Abstract:
The electrochemical reduction of alumina to produce aluminium is accompanied by the evolution and emission of fluoride vapours and HF gas. About 0.1 – 1.2kg of gaseous and particulate fluorides (condensed fumes) are emitted per tonne of aluminium. By contacting the primary alumina feed with the exhaust gases from the Hall-Heroult cells, smelter gas treatment centres (dry scrubbers) return the captured fluorides to the process as a means of protecting the environment and minimizing the cost of aluminium fluoride additions. The HF scrubbing capacity of alumina has been shown by earlier researchers to correlate directly to its specific surface area. However, irrespective of the BET surface area, it is common for dry scrubbers to record higher stack emissions when gas concentration, temperature and humidity increase individually or simultaneously. The mechanism of reaction and the kinetic rate limiting steps need better understanding in order to explain the role of alumina microstructural properties and the HF gas conditions that determine fluoride capture capacity in a dry scrubber. In this research, alumina was reacted with HF gas in a laboratory scale fluidized bed reactor in sets of statistically designed experiments under regulated conditions of concentration, temperature and humidity until a definite, predetermined exit HF concentration was measurable with the laser sensing equipment. This point in the experiment is the breakthrough point. Fluoride loadings were estimated by numerical integration of the area under the breakthrough curves and confirmed by 19F solid state NMR measurements. XRD, SEM, XPS and nitrogen physisorption measurements provided valuable information about the fluorination product and the related alumina microstructural modifications. Statistical variance analyses (ANOVA) of the effects of temperature, concentration and humidity on fluoride loading revealed that the gas temperature was the most significant determinant of fluoride loading at breakthrough. The breakthrough loading is important because smelter GTCs operate in the kinetic equivalent zone preceding the breakthrough point. The HF concentration had no influence whatsoever while the effect of humidity on the breakthrough fluoride loading was non-linear and therefore could not be resolved statistically. Depending on reaction temperature and humidity, breakthrough fluoride loadings of about 0.8 – 3.5 wt% were attained. Post-reaction analyses of the fluorinated aluminas by nitrogen adsorption showed that the pore size distribution of SGA is important in the dry scrubbing of HF. A ‘Three-Zone Model’ developed in the course of this work adequately described a proposed surface area and pore volume loss mechanism as involving the rapid blocking and attenuation of pores <1.5nm by the reaction product — an initially amorphous, octahedrally-coordinated aluminum hydroxyfluoride (Al(OH)3-xFx). The reaction product ages into a crystalline phase in the presence of moisture during storage of the fluorinated SGA. Additionally, the model results suggested that pores <2.5nm were kinetically inaccessible while a relatively slower Knudsen diffusion was likely at play in pores <10nm. By calculating alumina effectiveness factors from experimental N2 physisorption data and extrapolating the Thiele modulus for alumina fluorinated in a time sequence, it was shown that pore diffusion limited the overall rate of reaction after an initial transient period of a chemical reaction control. Effectiveness factor is an estimate of the residual capacity of the alumina to capture more HF while the Thiele modulus approximates the evolving gas concentration profile. The importance of temperature was underpinned by the increasing trend of Thiele modulus with temperature, which implied that the rate of HF diffusion decreased with temperature. It is postulated that temperature increases promote a higher rate of product formation (chemical reaction) through increased pore wall collisions (Knudsen diffusion). The combined effects of rapid pore blocking/attenuation and slower Knudsen diffusion kinetics hinder the effective utilization of all the internal reactive sites within the alumina microstructure. The effect of humidity on the efficiency of HF scrubbing with alumina was similarly elucidated from effectiveness factor versus Thiele modulus plots. There was a decreasing trend of Thiele modulus with humidity, indicating that chemical reaction assumes importance when moisture content of the gas increases. This suggests that water molecules modify the surface chemistry of the alumina in an unclear manner, thereby possibly reducing the population and strength of available sites for HF reaction. These findings emphasize the pore size distribution of SGAs as the more important determinant of alumina scrubbing capacity instead of the industrially preferred BET surface area. Additionally, an important interplay between temperature and pore diffusion limitations has been shown to prevent the full utilisation of the alumina HF capture capacity. As environmental legislations regarding fluoride emissions tighten, the aluminium smelting industry would need to give more attention to low temperature dry scrubbing as well as sourcing aluminas with the adequate pore size distribution for capturing all the fluorides generated in the smelting process.