| To elevate the economic benefit of the calcium enterprises and to strengthen the ability of competition in the marketing, the calcium enterprises must reduce the production costs by all the means, that is, decrease the loss of the raw and auxiliary materials and energy consumption and so on in the process of calcium molten salt electrolysis. Graphite anode is one of the important consumption in the process of calcium molten salt electrolysis, so it is important to research damage mechanism of the graphite anode and reduce its spoilage.The spoilage of the graphite electrode is analyzed in this thesis. It is thought that the chemical consumption and mechanical damage are the main factors. The oxidation kinetics of the graphite electrode is especially analyzed. The damage mechanism of the graphite anode in the calcium molten electrolysis process is investigated. In theory, The Cl2, HCl, CO2 produced in the calcium molten electrolysis process and the electrolyte can't corrupt the graphite anode. With the analysis of SEM and electron probe, the destroying mechanism is put forward. The electrolyte volatilizes and condenses in the aperture in anode, then partly hydrolyzes and dewater, then the stress are produced by expansion in this process. The expansion stress is the dominant cause of the graphite anode damage, and the oxidation of the active graphite particles on the anode surface is subsidiary for the anode damage. The measures of prolonging the anode service life are to enhance the quality of the anode such as increasing the forming pressure in the preparation and surface treatment before application. So we can use the coating to deal with the surface of the graphite anode, that is, to smear a thin coating material on the surface of anode to fill in the holes and eliminate the contact between the active particles and the oxygen and the electrolyte. The protection coating can separate the graphite from the oxidization air to prevent from oxidization.An overview of current status of research graphite electrode to high temperature oxidation resistant is provided in this thesis. The main meansare impregnating and coating electrode, the methods of the preparation of the coating includes impregnation method, chemical vapor deposition, plasma spraying and the sol-gel method. The requirement of the coating materials of the graphite anode in the calcium molten electrolysis are that the coating used must resist the corrosion to Cl2, H2O and hydrochloric acid, while it has not affect on the calcium molten salt electrolysis, and that the price of the coating itself is low.The protection coating, which has be used on graphite anode of calcium molten salt electrolysis, is developed. The fitting coating materials are SiO2, B2O3 and soluble glass. It is found that the coating is compact without crack and with strongly adhesive. It also can be concluded that to control the temperature of the coating heat treatment is very important.The XRD, SEM analysis, the oxidation weigh loss and annealing circulation experiment with the coating are done. The results indicates the coating is morphology of boric glass which can permeate into the holes of the graphite anode and seal the cracks developed by the coating, and it is adhesive strongly to the graphite anode. It demonstrates that the coating had an excellent oxidation resistance characteristic. The contrast of the oxidation weight loss of three kinds of samples is done. The graphite anode sample without coating oxidizes severely. The oxidation rate is quick with the hoist of the temperature and the weight loss increased with the time pronging at certain temperature. The sample with SiC coating can only improve the anti-oxidation ability a little. The boric glass, however, can improve the anti-oxidation ability evidently, whose oxidation weight loss and rate are change slightly. The annealing circulation with the boric glass coating is done at 800C and 850C, and the results demonstrate the sample has a good impulsion ability. The results show the coating is effect... |
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