Although aluminum is the element next most abundant to oxygen and silicon in the earth's crust, the recovery of the metal is a distinctively modern metallurgical feat.
For nearly 100 years aluminum has been known in the metal state, but only since 1886 has the production of the metal been 4 such a scale that it has been of commercial use. The reason why we have not had aluminum before is because of the difficulty 4 freeing it from the oxygen with which it is so intimately associated in nature.
Aluminum is one of the most useful of the common metals, and with its application to a great variety of purposes, the necessary facilities for its production by low-cost electricity continually are being augmented.
We are able to reduce aluminum from a mixture of the double chlorides of aluminum and sodium by the use of metallic sodium at a red heat. If a mixture of chemically prepared anhydrous chlorides with cryolite - a sodium-aluminum fluoride - is mixed with metallic sodium and charged suddenly into a hot reverberatory furnace, the reduced aluminum finally can be tapped out of the furnace into ingots. This entire process is very expensive and probably is not used at the present time.
The Hall process is the one most extensively used. This process consists in electrolizing aluminum oxide in a molten bath of cryolite with large carbon anodes to supply the current. The carbon electrodes necessarily are oxidized by the oxygen freed from the reaction; the aluminum collects in a layer below the fused bath and is tapped out at intervals into molds.
The process for making aluminum is extremely simple but is operated only with much attention to many critical details. The electric current through the furnace must be exactly right to keep the bath molten, as well as to effect the electrolytic decomposition of the aluminum. Only since we have been able to make suitable carbon electrodes has the process been possible commercially. Another extremely important detail is the exact chemical composition of the bath, which must be lighter than the fused metal; the two substances are of almost the same density in the molten state, and, unless the fused salt is kept slightly less dense than the aluminum, the latter would float, and not only burn but, of course, would disrupt the process entirely.