Article pubs.acs.org/IECR
Treatment of Spent Pot-lining with Aluminum Anodizing Wastewaters: Selective Precipitation of Aluminum and Fluoride as an Aluminum Hydroxyfluoride Hydrate Product Diego F. Lisbona,† Christopher Somerfield, and Karen M. Steel* School of Chemical and Environmental Engineering, The University of Nottingham, Nottingham, NG7 2RD, United Kingdom. S Supporting Information *
ABSTRACT: Wastewaters generated by the aluminum anodizing industry have been identified as a source of acid, Al3+, and caustic values that could be used for the treatment of spent pot-lining (SPL) generated in primary aluminum smelting. In this paper, recovery of aluminum and fluoride values as an aluminum fluoride product that could be converted into smelter grade (SG) AlF3 for recycling in aluminum smelting is reported. Experimental observations were contrasted with a thermodynamic model of the solution equilibria and new values for the solubility product of AlF2OH·1.4H2O, 10−15 at 30 °C and 10−23.8 at 50 °C, are reported. Neutralization of leachates obtained from SPL treatment with aluminum anodizing acid wastewaters was performed using 2 M NaOH and, alternatively, waste caustic solutions from the aluminum anodizing industry. pH-static precipitation with 2 M NaOH successfully recovered 92 wt % of the solubilized fluoride as AlF2OH·1.4H2O at pH 4.0 and 70 °C, although a hydrolyzed Na5Al3F14-like product (3−5 wt % Na) coprecipitated. Seeded heterogeneous nucleation of AlF2OH·1.4H2O on SG Al(OH)3 yielded a product that could be acceptable to AlF3 manufacturers, meeting specifications for particle size and phosphorus and silicon content. Caustic wastewaters from the aluminum anodizing industry were identified as suitable to provide the caustic values required for precipitation, providing that Al concentration is decreased by aging the solutions under seeded conditions. The use of caustic waste from the aluminum anodizing industry to treat the SPL leachates obtained with acidic anodizing waste totally removes the need of chemical reagents for SPL treatment and fluoride recovery. Table 1. Product Specification Values of SG AlF3 5
1. INTRODUCTION A two-step leaching treatment of spent pot-lining (SPL) comprising a water wash to dissolve water-soluble NaF followed by leaching with aluminum anodizing acid wastewaters to dissolve Na3AlF6 and CaF2 was presented by Lisbona et al.1 Water wash and SPL leachates can be mixed together to raise F/Al ratios toward values of 2−3 that are suitable for AlF3 precipitation. However, the metastability of supersaturated AlF3 aqueous solutions2 hinders precipitation while the relatively high solubility of AlF3 would mean low recovery yields.3 On an industrial scale, AlF3 for aluminum smelting is produced via two main routes: the reaction of aluminum hydroxide at 500−600 °C with gaseous HF under fluidized-bed conditions, and by reaction of fluosilicic acid solutions from the phosphate industry (15 wt %) with aluminum hydroxide at 60 °C.4 Fluidized-bed AlF3 production by reaction of HF with Al(OH)3 is an energy intensive process: HF is typically produced onsite by treatment of acid grade fluorspar (fluorite) CaF2 with concentrated H2SO4 (98−99 wt %) in a rotary furnace at temperatures between 100 and 300 °C. CaSO4 is formed as a byproduct. The gaseous HF generated undergoes further treatment to remove solid particles carried over from the furnace, SiF4, SO2, and H2S. The final HF product can be condensed and purified by distillation and/or reacted as a gas with aluminum hydroxide. In the work presented here, aluminum and fluoride recovery from SPL leachates was approached with the aim of obtaining smelter grade (SG) aluminum fluoride (AlF3), meeting the specifications described in Table 1, or as an aluminum hydroxyfluoride hydrate product that could be fed with © 2012 American Chemical Society
avg particle size μm
AlF3, wt %
Al2O3, wt %
Fe2O3, wt %
SiO2, wt %
P2O5, wt %
bulk density (kg/m3)
40−60
>90
