Overview: Secondary Aluminum Secondary Aluminum Description

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Overview Secondary Aluminum
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Secondary Aluminum Description

• Secondary aluminum smelting involves the
  production of aluminum from used aluminum
  products or process waste to recover metals by
  pretreatment, smelting and refining.
• Most secondary aluminum recovery facilities use
  batch processing in smelting and refining
  operations.
• Pretreatment, furnace type, feed and fluxes used
  will vary with each installation.

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Secondary Aluminum Steps

• Scrap Pretreatment
• Methods Mechanical, Pyrometallurgical,
  hydrometallurgical cleaning
• Feed process scrap, used beverage cans, foils,
  extrusions, commercial scraps, turnings, old
  rolled or cast metal, and recycled skimmings from
  the secondary smelting process.
• Presorting of scrap into desired alloy groups
  reduces processing time.
• Scrap contaminated with oil or coatings requires
  removal of oil to reduce emissions and improve
  melting rate
• Salt slag is treated to recover the salt, which
  is reutilized as flux in rotary furnaces. Salt
  slag treatment residue has a high aluminum oxide
  (Al2O3) content and can be recycled using the
  Bayer process or used as an additive in the
  cement industry.
• Charging
• Pretreated aluminum scrap placed into a melted
  aluminum pool (heel) that is maintained in
  melting furnaces. The scrap, mixed with flux
  material, is placed into the furnace charging
  well, where heat from the molten aluminum
  surrounding the scrap causes it to melt by
  conduction.

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Secondary Aluminum Steps

• Smelting Furnaces
• Reverberatory a smelting chamber heated by a
  heavy oil burner and an open well where aluminum
  scraps of various sizes are supplied
• Rotary furnaces -horizontal cylindrical shell
  mounted on rollers and lined with refractory
  material. The furnace is fired from one end,
  usually using gas or oil as the fuel
• Induction furnaces used to smelt cleaner aluminum
  feed materials
• Fluxing
• Flux materials combine with contaminates and
  float to the surface of the aluminum, trapping
  impurities and providing a barrier (up to 6
  inches thick) that reduces oxidation of the
  melted aluminum.
• To minimize aluminum oxidation (melt loss),
  mechanical methods are used to submerge scrap
  into the heel as quickly as possible.
• Demagging is necessary
• Reduces the magnesium content of the molten
  charge.
• Is accomplished by addition of chlorine, aluminum
  chloride or chlorinated organics
• Process Cl2 (or other compounds) gas is metered
  into the circulation pump discharge pipe.

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Secondary Aluminum Steps

• Degassing
• Process used to remove gases entrained in molten
  aluminum.
• High-pressure inert gases are released below the
  molten surface to violently agitate the melt.
  This agitation causes the entrained gases to rise
  to the surface to be absorbed in the floating
  flux.
• Alloying
• Combines aluminum with an alloying agent in order
  to change its strength and ductility.
• Skimming
• Removes contaminated semisolid fluxes (dross,
  slag, or skimmings) by ladling them from the
  surface of the melt.
• Pouring

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Secondary Aluminum Processes
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What are Sources of Dioxin Air Emissions?

• Potential air pollutants Dioxins/Furans, PM,
  metal compounds, chlorides, NOx, SO2, CO,
  Ammonia, and organic compounds
• Sources of dioxin/furan emissions
• Incomplete combustion
• De novo synthesis
• The presence of oils and other organic materials
  on scrap or other sources of carbon (partially
  burnt fuels and reductants, such as coke), can
  produce fine carbon particles which react with
  inorganic chlorides or organically bound chlorine
  in the temperature range of 250 to 500 C to
  produce PCDD/PCDF.
• Contaminants in feed include organic and chlorine
  compounds such as fluxes, hexachloroethane,
  chlorine, unburnt fuel, oils and plastics
• Catalyzed by the presence of metals such as
  copper or iron.
• Chemical additions - chlorine mixtures for
  degassing and demagging and chlorides in salt
  fluxes provide chlorine for potential formation
  of dioxins. Chemical additions combined with
  process conditions favorable to formation of
  dioxin/furans at temperatures between 250 and
  500 C result in emissions.

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How are air emissions reduced?

• Best Environmental Practices
• Performance levels associated with best available
  techniques and best environmental practices for
  secondary aluminum smelters lt 0.5 ng I-TEQ/Nm3
  (at operating oxygen concentrations).
• Eliminate use of artesianal and other small-scale
  aluminum recovery processes. Achievable
  performance limits are not applicable to
  artesianal and small-scale aluminum recovery
  processes.

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How are air emissions reduced?

• Primary Measures
• Presorting of scrap material
• Avoid presence of oils and other organic
  materials on scrap
• Current Methods swarf centrifuge, swarf drying,
  or thermal decoating followed by afterburning
• New Method being tested employ laser and eddy
  technology
• Use high-temperature advanced furnaces
• Reverberatory furnace
• Rotary and tilting rotary furnace
• Induction furnace
• Shaft furnace

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How are air emissions reduced?

• Primary Measures
• Good operation conditions in furnaces
• Maintain furnace temperatures gt 850 C to destroy
  dioxins/furans
• Monitor emissions if possible, temperature,
  residence time, gas components, and fume damper
  controls
• Choice of Demagging and Degassing Agents
• Minimize formation of chlorine compounds, ex. use
  mixtures of chlorine and inert gases
• Avoid use of hexachloroethane and maintain
  careful control over demagging
• Use potassium fluoride or potassium aluminum
  fluoride as a degassing agent instead of chlorine

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How are air emissions reduced?

• Secondary Measures Pollutant control measures
• Fume and gas collection for all processes
• Sealed feeding systems and sealed furnaces
• Control of fugitves by maintaining negative air
  pressure in furnace to prevent fugitive leaks
• Gas collection use of furnace or reactor
  enclosures
• Use of hooding if sealed enclosures are not
  possible
• High efficiency PM removal dioxin/furan adsorb
  on PM
• Collected PM should be treated in high
  temperature furnaces to remove dioxins/furans
• Methods high-efficiency fabric filters, ceramic
  filters, wet and dry scrubbers
• Use of catalytic coating on fabric filter bags
  destroys dioxins/furans by oxidation
• Afterburners and Quenching
• Afterburners with rapid quench are used to
  destroy organic materials that escape the
  combustion zone
• Operate afterburners at temperatures gt 950 C
  followed by rapid quenching to temperatures lt
  250C to prevent dioxin reformation. High
  temperature afterburners destroy dioxins/furans
  rapid quenching prevents reformation of
  dioxins/furans.
• Rapid quenching also reduces amount of wastes to
  be treated by scrubbers resulting in economic
  savings

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How are air emissions reduced?

• Secondary Measures Pollutant control measures
• Adsorption on Activated Carbon
• Dioxin/furans adsorb onto activated carbon Ideal
  due large surface area for adsorption
• Treatment with activated carbon using fixed or
  moving bed reactors
• Injection materials such as lime, sodium
  bicarbonate and carbon into gas stream followed
  by high-efficiency PM removal measures such as
  fabric filters
• Catalytic oxidation emerging research
• Transforms organic compounds into H2O, CO2, and
  HCl using a precious metal catalyst
• Off-gases should have PM removal prior to this
  measure
• 99 effective, shorter residence times, lower
  energy consumption