GB2302168A

GB2302168A - Scrap preheating method and apparatus

Info

Publication number: GB2302168A
Application number: GB9512001A
Authority: GB
Inventors: Michael Pownall
Original assignee: NOTTINGHAM METAL RECYCLERS LIM
Current assignee: NOTTINGHAM METAL RECYCLERS LIM
Priority date: 1995-06-13
Filing date: 1995-06-13
Publication date: 1997-01-08
Anticipated expiration: 2015-06-13
Also published as: GB2302168B; GB9512001D0

Abstract

A method of preheating scrap material 46 comprises enclosing a large quantity of scrap material within a semi-scaled enclosure 40 and subjecting the scrap material over a prolonged period of time to the passage of air which has been heated by passage through wall members of a furnace for cooling the furnace. The semi-sealed enclosure may be a warehouse 40. A sealed preheating chamber (18, Figure 3) may be provided for further preheating of the material immediately prior to feeding into the furnace. In the semi-sealed enclosure the material is heated to a relatively low temperature at which no hazardous gases are produced (e.g. 60{C). In the sealed chamber (18) the material is heated by furnace flue gases to a higher temperature which may cause hazardous gases to be emitted.

Description

FURNACE PREHEATING METHOD AND APPARATUS The present invention relates to furnace preheating methods and to apparatus for preheating scrap material which is subsequently to be fed into a furnace for melting.

In all furnace systems it is firstly important that the system is as efficient as possible in order to reduce the energy required to melt each unit (e.g. ton) of scrap. Secondly, it is important that no innocuous gases escape into the atmosphere.

To increase efficiency within a furnace it is known to preheat scrap with exhaust flue gas from the furnace. This is usually done in a sealed unit which is attached to the furnace. This is advantageous but has a number of problems. Firstly if the scrap in the preheating chamber is exposed to gases which are too hot, then any contaminants, e.g. paint on the scrap can explode and create possible escape of hazardous gases.

However, if a lower temperature flue gas is used then the time taken for the scrap to heat up to a reasonable temperature can be lengthy.

If too long then the scrap will enter the furnace at a low temperature, the furnace will then have to take longer to heat the scrap and the furnace temperature will drop from its optimum level. This results in a substantial loss of throughput for the furnace and lowers the furnace efficiency, thereby rendering the furnace less environmentally acceptable.

The preheating of scrap by flue gases is very inefficient, especially if the scrap is in the form of a bale of, for example, compressed old drink cans. The gases do not readily penetrate into the centre of the bale and thus through the extremities of the bale may be heated to several hundred degrees, the centre of the bale may be at ambient temperature.

Thus the placing of a bale in a preheating chamber for a time period of several minutes will not substantially increase the efficiency of the furnace.

It is an object of the present invention to provide a method and apparatus for preheating scrap material prior to such scrap material being fed into a furnace. The method is environmentally friendly, results in a greater throughput of scrap material through the furnace and by maintaining the furnace at a more constant operating temperature substantially increases the efficiency of the furnace.

The present invention provides a method of preheating scrap material comprising enclosing a large quantity of scrap material within a semi-sealed enclosure, subjecting the scrap material over a prolonged period of time to the passage of cooling air which has been heated by passage through wall members of a furnace.

Preferably the time period is sufficient to completely heat the scrap material to a relatively low temperature at which temperature there is no risk of any hazardous gases being created.

Preferably the method further comprises transferring the heated scrap to the furnace without substantial cooling.

In an embodiment of the invention the scrap material is heated to a temperature of approximately 60"C.

Preferably the scrap material is held in the semi-sealed enclosure on a plurality of pallets of such a size as to be fed into the furnace.

The present invention also provides apparatus for preheating scrap material including a semi-sealed enclosure, means for supplying the enclosure with preheated cooling air, the enclosure being shaped to ensure passage of the preheated cooling air over scrap material stored in the semi-sealed enclosure.

Preferably the semi-sealed enclosure comprises a warehouse provided with entrance means suitable for the passage of pallets loaded with scrap material.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows schematically a furnace preheating apparatus according to the present invention; Figure 2 shows a cross-section on line X-X of Figure 1; and Figure 3 shows a further schematic drawing showing an alternative embodiment according to the present invention.

With reference now to Figure 1, the apparatus comprises a furnace 10 which may be of the closed well type. The furnace in known manner comprises an inlet door 12 for input of scrap and an outlet door 14 for removal of dross.

Parts of the furnace require cooling and in this design of furnace an interior curtain wall 16 requires cooling.

In accordance with the present invention, the curtain wall 16 is cooled by passage of cooling air supplied from atmosphere by a blower 20 through pipework 22.

The air at ambient temperature is heated to a higher temperature by the curtain wall 16 and then passes via outlet holes 24 (see Figure 2).

The heated air then passes via further pipework 26 and a valve 28 to a holding warehouse type structure 40 which is in effect a semi-sealed enclosure. The heated air enters the enclosure 40 via an inlet 42 and exits via an outlet 44 on the opposite corner of the enclosure.

Within the enclosure 40 scrap material 46, possibly scrap aluminium in pallets, is stored. The material can be in the form of bales compressed in known manner in a baler.

The heated air increases the air temperature inside the enclosure 40 to a temperature substantially above ambient but below that at which hazardous gases are produced. The temperature may, for example, be approximately 60"C.

The scrap material 46 may be stored in the enclosure 40 for a substantial time period, sufficient to ensure that the whole of a bale is heated completely through. In the case of a tightly packed bale this may take many hours of exposure to the heated air. Thus, the enclosure should preferably be large enough to enable bales to be moved round, for example, by fork lift truck within the enclosure.

With reference to Figure 3, the enclosure 40 is shown having at least two doors 402,404. These doors are large enough to allow entry of a fork lift truck loaded with a pallet sized quantity of scrap material 46.

The pallets 460,462 can be moved within the enclosure 40 to ensure that each remains within the enclosure for a time sufficient to ensure complete heating of the scrap material. This is exemplified by a theoretical path for a pallet 464 which enters via door 402 and is initially positioned at 4641 moving to 4642 and then to 4643 just prior to entry into the furnace 10.

In a preferred embodiment a sealed preheating chamber 18 shown only diagrammatically is positioned between the semi-sealed enclosure 40 and the furnace 10. Entrance from the enclosure 40 is by means of a gas tight door 406. The chamber 18 is separated from the furnace by furnace door 12.

Flue gases may be circulated within chamber 18 once all doors have been sealed and the scrap material 4644 positioned within the chamber 18 following its first preheating cycle 4641-4643 within the enclosure 40 is further preheated within the chamber 18. This further preheating is, however, carried out at a higher temperature and may cause hazardous gases to be emitted. Thus, as stated, chamber 18 is completely sealed from atmosphere only flue gas circulation being allowed.

The cycle of the furnace 10 may be such that the scrap material 4644 can only be held in preheating chamber 18 for a relatively short period of time. However, the whole of the scrap material will have been completely preheated in enclosure 40 and therefore on entrance into the furnace 10 will be substantially hotter than if the scrap had entered chamber 18 from ambient.

With reference to Figure 2 and 3, the air circulation into enclosure 40 can be controlled by a control system 50. This can be used to set the temperature within enclosure 40 by means of a thermostat 52.

To enable this to be done the blower 20 may be controllable to switch off from the period that any door 402,404 is open, subject to any overriding which may be necessary in order to ensure correct maintenance of the curtain wall temperature.

If insufficient air is being provided via the supply pipework 26 then outlet 44 may be controlled to keep more warm air within enclosure 40.

Alternatively, if too much air is being supplied then this can be dumped directly to atmosphere via valve 54.

To isolate the system a valve 56 may be included in pipe 26 and to isolate enclosure 40 in conjunction with valve 54 a valve 58 may be included just prior to the entrance point 401 of heated air into enclosure 40.

By careful control of blower 20, valves 54,56 and 58 and outlet 44 the temperature in enclosure 40 can be maintained at a required level thereby preheating scrap material 46 to a desired temperature.

The introduction of scrap material into furnace 10 cools down the furnace which for ideal melting should operate a fixed level temperature, e.g. 600"C. To heat scrap from a low temperature within furnace 10 takes a substantial time period since with liquid material contact or hot air contact only transfers heat at a slow rate. Thus, by increasing the average intake temperature of scrap by up to 600C (from zero in winter conditions) the efficiency of the furnace is substantially increased.

Claims

1. A method of preheating scrap material comprising enclosing a large quantity of scrap material within a semi-sealed enclosure, subjecting the scrap material over a prolonged period of time to the passage of cooling air which has been heated by passage through wall members of a furnace.

2. A method of preheating scrap material as claimed in claim 1 in which the time period is sufficient to completely heat the scrap material to a relatively low temperature at which temperature there is no risk of any hazardous gases being created.

3. A method of preheating scrap material as claimed in claim 2 in which the method further comprises transferring the heated scrap to the furnace without substantial cooling.

4. A method of preheating scrap material as claimed in claim 3 in which the scrap material is heated to a temperature of approximately 60"C.

5. A method of preheating scrap material as claimed in claim 4 in which the scrap material is held in the semi-sealed enclosure on a plurality of pallets of such a size as to be fed into the furnace.

6. Apparatus for preheating scrap material including a semi-sealed enclosure, means for supplying the enclosure with preheated cooling air, the enclosure being shaped to ensure passage of the preheated cooling air over scrap material stored in the semi-sealed enclosure.

7. Apparatus as claimed in claim 6 in which the semi-sealed enclosure comprises a warehouse provided with entrance means suitable for the passage of pallets loaded with scrap material.

8. Apparatus as claimed in claim 7 in which the warehouse is provided with air outlet control means for controlling the air temperature within the warehouse structure.

9. Apparatus as claimed in claim 8 in which the air outlet control means comprises a grille sited at an opposite wall to an air inlet from the furnace.