Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF AND SYSTEMS FOR PREPARING A HEAT RESISTANT
ACCELERANT SLURRY AND ADDING THE ACCELERANT SLURRY TO A POST-
MIXER AQUEOUS DISPERSION OF CALCINED GYPSUM
BACKGROUND OF THE INVENTION
[0001] Set gypsum, which comprises calcium sulfate dihydrate, is a well-known
material that is included commonly in many types of products, such as gypsum
boards employed in typical drywall construction of interior walls and ceilings
of
buildings. Typically, such gypsum-containing board is prepared by forming a
mixture
of calcined gypsum, that is, calcium sulfate hemihydrate and/or calcium
sulfate
anhydrite, and water, as well as other components, as desired. The mixture
typically
is cast into a pre-determined shape on the surface of a conveyor or in a tray.
As it
travels along the conveyor, the calcined gypsum reacts with water to form a
matrix of
crystalline hydrated gypsum or calcium sulfate dihydrate. The desired
hydration of
the calcined gypsum is what enables the formation of an interlocking matrix of
set
gypsum crystals, thereby imparting strength to the gypsum structure in the
gypsum-
containing product. Mild heating can be used to drive off unreacted water to
yield a
dry product. Gypsum mixers and methods of producing gypsum products are
described, for example, in U.S. Patent Nos: 1,767,791; 2,253,059; 2,346,999;
4,183,908, 5,683,635; 5,714,032; and 6,494,609.
[0002] Accelerator materials are commonly used in the production of gypsum
products to enhance the efficiency of hydration and to control set time.
Accelerators
are described, for example in U.S. Patent Nos: 3,573,947; 3,947,285;
4,054,461; and
6,409,825. Some accelerators include finely ground dry calcium sulfate
dihydrate,
commonly referred to as "gypsum seeds." The gypsum seeds enhance nucleation of
the set gypsum crystals, thereby increasing the crystallization rate thereof.
Traditionally, accelerators have been added to the same mixer chamber as that
used
to combine water with calcined gypsum. While addition of accelerator to the
mixer
has the advantage of mixing the accelerator well and evenly throughout the
water
and calcined gypsum mixture, the accelerator can also cause the gypsum to
begin
setting prematurely. This premature setting causes the mixer to clog, can
cause
damage to the mixer, limits efficiency, and necessitates more frequent mixer
cleaning. Mixer cleaning requires shutting down a board line with a serious
detriment to productivity. Although additives including retarders have been
used in
the mixer to combat premature setting, such additives contribute additional
costs and
considerations.
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[0003] Conventional gypsum seed accelerator materials progressively lose their
effectiveness upon aging, even under normal conditions. In this respect, some
efficiency of the accelerator is lost even as it is ground, and the gypsum
seeds
dramatically lose potency over time during handling or storage. The loss of
acceleration efficiency of conventional accelerator materials is exacerbated
when the
accelerator is exposed to heat and/or moisture. To combat the loss of
efficiency of
the gypsum seeds over time, particularly under conditions of heat, it is
customary to
coat the calcium sulfate dihydrate accelerator material with any of a number
of
known coating agents, such as, for example, sugars, including sucrose,
dextrose
and the like, starch, boric acid, or long chained fatty carboxylic acids
including salts
thereof. Conventional heat resistant accelerator materials are both ground and
provided in dry form inasmuch as accelerator loses efficiency upon contact
with
moisture, for example, because the accelerator particles undesirably
agglomerate
and/or because the coating agents often are soluble in water.
[0004] New materials and methods are needed to overcome the deficiencies of
heat resistant accelerator that still preserve the benefits of using such an
accelerator.
BRIEF SUMMARY OF THE INVENTION
[0005] According to one aspect of the present invention, a method of preparing
a
heat resistant accelerant slurry and introducing the slurry into a post-mixer
aqueous
dispersion of calcined gypsum in a discharge apparatus is provided. A heat
resistant
accelerator (HRA) is added into a first mixing device. A liquid medium is
added into
the first mixing device. The HRA and liquid medium are mixed in the first
mixing
device to form the HRA slurry. The aqueous dispersion of calcined gypsum is
formed in a second mixing device. The aqueous dispersion is discharged from
the
second mixing device into a discharge apparatus. The HRA slurry is transferred
from the first mixing device into the discharge apparatus.
[0006] According to another aspect of the present invention, a method is
provided
for introducing a heat resistant accelerator (HRA) slurry into a post-mixer
aqueous
dispersion of calcined gypsum in a discharge apparatus. The aqueous dispersion
is
discharged from the second mixing device into a discharge apparatus. The HRA
slurry is introduced into the discharge apparatus.
[0007] A system for forming a heat resistant accelerant (HRA) slurry and
adding
the slurry to a post-mixer aqueous dispersion of calcined gypsum is provided
as an
aspect of the present invention. The system comprises a source of HRA; a
source of
liquid medium; a first mixing device; the sources operatively associated with
the first
mixing device; a second mixing device; a discharge apparatus operatively
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associated with an outlet of the second mixing device; a delivery device; the
first
mixing device and the discharge apparatus operatively associated with the
delivery
device.
[0008] For example, the present invention has particular utility in the
preparation
of gypsum board such as wallboard or ceiling tile. In such embodiments, after
the
HRA slurry is added to the aqueous dispersion of calcined gypsum, the
dispersion is
deposited onto a moving coversheet. In the case of wallboard, a second
coversheet
is applied to the deposited contents prior to drying. In some embodiments,
such as
some ceiling tile, a second coversheet is not employed.
[0009] The present invention's methods, systems, and elements thereof are
further described in the drawings and detailed description, which provide
representative embodiments.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0010] Figure 1 shows a schematic plan view of one embodiment of a system for
preparing a heat resistant accelerant slurry and adding the accelerant slurry
to a
post-mixer aqueous dispersion of calcined gypsum.
[0011] Figure 2 shows a schematic plan view of another embodiment of a system
for preparing a heat resistant accelerant slurry and adding the accelerant
slurry to a
post-mixer aqueous dispersion of calcined gypsum.
[0012] Figure 3 shows a schematic cross-sectional view of one embodiment of a
heat resistant slurry injection subsystem.
[0013] Figure 4 shows a schematic cross-sectional view of another embodiment
of a heat resistant slurry injection subsystem.
[0014] Figure 5 shows a partial perspective view of one embodiment of a heat
resistant slurry subsystem.
[0015] Figure 6 shows a partial perspective view of another embodiment of a
heat
resistant slurry subsystem.
[0016] Figure 7 shows a partial perspective view of still another embodiment
of a
heat resistant slurry subsystem.
[0017] Figure 8 shows a partial perspective view of a mixer and discharge
apparatus.
[0018] While the invention is susceptible to various modifications and
alternative
constructions, certain illustrative embodiments thereof have been shown in the
drawings and will be described below in detail. It should be understood,
however,
that there is no intention to limit the invention to the specific embodiments
disclosed,
but on the contrary, the intention is to cover all modifications, alternative
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constructions, and equivalents falling within the spirit and scope of the
invention as
defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention is premised, at least in part, on the surprising
discovery that the problems associated with use of heat resistant accelerator
(HRA)
can be minimized by forming a HRA slurry and then adding the slurry to an
aqueous
dispersion of calcined gypsum. Preferably, the HRA slurry is added to the
aqueous
dispersion after it has left a stucco mixer, for example, a pin, multipass, or
other
conventional mixer. Advantageously, the discharge apparatus according to the
invention does not require a separate power source in order to mix the high
viscosity
production additive with the aqueous dispersion of calcined gypsum as the
dispersion passes from the stucco mixer through the discharge apparatus.
[0020] In accordance with the present invention, Fig. 1 shows a system 12 for
preparing a heat resistant accelerant (HRA) slurry and adding it to a post-
mixer
aqueous dispersion of calcined, gypsum. The system comprises a first mixing
device
15 for preparing the HRA slurry, and a second mixing device 17, for example, a
stucco mixer such as a pin mixer, multipass mixer, piniess mixer, or other
mixer that
can be used to prepare aqueous gypsum dispersions, with interior 18 for
preparing
the aqueous dispersion of calcined gypsum. Operatively associated with the
first
mixing device 15 are a HRA source 21 and liquid medium source 24. Control
meters
27, 30 can be further operatively associated with the sources 21, 24, for
controling
the flow of HRA and liquid medium into the first mixing device 15. The
positioning of
the control meters, 27, 30, can be varied, and can be configured in any
position that
allows for metering of the source materials.
[0021] The HRA slurry formed in the first mixing device 15 is operatively
connected to a discharge apparatus 33, which is operatively associated with a
mixer
outlet 36, and terminating in an outlet 39. In some embodiments, the outlet
comprises a boot. A boot is appropriate for use on the discharge apparatus
used for
depositing of the main field slurry--as opposed to the densified layer slurry.
In other
embodiments, the outlet is provided as a conduit such as hose. A conduit or
hose
outlet is appropriate for a densified layer discharge apparatus.
[0022] The first mixing device 15 can be operatively associated with the
discharge device 33 via a transfer line 42, which can have a plurality of
subsections,
for example, 45, 48. A delivery device 51 can be operatively associated with
the first
mixing device 15 and discharge apparatus 33 so as to permit flow of HRA
slurry. In
some embodiments, the delivery device 51 is a pump, for example, a positive
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displacement pump. Appropriate pumps for use in the systems of the invention
are
discussed in more detail in relation to the methods of the invention. While
the
system 12 need only comprise a single discharge apparatus 33, as is shown in
Fig.
1, the system 12 can also comprise one or more additional discharge apparatus,
for
example, 133, 233, operatively associated with second and third mixer outlets
136,
236, and can terminate at 139, 239. The second and third discharge
apparatuses,
133, 233, can be operatively associated with the first mixing device 15 with
transfer
lines 142 with, for example, subsections 145, 148, 242, with, for example,
subsections 245, 248), and can further incorporate delivery devices 151, 251,
in a
manner analogous to that described for the operative association of the first
mixing
device 15 with the discharge apparatus 33.
[0023] As described above, the system 12 is configured so that the HRA slurry
can be transferred from the first mixing device 15 to the discharge apparatus
33.
The discharge apparatus can comprise an injection ring 54 comprising at least
one
injection port 57. Any additional discharge apparatuses provided in the
subsystem
12, for example, 133, 233, can further comprise injection rings, for example,
154,
254, and ports, for example, 157, 257. A more detailed discussion of the
injection
ring 54, injection port 57, and associated elements is provided below in
respect to
Figs. 3 and 4. While injection rings are discussed in the context of both
systems
and methods of the present invention, other means of injection can be employed
in
addition or in the alternative to an injection ring. For example, in some
embodiments, a needle on a transfer line can be used for transfer into the
discharge
apparatus. In some embodiments, a nipple is provided in the discharge
apparatus to
allow transfer into the apparatus.
[0024] Fig. 2 shows a system 112, which is a variation on the system 12 of
Fig. 1.
System 112 can comprise a delivery device 51 to assist in the transfer of HRA
slurry
into multiple discharge apparatuses, for example, 33, 133, 233. Use of the
delivery
device 51 as a common delivery device can be accomplished by branching the
transfer line 42 using an udder, manifold, or other device with branching
capability 60
that branches at 63 to provide multiple branch lines, for example, 66, 69, and
72.
The branches lines and/or udder, manifold, or other device with branching
capability
can incorporate control valves or like devices, for example, 67, 70, and 73,
to control
the flow of HRA slurry through the branch lines, for example, 66, 69, and 72,
such
valves can also or in the alternative be associated with the branching device
60.
These branch lines can be connected to the discharge apparatuses 33, 133, 233,
via
injection rings, for example, 54, 154, 254, and injection ports, 57, 157, 257,
in a
manner analogous to that described above for system 12.
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[0025] Fig. 3 shows an embodiment in which the transfer line 42 comprises an
udder, manifold, or other device with branching capability 75 that splits the
transfer
line 42 into a plurality of branch lines 78, 81, and 84. Three branch lines
are shown
for illustrative purposes only. The injection ring 54 of Fig. 3 is shown with
a plurality
of injection ports 57, 57', and 57", but again that number is shown for
illustrative
purposes only. The branch lines 78, 81, and 84 are feed into injection ports
57, 57",
and 57' respectively. In some embodiments, additional injection rings, for
example,
154, 254, as depicted in Fig. 1, can also incorporate the aforementioned
features.
[0026] Fig. 4 shows a variation on the embodiment shown in Fig. 3, which
incorporates a tee 87 that allows mixing of two or more production additives
prior to
injection into the discharge apparatus 33. The_tee 87 comprises a junction 90
at
which HRA slurry and a second additive converge from inlets 93, 96,
respectively.
While Fig. 4, only shows a tee 85 for one of the injection ports 57 that is
for
illustrative purposes only. Any number of the injection ports can have a tee
87
associated with them.
[0027] Fig. 5 shows a HRA mixing subsystem 315, which is an example of a form
that the first mixing device can take. The HRA mixing subsystem 315 can be
incorporated into the systems, for example, 12 and 112, and utilized in the
methods
of the invention. The HRA mixing subsystem 315 comprises a bottom discharge
tank 320. The bottom discharge tank 320 comprises an interior 323 and an
interior
perimeter 326. One or more baffles, for example, 329, 329', 329", can be
arrayed
about the interior perimeter 326. HRA and liquid medium sources, 21, 24, are
operatively associated with the bottom mixing tank 320. The HRA subsystem also
can comprise an agitator 332 positioned so as to facilitate mixing of the HRA
and
liquid medium. While agitator 332 is shown as a motor/propeller type device,
that is
for illustrative purposes only as the agitator can take on a number of
different forms
provided that a particular form facilitates mixing. Examples of suitable
mixer/agitator
devices and methods also include static mixers, spraying the liquid medium at
the
HRA, and a rotating cement mixer type mixer that can also comprise baffles. In
some embodiments an approximately 1750 rpm motor is used to spin the propeller
for the agitator. The cylindrical/frustoconical representation of the bottom
discharge
tank as shown in Fig. 5 is for illustrative purposes only, as it can take on a
number of
different shapes. The bottom discharge tank 320 is operatively associated with
a
discharge apparatus, for example, 33, 133, and 233 as shown in Figs. 1 and 2.
A
delivery device 51, for example, a pump, can be provided to assist in the
transfer of
the HRA slurry from the bottom mixing tank 320 and the discharge apparatus 33.
An example of such a pump is a progressive cavity pump from Moyno.
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[0028] Fig. 6 shows an HRA eductor subsystem 415, which is an example of a
form that the first mixing device can take. The HRA eductor subsystem 415 is
operatively associated with HRA and liquid medium sources, 21 and 24
respectively.
The eductor subsystem 415 comprises an eductor 450 and an entrance chamber
453. The entrance chamber 453 comprises an entrance 456 to allow insertion of
the
HRA from the source 21. The entrance chamber 453 can also include one or more
inlet ports 459 for insertion of the liquid medium from the source 24 via a
source line
461 and liquid medium entrance line 462. In addition or in the alternative to
the
liquid medium entering the entrance chamber 453 through the inlet port 459,
the
source line 461 can branch at 465 to enter the eductor 450 at 468. In the
absence of
the entrance line 462, a branch at 465 is not necessary. A valve 471 can be
incorporated into the HRA eductor subsystem 415 between the entrance chamber
453 and the eductor 450. A delivery device 51 can be provided to facilitate
transfer
of the HRA slurry to the discharge apparatus 33. Any type of eductor can be
used in
the present invention. In some embodiments, an inductor is substituted for the
eductor. Examples of suitable eductors and inductors are available from Fox
Valve
(Dover, N.J.).
[0029] Fig. 7 shows a HRA eductor subsystem 515, which is a variation on the
subsystem 415 shown in Fig. 6. The subsystem 515 can generally have the same
attributes as those described for the subsystem 415. The subsystem 515
comprises
some additional elements. A source pump 551 is operatively associated between
the eductor 450 and liquid medium source 24. A holding tank 574 is operatively
associated between the eductor 450 and the delivery device 51. The holding
tank
574 allows for positioning of the source pump 551 so the eductor will properly
function based on the Venturi principle given the back pressure the HRA slurry
can
experience when entering the discharge apparatus 33.
[0030] Fig. 8 shows a discharge apparatus 633, which is one embodiment for the
discharge apparatus 33, 133, 233, etc. The discharge apparatus 633 also
displays a
number of different elements and attributes that can be shared with a
discharge
apparatus generally. The discharge apparatus comprises a gate 680 with gate
opening 682, a series of hose sections 683, 685 and 688, a cage valve 691, and
two
injection rings 54, 654 with injection ports 57, 657, and an outlet 639. The
gate 680
acts as an adapter operatively associated with both the second mixing device
and
the discharge apparatus that allows the discharge apparatus' conduit to attach
to the
second mixing device 17 at the mixer outlet 36. The gate 680 is shown with an
injection port 757. The injection ports 57, 657, and 757 are examples of
possible
locations for entry of HRA, foam, or other production additive. Other
additives, such
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as sodium trimetaphosphate and other phosphates, that can be used include
those
described in concurrently filed and co-owned application "METHODS OF AND
SYSTEMS FOR ADDING A HIGH VISCOSITY GYPSUM ADDITIVE TO A POST-
MIXER AQUEOUS DISPERSION OF CALCINED GYPSUM" (Attorney Reference
No. 234910), U.S. Patent Application . The rings 54, 654 and gate
680 can be configured to have multiple injection ports, for example, as
illustrated in
Figs. 3 and 4. In some embodiments, the hose section 685 separating the rings
54,
654 is about 15 to about 16 inches long. The transfer line 42 or other
transfer lines
can be connected at any of the injection ports. The positioning of the cage
valve 691
can be varied along the length of the discharge conduit 633 and allow for
control of
flow in the discharge conduit. The discharge apparatuses and systems of the
invention can incorporate elements and subsystems as described in co-owned
U.S.
Patent No. 6,494,609.
[0031] The methods of the invention comprise the formation of a HRA slurry
from
HRA and a liquid medium. The HRA slurry formation can also comprise additional
ingredients. The liquid medium generally comprises at least water. Additional
ingredients can be added together with one or both of the HRA and liquid
medium
source streams. Additional ingredients can also be added in other streams,
either
alone or in combination with one another.
[0032] HRAs are generally known in the art, and any appropriate HRA can be
utilized in the slurry formation of the present invention. Suitable HRAs and
methods
of producing the same are described, for example, in U.S. Patent No.
3,573,947.
HRA can be produced using a ball mill or other suitable grinding device by
grinding
calcium sulfate dihydrate in a substantially dry state. Preferrably, the
calcium sulfate
is ground to achieve smallest particle size while maintaining a large overall
surface
area, but not so small that the slurry to be formed would have undesirable
properties,
for example, excessive viscosity. HRAs for use in the present invention are
also
referred to as ball mill accelerators (BMAs) and coated accelerators (CAs).
HRA for
use in the present invention has a coating that aids in maintaining the
effectiveness
of the HRA when stored for extended periods of time. HRA coatings can comprise
without limitation one or more of the following: sugars, including sucrose,
dextrose
and the like, starch, boric acid, and long chain fatty acids including salts
thereof.
While a heat resistant accelerator for use in the present invention preferably
has
heat resistant attributes, there is no requirement that the HRA pass any type
of heat
resistance test. HRA applicable to this invention also includes coated calcium
sulfate dihydrate that has-been subjected to one or more drying steps to
improve the
attributes of the accelerator. One example of such an accelerator is climate
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stabilized accelerator (CSA). Because aqueous solutions speed the degradation
of
HRA, additives can be included in the HRA slurry to help combat these
problems. In
some embodiments, organic phosphonates such as DEQUESTO phosphonates
commercially available from Solutia, Inc., St. Louis, Missouri, are utilized.
Examples
of DEQUEST phosphonates include DEQUESTO 2000, DEQUESTO 2006,
DEQUESTO 2016, DEQUESTO 2054, DEQUESTO 2060S, DEQUESTO 2066A, and
the like. In some embodiments, addition of one or more phosphorous containing
compounds such as phosphates and preferably sodium trimetaphopshate can also
be employed. Methods of preserving the efficacy of the HRA slurry also include
the
use of a gypsum solution including calcium sulfate dihydrate, and preferably a
saturated calcium sulfate dihydrate solution. One of skill in the gypsum art
will be
able to identify the appropriate type of HRA for a given gypsum application
based on
the teachings of the present invention and the knowledge available in the art.
[0033] The methods of the present invention can utilize one or more systems,
subsystems, and elements as described herein, for example, as described in
respect
to the figures. However, the methods can employ various other systems,
subsystems, and elements. While the methods are described in relation to such
systems, subsystems, and elements, such description is provided to assist the
reader in appreciating the invention, and not to limit the invention as set
forth in the
appended claims. Moreover, one or more additional accelerators can be used.
Examples of such accelerators include potash, wet gypsum accelerator (WGA),
climate stabilized accelerator (CSA), and any accelerator known in the gypsum
art.
In those embodiments where one or more additional accelerators are employed,
the
additional accelerant can be added in the aqueous dispersion of calcined
gypsum
mixer or outside of that mixer, that is, in the discharge apparatus. In some
embodiments, potash, in granule and/or powder form, is used as an additional
accelerator.
[0034] According to one aspect of the invention HRA and liquid medium are
introduced into the first mixing device 15 from sources 21 and 24
respectively, the
rate, volume, and other parameters of which can be controlled using meters 27
and
30 respectively. In some embodiments, the introduction of the HRA and liquid
medium to the first mixing device comprises separately metering of the HRA and
liquid medium. In some embodiments, the addition of the HRA and liquid medium
to
the first mixing device is continuous. In some embodiments, a feed system and
method similar to that described in U.S. Patent No. 3,262,799 is utilized. A
method
of preparing a heat resistant accelerant slurry and introducing it into a post-
mixer
aqueous dispersion of calcined gypsum in a discharge apparatus, in accordance
with
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the present invention comprises introducing a heat resistant accelerator (HRA)
into a
first mixing device; adding a liquid medium into the first mixing device;
mixing the
HRA and liquid medium in the first mixing device to form the HRA slurry;
forming the
aqueous dispersion of calcined gypsum in a second mixing device; discharging
the
aqueous dispersion from the second mixing device into a discharge apparatus;
transferring the HRA slurry from the first mixing device into the discharge
apparatus.
In some embodiments, the HRA and liquid medium are separately introduced into
the first mixing device. In some embodiments, the liquid medium comprises
water.
In some embodiments, the liquid medium comprises phosphate. In some
embodiments, the liquid medium comprises a gypsum solution, including calcium
sulfate dihydrate, and the gypsum solution can be saturated.
[0035] In some embodiments, the method of HRA slurry preparation comprises
disrupting vortex formation in the first mixing device, for example, when the
mixing
device comprises a bottom discharge tank. The disruption can be achieved using
a
plurality of baffles arrayed around an interior perimeter of the first mixing
device.
[0036] In some embodiments, the transferring step of the method comprises
pumping the HRA slurry into the discharge apparatus. In some embodiments, the
pumping comprises the use of a positive displacement pump.
[0037] In some embodiments, a substantial percentage of an added amount of
HRA and an added amount of liquid medium is retained in the first mixing
device less
than less than 24 hours, less than 18 hours, less than 12 hours, less than 6
hours,
less than three hours, less than two hours, less than one hour, less than 30
minutes,
less than 25 minutes, less than 20 minutes, 15 minutes, less than 10 minutes,
and/or
less than 5 minutes. In some embodiments, the substantial percentage of the
added
amounts is greater than 99%, 98%, 97%, 96%, 95%, 90%, 85%, 80%, 75%, and/or
50%. In general, the time between formation of the HRA slurry and its
introduction
into the discharge apparatus is minimized so as to minimize the degradation of
the
HRA. One of skill in the art will understand that even in a first in, first
out type
device, such a bottom discharge tank, a certain percentage of the "first in"
can
persist in the device due to mixing or other disruption.
[0038] In some embodiments, the HRA slurry is formed with a percent solids of
between about 30% and about 60% percent solids. In some embodiments, the HRA
slurry is formed with a percent solids of between about 40% and about 50%
percent
solids, which allows for the HRA slurry to be easily pumped by means of a
progressive cavity pump. The higher the percent solids HRA the lower amount of
slurry that needs to be pumped into the gypsum slurry to achieve the required
set
time at the knife. This set time can vary depending on the type of gypsum rock
being
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used by a plant, the calcination ratio of the converted gypsum dehydrate to
stucco,
the water stucco ratio of the slurry, the line speed/distance to the knife at
a given
plant, the efficiency of the HRA prior to being made into a slurry and a
number of
other variables particular to each plant. Because board line speeds can vary
and
the distance from the knife can vary greatly, set times from the mixer to
knife can
vary as well. Therefore accelerator usage to set the board at the knife can
vary over
a wide range. One of skill in the art will appreciate the amount of
accelerator used
can be tailored to individual plants and production lines on a case by case
basis.
[00391 A method in accordance with the present invention comprises delivering
a
HRA slurry from a first mixing device 15 to a discharge apparatus 33 where the
slurry is introduced into an aqueous dispersion of calcined gypsum that has
been
discharged from a second mixing device, for example, a stucco mixer such as a
pin
mixer, multipass mixer, pinless mixer, and other mixers that can be used to
prepare
gypsum dispersions, wherein the aqueous dispersion had been mixed. While
gravity
transfer is contemplated, the HRA slurry can be moved from the first mixing
device
15 to the discharge apparatus 33 with the assistance of one or more delivery
devices, for example, a pump. In some embodiments, the pump is a positive
displacement pump, but other pump types can be used in addition or in the
alternative can be employed, for example, a centrifugal pump. Examples of
suitable
positive deplacement pumps include progressive cavity, gear, and peristaltic
pumps.
The pressure of the HRA slurry in the transfer line 42 between the first
mixing device
15 and discharge apparatus 33 can be measured using a pressure gauge. However,
use of such a gauge is not necessary if the pump employed is self-regulating.
The
pressure of the slurry entering the discharge apparatus should be maintained
at a
pressure greater than that of the contents of the discharge apparatus so as to
minimize back pressure and allow efficient transfer of the HRA slurry. In some
embodiments, the pressure in the discharge apparatus is between about 5 and
about
15 p.s.i. Pressure gauges can be incorporated into the systems and used in the
methods of the present invention in a manner analogous to that described in
concurrently filed and co-owned application "METHODS OF AND SYSTEMS FOR
ADDING A HIGH VISCOSITY GYPSUM ADDITIVE TO A POST-MIXER AQUEOUS
DISPERSION OF CALCINED GYPSUM" (Attorney Reference No. 234910), U.S.
Patent Application . The HRA slurry can be discharged into the
discharge apparatus 33 through an injection port 57, which can be associated
with
an injection ring 54. In some embodiments, the HRA slurry is split into
multiple
branches to allow for multiple entries into the discharge apparatus 33. Such
multiple
entries can be achieved by providing multiple inlets, for example, 57, 57',
and 57" in
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12
the injection ring 54. In some embodiments, the HRA slurry is combined with
one or
more additional additives, for example, foam, before being introduced into the
aqueous dispersion of the discharge apparatus 33. Such combining can be
accomplished using a tee 90 providing for entry of HRA slurry 93 and another
additive 96. In some embodiments, the HRA slurry and one or more additional
additives are combined approximately three inches from the point of injection
into the
discharge apparatus. . In some embodiments, the HRA is transferred into the
discharge apparatus downstream of a pinch valve operatively associated with
the
discharge apparatus. In some embodiments, a dispersant is added to the
discharge
apparatus such as lignin, napthelene sulfate or other suitable dispersant.
[0040] For a particular gypsum product, multiple discharge apparatuses can be
used. For example, if the intended product is a wallboard and top and bottom
densified layers are desired, second and third discharge apparatuses, that is,
densified layer extractors 133, 233 can be provided. For certain wallboard
products
as well other board products such as ceiling tile, see co-owned, co-pending
U.S.
Patent Application No. 10/804,359, only a single densified layer is applied.
In some
embodiments a separate delivery devices 51, 151, and 251 are utilized for
transfer of
the HRA slurry from the first mixing device 15 to the discharge apparatuses
30, 133,
and 233. In other embodiments, there is a single delivery device 30 for
transfer of
the HRA slurry to all three discharge apparatuses. In still other embodiments,
a
delivery device 51 is used for the discharge apparatus 33, and a delivery
device 151
is used for the discharge apparatuses 133 and 233. Regardless of the number or
presence of delivery devices, the HRA slurry can split into branch transfer
lines using
an udder, tee, manifold, or other device allowing branching of the transfer
line.
Control of HRA slurry flow into particular branch lines can be controlled
using a valve
or other element of like function.
[0041] HRA slurry is generally introduced to the post-mixer aqueous dispersion
in
a stream perpendicular to the flow of the dispersion in the discharge
apparatus.
However, other orientations of HRA slurry introduction are also possible. For
ideal
incorporation into the aqueous dispersion, the HRA slurry is introduced into
the
discharge apparatus closer or close as possible to the mixer outlet 36 than
the
discharge outlet 39. In some embodiments, the introduction occurs from about
2.5
inches to 3 inches from the mixer outlet 36. In some embodiments, the
introduction
occurs about 1 inch from the mixer outlet. Generally, moving the introduction
of the
HRA slurry downstream in the discharge apparatus will serve to delay setting
acceleration.
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13
[0042] When using the present methods to make a wallboard product with first,
for example, bottom, and second, for example, top, densified layers, each
densified
layer discharge apparatus 133, 233 can comprise and/or be operatively
associated
with one or more of the following: a hose and a ring, for example, 154, 254.
The
percentage of HRA slurry to provide a proper set is dependent on the amount of
aqueous slurry that is being applied to the densified layer layer of the
board. For
example, if 10% of the main gypsum slurry, aqueous dispersion from the second
mixing device 17, is being applied to the first, for example, bottom,
densified layer,
then preferably approximately 10% of the HRA is directed to the bottom
densified
layer through the bottom discharge apparatus 133. If a second, for example,
top,
densified layer is being utilized, the proportion of HRA slurry would again
preferably
approximately match the percentage of gypsum slurry being applied to the top
densified layer. Percentages of gypsum slurry from the second mixing device 17
generally range from about 5% to about 20%. The terms top and bottom, as well
as
face and back and other equivalent terms, are relative terms in respect to
which
orientation of the gypsum product one is referring to. For purposes of
illustration
only, bottom refers to a first paper, that is, cover sheet that travels
beneath the
gypsum mixer and the densified layer that is applied to that first paper. Top
refers to
a second paper that is applied after addition of the gypsum slurry through the
main
discharge apparatus 33 to the bottom paper, as well as the densified layer
applied to
the second paper.
[0043] In some embodiments, the first mixing device 15 comprises a bottom
discharge mixing tank, and the mixing step comprises use of the mixing tank.
In
such embodiments, the bottom discharge mixing tank further comprises an
agitator,
and the mixing step can comprise agitation of the HRA and liquid medium. An
example of bottom discharge tank 320 is illustrated in Fig. 5 and discussed
herein.
[0044] In some embodiments, the method utilizes an eductor as the first mixing
device, and the mixing step comprises use of the eductor. Exemplary eductor
subsystems 415, 515, can be used and are illustrated in Figs. 6 and 7,
respectively.
When the method uses an eductor 450, and a pump as a delivery device 551
upstream of the eductor, the HRA slurry formed is transferred initially to a
holding
tank 574 and then pumped with a delivery device 51 to the discharge apparatus
33.
By using the holding tank, proper pressure is retained for the eductor 450 to
operate
correctly. Any of the methods described herein can also involve a holding tank
574
for HRA slurry, provided that the time that the HRA slurry spends in the tank
is
minimized. In some embodiments, the HRA slurry is kept in the tank less than
about
minutes.
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[0045] The systems and method of the present invention have the benefit of
delaying setting of an aqueous dispersion of calcined gypsum by delaying the
introduction of HRA slurry until after the dispersion has left the stucco
mixer, that is,
the second mixing device, 17. In some embodiments, the methods allow for
addition
of less water to the stucco mixer resulting in a lower water-stucco ratio,
because
setting in the mixer because of absence of accelerator in the second mixing
device
interior 18. Methods and systems are also contemplated for introducing the HRA
slurry once formed directly into the second mixing device 17 instead of or in
addition
to introduction into the discharge apparatus.
[0046] All references, including publications, patent applications, and
patents,
cited herein are hereby incorporated by reference to the same extent as if
each
reference were individually and specifically indicated to be incorporated by
reference
and were set forth in its entirety herein.
[0047] The use of the terms "a" and "an" and "the" and similar referents in
the
context of describing the invention, especially in the context of the
following claims,
are to be construed to cover both the singular and the plural, unless
otherwise
indicated herein or clearly contradicted by context. The terms "comprising,"
"having,"
"including," and "containing" are to be construed as open-ended terms, that
is,
meaning "including, but not limited to," unless otherwise noted. Recitation of
ranges
of values herein are merely intended to serve as a shorthand method of
referring
individually to each separate value falling within the range, unless otherwise
indicated herein, and each separate value is incorporated into the
specification as if
it were individually recited herein. All methods described herein can be
performed in
any suitable order unless otherwise indicated herein or otherwise clearly
contradicted by context. The use of any and all examples, or exemplary
language,
for example, "such as", provided herein, is intended merely to better
illuminate the
invention and does not pose a limitation on the scope of the invention unless
otherwise claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of the
invention.
[0048] Preferred embodiments of this invention are described herein, including
the best mode known to the inventors for carrying out the invention.
Variations of
those preferred embodiments may become apparent to those of ordinary skill in
the
art upon reading the foregoing description. The inventors expect skilled
artisans to
employ such variations as appropriate, and the inventors intend for the
invention to
be practiced otherwise than as specifically described herein. Accordingly,
this
invention includes all modifications and equivalents of the subject matter
recited in
the claims appended hereto as permitted by applicable law. Moreover, any
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combination of the above-described elements in all possible variations thereof
is
encompassed by the invention unless otherwise indicated herein or otherwise
clearly
contradicted by context.
