Measurement of SO2 concentration in effluent gas from aluminum electrolysis cell

由于用于生产阳极的原材料(主要是焦炭，但也包括沥青)中存在硫，所以在铝生产过程中会排放二氧化硫(SO2)。由于铝产量的增加和原材料质量的下降，二氧化硫排放量继续上升。目前，电解排放的二氧化硫和氟化氢(HF)是在气体处理中心(GTC)用氧化铝处理的。HF和SO2的吸附是竞争吸附，当HF被吸附时，SO2被解吸。因此，氧化铝对SO2的脱除效果不是很好。虽然含有氟化氢的氧化铝被回收回电解槽，但大多数二氧化硫仍留在废气中。因此，在气体处理中心之后安装第二个气体净化装置，它将使用消石灰作为试剂，可能是一个可行的解决方案来消除二氧化硫。石灰和其他化学品用于不同的过程，如火力发电厂，以消除二氧化硫。然而，这些工艺的操作条件和气体成分与铝电解槽出口的操作条件和气体成分有很大的不同。与力拓铝业合作，获得了名为“铝冶炼厂消石灰脱除二氧化硫”的合作研究与发展资助项目(CRDPJ 535950-18)。本项目涉及对在特定工艺条件下使用消石灰从电解槽排出的废气中脱除二氧化硫的基础研究。实验室和工厂将研究温度、湿度、颗粒大小、颗粒孔隙率、SO2浓度、气体中HF的存在、消石灰颗粒在过滤器表面的积累以及气体流量对消石灰捕获SO2的影响。吸收SO2的动力学将通过实验确定。该项目涉及三个HQP。 该项目需要在实验期间对SO2浓度进行连续监测。目前，一台气相色谱仪正被用于这一目的。然而，这台设备并不是很可靠。HQP在努力让这台设备正常运行方面浪费了很多时间。此外，GC需要定期手动注射样品，只要实验仍在继续，样品就必须出现在设备旁边。当实验持续时间超过工作时间时，这一点很难做到，特别是由于新冠肺炎的原因，限制很多，工作时间有限。此外，一些实验必须在气体中存在空气的情况下进行，空气代表工厂中气体的实际组成。然而，GC不能处理这种气体。因此，需要一种可靠的、能在有空气存在的情况下自动、连续测量SO2浓度的设备。购置这类设备是目前信息技术创新拨款提案的主题。

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.