1. Field of the Invention
The composition and the process which are the subject of the present invention concern a novel type of carbonaceous paste which is suitable for cold shaping, and its mode of use.
Generally, it is known for carbonaceous pastes to be used for forming a lining of carbonaceous materials in chambers which are to be used for containing all kinds of solid, liquid or gaseous materials which are often raised to high or very high temperatures, which materials comprise solid, liquid or gaseous alloys or metals, molten slags, molten electrolytes, etc. The carbonaceous pastes can also be used for forming vessels of carbonaceous materials, which are intended to contain corrosive materials such as acids, bases or salts and all kinds of corrosive compounds, in the solid or liquid state, at ambient temperature or at a temperature close to ambient temperature or at a relatively low temperature.
The vessel linings comprising carbonaceous materials may be formed entirely by applying a layer of carbonaceous paste according to the invention, in a suitable thickness, or by setting carbonaceous blocks, the jointing of which is effected by means of the carbonaceous paste according to the invention.
The jointing process is often referred to as luting, particularly when producing the carbonaceous coatings which line the bottom and the walls of aluminum electrolysis cells. The term luting is then used to denote the carbonaceous paste employed. The luting must be able to penetrate to the bottom of the spaces which occur between the carbon blocks. It must also wet the surface of the blocks, by penetrating into all their interstices. Finally, during a baking operation, the luting must be progressively converted into a carbonaceous residue which, in the manner of a cement, provides a solid bond between the adjacent carbonaceous blocks. It must also be possible for such a paste to be applied to all sorts of substrates, for example non-carbonaceous refractory materials or metal walls and, in this case also, it must be possible for the paste after baking to be converted into a stable self-sustaining rigid carbonaceous layer which adheres to the substrate on which it is deposited.
2. Description of the Prior Art
For making such linings, it is known to use carbonaceous pastes comprising a mixture of particles of coke, graphite and/or calcined anthracite, and/or other forms of calcined carbon, with a carbonaceous binding agent containing a petroleum and/or coal tar pitch and possibly a tar, with a softening point which is higher than ambient temperature. In general, so that such pastes can be compacted, they must be pre-heated to a temperature above 100.degree. C. Achieving good densification of the paste, which is absolutely necessary to achieve an optimum level of properties after baking, requires the use of suitable tools such as hammer-type or vibrating plate-type tools, and careful work. In particular, the paste must be maintained at a sufficient temperature for it to remain plastic throughout the period that it is being set in position.
Experience shows that this requirement for keeping the carbonaceous paste at a temperature higher than ambient temperature gives rise to serious difficulties.
Firstly, the hot paste gives off vapors which originate from the hydrocarbon binding agents which it contains. The vapors emitted by the tars and pitches are generally considered as being toxic and the safety and health regulations which prevail in various countries are becoming increasingly stringent as regards levels of concentration of such vapors in the atmosphere of the workshops and acceptable periods of time of exposure thereto. This means that it is necessary to have recourse to means for protecting the workers who are required to put the hot pastes in place, and such means often interfere with the careful and proper production of the linings.
In addition, it is necessary to have the required amount of paste at the desired temperature, at the moment at which it is required, and this necessitates careful pre-heating and mixing for the appropriate temperature level to be achieved throughout the quantity of paste involved. As the paste is being set in position, it rapidly cools in contact with the ambient air and the materials with which it is to be joined, e.g., carbonaceous blocks, metal walls or the like. In many cases, such localized cooling prevents the paste from penetrating into the interstices of the walls with which it is in contact, and limits its densification. This results in a defective joint which will subsequently result in cracking and perhaps even a rupture in the lining. Such phenomena can be particularly serious, for example when jointing carbonaceous blocks which line the bottom of an aluminum electrolysis cell. Indeed, during the operating life of the cell, the lining is in contact with liquid aluminum which is heated up to about 300.degree. C. above its melting point, and which therefore takes every opportunity to penetrate through the slightest crack. In order to remedy these difficulties, it is often necessary for not only the paste but the whole mass of the cathodic chamber and the carbon blocks which line the chamber to be pre-heated. This however is a long and expensive operation.
Finally, at least a part of the paste which has been pre-heated for example for the purpose of producing a plurality of joints between carbonaceous blocks will not be used in the required period and will consequently undergo cooling so that it cannot be used.
In order to remedy some of these difficulties, it has been proposed that a solvent which permits the pitch contained in the carbonaceous paste to remain plastic at ambient temperature, may be mixed with the pitch. Thus, U.S. Pat. No. 4,032,653 proposes producing the luting in aluminum electrolysis cells by using a carbonaceous paste whose pitch-base binding agent is made plastic at temperatures close to ambient temperature, by the addition of a solvent. The solvents used for this purpose are aromatic hydrocarbons with a boiling point of from 150.degree. to 350.degree. C., such as methylnaphthalenes. The examples show that a carbonaceous paste of this kind can effectively be used for luting carbonaceous blocks, at temperatures of the order of about 25.degree. C. Unfortunately, even at these relatively low temperatures, methylnaphthalenes have relatively substantial vapor pressures and using a carbonaceous paste containing same encounters serious restrictions set by the health and safety regulations in force. In addition, when the ambient temperature falls substantially below 25.degree. C., the plasticity of such paste falls fairly rapidly. Later, at the time of heating, i.e., baking, the linings which are thus formed all the aromatic solvent is given off into atmosphere and the problem of collecting it to minimize pollution again arises.
French Pat. No. 2,255,395 proposes a carbonaceous paste which can be shaped at ambient temperature and which contains a furane derivative and a pitch, with a high softening point, which is in powder form. Partial dissolution of the pitch in the furane gives the paste a sufficient degree of plasticity for it to be used at ambient temperature. After a certain period of time the paste hardens under the action of a catalyst which is contained in the dry materials. Just as when using a methylnaphthalene as the plasticizing agent, using furane derivatives also encounters the restrictions which are imposed by health and safety considerations. Indeed, the vapors emitted by furanes are dangerous to inhale, even at ambient temperature.
In addition, the presence of a catalyst in the carbonaceous paste, which causes the furane derivative to harden by polymerization, means that the paste must be shaped in a very short period of time after mixing. All this explains why the only use for such a carbonaceous paste, which is described in French Pat. No. 2,255,395, is the production of carbonaceous blocks by pressure molding a mixture comprising carbon grains which are partially impregnated with alkaline materials which act as a catalyst for polymerization of the furane derivative. It is known that, in practice, time restrictions would not make it possible to use a paste of this type for lining a vessel or for jointing carbonaceous blocks.