Mixing device

Abstract: The invention relates to a mixing device (10) for mixing inter-reactive fluids comprising a generally cylindrical vessel having a stirring device (20, 22) rotatable about the central axis thereof. When the stirring device is rotated about said central axis the resulting mixing and turbulence of the fluid contained within the vessel is such that radial displacement of any particle of fluid is minimised as compared with displacements perpendicular to the radius.

Description: This invention relates to a mixing device which is of particular utility for the mixing of fluid components whilst they undergo some species of chemical or physical reaction, or other time-related process, such that it is important to control the residence time of the fluid components in the mixer. The mixer which forms the subject of the present application has been evolved primarily for use in formulating and mixing photographic emulsions, although the mixer may also find utility in other chemical processes or in the manufacture of pharmaceuticals, for example. In a preferred embodiment of the invention, utilised in the manufacture of photographic emulsion, the mixer being utilised at a stage in the process in which the input to the mixer is provided by a nucleation device, such as that disclosed in EP-A-0 734 761 which provides, to an input of the mixer of the invention, a fluid emulsion precursor comprising a fluid gelatine solution carrying minute crystals or "seeds" of silver halide, the solution further comprising a mixture of as yet unreacted components of the emulsion, principally halogen salts and silver nitrate. The function of the mixer embodying the invention, in this scenario, is to mix the components of the emulsion continuously so as to ensure, so far as possible, uniform silver halide crystal growth over a predetermined reaction time and to supply further halogen salts and silver nitrate solution in a controlled manner to the emulsion passing through the mixer so as to ensure.a progressive and controlled growth of the silver halide crystals or "grains". According to the invention there is provided a mixing device, particularly suitable for mixing inter-reactive fluids over a predetermined period on a continuous flow basis, the device comprising a vessel which is generally cylindrical about a central axis thereof and has an end wall perpendicular to said axis, and a cylindrical peripheral wall coaxial therewith, the depth of said vessel measured along the axis being small in relation to the diameter measured perpendicular to said axis, the device including a stirring device which is rotatable within the vessel about said central axis the stirring devices comprising a central portion and a plurality of bars extending radially from the central portion, each bar extending from the central portion to a position closely adjoining the peripheral wall and having a height approximately half that of the peripheral wall, such that when the stirring device is rotated about said central axis through fluid contained in said vessel, the resulting mixing and turbulence is such as to minimise radial displacement of any particle of such fluid by the stirring device, as compared with displacements perpendicular to a radius through such particle relative to said axis, said vessel having an inlet port opening into the vessel and an outlet port from the vessel, one of said ports being disposed substantially on said axis and the other of said ports being disposed at the periphery of the vessel, whereby, in operation, with the stirrer rotated about said axis, the fluid flowing from one said port to the other is caused to follow a spiral path, about said axis, in the vessel. An embodiment of the invention is described below by way of example with reference to the accompanying drawings, in which:- Figure 1 is a schematic perspective view, partly in phantom, illustrating a mixer embodying the invention, Figure 2 is a schematic axial section view of the mixer, Figure 3 is a schematic plan view of a part of a photographic emulsion manufacturing plant incorporating the mixer, Figure 4 is a partial, less schematic view, in axial section, through a preferred embodiment of the mixer, Figure 5 is a fragmentary plan view of a portion of the mixer of Figure 4 with a cover portion thereof removed, Figure 6 is a fragmentary side elevation view, as viewed in the direction of the arrow VI in Figure 5, Figure 7 is a plan view of a solution injector incorporated in the mixer of Figures 4 to 6, Figure 8 is an end view of the injector of Figure 7, Figures 9 and 10 are respectively an underneath plan view and a side view of a component of the injector, Figure 11 is a schematic axial section of the interior of the mixer, Figure 12a shows a modified mixing bar; Figure 12b is a cross section through Figure 12a Figure 13 is a schematic axial section view of the interior of the mixer. Figure 14 is a fragmentary plan view of the mixer with the lower portion removed, and Figure 15, 16 and 17 illustrate further shapes and cross sections of bars that may be used. Referring to the drawings, the mixer comprises a hollow casing having the general form of a flat circular disc or plate, within which rotates, about the central axis of the disc, a stirring device comprising a plurality of bars extending radially within the casing from a central shaft rotatably mounted in the casing to adjacent a peripheral wall of the casing. For convenience, the mixer illustrated is hereafter referred to, for convenience, as a "plate mixer". Referring more particularly to Figures 1, 2 and 4, the mixer, indicated generally at 10, comprises top and bottom walls 12 and 14 each in the form of a flat circular metal plate, the top and bottom walls being arranged parallel with one another and spaced apart axially by a cylindrical side wall 16 coaxial with the walls 12 and 14. The top and bottom walls, 12 and 14, may be provided with ribs as shown in Figure 11. These ribs stop radial flow at the surfaces of the walls. As illustrated in Figure 4, the side wall 16 may be welded along its lower edge to the bottom wall 14 and the top wall 12 may be detachably secured to the top edge of the side wall, with the interposition of a suitable gasket or sealant, by tie bolts, (not shown), extending through the walls 12 to 14 adjacent their peripheries and arranged at intervals around the mixer, so that when necessary, e.g. for access for cleaning or repair, the top plate may be detached from the remainder. As will be noted, the top, bottom and side walls thus define between them a chamber of generally cylindrical configuration which is of a diameter substantially greater than its axial length, for example ten or more times greater. Mounted within this chamber is a stirrer assembly comprising a plurality (four in the example illustrated) of cylindrical stirrer bars 20 extending radially from a central shaft 22, which is coaxial with the mixer casing and is mounted for rotation with respect to the casing about the axis of the casing. Each stirrer bar extends from the shaft 22 to a position closely adjoining the side wall 16 and may, as shown, be spaced equally from the top and bottom wall. Each bar 20 is of a diameter around half the axial spacing between the top and bottom walls. The bars 20 may be tapered to compensate for pressure variations across the radius of the mixer. Pressure variations may induce unwanted flow paths. Other shapes of bars, with varying cross-sections may be used, see for example Figures 12a, 12b, 15, 16 and 17. As illustrated in Figure 4, the shaft 22 may be rotatably mounted in ball bearings supported in a bearing housing 26 fitted at its upper end in a central aperture in the bottom wall 14 and welded or brazed to the wall 14. The bars 20 may, as illustrated, be integral with (e.g. welded to) to shaft 22 and are preferably arranged at regular intervals about the mixer axis, (e.g. at 90° intervals, where, as in the Figures, there are four such bars extending from the shaft 22). The top wall 12 has a central discharge port from which extends upwardly a coaxial discharge duct 28 welded or brazed to the top wall. As shown in Figure 6, the mixer has a main inlet port in the side wall 16, which communicates with an inlet pipe 32 which may, as shown in Figure 3, extend tangentially with respect to the periphery of the mixer. Referring to Figure 3, in an installation for making photographic emulsion for application to film or paper the fluid precursors of the emulsion are first treated in a nucleation device 11 such as described in EP-A-0 734 761 in which small crystal nuclei or "seeds" of silver halide are formed in the fluid, which is then passed to the mixer 10 for growing or "ripening" of the "seeds", to form silver halide grains of the desired size. The fluid from the nucleation device 11 comprises, inter alia , a solution, in water of gelatine, silver nitrate and halide salt, with the aforementioned silver halide "seeds" or nuclei in suspension. Nuclei produced in the nucleation device 11 are extremely volatile in numbers and size. Where solution conditions in such an installation are suitable for growth of seed grains, it is found that whereas large grains increase in size by the "ripening" process, the smaller grains tend to dissolve so that the larger grains grow at the expense of the smaller ones. This effect tends to distort the grain size distribution in ways which are not always desirable. The effect is exaggerated where the solution conditions are different in different regions of the volume of fluid being processed, and/or where some portions of the fluid processed are subjected to the "ripening" process for longer than other portions. In order to preserve the number of grains produced by the nucleation device 11, the plate mixer 10 enables the grains to be continually supplied with silver, salt and halide solutions in order that the grains may ripen without losing numbers. The fluid emulsion/solution supplied to the plate mixer flows, in the mixer 11, in a path which, on a macroscopic scale, has the form of a spiral centred on the mixer axis as illustrated by the arrows in Figure 1, so that each volume of fluid entering the mixer via the inlet 32 travels around the mixer axis several times during its passage from the periphery of the mixer 10 to the outlet 28 at its centre. Nuclei or grains entering via inlet 32 thus progress naturally to the outlet port situated in the centre upper plate of the cylinder. The rotational spiral movement of the fluid within the mixer from inlet 32 to outlet 28, engendered and maintained by the rotation of the stirrer bar assembly, is quasi-laminar as viewed in plan so that a "first in first out" pattern of flow is thus established, i.e. so that each elementary volume of fluid introduced via inlet 32 follows substantially the same spiral path to the outlet 28 as the preceding and succeeding such elementary volumes and spends the same time in the mixer as the preceding and succeeding such elementary volumes. However, as illustrated in Figure 2 the stirrer bars 20, in passing through the fluid in the mixer, produce turbulent flow in the form of trains of vortices about axes parallel with and close behind the bars 20 causing them, so that each bar causes mixing, at each radial position along the bar, substantially only in a plane perpendicular to the radius along which the bar 20 extends (or rather in a localised region of a notional cylindrical surface coaxial with the mixer 11). There is thus substantially no stirring in a direction radially of the plate mixer, so that "first in first out" flow path is not destroyed. The angular velocity of the stirrer bars may be around twice that of the fluid passing through the mixer. As the fluid emulsion passes through the mixer, the silver halide grains therein, as they grow, of course remove silver and halide ions from the solution and these must be replaced in order to allow the grain growth or ripening to continue. To this end, the top and bottom walls of the mixer are provided with arrays of inlets 40 and 42 respectively through which fresh silver solution and halide solution respectively are introduced into the mixer and added to the emulsion already circulating therein. In view of the fact that, as noted above, there is no or very little mixing of the fluid within the mixer within the plane of the mixer, in order to avoid the occurrence of spiral bands of different silver and/or halide concentrations in the fluid flow through the mixer, an ideal arrangement would be for each of the fresh solution inlets to the mixer to extend continuously along a radius thereof. However, it is also desirable to control the rate of addition of fresh solution at different radial distances from the centre of the mixer, which is difficult with a single slot extending radially from the periphery to the centre of the mixer and fed from a single source. Accordingly, the fresh solution inlets are contrived of linear arrays of elongate radially extending slots 40, 42, each of a length small in relation to the radius of the mixer, but with a radial spacing between adjacent such slots being even smaller, each of said slots being independently supplied with the respective solution at a controlled rate. In order to avoid any temporary preponderance of silver solution over halide solution or vice versa , the inlets 40, 42, for both solutions are preferably disposed at the same angular position about the axis of the mixer, the preferred arrangement being, as illustrated, with the inlets 42 for one solution being provided in the bottom wall and the inlets 40 for the other solution being provided directly above these inlets, in the top wall. To ensure thorough mixing of the fresh solutions paddle 5 bars may be mounted at different heights along the bar, as illustrated in Figure 13. It will be understood that, in order to prevent premature interaction between the fresh silver solution and the halide solution fed to the mixer, without the presence of gelatine, separate inlets are provided for, on the one hand, the silver solution and, on the other hand, the halide solution. In the arrangement illustrated, (cf. Figure 2) a single such radial array of radially extending slots 40 is provided in the top wall 12 of the mixer, for the silver solution and a similar radial array of radially extending slots 42 is provided in the bottom wall 14 of the mixer for the sodium or potassium halide solution. In the preferred embodiment, each individual slot 40 or 42 is supplied with the respective solution by a respective constant delivery pump, such as a peristaltic pump (not shown). In this preferred embodiment, the peristaltic pumps comprise respective rotors mounted on a common driving shaft and co-operating, via rollers carried by the rotors, with respective lengths of resilient, (e.g. rubber or plastics) tubing. In this arrangement, the delivery rate of each peristaltic pump is determined by the internal diameter of the respective length of tubing which is selected, (and the fittings, etc. for such tubing correspondingly selected) to provide the desired flow rate. However, it will be appreciated that other arrangements may be equally effective, for example, individual peristaltic or other constant delivery pumps may be driven by respective electronically controlled motors at appropriately selected speeds. 5 Referring to Figure 5 and Figures 7 to 10, it is convenient to provide the inlets 40 or 42 for each of the fresh solutions in a respective single radially elongate manifold 50 having an insert portion 54 engaged in a correspondingly shaped radially elongate slot 52 in the respective top or bottom wall 12, 14 of the mixer. These slots 52 are indicated in Figure 4 (in which the manifolds 50 are not shown), and in Figure 5. As illustrated in Figures 7 and 8, the insert portion 54 may extend from a wider portion of the manifold block 56 so that a flat face or shoulder 58 is provided which entirely surrounds the periphery of the insert portion 54 and engages, with the interposition of a sealing gasket or adhesive/sealant (not shown) the underside of the bottom wall 14, (where the manifold is fitted in the latter with the insert portion 54 extending upwardly) or the upper surface of the top wall 12, (in the case of the manifold fitted to the top wall with its portion 54 projecting downwardly from the block 56 into the respective aperture 52). As illustrated in Figures 7 to 10, the manifold may comprise three parts, namely a unitary backing block 60 formed with the plurality of vertical through bores 61 arranged in series along the block 60 and two mating blocks 62 and 64 which between them define a plurality of inlet passages each extending from a respective said vertical bore 61 to a respective elongate slot 66 in the face of the composite block (62, 64) remote from block 60. For convenience, in the illustrated construction, the face of the block 62 facing the block 64 is a plane flat face, the inlets and slots 66 being formed by grooves or channels cut in the face of block 64 facing block 5 62. In view of this, the row of vertical through bores 61 is displaced laterally towards the block 64. It will be noted that each groove defining, with the opposing planar face of block 62, a respective solution inlet terminating in a respective slot 66, is widened and flattened progressively from the face of block 60 to the respective slot 66, allowing sufficient spacing between the vertical bores in block 60 to accommodate appropriate connectors leading from the respective pumps, etc. whilst minimising the radial spacing between adjacent ends of adjacent slots 66 in the manifold. It will be understood, of course, that the manifold could be constructed in any of a number of different ways so as to achieve the same or a similar desired configuration. The peripheral wall 16 is ideally internally radiused rather than circular. In practice, provided that the operating conditions are such that the radial depth of each "turn" of the spiral flow within the mixer is relatively small, appropriate spiral flow conditions can be maintained with a wall 16 of simple circular shape (as viewed in plan) using a simple corrective structure at the inlet 32 to the mixer, for example by arranging.for the inlet pipe to extend tangentially to the peripheral wall of the mixer as illustrated in Figure 3 and/or by providing, as shown in Figures 5 and 6, a flexible resilient shim 70 secured at an upstream end thereof to the interior surface of the wall 16, just upstream of the inlet 32, the upper and lower edges of the shim being closely adjacent the interior surfaces of the top and bottom walls 12 and 14 respectively and the shim 70 extending in the circumferential direction for some distance past the inlet 32, the shim 70 being further so configured that, in an unstressed position, it is spaced progressively from the wall 16 circumferentially in the direction of flow within the mixer. With this arrangement, the shim 70 can, if necessary, be deflected radially outwardly sufficiently by the outer ends of the stirrer bars to allow the latter to pass during their rotation within the mixer, but nevertheless defines, with the opposing surface of wall 16, an appropriate volute-like inlet passage for the emulsion fluid. As indicated in broken lines in Figure 5, the inner surface of the wall 16 facing the free portion of the shim may be relieved or recessed so as to define with the shim 70 an inlet passage of substantially constant flow cross section. Alternately, and preferably, the structure of the inlet to the mixer may be configured so that the outer wall of the mixer upstream of the inlet 32 is tapered in a circumferential direction past the inlet to form a rigid member 68 to guide the inward flow of the emulsion fluid. The inner side of the outer wall of the mixer downstream of the inlet and facing the tapered portion 68 may be relieved or recessed as described above. This arrangement is illustrated in Figure 14. Correspondingly, in order to ensure a relatively smooth flow of the fluid emulsion from the mixer via the outlet 28, the bottom interior surface of the mixer, in the region immediately opposite the outlet 28, is configured as a chamfered boss or dome projecting slightly upwardly relative to the major part of the bottom wall. In the arrangement 5 illustrated, this dome or boss is provided by the upper end of the bearing mounting block 26 which is fitted in a central aperture in the plate which forms the major part of the bottom wall 14. As illustrated, in order to prevent leakage of the emulsion between the shaft 22 and the bearing block 26, upper and lower seals 82 and 84 are provided, the space between the upper and lower seals being supplied with pressurised water via ducting 86, with a drain 88 being provided for the escape of any water passing the lower seal 84. In operation of the apparatus described, silver nitrate solution and halide solution are supplied to the inlet slots 66 in order to ripen the nuclei as they progress through the plate mixer. The flow rate from the inlet slots 66 increases progressively towards the centre of the plate to keep pace with the maximum possible growth rate which follows a cube law. (As noted above, these flow rates are determined by the respective peristaltic pumps). In operation, fluid emulsion incorporating gelatine and silver halide grains or seeds is supplied to mixer 11 via inlet 32 whilst the stirrer 20 is rotated continuously. Fresh silver and halide solutions are added continuously, as described above, via inlets 40 and 42. Thus, as the mixture is stirred rapidly by the stirrer bars 20, each elementary volume of fluid in the mixer passing the inlets 40, 42, at one radial position will have corresponding fresh amounts of the silver and halide solutions added thereto and will return rapidly, through the spiral circulation of the fluid in mixer 10, for a further dose of the fresh solutions, until the respective 5 volume, with its complement of "grown" silver halide grains, leaves the mixer via outlet 28. Provided the bars 20 move at a sufficiently high RPM, the function of the plate mixer will be independent of such bar RPM and only dependent on flow rate of nuclei and flow rate of silver and halide nuclei for given solution strengths. Whilst, for simplicity, reference has been made herein to silver solution, halide solution, and gelatine, it will be understood by those skilled in the art that many other significant components will be present in, or added to the emulsion in any practical emulsion making process, and that some of these components will be present in the fluid supplied via inlet 32 and some will be present in the fresh solutions added via inlets 40 and 42 for example. It is anticipated that the plate mixer 11 described will provide nuclei in the desired numbers, grown to a stable size, in a very repeatable way. The mixer described is compact, as compared with 5 the multiplicity of processing vessels which would traditionally be provided for a similar output, and should require only minimal maintenance, since the continuous flow of product through the mixer avoids the need for a regular cleaning phase in the operating cycle. If desired, a plurality of mixers 11 may be connected in series. Where, as in the manufacture of photographic emulsion, the process carried out in the mixer 11 is temperature sensitive, the temperature of the mixer and its contents may be maintained readily by housing the mixer within an air chamber with which is associated air-temperature regulating means, such as a thermostatically controlled air heater, the conduits leading to inlet 32, from outlet 28 and to inlets 40 and 42 passing through the wall of such air chamber to the mixer.

Claims: A mixing device, particularly suitable for mixing inter-reactive fluids over a predetermined period on a 5 continuous flow basis, the device comprising a vessel which is generally cylindrical about a central axis thereof and has an end wall perpendicular to said axis, and a cylindrical peripheral wall coaxial therewith, the depth of said vessel measured along the axis being small in relation to the diameter measured perpendicular to said axis, the device including a stirring device which is rotatable within the vessel about said central axis the stirring devices comprising a central portion and a plurality of bars extending radially from the central portion, each bar extending from the central portion to a position closely adjoining the peripheral wall and having a height approximately half that of the peripheral wall, such that when the stirring device is rotated about said central axis through fluid contained in said vessel, the resulting mixing and turbulence is such as to minimise radial displacement of any particle of such fluid by the stirring device, as compared with displacements perpendicular to a radius through such particle relative to said axis, said vessel having an inlet port opening into the vessel and an outlet port from the vessel, one of said ports being disposed substantially on said axis and the other of said ports being disposed at the periphery of the vessel, whereby, in operation, with the stirrer rotated about said axis, the fluid flowing from one said port to the other is caused to follow a spiral path, about said axis, in the vessel. A mixing device according to claim 1 wherein each bar is of constant shape in cross-section perpendicular to the respective radius. A mixing device according to any preceding claim wherein said outlet is disposed on the axis of the vessel and said inlet is disposed on the periphery of the vessel. A mixing device according to claim 3 wherein said vessel has at least one further inlet in said end wall at a position intermediate said axis and the periphery of the vessel. A mixing device according to any preceding claim wherein said vessel is a closed..vessel having opposing end walls perpendicular to said axis and a cylindrical peripheral wall extending between said end walls. Apparatus for processing photographic emulsion including a mixing device according to any of claims 1 to 6, having a first supplementary inlet or array of inlets in a said end wall and a second supplementary inlet or array of inlets in a said end wall, means for supplying to said inlet at the periphery of the vessel a fluid comprising silver halide grain "seeds", means for supplying to said first supplementary inlet or array of inlets a fluid component including a soluble halide solution, and means for supplying to said second supplementary inlet or array of inlets a fluid component including silver nitrate. Apparatus according to claim 6 wherein said means for supplying said first and second supplementary inlets comprise constant-mass flow delivery means. Apparatus according to any preceding claim including an air-chamber, means for maintaining the temperature within said air-chamber at a predetermined value, said mixing device being disposed within said air chamber, whereby the temperature of the fluid or fluids within the vessel is correspondingly maintained.

Inventors: BRYAN MICHAEL, GORECKI MICHAEL TADEUSZ, HAYNES TERRY DOUGLAS, BRYAN, MICHAEL, GORECKI, MICHAEL TADEUSZ, HAYNES, TERRY DOUGLAS
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1999
Main classification: B01F7
IPC codes: B01F3, B01F7, G03C1, B01F5, B01F15