AU2018219975A1 - Method and apparatus for plasterboard production - Google Patents

Abstract

- 19 A method and apparatus for use in plasterboard production is disclosed. The method and apparatus enable the introduction of surfactant foam with respect to a mixer for a gypsum-based slurry during the production of plasterboard. The method comprises 5 introducing the foam directly into a mixer discharge line for forming a core of the plasterboard. The method also comprises introducing the foam directly into either or both of the mixer and an extractor line for forming a hard-facing layer of the plasterboard. The method and apparatus are configured such that the foam, when introduced directly into the mixer, is introduced at a location outside of a lump ring of 10 the mixer. 10577725_1 (GHMatters) P105591.AU

Description

METHOD AND APPARATUS FOR PLASTERBOARD PRODUCTION

Technical Field

This disclosure relates to a method and apparatus for use in plasterboard production.

Background Art

It is known in the art to introduce surfactant foams into stucco during plasterboard production, in order to reduce the weight of the plasterboard, while maintaining sufficient mechanical strength. For example, US patent No. 5643510 discloses a method for producing gypsum board by blending two different foaming 10 agents. In particular, the adjustment of the ratio of foaming agents in order to control the size and distribution of voids in the core of the plasterboard is disclosed.

While the prior art discloses the use of blended foams for the controlled production of voids in the plasterboard core, the present methods are considered inefficient, and lack the flexibility to allow for distinct void distribution control within both the plasterboard 15 core and surface layers thereof.

The above references to the background art do not constitute an admission that the art forms part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the method and apparatus as disclosed herein.

Summary

Disclosed herein is a method of introducing surfactant foam with respect to a mixer for a gypsum-based slurry during the production of plasterboard. The method comprises introducing the foam directly into a mixer discharge line for forming a core of the plasterboard; and introducing the foam directly into either or both of the mixer 25 and an extractor line for forming a hard-facing layer of the plasterboard. Introducing the foam directly into the mixer discharge line enables greater foam efficiency, as air entrained in the foam is not lost by the mixing action of the mixer, and thus is better retained by the slurry that forms the core of the plasterboard.

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-2In accordance with the disclosure, the foam can be introduced directly into the mixer at a location outside of a lump ring of the mixer. Introducing the foam outside of the mixer lump ring enables the air contained in the foam to be better retained by the slurry, as opposed to foam introduced inside of the mixer lump ring, as less vigorous mixing occurs in this portion of the mixer. Additionally, the absence of foam in the mixer at a location inside of the lump ring of the mixer, where more vigorous mixing occurs, enables better mixing of the gypsum and water, and reduces the amount of water being consumed from the foam, thereby enhancing production of the gypsum-based slurry.

On the other hand, introducing the foam directly into the hard-facing extractor line enables greater foam efficiency to be achieved, as air and/or water is not otherwise lost in this part of the process as it is in the mixer, whilst optionally introducing foam into the mixer. In addition, foam structure is preserved. However, this enhanced foam efficiency can also require more precise control of foam addition, in order to produce a product for the hard-facing portion of the plasterboard that is of the required density.

In further accordance with the disclosure, the foam can be introduced into the hardfacing extractor line at a location adjacent to the mixer. Introducing the foam directly into the hard-facing extractor line adjacent to the mixer allows for a shorter period of mixing of the foam with the slurry leaving the mixer, with the foam being mixed in a more targeted manner.

In an embodiment of the method, the amount of foam introduced into the mixer discharge line, and directly into either or both of the mixer and extractor line may be varied based on a control procedure. For example, the control procedure may determine the relative amounts or ratios of foam to be added to either or both of the mixer and extractor line in order to achieve the required relative densities of the hard25 facing and core layers for a given plasterboard product.

By way of further example, the control procedure may comprise measuring and/or controlling one or more of: density of the slurry that forms the hard-facing layer and/or core of the plasterboard; foam density; flow rates of air and/or water for the foam; proportion of foam concentrates in the foam; pressure at various points within the foam 30 introduction system; etc.

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-3 In an embodiment of the method, a solution for the foam may comprise a mixture of a first foam concentrate and a second foam concentrate. The first foam concentrate may constitute a major proportion of the foam and the second foam concentrate may constitute a minor proportion of the foam. The mixing of varying proportions of two foam concentrates allows the composition of the resulting foam to be tailored to the introduction requirements at the mixer or extractor line, and to suit a broad range, of gypsum plasterboard product.

In this regard, the first foam concentrate may comprise at least one surfactant component selected from alkyl sulphates and blends thereof, and the second foam concentrate may comprise at least one surfactant component selected from alkyl ether sulphates and blends thereof. In one embodiment, the alkyl sulphate component may be sodium decyl sulphate, and the alkyl ether sulphate component may be ammonium ether sulphate.

Further in this regard, the first foam concentrate may comprise from about 70% to 15 about 98% of the total foam concentrate weight, while the second foam concentrate may comprise from about 30% to about 2% of the total foam concentrate weight.

Also disclosed herein is an apparatus for introducing surfactant foam with respect to a mixer for a gypsum-based slurry during the production of plasterboard. The apparatus comprises a mixer discharge foam line arranged to introduce foam directly into a main 20 mixer discharge for forming a core of the plasterboard; and a foam introduction line arranged to selectively introduce the foam directly into either or both of the mixer and an extractor line for forming a hard-facing layer of the plasterboard.

In accordance with the apparatus, the foam when introduced into the mixer can be introduced at a location outside of a lump ring of the mixer. Less vigorous mixing occurs in this portion of the mixer, allowing air contained in the foam to be better retained by the slurry. In addition, if there is an absence of foam in the mixer at a location inside of the mixer lump ring, this can enable better mixing of the gypsum and water, which can enhance production of the gypsum-based slurry for the plasterboard.

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-4In an embodiment of the apparatus, the foam that is introduced into the hard-facing extractor line may be introduced into that line at a location that is adjacent to the mixer, with the attendant advantages as set forth above for the method.

In an embodiment of the apparatus, one or more foam generators may be employed to generate the foam from a surfactant concentrate. The generators may be arranged and configured to pass the generated foam at sufficient pressure to overcome slurry pressure in the mixer, so as to better enable said foam introduction into the mixer. The generators may incorporate sufficiently tight internal clearances to thoroughly work the foam concentrate mix, improving the consistency of the resulting foam.

One or more density measurement devices may also be arranged to measure the density of the generated foam. The one or more density measurement devices may optionally be arranged to enable control of the density of the generated foam. For example, the one or more density measurement devices may form part of a feedback loop, such as by providing a signal that the ratio by which the first and second foam concentrates are employed to produce the foam solution, needs to be varied.

In an embodiment of the apparatus, a main foam line from the one or more foam generators may be split. A proportion of the foam may be fed via the mixer discharge foam line to the mixer main discharge that is located adjacent to the mixer for production of the core of the plasterboard. Another proportion of the foam may be fed 20 via the foam introduction line to either or both of the mixer and the extractor line for production of the hard-facing layer of the plasterboard.

In a known foam introduction procedure, the mixer foam line is employed to introduce foam into the mixer at a location inside of the lump ring (i.e. into the more vigorous mixing portion of the mixer). Here, a greater proportion of the air within the foam is lost due to the increased mixing action and the slurry hydration process. This foam introduction has been employed as a processing aid, for example, the foam surfactant may act as a lubricating agent within the mixer for the gypsum-based slurry.

In accordance with the present disclosure, the foam introduction line can instead be split into the mixer foam line and the extractor foam line. The mixer foam line may be selectively employed to introduce foam into the less vigorous mixing portion of the

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-5 mixer, outside of the lump ring, whereby less of the air within the foam is lost due to the mixing action. The extractor foam line may be selectively employed to introduce the foam directly into the extractor line, whereby significantly reduced mixing of the foam occurs, and hence foam structure is preserved. The ratios between foam split into 5 the mixer foam line and the extractor foam line may be varied to achieve variations in foam bubble structure, and to also take account of varying process setups in different factories.

In a further embodiment of the apparatus, the main foam line from the one or more foam generators may be split by a control valve. The valve may allow the density of the 10 final plasterboard product to be controlled.

In a further embodiment of the apparatus, foam introduction via the foam introduction line may be assisted by a progressing cavity pump. The progressing cavity pump may be fluidly connected to the foam introduction line. Such a pump can increase the level of control over the amounts of foam as well as the pressure of foam directed to each 15 foam line.

In a further embodiment of the apparatus, one or more density measurement devices and flow rate meters may be arranged within the system to determine the proportion of foam to be selectively introduced into the mixer discharge line, and directly into either or both of the mixer and the extractor line for the plasterboard hard-facing layer. Such devices and/or meters may allow for more precise control over the densities of the final plasterboard product.

For example, the one or more density measurement devices may be arranged to measure one or more of the density of the slurry that forms the hard-facing layer and/or core of the plasterboard. The one or more density measurement devices may optionally be arranged to enable control of the density of such slurries. For example, the one or more density measurement devices may form part of a feedback loop, such as by providing a signal or indication that the ratio by which the surfactant foam introduced with respect to the mixer is split between the mixer discharge line, and either or both of the mixer and extractor line needs to be varied.

Brief Description of the Drawings

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-6Embodiments will now be described by way of example only, with reference to the accompanying drawings in which:

Fig.l shows a flow diagram for an embodiment of a method for manufacturing plasterboard;

Fig. 2 shows a flow diagram for an embodiment of a method for producing a solution for a surfactant foam, from a first and a second foam concentrate;

Fig. 3 shows a flow diagram for an embodiment of a method for generating a surfactant foam from a solution of a first and a second foam concentrate;

Fig. 4 shows a flow diagram for an embodiment of a method for introducing a 10 foam to a gypsum-based slurry.

Detailed Description of Specific Embodiments

In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments 15 described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, 20 separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

Disclosed herein is a method of introducing surfactant foam to a gypsum-based slurry during plasterboard production. An exemplary formulation for preparing plasterboard, including various ranges of additives in grams per square meter, is set out in Table 1.

The general formulation provided in Table 1 can be used in the manufacture of plasterboards of 10mm thickness in accordance with the process flow diagrams, shown in Figures 1 to 4, as well as for ceiling board.

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Table 1

Component	Approx, gsm
Stucco	4000 - 5500
Accelerator	5 - 100
Retarder	0.1-2.0
Potassium Sulphate	0-50
Starch	20-80
Foaming Agent	2-8
Paper Pulp	0-25
Water Reducing Agent	4-25
Boric Acid	0-22
Fibreglass	0-15
Tartaric Acid	0-2

The ranges provided in Table 1 illustrate indicative ranges of additives suitable for inclusion in a foamed gypsum slurry for manufacturing a gypsum product, such as 5 plasterboard. Those skilled in the art will readily understand that different additives may interact in the foamed gypsum slurry in different ways, and that processing conditions may alter the amount of a specific additive required. For example, it is known that the way in which gypsum is calcined imparts different properties to the resulting stucco. As a consequence of those different properties, the amount of e.g.

accelerator, retarder, water, etc., required can vary. Those skilled in the art will also readily understand that the amount of the different additives may also be varied, depending on the properties required of the resultant plasterboard.

General reference will now be made to the process flow diagram shown in Figure 1. Individual components of the process will, at the same time, be described with reference to Figures 2, 3 and 4, noting that the embodiments illustrated in these figures may be incorporated into an integrated process, as illustrated in Figure 1. The integrated process illustrated in Figure 1 comprises generally a surfactant mixing unit 12, a foam generation unit 14, and a gypsum-foam mixer unit 16, each of which comprise individual components as illustrated in the accompanying figures.

In general, the foam introduction process first comprises generation of a surfactant foam. In an embodiment of the method illustrated by the flow diagram of Figure 2 (which details the surfactant mixing unit 12), a solution for the foam may comprise a

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-8mixture of a first foam concentrate 20 and a second foam concentrate 21. The first foam concentrate may constitute a major proportion of the foam and the second foam concentrate may constitute a minor proportion of the foam. The mixing of varying proportions of the two foam concentrates is controlled by a pump and flow meter arrangement 22 in fluid connection with the first foam concentrate 20, and a subsequent pump and flow meter arrangement 23 in fluid connection with the total flow of the combined foam concentrates, allowing the composition of the resulting foam solution to be tailored to the requirements, and to suit a broad range, of gypsum plasterboard product. In this regard, the density of the resulting foam is measured by a density meter 10 to determine the required proportions of each of the first and second foam concentrates to produce a required foam density. The density meter forms part of a feedback loop, by providing a signal that the ratio by which the first and second foam concentrates are employed to produce the foam solution, needs to be varied.

In this regard, the first foam concentrate may comprise at least one surfactant component selected from alkyl sulphates and blends thereof, and the second foam concentrate may comprise at least one surfactant component selected from alkyl ether sulphates and blends thereof. In one embodiment, the alkyl sulphate component may be sodium decyl sulphate, and the alkyl ether sulphate component may be ammonium ether sulphate.

Further in this regard, the first foam concentrate may comprise from about 70% to about 98% of the total foam concentrate weight, while the second foam concentrate may comprise from about 30% to about 2% of the total foam concentrate weight.

Water is introduced to the solution of the first and second foam concentrates by means of one or more pumps 24, 25 in fluid connection with a flow meter 26. In this regard, 25 the one or more foam water pumps 24, 25 supply water at sufficient pressure to cause the pressure of the generated foam to overcome slurry pressure in the mixer, to enable said foam introduction into the mixer. Thus, the foam water pumps force water through the foam generators and into the mixer.

In some embodiments, feedstock for the foam concentrates may be stored in storage tanks 27. The storage tanks 27 may comprise a plurality of pumps and flow meters,

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-9which may be employed to distribute varying mixtures of the first and second foam concentrates, for use in different aspects of the plasterboard production process.

In an embodiment of the apparatus illustrated in Figure 3 (which details the foam generation unit 14), the solution of the first and second foam concentrates is preferably fed through a static mixer 30, where water is introduced, prior to admission to the one or more foam generators 32, 33, 34. Compressed air is also introduced to the one or more foam generators by means of a flow meter 31 that is in circulating fluid connection with the first foam generator 32. The generators may be arranged and configured to pass the generated foam at sufficient pressure to overcome slurry pressure in the mixer, so as to better enable said foam introduction into the mixer. The generators may incorporate sufficiently tight internal clearances to thoroughly work the foam concentrate mix, improving the consistency of the resulting foam. The density of the resulting foam may be measured by means of a liquid density transducer 40.

The main foam line 35 carrying the resulting foam from the one or more foam generators may then be split into a foam introduction line 36, a mixer discharge line 37, a sample line 38 and a dump line 39. In an embodiment of the method, the amount of foam introduced into each of these lines may be varied based on a control procedure. For example, the control procedure may determine the relative amounts or ratios of foam to be added to each of the mixer discharge, foam introduction and extractor lines, in order to achieve the required relative densities of the hard-facing and core layers for a given plasterboard product. In a further embodiment, the control procedure may optionally employ a progressing cavity pump 41, fluidly connected to the foam introduction line 36, to increase the level of control over the amounts of foam as well as the pressure of foam directed to each foam line. Foam introduction via the foam introduction line may be assisted by such a pump.

By way of further example, the control procedure may comprise measuring and/or controlling one or more of: foam density; slurry density; flow rates of air and/or water for the foam; proportion of foam concentrates in the foam; and pressure at various points within the foam introduction system.

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-10With reference to the flow diagram of Figure 4 (which details the gypsum-foam mixer unit 16), a proportion of the foam may be fed via the mixer discharge foam line 37 to a main mixer discharge 47 that is located adjacent to the mixer 42 and the cannister 48 for production of the core of the plasterboard, via the boot 49. Another proportion of the foam may be fed via the foam introduction line 36 to the mixer 42 via a mixer foam line 45, or to an extractor line 43 via an extractor foam line 46, for production of the hardfacing layer of the plasterboard.

The foam introduction line 36 is further split into:

a mixer foam line 45 that is arranged to introduce foam into the mixer at a location outside the lump ring 44; and an extractor foam line 46 that is arranged to introduce foam into the extractor line 43 for the plasterboard hard-facing layer.

The mixer foam line 45 is selectively employed to introduce a proportion of the foam in line 36 at a location outside of the lump ring 44 of the mixer. This is the less vigorous 15 mixing portion of the mixer, whereby less of the air within the foam is lost due to the mixing action. The structure of the foam introduced in this location is better retained in the gypsum slurry, thereby improving bubble formation in the resulting hard-facing layer. Additionally, the relative absence of foam in the mixer at a location inside of the lump ring 44 of the mixer, where more vigorous mixing occurs, enables better mixing 20 of the gypsum and water in this location, thereby enhancing production of the gypsumbased slurry.

The extractor foam line 46 is selectively employed to introduce the foam directly into the extractor line 43 for the hard-facing layer. In this location, significantly reduced mixing of the foam occurs, and hence foam structure is substantially preserved.

Introduction of the foam directly via the extractor foam line 46 into the hard-facing extractor line 43 enables greater foam efficiency to be achieved, as air is not otherwise lost in this part of the process, whereas it is lost in the mixer 42. However, this enhanced foam efficiency can also require more precise control of foam addition, in order to produce a product for the hard-facing portion of the plasterboard that is of the required density. This foam can also be introduced directly into the hard-facing

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-11 extractor line at a location adjacent to the mixer, allowing for a longer period of mixing of the foam with the slurry leaving the mixer.

The selective and varying introduction of foam at either of lines 45 or 46, for the production of the hard-facing layer, allows optimum control over the density distribution within the final product and additionally improves the efficiency with which the gypsum-based slurry required for the hard-facing layer is produced. The ratios of the foam split between either of lines 45 or 46 is able to be varied to achieve variations in foam bubble structure in the hard-facing layer, and to also take account of varying process setups in different factories/applications.

One or more density meters, other density measurement devices (e.g. a standardised cup weight measurement device), and flow rate meters can be arranged within the system to determine the proportion of foam to be selectively introduced into either of the mixer or the extractor line for the plasterboard hard-facing layer. Such meters may allow for more precise control over the densities of the final plasterboard product, allowing for example, a density ratio between the light core and dense hard-facing products of 1:1.3 to be maintained.

Examples

Non-limiting examples will now be described. Example 1 outlines exemplary foam compositions and Example 2 describes the introduction of such foam in the preparation of gypsum boards to exemplify their suitability to form lightweight gypsum boards.

Example 1

Foam compositions A to D were prepared as solutions of a first foam concentrate, sodium decyl sulphate, and a second foam concentrate, ammonium ether 25 sulphate, in accordance with the present disclosure. The compositions are shown in Table 2, with the weight percentage of each component given as that of the total foam weight.

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Table 2

Composition	sodium decyl sulphate wt. %	ammonium ether sulphate wt. %
A	90	10
B	80	20
C	75	25
D	70	30

Foam compositions within the ranges outlined above were observed to be suitable for use with the gypsum-based slurries used to produce the core and hard-facing layers of 5 the plasterboard. Such compositions were able to produce plasterboard having both decreased weight and adequate strength characteristics. The sodium decyl sulfate and ammonium ether sulfate concentrates used in the preceding example are each commercially available from Stepan Company, under the trade names ‘POLYSTEP B25’ and ‘CEDEPAL FA-403’ respectively.

Example 2

Sample plasterboards A to E were prepared in accordance with the process flow diagram for a plasterboard manufacturing process shown in Figure 1, using Formulation A as shown in Table 3. The stucco in Formulation A had been prepared by flash calcination, using the Calcidyne™ process.

Table 3

	Formulation A	Formulation B	Formulation C	Formulation D	Formulation E
Stucco	4550 gsm	4550 gsm	4250 gsm	4550 gsm	4500 gsm
Accelerator	8 gsm	7 gsm	7 gsm	40 gsm	75 gsm
Retarder	0.7 gsm	0.9 gsm	0.9 gsm	1.1 gsm	1.4 gsm
Potassium Sulphate	10 gsm	10 gsm	10 gsm	31 gsm	20 gsm
Starch	50 gsm	50 gsm	50 gsm	50 gsm	45 gsm
Foam	4 gsm	3.5 gsm	3.5 gsm	4 gsm	5.3 gsm
Paper Pulp	20 gsm	20 gsm	20 gsm	20 gsm	15 gsm
Water Reducing Agent	15 gsm	15 gsm	15 gsm	18 gsm	20 gsm
Boric Acid	18 gsm	18 gsm	18 gsm	18 gsm	18 gsm

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- 13 Formulation differences (such as the amount of various additives employed) were attributable to, amongst other things, the way in which the stucco was prepared. The stucco used in Formulations A, B and C was prepared by flash calcination, using the 5 Calcidyne™ process. In the Calcidyne™ process the gypsum is ground into a powder prior to being calcined. The stucco used in Formulation D was prepared by flash calcination, using an impact (imp) mill process where grinding and calcining of the gypsum occur in one step. The stucco used in Formulation E was prepared by the continuous kettle calcination of ground gypsum. This is a slower process than the two 10 different flash calcination methods identified above.

It was noted that the different calcination methods can result in different ratios of stucco constituents (unburnt gypsum, hemihydrate, soluble anhydrite and insoluble anhydrite), which also results in different properties of the stucco, including acceleration rates and water requirements.

Example 3

The foam compositions of Example 1 were selectively introduced to the gypsum-based slurries of Example 2, in accordance with the flow diagram of Figure 4, as part of the plasterboard manufacturing flow diagram shown in Figure 1. The proportions of foam 20 introduced at each introduction location of Figure 4, were selected so as to produce the resulting density ratios of the core and hard-facing layers as given in Table 4

Table 4

	Core wt. %	Hard-facing wt. %
A	1	1.20
B	1	1.25
C	1	1.30
D	1	1.35

Samples A to D were prepared using 220 gsm face paper sheet and 160 gsm back paper sheet. Boards 10 mm thick were prepared, and the board weight for each sample was

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- 14determined. Various properties of the resulting plasterboard Samples A to D are shown in Tables 5 to 8, including results for nail pull resistance (AS/NZS 2588 minimum of 270N), penetrometer (AS/NZS 2588 minimum of 45N), bending strength in the machine direction (AS/NZS 2588 minimum of 360N), and bending strength in the cross 5 direction (AS/NZS 2588 minimum of 150N). The tests were conducted, and results provided in these tables, merely to indicate that Samples A to D prepared in this example meet various AU/NZ Standards for gypsum plasterboard.

Table 5

Sample	Brd Wt kg/m2	Nail Pull Resistance (N)
1	2	3	4	5	6	Avg
A	5.66	276.7	312.7	274.9	266.7	301.5	282.1	285.8
B	5.67	294.6	295.7	285.2	291.5	301.0	260.7	288.1
C	5.66	269.8	275.0	291.1	280.8	291.4	287.4	282.6
D	5.70	305.7	294.1	308.5	284.9	317.9	276.8	298.0

Table 6

Sample		’enetrometer (h	)
Top	Top	Top	Avg	Bot	Bot	Bot	Avg
A	67.3	73.9	73.9	71.7	66.3	71.9	71.9	70.0
B	71.9	79.1	84.7	78.6	68.3	75.5	75.5	73.1
C	68.0	72.6	72.6	71.1	69.0	69.3	69.3	69.2
D	68.3	74.8	85.0	76.0	73.5	76.8	76.8	75.7

Table 7

Sample	Brd Wt kg/m2	Bending Strength (Machine Direction) (N)
Face Up	Face	Down	Avg
A3	5.66	424.9	423.8	446.5	456.2	437.9
A4	5.70	410.5	410.7	438.7	436.8	424.2

Table 8

Sample	Brd Wt kg/m2	Bending Strength (Cross Direction) (N)
Face Up	Face	Down	Avg
A3	5.66	166.9	175.2	200.6	192.7	183.9
A4	5.70	162.8	162.4	197.2	199.2	180.4

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- 15 In the claims which follow and in the preceding summary except where the context requires otherwise due to express language or necessary implication, the word “comprising” is used in the sense of “including”, that is, the features as above may be associated with further features in various embodiments.

Claims (15)

1. Claims

   1. A method of introducing surfactant foam with respect to a mixer for a gypsumbased slurry during the production of plasterboard, the method comprising:

   - introducing the foam directly into a mixer discharge line for forming a core

   5 of the plasterboard; and

   - introducing the foam directly into either or both of the mixer and an extractor line for forming a hard-facing layer of the plasterboard, wherein the foam, when introduced directly into the mixer, is introduced at a location outside of a lump ring of the mixer.

   10
2. 2. A method according to claim 1, wherein the foam is introduced into the hard-facing extractor line at a location adjacent to the mixer.
3. 3. A method according to claim 1 or 2, wherein the amount of foam introduced into the mixer discharge line, and directly into either or both of the mixer and extractor line is varied based on a control procedure.

   15
4. 4. A method according to claim 3, wherein the control procedure comprises measuring and/or controlling one or more of: density of the slurry that forms the hard-facing layer and/or core of the plasterboard; foam density; flow rates of air and/or water for the foam; proportion of concentrates in the foam; pressure at various points.
5. 5. A method according to claim 3 or 4, wherein the control procedure comprises

   20 varying a ratio by which the surfactant foam introduced with respect to the mixer is split between the mixer discharge line, and either or both of the mixer and extractor line.
6. 6. A method according to any one of the preceding claims, wherein a solution for the foam comprises a mixture of a first foam concentrate and a second foam

   25 concentrate.
7. 7. A method according to claim 6, wherein the first foam concentrate constitutes a major proportion of the foam and the second foam concentrate constitutes a minor proportion of the foam.

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8. 8. A method according to claim 6 or 7, wherein the first foam concentrate comprises at least one surfactant component selected from alkyl sulphates and blends thereof, and the second foam concentrate comprises at least one surfactant component selected from alkyl ether sulphates and blends thereof.

   5
9. 9. A method according to claim 8, wherein the alkyl sulphate component is sodium decyl sulphate, and the alkyl ether sulphate component is ammonium ether sulphate.
10. 10. A method according to any one of claims 6 to 9, wherein the first foam concentrate comprises from about 70% to about 98% of the total foam concentrate weight and the second foam concentrate comprises from about 30% to about 2% of the total

    10 foam concentrate weight.
11. 11. Apparatus for introducing surfactant foam with respect to a mixer for a gypsumbased slurry during the production of plasterboard, the apparatus comprising:

    - a mixer discharge foam line arranged to introduce foam directly into a main mixer discharge for forming a core of the plasterboard; and

    15 - a foam introduction line arranged to selectively introduce the foam directly into either or both of the mixer and an extractor line for forming a hardfacing layer of the plasterboard, wherein the foam when introduced into the mixer is introduced at a location outside of a lump ring of the mixer.
12. 12. Apparatus according to claim 11, wherein the foam that is introduced into the hard-

    20 facing extractor line is introduced into that line at a location that is adjacent to the mixer.
13. 13. Apparatus according to claim 11 or 12, further comprising one or more foam generators, wherein the one or more foam generators generate the foam from a surfactant concentrate, and wherein the one or more foam generators pass the

    25 generated foam at sufficient pressure to overcome slurry pressure in the mixer to enable said foam introduction into the mixer.
14. 14. Apparatus according to claim 13, wherein a main foam line from the one or more foam generators is split, with a proportion of the foam being fed via the mixer

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    - 18 discharge foam line to a main mixer discharge adjacent to the mixer for production of a core of the plasterboard, and with another proportion of the foam being fed via a foam introduction line to either or both of the mixer and the extractor line for production of the hard-facing layer of the plasterboard.

    5 15. Apparatus according to claim 14, wherein the main foam line from the one or more foam generators is split by a control valve.

    16. Apparatus according to any one of claims 12 to 16, wherein foam introduction via the foam introduction line is assisted by a progressing cavity pump, attached to the foam introduction line.

    10 17. Apparatus according to claims 11 to 16, further comprising one or more density measurement devices and flow rate meters arranged to determine the proportion of foam to be selectively introduced into the mixer discharge line, and directly into either or both of the mixer and the extractor line for the plasterboard hard-facing layer.
15. 15 18. Apparatus according to claim 17, wherein the one or more density measurement devices are arranged to measure and optionally enable control of one or more of the density of the slurry that forms the hard-facing layer and/or core of the plasterboard.