Electrolysis: processes – compositions used therein – and methods – Electrolytic process involving actinide series elements or... – Uranium
Reexamination Certificate
2000-08-01
2003-09-09
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic process involving actinide series elements or...
Uranium
C204S242000, C204S267000, C204S291000, C204S292000, C204S293000
Reexamination Certificate
active
06616826
ABSTRACT:
TECHNICAL FIELD
The present invention is in the field of electrodes and electrolysis apparatus, and includes inventions pertaining to the production of metals such as aluminum, lead, magnesium, zinc, zirconium, titanium, silicon, and the like by the electrolytic reduction of oxides or salts of the respective metals. More particularly, the invention relates to an inert type electrode composition, and methods for fabricating the electrode composition, useful in the electrolytic production of such metals.
This invention also relates to an inert-type electrode composition, and methods for fabricating the electrode composition, used in processes for generating energy from fossil fuels.
BACKGROUND
A number of industrial manufacturing processes require the use of a high-temperature, corrosion-resistant electrodes. Examples include the Hall-Heroult process for producing aluminum from alumina-bearing ores, and electric arc furnaces for the manufacture of steel and for the melting of refractory metals and ceramics. Industrial processes for energy generation from fossil fuels also require the use of electrode materials that are high-melting and resistant to degradation. Examples of the latter include energy production by the magnetohydrodynamic (MHD) process and solid oxide fuel cells.
It is known to produce aluminum by molten salt electrolysis of aluminum oxide dissolved in a bath of aluminum sodium fluoride (AlF
3
.3NaF) or so-called cryolite, by using a carbon anode. This electrolysis is usually conducted at about 900°-1000° C.
When aluminum is produced by using a carbon anode, the carbon anode is oxidized and consumed by about 330 kg theoretically and about 400-450 kg actually per ton of aluminum due to oxygen produced through the decomposition of aluminum oxide. For this reason, it is necessary to continuously adjust the position of the electrode to maintain it at a constant level, and it is also required to replace the anode by a new one before it is completely consumed. These are economical and operational defects.
In the electrolytic production of aluminum by the Hall-Heroult process a cryolite melt with Al
2
O
3
dissolved in it is electrolyzed at 940°-1000° C. The aluminum which separates out in the process collects on the cathodic carbon floor of the electrolysis cell whilst CO
2
and to a small extent CO are formed at the carbon anode. The anode is thereby burnt away.
For the reaction
Al
2
O
3+
3/2C→2Al+3/2CO
2
this combustion should in theory consume 0.334 kg C/kg Al; in practice however, up to 0.5 kg C/kg Al is consumed.
The burning away of the anodes has a number of disadvantages. In order to obtain aluminum of acceptable purity, a relatively pure coke with low ash content has to be used to produce the anode carbon. The pre-baked carbon anodes have to be advanced from time to time in order to maintain the optimum inter-polar distance between the anode surface and the surface of the aluminum. Periodically the pre-baked anodes when consumed have to be replaced by new ones. Soderberg anodes have to be repeatedly charged with new material. In the case of pre-baked anodes a separate manufacturing plant typically is necessary.
Accordingly, the manufacture of carbon anodes and their use in aluminum production is laborious and expensive.
The direct decomposition of A
2
O
3
to its elements:
Al
2
O
3
→2Al+3/2O
2
using an anode where no reaction with the oxygen takes place is therefore of greater interest. With non-reactive anodes, oxygen, which can be re-used industrially, is released, and the above mentioned disadvantages of the carbon anodes also disappear. This, anode is particularly favorable for a sealed furnace the waste gases of which can be easily collected and purified. Accordingly, in order to reduce greenhouse gas emissions and to allow for lower energy costs, manufacturers of aluminum have long sought inert anode materials to replace carbon in the Hall-Heroult Cell for aluminum production.
There are many concomitant requirements and considerations that must be satisfied in or to produce such replacement material:
1. It must be thermally stable up to 1000° C.
2. The specific electrical resistivity must be very small so that the voltage drop in the anode is a minimum. At 1000° C. the specific resistivity should be comparable with, or smaller than that of anode carbon. The specific resistivity should also be as independent of temperature as possible so that the voltage drop in the anode remains as constant as possible even when temperature changes occur in the bath.
3. Oxidizing gases are formed on the anode therefore the anodes must be resistant to oxidation.
4. The anode material should be insoluble in a fluoride or oxide melt.
5. The anode should have adequate resistance to damage from temperature change so that on introduction into the molten charge or when temperature changes occur during electrolysis it is not damaged.
6. Anode corrosion should be negligibly small. If nevertheless some kind of anode product should enter the bath then neither the electrolyte, the separated metal nor the power output should be affected.
7. On putting the anodes into service in the industrial production of aluminum, they must be stable when in contact with the liquid electrolyte, have no influence on the purity of the aluminum obtained, and operate economically. Obviously the number of materials which even approach fulfilling these extremely severe criteria is very limited.
8. The anode should have adequate mechanical strength.
In applications directed toward the electrowinning of metals such as aluminum or similar electrolysis reactions conducted at high temperature an inert anode thus must first be resistant to dissolution by cryolite-based melts. It must also be electrically conductive and mechanically robust. The replacement material must likewise be resistant to reduction by molten species, such as molten aluminum.
As an approach to obviate the above-mentioned defects in the carbon electrode, various non-consumable anodes have been developed. For example, a method using an oxygen ion-conductive anode consisting mainly of zirconium oxide has been proposed (British Patent Specification No. 1,152,124). This method, however, is disadvantageous in that it requires an apparatus for removing oxygen produced and the operation is complex. A method using an anode consisting of electronic conductive metal oxide containing at least 80% by weight of tin oxide has also been proposed (British Pat. Specification No. 1,295,117). This method is also disadvantageous in that the anode has poor chemical resistance to the molten salt.
In the Swiss Pat. No. 520 779 an anode made of ceramic oxide material in particular 80-99% SnO
2
is described. However this anode was shown to be problematic in that it showed a certain amount of loss and as a result of this the aluminum obtained amongst other things was made impure by the inclusion of tin which in most cases is undesirable.
As an improvement, U.S. Pat. No. 3,960,678 to Alder disclosed a process for operating a cell for the electrolysis of aluminum oxide with one or more anodes, the working surface of which is of ceramic oxide material. However, according to the patent, the process requires a current density above a minimum value to be maintained over the whole anode surface which comes in contact with the molten electrolyte to minimize the corrosion of the anode. This patent discloses SnO
2
, Fe
2
O
3
, Fe
3
O
4
, Cr
2
O
3
, Co
3
O
4
, NiO or ZnO as base materials. Without additives, SnO
2
cannot be made into a densely sintered product and it exhibits a relatively high specific resistivity at 1000° C. Additions of other oxides in a concentration of 0.01-20%, preferably 0.05-2% have to be made in order to improve such properties of pure tin oxide. To improve the sinterability, the compactness and the conductivity of the SnO
2
, Alder teaches additions of one or more of the oxides of the following metals are found to be useful: Fe, Cu, Mn, Nb, Zn, Co, Cr, W, Sb, Cd, Zr, Ta, In, Ni, Ca, Ba, Bi.
Numerous efforts have been made to p
Sandhage Kenneth H.
Snyder Robert L.
Standley & Gilcrest LLP
The Ohio State University
Valentine Donald R.
LandOfFree
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