PCCell GmbH
PCCell GmbH Lebacher Strasse 60 66265 Heusweiler Germany

Process-integrated acid recovery

Process-integrated acid recovery

Process flow of pickling- and etch lines

The hot-dip zinc plating process of steel involves a pretreatmet with various aqueous solutions before the actual coating process. The workpiece is degreased in one bath and is derusted in another bath. Intermediary rinse baths accumulate the used pickling acid and finally have to be disposed.

A further example is the pickling and passivating of stainless steels [1] with mixed acid. After a workpiece is passivated in mixed acid, adhering pickling acid is removed in a rinse bath. The pickling bath is exposed to a high input of metals and has to be operated with a high acid excess, so that it has to be replaced regularly. For this, the pickling acid is neutralized, the metal oxides are filtered off and the salt solution is disposed as waste water.

This procedure not only causes high costs, because the neutralization chemicals have to be purchased and the neutralized acid is lost, but also leads to an environmental problem, since the neutralized rinse water contains besides traces of toxic heavy metals also nitrate and fluoride.

 
Closing the loop by electrodialysis

By application of electrodialysis, dragged-out hydrochloric acid and metal salts from the pre-treatment process (“pickling”) are separated and are re-circulated to the process flow in a re-concentrated state.

The water balance at hot-dip zinc plating is thus reduced by half, the disposal and new preparation of the rinse and flux bathes is no longer necessary, and the lifetime of the pickling baths is prolongated.

The rinsing bath is enriched with dragged-out pickle, leading to gradual degradation of the rinsing efficiency.

The pickle is selectively removed from the rinse bath by acid electrodialysis.

By special membranes and process design the dragged-out pickle is recovered in a re-concentrated state.

Figure: Electrodialysis unit behind a rinsing unit at a hot-dip zinc plating plant.

Lifetime extension of the rinse bath

The diluted HCl pickling solution can be removed from the rinse bath by electrodialysis.

The concentration of the rinsing bath is kept constant at a low level.

PCA has developed high-performance ED-cells, which enrich the dragged out pickle to its original concentration.

In this process, the concentrate is recirculated directly to the pickle.

Recycling of spent acid

This problem could be solved for example by acid electrodialysis (process principle) of the rinse bath, by keeping the pickle concentration in the rinse below a limit value ad simultaneously re-concentrating the acid so high that it ca be re-circulated to the pickle bath. Furthermore, the metal salts enriched in the pickle acid could be removed by selective electrodialysis, so that also the lifetime of the pickle could be considerably extended.

In similar application cases, membrane electrodialysis has been suggested for recovery of hydrofluoric acid contaminated with metal salts [2]. Also for the production of HCl [3], HIx [4], sulforic acid [5], phosphoric acid [6], hypo phosphoric acid [7] and chloric acid [8] electrodialysis processes have been proposed and investigated.

The applicability of these processes is limited by the properties of the electrodes and separators (membranes); Their development and design is therefore of central importance. The main limitation of the applicability results from the insufficient properties of the necessary anion exchange membranes (AAM’s), the so-called proton leakage. It concerns the lower permselectivity of AAM’s in contact with protons as in contact with other cations.

A further, presently rising application area [9] of electro membrane processes is found in the pharmaceutical industry and in the bio technology for recovering of spent organic acids:

Advantages:

  • Re-circulation of the pickle
  • Effectively unlimited rinse lifetime
  • Reduced Fe-input into the flux bath
  • Reduced formation of had zinc

Literature

[1] a) A.T. Cherif, J. Molenat, A. Elmidaoui, J. Appl. Electrochem. 27 (1997) 1069 b) R. Audinos, A. Nassr-Allah, J. R. Alvarez, J. Membr. Sci. 76 (1993) 147

[2] K. Sato, Y. Kurauchi, Y. Miyaki, M. Akazawa, Jp. Kokai Tokkyo Koho JP 01,123,606 (CA 112,121622u)

[3] a) C. Gavach, P. Amblard, J. F. Diaz, 13 th. Int. Congr. Electrochem. Appl. 1996, 2 b) A. Lindheimer, M. Boudet-Dummy, C. Gavach, Desalination 94 (1993) 151 c) S. Mazrou, H. Kerdjoudj, A. T. Cherif, J. Appl. Electrochem. 27 (1997) 558

[4] K. Onuki, H.Nakajima, S. Shimizu, Kagaku Kogaku Ronbunshu 23 (1997 ) 289 (CA 126,321555a)

[5] a) K. N. Mani, F. P. Chlanda, C. H. Byszaewski, Desalination 68 (1988) 149 b) D. Raucq, G. Pourcelly, C. Gavach, Desalination 91 (1993) 163 c) Y. Moma, T. Maehashi, Jp. Kokai Tokkyo Koho JP 07,126,997 (CA 123,115977)

[6] D. Touaibia, A. T. Cherif, J. Appl. Electrochem. 26 (1996) 1071

[7] F.I. Nobel, D. J. Vaughan, US Patent Nr. 5,578,182

[8] J. Landfors, R. Hammer-Olsen, PCT Int. Appl. WO 95 09,935

[9] K. N. Mani, D. K. Hadden, US Patent Nr. 5.814498

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