Preparation Of TiB₂ Wettable Cathode Coating For Aluminum Electrolysis By Plasma Spraying

There are some superior properties for wettable cathode used in aluminum electorlysis, such as good wetting, shertening polar distance, extending cell life etc. Thus, there are immense potential on asving energy by application of wettable bathode on aluminum electrolysis. There is not carbonaceous elements in TiB₂ coating, which can avoid corrosion of TiB₂/C coating by electrolyte becasuse of containing carbonaceous element in TiB₂ coating, by application of plasma spraying technology. Futhermore, there are better conductivity, stronger adhesive strength, and good thermal shock resistance for TiB₂ coating prepared by application of plasma spraying technology. Therefore, the aim of the thesis is to prepare TiB₂ wettable cathode coating used on raphitiferous cathode block by application of plasma spraying technology in detail and reveal the mechanism and theory study on it. The results can guide aluminum industrial production and provide theorety for it.Firstly, the theory in the high temperature oxidation process of TiB₂ by application of plasma spraying thechnology has been investigated in the paper. Then the influence of parameters of plasma spraying on the deposition efficiency of TiB₂ coating has been studied. The microstructures, phase composition, bonding of coating and cathode carbon block have been investigated. Finally, the adhesive strength, electrical conductivity, porosity, wear resistance, microhardness, thermal shock resistance, wettability of aluminum liquid, and resistance to sodium infiltration, aluminum liquid and electrolyte were measured. And the influence factors have been analyzedand the effect of these factos on properties of TiB₂ coating have been studied. The main results have been obtained as following:?1? The thermodynamics and TG-DSC in the TiB₂ oxidation process have been studied. The results show that the resultants are TiO₂ and B₂O₃ both as solid phase at 120-492 ? and TiO₂ as solid phase and B₂O₃ as liquid phase at 492-1177 ?. In the above both conditions, the surface of TiB₂ covered by the oxidation products, which prevent oxidation of TiB₂ powder. However, when the temperature is higher than 1177 ?, the oxidation resultants are TiO₂ as solid phase and B₂O₃ as gas phase. The pores are generated because of escaping of B₂O₃, which decreases the oxidation resistance since TiB₂ reacts with O₂ transfer though the genarated pores.?2? The effect of spraying parameters on deposition efficiency have been investigated and the results show that deposition efficiency of TiB₂ coatings increases with increasing spraying distance. It is noted that will the loose coating structure will be obtained when spraying distance is bigger than 100 mm. The deposition efficiency of TiB₂ coatings increases firstly and then decrases with a maximum value with increasing primary flow rate, powder feeding rate, and spraying power. The deposition efficiency of TiB₂ coatings increases with decreasing particle size. The order of spraying factors effecting on adhesive strength and deposition efficiency have been obtaiend as:primary gas flow rate, spraying voltage, spraying distance, spraying current. The orthogonal experiment have been employed to obtaine the optimum conditions as primary gas flow rate of 2000 L·h-1, spraying voltage of 72 V, spraying distance of 90 mm, spraying current of 580 A, powder feeding rate of 27.34 g·min-1, carrier gas flow rate of 120 L·h-1, particle diameter ?d50? of 18 ?m, spray gun movement speed of 100 mm·s-1. Under the conditions, the adhesive strength and deposition efficiency of TiB₂ coatings are 7.2 N·mm-2 and 65.63%, respectively.?3? The oxidation process has been investigated by application of plasma spraying technology. The results show that the products of TiB₂ reacts with O₂ are TiO₂, B₂O₃, Ti ?BO3?, Ti2O3, Ti3B4, Ti3O5 etc. The amount of oxygen available decreases from 15.91% to 10.05% when Ar is used as protective atmosphere during sparing process. The large particles in the process of spraying are semi-melted and shape as dish. The smaller particles in the process of spraying are full-melted adm play the role of connecting the large particles of TiB₂. The mian bonding mechanism of TiB₂ coating and cathode blocks is mechanical bonding and metallurgically bonding in in partial areas. The adhesive strength ?7.2 N-mm-2? of TiB₂ coatings and carbon, which is produced by applicaition of plasma spraying, is significantly higher than TiB₂/C carbon adhesive coating ?3.10 N·mm-2?.?4? The high temperature resistivity have been studied and the resuts show that the resistivity of TiB₂ coating decreases with increasing temperature. The electrical resistivity of the TiB₂ coating decreases 0.0257 ?·cm to 0.018 ?·cm when Ar is used as protective atmosphere during sparing process at 960 ?. It give an evidence that the protection of the inert gas can reduce the resistivity of the coating significantly. The effect of particle size on the microhardness of the coating has been studied and the results show that microhardness of TiB₂ coating increase with decreasing of particle diameter. And the thermal shock resistance of TiB₂ coating has been studied. After 9 thermal cycles, there are some small part flaked and damaged of TiB₂ coating. The TiB₂ coating was immersed in the molten aluminum for 48 h and the results show that titanium content in aluminum increases from 0.0026 wt.% to 0.0042 wt.%.?5? Comparing the wettability of the TiB₂ coating with carbon block, the wettability of the TiB₂ coating with aluminum liquid is better than the graphite cathode block. The static corrosion contrast experiments are performed to investigate the the corrosion resistance of TiB₂ coating against sodium and melting cryolite. The results show that it is better than the graphitiferous cathode block. Only a small amount of F, Na, Al penetrated into the inner surface of the TiB₂ coating after electrolying for 4 hours. The TiB₂ coating prepared by plasma spraying technology is used in a 220 kA cell. The cost of TiB₂ coating ?1441 yuan/m2? by application of plasma spraying technology is lower than that of TiB₂/C coating ?2970 yuan/m2?. Furtheremore, plasma spraying technology performs environment friendly.Currently, the time efficiency of desposition TiB₂ coating by plasma spraying is poor, which limits its large-scale industrial applications. It should be further improved plasma spraying technology. Found higher efficiency and lower cost technology to deposit TiB₂ wettable cathode coating by plasma spraying technology, obtaining more promising of industrial application.