WO1994012694A1

WO1994012694A1 - Casing for a self-baking anode for electrolytic cells for production of aluminium

Info

Publication number: WO1994012694A1
Application number: PCT/NO1993/000177
Authority: WO
Inventors: Odd Olsen; Adolf Karsten Syrdal
Original assignee: Elkem Aluminum ANS
Current assignee: Elkem Aluminum ANS
Priority date: 1992-11-30
Filing date: 1993-11-25
Publication date: 1994-06-09
Anticipated expiration: 1995-05-30
Also published as: AU5660494A; NO924609D0; NO924609L; NO176769C; NO176769B

Abstract

The present invention relates to an anode casing for Søderberg anodes for use in electrolytic aluminium reduction cells, wherein at least the lower and inner part of the anode casing is made from a ceramic material or refractory castable which is resistant against the atmosphere and the temperatures in the electrolytic aluminium reduction cell.

Description

Title: Casing for a self-baking anode for electrolytic cells for production of aluminium.

Technical field

The present invention relates to an anode casing for self-baking anode, a so-called Søderberg anode, for use in electrolytic cells for production of aluminium.

Background art

Electrolytic cells or furnaces for production of aluminium according to the Hall- Herould method, comprises a generally rectangular, low, flat shell with refractory material and carbon blocks in its sides and bottom. The carbon blocks constitutes a vessel for the produced aluminium and for the molten electrolyte. The carbon blocks in the bottom of the vessel are equipped with steel bars for electric coupling of the bus bars for the electric current. The bottom carbon blocks thus form the cathode for the electrolytic cell.

The molten electrolyte, which has a lower density than molten aluminium, consists of molten cryolite, certain inorganic salts, such as for example aluminium fluoride and calcium fluoride, and dissolved aluminium oxide. Aluminium oxide is consumed during the electrolysis and aluminium oxide therefore has to be added to the electrolyte quite frequently. During the electrolysis, the molten bath is covered by solidified electrolyte, called crust. The top of the crust is covered by aluminium oxide and/or other materials which are to be added to the electrolyte. The crust reduces the heat loss from the molten electrolyte, but is hard and therefore has to be broken in by means of a crust breaker in order to allow the oxide to be fed to the molten bath.

In electrolytic cells equipped with Søderberg anodes, each cell usually has one rectangular anode.

The Søderberg anode consists of a permanent outer casing made from cast iron or steel, which casing surrounds the self-baking carbon anode. Unbaked carbonaceous electrode paste is charged at the top of the anode and this unbaked electrode paste is baked into a solid carbon anode due to the heat which evolves during the supply of electric operating current to the anode and the heat from the molten bath. A major feature of the Søderberg anode is thus that the baked solid anode moves relatively to the permanent anode casing.

The Søderberg anode is suspended or supported by a number of vertically arranged anode studs, which also are used for supplying electric operating current to the electrolytic cell. The anode studs, which usually are made from steel are forced down into the top of the anode and by baking of the anode paste, they are connected to the anode. The anode studs follow the downward movement of the anode until the lower ends of the anode studs reach a certain distance from the bottom of the anode. The anode studs are then withdrawn from the anode and placed in a higher position in the anode.

By keeping the lower ends of the anode studs at different vertical level there will always be a sufficient number of studs which have such a position in the anode that a sufficient holding force and an even current distribution is obtained.

A problem with the above described Søderberg anodes is so-called "pocket formation". The pocket formation consists of cracks or openings in the baked carbon anode extending from the outside of the baked carbon anode near the lower end of the casing and inclined into and upwards in the baked anode. The cracks or openings seem more or less to follow the profile of the baking zone and in many instances these cracks or openings extend into the outer row of studs and in some cases also into the inner row of studs. These cracks and openings are usually discovered either due to flow of so-called stud paste through the anode and into the electrolyte during withdrawing or setting of anode studs, or by formation of a characteristic bump on the top of the anode above the crack or opening, resulting from stop of the downward movement of that part of the anode relative to the anode casing.

It is believed that the reason for the development of the above described cracks and openings is that at the lower end of the anode casing there is formed a layer of FeS/FeF2 at the boundary between the steel in the anode casing and the baked carbon in the anode. It is further believed that boundary layer is formed by reaction between the iron in the casing and gases from the electrolytic process such as COs and HF. This layer adheres strongly both to steel and carbon and has probably a thermal expansion coefficient which is between the thermal expansion coefficients for steel and carbon. The layer can thereby give an equalizing of the tension which forms during cooling. Depending on the degree of cracks in the anode, the efficiancy of aluminium electrolyte cells will be reduced. Further big cracks may give an increased soot- formation due to flow of unbaked anode paste into the electrolyte.

Disclosure of invention

It is thus an object of the present invention to provide an anode casing for electrolytic aluminium reduction cells whereby the formation of cracks and openings in the baked anode are avoided or at least substantially reduced.

Thus, the present invention relates to an anode casing for Søderberg anodes for use in electrolytic aluminium reduction cells, wherein at least the lower and inner part of the anode casing is made from a ceramic material or refractory castable which is resistant against the atmosphere and the temperatures in the electrolytic aluminium reduction cell.

According to a preferred embodiment the anode casing is made from steel or cast iron where the lower, inner part of the casing has a layer of ceramic material and/or a refractory castable.

The refractory castable is preferably a so-called super concrete based on an hydraulic cement such as Portland cement or calcium aluminate cement to which have added densifyers such as microsilica. The super concrete used int he anode casing according to the present invention has a compression strength of at least 70 MPa and preferably at least 120 MPa measured according to ASTM standard C-39-86.

It is particularly preferred to use a refractory castable consisting of 15 - 30 % weight calcium aluminate cement, 5 - 10 % by weight microsilica, the remaining being a refractory filler, preferably AI2O3.

Microsilica is amorpheous silica particles collected from the off-gas from electrothermic smelting furnaces for production of ferrosilicon or silicon. It is also possible to obtain microsilica as a main product from these furnaces by adjustment of the operating parameters. Amorpheous silica of this kind can also be produced synthetically without reduction or reoxidation. Finally a microsilica generator can be used for production of fine particulate silica or silica can be producing by precipitation from aquous solutions. Microsilica may contain 60 - 100 % by weight of Siθ2 and has a density between 2.00 and 2.40 g/cπ and a specific surface area of 15 - 30 m-^/g. The particles are of a substantially spherical shape and have a particle size substantial between lμm. Variation in these values are possible. The microsilica may have a lower Siθ2 content and the particle size distribution can be adjusted be removing coarse particles.

It has surprisingly been found that by use of an anode casing according to the present invention sticking of anode paste to the casing and thus formation of cracks and openings in the anode is avoided or strongly reduced. Further it has been found that d e anode casing according to the present invention is very durable at the temperature and atmosphere of an electrolytic cell for production of aluminium.

By reducing the cracks due to sticking of anode paste to the anode casing, an improved and more even anode operation with a reduced soot formation is obtained.

Brief description of the drawing

Figure 1 shows a vertical cut through an anode casing according to the present invention.

Detailed description of the invention

In figure 1 there is shown a vertical cut through an anode casing for an electrolytic reduction cell for production of aluminium.

The casing has an upper part made from steel. To the upper part 1 there is connected a flange 2 made from steel which constitutes the outer part of the lower part of the anode casing. The inside of the flange 2 has a layer 3 of a refractory castable consisting of 23 % by weight of calcium aluminate cement, 6 % by weight of microsilica and 71 % of aluminium oxide having a particle size less than 4 mm.

In order to ensure a sufficient stiffness of the anode casing, the casing is on its outside provided with vertical flanges 6 and a horizontal flange 7. In order to ensure a good connection between the flange 2 anode the layer 3 of refractory castable, reinforcement bars are affixed to the flange 2 as indicated by reference numeral 8.

The refractory castable was made using 5 % by weight of water based on dry material and a phosphorous-based water-reducing agent sold under the trademark Calgon was added. The mix was cast on the flange 2 and allowed to cure. The produced anode casing was installed in an electrolytic reduction cell for production of aluminium and after 6 months operation no sign of horizontal cracks could be observed. No wear or other damage on the casing was found.

Claims

Claims:

1. Anode casing for Søderberg anodes for use in electrolytic aluminium reduction cells, characterized in that at least the lower and inner part of the anode casing is made from a ceramic material or a refractory castable which is resistant against the atmosphere and the temperatures in the electrolytic aluminium reduction cell.

2. Anode casing according to claim 1, characterized in that the anode casing is made from steel or cast iron where the lower, inner part of the casing has a layer of ceramic material and/or a refractory castable.

3. Anode casing according to claim lor2, characterized in that the refractory castable is a super concrete based on an hydraulic cement such as Portland cement or calcium aluminate cement to which have added densifyers such as microsilica.

4. Anode casing according to claim 3, characterized in that the super concrete has a compression strength of at least 70 MPa and preferably at least 120 MPa measured according to ASTM standard C-39-86.

5. Anode casing according to claim lto4, characterized in that the refractory castable consists of 15 - 30 % by weight calcium aluminate cement, 5-10 % by weight microsilica, the remaining being a refractory filler, preferably AI2O3.