MELTING FURNACE

Abstract: The invention relates to a furnace for producing a melt for mineral wool production comprising a shaft (2) for preheating and melting of the raw material (6); a water-cooled grate (5) disposed in the bottom portion of the shaft, which supports a bed of ceramic filling bodies (4) as well as the raw material (6); a combustion chamber (3) disposed underneath the shaft (2), which has a bottom portion (11) for collecting the melt (14) dripping from the shaft (2) and an outlet (12) for melt tapping; and at least one main burner (9) disposed in the combustion chamber (3); as well as auxiliary burners (10) disposed above the grate (5). The bottom surface of the combustion chamber (3) is larger, preferably 20 to 400% larger than the cross area of the shaft (2).

Description: Melting furnace The present invention relates to a furnace for producing a melt for mineral wool production In mineral wool production, minerals of silicon and metal oxide or carbonates and/or slag are used as raw material. The raw mat rial is melted in a furnace and the melt is fed into a fiberizi unit, which converts the mineral melt into fibres. During the f berizing, a binding agent is added, which upon a thermal treatm fixes the fibres into each other so as to provide a shape perma nent product. In order to provide a product of good quality, it vital that the melting is regular and that the viscosity of the melt and the flow from the furnace to the fiberizing unit is constant. The most commonly used type of melting furnace is the watercool cupola furnace, which is charged with a mixture of mineral raw material and coke. An inlet for combustion air is provided in t cupola furnace, and the combustion air is usually preheated to o about 500 C. Stone raw material of a basalt or diabase type o melts at a temperature of about 1200 C. The melt is overheated o in the furnace so as to flow out of the furnace at about 1450 C Due to the coke intermixed with the mineral raw material , the melting in the cupola furnace takes place in a reducing atmosph re. The exhausted flue gases then typically contain 8 to 10% of unburnt carbon monoxide (CO) and a small amount of hydrogen sul hide (H S) and also sulphur dioxide (SO ). 2 - 2 A serious drawback of the cupola furnaces is that they in most cases for environmental reasons have to be equipped with a . gas purifier, which separates the dust and afterburns the flue gase and which also includes preheating of the combustion air. Such a gas purifying and afterburning equipment is usually more complicated and expensive than the very cupola furnace for the melting. Another drawback involved with the reducing atmosphere when using cupola furnaces charged with coke is that the iron oxide present in the raw material is reduced into metallic iron, whereby the furnace has to be equipped with a device for iron tapping. The iron tapping, which takes place every four hours, also causes a production stop of about 15 minutes. In addition, cupola furnaces using natural gas as additional fue have been manufactured for energy saving reasons, whereby a maxi mum of 25 to 30% of the coke has been replaced with gas. However the problem of incomplete burning remains, requiring afterburnin waste gas purification and iron tapping, which again involves a production break. Other types of melting devices are electric furnaces of a tank type, in which the melting energy is supplied by three graphite electrodes fed with 150 to 200 V alternating current and 10 000 20 000 A current. " The amounts of exhaust gas from an electric fu nace are normally small enough not to need purifying. Besides th environmetal advantage, the electric furnace also has the advan¬ tage of allowing more freely the choice of raw material (finely divided) and also yields a more regular melt flow and temperatur than the cupola furnace. However, the electric furnace has the drawback of incurring about four times higher equipment expenses than a cupola furnace with gas purification, and that of requiri a very refined and expensive energy form. Moreover, gas or oil heated melting tanks of a similar type as the ones used in glass industry are being used, possibly with a 5 to 10% energy addition through molybdenic electrodes. Gas heated tanks incur somewhat more expensive equipment costs than electric furnaces of a tank type, and comprise approxima¬ tely twice as large a melt bath surface for the same capacity. Gas and oil have the advantage of being easily available fuels, whereas αas or oil heated tanks have the drawback of a linincr en durance of only 3 to 4 years and the reconstruction costs repr sent about 30 to 40% of a newly constructed tank. All the above furnace types have their own advantages and draw backs, preventing any of them from being ideal, however the tre today is to use electric or gas heated furnaces for environmet reasons (mainly sulphur) . As noted above, the latter furnaces have the drawback of being markedly more expensive than cupola furnaces with flue gas puri fication, being at the same time less flexibly adaptable to a discontinuous operation during weekends or for instance 1 or shift operation. The cupola furnace comprises only a few hundre kilograms of melt at its bottom, whereas the tank furnaces have melt quantity of several tens of tons. For that reason the cupo furnace may run down and be emptied of melt very rapidly withou noticeably loosing production time. Likewise, the cupola furnac may be run up in a very short time in 1 or 2 shift operation, usually in about 1 hour. On the other hand, the running up of t furnaces lasts 1 to 3 days, signifying that they cannot be made flexible in view of a discontinuous operation. GB patent specification 1 326 884 discloses among others a gas heated iron melting furnace of a cupola furnace type, in which the melting takes place in a shaft, the bottom part of which comprises a water-cooled grate, which supports the iron raw mat rial to be melted, and a bed of ceramic graphite blended fillin bodies, which ■ slowly melt down and are mixed with the melted slag, the iron melt being simultaneously carburized. The iron melt deriving from such a furnace has a temperature of about o 1350 C and requires the temperature to be raised to the proper casting temperature, which usually is done in a separate induct furnace. The object of the present invention is to provide a furnace for producing a melt for mineral wool production, by which the above drawbacks of prior known mineral melting furnaces have been eliminated. Consequently, the present invention relates to a mel ting furnace that is not detrimental to the environment, yields regular flow, the temperature of which equals the final working temperature, incurs fairly low equipment costs and is adaptable a discontinuous operation. The characteristics of the invention appear from the enclosed claims. The present invention relates in particular to a melting furnace comprising an essentially vertical shaft for receiving, prehea¬ ting and melting of the mineral raw material? a water-cooled gra te disposed in the bottom portion of the shaft, which supports said raw material and ce ' ramic filling bodies; a combustion chamb disposed underneath the shaft comprising a bottom portion for collecting and overheating of the mineral melt dripping from the shaft and an outlet for tapping off the melt; and at least one main burner disposed in the combustion chamber. The main burner may use gaseous fuel, liquid fuel or pulverized coal fuel. A melting furnace of the above type is disclosed by the GB patent specification 1 326 884. However, this prior known meltin furnace is exclusively intended and used for iron melting. Thus, the present invention relates to a melting furnace of the above basic type, however modified according to the invention so as to be useable for producing a melt for mineral wool production. - The ceramic filling bodies, which form a porous bed permeable to the hot exhaust gases from the subjacent combustion chamber, sup port the raw material to be melted and enlarge the surface of t mineral melt flowing down along them, to obtain an increased s face of contact between the melt and the gases for an initial overheating of the melt. The filling bodies consist of a refra refractory material, which has to resist the attack of the min melt as well as possible. Appropriate melt resisting materials for instance silicon carbide and various chromium oxide bound aluminium oxide containing materials. The bodies are heated by hot exhaust gases and cooled again by the melt flowing down ov them. At the same time they also maintain the porosity of the They are preferably spheres having a diameter of 100 too 200 m According to a feature of the the invention, the bottom surfac the combustion chamber, i.e. the surface of the melt bath, is larger, preferably 20 to 400% larger than the cross area of th shaft, to enable the heat from the flames to furtherraise the o temperature of the melt to about 1450 C before the exhaust gas o flow upwards through the shaft at about 1600 C. Thus, there wi be produced a melt in the furnace, which can be directly led t the fiberizing unit. A further advantage of the larger bottom of the combustion chamber is that the melt is prevented from f wing down along the combustion chamber walls and causing erosi of the wall material. The bottom of the combustion chamber may of a square, hexagonal, oval or circular shape, other shapes be equally possible. The walls of the combustion chamber may be pe pendicular or oblique. According to a preferred embodiment, the melting furnace compri in addition to the main burners also auxiliary burners, which a disposed annularly above the water-cooled grate. The fuels used for these are gaseous fuels, like natural gas or liquid gas and the auxiliary burners preferably operate with cold, possibly ox gen-enriched air. The task of the auxiliary burners is especial to ensure a regular melting off, thus avoiding the forming, of tunnels in the shaft. The auxiliary burners are also crucial fo draining the shaft before production stops during weekends and like . The energy amount supplied by the auxiliary burners is preferabl in the range of 15 to 30% of the total -amount of required energy The number of auxiliary burners is preferably 10 to 30. The shaft and the combustion chamber are entirely lined with appropriate fireproof lining materials. Various kinds of stones, slag or mixtures of these may be used a raw material. In addition, dolomite or limestone may be added. An appropriate fraction of the raw material is 40 to 80 mm, and with this fraction, the raw material bed acts as a dust filter i the shaft, and thus the dust amount from this type of furnace is in most cases small enough not to require any dust filter. The heating of the raw material to the melting temperature of o about 1200 C is done by convection heat transmission from the hot exhaust gas to the lump raw material. The melting also takes place in a neutral or mildly oxidating atmosphere and hence no unburned gases flow out of the furnace. There is no reduction of the iron oxide portion of the raw material, and thus no iron tap ping device needs to be provided in the furnace, which is the ca with coke heated cupola furnaces and electric furnaces with grap hite electrodes. The combustion chamber is equipped with one or preferably a plu¬ rality of main burners of a conventional type. A gaseous fuel, like natural gas or liquid gas or a liquid fuel, like oil, is preferably used, but also pulverized coal is usable. In order to achieve a higher combustion temperature, preheated combustion ai may be used. Oxygen gas may also be intermixed in the combustion air. The burners are preferably directed towards the surface of the melt bath. The melting furnace may also be equipped with various devices fo overheating the melt by electric energy through electrodes of a known type, such as for instance molybdenic electrodes. The overheating of the melt in the bottom section of the furna can also be accomplished with plasma energy directly in the co bustion chamber, whereby the plasma energy passes from a water cooled electrode in the arch of the furnace through the melt t electrode in the furnace bottom. The electric additional energy supplied by electrode or plasma tecnique represents a maximum of 20% of the total energy required. In order to improve the energy recovery of the melting furnace the outlet duct of the exhaust gases having a temperature of a o 700 to 800 C may be provided with a recuperator for preheating the combustion air . A significant advantage of the structure of the furnace accord to the invention is the posibility of refeeding directly into melt space the solidified waste melt, which has not been fiber during the fiberizing process, and which thus has been removed from the fibre field. This waste melte is mostly a high qualit material, which advantageously is to be returned into the proc When using cupola furnaces, this is not possible without briqu A further advantage is achieved by the fact that this structur of the furnace allows the feeding of finely divided additional material directly onto the surface of the melt bath. The invention is described in detail below with reference to t enclosed drawing, in which figure 1 presents a vertical section through a preferred embod ment of the melting furnace according to the present invention and ' figure 2 presents a horizontal section along the line A-A.of f gure 1 on an enlarged scale, figure 3 shows a vertical section through another preferred em bodiment of the melting furnace according to the present inven tion, .. " ■ ' figure 4 shows a vertical section through the combustion chamber of the melting furnace along the line B-B of figure 3, and figure 5 shows the same melting furnace as figure 3 seen from above. The furnace shown in figures 1 and 2 comprises a vertical prehea ting and melting shaft 2 for the charge, i.e. the stone material to be melted (appropriate fraction 40 to 80 mm) and a combustion chamber 3 disposed underneath the shaft 2. The stone material 6 charged into a water-cooled feeding shaft 1 disposed at the top the shaft 2. At the bottom of the shaft 2 a water cooled grate 5 is provided, which supports a porous bed of ceramic spherical filling bodies 4 having a diameter of 100 to 200 mm and the ston material 6. The main burners 9 using gas, oil or pulverized coal as fuel are disposed annularly in the combustion chamber 3. The stone material 6 in the shaft 2 is preheated and heated to the melting temperature by heat transmission from the hot exhaust ga ses from the main burners 9. The melt 14 dripping from the sha t is further collected in the bottom portion 11 of the combustion chamber and is overheated to the working temperature by heat transmission from the exhaust gases and by energy supply through the molybdenic electrodes 8 and is tapped off through an outlet which is disposed at a certain height above the bottom of the combustion chamber 3, so that a melt bath 7 is formed in the sai bottom portion 11. The great electric current from the molybdeni electrodes 8 through the melt cause the melt to be brought into motion, resulting in a homogenization of the melt. The main burners 9 are directed towards the surface of the melt bath 7, whereby the heat from the fuel flames transferred to the o melt before the exhaust gases at about 1600 C flow up into the shaft 2. The hot exhaust gases are turned away from the shaft 2 through the duct 13. A recuperator 15 for the hot exhaust . gases can be disposed in the duct 13 for preheating the combustion air used in the main burners 9. Auxiliary burners 10 using gas as fuel are disposed annularly above the water-cooled grate 5. The auxiliary burners 10 contri bute to the melting of the stone material 6. However their main task is to prevent the forming of tunnels in the melting stone material and to purify the bed of ceramic filling bodies 4 duri run-down. The bottom portion 11 of the combustion chamber is provided wit a ceramic lining, which is particularly exposed to the hot melt whereby the lining has to be renewed from time to time, and hen the portion 11 may be formed so as to be removable for a rapid exchange with another newly lined bottom portion. The melting furnace according to the invention can also be equi ped with a device, for instance a feed screw 16, for refeeding fiberizing residues directly into the combustion chamber. An appropriate bottom surface of the combustion chamber 3 is m t to 10 and an appropriate cross area of the shaft 2 is about 2 3 m for a furnace having a melting capacity of about 5 t/h. The furnace shown ' in figures 3 to 5 comprise a vertical shaft 2 having a square cross area. The shaft 2' comprises at its botto a water-cooled grate 5', which supports a porous bed of ceramic spherical filling bodies 4' having a diameter of 100 to 200 mm. The stone material 6' (preferably the fraction 40 to 80 mm) is charged -at the top of the shaft 2' (not shown). The stone mater 6' is preheated and heated to the melting temperature by heat transmission from the hot exhaust gases from the burners 17 to disposed in the combustion chamber 3'. The burners 17 to 22, us gas, oil or pulverized coals as fuel, are directed towards the surface of the melt bath 7', whereby the heat from the fuel fla mes is trasmitted to the melt before the exhaust gases, at. o temperature of about 1600 C, flow upwards in the shaft 2 . The melt tapping outlet 12 ' is disposed at a certain height above t bottom of the combustion chamber 3' so as to form a melt bath ' in this bottom portion 11 ' . The lining of this bottom portion 1 is particularly exposed to the hot melt and has to be renewed f time to time, and for this reason this portion 11' can be forme so as to be removable for a rapid exchange with another newly l ned bottom portion. An appropriate bottom surface of the combustion chamber 3 ' is 4 10 m and an appropriate cross area of the shaft 2' is about 3 for a furnace having a melting capacity of about 5 t/h. Only a number of preferred embodiments of the invention have be described above, and the melting furnace according to the inven tion can of course be modified within the scope of protection o the claims.

Claims: Claims 1. A melting furnace comprising a shaft (2, 2') for preheating and melting of the raw material (6, 6') to be melted; a water- cooled grate (5, 5 * ) disposed in the bottom portion of the sha which supports a bed of ceramic filling bodies (4, 4') as well the raw material (6, 6'); a combustion chamber (3,3') disposed derneath the shaft (2,2') comprising a bottom portion (11, 11' for collecting the melt (14) dripping from the shaft (2, 2') and an outlet (12, 12') for melt tapping; and at least one mai burner (9, 17-22) disposed in the combustion chamber (3,3'), characterized in that the melting furnace is intended to be used for melting raw materials (6,6') for mineral wool product and in that the bottom surface of the combustion chamber (3,3' is greater than the cross area of the shaft. 2. A melting furnace according to claim 1, characterized in th it comprises auxiliary burners (10) disposed above the grate ( 3. A melting furnace according to claim 1 or 2, characterized in that the ceramic filling bodies (4, 4'), whic are supported by the grate (5, 5'), consist of chromium oxide bound aluminium oxide and have a diameter of 100 to 200 mm. 4. A melting furnace according to any of the preceding claims, characterized in that the bottom surface of the combustion chamber (3,3'), i.e. the surface of the melt bath (7,7') is 20 400% larger than the cross area of the shaft (2,2'). 5. A melting furnace according to any of the preceding claims, characterized in that the melt outlet (12, 12') is disposed at an appropriate height above the bottom of the combustion chamb so as to form a melt bath { 1 , 1 ' ) . 6. A melting furnace according to any of the preceding claims, characterized in that it comprises devices (8) for supplying electric additional energy by electrode or plasma technique for further overheating of the melt in the combustion chamber ( to the working temperature. 7. A melting furnace according to any of the preceding claims, characterized in that it comprises a recuperator (15) in the duct (13) for the hot exhaust gases for preheating of the combu tion air. 8. A melting furnace according to any of the preceding claims, characterized in that the bottom portion (11, 11') of the com¬ bustion chamber, which is equipped with a ceramic lining, is fo med so as to be removable. 9. A melting furnace according to any of the preceding claims, characterized in that it is equipped with a device (16) for re¬ feeding of fiberizing residues directly into the combustion cha ber.

Inventors: LAUREN HENNING JOHN-EMIL, LAUREN, HENNING, JOHN-EMIL
Applicants: PARTEK AB, OY PARTEK AB
Application year: 1988
Main classification: C03B5
IPC codes: C03B3, C03B5, F27B1