EP0512097A1

EP0512097A1 - Method and installation for treating an aqueous galvanization effluent

Info

Publication number: EP0512097A1
Application number: EP91920764A
Authority: EP
Inventors: Jean-Paul Laurent; Gérard Colin
Original assignee: Krebs & Cie Sa; Sollac SA; Lorraine de Laminage Continu SA SOLLAC
Current assignee: Krebs & Cie Sa; Sollac SA
Priority date: 1990-11-23
Filing date: 1991-11-12
Publication date: 1992-11-11
Also published as: PL295548A1; FR2669646A1; WO1992009725A1; FR2669646B1; JPH05505425A; KR927003887A; CS227392A3

Abstract

Procédé de traitement d'un effluent aqueux issu d'un procédé d'électrozingage à anode soluble et en milieu chlorure, caractérisé en ce que l'on soumet l'effluent à une extraction par une phase organique pratiquement insoluble dans l'eau, contenant un échangeur d'anions organique.Process for the treatment of an aqueous effluent from an electro-zincning process with a soluble anode and in a chloride medium, characterized in that the effluent is subjected to extraction with an organic phase practically insoluble in water, containing an organic anion exchanger.

Description

METHOD AND INSTALLATION FOR TREATING AN AQUEOUS ELECTROZINGING EFFLUENT

The present invention relates to a process for treating an effluent from a process of electro-zinc plating with a soluble anode and in a chloride medium as well as a process of electro-zinc plating comprising application.

Market supply needs

(automobile construction, household appliances) made of sheets offering great resistance to corrosion have led to the increasing development of electro-galvanizing of steel strips.

The electrozincing is carried out continuously in lines successively implementing the operations of preparing the sheets (alkaline degreasing and acid brightening), of electrozincing proper and of finishes (passivating finishes or organic coatings and rinses).

There are two main industrial processes for electrozincing which are well differentiated: that from aqueous solutions of zinc sulphate with insoluble metal anodes, that with metallic anodes soluble in an aqueous bath containing chlorides called "in chloride medium".

Like any process for treating surfaces by chemical and electrochemical means, electro-zinc plating leads to a certain number of losses and discharges which it is necessary to treat in order to satisfy the requirements of the environment. Conventional methods for treating various pollutant flows use methods for destroying effluents by insolubilizing metals (neutralization) with, on the one hand, a large production of cakes of metal hydroxides which are difficult to recover, and on the other hand hand, a fatal contamination liquid discharges by high salt concentrations.

In general, these techniques lead to high investment and operating costs to which must be added the costs induced by losses in raw materials (metals, salts, water, etc.).

The present invention aims in particular to provide a "clean" process for treating rejects adapted to the electrozincing process in chloride medium.

By "clean" process is meant a process which allows both: a - to minimize the various releases and losses, b - to continuously regenerate the maximum of releases with recovery of by-products (water, salts, metals) and more generally, to reduce operating costs while improving the consequences for the environment. The * present invention aims very particularly to provide an electrogalvanizing process in which there is practically no rejection of zinc.

It should be noted that electrozinc electrolytes are adaptable to market demands and can, in addition to zinc alone, lead to codepositions of zinc with other metals (Fe, Ni ...), in order to improve the properties of 'appearance or anti-corrosion of the sheets. The electrolytes then contain concentrations of chlorides in these addition metals.

A typical composition of electrolytic electrolysis in chloride medium is as follows:

Zn 100 - 105 g / 1

K ⁺ > 100 g / 1

HC1 S for pH = 4.5-5 Electrolytes of the same type can also be used, which also contain nickel ions.

The common characteristics of these electrolytes are: - a high concentration of metal and alkali chlorides to meet the requirements of electroplating and energy costs (conductivity of electrolytes), the need to maintain metal concentrations within a range of narrow variation (of the order of a few percent), the need to maintain the pH of the electrolyte in an equally narrow range (+ 0.25).

We know that as a result of a faradic yield difference between the anodic reaction of dissolution of the zinc anode (Zn ^* > Zn + 2e ^~ ) and the cathodic reaction of deposition of zinc on the sheet (Zn ++ + 2e -> Zn ^* ), the concentration of soluble zinc increases during electrolysis. This excess zinc corresponds to the parasitic reaction 2H + 2e> H- which is released at the cathode in competition with the deposit of zinc. This parasitic reaction is responsible at the same time: for the 100% difference in cathodic yield,

- the increase in the Zn content in

1'electrolyte, the consumption of H + (therefore the increase in pH) which requires permanent readjustment by adding HCl in the electrolyte.

In addition, a certain chemical dissolution of the anode in the electrolyte at pH = 4 is also responsible for the increase in the zinc concentration. In the conventional process, the Zn content is maintained in the concentration range targeted by periodic purging of electrolyte and reconsti¬ tution of new electrolyte. In addition, in industrial processes it is generally necessary to water the conductive rollers and the sheet with an aqueous solution. Although there is a entrainment by the sheet (which one seeks to reduce to the minimum by wringing) and that there occurs an evaporation of the bath, there is generally an increase in the quantity of water, so apart from the excess zinc in solution there is also an excess of water which must be drained and treated. The present invention aims to completely eliminate the consequences of these rejected surpluses by regenerating the electrolyte by selective extraction of the excess zinc and recovery of the latter.

The present invention thus relates to a process for the treatment of an aqueous effluent derived from an electrozincing process with a soluble anode and in a chloride medium, characterized in that the effluent is subjected to extraction by a phase organic practically insoluble in water, containing an organic anion exchanger.

It also relates to an electrozincing process with an anode soluble in an aqueous electrolyte containing chlorides, characterized in that an electrolyte current is drawn off, this current is subjected to an extraction with a practically insoluble organic phase in water containing an organic anion exchanger and the extracted current is recycled.

The organic anion exchanger can be advantageously constituted by an amine salt, in particular a salt formed with hydrochloric acid.

The organic amine can in particular be a tertiary amine of formula R ₃ N in which R is in particular a C ₁ to C ₁ alkyl group _? , for example an isooctyle group.

The salts of organic amines allow a selective extraction of zinc in the form of an anionic complex ZnCl. 2- which is present in the bath due to the high chloride concentration.

This extraction is selective and there is no extraction of the other metals possibly present (Ni, Fe) which are not in the form of anionic complexes. The organic phase advantageously contains an organic diluent insoluble in water and of low density which facilitates the separation operations (which allows for example the separation by decantation of the aqueous phase). In addition, it advantageously contains an alcohol insoluble in water which contributes to the stability (non-demixing) of the organic phase.

The amine is preferably used in a proportion by volume of 5 to 50 °., Preferably from 15 to 35%.

By way of example, the organic phase can have the following composition: amine (in the form of a salt) ... 30% by volume alcohol (isodecanol) 20% by volume diluent (kerosene) 50% by volume.

The extraction can be generally performed at a temperature from 15 to 70 ^* C. prati¬ that is performed at a temperature close to that of 1'electrozingage. In practice, one operates continuously against the current and thus continuously treats an electrolyte current which is recycled to the electrolytic cell after extraction. Zinc can be re-extracted from the organic phase with an aqueous phase. This phase can be a concentrated alkaline aqueous phase (preferably a concentrated KOH solution) leading to the obtaining of an alkaline zincate or water leading to the production of pure zinc chloride.

The recovery of pure re-extracted zinc is possible from either zinc hydroxide (acidification of the zincate or neutralization of the chloride) or directly from the chloride itself, as is given below.

In the case of a tertiary amine which is in the form of hydrochloride, the extraction reaction is written:

ZnCl ₄ ^" 4- 2R ₃ NH ⁺ C1 ^{~ <} ZnCl ₄ , 2R ₃ NH + 2C1 ^~ ^> and the re-extraction reaction with water: ZnCl ₄ , 2R ₃ NH ^< 2R ₃ NH ⁺ C1 ^" + ZnCl ₂

Before re-extraction, the organic phase is advantageously washed in order to remove the fine aqueous droplets possibly entrained by the organic phase and thus improve the purity of the zinc re-extracted in the next step.

The number of stages and the ratio of the phases present (organic phase 0 and aqueous phase A) are chosen as a function of the isothermal equilibrium curves. There is shown respectively in FIGS. 1 and 2 the isothermal equilibrium curves during extraction and during re-extraction with as organic phase that consisting of 30% by volume of triisoctylamine (hydrochloride), 20% by volume of isodecanol and 50% by volume of kerosene.

It should be noted a health-specific feature of the invention in that the process has great flexibility with regard to the objective set which is to extract from the electrolyte a given quantity of zinc by unit of time, according to faradic yield. In fact, this quantity of zinc is the product of the flow rate treated by the difference in concentrations between that of the electrolyte to be regenerated and that of the regenerated electrolyte (extraction raffinate). The flexibility thus lies in the fact that by playing at the same time on:

- the electrolyte flow to be regenerated,

- The number of extraction stages, the volume organic phase / aqueous phase volume ratio, it is possible, with a given quantity of zinc extracted per unit of time, to obtain an extraction raffinate more or less concentrated in zinc. This remark can advantageously be used to obtain from an electrolyte to be regenerated, a saline solution practically free of zinc (<100 mg / 1) usable in watering sheets or rolls before restoring to electrozincing cells.

The following example illustrates the flexibility mentioned above: for 50 kg / h of zinc to be extracted and a payload of the organic phase of 9 g / 1 Zn, one can obtain extraction operating points of several types : Type I Type II Type III

Electrolyte flow to re-regulated -near rrreerr (m3 / h) (100 g / 1 Zn) 0.5

Organic flow (m3 / h) 5.55 5.55 5.55

Ratio 0 / A 1.11 5.55 11

Zn concentration of the raffinate to be recycled (Zn g / 1) 90 50 0.1

All solvent extraction technologies can be used for the implementation of the process.

In the particular case of mixer-settlers, in order to ensure a phase ratio 0 / A close to 1/1 for phase balancing, it is advantageous to ensure if necessary an adjustment by internal recycling of aqueous phase on each floor. For the different types above, only the number of stages to be used varies according to the isothermal equilibrium curve: for example 1 stage for type I, 2 stages for type II and 5 stages for type III.

In addition, the excess water in the electrolyte can be removed by evaporation under vacuum. We then take advantage of the excess calories produced by the Joule effect in the electrozincing cell. For this purpose, it is possible to provide a closed loop at high flow where the evaporation under vacuum of the electrolyte takes place at the electrogalvanizing temperature. This evaporation has very little effect on the concentration of the electrolyte, given the low ratio between the water removed and the circulation rate (1 to 2). According to the general thermal balance a thermal reheating of the electrolyte can be carried out before evaporation. The condensation of water vapor leads, depending on the technology selected, by mixing or by surface, to recover a purge of deconcentration water or a condensate usable for rinsing of brightening.

Excess water in the electrolyte can also be removed by other techniques such as reverse osmosis. It should be noted that in the conventional technique, there is also a loss of electrolyte by entrainment by the electro-galvanized strip at the outlet of the electrolysis cells.

In this conventional technique, taking into account the non-regeneration of the electrolyte, any action aimed at reducing the entrainments (therefore the pollution diluted by rinsing) by better wringing the strip in particular, only transfers the same polluting flow d 'a diluted form to a concentrated form (equal amount of the constituents of the electrolyte).

In the conventional technique, the quantity of electrolyte purged to keep the zinc concentration constant results from the difference in balance between the excess zinc formed in the electrolysis cell and the zinc eliminated by the entrainments. Reducing the latter necessarily means increasing the quantity to be purged. It is therefore only a transfer between two treatments of rejections. One could therefore wonder about the interest of reducing training.

In the present invention, thanks to the regeneration of the electrolyte, any saving in training will be an absolute saving in electrolyte.

Given the possible reduction of the flow entrained at the cell outlet, the rinsing of the strip can be ensured in the following manner, making it possible to recycle at a low cost from 40 to 90% of this reduced flow.

To do this, a rinsing and brushing station is created after the last electrolysis cell supplied with a low flow of make-up water. The resulting diluted effluent which collects a high percentage of the entrained initial flow is returned to the electrolyte evaporation loop at the cost of a modest increase in its capacity, the equivalent of additional operating cost of evaporation being largely compensated by the subsequent savings made on the reduction of the effluent treatment station.

The numerical example below illustrates and justifies this.

From a drive of 150 l / h of electrolyte on the strip, the first brushing rinsing is supplied with an additional water of V 1 / h. The rinsing brushing is watered by a current flowing in a closed circuit to ensure the hydraulic asper¬ sion. Under these conditions, it is withdrawn V 1 / h of diluted rinse having the composition C g / 1 Zn.

The zinc material balance gives: 100 x 150 = (V + 150) C hence the values of C corresponding to various possible operating points with the recovery rates corresponding training

The electrolyte entrainment by the strip towards the demineralization loop of the subsequent rinses proposed in accordance with the invention is, in g / h Zn: 150 x C.

The optimization of V, therefore of C, for a given training, is done by comparing the cost of evaporation of V and the cost of demineralization of the pollutant load 150 x C.

The final rinsing of the strip is therefore economically feasible after the rinsing-brushing described above, by a closed water circuit on a demineralization unit by ion exchangers (a cation exchanger and an anion exchanger). The rinsing tanks are advantageously arranged in cascade with supply of demineralized water against the current of the strip.

In these overall rinsing conditions: the consumption of rinsing water (from 10 to ₃ 40 m / h in conventional technologies) is

3 reduced to a few m / h only (in the form of make-up V, make-up reagents for elution and rinsing of ion exchange resins), the treatment facilities for effluents (neutralization and sludge treatment) are reduced in very large proportions (at least 3 to 4).

Example of installation according to the invention

An elec¬ trozingage installation according to the invention will be described below with reference to FIG. 3 which represents a diagram of such an installation.

A raw sheet arrives at 1 in a degreasing station 2. It then passes successively through a station 3 for rinsing with water, a station 4 for acidic living, a station 5 for rinsing with water, a station 6 for electrozincing, a station 7 for rinsing-brushing, and finally in a station 8 for rinsing with demineralized water.

The installation further comprises a station 9 for evaporation of the electrolyte under vacuum, a station 10 and extraction of the electrolyte in an organic phase, a station 11 for washing the organic phase and a station 12 of re-extraction with water and a station 13 for concentrating the ZnCl 3 product.

In this installation, the flows are as follows: from the degreasing station 2 it is discharged periodically through a conduit 21 from the spent degreasing bath to an installation 22 for treating effluents. From the rinsing station 3, the rinsing water is discharged through a conduit 23 to the installation 22 for treating effluents.

From the acid brightening station 4 is periodically removed via a conduit 24 from the spent acid brightening bath to the installation 22 for treating effluents.

In the electrozincing station 6, sprinkling water is supplied via a pipe 25. From the electrozincing station 6, the electrolyte is permanently drawn off through a conduit 26 towards the evaporation station 9. The electrolyte is brought back to the electrozincing station 6 by a conduit 27. The condensates of the evaporation are evacuated from the station 9 by a conduit 28 which brings part of it by a current 28a to the rinsing station 5 and another part by a conduit 28b at station 7 of rinse-bros¬ sage. The rinsing water from station 5 is returned via a line 29 as rinsing water supplying the rinsing station 3.

Electrozincing station 6 is also continuously withdrawn via a conduit 30 from the electrolyte to bring it to the extraction station 10 with an organic phase 15 containing an amine salt. This station 10 comprises for example two stages of mixer-settlers.

The electrolyte having undergone the extraction is returned by a conduit 31 to the station 6 of electro-zinc plating.

The organic phase enriched in zinc in the extraction station 10 undergoes washing in the washing station 11 comprising for example 3 stages of mixer-settlers. The washing solution is returned via a line 32 to the electro-zincing station 6.

The washed organic phase is then sent to station 12 for re-extraction with water. The washing water is constituted on the one hand by water supplied by a conduit 33 coming from the station 8 for rinsing with demineralized water and on the other hand, by condensed water brought by a conduit 34 from the ZnCl- concentration station 13.

The aqueous solution of ZnCl 4 obtained by re-extraction is sent via a conduit 35 to the concentration station 13. Part of this solution is taken through a conduit 36 to constitute the washing solution for the washing station 11. The concentrated solution of ZnCl _? is evacuated via a conduit 37 to a use 38 allowing the ZnCl _{2 to be} valorized

The rinsing-brushing station 7 is supplied as indicated above by a conduit 28b bringing water leaving the evaporation station 9. The effluent is recycled through a pipe 39 to the loop of the evaporation circuit.

The station 8 for rinsing with demineralized water is supplied with water in a closed circuit coming from a station 14 for demineralization via a conduit 40. The rinsing water is returned by a conduit 41 to the station 14. As indicated previously, a part is withdrawn. through the conduit 33 for the supply of the station 12 for re-extraction. The resins used in the post; e- 14 are periodically eluted and rinsed, the effluents leaving the anionic resin are discharged to an installation 42 for treating effluents while the effluents leaving the cationic resin are returned via a conduit 43 towards the duct 26 of the evaporation loop.

Numerical example

Numerical data will be given below on the extraction and re-extraction conditions in an installation according to the invention.

In extraction station 10:

3 Supply via line 30: 0; 98 m / h

Mixer contact time: 2 minutes Flow rate of circulation of the organic phase

15: 6.7 m ³ / h

O / A ratio = 6.7 / 1

Concentration of the organic phase at the extraction inlet (or re-extraction outlet): 6 g / 1 Zn.

Concentration of the organic phase at the end of the extraction: 14.8 g / 1 Zn. Exit via line 31: Raffinate 0.98 m ³ / h at 40 g / 1 Zn.

In washing station 11:

Washing with a fraction of the re-extracted chloride ZnCl ₂ supplied via line 36: 0.22 m ³ / h at 27 g / 1 Zn. Contact time: 2 minutes

Ratio 0 / A = 30/1. Exit via conduit 32:

Washing solution recycled by electro-zinc coating 0.22 m ³ / h at 40 g / 1 Zn. In water re-extraction station 12: - -

Water flow 2.07 m ³ / h

O / A ratio = 3.2 / 1.

Contact time: 2 minutes.

The re-extraction is partly fed with the condensate from the pure ZnCl evaporation.

(duct 34) 1.52 m 3 / h, for part by one

3 water make-up (line 33) 0.55 m / h.

3

There is obtained 2.07 m / h of solution (evacuated via line 35) at 27 g / 1 Zn (56.4 g / 1 ZnCl), part of which (via line 36)

₃ 0.22 m / h is used for washing the solvent.

In station 13 for concentrating the solution of

ZnCl ₂ : Concentrated solution (evacuated via line 37): 0.33 m ³ / h at 150 g / 1 Zn, i.e. 313.8 g / 1 ZnCl ₂ .

Claims

1. Process for treating an aqueous effluent from an electrozincing process with a soluble anode and in a chloride medium, characterized in that the effluent is subjected to extraction with an organic phase practically insoluble in water , containing an organic anion exchanger.

2. Electrozincing process with anode soluble in an aqueous electrolyte containing chlorides, characterized in that a current of electrolyte is drawn off, this current is subjected to an extraction with an organic phase practically insoluble in 1 water containing an organic anion exchanger and the extracted stream is recycled.

3. Method according to claim 1 or claim 2, characterized in that the anion exchanger is an amine salt.

4. Method according to claim 3, charac¬ terized in that the organic amine is a tertiary amine.

5. Method according to claim 4, charac¬ terized in that the organic amine is an amine of formula R_N where R is a C ₁₂ -C ₁₂ alkyl group.

6. Method according to any one of claims 1 to 5, characterized in that the organic phase comprises a diluent insoluble in water and of low density.

7. Method according to claim 6, charac¬ terized in that the organic phase comprises an alcohol insoluble in water.

8. Method according to claim 7, charac¬ terized in that the amine represents from 5 to 50% by volume of the organic phase.

9. The method of claim 8, cara- terized in that the amine represents from 15 to 35% by volume of the organic phase.

10. The method of claim 9, charac¬ terized in that one carries out a re-extraction of the organic phase with an aqueous phase.

11. Method according to claim 10, characterized in that a washing is carried out prior to re-extraction.

12. Method according to any one of claims 2 to 11, characterized in that part of the water contained in the electrolyte is removed by evaporation under vacuum.

13. The method of claim 12, characterized in that the evaporated water is condensed and used for rinsing operations.

14. Method according to claim 13, characterized in that the condensed water is used for a rinsing-brushing operation after the elec¬ trozingage.

15. Electrozincing installation for the implementation of a method according to claim 2, characterized in that it comprises a circuit (30, 10, 31) for extracting the electrolyte current by an organic phase.

16. Installation according to claim 15, characterized in that it comprises a station (12) for re-extracting the organic phase with an aqueous phase.

17. Installation according to claim 16, characterized in that it comprises a station (11) for washing before the station (12) for re-extraction.

18. Installation according to any one of claims 15 to 17, characterized in that it comprises a circuit (26, 9, 27) for evaporation of a part of the electrolyte water.

19. Installation according to claim 18, characterized in that it comprises, after the electrozincing station (6), a rinsing-brushing station (7) supplied with condensed water from the circuit (26, 9, 27) evaporation.

20. Installation according to any one of claims 15 to 19, characterized in that it comprises a station (8) for final rinsing with demineralized water regenerated in a closed circuit (41, 14, 40).