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2 Secondary Metal Processes

2.1 Ferrous foundries

Iron and steel foundries produce castings from scrap iron, pig iron, internal returns and alloying materials. The main operations are raw material handling and preparation, melting, mould production, casting and finishing.

Alloying elements may be added to the charge, including ferro-alloys and pure elements such as ferrous silicon, ferrous manganese, ferrous chromium, copper, carbon, and nickel. Fluxes may also be added to assist refining.

Five furnace types are generally used: cupola, arc, induction, reverberatory and crucible. Cupola furnaces are the most common in the USA and Europe (US EPA, 1997; Quass et al, 2000) at least for iron, while electric arc and induction furnaces are usually used for steel. In New Zealand, cupolas are rarely used and furnaces are either electric arc or induction (Newby, 2001). Induction furnaces use a magnetic field to induce heating currents in the metal and require cleaner scrap than electric arc furnaces (EAFs).

Figure 1 provides a diagrammatic representation of an electric arc furnace and an induction furnace.

EAFs are used for melting iron and steel scrap. They allow for the melting of lower-grade scrap, but as a consequence requires appropriate flue-gas capture and cleaning. Typical EAFs have a capacity of around 200 tonnes and are operated as batch processes.

2.2 Secondary copper and alloys

Secondary copper processes generally involve scrap pre-treatment, smelting, refining, alloying and casting of copper, brass and bronze materials.

A variety of furnaces can be used. Induction furnaces are suitable where the alloy composition can be achieved without significant refining. Gas-fired rotary or reverberatory furnaces may be appropriate where degassing is needed or where there are significant quantities of slag from impurities.

In smelting operations, scrap is heated to separate the various metals by their different melting points. Smelting of low-grade copper scrap begins with melting in either a cupola blast or a rotary furnace, resulting in slag and impure copper. Copper is then charged to a converter, where the purity is increased to about 80 to 90%. A reverberatory furnace is then used to produce copper of about 99% purity.

Figure 1: Rotary electric arc furnace (a), and crucible induction furnace (b)

Flux may be added to the copper to oxidise impurities, which are then removed as slag.

Alloying involves the addition of one or more other metals to the copper to obtain desirable qualities in the metal. Copper can be alloyed with zinc, tin, nickel, aluminium, lead and other elements to make a wide range of alloys. Common alloys are brass (copper and zinc), bronze (copper and tin), cupro-nickel, aluminium bronze and gun metal (copper with tin or zinc).

2.3 Secondary aluminium and alloys

Secondary processing of aluminium and aluminium alloys includes melting aluminium ingots or clean aluminium scrap, and melting contaminated scrap and scrap containing other metal alloys. Secondary aluminium processes may include the use of chlorine gas as a degassing agent and fluoride- or chloride-containing fluxes.

Scrap metal sources include beverage cans, slags, drosses and engineering wastes such as swarf millings and turnings.

Furnaces used include induction, rotary, resistance and reverberatory furnaces. Induction furnaces are generally used for higher grades of scrap with limited contamination. Unwanted metal contaminants may be removed by using chlorine additions and other fluxes to form a slag that can be mechanically separated. Rotary furnaces can be used for metal reclaimed from a range of less clean raw materials.

In Europe (Farrell, 2001) rotary and reverberatory furnaces are the two main types of furnace used for secondary aluminium smelting. These furnaces are heated from fuel combustion, and the molten aluminium may be pumped to one or more holding furnaces or converters for final alloying.

The reverberatory furnace can operate without salt as a flux, particularly if a metal pump is used to recirculate molten metal. This type of furnace can smelt most secondary materials and is capable of handling larger pieces of scrap. Rotary furnaces may use salt flux to reduce oxidation and/or remove some impurities (e.g. sodium, magnesium, calcium and lithium). Hearth furnaces and induction furnaces normally do not require salt fluxes.

Induction furnaces can produce hardened aluminium by blending with agents such as manganese and silicon. Crucible smelting and refining processes are used to melt small batches of aluminium scrap, typically 500 kg or less.

2.4 Other non-ferrous foundries

Non-ferrous foundries generally melt internal return and alloying ingots. Non-ferrous metal processes, other than aluminium and copper, include tin, lead and zinc.

Crucible furnaces are common for non-ferrous metal melting, which are generally smaller-scale operations at less than 500 kg per batch. Crucibles are externally heated with either combustion gases or with a thermal fluid (heated using electricity). Crucibles operate at lower temperatures than those required for melting iron and steel.

2.5 Thermal pre-treatment of scrap

Scrap metal can be contaminated with paint, organic matter (e.g. oil and cutting fluids). Organic and inorganic forms of chlorine may also be present. Scrap may be thermally pre-treated to remove contamination by burning, drying to evaporate water, or by partial pyrolysis. Burning and pyrolysis of contaminated scrap give rise to products of incomplete combustion such as particulate matter, carbon monoxide and organic compounds, and may emit PCDD and PCDF. Pyrolysis is burning without oxygen, which may suppress formation of PCDD and PCDF because of the lack of oxygen. Pyrolysis and burning units usually employ an afterburner or fabric filtration to control incomplete combustion products. Drying generally has low potential for emissions of PCDD and PCDF depending on the temperature.

Wire-stripping processes and chemical cleaning methods provide alternatives to thermal treatment.