Coating processes – Foraminous product produced
Reexamination Certificate
1998-07-09
2001-05-08
Beck, Shrive (Department: 1762)
Coating processes
Foraminous product produced
C118S050000, C118S064000, C205S109000, C205S161000, C427S113000, C427S180000, C427S294000
Reexamination Certificate
active
06228424
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to a method of treating open-pored porous bodies which are to be exposed to oxidising conditions at high temperatures so as to protect the bodies against oxidation or other chemical attack in said high temperature oxidising conditions, in particular the treatment of prebaked carbon components of aluminium production cells, such as anode blocks or cathode blocks or cell side walls.
The invention also relates to an apparatus for carrying out this method and use of the apparatus for applying a treating liquid to a prebaked carbon component of an aluminium production cell.
BACKGROUND OF THE INVENTION
The treatment of prebaked carbon components of aluminium production cells, such as anode blocks, cathode blocks or cell sidewalls to improve their resistance to the conditions prevailing in the cell has already been proposed.
U.S. Pat. No. 5,486,278 (Manganiello et al) discloses treating a prebaked carbon-based anode of an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte, over its sides and top to improve the resistance thereof to erosion and corrosion during operation of the cell by air and oxidising gases released at the anode, by immersing the anode in a boron-containing solution containing 5-60 weight % of H
3
BO
3
or B
2
O
3
in methanol, ethylene glycol, glycerin or water with a surface-active agent, e.g. at 80° to 120° C. After immersion, lasting up to an hour, the boron-containing solution is impregnated to a depth of usually about 2-5 cm over the top and side surfaces of the anode to be protected, producing a concentration of boron in the impregnated surface from 100 ppm to 0.35%. The same treatment can be applied to cell sidewalls.
It was found advantageous to carry out this treatment with a heated solution, but this involved heating of the anode, which consumed large quantities of energy. Attempts were therefore made to carry out the process at ambient temperature because no special heating equipment would be required. Low temperature application however required the careful choice of solvents and surfactant agents in order to reduce the treatment time as far as possible.
For prebaked anodes, only the top and top side surfaces need to be protected, so it was suggested to dip the anode upside down into the solution. But this is impractical when the anodes are fitted with rods for connection to a suspension device which also serves as a current lead-in. Furthermore, it is inconvenient to treat the anodes first and then fix the suspension rods.
To overcome this difficulty it would be possible to dip the anode in the treating solution with the rodded top side up, and protect the bottom part of the anode by blocking its pores with a fugitive agent that prevents impregnation with the boron-containing compound, and can be removed afterwards. This however entails additional operations and careful selection of the fugitive agent.
To speed up the process, it was suggested to assist the impregnation by the application of a pressure differential, by pressure or vacuum. However, no practical way of doing this was disclosed.
U.S. Pat. No. 5,534,130 (Sekhar) describes the protection of the cell sidewalls of aluminium production cells by impregnating them with agents based on aluminium phosphate. Again, it would be desirable to perfect ways of applying this method in an efficient manner.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the above-described problems and shortcomings of the available methods and apparatus.
It is another object of the invention to provide a method of treating an open-pored porous body which is to be exposed to oxidising conditions at high temperatures so as to protect the body against oxidation or chemical attack in said high temperature condition, in particular carbon components of aluminium production cells such as anode blocks without necessarily pre-heating the anode blocks or other bodies, while assuring an effective impregnation of the treating liquid into the pores of the treated part, and enabling the treatment of large numbers of the bodies in an efficient manner.
A particular object of the invention is to provide a method which can use a hot treating liquid in a very efficient manner, without necessarily heating the treated body substantially above ambient temperatures.
In general terms, the invention provides a method of impregnating an open-pored porous body with a hot treating liquid containing a dissolved treating agent to produce within the body a protective layer to a desired depth. This method comprises impregnating a hot non-saturated treating liquid into the surface of the porous body assisted by the application of a pressure differential. The applied treating liquid is at a temperature above that of the body and the concentration of the treating agent in the applied liquid is such that cooling of the applied liquid as it impregnates the pores of the body causes precipitation of the treating agent in the pores to the desired depth.
More specifically, the method according to the invention comprises firstly bringing the surface of the body into contact with a hot liquid containing a dissolved treating agent at a concentration below saturation and at a temperature well above the temperature of the body. The concentration of treating agent in the hot liquid is such that when the liquid is cooled down to the temperature of the body the liquid saturates and treating agent precipitates.
A pressure differential is then applied to cause the hot non-saturated treating liquid contacting the body to impregnate into the surface pores of the body. As the treating liquid impregnates the pores of the body, it cools and deposits, within pores of the body underneath the surface, a layer of the treating agent precipitated from the impregnation liquid. Some treating agent may also cover the outside of the body's surface.
When, later, during use, the body is exposed to high temperatures particularly under oxidising conditions, this layer of the treating agent precipitated inside the body underneath its surface forms a protective layer in the surface pores which protects the body against oxidation or other chemical attack. Advantageously, the treating agent contains a soluble boron compound which forms an impervious viscous protective layer which is self forming above about 400° C.
The treating liquid in contact with the body is usually at least 5° C. and in many cases at least 10° C. above the temperature at which the treating liquid is saturated with the dissolved treating agent for its given concentration.
Moreover, the treating liquid is at a temperature well above that of the body to be treated, i.e. usually at least 20° C. above and conveniently 40° C. or more above. For example the treating liquid is at a temperature in the range 60° to 120° C., whereas the body can be at ambient temperature or just above, say from about 20° C. to 50° C. The applied treating liquid which is in contact with the body can be maintained at a more-or-less constant temperature by heating it to compensate for heat loss due to contact of the treating liquid with the body.
The treating time can be reduced to a few minutes, which is a great improvement over the previous impregnation techniques.
There is no need to heat the body to be treated which makes the method energy efficient.
The method permits treatment of only a selected part of the body, in particular by using a specially designed apparatus, described below.
Due to the rapidity of the impregnation process, the treated anode or other body only takes up a small quantity of heat, so the process is very energy efficient.
The treated carbon bodies typically have an overall porosity in the range 20-24%, of which about 10-12% is open porosity. The penetration depth can be of the order of 0.5 to 3 cm. However, the treating agent may also extend to and even cover the bodies' outer surface.
In one embodiment of the method, treating liquid is circulated by the following
Berclaz Georges
De Nora Vittorio
Duruz Jean-Jacques
Beck Shrive
Deshmukh Jayadeep R.
Moltech Invent S.A.
Strain Paul D.
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