Measurement of SO2 concentration in effluent gas from aluminum electrolysis cell

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Sulfur dioxide (SO2) is emitted during the production of aluminum due to the presence of sulfur in raw materials (mainly coke, but also pitch) used in the production of anodes. As a result of increasing aluminum production and declining raw material quality, SO2 emissions continue to rise. Presently, SO2 and hydrogen fluoride (HF) emissions coming from electrolysis are treated with alumina at the gas treatment centers (GTC). The adsorption of HF and SO2 is competitive, and SO2 is desorbed as HF is adsorbed. Therefore, the removal of SO2 with alumina is not very effective. While the alumina containing HF is recycled back to the electrolysis cells, most of the SO2 stays in the effluent gas. Thus, the installation of a second gas purification unit, which will use hydrated lime as a reagent, after the gas treatment center could be a viable solution to remove SO2. Lime and other chemicals are used in different processes such as thermal power plants to remove SO2. However, operating conditions and gas compositions of these processes are quite different from those at the outlet of aluminum electrolysis cells. A Collaborative Research and Development Grant - Project (CRDPJ 535950 -18) entitled “Removal of SO2 with hydrated lime in aluminum smelter" was obtained in collaboration with Rio Tinto Aluminum. This project involves a fundamental study on the efficiency of SO2 removal from the effluent gases coming from the electrolysis cells using hydrated lime under the conditions specific to this process. The effects of temperature, humidity, particle size, particle porosity, SO2 concentration, presence of HF in the gas, accumulation of hydrated lime particles on the filter surface, and gas flowrate on SO2 capture by hydrated lime will be studied in the laboratory and at the plant. The kinetics of SO2 uptake will be determined experimentally. Three HQP are involved in the project. The project requires the continuous monitoring of SO2 concentration during the experiments. Presently, a gas chromatograph is being used for this purpose. However, this equipment is not very reliable. HQP is losing lots of time trying to get this equipment functioning properly. In addition, GC necessitates periodic manual injection of the samples which requires their presence next to the equipment as long as the experiment is continuing. This is difficult when the duration of the experiment exceeds the working hours, especially when there are many restrictions and limited working hours due to COVID-19. Also, some experiments have to be carried out in the presence of air in the gas, which represents the actual composition of the gas in the plant. However, GC can't handle this type of gas. Due to all the reasons given above, a reliable equipment which can measure the SO2 concentration automatically and continuously in the presence of air is necessary. The acquisition of such an equipment is the subject of the present RTI grant proposal.