Aluminum - Production and Transformation

Aluminium is the earth's most abundant metallic element, making up approximately eight per cent of the planet's crust. While aluminium never occurs naturally in its pure form, it is commonly found in the form of oxides.

The most commercially viable source of aluminium is bauxite, which is predominantly found in tropical and sub-tropical regions of the world. Bauxite can be found as granules or rocks and it can vary in color, depending on its exact composition.

While the ore is generally easy to mine, the process to extract the aluminium from the bauxite is quite complex. The process of making metallic aluminium is carried out in two successive stages: a chemical process to extract alumina or anhydrous aluminium oxide (A1₂O₃) from the bauxite, and an electrolytic process to reduce the alumina to aluminium.

The bauxite, which contains about 75 per cent hydrated alumina
(A1₂O₃ • 3H₂O and A1₂O₃ • H₂O) is crushed and grounded into a powder and mixed with a solution of caustic soda. 1. This paste is mixed with further amounts of caustic soda solution in autoclaves or digesters. 2. There, under pressure and at a high temperature, the caustic soda combines with the hydrated alumina to leave a solution called sodium aluminate. Any impurities remain as undissolved residue.

The residue or "red mud", mainly the oxides of iron, silicon and titanium, is removed by sedimentation and filtration. 3. 4. The inert red mud is washed to recover the chemicals and is disposed of by a "wet stacking" technique over a specially prepared land area.

The sodium aluminate solution is then pumped into precipitator tanks where very fine and pure alumina trihydrate is added as "seed".

Under agitation and with gradual cooling the alumina trihydrate contained in the solution precipitates on the "seed". 5. The trihydrate solids are then separated from the caustic soda solution by settling and vacuum filtering. The caustic soda solution is recovered and returned to the start of the process to be reused in autoclaves.

The trihydrate solids are then passed through high-temperature (900-1100^°C) calciners that extract the chemically combined water that they contain. 6.

The aluminium oxide (Al₂O₃) that results is a white powder, like table salt in appearance, known as calcined alumina.

Four to five tonnes of bauxite are required to produce approximately two tonnes of alumina which, in turn, yield one tonne of aluminium.

On to Aluminium

An aluminium smelter comprises three main sectors: Carbon, Potlines and Casthouse.

This is where anodes are produced. Anodes are designed to be hung over electrolytic pots to carry the required electrical current. These anodes are a mixture of coke and petroleum pitch. Coke is crushed to a very precise granulometry, mixed with pitch to form a paste which is molded in vibro-compactors to produce raw anodes or "green" anodes. Raw anodes are then transferred to gas or oil-fired furnaces where they are baked for several days at high temperature (1100^° C).

Once baked, anodes are rodded, that is attached to aluminium stems from which they will hang over electrolytic pots.

Anodes burn and must be replaced about every twenty days; the carbon sector is also responsible for the recovery of carbon content in spent anodes (or butts) for recycling as well as for the cleanup of stems which will be reused. The smoke given off by the anode baking process is thoroughly treated in highly sophisticated systems.

POTLINES

Aluminium is obtained from alumina by electrolytic reduction - a chemical term meaning the removal of oxygen atoms from aluminium oxide. The calcined alumina is reduced to aluminium metal in electrolytic cells, or "pots", connected in series to a direct current power source. The cells are rectangular steel pots lined with refractory bricks and carbon blocks acting as the cathode.

The pot contains a molten electrolyte, called "bath" in which alumina is dissolved. The electrolyte is a mixture of cryolite (Na₃AlF₆), a molten salt, and certain additives to give it appropriate density, conductivity and viscosity. The principal additive is aluminium fluoride (AlF₃) which must be replaced from time to time due to losses through evaporation and a chemical reaction converting it into more cryolite. The emitted fluorides are collected and treated.

Suspended in the electrolyte are a number of anodes (positive electrodes) which act as electrical conductors for a high intensity direct current.

Electrical current passing from the anode through the electrolyte to the pot, which acts as a cathode, reduces the alumina molecules into aluminium and oxygen at a temperature of approximately 950^°C. This process is called electrolysis. The oxygen is released on the carbon anode, where it combines with the carbon to form carbon monoxide and carbon dioxide (CO and CO₂). The aluminium, being heavier than the bath, settles to the bottom of the pot. Considerable electrical energy, between 13 and 17 kilowatt-hours per kilogram of aluminium, is consumed in the process.

Each pot is tightly closed to achieve greater energy efficiency and to capture the pollutants emitted; gas treatment centers provide a very effective environmental protection.

At regular intervals, the molten aluminium is tapped from the bottom of the pot into large laddles and transferred to holding furnaces for casting.

CASTHOUSE

Molten metal from potlines is transferred to huge holding furnaces with a capacity of up to 60 tonnes, where it is refined and optionally mixed with metal additives to produce aluminium alloys.

While aluminium is sometimes used as is in its commercially pure form, most applications involve the addition of small quantities of other metals to create alloys with special properties. Certain alloying elements will increase strength or corrosion resistance, while others enhance such properties as machinability, ductility, weldability and strength at high temperature.

Recyclable aluminium beverage cans, for example, are made from an alloy containing manganese and magnesium that provide strength and formability. Magnesium is added to aluminium to create an alloy that provides the added strength required for the can top.

Alloys containing magnesium and silicon are very corrosion-resistant and suitable for use in window frames, doors and pleasure boats. Copper and zinc are added to produce alloys of the highest strength, while chromium, manganese or titanium are added for grain-size control. Recent research with metal matrix composite, a combination of aluminium and ceramic particles, promises stronger materials that are more cost-effective, stiffer and provide better resistance to wear.

Aircraft components are made from high-strength alloys containing copper, magnesium, silicon and zinc as the principal alloying elements. The aerospace industry employs the new aluminium-lithium alloys which provide significant weight savings over alloys of similar strength. Aluminium-magnesium-silicon alloys are used in architectural applications where pleasing, corrosion-resistant surface finishes are required. Automotive castings, strong and machinable, are made from aluminium-copper-silicon alloys.

Once the exact content of the molten aluminium has been analyzed, it is either cast into ingots, slabs or billets using the DC casting (direct chill) method, or cast directly into semi-finished products.Aluminium is cast into shapes that vary depending upon the type of equipment that will be used to process the metal. For example, very large ingots of rectangular shape, also called slabs, are intended for hot-rolling to produce plate, sheet and foil. Cylindrical ingots, also called billets, are for extrusion while metal for remelting can be cast into large blocks called sows, as well as tri-lock shapes or T-ingots, depending upon the shape. Each ingot can weigh up to 25 tonnes.

The process of flattening the ingot or slab is carried out by either hot- or cold-rolling the metal. In hot-rolling, the ingot is preheated so as to soften and/or homogenize it, and is then passed back and forth through massive rolls that reduce the ingot's thickness while increasing its length. The width remains unchanged. Hot-rolling improves the metallurgical qualities of the metal without appreciable work-hardening. Subsequent cold-rolling gives the strength characteristics that result from work-hardening, and the metal can be rolled to tighter dimensional tolerances.

Plate, which is hot-rolled, is 6.30 millimeters or more in thickness, while sheet, which is hot-and-cold-rolled, varies in thickness from 6.30 down to 0.15 millimeters. Foil is also cold-rolled to gauges below 6 microns. A continuous length of foil, 450 kilometers long, could be rolled from a single slab. The aluminium beverage can market is the primary end-user for aluminium sheet.

EXTRUDING

The extrusion process consists of pushing a pre-heated cylindrical aluminium ingot through a steel die. The ingot is formed into a continuous length of uniform cross-section by forced flow through the steel die. The outline of the die opening is reproduced in the extruded aluminium in much the same way as decorative icing is forced from a pastry pouch. Extruded tubing and hollow shapes are formed by placing a steel mandrel inside the die opening. The aluminium is forced to flow between the mandrel and the die, reproducing the shape of the mandrel on the inside of the section and the shape of the die on the outside.

Extruded tubing is used in the manufacture of such items as doors, window frames, building wall cladding, highway lighting standards and garden furniture. Larger extrusions are also used in the manufacture of railcars, truck trailers, aircraft and ship super-structures.

OTHER METHODS

Aluminium may be cast into various shapes by pouring molten metal into molds. The methods used are die casting, permanent mold casting or sand casting.

Forging - In this process, the desired part is formed in a confined die from a hot metal slug. This is achieved by applying force which causes the metal to flow and fill the die cavity.

Drawing - All aluminium wire and some tubing and rod products are manufactured by a cold-rolling process called drawing; a starter stock is pulled through a die in which it is both shaped and reduced. In the production of tubing, lengths of extruded, thick-walled tube are drawn through progressively smaller dies until reduced to the desired diameter.

Impacting - Also known as impact extruding, impacting is a combination of both extruding and forging. A disc-shaped slug of metal is placed in a die and struck by a punch; part of it is forged into a base, flange or hub, and the remainder is extruded upwards, downwards or sideways from the forged section.

Anodizing - Aluminium, particularly when intended for architectural purposes, may be anodized. Anodizing is an electro-chemical process whereby the natural oxide film on aluminium is thickened by passing an electric current through certain acid electrolytes with the aluminium part as the anode. Anodizing also provides a means of coloring the metal with dyes. This process also increases aluminium's hardness and corrosion resistance.