The Pivotal Role of Alumina Pore Structure in HF Capture and Fluoride Return in Aluminum Reduction

Abstract

Fluoride emissions during primary aluminum production are mitigated by dry scrubbing on alumina which, as the metal feedstock, also returns fluoride to the pots. This ensures stable pot operation and maintains process efficiency but requires careful optimization of alumina for both fluoride capture and solubility. The Brunauer-Emmett-Teller (BET) surface area of 70-80 m 2 g À1 is currently accepted. However, this does not account for pore accessibility. We demonstrate using industry-sourced data that pores <3.5 nm are not correlated with fluoride return. Reconstructing alumina pore size distributions (PSDs) following hydrogen fluoride (HF) adsorption shows surface area is not lost by pore diameter shrinkage, but by blocking the internal porosity. However, this alone cannot explain this 3.5 nm threshold. We show this is a consequence of surface diffusion-based inhibition with surface chemistry probably playing an integral role. We advocate new surface area estimates for alumina which account for pore accessibility by explicitly ignoring <3.5 nm pores.

The Adsorption Behavior of Moisture on Smelter Grade Alumina during Transportation and Storage—for Primary Aluminum Production

“…However, the chemical adsorption of gaseous HF on SGA is believed to be more complicated based on a subsequent chemisorption process after the diffusion of HF molecules into the micropores of the SGA particle. Many earlier studies have investigated the kinetics of HF adsorption on SGA, demonstrating that the HF adsorption rate is highly related to the moisture content [6], sodium content [13], specific surface area, and pore size distribution [14]; however, the adsorption mechanism of HF remains unclear [8]. Nevertheless, given the similar diffusion dynamics of gas molecules in mesoporous materials, some initial adsorption processes of HF may resemble the mass transfer process of H 2 O from the outside to the internal pore surface of SGA.…”

Probing hydrogen bonding interactions and impurity intercalation in gibbsite using experimental and theoretical XANES spectroscopy

“…[7][8][9][10][11] However, by far the largest consumer is the primary aluminium production industry that, in addition to using alumina as the primary feedstock, employs it as the dry scrubbing agent for hydrogen fluoride generated in the reduction cell. [12][13][14] With many of alumina's applications crucially dependent on its surface chemistry, it is necessary to understand the evolution of these properties from the precursor to final product.…”

The surface chemistry of metallurgical aluminas

“…In a typical smelter, the dry‐scrubbing units clean thousands of cubic metres per second of the cell off gases to both limit environmental emissions, particularly of fluorides, and to stabilise cell chemistry through the recycling of the recovered fluorides to the reduction cell . The introduction and evolution of point feeders and dry‐scrubbing technology over the past 30 years have caused alumina properties to converge on a specific surface area (SSA) between 70 and 80 m 2 g −1 and particle size distribution centred around 70 μ m.…”

“…Firstly, undercalcination results in increased residual structural hydroxide content bound to the alumina surface, representing an important source of HF generation. Secondly, increased surface area moves the pore size distribution to narrower, more constrained pores, which limit the kinetic capacity of injector type scrubbers . Recent work suggests that pores below around 3 nm in diameter make a significant contribution to the SSA, but very little contribution to the adsorption of HF .…”

“…Secondly, increased surface area moves the pore size distribution to narrower, more constrained pores, which limit the kinetic capacity of injector type scrubbers . Recent work suggests that pores below around 3 nm in diameter make a significant contribution to the SSA, but very little contribution to the adsorption of HF . Thus, a gibbsite precursor and calciner technology that favour these small pores produce an alumina that is not optimised for dry‐scrubbing.…”