FIELD OF THE INVENTION
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
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.
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.
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.
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.
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 für 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.
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.
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
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.
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.
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/m3.
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
These objects are attained, in accordance with the invention, in a method of preparing a porous paste which comprises the steps of:
(a) introducing a paste-forming binder and mixing water forming a settable composition with the binder into a mixing chamber;
(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
(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.
The mixer for producing the porous paste can comprise:
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 the binder into the mixing chamber;
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
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.
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.
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 μm and preferably between 3 and 100 μm with a still more preferred range being 10 to 30 μm.
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.
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/m3 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” (Wässrige Schaüme) 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.
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.
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/m3. 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.
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.
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 μm and smaller than 48 μm.
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.
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.
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 μm, especially 10 to 30 μm 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.