The commercial production of the aluminum in the world has been by the Hall-Heroult process. In this well-known process a purified source of alumina is dissolved in a molten all fluoride salt solvent, primarily consisting of cryolite and then reduced electrolytically with a carbon anode according to the reaction EQU 1/2Al.sub.2 O.sub.3 +3/4C+3e.fwdarw.Al+3/4CO.sub.2
and EQU 1/2Al.sub.2 O.sub.3 +3/2C+3e.fwdarw.Al+3/2CO.
Three characteristics of this system which are inherent in the Hall-Heroult process include: first, carbon dioxide is produced and the carbon anode is consumed at the rate of 0.33 to 1 pound of carbon per pound of aluminum produced which results in a required continual movement of the carbon anode downwardly toward the cathode aluminum pool at the bottom of the cell to maintain constant spacing for uniform aluminum production and thermal balance in the cell; second, the need to feed intermittently and evenly the solid alumina in a limited concentration range to the "open type" cell to maintain peak efficiency of operation in order to avoid "anode effects"; third, severe corrosion of cell materials due to the high temperatures of 950.degree.-1000.degree. C. and the fluoride salts resulting in low cell life and increased labor.
A fourth characteristic not inherent in the system but present nonetheless is that the cell power efficiency is limited to less than 50% due to the practical requirement of maintaining a carbon anode to liquid aluminum distance greater than one inch to reduce the magnetic fields' undulation of the aluminum layer causing intermittent shorting with resultant Faradaic losses due to the back reaction of aluminum droplets with carbon dioxide, EQU Al+3/2CO.sub.2 .fwdarw.1/2Al.sub.2 O.sub.3 +3/2CO.
The first three inherent limitations of the conventional Hall-Heroult process can potentially be overcome either by use of an aluminum chloride electrolysis process which in the prior art would directly produce aluminum and chlorine gas or through the use of all fluoride bath at temperatures of 700.degree.-750.degree. C. for the direct reduction of aluminum oxide.
The potential advantages of an aluminum chloride salt electrolysis process include: (1) chloride salts which are more economical than the fluorides of the Hall-Heroult salts, have a lower operating temperature of 700.degree.-800.degree. C. and are much less corrosive. This results in more economical cell construction materials with an attendant longer cell life; (2) the aluminum chloride electrolysis process requires a closed system reducing air pollution problems; (3) the chloride electrolytes, even at the lower operating temperature of 700.degree.-800.degree. C., have higher conductivities than that of the Hall-Heroult fluoride salts at 950.degree.-1000.degree. C. This results in the production of aluminum at lower energy consumption and at higher power and current efficiencies; (4) the use of the aluminum chloride electrolysis process has a very broad operating range of aluminum concentration which results in no "anode effect"; (5) it is possible to design the aluminum chloride electrolytic process cell with bipolar electrodes which result in a much more compact cell with increased production potential per unit volume.
There are, however, potential advantages to the use of an all fluoride bath if it is possible to use the Hall-Heroult system and yet continues to deposit metal. The all fluoride bath potentially: (1) avoids substantial structural changes in the cell if the aluminum oxide can be directly reacted thereby making unnecesary the requirement of the chloride system to close the top of the cell and (2) does not evolve any corrosive, noxious gas, merely CO.sub.2. To achieve these advantages the all fluoride bath must be used at low temperatures of 700.degree.-800.degree. C. but such is not possible in accordance with prior art techniques because alumina, unlike aluminum chloride, will not readily dissolve at such low temperatures.
In the comparison of the commonly used Hall-Heroult alumina-fluoride process and the much less familiar aluminum chloride process, there appear to be significant benefits in the use of the aluminum chloride process, but a fair comparison should not overlook the significant disadvantage of the aluminum chloride electrolytic process in producing large quantities of the corrosive gas chlorine liberated at the anode. The chlorine entrains the chloride electrolyte to clog the exit ports and deplete the bath. This entrained electrolyte must be collected and returned to the cell and the liberated chlorine must be recycled to produce further aluminum chloride.
Although the potential advantages of utilizing an aluminum chloride electrolysis process for the electrolytic production of aluminum have been recognized for well over a century, commercial realization of such a process has not occurred.
In general, the usual process known to the prior art for producing aluminum chloride has been the conversion of an alumina-containing material with chlorine in the presence of carbon to yield aluminum chloride and a mixture of the gases carbon dioxide and carbon monoxide. This reaction EQU (Al.sub.2 O.sub.3 +C+Cl.sub.2 .fwdarw.AlCl.sub.3 +CO.sub.2 and CO)
has been carried out under a wide range of conditions, each variation having some alleged advantage. All of these procedures for producing aluminum chloride have a common thread however. Each involves the use of a source of carbon, a source of chlorine, and an aluminum chloride reactor separate from the electrolytic cell in which the metallic aluminum is electrolytically produced.
The normal reaction temperature for the production of aluminum chloride is generally in the range of 400.degree. C. to 1000.degree. C. depending upon the form of the reacting agents. Unless a high purity alumina source is used, other elements that are generally present such as iron, silicon, and titanium, are also chlorinated and must undergo difficult separation from the aluminum chloride. This contributes to the size and cost of the aluminum chloride producing plants.
The aluminum chloride electrolytic process would have an unusual advantage beyond those advantages heretofore cited if it were possible to avoid both the chlorine collection and the independent production of aluminum chloride in a plant separate from the electrolysis plant.
The electrodeposition of aluminum by the direct reduction of alumina in an all fluoride bath is an attractive alternative to the aluminum chloride system provided that the alumina would dissolve at the low temperatures of 700.degree.-800.degree. C. rather than the 950.degree.-1000.degree. C. considered to be required for dissolution. Existing Hall-Heroult cells could be used without substantial capital expenditures and great energy savings would be possible with such an all fluoride bath but no such process for the electrodeposition of aluminum is available to those skilled in the art.
The fourth disadvantage of the Hall-Heroult cell, cell power efficiency, has been considered by those skilled in the art but it appears that the practical limit to energy savings and efficiency in present Hall-Heroult cells has been reached through careful design and operation of 150 to 225 Kamp cells at anode current densities between 4.5 and 5.5 amps/in.sup.2. The lower energy limit appears to be about 5.6 to 6.0 Kwh/lb utilizing the most advanced designs, computer controls, bath modification and other improvements.
A serious penalty from decreasing the anode current density in the high amperage cells is that less aluminum is produced per unit size although it is at lower energy. This results in a higher capital cost per ton of capacity and a slower return on investment, even though lower fixed cost of operation is achieved. Thus, the greater energy efficiency no longer offsets the increased capital.
As the cell size has increased from about 60 to 100 K amp to beyond about 225 K amp severe problems adversely affecting voltage stability, current efficiency, and cell lining life occur because of the large electro-magnetic effects and heat dissipation problems.
Because of the above factors, further increases in energy efficiency of Hall-Heroult cells comparable to those attained in the past should not be expected without radical changes in cell design and chemistry of the basic reation. Due to the large capital investment in existing cells there is disincentive to make any radical changes that cannot be readily accomplished within the existing cells. Examination within these criteria reveals that the area which will yield the greatest benefits for energy reduction is to reduce the substantial IR loss in the electrolyte between the anode and cathode. Currently, this spacing is about 1.75 inches and accounts for about 50% of the overall ohmic losses. Reducing this spacing from 1.75 inches to 0.75 to 0.5 inch spacing range could reduce overall energy consumption in the range of 20% to 25%. However, with close spacing, the large magnetic field effects inherently present produce undulations at the surfaces of the cathode aluminum pool, resulting in intermittent contact with the anode and short circuiting. It is possible to utilize a drained cathode made from titanium diboride (TiB.sub.2) because of its wettability. This concept utilizes TiB.sub.2 in various configurations to achieve a narrow spacing between the anode and cathode, and the reduced aluminum wets the TiB.sub.2 draining off into a pool. The subject of drained cathodes has received renewed interest due to its potential energy savings as shown by the following patents and publications:
British Patent Nos. 784,695; 784,696; 802,471; 802,905; PA1 U.S. Pat. Nos. 3,028,324; 3,400,061; 4,071,420; PA1 C. E. Ransley, "The Application of Refractory Carbides and Borides to Aluminum Reduction Cells". The Extractive Metallurgy of Aluminum (Vol. 2, 1963) at 487-507; PA1 R. A. Alliegro, "Boride and Boride-Steel Cathode Leads", Ibid. at 517-524; PA1 D. J. McPherson, "Changing Aluminum for the Nineties", J. Metals (August 1978) at 19-20.
Such approaches will reduce energy consumption in Hall-Heroult cells. However, there are two major disadvantages to such approaches. First, precision TiB.sub.2 shapes are required which are expensive and, second, in large cells the bottom will move due to expansion and contraction, which in conjunction with salt and aluminum absorption will result in a spacing change between the anode and cathode along the length of a cell and in some cases within the dimensions of a single anode. Such spacing changes cannot be tolerated within the limits of the 0.75-0.5 inch spacing. Such irregularities will cause some anodes to dissolve much faster than others and cause localized current concentrations resulting in unstable conditions in cell operation particularly with respect to temperature conditions.