Electrochemical Recovery of Zinc from the Spent Pickling Solutions Coming from Hot Dip Galvanizing Industries. Galvanostatic Operation

The performance of an electrochemical reactor for zinc recovery from the spent pickling solutions coming from the hot dip galvanizing industry in galvanostatic operation is studied in this paper. These solutions mainly contain ZnCl2 and FeCl2 in aqueous HCl media. The effect of the applied current on the figures of merit (fractional conversion, current efficiency, space-time yield and specific energy consumption) of the electrochemical reactor is analyzed and compared with the values obtained in the potentiostatic operation. The applied current was selected in order to obtain similar electrode potential values to those applied in the potentiostatic operation and ranged between -150 and -700 mA. In the absence of iron in solution, as the current value was shifted towards more negative values, the spacetime yield of zinc and its fractional conversion increased because of the increase in the zinc reduction rate, the electrode roughness and the hydrogen turbulence-promoting action. However, the increase in current values makes current efficiency to decrease due to the hydrogen evolution reaction (HER), which causes an increment in the specific energy consumption. The presence of iron in synthetic solutions led to a decrease in current efficiency associated with the reverse redox Fe2+/Fe3+ system and to the enhancement of the HER, which also induced increments in the local pH and the subsequent zinc redissolution. These adverse effects related to the presence of iron are greater in galvanostatic mode than in potentiostatic one. On the contrary, the additives present in the real spent pickling baths improve the zinc electrodeposition rate compared with the behavior of synthetic mixtures of zinc and iron solutions but the zinc redissolution is enhanced.