Chemical and Biomolecular Engineering - Theses
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ItemThe recovery of zinc salts from hot-dip galvanizing effluent streams( 2012)Hot-dip galvanizing is used to coat a range of steel products with a protective layer of zinc. There is a growing interest in the recovery of metals and hydrochloric acid recycling from spent pickling liquor of these hot-dip galvanizing plants. This research aims to explore and develop a process for zinc recovery from such waste using solvent extraction with tri-n-butyl phosphate (TBP) and di-2-ethylhexyl phosphoric acid as extractants (D2EHPA). The proposed recovery of zinc from spent pickling liquor involves two solvent extraction stages: first to selectively recover zinc chloride from the waste using TBP, and second to extract zinc cations into a sulphate media using D2EHPA. The ultimate aim is to produce a zinc sulphate solution that meets the requirements for sale to electrowinning plants. HCl and ZnCl2 extraction from model solutions (0–120 g/L ZnCl2, 0.5–2 mol/L HCl, 5 mol/L total Cl) using TBP diluted in ShellSol 2046 (30–70 vol% TBP) was investigated. Experimental results showed that HCl and ZnCl2 are extracted simultaneously by TBP; in the presence of zinc, HCl extraction increased, likely to be due to the extraction of acido-metal complexes (HZnCl3 or H2ZnCl4). However, zinc is still predominantly extracted as ZnCl2. The effect of dilute acid or alkali in the stripping solution was found to have a negligible effect on zinc and HCl back-extraction from loaded TBP; the presence of chloride ion in the stripping solution yielded a greater effect on the stripping of zinc and HCl. The production of zinc sulphate solution with low chloride content can be achieved using the proposed process provided that the entrained aqueous phase is scrubbed from the organic phase in the second stage solvent extraction. A model of the HCl-ZnCl2-TBP-diluent system is presented using the intrinsic equilibrium constant of each reaction. The Pitzer and Hildebrand-Scott models were used to estimate the aqueous and organic phase activity coefficients respectively. The equilibrium constants and solubility parameters of the extracted species were then determined from experimental and literature data. The model was able to predict co-extraction of water, HCl and ZnCl2 by TBP diluted in ShellSol 2046 up to 2 mol/L Zn (130 g/L) and 1 mol/L HCl. A model for the Zn-D2EHPA system reported in the literature was examined and found to agree with experimental results available. The model developed was tested against HCl and ZnCl2 extraction by TBP from spent pickling liquor obtained from a hot-dip galvanizing plant. Modelling results obtained were promising given that the Pitzer mixing parameters for Zn-Fe(II) were not available and had to be estimated. The extraction of HCl was found to be enhanced due to the high Fe(II) concentration in the spent pickling liquor. This increase can be attributed to an increase in acido-metal extraction. Simulation of the two stage extraction process using the model indicated that approximately 80 g/L Zn can be extracted from a feed solution containing 120 g/L Zn and 1 mol/L HCl. The modelling was also able to predict both ZnCl2 and HCl extraction using a hollow fiber membrane contactor; no additional fitted parameters were used in this modelling. The results of this simulation indicate that a contactor of 3.6 m length is required to achieve 99 % zinc extraction.