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Abstract
In this work, the failure of the pipelines used for recycling the enriched alumina to self-baked (Soderberg) cells is investigated. The study includes also the deterioration of the pipelines that is recycling the enriched alumina to pre-baked cells and that recycling fresh alumina. Systematic measurements for the loss in the pipeline –wall thickness with time are carried out. The gaseous emissions from the cells and the characteristics of enriched alumina are analyzed. Based on different analyses and visual inspections, a corrosive- wear sequence was identified, for the failure of pipelines of Soderberg system. The scales formed on the pipe inner wall are easily destroyed by the alumina impact due to the weak adherence of such scales. This resulted in the exposure of the fresh steel surface to the highly corrosive environment that prevails inside the pipelines, which accelerates the corrosion of the metal. For the pipelines recycling enriched alumina to pre-baked system, hard and dense layers were formed, which are strictly adherent to substrate. The loss in the pipelines wall thickness of the fresh and pre-baked alumina pipelines is negligible compared to Soderberg enriched lines (0.5 versus 5.0 mm/year); however, the operating conditions of the particles impact (velocity, particle size, flux) were close to each other in value. Therefore, the failure mode of pipelines in the Soderberg system is an erosion-corrosion process. Field tests were designed to evaluate alternative materials of construction. It was found that the alteration of chemical compositions of a material may affect both erosion and corrosion resistance. The results showed that under severe erosion-corrosion conditions, increasing corrosion resistance were beneficial to decreasing synergistic weight loss rate. Steel with 1% chromium and relatively high percentage of Silicon 0.3% and Manganese 1.3% was the more resistant. Steels with minor alloying elements of Molybdenum, Niobium have resistance more than carbon steel. It was found also that the corrosive rate of carbon steel increases with increasing carbon content. Horizontal parts of the pipelines were the more deteriorated than vertical parts because of alumina accumulated on the bottom during the stagnant periods. Methods of corrosion protection could be employed to protect the pipelines like proper materials selection and improving the design by reducing air velocity carrying alumina with reducing alumina flow rate to avoid saltation problems.