METHOD AND FURNACE FOR PREPARING A MELT FOR MINERAL FIBRE PRODUCTION

Abstract: A method and a furnace for preparing a melt for mineral fibre production, the method comprising introducing a starting material in the form of solid minerals at the upper end of a shaft which shaft at its lower end has at least one extension in the lateral direction to form a column of material forming an inclined surface in the extended part of the shaft furnace, and heating the starting material to melting by one or more burners from which a hot gas stream is emitted towards the inclined surface, and discharging melt from the bottom of the shaft furnace, and introducing mineral fibre waste material onto the inclined surface of the column of material.

Description: Method and furnace for preparing a melt for mineral fibre production The invention relates to a method of preparing a melt for mineral fibre production comprising introducing a starting material in the form of solid minerals at the upper end of a shaft, which shaft at its lower end has at least one extension in the lateral direction to form a column of material forming an inclined surface in the extended part of the shaft furnace, and heating the starting material to melting by one or more burners from which a hot gas stream is emitted towards the inclined surface, and discharging the melt from the bottom of the shaft furnace. In the production of mineral fibre products considerable amounts of waste materials are formed. Such waste materials may e.g. consist of cut off parts (edge cuts) of mineral fibre sheets, discarded mineral fibre sheets or parts of such sheets. Such waste materials ordinarily contain binder, but apart from the binder they have a composition desired for the melt production. Up to now mineral fibre waste material has been disposed of by deposition on deposit sites suitable for such purposes. However, it has gradually become a serious problem to find suitable deposit sites which is partly due to the fact that the amount of waste material is very large and partly that the waste material may contain non-cured binder which contains the toxic substance phenol. Furthermore, the costs of deposition is heavily increasing. Instead of disposing of such waste materials by depositing them on such suitable deposit sites it will, therefore, be desirable to convert them into a melt and use the melt thus formed for mineral fibre production. Therefore, it has been attempted to introduce waste material into the shaft furnace in mixture with the ordinary starting materials but this has caused problems in the operation of the shaft furnace. The reason for this is that the waste material in mixture with ordinary starting materials for a shaft furnace adversely affects the desired flow of hot gases up through the shaft furnace. Furthermore, in this method a complete decomposition of the binder is not achieved and instead the binder is pyrolized to form unacceptable emission products which consequently must be disposed of in a subsequent, highly cost-raising afterburning. It is well known to comminute waste materials from mineral fibre production by introducing them into a hammer mill and by mixing the product thus grinded with a binder, e.g. in the form of clay, and to produce briquettes thereof and to use these briquettes in mixture with ordinary starting materials for feeding conventional shaft furnaces. However, the use of such briquettes causes various nuisances. Thus, they increase the formation of a frothy melt, and the problem of avoiding undesired emission products originating from the binder is not solved. Beyond this the production of briquette is expensive as the production involves a laborious and cost-raising process and as a costly separate briquetting plant is required for exercising the process. The object of the invention is to utilize mineral fibre waste from mineral fibre production in the production of a mineral melt in a simple and effective manner. According to the invention this object is obtained by introducing mineral fibre waste material onto the inclined surface of the column of material . The invention is based on the discovery that the introduction of the waste material onto the surface of the starting material does not adversely affect the passage of hot gases through the column of material and that the temperature on the inclined surface of the column of material is very high, viz. about 1,300°C, and thus considerably higher than the 800°C which is required to decompose the binder completely and to convert it into unharmful compounds, viz. primarily CO- and water vapour, so as to eliminate the need for particular afterburning means. The reason why the introduced waste material does not adversely affect the passage of the hot gases through the column of material is that the waste material is quickly melted partly because it has a large surface (the surface area per unit of volume of the fibrous waste material is up to 20,000 times higher than the corresponding value of the starting material), partly because there is a high radiation intensity in the part of the furnace into which the waste is introduced, and partly because the temperature at that site is very high as mentioned above. Furthermore, it has surprisingly been found that the melt from the waste converges in small streams of melt which cover only a small part of the inclined surface and, therefore, do not prevent the hot combustion gases from flowing through the inclined surface and up through the column of material in order thereby to generate the necessary heating of the starting material which is a prerequisite for causing the starting material to melt in the extended part of the shaft furnace. Another advantage of the method of the invention is that the production of briquettes is avoided and, therefore, the waste material need not be grinded, which is required when the waste material is to be used for the production of briquettes. As a result thereof the dust nuisances in connection with the pre-treatment of the waste material are eliminated or reduced. A further advantage of the method of the invention is that according to it minimum dust emission in the flue gas is obtained due to the rapid smelting. The best results are obtained by introducing the waste material onto the inclined surface at several sites, e.g. at the same level, at the same time. The introduction may be effected in a number of different ways, e.g. by means of cell sluices, screw conveyors or pistons, or pneumatically. Instead of using several means for introducing the waste material into the furnace, the introduction can also be effected at one site and means may be provided for dispersing the introduced material over the said inclined surface. According to a preferred embodiment of the method of the invention the waste material is comminuted into pieces prior to the introduction into the furnace, the pieces having such a size that they have a length below 100 mm, preferably below 50 mm, in their greatest extension. The comminution of the waste material used for introducing into the furnace may e.g. be effected in a hammer mill or in a reaming apparatus. Furthermore the invention relates to a furnace for exercising the method described above. The furnace according to the invention comprises a shaft which at its upper end is provided with means for introducing starting material and means for discharging flue gas and which has at least one extension in the lateral direction at its lower end, said extension being formed in such a manner that the starting material therein forms a column of material having an inclined surface and wherein one or more burners are located for emitting a hot gas stream towards the inclined surface and in which the furnace has means for discharging melt. The furnace according to the invention is characterized in that it further comprises means for introducing mineral fibre waste material onto the inclined surface of the column of material . The melting of starting material, including waste material, is preferably effected by means of one or more gas and/or oil burners which are located in such a manner that the stream of hot combustion gases generated therein is directed substantially perpendicularly towards the said inclined surface, thereby mainly generating melting in proximity to the inclined surface. In case the mineral fibre content contains binder and/or other organic substances the flow of hot combustion gases may contain a surplus of oxygen or oxygen may be supplied separately to ensure that the binder and other organic substances, if any, are completely converted into unhar ful products. The binder typically constitutes about 2% by weight of the waste material. A preferred embodiment of the furnace according to the invention comprises a melt collecting chamber connected with the extended part of the furnace, and this chamber is preferably provided with means, e.g. in the form of gas and/or oil burners, for conditioning the melt. In the following the invention will be described in further detail with reference to the drawing showing a preferred embodiment of a furnace according to the invention as Fig. 1 shows a vertical cross-sectional view along the line I-I through the furnace according to Fig. 2 and Fig. 2 shows the furnace according to Fig. 1 as seen from above. The furnace shown in the drawing comprises a shaft 1 having dosing means 2 for introducing waste material 3 into the upper part of the shaft 1. A discharge passage 4 for flue gas is furthermore provided in the shaft 1. Moreover the furnace comprises a furnace bottom 5 extending from the shaft 1 to form a furnace chamber 6 which is delimited by a top wall 7 and an end wall 8. The furnace further comprises a melt collecting chamber 9 connected to the melt chamber 6, said melt collecting chamber 9 being provided with a melt discharge 10 having a pneumatically controlled discharge valve 11 and a top wall 12. Three supply ducts 13 are formed in the top wall 7, each duct being connected to a dosing apparatus in the form of a screw conveyor 14 having a motor 15 and a supply funnel 16. The furnace further comprises three gas burners 17 which are mounted obliquely in the end wall 8 and an additional gas burner 18 which is mounted vertically in the top wall 12. The mode of operation of the described furnace is as follows: The starting material is introduced into the shaft 1 by means of the dosing means 2 thereby resulting in the formation of a column of material 20 having an inclined surface 21 located in the melt chamber 6. On combustion of gas in the burners 17 streams of hot combustion gases are formed which upon contact with the inclined surface 21 heat the starting material 3 and waste material which has been introduced and distributed onto the inclined surface 21 by means of the screw conveyor 14 and the supply ducts 13 after introduction into the supply funnel 16. Thus, melting of the materials is effected and the melt flows down towards the furnace bottom 5, then into the chamber 9 and proceeds to the melt discharge 10. During the stay in the chamber 9 the melt is maintained heated by means of hot combustion gases formed in the burner 18. The melt is discharged from the chamber 9, which discharge is regulated by means of the discharge valve 11.

Claims: P a t e n t C l a i m s 1. A method of preparing a melt for mineral fibre production comprising introducing a starting material in the form of solid minerals at the upper end of a shaft ; which shaft at its lower end has at least one extension in the lateral direction to form a column of material forming an inclined surface in the extended part of the shaft furnace, and heating the starting material to melting by one or more burners from which a hot gas stream is emitted towards the inclined surface, and discharging the melt from the bottom of the shaft furnace, c h a r a c t e r i z e d in that mineral fibre waste material is introduced onto the inclined surface of the column of material . 2. A method according to claim 1, c h a r a c t e r i z e d in that the waste material is introduced onto the inclined surface at several sites at the same time. 3. A method according to claim 1, c h a r a c t e r i z e d in that all the waste material is introduced into the extended part of the shaft furnace at one site and that the material introduced is dispersed over the inclined surface. 4. A method according to claim 1, c h a r a c t e r i z e d in that prior to the introduction into the furnace the waste material is comminuted into pieces having such a size that the pieces have a length of below 100 mm in their greatest extension. 5. A method according to claim 1, c h a r a c t e r i z e d in that prior to the introduction into the furnace the waste material is comminuted into pieces having such a size that the pieces have a length of below 50 mm in their greatest extension. 6. A furnace for preparing a melt for mineral fibre production, which furnace comprises a shaft which at its upper end is provided with means for introducing starting material and means for discharging flue gas and which has at least one extension in the lateral direction at its lower end, said extension being formed in such a manner that the starting material therein forms a column of material having an inclined surface and wherein one or more burners are located for emitting a hot gas stream towards the inclined surface and in which the furnace has means for discharging melt, c h a r a c t e r i z e d in that it further comprises means for introducing mineral fibre waste material onto the inclined surface of the column of material. 7. A furnace according to claim 6, c h a r a c t e r i z e d in that the extended part of the shaft furnace is connected with a melt collecting chamber. 8. A furnace according to claim 7, c h a r a c t e r i z e d in that at least one burner is located in the melt collecting chamber.

Inventors: MOELLER JENSEN LEIF, MOELLER, JENSEN, LEIF
Applicants: ROCKWOOL INT, ROCKWOOL INTERNATIONAL A/S
Application year: 1991
Main classification: C03B3
IPC codes: C03B3, C03B5