Minimizing Melt Loss in Aluminum Recycling
The handling of dross is an unfortunate but necessary activity of melting aluminum in a reverberatory (reverb) furnace. Recyclers have been working on ways to minimize melt loss and improve metal recovery for over 100 years. Even so, as much as 3% of the scrap can still end up as dross. Big savings can accrue if this metal can be recovered, but this is only possible if dross is properly collected and handled. A side benefit of proper dross handling is that the workplace environment is improved and disposal costs are minimized.
Aluminum melt shops produce three types of dross. First is white dross, which is mainly a mixture of aluminum oxide and aluminum skimmed from the melt surface before tapping the furnace. White dross may contain up to 80% metallic aluminum. Second is black dross from the charging well, which is a mixture of salt flux, aluminum oxide and metal. The salt flux must be replenished because impurities in the scrap create sludge in the salt, which accumulates and hinders the settling of small droplets of molten aluminum. Third is salt cake, which arises from the treatment of white and black dross to recover their metal content. Most melt shops collect their dross and send it out to recyclers for treatment to recover the metal values.
White Dross Control
The exposure of molten aluminum to oxygen in the POC of a reverb furnace causes the formation of aluminum oxide (Al2O3). This reaction is very exothermic, producing 13,360 BTU per pound of aluminum oxidized. This heat is enough to bring 25 pounds of scrap aluminum from 77°F to the 1420°F tap temperature. In addition, some aluminum nitride forms, which gives off ammonia on exposure to air. Skimming of a reverb furnace produces 3 to 4 tons/day of white dross, which costs at least $5,000/day in metal loss. This gives a significant incentive to minimize dross production and to recover as much metal as possible from the dross that is produced.
Metal temperature is the single most controllable factor that determines the aluminum oxidation rate. The rate is tolerable below about 780°C (1440°F) but increases sharply for higher melt temperatures. The entire heat flux from the POC must be transferred to the melt via the bath surface, which causes it to be hotter than the bulk melt temperature. This has two serious side effects. First, the process is less efficient because energy is spent in overheating the top layer. Second, dross on the surface impedes heat transfer from the POC because dross is an insulator.
This gives rise to the first rule in managing the dross issue: minimize dross formation by maintaining a lower melt surface temperature. This is done by melt stirring to enhance heat transfer from the surface to the bulk. This makes the hotter areas cooler and the cooler areas hotter, thus improving heat conduction from the surface to the charging well where scrap is being added. A pump is used to circulate freshly melted metal to the main furnace chamber. Workbook AlMeltCalc (www.industrialheating.com/AlMeltCalc) shows that while melting 256 pounds/minute, a 40°F drop in Al temperature created by melt stirring causes about an 18% decrease in dross. Stirring also saves energy by avoiding surface overheating.
The second rule is to minimize dross by keeping the oxygen content of the POC as low as possible. The burner’s air-fuel ratio should be controlled close to stoichiometric and leak air minimized if not eliminated. The burner flame should not impinge directly on the melt surface unless it is designed to produce a slightly fuel-rich flame. A side benefit of cutting the POC oxygen content in half is energy savings of up to 35,000 cubic feet of natural gas per day.
The third rule is to rapidly cool skimmed dross to minimize exposure of hot dross to air. Otherwise, prills of aluminum in the dross will react, forming aluminum oxide and aluminum nitride (thermiting). This highly exothermic reaction leads to a rapidly escalating dross temperature and a corresponding loss of the metal content. One technique is to skim the dross into a thick-walled steel trough and cover it with a fitted lid. The trough is set aside until the contents are below about 500°C (900°F). Another technique is to skim the dross into a pan fitted with a shaped plunger lid that presses the dross into a cake. Much of the metal content is thereby squeezed out into a casting crucible. A dross press speeds up the cooling process and produces ingot aluminum that can be returned to the melting furnace. Without proper cooling, thermiting dross generates large quantities of harmful fume and smoke.
There are a number of methods for recovering metal from dross. One method is to crush the dross cake to flatten the aluminum globules so they can be screened out. Another method treats dross with an aqueous reagent to produce a marketable aluminum compound. The most common process is heating a mixture of dross and chloride flux (NaCl plus KCl) to about 725°C (1340°F) in a natural-gas-fired rotary furnace. Figure 1 shows a rotary furnace in the dross heating position. The molten salt wets the oxide and assists coalescence of the molten aluminum prills to a pool. The rotation action breaks up oxide particles that would otherwise entrap metal. Charging continues until the furnace reaches capacity, after which it is tilted to pour the contents into molds. A rotary furnace can also process black dross from the charging well.
The resultant rotary-furnace salt cake will be about half oxide and half salt, but it may still contain 3-10% aluminum. Within the U.S., there are no restrictions preventing the land-filling of salt cake with appropriate controls. In Europe, regulations have been put in place that restrict land-filling of salty products. These rules require the salt cake to be further processed before final disposal.
The aluminum recycling industry has matured over the past century as has its understanding of the factors that affect energy consumption and byproduct generation. The industry is a good example of how improved equipment is developed and processes adopted to minimize melt loss and save energy. A complex source material is treated efficiently to return aluminum products to market while minimizing the production of waste.
1. James Herbert, “The Art of Dross Management,” Aluminium Times, April/May 2007, p. 44
2. Ray Peterson and Lee Newton, “Review of Aluminum Dross Processing,” Light Metals 2002, W. Schneider, Ed., TMS, Warrendale, PA
3. Gary Brugger, “New EPA Rule May Promote Recycling,” Industrial Heating, May 2009
4. Jim Bintz, Jim Feese, and Felix Lisin, “TriOx Combustion System Provides Low Dross Formation in Side-Well Aluminum-Melting Furnace,” Industrial Heating, Feb. 2009