Choosing sorbent for fluoride ion removal from zinc sulfate solutions

The process of zinc sulfate solution purification determines process, economic and environmental production results. Since recently there has been a constant increase in the content of halides in pregnant solutions of zinc production due to the processing of technogenic zinc-containing raw materials, it is relevant to search for methods for removing halides, in particular fluorine, from zinc solutions using a variety of materials. The purpose of this paper was to investigate the effectiveness of akaganeite as an sorbent for fluoride ion removal from zinc sulfate solutions. When using akaganeite, it is especially important to choose a carrier for the sorbent since the nanosized particles of akaganeite make it difficult to clean the solution from the sorbent. Most suitable carriers for this purpose in terms of surface characteristics and physicochemical properties are gypsum and red mud of alumina production. Experiments used a zinc sulfate solution (100 g/dm3 Zn2+, pH = 4.5) containing 26.8–111.4 mg/dm3 F–. The maximum fluoride ion capacity was shown by red mud due to the formation of F–Al complexes. The highest fluorine recovery was demonstrated by red mud with impregnated akaganeite at elevated temperature that facilitates showing akaganeite properties and accelerates the surface ОН– ↔ F– exchange process. The gypsum-based adsorbent successfully removed fluorine due to calcium ions released and calcium fluoride formed. The amount of fluorine removed depends on the sorbent material, its consumption, sorption duration and temperature. The optimal processing conditions were (at pH = 5.5): temperature – 60 °C, process duration – 120 min, composite sorbent consumption – 20÷30 g/dm3. It was shown that composite sorbents based on red mud or gypsum with impregnated akaganeite (β-FeOOH) are most suitable for cleaning zinc solutions from halides. These adsorbents make it possible to achieve the greatest capacity and degree of fluoride ion removal (up to 98–99 %) in the actual pH range of process solutions. The abovementioned sorbents can be regenerated in an alkali solution, and then reused (up to 3–4 cycles).