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

Ntuk, Ubong Ubong (2016). Crystallization of AlF2OH.H2O, for application in the recycle of fluoride from Spent Pot Lining (SPL) PhD Thesis, School of Chemical Engineering, The University of Queensland. doi:10.14264/uql.2016.360

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Author Ntuk, Ubong Ubong
Thesis Title Crystallization of AlF2OH.H2O, for application in the recycle of fluoride from Spent Pot Lining (SPL)
Formatted title
Crystallization of AlF2OH.H2O, for application in the recycle of fluoride from Spent Pot Lining (SPL)
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
DOI 10.14264/uql.2016.360
Publication date 2016-06-20
Thesis type PhD Thesis
Supervisor Karen Steel
Edward White
Total pages 304
Total colour pages 64
Total black and white pages 240
Language eng
Subjects 0914 Resources Engineering and Extractive Metallurgy
0302 Inorganic Chemistry
0904 Chemical Engineering
Formatted 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 oC 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 ions. The heat of reaction (ΔHR) was obtained as −25.0 ± 2.9 kJ/mol while the enthropy (ΔS) was obtained as -549 ± 9 J/K.

The metastable zone width (MSZW) in the mother liquor was found to be narrow. The nucleation thresholds, just like the solubility was best expressed in terms of the solution pH. The corresponding supersaturation was however obtainable. At the conditions tested, typical starting solutions were saturated from pH 3.0 ± 0.5. At 25 oC, the 3 hour nucleation threshold was reached at pH 4.3 ± 0.2 which corresponded to a dimensionless relative supersaturation of 23 ± 2. The results strongly suggested that the secondary nucleation threshold (SNT) was very close to the primary nucleation threshold (PNT) and was treated as inseparable. The metastable zone width was also found to narrow with increasing temperatures.

Measurements were made of the crystal nucleation and growth kinetics. Two methods of generating supersaturation were found to be; pH control (partial neutralization) and by water evaporation. Partial neutralization promoted crystal nucleation over growth. The nucleation rate had a first order dependency on solution supersaturation. The nucleation rate constant Kn was obtained as 5.0 × 1010 #/min/ (kg slurry)/ (unit α). The growth constant Kg was obtained as 0.040 ± 0.002 μm/hr/unit α.

Interestingly, water evaporation promoted crystal growth over nucleation whereby zero nucleation was measured. The linear growth rate was obtained as 16 ± 3 μm/hr unit α which was comparable to the value obtained from partial neutralization. The evaporation method had the advantage of producing crystals with better developed morphology but with higher energy demand and lower yield. While partial neutralization had the advantage of higher yields, but produces crystals with lesser developed morphology. The crystal morphology is hexagonal, but starts from a flat cubic-like shape that exhibits a crystal habit of faster vertical growth, in comparison with growth in the horizontal direction.

Agglomeration of particles containing a few micron diameter crystals was encountered. The particles aggregated further upon drying leading to an increase in the volume median size from 80 ± 10 μm to 200 ± 50 μm. Sample calculation have been provided to show how the obtained crystal properties can be used in designing an industrial crystallization process, to produce AHF of desired properties from spent pot lining.
Keyword Aluminium hydroxyfluoride hydrate
Spent pot lining
Crystallization
Solubility
Solubility product constant
Metastability
Nucleation thresholds
Nucleation rate kinetics
Crystal growth kinetics
Speciation modelling

Document type: Thesis
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Created: Sun, 12 Jun 2016, 18:06:59 EST by Ubong Ubong Ntuk on behalf of Learning and Research Services (UQ Library)