METHOD AND APPARATUS FOR REMOVING SILVER IN THE FORM OF SILVER SALTS FROM A LIQUID

Abstract: By using a method of removing silver in the form of sparingly soluble silver salt, especially silver sulfide, from a solution containing another silver compound, such as a silver thiosulfate complex in a photographic fix or rinsing fluid by precipitation, the solution is subjected to electromagnetic irradiation with a wavelength below 375 nm. The irradiated liquid is led through a filter-like means 33 which binds the silver sulfide to its surface as crystalline silver sulfide. As a result, the silver sulfide is precipitated and deposited on the surface of the means as solid silver sulfide which in its turn acts as a catalyst for further precipitation and depositing of solid silver sulfide on the surface of the means. By binding the solid silver sulfide to the surface of the means, the required capacity of a possible subsequent filter is reduced.

Description: Method and apparatus for removing silver in the form of silver salts from a liquid Technical field The invention relates to a method for removing silver in the form of sparingly soluble silver salt, especially silver sulfide, from a solution containing another silver compound, such as a silver thiosulfate complex in a photo- graphic fix or rinsing fluid solution, by means of precipi¬ tation, which method comprises subjecting said solution to electromagnetic radiation with a wavelength of 375 n or less . Background Art US Patent No. , 088 , 552 discloses the conversion of silver thiosulfate complexes in a liquid into silver sulfide by irradiating a liquid by means of UV radiation or other high-energy radiation. Said patent specification discloses how by irradiation of the liquid with a relatively large amount of energy it is possible to precipitate silver sulfide as a solid matter which is subsequently removed by filtering. It is stated by way of example that by irradiat- ing 50 g of fix solution with 1200W UV radiation for 12 minutes 90% of the silver will precipitate. It is further described how by irradiating liquids containing .silver thiosulfate complexes with electron radiation from a 0.3 to 2 MW accelerator an almost momentary precipitation of silver sulfide is achieved. None of the above methods are however suited for conventional use because of the very large amount of energy necessary to carry out the methods . Disclosure of Invention The object of the invention is to provide a method of the type mentioned above which enables a rapid and safe removal of silver compounds from a photographic solution while using only small amounts of energy. The method according to the invention is characterised in that the irradiated liquid is passed through a filter-like means, which binds the silver sulfide to its surface as crystalline silver sulfide. As a result, the silver sulfide is precipitated and deposited on the surface of the means as solid silver sulfide which in its turn acts as a cata¬ lyst for further precipitation and depositing of solid silver sulfide on the surface of the means. By binding the silver sulfide to the surface of the means, the re¬ quired capacity of a possible subsequent filter is reduced. A filter-like means is broadly to be understood as a means by which the relationship between the surface area in con¬ tact with the liquid and the flow area is large or ex¬ pressed differently a means with a very large surface. Moreover according to the invention the liquid may be irradiated with such an amount of energy that substantially no silver sulfide is formed. It has surprisingly been found that by irradiating a rinsing fluid containing silver thio sulfate complexes by means of UV light at for instance 254 nm and by using energy levels in the order of 1% of the levels stated in US Patent No 4088552, the silver thio¬ sulfate solution is substantially completely converted into an oversaturated solution of silver sulfide in which substantially no silver sulfide is found in solid form. Tests have thus shown that by filtering the irradiated liquid through a 1 micrometer filter immediately after the irradiation, substantially no silver sulfide will be removed by filtering. Furthermore according to the invention the liquid may be filtered after having passed through the means. Crystal¬ line silver sulfide, which has not been deposited on the means or which has been detached therefrom, is thereby removed from the liquid. In addition, according to the invention the means may be formed of a granular or fibrous material. In practice said embodiment has proved especially advantageous for obtaining a very large surface. Further according to the invention the means may be of glass. Said embodiment has proved particularly advantageous in practice, especially' when the means comprises glass wool . The apparatus according to the invention for carrying out the method according to the invention and which comprises a irradiation means adapted to irradiate a liquid by means of electro magnetic radiation to convert silver thiosulfat dissolved in the liquid into silver sulfide and which apparatus has an outlet for the irradiated liquid is characterised in that the outlet of the irradiation means is connected to an inlet associated to a crystallization device, said crystallization device further comprising a housing with a flow passage in which a filter-like means is placed, said filter-like means binding the silver sulfide to its surface, and an outlet for liquid from which silver has been removed. According to a preferred embodiment of the invention the inlet of the crystallization device is situated at a first end of a tube which has a closed second end and near the latter at least one outlet opening, that the housing en¬ circles the tube at a jjadial distance thereto and that the filter-like means is placed between the tube and the housing, the outlet of said housing being situated near the same end as the inlet. This above embodiment has proved especially advantageous in practice. Finally according to the invention the filter-like means may be an interchangeable cartridge comprising glass wool. As it has been found that glass has a particularly good ability to increase the depositing and crystallization of silver sulfide and as glass wool also has a very large surface, said material is very suitable. Glass wool also being rather inexpensive and easy to handle and having large chemical resistance, it is advantageous to form said material into a cartridge to be interchangeably placed in the housing. When a predetermined amount of silver sulfide has been deposited in the cartridge, it is removed from the housing and replaced by a new one. It is thereby possible to choose to discard the used cartridge or to reuse the cartridge after having removed the silver sulfide by rinsing with an acid. Brief Description of Drawings The invention is explained in more details below with reference of the accompanying drawing in which the Figure illustates an apparatus for carrying out the method accord¬ ing to the invention of removing silver sulfide from a photographic fix or rinsing fluid containing a silver thiosulfate complex. Best Mode for Carrying Out the Invention The apparatus comprises an irradiation means 1 for ir¬ radiating a photographic fix or rinsing fluid containing silver salts with electro magnetic radiation and a crystal- lization device 2 placed below the irradiation means for precipitation and crystallization of sparingly soluble silver salt formed by irradiation. The irradiation means 1 comprises a low vessel or tray 3 with a stepped bottom 4 formed by an upper bottom portion 5 and a lower bottom portion 6. The liquid to be treated, and which may be rinsing fluid from the rinsing vessels of a photographic developing apparatus, is fed to the vessel through a feeding line 7 connected to an inlet 8 in the upper bottom portion 5. The irradiation means 2 further has an outlet 9 in the lower bottom portion 6 of the vessel 3. A number of UV lamps 10 are placed above the vessel 3, for instance low pressure mercury tubes emitting ultraviolet light with a wavelenght of substantially 254 nm. These lamps 10 are surrounded by a reflector 11 at the top and to the sides to make better use of the emitted light. The liquid flowing into the vessel 3 through the inlet 8, flows over the bottom 4 of the vessel 3 while being evenly distributed thereon and then out through the outlet 9. The influx of liquid is controlled in such a manner in relation to the quantity of light emitted that when the liquid flows from the inlet 8 to the outlet 9, a substan¬ tially complete conversion of the silver thiosulfate complexes present in the liquid into silver sulfide takes place. Further, said control is preferably carried out in such a manner that substantially no crystalline silver sulfide is formed, but on the contrary, an oversaturated solution of silver sulfide. The crystallization device 2 is placed below the irradia¬ tion means 1 inside a container 12 having an upper rim 13, releaseably engaging the outer periphery 14 of the lower bottom portion 6 of the low vessel or tray 3. The crystallization device 2 comprises a vertical tube 15 with an upper open end 16 , said open end forming the inlet 2 of the crystallization device and releaseably engaging the outlet 9 of the vessel 3. The tube 15 has a lower end, which is closed by means of a plug 17 and is near its lower end provided with a number of outlet openings 18. Furthermore, the tube 15 is placed centrally inside a cylindrical housing 19, the plug 17 resting on the bottom 20 of the housing, said bottom 20 in its turn resting on the bottom 22 of the container 12 via an intermediary layer or cushion 21. The bottom 20 of the housing 19 is provided with an opening 35 which is 5 closed by the intermediary layer 21 in such a manner that at conventional use the housing 19 is water-tight. By lifting the housing 19, said housing 19 is emptied through the opening 35, whereby the crystallization device 2 is more easily lifted out of the container 12. Glass wool 33 10 has been provided between the cylindrical wall 23 of the housing 19 and the tube 15. The tube 15, the plug 17 of said tube and the glass wool are wrapped in a filter bag and jointly said parts form an interchangeable cartridge. The housing 19 is open at the top thereby forming an outlet 15 24 for liquid which has passed through the crystallization device 2. From the outlet 9 of the vessel 3 the irradiated liquid flows downwards through the tube 15 and via the outlet 20 openings 18 near the lower end of the tube 15 into the glass wool 33 at the lower end thereof. From here the liquid flows through the glass wool towards the outlet 24 at the upper end of the housing 19 and the tube 15, res¬ pectively. While passing through the glass wool 33, the 25 silver sulfide in the oversaturated solution of silver sulfide is deposited and crystallized as a solid substance on the surface of the fibres of the glass wool and when the liquid reaches the outlet, the silver sulfide is substantially precipitated. Precipitated silver sulfide 30 which has not been deposited on the glass wool or which has been detached, is held back by the filter bag 34. From the outlet 24 the liquid flows into the container 12. The container 12 is provided with an outlet 25 in the bottom 22 and a cylindrical filter 26 is placed around 35 the outlet inside the container thereby removing solid silver sulfide possibly carried along in the liquid. At the top the cylindrical filter 26 is closed by means of a plate 27, wherein an overflow pipe 28 is provided, said overflow pipe communicating with the interior of the filter 26 and extending upwards in the container 12 to a distance below the outlet 24 of the crystallization device. Further¬ more, a level sensor 30 is placed inside the container immediately below the upper end 29 of the overflow pipe, said level sensor providing a control or alarm signal when the level of liquid in the container 12 reaches a predetermined level. At the same time, the overflow pipe 28 ensures that the level of liquid in the container 12 is never above the level of the upper end of the overflow pipe 28, as in that case the liquid will flow directly out through the outlet 25 via the overflow pipe 28 and the interior of the cylindrical filter 26. The outlet 25 of the container 12 is connected to a discharge pipe 31 for discharging the treated liquid and wherein a pump 32 is provided. The treated liquid may be fed back to the fix or rinsing fluid vessel from where it was originally removed. The invention may be varied in many ways without thereby deviating from the scope thereof. Thus glass beads, sin¬ tered glass dust or granules etc and/or other materials than glass capable of binding silver sulfide to its surface and which has a large surface may be used as filter-like material.

Claims: C 1 a i s 1. A method for removing silver in the form of sparingly soluble silver salt, especially silver sulfide, from a solution containing another silver compound, such as a silver thiosulfate complex in a photographic fix or rinse fluid solution by means of precipitation, which method comprises subjecting said solution to electromagnetic radiation with a wavelength of 375 mm or less, c h a - r a c t e r i s e d in that the irradiated liquid is passed through a filterlike means which binds the silver sulfide to its surface as crystalline silver sulfide. 2. The method as claimed in claim 1, c h a r a c t e r- i s e d in that the liquid is irradiated with such an amount of energy that substantially no crystalline silver sulfide is formed. 3. The method as claimed in claim 1 or 2, c h a r a c - t e r i s e d in that the liquid is filtered after having passed through the means. 4. The method as claimed in claim 1 or 2 , c h a r a c - e r i s e d by the means being formed of a granular or s fiberous material. 5. The method as claimed in claim 1, 3 or 4, c h a - r a c t e r i s e d by the means being of glass. 6. An apparatus for carrying out the method according to claim 1 and comprising an irradiation means (1) adapted to irradiate a liquid by means of electro magnetic radia¬ tion to convert silver thiosulfat dissolved in the liquid into silver sulfide and which means has an outlet (9) for the irradiated liquid, c h a r a c t e r i s e d in that the outlet (9) of the irradiation means (1) is con¬ nected to an inlet (16) associated to a crystallization device (2), said crystallization device further comprising a housing (19) wich a flow passage in which a filter-like means is placed, said filter-like means binding the silver sulfide to its surface, and an outlet (24) for liquid from which silver has been removed. 7. The apparatus as claimed in claim 6, c h a r a c ¬ t e r i s e d in that the inlet (16) of the crystal¬ lization device (2) is situated at a first end of a tube (15) which has a closed second end and near the latter at least one outlet opening (18) and that the housing en¬ circles the tube at a radial distance thereto and that the filter-like means (33) is placed between the tube (15) and the housing (19) , the outlet (24) of said housing being situated near the same end as the inlet (16) . 8. The apparatus as claimed in claim 6 or 7 , c h a - r a c t e r i s e d by the filter-like means being an interchangeable cartridge comprising glass wool (33) .

Inventors: BUSCH-SOERENSEN THOMAS, BUSCH-SOERENSEN, THOMAS
Applicants: ESKOFOT AS, ESKOFOT A/S
Application year: 1991
Main classification: G03C5
IPC codes: C01G5, C02F1, C02F9, G03C5, G03D3