Industrial Process Description

This section specifically contains a description of commonly used production processes, associated raw materials, the byproducts produced or released, and the materials either recycled or transferred offsite. This discussion, coupled with schematic drawings of the identified processes, provides a concise description of where wastes may be produced in the process.

Primary aluminum processing

Primary aluminum producers generally use a three step process to produce aluminum alloy ingots. First, alumina is extracted from bauxite ore using the Bayer process (Figure 3.1). In the Bayer process, finely crushed bauxite is mixed with an aqueous sodium hydroxide (caustic soda) solution to form slurry. The slurry is then reacted at a high temperature under steam pressure in a vessel known as a digester, and creates a mixture of dissolved aluminum oxides and bauxite residues. During the reaction a majority of the impurities such as silicon, iron, titanium, and calcium oxides drop to the bottom of the digester and form sludge. The remaining sodium aluminate slurry is then flash cooled by evaporation and sent for clarification. During clarification, agents such as starch are added to help any fine impurities that remain in the slurry, such as sand, to drop out, further purifying the sodium aluminate solution. The solution is then fed into a precipitation tank to be crystallized. In the precipitator the solution is allowed to cool with the addition of a small amount of aluminum

FIGURE 3.1 Bayer process for alumina refining. (From U.S. EPA, Profile of the Nonferrous Metals Industry, publication EPA/310-R-95-010, U.S. EPA, Washington, DC, September 1995.)

hydroxide "seed." The seed stimulates the precipitation of solid crystals of aluminum hydroxide and sodium hydroxide.

The aluminum hydroxide crystals settle to the tank bottom, and are removed. The crystals are then washed to remove any caustic soda residues, vacuum dewatered, and sent on for calcination. In the calciners (a type of rotating kiln) the aluminum hydroxide is roasted for further dewatering.

In the second step, the aluminum oxide (alumina) produced during the Bayer process is reduced to make pure molten aluminum. Alumina is a fine white powder, and consists of about equal weights of aluminum and oxygen. The strong chemical bond that exists between the aluminum and oxygen makes separating them difficult—pyrometallurgical separation requires a temperature of about 1980°C (3600°F). However, alumina will dissolve when placed in the molten metal cryolite at around only 950°C (1742°F). Once dissolved, the aluminum oxide is readily separated into aluminum and oxygen by an electric current. The Hall-Heroult process, as this type of electrolytic reduction is known begins with the placement of the alumina into electrolytic cells, or "pots," filled with molten cryolite (Figure 3.2). Although the process requires large amounts of electricity (13 or 15 kW of electricity per kg of aluminum produced), only a low voltage is needed. This allows the pots to be laid out in a series along one long electrical circuit to form what is known as a "potline." Within each pot a positive electric current is passed through the cryolite by means of a carbon anode submerged in the liquid cryolite. The oxygen atoms, separated from aluminum oxide, carry a negative electrical charge and are attracted to the carbon anodes. The carbon and the oxygen combine immediately to form carbon dioxide and carbon monoxide. These gases bubble free of the melt. The aluminum (which is more than 99% pure) collects at the bottom of the pot, is siphoned off, placed into crucibles, and then transferred to melting/holding furnaces.

The third step consists of either mixing the molten aluminum with other metals to form alloys of specific characteristics, or casting the aluminum into ingots for transport to fabricating shops.1,13 Casting involves pouring molten aluminum into molds and cooling it with water. At some plants, the molten aluminum may be batch treated in furnaces to remove oxide, gaseous impurities, and active metals such as sodium and magnesium before casting. Some plants add a flux of chloride and fluoride salts and then bubble chlorine gas, usually mixed with an inert gas, through the molten mixture. Chloride reacts with the impurities to form HCl, Al2O3, and metal chloride emissions. Dross forms to float on the molten aluminum and is removed before casting.

Two types of anodes may be used during the reduction process: either an anode paste or a preb-aked anode. Because the carbon is consumed during the refining process (about 0.5 kg of carbon is consumed for every kg of aluminum produced), if anode paste (Soderberg anode) is used, it needs to be continuously fed through an opening in the steel shell of the pot. The drawback to prebaked anodes is that they require that a prebaked anode fabricating plant be located nearby or onsite. Most aluminum reduction plants include their own facilities to manufacture anode paste or prebaked anode blocks. These prebaked blocks, each of which may weigh about 300 kg, must be replaced after 14 to 20 d of service.

The waste materials produced during the primary production of aluminum are fluoride compounds. Fluoride compounds are principally produced during the reduction process. One reason that prebaked anodes are favored is that the closure of the pots during smelting facilitates the capture of fluoride emissions, although many modern smelters use other methods to capture and recycle fluorides and other emissions.

The pots used to hold the aluminum during smelting range in size from 9 to 15 m (30 to 50 ft) long, 2.7 to 3.6 m (9 to 12 ft) wide, and 0.9 to 1.2 m (3 to 4 ft) high, and are lined with refractory brick and carbon. Eventually, the carbon linings crack and must be removed and replaced. However, during the aluminum reduction process iron cyanide complexes form in the carbon portion of the liners. When the linings are removed they are "spent," and are considered to be RCRA-listed hazardous waste.

Secondary aluminum processing

In the secondary production of aluminum, scrap is usually melted in gas- or oil-fired reverberatory furnaces of 14,000 to over 45,000 kg capacities. The furnaces have one or two charging wells separated from the main bath by a refractory wall that permits only molten metal into the main bath. The principal processing of aluminum-base scrap involves the removal of magnesium by treating the molten bath with chlorine or with various fluxes such as aluminum chloride, aluminum fluoride,

Anode bus

Anode bus

Vertical stud soderberg cell Anode beam

Center-worked prebake cell

FIGURE 3.2 Aluminum anodes. (From U.S. EPA, Profile of the Nonferrous Metals Industry, publication EPA/310-R-95-010, U.S. EPA, Washington, DC, September 1995.)

Center-worked prebake cell

FIGURE 3.2 Aluminum anodes. (From U.S. EPA, Profile of the Nonferrous Metals Industry, publication EPA/310-R-95-010, U.S. EPA, Washington, DC, September 1995.)

or mixtures of sodium and potassium chlorides and fluorides. To facilitate handling, a significant proportion of the old aluminum scrap, and in some cases new scrap, is simply melted to form sweated pig that must be processed further to make specification-grade ingot.

Another method of secondary aluminum recovery uses aluminum drosses as the charge instead of scrap. Traditionally, the term dross was defined as a thick liquid or solid phase that forms at the surface of molten aluminum, and is a byproduct of melting operations. It is formed with or without fluxing and the free aluminum content of this byproduct can vary considerably. Most people in the industry have generally referred to dross as being lower in aluminum content, while the material with higher aluminum content is referred to as "skim," or "rich" or "white dross." If a salt flux is used in the melting process, the byproduct is usually called a "black dross" or "salt cake." Drosses with about 30% metallic content are usually crushed and screened to bring the content up to about 60 to 70%. They are then melted in a rotary furnace, where the molten aluminum metal collects on the bottom of the furnace and is tapped off. Salt slags containing less than 30% metallic may be leached with water to separate the metallic. In addition to this classic dross-recycling process, a new dross treatment process using a water-cooled plasma gas arc heater (plasma torch) installed in a specially designed rotary furnace has been patented recently. The new process eliminates the use of salt flux in the conventional dross treatment process, and reports recovery efficiencies of 85 to 95%.

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