WO1988004949A1

WO1988004949A1 - A method and apparatus for separating water and solvent from a mixture thereof

Info

Publication number: WO1988004949A1
Application number: PCT/SE1987/000631
Authority: WO
Inventors: Orvar Karlsson
Original assignee: Holmstrands Products Ab
Priority date: 1986-12-29
Filing date: 1987-12-22
Publication date: 1988-07-14
Also published as: SE8605574D0; SE8605574L; SE459379B

Abstract

A method and apparatus for separating water and solvent from a mixture of said liquids used in a developing machine (1) for printed boards in which the printed boards are treated with a solvent as a developing liquid and thereafter with clean water to interrupt the developing process by removing the solvent from the printed boards, the solvent having a density above 1.2 g/cm3, being insoluble in water and occuring in the mixture as small particles, said water-solvent mixture being treated in a main filter comprising a filter house (12) and a filter body (17) arranged therein in the form of a coalescence filter. According to the invention said water-solvent mixture is fed continuously from the developing machine (1) to the main filter in a circulation system simultaneously as a water phase, which is free from particles and drops of solvent, is continuously discharged from the filter house (12) through an upper outlet (27) and is directly returned to the developing machine (1) to be re-used in a closed cycle.

Description

A method and apparatus for separating water och solvent from a mixture thereof

The present invention relates to a method for separating water and solvent from a mixture of said liquids used in a developing machine for printed boards in which the printed boards are treated with a solvent as a developing liquid and thereafter with clean water to interrupt the developing process by removing the solvent from the printed boards, the solvent having a density above 1.2 g/cm³, being insoluble in water and occurring in the mixture as small particles, said water-solvent mixture being treated in a main filter compris¬ ing a filter house and a filter body arranged therein in the form of a coalescence filter having a wall structure permeable to the water-solvent mixture and comprising a first material to which said small particles of solvent adhere, and a second material which is wetted by water, said small particles of solvent being combined by said adhesion to form drops which gradually become detached from the coalescence filter and, due to gravity, fall down to form a lower layer of separated solvent phase which is removed from the filter house. The invention also relates to an apparatus for per¬ forming the method.

Coalescence filters are known through a plurality of patent specifications, see FR-A-2 177464 and US-A-3 239 452, for in¬ stance.

CH-A5-570 187 describes a method for treatment of a used cleaning liquid consisting of a mixture of water and a solvent. Said mixture is allowed to stand in a vessel during an extended time so that an upper water phase and a lower solvent phase are obtained. The emptying of the vessel is controlled by signals delivered by a liquid detector which senses the type of phase being discharged. The water phase is fed to the sewage system and the discontinuous procedure is repeated. In developing machines for printed boards a specific solvent is applied which acts as developing liquid. Developing is terminated by spraying clean water onto the printed boards so that the solvent s rinsed off. A mixture of water and solvent is thus obtained which has hitherto caused a number of problems. The water-solvent mixture is often discharged directly to the sewage system without purification, and thereby contributing to increased environment problems. Efforts have therefore been made to separate the solvent by various methods, such as blowing air through the water-solvent mixture so that the solvent disappears with the air into the atmos¬ phere. Although the water discharged into the sewage system is then cleaner, there remains the problem of solvent being discharged to the surroundings. Furthermore, the constant supply of new solvent and fresh water in relatively large quantities without the opportu- nity of recirculating these treatment liquids has also resulted in considerable costs. Transferring the water-solvent mixture to special vessels in which the heavier solvent is allowed to sink to the bottom to form a solvent phase has therefore been suggested to recover the solvent, the solvent phase being separated and re-used after distillation. However, a plurality of vessels have to be utilized and the time the mixture spends in them is considerable since the flow must be slow to allow sedimentation. The apparatus for carrying out this method therefore takes up considerable space in the vicinity of the developing machine and its capacity is low. A solvent phase is obtained but this contains a small quantity of water which must be removed before the solvent can be re-used.

The object of the present invention is to eliminate all the prob¬ lems mentioned above and provide an improved continuous method of separating water and solvent from a mixture of said treatment liquids so that at least the water can be continuously re-used repeatedly in a closed cycle without greater amounts of fresh water having to be added to the system and without any quantity or por¬ tion of the water-solvent mixture at all being discharged to the outer environment during operation. An apparatus for carrying out this separation requires an extremely little space at the side of the developing machine. The method according to the invention is substantially charac¬ terised in that said water-solvent mixture is fed continuously from the developing machine.to the main filter in a circulation system simultaneously as a water phase, which is free from particles and drops of solvent, is continuously discharged from the filter house through an upper outlet and is directly returned to the developing machine to be re-used in a closed cycle.

The apparatus according to the invention is substantially charac¬ terised in that said main filter is connected to the developing machine in a circulation system through pipes to convey continuous¬ ly the water-solvent mixture to the main filter, and that the filter house is through an upper outlet and pipe connected to the developing machine to discharge continuously from the filter house and return directly to the developing machine in a closed cycle of a water phase free from particles and drops of solvent.

The invention will be described further in the following with reference to the accompanying drawing. Figure 1 therein shows schematically an apparatus for separating a water-solvent mixture in accordance with the present invention, the apparatus being connected to a developing machine.

In Figure 1 the reference number 1 denotes a developing machine for treating printed boards in a plurality of consecutive steps includ¬ ing the supply of a specific solvent as developing liquid for the printed boards and thereafter the supply of clean water to rinse the solvent off the printed boards so that the developing process is interrupted in a desired manner.

A special apparatus is connected to the developing machine to treat the mixture of water and solvent thus obtained from the process in order to separate these two treatment liquids so that they can be re-used in the process and particularly to recover the water in such a degree of purity that it can immediately be re-used as rins¬ ing agent in the developing machine 1 in a closed cycle without additional clean water having to be added except those minor amounts which can evaporate, for instance, or in some way disappear from the process.

The special apparatus comprises a circulation tank 2 which receives the water-solvent mixture from the developing machine 1 via a pipe 3. Fresh clean water can be supplied to the circulation tank 2 through a pipe 4 when necessary, the pipe 4 being provided with a valve 5 connected to a level control means 6 with regulating means for automatic supply of fresh clean water. This supply is carried out at the start and during operation in order to replace such water which disappears from the system for some reason. A pipe 7 provided with a circulation pump 8 is connected to the bottom of the circulation tank 2. A level guard 9 is mounted in the circula¬ tion tank 2 and connected to the circulation pump 8 to prevent dry running of the pump. The supply of liquid to and withdrawal from the circulation tank 2 are sufficient to maintain movement of the water-solvent mixture and mix it in the circulation tank 2.

The pipe 7 is connected to a pre-filter 10 of the type which filters out solid impurities having a particle size greater than 5 micron. Some of the water-solvent mixture can be returned to the circulation tank 2 via a return pipe 11.

After the pre-filter 10 there is a main filter, to which the pre-filter 10 is connected by a pipe 13 provided with valve 14 and nonreturn valve 15. The main filter comprises a vertically extend- ing filter house 12 and a vertically extended cylindrical filter body 17 mounted therein. The filter body 17 is in the form of a coalescence filter. Said pipe 13 extends into the centre of the filter house 12 and continues upwardly in a vertical section 16 to join and cooperate with the coalescence filter 17. The pipe section 16 extends through the lower end of the coalescence filter 17 and suitably continues to its upper end, the pipe section 16 situated in the coalescence filter 17 being slotted or provided in some other way with openings to distribute the water-solvent mixture along the entire inner surface of the vertically mounted coa¬ lescence filter 17. A partition body 18 is mounted outside the coalescence filter 17 and comprises a surrounding partition wall 19 completely enclosing the coalescence filter 17 and located a prede- termined distance therefrom to define a space 20 between filter 17 and body 18. The partition wall 19 is preferably shaped as a trun¬ cated cone and extends downwardly past the coalescence filter 17 so that its wider, lower end is located a suitable level above the bottom of the filter house 12 to form a separation chamber 21 therebetween in which the water phase and solvent phase are sepa¬ rated from each other. In connection to the lower part of the sep¬ aration chamber 21 the filter house 12 is provided with a first outlet 22 for the separated layer 38 of solvent phase which is drained off to a container 23 via a pipe 24. The outer casing 25 of the filter house 12 and the partition wall 19 define therebetween an annular space 26 for removal of the water phase in upward direc¬ tion from the separation chamber 21. In connection to the upper part of the space 26 the filter house 12 is provided with a second outlet 27 for the purified water phase which is then returned to the developing machine 1 via a pipe 28 for repeated use. By means of a flow gauge 29 in the pipe 28 the quantity of water supplied to the developing machine in a closed cycle is controlled.

In the embodiment shown a carbon filter 30 is also mounted in a branch pipe 31 connected to said pipe 28 from the main filter. The carbon filter 30, normally disconnected by means of a valve 32 in the branch pipe 31, contains various layers of sand and gravel in addition to a carbon layer. The water treated by the carbon filter 30 is withdrawn via a pipe 33 leading into a drain 34.

In the developing machine there is a plurality of nozzles 35 mounted in a spray chamber 39 in order to distribute the purified water on to the upper and lower surfaces of the printed boards while they are fed through the machine. The solvent applied in a previous step in a spray chamber is thus washed away from the printed boards. The water thus polluted is caught by an inclined plate 36 and runs down the plate into the pipe 3. In this water- -solvent mixture the solvent is distributed in the form of very small particles of such a dimension that they sink to the bottom only very slowly if the mixture is allowed to stand. The sinking speed is thus so low that it is practically impossible to achieve effective separation of the solvent.

The apparatus according to the invention requires a floor area outside the developing machine of only about 1x1.5 .

The developing machine and main filter of the apparatus, and the communicating pipes with their various equipments form a circu¬ lation system for the two treatment liquids, of which at least the water can be used immediately the solvent has been separated out in the main filter. The circulation system contains about 300 liter liquid. About 1-2.5 liter solvent per hour is supplied in the developing machine and about 1200-1500 liter water per hour is supplied in the spray chamber 39. Normally about 2-3 liter water per hour disappears from the circulation system as described above. However, by means of the level control this loss is supervised so that equivalent quantities of fresh water automatically is supplied to the circulation tank 2.

The filtering process in the main filter is based on the wetting properties of different liquids with respect to solid filter means which are in the form of surface-enlarging contact elements in the filter house. The cartridge-like filter body 17 is knitted or wound from two functionally different materials, one of which has the property that very small particles of solvent are adhered to the surface of said one material while the other material is wetted by water. In the water-solvent mixture entering, the solvent is in the form of very small particles having such a small dimension that they sink very slowly to the bottom in a stationary liquid mixture. In the embodiment described the particles of solvent have a dimen¬ sion in the order of 0.0001 mm. When the water-solvent mixture is brought to pass through the coalescence filter 17, which is provid- ed with a great number of small through passages or channels, the small particles of solvent will adhere to said first material and will gradually combine to form larger, drop-like bodies. When these have attained a certain larger dimension, they will no longer be retained by the adhesive power of the first material and will be¬ come detached from the outer surface of the coalescence filter 17 and, due to the force of gravity, will fall through the water phase in space 20 and separating chamber 21 to be collected at the bottom of the filter house 12 and subsequently withdrawn to the container 23 while the purified water phase, which is free of any particles or drops of solvent, flows up along the inside of space 26 in the filter house 12 and is removed through the upper, second outlet 27 to be directly pumped back to the developing machine 1 for con¬ tinued use without further purification steps. The drops have a size of up to about 4 mm. The solvent 38 discharged from the filter house 12 is suitably subjected to distillation before being re¬ turned to the developing machine 1 for re-use. Furthermore, it is also often desirable to treat the solvent with a suitable stabi¬ lizing agent to prevent it from disintegrating.

A solvent currently in general use as developing liquid in the de¬ veloping machine described above is 1 ,1 ,1-trichlorethane (CH-_jCC ) which has a density of 1.31 g/cm³ at 20°C and a solubility in water of only 0.035 per cent by weight. In the present context, there¬ fore, it can be regarded as practically insoluble in water. Its boiling point is 74.0°C at 760 mm Hg.

In experiments performed in an apparatus of the type described above, using 1 ,1 ,1-trichlorethane, the water leaving the filter house 12 surprisingly contains only 0.5 g/1 solvent, corresponding to 500 ppm, which is considered an extremely good value. This low content of solvent is negligible and therefore the separated clean water phase can be considered as being free from particles and drops of solvent. It is suitable for the water-solvent mixture to be kept at a low temperature in order to impede the growth of algae in the cir¬ culation system. A suitable temperature range is 15-18°C. For this purpose a suitable cooling means is arranged in the circulation tank 2, in which the water-solvent mixture is kept in movement as described earlier so that it remains homogenous.

The apparatus can be run using the same water for a considerable time. When the circulation system for some reason must be emptied and cleaned due to too high algae growth and/or too high salt con- tent, for instance, the valve 37 in pipe 28 is closed and the valve 32 in the branch pipe 31 opened so that the water is passed through the carbon filter 30 for cleaning before it is discharged in the drain 34.

In order to further increase the running time between water changes in the system, the fresh water originally supplied is preferably deionized. It is also suitable to use deionized water to replace the small quantities of water which disappear from the system due to evaporation, accompanying the printed boards, etc. Thus, accord¬ ing to a preferred embodiment, the apparatus includes a suitable ion exchanger to which the pipe 4 is connected.

Claims

1. A method for separating water and solvent from a mixture of said liquids used in a developing machine (1) for printed boards in which the printed boards are treated with a solvent as a developing liquid and thereafter with clean water to interrupt the developing process by removing the solvent from the printed boards, the sol¬ vent having a density above 1.2 g/cm³, being insoluble in water and occurring in the mixture as small particles, said water-solvent mixture being treated in a main filter comprising a filter house (12) and a filter body (17) arranged therein in the form of a coa- lescence filter having a wall structure permeable to the water- -solvent mixture and comprising a first material to which said small particles of solvent adhere, and a second material which is wetted by water, said small particles of solvent being combined by said adhesion to form drops which gradually become detached from the coalescence filter and, due to gravity, fall down to form a lower layer (38) of separated solvent phase which is removed from the filter house (12), characterised in that said water-solvent mixture is fed continuously from the developing machine (1) to the main filter in a circulation system simultaneously as a water phase, which is free from particles and drops of solvent, is con¬ tinuously discharged from the filter house (12) through an upper outlet (27) and is directly returned to the developing machine (1) to be re-used in a closed cycle.

2. A method according to claim 1, characterised in that the separated solvent phase (38) is withdrawn from the filter house

(12) and is returned directly or after distillation to the developing machine (1) in a closed cycle for re-use.

3. A method according to claim 1 or 2, characterised in that minor water losses normally arising during operation are controlled and replaced by fresh water which is automatically supplied to the circulation system.

4. A method according to any of the preceding claims, character¬ ised in that the water-solvent mixture is maintained at a tempera¬ ture of 15-20°C, preferably 17-18°C.

5. A method according to claim 1, characterised in that the solvent is 1,1,1-trichlorethane.

6. A method according to any of the preceding claims, character¬ ised in that, when the water in the system is to be changed, the used water is passed through a carbon filter (30) before discharge to a drain (34).

7. A method according to any of the preceding claims, character¬ ised in that the water initially supplied into the system, and the replacement water added during operation, is deionized water.

8. An apparatus for performing the method according to claim 1 for separating water and solvent from a mixture of said liquids used in a developing machine (1) for printed boards in which the printed boards are treated with a solvent as developing liquid and thereafter clean water to interrupt the developing process by removing the solvent from the printed boards, said solvent having a density above 1.2 g/cm³, being insoluble in water and occurring in the mixture as small particles, said apparatus comprises a main filter having a filter house (12) and a filter body (17) arranged therein in the form of a coalescence filter having a wall structure permeable to the water-solvent mixture and comprising a first mate¬ rial to which said small particles of solvent adhere, and a second material which is wetted by water, said small particles of solvent are combined by said adhesion to form drops which gradually become detached from the coalescence filter (17) and, due to gravity, fall down to form a lower layer (38) of separated solvent phase, said filter house (12) having a lower outlet (22) for the discharge of said separated solvent phase, characterised in that said main filter is connected to the developing machine (1) in a circulation system through pipes (3, 7, 13) to convey continuously the water- n

-solvent mixture to the main filter, and that the filter house (12) is through an upper outlet (27) and pipe (28) connected to the developing machine (1) to discharge continuously from the filter house (12) and return directly to the developing machine (1) in a closed cycle of a water phase free from particles and drops of solvent.

9. An apparatus according to claim 8, characterised in that the lower outlet (22) is connected to the developing machine (1), directly or via a distilling apparatus.

10. An apparatus according to claim 8 or 9, characterised in that it comprises cooling means to maintain the water-solvent mixture at a low temperature in the circulation system.

11. An apparatus according to any of claims 8-10, characterised in that it comprises a pre-filter (10) mounted upstream the main filter, to remove solid particles from the water-solvent mixture.

12. An apparatus according to any of claims 8-11, characterised in that it comprises means for control of minor water losses normally arising during operation and replacement thereof with fresh water which is automatically supplied to the circulation system.

13. An apparatus according to any of claims 8-12, characterised in that it comprises an ion exchanger for the supply of deionized water to the circulation system when starting up and as required during operation.

14. An apparatus according to any of claims 8-13, characterised in that it comprises a carbon filter (30) mounted in a branch pipe (31) downstream the main filter (12, 17), to purify the used water before it is discharged to a drain (34) when the water in the circulation system is to be changed.