The present invention relates to a process for producing a coating having a titanium boride content of at least 80% by weight, a thickness of from 0.1 mm to 1 mm, a porosity of not more than 10% by volume and an oxygen content of less than 1% by weight by plasma spraying.
For the isolation of aluminum by electrolysis of aluminum oxide, aluminum oxide powder is dissolved in an electrolyte, known as the fluoride melt, which consists predominantly of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the bottom of the cell which consists essentially of carbon blocks, with the surface of the liquid aluminum or a solid which can be wetted by this forming the cathode. The electric current flows in on the cathode side through the carbon blocks which are joined to one another by heat- and corrosion-resistant adhesive or tamped compositions and are enclosed by a metal tank or a container. The electric current is supplied to the carbon blocks via conductor rails or bars which are let into recesses in the carbon blocks and are connected to the latter. Anodes consisting essentially of carbon in conventional processes and fixed to the anode bar dip into the electrolyte from above. At the anodes, the electrolytic decomposition of the aluminum oxide forms oxygen which reacts with carbon anodes to give carbon dioxide and carbon monoxide. The electrolysis generally takes place in a temperature range from 940 to 970xc2x0 C.
An important disadvantage of the cathode blocks made of carbon is that they are not readily wetted by the molten aluminum formed during operation of the electrolysis cell. For this reason, a comparatively thick aluminum metal layer covering the carbon blocks is necessary for operation of the cell. Since thick metal layers are considerably deformed by electromagnetic forces and convection currents, a comparatively large spacing is necessary between the carbon blocks and the carbon anodes located above the blocks in order to avoid possible short circuits. This leads to a higher electric power consumption of the cell. Furthermore, the metal flow generated at the phase interface between liquid aluminum/electrolyte leads, owing to the low interfacial tension, to increased chemical dissolution or to fine dispersion of the aluminum in the electrolyte. All dispersed aluminum which gets to the vicinity of the anode is reoxidized in contact with the anodically generated carbon monoxide and carbon dioxide to form aluminum oxide. This results in noticeable losses in the current yield. For these reasons, one is forced to employ an electrode spacing of from 4 to 6 cm, which is quite unusually wide for a process which usually operates at high current densities, so that the current yield losses do not become too high. To reduce the consequential higher voltage drop and energy consumption, the use of cathodes which are wetted by aluminum and allow smaller electrode spacings (interpolar distances) has therefore been proposed.
Although virtually all metals which are solid at the melting point of aluminum are readily wetted by aluminum, most of them have good to very good solubility in liquid aluminum or they form at least intermetallic phases with this very reactive metal. Only intermetallic compounds such as TiB2 and ZrB2 which have a high negative free enthalpy of formation, i.e. a high lattice energy, are resistant to liquid aluminum and are dissolved only slightly. Apart from wettability by liquid aluminum and resistance to liquid aluminum and cryolite/alumina melts, the ideal cathode material should meet further requirements: it has to have a sufficiently high mechanical strength to be resistant to thermal shock, be sufficiently electrically conductive and have sufficiently good adhesion to the underlying cathode blocks if it is in the form of a coating.
The potential advantages of the use of electrically conductive titanium diboride for this application have been known for over 25 years. However, attempts to use titanium diboride cathodes in commercial electrolysis cells have up to now foundered on their short life. The titanium diboride materials which were available were sensitive to intergranular penetration by aluminum metal, which finally ended in complete dissolution or destruction of the material. Other material properties completed the problem: titanium diboride is a very brittle material which is sensitive to thermal shock and has poor resistance to impacts or knocks.
However, today""s energy situation combined with developments and improvements in materials technology in the last 15 years has led to renewed attempts to develop titanium diboride cathodes. Owing to the high cost of titanium diboride and the problems in producing a cathode of solid titanium diboride, various cell linings or coatings have been developed. Processes for producing current-carrying titanium diboride elements in electrolysis cells for aluminum melt flux electrolysis are described in the following prior art:
One possibility is the application of tiles or other prefabricated parts of titanium diboride or material comprising titanium diboride. The titanium diboride tiles can be produced by hot pressing or sintering. DE-A-36 38 937 describes the fitting of parts shaped in the manner of a dovetail into the surface of the carbon cathodes in order to avoid adhesively bonded joints.
In U.S. Pat. No. 5,286,359, specially shaped parts instead of tiles are fixed in the cathode surface. These parts comprise titanium diboride, TiB2xe2x80x94Al or TiB2-graphite.
WO-A-8201018 describes a process in which TiO2, B2O3, petroleum coke and a binder are mixed, elements are shaped and subsequently calcined. The porous body formed here is impregnated with a boron compound and reheated to form a graphite-titanium diboride composite. These elements are preferably mushroom-shaped with a horizontal surface facing the underside of the anode.
Coating processes which apply a coating directly to the cathode surface are likewise used. U.S. Pat. No. 4,466,995 discloses a method of coating the cathode surface with a mixture of refractory hard material (RHM), heat-curable binder, solvent and carbon-containing filler material; the RHM is one of the compounds TiB2, TiC, ZrB2 or ZrC.
EP-A-0 021 850 describes, as coating process, the electrolytic deposition of TiB2 from a molten electrolyte which comprises titanium dioxide or a titanate as source of titanium and a borate as source of boron.
DE-A-23 05 281 describes a process for producing cathodes by applying a coating or an overlay of molten or highly sintered, dense refractory hard material to a surface. Hard materials used are the borides, nitrides, carbides and silicides of transition metals of groups IV to VI of the Periodic Table. This melt layer can be obtained either by heating to temperatures of from 2200 to 2300xc2x0 C. or by plasma spraying.
DE-A-23 12 439 describes a process for coating a container for a cathode tank. A thin coating of electrically conductive ceramic material is applied to this container by introducing the ceramic material into an ionized gas jet of high energy content and applying the material in a molten state. To provide protection against oxidation, the ionized gas jet is surrounded by a protective shroud of inert or reducing gas.
Thesis No. 22, Jan. 19, 1989, of the Rheinisch-Wesffxc3xa4lischen Technischen Hochschule Aachen, xe2x80x9cSpritztechnische Verarbeitung von Refraktxc3xa4rmetallen und Refraktxc3xa4rhartstoffen fxc3xcr Korrosions- und Verschleixcex2schutzanwendungenxe2x80x9d by Doris Jxc3xa4ger describes the vacuum plasma spraying of refractory metals and refractory metal compounds. 2 mm thick steel plates having an area of a few square centimeters are coated, inter alia, with titanium diboride, using a pressure of from 150 to 350 mbar. No indications of the oxygen content of the spraying powder used and the coating produced therefrom are given. The thin plates are cooled by means of argon during the coating process.
It can thus be seen that despite the fairly comprehensive efforts which have been made up to the present time and the potential advantages of using titanium diboride as current-carrying element, titanium diboride is up to now not commercially used as electrode material in the aluminum industry. The lack of acceptance is attributable to the short life in cell operation. It has been reported that such current-carrying elements have failed after only very short operating times or have not displayed the desired effect in terms of giving an energy saving.
In the case of sintered parts, this failure is linked to penetration by electrolyte or aluminum. It is known that penetration of liquid phases into the grain boundaries of sintered solids can have very undesirable effects. Titanium diboride tiles, for example, are susceptible to rapid attack by aluminum and/or cryolite melts at the grain boundaries, since the oxygen impurities preferentially accumulate at the grain boundaries during the sintering process. Past attempts to prevent the dissolution of titanium diboride tiles led to the use of high-purity titanium diboride powder. The oxygen content was less than 50 ppm and the price was three to four times higher than that of normal powder having an oxygen content of about 3000 ppm. A further disadvantage is that the production costs for the tiles increase considerably since particular sintering conditions have to be employed. No cell which has operated successfully with titanium diboride tiles for a relatively long time is known. The titanium diboride has either dissolved or the tiles have become detached from the substrate carbon owing to the different coefficients of thermal expansion of titanium diboride and carbon. Other reasons for the failure of titanium diboride tiles or coatings are the lack of mechanical strength or the low resistance to thermal shock.
Cathodes or cathode coatings made of a composite of titanium diboride with carbon, graphite and/or titanium carbide or similar hard materials are not particularly readily wetted by aluminum, so that no great energy saving effect is to be expected here. The additional constituents reduce the electric conductivity and the material which has been mixed in usually also reduces the durability since it is usually not very resistant to aluminum.
Thin Solid Films, 224, 1993, 148-152 teaches that titanium diboride coatings which have been produced by means of atmospheric plasma spraying can, owing to the method by which they are produced, contain up to 14% by weight of oxygen when the starting powder contained 2% by weight of oxygen. Thus, these coatings are very susceptible to corrosion by liquid aluminum. The use of a protective gas shroud around the plasma burner reduces the oxygen uptake by the reactive titanium diboride only to a still unsatisfactory 9% by weight.
Inert gas plasma spraying in which both the body to be coated and also the plasma burner are located totally in an inert gas atmosphere avoids the introduction of oxygen, but gas inclusions in the coating lead to a quite porous coating which offers a very large attack area for corrosion and erosion.
In our own experiments (Table I) plasma spraying under reduced pressure at from 10 to 200 mbar gave, in contrast to the thesis by Jxc3xa4ger, a minimal deposition efficiency, thus drastically reducing the economics of the process. The coating of small, thin plates leads to a completely different temperature distribution than in the case of large-area substrates, as indicated by the cooling with argon required by Jxc3xa4ger. This also results in a different coating structure, adhesion and a different deposition efficiency.
It is an object of the invention to provide a process for producing a coating comprising titanium boride for large-area substrates, in particular for carbon and/or graphite bodies, by means of which corrosion- and erosion-resistant, strongly adhering coatings which are suitable for use in an aluminum melt flux electrolysis cell can be produced. A further object is to propose an economically operating process which can be used for an industrial process.
This object is achieved by a process for producing a coated substrate in which the coating has a titanium boride content of at least 80% by weight, which process comprises applying a coating having a thickness of from 0.1 mm to 1 mm, a porosity of not more than 10% by volume and an oxygen content of less than 1% by weight to the surface of a substrate by plasma spraying in an atmosphere which is virtually or completely free of oxygen, with no metallic powder being added to the spraying powder.