DE3625726A1 - Process for producing a temperature-resistant bond between shaped carbon parts - Google Patents

Abstract

To produce an integral bond between porous carbon and graphite bodies, the bodies are impregnated with pitch, curable resins or other carbonisable materials, the surfaces to be bonded are brought into contact and the bodies are heated. At a first temperature a part of the impregnation agent flows from the pores, at a second temperature the impregnation agent is carbonised and a layer of coke is formed between the bodies.

Description

The invention relates to a method for manufacturing a cohesive connection between porous Carbon moldings exposed to elevated temperatures will.

Carbon moldings, under this term are described below also understood graphite and semigraphite moldings, are used as linings for furnaces and electrolysis cells or used as electrodes, the higher temperatures or are also exposed to the attack of corrosive substances. At In these applications, there are several moldings with each other connected to a larger unit, e.g. a furnace wall or the bottom of an electrolytic cell. To connect the Moldings are the because of the temperature stress technically usual glue and putty and also solders not suitable. Other joining or joining methods, such as Welding, result only under special conditions, e.g. high pressures and temperatures, technically usable Connections between the carbon bodies. One uses hence for making connections between Carbon moldings almost exclusively stamped or Putties containing a carbonizable binder and the contain appropriate fillers for each intended use. Binder are primarily pitches, tars and hardenable plastics; the spectrum of fillers contains almost all substances that  at the use temperature of the carbon moldings are thermally stable. Correspondingly, coke is preferred, Anthracite, carbon black, possibly with additives from hard ceramic materials or metal chips Abrasion resistance and thermal conductivity of the connection to increase. Masses of simpler composition exist made of coal tar pitch and coke powder (DE-PS 8 63 999). The components of the putty or ramming compounds carefully mixed, if necessary with an increased Temperature in the joint between those to be joined Introduced surfaces of the moldings. Moldings and putty are at least on the carbonization temperature of the Binder heated and a coke connecting the moldings educated. Because pitch binders are below the coke formation temperature soften, the bond initially loses with heating their firmness, and the moldings are during the annealing fix carefully to prevent displacement. This is a disadvantage, especially of wider kite layers Outflow of the binder in the temperature range between Softening the binder and coke formation. It is has been proposed to drain the binder through Seal the putty joints with impermeable tapes prevent (US-PS 32 75 488). Other putties included catalytically curable resins alone or in a mixture with pitches or tars as binders (GB-PS 7 46 516). Because these putties only slightly soften when heated, you can Connection with a comparatively high heating rate heat without losing binder and an expensive Fixing the moldings is necessary. Another Putty that has become known contains furfural as a binder or furfuryl alcohol, a hardener and anthracene oil (DE-OS 16 71 167). Without the essentially carbon and containing graphite powder and hard material compounds  The filler is used as a waterproofing agent for Carbon bodies are used, mainly for their strength and increase conductivity.

When applying the plastic putty to the connecting surfaces of the porous moldings by brushing, Filling or spraying or with wide putty joints too stamping forms a binder film on the Surface of the moldings and a smaller part of the Binders penetrates the pore system of the moldings, their Porosity is about 10 to 20%. The firmness the connection between the moldings essentially determining wetting and infiltration of the surface is determined by the quality of the putty or ramming paste and the application or introduction of the putty and ramming compounds certainly. Experience has shown that there are major fluctuations, so that the connection layers are not always technical Requirements met. In particular, replacing the Connection layer from the adjacent surface of the Moldings are common causes of failure Connection. Another disadvantage is that the putty or Ramming mass in general in the delivery of furnaces, Electrolysis cells and other apparatus can be processed must, for which not only a particularly high effort is necessary, but because of the generally confined spaces favored and when using tarry masses personnel and the environment are polluted by tar emissions.

The invention is therefore based on the object Process for joining carbon and To create graphite moldings in which the putty containing binder not in a special one Process step on the surface of the connecting moldings must be applied.  

The object is achieved in that the moldings with a carbonizable softener when heated Impregnating agent impregnated, the to be joined Surfaces of the impregnated moldings against each other pressed and the moldings to a temperature above the carbonization temperature of the impregnating agent be heated.

The method according to the invention uses the pore volume of the carbon moldings as storage for the putty. The molding is impregnated with the cement that is used in the Heating the carbon molding softens and has an effect one that is formed by the release of volatile substances Pressure at least partially through the surface of the Moldings in the gap between neighboring moldings flows, is pyrolyzed on further heating and one from the marginal pore system of one to the pore system of the other molding, the moldings together connecting coke zone forms. To secure the between the Sections of a graphite electrode strand with Connections made screw nipples it is known store in recesses of the coal tar pitch, that melts when the electrode strand is heated and that fills free space between the thread flanks (U.S. Patent No. 27 35 705). Storage space is not the pore system of graphite, but special recesses in the nipple that have to be incorporated in a special operation, e.g. Bores that are perpendicular to the nipple axis extend. Because the recesses are only part of this Taking up area, the pitch must be essentially below Distribute the effect of gravity, which is only beneficial Heating conditions is possible. To connect larger ones Moldings with essentially parallel surfaces is this Process hardly suitable.  

The method according to the invention is based on the manufacture a connection between carbon blanks with a accessible pore volume is limited. Impermeable Carbon body used, for example, to manufacture used by chemical apparatus, or glassy carbon, that contains only closed pores are for this Process not suitable. Another limiting factor is the average pore size that should be as large as possible should. Carbon moldings with a accessible pore volume from 10 to 20%. For the Flowability of the putty in the pore system of the moldings is its toughness and especially the diameter of the Pore channels of particular importance. Since the Flow rate of putty or waterproofing agent decreases significantly with decreasing pore size Carbon moldings preferred, the pores with a Diameters above 0.01 mm included.

Suitable as putty and impregnating agents are all carbonisable substances which have the greatest possible fluidity in a temperature range below the coke formation temperature. Examples are - coal tar pitches and petroleum pitches - hereinafter referred to briefly as peche - , thermosetting resins such as phenol formaldehyde resins, furan resins or epoxy resins, thermoplastic resins such as polyvinyl alcohol or polyvinyl chloride, and mixtures of these groups of substances. Preference is given to pitches which give a comparatively large coke residue when tempered. It is expedient to adapt the toughness of the impregnating agent to the pore size spectrum of the moldings by adding solvents. Impregnating agents with a comparatively low viscosity should be used for small-pored moldings, and agents with a large viscosity for large-pored bodies. Solvents are, for example, tars, tar oils, alcohols or ketones. The amount of impregnating agent required to produce the connection, based on the unit area, is determined by the mean distance ( x ) of the moldings to be connected. It is per square centimeter, if d is the density of the impregnating agent, m = xd grams. This amount of impregnating agent is contained in each of the two molded parts to be connected in a disc, the thickness of which is x d / 2 p (p = porosity). Since only part f of the impregnating agent actually flows out and is available for the formation of the connection, the thickness of the disk is x d / 2 pf. Experience has shown that f is approximately 0.2 to 0.5, mean approximately 0.3.

To fill the storage layer with the thickness D = xd / 2 pf, the carbon moldings are immersed in the impregnating agent and the storage layer is filled with the impregnating agent under increased pressure, if necessary. To produce a connection that meets the requirements, it is necessary to fill at least one layer with the thickness D. The penetration depth can extend beyond the thickness D to a larger volume and in particular can encompass the entire volume of the moldings. Due to the complete or largely complete filling of the pores contained in the moldings, bodies are obtained after the temperature treatment which have better volume properties than the non-impregnated bodies, for example greater strength and electrical and thermal conductivity. However, since the amount of volatile pyrolysis products released during carbonization and to be removed by diffusion through the surface is proportional to the amount of the impregnating agent, the permissible rate of heating of these bodies is comparatively low. For complete impregnation, the moldings are expediently placed in an autoclave, which is first evacuated and pressurized with the impregnating agent after flooding. The residence time depends on the pore size of the molding and the toughness of the impregnating agent. After impregnation has ended, impregnating agent adhering to the surface of the molded articles is removed with a solvent from previously processed molded articles, the impregnating agent adhering to unprocessed molded articles expediently when they are processed by sawing, turning, milling or another cutting process.

To establish the connection between the moldings, the surfaces to be connected are brought into contact, the average distance x of the surfaces defined above being as small as possible. The distance x is determined by the machining tolerance t related to a reference plane, in the first approximation x = 2 t . The weight is generally sufficient to fix the moldings stacked one above the other as part of a furnace wall, for example. When horizontally juxtaposed moldings are connected, a pressure force is generated by heating the outer moldings. The connections between the moldings essentially absorb the thermal expansion of the moldings.

For flowing the impregnating agent out of the pores formed memory in the gap between the moldings, are at least the areas to be connected to one heated to the first temperature at which the impregnating agent melts or softens. The temperature becomes essentially determined and must by the type of impregnating agent lower than the temperature of the beginning pyrolysis. It is more gaseous due to the development of larger quantities Pyrolysis products defined in the impregnating agent. These Temperature is held until the filling of the gap between the moldings quantity has flowed out. One expediently determines that Time required for each carbon type and each Impregnation agent through tests. The temperature of the impregnated moldings is then increased until pyrolysis  completed and a firm connection between apart moldings has been formed. It is useful to start up furnaces with Carbon moldings are delivered to soften the, Flow out of the pore storage and carbonization of the To produce impregnating agent necessary temperature, so that a special heat treatment to make the connection is not necessary. The rate of heating is included essentially uncritical and can be up to 100 K / h. The heating of the bonded together by putty Moldings are made by direct current passage (Resistance heating), radiation or in direct contact with open gas or oil flames. In the last procedure become volatile in the compound Pyrolysis products burned and used as an energy source.

The method is particularly suitable for connecting Carbon stones and blocks used as lining metallurgical furnaces and cells for the Melt flow electrolysis can be used. The connections are characterized by great strength, even with the usual Operating temperatures between about 800 and 2000 ° C out. The compounds are impermeable to melts and largely corrosion-resistant.

The invention is described below by way of example:

Example 1

Intended for lining a blast furnace frame cuboid carbon blocks with an accessible Pore volume of 17%, an average pore diameter of about 0.05 mm, a maximum pore diameter of about 0.5 mm and an average bending strength of 7 MPa were impregnated with coal tar pitch Softening point (KS) was 60 ° C. The to about 100 ° C  heated blocks were placed in a heated autoclave introduced, the pressure lowered to about 10 mbar and after overlaying the blocks with the impregnating agent 8 bar was increased. The impregnation time was about 2 hours. The blocks were removed from the autoclave and after Cooling down to room temperature and milling the parts to be glued Surfaces stacked up to a furnace lining, which with an average temperature gradient of about 50 K / h 1200 ° C was heated. By coking the from Pore from the block of outflowing pitch formed solid, pitch coke bridges connecting the blocks. The was the strength of the connection measured on comparative samples greater than 7 MPa.

Example 2

To line the bottom of a cell for aluminum melt flow electrolysis, cuboid shaped blocks made of semigraphite with an accessible pore volume of 20% and milled to size were impregnated with a tar-resin mixture. The average pore diameter was about 0.03 mm, the maximum pore diameter was about 0.1 mm. The impregnating agent mixture contained 60% coal tar with a viscosity at room temperature of about 1 Ns / m ² , 10% furfuryl alcohol and 30% phenol formaldehyde resin. The processed blocks of semigraphite were overlaid with the impregnating agent mixture, removed from the impregnating agent after a dwell time of 3 hours, and the surface of the blocks was rubbed off with methanol.

The mean penetration depth of the impregnating agent was about 5 cm under these conditions. The blocks are on with grooves on the underside, into the busbars or bars are inserted. The "pouring" of the bars cast iron is not possible with these blocks because the Heating to the casting temperature pyrolyzed the impregnating agent. The bars were therefore made with a carbonaceous  Putty attached (EP-PS 00 27 534).

With the impregnated blocks became the bottom of a cell for the melt flow electrolysis of the aluminum, the blocks are fixed against each other, the middle one Gap width between the blocks was about 1 mm. The Cell floor was made with an open oil burner Carbonization of the compound to about 600 ° C and by resistance heating to the operating temperature heated to approximately 1000 ° C. Connections formed layers between the blocks for the electrolyte and the molten aluminum is impermeable and their strength is about the strength of the blocks corresponds.

Claims (4)

1. A process for producing a cohesive connection between porous carbon moldings which are exposed to elevated temperatures, characterized in that the moldings are impregnated with a carbonizable impregnating agent which softens when heated, the surfaces of the impregnated moldings to be bonded are pressed against one another and the moldings are heated to a temperature above them the carbonization temperature of the impregnating agent are heated. 2. The method according to claim 1, characterized characterized in that moldings with a accessible pore volume used from 10 to 20% will. 3. The method according to claim 1 and 2, characterized characterized that pores with a Moldings containing diameters above 0.01 mm be used. 4. The method according to claim 1 to 3, characterized characterized that coal tar pitch as Impregnant is used.