Hall–Héroult Process - Process

Process

Aluminium cannot be produced by the electrolysis of an aluminium salt dissolved in water because of the high reactivity of aluminium with the protons of water and the subsequent formation of hydrogen. As, in an aqueous solution, protons (H+) are preferentially reduced to atomic hydrogen before Al3+ ions, the reduction of Al3+ is done by electrolysis of a molten aluminium salt. This is a water free medium, and hence, H+ reduction is avoided.

In the Hall–Héroult process alumina, Al₂O₃, is dissolved in an industrial carbon-lined vat of molten cryolite, Na₃AlF₆ (sodium hexafluoroaluminate), called a "cell". Aluminium oxide has a melting point of over 2,000 °C (3,630 °F) while pure cryolite has a melting point of 1,012 °C (1,854 °F). With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1,000 °C (1,830 °F). Some aluminium fluoride, AlF₃ is also added into the process to reduce the melting point of the cryolite-alumina mixture.

The molten mixture of cryolite, alumina, aluminium fluoride is then electrolyzed by passing a direct electric current through it. The electrochemical reaction causes liquid aluminium metal to be deposited at the cathode as a precipitate, while the oxygen from the alumina combines with carbon from the anode to produce carbon dioxide, CO₂. An electric potential of three to five Volts is needed to drive the reaction, and the rate of production is proportional to the electric current. An industrial-scale smelter typically consumes hundreds of thousands of Amperes for each cell.

The oxidation of the carbon anode reduces the required voltage across each cell, increasing the electrical efficiency, at a cost of continually replacing the carbon electrodes with new ones, and also the cost of releasing carbon dioxide into the atmosphere. Hundreds of Hall-Heroult cells are usually connected electrically in series, and they are supplied with direct current (DC) from a single set of rectifiers that convert the alternating current (AC) supplied to the factory into direct current. The very high electric current is supplied to the cells through heavy, low electrical resistance metal busbars made of pure aluminium or copper. The cells are electrically heated to reach the operating temperature with this current, and the anode regulator system varies the current passing through the cell by raising or lowering the anodes and changing the cell's resistance. If needed any cell can be bypassed by shunt busbars.

The liquid aluminium is taken out with the help of a siphon operating with a vacuum, in order to avoid having to use extremely high temperature valves and pumps. The liquid aluminium then may be transferred in batches or via a continuous hot flow line to a location where it is cast into aluminium ingots. The aluminium can either be cast into the form of final cast-aluminium products, or the ingots can be sent elsewhere such as a rolling mill to be pressed into sheets, or a wire-drawing mill producing aluminium wires and cables.

While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser than the molten cryolite at temperatures around 1,000 °C (1,830 °F), and the aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The tops and sides of the cells are covered with layers of solid cryolite which also act as thermal insulation. The unavoidable electric resistance within each cell produces sufficient heat to keep the cryolite-alumina mixture molten.

With the percentage of aluminium dissolved in each cell being depleted by the electrolysis in the molten cryolite, additional alumina is continually dropped into the cells to maintain the required level of alumina. Whenever a solid crust forms across the surface of the molten cryolite-alumina, this crust is broken from time to time to allow the added alumina to fall into the molten cryolite and dissolve there.

The electrolysis process produces exhaust which escapes into the fume hood and is evacuated. The exhaust is primarily CO₂ produced from the anode consumption and hydrogen fluoride (HF) from the cryolite and flux. HF is a highly corrosive and toxic gas, even etching glass surfaces. The gases are either treated or vented into the atmosphere; the former involving neutralization of the HF to its sodium salt, sodium fluoride. The particulates are also captured and reused using electrostatic or bag filters. The remaining CO₂ is usually vented into the atmosphere.

The very large electric current passing through the electrolytic cells generates a powerful magnetic field, and this can stir the molten aluminium with magneto-hydrodynamic forces in properly-designed cells. The stirring of the molten aluminium in each cell typically increases its performance, but the purity of the aluminium is reduced, since it gets mixed with small amounts of cryolite and aluminium fluoride. If the cells are designed for no stirring, they can be operated with static pools of molten aluminium so that the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle.

Aluminium smelters are usually sited where inexpensive hydroelectric power is available. For some European smelters, the electric power produced by hydroelectric plants in countries such as Norway, Switzerland, and Austria is transmitted by high-voltage power lines to such places as Denmark, Sweden, Germany, and Italy to be used by aluminium and magnesium factories. Since aluminium factories require nearly-uniform supplies of electric current, they make the most of nearly-constant supplies of electric power, and these are also available close to many hydroelectric power plants. To give an example of such use of hydroelectric power, the three main regions for aluminium production in North America have always been in the Tennessee River Valley of the Southeastern United States, the Columbia River Valley of Washington and Oregon, and the St. Lawrence River Valley of southeastern Canada and the Northeastern United States.

Many decades ago, before the existence of the Tennessee Valley Authority, aluminium companies such as Alcoa even built their own hydroelectric dams and powerhouses in the Appalachian Mountains of North Carolina and Tennessee.