Estudo e análise de instabilidade na mudança tecnológica de células electrolíticas de produção de alumínio, do modelo AP35 para o modelo AP3XLE: caso de estudo Mozal

Abstract

The production of primary aluminum is carried out through a process known as the Hall-Héroult process, which involves the electrochemical reduction of alumina into aluminum in electrolytic cells, commonly referred to as pots or electrolytic cells. The electrolytic cell technology used was designed by Aluminium Pechiney, a company that has invested significantly in developing models of electrolytic cells aimed at increasing production capacity while reducing aluminum production costs, minimizing the anode effect (caused by low alumina concentration in the electrolytic bath), and reducing greenhouse gas emissions. This study analyzes the instability resulting from the technological transition of electrolytic cells from the AP35 model to the AP3XLE model at MOZAL. The specific objectives of the study were to: identify the factors contributing to the instability of the electrolytic cells, identify and quantify the types of instability, and propose potential solutions to reduce instability during amperage increase. To address these specific objectives, data collection was conducted with the support of the laboratory in collaboration with the measurement team. Due to the instability crisis particularly evident in pot line 1 a detailed analysis was carried out on several operational quality parameters of the electrolytic cells, namely: iron and silicon concentration in the aluminum, temperature of the electrolytic bath, metal and bath height in the pot, pot superheat, energy efficiency, and other parameters used to monitor cell stability such as WRMI, LF (low frequency), and HF (high frequency). It was observed that the instability was most pronounced between mid-November 2023 and January 2024, resulting in a reduction of energy efficiency by approximately 4.85%, indicating low productivity and significant energy loss. During the crisis period, contamination of aluminum was recorded with peaks of 7650 ppm of iron and 750 ppm of silicon. As a result, most of the aluminum produced was not sold but rather stored in the harbour, at Matola. However, it was concluded that the electrolytic cells have dynamic behavior and that increasing the amperage without conditioning the cells to handle the increase was the main cause of the instability— especially anodic instability.