Selective recovery of Zn and Fe from spent pickling solutions by the combination of anion exchange and membrane electrowinning techniques

Pérez-Herranz

2014

to reach the equilibrium between the zinc transferred through the membrane and that deposited on the cathode. Therefore, the synthetic cathodic zinc is not consumed at any time. Moreover, under this circumstances iron codeposition is also avoided. INTRODUCCTIONOne of the most well known uses of metallic zinc is to protect iron or steel pieces from corrosion processes by coating them with it [1]. In order to coat the pieces, the oldest technique used is the hot dip galvanizing, which is based on dipping the pieces into molten zinc. Previously to the dipping process, these pieces must be cleaned by means of different pretreatments. The present paper focuses on the effluents coming from the pickling process, which consists of attacking the pieces surface with HCl for cleaning them from rust and impurities. The effluents coming from the pickling process contain high concentrations of Zn, Fe and HCl together with low concentrations of organic compounds, such as hydrogen evolution reaction inhibitors, and other heavy metals [2].Therefore, spent pickling baths have to be treated before their disposal to accomplish with the environmental restrictions. However, the development of an adequate treatment for this effluent is very difficult because of the high complexity usually encountered in the hydrochloric acid effluents, where the target species are present in a heterogeneous mixture with different amounts of non-desirable compounds [3].Owning to the inefficiency of the traditional methods for the treatment of spent pickling baths, as the precipitation-filtration process [4], many different techniques such as liquid-liquid extraction [5] or anionic resins [6] have been suggested [7]. In this way, the electrolysis in a membrane reactor is presented in this paper as an alternative for the treatment of the spent pickling baths in one single step. In a previous work [8], the authors performed an electrochemical study of the solution to obtain the kinetics of the electrochemical processes and, then, an undivided electrochemical batch reactor was used in potentiostatic and galvanostatic mode [9,10] to determine the viability of zinc recovery from spent pickling baths. During these experiments zinc redissolution was observed at high time values for all the experimental conditions. This process is related to the synergic effect of iron ions and dissolved chlorine gas that attacks zinc deposits causing their oxidation [11,12]. 3In order to prevent the zinc redissolution phenomena, an anion-exchange membrane (AEM) was initially used [13] to avoid chlorine presence in the cathodic compartment.This membrane permitted zinc conversion values closer to 100% and higher current efficiencies. However, iron began to codeposit with zinc as the latter was being removed from the solution since the iron-zinc system deposits following the anomalous codeposition phenomenon [14][15][16], in which the less noble metal (zinc) deposits preferentially, and iron deposition depends on the zinc-iron ratio, the applied current and the pH value.Therefore...

Section: Introducctionmentioning

confidence: 99%

Study of the zinc recovery from spent pickling baths by means of an electrochemical membrane reactor using a cation-exchange membrane under galvanostatic control

Pérez-Herranz

2014

“…The most widely used is the precipitation-filtration technique. Although the required equipment is relatively inexpensive, there are a number of disadvantages such as a large consumption of 4 chemicals and the expensive disposal of the generated hazardous waste [3] that makes this technique inappropriate.…”

Section: Introductionmentioning

confidence: 99%

“…The diffusion dialysis has proven successful in regenerating mixed acids until 80-85% of the original acid, but the high metal containing solution must be treated as hazardous waste. Its advantage is its low operating costs, but its investment costs are significant and it has low productivities [3].…”

Section: Introductionmentioning

confidence: 99%

Electrochemical recovery of zinc from the spent pickling baths coming from the hot dip galvanizing industry. Potentiostatic operation

Ortega

et al. 2011

ElsevierCarrillo Abad, J.; García Gabaldón, M.; Ortega Navarro, EM.; Pérez-Herranz, V. (2011). Electrochemical recovery of zinc from the spent pickling baths coming from the hot dip galvanizing industry. Potentiostatic operation. Separation and Purification Technology. 81 (2) Keywords: electrochemical deposition, iron, hydrogen evolution reaction, pickling solutions, zinc electrodeposition AbstractAn electrochemical reactor was developed to recover zinc from the spent pickling solutions coming from the hot dip galvanizing industry. These solutions mainly contain ZnCl 2 and FeCl 2 in aqueous HCl media. The effect of the applied potential on the figures of merit (fractional conversion, current efficiency, space-time yield and specific energy consumption) of the electrochemical reactor was analysed. Voltammetric experiments were performed previously in order to select the optimum conditions to be applied in the electrolysis experiments. From the i-V curves it was inferred that bulk zinc deposition started from potential values more cathodic than -0.99V. The hydrogen evolution reaction (HER) appeared from -0.45V and masked the zinc cathodic peak C1, 2 related to bulk zinc deposition, at high HCl concentrations. The presence of HCl inhibited iron deposition in synthetic samples. The additives present in the real baths, which diminish the massive hydrogen generation, allowed the observation of peak C1.The potential values to be applied in the electrolysis experiments were chosen from the voltammetric experiments and ranged between -1V and -1.75V. In the absence of iron in solution, as the electrode potential was shifted towards more negative values, the space-time yield of zinc and its fractional conversion increased because of the increase in the electrode roughness and the hydrogen turbulence-promoting action.Simultaneously, the specific energy consumption decreased initially due to the increase in the zinc conversion rate but decreased for the most cathodic potential value due to HER. The presence of iron in synthetic solutions led to a decrease in current efficiency associated with the reverse redox Fe 2+ /Fe 3+ system and to the enhancement of the HER, which also induced increments in the local pH and the subsequent zinc redissolution for the most cathodic potential values. On the contrary, the additives present in the real spent pickling baths avoided the adverse effects of iron, and zinc electrodeposition was possible even at high cathodic potential values. In fact, a potential value of -1.75V was selected as the optimum since the conversion, the current efficiency and the space time yield obtained in the real baths were relatively high.3

“…This process consists of the following steps: HCl. This gives rise to an environmental problem that has to be solved due to the hazardous nature of the effluents [2,3]. In addition, the decrease of natural reserves of non-ferrous metals and the requirement of environmental protection make zinc electrowinning from spent pickling solutions an interesting alternative [4,5].…”

mentioning

confidence: 99%

Recovery of zinc from spent pickling solutions using an electrochemical reactor in presence and absence of an anion-exchange membrane: Galvanostatic operation

Ortega

et al. 2012

ElsevierCarrillo Abad, J.; García Gabaldón, M.; Ortega Navarro, EM.; Pérez-Herranz, V. (2012). Recovery of zinc from spent pickling solutions using an electrochemical reactor in presence and absence of an anion-exchange membrane: Galvanostatic operation. Separation and Purification Technology. 98:366-374. doi:10.1016/j.seppur.2012.08.006. Keywords: electrochemical deposition, chlorine, iron, redissolution, pickling solutions, zinc electrodeposition 1 RECOVERY OF ZINC FROM SPENT PICKLING SOLUTIONS USING AN ELECTROCHEMICAL REACTOR IN PRESENCE AND ABSENCE OF AN ANION-EXCHANGE AbstractThe performance of a one and two-compartment electrochemical reactor under galvanostatic control for zinc recovery present in the spent pickling solutions is studied in this paper. These solutions, which mainly contain ZnCl 2 and FeCl 2 in aqueous HCl media, come from the hot dip galvanizing industry. The effect of the anion-exchange membrane (AEM) on the figures of merit of the electrochemical reactor is analyzed.In the absence of iron in solution, as the current value was shifted towards more negative values, the zinc fractional conversion increased because of the increase in the 2 zinc reduction rate. However, the increase in current values made current efficiency decrease due to the hydrogen-reduction side reaction, which caused 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 Fe 2+ /Fe 3+ 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 could be minimized by the interposition of an AEM. In this case, the zinc redissolution was eliminated which enabled zinc conversion values close to 100% together with higher current efficiencies as the consumption of current by the system Fe 2+ /Fe 3+ was diminished.3 1-IntroductionHot dip galvanizing processes offer a simple and effective way for corrosion protection of steel parts. This protection is based on covering the steel with a thin layer of metallic zinc that protects steel from corrosion by two ways: surface coating and sacrificial anode as zinc is lees noble than iron. This process consists of the following steps: HCl. This gives rise to an environmental problem that has to be solved due to the hazardous nature of the effluents [2,3]. In addition, the decrease of natural reserves of non-ferrous metals and the requirement of environmental protection make zinc electrowinning from spent pickling solutions an interesting alternative [4,5]. Moreover, the electrolytic recovery of zinc present in the spent pickling baths is a suitable technique because zinc electrowinning has been practiced for many years [6]. In fact, 80% of world zinc production is obtained by electrowinning [1].In our previous paper [7], the potentiostatic recovery of zinc present in the spent pickling bath using an electrochemical reactor was studied. The major interfering re...