PROCESS AND DEVICE FOR RETURNING SCRAP INTO CUPOLA FURNACE

Abstract: The invention concerns a process for returning scrap into a cupola furnace for preparing silicate melt for the production of rock wool, and a device for returning scrap into the furnace. In accordance with the invention, the problem of scrap returning is solved through a process for returning scrap into a cupola furnace by which scrap is fed against the wall of the lower portion (1) of the furnace simultaneously with charging the furnace with raw material and fuel, and by means of a device having a scrap feeder (10) on a revolving portion (4) which feeds the scrap into the furnace. The device under the invention and in accordance with an embodiment consists of a revolving portion (4) with feeder (10) and supporting structures (13), and a guiding ring (11) and a driving ring (12) along the outer circumference of the upper edge of the lower portion (1) of the furnace.

Description: PROCESS AND DEVICE FOR RETURNING SCRAP INTO CUPOLA FURNACE The invention concerns a process for returning scrap into a cupola furnace for preparing silicate melt for the production of rock wool, and a device for returning scrap into a cupola furnace for preparing silicate melt for the production of rock wool. The invention relates to returning the scrap produced during the production of rock wool back into the process. Silicate melt, which is produced in cupola furnaces out of lump raw materials of the usual grain size of 30 to 200 mm, is most often used for the production of rock wool. The scrap produced in this process after the defibration of silicate melt, amounts to 30 % of the weight of silicate melt, depending on the defibration procedure. Scrap is also produced during further processing of mineral rock, i.e. throughout the production line. Scrap usually appears in fibrous or pearly form, and represents an ecological and economic burden. Silicate melt is produced in cupola furnaces. A cupola furnace for the production of silicate melt is a special version of shaft furnace consisting of an upright, mainly double-walled, water-cooled iron tube, which is closed at the bottom by a removable lid lined with refractory mixture. The charging with raw material and heat producing fuel is performed from the top by means of a dosing device. Coke is normally the fuel which produces the necessary heat for melting. The oxygen or air needed for coke combustion is brought in at the lower portion of the cupola via a row of tuyeres. Heat is released during coke combustion in the cupola, which is reflected in reactive temperatures above 1500 °C, the result being the melting of raw material. Due to gravity, the melt accumulates in the lower portion of the furnace and flows away through a side opening into a defibration machine. Flue gases which are produced during coke combustion flow away through the intermediate space between pieces of coke and raw material towards the upper portion of the furnace, where a stack or a fume exhausting pipeline is installed. In this way, flue gases preheat the pieces of raw material lying above the melt. The usual raw materials for the production of silicate melt are: diabase, basalt, amphibolite and other convenient alumino-silicate magmatic minerals, or various types of metallurgical slags. It has been known in prior art that only raw materials and fuels in lump form may be used for the production of melt in cupola furnaces. No other approach has so far been used, the state-of-the- art rule being that only the material in lump or core form larger than 30 mm can be used in cupola furnaces, because this has been the only way of enabling flue gases to flow - after the insertion of raw material and fuel - from the place of origin to the furnace outlet. The problem, however, consists in the fact that scrap is normally or substantially in a fibrous or pearly form, i.e. in a non- lumpy form, therefore it could disturb the flow of flue gases, if added to the raw material and fuel in an uncontrolled way. Processes for returning scrap into furnace have already been developed for ecological and economic reasons. The most usual process is the briquetting process, where scrap is ground to obtain the desired granulation and compressed with the addition of cement into briquets which, when the cement hardens, give scrap the necessary lumpy form, size and hardness. Another known process is to blow ground scrap together with the air needed for combustion through tuyeres. In this case, however, scrap needs grinding, and the solution is ecologically questionable, since a considerable quantity of blown-in and unmelted scrap powder can be found in the exhaust flue gases. A known solution is disclosed in EP 0 625 485, according to which ground scrap is filled into bottles. In accordance with PCT WO 95/04003, scrap and raw materials are processed separately, and only when melted they are processed together in a special melting unit. The invention seeks to find a solution to the problem of returning scrap easily into the furnace without its preliminary processing, and without interrupting the flow of flue gases or causing undesired emissions into the environment. In accordance with the present invention, this problem is solved by a process for returning scrap into a cupola furnace according to which scrap is fed against the wall of the lower portion of the furnace at the same time as the furnace is being charged with raw material and fuel, by means of a device consisting of a revolving portion with a scrap feeder installed between the lower and the upper portion of the furnace. By means of inventive thought and with the support of tests, it has been established that when scrap is added in a non-processed form, i.e. substantially in the form of fibres, but also pearls or smaller pieces, close to the inner wall of the lower furnace portion, in a layer of defined thickness, and simultaneously with the charging of furnace with raw material and fuel, it does not hinder the transition of flue gases, while the scrap melts into a useful melt. A side effect of the layer of scrap against the wall of the lower portion of the furnace being properly dimensioned is that there is less need for (usually water) cooling of the furnace wall, which results in significant power saving. By means of this method of charging, and due to a reduced light cross-section of the charge of raw material and fuel, the quantity of flue gases and - indirectly - also of the air needed for coke combustion is reduced; however, the lacking quantity of air is replaced by oxygen, so that the initial capacity of the furnace is maintained. An oxygen adding device belongs to the standard furnace equipment, and it is basically intended for regulating the temperature of the melt, the capacity of the cupola, starting the furnace, and the like. The feeding of scrap against the inner wall of the furnace results in a reduced direct contact of fuel with the wall. Hence the wall, which is usually water-cooled, heats less. It has been established that the need for cooling has been reduced by as much as a half, so that the thermal energy that would otherwise be removed by cooling, stays in the furnace and serves for melting the raw material. With the scrap thus loaded, partial insulation of the inner wall of the cooled cupola furnace, and thereby fuel saving and, as a consequence, reduced harmful emission into the environment is achieved. With the scrap layer being placed against the inner circumference of the wall, and with the scrap being impermeable for flue gases, it is not carried out of the furnace into the environment by them. A device for returning scrap into cupola furnace in accordance with the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic elevation view illustrating a cupola furnace; Figure 2 is a schematic cross-sectional view illustrating the charging of the furnace; Figure 3 is a cross-sectional detail illustrating a cupola furnace and a scrap feeder with drive; Figure 4 is a detail of the first embodiment illustrating the control of the revolving portion of a furnace; Figure 5 is a detail of the second embodiment illustrating the control of a revolving portion of the furnace; and Figure 6 is a cross-sectional/plan view illustrating the revolving part with a feeder and control. The cupola furnace has a lower portion 1 with a (usually) water-cooled wall 8, where air is blown through tuyeres 6, the air being enriched with oxygen when needed for the combustion of fuel, this usually being coke. Said lower portion 1 has an outlet 7 for the melt. In the lower portion 1 , the process of fuel combustion and raw material melting takes place in the area of tuyeres 6, while above it, the pre-heating of raw material with flue gases is taking place. The under the invention is located between said lower portion 1 and an upper portion 2. Said upper portion serves for charging the furnace with raw material and fuel via a conveyor 5, and for removing flue gases through a stack 3. As a rule, cupola furnaces for extraction of silicate melt are not free-standing but clamped in a supporting structure and raised from the floor. Figure 1 illustrates how the lower portion 1 and the upper portion 2 are fixed by supporting frames 22, which, in the pictures, are shown only schematically to allow better clearness. The device in accordance with the present invention and with the embodiment consists of a revolving portion 4 with feeder 10 and supporting structures 13, and a guiding ring 11 and a driving ring 12 on the periphery of the upper edge of the lower furnace portion 1 The arrangement of said supporting structures 13 along the circumference of the revolving portion 4 is shown differently in Figure 1 than it is shown correctly in Figure 4, all for the sake of better clearness. The revolving portion 4 is installed, in as much as possible gas-proof way, between the lower portion 1 and the upper portion 2. A scrap 24 feeder 10 consisting of worm conveyors with a drive 17 is fixed to the revolving portion 4. The upper portion of the feeder 10 is a scrap 24 collector tank 18 At the side of the feeder 10 there is a drive 14 with a toothed wheel 21. Inside the furnace, a guiding device 15 is arranged on the revolving portion 4 above the feeder 10, which directs the scrap 24 which is pushed by a worm conveyor against the inner wall of the lower portion 1 of the furnace The guiding device 15 is carried out as a screen lying in acute angle to the wall of the revolving portion 4 over the outlet of the feeder 10. Along the periphery of the revolving portion 4 there are at least three supporting structures 13 fitted, in accordance with the embodiment and as shown in Figures 1 and 4, with horizontal wheels 19 and vertical wheels 20 and 24 The number of said supporting structures 13 should, of course, meet the static and stability requirements, depending on the dimensioning of the structure. The supporting structures 13 enable the rotating of the revolving portion 4 alongside the guiding ring 11 mounted at the upper circumference of the lower portion 1 of the furnace The horizontal wheels 19 guide the revolving portion 4 in horizontal direction, whereas the vertical wheels 20 and 24 carry the revolving portion 4, guiding it in vertical direction, namely all the wheels: 19, 20 and 24, alongside the guiding ring 11 which is mounted at the upper circumference of the lower portion 1 of the furnace. The guiding ring 11 is structurally linked with a driving ring 12, which is toothed along its periphery in order to match the toothed wheel 13 of the drive 14 on the feeder 10. Other arrangements of rings and supporting structures with wheels are also possible. In accordance with the second embodiment illustrated by Figure 5, a guiding ring 23 is added at the lower circumference of the upper portion 2 of the furnace for vertical guiding, and the supporting structure 13 is shaped in such a way as to allow the vertical wheel 25 to sit onto the ring 23. Further embodiments are possible with the supporting structures or parts thereof on the lower portion 1 and/or the upper portion 2 of the furnace, and with one or more than one guiding rings on the revolving portion 4. Combinations are also possible where parts of supporting structures are both on the lower portion 1 and on the upper portion 2 as well as on the revolving portion 4, and with one or more guiding rings on the lower portion 1 as well as on the upper portion 2 and the revolving portion 4. Another possible embodiment is with the driving wheel on the periphery of the revolving portion 4 and with the drive 14 on the lower portion 1 or the upper portion 2. The charging of the furnace proceeds as follows: in idle state, the tank 18 of the feeder 10 is filled with scrap 24 via charging unit 9. The charging of the furnace begins when the level sensor senses that the furnace needs re-charging. A drive 17 starts pushing the scrap 24 over a worm conveyor into the furnace, while simultaneously the drive 14 starts rotating the revolving portion 4 over the toothed wheel 13 in engagement with the driving ring 12. The guiding device 15 directs the scrap 24 close to the inner wall 8 of the lower portion 1. Experience has shown that it is most convenient when for one charge of the scrap 24, the feeder 10 rotates by two cycles. Experience has also shown that it is most convenient if the charging of raw material and fuel or a mixture 26 of raw material and fuel starts when scrap 27 has already been fed into the furnace. The charging of furnace with raw material and fuel or a mixture 26 of raw material and fuel may proceed in the known way, and there is no need to modify the furnace in accordance with the invention. Of course, the invention is not limited by the stated sequence of charging the furnace, and it is possible, in accordance with the invention, to use any consecutive or simultaneous charging of the furnace with the scrap 27 and the mixture 26, however, always with the scrap 27 being charged against the inner portion of the wall 8 of the lower portion 1 of the furnace. Figure 2 shows, in a schematical cross-sectional view of the furnace, consecutive charges with the scrap 27, raw material and fuel, or a mixture 26 of raw material and fuel. As the scrap 27 is being charged, the thickness of the scrap 27 layer against the wall 8 is between 8 and 0 cm, and approximately 25cm.

Claims: CLAIMS 1. A process of returning scrap into cupola furnaces for preparing silicate melt for the production of rock wool, characterised in that scrap (27) is fed against the wall of the furnace. 2. A process as claimed in Claim 1 , characterised in that the layer of scrap (27) against the wall of the furnace has a thickness of up to 25 cm. 3. A process as claimed in Claim 1 , characterised in that during furnace charging, first the scrap (27) is fed, and then raw material and fuel or a mixture thereof (26). 4. A process as claimed in Claim 1 , characterised in that during furnace charging, first raw material and fuel or a mixture thereof (26) is charged and then the scrap (27) is added. 5. A process as claimed in Claim 1 , characterised in that the furnace is simultaneously charged with the scrap (27) and with raw material and fuel or a mixture thereof (26). 6. A device for returning scrap into cupola furnaces for preparing silicate melt for the production of rock wool, characterised in that a revolving portion (4) is guided between the lower portion (1 ) and the upper portion (2) of the furnace fitted with a scrap (27) feeder (10). 7. A device as claimed in Claim 6, characterised in that said revolving portion (4) is guided by supporting structures (13) and at least one guiding ring. 8. A device as claimed in Claim 7, characterised in that said supporting structures (13) have horizontal wheels (19) and vertical wheels (20, 24, 25). 9. A device as claimed in Claim 6, characterised in that a scrap (24) feeder (10) is fixed to said revolving portion (4). 10. A device as claimed in Claim 9, characterised in that said feeder (10) consists of worm conveyors, a collector tank (18), a guiding device (15) and drives (14, 17). 11. A device as claimed in Claim 10, characterised in that said guiding device (15) is carried out as a screen in acute angle to the wall of the revolving portion (4) over the output of said feeder (10).

Inventors: DOLENEC ROMAN, DOLENEC, ROMAN
Applicants: TERMO D D IND TERMICNIH IZOLAC, TERMO D.D., INDUSTRIJA TERMICNIH IZOLACIJ, SKO
Application year: 1997
Main classification: C03B3
IPC codes: C03B3, C03B37, C21B5