FR2839984A1 - Treatment of a spent pickling bath in an electrolysis cell with a cathodic compartment and an anodic compartment separated by an anion exchange membrane permeable to chloride ions and impermeable to iron ions - Google Patents

Abstract

The treatment of a spent pickling bath, containing trace metal ions, Fe2+ ions, chloride ions and hydrochloric acid, is carried out in an electrolysis cell including a cathode (34), an anode (33) and an anion exchange membrane (35) separating the cell into a cathodic compartment (37) connected to the cathode and an anodic compartment (36) connected to the anode. The spent pickling bath is introduced to the anodic compartment and an aqueous solution of potassium chloride is introduced to the cathodic compartment. The membrane is permeable to chloride ions allowing the oxidation of the Fe2+ ions to Fe3+ in the anodic compartment and impermeable to Fe2+ and/or Fe3+ ions to prevent their migration to the cathodic compartment.

Description

of 4lectrolyse.

  ELECTROCHEMICAL TREATMENT OF USED STRIPPING BATHS

  The invention relates to a method for the electrochemical treatment of used pickling baths resulting from the surface treatment of metallic materials.

  with aqueous pickling solutions.

  There are already known methods for reprocessing pickling baths resulting from a treatment of metallic materials using aqueous pickling solutions. These methods include among others methods of electrolysis in a cell comprising a cathode and an anode. The used aqueous pickling bath comprises metal ions, in particular ions

  Fez + and Fe3 +, chloride ions and hydrochloric acid.

  In Figure 1 in the appendix, there is a diagram showing such a

electrolysis process.

  The electrolysis bath 1 is a used pickling bath resulting from the surface treatment of metallic materials with aqueous pickling solutions, more particularly aqueous solutions with a high concentration of HCl. The used bath therefore contains Fez + ions, Fe3 + ions, Cl- ions. The electrolysis bath 1 is placed in an electrolysis cell 2 in which an anode 3 and a cathode 4 are immersed. The ions Fez + and Fe3 + migrate in the bath from the anode in the direction of cathode 4 (see arrow) F1) and unload on its extremity to precipitate. The chloride ions migrate in the opposite direction of the Fez + and Fe3 + ions, that is to say from the cathode towards the anode (see arrow F2), and discharge to form chlorine gas

which escapes into the atmosphere.

  Such processes have insufficient yields due to the presence

  in the electrolyte solution of several metal ions and chloride ions.

  Indeed, the chloride ions at the end of the cathode tend to attack the iron which is deposited thereon to reform ferrous chloride, more stable than ferric chloride. Such a procedure therefore presents the drawback

  to have an insufficient iron yield and clearly lower than 90%.

  Patent application EP-A1-0 435 382 describes a process consisting in separating the cathode from the anode by an anion exchange membrane and

  keep the adjustable potential of the cathode or anode constant.

  In Figure 2, there is a cell for increasing the

  yield of the traditional electrolysis process described with reference to FIG. 1.

  The electrolysis cell 22 contains an electrolysis bath 21 in which an anode 23 and a cathode are soaked. An anion exchange membrane 25 separates the cell 22 into an anode compartment 26 and a cathode compartment 27. The cathode compartment 27 is filled with the pickling solution containing the ions Fez +, Fe3, ions Nj2 and Cl- and H +, this solution coming from the pickling bath 31 being introduced via the pipe 30 into the cathode compartment 27. The membrane 25 allows the passage of the Cl ions towards cathode 24 without preventing the migration of ferric or ferrous ions towards cathode 24. The anions of the pickling bath 31 pass through the membrane 25 to reach the anionic space and to concentrate there. Metals precipitating at the cathode. The reactions are as follows: At the anode: Fez + + 2e Fe At the cathode: 2 Cl- Cl2 + 2e 11 there is therefore a release of chlorine at the 'anode' without the chlorine coming to attack the iron deposited at the cathode. The yield of metallic iron

is therefore increased.

  At the exit from the cathode space 27, the demetallized pickling solution containing ferric chloride is conveyed to the anode space by a pipe 28. The chlorine gas circulates by another pipe 32 and is reintroduced into the pickling bath 31, while ferric chloride is taken from the anode compartment 26 through line 29 to be reintroduced into the pickling bath 31. In the pickling bath 31, and therefore outside the electrolytic cell, chlorine which comes into the pickling bath

  oxidizes ferrous ions to ferric ions.

  Out of the bath, FeCI2 + '/ Ci2 FeCI3 is produced Ferrous chloride is transformed into ferric chloride by oxidation by chlorine. Ferrous chloride is transformed into ferric chloride by oxidation by

chlorine.

  The role of the membrane is to allow the circulation of Cl- ions from the cathode department to the anode department and to avoid the reduction of Fe3 + to Fez +.

  The disadvantage of such a process is the release of chlorine.

  We therefore wish to avoid the use of chlorine to transform ferrous chloride into ferric chloride, chlorine being a toxic and corrosive gas. The purpose of the prior art described in this document is to treat a used pickling bath to obtain a bath which can be used again to pickle metal parts. Another aim of the prior art is to recover metals such as iron and nickel from a used pickling bath containing Fez + ions at a concentration of 5 g / l and

  Fe3 + ions at a concentration of 260 9 / l.

  The method according to the present invention is a method of treating a pickling bath used so as to obtain a flocculant, ie a

  high concentration aqueous FeCI3 solution.

  Another object of the present invention is to treat a pickling bath

  used without giving off chlorine gas.

  The objects of the invention are achieved by a method of treating a spent pickling bath in an electrolytic cell comprising a cathode, an anode and an anion exchange membrane, said used pickling bath containing metal ions to the trace state, Fez + ions, chloride ions and hydrochloric acid, the membrane separating the cell into a first cathode compartment connected to the cathode and a second anode compartment connected to the anode, characterized in that it introduces in said anode compartment said pickling bath uses and in that an aqueous solution of KCI is introduced into said cathode compartment, the membrane being permeable to chloride ions to allow the oxidation of Fez + ions to Fe3 + ions in the anode compartment and being impermeable to Fez + and / or Fe3 + ions to prevent migration of Fez + eVou Fe3 + ions to the cathode compartment.

  The following description, with reference to the accompanying drawings under

  of nonlimiting examples, will make it possible to understand how the invention can

be put into practice.

  Fig ure 1 is a schematic view of a process for treating a pickling bath used according to the prior art; Figure 2 is a schematic view of an improved method of treatment of a pickling bath according to document EP-A1-0 435 382; and Figure 3 is a schematic view of the method according to the present invention. In FIG. 3, an electrolytic cell 32 is represented comprising an anode 33, a cathode 34 and an anionic membrane 35 which separates the

  cell 32 into an anion compartment 36 and a cathode compartment 37.

  The anion exchange membrane 35 allows the passage of negatively charged anions in the direction of the anode but prevents the passage of cations

positively charges towards the anode.

  According to the invention, the used pickling solution is introduced into the anode compartment, whereas according to the prior art, the pickling solution is introduced into the cathode compartment. According to the invention,

  therefore performs the opposite of what is practical in the prior art.

  The used pickling solution which is treated according to the invention is a solution originating from the pickling of metallic parts with an aqueous solution of HCl. This used solution comes from the Klinghart pickling method according to which HCI pickling bath is used with FeCI2 as catalyst. This continuous method is such that a small quantity of HCI-FeCI2 is removed and that HCI is introduced pun The used bath contains 250

  350 9 / l of Fez + and a low concentration of HCI (less than 20 g / l).

  Furthermore, according to the present invention, the compartment is introduced into the compartment.

  cathodic an aqueous solution of KCI.

  The bivalent iron is then oxidized to trivalent iron at the anode. The Cl ions cross the anionic membrane to enter the anode compartment 36. The chemical reactions at the anode are as follows: FeCI2 + KCI + H20 - FeCI3 + KOH- +% H2

KOH + HCI KCI + H20

  According to the invention, the metal is not precipitated at the cathode. The

  FeCI3 concentration increases. It evolves from hydrogen at the anode.

  The final concentration of FeCI3 is approximately at least 40%. The electrolytic cell comprises a reservoir of aqueous KCI solution, a reservoir of aqueous mixture of ferrous chloride / ferric chloride (used pickling bath) and a reservoir of ferric chloride in aqueous solution (final product). The anionic membrane separates the cell into two sealed compartments allowing it to operate with two independent flows for each compartment. The anode and cathode compartments are powered by a pump, each of which is independent and returns from the

  cell with the corresponding tanks are made by gravity and without overpressures.

  Once the oxidation of ferrous chloride to ferric chloride has started, a pump sends the ferric chloride to the reservoir of final product. The automatic triggering of this pump is done using a redox potential control located in the ferrous / ferric chloride tank

(pickling bath used).

  The two supply circuits of the cell each have a flow meter and a control valve allowing them to fix the flow in each compartment. Two heat exchangers maintain the

  constant temperature during cell operation.

Claims (3)

REVEN DICATIONS   1 Method for treating a used pickling bath in an electrolytic cell comprising a cathode (34), an anode (33) and an anion exchange membrane (35), said used pickling bath containing metal ions with trace state, Fez + ions, chloride ions and hydrochloric acid, the membrane separating the cell into a first cathode compartment (37) connected to the cathode (34) and a second anode compartment (36) connected to [' anode (33), characterized in that one introduces into said anode compartment (36) said pickling bath used and in that one introduces into said cathode compartment (37) an aqueous solution of potassium chloride, the membrane being permeable with chloride ions to allow the oxidation of Fez + ions into Fe3 + ions in the anode compartment (36) and being impermeable to Fez + and / or Fe3 + ions to prevent ion migration   Fez + and / or Fe3 + to the cathode compartment (37).   2. Method according to claim 1, characterized in that hydrogen is clearance at the anode.   3. Method according to claim 1, characterized in that the concentration