The present invention relates to a furnace for the production of a melt for the manufacture of mineral fibres, said furnace comprising an opening for the introduction of starting material, a furnace chamber having such a shape that the introduction of starting material through the inlet opening results in the formation of a material column having at least one inclined surface, at least one burner capable of ge¬ nerating a flow of hot gasses directed towards the inclined surface of the material column to effect melting of the starting material, means for promoting the movement of starting material downwardly over the inclined surface of the material column, and means for collecting and discharging melt from the furnace bottom.

WO 92/04286 describes a shaft furnace of the above-mentioned type. This known shaft furnace comprises an inlet shaft having a bottom debouching into a furnace chamber provided with a number of gas burners. During operation of such known shaft furnace, a shell of partially molten starting material is formed on the inclined surface of the material column and such shell, which may extend all the way to the top of the furnace chamber, prevents continuous flow of new starting material downwardly over the inclined surface of the material column concurrently with the melting. This causes the production of melt to be reduced and causes that major temperature differences are generated in the uppermost portion of the furnace chamber which temperature differences substantially reduce the lifetime of the furnace lining.

US patent No. 4,328,019 describes a preheating furnace for an electric furnace. In the preheating furnace, hydraulically operated pushers are arranged which perform forward and backward movements across the furnace bottom to ensure that

preheated raw material is conveyed through a duct into the electric furnace.

EP 0347 047 describes a glass melt furnace comprising a melt bath which is heated by means of a vertical, downwardly oriented, hot flow of gas. The raw material to the melt bath is supplied from a material column from which raw material is pushed into the melt bath by means of pushers. In the known glass melt furnace no or only partial melting of the raw mateial is effected on the inclined surface of the material column.

It has been proposed to prevent shell formation by acting upon the material column from the inside by means of comb- like means arranged outside the furnace chamber and provided with teeth arranged in the same horizontal plane and ex¬ tending into the material column. This solution is only useful in comparatively small furnaces.

EP application No. 9311071.4 describes a melt furnace wherein means are also provided to eliminate the above-described shell from the inside, viz. in the form of tubes which may contain a screw conveyor which is rotatable and which may be caused to move forward and backward so that its front end is caused to thrust against the above-described shell from the inside. Additionally the screw conveyor serves to introduce waste material produced in the mineral fibre production into the material column. The tube and the cooperating screw conveyor are so arranged that a portion of the inclined surface of the material column is disintegrated in an area approximately halfway between the furnace bottom and top.

Tests have shown that a furnace of the above-mentioned type does not have such an influence on the inclined surface of the material column that the mentioned differences in temperature at the furnace chamber top are avoided.

It is the object of the present invention to provide a steady movement of starting material downwards across the material column to avoid undesirable high temperature differences in the uppermost portion of the furnace chamber and thereby to prolong the lifetime of the furnace lining.

This object is obtained according to the invention by the furnace according to the invention which furnace is charac¬ terized in that the means for promoting the movement of material downwardly over the inclined surface of the material column comprise a row of tubes in which there are provided individually controllable pistons each of which being capable of performing forward and backward axial movements relative to the surrounding tube.

By means of such a row of individually controllable forwardly and backwardly moving pistons to influence the inclined surface of the material column, an impact pattern may be obtained which is on the one hand adapted to the properties of the starting material used and on the other to the shape of the furnace chamber and which provides optimum supply of starting material to the area in which melting takes place.

Thus it is possible to increase the melt production and thus the furnace capacity without substantially changing the furnace temperature which is essential for both fuel economy and wear reasons.

At the same time material movement downwardly over the inclined surface is improved and this contributes to maintai¬ ning that portion of the surface, on which melting takes place, constant.

The row of tubes with their pistons are preferably so arranged that their ends are disposed at approximately the same distance from the furnace chamber ceiling and so that the material column may be uniformly influenced across its

entire width. Thus, in a furnace with an arched furnace chamber, the ends of the tubes are arranged along a curved line.

The pistons may be made of steel and need not be made of a particular temperature-resistant material due to the fact that most of the time, they are lying protected within the respective tubes which are not exposed to intensive heat influences. In order to prevent the pistons from being stuck within the tubes, they preferably have such diameter that a clearance of a few mm is obtained between the outside of the piston and the inside of the tubes.

The number of conduits with the cooperating pistons is preferably adjusted to the diameter of the pistons. For example, four pistons may be used with a diameter of 400 mm, six pistons with a diameter of 250 mm or 6 pistons with a diameter of 150 mm.

Preferably, the conduits are disposed at a distance from each other which corresponds to 1.5-3 times the diameter of the piston.

As mentioned above, the pistons are connected to control means which control the movements of the pistons in an adjustable manner. The control means may e.g. be so construc¬ ted that the movements of the individual piston, i.e. stroke length and the speed of movement, are adjustable. For example, the advancement speed may be adjusted to another value than the retraction speed.

The mutual movements of the pistons are preferably effected in a predetermined pattern. For instance, the movement pattern of the pistons in the row of pistons may be such that the movement of a piston is not initiated until both the advancement and the retraction of the preceding piston have

been discontinued. The movements may also be partially over¬ lapping or effected in pairs.

The front ends of the pistons are preferably dish-shaped since pistons of such design perform a particularly powerful influence on the starting materials with a comparatively low wear on the pistons. Pistons without a dish-shaped front end have a tendency to slide through the starting material substantially without pushing it forwards and the relative movement thus occuring causes strong wear on the piston along its edge.

By using dish-shaped pistons the material is collected at the front end of the piston, and the material protects the front end of the piston against wear during advancement of the piston. At the same time, the material located in the dish- shaped front end causes the area influenced during the advan¬ cement of the piston to be enlarged.

The tubes and the cooperating pistons may be arranged so that their axes extend substantially perpendicular to the inclined surface of the material column and such that they influence the latter at a relatively short distance from the furnace arch.

However, the conduits may also be provided with horizontal axes. As mentioned, the object of influencing the inclined surface of the material column is to promote the movement of material downwardly over the latter. However, there is also a need to break a shell already formed. This is particularly relevant following discontinuation of the material supply to the furnace, e.g. due to a stop of a cooperating fibre production line.

The invention has been explained above with reference to a furnace wherein only the one surface of the material column is inclined.

However, it may also be used in furnaces where the furnace chamber extends to two opposite sides relative to the inlet opening and where the material column has two opposite, inclined surfaces.

In this case two oppositely oriented rows of tubes with cooperating pistons are used, said tubes in the one row being arranged alternatingly with the conduits of the other row.

In this manner both inclined surfaces of the material column may be influenced simultaneously.

The invention will now be described in further detail with reference to the drawings, in which

Figure 1 is a schematical, vertical, sectional view of a portion of the inlet opening and the furnace chamber of a shaft furnace without means for influencing the material column.

Figure 2 is a vertical, sectional view through a preferred embodiment of a shaft furnace according to the invention and wherein the furnace chamber extends outwardly to both sides of the inlet opening for starting material,

Figure 3 is a side view of the two rows of pistons and tubes shown in Figure 2, and

Figure 4 is a perspective view of the front end of a piston as shown in Figures 2 and 3.

The portion of the shaft furnace shown in Figure 1 comprises an inlet shaft 1 arranged above a furnace arch 2, which shaft cooperates with a furnace vault 3 to delimit a furnace chamber 4 wherein a material column 5 with two inclined surfaces 6 is arranged, of which, however, only one is shown.

As will appear from Figure 1, a shell 7 is formed on the inclined surface 6 of the material column 5, which shell inhibits continuous movement of unmolten starting material downwardly over the inclined surface 6 of the material column 5.

The shaft furnace shown in Figures 2 and 3 comprises a furnace chamber which is delimited by a vault 10, two vertical side walls 11 and 12, two end walls 30 and 31 and a furnace bottom 13.

Above the central portion of the furnace chamber and in connection therewith, a shaft 14 is provided below which a furnace arch 15 is arranged.

Two burners 16 and 17 are built into the side walls 11 and 12 said burners being capable of generating flows of hot combustion gases which are directed towards the inclined surfaces 18 and 29 of a material column 20 formed by the introduction of starting material through the shaft 14. Collection grooves 21 and 22 for melt are provided at the side walls 11 and 12, and means (not shown) for conveying melt out of the furnace chamber are provided in said chamber.

The central portion of the furnace bottom 14 comprises a support 24 insulated with a lining layer 23 for two rows of inclined movement means each comprising a piston 25 arranged axially displaceably within a tube 26. At the end of the pistons 25 which is below the support 24, they are in communication with control means 27 capable of imparting to the pistons forward and backward movements relative to the surrounding tubes 26 that are secured to the support 24 by means of socket pipes 28.

During the manufacture of melt in a shaft furnace as shown in Figures 2 and 3, backward and forward movements relative to the surrounding tubes 26 are imparted to the pistons 25 by

means of the control means and in a predetermined, mutually coordinated pattern of movement.

During advancement of the pistons 25, they convey material in a direction towards the inclined surfaces 18 and 19 of the material column 20 and thereby they contribute to break any shell of partially molten material which may have been formed thereon and to promote the movement of unmolten starting material downwardly over the inclined surfaces 18 and 19.

At the front end, the piston shown in Figure 4 has a peri¬ pherally protruding edge 32 that delimits a cavity 33. During the forward movement of such piston, a material layer will be trapped in this cavity 33 which on the one hand increases the pressure plane of the piston and on the other protects the front end of the piston against wear.