Crystallization of AlF2OH.H2O, for application in the recycle of fluoride from Spent Pot Lining (SPL)

Abstract

Spent pot lining (SPL) is a major waste generated in the process of producing primary aluminium (7-30 kg SPL per tonne of primary aluminium). It is the discarded carbon cathode and refractory lining of the smelting pots in the electrolytic process. This waste is classified as hazardous, mainly due to its leachable cyanide and fluoride content. This has led to a ban from disposal in landfills or water bodies by several environmental laws. Currently, about 1 million tonnes per annum of this waste is produced globally. This figure is projected to increase by 6 % per year by 2016.Over the years, much research has been carried out to offer an economic method of handling this waste, but to date no such method is widely accepted. Previous methods have aimed at inertization, but current efforts are driven towards recycling. Chemical leaching, followed by product recovery has been found to be a promising option. Previous work has successfully recovered up to 86 % of the fluoride values in the form of aluminium hydroxyfluoride hydrate, AlF2OH (AHF). Which could be subsequently converted to AlF3 for re-use in the smelters. There is currently very little information available on the properties or methods for obtaining AHF that is suitable for subsequent conversion.This project aimed at developing a controlled crystallization process for aluminium hydroxyfluoride hydrate from model solutions using established techniques of crystallization to improve the crystal properties. It also aimed to provide information on this compound for standard references. Model solutions were made to mimic the spent pot lining leachate solution and used throughout the studies.AHF was isolated and studied, and useful information of its solubility, supersaturation, metastability, crystal habit, nucleation and crystal growth kinetic are obtained. The obtained data is needed for the design of a suitable crystallizer/crystallization process.Co-precipitation of impurities is a major problem in crystallizing AHF. Major co-precipitates are cryolite (Na3AlF6), gibbsite (Al(OH)3) and sodium sulphate (Na2SO4). Results from this study have found that when NaOH is used for pH adjustment, a high fluoride to aluminium (F:Al) molar ratio favoured cryolite co-precipitation due to increased sodium (Na) in solution. While a low F/Al molar ratio favoured gibbsite co-precipitation due to the excess Al in solution. An optimum F/Al molar ratio was 2.0 ± 0.2. Solubility tests showed that AHF solubility was dependent mainly on the solution pH and to a lesser extent on temperature. It decreased with increasing pH/temperature and vice versa. The solubility product constant (log Ksp) at 25 o C was obtained as -24.3 ± 0.05. This value holds true for the definition of the solubility constant product in terms of the dissociation of AHF into its composing III ions. The heat of reaction (ΔHR) was obtained as −25.0 ± 2.9 kJ/mol while the enthropy (∆S) was obtained as -549 ± 9 J/K.The metastable zone width (MSZW) in the mother liquor was found to be narrow. The nucleation ...

A Zero-Waste Process for the Treatment of Spent Potliner (SPL) Waste

“…Aluminum hydroxyfluoride (AlF 2 OH) is of particular importance among the produced fluorides. It has a high market value and can be converted to aluminum fluoride (AlF 3 ), which is one of the important key materials for aluminum metal production and constitutes a major cost in it [19].…”

“…The majority of the chemical leaching processes of the SPL targeted fluoride recovery in the form of metal fluorides such as sodium fluoride (Villiaumite, NaF), calcium fluoride (CaF 2 ), sodium aluminum fluorides [e.g. cryolite (Na 3 AlF 6 ) and 5NaF.3AlF 3 complex], aluminum fluoride (AlF 3 ), aluminum hydroxyfluoride (AlF 2 OH) or aluminum hydroxyfluoride hydrate (AlF x (OH) (3-x) .xH 2 O, x = 1 or 2) [19]. The most valuable fluoride among these are AlF 3 and AlF 2 OH.…”

“…Around 76-86% of the fluoride was recovered from the SPL. It should also be noted that AlF 2 OH can be easily converted to AlF 3 by its reaction with HF [19].…”

“…An alternative to this two-step leaching process expressed by Eqs. ( 16) and ( 18), the Na 3 AlF 6 can be leached with an acidic Al 3+ solution comprised of Al(OH) 3 and H 2 SO 4 , which was found to be more effective than leaching with an acid only or an alkali only [41,19]. This acidic Al 3+ solution can be prepared according to Eq.…”

“…The resulting SPL fines are fed to an agitated semi-batch reactor filled with a preprepared H 2 SO 4 solution. To ensure that all the SPL particles are sufficiently exposed to the solution, a 2.5 M H 2 SO 4 (with 5 wt% excess) is used along with a recommended L/S ratio of 2.52 liters of H 2 SO 4 acid solution per kg of SPL [19]. The reactor contents should be kept under agitation for 2-4 h. A 40,000 tons of SPL is assumed to be processed annually (or 5930 kg/h based on a stream factor of 0.77).…”