Electrochemistry of the Hall-Heroult Process for Aluminum Smelting W. E. Haupin
Aluminum Corp. of America, P.O. Box 772, Freeport Road, New Kensington, PA 15068
Aluminum is the most abundant metallic element on the surface of the earth, hut it is never found free in nature. Its many desirable physical, chemical, and metallurgical properties make i t the most widely used nonferrous metal. The aluminum industry, second only to steel in volume of metal production, is the world's largest electrochemical industry. Bauxite, which contains 20-30% aluminum as hydrated oxide and hydroxide, is the principal ore. I t is chemically purified by the Bayer process to 98+% pure aluminum oxide (alumina). The Bayer process ( I ) makes a very interesting chemical engineering study. The process employs the unit operations: crushing and grinding, digestion, heat exchange, clarification, filtration, precipitation, evaporation, and fluid bed calcination. Electrolysis of Alumina Nearly all aluminum is produced by the electrolysis of alumina dissolved in a molten cryolite-based electrolyte, the Hall-Heroult Process. A typical modern aluminum reduction cell (2) consists of a rectangular steel shell, 9-12 m long by 3 4 m wide and 1-1.2 m high. The shell is lined with refractory thermal insulation that surrounds an inner lining of carbon to contain the highly corrosive fluoride electrolyte and molten aluminum. The thermal insulation is adjusted to provide sufficient heat loss to form a protective ledge of frozen electrolyte on the inner walls hut not on the bottom of the cell cavity, which must remain bare to provide electrical contact with the molten aluminum cathode. Electric current enters the cell through prehaked carbon anodes (Fig. 1) or through a continuous self-baking Soderberg anode (Fig. 2). A crust of frozen electrolyte and alumina covers the top of the cell around the anodes. The anode-to-cathode spacing ranges from 3-6 cm. Steel current collector bars keyed into the carbon lining carry the electric current from the cell. Aluminumcontaining ions discharge into a molten aluminum pool resting on the carbon lining that also holds the melt. Simultaneously, oxygen-containing ions discharge on and consume the cell's carbon anodes. Today's cells have a current capacity in the range of 50-250 X lo3 amp. Prehaked anodes are molded from petroleum coke and coal tar pitch binder into blocks typically 70-cm wide by 125-cm long and 50-cm high. These blocks are then baked to 10001200°C. Petroleum coke is used because of its high purity. Impurities such as iron andlsilicon deposit on the aluminum, while less noble impurities such as calcium and magnesium tend to accumulate as fluorides in the bath. Steel stubs keyed into the anodes, either with cast iron or with a carbonaceous ramming mix, both support and conduct electric current into the anodes. The electrical resistivity of prehaked anodes ranges from 0.005-0.006 ohm cm. Anode current density ranges from 0.6-1.3 A Soderberg anodes are formed continuously from a paste of petroleum coke and coal tar pitch. This mixture is added to the top of a rectangular steel casing, typically 6-8 m long by 2 m wide and 1 m high. While passing down through the casing, the paste bakes forming carbon to replace the carbon being consumed a t the bottom. A baked portion extends past the casing into the molten electrolyte. Electric current enters the anode through vertical
-
Figure 1. Aluminum electrolysis cell with prebaked anode. current Supplying Pi".
Figure 2. Aluminum 6lectrolysis cell with Soderberg anode.
or sloping steel pins or spikes. Periodically the lowest mikes density typically is lower, ranging from 0.60.9 A ~ m - ~ . Alumina is added in appropriate increments, either manually or by automatic feeders. Molten aluminum is removed from the cells, generally daily, by siphoning into a crucible. Electrolyte Chemistry The electrolyte is a solution of aluminum oxide in molten cryolite containing an excess of aluminum fluoride over the 3NaF.AlF3 cryolite composition. The mole ratio NaF/A1F3 is defined as the cryolite ratio while the mass ratio of NaF/A1F3 is called the bath ratio. Numerically, the bath ratio is half the cryolite ratio. Pure cryolite melts at 10IZ°C, hut alumina and aluminum fluoride lower the melting point as shown in Firmre
.
and emitted in the off-gases at a rate equal to its introduction. Calcium fluoride lowers the liquidus temperature about 29°C Volume 60 Number 4
April 1983
279
however, using cyclic voltametry, stationary electrode polarography, differential pulse polarography, and chronopotentiometry found evidence only for three electron transfer processes. He found no evidence for a chemical reaction following or preceding the electron transfer process. This ruled out both discharge of Na+ at low activity followed hy chemical reaction to form aluminum and also dissociation to form'A13+ ions prior to discharge. A1Fe3- = AI3+
+ 6F-
(6)
T h e latter mechanism is, however, supported by Rolin (9). Bowman's findings support the cathode reaction Figure 3. The Na3AIFs-AiF3-Also3system
with the F- ions neutralizing the charge of the current carrying Na+ ions. per weight percent calcium fluoride. Sometimes lithium fluoride is added to increase the electrolyte conductivity and further lower the liquidus temperature. Molten cryolite ionizes
Anode The primary anode reaction may he represented simplistically hy C
The hexafluoroaluminate ion undergoes further dissociation A l F e 3=A I F ,
+ 2F-
Dissolution of Alumina Virtually any addition to molten cryolite reduces alumina solubility. Since rapid dissolution and a high activity of alumina are desirable, additives usually are limited to less than 10 weight percent. When alumina dissolves, a strong chemical interaction takes place between the solute and solvent causing melt properties to change (5).The ev~dencesuggests several oxygen-containing anions. Complex, rather bulky oxyfluoroaluminate ions, A120,F,6-2i-Y, are likely rather than the simple aluminates such as AlOa-, A10+, A12042-, or A10&. Prohably a single oxygen atom is present per complex ion a t low concentrations of alumina and two a t higher concentrations. Gilbert et al. (6) found by Raman spectroscopy that all oxygen species bad bridging AI-0-A1 bonds. Reactions (4) and (5) present dissolution mechanisms consistent with the above.
The solution mechanism of eqn. (4) would be favored by low alumina (
