Abstract:
A process is disclosed for making a blend of alpha- and beta-stucco including a slurry calcination step to produce alpha calcium sulfate hemihydrate followed by a fluidized bed calcination step to produce beta calcium sulfate hemihydrate. The process starts with 50-75% gypsum-containing solids slurry, and then steam calcines the slurry in a first reactor to form partially calcined gypsum slurry which contains calcium sulfate dihydrate and alpha calcium sulfate hemihydrate. The partially calcined slurry is then dewatered. Then the filter cake is fed into a kettle to complete the calcination process by converting the calcium sulfate dihydrate of the filter cake material into beta calcium sulfate hemihydrate.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an improved method of making calcined gypsum which results in an ultra-low consistency alpha- and beta-blend stucco. In particular, the present invention provides a process which comprises a slurry calcination step in a first reactor to produce alpha calcium sulfate hemihydrate followed by a calcination step, for example a fluidized bed calcination step, in a second reactor to produce beta calcium sulfate hemihydrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gypsum calcium sulfate dihydrate, CaSO 4 .2H 2 O comes from a variety of sources. Land plaster is a term for natural gypsum which is any mixture containing more than  50 % calcium sulfate dihydrate, CaSO 4 .2H 2 O (by weight). 
         [0003]    Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum phase (i.e., calcium sulfate hemihydrate and/or calcium sulfate soluble anhydrite) and water, and, optionally, other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product. 
         [0004]    Stucco is defined as chemically calcium sulfate hemihydrate and is a well-known building material used to make building plasters and gypsum wallboard. Stucco is typically made by crushing the gypsum rock with and then heating the gypsum at atmospheric pressure to calcine (dehydrate) the calcium sulfate dihydrate into calcium sulfate hemihydrate. In addition to natural gypsum rock the use of Flue Gas Desulphurization gypsum or gypsum from chemical processes can be used as well. Traditionally, the calcining of gypsum has occurred in a large atmospheric pressure kettle containing a mixture of the various phases of the gypsum 
         [0005]    U.S. Pat. No. 5,927,968 to Rowland et al., incorporated herein by reference in its entirety, discloses its own method and apparatus for continuous calcining of gypsum in a refractoryless atmospheric kettle. However, U.S. Pat. No. 5,927,968 to Rowland et al. also discloses a variety of kettles for calcining stucco. One such kettle has a thickened dome-shaped bottom, against which a gas-fired flame is directed, with the kettle and burner flame being enclosed in a suitable refractory structure. There is usually an associated hot pit into which the calcined material is fed. The kettle must withstand temperatures in the 2,000-2,400° F. (1093-1314° C.) range. U.S. Pat. No. 5,927,968 to Rowland et al. states U.S. Pat. No. 3,236,509 to Blair typifies this type construction. 
         [0006]    U.S. Pat. No. 3,236,509 to Blair, incorporated herein by reference, discloses continuous fluidized kettle calcination in which dried mineral gypsum powder is fed to a covered, but air vented and lightly vacuum exhausted, calcination vessel. After a steady state of operation is attained in the vessel, a substantially continuous stream of cold gypsum that has been pre-dried and ground to a finely divided state and with a wide distribution of fragmented particle sizes, is added on top of the fluidized, boiling mass in the kettle. Under such conditions, the thermal shock upon the cold, dry mineral being dropped into the already boiling mass radically fractures the ground gypsum rock fragments, and the resultant stucco (beta hemihydrate) is highly fractured and fissured, as well as being widely distributed in particle size. This causes the stucco to disperse very rapidly in water, and requires high amounts of gauging water to be mixed with the stucco for rehydration to gypsum at customary use consistencies. 
         [0007]    This “dispersed consistency”, also known in the art as “consistency” or “water demand”, is an important property of stucco. Stuccos of lower consistency generally result in stronger casts. 
         [0008]    The normal consistency of stucco (gypsum plaster) is a term of art and is determinable according to ASTM Procedure C472, or its substantial equivalents. It is defined as the amount of water in grams per 100 grams of stucco. 
         [0009]    U.S. Pat. No. 4,533,528 to Zaskalicky, incorporated herein by reference in its entirety, discloses directly feeding wet chemical gypsum cake to a continuous kettle calciner to produce beta hemihydrate of lower consistency. As explained in Zaskalicky, and also for purposes of the present description, “dispersed consistency” may be defined as the water volume required to give a standard viscosity or flow when a standard amount by weight of stucco is dispersed by mechanical mixing in a laboratory mixer at high shear intensity and for a standard time to equal mixing encountered in the gypsum board forming line, e.g., 7 seconds, or in an industrial plaster formulation casting mixer, e.g. 60 seconds. 
         [0010]    For example, as explained in U.S. Pat. No. 4,201,595 to O&#39;Neill, incorporated herein by reference in its entirety, calcined gypsum made by continuous calcination may have a dispersed consistency of about 100-150 cc. “Dispersed consistency” for purposes of gypsum board manufacture may be defined as the water volume required to give a standard viscosity or flow when 100 grams of stucco is dispersed by mechanical mixing in a laboratory high speed blender at high shear intensity and for 7 seconds which is equivalent to the mixing encountered in the board forming line. While the dispersed consistency may be expressed in a particular numerical figure, it will be appreciated that any particular number is variable from one process to the next depending on the particular stucco and the rate of production. 
         [0011]    Low consistency stucco is particularly advantageous in automated gypsum board manufacture, in which a large portion of the processing time and processing energy is devoted to removing excess water from the wet board. Considerable excess water is required in gypsum board manufacture to properly fluidize the calcined gypsum and obtain proper flow of the gypsum slurry. 
         [0012]    A dispersed consistency value of 100-150 cc. indicates a water requirement of about 85-100 parts of water per 100 parts of the calcined gypsum for a typical slurry in a gypsum wallboard plant. The theoretical water required to convert the calcined gypsum (calcium sulfate hemihydrate or stucco) to set gypsum dihydrate is only 18.7% by weight on a pure basis. This leaves about 67 to about 82% of the water present in the gypsum slurry to be removed in drying the board. Ordinarily, gypsum board dryers in a gypsum board manufacturing line will remove this water, for example, by maintaining the air temperature at about 400° F. (204° C.) and requiring a drying time of about 40 minutes. 
         [0013]    U.S. Pat. No. 4,201,595 (also mentioned above), U.S. Pat. Nos. 4,117,070 and 4,153,373 to O&#39;Neill, all incorporated herein by reference in their entirety, teach to lower the dispersed consistencies of continuously calcined kettle stuccos by an after calcination treatment of the stucco with small amounts of water or various aqueous solutions, resulting in a damp but dry appearing material and allowing the small amounts of free water to remain on the calcined gypsum particle surface for a short period of time, about 1-10 minutes for the treated stucco to “heal” . 
         [0014]    U.S. Pat. No. 3,410,655 to Ruter et al., incorporated herein by reference in its entirety, teaches producing alpha calcium sulfate hemihydrate. Ruter et al. states the alpha-hemihydrate forms non-needle like crystals, as opposed to the beta calcium sulfate hemihydrate which forms needle-like crystals. Ruter et al. also states the usual plaster of Paris (calcium sulfate hemihydrate) is the beta calcium sulfate hemihydrate. However, depending on the manner of preparation, the plaster of Paris still contains more or less anhydrous calcium sulfate, and/or alpha calcium sulfate hemihydrate. Moreover, plasters with definite alpha-hemihydrate content exhibit higher strengths. Ruter et al. teaches to make alpha calcium sulfate hemihydrate in the form of non-needle-like crystals by elutriating the dihydrate with water to remove organic impurities and fine and slimy crystal portions, forming an aqueous suspension of the dihydrate at a pH about 1.5-6, and subsequently heating under closely controlled conditions. 
         [0015]    U.S. Pat. No. 2,907,667 to Johnson, incorporated herein by reference in its entirety, states alpha-hemihydrate is prepared by heating the dihydrate under controlled vapor pressure conditions in the presence of steam or in an aqueous solution. 
         [0016]    U.S. Pat. No. 4,234345 to Fässle discloses fast-setting alpha calcium sulfate hemihydrate made from calcium sulfate dihydrate by hydrothermally recrystallizing calcium sulfate dihydrate to form a mixture containing 95%-99% by weight alpha calcium sulfate hemihydrate and 5 to 1% calcium sulfate dihydrate. The dihydrate in this mixture is then converted to beta calcium sulfate hemihydrate by calcining, except for a remainder of up to 0.5 percent of dihydrate, which remains in the mixture. 
         [0017]    There is a need for stuccos having low consistency and good strength characteristics. 
       SUMMARY OF THE INVENTION 
       [0018]    It is an object of the invention to provide a process for making a stucco composition comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate. 
         [0019]    The present process starts with 50-75% gypsum-containing solids by weight in aqueous slurry. 
         [0020]    Direct injection of steam of a quality between 100 to 200 psig, into the slurry in or prior to the first continuous stirred tank reactor, at 60 psig, converts 50 to 95% wt. % of the gypsum solids to alpha calcium sulfate hemihydrate. This forms partially calcined gypsum slurry which contains calcium sulfate dihydrate and alpha calcium sulfate hemihydrate. In particular, about 80-90% wt. % or 70-85 wt. % of the gypsum is calcined to alpha calcium sulfate hemihydrate. The partially calcined gypsum slurry is then dewatered, for example in a filter press to produce a filter cake of dewatered solids of 95 to 98% solids. The filter cakes temperature is maintained above 170° F. (77° C.) during the separation. Then the dewatered hot solids are fed to an atmospheric kettle to complete the calcination process by converting the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The hot water (recovered without significant cooling) is returned to the feed of the process to minimize the energy used in the process. Alternately, the heat from the water can be used along with the waste heat from the kettle process to preheat the feed slurry of gypsum at the start of the process. 
         [0021]    The present process for making a blend of alpha- and beta-stucco results in near theoretical water demand for use in a board manufacturing process. The theoretical amount of water to hydrate 100% pure CSH (calcium sulfate hemihydrate) to the gypsum form would be 21 parts of water to 100 parts of CSH. This process results in a water demand down to 21 parts with a minimum amount of dispersants or fluidizers required. Beta stucco alone has a water demand up to 140 parts of water and requires a large amount of dispersant to reach the flow characteristic of the alpha-beta blend stucco. Alternately a blend of alpha and beta hemihydrate can be made using powders. The resulting material requires more total energy if made by stand alone processes. Also, the resulting material requires a higher percentage of alpha to beta to achieve the same results. Therefore, the present invention provides a more economical calcining method to produce the alpha-beta stucco. 
         [0022]    The alpha hemihydrate aids in fluidity while the beta-hemihydrate aids in reactivity. The process can also be energy efficient because it can recycle hot water recovered from dewatering. Also, the solids are kept hot during dewatering to ensure the material does not hydrate back to gypsum. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0023]      FIG. 1  is a process flow diagram of an embodiment of the process of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIG. 1  shows an embodiment of an apparatus for performing the process of the present invention. Gypsum (calcium sulfate dihydrate) and water are mixed in a mixer (not shown) to form a 50-75% solids gypsum slurry  10 . Gypsum slurry  10  is fed to a jacketed reactor  12  (autoclave). Steam  13  is also fed to the reactor  12  to provide heat. Other forms of heat may also be provided to the reactor  12  as appropriate. The feed gypsum may be any form of gypsum, such as land plaster, gypsum mineral from ground or unground sources, synthetic gypsum from flue gas desulfurization processes in power plants, or other chemical gypsum as by-products of the titanium dioxide industry. Traditionally the feed gypsum is land plaster manufactured by grinding gypsum rock to a fine particle size in a roller mill. The fineness of the land plaster is 95 to 98% less than 100 ASTM mesh. Land plaster gypsum purity can range from 80 to 99 wt. % calcium sulfate dihydrate. 
         [0025]    A crystal modifier  14  may also be fed to the reactor  12  if desired. The crystal modifier  14  controls the crystal morphology of the calcium sulfate alpha hemihydrate to achieve a desired particle size, e.g., 50 to 20 microns (d 50 ) average particle size. Prior to the dewatering of the alpha hemihydrate slurry additives may be added that will aid in the filtration, act as a hydration accelerator, and/or provide added fluidity to the final material. 
         [0026]    The slurry  10  is held in the reactor  12  at conditions for calcining the gypsum to partially convert it to alpha calcium sulfate hemihydrate, for example 55 psig at 300° F. (149° C.). Typically, 50 to 95%, or 80 to 95%, or 80 to 90% of the gypsum is converted by calcination to alpha calcium sulfate hemihydrate, alpha-CaSO 4 .0.5 H 2 O with a residence time of 5 minutes. The conversion can be controlled by changing the residence time or temperature of the reactor discharge. The higher the temperature the faster the conversion takes place. The longer the residence time the higher the conversion rate is achieved. 
         [0027]    Typically, the reactor  12  is a continuous stirred tank reactor (CSTR) operating at a pressure of 15 to 100 psig (29.7 to 114.7 psia, 2.0 to 7.9 bar), preferably 25 to 75 psig (39.7 to 89.7 psia, 2.7 to 6.2 bar) or 35 to 55 psig (49.7 to 69.7 psia, 3.4 to 4.8 bar). The temperature of the reactor  12  corresponds to the temperature of saturated steam at the operating pressure. For example, a pressure of about 52 psig (66.7 psia, 4.6 bar) corresponds to a temperature of about 300° F. (149° C.). The residence time of the slurry in the reactor  12  generally ranges from 2 to 30 minutes, preferably 5 to 15 minutes. 
         [0028]    For example, in a typical embodiment, after the reactor  12  is closed, hot steam  13  is delivered to the jacket around the reactor  12  to heat the reactor  12  for about  5  minutes. The change in temperature and pressure inside the reactor are monitored as a function of time. Then after about 10 minutes, the delivery pressure of the steam  13  was increased to bring the reaction to completion in about 5 additional minutes. The crystal modifiers  14  could, for example, be added to the slurry  10  before heating begins or while the slurry  10  is being heated or maintained at a desired temperature in the reactor  12 . 
         [0029]    The partially calcined gypsum product  16  discharges from the reactor  12  as a slurry comprising calcium sulfate dihydrate and alpha calcium sulfate hemihydrate and feeds an accumulator tank  20 . Accumulator tank  20  acts as a holding tank and permits release of the steam as the slurry&#39;s pressure drops to atmospheric pressure. If desired the accumulator tank  20  may be omitted if the separation stage (dewatering unit  30 ) is direct coupled. 
         [0030]    The slurry  24  discharges from the accumulator tank  20  and feeds a dewatering unit  30  which removes water to produce a dewatered solids-containing product  32  and a removed water stream  34 . 
         [0031]    All or a portion of the removed water  34  may be recycled as a stream  38  to be part of the slurry  10  to assist in recycling water, heat and chemicals (such as the crystal modifiers or other additives) used in the process. Typically the stream  38  is recycled at an elevated temperature, such as 100 to 200° F. (38 to 93° C.). The partially calcined gypsum product  16 , the accumulator tank  20 , the stream  24 , the dewatering unit  30  and the dewatered product  32  are kept at a temperature sufficiently high to prevent the alpha hemihydrate from rehydrating, e.g., kept at elevated temperature of 160-212° F. (71-100° C.). 
         [0032]    Typically the dewatering unit  30  is a filter press and/or centrifuge and the dewatered product  32  has a 2 to 6 wt. %, typically 4%, free water moisture content. A typical filter press employs steam to press down on a plate over the partially calcined gypsum product slurry to drive out the water. If desired the process of Baehr U.S. Pat. No. 4,435,183 may be employed for dewatering and drying calcium sulfate hemihydrate in a centrifuging and flash drying operation by ejecting the wet solids from the centrifuge bowl directly into the flash dryer&#39;s high velocity, high volume, heated air stream. Baehr U.S. Pat. No. 4,435,183 is incorporated herein by reference. 
         [0033]    The dewatered product  32  is fed to a board stucco kettle calciner  40  at conditions to convert the majority or all of the gypsum in the dewatered product  32  to beta calcium sulfate hemihydrate. The kettle calciner  40  typically is indirectly heated at atmospheric pressure by use of natural gas heating on the bottom and direct fired heated air  42 . The material behaves as a fluidization bed due to the free water vapor leaving the solids fed to the kettle reactor  40  as well as the bound water released as the gypsum (calcium sulfate dihydrate) converts to calcined beta gypsum (beta calcium sulfate hemihydrate). Fluidization gas may also be provided by the indirect fired gas heated air or use of direct fired heated air  42 . The kettle  40  typically operates at atmospheric pressure, and a temperature of from 150 to 1000° F. (66 to 538° C.), preferably 250 to 650° F. (121 to 343° C.) or 400 to 500° F. (204 to 260° C.) or 285 to 300° F. (140 to 149° C.). 
         [0034]    The kettle  40  discharges a dry product  44  comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate (also known as an alpha and beta stucco blend). Optionally, the dry product  44  is sent to grinding  50  to reduce the particle size of the material. 
         [0035]    Typically the dry product  44  has less than 5 wt. %, preferably less than 2 wt. %, calcium sulfate anhydrite and less than 5 wt. %, preferably less than 2 wt. %, calcium sulfate dihydrate. 
         [0036]    Typically the calcium sulfate of the final product is 50-95 wt. % alpha hemihydrates and 50 to 5 wt. % beta hemihydrate; for example, 70-85 wt. % alpha hemihydrates and 30-15 wt. % beta hemihydrate; or 80-90 wt. % alpha hemihydrates and 20-10 wt. % beta hemihydrates. 
         [0037]    The crystal modifier  14 , if employed, is in the solution during the period of calcination to alpha hemihydrate. The pH of the solution is in the neutral range between 6 and 8. The crystal modifiers  14  act in reducing the number of nuclei that form in the solution and also restrain the growth of the crystal in one of its axis. The result is control of the particle size through control of the number of crystals forming and growing. The other result is that the shape of the crystal is cubic like in aspect ratio. With no modifiers in the solution the shape of the alpha hemihydrate would be a long acicular needle shaped crystal of aspect ratio up to 100:1 in length to diameter. 
         [0038]    The resulting alpha-beta-stucco blend typically has a number of desirable properties of consistency, compressive strength and density. 
         [0039]    For example, the typical dry product has a normal consistency of about 30 to 36 as measured by a handmix drop consistency determination. 
         [0040]    In contrast to normal consistency measured according to ASTM Procedure C472, normal consistency as measured by a handmix drop consistency method is not ASTM Procedure C472 test. The test method for measuring normal consistency by a handmix drop consistency method is as follows. 
         [0041]    Weigh a 50 gram sample of the plaster to be tested at 70-80° F. (21-27° C.) to 0.1 gram accuracy. Drained the mixing cup and spatula before using such that the mixing cup and spatula contain a maximum of ¼ cc of adhering droplets of water or are wiped dry. Add water to the mixing cup from a burette (deionized or distilled at 70-80° F. (21-27° C.) unless otherwise specified) in the estimated quantity to produce the proper flow. Sift the plaster into the water and allow the sample to soak undisturbed for 60 seconds. Mix thoroughly for 30 seconds, stirring 90 to 100 complete revolutions with the spatula. Pour the slurry immediately after mixing on to a clean, dry, unscratched PLEXIGLASS sheet from a height of 1½ inch. At the correct consistency, the mix will flow out of the cup without the aid of the spatula. 
         [0042]    The mix should form a round patty of reasonably uniform thickness. The patty diameters for each specific consistency range are as follows in TABLE 1 (when measured in at least two directions and averaged): 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Normal Flow Formulations 
                   
               
               
                 Consistency Range (cc) 
                 Average Patty Diameter 
               
               
                   
               
             
             
               
                 30–39 
                 3⅛ ± 1/16 inch (7.9 ± 0.16 cm) 
               
               
                 40–49 
                 3¼ ± 1/16 inch (8.25 ± 0.16 cm) 
               
               
                 50–59 
                 3½ ± 1/16 inch (8.9 ± 0.16 cm) 
               
               
                 60–89 
                 3¾ ± 1/16 inch (9.5 ± 0.16 cm) 
               
               
                  90–140 
                 4 ± 1/16 inch (10.2 ± 0.16 cm) 
               
               
                   
               
             
          
         
       
     
       Crystal Modifiers 
       [0043]    TABLE 2 presents typical crystal modifiers. Also, U.S. Pat. No. 2,907,667 to Johnson, incorporated herein by reference, discloses a number of chemicals which impact reactions in reactors for making alpha calcium sulfate hemihydrate. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Typical Crystal Modifiers 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Maleic Acid 
                 Tartaric Acid 
               
               
                 Succinic acid 
                 Polyacrylic acid 
               
               
                 Lactic acid 
                 Aspartic acid 
               
               
                 Citric acid 
                 Monosodium gluconate 
               
               
                 Tartaric acid 
                 Tri-polyphosphate 
               
               
                 Monosodium gluconate 
                 Gelatin 
               
               
                 Ethylene diamine tetra-acetic 
                 DEQUEST 2006 
               
               
                 acid or sodium salt thereof 
                 (penta-sodium salt of amino 
               
               
                   
                 trimethylene phosphonic acid) 
               
               
                 Aspartic acid 
                 Ethylene diamine tetra-acetic acid or 
               
               
                   
                 sodium salt thereof 
               
               
                 Citric acid 
                 Diethylene triamine penta-acetic acid 
               
               
                   
               
             
          
         
       
     
         [0044]    The stucco composition of the invention can be used in both the manufacture of gypsum wallboard and stucco for production of a plaster for interior and exterior applications. One or more additives can be added to the stucco composition to facilitate the desired viscosity, and other optional additives may be added to achieve desired physical characteristics in the final set product, such as, for example, flexural strength, abuse resistance (e.g., chip resistance), water resistance, flame resistance, and the like, or combinations thereof. 
       EXAMPLES 
       [0045]    A plant control and three plant trial examples of the present invention were conducted. In the Control and Examples, 75% solids slurry was fed to one continuous stirred tank reactor (CSTR) of 275 gallons (1041 liters) in size used for the Alpha-portion of the calcinations. A high temperature Tube mill was used for the Beta-portion of the calcinations of the Examples. The Tube mill was a heated ball mill. 
       Control  
       [0046]    At a reactor temperature of 298° F. (148° C.), 99% of the gypsum of the feed slurry was calcined to Alpha calcium sulfate hemihydrate, which had a normal consistency of 32 to 34 cc. Normal consistencies in the Control and the following Examples were measured by the above-described hand drop test. 
       Example 1 
       [0047]    At a reactor temperature of 285° F. (141° C.), 90% of the gypsum fed to the first reactor was calcined to Alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the Tube mill at 300° F. (149° C.). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212° F. (71-100° C.). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. Thus, the resulting product had 90% alpha calcium sulfate hemihydrate and 8.5%-9% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the amount of gypsum of the feed slurry. In other words, 90% of the gypsum of the feed slurry converted to alpha calcium sulfate hemihydrate and 8.5%-9% converted to beta calcium sulfate hemihydrate. The normal consistency of the resulting product was 32 cc. 
       Example 2 
       [0048]    At a reactor temperature of 280° F. (138° C.), 85% of the gypsum fed to the first reactor was calcined to alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the tube mill at 300° F. (149° C.). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212° F. (71-100° C.). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The resulting product had 85% alpha calcium sulfate hemihydrate and 13.5%-14% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the amount of gypsum of the feed slurry. The normal consistency of the resulting product was 34 cc. 
       Example 3 
       [0049]    At a reactor temperature of 275° F. (135° C.), 80% of the gypsum fed to the first reactor was calcined to alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the tube mill at 300° F. (149° C.). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212° F. (71-100° C.). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The resulting product had 80% alpha calcium sulfate hemihydrate and 18.5%-19% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the gypsum of the feed slurry. The normal consistency of the resulting product was 32 cc. 
         [0050]    The data shows the present inventive process has the advantage that it results in a combined alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate product that has a normal consistency similar to that of an alpha calcium sulfate hemihydrate product. 
         [0051]    Although we have described the preferred embodiments for implementing our invention, it will be understood by those skilled in the art to which this disclosure is directed that modifications and additions may be made to the invention without departing from the spirit and scope thereof.