Electrode lowers cost of aluminium smelting

An engineer at Ohio State University has patented a potentially major improvement in aluminium production: an electrode that could cut smelting costs by one-fourth and air pollution by half.

Robert Rapp, Distinguished University Professor Emeritus of Materials Science and Engineering, leads a team of investigators who are using fuel cell technology to design a new electrode – specifically, an anode – for aluminium smelting.

The engineers have yet to build a model of the anode, and Rapp cautions that perfecting the technology would take many years and cost many millions of dollars.

‘This is a long shot with a very big potential payoff,’ he said.Today’s smelting operation works by electrolysis, Rapp said. Refrigerator-sized carbon anodes are submerged in a bath of very hot fused fluoride salt solution containing refined aluminium oxide, called alumina. The 1,000C-bath dissolves the alumina, and the anodes’ strong electric current plates out pure metallic aluminum.Today’s carbon anodes react with oxygen to release greenhouse gases – carbon dioxide and fluorocarbons – directly into the atmosphere. For each pound of aluminum, the process produces almost 1.5 pounds of carbon dioxide.

Rapp’s new anode concept would replace the carbon with zirconia tubes carrying natural gas. An applied DC voltage would draw oxygen through the tubes to oxidize the natural gas, which is mostly methane. The design would eliminate fluorocarbon production, cut carbon dioxide production by half, and use 40 percent less electricity.

The US Department of Energy sponsors Rapp’s collaboration with Kaiser Aluminum Corp., Siemens-Westinghouse Power Corp., and the Gas Research Institute. The goal is to develop an anode that smelters could retrofit into existing equipment.

Smelting is a power-hungry process, Rapp said. The United States, which accounts for only about 17 percent of the world’s aluminum production, manufactures about 3.4 billion kilograms of aluminum per year, and uses 44.2 billion kilowatt-hours of power to do it — close to the annual power consumption of New York City. Eliminating carbon anodes and reducing power consumption by 40 percent would cut aluminum smelters’ total production costs by 25 percent, Rapp said.

Whether Rapp’s design will work in practice depends on researchers resolving a number of issues, he said. One problem to be confronted is how to adjust the chemistry of the salt bath so that it continues to dissolve alumina, but not the zirconia tubes of the proposed new anode.

‘We’re between a rock and a hard place there, but through our studies we’ve really been learning more about the acid-base chemistry of this aggressive fused salt,’ Rapp said.Rapp hopes to be able to reformulate the composition of the salt bath so that his anode will survive immersion. Siemens-Westinghouse will fabricate and provide zirconia tubes for testing, he said.

Aside from its more obvious uses – beverage cans, home siding, gutters and downspouts and more – aluminium is commonly used in parts for trains, trucks, boats, airplanes, and – increasingly – cars. A lower cost for aluminum will benefit all these industries, Rapp said.

‘Even if my design doesn’t find its way into industry, I am optimistic that we will advance the knowledge on the subject of aluminum production, and explain certain questions that will be useful in general,’ such as the mechanism by which the salt dissolves alumina and other oxides, he said.