Aluminum is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperature up to around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminum pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminum pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminum through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks, aluminum reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminum metal produced.
A major drawback of carbon as cathode material is that it is not wetted by aluminum. This necessitates maintaining a deep pool of aluminum (at least 100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminum and, to avoid short-circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminum pool and the anode. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals or Refractory Hard Materials (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminum, have a low electrical resistance, and are wetted by aluminum. This should allow aluminum to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminum pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminum, resistant to the corrosive environment of an aluminum production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the savings of energy by reducing the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminum production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,214,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminum industry.
The non-acceptance of tiles and other methods of applying layers of TiB.sub.2 and other RHM materials on the surface of aluminum production cells is due to their lack of stability in the operating conditions, in addition to their cost. The failure of these materials is associated with penetration of the electrolyte when not perfectly wetted by aluminum, and attack by aluminum because of impurities in the RHM structure. In RHM pieces such as tiles, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminum metal and/or by cryolite. To combat disintegration, it has been proposed to use highly pure TiB.sub.2 powder to make materials containing less than 50 ppm oxygen. Such fabrication further increases the cost of the already-expensive materials. No cell utilizing TiB.sub.2 tiles as a cathode is known to have operated for long periods without loss of adhesion of the tiles, or their disintegration. Other reasons for failure of RHM tiles have been the lack of mechanical strength and resistance to thermal shock.
Various types of TiB.sub.2 or other RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the RHM material and the carbon cathode block.
U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride and other Refractory Hard Metals to a conductive substrate such as graphite. But large differences in thermal expansion coefficients between the RHM tiles and the substrate cause problems.
U.S. Pat. No. 5,320,717, the content which is incorporated herein by way of reference, provides a method of bonding bodies of Refractory Hard Material (RHM) or other refractory composites to carbon cathodes of aluminum protection cells using a colloidal slurry comprising particulate preformed RHM in a colloidal carrier selected from colloidal alumina, colloidal yttria and colloidal ceria as a glue between the bodies and the cathode or other component. The slurry is dried to bond the bodies to the cathode or other component, the dried slurry acting as a conductive thermally-matched glue which provides excellent bonding of the bodies to the cathode or other component.
U.S. Pat. No. 5,310,473 discloses a method of producing a protective refractory coating on a substrate of, inter-alia, carbonaceous materials, by applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants in a colloidal carrier, and initiating a micropyretic reaction. The micropyretic slurry optionally also contains some preformed refractory material, and the micropyretic slurry may be applied on a non-reactive sub-layer.
U.S. Pat. No. 5,364,513 (MOL0513) discloses a body of carbonaceous or other material for use in corrosive environments such as oxidizing media or gaseous or liquid corrosive agents at elevated temperatures, coated with a protective surface coating which improves the resistance of the body to oxidation or corrosion and which may also enhance the body's electrical conductivity and/or its electrochemical activity. This protective coating--in particular silica-based coatings--is applied from a colloidal slurry containing particulate reactant or non-reactant substances, or a mixture of particulate reactant and non-reactant substances, which when the body is heated to a sufficient elevated temperature form the protective coating by reaction sintering and/or sintering without reaction.
PCT application PCT/US93/05142, (MOL0521) (the content of which is incorporated herein by reference), claiming priority from U.S. Ser. No. 07/898,052 (now U.S. Pat. No. 5,364,513), discloses a carbon containing component of an aluminum production cell, which is protected from chemical attack by liquid and gaseous components of cryolite by a coating of pre-formed refractory hard metal boride in a dried colloid. It has been discovered, quite unexpectedly, that the same coating used in the PCT/US93/05142 is useful in preventing erosion due to movement of the cryolite and of alumina. Erosion as used herein does not include any chemical attack or reaction. Very few prior art documents have been found which specifically address the problem of non-chemical erosion, as described above. U.S. Pat. No. 4,308,113 describes a method of reducing erosion by incorporating alumina and titanium dioxide into the cathode. U.S. Pat. No. 4,333,813 recognizes non-chemical erosion as being caused at least in part by turbulence of the aluminum pad.