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CROSS-REFERENCE TO RELATED APPLICATION
This application is related, in subject matter, to Serial No. 554,226,
Finnicum et al, filed Feb. 28, 1975, now U.S. Pat. No. 4,147,551.
DESCRIPTION
1. Technical Field
This invention relates to processes for the preparation of photographic
emulsions and more particularly to processes in which precipitation of the
silver halide grains is carried out in a carefully controlled constant pAg
environment.
2. Background Art
It is recognized that the preparation of silver halide emulsions should be
carried out under carefully controlled conditions. For example, the
accurate control of rates of addition of reactants, pAg, pH, and the
duration of the precipitation as well as temperature and the relative
mixing uniformity of reactants added from two separate sources is
desirable. It is equally recognized that in the preparation of
monodisperse (narrow grain size distribution) silver halide emulsions by
double jet precipitation, rapid dilution and mixing of reactants in the
precipitation vessel play an important role in determining the final mean
grain volume (MGV) and grain size distribution (GSD). Prior art processes
usually employ a high speed agitator, and in some cases a dispersator such
as disclosed in Frame and Johnson, U.S. Pat. No. 3,415,650, to effect
rapid dispersion of reactants and to maintain uniform halide or silver ion
concentration in the precipitation vessel.
Such rapid mixing has not been possible in vessels usually employed to make
conventional, polydisperse silver halide emulsions where grain growth
takes place by Ostwald ripening and where control of pAg or pBr is of no
consequence. Typically, a paddle agitated, unbaffled vessel is employed
where the primary purpose of agitation is to prevent the grains from
settling and to maintain uniform ripening temperature in the vessel. One
object of this invention is to provide a process which can make use of
such a conventional vessel to prepare monodisperse silver halide emulsions
.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, forming a material part of this disclosure:
FIG. 1 is the diagram of an apparatus, partially in cross-section, for
carrying out the process of the invention.
FIG. 2 is a top view of FIG. 3, which in turn is a cross-section of
precipitation vessel 1 in FIG. 1 as adapted to the single recycle loop
arrangement.
FIG. 4 is a top view of FIG. 5, which in turn is a cross-section of
precipitation vessel 1 of FIG. 1 as adapted to the double recycle loop
arrangement.
FIG. 6 illustrates diagrammatically two turbulent jets emerging from
submerged nozzles Nos. 6a and 7a of FIGS. 4 and 5 into precipitation
vessel 1.
DISCLOSURE OF INVENTION
This invention provides an improved process for preparing a photographic
emulsion having a controlled silver halide grain size, structure, and size
distribution, which includes the steps of:
1. adding silver nitrate to a stream supplied from a precipitation vessel
containing gelatin solution, and recycling this stream back to the vessel,
2. adding a single halide or mixed halide solution to another stream
supplied from the vessel, and recycling this stream back to the vessel or
adding the halide solution or solutions directly to the vessel, to
precipitate silver halide grains therein, and
3. recycling the contents of the vessel into the stream or streams whereby
additional silver halide grains are precipitated on the initially
precipitated grains to form silver halide grains of controlled structure,
size and size distribution,
and an important feature of the process being that said recycle streams are
injected into the precipitation vessel in downwardly directed jets at a
velocity of 8-30 ft./sec. (2.44-9.14 m/sec.) so as to effect uniform, high
velocity, jet mixing in a defined region and maintain pAg constant.
The process of the invention, which is adaptable to both single and double
recycle type processes, provides advantages over either the conventional
double jet or recycle stream processes in that it provides for adequate
dilution of the reactants and uniformity of mixing, generally confines the
chemical reaction to a specific region of the precipitation vessel, and
shortens precipitation times. The high velocity jets emerge downwards and
expand in a conical envelope of about 20.degree. with a high degree of
turbulent mixing regardless of vessel size or shape.
As illustrated in the drawings, precipitation vessel 1 is jacketed so as to
permit the circulation of heating and cooling water in direct contact with
the vessel wall, and is provided with an agitator or paddle 2, and an
initial charge of gelatin solution 3. Storage vessels 22 and 23 are
provided for storing and supplying aqueous silver nitrate solution and
aqueous alkali metal halide solution, respectively, to vessel 1. A mixed
halide solution storage and supply vessel 25 is also provided.
A discharge line 1a equipped with control valve 4 is provided in order to
recirculate the contents of vessel 1 through either recycle line 6 or
recycle line 7, or both; these are each referred to at various times
hereafter as a "recycle path" or "loop".
Referring first to recycle line 6, the latter is provided with a rotameter
8 for measurement of flow rates, a recycle pump 9, a heat exchanger 10, a
mixer 14, and an ion monitor 27 to monitor the silver ion concentration in
recycle line 6, and it terminates in a vertical downpipe 6a which extends
below the liquid level in vessel 1.
Mixer 14 in line 6 is also connected to conduit 12, which serves to add
silver nitrate solution from vessel 22. Conduit 12 is equipped with
metering pump 16 for controlling the flow rate of silver nitrate solution,
and with valve 17 and rotameter 18 for adjusting and measuring the flow
rate.
At some point in the line, between rotameter 18 and mixer 14, conduit 12
connects with a water line 20, the volume of which is adjusted by valve 21
in line 20, so that downpipe 6a serves to return to vessel 1 the mixture
formed by combining the recycle in line 6 with the silver nitrate from
vessel 22 and the water from line 20.
The right hand side of FIG. 1 illustrates an alternate recycle conduit 7
which is connected to the discharge conduit 1a and is provided with
control valve 5, rotameter 8a, recycle pump 9a, heat exchanger 10a, and
mixer 14a, and it, too, terminates in a vertical downpipe 7a which extends
below the liquid level in vessel 1. Heat exchangers 10 and 10a serve to
control the temperature of the recycle by means of circulation of hot or
cold water. Mixer 14a is connected through conduit 13 to vessel 23 for
direct addition of aqueous alkali metal halide solution to recycle line 7.
Conduit 13 is equipped with a metering pump 16a and a two-way valve 17a
for controlling the flow rate of alkali metal halide solution from vessel
23, and with a three-way valve 24 which permits mixed halide solution in
vessel 25 to drain through discharge conduit 26 and metering pump 26a into
conduit 13. Conduit 13 is also provided with rotameter 18a for measuring
flow rates therein, and with three-way valve 28 for selective addition of
aqueous alkali metal halide or mixed halide solution to mixer 14a in
recycle line 7, or directly to vessel 1. Conduit 13 terminates in a
vertical downpipe 13a which extends below the liquid level in vessel 1.
Provision is also made for the addition of water to conduit 13 at a point
between rotameter 18a and valve 28, the water being admitted through line
30 and control valve 31.
In addition to the valves illustrated in FIG. 1, another valve (not shown)
may be provided in conduit 1a from vessel 1 for recycling the contents
back to vessel 1, or to another vessel, and a valve (not shown) may be
provided in conduit 12 to provide for the direct addition of silver salts
to the vessel 1. Also, in addition to ion monitor 27 additional ion
monitors (not shown) may be provided for monitoring the silver ion
concentration at other locations in the process. The rotameters and ion
monitors may be used to generate control signals for controlling flow
rates in the process and controlling pAg or excess halide concentrations
at various points in the process.
The mixers 14 and 14a are preferably tee-mixers, although other types of
static or dynamic mixers may be used. Using a conventional side tee-mixer,
the main stream of the mixer is used for the recycle path of the process
and the side stream of the mixer is used as the addition path for aqueous
silver and halide solutions. The side tee-mixer provides highly efficient
mixing and precipitation in a reasonably short length of the recycle path,
e.g., substantially 100% mixing is accomplished within 3 to 7 stream
diameters of the silver solution tee addition when the mass velocity ratio
of the main path to the side path is 2.7. The optimum mass velocity ratio
of 2.7 must be maintained for efficient mixing of the components in a
tee-mixer. However, mixing times may be controlled by varying the recycle
flow rate and proportionally changing the side path flow rate.
The recycle ratio for the process is defined as the ratio of the recycle
flow rate to the silver or halide solution flow rate. Although any recycle
ratio may be used, it is preferred to maintain the recycle ratio equal to
or greater than 10.
In operating the process of the invention, an aqueous solution of gelatin 3
is recycled from the bottom of the vessel 1 through a single or double
loop. Where a single recycle loop is used, aqueous silver nitrate can be
injected into the recycle line 6 via a tee-mixer at 14 and halide
solutions fed directly to the vessel 1, much as described in copending
U.S. Pat. Application Ser. No. 554,226, filed Feb. 28, 1975, now U.S. Pat.
No. 4,147,551, but in the process of the invention the recycle path
reentering the vessel 1 emerges vertically downward and at substantially
high velocities, usually in excess of 8 ft./sec. (2.44 m/sec.), preferably
20, and up to 30, through a nozzle or restricted orifice in vertical
downpipe 6a. Vertical downpipe 13a also discharges in the same manner as
6a and at a velocity of about 1-12 ft./sec. (0.3-3.66 m/sec.) into vessel
1, but upstream or countercurrent from the direction of paddle rotation,
and a quadrant away from vertical downpipe 6a, as illustrated in FIGS. 2
and 3. In a typical embodiment, downpipes 6a and 13a are 1" (2.54 cm)
pipes with a jet orifice of about 0.19" (4.826 mm) ID.
These high velocity jets expand in a 20.degree. angle conical envelope with
a high degree of turbulent mixing as illustrated in FIG. 6, and regardless
of the size or shape of precipitation vessel 1, they provide adequate
dilution of the reactants and uniformity of mixing, and generally confine
the chemical reaction to one quadrant of the vessel. After initial
precipitation of silver halide grains, further precipitation occurs on top
of existing stable grains continuously brought into the reaction zone by
(a) the recycle path, (b) the path entrained by the expanding jets, and
(c) the path swept by the swirling action of the paddle 2. This assures
uniform pAg in the precipitation vessel 1.
In the double recycle loop or path process, silver nitrate and alkali metal
halide are injected into separate tee-mixers 14, 14a (one in each loop or
stream). The two recycle loops or paths reenter vessel 1 in close
proximity, as shown in FIGS. 1, 4 and 5, and emerge vertically downward at
6a and 7a with velocities in the range previously stated above. The jets
expand in a 20.degree. angle conical envelope (FIG. 6) to promote the high
degree of turbulence and intense agitation needed to obtain nearly
stoichiometric or equal strength volumetric mixing. They also confine the
chemical reaction to volumes directly below and adjacent to the reentry
points of the jets. Precipitation occurs on top of existing stable grains
as described in the single recycle process.
The invention is illustrated in the Examples which follow, of which
Examples 2 and 6 constitute the best mode.
EXAMPLE 1
A gelatin solution (A) comprising gelatin, ammonium hydroxide, ammonium
nitrate, and distilled water, was prepared and digested in a conventional
manner, and was placed in precipitation vessel 1, heated to 110.degree. F.
(43.3.degree. C.) and agitated.
An aqueous solution of 3 molar silver nitrate (B) was prepared in a
conventional manner, and placed in supply vessel 22, maintained at room
temperature.
An aqueous solution of ammonium bromide (C), prepared from 2058 g, NH.sub.4
Br and 6062 ml. H.sub.2 O was placed in supply vessel 23, maintained at
room temperature.
The digested gel solution (A) was fed into the recycle paths 6,7 at a flow
rate of 1.6 gallons per minute (6.06 liters per minute) in each path.
Silver nitrate solution (B) and ammonium bromide solution (C) were
simultaneously added to respective recycle paths 6,7 at flow rates of 15
ml/min. through tee-mixers 14, 14a. A dispersion of initially precipitated
silver bromide grains was formed in vessel 1 and continuously recycled at
a recycle ratio of 253. Silver nitrate and ammonium bromide solution were
continuously added to the recycle paths 6,7 for 15 minutes at a
temperature of 110.degree. F. (43.3.degree. C.) and the pAg in
precipitation vessel 1 was maintained at 8.5. The reentry velocity in
vertical downpipes 6a and 7a was 8 ft./sec. (2.44 m/sec.). Continuous
addition of silver nitrate and ammonium bromide solution to the respective
recycle paths was continued for another 45 minutes while the pAg in vessel
1 was maintained at 9.0, the recycle flow rate in each path was increased
to 2.5 gal./min. (9.46 liters/min.) and the reentry velocity of the jets
was increased to 12.5 ft./sec. (3.81 m/sec.). The dispersion in vessel 1
was continuously recycled at a recycle ratio of 79.
At the completion of the above-described addition of silver nitrate and
ammonium bromide solution, the dispersion of silver halide grains in
vessel 1 was quenched by the addition of distilled water at approximately
72.degree. F. (22.2.degree. C.) and by circulating chill water at
55.degree. F. (12.8.degree. C.) in the jacket of vessel 1 till the
temperature of the contents of the vessel was lowered to 84.degree. F.
(28.9.degree. C.). The vessel contents were coagulated and washed in a
conventional manner to produce emulsion curds.
Subsequently, the curds were redispersed, sensitized and coated on a
photographic support as is well known in the art of manufacturing
photographic films, to produce a film having high sensitivity and good
image quality. The grain structure, determined by electron micrographs of
the silver halide grains, was cubic.
The mean grain volume (MGV) and grain size distribution (GSD), determined
by using histograms obtained from a Particle Size Analyzer which is
discussed in "Photographic Science and Engineering", Vol. 17, Number 3,
May/June 1973, pgs. 295-298, were 1.4 cubic microns and 1.0 to 1.9 cubic
microns, respectively.
EXAMPLE 2
An aqueous gelatin solution (A) was prepared and digested in a conventional
manner, and was placed in precipitation vessel 1, heated to 110.degree. F.
(43.3.degree. C.), and agitated.
An aqueous solution of 3 molar silver nitrate (B) was prepared in a
conventional manner, placed in supply vessel 22, and heated to a
temperature of 110.degree. F. (43.3.degree. C.).
An aqueous solution of ammonium bromide (C), prepared from 3800 g. NH.sub.4
Br and 11,200 ml. H.sub.2 O, was placed in supply vessel 23, and heated to
a temperature of 110.degree. F. (43.3.degree. C.).
The digested gelatin solution was fed into the recycle paths 6,7 at a flow
rate of 2 gallons per minute (7.57 liters per minute) in each path.
Aqueous silver nitrate and ammonium bromide solutions were simultaneously
added to these recycle paths as described in Example 1, through the side
stream of tee-mixers 14, 14a for 80 seconds while the pAg was maintained
at 8.5 in the precipitation vessel. The reentry velocity for each path was
24 ft./sec. (7.32 m/sec.) during the entire precipitation. An aqueous
solution of ammonium hydroxide was then added to the precipitation vessel
and precipitation continued at a pAg of 9.5. After 3 moles of the aqueous
silver nitrate solution were added over 6.5 minutes, a mixed halide
solution (KCl, KI, and NH.sub.4 Br) was introduced at the tee-mixer in
place of the ammonium bromide solution and precipitation continued at the
same rate for a further 10 minutes. The remaining silver nitrate and
aqueous halide solutions were then added in 16 minutes, so as to form a
dispersion of mixed crystal silver halide grains. These were immediately
quenched, cooled, coagulated and washed, redispersed, sensitized, and
coated in the manner described in Example 1.
The resulting film was a useful X-ray type film having high sensitivity and
good image quality. The grain structure, determined by electron
micrographs of the chloride-modified core-shell type silver iodobromide
grains, showed the emulsion consisted of rounded cubic grains. The grain
size distribution (GSD), determined by using histograms obtained from the
Particle Size Analyzer (PSA), was between 0.6 and 1.8 cubic microns and
the mean grain volume (MGV) was found to be 1.07 cubic microns.
EXAMPLE 3
A chloride-modified core-shell type silver iodobromide emulsion was
prepared by the procedure described in Example 2, with the exception that
the reentry jet velocity of the recycle paths was changed to 9 ft./sec.
(2.74 m/sec.) while maintaining the same flow rate.
A similar emulsion resulted, having a GSD of between 0.45 and 1.8 cubic
microns and a MGV of 0.95 cubic micron.
EXAMPLE 4
A chloride-modified core-shell type iodobromide emulsion was prepared using
the procedure described in Example 2, with the following exceptions: (1)
the halides were added directly to the vessel 1 and the silver nitrate was
added to a single recycle path through a tee-mixer, (2) the recycle flow
rate was maintained at 3.8 gallons per minute (14.38 liters per minute)
and the recycle path jet reentry velocity was 18 ft./sec. (5.49 m/sec.),
and (3) the halide path velocity ranged from 1-2 ft./sec. (0.3-0.61
m/sec.) at nucleation to 12 ft./sec. (3.66 m/sec.) at full growth.
The resulting film was a useful X-ray type film having high sensitivity and
good image quality. Electron micrographs showed the emulsion consisted of
rounded cubic grains, with a GSD of between 0.6 and 1.5 cubic microns, and
a MGV of 0.93 cubic micron.
EXAMPLE 5
An aqueous gelatin solution (A) was prepared and digested in a conventional
manner, and was placed in vessel 1, heated to 110.degree. F. (43.3.degree.
C.), and agitated.
An aqueous solution of 3 molar silver nitrate (B) was prepared in a
conventional manner, placed in the supply vessel 22, and heated to a
temperature of 110.degree. F. (43.3.degree. C.)
An aqueous solution (C) of ammonium bromide, prepared from 95,000 g.
NH.sub.4 Br and 280,000 ml. H.sub.2 O, was placed in the supply vessel 23,
and heated to a temperature of 110.degree. F. (43.3.degree. C.).
The digested gelatin solution was fed into the recycle path 6 at a flow
rate of 50 gallons per minute (189.27 liters/minute). Aqueous silver
nitrate solution was added to the recycle path as described in Example 1,
through the side stream of a tee-mixer 14, and the aqueous ammonium
bromide solution was added directly to vessel 1 for 80 seconds while the
pAg was maintained at 8.5. The reentry velocity of the recycle path was 28
ft./sec. (8.53 m/sec.) during the entire precipitation, while the halide
reentry velocity varied from 1.2 ft./sec. (0.37 m/sec.) at nucleation to
12 ft./sec. (3.67 m/sec.) at full growth. Aqueous ammonium hydroxide was
then added to the conversion vessel and precipitation continued at a pAg
of 9.5. After 60 moles of aqueous silver nitrate solution were added over
6.5 minutes an aqueous solution of mixed halides (KCl, KI, NH.sub.4 Br)
was added directly to vessel 1 in place of the aqueous bromide solution
and precipitation continued at the same rate for a further 10 minutes. The
remaining silver nitrate and aqueous bromide solutions were then added in
16 minutes to form a dispersion of mixed crystal silver halide grains.
These were immediately quenched, cooled, coagulated and washed,
redispersed, sensitized and coated in a manner described in Example 1.
The resulting film had high sensitivity and good image quality. The grain
structure, determined by electron micrographs, comprised round cubic
grains having a MGV of 1.25 cubic microns and a GSD of between 0.6 and 2.1
cubic microns.
EXAMPLE 6
An aqueous gelatin solution (A) containing traces of rhodium was prepared
and digested in conventional manner, and was placed in precipitation
vessel 1, heated to a temperature of 120.degree. F. (48.9.degree. C.), and
agitated.
3 molar silver nitrate (B) was prepared in a conventional manner, placed in
a supply vessel 22, and heated to 120.degree. F. (48.9.degree. C.).
An aqueous solution (C) of a mixed halide (NaCl-NaBr) was prepared and
heated to 120.degree. F. (48.9.degree. C.).
The digested gelatin solution (A) was fed into the recycle paths 6,7 at a
flow rate of 2 gallons per minute (7.57 liters per minute) in each loop.
Aqueous silver nitrate (B) and mixed halide solution (C) were
simultaneously added to their respective recycle paths as in Example 1
through the side stream of tee-mixers 14, 14a. The dispersion of the
initially precipitated grains was continuously recycled at a recycle ratio
of 79. Silver nitrate and the mixed halide solution were continuously
added to the recycle paths for 5 minutes at a temperature of 120.degree.
F. (48.9.degree. C.) and the pAg in the conversion vessel maintained at
5.1. The reentry velocity of the jets was 26 ft./sec. (7.93 m/sec.).
Continuous addition of silver nitrate and the mixed halide solutions to
the respective recycle paths was continued for another 23 minutes at
120.degree. F. (48.9.degree. C.), while the pAg in the precipitation
vessel was maintained at 6.7. The dispersion in the precipitation vessel
was continuously recycled at a recycle ratio of 20. This dispersion of
silver halide grains was immediately quenched, cooled, coagulated, and
washed, redispersed, sensitized and coated in the manner described in
Example 1.
The resulting film was a useful lithographic film having high sensitivity
and good dot quality. Electron micrographs of the silver chlorobromide
grains revealed that they were rounded cubic grains with a GSD of between
0.0016 and 0.013 cubic micron and a MGV of 0.0058 cubic micron.
Industrial Applicability
The process of the invention is useful for preparing silver halide
emulsions for photographic films having controlled grain structure, size
and size distribution over a wide range, for example, cubic, mixed cubic
and octahedral, or octahedral grains, having median particle sizes in the
range of 0.004 to 3.0 cubic microns and a size distribution .alpha. in the
range of 0.13 to 0.45 cubic micron. Temperature and pAg may be varied over
a wide range in the recycle paths, and precipitation vessel to achieve the
desired grain structure, size and size distribution. For example,
temperature in the path and precipitation vessel may be separately
controlled within the range of 100.degree. F.-160.degree. F. (37.8.degree.
C.-71.1.degree. C.), and pAg may be controlled within the range of 5-11 at
precipitation, to achieve the grain structure desired.
In summary, the ranges of parameters for the double loop systems are:
1. Jet reentry velocity--8-30 ft./sec. (2.44-9.14 m/sec.).
2. Jet reentry point--0.25-0.35 times the vessel diameter from the bottom
of the vessel 1.
3. Jet reentry point location--radially from the center of the vessel 1 at
a distance of 0.3-0.4 times vessel diameter.
4. Jet orifice size--3-5% of distance mentioned in item 2.
5. Distance between jet tips at reentry point--0.5-1.25 inches (1.27-3.175
cm) and independent of vessel size.
The process of the invention has been found particularly useful for
producing fine grain, rounded, cubic structure silver halide emulsions
useful in lithographic and X-ray type films.
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