The Pivotal Role of Alumina Pore Structure in HF Capture and Fluoride Return in Aluminum Reduction
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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 m2 g−1 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.
KeywordsPore Size Distribution Hydrogen Fluoride Pore Blocking Knudsen Diffusion Hydrogen Fluoride
Maintenance of the fluoride balance in an aluminum smelter is extremely important. Fluorides, which form a vital component of the electrolyte, may be lost and emitted to the environment if converted to either volatile particulates or hydrogen fluoride (HF). These compounds are generally toxic.1, 2, 3, 4, 5 Additionally, fluctuations in fluoride level are associated with losses in cell stability and ultimately energy efficiency.6 These losses are typically compensated for by AlF3 addition to the cells, but this constitutes a materials input and cost to the smelter.
To close the fluoride balance and reduce the impact of the smelter on the local environment, alumina is used as an HF dry scrubbing agent prior to being fed to the cell for electrolysis. As a consequence of the strength of the Al-F bond, HF binds irreversibly to the alumina surface.7 It is therefore an excellent medium for scrubbing and fluoride return. Within the smelting community, surface area is typically the parameter by which the efficacy of an alumina as a scrubbing agent is judged: the higher the specific surface area (SSA), the more sites available for gas capture and the more effective the material is presumed to be. The Brunauer-Emmett-Teller (BET) method.8 is universally employed for SSA determination. While aluminas may achieve SSAs up to ~400 m2 g−1, balancing other smelter properties has led the consensus to settle on 70–80 m2 g−1 as a target BET SSA.
However, very little attention has been paid to the microstructure which generates this surface area and the accessibility of the pores which contribute to this. A high SSA material is an ineffective scrubbing agent if much of the porosity cannot be accessed fully on time scales relevant to dry scrubbing. This consideration has been recently been identified by Perander et al.9,10 who suggest that the finer porosity may be difficult to access. This becomes relevant when it is recognized that the SSA of a material of under- and over-calcined aluminas blended together may have a comparable BET SSA to a more evenly calcined material (so-called bimodal aluminas). However, the blended sample will have a much higher proportion of the porosity associated with these small pores.
Our alumina group within the Light Metals Research Centre has recently performed a number of studies examining the role of pore size on HF capture efficiency11,12 and its role in fluoride return to the reduction cells.13 In this work, we review key elements of these studies, and address the overall implications when these results are taken together. These are supplemented with preliminary results from a model we are currently developing to describe the aging of alumina porosity during reaction with HF. We show that pores smaller than approximately 3.5 nm are not efficiently utilized in fluoride capture during dry scrubbing. In this region of porosity, HF enters a surface diffusion regime that for kinetic reasons inhibits HF from reaching adsorption sites. The mechanism of pore evolution as scrubbing proceeds is also detailed. As a consequence, the universally used BET SSA predictor cannot be used to accurately determine HF scrubbing capacity or fluoride return efficacy. In terms of scrubber performance, there is the potential to introduce a new, more useful SSA predictor to replace this value on alumina specification sheets.
The Impact of Alumina Pore Size on Fluoride Balances in an Operating Smelter
We have correlated several alumina properties with important parameters relevant to smelter operation such as HF generation,13 current efficiency and energy consumption.6 In these studies, data from a wide range of smelter parameters were assimilated into 572-point-long vectors, one vector for each parameter. Each data point in a vector represents a daily mean value of that parameter, and was obtained after averaging over 24 h of observation per cell, and further averaged over 60 cells running a common alumina. Each vector is referred to as a predictor. This set of predictors was supplemented with analogous vectors associated with weather conditions (in particular, relative and absolute humidity) and alumina properties (gibbsite and α-Al2O3 phase fractions, and SSAs). The collection of vectors less one, the subject of the model, served as the basis for multiple regression analyses where insignificant predictor vectors (at the 95% confidence level) were removed to produce a final multiple regression model. A full overview of model construction is given in Refs. 6 and 13. Studies were performed at TRIMET’s Essen smelter employing an injector-type dry scrubber; for the interested reader, full details of the gas treatment centre (GTC) system is provided by Iffert et al.14
One of the models of particular relevance to the current work is that of the excess AlF3 levels of the cell (XsAlF3). It is reasonable to assume that bath fluoride levels should correlate with the surface area of the feed alumina as one of the predictors: the higher the surface area, the greater its ability to capture fluoride and return this to the pots, thereby increasing the XsAlF3 levels. That is, XsAlF3 should exhibit a positive correlation with alumina SSA. However, as the potline ran two different alumina sources concurrently, both competing for fluoride, SSA could not be used directly. Instead, a differential SSA, ΔSSA(a), was used in recognition of this competitive adsorption, defined as the difference in SSA of concurrently running aluminas a and b, where alumina a is in the end of the potline being studied.
PSD Reconstruction Models
In a series of related studies, we have examined changes induced by HF in the PSDs of smelter-grade alumina through carefully controlled laboratory studies. These employ an HF source in a purpose-built dosing apparatus.11,12 SSA decreases are observed with preferential losses from the smaller pores (typically < 20 nm).11,12 From this, it is possible to develop a model describing PSD changes and benchmark this against aluminas dosed under both laboratory and industrial conditions.
PSDs effectively provide the total surface area of a pore at a particular pore diameter D p. If an explicit pore model, typically assuming either cylindrical or slit pore geometries, is adopted, this is sufficient to determine all pertinent geometric parameters of an idealized pore. In particular, an effective pore length, L p, can be assigned. Perturbation of the pore geometry after assuming a particular model of gas transport and surface area loss provides a powerful tool by which the mechanism of pore aging can be assessed. Note that such manipulations do not make sense with the common BET method. This approach provides a single value for the total surface area and ignores surface geometry entirely. As such, there is no means of extracting any pore diameter dependence.
Note that this has an identical form to Eq. 2 but with different fitting parameters.
With the porosity partitioned appropriately, a model of surface area loss must be applied to each partitioned subset, after which the subpores may be rebinned to give a simulated post-dosing PSD. In the smallest pores, all surface area is set to zero. This is a consequence of porosity being blocked following fluoride exposure. In the middle region, associated with kinetically inhibited access, no model of loss is imposed as it is assumed that HF cannot reach the surface to react.
The term A([HF]) is the constant of proportionality. This will also subsume any small corrections associated with slit versus cylindrical geometry and therefore this approach is applicable regardless of the underlying pore geometry. We have derived this term explicitly in previous work,11 but its exact form is not particularly instructive here and will not be discussed further. However, it has been shown to be a function that increases with HF exposure and its role is to account for the fact that a blocking event becomes increasingly likely as more adsorbate deposits on the alumina surface. The pore length of the secondary alumina, L s, may be calculated as L s = L p P blocked(D p), and the new pore surface area computed from this perturbed pore length.
Previous work21 on HF capture in dry scrubbing has focused on determining monolayer thickness t m of the adsorbate on the alumina surface. These studies have implicitly assumed a shrinking-core model of a cylindrical pore that the results above have demonstrated does not comport with observation. However, the thickness of the adsorbate phase is still an important parameter. Our earlier results demonstrate that small pores are ineffective in fluoride return to the reduction cell through the secondary alumina. The blocking of pores, removing useful surface area, is an obvious possible mechanism to describe this phenomenon, as the smallest pores will be the most susceptible. The pore partitioning/attenuation model provides two independent estimates of t m for a given alumina. The first is determined when fitting the partitioning between kinetically inaccessible and surface blocked pores (Eq. 4), the second in determining the new pore length after attenuation (Eq. 5). In a previous study, we reported an average value11 of t m = 0.74 ± 0.16 nm. This was determined over a range of very different aluminas similar to those depicted in Fig. 6 subject to a considerable variation in fluoride loadings from both industrial and laboratory fluoride sources. The low error demonstrates the consistency of these values despite the varied alumina and fluoride source conditions. This indicates that this parameter is fundamental to the underlying chemistry and not simply a fitting artefact. This value is also in excellent agreement with previously determined estimates of 0.65–0.85 nm.21
These results, taken together, provide very strong evidence that diffusion effects, and not simply pore blocking, must play the decisive role in determining HF capture. This therefore requires that useful surface area be associated with pores significantly larger than one may predict on the basis of the small size of the HF molecule.
The total diffusion coefficient, Ď tot, is a function of pore diameter; note the accent used to distinguish diffusivities Ď from pore diameters D. It is in this way that different diffusion models may be explored.
The term α gives the jump distance, with Z surrounding sites to which HF may jump. The key feature is that this is an activated process, dramatically slowing diffusion (by several orders of magnitude) relative to Knudsen diffusion.27 Study into zeolites indicates that this regime is initiated in pores 1–5 nm in diameter;26 the 3.5 nm threshold advocated here falls squarely in this range.
The results of recent studies undertaken by our group have led to several conclusions that have a tangible impact on predicting the nature of fluoride cycling in the smelting process. First, from data taken exclusively from an operating smelter, a pore diameter threshold of 3.5 nm is identified below which the pores are ineffective in fluoride return through HF capture. We have previously demonstrated that this affects the stack emissions,13 and also influences the fluoride balance of the pot. We have also linked this to energy efficiency.6 This indicates that the BET SSA, as reported on alumina specification sheets, is not ideal in predicting scrubbing or fluoride return efficiency, as this method necessarily includes this small pore surface area. Rather, the authors advocate a BJH-based estimate, requiring PSD knowledge, where pores ≤3.5 nm are explicitly omitted from the surface area. We note that current technology may make alumina quality concerns secondary. However, trends to decrease energy consumption28 look to utilizing every available means to gain every bit of energy efficiency, and alumina quality is useful here. A close examination of the fundamental processes is therefore needed. Further, these insights will be invaluable in meeting future challenges. For example, phenomena such as low voltage anode effects29 indicate alumina solubility is increasingly problematic, particularly as cells get larger. A shift to lower degrees of calcination may be one means to aid solubility, but our studies demonstrate that we cannot indefinitely lower this calcination degree with the expectation that the increased SSA will compensate for the increased emissions. The pore diameter threshold advocated here represents a fundamental limit that GTC operations cannot circumvent. This may also become a problem in distributed pot suction methods for raw gas treatment;28 the lower gas volume is associated with higher HF concentrations and temperatures, the latter both effectively “shrinking” the GTC bringing alumina quality back to prominence. These future trends may well benefit from knowing the “true” accessible SSA, with the BJH (<3.5 nm) method a direct means of achieving this.
PSD reconstructions of aluminas after the dry-scrubber reveal insights into the fundamentals of this process. We have demonstrated that surface area loss is not due to mono/multilayer formation (a shrinking core model), but rather due to pore shortening, a consequence of blocking along the pore length. From this, we have extracted a monolayer thickness (the size of the hydroxyfluoride phase formed on reaction) of 0.74 ± 0.16 nm, in good agreement with previous results.21 This indicates that pore blocking alone cannot be responsible for the slowed uptake as this only accounts for pores smaller than 2t m = 1.48 nm. More sophisticated models demonstrate that this is very likely controlled by surface diffusion. This, and the model input requiring population of sites with accessible or inaccessible regions, indicates that a strong surface chemistry component is active in dry scrubbing. This may allow for some degree of tuning around which improved scrubbing performance could be obtained, and work is proceeding towards this objective.
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