Process and device for the separation of substances dissolved in the washing water used in a processing bath of a photographic film

Abstract: The present invention concerns a process and a device for the separation of substances dissolved in the washing waters used in a processing bath (20) for a photographic film (21). The photographic film, after passing through the processing bath, is caused to pass successively through at least two washing tanks (22, 23, 24), the process consisting of causing the waste water from the washing baths (22, 23, 24) to circulate through a nanofiltration membrane device (27), the permeate (P) from the said nanofiltration device (27) being recirculated to the washing tank or tanks (22, 23) other than the last (24) of the tanks in which the film passes, the last of the tanks being fed with clean water from an auxiliary source (31). Application to the recovery of photographic effluents.

Description: The present invention concerns a process for the separation of photographic chemical products from washing water used downstream of a processing bath, and for their recovery. The invention also concerns a device for implementing the process. Many manufacturing or conversion processes produce waste water which, on the one hand, cannot be discharged directly to the drains because of their composition and, on the other hand, contain precious substances whose recovery and reuse would be economically advantageous. One example is the photographic processing industry, in which films and exposed photographic papers pass through several processing baths, after which the chemical products must to a great extent be removed from the finished products. Such photographic film processes are well known (see for example Chimie et Physique Photographiques; Pierre Glafkides; Vol 2; Chap XL, pages 947-967) and consequently do not require any further description. These processes produce washing water which contains, in relatively low concentrations, chemical products whose separation was very expensive with the processes known up till now. With photographic development processes, in fact, the processing usually had to take place in two stages, the salts being eliminated from the solution by ion exchange and the organic chemical products by absorption, for example using active charcoal. By means of a subsequent process using additional chemical products, the substances eliminated from the solutions then had to be separated once again from the resins used for elimination, or from the active charcoal. Evaporation or distillation are also used for separating dissolved substances. However, for highly dilute solutions, these processes are expensive because of the high expenditure of energy which they involve. The patent application FR-A-2 684 024 describes a process for separating components essentially dissolved in a liquid, for the purpose of recycling the liquid and/or the components. Figure 1, to which reference is now made, illustrates diagrammatically the device used for implementing the process described in the above-mentioned application. As is clear from Figure 1, the waste water coming from a processing area 1 and containing, at a relatively low concentration, dissolved constituents, must be purified and reprocessed. From the processing area 1, the waste water arrives at a feed or a first reservoir 2, from which a set of membranes is fed, consisting of two filters with separate membranes. It is the membrane filter 6, intended for separating constituents at a high flow rate and low concentration, which is fed first. The filtration residue is returned, from the first membrane filter 6, to the feed, that is to say to the first reservoir 2, as a result of which the waste water or liquid contained in the latter have a higher concentration. This filtration process is repeated until the concentration in the reservoir 2 has reached a certain value, after which the liquid or waste water is taken to the second membrane filter 7, suitable for a reduced flow rate whilst providing separation to a higher concentration. A reduction in the flow rate is possible, since a large part of the waste water to be treated is already purified. The filtration residue arriving in the second membrane filter is returned to the reservoir 2, after which the concentration in the latter increases still further. This increase in the concentration continues until the concentration reaches a value at which the dissolved components can be reused in this form as chemical products, or at which they can be separated from the solvent by means of known suitable and inexpensive processes. Such a process, even if it satisfactorily assists in resolving the problems mentioned above, has other drawbacks. The first lies in the fact that the water recirculated and transferred to a reservoir 13 before being re-routed to the processing machine, is readmitted into the washing circuit at a rinsing tank through which the photographic film is being drawn. One of the problems in such an approach relates to the fact that the degree of cleanliness of the recirculated water admitted into the last rinsing tank in which the film is passing may be insufficient. This entails, in the case of a processing bath preceding another bath, a contamination of the second processing bath by drawing chemical products from the first bath to the second. In addition, when the last treatment bath is involved, the photographic film is not perfectly clean, which poses problems of preserving the physical or sensitometric qualities of the film. DE-A-3,246,897 discloses a color photographic processing apparatus provided with color developing blixing and wash water tanks through which photographic materials are sequentially processed on passing the tanks. Said apparatus comprises a reverse osmosis device having a wash water inlet connected with the wash water tank. The permeate containing less dissolved substances is returned the wash water tank except the rearmost one which is provided with clean water. One of the objects of the present invention is to provide a device and a process for the separation of substances dissolved in the rinsing waters of a processing machine which have improved yields and throughputs. These objects are achieved by a process, according to claim 1, consisting of circulating the waste water through a nanofiltration membrane device, the permeate from said filtration device being recirculated in the washing tank or tanks other than the last of the tanks through which the film passes, the last of the tanks being fed with clean water from an auxiliary source. In said process the waste water is circulated in a plurality of nanofiltration modules serially connected to one another, each subsequent downstreams module comprising a decreasing number of nanofiltration membranes connected in parallel within the same nanofiltration module. According to a particular embodiment of the present invention, the process comprises recirculating the residue coming from the nanofiltration device in a buffer reservoir receiving waste water from the washing tanks and discharging a concentrated liquid from said buffer reservoir, when the concentration in the buffer reservoir reaches a given value. The invention is also directed to a device for the separation of substances dissolved in the rinsing waters of a photographic processing machine according to claim 5. During the following description, reference will be made to the drawing in which: Fig 1 shows diagrammatically a conventional device for separating constituents dissolved in a liquid; Fig 2 shows diagrammatically an embodiment of the separation device according to the present invention; Fig 2, to which reference is now made, shows diagrammatically the device used for implementing the process according to the present invention. The waste water from the washing tanks 22, 23, 24 of a photographic processing device 20 is discharged to a buffer reservoir 25. The device according to the invention can be used at the outlet from any washing tank. For example, it may be a washing tank placed downstream of a chromogenic developer bath, a fixing bath or a bleaching bath. It is, however, particularly advantageous to use it at the outlet from the last fixing bath. From the buffer reservoir the waste water is routed through a nanofiltration membrane device 27 by means of a highpressure pump 29. In fact, the process on which the present invention is based basically uses membranes for separating the dissolved substances or chemical products from dilute solutions. Such membranes in principle behave as large-surface sieves having, as "holes", pores of microscopic or molecular size, the dimensions of which must be highly regular so that, as from a given size, molecules are retained, whilst smaller molecules or the ions of simple salts pass through the membrane. Membranes for ultra-filtration generally allow molecules to pass whose molecular weight is less than approximately 2000, whilst the molecules whose molecular weight is higher are retained. With nanofiltration, which is important for the present invention, this limit is situated at a molecular weight of around 200. Membranes for nanofiltration of this type are characterised by high selectivity and have higher flow rates. The purified water (the permeate P) at the outlet from the nanofiltration membrane device 27 is discharged to an auxiliary reservoir 28 in order to be readmitted into the processing device by means of a pump 30. According to one important characteristic of the present invention, the purified water is admitted into the washing tank or tanks 22, 23 other than the last washing tank 24 in which the photographic film 21 passes, before leaving the corresponding processing station. The washing tanks can be arranged in different ways. In the embodiment shown, the device comprises three washing tanks 22, 23, 24. The recirculated purified water is introduced either into the first two tanks 22, 23 or into the second tank only, the first tank and the second tank communicating with each other in the direction indicated by the arrow 32 by means of suitable pipes. Likewise, depending on the connections existing between the different washing baths 22, 23, 24, the discharge of the waste water to the buffer reservoir 25 can take place through one, two or three washing baths. According to another characteristic of the present invention, clean water is added to the last of the washing tanks from an auxiliary source 31. A commonly accepted definition of the concept of clean water is given for guidance in Photographic Science and Engineering, Volume 9, No 6, November-December 1965, pages 398-413. In this extract, examples of proportions of different constituents of clean water are given. This approach makes it possible to have a last washing tank which is also clean, which limits the entrainment of chemical products from one bath to another and, in the case of the use, for the last processing bath of the photographic development process, to preserve the physical and sensitometric characteristics of the film. The quantity of water added depends partly on the level of residue from the nanofiltration device. For example, in the case of a filtration process with a throughput of 80% and a residue level of 20%, 20% of water is added. The buffer reservoir 25 is equipped with appropriate means (such as overflow discharge means) for compensating for the variations in volume in the separation circuit. The nanofiltration membrane device 27 will now be described. The membrane filter device 27 comprises a plurality of nanofiltration modules 270, 280, 290 disposed in series. The first module 270 comprises three membrane filters in parallel, from which the purified water (the permeate) 271 is discharged to the auxiliary reservoir 28. The residue 272 from the filter 270 is sent to the inlet of the second module 280 comprising two membrane filters disposed in parallel, from which the purified water 281 is discharged to the auxiliary reservoir 28. The residue from the filter 280 is sent to the inlet of the third module 290 comprising a membrane filter, from which the purified water 291 is discharged to the auxiliary reservoir 28. The residue 292 is discharged in order to be reprocessed in a suitable manner or to be recirculated to the buffer reservoir 25. The advantage of using nanofiltration filters in series lies in the fact that the throughputs and yields of the device can be improved considerably. By way of example, the following values of throughputs can be used: rate of discharging waste water to the buffer reservoir: 1500 l/h; rate of feed of each of the filters in the module 270: 500 l/h; flow rate of permeate at the discharge from each of the filters in the module 270: 200 l/h; flow rate of the residue at the discharge from each of the filters in the module 270: 300 l/h; feed rate of each of the filters in the module 280: 450 l/h; flow rate of the permeate at the discharge from each of the filters in the module 280: 200 l/h; flow rate of the residue at the discharge from each of the filters in the module 280: 250 l/h; feed rate of the filter in the module 290: 500 l/h; flow rate of the permeate at the discharge from the filter in the module 290: 200 l/h; flow rate of the residue at the discharge from the filter in the module 290: 300 1/h. The invention has just been described with reference to a preferred embodiment. Obviously this embodiment is not to be construed as limiting the present invention, the scope of which is only limited by the appended claims.

Claims: A process for the separation of substances dissolved in the washing water used in a photographic processing machine (20) for processing a film (21), said photographic film, after passing through a processing bath, being made to pass in succession through at least two washing tanks (22, 23, 24), the process consisting of circulating the waste water through a filtration membrane device, the permeate (P) from said filtration device (27) being recirculated in the washing tank or tanks (22, 23) other than the last (24) of the tanks through which the film passes, the last of the tanks being fed with clean water from an auxiliary source (31), said process being characterized in that the filtration device is a nanofiltration device, and waste water from the washing tanks circulates in a plurality of nanofiltration modules (270, 280, 290) serially connected to one another, each subsequent downstreams module comprising a decreasing number of nanofiltration membranes, connected in parallel within the same nanofiltration module. A process according to Claim 1 comprising the steps of: discharging the washing tanks into a buffer reservoir (25) : circulating the waste water from the buffer reservoir (25) through said nanofiltration membrane device (27), the permeate from the said nanofiltration device being discharged to an auxiliary reservoir (28). A process according to Claim 2 further comprising the step of: recirculating the residue coming from the nanofiltration device in the buffer reservoir, means being provided for discharging a concentrated liquid from said buffer reservoir, when the concentration in the buffer reservoir reaches a given value. A process according to Claim 3 comprising the step of: causing said concentrated liquid to pass through an ultra filtration membrane, the residue being returned to the buffer reservoir, and the permeate obtained being discharged to said auxiliary reservoir. A device for the separation of substances dissolved in the washing water used in a photographic processing machine, said device comprising: a) at least two washing tanks wherein said photographic film is circulated; b) a filtration membrane device receiving waste water from the washing tanks in order to recirculate the permeate in the washing tank or tanks other than the last of the tanks; c) an auxiliary source of clean water to provide clean water to the last of the tanks; said device being characterized by: d) the filtration membrane device is a nanofiltration membrane device having a plurality of nanofiltration modules serially connected to one another, each module having a plurality of nanofiltration membranes connected in parallel and each subsequent downstreams module having a decreasing number of nanofiltration membranes, wherein the last module can have one membrane. A device according to Claim 5, further comprising: a) a buffer reservoir (25) for collecting the waste water from the washing tanks (22, 23, 24); b) means for discharging the waste water from the washing tanks (22, 23, 24) to said buffer reservoir (25); c) means for circulating the waste water from the buffer reservoir through the said nanofiltration membrane device; d) an auxiliary reservoir for discharging the permeate from the nanofiltration device. A device according to Claim 6, further comprising: i) means for recirculating the residue from the nanofiltration device to the buffer reservoir; and ii) means for discharging, when the concentration in the buffer reservoir reaches a given value, some of the contents of said buffer reservoir. A device according to Claim 7, further comprising: a) an ultra filtration membrane device; and b) means for selectively causing the liquid from the buffer reservoir to pass through the nanofiltration membrane device, or through the ultra filtration membrane device. A device according to Claim 8, wherein said means for selectively causing the liquid from the buffer reservoir to pass through the nanofiltration membrane device, or through the ultra filtration membrane device comprises means for measuring the concentration in the buffer reservoir.

Inventors: BERNARD PATRICK, MANAS GUY, BERNARD, PATRICK, MANAS, GUY, BERNARD, PATRICK, C/O KODAK-PATHE, MANAS, GUY, C/O KODAK-PATHE
Applicants: EASTMAN KODAK CO, KODAK PATHE, EASTMAN KODAK COMPANY, KODAK-PATHE
Application year: 1995
Main classification: B01D61
IPC codes: B01D61, G03C5, C02F1, G03C7, G03D3