1. Overview: Secondary Aluminum

2. 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.

3. 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.

4. 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.

5. 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

6. Secondary Aluminum Processes

7. 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.

8. How are air emissions reduced?
Best Environmental Practices
Performance levels associated with best available techniques and best environmental practices for secondary aluminum smelters: < 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.
Artisanal and other small-scale aluminum recovery processes are used in a number of countries.
Achievable performance limits are not applicable to artisanal and small-scale aluminum recovery
processes as the processes used cannot be considered best available techniques or best environmental
practices and ideally would not be practiced at all. Sometimes largely unsorted scrap is melted in a
small crucible or furnace housed in a building or a roofed space, most often inadequately ventilated.
This device may be fired with charcoal, oil, waste oil or coal, depending on economic factors and the
local fuel supply situation. In larger furnaces, the melt may be treated with fluxes and degasifying
chemicals to improve the quality of the molten metal. Artisanal and other small-scale aluminum
recovery processes may release many chemicals into the environment, including persistent organic
pollutants. These processes should be discouraged in favor of using the proper air pollution controls
on larger-scale secondary aluminum smelting operations.
However, where artisanal and other small-scale aluminum recovery processes are practiced, certain
measures can be put in place in order to reduce the amount of pollutants released into the
environment. Measures to reduce emissions of persistent organic pollutants and other pollutants from
artisanal processes include presorting of scrap material, selecting a better fuel supply (oil or gas fuels
instead of coal), adequate ventilation, filtration of exhaust gases, proper management of wastes and
proper choice of degasifies. These measures can be achieved by education and outreach programs
working with craft groups and town authorities.

9. 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

10. How are air emissions reduced?
Primary Measures
Good operation conditions in furnaces
Maintain furnace temperatures > 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

11. 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 > 950 C followed by rapid quenching to temperatures < 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

12. 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