Liquid–liquid extraction of sulphuric acid from zinc bleed stream

Recovery of both sulfuric acid and zinc from the spent zinc bleed stream(SZBS) generated during the electrowinning of zinc in the zinc refineries using tris-(2-ethylhexyl) amine (TEHA) dissolved in kerosene has been investigated in detail. Extraction was studied under various experimental conditions such as concentration of TEHA in the organic phase and temperature, concentration of acid and zinc present in the initial aqueous feed, temperature of extraction, equilibrium time. Increasing the experimental temperature led to a decrease in the extraction of acid. The extraction behaviour of zinc present in the spent liquor was examined under the optimized conditions for its co-extraction with the acid. It was found that in the sulphate medium, zinc is not extracted along with the acid and the raffinate can be used for the preparation of zinc oxide. Thus under the optimised conditions acid extraction was found to be 182 g/L of H2SO4 with 75% TEHA in 3–5 min of phase contact. Extraction increases with the increase in O/A ratio and was found to be about 94% at O/A of 5 with the initial feed concentration of 173.5 g/L of sulphuric acid. After extraction process, stripping was performed to recover the loaded acid and regenerate the solvent for recycling. Resultant regenerated organic phases can be reused in succeeding extraction step with the yield ranging from 94.8% to 99.7% depending on composition of the initial feed solution. Effect of temperature was studied at 30, 45 and 60 °C. The effect of the number of extraction and stripping stages and the volume phase ratios in extraction and stripping upon sulphuric acid recovery is discussed.