Overview: Secondary Aluminum

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.

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.

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.

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

Secondary Aluminum Processes

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.

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.

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

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

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

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