Advances in Industrial Energy-Efficiency Technologies
Prepared for:
U.S. Department of Energy
Office of Industrial Technologies
Washington, DC 20585
Produced by:
National Renewable Energy Laboratory
Golden, Colorado 80401-3393
In conjunction with:
Energetics, Inc.
Columbia, Maryland 21046
DOE/CH10093-140
DE92001232
September 1992
For the past 100 years, all primary aluminum in this country has been produced in Hall-Heroult electrolytic cells that electrochemically reduce alumina to aluminum metal via carbon anodes and molten aluminum cathodes. This very capital- and energy-intensive process accounts for 2% and 3% of the electricity used in the United States every year. The aluminum industry has suspected for several decades that cathodes made from a compound called titanium diboride would be superior to molten aluminum, but has been frustrated by technical and economic difficulties in its efforts to develop them.
Despite these difficulties, interest in titanium diboride cathodes has remained high because of their great potential for significant energy and cost savings in the production of primary aluminum. These savings could in turn improve the position of the U.S. aluminum industry, which faces intense competition in both domestic and international markets. In particular, use of the new cathode would increase the competitive advantage of U.S. producers over major foreign producers in countries where electricity is much cheaper, like venezuela and Brazil.
Photo: Scientists have developed a new material-titanium diboride graphite-for the cathodes used in aluminum production. Cathodes made with this material could save 20% of the energy used by conventional technology. [provided in source document]
The unique properties of titanium diboride allow engineers to make several improvements in the cathode. One of the most important improvements is the ability to decrease the voltage difference between the anode and cathode.
The molten aluminum pool that acts as the cathode in the conventional Hall-Heroult cell fluctuates in height due to waves formed by electromagnetic and hydrodynamic effects. This unstable surface of the cathode can cause short-circuiting of the cell. To avoid this effect, conventional cells separate the anode and cathode by a large gap, resulting in high voltage differences and low average energy efficiencies for the cells of less than 50%.
In contrast, the titanium diboride cathodes can be wetted with a thin film of aluminum, which then drains to a sump. This provides a stable cathodic surface, allowing the cell to operate with a much narrower anode-cathode gap. The narrower gap results in a significant decrease in the voltage difference between the anode and the cathode, thus improving energy efficiency. This and other improvements result in the potential to conserve up to 20% of the energy used by a conventional cell.
To date, a major cause of the lack of success in producing titanium diboride cathodes has been material failure. In 1988, the U.S. Department of Energy's Office of Industrial Technologies (OIT) signed a cooperative agreement with Great Lakes Research Corporation (GLRC) to test and evaluate potential cathode materials made from titanium diboride and graphite (TiB2-G), a combination that researchers believed would be superior to the titanium diboride materials previously investigated. In late 1990, a follow-up effort among OIT, Reynolds Metals, and GLRC was initiated to produce and test cathode elements made from the new material.
Photo: Existing Hall-Heroult reduction cells can be retrofitted with the new titanium diboride-graphite cathodes, with a potential energy savings nationwide of 1500 megawatts, the size of a large power plant. [provided in source document]
The new TiB2-G cathode material developed by GLRC represents a significant improvement over other titanium diboride materials in both lifetime and implementation costs. As part of the research, a number of different cathode shapes were designed, produced, and tested. The researchers conducted long-term testing (lasting from 3 to 21 months) of sample cathodes in an operating Reynolds plant to analyze cell operation and cathode life. More than 50 cathode samples representing a variety of shapes, two manufacturing techniques, and several alternative sources of titanium diboride powder were tested. The results of these tests helped the researchers determine the best cathode shapes and production methods.
Scientists then tested the cathodes in a pilot Hall-Heroult cell at a Reynolds Metal Company laboratory. The purpose of these tests was to determine operating data for several of the more successful shapes tested in the plant. By using the pilot cell, researchers were able to test full-size cathode shapes to determine voltage and energy savings, economics, cell design, and optimum operating conditions. The results of all of the tests demonstrated that the TiB2-G materials were superior to other titanium diboride materials in terms of:
Reynolds and GLRC estimate that TiB2-G cathode technology has the potential to lower the power consumption of Hall-Heroult cells by 1.5 kilowatt hours per pound of aluminum, equivalent to approximately $0.037 per pound. The cost of retrofitting a commercial cell with the new cathode is projected to be $0.024 per pound, resulting in a production cost savings of from $0.008 to $0.013 per pound. Applied to the total U.S. annual production of primary aluminum, use of the new cathode would reduce the electric power consumption of the aluminum industry by the equivalent of a large (1500 megawatt) power plant each year. This translates into a dollar savings as high as several hundred million dollars. Because these cathodes are a retrofit technology, their implementation would also enhance the ability of the U.S. aluminum industry to optimize the use of existing equipment and facilities.
At the conclusion of the pilot-cell tests, the new cathodes were ready to be installed in a commercial facility, with start-up scheduled for late 1991. At that time, however, the manufacturer was forced to close this facility because of economic conditions. GLRC and the manufacturer are now considering relocating the retrofitted cells to another site for further testing.
For More Information:
Materials Processing Program
CE-231
Office of Industrial Technologies
U.S. Department of Energy
1000 Independence Ave., SW
Washington, DC 20585
(202) 586-6750
Last Updated: February 13, 1996