by Mark E. Schlesinger
Chapter 1 – Introduction
- A Brief History of Aluminum Recycling
- Advantages (and Challenges) of Recycling
Chapter 2 – The Ore Body
Chapter 3 – Scrap Collection
- The Materials Life Cycle
- Scrap Collection Practice
- New Scrap
- Old Scrap
- Wire and Cable
Chapter 4 – Aluminum Recycling Economics
- History of Aluminum Production
- Production of Primary Aluminum
- Aluminum Recycling
- Recycling of Used Aluminum Beverage Cans
- Aluminum Recycling: An Economic Perspective
Chapter 5 – BeneficiationTechnology
- Hand Sorting
- Air Classification
- Magnetic Separation
- Eddy-Current Sorting
- Heavy-Media Separation (HMS)
- The Alloy Separation Problem
- Thermal Processing
- Pyrolysis of Composite Packaging
- Paint Removal
- Partial Melting
- Sweat Melting
Chapter 6 – Beneficiation Practice
- New Scrap
- Municipal Solid Waste
- UBCs and Other Packaging
- Electrical and Electronic Scrap
Chapter 7 – Melting Furnace Fundamentals
- Heat-Transfer Kinetics
- The Chemistry of Fluxing
- Refractory Interactions
Chapter 8 – Melting Furnace Parts and Accessories
- Use of Enriched Air
- Flame Manipulation
- Environmental Equipment
Chapter 9 – Fossil-Fuel Furnaces
- Single-Chamber Furnaces
- Multiple-Chamber Furnaces
- Small-Volume Melters
- Rotary Furnaces
- Holding and Dosing Furnaces
Chapter 10 – Electric Furnace Melting
- Induction Furnaces
- Coreless Furnaces
- Channel Furnaces
- Resistance Furnaces
Chapter 11 – The Recycling Industry
- Who Recycles Aluminum?
- Influences on the Aluminum Recycling Industry
- The Impact of Government
- The Impact of Demand
- The Impact of Cost
- The Impact of Technology
Chapter 12 – Metal Refining and Purification
- Common Impurities in Molten Aluminum
- Fundamentals of Impurity Removal
- Reactive Metals
- Refining Strategy
Chapter 13 – Dross Processing
- Types of Dross
- Processing Options for Dross
- Hot Processing
- Dross-Cooling Options
- Melting Options: The Rotary Salt Furnace
- Salt-Free Processes
- Saltcake and Salt Dross Processing
Chapter 14 – Safety and Environmental Considerations
- Collection and Beneficiation
- Thermal Processing and Melting
- Responses to Hazards
- Scrap Acquisition and Storage
- Charging and Melting
Some very usuful figures and texts from this book
The melt loss from various types of coated and uncoated scrap
charged directly to a furnace (McAvoy et al.).
- The painted or lacquered scrap oxidizes more than bare scrap regardless of alloy.
- Melting under a salt flux helps somewhat, but melt losses from laminated material are still over 5%.
Figure 7.1 – Cross section of reverberatory melting furnace,
illustrating radiation heat transfer.
(Alchalabi, R., Meng, F., and Peel, A.)
- The reverberatory melting furnace is typically fired with natural gas.
- Heat is transferred from the burner to the mixture of solid and/or molten aluminum in the hearth (furnace bottom) below.
- Much of the heat is transferred directly from the flame to the metal.
- Some of heat is transferred to the metal by indirect transfer from the flame to the refractory walls.
- The heat reverberates (bounces) off the walls and impinges on the aluminum in the hearth.
- Heat may bounce back and forth between the metal and the walls several times before it is absorbed by the metal, since aluminum reflects heat better than most other metals.
- As a result, reverb furnaces feature shallow baths, maximizing the surface area per ton of contained metal.
- The burners are mounted at opposite ends of the furnace. Each burner features a regeneration bed, comprised of a nonreactive ceramic.
- When the burner at the left is in operation, exhaust gas is drawn through the regeneration bed at the right. The bed absorbs heat from the exhaust gas, heating up in the process.
- Eventually, the burner at the left is shut down and the one at the right is fired up. When this happens, combustion air is drawn through the heated regeneration bed. The air is preheated by the bed, which loses its energy in the process.
- At the same time, the regeneration bed at the left is opened to receive exhaust gas, and is heated up in turn.
- When the regeneration bed at right has given up most of its stored heat, the left burner becomes operational again and the process is repeated.
- By preheating the air, regeneration increases flame temperatures, improving furnace productivity.
- The recovery of heat from the off-gas means that less heat is lost from the furnace; this decreases fuel costs.
- Because the impact of regeneration is similar to that of oxygen enrichment, the two techniques are rarely used at the same time.
- Scrap is simply dumped or loaded into this type of furnace, the furnace opening is closed, and melting begins.
- When the molten metal reaches the desired temperature, it is either pumped out or tapped from a hole in the bottom.
- Burners are mounted at the opposite end of the furnace from the charge well; mounting the flue near the charge well gives hot combustion gases more chance to transfer energy to the melt as they head toward the flue.
- Burners can be roof- or sidemounted.
- Common practice is to leave a heel of molten metal in the furnace bottom after tapping. It makes melting easier for the next charge and to reduce damage to the bottom refractories from the shock of the scrap being dumped in.
- The dry-hearth furnace features a sloping hearth at the right, onto which solid scrap is placed for initial heating.
- As the metal melts, it drains down the hearth into the bath at the left, leaving other metallic materials behind.
- The scrap also dries during the heating process, reducing the possibility of explosions and reducing the potential for melt loss from interaction between the metal and water vapor.
- The reduction in melt loss caused by the use of different types of flame over the solid scrap and the metal bath. Convective flames are used in the dry hearth area to maximize heat transfer to the solid scrap; flat luminous flames work better to heat the molten metal.
- Top-loading dry-hearth furnaces are widely used to maximize capacity.
- Dry-hearth furnaces are the most popular approach for melting large or bulky scrap.
- Refractories in the elevated portion of the hearth are more prone to breakage by the solid scrap being dumped on them without a cushion.
- The lack of direct contact between combustion gases and the charge means that thermal efficiencies are still low (30 to 35%).
- Scrap size is still a limitation as well, since smaller scrap piled on the hearth is less likely to stay there.
- In stack melters, scrap is input directly into the exhaust stack, forcing the exhaust gas to go through the scrap as it leaves the furnace.
- As the heated scrap descends to the sloping hearth, additional burners melt it, causing it to flow into the molten bath. This in turn allows more scrap to descend to the hearth, creating a semicontinuous melting operation.
- Stack melting has several advantages over traditional wet- or dry-hearth melting. The most significant is improved efficiency. The preheating of the scrap by the exhaust gas also reduces melting time on the hearth, improving furnace productivity. The elimination of the open charging well reduces melt loss and minimizes emissions.
- Stack melting is not suitable for very large scrap. But a stack can be combined with a dry hearth to accommodate a range of charge materials.
- Stack melting has a few disadvantages. The foremost is controllability; scrap descends as quickly as it melts, which makes slowing down the melting rate difficult. The pressure of the scrap on the stack limits the depth to which it can be stacked, and this in turn limits the length of time that it can be preheated.
- Uneven heating is also a concern if the charged scrap has a range of sizes.