Title:
PROCESS FOR THE PRODUCTION OF SILVER HALIDE DISPERSIONS
United States Patent 3790386
Abstract:
The preparation of a photo-sensitive salt dispersion having salts of sparing solubility in water which results in a material having a characteristic curve of the flatter gradient type by precipitating a sparingly soluble salt in a small first volume chamber which permits the reactants to be maintained with constant uniformity through the first chamber by stirring. Conducting the resultant dispersion through a conduit to a separated ripening chamber of a second volume many times that of the precipitating first volume. The silver halide emulsion is partially recycled from the separated ripening chamber through a conduit to the precipitation reaction which is outside of the second volume and where the recycled emulsion serves as reaction medium for the further precipitation of further silver halide salts. The amount of recycled silver halide emulsion is variable to provide an influence on the flatness of the gradient of the characteristic curve of the final emulsion.
US Patent References:
Method of making fine, uniform silver halide grains
Frame - December 1968 - 3415650
Method of making fine, uniform silver halide grains
Frame - December 1968 - 3415650
Inventors:
Posse, Rolf-fred (Cologne-Flittard, DT)
Saleck, Wilhelm (Schildgen/Bergisch-Gladbach, DT)
Muller, Herbert (Leverkusen, DT)
Randolph, August (Leverkusen, DT)
Moll, Franz (Cologne-Stammheim, DT)
Saleck, Wilhelm (Schildgen/Bergisch-Gladbach, DT)
Muller, Herbert (Leverkusen, DT)
Randolph, August (Leverkusen, DT)
Moll, Franz (Cologne-Stammheim, DT)
Application Number:
05/200507
Publication Date:
02/05/1974
Filing Date:
11/19/1971
View Patent Images:
Export Citation:
Assignee:
Agfa-Gevaert Aktiengesellschaft (Leverkusen, DT)
Primary Class:
Other Classes:
430/567, 430/569
International Classes:
G03C1/015; G03C1/02
Field of Search:
96/94,114.7 423/34,38,42,46,491
Primary Examiner:
Brown, Travis J.
Assistant Examiner:
Schilling, Richard L.
Attorney, Agent or Firm:
Arthur, Connolly Et Al G.
Parent Case Data:
This application is a continuation-in-part of applicants' copending U.S. application Ser. No. 771,545 entitled "A Process for the Production of Silver Halide Dispersions" filed Oct. 29, 1968, now abandoned.
Claims:
1. In a process for the production of a predetermined volume of silver halide emulsions having a controlled characteristic curve of gradation providing a substantial density upon exposure to light, the steps of first mixing components including an aqueous solution of silver nitrate and an aqueous solution of an alkali-metal halide in proportion which form a precipitate of a silver halide and in a first precipitate volume of between 0.1 to 10 liters and in which the concentrations of at least one of the components is maintained constant throughout the volume of said silver halide, stirring the components, precipitating and forming an aqueous dispersion of a sparingly soluble silver halide, immediately transferring the aqueous dispersion from the precipitate volume to a separate second ripening volume which is at least 1 liter and at least 10 times greater than the first volume of precipitate forming components, continuously recycling back to the separate first precipitate volume of precipitate forming components an amount of the ripened emulsion to serve as a medium for further precipitation in the precipitate forming components and to serve as a control of the emulsion gradation, said recycling of said ripened emulsion being at an hourly volume rate of 30 to 80 times greater than the predetermined volume product of emulsion, continuously removing the emulsion from the first precipitate volume and transferring it to the second ripening volume, so as to continually provide the recycled emulsion as a medium for further precipitaton and control of emulsion gradation and continuing recycling the silver halide emulsion formed to obtain said predetermined volume of silver halide emulsion with a controlled characteristic curve of gradation.
Description:
BACKGROUND OF THE INVENTION
This invention relates to a process for the production of dispersions of sparingly soluble silver salts.
It is common practice to prepare silver halide dispersions in the presence of a protective colloid, e.g. by adding at a fixed temperature with stirring a silver nitrate solution to a gelatin alkali metal halide solution in a vessel which can be heated externally. The two precipitate-forming components may also be mixed together in a different sequence, e.g. the alkali metal halide and silver nitrate solutions may be run simultaneously into a gelatin solution placed in the reaction vessel or the silver nitrate and halide solutions may be added alternately, or the silver salt solution may be placed in the empty vessel and the alkali metal halide solution added to it. The four methods of introducing the components are often combined. These methods are used both for the preparation of both the so-called neutral or boiled emulsions and for ammonium silver halide emulsions.
Generally the precipitation and the physical ripening, or Ostwald ripening are carried out in the same vessel. Whereas precipitaiton involves a rapid chemical reaction, ripening is a relatively slow physical process which influences the particle size distribution and the size of the crystallites.
Apparatus required for the conventional process customarily includes a vessel for precipitation and ripening, and supply vessels for the solutions which are to be introduced and at least one stirrer to ensure thorough mixing.
Conventional manufacturing processes are particularly unsatisfactory for the production of large quantities of emulsion. Thus, for example, it is extremely difficult to provide sufficiently thorough stirring in large reaction vessels of 1,000 liters capacity or more. Hence, extremely large stirring arrangements or alternatively several stirrers have to be used, the installation of which is expensive. Since essentially the photographic properties of a silver halide emulsion are determined at the preparation stage when the sparingly soluble silver salts are formed, it is particularly important to have a reliable and reproducible process for precipitation. One of the requirements that has to be satisfied if precipitation is to be reproducible is that the concentrations of the precipitate forming components should be kept constant during the precipitation process. This can only be done by intensive stirring. When working with large volumes, it is no longer possible to introduce the solution of the precipitation component through only one pipe. Several such pipes with a corresponding number of valves are needed. This again complicates the control of the process, and makes the problem of stirring even more difficult. Very efficient stirrers must also be provided at the inlets in order to prevent formation of agglomerates of precipitate there.
Such vessels for very large volumes, which usually contain several stirrers, can only be heated or cooled externally. Also since any increase in the dimensions of the apparatus increases the volume to the third power, but the surface being to the second power, the problem of rapid cooling or heating is difficult to overcome. Furthermore, the vessel is difficult to cover on account of the number of stirrers and inlet pipes, so that it is equally difficult to reduce heat losses.
Another serious disadvantage is the difficulty to transfer recipes which are suitable on an experimental scale to a large manufacturing scale. Similarly, it is often difficult to transfer a known production recipe from one plant to another plant.
Furthermore, it is known to make silver halide emulsions in two stages by mixing of the aqueous silver nitrate solution and alkali metal halide solution nonturbulently by means of upwardly concave vessels disposed in nesting relation with each other, each mounted for rotation about an axis. The aqueous dispersion of the silver halide climbs the inside wall of the cone-shaped outer vessel by the action of centrifugal force to its upper edge over which the dispersion passes into the larger ripening vessel. The above method, however, is only of limited utility, since only small amounts of silver halide emulsions can be made due to the limited volume of the cone-shaped rotating vessels. Moreover, only certain types of emulsions can be prepared since the emulsions obtained with that process are relatively uniform with respect to the grain size distribution. Emulsions of that type have a very steep characteristic curve. Silver halide emulsion with a flat gradation, for example, emulsions useful for negative materials cannot be made with that apparatus.
The object of the present invention is to provide a process for the production of dispersions of sparingly soluble silver salts by means of which large quantities of these dispersions can be produced in a simple and reproducible manner.
These and other objects of this invention will become more apparent upon consideration of the following descriptions taken together with the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic illustration of the means for carrying out the process of this invention;
FIG. 2 is a schematic illustration of a modified means;
FIG. 3 is a graph of a density curve of an emulsion according to this invention; and
FIG. 4 is a graph of a density curve of another emulsion according to this invention.
SUMMARY OF THE INVENTION
This invention provides a method of producing silver halide emulsions of higher sensitivity with the same or similar grain size by precipitaitng sparingly soluble silver salts in a first volume of between 0.1 and 10 liters, continuously removing the resultant dispersion through a conduit to a second volume outside of and separated from the first volume and at least 10 times greater than the first volume and physically ripening the dispersion in the second volume while continuously recycling at least a portion of this ripened silver salt dispersion from the separated second volume through a conduit to the first volume which is outside of the second volume. The removal of the dispersion fron one volume to the other may be accomplished by the rotating parts of a pump which simultaneously performs the mixing and recycling of the dispersion. The recycling is continued to form a silver salt emulsion having a predetermined sensitivity.
DETAILED DESCRIPTION
We now have found that disperions of sparingly soluble silver salts can also be produced simply and reliably even in large quantities by carrying out the process in two stages, whereby the fast precipitation reaction is performed with intensive stirring in a relatively small precipitation chamber whilst physical ripening is subsequently carried out in a ripening chamber of very large volume. The two volumes are physically spaced apart and separated. Further, the disperions of the ripening chamber is recycled in order to serve as medium for the precipitation of the sparingly soluble silver salt, more particular silver halide, in the precipitation chamber. The recycling is by moving the dispersion from one volume to the other through suitable conduits such as pipes.
Examples of apparatus suitable for carrying out the reaction is diagrammatically illustrated in FIGS. 1 and 2. In these Figures, 1 indicates a relatively small precipitation chamber equipped with an efficient stirrer, 2 is the ripening chamber of considerably larger volume. The solutions of the precipitate forming components enter the precipitation chamber from the storage vessels 3 and 4. Precipitation takes place there and the resulting dispersion of the sparingly soluble silver salt is immediately fed to the ripening chamber through the pipe 5 if desired by means of a pump 7.
The silver halide emulsion formed is repumped through the pipe 6 if desired by the aid of a pump of any construction into the precipitation chamber 1 in order to serve as medium for the further precipitation.
According to another embodiment, one of the precipitate forming components may already be dissolved in a solution present in the ripening chamber. The liquid in the ripening chamber may be fed through the pipe 6 back into the reaction chamber. In this case, one of the storage vessels is redundant. Any number of additional storage vessels may of course be connected to the system if it is desired to introduce additional components into the reaction zone.
Mechanical stirrers, vibrators or flow-mixers may be used to ensure thorough mixing in the precipitation chamber so intensively that the concentration of at least one of the precipitate forming components remains substantially constant throughout the dispersion present in the precipitation chamber so that "monocrystallites" are formed. "Monocrystallites" are crystals which are initially formed as nuclei during the ionic reaction. They either act as crystallization nuclei for the formation of larger crystals or may be dissolved and recrystallized during the Ostwald ripening stage when larger crystals are grown.
The pump 7 which causes recirculation may be one of a number of types of pumps, e.g. a reciprocating pump, centrifugal pump, gear pump or vacuum pumping systems. The pump system removes the silver salt disperions continuously from the reaction chamber and transfers it into the ripening chamber of large volume.
This ripening chamber is used essentially for Ostwald ripening during which the grain size and grain distribution are adjusted as required.
FIG. 2 shows a preferred and particularly simple embodiment. The reaction chamber 3 is the inner volume of a rotary pump. In this case, the feed pipes for the precipitate forming components are guided into the pump intake, extending as for as possible into the reaction chamber, i.e. terminating just before the inner edge of the blades of the rotor of the pump.
The turbulence created by the blades of the high-speed turbine or rotor disperses the liquids flowing in, with the result that the precipitate forming components are kept at a constant concentration during the precipitation. The dispersion is prevented from settling by the intensive mixing to which the contents of the pump chamber are subjected. There is no need for a recirculating pump as there is in the apparatus shown in FIG. 1.
The rotary pump may also be replaced by rotary dispersing and emulsifying machines.
The speed of the rotary pump may be from 750 to 3,000 r.p.m. preferably about 1,500 r.p.m. Rotational speeds above 3,000 r.p.m. may be used in other rotary dispersing machines. Grain size and grain size distribution may both be influenced by the speed of rotation used. Very fine-grained silver halide emulsions of a steep gradation are obtained at high speeds of rotation.
Storage vessels for the precipitate forming components are indicated by the reference numbers 9 and 10, 8 is the ripening and 11 and 12 are the feed and return pipes between the precipitation and the ripening chambers. The preparation of silver halide emulsions may be modified in the same way when using the apparatus shown in FIG. 2 as described above for the apparatus shown in FIG. 1.
Ostwald ripening takes place in the ripening chamber. The contents can be thoroughly mixed by tangential introduction of the recycled emulsion in the ripening chamber. If the inlet pipe is placed tangentially in the vessel, there is usually no need to provide an additional stirrer in the ripening chamber. However, additional stirring may, of course, be provided in special cases.
The characteristic feature of the inventive process as shown, for example in FIGS. 1-2, is the repumping of a part of the silver halide dispersion from the ripening chamber to the precipitation chamber. In other words, the silver halide emulsion is partially recycled, whereby the recycled emulsion serves as reaction medium in the precipitation chamber for the precipitation of further silver halide. The amount of the recycled silver halide emulsion is not critical and depends on the desired photographic properties of the emulsion to be made. In particular the gradation can be influenced by the rate of recycled emulsion in the precipitation chamber, whereby generally a higher amount of recycled emulsion results into a final emulsion with flatter gradation. The rate of recycled emulsion required for certain photographic properties can be readily found out by some simple tests.
It has proved sufficient to recycle a volume of emulsion per hour which is 30 to 80 times greater than the final volume of the emulsion.
By the present invention all disadvantages of the known two-stage processes are overcome. Due to the very fast and highly effective mixture of the precipitation components in the precipitation chamber, large amounts of silver halide emulsion can be made in spite of the small volume of the precipitation chamber, for example, the inner volume of a centrifugal pump. The circulation of a part of the silver halide emulsion formed, it is easily possible to modify the photographic properties of the silver halide emulsion as desired. It is possible, for example to make emulsions with very flat as well as extremely steep gradation. Moreover, it is possible to make highly concentrated silver halide emulsions.
Further advantages of the process of the invention are evident. Precipitation of the sparingly soluble silver salts are carried out in a relatively small reaction chamber; preferably of 0.01 to 10 liters capacity, the contents of which can be mixed thoroughly without difficulty. Fairly gentle stirring is adequate for the physical ripening so that this stage in the process does not present any difficulties. The ripening chamber has a volume of some 10 times and preferably some 100 times, in particular 100 to 1,000 times greater than the volume of the precipitation chamber. The dispersion is continuously transferred from one separated vessel to the other by transfer and recycling through connecting pipes.
The shape of the ripening vessel is immaterial, although vessels which taper semi-circularly or conically at their lower end are preferred. The ripening vessel may be heated or cooled to the required temperature from inside or outside by cooling or heating coils. This considerably shortens the cooling and heating times, a factor of considerable importance when using large vessels. It is also possible to use an enclosed vessel, thus avoiding further heat losses.
Since both the reaction and the ripening chambers can be closed so that exposure to light is avoided, it is possible for the process according to the invention to be carried out under artificial light or even in daylight.
In conventional processes, a protective colloid, preferably gelation, must always be present in order to stabilize the dispersion and to prevent the formation of fairly large agglomerates. However, it is possible with the process according to the invention to disperse almost insoluble silver salts in the absence of a colloid, with the result that the colloid, which acts as binder for the photographic layers, can be added subsequently.
Surprisingly, the emulsions prepared by the process according to the invention also have superior photographic properties because the particle growth and particle size distributions are much better controlled than has been obtained previously through the separation of crystal formation from the physical or Ostwald ripening.
Also much more stable emulsions are obtained after ripening and the incorporation additives whatsoever can be controlled with considerable precision and reliability.
EXAMPLE 1
Solution A is prepared by dissolving 150 g. of gelatin in 6 liters of water at 50° C. Solution B is obtained by dissolving 2,000 g. of silver nitrate in 5 liters of water at 45° C. Solution C is obtained by dissolving 400 g. of sodium chloride and 800 g. of potassium bromide in 5 liters of water at 50° C.
Solution A is introduced into the ripening chamber. The apparatus shown in FIG. 1 is used. Solution A which is already in the ripening chamber is recirculated by pumping so that it flows through the precipitation chamber.
Solutions B and C are then allowed to run in simultaneously from two storage vessels 3 and 4 over a period of 15 minutes, the introduction of solution C being commenced 15 seconds earlier. During the precipitation a part of the silver halide emulsion already formed is recycled and serves as reaction medium in the precipitation chamber. 1.5 kg. of gelatin are then added and after 20 minutes' digestion at the same temperature it is hardened, converted into noodled or shredded form and rinsed with water. After the chemical ripening additives have been added, the emulsion is finally after-ripened.
A fine-grained emulsion of a medium gradation is obtained.
The characteristic density curve of the silver halide emulsion is shown in FIG. 3 in which density is shown on the ordinate and the light exposure on the abscissa.
For comparison another emulsion was prepared from the above solutions A, B and C. According to common practice solutions B and C were added simultaneously to solution A in a light-proof vessel with stirrer.
Both emulsions were tested in a sensitometer customarily employed in the art and the average grain size measured. The results are shown in the following table
Emulsion Sensibility Average grain size (relative value in μ m ______________________________________ according to Example 1 100 0.64 comparison sample 100 0.73 ______________________________________
It is readily apparent from the above table that both emulsions have the same sensitivity whereby, however, the emulsion prepared according to the present invention has an unexpectedly smaller average grain size. This results upon exposure and photographic processing into a finer image silver with considerably improved resolution.
Another highly desirable advantage of the process of the present invention is the excellent consistency of photographic properties of emulsions prepared according to the present process.
For example if the preparation of the above emulsion is repeated 100 times the sensitivity only varies within a range of 100 ± 3. With the comparison emulsion produced according to common practice the sensitivity of the emulsion batches varies within 100 ± 20.
EXAMPLE 2
Solution A is obtained by dissolving 100 g. of gelatin, 640 kg. of KBr and 48 g. of KI in 10 liters of water at 65° C. For solution B 800 g. of silver nitrate are dissolved in 10 liters of water at 60° C. The apparatus as shown in FIG. 2 is used.
Solution A is again placed in the ripening chamber. Before solution B is added, the rotary pump is switched on. Solution B, is then run in from the storage vessel 10 over a period of 15 minutes. During the precipitation A part of the silver halide emulsion already formed is recycled and serves as reaction medium in the precipitation chamber. 2 kg. of gelatin are then added and, after 20 minutes' digestion, the emulsion is solidified, converted into noodle form and rinsed with water.
The usual chemical stabilizers, such as sulfur ripeners and gold salts are added for after-ripening, and the emulsion is then ripened to obtain maximum sensitivity.
An emulsion of average sensitivity and flat gradation is obtained.
The characteristic density curve of the above emulsion is shown in FIG. 4 in which the density is shown on the ordinate and the light exposure on the abscissa.
A comparison emulsion was prepared according to common practice from the above solutions A and B. Solution B was added to solution A in a light-proof vessel with stirrer.
Both emulsions were tested in a sensitometer customarily employed in the art and the average grain size measured. The results are shown in the following table
Emulsion Sensibility Average grain size (relative value in μm ______________________________________ according to Example 2 175 0.9 comparison sample 100 0.9 ______________________________________
As shown in the above table both emulsions have the same average grain size; the sensitivity of the emulsion prepared according to the present invention however is considerably higher.
Another highly desirable advantage of the process of the present invention is the excellent consistency of photographic properties of emulsions prepared according to the present process.
For example if the preparation of the above emulsion is repeated 100 times the sensitivity only varies within a range of 100 ± 5. With the comparison emulsion produced according to common practice the sensitivity of the emulsion batches varies within 100 ± 25.
The examples show that the process and the apparatus of the invention may be used for the production of all types of silver halide emulsion. Fine-grained, less sensitive and coarse-grained high-sensitive emulsions can be obtained. It is also possible to obtain emulsions of a steep or a flat gradation.
The process according to the invention may also be used for the production of ammonium, semi-ammonium or neutral emulsions.
This invention relates to a process for the production of dispersions of sparingly soluble silver salts.
It is common practice to prepare silver halide dispersions in the presence of a protective colloid, e.g. by adding at a fixed temperature with stirring a silver nitrate solution to a gelatin alkali metal halide solution in a vessel which can be heated externally. The two precipitate-forming components may also be mixed together in a different sequence, e.g. the alkali metal halide and silver nitrate solutions may be run simultaneously into a gelatin solution placed in the reaction vessel or the silver nitrate and halide solutions may be added alternately, or the silver salt solution may be placed in the empty vessel and the alkali metal halide solution added to it. The four methods of introducing the components are often combined. These methods are used both for the preparation of both the so-called neutral or boiled emulsions and for ammonium silver halide emulsions.
Generally the precipitation and the physical ripening, or Ostwald ripening are carried out in the same vessel. Whereas precipitaiton involves a rapid chemical reaction, ripening is a relatively slow physical process which influences the particle size distribution and the size of the crystallites.
Apparatus required for the conventional process customarily includes a vessel for precipitation and ripening, and supply vessels for the solutions which are to be introduced and at least one stirrer to ensure thorough mixing.
Conventional manufacturing processes are particularly unsatisfactory for the production of large quantities of emulsion. Thus, for example, it is extremely difficult to provide sufficiently thorough stirring in large reaction vessels of 1,000 liters capacity or more. Hence, extremely large stirring arrangements or alternatively several stirrers have to be used, the installation of which is expensive. Since essentially the photographic properties of a silver halide emulsion are determined at the preparation stage when the sparingly soluble silver salts are formed, it is particularly important to have a reliable and reproducible process for precipitation. One of the requirements that has to be satisfied if precipitation is to be reproducible is that the concentrations of the precipitate forming components should be kept constant during the precipitation process. This can only be done by intensive stirring. When working with large volumes, it is no longer possible to introduce the solution of the precipitation component through only one pipe. Several such pipes with a corresponding number of valves are needed. This again complicates the control of the process, and makes the problem of stirring even more difficult. Very efficient stirrers must also be provided at the inlets in order to prevent formation of agglomerates of precipitate there.
Such vessels for very large volumes, which usually contain several stirrers, can only be heated or cooled externally. Also since any increase in the dimensions of the apparatus increases the volume to the third power, but the surface being to the second power, the problem of rapid cooling or heating is difficult to overcome. Furthermore, the vessel is difficult to cover on account of the number of stirrers and inlet pipes, so that it is equally difficult to reduce heat losses.
Another serious disadvantage is the difficulty to transfer recipes which are suitable on an experimental scale to a large manufacturing scale. Similarly, it is often difficult to transfer a known production recipe from one plant to another plant.
Furthermore, it is known to make silver halide emulsions in two stages by mixing of the aqueous silver nitrate solution and alkali metal halide solution nonturbulently by means of upwardly concave vessels disposed in nesting relation with each other, each mounted for rotation about an axis. The aqueous dispersion of the silver halide climbs the inside wall of the cone-shaped outer vessel by the action of centrifugal force to its upper edge over which the dispersion passes into the larger ripening vessel. The above method, however, is only of limited utility, since only small amounts of silver halide emulsions can be made due to the limited volume of the cone-shaped rotating vessels. Moreover, only certain types of emulsions can be prepared since the emulsions obtained with that process are relatively uniform with respect to the grain size distribution. Emulsions of that type have a very steep characteristic curve. Silver halide emulsion with a flat gradation, for example, emulsions useful for negative materials cannot be made with that apparatus.
The object of the present invention is to provide a process for the production of dispersions of sparingly soluble silver salts by means of which large quantities of these dispersions can be produced in a simple and reproducible manner.
These and other objects of this invention will become more apparent upon consideration of the following descriptions taken together with the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic illustration of the means for carrying out the process of this invention;
FIG. 2 is a schematic illustration of a modified means;
FIG. 3 is a graph of a density curve of an emulsion according to this invention; and
FIG. 4 is a graph of a density curve of another emulsion according to this invention.
SUMMARY OF THE INVENTION
This invention provides a method of producing silver halide emulsions of higher sensitivity with the same or similar grain size by precipitaitng sparingly soluble silver salts in a first volume of between 0.1 and 10 liters, continuously removing the resultant dispersion through a conduit to a second volume outside of and separated from the first volume and at least 10 times greater than the first volume and physically ripening the dispersion in the second volume while continuously recycling at least a portion of this ripened silver salt dispersion from the separated second volume through a conduit to the first volume which is outside of the second volume. The removal of the dispersion fron one volume to the other may be accomplished by the rotating parts of a pump which simultaneously performs the mixing and recycling of the dispersion. The recycling is continued to form a silver salt emulsion having a predetermined sensitivity.
DETAILED DESCRIPTION
We now have found that disperions of sparingly soluble silver salts can also be produced simply and reliably even in large quantities by carrying out the process in two stages, whereby the fast precipitation reaction is performed with intensive stirring in a relatively small precipitation chamber whilst physical ripening is subsequently carried out in a ripening chamber of very large volume. The two volumes are physically spaced apart and separated. Further, the disperions of the ripening chamber is recycled in order to serve as medium for the precipitation of the sparingly soluble silver salt, more particular silver halide, in the precipitation chamber. The recycling is by moving the dispersion from one volume to the other through suitable conduits such as pipes.
Examples of apparatus suitable for carrying out the reaction is diagrammatically illustrated in FIGS. 1 and 2. In these Figures, 1 indicates a relatively small precipitation chamber equipped with an efficient stirrer, 2 is the ripening chamber of considerably larger volume. The solutions of the precipitate forming components enter the precipitation chamber from the storage vessels 3 and 4. Precipitation takes place there and the resulting dispersion of the sparingly soluble silver salt is immediately fed to the ripening chamber through the pipe 5 if desired by means of a pump 7.
The silver halide emulsion formed is repumped through the pipe 6 if desired by the aid of a pump of any construction into the precipitation chamber 1 in order to serve as medium for the further precipitation.
According to another embodiment, one of the precipitate forming components may already be dissolved in a solution present in the ripening chamber. The liquid in the ripening chamber may be fed through the pipe 6 back into the reaction chamber. In this case, one of the storage vessels is redundant. Any number of additional storage vessels may of course be connected to the system if it is desired to introduce additional components into the reaction zone.
Mechanical stirrers, vibrators or flow-mixers may be used to ensure thorough mixing in the precipitation chamber so intensively that the concentration of at least one of the precipitate forming components remains substantially constant throughout the dispersion present in the precipitation chamber so that "monocrystallites" are formed. "Monocrystallites" are crystals which are initially formed as nuclei during the ionic reaction. They either act as crystallization nuclei for the formation of larger crystals or may be dissolved and recrystallized during the Ostwald ripening stage when larger crystals are grown.
The pump 7 which causes recirculation may be one of a number of types of pumps, e.g. a reciprocating pump, centrifugal pump, gear pump or vacuum pumping systems. The pump system removes the silver salt disperions continuously from the reaction chamber and transfers it into the ripening chamber of large volume.
This ripening chamber is used essentially for Ostwald ripening during which the grain size and grain distribution are adjusted as required.
FIG. 2 shows a preferred and particularly simple embodiment. The reaction chamber 3 is the inner volume of a rotary pump. In this case, the feed pipes for the precipitate forming components are guided into the pump intake, extending as for as possible into the reaction chamber, i.e. terminating just before the inner edge of the blades of the rotor of the pump.
The turbulence created by the blades of the high-speed turbine or rotor disperses the liquids flowing in, with the result that the precipitate forming components are kept at a constant concentration during the precipitation. The dispersion is prevented from settling by the intensive mixing to which the contents of the pump chamber are subjected. There is no need for a recirculating pump as there is in the apparatus shown in FIG. 1.
The rotary pump may also be replaced by rotary dispersing and emulsifying machines.
The speed of the rotary pump may be from 750 to 3,000 r.p.m. preferably about 1,500 r.p.m. Rotational speeds above 3,000 r.p.m. may be used in other rotary dispersing machines. Grain size and grain size distribution may both be influenced by the speed of rotation used. Very fine-grained silver halide emulsions of a steep gradation are obtained at high speeds of rotation.
Storage vessels for the precipitate forming components are indicated by the reference numbers 9 and 10, 8 is the ripening and 11 and 12 are the feed and return pipes between the precipitation and the ripening chambers. The preparation of silver halide emulsions may be modified in the same way when using the apparatus shown in FIG. 2 as described above for the apparatus shown in FIG. 1.
Ostwald ripening takes place in the ripening chamber. The contents can be thoroughly mixed by tangential introduction of the recycled emulsion in the ripening chamber. If the inlet pipe is placed tangentially in the vessel, there is usually no need to provide an additional stirrer in the ripening chamber. However, additional stirring may, of course, be provided in special cases.
The characteristic feature of the inventive process as shown, for example in FIGS. 1-2, is the repumping of a part of the silver halide dispersion from the ripening chamber to the precipitation chamber. In other words, the silver halide emulsion is partially recycled, whereby the recycled emulsion serves as reaction medium in the precipitation chamber for the precipitation of further silver halide. The amount of the recycled silver halide emulsion is not critical and depends on the desired photographic properties of the emulsion to be made. In particular the gradation can be influenced by the rate of recycled emulsion in the precipitation chamber, whereby generally a higher amount of recycled emulsion results into a final emulsion with flatter gradation. The rate of recycled emulsion required for certain photographic properties can be readily found out by some simple tests.
It has proved sufficient to recycle a volume of emulsion per hour which is 30 to 80 times greater than the final volume of the emulsion.
By the present invention all disadvantages of the known two-stage processes are overcome. Due to the very fast and highly effective mixture of the precipitation components in the precipitation chamber, large amounts of silver halide emulsion can be made in spite of the small volume of the precipitation chamber, for example, the inner volume of a centrifugal pump. The circulation of a part of the silver halide emulsion formed, it is easily possible to modify the photographic properties of the silver halide emulsion as desired. It is possible, for example to make emulsions with very flat as well as extremely steep gradation. Moreover, it is possible to make highly concentrated silver halide emulsions.
Further advantages of the process of the invention are evident. Precipitation of the sparingly soluble silver salts are carried out in a relatively small reaction chamber; preferably of 0.01 to 10 liters capacity, the contents of which can be mixed thoroughly without difficulty. Fairly gentle stirring is adequate for the physical ripening so that this stage in the process does not present any difficulties. The ripening chamber has a volume of some 10 times and preferably some 100 times, in particular 100 to 1,000 times greater than the volume of the precipitation chamber. The dispersion is continuously transferred from one separated vessel to the other by transfer and recycling through connecting pipes.
The shape of the ripening vessel is immaterial, although vessels which taper semi-circularly or conically at their lower end are preferred. The ripening vessel may be heated or cooled to the required temperature from inside or outside by cooling or heating coils. This considerably shortens the cooling and heating times, a factor of considerable importance when using large vessels. It is also possible to use an enclosed vessel, thus avoiding further heat losses.
Since both the reaction and the ripening chambers can be closed so that exposure to light is avoided, it is possible for the process according to the invention to be carried out under artificial light or even in daylight.
In conventional processes, a protective colloid, preferably gelation, must always be present in order to stabilize the dispersion and to prevent the formation of fairly large agglomerates. However, it is possible with the process according to the invention to disperse almost insoluble silver salts in the absence of a colloid, with the result that the colloid, which acts as binder for the photographic layers, can be added subsequently.
Surprisingly, the emulsions prepared by the process according to the invention also have superior photographic properties because the particle growth and particle size distributions are much better controlled than has been obtained previously through the separation of crystal formation from the physical or Ostwald ripening.
Also much more stable emulsions are obtained after ripening and the incorporation additives whatsoever can be controlled with considerable precision and reliability.
EXAMPLE 1
Solution A is prepared by dissolving 150 g. of gelatin in 6 liters of water at 50° C. Solution B is obtained by dissolving 2,000 g. of silver nitrate in 5 liters of water at 45° C. Solution C is obtained by dissolving 400 g. of sodium chloride and 800 g. of potassium bromide in 5 liters of water at 50° C.
Solution A is introduced into the ripening chamber. The apparatus shown in FIG. 1 is used. Solution A which is already in the ripening chamber is recirculated by pumping so that it flows through the precipitation chamber.
Solutions B and C are then allowed to run in simultaneously from two storage vessels 3 and 4 over a period of 15 minutes, the introduction of solution C being commenced 15 seconds earlier. During the precipitation a part of the silver halide emulsion already formed is recycled and serves as reaction medium in the precipitation chamber. 1.5 kg. of gelatin are then added and after 20 minutes' digestion at the same temperature it is hardened, converted into noodled or shredded form and rinsed with water. After the chemical ripening additives have been added, the emulsion is finally after-ripened.
A fine-grained emulsion of a medium gradation is obtained.
The characteristic density curve of the silver halide emulsion is shown in FIG. 3 in which density is shown on the ordinate and the light exposure on the abscissa.
For comparison another emulsion was prepared from the above solutions A, B and C. According to common practice solutions B and C were added simultaneously to solution A in a light-proof vessel with stirrer.
Both emulsions were tested in a sensitometer customarily employed in the art and the average grain size measured. The results are shown in the following table
Emulsion Sensibility Average grain size (relative value in μ m ______________________________________ according to Example 1 100 0.64 comparison sample 100 0.73 ______________________________________
It is readily apparent from the above table that both emulsions have the same sensitivity whereby, however, the emulsion prepared according to the present invention has an unexpectedly smaller average grain size. This results upon exposure and photographic processing into a finer image silver with considerably improved resolution.
Another highly desirable advantage of the process of the present invention is the excellent consistency of photographic properties of emulsions prepared according to the present process.
For example if the preparation of the above emulsion is repeated 100 times the sensitivity only varies within a range of 100 ± 3. With the comparison emulsion produced according to common practice the sensitivity of the emulsion batches varies within 100 ± 20.
EXAMPLE 2
Solution A is obtained by dissolving 100 g. of gelatin, 640 kg. of KBr and 48 g. of KI in 10 liters of water at 65° C. For solution B 800 g. of silver nitrate are dissolved in 10 liters of water at 60° C. The apparatus as shown in FIG. 2 is used.
Solution A is again placed in the ripening chamber. Before solution B is added, the rotary pump is switched on. Solution B, is then run in from the storage vessel 10 over a period of 15 minutes. During the precipitation A part of the silver halide emulsion already formed is recycled and serves as reaction medium in the precipitation chamber. 2 kg. of gelatin are then added and, after 20 minutes' digestion, the emulsion is solidified, converted into noodle form and rinsed with water.
The usual chemical stabilizers, such as sulfur ripeners and gold salts are added for after-ripening, and the emulsion is then ripened to obtain maximum sensitivity.
An emulsion of average sensitivity and flat gradation is obtained.
The characteristic density curve of the above emulsion is shown in FIG. 4 in which the density is shown on the ordinate and the light exposure on the abscissa.
A comparison emulsion was prepared according to common practice from the above solutions A and B. Solution B was added to solution A in a light-proof vessel with stirrer.
Both emulsions were tested in a sensitometer customarily employed in the art and the average grain size measured. The results are shown in the following table
Emulsion Sensibility Average grain size (relative value in μm ______________________________________ according to Example 2 175 0.9 comparison sample 100 0.9 ______________________________________
As shown in the above table both emulsions have the same average grain size; the sensitivity of the emulsion prepared according to the present invention however is considerably higher.
Another highly desirable advantage of the process of the present invention is the excellent consistency of photographic properties of emulsions prepared according to the present process.
For example if the preparation of the above emulsion is repeated 100 times the sensitivity only varies within a range of 100 ± 5. With the comparison emulsion produced according to common practice the sensitivity of the emulsion batches varies within 100 ± 25.
The examples show that the process and the apparatus of the invention may be used for the production of all types of silver halide emulsion. Fine-grained, less sensitive and coarse-grained high-sensitive emulsions can be obtained. It is also possible to obtain emulsions of a steep or a flat gradation.
The process according to the invention may also be used for the production of ammonium, semi-ammonium or neutral emulsions.