FR2573221A1 - METHOD AND DEVICE FOR RECOVERING SILVER FROM FASTENING BATHS - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS A PROCESS FOR RECOVERING THE SILVER CONTAINED IN A FIXING BATH BY ELECTROLYSIS OF THE SAID SOLUTION IN AN ELECTROLYZER WHOSE CATHODIC COMPARTMENT CONTAINS A VOLUME CATHODE AND IS SEPARATED FROM THE ANODIC COMPARTMENT BY AN EXCHANGE MEMBRIONS; IT ALSO CONCERNS A DEVICE FOR IMPLEMENTING THE SAID PROCESS, THOSE DEVICE INCLUDING: -AN ANODIC COMPARTMENT 1 PREFERABLY INCLUDING A VOLUME ANODE AND SUPPLIED WITH ELECTROLYTE FROM A BAC7 AND A PUMP6; -A CATHODIC COMPARTMENT 2 INCLUDING A VOLUME CATHOD SUPPLIED AS A FIXING BATH TO BE TREATED FROM A BAC4 AND A PUMP5; -AND AN ION EXCHANGER MEMBRANE 3.

Description

Method and device for recovering silver from baths

fixing.

  The present invention relates to a method and a

  for the recovery of the silver contained in the fixing baths. In black and white photographic processes, only a fraction of the silver contained on the film remains on the plate after revelation and fixing. Indeed, during the fixing operation, the silver halide crystals not exposed to light, and therefore not reduced, are dissolved in a complexing solution, often

  consisting of hyposulphite called fixative.

  This hyposulphite solution is progressively

  in money and its activity is gradually decreasing.

  When the silver concentration reaches several g / l (for example 5 to 10 g / l), it is necessary either to eliminate the fixer after having recovered all the money, or to eliminate only a part of the money and that without alter the properties of the bath, which allows the recycling and reuse of the fixer, so

  & a substantial economy of chemicals.

  To extract the silver from a spent fixer bath, electrochemical techniques are very widely used, insofar as they make it possible both to treat solutions with a silver concentration of several g / l, to preserve all the regenerative bath fixing properties and finally not to introduce foreign ions such as chemical reduction processes.

  The recovery of money by electro-

  lysis from the fixer baths is commonly used, but it generally has the disadvantage of not being able to lead to total recoveries of silver without degradation of the fixer and without the appearance of sulfide products at the cathode, or even

  a release of sulphurous vapors.

  It has been found, and this is the object of the present invention, that the known methods for recovering silver contained in a fixing bath of the plates by electrochemical techniques could be considerably improved by using a

  perpendicular configuration electrolyser comprising a comparator

  cathode feed supplied with said fixing bath comprising an electrode

2 5-73221

  high porosity fixed volume, said cathode compartment being separated from the adjacent anode compartment by means of a

ion exchange membrane.

  The process according to the invention is characterized by the use of the following techniques:

  1) said electrolyser is separated into two compartments

  by an ion exchange membrane; the existence of this membrane makes it possible in particular to obtain, by treatment of a given bath, very low residual silver concentrations (of the order of a few parts per million), and this for a

  duration of treatment compatible with an industrial operation ,.

  and avoids any pollution of the bath by the anodic oxidation products which are normally released during the treatment. The exact nature of the ion exchange membrane is small and

  may use membranes of anionic or cationic type.

  2) The cathode compartment of the electrolyser comprises - or consists of - a fixed volumetric electrode: a volumetric electrode is called an electrode in the volume of which the electrolyte bath circulates; such an electrode opposes the surface electrodes in. lesquees only the outer surface

  electrode is in contact with the bath. This large electrode

  is fixed. It is known that there are volume electrodes produced by a fluidized bed of a conductive material in the electrolysis bath; these fluidized bed electrodes present

  large specific areas (5,000 m / m), good

  material transfer tees, but the continual movement of the grains

  night the propagation of electric current in the metal matrix

  lic; this results in a non-uniformity of the electron reactivity

  local chemical related to potential distributions within the

  metal matrix as well as electrolyte; however, such elections

  fluidized bed trodes are not suitable in the present invention and this is why fixed solid electrodes have been chosen. These fixed voluminal electrodes are preferably constituted by a stack of expanded metal metal grids. The porosity of a three-dimensional electrode (fraction of volume occupied by the liquid phase) plays an essential role both in the phenomena of clogging

  (during the cathodic deposition operation and / or in the presence of

  solid purities present in baths) and on the distribution of

  the current and therefore the electrochemical surge, which

  value imposes the selectivity of the process.

  High porosity volume electrodes have a higher capacity for silver recovery and powers

  mechanical requirements for pumping the electrolyte len are reduced.

  It is well known that the equivalent electrical conductivity of an electrolyte present in a porous structure can be connected to the true conductivity yo via 2e

  the porosity 6 according to the relation y = 'Yo 3-

  Therefore, the higher E is, the closer it is to yo-: a high value of the porosity thus makes it possible to limit the ohmic drops of potential within the electrolyte and

reduce overvoltage discrepancies.

  Fixed electrodes will therefore be used

with high porosity.

  3) The anode compartment of the electrolyser can be any; but it is preferably also constituted

  of a porous voluminal electrode which is traversed by an electro-

  suitable lyte preferably containing sodium thiosulfate and

sodium bisulphite.

  4) When operating the electrolyser as defined above, it was found that it was very desirable that said electrolyser be perpendicular type, that is to say

  that the direction of circulation of the electrolytes is then per-

  pendicular to the direction of the electric current, which allows,

  among other benefits, to increase the residence time of electro-

  cathodic lyte in the reactor, so the conversion of the reagent.

  This structure also has the advantage of limiting the variations of the electrochemical overvoltage, thus to avoid the appearance of parasitic reactions on the cathode, but also to increase the

  material transfer rates, thus reducing the time required

  electrochemical treatment.

  The method and the device described above can be used in two ways, namely: - either recover the silver from the photographic fixing baths until the silver concentration of these baths

  reaches 1 to 2 g / l; at this stage, given the non-

  bath pollution, we can recycle the treated bath and reuse it directly; - either treat the photographic fixing bath to the

  complete recovery of money.

  The present invention also relates to the entire device that can be used for carrying out the method. This assembly is represented, in a nonlimiting manner, in the figures

, 11 and 12.

  FIG. 10 represents an assembly device according to the invention.

  Figures 11 and 12 show a particular arrangement

  the desirability of the electrolyser and the overall device

(diagram 1).

  The electrolyser comprises two compartments (1) and (2) where are housed the electrodes which are constituted by a porous conductive material of very high porosity. The two compartments are separated by an ion exchange membrane (3). The fixing solution containing the silver is stored in the tray (4). A pump (5) ensures the continuous circulation of this solution in the cathode compartment (2). In the anode compartment (1) a second pump (6) allows the circulation of another solution from a second storage tank (7). During electrolysis, the metallic silver is deposited on the porous structure produced by a stack of cathode grids (8). The management of

  electrolyte circulation is then perpendicular to the direction

  electrical current. In (9), a debit

metre.

  More specifically, an example of electronic construction

  The lyser can be as follows: the anode compartments (1) and the cathode compartments (2) are rectangular and have a height of 45 cm and a width of 15 cm, the thickness of each of the compartments

  being 0.6 cm with a rising speed of electronic circulation.

  lytes a few centimeters per second. The separator (3) is an ion exchange membrane passing through certain anions: it is an electrodialysis membrane of thickness 500 μm. The electrodes (1) and (2) are formed by a stack of 8 grids of metal expanded mesh 6 x 50 x 40 stainless steel ensuring a porosity of 80%. The current leads are placed on the outer walls of the cell and ensure the clamping of the stack. At the bottom of the stack, the uniform distribution of the electrolyte is provided by a perforated plate. When carrying out

  the stack, the expanded metal grids with an anodizing

  tropie are placed perpendicular to each other. A possible composition of the anolyte can be: sodium thiosulfate 250 g / l, sodium bisulfite 12.5 g / l and sodium bromide

1.63 g / l.

  The recovery capacity of such an electrolyzer is about 4 kg of silver per liter of cathode compartment, which corresponds for the standard configuration to a recovery of 1.9 kg of silver by treating 380 1 of 5% solution. g / l, this

  for an energy consumption of 0.4 kWh / kg of silver.

  A specific electrolyser may be arranged as shown in FIG. 11, and the overall scheme may be

type shown in Figure 12.

  On these figures, it has been indicated: - PVC watertight tank containing all the apparatus of

operation.

  11 - Compartment reserved for electric parts and pumps.

  12 - Support plate receiving the anode wire with the inputs.

13 - Anodic wire mesh.

  14 - Membrane separating the anode compartments

and cathodic.

  - Support plate receiving the cathode grid with the inputs

and outputs of the cathode liquid.

16 - Cathode grid.

  17 - Power supply of the cathode.

  18 - Electrical supply of the anode.

19 - Release of the anode liquid.

- Cathode liquid outlet.

  21 - Cathode liquid inlet piping between the plate

support (15) and the pump (25).

  22 - Anodic liquid inlet piping between the support plate (12) and the pump (26).

  23 - Tank of cathode liquid.

  24 - Anodic liquid tank.

  - Cathode liquid circulation pump.

  26 - Anodic liquid circulation pump.

  27 - Cathode liquid level detector for filling

  by the pump (30) of the reservoir (23).

  28 - Minimum cathode level liquid level detector

  the stop information of the drain pump (35) and the replacement

  pleating by the pump (30) of the tank.

  29 - Tank volume buffer greater than the cathode tank.

  - Circulation pump on buffer tank.

31 - Current rectifier.

32 - Draining on anode tank.

33 - Draining on buffer tank.

34 - Draining on cathode tank.

- Cathodic drain pump.

  36 - Level detector on buffer tank.

37 - Cathode grid handle.

  The circulation of liquids is as follows:

  a) - The circulation pump of the cathode liquid (25) is fed

  - mented by the tank (23). The liquid passes through the cathode grid (16) via the cathode liquid inlet pipe (21). After passing through the mesh, the cathodic liquid flows into the tank through the

output (20).

  b) - The circulating pump of the anode liquid (26) is fed

by the tank (24).

  The liquid passes through the anode grill (13) through

  through the inlet pipework of the anode liquid

  dique (22). After passing through the fence, the liquid

  anodic flows into the tank through the outlet (19).

  c) - The flow rate of the cathode pump is 30 1 / min, that of

the anode pump is 10 1 / min.

  The active recovery zone is 500 x 250 mm.

  The device can operate automatically by operating as follows: The anode liquid is placed manually in its compartment. The solution to be treated from the buffer tank (29) is transferred to the cathode compartment by

via the pump (30).

  When the cathode compartment is full, the

  level sensor (28) stops the pump (30).

  The device is then ready for operation. The pumps (25) and (26) and the rectifier (31) are put into operation. A

  Delay adjusts the duration of the operating cycle.

  At the end of this cycle, the cathode compartment is emptied via the pump (35), the cathode compartment can then receive another load waiting in the

buffer tank.

  This solution will be brought automatically when the level of the liquid in the tank is full, that is to say that the

  level detector (36) will be solicited.

  The reservoir (29) is larger than the reservoir (23).

  The emptying of the anode circuit is done manually. Depending on the silver richness of the treated solutions, the number of cycles can vary before extracting the cathode grids

  to recover the silver particles.

  The operation is carried out by pulling on the handle (37)

  of the grid, after disconnecting the power supply (17).

  Removable cathode grates are separated and replaced after money recovery, performed by

shaking and brushing.

  The following non-limiting examples illustrate the

  main performance of the method and device according to the invention.

Example 1

  Processing of 10 1 of solution from a graphic art workshop.

  FIG. 1 represents the evolution of: the concentration as a function of time during electrolysis with a potential difference between the electrodes constant, equal to 1.8 V, for an initial concentration solution of approximately 1500 mg / l . After 30 minutes of electrolysis, the silver concentration is less than

  ppm, it is close to 10 ppm after 1 h of electrolysis (Figure 2).

  In this experiment, the cathode electrode consists of a stack of 8 expanded metal grids, the flow velocity of the electrolyte in the compartment.

  presence of grids is about 11 cm / s.

Example 2

  Treatment of 8 1 of solution from a radio laboratory

graphy.

  For the same operating conditions: grids, voltage between electrodes: 1.8 V, electrolyte circulation rate u

  11 cm / s, the decrease in concentration is even faster.

  After 60 min of treatment, the concentration is less than 20 ppm whereas the initial concentration of

solution was 3.6 g / l.

  Figure 3 shows the evolution of the concentration

  in silver depending on the electrolysis time. After 4 hours of

  lysis, at constant cell voltage, the residual concentration

in silver is less than 1 ppm.

- Example 3

  Treatment of 13.5 1 of synthetic solution, influence of inten-

current.

  These solutions of the same composition as the anolyte have

  an initial concentration close to 200 mg Ag / l.

  FIG. 4 represents the temporal evolution of the concentration for electrolyses in the same apparatus configuration (8 cathode grids, anion exchange membrane, 8 anode grids) for a catholyte circulation rate.

  weaker than 4.4 cm / s and for electrolyses at constant intensity

  aunt. This figure clearly shows that at the beginning the concentration

  tion decreases linearly with time and that, for sufficiently long processing times, the amplitude of

  changes in concentration gradually decreases.

  It is also observed that an increase in

  operating voltage decreases the treatment time required for

  get a given concentration in money.

Example 4

  Treatment of 14 1 of synthetic solution, influence of speed

of circulation.

  This time, the initial concentration of the solution is about 120 mg / l, the intensity of the operating current being equal to 1.75 A. The lowering of concentration is even faster than the circulation speed of the electrolyte is great

(see Figure 5).

Example 5

  Influence of the circulation rate of the catholyte.

  This example and the following ones correspond to operations carried out at controlled intensity, variable at

  course of time. At any moment, the intensity is proportional

  the initial intensity, the concentration in silver -

  -15 measured by a sensor - and conversely porportional to the

initial concentration.

  The set of FIGS. 6, 7 and 8 corresponds to a standard electrolyser configuration but at three speeds of

  circulation of the electrolyte in the cathodic

  respectively 0.5 - 3.5 and 11.1 cm / s within the grids.

  Examination of these figures clearly shows that the higher the intensity of operation, the faster the decline in

concentration in money.

  Moreover, the evolution of the current efficiencies corresponding to these three tests makes it possible to determine for each flow rate of the electrolyte an order of magnitude of the intensity of the diffusional limit current; we find Interstitial velocity Limit intensity A cm / s

0.5 0.96 <IL <1.6

3.5 3.6 <IL <4.93

11.1 4.83 <I <6.45

  The faster the flow velocity of the electrolyte is

  large, the maximum permissible maximum intensity without the risk of

  ration is high, so faster is the electrochemical treatment.

Example 6

  Influence of the thickness of the cathode electrode.

  From the same initial conditions (bath volume, silver concentration), it was found that the larger the surface of the electrode, the higher the operating intensity. However, the thicker the electrode in the direction of the current lines, the worse the

  distribution of the electrochemical overvoltage.

Example 7

  Influence of the nature of the ion exchange membrane.

  Figure 9 shows the changes in the concentration

  silver tration versus electrolysis time for two electrolysers differing only in the nature of the ion exchange membrane, one passing the anions, the other the cations. There is no significant difference in the decay of the silver concentration, but on long-term experiments, one or the other of the membranes may be more

  effective in preventing sulphidation of the deposit.

Claims (7)

  1. A method for recovering the silver contained in a fixing bath of the plates used in particular in photography and radiography by electrochemical techniques using a electrolyser perpendicular configuration characterized in that said electrolyseurcomportun cathodic compartment, powered by said fixing bath, comprising a high porosity fixed-density electrode, said cathode compartment being separated from the neighboring anode compartment by means of an ion-exchange membrane. 2. Method according to claim 1, characterized in that said voluminal electrode is constituted by a stack expanded metal grilles.   3. Method according to one of claims 1 and 2, characterized   characterized in that said fixing bath is treated until exhaustion full of the money it contains.   4. Method according to one of claims 1 and 2, characterized   in that said fixing bath is treated so that it contains a sufficiently small amount of silver to be able to to be recycled.   5. Method according to one of claims 1 to 4, characterized   in that the liquid circulating at the anode contains thiosulphate   of sodium, sodium bisulfite and sodium bromide.   6. Electrolysis device usable for the implementation   process according to one of claims 1 to 5, characterized   in that it. comprises: - an anode compartment (1) preferably comprising a volumic anode and supplied with electrolyte from a tank (7) and a pump (6) - a cathode compartment (2) comprising a volumic cathode and supplied with fixing bath to be treated from a tank (4) and a pump (5), and   an ion exchange membrane (3).   7. Device according to claim 6, characterized in that the volumic cathode is made by stacking grids in expanded metal.