Measurement of SO2 concentration in effluent gas from aluminum electrolysis cell

在铝的生产过程中，由于阳极生产中使用的原材料（主要是焦炭，但也有沥青）中存在硫，因此会排放二氧化硫（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，来自电解的SO2和氟化氢（HF）排放物在气体处理中心（GTC）用氧化铝处理。HF和SO2的吸附是竞争性的，SO2在吸附HF的同时脱附。因此，用氧化铝去除SO2不是很有效。虽然含HF的氧化铝被再循环回电解槽，但大部分SO2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（将使用熟石灰作为试剂）可能是去除SO2的可行解决方案。石灰和其他化学品用于不同的过程，如热电厂，以去除SO2。然而，这些过程的操作条件和气体组成与铝电解槽出口处的操作条件和气体组成有很大不同。与Rio Tinto Aluminum合作获得了一项名为"在铝冶炼厂中用熟石灰去除SO2"的合作研究和开发资助项目（CRDPJ 535950 - 18）。该项目涉及在特定于该工艺的条件下使用熟石灰从来自电解槽的废气中去除SO2的效率的基础研究。将在实验室和工厂研究温度、湿度、颗粒尺寸、颗粒孔隙率、SO2浓度、气体中HF的存在、过滤器表面上熟石灰颗粒的积累以及气体流速对熟石灰捕获SO2的影响。将通过实验确定SO2吸收的动力学。三名HQP参与了该项目。 该项目要求在实验期间连续监测SO2浓度。目前，气相色谱仪正用于此目的。但是，这种设备并不可靠。HQP正在浪费大量的时间试图让这个设备正常工作。此外，GC需要定期手动注入样品，只要实验继续进行，样品就需要靠近设备。当实验的持续时间超过工作时间时，这是困难的，特别是当由于COVID-19而有许多限制和有限的工作时间时。此外，一些实验必须在气体中存在空气的情况下进行，这代表了工厂中气体的实际组成。然而，GC不能处理这种类型的气体。由于上述所有原因，需要一种可靠的设备，可以在空气存在的情况下自动连续测量SO2浓度。这种设备的采购是目前RTI赠款提案的主题。

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.