The present invention relates to an apparatus for reco¬ vering silver from liquids, preferably photographic liquids, and more specifically to an apparatus of the ion exchanger type for silver recovery.

Up to now there have been no definite regulations con¬ cerning the amount of silver which may be released into waste water from film processing plants and the like, this having been more a question as to how much silver it has been economically feasible to recover. Some kind of recovery device of the electric catalyser type has there¬ fore been most often used, where the silver has been de¬ posited on electrically charged electrodes.

These silver separators or recoverers normally leave be¬ hind silver to an amount of about 5-20 mg per liter waste water, and this has been regarded as acceptable, since further recovery has required apparatus which, primarily due to operational disturbances, has been too expensive to be financed by the amount of silver regained. However, there is now expected a general regulation according to which a maximum of 1 mg silver per liter waste water will be allowed, and then more rigorous separation of silver will be compulsory.

In order to get down to as low values as 1 mg silver per liter waste water and preferably even lower, ion ex¬ changers have been used with a filter material in the form of granules in a closed column, the waste water being urged through the filter material at excess pressure.

This type of apparatus for recovering silver has not functioned satisfactorily, primarily due to the gelatine used for retaining the silver salt on a carrier having gummed up in the filter material and the region around

t, to ^" heavily limit the through flow of the waste water, in some cases even cutting the latter off entirely. Removal of the gelatine lumps has been troublesome and has required dismantling of at least some parts of the apparatus as well as cleaning the column and filter material, and in some cases further parts of the apparatus .

The present invention has the object of dispensing with the above-mentioned problems. This object is achieved by an apparatus of the kind disclosed in the claims, wherein the features particularly distinguishing the invention are also disclosed.

The invention will now be described in detail with refe¬ rence to the accompanying drawing illustrating a schema¬ tic section through a possible embodiment of the appara¬ tus in accordance with the invention.

The recovery apparatus 1 illustrated on the drawing is of a compact type, which means that tanks for liquid have been placed one inside the other. One skilled in the art will, however, realise that the different tanks can be placed in other ways _*

In the illustrated, preferred embodiment the recovery ap¬ paratus comprises a bottom 2, in which an array of pipes is laid, and on which an outer tank 3 is placed, the latter being intended for the reception of fixing liquid and which constitutes a buffer tank for balancing fluc¬ tuations in the supply of fixing liquid, so that the re¬ covery apparatus can operate continuously. An inner tank 4 is placed in the outer tank 3, the former being inten¬ ded for the reception of washing water and also consti¬ tuting a buffer tank. A filter container 5 is placed in the inner tank 4.

As will be seen from the drawing, the different tanks

have increasing height, such that the tank 3 for the fixing liquid is lowest, the washing water tank 4 being higher, with the filter container 5 being highest. All the tanks are upwardly open for the free supply of air, but for preventing objects "and other debris falling down into the different containers they are suitably provided with some simple form of cover.

A pipe 6 extends from an unillustrated initial separator for silver, e.g. a separator working on the electrolysis principle, through the bottom 2 and into the fixing liquid buffer tank 3, where the pipe opens out in the vicinity of the upper end of the tank. A pipe 7 extends from the bottom of the tank 3 to a metering pump 8, and from there further into the washing water tank 4.

From the washing side of an unillustrated developing plant a pipe 9 extends to the washing water tank 4 for the supply of washing,water thereto. From the tank 4 a pipe 10 extends via a circulation pump 11 into the filter container 5 and opens out at the bottom thereof. In the pipe 10 there is a flow monitor 12 for supervising the flow and a non-return valve 13 for preventing flow from the filter container 5 back to the tank 4.

For about a third of its volume the filter container 5 contains a filter material 14 in the form of plastics granulate, where the balls have a diameter of 0.5 - 1 mm and a specific gravity of about 11 - 11.5 kg per m . At the bottom of the filter container 5 there is an air nozzle 15 of the type where air is released through a plurality of small holes over a large area. A supply pipe 16 for compressed air extends from an unillustrated compressor to the nozzle 15, and in this pipe there is a solenoid valve 17 for opening and closing the supply of compressed air to the nozzle 15.

A circular collection container 18 is placed at the upper

end of the filter container 5 so that it surrounds the filter container 5, the end edge 19 of which projects into the collection container. There is thus formed an annular trough 20 for the reception of liquid which flows over the edge 19, and from this trough 20 there extends a waste pipe 21 to an unillustrated waste water system.

A waste pipe 22 also extends from the upper end of the filter container 5 to the unillustrated waste water system, and a solenoid valve 23 is adapted to open and close this pipe 22.

Some level monitors 24 are shown in different places, and these are arranged to a necessary extent for signaling that the recovery apparatus is beginning to contain too much or too little liquid in the different tanks to enable satisfactory continuation of operations.

The recovery apparatus functions in the following manner: From the initial separator or directly from the fixing part of the developing plant the fixing liquid is taken in batches to the buffer tank 3 via the pipe 6. In the illustrated embodiment the buffer tank 3 has a volume sufficient for 86 liters of fixing liquid. The majority of the silver ions in the fixing liquid have formed a complex with thiosulphate, a negative charge thus being present.

The washing water is led from the developing plant via the pipe 9 into the washing water tank 4 which has a volume of 163 liters in the illustrated embodiment. The washing water contains a minor quantity of silver ions and has no real interest from the recovery aspect. The washing water is used for diluting the fixing liquid, but it must also be relieved of its silver content so that the effluent does not contain silver above the stipulated value of 1 mg/l. The metering pump 8 sucks fixing liquid through the pipe 7 in a predetermined quantity from the

buffer tank 3 and supplies it to the washing water tank 4. A circulation pump 11 with a capacity of 20 1/m, in the illustrated embodiment, continuously sucks the washing water mixed with fixing liquid from the washing water tank 4 and takes it to the bottom of the filter container 5 via the pipe 10. The mixture of fixing liquid and washing water is then allowed to pass up through the filter material 14. In the illustrated embodiment the latter has a volume of 38 liters, while the filter con¬ tainer 5 has a volume of 105 liters.

The granules or balls of the filter material 14 are charged such that the silver thiosulphate complex dis¬ places the ions on the granules and takes the place of these ions. The process, as well as the regeneration process, are known to one skilled in the art and are no part of this invention.

However, both the fixing liquid and the washing water contain gelatine used for attaching the silver to the film itself. This dissolved gelatine accompanies the liquid to the filte-r container 5, but the flow rate is decreased here, and the gelatine, which is normally in small lumps, adheres to the walls and the filter mate¬ rial, but above all the lumps adhere to each other. After the recovery apparatus 1 has been in use for some time, the flow through the filter material and the filter con¬ tainer becomes restricted to a troublesome degree, and it is precisely this problem which has made this type of recovery apparatus less attractive.

By blowing in compressed air at regular intervals through the pipe 16 to the nozzle 15 in the bottom of the filter container 5 a turbulent flow is obtained through the filter material 14 and up through the container 5. The liquid flowing through the filter material 14 has thus deposited its silver conventionally on the filter granules and the de-silvered liquid, which then has a

very minor silver content of at most 1 mg/1, flows conti¬ nuously over the upper edge 19 of the filter container 5 and down into the trough 20, from where the liquid is re¬ moved via the waste pipe 21. The liquid then contains harmless pollutants such as gelatine etc. At the turbu¬ lent flow created by the compressed air the gelatine lumps are broken away and disintegrate to flow with the waste water in the manner mentioned.

It has thus been found that blowing in at an air pressure of between 2 and 3 kp/cm the granules of the filter material 14 do indeed whirl round and upwards a distance in the container 5, but they do not reach up to the edge 19, while the gelatine does accompany the liquid over this edge. In the illustrated embodiment, the filter con¬ tainer has a volume of 105 liters and the filter material 14 a volume of 38 liters , which means that the filter material 14 takes up approximately a third of the height of the filter container 5. As previously mentioned, the filter material has a density of between 11 and 11.5 kg/m . However, to ensure that the granules in the filter material 14 will not flow over the edge 19 when compres¬ sed air is blown in, the liquid level in the container 5 is suitably lowered by opening the valve 23 before blow¬ ing with compressed air, de-silvered liquid then flowing out through the waste pipe 22.

By the filter container 5, in particular, being open there is no great excess pressure which has to be over¬ come, and the filter material 14 will be easily acces¬ sible for regeneration and possible exchange, as well as all the other parts in the filter container 5 being easily accessible, such as the nozzle 15 with its supply pipe 16 and the inlet pipe 10 with its spreader nozzle. By the open construction, the pressures which the pumps 8 and 11 operate at will be comparatively low, and these will thus be moderately dimensioned as well as requiring minor power for their operation.

It will be understood from the above that the obj ect dis¬ cus sed in the introduction has been achieved. For one skilled in the art it is clear, however, that the inven¬ tion may be embodied in other ways than the one described here, but such embodiments using the principles according to this invention are intended to fall within the scope of the accompanying claims .