- Ceramics & Refractories/Insulation
- Combustion & Burners
- Heat Treating
- Heat & Corrosion Resistant Materials/Composites
- Induction Heat Treating
- Industrial Gases & Atmospheres
- Materials Characterization & Testing
- Process Control & Instrumentation
- Sintering/Powder Metallurgy
- Vacuum/Surface Treatments
A new idea for a melting furnace has been developed to challenge the present-day rotary, reverberatory, crucible and stack furnaces currently used in the scrap-metal industry. This new continuous melting furnace achieves the best efficiency and cost effectiveness to date because of its smaller size.
The major concept of the new continuous melting furnace is a design change from the present batch melting and continuous melting furnace operations. This dramatically changes the working operations for melting nonferrous metals, making the melting operations more energy efficient. The furnace was developed mainly for small to medium scrap yards to add value to the scrap metal by being able to sell the metal at an ingot price. The furnace is compact in size, and the height of the unit is designed to fit inside a typical industrial building. Most of the present-day furnaces are bulky in size with extensive energy and metal losses.
Scrap-yard personnel want a melting furnace that is simple to operate, safe and energy efficient. It should also be small in size and meet all the state and federal environmental regulations.
The food and chemical process industries have already upgraded their batch operations to continuous operations. They were driven by the need to reduce operating costs and improve efficiencies. One of the major savings from the continuous melting furnace is from energy reduction. Additional cost benefits are derived from non-energy operations such as life-cycle cost reduction, which will be discussed later.
The continuous melting furnace has a minimum energy efficiency of 55% for aluminum. Preheating the scrap using the exhaust gases from the furnace can increase the furnace efficiency up to 70% (reverbertory-type melting furnaces are not as efficient because the scrap metal cannot be preheated). The scrap metal charged into the furnace can either be charged by hand or by using a forklift with a self-dumping hopper. Charged material size can be scraps of up to 3 feet in length.
The furnace is designed for dirty materials (i.e. iron-y aluminum) or metals that are painted. The dirt will rise to the top of the molten heel surface, and any iron will settle to the bottom of the furnace. The dirt layer, metal oxides and impurities (dross) that rise to the surface of the molten heel provide a heat blanket to retard the flow of heat into the atmosphere. This helps to keep surface energy losses to a minimum. The metal oxides due to oxidation on the molten surface are negligible, and any metal loss due to dross formation is small. There is no contamination of the poured metal due to dross formation and dirt since the molten metal is withdrawn below the surface of the molten bath.
The continuous melting furnace is simple to operate, small in size and efficient in operation. The furnace is made of high-temperature internals, and no refractory is used. The small size of the furnace provides for a rapid heating start-up operation. Upon start-up, the furnace internals are heated only once for the continuous operation of the furnace.
Upgrading to a continuous melting operation is fairly simple. The major change would be the type of heat transfer used to heat and melt the scrap metal. There are three different types of heat transfer: conduction, convection and radiation.
As an example, the reverberatory furnace uses radiation and conduction to transfer the heat to the metal. This type of operation is energy-intensive with heat loss through the refractory walls, ceiling and doors. Only a small portion of the radiation type of heat transfer is actually available to melt the scrap metal. The continuous melting furnace uses only conduction and convection to transfer heat to the metal.
To fully understand the operation of the continuous melting furnace, an operating procedure will now be provided. The scrap metal is charged into the furnace from its side, filling the furnace and the area above the furnace (hopper). The direct-fired burners continuously melt the scrap. The exhaust gases leaving the furnace preheat the scrap metal in the hopper. The furnace has a continuous molten heel between the bottom of the furnace and the dross level. The molten metal is withdrawn from the furnace between these two levels, always below the surface of the dross layer. The predetermined molten level is designed to always maintain a constant temperature. Molten metal will flow from the furnace continuously by gravity as long as heat and scrap metal are supplied.
The weight of the scrap in the hopper above the molten level submerges the solid metal into the molten heel, keeping it from floating to the bath surface. The continuous removal of the molten metal from the furnace provides constant movement of the molten heel. The small sump and the high flow of molten metal provide good heat transfer to melt the submerged solid metal in the heel. This provides a self-regulating feature. The temperature of the molten metal stays constant and does not go above its pouring temperature of approximately 1400°F for aluminum. Depending on the height and density of the charge of material above the burners, the furnace efficiency will be increased by preheating the cold material, and efficiencies up to 70% can be obtained.
With this design, there is no need for tilting and holding furnaces. The furnace can also be used for a batch-type operation if necessary. Flow of molten metal can be stopped and started again safely by an operator without shutting down the operations. This would eliminate any idling of the furnace with no cold restarts.
The size of the continuous melting furnace is rated based on pounds of metal melted per hour. For aluminum, the furnace sizes range from a 4 foot diameter for melting 2,500 pounds/hour to a 6 foot diameter for melting 6,000 pounds/hour. Considering energy savings, lower metal losses and more efficient operations, payback for these units is less than a year. The shape of the furnace can either be circular or rectangular. The rectangular sizes will accommodate higher melting rates.
The Total Cost of Ownership
The next topic of discussion is the life-cycle costs associated with the furnace. These are non-energy benefits called “The Total Cost of Ownership.” These are used to evaluate the improvements over existing furnaces. Converting the melting process from a batch process to a continuous operating furnace provides a number of non-energy benefits. They include the following:
1. Low initial purchase costs, installation costs and commissioning costs
2. High efficiency and good yields
3. Reduced production and operating costs
4. Reduced downtime costs – no downtime for charging and emptying the furnace
5. Low cleanup costs, low waste disposal costs, low dross waste
6. Low maintenance costs
7. Small floor space
8. Reduced exhaust air pollution and environmental costs for environmental compliance
9. Increased productivity
10. Improved capacity utilization
The small size of the continuous melting furnace has a much lower initial purchase cost than a comparable reverberatory or stack furnace. Installation and start-up costs are also low. With a smaller footprint there is a major improvement in capacity utilization and higher energy efficiency per pound of melted metal. The exhaust gases leaving the furnace are lower in temperature and exhaust air volume. If the exhaust gases are dirty, a baghouse is required to meet all state and federal environmental regulations for particulate control.
Furnace utilization is increased to a large extent by continuous operation. There is no downtime between batches to charge scrap metal and unload molten metal from the furnace. There are no cool-down or heat-up times between batches. No additional energy is required to preheat the furnace each time there is a batch operation. There is no problem if the operation needs to be stopped for a short amount of time (e.g., 10 minutes). The sump (heel) has the capacity to hold additional molten metal.
Cleanup time between batches is eliminated. The increased production improvements are realized by reduced cleanup time and furnace internals being heated only once.
Cleanup from dross formation is small for a number of reasons. The surface area of the continuous melting furnace is small compared to a reverberatory furnace. The small surface area and the continuous operation of the furnace reduce slag formation and lower waste disposal costs. Minimal metal oxides are formed due to oxidation at the molten surface. The furnace runs as a continuous steady-state operation with no excessive temperature spikes to generate additional dross.
Maintenance costs for the continuous melting furnace are low. There are no doors or openings, no refractory lining problems, no furnace air tightness problems, no internal furnace draft problems, no rebuilding of the furnace refractory, and no dross formation and molten-metal accumulation inside the furnace. With the inside surface of the furnace being very smooth, molten metal does not adhere to its internal surface. The use of metal-to-metal surfaces fully drains the furnace with no buildup of metal on the internal surfaces. The furnace is designed to completely drain without tilting. The simplicity of the furnace operation and design greatly reduces maintenance costs. Although this type of furnace was designed to melt lower-temperature metals, the same idea can be used for melting higher-temperature metals (e.g., steel).
A Better Furnace Design
It’s not possible to operate any of the present-day melting furnaces in an efficient manner. The design of the furnaces and their batch operation has limited their efficiency. Although it has never been tried before, depending on the design of the present-day reverberatory furnaces, these furnaces could be modified to be more efficient using the principles of the new continuous melting furnace design. There would be a reduction of dross formation on the surface of the molten metal. The chimney effect generating negative pressure inside the furnace would be eliminated, and infiltration of cold air into the furnace would be a thing of the past. The efficiency of the modified furnace would increase, and the exhaust flue-gas temperature and exhaust air volume (acfm) would be reduced. The modified furnace would eliminate the need for a tilting design.
Much has been written over the past years about improving the energy efficiency of the existing melting furnaces. The industry has tried to optimize the currently used furnace designs with little success. The present-day melting furnaces have fundamental limitations due to their size, shape and operation. The mind-set has been to incrementally improve existing designs rather than start from scratch and develop a new type of melting furnace. Industry continues to use melting technologies with poor efficiencies.
I have taken a different approach to achieve the best energy-efficient furnace to date. The continuous melting furnace eliminates the major problems associated with present-day melting furnaces. It is smaller in size with reduced height and footprint and smaller price tag than the existing stack furnaces. The initial capital investments for the existing batch-type furnaces are very high, making it difficult to invest in new technology. With its energy efficiency and reduced life-cycle costs, the continuous melting furnace has a return on investment of less than one year.
The technology and innovations presented in this article are currently patent pending. IH
For more information: Contact John E. Tobolski, P.E., consultant, from Reading, Pa.; tel: 610-777-6989; e-mail: firstname.lastname@example.org