This invention relates to a method and a device for pre-heating waste metal for furnaces, flue gases generated in a furnace being supplied simultaneously or sequentially to two pre-heating chambers with waste me¬ tal containers.

In the electric steel industry waste metal can be utilized as main raw material in electric arc urnaces. Moreover, waste metal can be used as cooling agents in basic oxγgene furnaces.

In both cases a controlled pre-heating of the waste metal will bring an improved operation economy. Methods and devices of the kind mentioned by way of introduction are previously known where the flue gases after flowing through the pre-heating chambers are recycled to a combustion chamber or dust collector directly connected to the furnace for post-combustion of unburnt flue gases. However,""*it has been found that the combustion of flue gases in such a combustion chamber will be incomplete and uncontrollable due to a temperature from the furnace varying in time. Moreover, the waste metal cannot be heated as cold flue gases are generated in the furnace e.g. at times with no con¬ nected power.

It is the object of this invention to provide an improved method and an improved device of the kind in¬ dicated above, and this object has been achieved by means of the characteristic features of the method and the device defined in the claims.

One advantage of the invention in comparison with previous solutions is that the invention suggests a combustion chamber provided with a controlled burner of its own, by means of which unburnt gases from the elβc-

trie arc furnace, on one hand, and, on the other hand, from the pre-heating chambers are post-combusted and destructed at a temperature controllable in time. More¬ over, a larger amount of energy is added to the flue gases which is then recycled to the process via a hot¬ ter waste metal. The location of the combustion chamber is independent of the position of the electric arc fur¬ nace. In other words, it can thus be placed in a direct connection with the pre-heating chambers. Moreover, the waste metal can also be heated at times with cold flue gases from the furnace bγ means of the burner.

An illustrative exemple of the invention will be described below with reference to the enclosed drawing which shows schematically a device according to the in- vention.

A pre-heating device according to the invention is shown with reference to the figure. Flue gases gene¬ rated in an electric arc furnace/basic oxygene furnace are evacuated via a dust collector 2 to a combustion chamber 19 provided with a burner 20 and thereafter further to two pre-heating chambers 3 and 4.

The burner 20 is intended to produce hot flue ga¬ ses for admixture with flue gases from the furnace 1 and for post-combustion of unburnt gases from the fur- nace 1 and from the pre-heating chambers 3 and 4.

The burner 20 can be fired e.g. by means of oil, carbon powder, gas or biofuel (powder) . In the figure

21 designates the fuel supply system of the burner and

22 its air supply system. It is also possible to have the burner 20 consist of an electric plasma burner for production of hot air which can be mixed with the flue gases from the furnace 1 and/or the chambers 3 and 4.

The combustion chamber 19 is provided with con- necting ducts for flue gases; an inlet duct 23 for flue gases from the furnace 1, an outlet duct 24 for super-

heated flue gases to the pre-heating chambers 3 and 4, an inlet duct 25 for unburnt flue gases from the cham¬ bers 3 and 4 and an outlet duct 26 for burnt flue gases to the gas cleaning plant 5 for flue gases. A by-pass duct 27 for flue gases is arranged between the inlet duct 23 and the outlet 10 and a by-pass duct 30 is ar¬ ranged between the ducts 25 and 26.

The pre-heating chambers 3 and 4 are each provided with an inlet duct 6 and 7, respectively, and an outlet duct θ and 9, respectively. The outlet ducts 8 and 9 are connected to a common outlet 10.

In order to achieve evacuation of the flue gases flue gas fans 12 with a variable speed are arranged in the outlet 10 and immediately upstream the plant 5, rβ- spectively.

The pre-heating chambers 3 and 4 comprise each a liftable roof 13 which is intended to be sealingly adapted on top of a waste metal container 14 in the re¬ spective chamber 3 and 4. Said containers 14 which can constitute for example a basket or a chute placed on a carriage are preferably provided with a water-cooled jacket and a water-cooled bottom with automatic con¬ nection of cooling water and emptying of the water cir¬ cuits upon charging of the waste metal in the furnace in order to be able to withstand the high gas tempe¬ ratures <500-1000°C) . The sealing between the covers 13 and the chambers 3 and 4, respectively, can be achieved for instance by means of an annular waterseal 15.

The pre-heating chambers 3 and 4 are mutually con- nected by means of a crosswise connection system, i.e. the chamber 3 is connected to the inlet duct 7 by means of a passage 16 while the chamber 4 is connected to the inlet duct 6 by means of a passage 17.

In order to enable control of the flue gas flow the ducts included in the device are provided with ad¬ justable and controllable flue gas dampers 18.

The flow of the flue gases through the chambers 3 and 4 will be described in the following. The flue ga¬ ses are led via the inlet duct 6 to the chamber 3 con¬ taining a waste metal basket 14 next in turn to be charged. The flue gases flow through the waste metal container 14 and further through holes in the bottom of the container 14 into the chamber 3. The crosswise con¬ nection system makes it possible to lead the flue gases via the ducts 16 and 7 to the pre-heating chamber 4 in which the flow is in the same way as in the chamber 3. The flue gases are thereafter evacuated through the outlet duct 9 and the outlet 10 either to the flue gas cleaning plant 5 or to the combustion chamber 19.

The flue gases can also be led directly from the chamber 3 to the outlet 10 without passing the chamber 4.

Of course the flue gases can first be made to pass the chamber 4 if it is found that the waste metal con¬ tainer 14 therein is next in turn for charging. The crosswise connection system is then utilized in the same way as described above.

Normally the flue gases are first supplied to the chamber containing the hot waste metal basket, so-call¬ ed cascade heating. However, it is also possible first to supply the flue gases to the chamber containing the cold waste metal basket, for instance a new inserted basket. In this latter heating cycle, so-called revers¬ ible heating, a high temperature <800-1000°C) is ob¬ tained by means of the hot flue gases in the upper part of the cold waste metal basket. The flue gases are thereafter made to flow via the crosswise connection system through the next chamber with a waste metal basket previously heated in the same way. The possible unburnt flue gases after heating the waste metal in the waste metal basket colder from the beginning will then be ignited, post-combusted and destructed to some ex-

tent in the waste metal in the waste metal basket hot from the beginning. Thus, in this way a started poβt- -combustion of the flue gases already in the waste metal basket with a following post-combustion in the combustion chamber 19 provided with the burner 20 is achieved.

In case pre-heating is not concerned the gases can be evacuated from the furnace 1 directly via the by¬ pass duct 27 to the gas cleaning plant 5. When the device is in operation the flue gases ge¬ nerated in the furnace 1 stream into the combustion chamber 19 via the duct 23. The superheated flue gases leave the chamber 19 via the duct 24 and flow there¬ after through the pre-heating chambers 3 and 4 in the way previously described. After flowing through the chambers 3 and 4 the flue gases can be led directly to the plant 5 via the ducts 10 and 30 or be recycled through the combustion chamber 19 via the ducts 25 and 26. In order to increase the temperature in the com¬ bustion chamber 19 and to improve the power efficiency a heat exchanger 31 is arranged between the ducts 25 and 26.

The ratio of gases generated by the burner 20 to flue gases generated in the furnace 1 can be varied thanks to the fact that the capacity of the burner 20 is adjustable steplessly. If the furnace 1 does no ge¬ nerate any flue gases at all the burner 20 can generate alone a sufficient amount of flue gases for pre-heating the waste metal.

When the furnace 1 is an basic oxγgene furnace there will be difficulties in leading the hot unburnt gases to the chambers 3 and 4 and the combustion cham¬ ber 19, respectively, and therefore the burner 20 in this case accounts alone for the pre-heating of the waste metal. On the other hand, the hot flue gases are

more suitable for use as fuel. For example, the gases can be supplied to the fuel supply system 21 via the duct 29 after treatment.

Process control of the pre-heating device can be carried out by means of a microprocessor 28 which also controls the part systems included in the device in ad¬ dition to the control of dampers 18, roof lifting de¬ vice (not shown) and fans 12.

The invention is not restricted to what has been shown and described, but amendments and modi ications thereof are possible within the scope of tle inventive idea defined in the claims.