Modelling HF Generation: The Role of Ambient Humidity

HF generation in aluminium reduction cells is known to occur by the reaction between fluorides (in a variety of forms) and a source of water or hydrogen. Studies on the generation of HF indicate that the major sources of moisture for hydrogen fluoride generation appear to be alumina, in the form of structural hydroxides (LOI 300–1000) and adsorbed moisture (LOI 25–300), and the ambient humidity. Several models have been developed to estimate the amount of hydrogen fluoride generated but these do not consider the individual sources of fluoride generation and cannot be easily used to assess the impact of changes in relative humidity. This paper presents a new model, still under development, which simulates the hydrolysis of pot gas from an open feeder hole. Initial results suggest that, in an open feeder hole, the high background concentration of hydrogen fluoride (3000–5000 ppm) presents, inhibiting the hydrolysis of pot gas, and thus the ambient humidity has relatively limited influence on the total fluoride emission. While the influence of ambient humidity has been the focus of this study, the model developed is intended to provide a framework that can be used to assess the sensitivity of fluoride emissions to other factors.