Evaluation of saponins as acid mist suppressants in zinc electrowinning

Generation of acid mist is a major concern in zinc electrowinning (EW) due to its detrimental effects on worker's health, the environment and plant infrastructure. One of the effective methods for acid mist control is to form a foam layer above the electrolyte surface by using surfactants. In this work, two saponin-rich products (Mistop® and QLZinc®) and a commercial Licorice product were evaluated and compared as acid mist suppressants in zinc EW. Foam properties, acid mist suppression, phase separation in solvent extraction, cathodic current efficiency (CE) and cathode quality were correlated to the addition of these surfactants. Foam properties studied included minimum soluble solids dosage for 100% foam coverage, time required to achieve 100% foam coverage and foam persistence time. The two saponin-rich products were found to be more effective foaming agents than Licorice. Based on foam property tests, the recommended soluble solids dosage was 1.8, 3.8 and 21.6 ppm for Mistop®, QLZinc® and Licorice, respectively. At these dosages, acid mist suppression was 99% for both Quillaja products and 94% for Licorice. Foam coverage of 100% was achieved with 1.0 ppm of saponins when Mistop® was added, compared with 0.5 ppm of saponins when QLZinc® was added, suggesting that QLZinc® contains other Quillaja solids which are surface active and positively contribute to improve the quality of foam produced. For all three acid mist suppressants tested, no significant changes in phase disengagement time and interfacial tension were detected when an organic solution was separately contacted with the zinc electrolytes containing the three agents. The CE for the solutions containing 1.8, 3.8 and 21.6 ppm soluble solids of Mistop®, QLZinc® and Licorice were 91.4%, 92.5% and 92.6%, respectively. These values were slightly lower than for the blank solution (92.8%). Zinc deposit quality was also affected slightly by all three agents tested. Overall, it is concluded that under the current testing conditions, Quillaja products, at much lower soluble solids dosage, performed very well and better than Licorice.