Galvanic stripping

C. Chang, H. Gu and T.J. O Keefe, Galvanic Stripping of Iron from Solvent Extraction Solutions from Zinc Residues Leaching , Iron Control and Disposal. J.E. Dutrizac and G.B. Harris, Eds., Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, 1996, 417-428. 

THE GALVANIC STRIPPING TREATMENT OF ZINC RESIDUES FOR MARKETABLE IRON PRODUCT RECOVERY... 

In this rqrplication to recover iron from a neutral leach zinc residue, a DEHPA extractant is used in conjunction with metallic zinc as the active metal to reduce the ferric ion. Although DEHPA is an excellent solvent for the separation of ferric ion, the subsequent stripping stq> is difficult. Conversely, the stripping of ferrous ion from DEHPA is easily accomplished, even with dilute acid solutions in the range of pH 1.5 to 2.0. However, the concentration of ferrous ion that could be obtained using galvanic stripping was not known. Consequently, one of the primary objectives of this study was to determine if a relatively concentrated iron solution could be produced. 

The galvanic stripping reactions involved in separating iron from a zinc sulphate electrolyte using solvent extraction can be assumed to comprise the following half cell reactions. The major anodic and cathodic steps are ... 

As indicated by reaction (3), one mole of zinc will theoretically reduce two moles of ferric ion to ferrous ion to give an ideal stoichiometry number of one. The actual value obtained in practice is always higher because of the non-productive side reactions, in particular hydrogen evolution. Because of this, one important economic aspect of the galvanic stripping process is to identify the operating conditions which minimize the stoichiometry number. 

Simultaneous galvanic stripping was used in the continuous flow tests, but some batch tests were conducted to supplement these studies. Zinc dust was the only metal used as a reductant in the continuous runs, but scrap iron has also been used successfully in other batch experiments. Alternative stripping solutions based on acidic chloride media may also be used, but sulfate was employed in the majority of the work. 

Figure 1 - Bench Scale, Simultaneous Galvanic Stripping Flowsheet Arrangement...

Two stripping solutions were prepared from deionized water and reagent grade sulfuric acid of either 400 or 250 g/1 H2SO4 concentration. This solution made up stream Al 1 which was fed directly into the galvanic stripping reactor with the return strip solution A9. A high acid concentration was utilized to produce a more concentrated iron solution and the low concentration was utilized to study the effect of the pH. 

After stripping, the A10 aqueous exit solution containing mainly ferrous sulfate, was split into the product stream, Al 2, and the return stream, AlO, going into the galvanic stripping reactor. Therefore streams A9, AlO, and A12 have the same chemical composition but different volumes and flow rates. The iron was allowed to build up in these streams until a steady state was reached. 

Table I - Average Response Variables for Continuous Galvanic Stripping of Iron...

Figure 6 - Electrolytic Iron Deposited From Galvanic Strip Solution...

Using the galvanic stripping process, it is possible to remove iron from an acidic zinc sulfate leach electrolyte and to produce a ferrous sulfate solution having an iron concentration as high as 130 g/L. A zinc concentration as low as 250 mg/1 is possible, but the pH of the strip... 

The acidity of the aqueous strip solution is a very strong variable whose value should be carefully selected in order to effect a compromise among the iron removal, the zinc concentration in the iron product, the stoichiometry of the process, the process rate and the iron-zinc separation index. As demonstrated, it is possible to separate iron from zinc sulfate electrolyte and recover a concentrated, potentially usable form of iron. It is interesting to contemplate the use of galvanic stripping for the removal of other less concentrated impurities or valuable by-products from zinc processing solutions or from other hydrometallurgical streams, eith in conjunction with iron removal or separately. 

Overall, the data obtained were promising, but some of the limitations of the galvanic stripping that were identified require additional evaluation. On the positive side, it was demonstrated that running in a continuous mode is possible and that a concentrated ferrous sulfate solution can be produced. A number of by-product uses appear to be potentially possible and include water treatment chemicals, electrolytic iron or a precipitated or crystallized solid iron salt. 

L.M. Chia, M.P. Neira, C. Flores,.and T.J. O Keefe, "Overview of Galvanic Stripping of Organic Solvents in Waste Materials Treatment, Extraction and Processinu for the Treatment and Minimization of Wastes. J.P. Hager, B.J. Hansen, J.F. Pusateri, W.P. Imrie and V. Ramachandran, Eds., The Minerals, Metals and Materials Society, Watrendale, PA,... 

M. Moats, C. Chang and T.J. O Keefe, "Recovery of Zinc from Residues by SX-galvanic Stripping Process", Third Int. Svmn. on Recycling of Metals and Engineered Materials. P. Queneau and R. Peterson, Eds., Hie Minerals, Metals and Materials Society, Warrendale, PA, U.S.A.,1995,545-562. 

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