WO1990011256A1

WO1990011256A1 - Process and apparatus for producing calcium sulfate microfibers

Info

Publication number: WO1990011256A1
Application number: PCT/US1990/001406
Authority: WO
Inventors: Michael R. Lynn; James R. Wittbold; Thomas E. Rehder
Original assignee: The United States Gypsum Company
Priority date: 1989-03-16
Filing date: 1990-03-14
Publication date: 1990-10-04
Also published as: EP0416089A1; CA2024146A1; EP0416089A4; JPH03504596A

Abstract

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

PROCESS AMD APPARATUS FOR PRODUCING CALCIUM SULFATE MICROFIBERS

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION AND PRIOR ART

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

recrystallized from a pressure calcined 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 of the applications for their use, is disclosed in U.S.

Patents 3,822,340; 3,961,105; and 4,152,408. 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 separated from the menstruum, dried and stabilized, if desired.

While 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 limited amount, say for example about 35 pounds, of microfibers per cycle, which 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. Typically, 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.

As a result, the plug-flow process, which is the subject 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, 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

consistently produced.

While satisfactorily 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. 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.

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 is the principle object of this invention to provide an improved process 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 object to provide such an apparatus and process which has the capability of producing microfibers having larger diameters without sacrifice of high aspect ratio, and which can be manipulated to control the dimensional growth of such microfibers.

These and other objectives 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 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

temperature on the order of 285°F 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 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.

Gradually 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. 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 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.

Having thus briefly described the invention, a more detailed description of it, along with further discussion of its features and advantages, now follows with reference to the accompanying drawings, which form part of the specification, and of which:

BRIEF DESCRIPTION OF THE DRAWINGS 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; and

FIG. 5. is a cross-sectional view taken through the apparatus of FIG. 4, along the line 5-5 in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

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. 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 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.

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 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.

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 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 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

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 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

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 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 blocks 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

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 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. Preferably, 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. Again, the reactor 40 is pressure rated and insulated to maintain the internal process temperature and autogeneous pressure.

Because of the substantially greater volume of the tank reactor 40, 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. However, 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 particles and grow both radially as well as axially. This enables the production of the larger diameter microfibers without substantial sacrifice in aspect ratio.

A series of vertically extending 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.

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 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

coefficient and/or speed of the impeller to regulate crystal length to suit 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 is 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 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.

As an optional 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 chemically 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 microfibers are collected, as indicated at 66, and put into bags 70 or other suitable packages for distribution.

The foregoing process and apparatus represent significant

improvement over the prior art batch and continuous processes, and 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 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.

As part of an experiment to produce larger diameter, high aspect ratio microfibers, a pilot plant was set up in accordance with the apparatus and process 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 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.

NOTE: As used throughout this specification, including the foregoing Examples, "aspect 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

accuracy.

Step 2: The specimen 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 cylinder 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 determined according' to the

following formula:

TAPPED DENSITY ASPECT RATIO=[(9.6 * V) + 72]^1/2- 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 on 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 is 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.