Treatment of Acid Pickling Baths by Means of Anionic Resins

This paper is a study of the use of two anionic ion-exchange resins, Lewatit MP-500 and Lewatit M-504, to remove iron and zinc from acid pickling baths employed in the galvanizing process. Ion-exchange assays were carried out with HCl baths containing Zn(II) and Fe(III), obtained after oxidation with H2O2, both of which are present in the form of chloro complexes. The breakthrough curves obtained demonstrate that both resins exhibit a greater capacity for Zn. Two alternatives were studied, employing one column (simultaneous removal of Zn and Fe) or two columns in series (sequential removal) with the aim of designing the most adequate treatment process. A series of pilot-plant-scale assays were carried out that verified the reproducibility of the results obtained in the laboratory. The retained metals were eluted with water, with 3 bed volumes (BV) being needed for iron and greater amounts, 6 BV, for zinc. However, when 12% NH4OH is used, the elution of Zn is achieved with only 2 BV. In both cases, decomposition of the retained chloro complexes took place, releasing ZnCl2 and FeCl3; the resins then dissociated to the chloride form, ready to be used again.