Phosphate solution wetting of graphite blocks for magnesium electrolysis to enhance their oxidation resistance. Part 1

Abstract

Graphitized electrodes are broadly used in industry. However, when they are used in high-temperature operating environments, they are subject to oxidation, which can lead to abnormal operation or premature failure of an electrolytic cell. Use of protective coatings or special wetting solutions (melts) help increase the oxidation resistance of a wide assortment of components. It is obvious that a coating that covers an electrode completely will hinder or stop the electric current from flowing at the electrode/electrolyte boundary, which makes this technique inapplicable to anodes for magnesium electrolysis. Aqueous solutions of phosphates are widely used around the world to make materials more resistant to high-temperature oxidation due to the formation of glassy phases during drying. This paper examines the efficiency of using a mixture of zinc and aluminium dihydrophosphates dissolved in an aqueous solution of orthophosphoric acid to enhance the oxidation resistance of the graphite electrode EGP (NR). A comprehensive thermal analysis was carried out to examine the solution for suitability. And X-ray diffractometry helped verify the formation of crystals after the solution had been dried. Cube-shaped specimens with the side length of 50 mm were used in the experiments aimed at identifying optimum graphite wetting and drying conditions. Isopropanol was used as a surfactant to ensure proper wetting. The specimens were first subjected to vacuum degassing for air to be removed from the pores, and then they were soaked in a rarefied solution. A kinetic model was selected to describe the post-wetting drying procedure. The oxidation resistance was analyzed in a dynamic air flow. The experiments were carried out at 700 oC as it is the highest possible temperature for magnesium electrolysis. The results of the experiments showed that the above wetting technique, when applied in a laboratory environment, helped achieve a five-fold increase in the oxidation resistance of the model graphite electrodes. The authors looked at the feasibility of scaling the experiments and developing process circuits to produce graphite with high oxidation resistance.