wall.

These and further objects will be evident from a study of the following disclosure and the accompanying drawings which illustrate two preferred modifications of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinally sectioned view of a closed electric smelting furnace according to an embodiment of this invention;

FIG. 2 is an plan view of FIG. I in a reduced scale;

FIG. 3 is a longitudinally sectioned view of a closed electric smelting furnace according to another embodiment of this invention;

FIG. 4 is an plan view of FIG. 3 in a reduced scale; and

FIG. 5 is a chart showing the relation between the air flow rates and the atmospheric temperatures in the central and surrounding chambers of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION The space under the lid of a closed electric smelting furnace is divided into two chambers, that is, central and surrounding ones by a cylindrical partition wall substantially almost as high as the furnace lid. The cylindrical wall is fixed to the underside of the ceiling of the lid so as to surround three electrodes and several chutes for feeding mixed raw materials. The raw materials are charged into the furnace body through the chutes and heaped up inside of the partition wall. Most of the furnace gas mainly consisting of carbon monoxide speedily goes up the electrodes and accumlates in the central chamber of the furnace lid. Then the gas now rich in dust slowly travels under its own pressure through the top layer of the heaped raw materials and those under the partition wall of the central chamber into the surrounding chamber. The gas cleaned by having its dust filtered away by the heaped raw materials collects in the surrounding chamber and is delivered outside of the furnace through a gas outlet duct to be utilized for any purpose.

The small remainder of the furnace gas slowly rises through the raw materials disposed apart from the electrodes and, after having small amounts of dust contained therein trapped by the raw materials, is also conducted to the surrounding chamber. The gas thus cleaned is discharged out of the furnace together with the main parts of the furnace gas introduced from the central chamber.

There will now be further described by reference to FIGS. 1 and 2 a closed electric smelting furnace according to an embodiment of this invention. A furnace body 1 is covered with a lid 2 shaped like an inverted cup. The lid has three holes each fitted with a packing ring 5, through which there are inserted into the furnace body 1 three electrodes supported by electrode holders 4 connected to a power source (not shown). Several chutes 8 for feeding mixed raw materials 6 are fitted to the furnace lid 2 so as to charge the mixed raw materials stored in storage tanks 7 into the furnace body 1. The reaction among the raw materials takes place in a high temperature reaction zone 9, producing a liquid product 10, which collects at the furnace bottom to be drawn out intermittently through a tapping hole 1 1.

A cylindrical partition wall 12 substantially as high as the furnace lid 2 is fixed to the underside of the ceiling of the furnace lid so as to surround three electrodes and the chutes 8. As shown in FIG. 2, however, some of the chutes may be disposed outside of the partition wall 12 for design convenience. It does not matter whether the cylindical partition wall has a circular or angular cross section. As previously described, the upper space of the furnace over the top level of furnace body is divided into a central chamber 13 and a surrounding chamber 14 by the cylindical partition wall 12. The surrounding chamber 14 is provided with a gas outlet duct 15 and several peep windows 16 for observing the internal condition of the furnace.

The furnace gas G0 evolved from the reaction zone 9 rises through the furnace in two streams, that is, the main stream Ga and the minor stream Gb. As mentioned before, the main stream Ga speedily goes up the electrodes 3 with large amounts of dust and accumulates in the central chamber 13, while the minor stream Gb very slowly rises through the raw materials disposed apart from the electrodes and, after having small amounts of dust contained therein filtered away by the raw materials, is conducted to the surrounding chamber 14. The amount of the main stream Ga accounts for 80 to 90 percent by volume of the total furnace gas G0. The main stream Ga collected in the central chamber 13 contains 50 to 100 g/m of dust.

The main stream Ga slowly travels downward through the section of raw materials disposed near the inner wall of the cylindrical partition wall 12 due to a pressure difference of about 25 mmAq naturally occuring between the central and surrounding chambers 13 and 14, and further proceeds through the section of raw materials charged under the partition wall 12 into the surrounding chamber 14 so as to have the dust contained in said main stream Ga filtered away by these sections of raw materials. The cleaned main stream Ga is mixed with the purified minor stream Gb in the surrounding chamber 14, the mixed stream Gc leaving the furnace through the gas outlet duct 15. The amount of dust which has already been removed from the mixed outlet stream Gc accounts for more than 90 percent of the original weight of dust contained in the main stream Ga collected in the central chamber. When, therefore, the outlet stream gas Gc is to be further stripped of dust as occasion demands, there will be only required a dust collecting device having a very small capacity, offering a great economical advantage.

The cylindrical partition wall can be so easily fixed to the ceiling of the furnace lid that it is well adapted to be fitted to any electric smelting furnace in existence. The partition wall is built of metal with a water or stream cooling jacket provided inside and a heat resistive cover mounted thereon. It withstands long use without any local thermal deformation because of its simple construction.

There will now be described by reference to FIGS. 3, 4 and 5 a closed electric smelting furnace according to another embodiment of this invention. In this embodiment, the aforementioned main stream gas Ga and minor stream gas Gb are burnt in the central and sur- 'rounding chambers 13 and 14 respectively so as to preheat the mixed raw materials charged therein by the resulting combustion heat, the burnt mixed stream gas Gd leaving the furnace through the gas outlet duct 15. Throughout the figures, the same numerals and symbols of reference indicate the same parts and gases as in FIGS. 1 and 2.

Heretofore the furnace gas mainly consisting of carbon monoxide which is obtained during the operation of a closed electric smelting furnace has been delivered outside of the furnace without being burnt therein to be used as a fuel gas. Already known is the attempt to introduce additional air into the electric furnace to burn the gas therein so as to preheat the charged raw materials by the resulting combustion heat, thereby saving power consumption for smelting. If, in this case, the furnace atmosphere has a considerably higher temperature than the ignition point of the furnace gas, then there will be no possibility of explosion. Occasionally, however, the furnace atmosphere has a lower temperature than said ignition point, most likely leading to the occurrence of explosion. Accordingly, this internal combustion method has been rarely practised.

If there are provided within an electric furnace some pilot burners using auxiliary fuel such as town gas even when the furnace atmosphere has a lower temperature than the ignition point of the furnace gas, then the furnace gas will be induced by flames delivered from the pilot burners to burn smoothly, thus removing the danger of explosion. However, such pilot burners are of complicated construction and require troublesome operation, still failing to reach the stage of practical application.

For the reason given above, the furnace gas from most of the closed electric smelting furnaces now in use is drawn outside without being burnt therein. In this case, the construction of the furnace lid is generally defective in gastightness, so that to avoid the explosion resulting from the intrusion of external air, the furnace is operated with its internal pressure always controlled to have a slightly higher level than the atmospheric. Accordingly, when there are opened the peep windows for observing the internal condition of the furnace, there are expelled outside large volumes of furnace gas, making it impossible to poke the raw materials placed at the top surface of the furnace by fully opening said peep windows. Therefore, the closed electric furnace developed to date has not attained much more improved operation results than the open type.

In view of the aforementioned circumstances, the present inventors have slightly modified the construction of an electric smelting furnace according to the first embodiment and succeeded in safely burning the furnace gas in the lid chamber to preheat raw materials charged at the furnace top, and removing most of the dust contained in the gas as effectively as in the first embodiment. This modification consists in boring the side of the cylindrical partition wall 12 fixed to the ceiling of the furnace with several small through holes 17 for introducing pilot flames therethrough, and providing primary and secondary air inlet pipes 18 and 19 in the central and surrounding chambers 13 and 114 respectively. The air inlet pipes 18 and 19 have control valves 18a and 19a respectively.

There will now be described the operation of a closed electric smelting furnace thus modified. The furnace gas collected in the central chamber 13 has a temperature of 700 to 800 C, an appreciably higher level than its ignition point of from 500 to 600 C. Therefore, additional introduction of smaller amounts of air than theoretically required for the full combustion of the furnace gas into the central chamber 13 through the air inlet pipe 18 enables the partial combustion of the gas to be safely effected without the danger of explosion, forming stable combustion flames on the surface of the charged raw materials. Further, control of the opening of the control valve 18a permits the adjustment of the extent of said partial combustion, maintaining the atmospheric temperature in the central chamber 13 at any desired level. The furnace gas which has been subjected to partial combustion travels over the surface of the raw materials and then through that section of the raw materials charged under the cylindrical partition wall 12 to the surrounding chamber 114. During the passage, the furnace gas is stripped of dust, and its sensible heat is applied to preheat raw materials charged in succession. if the atmospheric temperature in the central chamber 13 is unduly high, then the raw materials will be sintered at the top to obstruct their own descent and the passage of furnace gas therethrough, rendering the furnace condition unstable. The aforesaid partial combustion is intended to control the atmospheric temperature in the central chamber ll3.'Since, as the result, there prevails a reducing atmosphere in the chamber, the carbon in the charged raw materials and the surfaces of electrodes are protected from oxidation depletion.

After the above-mentioned partial combustion, there are introduced into the surrounding chamber 14 the main stream Ga of partially burnt furnace gas passing through that section of the raw materials disposed under the cylindrical partition wall K2, and the minor stream Gb of the furnace gas slowly traveling through that section of the raw materials charged apart from the electrodes. Since both streams Ga and Gb have already given off their sensible heat to the raw materials, the mixed stream gas Gd has a lower temperature than the ignition point of the furnace gas. If, therefore, said mixed stream gas Gc is again burnt by introducing air through the secondary air inlet pipe 19 into the surrounding chamber M, then there will occur the possibility of explosion taking place as previously described. According to the present embodiment, however, the side of the cylindrical partition wall 12 are bored with several small through holes 117, which enable very small portions of the flames generated in the central chamber 13 to be always ejected into the surrounding chamber M in the form of minor flames 20, which play the role of a pilot, removing the danger of explosion taking place in the surrounding chamber 14. Control of the opening of the secondary air valve 19a enables full as well as partial combustion to be conducted in the surrounding chamber M as in the central chamber l3.

Since, in this embodiment, there is introduced external air into the furnace to burn the furnace gas, there is no need to maintain the internal pressure of the lid chamber at a higher level than the atmospheric, permitting the use of a positive or negative pressure. Particularly where the electric furnace is operated with a negative pressure, not only the supply of some amounts of air to the furnace presents little harmful effect, but also the opening of the peep windows 16 even during operation permits the careful observation of the internal condition of the furnace and the full poking of the top layer of the charged raw materials through said windows 16. Therefore, there can be attained better operation results than when there is not performed the combustion of the gas in the furnace.

This invention will be understood more fully by reference to the examples which follow.

EXAMPLE 1 There was manufactured high carbon ferromanganese containing 6 C and Mn, using a closed electric smelting furnace with a capacity of 1500 KVA which was manufactured according to the first embodiment of this invention and operated with a load of 1380 KW, obtaining the operation results described below.

The furnace body was 500 and 400 cm in outer and inner diameters respectively and the furnace lid ceiling was 70 cm high as measured from the top of the furnace body. The cylindrical partition wall was 280, 250 and 70 cm in outer and inner diameters and height respectively. The partition wall was provided with a water cooling jacket and had its peripheral surface covered with a castable heat resistive material. When the furnace was fully charged with mixed raw materials, that portion of the heaped raw materials which contacted the inner wall of the partition wall had a height of from 35 to 60 cm, averaging 47.5 cm, the rest angle of said heaped raw materials defining 43 C on average with respect to a vertical plane. The average rate at which there was generated furnace gas during one tapping period was about 290 Nm per hour, and the furnace gas consisted of 42 CO, 47 C0 8 H and 3 H O. Determination was made of the dust content and pressure of furnace gas in the central chamber and the gas outlet duct 9 times at an interval of about 30 minutes during one tapping period of 4 hours, the results being presented in Table 1 below.

Time of determi Dust Content Pressure nation mm Aq) after completion of (g/Nm) tapping (min.) Central Gas Central Gas Chamber Outlet Chamber Outlet 5 25 4.25 21 3 30 43 5.75 25 3 60 52 4.76 28 4 67 5.30 31 3 83 5.47 34 2 I50 91 5.28 30 4 95 5.43 35 5 210 97 5.62 38 3 240 48 6.64 13 2 (during tapping) Average 66.7 5.4 28.3 3.2

The rate of dust collection accounts for 92 percent as calculated from the average value given in Table 1 above.

Example 2 There was manufactured high carbon ferromanganese containing 6 C and 75 Mn in a closed electric smelting furnace of the same size and capacity as in Example 1 under the same conditions used therein excepting that there was burnt the furnace gas in the furnace obtaining the undermentioned operation results. The partition wall 12 was bored on the side with six equally spaced through holes mm in inner diameter. There was introduced additional air into the central and surrounding chambers 13 and 14 through the air inlet pipes 18 and 19 respectively. Determination was made of the atmospheric temperature of said chambers 13 and 14 during one tapping period of4 hours with the introduced amounts of air varied each time, the results being presented in FIG. 5. In this figure, the point at which the line representing the atmospheric temperature of the surrounding chamber is bent denotes the stage of full combustion. The straight portion of the line extending from said point to the right indicates that the atmospheric temperature of the surrounding chamber fell due to the introduction of surplus amounts of air, though the combustion was carried to the full. The average length of small flames 20 derived from the through holes 17 was about 30 cm.

The electric furnace was operated with the supply of air to the central and surrounding chambers so controlled as to maintain the atmospheric temperature therein at 1000 C throughout each tapping period. In this case, power consumption per 1000 Kg of product was 2200 KWH, while Example 1 where there was not used any additional air consumed 2850 KWH. The difference in these power consumption is derived from the fact that in Example 2, the charged raw materials were preheated by the combustion of furnace gas effected by additional supply of air. The dust content and pressure in the chambers gave substantially the same values as shown in Table 1.

What we claim is: 1. An electric smelting furnace of the closed type comprising;

a body for holding the raw materials to be smelted; an inverted cup-shaped closed cover for said body, supporting the furnace electrodes and feed chutes for charging raw material into said body; cylindrical partition extending downwardly from the inner upper surface of said cover, said partition surrounding the electrodes and at least some of the feed chutes and separating the upper area of said furnace into outer and inner portions; and a gas vent extending through said cover and communicating with said outer portion; whereby when raw material is charged into said body to a level extending above the lower level of said partition, the majority of gases evolved during smelting pass rapidly upwardly along said electrodes, accumulate in said inner portion and then bleed slowly into said outer portion through the raw material thus filtering a substantial portion of dust particles from the gases before they pass out through said vent. 2. A furnace as defined by claim 1 including a pair of valved air inlet pipes extending through said cover one communicating with said inner and outer portions respectively and a series of openings extending radially through said partition functioning as pilot flame holes conducting flames from said inner to said outer portions.

Claims (2)

1. An electric smelting furnace of the closed type comprising; a body for holding the raw materials to be smelted; an inverted cup-shaped closed cover for said body, supportIng the furnace electrodes and feed chutes for charging raw material into said body; a cylindrical partition extending downwardly from the inner upper surface of said cover, said partition surrounding the electrodes and at least some of the feed chutes and separating the upper area of said furnace into outer and inner portions; and a gas vent extending through said cover and communicating with said outer portion; whereby when raw material is charged into said body to a level extending above the lower level of said partition, the majority of gases evolved during smelting pass rapidly upwardly along said electrodes, accumulate in said inner portion and then bleed slowly into said outer portion through the raw material thus filtering a substantial portion of dust particles from the gases before they pass out through said vent.

2. A furnace as defined by claim 1 including a pair of valved air inlet pipes extending through said cover one communicating with said inner and outer portions respectively and a series of openings extending radially through said partition functioning as pilot flame holes conducting flames from said inner to said outer portions.

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