Patent Document

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&#39;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&#39; outer surface. 
     In one embodiment of the method, treating liquid is circulated by the following arrangement. Hot treating liquid is supplied from a reservoir and, after treatment of the body, non-impregnated treating liquid is returned back to the reservoir. The treating liquid in the reservoir is stirred and heated to maintain a desired temperature. Components of the treating liquid (the dissolved treating agent; possible further materials in suspension and water) can be added to the reservoir to compensate for consumption of the treating liquid in the treating process. 
     Another inventive aspect is a method of supplying the treating liquid which comprises providing a saturated solution of the treating agent at a given temperature, and deriving, from said saturated solution, a non-saturated solution of the treating agent. The non-saturated treating solution is then supplied as the treating liquid to treat the body. 
     For example, the non-saturated solution is obtained by increasing the temperature of the solution. In this way, the non-saturated solution contains the treating agent at the same concentration as the saturated solution (which corresponds to the saturation concentration at said given temperature) but is simply at a higher temperature. 
     The treating liquid preferably contains an oxidation retardant agent for impregnating the part of the body to be treated, in particular at least one soluble compound of boron and/or phosphorous for improving the resistance to oxidation of the carbon. 
     Preferred oxidation retardant agents for application to aluminium production anodes are boron containing liquids based on B 2 O 3 , boric acid, tetraboric acid, salts of said acids or boron silicate. These boron containing agents such as B 2 O 3  act as catalytic oxygen inhibitors when impregnated/dispersed in carbon. However, when they are densely applied they form a vitreous impervious layer in the surface pores at temperatures above about 400° C. Such a layer acts as a barrier to protect the pores of a carbon body from oxidation. Such viscous layers act mainly by a barrier effect to inhibit oxidation. This is different to the effect of deeply impregnated boron compounds whose effect is principally as an anti-catalyst. 
     Various silicon containing compounds, in particular boron silicate and silica are also suitable for producing viscous protective layers at the operating temperatures, especially in combination with boron compounds where an excess of the boron compound (in particular boric acid) will lead to the formation of borosilicate glass. Though the silicon compounds alone are not effective against oxidation and provide no anti-catlytic effect, when combined with boron compounds the resulting glassy material provides improved protection. 
     Alternative treating liquids, specially suitable for protecting cell side walls, are liquids based on phosphates of aluminium for example selected from the group consisting of monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, aluminium metaphosphate, and mixtures thereof. 
     The boron, phosphorous and/or silicon-containing treating liquids can contain surfactants surfactant agents in particular tensio-active cationic agents. Anionic tensio-active agents can also be used. Such agents should be devoid of components that would undesirably contaminate the aluminium produced and components that promote oxidation of the carbon. These surface-active agents may possibly be present together with other solubility improving agents such as tartaric acid or citric acid, and the liquid may be heated to improve and to speed up the impregnation of the anode. 
     When water is chosen as solvent, a surfactant such as those available under the tradenames NONIDET P 40 and SPAN 85, from Fluka, and GLUCOPON 225, DEHYPON LS, QUAFIN LDM and QUAFIN CT, from Henkel, can be used in order to achieve an acceptable low treatment time. However, this is not essential. 
     Optionally, treatment with the treating liquid may be combined with the application of a suspension containing particles which block the surface pores of the body and form a surface coating, or the treatment may be followed by the application of a suspension containing particles which help reduce the pore size. Such a suspension may contain a colloid selected from colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate, or particles of the same materials. 
     For certain applications, for example for treating the surface of cathode blocks, this impregnation or top coating liquid may contain particulate refractory boride, such as TiB 2 , and/or aluminium powder, chips or cuttings. 
     A very advantageous treating liquid further contains fine carbon powder which helps keep the pores blocked and helps prevent migration of the oxidation retardant agent (which becomes viscous at the operating temperatures) towards the bottom of the impregnated component. 
     Thus, the term “treating liquid” should be understood as including non-saturated solutions of the treating agent and “quasi-solutions”. In addition to the dissolved treating agent at below-saturation concentration forming a true homogeneous solution, “quasi-solutions” further comprise components such as colloids and particles in suspension. 
     In all cases, when the treating solution is deposited in the pores of the treated body, the dissolved treating agent precipitates. Later, when the body is heated, removal of water will lead to a continuous protective barrier layer substantially impervious to oxygen. For this, an adequate amount of the treating agent must be deposited in the pores, which can precisely be achieved with the method according to the invention. Furthermore, to improve the action of closing of the pores by the precipitated treating material, the treating solution advantageously further comprises the above-mentioned suspensions or powders. 
     For bodies which are exposed to uneven wear over different parts of their surface due to different oxidising conditions, for example prebaked carbon anodes or cell sidewalls of aluminium production cells, it is possible to apply different treatments to different areas in order to optimize the protective effect against oxidation and in order to minimize the amount of oxidation retardant used. For example, it is possible to impregnate practically the entire sides of anodes with a dilute liquid of an oxidation retardant, as described in the aforementioned U.S. Pat. No. 5,486,278, followed by applying a surface layer in accordance with this invention only over the “top and shoulders” of the anode that will be recovered as the anode butt. Or the “top and shoulders” could be further protected by applying a top coating including inert particles. 
     Where special problems of oxidation occur, it is also possible to further top coat or spray with a slurry for example of boric acid or another protective material. 
     In a modified method, the surface of the body can first be impregnated to a depth of several centimeters by impregnation with a “dilute” solution of the treating agent, followed by a treatment according to the invention with a near-saturated solution which precipitates into the surface pores. 
     The treated body may be a pre-baked anode of an aluminium production cell, which anode is rodded, i.e. has a rod attached to its top face for connection of the anode to a suspension device which also serves as a current lead-in. Using the method of the invention and the described apparatus, rodded anodes can easily be treated with their rodded side up, which greatly facilitates handling. 
     When preformed carbon anodes are put into service in aluminium production cells, their bottom part immersed in the electrolyte is at a temperature usually about 960° C., whereas their top part exposed to oxidising gases is initially at a temperature of about 450-500° C., but which rises to 700° C. or more as the anode is consumed. 
     By protecting the top parts of the anodes in accordance with the invention with a boron-containing liquid, at the operating temperature this liquid loses water and forms a highly viscous non-solid coating in the surface pores of the top part of the anode to be protected. This viscous layer self-forms in the operating conditions and considerably reduces wear of the top part of the anode by oxidation. As a result, the anode butts remaining at the end of the anodes useful life are considerably more intact than those of non-protected anodes, or anodes protected by prior art methods. 
     The treated body may alternatively be part of a sidewall of an aluminium production cell, or a cathode block of an aluminium production cell. 
     The method of the invention can advantageously be carried out in a specially developed apparatus for applying a treating liquid to an open-pored porous body. Such apparatus comprises a treating chamber having at least one sealing member which is arranged to be applied to a body to be treated which is placed in the treating chamber. The sealing member or members is/are arranged so as to isolate an upper part of the treating chamber around the part of the body to be treated from a lower part of the treating chamber around a bottom part of the body which is not to be treated. 
     The apparatus also includes a reservoir containing a supply of hot non-saturated treating solution of a treating agent at a temperature above the temperature of the body to be treated. The concentration of treating agent in the hot solution is such that when the solution is cooled, before it reaches the temperature of the body, the solution will saturate so that treating agent precipitates. 
     Means are provided for filling the upper part of the treating chamber with a treating liquid to cover the part of the body to be treated. These means can include a pump for pumping treating liquid from a supply reservoir. 
     Means are also provided for applying a pressure differential to intake an amount of the treating liquid into pores in the part of the body to be treated, in particular by applying a vacuum to the lower part of the treating chamber. 
     Lastly, means are provided for removing remaining treating liquid, which has not been absorbed by the body during the treatment, from the treating chamber. These means can include another pump for pumping residual treating liquid back to the supply reservoir. 
     Further details of an advantageous apparatus are described in PCT application PCT/IB97/---- concurrently filed herewith (Ref. MOL0546-03). 
     In the method of the invention, a hot treatment liquid is to be used to treat bodies at ambient temperature. For this, the treating chamber can be provided with means for heating the treating liquid in the upper part of the treating chamber to compensate for heat loss due to contact of the liquid with the body. Other arrangements to achieve a thermal balance can be used, as appropriate. 
     An embodiment of the apparatus comprises a reservoir for treating liquid, from which hot treating liquid is supplied to the top part of the treating chamber and to which remaining treating liquid, which has not been absorbed by the body during the treatment, is returned from the upper part of the treating chamber. This reservoir comprises means for heating and means for stirring the treating liquid therein. A metering device can be provided for adding components of the treating liquid to the reservoir to compensate for consumption of the treating liquid in the treating process. 
     A preferred storage vessel or reservoir—which can be used for various treatments—comprises first and second compartments, the first compartment containing a reserve supply of the hot treating liquid in contact with a mass of the treating agent at a given temperature T 1 . In the first compartment, the treating agent is dissolved at a concentration which corresponds to the saturation concentration, at temperature T 1 . The second compartment contains a supply of non-saturated treating liquid in which the treating agent is dissolved at the same concentration but at a temperature T 2  above temperature T 1 , or at the same temperature T 1  but a lower concentration. The vessel further comprises: means for maintaining the hot treating liquid in each of the first and second compartments at the respective temperature; an outlet conduit for supplying the non-saturated hot treating liquid from the second compartment of the vessel to treat a body or material; and a conduit for supplying hot treating liquid from the first compartment to the second compartment to compensate for consumption/loss of the treating liquid by treatment of the body or material. 
     The apparatus preferably comprises means for applying heat to treating liquid in the upper part of the treating chamber to compensate for cooling of the liquid by contact with the body. 
     An advantage of the above-outlined method and apparatus is that it is possible to treat large bodies such as prebaked anodes without a need to pre-heat them. However, the bodies can be pre-heated if required, to just above ambient temperature. In particular, it can be useful to locally pre-heat the top part of the bodies to be treated, while maintaining a sufficient temperature differential with the treating liquid. 
     Another advantage is that the method and apparatus allow sequential treatments to be carried out on the top part of the same body, over the same area or over a different area, using the same treating liquid or different treating liquids/slurries. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram of an apparatus for carrying out the method according to the invention. 
     FIG. 2 is schematic cross-sectional view of a preferred type of reservoir for preparing and supplying the treating liquid. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically shows an apparatus for applying a treating liquid  10  to a porous body  11  by the method of the invention. The apparatus comprises a treating chamber  12  having an upper part  14  and a lower part  15 . The upper part  14  is open-topped and may be provided with a removable cover or lid. 
     A sealing member  13  consisting of several sections of elastomeric material is arranged to surround a body  11  to be treated when the body is placed in the treating chamber  12  by inserting it into the open top by means of an automatic handling device (not shown). In the case of square or rectangular bodies  11  such as prebaked carbon anodes of aluminium production cells, four sections of elastomeric material can be arranged around the four sides, each section being associated with a series of hydraulic, pneumatic or mechanically actuated cylinders, one such hydraulic cylinder  25  being shown in FIG.  1 . 
     This sealing member  13 , when it is tightened around the body  11 , isolates a space in the upper part  14  of the treating chamber around the part of the body  11  to be treated, from a lower part  15  of the treating chamber around a bottom part of the body  11  which is not to be treated. 
     Isolation of the upper and lower parts  14 ,  15  of chamber  12  can be achieved by means of a flexible skirt associated with the sections of the sealing member  13 , or by arranging the sections of the sealing member  13  to fluid-tightly protrude from a groove or the like around the chamber wall. 
     A supply conduit  16  for treating liquid leads into the upper part  14  of the treating chamber. Conduit  16  leads from a reservoir  32  of treating liquid  10  and has a supply pump  17  by means of which treating liquid  10  can be supplied to the upper part  14  of the treating chamber so as to cover the part of the body  11  to be treated, up to a level determined by a sensor  22 . 
     A Venturi or vacuum pump  18  is connected to the lower part  15  of chamber  12  for evacuating the space around the underside of the body  11 . When the Venturi or vacuum pump  18  is switched on, air filling the pores of body  11  is evacuated, which causes an amount of the treating liquid  10  in the upper part  14  of chamber  12  to be intaken into the part of the body  11  to be treated. A pressure detector  19  is provided in the lower part  15  of chamber  12 . This detector  19  is sensitive to the change in pressure which occurs when all of the pores of the part of body  11  being treated are filled. 
     An outlet conduit  20  is connected to the bottom of the upper part  14  of the chamber. This conduit  20  leads back to the reservoir  32  and has a pump  21  for returning treating liquid remaining in the chamber at the end of treatment of a body  11  back to the reservoir  32 . 
     A hydraulic system, comprising a hydraulic cylinder  25  controlled by a hydraulic pump  26 , is provided for adjusting the sealing member  13  which is connected to a piston of the hydraulic pump  26 . When this piston is pulled into its cylinder  25 , the member  13  is pulled out to allow a loose fit around a body  11 , permitting insertion and removal of the body  11  into or from the treating chamber  12 . When the piston is pushed out of its cylinder  25 , the sealing member  13  is tightened around the body  11  to provide a sealing fit, by elastic deformation of the elastomeric material making up the sections of sealing member  13  when they are applied against the body  11  under the pressure applied by the hydraulic control. 
     Alternatively, it is possible to control the sealing member  13  pneumatically, mechanically or electro-mechanically. 
     A position detector  30  is provided in the lower part  15  of the treating chamber for detecting when a body  11  introduced into chamber  12  reaches a predetermined position. This position depends on the size of the body  11  and corresponds to the level of the bottom of the body  11  when the top of the body to be treated is at a level where it will be covered by treating liquid  10  in the upper part  14  of the chamber. The detector  30  is arranged to actuate the hydraulic pump  26  and hydraulic cylinder  25  to bring the sealing member  13  to sealably engage with the body  11  when the body  11  has reached the given position. 
     The treating chamber  12  comprises a heater  31  for heating the treating liquid  10  in the upper part  14  of chamber  12 . The heater  31  can be an electric heater or can operate by circulating hot air or another heating fluid. This heater  31  can be adjusted to supply an amount of heat which compensates for heat loss due to contact of liquid  10  with body  11 , i.e. depending on the size and temperature of the body  11  and its thermal characteristics, and the operating temperature of the treating liquid  10 . If required, the heater  31  can be replaced by means for maintaining a proper thermal balance. 
     The reservoir  32  supplies hot treating liquid  10  to the top part  14  of the treating chamber and, after the end of the treatment of a body  11 , treating liquid remaining is returned to the reservoir  32  via conduit  20 . The reservoir  32  is externally insulated and is fitted with a heater  33  for maintaining the treating liquid  10  at a selected temperature controlled by a thermostat  37 . A stirrer  34  constantly or intermittently driven by a motor  35  stirs the treating liquid  10  contained in reservoir  32 . In the embodiment shown in FIG. 1, at the top of reservoir  32  is a metering device  36  for adding components of the treating liquid  10  to the reservoir  32  in an amount to compensate for consumption of the treating liquid  10  in the treating process. The reservoir  32  also includes a pressure sensor  38  for monitoring the level of liquid  10  and a sensor  39  measuring the density or the conductivity of liquid  10 , serving to control the metering device  36 . 
     The treating liquid  10  in reservoir  32  is maintained at a selected temperature, say from 60° C. to 120° C. and contains a treating agent at a concentration well below saturation. An advantageous reservoir for maintaining a supply of non-saturated treating liquid is described below with reference to FIG.  2 . 
     A preferred apparatus comprises a control panel schematically indicated at  40 . This control panel includes controls for all routine adjustments such as temperature of the liquid in reservoir  32  and in the top part  14  of chamber  12 , the liquid level to be controlled by sensor  22 , etc. The control panel also includes an overall control arranged to sequentially perform the following operations: 
     First, the sealing member  13  is actuated (by the position detector  30  and hydraulic control  26 / 25 ) to sealably engage with the body  11  when the body  11  to be treated has reached the given position. 
     Next, the pump  17  is switched on to fill the upper part  14  of the treating chamber  12  with a quantity of hot treating liquid  10  from the reservoir  32 , up to a level controlled by sensor  22 . 
     Then, the lower part  15  of the treating chamber is evacuated by switching on the Venturi or vacuum pump  18 , which evacuates air from the pores of body  11  and intakes treating liquid  10  into the surface pores. 
     During the vacuum treatment, if required the heater  31  is switched on to heat the treating liquid  10  in the upper part  14  of the chamber  12  in order to compensate for cooling of the liquid  10  by contact with body  11 , so that the treating liquid  10  in contact with the body  11  remains at more-or-less constant temperature above the saturation temperature. In this way, the treating liquid in the circulation system is always maintained non-saturated, thereby avoiding unwanted deposits of the treating agent in the circulation system. 
     When the vacuum is applied, hot treating liquid is intaken into the pores of body  11  which, for example, is at ambient temperature. As it is intaken into the pores, the treating liquid heats the surface part of body  11 , whereas the temperature of the impregnated treating liquid drops. As the pores fill with treating liquid, the deeper the liquid penetrates the cooler it becomes until it deposits precipitated treating agent firstly in some pores. The liquid is then diverted to fill those pores which still remain open. When all pores are full and blocked by deposited treating agent, the impregnation is sufficient and this is signaled by the pressure detector  19 , which immediately or after a given time automatically switches off the Venturi or vacuum pump  18  and then actuates pump  21  to remove residual treating liquid  10  from the upper part  14  of the treating chamber. 
     When all of the residual liquid  10  has been removed from the upper part  14  of the treating chamber, the hydraulic pump  26  is actuated to release the sealing member  13  and allow removal of the treated body  11  from the treating chamber  12  by an automatic handling device (not shown). 
     A typical body  11  to be treated is a prebaked carbon anode of an aluminium production cell. Such bodies usually have a porosity of about 20-24%, of which 10-12% is open porosity. A prebaked anode may weigh of the order of 1000 kg. Attached to its upper side of the anode is a steel rod for connection to a suspension device and which also serves as an electrical connection. 
     The treatment of prebaked anodes by known impregnation processes is difficult (because of the attached rod) and energy consuming (because of the need to heat the entire anode to a temperature at which the impregnation process will be effective). 
     With the method according to the invention, the treatment of prebaked anodes is advantageous, because the anode can be treated with its rodded side up and only the part which needs to be treated (the upper side shoulders and top) can be treated without preheating of the anode, in a simple treatment with the anode at ambient temperature and the treating liquid at a convenient temperature, say from 60° to 120° C. in a treatment lasting only a few minutes. 
     Moreover, 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. 
     In an alternative embodiment, the operating cycle is slightly modified, by removing used treating liquid from the bottom part  15  of the chamber after releasing the sealing members  13 . In this case, when the sealing members  13  are retracted, the used treating liquid flows down and is collected in the lower part of the treating chamber, in a channel or sump at the bottom of an inclined surface down which the liquid flows. The outlet  20  then leads from this channel or sump for return of the used liquid to the reservoir. Particularly when boron-based or phosphorous-based treating liquids are used, a sloping floor ensures continuous draining of the used liquid, which avoids unwanted deposits of the oxidation retardant. This simplifies maintenance of the apparatus and reduces operating costs. 
     FIG. 2 shows the main components of a very advantageous design of a storage vessel or reservoir  52  for supplying the treating liquid  10  via a supply conduit  16  and returning used treating liquid to the reservoir  52  via a return conduit  20 . 
     Reservoir  52  is separated by a horizontal inner divider wall  54  into an upper (or first) compartment  55  and a lower (or second) compartment  56 . The upper compartment  55  contains saturated treating liquid  10 ″ at a temperature T 1 , this saturated treating liquid  10 ″ being in contact with a mass  60  of undissolved treating agent on the divider wall  54 . The lower compartment  56  contains a supply of non-saturated treating liquid  10  which is at the same concentration as that in compartment  55  but is at a temperature T 2  which is higher than T 1 . 
     The reservoir  52  further comprises arrangements for maintaining the hot treating liquid in the compartments  55  and  56  at the respective temperatures T 1  and T 2 . As shown for compartment  55 , such an arrangement comprises an outlet  61  and a return inlet  62  for circulating the hot liquid via a standard type of heat exchanger  63  which heats the circulating liquid to the desired temperature T 1  (or T 2 ). A similar arrangement (not shown) is provided for compartment  56 . These heater/circulating arrangements also serve to stir the liquid in compartments  55  and  56 . Additional stirrers can be included if desired. 
     Compartment  55  has an outlet  64  and compartment  56  has an inlet  65  via which hot treating liquid  10 ″ from compartment  55  at temperature T 1  can be transferred into compartment  56  where the liquid is maintained at a temperature T 2  above T 1 . Heating the liquid from temperature T 1  to T 2  can be done between the outlet  64  and inlet  65 , or in a separate heating/circulating device (like  61 ,  62 ,  63 ). 
     The reservoir  52  also has a window  66  in one face, through which the level of the undissolved treating material  60  can be monitored. As shown for compartment  55 , a thermometer  67  and a manometer  68  are provided for monitoring the temperatures T 1  (or T 2 ) and the pressure in the respective compartment  55  (or  56 ). 
     At the top of reservoir  52  is a hinged cover  70  which can be manually opened for tipping into the compartment  55  a fresh supply of material  60  when needed, as can be visually ascertained by inspection via window  66 . The material  60  can thus be supplied at convenient times by emptying it, for example from a sack. The cover  70  is fitted with a seal to prevent the escape of fumes. 
     At the bottom of reservoir  52  is an optional manhole-type opening  71  that can be opened for manually removing debris etc. that may accumulate in the lower compartment  56 . For this purpose, the lower compartment is fitted with a perforated dividing wall  76  for restraining the debris. 
     The reservoir further comprises a hot water inlet  72  and a hot water outlet  73  at the top of compartment  55 . It thus possible to include, inside the main compartment  55  of the reservoir  52 , an internal hot water storage tank arranged so that, when needed, the stored hot water can be used to flush the conduits  61 ,  62  and the heat exchanger  63  to dissolve any deposits of the treating material deposited from the saturated liquid. 
     Also, each of the upper and lower compartments  55 ,  56  has a venting device for equalizing pressure therein. This consists of a vent tube  74 , which connects the compartment  56  to the outside, having a spring-actuated closure flap  74 ′ for venting in case of excess pressure in the compartment  56 . Likewise, compartment  55  has a vent tube  75  closed by a spring-actuated closure flap  75 ′. 
     The described improved reservoir  52  can be used for supplying hot treating liquid for various processes; i.e. even without using a vacuum. For instance, it can be used to spray or otherwise apply a topcoating of the treating material onto a treated body. This reservoir  52  is very advantageous from several points of view. 
     By maintaining the treating liquid at a given temperature T 1  in compartment  55 , the treating material therein is maintained dissolved, at a concentration which corresponds to the saturation concentration at that temperature T 1 . This is achieved without any complex control means and without a need to meter in selected amounts of the treating material, simply by maintaining an excess of undissolved treating material  60 , by adequate stirring, and by maintaining the temperature at the desired value T 1 . Dissolved at the same concentration as before, but is sufficiently below the saturation concentration that when the liquid is supplied for example to the upper part of chamber  14 , the risk of unwanted deposition of the treating material in the supply arrangement is reduced or eliminated, compared to when a liquid at or near saturation is used. Moreover, the concentration of the treating liquid is controlled in a very simple way. The reservoir  52  can be filled manually at convenient intervals with fresh treating material  60 . The simple arrangement with sealed cover  70  avoids the escape of undesirable fumes. 
     Instead of increasing the temperature in compartment  56 , the same effect could be achieved by maintaining the treating liquid at the temperature T 1  and adding selected amounts of hot water at temperature T 1 . 
     Instead of returning the used treating liquid into compartment  56  via the conduit  70 , it could be returned into compartment  55 . For certain applications of the reservoir  52 , the supplied treating liquid need not be returned to the reservoir. 
     When a solution of boric acid is used for the treatment of the upper parts of pre-baked carbon anodes of aluminium production cells, a saturated solution of boric acid can be contained in compartment  55  at a temperature T 1  in the range 50° C. to 110° C., for example. The sub-saturated treating solution  10  can be obtained by heating the solution in compartment  56  to a temperature say about 10° C. to 15° C. above T 1 . When this sub-saturated solution is impregnated into the surface of a carbon anode  11  at ambient temperature assisted by vacuum, and maintaining the external solution hot by means of the heater  31 , a boron-containing layer precipitates in the pores underneath the surface of the carbon body  11 . During use of the anode, the impregnated layer vitrifies and forms a dense viscous protective layer considerably reducing oxidation of the upper part of the anode which remains as an anode butt.

Technology Category: c