HK1190987B - Method and apparatus for manufacturing gypsum products - Google Patents

Description

Method and apparatus for making gypsum products

The present invention relates to a method and related apparatus for producing a gypsum plaster (mortar) product for construction purposes, such as for use in the manufacture of plasterboards and the like.

Gypsum is a naturally occurring form of calcium sulfate in the form of a stable dihydrate (CaSO)₄·2H₂O) form. The term "gypsum" as used herein refers to calcium sulfate in that stable dihydrate state and includes naturally occurring minerals, synthetically derived equivalents, and dihydrate materials formed by hydration of stucco (calcium sulfate hemihydrate) or anhydrite.

The properties of gypsum make it highly suitable for use in industrial and construction plasters as well as other building products such as gypsum wallboard. It is a plentiful and generally inexpensive raw material that, through successive dehydration and rehydration steps, can be cast, molded or otherwise formed into useful shapes. For example, gypsum wallboard (also known as plasterboard or drywall) is formed as a set gypsum core sandwiched between paper cover sheets.

Gypsum is typically prepared for use as stucco by grinding and calcining at relatively low temperatures (e.g., about 120-. This results in partially dehydrated gypsum, typically in the form of the beta crystalline form of hemihydrate, which generally has an irregular crystal structure. Beta hemihydrate can be used as a building or construction material by: it is mixed with water to form an aqueous stucco slurry, paste or dispersion, which is then solidified by recrystallization from an aqueous medium.

It is known to use foam in the manufacture of such gypsum products. The bubbles created by the foam help to reduce the weight of the gypsum product so that shipping and handling of the product is more cost effective. The size and distribution of the bubbles have an influence on the mechanical properties of the core of the produced plasterboard.

Foams are typically produced by utilizing a quantity of a foam-producing surfactant that is diluted with water and then combined with compressed air. This foam is then injected into a mixer, typically a high shear mixer.

A typical foam generator comprises a tube filled with a permeable porous medium with controlled pore space (e.g., a bead of porous glass or ceramic). The foam is then produced by injecting a blend of blowing agent and air stream into the tube. In this case, the structure of the foam produced is then controlled by optimizing the back pressure applied to the tube. Such foam generators are referred to as static foam generators. A typical static foam generator is described in US4455271, which is incorporated herein by reference.

Other foam generators include internal rotating mechanisms that thoroughly mix water with a blowing agent to produce a foam. In some cases, the rotating blades may be equipped with a mixing chamber that allows for nucleation of foam bubbles (foambubbles). These are commonly referred to as dynamic foam generators. A typical dynamic foam generator is described in US4057443, which is incorporated herein by reference.

The placement and use of foam generators within a gypsum product line, however, provides little control over the size of the foam structure and the bubbles that ultimately form part of the gypsum product structure (e.g., a plasterboard structure).

WO2005/080294 discloses the concept of producing a gypsum slurry with controlled bubble size and distribution of the added foam. It is proposed that a bimodal distribution can be created by separating the effluent from the mixer in two different air blenders. The separate streams of slurry may then be recombined into a slurry mixture before being deposited onto the line conveyor. WO2005/080294 relates to the concept of using gypsum slurry as liquid to produce a foamed slurry. It is proposed to provide foam without the addition of water, which is necessarily associated with pre-foaming. However, it is also difficult to control the size and distribution of foam bubbles within the slurry and thus the distribution and size of air pockets (airbubbles) created within the set plasterboard.

US5484200, which is incorporated herein by reference, discloses the use of a first mixer and a second mixer operating at relatively low shear conditions compared to the first mixer. The foam is introduced into the second mixer, which reduces uneven distribution of air and thus voids in the finished board product.

According to the invention, there is provided an apparatus for producing a gypsum product, comprising a mixing unit for mixing gypsum with water and at least two foam generators, each producing a different foam and each being arranged to provide a simultaneous separate foam feed into the mixing unit.

By providing foams from two different foam generators to mix with gypsum and water in a mixing unit, it is possible to independently vary certain characteristics of the foam, thereby achieving an improved combination of properties in the gypsum/water/foam slurry. Preferably, the device is configured to allow for independent control of physical parameters of the foam generating process (such as temperature, air flow and other parameters unrelated to the chemical composition of the foam) for each of the at least two foam generators. In addition, the device may also be configured to allow for independent control of certain chemical parameters of the foam-generating process (e.g., surfactant type and amount).

Control of the physical parameters of the foam-generating process is preferred over control of the chemical parameters of the foam-generating process, as control of the physical parameters does not require changes in the formulation of the foam (e.g., by using additional additives or changing the relative amounts of different chemical components of the foam). Furthermore, by controlling the physical parameters in preference to the chemical parameters of the foam generation process, it is possible to reduce the effect of material changes (e.g. impurities) on the foam bubble size.

Preferably, two foam generators are provided, each arranged to produce foam having a different air volume fraction. This can be achieved, for example, by: varying the rate of air flow into one or both foam generators, or operating the foam generators at different operating temperatures.

For example, a first of the two foam generators may be adapted to produce foam having a density of between 100g/l and 300g/l, preferably between 200g/l and 300 g/l. Such foams are typically referred to as high density foams or wet foams. Such foams are considered to be relatively stable, meaning that the size and size distribution of the air bubbles within the foam does not significantly change or develop after the foam has been produced.

The second of the foam generators may be adapted to produce foam having a density of between 20g/l and 100g/l, preferably between 30g/l and 50 g/l. Such foams are typically referred to as low density foams or dry foams. Such foams are considered to be relatively unstable, with the result that the bubbles initially formed within the foam tend to coalesce, and therefore large bubble sizes are typically obtained. Due to its low stability, such foams have until now been considered unsuitable for incorporation into gypsum slurries, especially during industrial-scale (rather than laboratory-scale) production. Furthermore, it has been found that when the foam introduced into the gypsum slurry is composed entirely of low density, unstable foam, it is difficult to achieve a uniform, lightweight gypsum product.

By providing the mixing unit with foams of different densities, it is possible to achieve an improved control of the porous structure of the resulting gypsum product. For example, a mixture of low density and high density foams can allow for the introduction of large voids into the final gypsum product while reducing the disadvantages associated with mixing only low density fugitive foams into the gypsum slurry. It is effective that the use of two different foams allows the stability of the high density foam to be combined with the high air content of the low density foam in order to provide a foam that can be readily mixed with the gypsum slurry while introducing a high level of porosity into the resulting gypsum product.

Moreover, the use of foams having different air contents and densities can provide improved bubble size distribution within gypsum products (e.g., plasterboards). In particular, it is believed that a mixture of small bubbles (produced in high density foams) and large bubbles (produced in low density foams) can result in a pore structure in the gypsum product in which the smaller pores fill the interstices between the larger pores, thereby potentially allowing the overall porosity of the gypsum product to increase.

In fact, it is believed that the improvement in the porous network achieved by using high and low density foams for a given gypsum product density can result in improved mechanical properties of the resulting gypsum product.

Typically, the high density foam and the low density foam are added to the gypsum slurry mixture in the following ratios: 1 part high density foam to 9 parts low density foam, preferably 2 parts high density foam to 8 parts low density foam, more preferably 3 parts high density foam to 7 parts low density foam.

The two foam generators may both be static foam generators. Alternatively, the two foam generators may both be dynamic foam generators. As another alternative, one of the two foam generators may be a static foam generator and the other foam generator may be a dynamic foam generator.

In the case where one or both of the two foam generators are dynamic foam generators, the variation in the size of the air bubbles within the foam can be achieved by varying the rotational speed of the blades. Typically, the rotational speed of the blades is maintained between 1500rpm and 3000 rpm. The higher the rotation speed, the smaller the foam bubble size.

In the case where one or both of the two foam generators are static foam generators, the variation in the size of the air bubbles within the foam may be achieved by varying the pore size and/or pore size distribution within the permeable porous medium (i.e., by varying the parameters of the porous spatial network within, for example, the porous glass or ceramic providing the permeable porous medium).

Where both foam generators are dynamic foam generators, the two foam generators may be configured to operate at different rotational speeds.

The surfactant used is typically a standard anionic foaming agent used in plasterboard production plants, for example sodium or ammonium alkyl ether sulphate having a carbon chain length between 8-12C.

The use of more than two foam generators may allow for control of the bubble size distribution within the gypsum product without changing the composition of the surfactant that generates the foam. That is, the structure of the foam injected into the mixer for forming the gypsum product can be substantially or completely controlled by changes in the physical parameters of the foam-generating process.

However, in some cases, a first of the two foam generators may be arranged to use a first surfactant and a second of the two foam generators may be arranged to use a second surfactant having a different composition than the first surfactant.

Also, a first of the two foam generators may use a foaming solution having a higher surfactant concentration than a foaming solution used by a second of the two foam generators. For example, a first of the two foam generators may be configured to use a surfactant concentration of 0.01-0.1 g per 100g of stucco, while a second of the two foam generators may be configured to use a surfactant concentration of 0.005-0.01 g per 100g of stucco.

Preferably, the mixing unit comprises a first mixer and a second mixer, the second mixer being located downstream of the first mixer so as to receive the gypsum slurry produced in the first mixer. Typically, the first mixer is arranged to receive foam from a first of the two foam generators, and the second mixer is arranged to receive foam from a second of the two foam generators, the second of the two foam generators being arranged to produce foam having a density different from that produced by the first of the two foam generators.

Typically, where two mixers are provided, the first, upstream mixer is arranged to operate at a higher shear than the second, downstream mixer. Typically, in this case, the foam provided to the upstream mixture has a higher density than the foam provided to the downstream mixer. By supplying the low density unstable foam only to the downstream mixer (which operates at a lower shear rate than the upstream mixer), it is possible to reduce damage to the structure of the unstable low density foam so that large bubbles of the low density foam remain within the gypsum slurry, resulting in large voids in the final gypsum product.

Alternatively, the foam produced by each of the two foam generators may be supplied directly to the second, downstream mixer without passing through the first, upstream mixer. In this case it is also possible to reduce damage to the low density unstable foam, so that the probability of large bubbles within the low density foam being retained is increased.

Also, according to the present invention, there is provided a method of producing a gypsum product, wherein calcined gypsum is mixed with water in a mixing unit, and at least two different foam feeds are simultaneously introduced into the mixing unit, the first foam feed comprising a different air volume fraction than the second foam feed.

Preferably, the method of producing a gypsum product comprises: the gypsum is mixed with water in two mixers providing the mixing unit, each mixer being simultaneously provided with a foam feed comprising foams of different bubble size distribution.

Certain features of the invention will now be described with reference to the following drawings.

Drawings

FIG. 1 is a schematic representation of one embodiment of the present invention.

FIG. 2 is a schematic representation of another embodiment of the present invention

Figure 3 is a schematic representation of another embodiment of the present invention.

Fig. 4 is a schematic view of another embodiment of the present invention.

Fig. 5 is a scanning electron micrograph of a gypsum polished cross section of the sample of example 1.

FIG. 6 is a scanning electron micrograph of a polished section of gypsum of the sample of comparative example 1

Fig. 7 is a scanning electron micrograph of a gypsum polished section of the sample of comparative example 2.

Fig. 8 is a scanning electron micrograph of a gypsum polished section of the sample of comparative example 3.

Fig. 9 is a graph showing the results of comparative example 4.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.

Referring to fig. 1, a gypsum product manufacturing process is generally shown at 10. The gypsum product manufacturing process is used in the manufacture of plasterboards and comprises a mixer 12 into which mixer 12 dry gypsum powder is fed from a source 14 via a conduit 16.

The water is conveyed to the mixer 12 via a further conduit 18.

The two foam generators 20 and 22 are arranged to provide two separate foam inputs 24, 26 which are combined prior to feeding into the mixer via foam feed conduit 28. In the embodiment shown in fig. 1, foam generators 20 and 22 and their respective feed conduits 24 and 26 are positioned parallel to each other.

In operation, powdered gypsum or gypsum plaster is continuously supplied through conduit 16 and water is provided through its conduit 18. The water and gypsum are thoroughly mixed together. The foams produced by foam generators 20 and 22 are mixed together via their respective conduits 24 and 26 to provide a "mixed" foam feed 28. The mixed foam feed 28 is then directed into the mixer 12.

Alternatively, the foam streams generated by the two foam generators may be injected separately into the slurry mixer, with each foam stream being injected at a respective injection point. Fig. 2 shows an arrangement in which foam generators 20a and 22a generate foam streams that are injected separately into mixer 12a without being premixed together.

When additives and other ingredients are required during the manufacture of gypsum products, as is often the case, they can be added at any stage through specially provided inlets.

Referring to FIG. 3, a gypsum board manufacturing process is shown generally at 110. In this embodiment of the invention, two mixers 112 and 113 are provided. Both mixers 112 and 113 produce a foamed gypsum slurry simultaneously. However, the first mixer 112 may preferably be relatively high shear compared to the second mixer 113. The gypsum mixture produced by the mixer 112 is fed into a second mixer 113 via a conduit (not shown).

Parallel foam streams are generated in foam generators 120 and 122. The foam stream is mixed and injected into a second (low shear mixer) 113.

High shear mixer 112 provides a highly reactive slurry, while low shear mixer 113 allows for the incorporation of foam into the mixer while avoiding damage to the foam structure.

Alternatively, one foam generator may generate a foam stream that is injected into an upstream mixer, while another foam generator may generate a foam stream that is injected into a downstream mixer. Such an arrangement is shown in fig. 4, where foam generator 120a generates a foam stream that is injected into high shear mixer 112a, while foam generator 122a generates a foam stream that is injected into low shear mixer 113a (which is downstream of high shear mixer 112 a).

Typically, the foam stream injected into high shear mixer 112a has a smaller bubble size than the foam stream injected into low shear mixer 113 a. Thus, damage to less stable foams (i.e., foam streams with larger bubble sizes) may be reduced.

Many other additives may also be added to the mixers 12, 12a, 112, 113, 112a, 113a, which are not discussed or illustrated herein, but are well known in the art. Such additives may include retarders (retarders), accelerators, fibrous materials, and starches. Each will be delivered via a suitable conduit at a designated stage in the mixing process.

Example 1

Samples for measuring nail pull resistance (nailpullresistance) and compressive strength were prepared using the following composition:

TABLE 1

Components	Weight (php for 100 parts stucco)
		Stucco (calcium sulfate hemihydrate)	100
Starch	0.5
		Retarder	0.004
Total water (C)Including foam water)	75
		Foam	See Table 2

Foam prefabrication:

The foaming agent was diluted in water to form a foaming solution (i.e. at 0.1/ww for 100 parts of stucco). Using a feed fromSTEOLDES32 (32% active).

-dividing the flow of foaming solution and the flow of air into two portions, both of which feed (aliment) the respective foam generators (foam generator 1 and foam generator 2). Both foam generators are dynamic foam generators.

Combining the foam produced by the foam generator 1 and the foam generator 2 to form the final foam. The foam is combined in a proportion of 30% to 70% volume fraction for the foam from foam generator 1 and foam generator 2, respectively.

The properties of the foam produced by foam generator 1 and foam generator 2, and of the final foam, are shown in table 2.

TABLE 2

Preparation of stucco slurries：

Stucco and dry ingredients (i.e. 1000g) were weighed.

-dry blending together the stucco and the dry component.

Weigh the required amount of water (i.e. 650 g). The water temperature needs to be approximately 40 deg.c.

-subjecting process water to Waring equipped with an electric speed controller^TMCommercial4L blender.

-then pouring the dry powder into the blender during a period of 30 seconds.

Wait a further 30 seconds to wet the stucco.

At 60 seconds, start mixing at 15000rpm for 10 seconds.

Sample preparation：

-stopping the blender and starting to add the desired amount of foam to the initial slurry (i.e. 100 g).

Mixing of the foam with the initial slurry can be done gently during 10 seconds using a spatula or using a blender at a lower speed level (i.e. about 6700 rpm).

-then pouring the slurry into the following moulds:

a) mini plate samples (i.e. 150mm X12.5 mm); wallboard cardboard envelopes (envelope) for measuring nail pull resistance.

b) Cylinder samples for measuring compressive strength (i.e. for measuring compressive strength))。

The sample is then held in the mould to set, dried in a ventilated oven at a high initial temperature and then a low final temperature until it dries.

Samples were conditioned at 40 ℃ for 24 hours, weighed and subjected to compression and nail pull tests. Nail pull tests were performed according to astm c473 (method B: constant rate cross head speed).

Comparative example 1

Samples for measuring nail pull resistance and compressive strength were prepared using the same method as in example 1, except that foam preparation was performed as detailed below.

Foam prefabrication：

The foaming agent was diluted in water to form a foaming solution (i.e. at 0.1/ww for 100 parts of stucco). Using a feed fromSTEOLDES32 (32% active).

The flow of foaming solution and the flow of air through the foam generator 1 (dynamic foam generator) using the parameters given in table 3.

TABLE 3

Comparative example 2

Samples for measuring nail pull resistance and compressive strength were prepared using the same method as in example 1, except that foam preparation was performed as described in detail below.

Foam prefabrication：

The foaming agent was diluted in water to form a foaming solution (i.e. at 0.1/ww for 100 parts of stucco). Using a feed fromSTEOLDES32 (32% active).

Passing the foaming solution flow and the air flow through the foam generator 2 (dynamic foam generator) using the parameters given in table 4.

TABLE 4

Comparative example 3

Samples for measuring nail pull resistance and compressive strength were prepared using the same method as in example 1, except that foam preparation was performed as described in detail below.

Foam prefabrication：

The foaming agent was diluted in water to form a foaming solution (i.e. at 0.1/ww for 100 parts of stucco). Using a feed fromSTEOLDES32 (32% active).

-injecting a flow of foaming solution and a flow of air into a first foam generator (foam generator 1). The generated foam was then re-injected into a second foam generator (foam generator 2) to generate a reference foam. Thus, the foaming solution is passed through the foam generator 1 and the foam generator 2 sequentially. Both the foam generator 1 and the foam generator 2 are dynamic foam generators.

The parameters used in the foam generation are given in table 5.

TABLE 5

As a result, mechanical testing

For 650kg/m³The equivalent dry core density of example 1 and the mechanical properties of comparative examples 1 to 3 are shown in Table 6.

TABLE 6

As a result: photomicrographs

Fig. 6 and 7 are scanning electron micrographs of polished sections of comparative examples 1 and 2, respectively. A comparison of fig. 6 and 7 shows: when foam is generated using foam generator 2 (rather than foam generator 1), a significantly larger pore size is formed.

Comparative example 4

Samples were prepared according to the method of comparative example 1 (i.e., using the foam preparation method detailed in comparative example 1 and the stucco slurry preparation and sample preparation method of example 1). The rotational speed of the mixer is varied during the step of mixing the foam and the initial slurry.

Figure 9 shows the pore size distribution of four gypsum samples prepared using this method as a function of the rotational speed of the mixer.

Curve 1 shows the pore size distribution for samples prepared using a mixer rotation speed of 6700rpm (revolutions per minute).

Curve 2 shows the pore size distribution for samples prepared using a mixer rotation speed of 8700 rpm.

Curve 3 shows the pore size distribution of samples prepared using a mixer rotation speed of 10400rpm (revolutions per minute).

Curve 4 shows the pore size distribution for samples prepared using a mixer rotation speed of 12000 rpm.

The pore size distribution chart shown in Table 7 was obtained by performing the process on a porous gypsum sample that was fully re-saturated with water¹Obtained by H-NMR relaxation measurement (relaxometric). Technical details regarding this analytical technique and its feasibility in characterizing gypsum-based porous structures are described in the following academic documents: [ Jaffel, H, et al, J.Phys.chem.B,2006,110(14), 7385-) -7391; song, K.M. et al, J.Mat.science,2009,44(18),5004-]. The results shown were obtained using the inverse laplace transform of the transverse NMR relaxation decay (which was obtained using the standard CPMG pulse sequence).

From these results, it can be seen that higher mixer speeds result in smaller final gypsum product pore sizes and lower foam efficiency factors (FoamEfficiencyFactor), as shown in table 7.

TABLE 7

Mixer speed (rpm)	Foam efficiency factor (%)
		12000	37
10400	48
		8700	88
6700	98

Claims (8)

1. A method of producing a gypsum product, wherein calcined gypsum is mixed with water;

characterized in that at least two different foam feeds are simultaneously introduced into a mixture of gypsum and water, a first foam feed comprising a different bubble size distribution than a second foam feed, wherein the first foam feed is generated in a first foam generator (20,120) using a first foam generating process and the second foam feed is generated in a second foam generator (22,122) using a second foam generating process, at least one physical parameter of the first foam generating process being controlled independently of the second foam generating process,

and further wherein the foam-producing surfactant used in the first foam-producing process and the foam-producing surfactant used in the second foam-producing process have the same composition.

2. The method of claim 1, wherein the at least one physical parameter is foam generation temperature.

3. The method of claim 1, wherein the at least one physical parameter is an air inflow rate.

4. The method according to claim 1, wherein at least one of said foam feeds is generated in a dynamic foam generator and said at least one physical parameter is the rotational speed of said dynamic foam generator.

5. The method of claim 1, wherein at least one of said foam feeds is generated in a static foam generator comprising a porous packing medium, said at least one physical parameter being the pore size of the porous packing medium.

6. The method according to claim 1, wherein the first foam generating process uses a foaming solution having a higher concentration of surfactant than the foaming solution used by the second foam generating process.

7. The process of claim 1, wherein the process comprises mixing calcined gypsum with water in two mixers, the mixers comprising a first mixer and a second mixer positioned downstream of the first mixer to receive the gypsum slurry produced in the first mixer, the first mixer receiving the first foam feed, and the second mixer receiving the second foam feed.

8. The process according to claim 7, wherein the first foam feed comprises smaller gas bubbles than the second foam feed.