Process for the preparation of stable coarse grain with compact structure of different calcium sulfate phases

Abstract

The present invention is concerned with a process for the processing of gypsum powder, wherein gypsum or stucco powder having a bulk density of 990 g / L or less is subjected to compression treatment at a pressure of at least 0.1 N / mm2, a calcination treatment to predominantly produce calcium sulfate anhydrite, and wherein calcium sulfate anhydrite thus obtained is subsequently subjected to a sintering treatment. By such processing, it is possible to significantly reduce the water demand of calcium sulfate anhydrite prepared from for recycled gypsum, which improves the processing speed and reduces the energy demands and / or drying times for compositions based on calcium sulfate anhydrite. The present application is further concerned with a respective use of the inventive process for reducing the water demand of calcium sulfate anhydrite made from recycled gypsum and the use of the product recycled calcium sulfate anhydrite for the production of gypsum articles.

Description

[0001] Knauf Gips KG

[0002] P-KGI-379-WO

[0003] 5 Process for the preparation of stable coarse grain with compact structure of different calcium sulfate phases

[0004] The present invention is concerned with a process for the processing of gypsum powder, wherein gypsum powder having a bulk density of 990 g / L or less is subjected to compression treatment at a pressure of at least 0.1 N / mm2, a calcination treatment to predominantly produce calcium sulfate anhydrite, and wherein the calcium sulfate anhydrite thus obtained is subsequently subjected to a sintering treatment. By such processing, it is possible to significantly reduce the water demand of gypsum powder having a low bulk density, as is often the case for recycled gypsum, which improves the processing speed and reduces the energy demands for respective gypsum powders. The

[0005] 15 present application is further concerned with a respective use of the inventive process for reducing the water demand of calcium sulfate anhydrite from recycled gypsum and the use of the product recycled calcium sulfate anhydrite for the production of gypsum articles.

[0006] State of the art

[0007] Recycled gypsum-based binders such as calcium sulfate anhydrite have a significantly

[0008] 20 increased water demand and a low bulk density compared to respective gypsum -based binders made from natural raw or FGD gypsum (i.e. gypsum obtained from the residues of flue gas desulphurisation plants). This is due to the tendency of gypsum to form aggregates or agglomerates with a significant proportion of voids (when the gypsum is initially processed), whereas natural raw gypsum or FDG gypsum is predominantly present as compact individual particles. This problem does not exclusively apply to recycled gypsum but is also observed for phosphogypsum and the fine fraction of FGD and natural gypsum.

[0009] Calcium sulfate anhydrite has important relevance e. g. in floor screeds including selflevelling floor screeds, e. g. according to EN13813 (2002), which are processed by mixing

[0010] 30 the dry screed composition with water. In addition, according to EP 2943447 B1 calcium sulfate anhydrite can be used in combination with Portland cement and zeolite and / or

[0011] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0012] P-KGI-379-WO

[0013] - 2 - metakaolin and an additional sulfate salt in construction materials such as tile adhesives, grout and mortars. Respective compositions are indicated to provide a water-resistant binder. Likewise, CN 1 1 1875336 A describes a high strength cementing material based on calcium sulfate anhydrite and limestone, which is said to provide high strength for a

[0014] 5 long time.

[0015] The increased water demand, for example in the production of floor screeds, leads to greater drying efforts and thus to longer drying times, which in construction are a relevant cost issue. In addition, the increased water demand leads to reduced strength of the corresponding products. Further in addition, the low bulk density leads to a reduction in the conveying capacity in the existing gypsum transport routes in production (e. g. limiting the conveying in a dosing screw that can only convey a certain volume). Also, the low bulk density leads to a reduction of capacity during calcination (e. g. because the calcination device is filled with less material at the same volume) and to reduced heat transfer in the gypsum fill and thus also to higher energy consumption. For this reason, for example

[0016] 15 recycled gypsum is currently only used in minor quantities together with FGD or natural stone gypsum.

[0017] On the other hand, FGD gypsum is obtained from the combustion of coal by reacting the waste gases from the combustion with a calcium oxide or calcium carbonate suspension, whereby the sulfur dioxide (SO2) contained in the flue gas is first converted to sulfur

[0018] 20 trioxide (SO3) in the presence of oxygen and then bound as calcium sulfate (CaSO4). Given the fact that many governments have decided to phase out coal-fired power generation in the coming years, it is to be expected that FGD gypsum will become an increasingly scarce raw material. Also, natural gypsum faces problems in the approval of mining sites by the local population, as gypsum is often produced in open pit mining, which is a concern with population which lives in the vicinity of the site.

[0019] Whereas recycled gypsum or the use of fine fractions of gypsum is an obvious substitute for these declining gypsum sources, for the reasons as given above they are not economically competitive with the use of natural or FGD gypsum.

[0020] However, in order to still be able to meet the existing demand for gypsum building

[0021] 30 materials, it can be assumed that the recycling of gypsum products will play an increasingly important role over time.

[0022] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0023] P-KGI-379-WO

[0024] - 3 -

[0025] In the prior art, some methods have been described, by which the calcium sulfate (including calcium sulfate dihydrate, hemihydrate and anhydrite) is compacted. E. g. DE 26 58 915 A1 describes a process for the production of lumpy calcium sulfate from finely divided calcium sulfate using roller pressing. In DE 26 58 915 A1 , the starting material

[0026] 5 calcium sulfate can be from a natural source or a synthetic source and it can be a dihydrate as well as a hemihydrate or anhydrite. For the processing, the free water content of the finely divided calcium sulfate is adjusted to a value between greater than 0 and 4% and the resulting mixture is pressed into a lumpy form at temperatures of between 0 and 60 ° C under a contact pressure of 1 to 5 Mp per cm (about 9.81 to 49.05 kN / cm) of roller width.

[0027] 10 The aim of the process of DE 26 58 915 A1 was to improve the handling (dosing) and to reduce the tendency to form dust of the material.

[0028] EP 0 534 458 A1 describes a similar process to DE 26 58 915 A1 , which differs therefrom in that the free water content of the finely divided calcium sulfate is adjusted to between greater than 5 to 12% and the starting material is calcium sulfate dihydrate from flue gas

[0029] 15 desulfurization.

[0030] US 2016 / 0214895 A1 describes a recycling process for plasterboards. In this process, after the plates are broken and ground into small particles, the material is sieved to remove paper residue. The different gypsum size fractions, which are thereby obtained, are then mixed in a defined manner in order to obtain a constant gypsum quality. After mixing, the

[0031] 20 gypsum mixture is compacted in a roller press to increase the density.

[0032] Whereas a compaction step may contribute to better processing efficiency for e.g. later converting the gypsum to calcium sulfate anhydrite in a calcination device, calcium sulfate anhydrite, which is processed accordingly (similar to calcium sulfate anhydrite, which is not processed by compaction processing), still suffers from the problem of a high water demand (i.e. the calcium sulfate anhydrite requires a higher quantity of added water to provide the same processing characteristics such as fluidity of a gypsum slurry than is required for the processing of FGD or natural stone gypsum).

[0033] In the prior art, there have been attempts to increase the grain size of the gypsum which involve a partial recrystallization thereof. E. g. EP 2 305 605 B1 describes a process for

[0034] 30 continuously modifying dihydrate gypsum, wherein the gypsum is subjected to a first dehydration step and a subsequent “recrystallization” in an aqueous environment at elevated temperature. This method is effective to increase the average particle size of the

[0035] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0036] P-KGI-379-WO

[0037] - 4 - gypsum dihydrate from e. g. about 28 pm to 86 pm and also provides a reduced water demand for a gypsum hemihydrate, which is obtained after calcination of the thus produced gypsum dihydrate.

[0038] The process of EP 2 305 605 B1 requires the use of a very large amount of energy (the

[0039] 5 material has to be calcined twice to obtain a hemihydrate with the desired properties). Accordingly, this process is hardly feasible for the processing of large gypsum quantities.

[0040] Accordingly, there is a demand for a process for the reprocessing of recycled gypsum and other gypsum types, which suffer from higher water demands on processing to gypsum articles such as gypsum-based screeds, wherein the process has improved economic feasibility and is associated with less costs and minimal adaptation of existing processing equipment for gypsum.

[0041] The present application addresses these needs.

[0042] Detailed description of the invention

[0043] In the investigations which are underlying the present invention it has unexpectedly been

[0044] 15 found that the water demand of calcium sulfate anhydrite, which is produced from a gypsum dihydrate starting material with a low bulk density (as an indication of a higher content of void volume in the gypsum dihydrate due to agglomerate and aggregate formation of smaller initial gypsum grains) via a compression treatment at a pressure of at least 0.1 N / mm2and which is subsequently subjected to a calcination treatment to

[0045] 20 predominantly produce calcium sulfate anhydrite and a subsequent sintering, provides a product, which has a water demand similar or even better (i.e. less water is required) compared to the water demand of a calcium sulfate anhydrite obtained from conventional FGD or natural stone gypsum. Thereby, the process allows for an economic use of recycled gypsum for the production of respective new gypsum articles, which may even be constituted from 100 % recycled gypsum after processing of the same via the herein described process.

[0046] Compared to the prior art processes as discussed above, the process of the invention solves the problems with significantly lower energy and process expenditure and provides a raw material (in the form of compact individual particles) that is comparable to calcium

[0047] 30 sulfate anhydrite made from natural stone or FGD.

[0048] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0049] P-KGI-379-WO

[0050] - 5 -

[0051] Accordingly, in a first aspect the present invention concerns a process for the processing of gypsum or stucco powder, wherein gypsum or stucco powder having a bulk density of 990 g / L or less is subjected to compression treatment at a pressure of at least 0.1 N / mm2, a calcination treatment to predominantly produce calcium sulfate anhydrite, and wherein

[0052] 5 the calcium sulfate anhydrite thus obtained is subsequently subjected to a sintering treatment.

[0053] As noted above, the bulk density of 990 g / L or less is a result of the gypsum or stucco material (e.g. gypsum powder or stucco powder) having a constitution of aggregates or agglomerates with a larger internal gas volume, which in turn influences the behavior of the calcium sulfate anhydrite (or calcium sulfate anhydrite obtained from respective gypsum) during the hardening / curing process. In addition, more water is required to convert the calcium sulfate anhydrite into a slurry with the required fluidity for a given processing, when the stucco has a larger internal gas volume. As calcium sulfate anhydrite on conversion into gypsum dihydrate takes up 2 water molecules, it is desired

[0054] 15 to add water in an amount which does not significantly exceed the amount of water that is bound to the calcium sulfate anhydrite, as such excess water later has to be removed / evaporated from the product by conventional drying. This drying results in higher energy demands or a longer drying time.

[0055] While the inventors do not want to be bound to any particular theory it is believed that

[0056] 20 during setting of gypsum hemihydrate to the respective dihydrate in the production of e.g. gypsum boards or plaster, a fine-pored sponge structure forms as a result of the excess water that is necessary to achieve the desired consistency; this excess water evaporates and leaves behind fine-pored cavities in respective gypsum products. When such products are later on calcined to calcium sulfate anhydrite, the fine pores are not eliminated and provide a material with a low bulk density and are found in all recycled calcium sulfate anhydrites. The sponge structure is believed to be the cause of the high water demand and the low bulk density of recycled calcium sulfate anhydrite. To reduce the water demand and increase the bulk density, this sponge structure has to be destroyed and coarse compact grains have to be produced. Such particles require less water for

[0057] 30 wetting and the bulk density increases due to the loss of void volume in the particles.

[0058] For the inventive process, it is not of relevant importance if the starting material is gypsum or stucco, which may be obtained e.g. as a waste byproduct from the production of gypsum boards. In the inventive process the compression is conducted as a first treatment

[0059] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0060] P-KGI-379-WO

[0061] - 6 - on a gypsum or stucco starting material, the calcination is conducted as a second treatment and the sintering is conducted as a third treatment.

[0062] In the inventive process, the “sintering treatment” designates a processing of calcined calcium sulfate anhydrite, where after calcination the calcium sulfate anhydrite is further

[0063] 5 thermally treated leading to a more stable grain, which can also be described as a sintering process of the material surface. This effect is reflected in the reduced water demand of the resulting calcium sulfate anhydrite.

[0064] “Gypsum”, as this term is used in the present invention designates the chemical compound with the formula CaSC ■ 2 H2O, i. e. calcium sulfate dihydrate. When gypsum is heated at elevated temperature, the dihydrate loses water and provides an active form as calcium sulfate with 1 / 2 units of crystal water as “CaSCh ■ 1 / 2 H2O” (also designated as calcium sulfate hemihydrate or CaSC ■ 0.5 H2O or hemihydrate) or without crystal water as “CaSCh” (also designated as calcium sulfate anhydrite). The “gypsum” for use in this invention may comprise at least 60 wt.-% of calcium sulfate dihydrate (CaSC ■ 2

[0065] 15 H2O), preferably at least 80 wt.-%. Further, gypsum may contain other calcium sulfate phases (hemihydrate (CaSCh ■ 0.5 H2O (“hemihydrate”) or calcium sulfate anhydrite (CaSC without H2O)) as well as carbonates or other minerals in small amounts.

[0066] “Stucco”, as this term is used in the context of this invention, is gypsum, which has been heat treated to convert at least a portion of the calcium sulfate dihydrate in the gypsum to

[0067] 20 calcium sulfate hemihydrate, e. g. at least 40 wt.-% and preferably at least 60 wt.-%. Stucco, next to the gypsum, which has not been converted to calcium sulfate hemihydrate, usually contains minor share of calcium sulfate anhydrite and carbonates or other minerals.

[0068] For the compression treatment in the inventive process, the pressure to which the gypsum or stucco powder is exposed is not subject to any relevant restrictions as long as the pressure is above 0.1 N / mm2as noted above. For practical reasons, it is however preferred that the pressure is not excessive (for cost reasons), and in most cases a pressure of up to 30 N / mm2will be sufficient to provide the desired compression and morphology change of the gypsum. Accordingly, in a preferred embodiment of the

[0069] 30 inventive process the gypsum powder is subjected to compression treatment at a pressure in the range from 0.1 to 30 N / mm2and preferably in the range 0.5 to 20 N / mm2.

[0070] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0071] P-KGI-379-WO

[0072] - 7 -

[0073] The compression treatment in the invention can in principle be affected by any means, which is available to compress a powdery material, i.e., by extrusion or processing in a stamp press. In a preferred embodiment, the compression treatment is effected by compacting the gypsum between two counter-rotating rolls (similar to a rolling process in

[0074] 5 steel production, where the roll may be rolls with or without recesses), as this is a very simple and cheap means for such processing. To this end, the gypsum or stucco mass to be compressed can be continuously fed to the rolls, where the mass is drawn in by the rolling movement of the rolls and compressed as a result of the space becoming less and less when the material gets right between the middle of the rolls.

[0075] The compression can involve a single pass of the gypsum or stucco to be compressed through a respective compression apparatus, or the compression can be conducted as multiple passes of the gypsum to be compressed through a respective apparatus, where the respective passes can be conducted with the same or different compression pressures. In one embodiment, the compression pressure is lower in a first pass of the

[0076] 15 gypsum or stucco through the compression apparatus and higher (e.g. by at least 1 N / mm2and preferably at least 3 N / mm2) in a second pass of the gypsum or stucco through the compression apparatus.

[0077] In the inventive process, the form, into which the gypsum or stucco is processed by the compression treatment, is not subject to any relevant restrictions. For example, the

[0078] 20 material can be processed by rolls which have a flat surface, where the form, in which the gypsum material exits the rolls is irregular as a result of the material breaking at uneven spots. On the other hand, it is also possible to produce compression processed gypsum and stucco by using rolls, which are shaped such that the gypsum produced by the processing of the gypsum or stucco is in the form of strips or briquettes (e.g. with a thickness of up to 50 mm). However, the result of the compression treatment with the rolls which have a flat surface can be better than the result of the compression treatment with the rolls which are shaped.

[0079] For the compression treatment, the gypsum can be either a dry gypsum (i.e. a gypsum with a moisture content of less than 1 % and preferably less than 0.5 % by weight), but it

[0080] 30 is also possible that the gypsum can have residual moisture, or moisture which has deliberately been added. In most cases, however, the gypsum powder will have a moisture content of at most 12 wt.-%. In one preferred embodiment, the gypsum powder used as starting material in the process of the invention has a moisture content of from 0 to 12 wt.-

[0081] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0082] P-KGI-379-WO

[0083] - 8 -

[0084] %. In a particularly preferred embodiment, the gypsum powder has a moisture content of 1 to 10 wt.-% and especially 3 to 8 wt.-%.

[0085] As noted above, the bulk density of the starting material gypsum or stucco is 990 g / L or less, which results from the gypsum being present as small grains, which form aggregates

[0086] 5 or agglomerates. In most cases, the gypsum or stucco powder will have a bulk density of 900 g / L or less, preferably 800 g / L or less, in particular in the range of 400 to 750 g / L and especially in the range of 550 to 710 g / L.

[0087] The gypsum material, which is used as the starting material for the inventive process, can be any gypsum material having the properties as indicated above, e.g. gypsum or stucco powder. However, the main gypsum materials, which have these properties, and which are therefore preferred for use in the context of this invention encompass recycled gypsum, citro gypsum, phosphogypsum or gypsum grinding dust. In a particularly preferred embodiment, the starting material is recycled gypsum from used gypsum boards (e. g. plasterboards, gypsum fiber boards) and / or gypsum plaster. In general, the recycled

[0088] 15 gypsum may contain at least 60 wt.-% of calcium sulfate dihydrate (CaSC ■ 2 H2O), preferably at least 80 wt.-%. Further, recycled gypsum may contain other calcium sulfate phases (hemihydrate (CaSCh ■ 0.5 H2O (“hemihydrate”) or calcium sulfate anhydrite (CaSC without H2O)) as well as carbonates or other minerals in small shares. Also, very small contents organic constituents or other materials used in drywall construction may

[0089] 20 be contained.

[0090] The process of the invention can be conducted without the addition of any additives, or it is possible that additives are added to the gypsum or stucco starting material to adjust its processing properties. In a preferred embodiment, an additive to slightly dissolve the gypsum on the surface and promote the formation of stable gypsum grains is added to the gypsum or stucco starting material before the material is subjected to compression treatment. Such additive is suitably selected from the group comprising formic acid or sulfuric acid.

[0091] Additives, which affect the processing properties of the calcium sulfate anhydrite are regularly not added before the gypsum has been calcined and sintered, as most such

[0092] 30 additives would not be stable under the calcination and sintering condition required. On the other hand, after the calcium sulfate anhydrite has been prepared, it is possible that

[0093] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0094] P-KGI-379-WO

[0095] - 9 - one or more additives are added thereto in order to enhance the dispersibility of calcium sulfate anhydrite in water at ambient or higher temperatures and / or for modulating the properties of an end product, which are known to the skilled practitioner. Such additives may be selected from one or more foaming agents, one or more accelerators, one or more

[0096] 5 thickeners, one or more defoamers, one or more retarders, and one or more liquefiers. Suitable foaming agents may be surface-active, amphiphilic substances, preferably tensides (also called surfactants), surface-active, amphiphilic polymers or surface-active, amphiphilic proteins. Suitable accelerators may be fine ground gypsum (also called ball mill accelerator (BMA)), potassium sulfate or aluminium sulfate. Suitable thickeners may

[0097] 10 be polysaccharides or a polyacrylamide. Suitable defoamers may be unipolar silicon- components. Suitable retarders may be hydroxycarboxylic acids, preferably tartaric acid malic acid or citric acid, phosphates or degraded or modified proteins. Suitable plasticizers may be melamine resins, polycarboxylates or cellulose partial hydrolysates.

[0098] The compression treatment can be conducted at ambient temperature, decreased

[0099] 15 temperature or elevated temperature. Mostly, however, the compression treatment will be conducted at a temperature of from 0 to 80°C, where a range of 15 to 50°C and in particular 20 to 40°C will be employed in most cases.

[0100] After the compression treatment, it is expedient in the process according to the invention, that the compacted gypsum or stucco from the compression treatment is converted into

[0101] 20 smaller parts, as in most cases the compression treatment will provide a size of the product, which is too large for regular calcination processing. Material, which has not or not sufficiently been compacted (e. g. because it has passed the compaction apparatus in the edge area of the rolls), may suitably be separated from the mixture and reintroduced into the compression processing. While in principle the larger compressed pieces of the processed gypsum or stucco can be reduced to smaller pieces by any means which is available to the skilled practitioner in the art, a particularly suitable means for this processing is grinding, preferably with a device selected from a condux mill, a ball mill, a hammer mill, a cross beater mill, a roller mill, a pendulum roller mill, a cutting mill, a bowl mill crusher or by grinding drying wherein the grinding can be before and / or after the

[0102] 30 stabilization treatment.

[0103] In one embodiment, the milling will be such that the predominant part of the resulting product will have a particle size less than 1 mm, preferably, at least 60 wt.-% of the product will have such particle size, more preferably at least 70 wt.-%. The milling is usually to

[0104] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0105] P-KGI-379-WO

[0106] - 10 - provide a particle size according to needs of the end product, and may be different for a floor screed, plaster, gypsum board, gypsum fiber board. The skilled practitioner will regularly choose respective milling conditions and resulting particle size distributions according to the product needs. In the context of the application the particle size in the

[0107] 5 range of 1 mm (as well as the ratio of particles having a particle size of less than 1 mm is determined by sieve analysis, where the length designates the mesh size of the sieve, i.e., a sieve to retain particles having a particle size of larger than 1 mm has holes of 1 mm x 1 mm).

[0108] As it will in most cases not be possible to avoid the formation of larger particles (i.e.,

[0109] 10 particle with a particle size of more than 1 mm) the share of particles in the desired range can be increased by sieving of the particles of the desired size. Smaller particles (which may alternatively be designated as fines), which are also obtained after the grinding, are likewise suitably removed by sieving and are then reintroduced into the gypsum compression treatment (where they are reprocessed to larger particles).

[0110] In a preferred embodiment, the compacted gypsum or stucco from the compression treatment may be ground, and preferably fractions with undesired particle size, which are obtained after the grinding, may be separated. Fractions finer than desired may be reintroduced into the gypsum or stucco compression treatment and / or fractions larger than desired may be reintroduced into the grinding treatment.

[0111] 20 The product produced in the compression treatment and subsequent grinding differs significantly in the properties for the product before this treatment. For example, it is preferred if the product of the compression and grinding treatment exhibits a bulk density of at least 950 g / L, and preferably in the range of from 1000 to 1200 g / L. Alternatively, or in addition it is preferred that the respective product has a tamped density of at least 1050 g / l and preferably in the range from 1 100 to 1400 g / L. In the context of this invention, the bulk density is determined according to DIN EN 459-2:2021 -09. The tamping density is determined by DIN EN ISO 787-1 1 :1995-10 and the measurement is done based on said DIN. Due to the fact, that some materials have quite a low bulk density the procedure differs from DIN EN ISO 787-1 1 :1995-10 in the amount of sample that is used, here 100 g

[0112] 30 unsieved material, and the number of stamps performed by the device, here 1000. The volume of the sample after stamping was read to + / - 1 ml. Alternatively, or in addition thereto, it is preferred that the product produced in the compression treatment has a bulk or tamping density, which is higher by at least 10% preferably at least 15% and even more

[0113] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0114] P-KGI-379-WO

[0115] - 1 1 - preferably at least 20% than the bulk or tamping density of the non-compressed starting material, where increases of up to about 120% have been observed.

[0116] The gypsum powder, which is to be processed to calcium sulfate anhydrite, in the calcination step is subjected to temperatures where the gypsum or stucco is converted to

[0117] 5 the respective calcium sulfate anhydrite which in most cases will be temperatures above 250°C. In this respect, the skilled practitioner is aware that a calcination at a higher temperature provides a higher conversion rate, but on the other hand, may be less cost effective if the temperature is excessive. In terms of a good compromise between conversion rate and costs of the formation of the calcium sulfate anhydrite, the heating

[0118] 10 during the calcination is preferably at a temperature of at least 300°C, more preferably of at least 350°C, even more preferably in a temperature range from 300°C to 850°C and even more preferably in a temperature range from 350°C to 700°C.

[0119] The product, which is obtained after the sintering treatment, preferably has a water gypsum value of less than 0.7, and more preferably in the range from 0.69 to 0.40, and even more preferably from 0.65 to 0.45. The water gypsum value in the context of this invention is determined according to the sprinkling quantity in accordance with DIN EN 13279-2:2014-03.

[0120] Suitable calcination times to provide calcium sulfate anhydrite are usually from 20 to 60

[0121] 20 Min. However, calcination times to provide calcium sulfate anhydrite of between 1 and 19 Min are possible also, e.g. in a flash calciner.

[0122] A sintering treatment, which is employed subsequent to the conversion of the predominant part of the gypsum to calcium sulfate anhydrite is preferably conducted at a temperature of from 250 to 750°C, and in particular from 300 to 650°C. However, both processes can be conducted directly subsequently even in the same unit.

[0123] In a further aspect, the present invention pertains to the use of a process as noted above for reducing the water demand of calcium sulfate anyhdrite obtained via calcination of gypsum, wherein the gypsum is preferably selected from recycled gypsum, citro gypsum, phosphogypsum or gypsum dust, and the calcium sulfate anhydrite is preferably calcium

[0124] 30 sulfate anhydrite obtained from the calcination of recycled gypsum, where the water demand is assessed on the basis of the water gypsum value as noted above.

[0125] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0126] P-KGI-379-WO

[0127] - 12 -

[0128] In a yet further aspect, the present invention pertains to the use of sintered calcium sulfate anyhdrite, which is produced according to the process as described above, for the production of floor screeds, as well as for gypsum boards, blocks, jointing compounds, finishing compounds, wall plaster, molding plaster or any other gypsum-based product.

[0129] 5 In relation to the prior art, the inventive process in particular provides the following advantages:

[0130] Compared to a process which only involves a compression treatment the inventive processing provides a product, which has a significantly reduced water demand, so that, for example, calcium sulfate anhydrite on the basis of recycled gypsum can be produced,

[0131] 10 which has an equivalent water demand to calcium sulfate anhydrite on the basis of natural or FGD gypsum. Also, only a compression treatment of grinding dust of gypsum cannot produce the advantageous properties that can be achieved with the inventive process.

[0132] Compared to a process, which only involves a stabilization treatment, also, the water demand of the product can be significantly reduced to obtain an equivalent water demand

[0133] 15 to stucco on the basis of natural or FGD gypsum. In addition, by subjecting the gypsum to a compression step, the bulk density increases to a relevant degree, which leads to further advantages as explained above, in particular for a calcination step.

[0134] In comparison to the process described in EP 2 305 605 B1 the process of the invention requires significantly less energy for the same or an even better result. As an example,

[0135] 20 approximately 160 kWh / t of recycled gypsum must be used for the additional calcination process required in the process of EP 2 305 605 B1 alone, where the energy required for additional process steps is not yet even taken into account. In comparison, the compression processing of the invention only requires an energy input of a maximum of about 14 kWh / t of recycled gypsum.

[0136] In addition, the implementation of the process of the invention requires significantly lower investment costs (low expenditure on equipment, small space requirement), since only an appropriately sized compression / compacting machine and simple sintering are required. The other process steps mentioned in the description of the process of the invention above, comminution and calcination, are mostly already components of calcium sulfate

[0137] 30 anhydrite production.

[0138] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0139] P-KGI-379-WO

[0140] - 13 -

[0141] The present invention is further described with reference to an example of embodiments which, however, is intended solely to illustrate the invention and is not in any way to be construed as limiting the scope of protection of the application. and non-

[0142] 5

[0143] Non-1calcium sulfate

[0144] Recycled gypsum was processed in a condux mill to a maximum particle size of 3 mm. The thus obtained gypsum material was calcined and directly sintered for 2 h at a temperature of 600 °C, followed by milling the resulting product to a maximum particle

[0145] 10 size of 0.75 mm.

[0146] Recycled gypsum was compacted in an apparatus with two counter-rotating rolls with a pressure of 200 kN (13.71 N / mm2), where the gypsum was compressed by passing the space between the two rolls.

[0147] 15 After the compression processing, the gypsum was processed in a condux mill to a maximum particle size of 3 mm. The thus obtained gypsum material was calcined and directly sintered for 2 h at a temperature of 600 °C, followed by milling the resulting product to a maximum particle size of 0.75 mm.

[0148] The water gypsum value of the final product was determined according to the sprinkling method in accordance with DIN EN 13279-2:2014-03 (SQ indicates “sprinkled quantity”). The calcium sulfate anhydrite samples obtained in this way were investigated for their bulk density. The bulk densities of the final product were determined according to DIN EN 459- 2:2021 -09. The respective results of these tests are given in the below table.

[0149] 25

[0150] Received at EPO via Web-Form on Dec 20, 2024 Knauf Gips KG

[0151] P-KGI-379-WO

[0152] - 14 -

[0153] Table

[0154] As is apparent from the above table, the processing according to the invention provides 5 significantly higher sprinkled quantities compared to respective samples which have not been processed by compression treatment.

[0155] Received at EPO via Web-Form on Dec 20, 2024

Claims

Knauf Gips KGP-KGI-379-WO- 15 -5 Claims1 . Process for the processing of gypsum or stucco powder, wherein gypsum or stucco powder having a bulk density of 990 g / L or less is subjected to compression treatment at a pressure of at least 0.1 N / mm2, a calcination treatment to predominantly produce calcium sulfate anhydrite, and wherein calcium sulfate anhydrite thus obtained is10 subsequently subjected to a sintering treatment.

2. Process according to claim 1 , wherein the gypsum or stucco powder is subjected to compression treatment at a pressure in the range from 0.1 to 30 N / mm2and preferably in the range 0.5 to 20 N / mm2.

3. Process according to claim 1 or 2, wherein the gypsum or stucco obtained after compression treatment is subjected to grinding, preferably with a device selected from a ball mill, a condux mill, a hammer mill, a cross beater mill, a roller mill, a pendulum roller mill, a cutting mill, a bowl mill crusher or by grinding drying, wherein the grinding can be before and / or after the stabilization treatment.

4. Process according to any one of claims 1 to 3, wherein the gypsum or stucco powder20 in the compression treatment is processed by rolls which have a flat surface or by rolls which are shaped, preferably processed by flat rolls.

5. Process according to any one of the preceding claims, wherein the gypsum powder used as starting material in the process has a moisture content of from 0 to 12 wt.-%.

6. Process according to any one of the preceding claims, wherein the gypsum or stucco25 powder has a bulk density of 900 g / L or less, preferably 800 g / L or less, in particular in the range of 400 to 750 g / L and especially in the range of 550 to 710 g / L. ceived at EPO via Web-Form on Dec 20, 2024Knauf Gips KGP-KGI-379-WO- 16 -7. Process according to any one of the preceding claims, wherein the calcination treatment to predominantly produce calcium sulfate anhydrite is done at a temperature of at least 250°C, preferably in a temperature range from 300 to 850°C and more preferably from 350°C to 700°C.5 8. Process according to any one of the preceding claims, wherein the calcium sulfate anhydrite, which is obtained after calcination, is subjected to a sintering treatment at a temperature of from 250 to 750°C, in particular 300° to 650°C.

9. Process according to any one of the preceding claims, wherein an additive to promote the formation of stable gypsum grains, preferably selected from formic acid and10 sulfuric acid, is added in the compression treatment.

10. Process according to any one of the preceding claims, wherein gypsum or stucco material, which has not been compacted into larger parts during the compression treatment is separated from the compacted gypsum or stucco and is reintroduced in the compression treatment.1 1 . Process according to any one of the preceding claims, wherein the compression treatment is affected by compacting the gypsum or stucco between two counterrotating rolls and / or wherein the compression treatment is conducted at a temperature of from 0 to 80°C.

12. Process according to any one of the preceding claims, wherein the compacted20 gypsum or stucco from the compression treatment is ground, and wherein preferably fractions with undesired particle size, which are obtained after the grinding, are separated and wherein fractions finer than desired are reintroduced into the gypsum or stucco compression treatment and / or wherein fractions larger than desired are reintroduced into the grinding treatment.25 13. Process according to claim 12, wherein the ground gypsum or stucco exhibits a bulk density of at least 950 g / L and preferably in the range from 1000 to 1200 g / L and / or a tamped density of at least 1050 g / l and preferably in the range from 1 100 to 1400 g / L. ceived at EPO via Web-Form on Dec 20, 2024Knauf Gips KGP-KGI-379-WO- 17 -14. Process according to any one of the preceding claims, wherein the calcium sulfate anhydrite after the sintering treatment exhibits a water gypsum value of less than 0.7, preferably in the range from 0.69 to 0.4, and more preferably from 0.65 to 0.45.

15. Process according to any one of the preceding claims, where subsequent to the5 sintering one or more additives to enhance the dispersibility of calcium sulfate anhydrite in water at ambient or higher temperatures and / or to modulate the properties of a slurry or an end product, preferably selected from one or more foaming agents, one or more retarders, one or more accelerators, one or more plasticizers, one or more thickeners and / or one or more defoamers, is added.10 16. Process according any one of the preceding claims, wherein recycled gypsum, citro gypsum, phosphogypsum or gypsum dust, in particular in the form of gypsum grinding dust, fine natural gypsum and fine gypsum residues from gypsum product production processes, is used as a starting material.

17. Process according to claim 16, wherein the recycled gypsum is recycled gypsum from gypsum boards and / or gypsum plaster.

18. Use of a process according to any one of claims 1 to 17 for reducing the water demand of calcium sulfate anyhdrite obtained via calcination of gypsum, wherein the gypsum is preferably selected from recycled gypsum, citro gypsum, phosphogypsum or gypsum dust, wherein the gypsum is more preferably selected from recycled gypsum.20 19. Use of stabilized calcium sulfate anyhdrite produced according to any one of claims 1 to 17 for the production of building boards, gypsum blocks, screeds, jointing compounds, finishing compounds, wall plaster, molding plaster or any other gypsumbased product, preferably for screeds. ceived at EPO via Web-Form on Dec 20, 2024

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