Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1999-04-13
2002-01-15
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C427S372200
Reexamination Certificate
active
06338785
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the starting up of cells for the electrowinning of aluminium by the electrolysis of alumina in a cryolite-based melt, which cell comprises a conductive cell bottom on which, in use, aluminium is produced and forms a layer or pool atop which is the molten cryolite electrolyte. The invention is particularly but not exclusively concerned with the start up of such cells where the cathode surface is protected by an aluminium-wettable refractory coating.
BACKGROUND OF THE INVENTION
Aluminium is produced conventionally by the Hall-Héroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950° C. A Hall-Héroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar or resins.
In Hall-Héroult cells, a molten aluminium pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation or sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
When they are put into service, aluminium electrowinning cells must be preheated. When the cell has reached a sufficient temperature, molten cryolite electrolyte is added and the start-up is continued until the cell reaches an equilibrium operating condition.
One known cell start-up procedure comprises applying a layer of coke or similar conductive material to the cell bottom and passing an electric current via anodes and through the coke into the cell bottom to heat the cell by the Joule effect. Another known cell start-up procedure uses flame burners. In U.S. Pat. No. 4,405,433 (Payne), it has been suggested that refractory fibrous materials of aluminium silicate be placed over refractory hard metal cathode assemblies prior to preheating of the cathode assemblies.
U.S. Pat. No. 5,651,874 (Sekhar/de Nora) has proposed coating the carbon cell bottom with particulate refractory hard material in a colloidal carrier to produce a hard adherent aluminium-wettable surface coating. These aluminium-wettable refractory coatings have by far outperformed all previous attempts to use such materials to protect carbon cell bottoms.
To facilitate cell start up, in particular when using these improved coatings, it has already been proposed to place an aluminium sheet on top of the coating before preheating (see Cathodes in Aluminium Electrolysis, 2nd Edition, 1994, M. Sorlie and H. Oye, published by Aluminium Verlag, page 70). The purpose of this aluminium sheet was to avoid possible hot-spots due to uneven current distribution. Because of the high current densities employed and the need to ensure an even heat distribution, aluminium sheets with a thickness of 1 to 5 mm were used. This aluminium sheet melts during the start-up procedure and is integrated into the pool of product aluminium.
However, it has been found that whereas use of such aluminium sheets has been effective in reducing hot-spots, they do not protect against oxidation of the cathode. The use of thick aluminium sheets has not addressed this problem.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a start-up procedure which is entirely reliable as regards the avoidance of any damage to the cathode surface by using a material which when applied to the cathode forms a thin and uniform protective layer.
Another object of the invention is to protect the cathode by covering it with a temporary protective material before preheating the cell. A further object of the invention is to provide a temporary protective material which is at least partially eliminated by the beginning of normal use of the cell, such that it does not contaminate the product aluminium with the temporary protective material.
The invention in particular relates to a method of starting-up a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based melt such as cryolite, the cell comprising a cathode on which, in use, aluminium is produced and forms a layer or pool. The start-up method comprises applying one or more aluminium-containing start-up layers on the cathode surface followed by preheating the cell, the start-up layers temporarily protecting the cathode surface during start-up.
The method of the invention comprises applying at least one pliable foil of aluminium which comes into and remains in intimate matching contact with the cathode surface during preheating the cell and/or applying at least one aluminium-containing metallization which is applied and remains in intimate matching contact with the cathode surface during preheating the cell. The start-up layer(s) temporarily protect(s) the cathode against chemical attack by reaction with gases and/or fluids such as melting electrolyte during cell start-up.
The start-up layer(s) form(s) a temporary protection against damage of chemical or chemical/mechanical origin to the cathode, this temporary protection being in intimate contact with the cathode surface and being usually at least partly eliminated before or during the initial normal operation of the cell. The temporary protection may be “washed away” by normal operation of the cell or permanently integrated into the cathode surface.
In contrast to the prior art, the temporary protection remains in intimate contact with the cathode surface below a layer of molten aluminium during the cell start-up. When only a thick sheet of aluminium is applied on the cell bottom, the applied layer melts during start-up and is merely integrated to the pool of product aluminium without preventing melting electrolyte from attacking the aluminium-wettable coating.
For the purpose of this invention, start-up layers may for example be obtained from the following materials: at least one pliable foil of aluminium having a thickness of less than 0.1 mm; and/or an applied metallization of aluminium or an alloy or an intermetallic compound comprising aluminium and at least one further metal selected from nickel, iron, titanium, cobalt, chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenum, cerium and copper.
In combination with the aluminium-containing start-up layers, additional start-up layers may be used such as:
a) a boron-containing solution forming a glassy layer;
b) a polymer or a polymer precursor;
c) a solution containing phosphates of aluminium;
f) a colloid; and combinations of the aforesaid.
Normally the cell bottom is made of carbonaceous material such as carbon blocks. The cathode mass can be made mainly of carbonaceous material, such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Pat. No. 5,362,366 (Sekhar et al), prebaked carbon blocks assembled together on the shell, or graphite blocks, plates or tiles.
It is also possible for the cathode to be made mainly of an electrically-conductive carbon-free material, of a composite material made of an electrically-conductive material and an electrically non-conductive material, or of an electrically non-conductive material.
Such non-conductive carbon-free materials can be alumina, cryolite, or other refractory oxides, nitrides, carbides or combinations thereof and the conductive materials can be at least one metal from Groups IIA, IIB, IIIA, IIIB, IVB, VB and the Lanthanide series of the Periodic Table, in particular aluminium, titanium
de Nora Vittorio
Duruz Jean-Jacques
Liu James Jeng
Sekhar Jainagesh Akkaraju
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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