Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B7/00—Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/18—Stationary reactors having moving elements inside
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/24—Stationary reactors without moving elements inside
  • B01J19/2415—Tubular reactors
  • B01J19/243—Tubular reactors spirally, concentrically or zigzag wound
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F11/00—Compounds of calcium, strontium, or barium
  • C01F11/46—Sulfates
  • C01F11/466—Conversion of one form of calcium sulfate to another
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B14/00—Use of inorganic materials as fillers, e.g. pigments, for mortars, concrete or artificial stone; Treatment of inorganic materials specially adapted to enhance their filling properties in mortars, concrete or artificial stone
  • C04B14/38—Fibrous materials; Whiskers
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/60—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape characterised by shape
  • C30B29/62—Whiskers or needles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00002—Chemical plants
  • B01J2219/00018—Construction aspects
  • B01J2219/0002—Plants assembled from modules joined together
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00761—Details of the reactor
  • B01J2219/00763—Baffles
  • B01J2219/00765—Baffles attached to the reactor wall
  • B01J2219/00768—Baffles attached to the reactor wall vertical
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/18—Details relating to the spatial orientation of the reactor
  • B01J2219/185—Details relating to the spatial orientation of the reactor vertical
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/19—Details relating to the geometry of the reactor
  • B01J2219/194—Details relating to the geometry of the reactor round
  • B01J2219/1941—Details relating to the geometry of the reactor round circular or disk-shaped
  • B01J2219/1943—Details relating to the geometry of the reactor round circular or disk-shaped cylindrical
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/54—Particles characterised by their aspect ratio, i.e. the ratio of sizes in the longest to the shortest dimension
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer

Definitions

• the present invention relates to an improved process and apparatus for the continuous production of calcium sulfate microfibers, and more particularly, to such a process and apparatus which enables closer control of the microfibers' dimensional development.
• Manmade calcium sulfate microfibers sometimes also referred to as "whisker fibers" are used for a variety of purposes, including: as fillers or reinforcements in plastics, asphalt, mineral cements, paper and paint. Generally speaking, they consist of high aspect ratio crystals of alpha hemihydrate or anhydrous calcium sulfate
• microfibers are commercially available under the trademark "FRANKLIN FIBER" from the United States Gypsum Company.
• calcium sulfate microfibers are made by heating a dilute aqueous slurry of calcium sulfate dihydrate (gypsum) particles under pressure. After going into solution, the dihydrate molecules give up one and one-half molecules of water and recrystalize as long thin crystals of calcium sulfate alpha hemihydrate. Once recovered from the slurry and dried, the hemihydrate microfibers are either heated further to drive off the remaining chemically bound water, or coated, to render them stable in the presence of moisture. Further discussion concerning the stabilizing of such microfibers is found in U.S. Patents 3,822,340 and 3,961,105.
• calcium sulfate microfibers have heretofore been produced by one of two processes.
• a finite amount of ground gypsum is mixed with sufficient water to form a charge of dilute slurry.
• the charge is heated in a pressure vessel until the gypsum is substantially converted to alpha hemihydrate crystals and then discharged to a filtering or separation device, where the microfibers are separated from the menstruum, dried and stabilized, if desired.
• microfibers per cycle typically might be about 45 minutes. While the output may be increased by increasing the solids concentration of the slurry charge, this has been shown to result in microfibers having a lower aspect ratio. Conversely, microfibers having a higher mean aspect ratio, say 100 or greater, can be produced by using a more dilute slurry charge, but with a corresponding reduction in output per cycle. Expanding the capacity of the batch reactor requires additional capital expenditure which offsets some or all of the savings that might be obtained through higher productivity.
• Calcium sulfate microfibers have been produced more economically by a continuous process. Again, finely ground gypsum particles and water are mixed together to form a slurry having, in this process, up to about 30% solids by weight.
• the slurry is fed through a progressive cavity pump into a continuous stirred tank reactor or a plurality of such reactors connected in series.
• the reactor consists of a cylindrical pressure vessel having 3-6 stages or zones connected in series, such as generally described in U.S. Patent 3,579,300. Steam is introduced, either premixed with the slurry or separately, into the first stage of the reactor to bring the temperature up to the desired conversion temperature and autogeneous pressure.
• Each stage of the reactor is provided with a separate agitator to maintain the calcium sulfate particles in suspension.
• the slurry passes progressively through the several stages of the reactor with a typical elapsed residence time in the range of 4-10 minutes, and is then depressurized to atmospheric pressure. Again, the microfibers are separated from the slurry menstruum and further processed as desired.
• the continuous process significantly improves productivity over the batch process. For example, a typical facility has produced microfibers at a rate in excess of 1200 pounds per hour. However, this continuous process facility has also, so far, been limited to producing microfibers having a maximum length to diameter ratio of up to about 45. Since, microfibers having a higher aspect ratio than that are needed for many applications, an improved continuous process capable of producing longer microfibers was needed.
• the aqueous slurry containing finely ground gypsum particles, is premixed with superheated steam and pumped under pressure through a smooth interior, continuous hollow conduit, under "plug-flow" condition.
• the length of the plug-flow reactor is determined to provide sufficient residence time for substantial conversion of the dihydrate to the hemihydrate crystals. Because there is no agitation or other disturbance of the generally laminar flow through the reactor, the dissolved calcium sulfate alpha hemihydrate nucleates and grows into long needle-like crystals. For example, using a plug-flow reactor having a residence time of about 90-100 seconds, microfibers having an aspect ratio above 80 have been
• the diameter of the microfibers made by this process usually do not exceed about 1.0 micron, although on occasion slightly larger diameters have been observed. Furthermore, because of the general absence of turbulence, and therefore little, or no, mixing of the slurry, it is difficult to achieve complete conversion of the feed gypsum material in the plug-flow reactor.
• An apparatus comprises, in combination: a pressure raising progressive cavity feed pump, a slurry/steam mixing value, a continuous plug-flow reactor, a continuous stirred tank reactor, and a pressure reducing progressive cavity let-down pump.
• the unique plug-flow reactor consists of an elongated conduit, such as a pipe, having a smooth uninterrupted interior of substantially constant cross-section. It is connected between the steam/slurry mixing valve and the stirred tank reactor.
• the continuously stirred tank reactor comprises one or more cylindrical vessels connected in series, each being substantially larger in diameter than the plug-flow reactor, and each having its own agitator for keeping the solids in suspension and promoting particle-to-particle contact.
• the stirred tank reactor is connected between the plug-flow reactor and the pressure let-down pump. All the components of this apparatus and the connections between them are pressure rated and capable of maintaining the slurry at a
• a preheated dilute aqueous slurry having about 1/2%-15% gypsum by weight, is fed through the pressure raising feed pump to the mixing valve where it is proportionally merged and mixed with saturated steam and very quickly raised to about 285°F.
• the turbulent hot slurry is subsequently quieted to substantially laminar flow, and maintained at the desired temperature and autogeneous pressure as it moves as "plug flow" through the pipe reactor long enough for the gypsum particles to convert to calcium sulfate alpha hemihydrate.
• the dissolved hemihydrate nucleates and forms slender, needle-like seed crystals.
• the residence time in the plugged-flow reactor is typically on the order of 30 seconds.
• the slurry which now contains high aspect ratio seed crystals of alpha hemihydrate plus some unconverted gypsum particles, is fed directly into the higher volume, stirred tank reactor. Owing to the greater volume of this reactor, the overall flow of the slurry is reduced and the residence time in the total reactor may be in the order of 4-10 minutes. During this time the slurry is constantly agitated to keep the solids in suspension and cause contact between suspended particles, including unconverted gypsum particles and seed crystals. This promotes both a radial and axial growth of the seed crystals, resulting in the larger diameter, yet high aspect ratio, microfibers.
• the slurry progresses through the successive zones of the stirred tank reactor until substantially all of the gypsum has been converted.
• the final product slurry passes from the stirred tank reactor to a reverse rotating progressive cavity pump which reduces the slurry to normal atmospheric pressure.
• the slurry is then filtered, or otherwise dewatered, leaving a filter cake of microfibers.
• the cake is broken up, and the microfibers are dried and packaged.
• the microfibers can be further calcined, or coated, to stabilize them against moisture absorption before packaging.
• the process, and implementing apparatus, according to this invention offers many advantages over the prior art processes. It produces calcium sulfate microfibers at a substantially greater rate and lower cost than the batch process. It can produce much higher aspect ratio fibers than the previous continuous stirred tank reactor process alone, and with a more complete conversion ratio than the improved plug-flow reactor alone. And, with the invention, it is feasible to produce high aspect ratio microfibers with larger diameters than were achievable in any of the previous processes. And, perhaps most importantly, giving consideration to the many control parameters that will be discussed below, the process according to the invention can be manipulated to effectively control both the nucleation of crystals and the growth and development of the crystals to produce specific
• microfibers within a generous range of predetermined dimensions.
• FIG. 1 is a schematic diagram of a process for making calcium sulfate microfibers in accordance with the invention
• FIG. 2 is a side elevational view, partially in section, of a plug-flow reactor suitable for use in the process of FIG. 1;
• FIG. 3 is a cross-sectional view taken through the apparatus of FIG. 2, along the line 3-3 in FIG. 2;
• FIG. 4 is a side-elevational view, partially sectioned, of a multi-stage, continuous stirred reactor suitable for use in the process of FIG. 1;
• FIG. 5. is a cross-sectional view taken through the apparatus of FIG. 4, along the line 5-5 in FIG. 4.
• the process illustrated in FIG. 1 begins with the mixing of ground gypsum and water in the mix tank 10 to form a dilute slurry.
• terra alba which is substantially pure gypsum ground to 99.9% minus 100 mesh, may be combined with sufficient water to form a slurry having between 1/2% and 15%, and preferably about 12%, solids by weight. It is also advantageous to heat the slurry up to about 200°F at the mixing tank stage.
• the slurry is fed from the mixing tank to the intake side of the feed pump 12.
• the progressive cavity pump 12 increases the pressure of the slurry up to about 80 psi and discharges it into the slurry/steam mixing valve 20.
• Saturated steam at about 350°F is also supplied to the valve 20 and metered directly into the hot slurry as shown more clearly in FIG. 2.
• the mixing valve 20 consists of a housing 22, a steam inlet 23, a slurry inlet 24, a needle valve comprising: valve seat 25 and axially adjustable valve stem 26 with handle 27, and a discharge outlet 28 which can be connected directly to the plug-flow reactor 30.
• the hot steam enters through the inlet 23 and is directed through the opening between the needle valve stem 26 and valve seat 25; the needle valve being adjustable to regulate the steam feed.
• the slurry enters through the inlet 24 which is provided with a restricted orifice 29.
• the slurry is brought up to a pressure on the order of about 80 psi, which is substantially higher than the pressure downstream of the orifice 29. Consequently, the slurry flashes into the mixing valve as a spray or fine stream directly in the path of the accelerated jet of steam coming through the needle valve.
• the incoming slurry is continuously metered with, directly impinged upon by, and mixed with the hotter steam.
• the result is an almost instantaneous transfer of heat to very quickly raise the temperature of the slurry and entrained calcium sulphate particles to the predetermined process temperature, for example, of about 285°F. It is believed that this flash heating is a very advantageous step in forming acicular seed crystals.
• the hot slurry flows from the mixing valve 20 through the outlet 28 directly to the plug-flow reactor 30 which consists of an elongated hollow conduit.
• the diameter of the plug-flow reactor is largely determined in order to provide a flow rate which is high enough to prevent settling of the suspended particles, but not so high as to cause turbulence.
• the length of the conduit is primarily determined to provide sufficient residence time during which the calcium sulfate dihydrate particles will be converted to calcium sulfate alpha
• the interior surface 31 of the plug-flow reactor be as smooth and frictionless as reasonably possible, since surface imperfections tend to provide nucleation and attachment sites for crystals which then cause a build-up and eventual plugging of the conduit.
• the plug-flow reactor 30 must, of course, be strong enough to withstand pressures autogenous with process temperatures in the range of 285°F or higher; such pressures typically being about 40-50 psi. It is also desirable to insulate the reactor against heat loss.
• the hot pressurized slurry moves through the pipe reactor 30 as "plug-flow", i.e. wherein each incremental part of the liquid moves almost like a solid, maintaining its place between surrounding
• the continuous stirred tank reactor 40 as shown in FIGS. 4 and 5, comprises one or more sections, or stages, 41a, 41b and 41c
• a typical section 41 of the reactor may be about 15 inches in diameter and 15 inches in height.
• the vertically stacked sections of the reactor are serially connected to each other by flanges 43.
• the hot slurry from the plug flow reactor is introduced through an inlet 45 at the bottom of the first stage of the continuous tank reactor and flows upward to exit through an outlet 46 at the top of the last section of the reactor.
• the reactor 40 is pressure rated and insulated to maintain the internal process temperature and autogeneous pressure.
• the flow velocity of the slurry through it is substantially lower than in the pipe reactor. This could lead to settling of the solids from the slurry menstruum.
• the rotary impellers 42 in each stage provide sufficient agitation to prevent settling and cause moderate turbulence. This not only keeps the solids in suspension, but promotes mixing and particle-to-particle contact.
• the seed crystals from the pipe reactor are built upon by previously unconverted or unattached particles and grow both radially as well as axially. This enables the production of the larger diameter microfibers without substantial sacrifice in aspect ratio.
• baffles 44 which typically consist of rectangular bars, extend the length of the reactor 40.
• the baffles 44 are spaced around the interior of the reactor and serve to prevent the impeller action from causing purely radial flow. Rather, because of the baffles 44, a toroided flow is induced, which maintains the particles in suspension.
• additional gypsum particles, or crystal growth aids such as potassium sulfate, or nucleation inhibitors such as succinic anhydride, can be added directly into the continuous stirred reactor to manipulate the physical development of the seed fibers.
• the turbulence caused by the rotating impeller can, of course, cause shear or breakage of the long slender seed crystals. This phenomenon is also turned to advantage by controlling the drag
• Pump 50 is also a progressive cavity pump, but in this instance is operated in reverse rotation in order to maintain the upstream pressure.
• the slurry After being restored to atmospheric pressure, the slurry is fed to a rotary pressure filter 60, or other dewatering device, for separation of a substantial portion of the free water, leaving a microfiber filter cake having about 40-50% water by weight. Water extracted by the filter can be recycled to the mixing tank 10. The resulting filter cake is broken up and the separated microfibers are fed through a dryer 62 to remove the remaining free moisture.
• a rotary pressure filter 60 or other dewatering device
• the dried calcium sulfate hemihydrate microfibers may be put through a dead burning oven 64 to remove some or all of the remaining chemically bound water and convert the hemihydrate to soluble or insoluble anhydrite.
• the fibers may be post treated with a coating, as taught by the prior art, in order to stabilize them against water absorption.
• the finished calcium sulfate microfibers are collected, as indicated at 66, and put into bags 70 or other suitable packages for distribution.
• the operating parameters in the first stage of the process i.e. the plug-flow reactor are: the feed slurry concentration, the steam pressure, the control temperature, the dispersion of steam into the feed slurry stream, the residence time within the plug-flow reactor and the dimensions of the reactor. These parameters effect the length of the initial alpha hemihydrate seed crystals as well as the degree of conversion that takes place.
• the additional parameter of agitation energy can be used to control not only radial growth, but the final axial dimension of the microfibers. Control of the agitation energy is accomplished through regulation of the drag coefficient of the impeller and/or by its rotational speed. Furthermore, additional gypsum particles, crystal modifiers and/or nucleation inhibitors can be introduced in the second stage to manipulate the build-up of the seed crystals from the first stage.
• a steam/slurry mixing valve was connected directly to a plug-flow reactor consisting of about 32 feet of nominal 1-1/2 inch diameter pipe. In-order to conserve space, the pipe reactor was arranged in four 7-foot straight sections disposed parallel to each other and connected by smooth curved sections, such as shown in FIG. 2. The outlet from the plug-flow reactor was connected directly to the inlet of a 3-section continuous tank reactor.
• Each section of the tank reactor consisted of a 15-inch inside diameter by 15 inch high cylindrical pressure vessel provided with a centrally disposed impeller on a vertical axis of rotation. The 3-sections were vertically stacked and the slurry progressed from the inlet near the lowest level in the first section to an outlet near the top level in the last section.
• the pilot plant was operated to make microfibers for comparison with microfibers made according to the multi stage continuous tank reactor process alone and by the plug-flow reactor process alone.
• the operating parameters of, and results from, the three processes are summarized in the following Table I.
• aspects ratio means Tap Density Aspect Ratio as an approximate indicator of the actual length to diameter ratio of the respective microfibers, and is expressed in terms of the mean aspect ratio for a given group or sample of microfibers. Tap Density Aspect Ratio measurements were determined using apparatus per ASTM standard test method D4164 and the following modified procedure:
• Step 1 A 14 gram spec microfiber sample to be tested is weighed to two place
• Step 2 The specimen is placed into a clean 100 ml
• Step 3 The filled graduated cylinder is placed
• Step 4 The unit is turned on, and stops
• Step 5 The graduated cylinder is removed, and
• Step 6 The aspect ratio of the microfibers in the specimen is determined according' to the
• TAPPED DENSITY ASPECT RATIO [(9.6 * V) + 72] 1/2 - 11.5

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• 1990
  • 1990-03-14 CA CA002024146A patent/CA2024146A1/en not_active Abandoned
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  • 1990-03-14 JP JP2505488A patent/JPH03504596A/ja active Pending
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