Salt-Free Aluminum Scrap Melting in Twin-Chamber Furnace
The Twin-Chamber Melting Furnace (TCF) was developed to melt aluminum scrap that contains different types of contamination, such as rubber, plastic materials, lacquers and oils, from various origins. The unique melting concept allows the scrap to be completely processed in the furnace without the need for pretreatment and without using salt. The charge normally consists of about 30% clean material and 70% scrap containing up to approximately 10% contaminants. The Schmitz + Apelt LOI Twin-Chamber Melting Furnace (Fig. 1) combines the advantage of salt-free melting of contaminated aluminum scrap with a heating concept that has been optimized with respect to energy and environmental aspects. It uses the combustible components included in the contaminants, a special charging method and a rotary generator to optimize energy consumption.
The furnace design consists of two separate processing chambers (a heating chamber and a scrap chamber) in one furnace casing. The two chambers are connected to one another by a common melt bath (Fig. 2).
Contaminated scrap is placed on the scrap chamber's bridge by means of a special charging machine, while clean material consisting of thick-wall parts is placed on the heating chamber's bridge. A continuously working liquid metal pump is used to improve the homogeneity of the melt temperature and chemical composition. This operation brings molten metal from the heating chamber through the charge well and into the scrap chamber, which assists the melting process at the same time. The melt flows the other way under the partition wall into the heating chamber. Liquid metal is taken out of the furnace either in batch quantities or continuously and transferred to a casting furnace or, via a launder system, directly to a casting machine or to transport ladles.
The furnace can be stationary or tiltable. Also, because the furnace is based on a modular design, it can be customized to meet special requirements of the customer with respect to the process and the furnace geometry; that is, fitting the furnace into existing buildings or a given environment. The footprint of the furnace can be made to fit three different design possibilities (Fig. 3) including opposite doors (I), doors positioned on an angle (II) and doors on one side of the furnace (III). The heating system also is fitted to the special requirements by adjusting it to the flow of pyrolysis gases and required heating capacity.
It is possible to charge large size scrap into the furnace through the wide doors that stretch over the total furnace width. Additional scrap preparation prior to the melting process, such as thermal or mechanical pretreatment, is not necessary. The design also simplifies cleaning of the furnace and the bath because there are no hidden corners inside the furnace and there is sufficient room to quickly and simply clean furnace walls and bath without much effort. This reduces the time the door is open, the personnel required, and, to a large extent, damage of the brick lining.
A special charging machine allows ideal charging of the furnace. The charging machine trough is filled at the scrap storage location and moved in front of the furnace. At the same time, the charging machine opposite to the suction hood of the scrap chamber is sealed in a way to prevent the furnace atmosphere from escaping into the workshop when the scrap chamber door is open. The scrap is charged into the furnace at the appropriate time, which is indicated and controlled by the PLC (Fig. 4).
While the furnace door is open, the exhausting furnace atmosphere is sucked off in a controlled way and passed on to a waste-gas treatment operation. After the furnace door is completely opened, the trough moves into the furnace and stops above the melting bridge. Withdrawing the trough bottom plate places the scrap equally and in a controlled way on the bridge. After the trough has moved out of the furnace, the furnace door is closed and the charging machine is available for a refilling. Swarf and shredded scrap is fed to the charge well using a belt conveyor. Feeding swarf and scrap is controlled and recorded via weighing cells (Fig. 5).
A charging and melting management system (CMMS) makes it easy to find the components for the required alloy in the storage area using recorded data of analyzed scrap. Use of the CMMS improves production in the Twin-Chamber Melting Furnace (as well as other types of melting furnaces) by:
- Optimizing charge calculation and charge tracking
- Minimizing material costs for scrap and virgin alloys
- Including the alloy calculation in the actual analysis
- Supervising pyrolysis process time
- Automating signals for stirring and skimming
The CMMS serves as an extension to the basic Twin-Chamber Melting Furnace control, and not only includes all of the scrap data of the remelting plant, but also it provides comprehensive information about the whole process.
In the scrap chamber, contaminants are first decomposed and then the aluminum is melted. This takes place in two different process steps to reduce melting losses. The pyrolysis process used to eliminate contaminants is carried out at temperatures to 550?C (1020?F). The atmosphere in the scrap chamber is set at an oxygen-free condition to reduce the melting losses. Two auxiliary burners are operated at under-stoichiometric conditions, and their flames are in intensive contact with the circulating chamber atmosphere, which supports the formation of the oxygen-free atmosphere. Thus, there is no reacting agent available to the aluminum from the waste gas for oxidation (metal loss), and the oxides originating from the contaminants can react with the chamber atmosphere.
The gas atmosphere in the scrap chamber is circulated by means of two fans, which are used to blow the atmosphere into the scrap lying on the bridge. This optimizes heat transmission and achieves fast heat up with excellent heat utilization. A partial stream of the gas is forwarded to the burners of the heating chamber and directly led into the flame reaction room via channels integrated in the burners. This completely burns the contaminants in the gas and the combustible components included therein are used to heat the furnace.
As a countermove to the gas stream to the heating-chamber burners, hot furnace atmosphere escapes from the heating chamber into the scrap chamber through an opening in the separation wall.
The burners installed in the heating chamber are mainly used to heat the furnace. They are fed with both combustible fuel and pyrolysis gas originating from the scrap chamber. On one hand, these burners serve to heat the bath in the heating chamber, while on the other hand, hot gas is made available to heat the scrap chamber to ensure the pyrolysis process occurs and to melt the scrap.
The burners are controlled by the PLC in such a way that the combustible/air ratio always is optimal. To accomplish this, both the mass flows of the reacting agents (combustible and air) and the oxygen content of the waste gas are measured and are involved in the control.
The waste gas exhausted from the heating chamber is guided via an EcoRegR (Schmitz + Apelt LOI/Jasper) regenerator, which functions according to the principle of a rotary-bed regenerator (Fig. 6). Waste gas exhausted from the furnace is streamed through the regenerator, during which time the ceramic filling of the regenerator bed that lies in the path of the stream is heated up while the waste gas is cooled down. The heated part of the regenerator bed is turned to the air-stream part and transfers its heat to the air. Combustion air heated in this way is supplied to the burners by means of thermally insulated pipelines, and the waste gas is led to the filter plant. The regenerator-bed filling can consist of either small balls or it can be a honeycomb construction.
Using this type of heat exchanger, the changeover between conjugated burners used in other conventional regenerators is cancelled. In a conventional regenerative burner system, two burners have to be installed because one is used for burning and heating the air, while the other is flue gas outlet and for cooling the flue. This function changes cycle by cycle. This requires double the burner capacity equipment installed than is necessary. The burner capacity required for the TCF is only installed once because there is no change of function necessary. Another benefit of this is that the burners can be arranged in the best manner to heat the metal bath.
The burners are constantly supplied with heated combustion air due to the continuous operation mode of the regenerator; that is, the conditions for combustion can be adjusted in the optimal manner. At the same time, conditions in the furnace room, such as the furnace room pressure, are steady.
In addition to the gas exchange between both chambers, liquid aluminum also is circulated from the heating chamber into the scrap chamber using an electromagnetic pump. The liquid metal is pumped into the charge well from which it flows into the scrap chamber. As a countermove, melt flows under the separating wall from the scrap chamber into the heating chamber. Thus, the scrap chamber is heated by the bath and homogenization of the entire bath (both temperature and chemical composition) is achieved.
The charge well allows melting of chips and thin, small part type scrap. Chips are immediately drawn under the bath surface due to the special melt flow in the charge well, and therefore, do not come into contact with oxygen in the furnace during melting. This prevents oxidation of the very large specific surface of the chips, and, consequently, the aluminum loss is lower compared with other melting procedures. Due to the arrangement of the charge well, the melt and any contaminants in the melt introduced from the chips is led into the scrap chamber. This keeps the heating chamber clean, and there is no need to clean the bath surface very often. Another advantage of this clean bath surface is that the heat transmission from the flame to the bath is not disturbed.
Rotary-bed regenerator process
Variations of furnace pressure caused by the switchover required when using conventional regenerators are avoided in the same way as the unsteadiness within the combustible/air control (which also is due to this switchover) is avoided. Furthermore, waste gas is completely exhausted via the regenerator, and there is no need for a bypass control to guide the waste gas out of the furnace room.
An essential feature of the regenerator is that the waste gas is cooled down so rapidly that reformation of harmful chemical compositions (DeNovo synthesis) is avoided. The result is proven waste gas values that are environmentally harmless. At the same time, considerably higher preheating of the air provides a degree of efficiency higher than that of plants using other types of heat-exchanger facilities (Table 1).
The regenerator allows higher preheating of the air and faster waste-gas cooling than a conventional recuperator, which considerably reduces energy consumption. Simultaneously, waste gas filter costs will decrease due to the reduced waste-gas temperature. There is no additional cooling required due to the low waste-gas temperature in front of the filter, which reduces the filter plant investment costs.
The charging machine reduces the time the furnace door is open, which not only results in savings in furnace operation, but also results in a positive affect on energy consumption, stabilizes process conditions and increases melt rate. The wide trough of the charging machine makes it possible to charge large scrap pieces without preparation. Switching over between burners is avoided by using the EcoReg(r), and at the same time, air can be preheated to a large extent. Better use of waste gas heat and the resulting lower waste-gas temperature allows better use of combustibles, which leads to lower fuel costs.
Continuous operation of the burners maintains an optimal combustible/air ratio setting, and the formation of harmful substances in the flame, as well as the possibility of an oxidation reaction at the bath surface (melting losses) are reduced.
Use of the combustible materials included in the contaminants not only eliminates the harmful substances, but also it exploits the energy that is included in the combustibles. Metal yield is increased due to the reducing atmosphere in the scrap chamber and the charge well. The salt-free operation avoids the costs related to the supply of and disposal of the salt.