JPH0892782A - Apparatus for electrolytic recovery of silver from solution containing silver - Google Patents

Abstract

PURPOSE: To prevent the detection of an incorrect potential at a reference electrode by easily attachably and detachably disposing a cathode in an electrolytic cell, disposing the inside surface of this cathode opposedly to an anode, disposing the outside surface thereof opposedly to the reference electrode and adding a means for suppressing the deposition of silver on the outside surface of the cathode.

CONSTITUTION: The cathode 30 is inserted in an easily attachable and detachable state into the electrolytic cell 10. The inside surface of the cathode 30 is provided with the anode 20 and the outside surface is provided with the reference electrode 45. A part of the outside surface of the cathode 30 is provided with a nonconductive coating to lessen the deposition of silver and to prevent the short-circuiting of the outside surface of the cathode 30 with the silver deposited by the reference electrode 45. If to the respective electrodes, voltage is applied to electrolyze, silver deposits in a considerable amt., the cathode 30 is taken outside and the deposited silver is recovered. A danger of making the potential detected by the reference electrode 45 incorrect is decreased.

COPYRIGHT: (C)1996,JPO

Description

Detailed Description of the Invention

FIELD OF THE INVENTION This invention relates to solutions containing silver, especially fixing solutions and bleach-
An apparatus for electrolytic recovery of silver from a used photographic solution such as a fixing solution.

BACKGROUND OF THE INVENTION Electrolytic silver recovery from used photographic solutions is a common method for extending the life of such solutions.

Control of the electrochemical processes that occur at the anode and cathode is important in silver recovery processes. In some devices a constant anode / cathode potential is applied. When the desilvering of the solution reaches its end, a decrease in the electrolysis current occurs, and the method is usually turned off when the current drops below a predetermined level. The disadvantages of this method are that the deposition potential is not precisely controlled in many practical situations, and that the potential difference between the cathode and the solution (cathode potential) changes unintentionally during desilvering, causing unwanted side reactions or optimal Not to produce a desilvering rate.

In the above-mentioned assembly apparatus, the actual fixing solution to be desilvered is not only the fixing solution but also the developing solution components carried from the developing tank, the replenishing solution, the additives, the reaction products of the development or the previous desilvering. It suffers from the drawback that the actual plating potential is often unknown when it is used in an actual application which also contains etc.

When reusing a desilvered fixer, it is desirable to minimize side reactions that occur at the anode and cathode that would give rise to undesirable byproducts.

Positioning of the reference electrode is important for the desilvering method. The potential of the reference electrode depends on its position due to the ohmic potential drop, which can be greater than 100 mV for electrolytic cells using high currents. In principle, the electrodes are best placed as close as possible to the cathode, between the cathode and the anode. However, this can lead to failure if silver gradually deposits on the cathode and thus grows thick. When the reference electrode is placed away from the cathode, the ohmic potential drop makes potentiostat desilvering truly potentiostat. It was therefore proposed to place the reference electrode farther from the anode, but closer to the cathode. An apparatus for the electrowinning of silver from solutions containing silver is known from the European patent application EPA 93200427.8 of Agfa Gevert N.V. dated 16 February 1993. The device includes an electrolytic cell and an anode, all located within the cell, a removable cathode and a reference electrode. The cathode has an inner surface facing the anode and an outer surface facing the reference electrode. In use, the silver from the silver-containing solution is deposited on the side of the cathode facing the anode.

The use of a reference electrode allows for very good control of the silver deposition process since the potential difference between the cathode and fixer solution can be controlled. With this arrangement, the potential difference between the cathode and the anode can be controlled by a feedback mechanism that keeps the potential difference between the cathode and the reference electrode constant (potentiostat control). This allows the best control of the plating reaction, since the reaction that takes place at the electrode is essentially controlled by the potential difference between the electrode and the solution.

Accurate control of the potential between the cathode and the reference electrode is important for the best performance of the cell. For example, when using an Ag / AgCl reference electrode, the potential between the cathode and reference electrode is about 400 mV. In optimizing the performance of the device, which means using the highest current without causing side reactions, the potential should be measured with an accuracy of a few mV. While a large amount of deposited silver is formed on the surface of the cathode facing the anode, some silver is likely to deposit on the surface of the cathode facing the reference electrode, and this deposited silver acts as the cathode. The inventors have found that it is possible to interfere with the measurement of the potential between the reference electrodes. In one example of such a cell, the cathode includes an opening extending from the outer surface to the inner surface, which opening is located near the reference electrode, ensuring that the reference electrode is located within the electric field of the cell. The inventors have found that silver becomes deposited on the outer surface of the cathode near the opening and interferes with the electric field in which the reference electrode is located.

The object of the present invention is to reduce or eliminate these drawbacks of such electrolytic silver recovery tanks.

SUMMARY OF THE INVENTION According to the present invention, there is provided an apparatus for electrolytically recovering silver from a solution containing silver, the apparatus comprising an electrolysis cell; an anode located in the cell; a reference electrode located in the cell. A removable cathode located within the cell, the cathode having an inner surface facing the anode and an outer surface facing the reference electrode;
A means for suppressing the deposition of silver on the outer surface of the cathode is included.

The inventors have found that a device constructed according to the present invention can reduce or avoid the risk of false potentials sensed by the reference electrode. We have further found that in another embodiment of the invention, the tendency for side reactions and cathodic poisoning to occur is reduced.

In a first embodiment of the present invention, the means for inhibiting silver deposition on the outer surface of the cathode comprises an electrically non-conductive surface provided on at least a portion of the outer surface of the cathode. You can A non-conductive coating is preferably provided on the outer surface of the cathode. Suitable coating materials include inorganic and organic materials, especially organic polymeric film forming materials such as polyethylene and polyacrylates. The coating material should ideally be inert to any components that may be present in the solution being treated and be substantially water insoluble. Suitable coating methods include silk screen printing, spraying, brushing and laminating methods.

In another embodiment of the present invention, the means for suppressing silver deposition on the outer surface of the cathode may include shielding means for shielding the outer surface of the cathode from the reference electrode.

The electrolytic cell is suitably formed from a non-conductive material,
Generally cylindrical, but other shapes are possible. The shape of the cylinder relative to the cell allows the cathode to be located close to the wall of the cell. The anode has a generally linear shape placed axially within the housing. The cathode has an open circular cross-sectional shape surrounding the anode. The reference electrode is placed in the side arm of the housing. Preferably the housing further comprises a liquid inlet and a liquid outlet for the electrolyte liquid to predetermine the liquid level in the bath. The non-conductive surface should be provided on the outer surface of the cathode below the liquid level. In one embodiment of the cell, a conductive contact surface is provided above the liquid level and the gripping means acts to grip the contact portion of the cathode against the contact surface to complete the electrical connection to the cathode. The contact area of the should have a conductive surface. The provision of a contact surface, in particular an annular contact surface, at the top of the electrolytic cell makes it possible to bring this surface above the level of electrolyte in the cell in use, thus reducing the risk of leakage and corrosion. Therefore, at least a portion of the outer surface of the cathode above the liquid level preferably has a conductive surface.

The cathode preferably includes an opening extending from the outer surface to the inner surface, the opening being located near the reference electrode. This ensures that the reference electrode is placed in the electric field of the cell, even when placed opposite the cathode from the anode. In such an example, the means for suppressing silver deposition on the outer surface of the cathode may include sealing means positioned between the housing and the cathode adjacent the opening to shield the outer surface of the cathode from the reference electrode. .

The cathode can generally be formed from a flat sheet of flexible material, with a conductive surface on one major surface thereof,
The non-conductive surface is provided on at least a portion of the other major surface and the fastening means is provided to allow the sheet to be folded and attached to the open circular cross-section formation. The cathode preferably and ideally has a truncated conical cross section,
Its large diameter end is at the top, i.e. facing the circular upper opening of the electrolytic cell. This shape facilitates removal of the cathode even after the silver deposit has accumulated on it after use. Cathode materials that can be used include stainless steel, silver, silver alloys, and other conductive materials, but stainless steel is preferred from a cost perspective. A non-conductive coating is preferably provided on the other major surface of the cathode. In this example, the non-conductive coating is preferably visually distinguishable to ensure that the cathode is formed on the outer, non-conductive surface.

The material used for the anode is not strictly limited, but titanium platinized is usually used. There are also platinum, graphite and precious metals. The anode is in the form of a rod, preferably on the shaft of the electrolyser, which is in the form of a cylinder.

Conveniently, the reference electrode is placed adjacent the outlet portion of the bath. Reference electrodes suitable for use in electrolytic desilvering include calomel-type electrodes or Ag / AgCl-type electrodes, but especially pH-sensitive electrodes such as glass electrodes, hydrogen electrodes,
The use of quinhydrone electrodes and antimony electrodes is particularly preferred, and the use of glass electrodes, which are relatively inexpensive to maintain and are not sensitive to hydrostatic pressure fluctuations, is most preferred. The potential at which sulfite reduction begins to occur is determined by the pH of the fixing solution. Therefore, the potential to be used for best desilvering depends on the nature of the fixer used and other parameters such as pH of the developer bath, presence or absence of intermediate washing, carry-over from developer to fixer, and developer and fixing. Determined by the buffer capacity of the solution.
In practice, this means that there is no common potential for best desilvering for various fixers with different pH values.
For the best desilvering, any fixing solution with a different pH requires a different potential difference between the reference electrode and the cathode.
Therefore, the pH electrode is preferable as the reference electrode. Suitable electrodes are described in the European patent application EPA 92233439.2, filed Nov. 11, 1992, under the title of invention of pH sensitive reference electrode in electrolytic desilvering.

Solutions containing silver that can be desilvered using the apparatus according to the invention are complexed or free silver as a result of the fixing step, sulphite ions as antioxidants, silver complexing agents such as thiosulfates. Alternatively, the apparatus, including a solution containing thiothianate ions, can also be used with a concentrated or diluted spent fixing solution, or a solution containing a carry-over developer or wash water. In addition to the essential ingredients, such solutions often also contain wetting agents, buffers, sequestering agents and pH adjusting agents.

The apparatus of the present invention can also be used to desilver bleach-fixing solutions which may additionally contain bleaching agents such as iron (III) and polyaminocarboxylic acid complexes.

The desilvering process can be carried out batchwise or continuously, the apparatus being connected to a fixing solution which forms part of a continuous processing sequence.

It will be appreciated that the apparatus of the present invention does not require precise potential control when desilvering the fixer to be discarded, for example.

Preferred Embodiments of the Invention The invention is explained below by way of examples with reference to the drawings.

FIG. 1A shows a partial cross-sectional view of a device incorporating a cathode according to the present invention.

FIG. 1 (B) is different from FIG. 1 (A) from different angles.
Figure 3 shows a cross-sectional view of a portion of the housing of the device shown in Figure 1 with the cathode and the housing lid removed.

FIG. 2 is a view of the cathode used in the electrolytic cell according to the invention in a flat state.

FIG. 3 is a view showing a state in which it is folded for use.
3 is a perspective view of the cathode shown in FIG.

FIG. 4 is a schematic representation of the use of the device according to the invention.

FIG. 5 is a view from one side of the housing of the device shown in FIG.

As shown in FIGS. 1A, 1B and 5, the device is made of a non-conductive material such as PVC,
It includes an electrolytic cell housing 17 including a bottom plate 15, sidewalls 16 and an upper portion 17. An electrolytic solution inlet 18 is provided toward the bottom of the tank, and an electrolytic solution outlet 19 is provided toward the top of the tank angularly diverging from the inlet 18.

The anode 20, which is in the form of a titanium platinized rod, is attached to the bottom plate 15 of the layer by means of bolts 21 which act as electrical connectors for the anode. Anode 20
Are along the axis of the housing 10. Reference electrode 45
Is in the side arm 24 of the outlet 19 and is located in the tank housing 10
It projects inside. A suitable reference electrode is Yokogawa SM2
PH sensitive glass electrodes such as 1 / AG2 or INGOLA HA265-S8 / 120 glass electrodes.

The upper part 17 of the bath is in the form of a neck part having an opening defined by a stainless steel tube 22. The contact surface of the ring 22 has a truncated conical shape and has a narrowing radius in the lower part. Stainless steel ring 22 is permanently fixed to one end of a bolt 31 that extends through the wall of the cell and provides a connector for cathode 30. Below the level of the annulus 22, there is a sealing annulus 14 at the neck of the bath.

The device further includes a lid 40 shaped to fit the neck portion of the layer. The lid 40 is formed of a non-conductive material such as PVC. The lower part of the lid 40 is made corresponding to the shape of the ring 22.

With particular reference to FIGS. 2 and 3, a cathode 30 formed from a flat stainless steel sheet 50 having, for example, a thickness of 100 μm, is wound in a truncated conical shape. The seat 50 has a slightly curved top edge 51, a concentrically parallel slightly curved bottom edge 52 and two flared opposite side edges 53 and 54. There is a pair of upper and lower mounting means including tong-shaped cut portions 55a, 55b formed along side edge 53 and slots 56a, 56b adjacent to opposite side edge 54, respectively. The elements 55a, 56a of the upper mounting means are approximately 1.02 times more spaced apart from each other than the elements 55b, 56b of the lower mounting means. Tongue-shaped parts 55a, 55b
Can be fitted into the corresponding slots 56a, 56b, allowing the seat 50 to be bent and mounted in an open trough as shown in FIG. In this case, the upper radius R ₁ is 1.05 times larger than the lower radius R _{2 in the} periphery. The cathode 30 has a deformable top edge 32. The protrusion 33 is formed at the upper edge of the cathode by providing a recess 34 extending longitudinally away from the edge. The ridges or tabs 33 together form the deformable upper edge portion 32 of the cathode so that the sheet material from which the cathode is formed allows the tab to bend outward in response to outwardly directed forces. It has sufficient elasticity.

The backside of the cathode 30 (as can be seen in FIG. 2)
Is electrically conductive, while the front surface is partially coated on the area 36 with an acrylic polymer applied by silk screen printing. The covered region 36 does not extend to the top edge of the cathode and includes the tab 33 therein.
7 which remains uncovered, ensuring good electrical contact between the stainless steel ring 22 of the housing 10 and the cathode.

The cathode is provided with a number of openings 57 extending therethrough. The cathode 30 is in the cell 10 and is supported at its bottom edge by a cathode support shelf 35 in the cell, the surface of which is separated from the side wall 16 by ribs 38 formed on the inner surface of the side wall 16. One of the openings 57 is placed near the reference electrode 45. The deformable upper edge portion 32 of the cathode is adjacent to the stainless steel ring 22. The lid is fixed
The tab 33 is tightly gripped by the lid against the tube 22, which establishes a good electrical contact between them.

In the closed position of the lid, the sealing ring 14 bears against the outer surface of the lid 40, thereby forming a seal. The electrolyte solution is now fed into the bath by means of the inlet part 18, fills the bath and leaves by means of the outlet part 19. Seal ring 1
The effect of 4 is to prevent electrolyte levels rising above the level of the outlet part 19, thus preserving the space above the liquid and preventing contact between the liquid and the annular contact surface 11. The risk of corrosion of the annular contact surface is reduced.

The bath is operated under normal conditions during which silver deposits accumulate on the cathode 30, primarily on its inner surface. After a period of time determined by the amount of silver to be deposited, the worker unscrews lid 40 and lifts cathode 30 out of the layer. To allow this, each of the tabs 33 is provided with a hole 58 (shown only in FIG. 2) into which a retractor can be inserted. Due to the truncated cross section of the cathode, the sides of the cathode do not become clogged with respect to the annulus 22, even when a small amount of silver deposit has accumulated on its outer surface. The silver deposit is then removed from the cathode, which can be reused if desired, or replaced by another same configuration for desilvering another electrolyte batch.

Referring to FIG. 4, the anode 2 of the electrolytic cell 10
0, the cathode 30 and the reference electrode 45 are connected to a potentiostat 41 which controls the application of power to the anode and the cathode. The tank 10 is supplied with the dirty fixer from the first fixer container 42 via a pump 43 provided with a filter (not shown). In practice, a representative of this dirty fixer solution contains 9 parts diluted fixer solution to 1 part diluted developer solution and 0.5 g silver per 1 liter. The diluted fixing solution is, for example, 137 g of ammonium thiosulfate, 10.8 g of sodium sulfite, 5 g of boric acid, 14 g of sodium acetate per liter.
It can contain hydrates and 8 ml of acetic acid. The diluted developer solution is, for example, for 1 liter, 2.5 g of hydroxyethylelenediamine triacetic acid, 23.7 g of sodium carbonate, 65.3 g of potassium sulfite, 1.3 g of sodium tetrapolyphosphate, 10 g of potassium bromide, 5.3 g potassium hydroxide, 20 ml ethylene glycol, 20
g hydroquinone, 0.48 g phenidone and 30
It can contain mg of 1-phenyl-5-mercaptotetrazole.

The dirty fixer solution is delivered from the second fixer container 44 with fresh fixer solution from moment to moment while the total solution volume is maintained at a constant level by the overflow 46.

[Brief description of drawings]

1A is a partial cross-sectional view of a device incorporating a cathode according to the present invention, and FIG. 1B is a cross-sectional view of a portion of the housing of the device shown in FIG. And with the removed housing lid.

FIG. 2 is a view of the cathode used in the electrolytic cell according to the invention in a flat state.

FIG. 3 is a perspective view of the cathode shown in FIG. 2 in a folded state for use.

FIG. 4 is a schematic representation of the use of the device according to the invention.

5 is a view from one side of the housing of the device shown in FIG. 1 (A).

[Explanation of symbols]

 10 Electrolyzer Housing 14 Sealing Ring 15 Bottom Plate 16 Side Wall 17 Upper Part 18 Inlet 19 Outlet 20 Anode 21 Bolt 22 Ring 24 Side Arm 30 Cathode 31 Bolt 32 Upper Edge Part 33 Tab 34 Recess 35 Support Shelf 36 Covering Area 37 Non-Coating Area 40 Lid 45 Reference Electrode 50 Flat Stainless Steel Sheet 51 Curved Top Edge 52 Curved Bottom Edge 53 Side Edge 54 Side Edge 55a Tong Type Part 55b Tong Type Part 56a Slot 56b Slot 57 Open 58 Hole

Front page continued (72) Inventor Werner van de Vancur, Septerrato, Mortère, Belgium 27 Agfa Gewert Naamrose, in the Bennaught Chap (72) Inventor, Franc Michel, Mortère, Belgium 27 Septart, Belgium・ In the Gewert Namrose Rose Bennault Chap

Claims (13)

[Claims] 1. An electrolysis cell; an anode located in the cell; a reference electrode located in the cell; a removable cathode located in the cell, the cathode facing the anode and the reference surface. An apparatus for electrolytic recovery of silver from a silver-containing solution, having an outer surface facing the electrode; and including means for suppressing the deposition of silver on the outer surface of the cathode. 2. The means for suppressing silver deposition on the outer surface of the cathode comprises a non-conductive surface on at least a portion of the outer surface of the cathode. apparatus. 3. The device of claim 2 wherein the outer surface of the cathode is provided with a non-conductive coating. 4. The apparatus of claim 1, wherein the cathode includes an opening extending from the outer surface to the inner surface, the opening being located near the reference electrode. 5. The means for inhibiting silver deposition on the outer surface of the cathode comprises screening means for screening the outer surface of the cathode from the reference electrode. The device of 1. 6. The electrolytic cell includes a generally cylindrical housing having side arms, the anode has a linear shape generally coaxially placed within the housing, and the cathode has an open circular shape surrounding the anode. The device of claim 1 having a cross-sectional shape and wherein the reference electrode is located on the side arm of the housing. 7. The housing includes a liquid inlet and a liquid outlet that predefine a liquid level in the bath, and the cathode is provided with a conductive surface on its outer surface under the liquid level. The device of claim 6, wherein 8. The apparatus of claim 7, wherein at least a portion of the outer surface of the cathode above the liquid level has a conductive surface. 9. The cell further comprises gripping means for gripping a contact portion of the cathode against the contact surface to complete an electrical connection to a liquid level upper conductive contact surface and the cathode, 9. The device of claim 8, wherein the contact portion of the cathode has a conductive surface. 10. The cathode has an opening extending from the outer surface to the inner surface, the opening being located near the reference electrode, the opening for suppressing silver deposition on the outer surface of the cathode. 2. The apparatus of claim 1, wherein the means includes sealing means disposed between the cathode and the housing adjacent the opening for screening the outer surface of the cathode from the reference electrode. 11. The cathode is formed from a generally flat sheet of flexible material, having a conductive surface on one major surface thereof and a non-conductive surface on at least a portion of the other major surface. 2. The apparatus of claim 1 wherein said fixing means is provided and the fixing means is fixed with an open circular cross-sectional shape such that the sheet can be folded therein. 12. The device of claim 11, wherein the other major surface of the cathode is provided with a non-conductive coating. 13. The device of claim 12, wherein the non-conductive coating is visually distinguishable.