Hoopes process

The Hoopes process is a metallurgical process, used to obtain aluminium metal of very high purity (about 99.99% pure). The process was patented by William Hoopes, a chemist of the Aluminum Company of America (ALCOA), in 1925.^[1]

Introduction

It is a method used to obtain aluminium of very high purity. The metal obtained in the Hall–Héroult process is about 99.5% pure, and for most purposes it is taken as pure metal. However, further purification of aluminium can be carried out by the Hoopes process. This is an electrolytic process.

The process

A Hoopes cell consists of an iron tank lined with carbon at the bottom, containing three layers of molten metal; an alloy of copper, crude aluminium and silicon on the bottom (serving as the anode), a mixture of fluorides of sodium, aluminium and barium (cryolite + BaF₂) in the middle, and pure aluminium at the top. A set of graphite rods dipped in molten aluminium serve as the cathode. During electrolysis, aluminium is oxidised to Al³⁺ ions in the bottom layer. These ions migrate up through the middle layer before being reduced back to aluminium at the top layer. There, the aluminium can be tapped off at roughly 99.99% purity.