U.S. patent application number 09/823130 was filed with the patent office on 2001-08-23 for method of producing a porous paste, especially a porous plaster slurry, and a mixer for preparing such paste or slurry.
This patent application is currently assigned to BABCOCK-BSH GmbH. Invention is credited to Bahner, Friedrich, Braun, Kurt, Eidam, Helmut, Hose, Horst, Maurer, Karl, Ullsperger, Frank.
Application Number | 20010015935 09/823130 |
Document ID | / |
Family ID | 23901138 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015935 |
Kind Code |
A1 |
Bahner, Friedrich ; et
al. |
August 23, 2001 |
Method of producing a porous paste, especially a porous plaster
slurry, and a mixer for preparing such paste or slurry
Abstract
A porous paste, especially a plaster paste or slurry for
producing sandwich-type plasterboard, is made in a disk-shaped
mixer having a rotor rotatable in a mixing chamber by introducing
compressed air or other pressurizable gas through a wall or bottom
segment directly into the chamber so that the incoming pressurized
gas meets the mixture with a shearing action along the wall or
bottom.
Inventors: |
Bahner, Friedrich;
(Rotenburg, DE) ; Braun, Kurt;
(Ludwigsau-Friedlos, DE) ; Eidam, Helmut;
(Schenklensfeld, DE) ; Hose, Horst; (Kassel,
DE) ; Maurer, Karl; (Bad Hersfeld, DE) ;
Ullsperger, Frank; (Alsfeld-Leusel, DE) |
Correspondence
Address: |
THE FIRM OF KARL F ROSS
5676 RIVERDALE AVENUE
PO BOX 900
RIVERDALE (BRONX)
NY
10471-0900
US
|
Assignee: |
BABCOCK-BSH GmbH
|
Family ID: |
23901138 |
Appl. No.: |
09/823130 |
Filed: |
March 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09823130 |
Mar 30, 2001 |
|
|
|
09478730 |
Jan 6, 2000 |
|
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Current U.S.
Class: |
366/65 ;
366/317 |
Current CPC
Class: |
B01F 23/23365 20220101;
B01F 23/23342 20220101; B01F 23/233 20220101; B01F 23/23362
20220101; B28C 5/1269 20130101; B01F 27/93 20220101; Y10S 261/26
20130101; B28C 5/0881 20130101; B01F 23/53 20220101 |
Class at
Publication: |
366/65 ;
366/317 |
International
Class: |
B01F 007/26 |
Claims
We claim:
1. A mixer for producing a porous paste, comprising: a disk-shaped
chamber provided with a peripheral wall and a bottom wall; means
for introducing a paste-forming binder and mixing water forming a
settable composition with said binder into said mixing chamber; a
mixing disk rotatable in said chamber and formed with a peripheral
array of teeth for mixing said binder and water to form a settable
composition; and a fine-porous element forming at least a segment
of at least one of said walls and bounding said chamber on one side
and a pressurizable compartment on an opposite side for introducing
a pore-forming gas into said composition at a supply pressure above
a pressure in said chamber during rotation of said mixing disk,
thereby forming said porous paste.
2. The mixer defined in claim 1 wherein said fine-porous element
has a pore width of 3 to 100 .mu.m.
3. The mixer defined in claim 2 wherein said fine-porous element
has a pore width of 10 to 30 .mu.m.
4. The mixer defined in claim 1 wherein said element is a sintered
metal wall portion having a thickness of 2 to 10 mm.
5. The mixer defined in claim 1 wherein said chamber and said disk
are horizontal, said disk is rotatable about a vertical axis, said
chamber has a top wall provided with inlets for said binder and at
least a portion of said water, and said chamber has a bottom wall
provide with an inlet for a tenside contained in 5 to 15% of the
water.
6. The mixer defined in claim 1 wherein said peripheral wall has an
outlet for said composition and said element is located along said
peripheral wall in a region of a first third of a rotation of said
disk past said outlet.
7. In a mixer for producing a porous paste wherein a rotor is
rotatable in a chamber provided with a peripheral wall and a bottom
wall for mixing a paste-forming binder and mixing water to form a
settable composition, the improvement which comprises a fine-porous
element forming at least a segment of at least one of said walls
and bounding said chamber on one side and a pressurizable
compartment on an opposite side for introducing a pore-forming gas
into said composition at a supply pressure above a pressure in said
chamber during rotation of said rotor, thereby forming said porous
paste.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a division of Ser. No. 09/748,730 filed
Jan. 6, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to the production of porous
pastes, especially settable pastes adapted to solidify into solid
porous compositions. More particularly the invention relates to a
method of making a porous plaster slurry and to a mixer for
producing such pastes or slurries.
BACKGROUND OF THE INVENTION
[0003] Porous plaster slurries or pastes are used, for example, in
the production of plasterboard and particularly plasterboard in
which the plaster layer is sandwiched between two cardboard or
paperboard sheets.
[0004] EP 0 305 702 A2 describes the production of the plaster
slurry in a plaster mixer in which the settable binder in the form
of gypsum powder, usually the calcium sulfate hemihydrate, is mixed
with water. The plaster slurry thus produced is spread onto a
continuously movable belt between two cardboard or paperboard webs.
After setting of the plaster to the calcium sulfate dihydrate, the
continuous strand is subdivided into boards and dried.
[0005] To produce the commercial density of the plaster which is
commonly marketed, for the set and dried plasterboard, the
composition is usually provided with an excess of water. The
drying, therefore, must eliminate the excess water and is a step
which involves high costs. To reduce the energy cost it is known to
reduce the density of the plaster layer, for example by introducing
a foam or, in a like manner, to produce pores in the hardenable
layer. A portion of the water which is to be mixed with the plaster
can then be diverted and combined with a foam concentrate, for
example, a surface-active agent or tenside and foamed with air
before the foam is blended into the mixture of the plaster powder
and the balance of the mixing water.
[0006] In the conventional process, the foam is produced in a
separate device and fed to the plaster slurry in a mixer to form
pores in the resulting slurry. This, of course, requires higher
capital cost for a separate apparatus for the generation of the
foam as well as increased operating costs to produce the foam
concentrate. The foam is partly broken down in the mixer and can
give rise to large pores which are seldom desirable.
[0007] In another process known from DE 196 51 448 A1, porous
gypsum is produced by introducing a foaming agent into the calcium
sulfate anhydrate or hemihydrate. To produce the sandwich-type
plasterboard in which the layer of plaster slurry is provided
between two paper or cardboard webs, a mixer has been described in
"Der Baustoff Gips", VEB Verlag fur Bauwesen, Berlin, pages 86-93.
This discontinuously operating mixer initially receives the water
and gypsum powder is stirred into the water and a foaming agent
concentrate is then metered into the composition. The mixing rotor
sucks air into the mixing chamber. The efficiency of pore formation
is not satisfactory in this system.
[0008] A method and apparatus (mixer) for producing porous
finishing mortar or plaster is known from DE-A 21 17 000. The
apparatus comprises a mixer supplied with a water feed and a device
for producing fine and generally stable gas bubbles in a uniform
distribution in the pasty mass of the finish-coat mortar. The
device includes elements for introducing compressed gas into the
mixer, the device utilizing a fritted gass porous element held by a
spring ring.
[0009] Through the porous element the gas is forced into the
previously formed mortar mixture. The fritted glass is located
externally and is connected via openings in the housing wall with
the mixing chamber. This arrangement has the drawback that the
pressure on the fritted glass must be comparatively high or there
is a danger of plugging the openings in the housing wall with the
mortar slurry. Excessive pressure can rupture the glass bubbles
downstream of the fritted glass and thus prevent uniform
distribution of gas bubbles in the composition.
OBJECTS OF THE INVENTION
[0010] It is the principal object of the present invention to
provide an improved method of preparing a porous paste, especially
a paste for the production of gypsum board or plasterboard of the
sandwich type, whereby drawbacks of earlier systems are
avoided.
[0011] Another object of the invention is to provide a mixer for
producing porous paste and especially a plaster slurry which is
capable of ensuring a uniform distribution of gas bubbles in the
paste.
[0012] Still another object of the invention is to provide a method
of making a porous paste or plaster slurry with a comparatively low
water content or proportion and which is particularly suitable for
manufacturing plasterboard or like structural materials with a dry
raw density of less than 1000 kg/m.sup.3.
[0013] Finally it is an object of the invention to provide a method
of mixing a plaster slurry and a mixer for producing such a slurry,
whereby prior art disadvantages are avoided.
SUMMARY OF THE INVENTION
[0014] These objects are attained, in accordance with the
invention, in a method of preparing a porous paste which comprises
the steps of:
[0015] (a) introducing a paste-forming binder and mixing water
forming a settable composition with the binder into a mixing
chamber;
[0016] (b) mixing the binder and water to form a settable
composition by displacing at least one mixing member in the chamber
relative to chamber walls defining the mixing chamber; and
[0017] (c) during displacement of the at least one mixing member
introducing a pore-forming gas into the composition at a supply
pressure above a pressure in the chamber through at least one
fine-porous element forming at least a segment of at least one of
the walls, thereby forming the porous paste.
[0018] The mixer for producing the porous paste can comprise:
[0019] a disk-shaped chamber provided with a peripheral wall and a
bottom wall;
[0020] means for introducing a paste-forming binder and mixing
water forming a settable composition with the binder into the
mixing chamber;
[0021] a mixing disk rotatable in the chamber and formed with a
peripheral array of teeth for mixing the binder and water to form a
settable composition; and
[0022] a fine-porous element forming at least a segment of at least
one of the walls and bounding the chamber on one side and a
pressurizable compartment on an opposite side for introducing a
pore-forming gas into the composition at a supply pressure above a
pressure in the chamber during rotation of the mixing disk, thereby
forming the porous paste.
[0023] In the method of the invention for producing a porous paste
or slurry, especially where a binder of gypsum or calcium sulfate
is combined with mixing water, initially the binder and mixing
water are combined to form a homogeneous paste. The paste is then
moved along walls of the mixing chamber and a gas is injected at an
overpressure, i.e. a pressure above that which prevails in the
mixing chamber, through a portion of a wall of this chamber via at
least one porous element forming that wall portion of the chamber.
The introduction of gas through a porous portion of the wall
directly, ensures a high degree of homogeneity of the gas bubbles
in the mixture of solids and water. Preferably the gas is fed into
the mixture as soon as the paste has a certain homogeneity.
[0024] According to the invention, the gas is supplied to the base
through at least one porous wall segment formed by a porous element
of the wall. The pore width of the fine-pore wall segment should be
smaller than 500 .mu.m and preferably between 3 and 100 .mu.m with
a still more preferred range being 10 to 30 .mu.m.
[0025] The supply of gas through a fine-porous wall segment
directly utilizes in addition to the pore formation by the gas
pressure, the shearing effect of the surface of the wall upon the
paste which is displaced along the wall to finely distribute the
gas in the paste. The shearing effect cuts the gas bubbles free at
the wall and blends them into a homogeneous paste and thus ensures
the homogeneous pore formation. The walls of the mixing chamber
which can be provided with the porous wall segment or element, can
include the peripheral wall, the roof and floor of the mixing
chamber and to the extent that the paste, upon mixing, is moved
along the roof or floor of the chamber.
[0026] According to a feature of the invention, the supply of gas
under pressure causes the foaming of the paste and thus produces a
porous paste. When the paste is used to fabricate building
materials like plasterboard, the porous paste can be such that
densities below 1000 kg/m.sup.3 are attained. The system of the
invention eliminates the need for separate foam-forming apparatus.
The mixing water is in part supplied together with foam formers.
The foam formers that are used can be of the type described in the
publication "Aqueous Foams" (Wssrige Schaume) Spektrum der
Wissenschaft, July 1986, pages 126,127 and 132 through 138, and can
include in addition smaller amounts of foam formers than are
necessary when a separate foaming part of the apparatus is used.
These foam formers are referred to generally here as surface-active
agents or tensides.
[0027] According to a feature of the invention the supply pressure
for the compressed air introduced through the porous wall segment
into the mixing chamber is 0.5 to 6 bar above the pressure in that
chamber.
[0028] Advantageously, the gypsum paste is produced by the mixing
of the calcium sulfate hemihydrate and the mixing water which can
contain foam formers, the water gypsum ratio being 0.6 to 0.8.
Gypsum or plaster paste with this water/gypsum ratio can produce
building materials with a density of less than 600 kg/m.sup.3. For
pastes capable of forming plasterboards with such low densities,
separate foam generation has hitherto been required. The foam
formers used for the production of a porous plaster paste can be
present in relatively small quantities, namely 10 to 500 ppm, for
example, about 100 g of the tenside to 1000 kg of the
hemihydrate.
[0029] It has been found that gypsum recovered from flue gas
cleaning operations can be used. The supply of gas through fine
porous wall elements has been found to yield an fine distribution
of bubbles in the plaster in this case. The especially fine
distribution of the bubbles is believed to be due to the
particularly fine structure of the gypsum waste product.
[0030] Especially good results for the production of porous plaster
compositions can be obtained when the gypsum has a particle size
distribution wherein 30 to 75% of the particles are larger than 12
.mu.m and smaller than 48 .mu.m.
[0031] For the production of porous gypsum in a disk-type mixer
having a rotor disk, the gas is preferably admitted through at
least one fine porous wall segment in the peripheral wall of the
mixing housing or in the housing bottom. This ensures that at least
initially a homogeneous mixture can be made from the hemihydrate
and the mixing water and only then is the gas fed to the outer
periphery of the disk mixer or through another fine porous wall
segment. A portion of the mixing water can be combined with the
tenside and added together therewith beneath the rotor disk to
improve pore formation in the gypsum paste. Preferably the tenside
is added at the region at which the gas is supplied.
[0032] According to another feature of the invention the supply
element for the pore-forming gas, especially pore-forming air, is
arranged on the walls of the mixing chamber. The porous element can
be at least one fine porous wall segment of the peripheral wall of
the chamber. The fine porous wall segment or wall segments formed
by the gas supply elements are directed toward the mixing
chamber.
[0033] According to another aspect of the invention, the mixer has
a mixing chamber which is directly defined by at least one porous
wall segment having a pore width of 3 to 100 .mu.m, especially 10
to 30 .mu.m and a preferred thickness of 2 to 10 mm when that wall
segment is composed of a sintered metal. The use of a sintered
metal porous structure has the advantage over other fine-pore
elements that it is sufficiently stable even at a thickness of 2 to
10 mm to feed the gas under pressure into the paste. Small wall
thicknesses, of course, ensure a small volume of the resulting
structure. According to a feature of the invention, the element is
located along the peripheral wall in a region of a first third of a
rotation of the disk past the outlet for the paste. The combination
of the porous element at this location with the teeth along the
periphery of the rotor disk ensures that the plaster paste will be
fully uniform before it leaves the mixer both in terms of the
combination of the water with the powder and the distribution of
the pores in the water/powder mixture.
BRIEF DESCRIPTION OF THE DRAWING
[0034] The above and other objects, features, and advantages will
become more readily apparent from the following description,
reference being made to the accompanying drawing in which:
[0035] FIG. 1 is a partial vertical section through a disk mixer
according to the invention as taken along the line I-I of FIG.
2;
[0036] FIG. 2 is a radial section, partially broken away and with
other parts in elevation through the mixer of FIG. 1; and
[0037] FIG. 3 is a section through another disk-shaped mixer for
use in the method of the invention.
SPECIFIC DESCRIPTION
[0038] FIGS. 1 and 2 show a disk mixer for mixing plaster powder
with water and comprising a generally flat cylindrical housing with
a bearing 13 in which a shaft 14 is journaled. The housing is
horizontal and the shaft 14 is vertical.
[0039] The shaft 14 carries a rotor disk 15 which is formed along
its periphery with large teeth 16. In the cover or roof 17 of the
housing, an inlet 18 is provided to admit water to the mixing
chamber within the housing, the calcium sulfate powder being
admitted through an inlet 19 for solids which is spaced further
from the shaft than the inlet 18.
[0040] A multiplicity of water inlets 18 can be provided in
angularly equispaced relationship around the shaft 14 in a crown
configuration in the preferred construction and preferably 12 such
water inlets are used.
[0041] The rotor disk 15 has a thick portion or hub in the region
of the shaft 14 and adjoining the zone at which the water inlets 18
are provided and this hub transitions into a thinner portion along
an outer annular zone.
[0042] The bottom 20 of the housing has at least one outlet 21 in
the region of the periphery of the mixing chamber. At least one
water feed inlet 22 is provided in the bottom 20 inwardly of the
annular region of the disk 15 formed with the teeth 16. In a
preferred embodiment, four outlets 21 can be provided along a
semicircle. Between the water supply inlet 22 and the annular
region provided with the teeth 16, a baffle or like arrangement can
be provided in the small gap between the bottom 20 of the housing
and the disk 15 to define an annular labyrinth-like constriction
22a.
[0043] At least one fine-pore wall segment 23 is provided as a feed
element for the gas and this element 23 forms a segment of the
housing wall 24 directly and can extend within the first third of a
rotation of the disk past the outlet 21 or the last outlet 21 in
the direction 25 of rotation of the disk. The housing forms the
mixing chamber between the cover 17 and the disk 15, between the
disk 15 and the cylindrical housing wall 24 and between the bottom
20 beneath the tooth portion 16 of the disk 15 up to constriction
22a.
[0044] In the embodiment shown in FIGS. 1 and 2, the fine pore wall
segment 23 extends over a small portion of the first third of the
peripheral wall 24, namely, in the angular region between the 55th
degree and the 80th degree past the last outlet 21.
[0045] The housing wall 24 is formed in the region of the fine-pore
wall segment 23 with a pressure chamber 29 defined by an outer wall
section 26 and radial end wall sections 27 connected to the wall
element 23. The end walls 27 and section are connected in a
pressure-tight manner with the segment 23 and a fitting 28 is
connected to the compressed air source. The fine-pore wall segment
23 is composed of sintered metal with a thickness of 6 mm and a
pore width of 30 mm. In FIG. 1 between the shaft 14 and the housing
cover 17, the seal 30 is provided. The diameter of the disk mixer
in the embodiment shown is 650 mm. An alternative to the mixer of
FIGS. 1 and 2 can have at an angle region between 90.degree. and
180.degree. in the direction 25 from the last outlet 21,
fine-porous wall segments which can be constructed as described for
the wall segment 23 and can be two in number and thus can be
located in the half of the rotation of the rotor beyond the last
outlet 21.
[0046] In still another alternative, the housing wall 24 can be of
a double-wall construction with an inner wall which is porous and
composed of sintered metal and an outer wall which defines an
annular pressure chamber with the inner wall. The inner wall can be
covered, for example, in the region of the outlets 21, so that it
does not introduce gas into the mixture in these regions. The cover
17 and the bottom 20 can be formed with grooves receiving the
peripheral wall or walls (see for example the grooves 17a and 20a
in FIG. 3).
[0047] In operation a calcium sulfate hemihydrate is mixed with
water in the mixer of FIGS. 1 and 2 with the rotor 15 operating at
speeds of 100 to 300 rpm. The mixer moves the mixture outwardly so
that the mixture is further blended by the teeth 16.
[0048] To avoid the collection of plaster which can set under the
disk 18, a small portion of the mixing water (5 to 15%, by weight,
preferably 7%) is supplied by the inlet 22 from below and flows
outwardly to be distributed uniformly by the labyrinth constriction
22a to mix with the plaster and push out any of the composition
which tends to flow beneath the rotor. In this region the pressure
is about 0.5 bar gauge, i.e. above the pressure within the mixing
chamber above the rotor. Thus if the gauge pressure above the rotor
is 0.5 bar, the water fed below the rotor is introduced at a gauge
pressure of 1 bar. The compressed air is introduced at 0.05 to 2
bar above the pressure in the mixing chamber.
[0049] The hemihydrate can be calcium sulfate collected from a flue
gas desulfurization plant and can have a particle size distribution
in which 30 to 75% by weight of the grains are larger than 12 .mu.m
and smaller than 48 .mu.m. The particle size distribution can be 92
to 97% greater than the 2 .mu.m, 77 to 91% greater than 6 .mu.m, 68
to 87% greater than 12 .mu.m, 59 to 79% greater than 24 .mu.m, 18
to 30% greater than 48 .mu.m and 5 to 9% greater than 64 .mu.m.
[0050] The hemihydrate and mixing water are combined in a
water/plaster ratio (by weight) of 0.6 to 0.8 and preferably 0.7.
10 to 500 ppm of a foaming agent tenside is used, preferably by
addition to the water supplied to the inlet. In a preferred case 91
ppm of the tenside is used.
[0051] The porous paste or plaster slurry which results can be used
to produce plasterboard with a density of 600 kg per m.sup.3.
Without the incorporation of air, the density of the plasterboard
would be 1000 kg per m.sup.3. A metric ton of the plaster slurry is
produced per hour.
[0052] The mixer of FIG. 3 corresponds to that of FIGS. 1 and 2
except that a fine-pore bottom segment 31 is provided to feed the
compressed air into the mixture. The rotor disk 15 has a plate 32
additionally affixed thereon and composed of
polytetrafluoroethylene so that its wear against the housing will
be reduced should the disk come to contact the housing. The porous
segment 23 extends over a small region, for example 55.degree. to
80.degree. over the first third of the path of the rotor beyond the
last outlet in the direction represented by arrow 25 of rotation of
the disk.
[0053] The porous bottom segment 31 abuts the cylindrical housing
wall 24 and can adjoin the latter at a seal 33 which can seal the
pressurizable chamber 35 along the periphery of the segment 31
against the wall 24 and the bottom 20.
[0054] Along the radial edges of the segment 31, the latter and the
disk 15 are so beveled that the edge of the disk can rest upon the
segment 31. Screws countersunk in the segment 31 or extending into
recesses in the disk 15 can hold the porous segment 31 in a
pressure-tight manner on the housing bottom 20 and radial edges of
the segment 31 can also be sealed. The segment 31 can also be
beveled along its inner periphery and sealed in a pressure-tight
manner at 34 with the housing bottom 20. The seal 34 can extend
over the entire inner edge of segment 31 and the disk.
[0055] Below the fine-pore bottom segment 31, therefore, the
pressure chamber 35 is formed in a recess 36 in the housing bottom
20 bounded by the seals previously mentioned. The recess 36 has the
form of a segmental cutout. A bore 37 and a fitting 38 for
connecting to a compressed-air source communicate with the chamber
35.
[0056] The radial extent of the disk and the fine porous bottom
segment 31 stretch from the housing wall 24 beneath tooth portion
16 of the rotor disk 15. Between the fine-pore bottom segment 31
and the rotor disk and the two flanks of the teeth 16, there is
only a small gap which can amount to about 1 mm. Between the plate
32 and the housing bottom 20 along the central portion of the rotor
disk 15 there is also only a small gap with a thickness of about 1
mm. The bottom 20 can be formed with a further recess 39 close to
the hub of the rotor and connected by a bore 40 with another
compressed air fitting 41. An alternative construction of this disk
mixer provides the fine-porous bottom segment in an angular region
of 90.degree. to 180.degree. from the outlet 21 in the direction of
rotation 25. The fine porous bottom segment can thus lie in the
half of the travel of the rotor beyond the last outlet 21.
[0057] In still another alternative of this disk mixer, the recess
36 can be covered by the housing bottom which is itself composed of
sintered metal and forms the porous segment. In this case, a disk
for the bottom segment and the plate 32 need not be provided.
[0058] The operation of this disk mixer corresponds to that of
FIGS. 1 and 2 with the compressed air being forced into the plaster
paste both through the porous segment 31 and through the gap 39.
The supply of air in this region prevents the accumulation of
hardenable plaster paste beneath the plate 32 and the formation of
an air cushion.
[0059] In an alternative, the fitting 41 can be connected to a
water line through which 5 to 15% of the mixing water, preferably
10 to 15% of the mixing water can be added with any tenside which
is to be introduced. The housing wall 20 can additionally be
provided with porous wall segments 23 as described in connection
with FIGS. 1 and 2 through which compressed air can also be added
to the mixture.
* * * * *