Process for electrolytic production of aluminum

Abstract

A new method for the electrolytic production of aluminum, using a new electrolyte composition and low temperature operation, is provided. The electrolyte comprises a mixture of aluminum fluoride-potassium fluoride with from about 2 wt. % to 6 wt. % of alumina. The new electrolyte allows for the electrolytical reduction of alumina at temperatures as low as 700° C. The lower temperature allows for the use of inert anodes, and is conducive to the use of wetted cathodes. The new electrolyte mixture has a higher solubility for alumina and remains entirely liquid, even with 5 wt. % of alumina present in the electrolyte during electrolysis. Oxygen (O2) is the only gas generated during alumina electrolysis with the new electrolyte and the inert anodes, thus eliminating the production of greenhouse gases. Anodes and cathodes can be mounted in either a vertical, horizontal, or some other configuration.

Description

CONTRACTUAL ORIGIN OF INVENTION [0001] The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and the University of Chicago, representing Argonne National Laboratory.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to an electrolyte and a method for producing aluminum at lower temperatures, and, more specifically, this invention relates to an electrolyte that enables a method for producing aluminum at lower temperatures using a myriad of electrodes, including inert anodes and wettable cathodes. [0004] 2. Background of the Invention [0005] Primary aluminum is produced by the electrolysis of alumina in a cell (or pot) containing a molten fluoride salt based on the AlF3—NaF system. During electrolysis, carbon dioxide gas forms at the anode and escapes from the cell, while liquid aluminum metal forms at the cathode at the bottom of the cell. The electrolytic proces...

AI Technical Summary

Benefits of technology

[0013] Another object of the present invention is to electrolyze alumina to aluminum at temperatures lower than 800° C. A feature of the invention is an electrolyte salt bath based on a mixture of aluminum fluoride (AlF3)—potassium fluoride (KF) containing alumina (Al2O3) for aluminum production by electrolysis. An advantage of this invention is that it allows for the electrolytic production of aluminum at lower temperatures, and thus lowers costs including capital, operational, and environmental costs.

[0020] Still another object of the present invention is to provide a method of alumina electrolysis whereby the generation of greenhouse gases, e.g., carbon dioxide and fluorocarbons, is eliminated. A feature of the invention is that the anodes used are inert. An advantage of this feature is that the only gas generated during the electrolysis of alumina is oxygen (O2). An additional advantage is that costs are lowered even more due to the elimination of the carbon plant for the production of carbon anodes, and a reduced need for any emission controls.

Problems solved by technology

The electrolytic process (called the Hall-Héroult process) operates at temperatures in excess of 900° C. Such high temperatures are detrimental for a number of reasons, including deterioration of cell components.

Another drawback to typical aluminum production processes is the use of consumable carbon anode materials.

Anodes made of ceramic or cermet materials cause some practical engineering problems, such as poor thermal shock resistance, electrical connection, operational challenges and scalability etc.

However, work on metal anodes at high temperatures (i.e., between 900° C. and 1000° C.) has had only limited success.

However, slurry-cell operation causes operational difficulties and problems with metal collection, as well as the fact that very strict requirements on cell design are needed to maintain the alumina in suspension.

Another drawback to typical aluminum production processes is the need to maintain a sufficient molten aluminum metal pool at the bottom of the cell.

Significant operation difficulties occur when the carbon blocks on the cell bottom are exposed to the molten salt electrolyte bath.

The presence of this deep aluminum pool leads to process inefficiencies due to fluctuations at the bath-aluminum interface resulting from magnetic fields generated in the metal pool during electrolysis.

Still another drawback to typical aluminum production is the large dimensions or “footprint” of the typical alumina electrolytic cell.

This is due to the horizontal alignment of the electrodes, necessitated by the lack of suitable inert anodes and wetted cathodes required to maintain a stable anode-to-cathode distance in cells with vertically aligned electrodes.

Images