Recovery of sulphuric acid from waste and process solutions using solvent extraction
Introduction
Solvent extraction (SX) is a well established technology to separate, purify and concentrate metals. It has been also applied for acid recovery from waste solutions and bleeding streams of copper, zinc and precious metals (Agrawal and Sahu, 2009, Agrawal et al., 2007, Agrawal et al., 2008, Gottliebsen et al., 2000a, Gottliebsen et al., 2000b). The use of SX to recover various acids has been reviewed by Ritcey (2006).
Extractants including TEHA (tris-2-ethylhexylamine), Alamine 336 (tri-octyl/decyl amines), TBP (tributyl phosphate) and Cyanex 923 (hexyl/octyl phosphine oxides) have been tested and applied to recover acids using solvent extraction (Agrawal et al., 2007, Alguacil and Lopez, 1996, Gottliebsen et al., 2000a, Gottliebsen et al., 2000b, Liao et al., 2002, Rickelton, 1993, Sarangi et al., 2006). Agrawal et al. (2008) studied the extraction of sulphuric acid from zinc electrowinning bleed stream containing 174 g/L H2SO4 using TEHA in kerosene. It was found that TEHA had very good acid extraction capacity even in the presence of a large amount of zinc. In a similar study, Haghshenas et al. (2009) investigated the extraction of sulphuric acid in terms of thermodynamics and other conditions. It was reported that the acid extraction increased with the increase in the TEHA concentration in the organic solution and reached a maximum depending on the initial acid concentration in the feed solution. Furthermore, a comparison of TEHA and Cyanex 923 for sulphuric acid extraction was studied in terms of thermodynamics. It was found that stripping of sulphuric acid from TEHA is more readily than from Cyanex 923 while the stripping of acid was easier from Cyanex 923 than from amine extractants (Wolter et al., 2002). Agrawal et al. (2007) studied the extraction of sulphuric acid using Alamine 336 and found that the extraction of sulphuric acid increased with the increase in the Alamine 336 concentration. Although, Alamine 336 showed higher extraction ability for sulphuric acid than Cyanex923, the acid extracted could not be stripped completely using hot water.
The acid concentrations in mining waste solutions are usually low and can be concentrated using membrane distillation (MD) technology in which the vapour of volatile constituents including water in the feed solution of the hot side can pass through the pores of the hydrophobic membrane to the cold side for condensation. As a result, fresh water is recovered and the other constituents are concentrated. MD offers advantages of low energy consumption, high efficiency and operating at low temperatures to recover fresh water and concentrate the acid and metals (Tomaszewska, 2000, Tomaszewska et al., 1995). After concentrating, solvent extraction can be applied to selectively recover the sulphuric acid and valuable metals.
In the current paper, the most suitable extractant for sulphuric acid recovery in terms of extraction and stripping was selected, optimised and characterised. The composition of the extracted species was investigated using slope analysis and a process flowsheet to recover water, acid and metals proposed.
Section snippets
Organic solutions
Organic solvents TEHA and octanol were purchased from Sigma-Aldrich. Alamine 336 was supplied by BASF, Cyanex 923 by Cytec and Shellsol A150, Shellsol 2046 and Shellsol D70 by Shell Chemicals, Australia. All reagents were used without further purification.
Aqueous solutions
The feed solution containing only sulphuric acid was prepared by diluting AR grade concentrated sulphuric acid using deionised water. The feed solution containing sulphuric acid and metals was prepared by adding required amounts of AR grade
Selection of organic system in terms of extraction
Three organic extractants including TEHA, Alamine 336 and Cyanex 923 were used to extract acid in the feed solution containing 200 g/L H2SO4 at an O/A ratio of 2 and 22 °C. Preliminary experiments were performed using extractants in the concentration range of 20–60% (v/v) TEHA and Alamine 336 and 20–100% (v/v) Cyanex 923 (Table 1). Initially, octanol was used as the diluent for all three systems. For the Alamine 336 system, gel was formed with octanol as the diluent. Agrawal et al. (2007) met the
Conclusions
Three extractants for the recovery of sulphuric acid from acidic waste solutions were tested including TEHA, Alamine 336 and Cyanex 923. Amongst the three extractants, TEHA performed best in terms of acid extraction and stripping. Therefore, the TEHA system was selected for optimisation.
A number of diluents and modifiers were tested with various TEHA concentrations. An optimum organic composition consisting of 50% TEHA, 40% octanol and 10% Shellsol A150 was determined. The acid extraction
Acknowledgements
Drs Zhaowu Zhu and Goutam Das and Mr Yoko Pranolo are thanked for assisting test work. One of the authors (U. Kesieme) would like to thank CSIRO and Victoria University for providing PhD scholarships. The support of CSIRO Process Science and Engineering and CSIRO Minerals Down Under National Research Flagship is gratefully acknowledged.
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