METHOD FOR RECYCLING GYPSUM MATERIAL

Abstract

Method for recycling gypsum board material containing calcium sulfate dihydrate (1), comprising the following steps: - introducing the gypsum board material (1) together with water (2) into a wet dissolving unit, in particular a pulper (3), and dissolving the gypsum board material (1) in the wet dissolving unit to produce a gypsum fiber suspension (19), and - converting the calcium sulfate dihydrate of the gypsum fiber suspension (19) to calcium sulfate alpha hemihydrate in a pressure vessel (12).

Description

[0001] The invention relates to a method for recycling gypsum board material containing calcium sulfate dihydrate.

[0002] Gypsum is the mineralogical and technical name for the chemical compound calcium sulfate dihydrate (CaSO₄·2H₂O). Through thermal treatment, gypsum can be converted into calcium sulfate hemihydrate (CaSO₄·1 / 2H₂O), during which the calcium sulfate dihydrate loses some of its bound water of crystallization. Calcium sulfate hemihydrate has the property that, at temperatures below its transformation temperature, it solidifies into a solid mass in the presence of water; this is the reverse reaction to calcium sulfate dihydrate. This property forms the basis for a wide variety of applications.

[0003] Calcium sulfate hemihydrate exists in two chemically and mineralogically identical forms: the alpha and the beta forms. These forms can be distinguished by their physical properties. Calcium sulfate alpha hemihydrate is less soluble in water, exhibits well-formed crystals under a microscope, and requires less water to form a workable gypsum mixture. The beta form of the hemihydrate is also known as plaster of Paris.

[0004] Natural gypsum or technical gypsum, which is a byproduct of industrial processes, is typically used as a raw material for the production of calcium sulfate alpha hemihydrate, also known as setting gypsum. The most important type of technical gypsum is flue gas desulfurization gypsum (FGD gypsum). In addition to technical and natural gypsum, chemical gypsum can also be used. Chemical gypsum is a byproduct of the production of phosphoric acid, citric acid, or titanium dioxide and is accordingly called phosphoric acid gypsum, citric acid gypsum, and titanium dioxide gypsum.

[0005] The planned energy transition will lead to the foreseeable shutdown of coal-fired power plants in the short to medium term, resulting in the loss of large quantities of FGD gypsum, which were previously used effectively by the gypsum industry. This will increase the demand for natural and chemical gypsum. However, the usable quantities of chemical gypsum are significantly lower than those of FGD gypsum, and natural gypsum cannot compensate for the lost quantities because deposits, for example in Germany, are insufficient, developing new gypsum deposits is costly, and permitting issues can arise. Consequently, gypsum is becoming a scarcer raw material.

[0006] Parallel to this trend, the costs for disposing of gypsum waste are rising continuously. This particularly affects the large quantities of gypsum plasterboard waste generated. Gypsum plasterboard consists of approximately 94% gypsum core and approximately 6% cardboard, which laminates the gypsum core on both sides. Gypsum plasterboard waste can be categorized as originating from manufacturing and processing in factories, processing on construction sites, and from the renovation or demolition of buildings. Manufacturing and processing waste includes rejects from gypsum plasterboard plants, as well as cutting and punching remnants generated during the fabrication of gypsum plasterboard (for example, in the production of perforated acoustic panels). Such gypsum plasterboard waste is typically clean and free of foreign matter.On construction sites, gypsum board waste is generated due to board breakage, for example, from improper handling or storage, which can also occur in DIY stores / building supply centers, and primarily from offcuts. Thus, at least 5% of offcuts are typically expected on construction sites. Regarding the renovation and demolition of buildings, gypsum board waste from existing structures must also be considered. This material is less defined in terms of both composition and unwanted foreign materials compared to the previously mentioned waste. For example, tiles, wood panels, remnants of stud walls and their fasteners, such as screws or nails, and other building debris may be adhering to gypsum board during building demolition or renovation.

[0007] Various approaches to recycling gypsum board material, particularly gypsum board waste, have already been proposed in the prior art. Typically, the first step involves mechanically separating the paperboard from the gypsum core. The gypsum core is then mechanically shredded in several stages and classified between each stage, with the paperboard, largely free of gypsum, being separated, for example, by sieving. The gypsum component typically still contains approximately 0.5 to 2% by weight of paper fiber. The gypsum particle size ranges from one millimeter to several centimeters, depending on the system. Both mobile and stationary systems are known.

[0008] These systems, however, have the disadvantage that mechanical separation and processing are complex. They require a large amount of space. Foreign materials frequently cause malfunctions, and contaminated boards must be sorted out manually. Due to the dry, mechanical separation and shredding process, processing generates significant dust, necessitating encapsulation of the system components and additional extraction systems and filters. Wet gypsum board waste poses problems in this recycling process, as the damp gypsum can clog the separation and sorting components. Furthermore, wet gypsum is difficult or impossible to handle in the silo and is problematic for subsequent calcination.

[0009] Recycled gypsum, mechanically processed using such methods, is frequently used in gypsum plasterboard plants. For this purpose, the gypsum is calcined into plaster of Paris in conventional calcining plants, such as rotary kilns and / or airflow calciners, and blended with other gypsum raw materials. The proportion of this calcined recycled material in a mixture for the production of gypsum plasterboard is limited because the gypsum from gypsum plasterboard recycling has significantly smaller crystals and a considerably larger surface area, and therefore a significantly higher water demand than commonly used FGD gypsum or natural gypsum. This effect persists after calcination into plaster of Paris and is sometimes further exacerbated by grain fragmentation during calcination and subsequent processing.

[0010] The high water content of calcined recycled gypsum plaster means that more water is bound in the gypsum core when processed into plasterboard, requiring more drying. This necessitates higher drying capacity compared to non-recycled gypsum plaster, resulting in longer drying times and higher drying energy consumption. The increased amount of bound water then leads to greater porosity in the gypsum core, resulting in reduced strength.

[0011] In summary, it must be noted that the utilization of gypsum from current gypsum board recycling is fraught with significant disadvantages, and the recycling rate is limited to a maximum of 30% based on the total gypsum content (see also "Life Cycle Assessment of Gypsum Board Recycling", research code 3715 343200, Texts 33 / 2017, Environmental Research Plan of the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety. UBA-FB 002501). This also applies to the use of such processed gypsum from gypsum board recycling in the production of gypsum fiberboards, which must be dewatered during production.

[0012] Another disadvantage of conventional gypsum board recycling processes is the poor recyclability of the cardboard waste separated during the recycling process. This cardboard waste is undesirable in the paper industry because gypsum residues, due to the sulfate-containing eluates they produce, pose a problem for the water cycle of paper mills. The sulfate contamination also makes its use in waste incineration plants and as a substitute fuel in cement production undesirable.

[0013] The applicant also unsuccessfully investigated the use of cardboard waste in the production of gypsum fiberboards. For use in gypsum fiberboard plants, where the boards are manufactured using the wet process, the cardboard is dissolved in water to form a fiber slurry, then mixed with additives and setting gypsum, and subsequently dewatered. However, this process presents the problem that the cardboard from gypsum board recycling contains undefined amounts of gypsum residues. Due to their fineness and the mechanical activation by the recycling process, these residues act as highly reactive nucleation sites for the reaction of calcium sulfate hemihydrate to calcium sulfate dihydrate. Consequently, the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate is accelerated in an uncontrolled manner, leading to an undefined and excessively rapid hardening reaction of the gypsum fiber mixture.This negatively impacts the production process even at very high doses of setting retarders, making reliable production impossible. Therefore, the practical use of cardboard waste in gypsum fiberboard production is not feasible.

[0014] The invention is therefore based on the objective of providing an improved method for recycling gypsum board material, especially gypsum board waste, which in particular allows for high-quality and simple recycling.

[0015] To solve this problem, a method with the features of claim 1 is provided. Advantageous further developments result from the dependent claims.

[0016] Accordingly, a process according to the invention for recycling gypsum board material containing calcium sulfate dihydrate comprises the following steps: Introducing the gypsum board material together with water into a wet dissolving unit, in particular a pulper, and dissolving the gypsum board material in the wet dissolving unit to produce a gypsum fiber suspension, and converting the calcium sulfate dihydrate of the gypsum fiber suspension to calcium sulfate alpha hemihydrate in a pressure vessel.

[0017] The resulting alpha hemihydrate gypsum fiber suspension can then, for example, be fed into a process for the production of gypsum plasterboard and / or gypsum fiberboard or, in particular after a cooling and / or dewatering and / or drying process, be stored for later use.

[0018] Within the scope of the present invention, it was surprisingly discovered that known wet dissolving units from the paper industry, unlike wet mixers, are ideally suited for creating a gypsum fiber suspension which, due to the small particle size of the calcium sulfate dihydrate it contains, can be easily further processed to obtain high-quality calcium sulfate alpha hemihydrate in a wet process. For this purpose, the gypsum board material is dissolved and comminuted with water in a pulper. To this end, pulpers, or wet dissolving units from the paper industry in general, typically have at least one stirring device to support the process. At the end of the dissolving process, a pumpable gypsum fiber slurry, namely the gypsum fiber suspension, is obtained, consisting of paperboard fibers separated in water and comminuted gypsum, particularly with particle sizes smaller than 10 mm.The calcium sulfate dihydrate of the gypsum fiber suspension thus produced can then be converted to calcium sulfate alpha hemihydrate in conventional pressure vessels, preferably pressure mixing vessels, under standard thermal conditions and optionally with known additives. A particular advantage of this process is that, after treatment and conversion in the pressure vessel, the previously coarse gypsum components from the gypsum board material are broken down and finely dispersed. This means that the conversion to alpha hemihydrate causes the coarse-grained component to break down into crystals or crystal agglomerates with grains significantly smaller than 1 mm.

[0019] The resulting gypsum fiber suspension with calcium sulfate alpha hemihydrate is suitable for direct further processing, alone or in mixtures with fibers and other gypsums / additives, into gypsum plasterboard or gypsum fiberboard. Alternatively or additionally, the aqueous phase of the alpha hemihydrate suspension can be separated, followed by drying of the solids.

[0020] In this way, proven and robust plant components, some of which originate from the paper industry, are used to enable high-quality yet simple recycling of gypsum board material. This gypsum board material can include, in particular, gypsum board waste. Gypsum board waste can originate, for example, from manufacturing and processing at the manufacturer, from processing on a construction site, and / or from the renovation and / or demolition of buildings.

[0021] The process according to the invention allows for the complete utilization of gypsum and cardboard in a single gypsum product, thus achieving a closed-loop system. In particular, the previously unusable, calcium sulfate-containing cardboard of the gypsum board material is also utilized to a high-quality extent in a gypsum product. Instead of multiple comminution and fractionation components in prior art recycling (i.e., dry, mechanical dismantling), preferably only one dissolution stage is required, which both comminsles the gypsum and fiberizes the cardboard. The space requirements and machinery costs are therefore low, as only a few system components are needed. Due to the wet processing, the process is largely dust-free and can be carried out without the installation of filter and extraction systems. Furthermore, the process also allows for the recycling of moistened gypsum board materials, which were previously inaccessible to prior art dry recycling.

[0022] In summary, both the gypsum core and the cardboard, which has not previously been usable in the gypsum industry, are processed and recycled economically and reliably from the gypsum board material. The fibers from the cardboard can then become a component of the reinforcement in the gypsum product manufactured from it. The gypsum is directly suitable for use in the gypsum industry. This applies in particular to its use in gypsum fiberboard production, whereby the rapid dewatering required for a wet process is provided within the scope of the present invention, as will be explained in more detail below.

[0023] It should also be noted at this point that the transformation conditions of the gypsum fiber suspension in the pressure vessel at temperatures especially greater than or equal to 120°C also sterilize microbiologically contaminated gypsum board material, e.g. caused by long storage with high humidity, and can therefore be reused in the building product without hesitation.

[0024] In particular, it should be noted that conventional methods, i.e., wet processes, can be used for the conversion of the calcium sulfate dihydrate in the gypsum plasterboard suspension to calcium sulfate alpha hemihydrate in the pressure vessel. A pressurized mixing vessel can be advantageously used as the pressure vessel. The conversion step preferably comprises heating the gypsum fiber suspension in the pressure vessel and maintaining the heated gypsum fiber suspension at the heating temperature, whereby the calcium sulfate dihydrate is converted to calcium sulfate alpha hemihydrate. The gypsum fiber suspension is heated to a temperature in the range of 105 to 150°C, particularly in the range of 105 to 135°C, preferably 120°C to 125°C. Of course, exceptions to this range are also conceivable.The holding time for the conversion to calcium sulfate alpha hemihydrate can range from 1 to 90 minutes, particularly 1 to 60 minutes, preferably 1 to 45 minutes. The vapor pressure is selected according to the temperature. With sufficient energy input corresponding to the energy required for the conversion, the conversion is completed after very short holding times, for example 8 to 12 minutes, particularly 10 minutes, after reaching the heating temperature, for example 120 to 125°C, preferably 125°C. The initially present calcium sulfate dihydrate is completely converted to calcium sulfate alpha hemihydrate after the conversion, including the solid adhering to the cardboard fibers.

[0025] In a further advantageous embodiment of the invention, the gypsum board material is introduced gravimetrically into the wet pulping unit such that a mixture of 30 to 60 wt.% gypsum board material with 40 to 70 wt.% water is obtained. For example, in a specific embodiment, the wet pulping unit, in particular the pulper, can be filled with a water reservoir, after which the gypsum board material is gravimetrically metered into the pulper. A panel weighing belt can be used for this purpose, for example. A stirring device (rotor) of the pulper is then activated.

[0026] It has been shown that a high solids content is advantageous for high throughput and low energy consumption. A mixture of 50 wt% gypsum plasterboard and 50 wt% water yields particularly good results.

[0027] The dissolution time is preferably 10 to 20 minutes, preferably 14 to 16 minutes. It has been shown that, regardless of the pulper type used, a pumpable and flowable gypsum fiber slurry is obtained as a gypsum fiber suspension after approximately 15 minutes of dissolution. This slurry consists of almost completely separated fibers and gypsum with particle sizes typically ranging from less than 1 mm to approximately 10 mm. The higher the solids content, the better the comminution of the gypsum. For example, it has been shown that 50 wt% solids results in significantly better comminution than 30 wt% solids. In particular, the friction between the gypsum plasterboards during the dissolution process has a beneficial effect.

[0028] At a solids content of 50 wt%, the material is still easily pumpable. Dissolution is possible up to approximately 60 wt% gypsum board material, although this may require a longer dissolution time. The maximum possible solids content also depends on the ratio of cardboard to gypsum in the gypsum board material. This ratio can vary slightly depending on the board type and thickness. Uniform circulation of the material in the wet dissolution unit has proven important. If this is not achieved visually or by other means, the solids content can be reduced. It should be noted that, depending on the subsequent processing, the solids content of the gypsum fiber suspension can, of course, be further reduced to the desired value by adding water after the dissolution time has elapsed.

[0029] A low-consistency, medium-consistency, or high-consistency (HC) pulper can be used as the wet pulping unit, with the use of a high-consistency pulper being preferred. High-consistency pulpers are used in the paper industry, for example, for fiber contents of 10 to 15%, and it has been shown that, within the scope of the present invention, solid contents of up to 60% can be processed by such a high-consistency pulper.

[0030] Pulpers used in the paper industry typically comprise a mixing vessel (a mixing tub) containing an agitator, the rotor shaft of which can be horizontal or vertical. In essence, a pulper functions like an oversized mixer with a powerful and efficient agitator. This agitator usually includes a rotor as the actuator in contact with the materials being mixed.

[0031] Within the scope of the present invention, particularly when using a low-consistency pulper and / or a medium-consistency pulper, it may be provided that a stirring device with shredding teeth is used in the wet dissolving unit. Such shredding teeth, especially sharp ones, particularly on the rotor, support the fiberization of the cardboard fraction, although it has been shown that this is not necessary in a high-consistency pulper.

[0032] With a high-consistency pulper with a capacity of 20 cubic meters, approximately 20 tons of gypsum board can be processed per hour, assuming 50% by weight, a dissolution time of 15 minutes, and a total loading and unloading time of 15 minutes. Such a high-consistency pulper with a capacity of 20 cubic meters typically has an installed power output of 315 kW for the mixing unit and a power consumption of approximately 220 kW. At the aforementioned throughput, this results in an electrical energy requirement of 11 kWh per ton of gypsum board. Even with a pulper only half the size, with a usable volume of 10 cubic meters, a throughput of 72,000 tons of gypsum board per year is achieved with 7,200 operating hours per year. This throughput is comparable to that of conventional, dry gypsum board recycling plants, which, for example, can process 50,000 tons of gypsum board waste per year.

[0033] If only contaminant-free gypsum board material is used, no further equipment for contaminant removal is necessary, so the gypsum fiber suspension can be conveyed directly or after at least one optional pre-processing step for conversion. In this case, the discharge opening for emptying the suspension can be designed without a screen in the pulper floor. If contaminants are present, the use of a screen in the discharge opening is conceivable, for example with a mesh size of 20 mm to separate coarse contaminants.

[0034] In a preferred embodiment of the present invention, when dealing with gypsum board containing at least one contaminant, the gypsum fiber suspension from the wet dissolving unit is passed through at least one contaminant separator. This applies particularly to gypsum board waste containing contaminants from the renovation and / or demolition of buildings, but may also be applicable in individual cases to gypsum board waste from construction sites. In this context, it was also surprisingly discovered within the scope of the present invention that known contaminant separators from the paper industry can be used to particular advantage. Contaminants differ from the clean material, i.e., the dissolved gypsum fiber suspension, by their dimensions and / or their specific gravity. Accordingly, the separation of such contaminants can be carried out by sieving and gravimetric separation.

[0035] In the paper industry, pulpers with a screen tray, over which the usable pulp is drawn, are frequently used for coarse cleaning. These are combined with a downstream disc sorter, consisting of a screen and a rotor that provides a pumping action and simultaneously keeps the screen clear for the coarse separation of contaminants. Disc sorters, also known as secondary pulpers or reject sorters, remove the majority of the impurities. Such disc sorters can also be used in combination with a pulper without a screen tray.

[0036] Further gravimetric separation of contaminants can be carried out using density separators, also called heavy material separators (or hydrocyclones or MC cleaners). These devices separate contaminants that are heavier than the fibers.

[0037] For the remaining contaminants after the initial sorting, cylinder sorters are used, which can consist of a screen basket and a rotating vane rotor. The screens in the basket can be perforated or slotted. Typical hole sizes are 1 to 2 mm for perforated screens and 0.1 to 0.5 mm for slotted screens. These cylinder sorters are also known as pressure sorters.

[0038] The contaminant separators listed after the pulper are typically used in the processing of waste paper with a solids content of 3 to 7%.

[0039] Within the scope of the present invention, these concepts can also be advantageously employed. Specifically, in the case of a gypsum fiber suspension containing contaminants, it can be provided that the at least one contaminant separator is selected from the group comprising a disc sorter (for the separation of coarse dirt, e.g., a reject sorter), a heavy material separator (hydrocyclone), and a cylinder sorter (pressure sorter, for the separation of finer dirt). The use of contaminant separators in the form of a disc sorter with at least one rotor and at least one screen can be particularly advantageous, since it has been shown that such a screen-rotor combination also has an additional comminution effect on larger gypsum particles. In other words, it can be provided that at least one of the at least one contaminant separator comprises a screen with a rotor that additionally comminses larger gypsum particles.However, in the case of contaminants that threaten to clog such sieves, it may also be advisable to use contaminant separators alternatively or additionally, which at least have a washing drum.

[0040] The disc sorter can be equipped with interchangeable screen sizes. For example, depending on the requirements, screens with mesh sizes of 7 mm, 10 mm, and / or 15 mm can be used. The advantageous effect of further reducing the size of coarser gypsum grains while simultaneously effectively separating contaminants larger than 10 mm has been particularly evident with a 10 mm mesh screen.

[0041] For effective removal of contaminants, it has proven beneficial to adjust the solids content of gypsum fiber suspension to 10 to 25 wt%. This reduces the risk of removing more of the desired material along with the contaminants, thus minimizing efficiency. Therefore, a suitable further development procedure involves diluting the gypsum fiber suspension to a solids content of 10 to 25 wt% before it passes through at least one contaminant separator.

[0042] A specific, advantageous embodiment of the present invention can, for example, provide that, in a first stage, contaminant particles larger than 10 mm are removed in a disc sorter with a screen and / or a washing drum, wherein, in a second stage, heavier contaminant particles are removed by a heavy material separator, in particular a hydrocyclone, and in a third stage, contaminant particles larger than 2 mm are removed by means of a cylinder sorter. Further separation of finer contaminants by pressure sorters with different screen mesh sizes has proven not to be absolutely necessary for the subsequent processes. Therefore, a disc sorter with variable screen sizes, for example using a screen size of 10 mm, is particularly advantageous as a contaminant separator in a first stage for separating coarse contaminants.The upstream wet pulping unit, in particular the pulper, can have a screen tray, preferably with a mesh size of 20 mm, although the discharge tray can also be open. For problematic contaminants, such as fine plastic films that tend to curl up and clog / smear the screen openings, a washing drum is more suitable. Separation of heavy particles heavier than the gypsum and fibers can be carried out in a second stage, for example, by a heavy material separator. In a further, third stage, if necessary, contaminants can then be sorted out using a pressure screen with a reduced mesh size, for example, 2 mm. These are all robust and proven system components from the paper industry that can be applied to a new, advantageous application within the scope of the present invention.

[0043] After treatment in the wet dissolving unit, particularly the pulper, the gypsum fiber suspension typically already has particle sizes smaller than 10 mm, or the dissolving time has been selected accordingly. In principle, the gypsum fiber suspension can then be directly fed to the conversion process, if necessary after passing through at least one contaminant separator. Nevertheless, in some applications, for example, when smaller particle sizes are more suitable for the conversion process, it can be advantageous to optionally pass the gypsum fiber suspension through a wet grinder before conversion, especially one that reduces the particle size to values ​​smaller than 1 mm. Reducing the particle size to less than 1 mm makes the gypsum fiber suspension, which can also be referred to as gypsum fiber slurry, even more homogeneous. Furthermore, this method can reduce the tendency for sedimentation during the conversion to calcium sulfate alpha hemihydrate.Because of these effects, the pressure vessel, especially the pressure mixing vessel, can be designed more simply and can be operated with agitators of lower installed power.

[0044] As has been shown, familiar components from the paper industry, such as deflakers, refiners, and / or dispersers, can be used to particular advantage as wet crushers. In a different configuration, very rapid crushing and a robust process can also be achieved with wet crushers typically used to crush crystal agglomerates and other mineral solids. Such wet crushers include, for example, macerators. These devices can also be used with a solids content of 50 wt% in the gypsum fiber suspension. A single pass is sufficient for adequate crushing, for example, to particle sizes smaller than 1 mm.

[0045] The wet crusher can be connected between the outlet of the wet pulper or the outlet of the last foreign matter separator and the subsequent pressure vessel for thermal treatment.

[0046] In a particularly advantageous embodiment of the present invention, at least one additional gypsum can be added to the gypsum fiber suspension prior to its conversion to calcium sulfate alpha hemihydrate. By adding additional gypsums with larger calcium sulfate dihydrate crystals, for example, synthetic gypsums such as FGD gypsum, chemical gypsum, and / or natural gypsum, the particle size of the calcium sulfate alpha hemihydrate crystals formed during the conversion, the specific surface area, and the dehydration behavior can be specifically influenced. Therefore, it can be provided, in particular, that 10 to 80 wt.%, especially 10 to 50 wt.%, of FGD gypsum and / or natural gypsum and / or chemical gypsum, based on the total gypsum content, is added to the gypsum fiber suspension.Depending on the desired properties of the calcium sulfate alpha hemihydrate to be produced from recycled gypsum, the gypsum fiber suspension can be blended with additional gypsum, for example, to achieve a total gypsum content of 10 to 80 wt% additional gypsum and 90 to 20 wt% recycled gypsum. As already mentioned, this allows for targeted control of the specific surface area, crystal size, particle size distribution, and water demand, as well as the dewatering behavior in a gypsum fiber mixture, which is particularly useful for the production of gypsum fiberboards from the gypsum fiber suspension. For example, tests have shown that adding up to 50 wt% chemical gypsum, natural gypsum, and / or FGD gypsum significantly improves the dewatering and / or filtration behavior, if desired.In particular, it can be shown that filtration and dewatering properties can be adjusted by using such additive gypsum to values ​​that essentially correspond to those of current suspensions used in gypsum fiber mixtures, thus avoiding or at least minimizing any reduction in dewatering time. The fact that only a maximum of 50 wt% of the additive gypsum is required demonstrates that extremely large quantities of the gypsum fiber slurry obtained from recycling gypsum board material—i.e., the gypsum fiber suspension with recycled gypsum—can be used. It should also be noted that the chemical gypsum used as the additive can be selected from the group including phosphoric acid gypsum, citric acid gypsum, and titanium dioxide gypsum. Citric acid gypsum is particularly preferred.

[0047] In a further embodiment, it can also be provided that at least one gypsum waste from the processing of gypsum products, in particular gypsum dust from the processing of gypsum fiberboards and / or crushed gypsum fiberboards and / or gypsum sludge, is added to the gypsum fiber suspension as additional gypsum. In this context, it may be advantageous to add at least one dicarboxylic acid, preferably succinic acid, or a salt of a dicarboxylic acid to the gypsum fiber suspension along with the gypsum waste. In other words, other gypsum-containing waste materials can be mixed into the aqueous gypsum fiber suspension, as proposed, for example, in EP 2 418 184 A2. Fine-grained, gypsum-containing waste is used for this purpose, for example, gypsum-containing dusts, in particular grinding, milling, and sawing dusts generated during the processing of gypsum fiberboards, or suspendable or crushed gypsum sludge, which, for example,Gypsum fiber waste or crushed gypsum board waste, or a mixture thereof, is used as waste material from the sedimentation of gypsum-containing waters. The statements made in EP 2 418 184 A2 can also be applied to the procedure described here, particularly with regard to the amounts of dicarboxylic acid to be added.

[0048] In addition to or as an alternative to the addition of gypsum, it is of course also possible to add at least one additive to the gypsum fiber suspension before or during its conversion to calcium sulfate alpha hemihydrate. Common additives suitable for influencing the crystal habit of calcium sulfate alpha hemihydrate include, besides the aforementioned succinic acid, maleic acid and its salts. Other additives that accelerate the conversion and influence crystal growth include, for example, aluminum sulfate, potassium sulfate, or potassium aluminum sulfate.

[0049] The addition can be carried out with particular advantage in the pressure vessel designed as a pressure mixing vessel. This means that the processes of mixing the gypsum fiber suspension with added gypsum and / or the addition of additives can take place directly in a pressure mixing vessel, in which the conversion to calcium sulfate alpha hemihydrate also occurs. Alternatively, it is also conceivable to carry this out, at least partially, in a separate mixing vessel.

[0050] As mentioned previously, the gypsum fiber suspension converted to calcium sulfate alpha hemihydrate can then be utilized. For example, after the conversion is complete, the product can be cooled to below 100°C, dewatered, dried, and stored. Alternatively, the gypsum fiber suspension cooled to below 100°C can be directly fed into a process for manufacturing gypsum plasterboard or gypsum fiberboard. In this process, the gypsum fiber suspension can be mixed with other raw materials such as setting gypsum, pulps, and known additives, and then fed into the manufacturing process for producing gypsum plasterboard or gypsum fiberboard.

[0051] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawing. The drawings show: Fig. 1 a schematic diagram to illustrate the method according to the invention, Fig. 2 a multi-stage embodiment for the removal of contaminants, Fig. 3 a view inside a pulper before the start of a dissolution process, Fig. 4 a view inside a pulper after 10 minutes of dissolution time, Fig. 5 a view inside a pulper after 15 minutes of dissolution time, and Fig. 6 a graph for determining a characteristic value for the filtration behavior.

[0052] Fig. 1Figure 1 shows a schematic diagram illustrating the process according to the invention. It starts with gypsum board material 1, which is to be recycled, in this case, gypsum board waste. The gypsum board waste can originate, for example, from the manufacturing and processing at the manufacturer, from processing on a construction site, and / or as waste from the renovation and / or demolition of buildings. Gypsum board waste from the manufacturing and processing of gypsum boards can include, for example, rejects from gypsum board plants, cutting and punching remnants generated during the fabrication of gypsum boards, and the like. From a construction site, gypsum board waste resulting from board breakage and offcuts can originate, for example, from gypsum board waste. Gypsum board waste from renovation and demolition is most likely to contain contaminants, such as tile remnants, screws, nails, and the like.This will be discussed in more detail below.

[0053] The gypsum board material 1 is gravimetrically introduced together with water 2 into a pulper 3 as a wet-dissolving unit, for example by means of a plate weighing belt, such that 50 wt.% gypsum board material 1 and 50 wt.% water 2 are present. Generally, the proportion of gypsum board material should be between 30 and 60 wt.%. The pulper 3 comprises an agitator 5 arranged in a mixing vessel 4 (mixing tank), which includes a spiral rotor 6, which in exemplary embodiments may be provided with, for example, sawtooth-shaped grinding teeth. Preferably, however, the pulper 3 is a high-consistency pulper in which such grinding teeth are not necessary.

[0054] Within the pulper, the gypsum board material is dissolved in such a way that the fibers and gypsum particles are separated as much as possible. The dissolution time can be, for example, 10 to 20 minutes, preferably 14 to 16 minutes, and, together with the design of the pulper 3, is selected so that the resulting gypsum fiber suspension contains, as far as possible, only particle sizes smaller than 10 mm. The usable volume of the pulper 3 depends on the desired throughput per unit of time. If the required usable volume is very small, it is advantageous to mechanically pre-shred the gypsum board in a simple form so that the large-format gypsum boards are not too bulky in the container and can be wetted with water more quickly. New gypsum boards typically have a width of 600 to 1250 mm and a length of 1250 to 3000 mm.If such boards are to be used as rejects, mechanical pre-shredding before feeding them into the pulper is recommended to speed up the dissolution process. Even with small pulpers of only 5 m³ usable volume, shredding to approximately 600 mm x 600 mm is sufficient, although shredding to smaller pieces, e.g., DIN A4 size, is not detrimental. Such shredding can be achieved with very little dust using conventional, slow-rotating single- or twin-shaft shredders. The boards fed into the shredder can then be conveyed directly, e.g., via a conveyor belt, to the weighing conveyor. Alternatively, the large-format gypsum board can be sufficiently pre-shredded in the warehouse very easily by driving over it with a tracked wheel loader, which is used for transporting the boards.

[0055] Due to a solids content of, for example, 50 wt%, the dissolved gypsum fiber suspension is readily pumpable. It should be noted that it is quite conceivable to add water downstream of Pulper 3 to produce a lower solids content for further processing.

[0056] If gypsum board material 1 containing contaminants is used, as described above, for example from the renovation and / or demolition of buildings, the gypsum fiber suspension from the pulper 3, which is also a pulper usable in the paper industry, can be passed through at least one contaminant separator in a contaminant separation process 7. Here, too, units known from the paper industry can be advantageously used.

[0057] Fig. 2Figure 7 shows an example of a three-stage design of the contaminant separation process. In a first stage, a disc screen 8 (only indicated here) with a screen size of 10 mm is used. Alternatively, a washing drum can be used instead of a screen if contaminants clog its holes. This is followed in a second stage by a heavy material separator (hydrocyclone) 9 for the separation of heavy particles. In a third stage, a cylinder screen (pressure screen) 10 can then sort out contaminants down to a size of 2 mm.

[0058] To improve sedimentation behavior and / or to meet other requirements regarding the resulting product, the gypsum fiber suspension can optionally be passed through a wet crusher 11 to reduce the overall particle size to less than 1 mm. This makes the gypsum fiber suspension even more homogeneous and less prone to sedimentation during the subsequent conversion of the calcium sulfate dihydrate to calcium sulfate alpha hemihydrate. It should be noted, however, that even without the use of the wet crusher 11, the gypsum fiber suspension is prepared in such a way that it can be converted to finely divided calcium sulfate alpha hemihydrate using the wet process, as it has been shown that larger gypsum particles are crushed during the subsequent conversion process. If a wet crusher 11 is to be used, conventional components from the paper industry, such as destippers, refiners, and / or dispersers, can be employed.Macerators or related aggregates known for reducing the size of crystal agglomerates and / or other mineral solids are also conceivable.

[0059] Regardless of whether one of the optional processes has been carried out or not, the gypsum fiber suspension is then fed into a pressure vessel 12 designed as a pressure-stirring vessel, where the gypsum fiber suspension can optionally be blended with an additive gypsum 13 and / or additives 14 can be added before or during the conversion of the calcium sulfate dihydrate contained in the gypsum fiber suspension to calcium sulfate alpha hemihydrate at temperatures between 110 and 150°C, preferably 120 to 125°C. The additive gypsum 13 can, for example, comprise FGD gypsum, natural gypsum, and / or chemical gypsum and can be added in a proportion of up to 80 wt.% (based on gypsum alone) in order to produce desired properties of the calcium sulfate alpha hemihydrate to be produced, such as the specific surface area, crystal size, particle size distribution, water demand, and dewatering behavior.

[0060] It is also conceivable, and possibly additionally, that the added gypsum consists of fine-grained gypsum waste, for example, gypsum dust, in which case a suitable additive 14 is a dicarboxylic acid, in particular succinic acid. Besides the already mentioned succinic acid and its salts, other, generally known additives 14 can of course be used to influence the crystal habit or crystal growth and / or to accelerate the transformation, for example, maleic acid and its salts, aluminum sulfate, potassium sulfate, or potassium aluminum sulfate. While in the present embodiment the added gypsum 13 and / or the additives 14 are mixed directly into the pressure stirred vessel 12, it is of course also conceivable in other embodiments to use a separate stirred vessel.

[0061] As already mentioned, the conversion to calcium sulfate alpha hemihydrate preferably occurs by heating the gypsum fiber suspension to a temperature between 110 and 150°C, preferably to 120 to 125°C, and holding it at this temperature for a specified time. With sufficient energy input corresponding to the energy required for the conversion, the holding times can be, for example, between 5 and 20 minutes, such as 10 minutes at 125°C. Then the calcium sulfate dihydrate of the gypsum fiber suspension is completely converted to calcium sulfate alpha hemihydrate with the desired properties, which also applies to any gypsum granules still adhering to cardboard fibers.

[0062] The resulting gypsum fiber suspension with alpha-calcium sulfate hemihydrate can, in a first utilization alternative 15, be cooled to a temperature below 100°C, dewatered, dried, and stored. In a second utilization alternative 16, the gypsum fiber suspension, also cooled to a temperature below 100°C, is directly fed into a process for the production of gypsum plasterboard. Finally, in a third, preferred utilization alternative 17, it is conceivable to feed the gypsum fiber suspension into a process for the production of gypsum fiberboard. In general, the suspension can be mixed with other raw materials such as setting gypsum, pulp, and known additives before being fed into the processing operations for the production of gypsum plasterboard or gypsum fiberboard (utilization alternatives 16 and 17).

[0063] The following will explain several examples that demonstrate the functionality of the present invention and can be used to derive properties of the existing product. Example 1: Production of a gypsum fiber suspension in a laboratory pulper

[0064] For the experiments described below, a laboratory pulper (Bi-Pulper DA 100E from A. Celli) was used, in which waste paper with a maximum density of approximately 13% can be processed. The pulper had a diameter of 550 mm and a height of 600 mm. It consists of a bottom rotor with a 5.5 kW drive motor and an upper rotor with a 2.8 kW drive motor, which is used for circulating and kneading the paper pulp. The lower rotor has a diameter of 300 mm and can be operated at 350 and 700 rpm. The upper rotor is driven via a universal joint, allowing for a pendulum motion. With the upper rotor, a maximum density of 13% is possible; without it, the maximum density is 7%. For the following experiments on the pulping of gypsum plasterboard, the upper rotor was removed from the pulper.In this version, the laboratory pulper is configured for a maximum pulp density of 7% when dissolving waste paper and is therefore to be understood as a medium consistency pulper.

[0065] For the experiments, 20 kg of water were used, followed by 20 kg of hand-crushed gypsum plasterboard as the gypsum plasterboard material. In the first test in the laboratory pulper, the pulper rotor was operated for 15 minutes at 350 rpm, then for another 5 minutes at 700 rpm. After the 20-minute stirring time, a sample was taken. The sample was placed on a laboratory filter basket lined with filter paper. Under vacuum, the water was separated from the solid. The solid remaining on the filter paper was dried. It was found that after the test time, the coarse particles were significantly reduced; the maximum grain size of the gypsum was 5 mm.

[0066] In a second test, the pulper rotor was operated for 5 minutes at 350 rpm, followed by another 5 minutes at 700 rpm. A sample was then taken. After just 10 minutes, a visually homogeneous gypsum fiber suspension was obtained. The results showed that after only 10 minutes of dissolution, the cardboard was completely dissolved and the gypsum was pulverized to a maximum particle size of approximately 10 mm. The majority of the gypsum had a particle size of <1 mm. Example 2: Production of a gypsum fiber suspension in a pilot plant spooler with a volume of 2.5 cubic meters

[0067] In this example, a pilot plant pulper with a capacity of 2500 liters / 2.5 cubic meters was used. The pulper features a rotor with a typical helical geometry, as commonly used in high-consistency pulpers for solids contents of 12 to 15% when dissolving waste paper, thus classifying it as a high-consistency pulper. The motor driving the rotor has an installed power of 37 kW and is sealed via a mechanical seal with a water inlet. For the tests, the gypsum plasterboard used as the pulping material was pre-shredded by hand into pieces of approximately 30 x 50 cm to prevent the boards from jamming in the pulper and to minimize their volume. In a preliminary test, 100 kg of gypsum plasterboard and 100 kg of water were added.During rotor operation, 6.5 l / min flowed into the pulper through the barrier water inlet, causing the water content to increase with increasing stirring time and the solids content to decrease. It should be noted that in an actual industrial implementation, if one exists at all, the barrier water inlet would be significantly lower, thus maintaining a much more precise solids content of 50 wt%.

[0068] After 10 minutes of dissolving time, a few pieces of plasterboard were visible; after 15 minutes, most of the plasterboard panels had dissolved. Taking the sealing water into account, the calculated solids content after 10 minutes is 37.7% by weight, and after 15 minutes it is 33.6% by weight.

[0069] In a second experiment, 200 kg of gypsum plasterboard was mixed with 200 kg of water, yielding a similar result. After 15 minutes, the suspension was visually similar to the suspension in the laboratory pulper after 10 minutes. After a dissolution time of 20 minutes, the pilot pulper was emptied. Taking the sealing water into account, the calculated solids content after 10 minutes was 43 wt%, after 15 minutes 40.2 wt%, and upon emptying 37.7 wt%. The gypsum fiber suspension produced after emptying is approximately comparable to the gypsum fiber suspension from the laboratory pulper (Example 1).

[0070] The Figs. 3 to 5 show the dissolution behavior of the plasterboard panels in the high-consistency pilot plant spool at different times.

[0071] Fig. 3 Figure 1 shows the gypsum plasterboard 18 used as gypsum board material 1 inside the pulper 3 before the start of the dissolving process. Fig. 4shows the state after 10 minutes of dissolving time, after which a large part of the gypsum plasterboard 18 has already been dissolved, but there are still undissolved gypsum plasterboard 18 in the edge area.

[0072] Fig. 5 Finally, the image shows the state after 15 minutes of dissolving time, where only visually uniform gypsum fiber suspension 19 remains around the rotor 6.

[0073] The drained, homogeneous gypsum fiber suspension 19 was used for further experiments to convert the gypsum components into calcium sulfate alpha hemihydrate in a pressure mixing vessel with a maximum total capacity of 164 liters with anchor agitator.

[0074] The container can be heated with steam in two ways: via jacket heating and, optionally, via direct steam heating in a pipeline through which the gypsum fiber suspension is circulated. The tests were conducted exclusively using jacket heating.

[0075] The comparative tests in the pressure stirred tank were conducted to maintain a water-to-solids ratio of 4:1. The tank was first filled with water from gypsum fiber production. Then, the sodium salt of succinic acid was added. With the stirrer running, the solids were then added until, after subtracting the moisture content of the gypsum product being converted, 10 kg of solids remained. The moisture content of the gypsum being converted was taken into account when calculating the amount of water.

[0076] To determine the characteristic properties of the generated calcium sulfate alpha hemihydrate, the hot gypsum fiber suspension was filtered after being removed from the pressure vessel and then repeatedly rinsed / filtered with alcohol. Finally, it was rinsed with Acton. The filter cake was dried at 40°C. This treatment prevents rehydration of the calcium sulfate alpha hemihydrate to calcium sulfate dihydrate. The resulting filter cake was used for further investigations.

[0077] Three different compositions were tested for conversion in the pressure stirred tank: Experiment 1: The gypsum fiber suspension produced in the pilot plant pulper was modified by adding 0.1% sodium succinic acid, based on the gypsum-based solids. Experiment 2: The gypsum fiber suspension produced in the pulper was blended with citric acid gypsum, resulting in a composition of 50 wt% citric acid gypsum and 50 wt% gypsum from the gypsum plasterboard, relative to the dry matter. Experiment 3: The gypsum fiber suspension produced in the pilot plant pulper was blended with FGD gypsum, resulting in a composition of 50 wt% FGD gypsum and 50 wt% gypsum from the gypsum plasterboard, relative to the dry matter. The suspension was modified by adding 0.1% sodium succinic acid, based on the gypsum-based solids.

[0078] The conversion took place at 120 to 125°C via jacket heating. Samples were taken after a holding time of 40 or 50 minutes.

[0079] The following investigations were carried out to evaluate the samples from experiments 1 to 3: Visual assessment of the gypsum fiber suspension after conversion. Determination of the specific surface area according to Blaine. Determination of the particle size distribution using laser granulometry (instrument: Coulter LS 13320 from Beckmann Coulter GmbH). Isopropanol was used as the measuring fluid; the samples were dispersed in it for 90 s using internal ultrasound. Testing of the filtration rate of a gypsum fiber suspension produced from the generated gypsum and a waste paper pulp of defined and, in all cases, identical composition and density.

[0080] For testing the filtration rate, a fiber content of 11 wt% based on fibers and gypsum was set. The fiber introduced via the gypsum board was included in the fiber content calculation. The gypsum fiber mixture was placed in a filtration vessel consisting of a steel sieve with 3 mm perforations, on which a black ribbon filter was placed, and dewatered downwards using a vacuum (0.4 bar negative pressure). The diameter of the filtration area was 102 cm. The filtrate was collected in a suction flask, which was positioned on a scale. The filtrate volume was recorded over time using a data acquisition system. For this purpose, a quantity of 114 g of the gypsum to be tested, containing 17 g of fiber (calculated on dry fiber), in the form of waste paper from group 5.03, was used.01 according to the European list of standard waste paper grades DIN EN 643-2014-5 and 500 g of distilled water mixed with one gram of Sika Retardan L200 gypsum setting retarder. The fiber content takes into account fibers introduced via the recycled gypsum. Accordingly, the fiber content is then reduced (17 g fiber = sum of waste paper of group 5.03.01 according to the European list of standard waste paper grades DIN EN 643-2014-5 and fibers from recycled gypsum).

[0081] Plotting the amount of water against the square root of time yields a straight line. This is shown schematically in Fig. 6 The slope of the line 20 shown there is a measure of the filtration rate and is referred to below as the K-value. The higher the K-value, the faster the gypsum fiber mixture filters.

[0082] In addition to the results of tests 1 to 3, several reference gypsums were also subjected to testing of the filtration rate, including commercially available calcium sulfate alpha hemihydrate (designated AGLF by Knauf Gips KG), standard gypsum mixtures for the production of gypsum fiberboards at the production site of Lindner Norit GmbH in Dettelbach (for board thicknesses between 20 and 40 mm), calcium sulfate alpha hemihydrate made from 100% gypsum fiber dust (see also EP 2 418 184 A2) and stucco gypsum made from 100% gypsum fiber dust.

[0083] As a result of the investigations, the mixtures of all three experiments exhibited an optically homogeneous suspension after conversion to calcium sulfate alpha hemihydrate. The coarser gypsum grains originally present in the gypsum fiber suspension taken from the pulper were no longer present in the suspension converted to calcium sulfate alpha hemihydrate and had thus been completely pulverized by the conversion. The remaining results are shown in the following table: plaster K-value [g / s 0.5< ] Blaine value [cm² / g] Median grain size [µm] Experiment 1: Alpha hemihydrate from 100% recycled gypsum from gypsum plasterboard waste 40,8 5402 24,6 Experiment 2: Alpha hemihydrate from 50 wt% recycled gypsum from gypsum plasterboard waste and 50 wt% citric acid gypsum 59,1 3262 44,2 Experiment 3: Alpha hemihydrate from 50 wt% recycled gypsum from gypsum plasterboard waste and 50 wt% FGD gypsum 55,4 3329 27,9 Comparison: commercially available calcium sulfate alpha hemihydrate: AGLF 65,9 Not determined Not determined Standard gypsum mixtures for the production of gypsum fiberboards at the Lindner Norit GmbH production site in Dettelbach (for board thicknesses between 20 and 40 mm) 43 to 60 Not determined Not determined Alpha-calcium sulfate hemihydrate from 100% gypsum fiber dust 29,1 Not determined Not determined Plaster of Paris made from 100% gypsum fiber dust 6,6 Not determined Not determined

[0084] Of particular interest here are the K-values ​​determined for the comparison gypsums. The material from test 1 (100% gypsum fiber suspension from gypsum board material) already comes close to the lower end of the value for the standard gypsum mixture used to produce gypsum fiberboards. Compared to calcium sulfate alpha hemihydrate made from 100% gypsum fiber dust, filtration is improved. Plaster of Paris produced from the same starting material, namely 100% gypsum fiber dust, cannot be filtered quickly enough; its K-value is only 6.6.

[0085] The mixture from experiment 2, in which calcium sulfate alpha hemihydrate was produced from 50 wt% recycled gypsum from the gypsum fiber suspension and 50 wt% citric acid gypsum, has a K-value of 59.1, which is very close to that of a suspension of 100% commercially available calcium sulfate alpha hemihydrate designated AGLF and is in the upper range of standard gypsum mixtures for the production of gypsum fiberboards at the Lindner Norit GmbH site in Dettelbach. The mixture from experiment 3, in which calcium sulfate alpha hemihydrate was produced from 50 wt% recycled gypsum from the gypsum fiber suspension and 50 wt% FGD gypsum, has a slightly lower K-value of 55.4 than the product from experiment 2. However, this still represents a K-value that is in the middle to upper range of standard gypsum mixtures for the production of gypsum fiberboards at the Lindner Norit GmbH site in Dettelbach.

[0086] This demonstrates that very high proportions of gypsum fiber suspensions derived from gypsum plasterboard can be processed in mixtures. Without any reduction in dewatering time, at least 50 wt% gypsum fiber suspension from gypsum plasterboard, particularly gypsum plasterboard waste, can be reliably incorporated into such a gypsum mixture if the gypsum fiber suspension is produced according to the present invention. For thinner gypsum fiberboards, for example up to 20 mm thick, which require shorter filtration paths, the use of the gypsum fiber suspension from Experiment 1 is also conceivable, i.e., the use of calcium sulfate alpha hemihydrate derived from 100% gypsum fiber suspension from gypsum plasterboard, particularly gypsum plasterboard waste.

Claims

1. A method for recycling gypsum board material containing calcium sulfate dihydrate (1), comprising the following steps: - introducing the gypsum board material (1) together with water (2) into a wet dissolving unit, in particular a pulper (3), and dissolving the gypsum board material (1) in the wet dissolving unit to produce a gypsum fiber suspension (19), and - converting the calcium sulfate dihydrate of the gypsum fiber suspension (19) to calcium sulfate alpha hemihydrate in a pressure vessel (12).

2. Method according to claim 1, characterized by the fact that The gypsum board material (1) is introduced gravimetrically into the wet dissolving unit in such a way that a mixture of 30 to 60 wt.% gypsum board material (1) with 40 to 70 wt.% water (2) is obtained, and / or that the gypsum board material (1) is held in the wet dissolving unit for a dissolving time of 10 to 20 minutes, preferably 14 to 16 minutes.

3. Method according to claim 1 or 2, characterized by the fact thata high-consistency pulper (3) is used as the wet dissolving unit and / or a stirring device having crushing teeth is used in the wet dissolving unit.

4. Method according to any of the preceding claims, characterized by the fact that in the case of a gypsum board material (1) containing at least one contaminant, the gypsum fiber suspension (19) from the wet dissolving unit is passed through at least one contaminant separator.

5. Method according to claim 4, characterized by the fact that which at least one contaminant separator is selected from the group comprising a disc sorter (8), a heavy material separator (9) and a cylinder sorter (10).

6. Method according to claim 4 or 5, characterized by the fact that at least one of the contaminant separators includes an additional sieve with rotor for crushing larger gypsum grains.

7. Method according to one of claims 4 to 6, wherein, for the purpose of separating contaminants, in a first stage in a coarse sorter, in particular the disc sorter (8), with a sieve and / or a washing drum, contaminant particles larger than 10 mm are removed, after which, in a second stage, heavy parts of the contaminant are removed by the heavy material separator (9) and, in a third stage, contaminant particles larger than 2 mm are removed by means of a cylinder sorter (10).

8. Method according to any one of claims 4 to 7, characterized by the fact that the gypsum fiber suspension (19) is diluted to a solids content of 10 to 25 wt.% before passing through at least one contaminant separator.

9. Method according to any of the preceding claims, characterized by the fact thatthe gypsum fiber suspension (19) from the pulper (3) is passed through at least one wet crusher (11), in particular a wet crusher (11) that reduces the particle size to values ​​less than one millimeter.

10. Method according to any of the preceding claims, characterized by the fact that at least one additional gypsum (13) and / or at least one additive (14) is added to the gypsum fiber suspension (19) before conversion to calcium sulfate alpha hemihydrate.

11. Method according to claim 10, characterized by the fact that 10 to 80 wt.% of FGD gypsum and / or natural gypsum and / or chemical gypsum, in particular citric acid gypsum, based on the total gypsum content, shall be added as additional gypsum (13) to the gypsum fiber suspension (19).

12. Method according to claim 10 or 11, characterized by the fact thatas additional gypsum (13) to the gypsum fiber suspension (19) at least a gypsum waste from the processing of gypsum products, in particular gypsum dust from the processing of gypsum fiberboards and / or crushed gypsum fiberboards and / or gypsum sludge, is added.

13. Method according to claim 12, characterized by the fact that at least one dicarboxylic acid, preferably succinic acid, or its salt is added to the gypsum fiber suspension (19) containing the gypsum waste.

14. Method according to any one of claims 10 to 13, characterized by the fact that The addition takes place in the pressure vessel (12) designed as a pressure mixing vessel.

15. Method according to any of the preceding claims, characterized by the fact that the gypsum fiber suspension (19) is fed into a process for the manufacture of gypsum plasterboard and / or gypsum fiberboard after conversion or is stored for later use, in particular after a cooling and / or dewatering and / or drying process.