CA2024146A1

CA2024146A1 - Process and apparatus for producing calcium sulfate microfibers

Info

Publication number: CA2024146A1
Application number: CA002024146A
Authority: CA
Inventors: Michael R. Lynn; James R. Wittbold; Thomas E. Rehder
Original assignee: Individual
Current assignee: United States Gypsum Co
Priority date: 1989-03-16
Filing date: 1990-03-14
Publication date: 1990-09-17
Also published as: JPH03504596A; EP0416089A4; WO1990011256A1; EP0416089A1

Abstract

ABSTRACT OF THE DISCLOSURE

Process and apparatus for producing larger diameter, high aspect ratio, calcium sulfate microfibers are provided. The process involves passing a steam heated dilute aqueous slurry, containing about 1/2% -15%. gypsum by weight, under pressure, first through a plug flow stage wherein slender, needle-like crystals are formed, and then through a stirred-flow stage wherein the crystals are grown both axially and radially. The implementing apparatus includes means for mixing the slurry with steam to rapidly raise the slurry temperature, and a plug-flow pipe reactor connected to a continuous stirred tank reactor.

Description

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J `' PROCESS AND APPARATUS FOR PRODUCING CALCIUM SULFATE MICROFIBERS

FIELD OF THE INVENTION

The present invention relates to an improved process and apparatus for the eontinuous production of calcium sulfate mierofibers, and more partieularly, to sueh a process and apparatus which enables closer eontrol of the mierofibers' dimensional development.

BACKGROUND OF THE INVENTION AND PRIOR ART

Manmade ealeium sulfate mierofibers, sometimes also referred to as "whisker ibers", are used or a variety of purposes, ineluding: as fillers or reinforeement~ in plasties, asphalt, mineral cements, paper and paint. Generally speaking, they consist of high aspect ratio crystals of alpha hemihydrate or anhydrous calcium sulfate reerystallized from a pressure caleined gypsum. Certain such microfibers are commercially available under the trademark "FRANKLIN
FIBER" from the United States Gypsum Company.
The basic technology for making calcium sulfate microfibers, along with many oE the applications for their use, is disclosed in U.S.
Patents 3,822,340; 3,961,105; and 4,152,408.

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Generally speaking, 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.
On a commercial basis, calcium sulfate microfibers have heretofore been produced by one of two processes. In the batch process, 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 ~eparated from the menstruum, dried and stabilized, if desired.
Whlle it has been used successfully to produce quality microfibers, the batch process has also proven to be somewhat costly. For a given facility, the batch process produces a llmited amount, say for example about 35 pounds, of microfibers per cycle, which typically mi8ht be about 45 mlnutes. While the output may be increased by increasing the ~ollds concentration of the slurry charge, thi~ 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 proces~, up to about 30% isolids 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. Typically, the reactor consists of a 6 ~
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cylindrical pressure vessel having 3-6 stages or æones connected in series, such as generally described in U.S. Patent 3,579,300. Steam i9; ,. ~ ' 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 i8 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 a~ desired.
The continuous process significantly improves productivity over the batch process. For example, a typical facility has produced microfibers ` -at a rate in exces3 of 1200 pounds per hour. However, this continuous 15 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.
As a result, the plug-flow process, which is the sub~ect of a copending U.S. application serial number 324,158, was recently developed. In this process, the aqueous slurry, containing finely ground gypsum particles, is premixed with superheated steam and pumped under pressure through a smooth interior, continuous hollow conduit, , 25 under "plug-flow" condition. The length of the plug-flow reactor is ; determined to provide sufficient residence time for sub?3tantial 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 consistently produced.
While satlsfactorily producing higher aspect ratio microfibers, it has been observed that 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. Furthérmore, 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.
Again, there is a need for calcium sulfate fibers of larger diameter, such as between 1.5 to 4.0 microns, but which still retain an aspect-ratio in excess of 45, and preferably in excess of 65.
Moreover, it is desirable to achieve substantially complete conversion of the gypsum feed material to microfibers in order to minimize costs and make such fibers more competitive with alternative fillers or reinforcements.

SUMMARY OF THE INVENTION

It i5 the prlnclple ob~ect of this invention to provide an improved proce~ and apparatus for continually producing high aspect ratio calcium sulfate microfibers at an economical production rate and with substantially total conversion of the gypsum feed stock.
It is a further ob~ect to provide such an apparatus and process whlch has the capability of producing microfibers having larger diameters without sacrifice of high a~pect ratio, and which can be manipulated to control the dimensional growth of such microfibers.
These and other ob~ectives are achieved in accordance with the invention by combining aspects of both the plug-flow process and the continuous stirred reactor process.
An apparatus according to the invention comprises, in combination:
a pressure raising progre~sive 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 consi~ts of an elongated conduit, such as a pipe, having a smooth uninterrupted lnterior of substantlally constant cross-section. It is connected bétween the steam/slurry mixing vàlve 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 pres~ure 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 temperature on the order of 285F and autogeneous pressure.
According to the process aspect of the invention, 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 i9 proportlonally merged and mixed with saturated steam and very quickly raised to about 285F. 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 ~ulfate 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 hi8h aspect ratlo seed crystals of alpha hemihydrate plus some uncon~erted 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 ~eed crystals.
Thls yromotes both a radial and axlal growth of the seed crystals, resulting in the larger diameter, yet hlgh aspect ratio, microfibers.
Gradually the slurry progresses through the successive zone~ 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 S

otherwise dewatered, leaving a filter cake of microfibers. The cake is broken up, and the microfibers are dried and packaged. Optionally, as in the case of prior processes, 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 preYiOUs 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 manlpulated to effectively control both the nucleation of crystals and the growth and development of the crystals to produce specific mlcroflbers wlthln a generous range of predetermlned dlmensions.
Havlng thus brlefly descrlbed the lnventlon, a more detailed descrlptlon of lt, along with further discussion of its features and advsntages, now follows wlth reference to the accompanylng drawlngs, whlch form part of the speclflcatlon, and of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic dia8ram of a process for making calcium ~ulfate microfibers ln accordance wlth the lnvention;
FIG. 2 19 a side elevational view, partially in sectlon, of a plug-flow reactor suitable for use in the process of FIG. l;
FIG. 3 is a cross-sectlonal view taken through the apparatus of FIG. 2, along the line 3-3 in FIG. 2;

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FIG. 4 is a side-elevational view, partially sectioned, of a multi-stage, continuous stirred reactor suitable for use in the process of FIG. l; and FIG. 5. is a cross-sectional view taken through the apparatus of FIG. 4, along the line 5-5 ~n FIG. 4.

DETAILED DESCRIPTION OF T~E INVENTION

The process illustrated in FIG. 1 begins with the mixing of ground gypsum and water in the mix tank lO to form a dilute slurry. For example, 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 l/2% and 15%, and preferably about 12%, ~olids by weight. It is also advantageous to heat the slurry up to about 200F 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 increa~e~ the pressure of the slurry up to about 80 psi and di~charges it into the slurry/steam mixing valve 20. Saturated steam at about 350F is al~o supplied to the valve 20 and metered directly into the hot slurry as ~hown more clearly in FIG. 2.
Referring to 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 Z5 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 i9 provided with a restricted orifice 29.
Between the feed pump 12 and the 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 2~2~

directly in the path of the accelerated jet of steam coming through the needle valve. By this arrangement the incoming slurry is continuously metered with, directly impinged upon by, and mixed with the hotter steam. The result is an almo~t in~tantaneous tran~fer of heat to very quickly raise the temperature of the ~lurry and entrained calcium sulphate particles to the predetermined process temperature, for example, of about 285F. 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 ~he 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 hemihydrate crystals and microfibers. As another important consideration, it is highly desirable that 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 pres~ures autogenous with process temperatures ln the range of 285F or higher; such pressures typically being about 40-50 psi. It is also desirable to insulate the reactor against heat 1099.
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 increments. In other words, the fluid stream moves without appreciable shear or mixing action. This condition permits the dissolved calcium sulfate hemihydrate to nucleate and grow to longer, higher aspect ratio seed crystals.
However, because of this "plug-flow"~ some calcium sulfate dihydrate particles will escape nucleation and/or attachment to other 2 ~

crystals, and therefore complete conversion of the gypsum to alpha hemihydrate crystals is not achieved in the plug flow reactor alone.
According to the present invention, that consequence is turned to advantage as these unconverted particles are carried into the continuous stirred tank reactor 40 where they are available as building bloc~ in the second phase of the process.
The continuous stirred tank reactor 40, as shown in FIGS. 4 and 5, comprises one or more sections, or stages, 41a, 41b and 41c j respectively, each consisting of a high volume pressurized cylindrical vessel having its own rotary impeller 42 or other method of agitation.
By way of example, a typical section 41 of the resctor 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. Preferably, the hot slurry from the plug flow reactor is introduced through an inlet 45 at the bottom of the first stage of the contlnuous tank reactor ant flows upward to exit through an outlet 46 at the top of the last sectlon of the reactor. Again, the reactor 40 is pressure rated and insulated to maintain the internal process temperature and autogeneous pressure.
Because of the substantially greater volume o the tank reactor 40, the flow velocity of the slurry through it is substantially lower than in the plpe reactor. This could lead to settling of the solids from the ~lurry menstruum. Nowever, 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. As a consequence, the seed crystals from the pipe reactor are built upon by previously unconverted or unattached partlcles and grow both radially as well as axlally. This enables the productlon of the larger dlameter microfibers wlthout substantial sacrlflce ln aspect ratio.
A series of vertically extending bafles 44, which typically consist of rectangular bars, extend the length of the reactor 40. The baffles 44 are spaced around the interlor 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.
Optionally, 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 phy~ical 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 coefficient and/or speed of the impeller to regulate crystal length to ~uit the desired product specification.
After leaving the last stage of the tank reactor 40, the product slurry is restored to atmospheric pressure through the let down pump 50. Pump 50 i9 also a progressive cavity pump, but in this instance is operated in reverse rotation in order to maintain the upstream pressure.
After being restored to atmospheric pressure, the slurry i9 fed to a rotary pres~ure filter 60, or other dewatering device, Por separation of a substantial portion of the free water, leaving a microfiber filter cake havin8 about 40-50% water by weight. Water extracted by the fllter can be recycled to the mixlng tank 10. The resulting filter cake ls broken up and the separated mlcrofibers are fed through a dryer 62 to remove the remainin~ ree moisture.
A~ an optlonal step in the process, as depicted by the broken line in FIG. 1, the dried calcium sulfate hemihydrate microfibers may be put through a dead burning oven 64 to remove some or all of the remaining chemlcally bound water and convert the hemihydrate to soluble or insoluble anhydrite. In any case, 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 microibers are collected, as indicated at 66, and put into bags 70 or other suitable packages for distribution.
The oregoing process and apparatus represent signi1cant improvement over the prior art batch and continuous processes, and ` 2~2~6 afford the producer many advantages. They enable the efficient production of high aspect ratio calcium sulfate microfibers at better production rates, with generally complete conversion of the raw feed material, and therefore lower cost . Moreover, they provide greater flexibility and control to enable the operator to regulate the physical development of the microfiber product.
For example, 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.
In addition, in the second stage of the process, i.e. the continuous tank reactor, the additional parameter of agitation energy can be used to control not only radial growth, but the final axial dlmenslon of the microfibers. Control of the agitation energy is accomplished through regulation of the drag coefficlent of the impeller and/or by ita 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 ~eed crystals from the first stage.
As part of an experiment to produce larger diameter, high aspect ratio microfibers, a pilot plant was set up in accordance with the 21 apparatus and proce~s described above. A steam/slurry mixing valve was connected directly to a plug-flow reactor consisting of about 32 feet of nominal 1-1/2 lnch 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 sectlons, 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 di~posed impeller on a vertical axis of rotation. The 3-sections were vertically stacked 2~2~

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 proces~ alone. The operating parameters of, and results from, the three processes are summarized in the following Table I.

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NOTE: As used throughout this specification, including the foregoing Examples, "aspect ratio", means Tap Density Aspect Ratio as an approximate lndicator of the actual length to diameter ratio of the respective microfiberq, and is expreqsed in terms of the mean aspect ratio for a given group or sample of microfibers. Tap Density Aspect Ratio measurementq were determined using apparatus per ASTM standard test method D4164 and the following modified procedure:

Step 1: A 14 gram specimen of the microfiber sample to be tested is weighed to two place --accuracy.
Step 2: The speclmen is placed into a clean 100 ml graduated cylinder which is then stoppered or sealed with parafilm to prevent spillage.
Step 3: The filled graduated cylinder is placed onto the holder of the tap density unit, and the counter set to 300.
Step 4: The unit is turned on~ and stops automatically after it has tapped 300 times.
Step 5: The graduated cyllnder is removed, and without tapping it further to level the contained specimen, the volume of the specimen, (estimating where it would be level, if necessary), is noted.
Step 6: The aspect ratio of the microfibers in the specimen is determlned according to the following formula:

TAPPED DE~SITY ASPECT RATIO=~9.6 * V~ + 721 - 11,5 0.244 where V = final volume in ml.

Claims

The subject matter claimed is:

1. Apparatus for use in continuously producing calcium sulfate microfibers, comprising in combination:

(a) feed means for continuously providing a dilute aqueous slurry containing discrete particles of calcium sulfate dihydrate at elevated temperature and pressure;
(b) a plug-flow reactor having an inlet connected to said feed means and an outlet, said reactor comprising between said inlet and outlet a continuous hollow conduit of substantially constant cross-section and having a smooth interior surface whereby the slurry passes through in substantial plug-flow condition, said reactor being of sufficient length to afford the slurry adequate residence time to convert the dihydrate particles to acicular seed crystals of alpha hemihydrate;
(c) a continuously stirred tank reactor having an inlet connected to the outlet from the plug-flow reactor, an outlet, and in between at least one fluid retaining vessel capable of holding sufficient slurry whereby the residence time of slurry passing through the vessel is sufficient to achieve both radial and axial growth of the acicular core crystals, said reactor being provided with slurry agitating means within said vessel to maintain the solid particles in the slurry in suspension; and (d) pressure reducing means connected to the outlet of the tank reactor and effective to restore the microfiber containing slurry to atmospheric pressure.

2. An apparatus as recited in Claim 1, wherein said plug flow reactor consists of a continuous pipe.

3. An apparatus as recited in Claim 2, wherein said pipe is on the order of 1-1/2 inches in nominal inside diameter.

4. An apparatus as recited in Claim 3, wherein said pipe is on the order of 32 feet in length.

5. An apparatus as recited in Claim 1, wherein said reactor comprises two or more fluid retaining vessels connected in series so that the hot slurry passes into, resides in for awhile, and passes out of each of the vessels before exiting the reactor.

6. An apparatus as recited in Claim 5, wherein said vessels are stacked vertically one upon another such that the slurry flows into the reactor through an inlet at or near the lowest level of the first vessel and upwardly through the vessels and out through an outlet at or near the highest level of the last vessel in the series.

7. An apparatus as recited in Claim 5, wherein said reactor comprises from 3 to 6 vessels.

8. An apparatus as recited in Claim 5, further including an agitating means in each of the vessels.

9. An apparatus as recited in Claim 6, further including separate rotary agitating means in each of said vessels, all of said agitating means being attached to a common rotatable shaft extending vertically through said reactor.

10. A process for the continuous production of high aspect ratio calcium sulfate microfibers , comprising in sequence the steps of:

(a) mixing ground particles of calcium sulfate dihydrate and water to form a dilute slurry;
(b) heating said slurry under pressure to raise its temperature to about 285°F;

(c) passing the hot slurry while under pressure through a conduit in plug-flow condition until a substantial portion of the dihydrate material is converted to alpha hemihydrate nucleated as acicular seed crystals;
(d) further passing the hot slurry under pressure through a continuously stirred tank reactor while agitating it sufficiently to keep the solids in suspension and to promote particle to particle contact for a sufficient time to foster both axial and radial growth of the acicular seed crystals until microfibers of the desired mean diameter and aspect ratio are produced;
(e) reducing the pressure of the microfiber containing slurry; and (f) separating the microfibers from the slurry menstruum.

11. A process as recited in Claim 10, wherein step (b) further comprises mixing said slurry with steam at a temperature an the order of 350°F in a pressure sealed mixing valve to effect a rapid rise in the slurry temperature.

12. A process as recited in Claim 10, wherein the temperature of said slurry is maintained at about 285°F throughout steps (b) through (f).

13. A process as recited in Claim 10, wherein said slurry initially includes about 1/2% to about 15% by weight calcium sulfate dihydrate particles.

14. A process as recited in Claim 10, wherein the residence time of the hot slurry in the plug flow conduit 19 on the order of 30 to 60 seconds .

15. A process as recited in Claim 10, wherein the residence time of the hot slurry in the continuously stirred tank reactor is on the order of between 4 minutes and 6 minutes.

16. A process as recited in Claim 10, further including in step (d) the introduction of additional finely ground calcium sulfate particles into the continuously stirred tank reactor for additional material to build upon the acicular seed crystals.

17. A process as recited in Claim 10, further including in step (d) the introduction of a crystal growth aid or a nucleation inhibitor into the continuously stirred tank reactor as a means to manipulate the physical development of the microfibers.

18. A process as recited in Claim 13, wherein said slurry initially includes on the order of 12% by weight calcium sulfate dihydrate particles.

19. Calcium sulfate microfibers made by heating an aqueous slurry, comprising between about 1/2% and about 15% by weight of finely ground calcium sulfate dihydrate particles, under pressure to a temperature of about 285°F, feeding said slurry through a continuous hollow conduit as plug flow for sufficient time to allow conversion of some of the calcium sulfate dihydrate to calcium sulfate alpha hemihydrate acicular seed crystals, passing said slurry containing said seed crystals through a continuous stirred tank reactor until said seed crystals grow to microfibers having a mean aspect ratio greater than 45 and a mean diameter greater than about 1.5 microns, reducing the pressure on the slurry to atmospheric, and separating the microfibers from the slurry menstruum.