Development of a process for the recovery of zinc sulphate from hot-dip galvanizing spent pickling liquor via two solvent extraction steps
Introduction
In the hot-dip galvanizing industry, pickling baths are used to remove rusts and scales from metal works before they are galvanized. However, the pickling liquor becomes ineffective as the concentration of dissolved metals, mainly iron and zinc, in the bath increases. The highly contaminated spent pickling liquor (5 to over 200 g/L zinc, 60 to 150 g/L iron, and 10 to 80 g/L HCl) (Cook et al., 2011, Kerney, 1994, Stocks et al., 2005) is usually disposed to specialised landfill even though the metals and acid can be recovered. With increasing environmental concerns, legislations and disposal costs, the galvanizing industry is gaining interest in recycling of their spent pickling liquor.
Zinc is the most valuable component of the waste liquor. This zinc can be easily recovered as zinc chloride and used as a fluxing agent within the galvanizing plant. However, only a small quantity of zinc chloride is required for this purpose. Zinc sulphate would have a greater commercial value as it can be readily sold to electrowinning plants for zinc metal production. The development of a process to recover zinc as zinc sulphate is the focus of this paper.
Authors have previously considered the recovery of zinc sulphate using a combination of solvent extraction and evaporation (Reinhardt, 1978, Van Veen, 1981). In this case, zinc chloride is first selectively extracted using tri-n-butyl phosphate (TBP) and then stripped into a sulphuric acid solution. Evaporation is then used to remove the chloride content as HCl vapour. However, this process poses safety concerns and exotic materials are required to resist corrosion by the acid vapours; making the process less economically viable.
Another option is to use a second solvent extraction step to transfer the zinc cations from the chloride to sulphate media, as noted by Kerney (1994) (though the choice of extractant was not reported). This option is similar to the Zincex process which also utilises two solvent extraction steps to produce zinc sulphate for electrowinning purposes from primary or secondary zinc sources (Díaz et al., 2001, Nogueira et al., 1980). The Zincex process uses a secondary amine as the first extractant to selectively extract zinc chloride followed by di-2-ethylhexyl phosphoric acid (D2EHPA) to convert to the sulphate form. However, this process was developed primarily for feed solutions with a low iron content. The presence of a high iron concentration such as in spent pickling liquor has been reported to cause precipitation and poor phase separation with amine extractants (Cierpiszewski et al., 2002).
By substituting the secondary amine with an extractant suitable for processing spent pickling liquor, such as TBP, and keeping the second solvent extraction step with D2EHPA, zinc sulphate can be produced from spent pickling liquor. This option is viable especially since both extractants are commercially available and well understood.
To the best of the authors' knowledge, this process scheme has not been previously examined. As such, there are many processing parameters that require study, such as the composition of the stripping solution for the first solvent extraction step and the appropriate aqueous-to-organic ratio for each stage.
Hence, this paper aims to examine and develop this process of zinc sulphate recovery from spent pickling liquors via a two step solvent extraction process. Additionally, a simulation of this process is included to demonstrate that both high purity and recovery can be achieved using this arrangement.
Section snippets
Experimental
The extractants TBP (Consolidated Chemical Co., purity 99.0%) and D2EHPA (Albright & Wilson Specialties, 97%), were used as supplied and ShellSol 2046 from Shell Chemicals (69 wt.% paraffin, 14 wt.% naphthenes napthtalenes, 17 wt.% aromatics) was used as diluent. Hydrochloric acid (Sharlau, 37 wt.% HCl) and ZnCl2 (AJAX Finechem Pty. Ltd., 95.0%) were used to prepare model solutions while sodium chloride (Chem Supply, 99.0%) was added to adjust total chloride concentration to 5 mol/L, unless
Process development
A generic scheme of the process under study is depicted in Fig. 1.
From Fig. 1, TBP extracts ZnCl2 and, to a lesser extent, HCl. These two extracted species are stripped into a stripping solution (Strip. Soln 1), producing a stream containing both ZnCl2 and HCl (Product 1). This is then fed into the second solvent extraction circuit where D2EHPA extracts Zn2 + via cation exchange with H+. As often the case with cationic extractants, the aqueous pH has to be regulated in order to maintain
Modelling and simulation
Having determined that the final zinc sulphate solution can meet the specification used in electrowinning applications, the process is then simulated in MATLAB to determine the optimum aqueous-to-organic ratio, extraction and stripping stages and estimate the stream compositions. The equilibrium reaction model for zinc and HCl extraction by TBP presented in earlier work (Lum et al., 2012, Lum et al., 2013) is used to simulate SX 1 while the model by Mörters and Bart (2000) for zinc extraction
Conclusion
Factors affecting the operation of a proposed process for zinc sulphate recovery from spent pickling liquor have been investigated. The use of dilute acid or alkali solutions as stripping solution does not affect either zinc or acid stripping significantly. Chloride concentration in the stripping solution has a notable effect on zinc stripping, but not acid stripping; increased chloride content decreases zinc stripping. This indicates that acid is easily stripped even with stripping solutions
Acknowledgements
The authors are grateful for support from the Australian Research Council through Linkage grant LP0989575, the Industrial Galvanizers (Australia) and the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council. The technical support given by Dr. Peter Hetherington of Industrial Galvanizers (Australia) is also greatly appreciated.
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