The History and Future Challenges of Calcined Petroleum Coke Production and Use in Aluminum Smelting
Calcined petroleum coke (CPC) has been used for more than 120 years to produce the carbon anodes used in the Hall-Héroult aluminum electrolysis process. Several other forms of carbon were tested in the early stages, but none proved to have the right combination of low impurity levels, relatively low cost, and ready availability. Prebaked anodes are produced with 55–65% CPC, 13–15% coal tar pitch binder, and 20–30% recycled anode butts. The anodes are consumed at a net consumption rate of approximately 400 kg carbon/ton aluminum for modern smelting cells according to reaction 1. They provide an important source of heat for the electrolysis process, reducing the cell voltage and overall energy requirement relative to cells operating with nonconsumable anodes.1
CPC is produced by heating or calcining green petroleum coke (GPC) at temperatures greater than 1200°C. The production of GPC has remained essentially the same since 1929 when the modern delayed coking process was born. This was followed in 1935 by the development of the rotary kiln calcining process, which is the most commonly used technology in the Western world. CPC production has increased significantly in China over the last 10 years, and shaft calcining is the dominant technology used in this region. The aluminum industry has had a ready supply of good-quality GPC and CPC for many years, but the situation has become more challenging over the last 10 years due to a general trend toward higher impurity levels resulting from changes in crude and refining economics.
This article will review the history of GPC and CPC production and its use in aluminum smelting and provide comments on current and future challenges.
An excellent review of the history of aluminum production is provided in the Hall-Héroult Centennial book,2 which also includes a chapter on anode production. It was fortuitous that petroleum coke had been discovered about 30 years before Hall and Héroult made their independent discoveries on producing aluminum. Work had already been done on developing coke as a filler material in carbon products by Charles Brush and Washington Lawrence in 1877.3 Brush discovered that calcining coke prior to forming pitch-bonded shapes produced better quality carbons, and Hall used electrodes supplied by Brush in his early work. In Europe, purified anthracite was believed to have been used as an alternative to CPC during these first years of production.