Process for preparing alpha hemihydrate of calcium sulfate.

Abstract

According to a first aspect, the invention relates to a process for the production, preferably continuous, of calcium sulfate alpha hemihydrate, the process comprising: - the preparation of an aqueous gypsum suspension comprising gypsum in particulate form, water and at least one superplasticizing polymer selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate); - the calcination of the aqueous gypsum suspension at a suitable temperature and pressure to obtain a suspension of calcium sulfate alpha hemihydrate; - optionally, the filtration of the calcium sulfate alpha hemihydrate suspension to separately obtain a concentrated alpha hemihydrate suspension and an aqueous filtrate.The invention also relates, according to a second aspect, to a process for preparing a gypsum-based product comprising the preparation of a plaster suspension comprising calcium sulfate alpha hemihydrate obtained by the process according to the invention. Furthermore, the invention relates to a plaster-based product obtained using a process according to the invention. The invention also relates to a gypsum-based product obtained using a process according to the invention, such as a plasterboard. Figure 2.

Description

Title of the invention: Process for preparing alpha hemihydrate of calcium sulfate. technical field

[0001] The invention relates to the field of gypsum-based product production, particularly plasterboard. More particularly, the present invention relates to a process for preparing alpha-type calcium sulfate hemihydrate, as well as a plasterboard obtained according to such a process. TECHNOLOGICAL BACKGROUND

[0002] Plaster is a versatile material widely used in the construction industry in various forms (plaster, mortar, moldings and plaster tiles, plasterboard for partitions, etc.). Plaster is obtained by heating or "calcining" gypsum, a mineral that can be natural or synthetic, essentially composed of calcium sulfate dihydrate (or CaSO4·2H2O). During the transformation of gypsum into plaster, the dihydrated calcium sulfate is partially dehydrated, resulting in a powder consisting mainly of calcium sulfate hemihydrate (CaSO4·2H2O). In the presence of water, the plaster (or calcined gypsum) undergoes a hydration reaction and transforms into a whitish, cohesive material that is relatively hard after drying. This material can be molded into the desired shape and is simply dihydrated calcium sulfate (gypsum). Thus, gypsum and plaster can be used and recycled indefinitely.

[0003] Rehydrating gypsum plaster during the manufacture of gypsum-based products in the desired shape (such as plasterboard or plaster tiles) requires adding large quantities of water to the plaster to obtain a suspension with sufficient fluidity to allow for homogeneous mixing and good spreading or shaping of the suspension. However, such quantities of water also result in longer drying times and greater energy consumption (and therefore higher CO2 emissions). Therefore, fluidizers or superplasticizers are generally added during rehydration to reduce the amount of water added while maintaining sufficient fluidity.

[0004] Furthermore, the composition and properties of the plaster depend strongly on the calcination process used. In particular, depending on the firing, the calcium sulfate hemihydrate crystals obtained can have very different structures or sizes. Two crystalline types of hemihydrate are distinguished: - the [3 (beta)] calcium sulfate hemihydrate, whose crystals are porous, cracked, and irregularly shaped; and - the a (alpha) hemihydrate of calcium sulfate, consisting of denser crystals and of regular prismatic shape.

[0005] Beta hemihydrate is the most common hemihydrate and can be obtained using simple, "dry" curing processes under atmospheric conditions. Alpha hemihydrate may be preferred for certain applications because it allows the production of gypsum-based products with better mechanical properties (harder materials with greater strength after rehydration). Furthermore, alpha hemihydrate has a lower water requirement (or mixing ratio) than beta hemihydrate, which reduces drying times for gypsum-based products (plasterboard or other) and lowers energy consumption. However, alpha hemihydrate requires more complex curing processes, using a "wet" method and under high pressure conditions.

[0006] In addition to these complex processes, further difficulties arise in the case of continuous alpha hemihydrate production compared to a batch production process. On the one hand, the gypsum particles introduced into the calcination reactor (or pressurized tank) have a residence time distribution (rather than a single residence time in the case of a batch process) that must be controlled. On the other hand, the continuous process requires sufficient fluidity of the gypsum suspension to optimize heat exchange in the calcination tank and to limit the risks of blockage and / or pressure drops in the calcination reactor. At the same time, the fluidity must not be too high to avoid sedimentation and / or settling phenomena and to minimize the amount of water and energy consumption used to heat the gypsum suspension.Finally, the production process involves dissolution and recrystallization phenomena during the transformation of gypsum into plaster, which impact the specific surface area and shape of the resulting alpha hemihydrate crystals and can be affected by various factors. Crystallization agents can also be added to influence these recrystallization phenomena.

[0007] Document WO2022 / 263217 describes an example of a continuous production process for calcium sulfate alpha hemihydrate in which various parameters are measured and controlled in real time to optimize the amount of crystallizing agent. Document WO2022 / 194499 describes a continuous calcination process in which the size of the gypsum particles is optimized in the gypsum suspension to balance fluidity requirements while allowing the formation of alpha hemihydrate crystals of satisfactory shape and size.

[0008] Research is ongoing to improve large-scale production processes in terms of stability and reproducibility, but also to reduce the quantities of water and energy required. Thus, there is always a need for efficient production. alpha hemihydrate for the production of plaster and / or gypsum-based products of the desired quality and conformity, while minimizing water quantities and energy consumption.

[0009] It is to the applicant's credit that she has proposed a process for producing alpha hemihydrate which, surprisingly, solves these problems. Summary of the invention

[0010] According to a first aspect, the invention relates to a process for the production, preferably continuous, of calcium sulfate alpha hemihydrate, the process comprising: - the preparation of an aqueous gypsum suspension comprising gypsum in particulate form, water and at least one superplasticizing polymer chosen from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate); - calcination of the aqueous gypsum suspension at a suitable temperature and pressure to obtain a suspension of calcium sulfate alpha hemihydrate; - optionally, filtration of the calcium sulfate alpha hemihydrate suspension to obtain separately a concentrated alpha hemihydrate suspension and an aqueous filtrate.

[0011] The invention also relates, according to a second aspect, to a process for preparing a gypsum-based product comprising: - the preparation of a plaster suspension comprising calcium sulfate alpha hemihydrate, obtained by the process according to the invention; and - shaping the plaster suspension onto a support of suitable shape to form a gypsum-based product.

[0012] The invention also relates to a plaster-based product comprising calcium sulfate alpha hemihydrate obtained using a process according to the invention.

[0013] The present invention also relates to a gypsum-based product obtained using a process according to the invention, such as a plasterboard.

[0014] Surprisingly, the process according to the invention allows the manufacture of alpha plaster (pressure calcination) while reducing the amount of water and energy required during calcination. Indeed, the inventors have demonstrated that the use of specific fluidizers makes it possible to lower the water / gypsum ratio while maintaining very good fluidity in the calcination chamber and at the chamber outlet at the required temperatures, and while allowing the formation and quality of alpha hemihydrate crystals, including in terms of size and dimension ratio. The invention therefore has the advantage of providing an improved process for the large-scale manufacture of alpha plaster and gypsum-based products. of quality and desired conformity, with reduced water and energy consumption. DETAILED DESCRIPTION

[0015] The general terms used in this text are defined below.

[0016] The expression "including" encompasses the expression "consisting of".

[0017] The expression "from ... to ..." should be understood inclusive of the limits.

[0018] The production process according to the invention includes a step of preparing an aqueous suspension of gypsum, preferably in a mixer, the aqueous suspension of gypsum comprising a mixture of gypsum in particulate form, water and at least one superplasticizing polymer selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate).

[0019] The gypsum used may be natural, synthetic, or recycled, preferably natural or recycled. It is used in particulate form, i.e., as a powder. The particles may, for example, have an average diameter of less than 1 mm. The particulate gypsum may advantageously have a Di0 value greater than or equal to 2 µm, a D90 value less than or equal to 90 µm, and a D50 value less than or equal to 25 µm. The process according to the invention may, for example, include, before the step of preparing the aqueous gypsum suspension, a preliminary step of grinding the gypsum to reduce the size of the particles it contains, and optionally, a step of sieving the gypsum to remove gypsum particles of undesirable sizes. The particulate gypsum may be supplied continuously to a mixer, for example, at a substantially constant mass per second.

[0020] For the purposes of this invention, a "superplasticizing polymer" is defined as a water-soluble polymer with a high water-reducing capacity. This type of polymer is generally used as an additive in cementitious materials, such as grouts, mortars, and concretes, to reduce the amount of water required during their use and / or to decrease the viscosity of these materials (thinning agent). In particular, this type of polymer disperses mineral particles and limits their agglomeration through electrostatic stabilization and / or steric repulsion. Various types of superplasticizers exist, such as polymelamine sulfonate (PMS), polynaphthalene sulfonate (PNS), modified lignosulfonates (MLS), polycarboxylates, etc.

[0021] Within the framework of the present invention, at least one superplasticizing polymer is selected from among phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium polystyrene sulfonate.

[0022] For the purposes of this invention, "phosphated polyether" means a polyether comprising at least one phosphate group. For the purposes of this invention, "phosphated polymer" or "phosphated" means a polymer or group comprising a phosphate or phosphonate group, preferably a group or polymer comprising a linear side chain including a phosphate or phosphonate group. This type of phosphated polymer forms polyanions in water, which provide the anionic repulsive function of the fluidizer. Preferably, in the process according to the invention, the phosphated polyether is a phosphated polyarylether, more particularly a polyarylether comprising a phosphated side chain. Polyarylethers, or aromatic polyethers (APEs), are a family of polymers that contain ether and aromatic groups in their main chain, more particularly aromatic ether groups.Such polymers are described, for example, in documents EP3197830, WO2012 / 049077, or US20110288244. The phosphate polyether used according to the invention may, for example, have a weight-average molecular weight of 4,000 to 200,000 Da, preferably 10,000 to 100,000 Da, for example, determined by Gel Permeation Chromatography (GPC). Preferably, the phosphate polyether used according to the invention is an aromatic polyether whose motif includes a polyether side chain, preferably a polyalkylene glycol side chain, for example, a polyethylene glycol side chain. Preferably, the phosphate polyether used according to the invention is selected from the products marketed under the name "Melfulx Plus 1085" or "Melfulx Plus 1087" by BASF.

[0023] "Sulfonated melamine formaldehyde" ("SMF"), also known as sulfonated melamine polycondensate ("SMP"), is a well-known type of anionic polymer comprising at least one sulfonated unit, and more particularly a unit comprising a sulfonate group, for example, a unit comprising a side chain comprising a sodium sulfonate group. Such polymers are described, for example, in document WO2023 / 205049. Preferably, in the sulfonated melamine formaldehyde used according to the invention, each unit (repeating unit) contains a sulfonate group. The sulfonated melamine formaldehyde used according to the invention may, for example, have a weight-average molecular weight of 4,000 to 60,000 Da, preferably 8,000 to 20,000 Da, for example, determined by Gel Permeation Chromatography (GPC).For example, the sulfonated formaldehyde melamine used in the context of the present invention may be the product marketed under the name "Melement F15 G" by BASF.

[0024] "Sodium polystyrene sulfonate" (or sodium poly(styrene sulfonate) or "PSS") is a substituted polymer of polystyrene in which a sodium sulfonate group (-SO3 Na+) is attached to the phenyl group of the styrene units. This polymer is known under CAS No. 25704-18-1. The sodium polystyrene sulfonate used according to the invention may, for example, have a weight-average molecular weight of 4,000 to 2,000,000 Da, preferably 20,000 to 500,000 Da, for example, determined by Gel Permeation Chromatography (GPC). For example, the sodium polystyrene sulfonate used in the context of the present invention may be a product marketed under the name "poly(sodium 4-styrenesulfonate)" or "poly(sodium 4-styrenesulfonate) solution," for example, a 30% by mass solution in water, from the Sigma Aldrich company.

[0025] More preferably, in the process according to the invention, the superplasticizing polymer is a phosphated polyether, preferably a polyarylether comprising a phosphated side chain.

[0026] The process for producing alpha hemihydrate of calcium sulfate according to the invention includes a step of calcining the aqueous suspension of gypsum, in particular in a calcination reactor, at a temperature and pressure enabling the obtaining of a suspension of alpha hemihydrate of calcium sulfate.

[0027] Preferably, the calcination temperature is from 100°C to 170°C, more preferably from 110°C to 150°C.

[0028] Preferably, the calcination pressure is from 0.15 MPa to 0.85 MPa, more preferably from 0.25 MPa to 0.50 MPa.

[0029] Preferably, the step of preparing the aqueous gypsum suspension includes the addition of a crystallizing agent.

[0030] The crystallizing agent is a substance that facilitates the transformation of gypsum into its hemihydrate, particularly alpha gypsum. This type of agent can, in particular, accelerate and / or influence crystal growth and allow control of the shape of calcium sulfate hemihydrate crystals during their formation (during the conversion of gypsum into the hemihydrate). The crystallizing agent can be, for example, a dicarboxylic acid, preferably aliphatic, or a neutral metal salt, particularly one whose anion can form complexes with the Ca2+ ion. Succinic acid is an example of a dicarboxylic acid. Examples of neutral metal salts include aluminum salts or potassium salts, such as potassium sulfate or potassium chloride.

[0031] Preferably, the crystallizing agent is a dicarboxylic acid, in particular aliphatic, more preferably succinic acid.

[0032] Preferably, the content of crystallizing agent in the aqueous gypsum suspension is 0.1% to 2% by mass per liter of suspension, more preferably 0.3 to 1.5% by mass per liter of suspension.

[0033] Advantageously, the process according to the invention may further comprise an optional step of filtering the alpha hemihydrate suspension of sulfate of calcium to separately obtain a concentrated alpha hemihydrate suspension and an aqueous filtrate. Preferably, the resulting concentrated alpha hemihydrate suspension comprises 4 to 8% water by mass. The filtration step allows for the separation of at least some of the aqueous solution contained in the alpha hemihydrate suspension, which can be recovered and reused, particularly in a subsequent step of preparing an aqueous gypsum suspension (a new batch of gypsum suspension in the case of a batch production process or a new fraction of gypsum suspension in the case of a continuous production process). The recovered aqueous filtrate comprises water, a residual amount of superplasticizer polymer (a portion of the superplasticizer polymer also remaining in the concentrated alpha hemihydrate suspension), and possibly a residual amount of crystallizing agent (when added during the preparation of the aqueous gypsum suspension).Thus, recovering the aqueous filtrate saves water and reduces the amount of superplasticizing polymer and crystallizing agent used. Furthermore, the process according to the invention preferably includes, after the filtration step, a recirculation step of the aqueous filtrate for the preparation of an aqueous gypsum suspension.

[0034] In the process according to the invention, the aqueous gypsum suspension can comprise at least 20% by mass of water relative to the mass of the aqueous gypsum suspension, for example from 20% to 60% by mass of water.

[0035] Preferably, the aqueous gypsum suspension comprises 30% to 60% by mass of water, preferably 30% to 50% by mass of water, more preferably 35% to 45% by mass of water, relative to the mass of the aqueous gypsum suspension.

[0036] In the aqueous suspension of gypsum, the water / gypsum mass ratio can be at most 1. Preferably, in the aqueous suspension of gypsum, the water / gypsum mass ratio is 0.3 to 2, preferably 0.4 to 0.9, more preferably 0.5 to 0.75.

[0037] The aqueous gypsum suspension may contain a superplasticizing polymer content of at most 2% by mass per liter of aqueous gypsum suspension. Preferably, the superplasticizing polymer content in the aqueous gypsum suspension is 0.1% to 1.5% by mass, more preferably 0.25% to 1% by mass, per liter of aqueous gypsum suspension.

[0038] Advantageously, in the process according to the invention, the calcium sulfate alpha hemihydrate suspension, optionally concentrated, comprises alpha hemihydrate particles having a length / width aspect ratio of 0.8:1 to 1.2:1.

[0039] The aspect ratio (length / width) of the alpha hemihydrate crystals is influenced by the elements and substances present in the suspension during the calcination process. Generally, the desired aspect ratio is a width / length ratio of approximately 1. Such a length / width ratio may be preferred to ensure good The fluidity of the suspension throughout the process is maintained. The process according to the invention advantageously allows for obtaining a good aspect ratio despite the use of superplasticizing polymers during the calcination step. The aspect ratio can also be improved using the previously mentioned crystallizing agent.

[0040] Preferably, in the process according to the invention, the aqueous gypsum suspension has a dynamic viscosity of 0.005 to 0.5 Pa·s, more preferably of 0.04 to 0.3 Pa·s, the viscosity being measured at 85°C, at a shear rate of 50 s⁻¹, using a Malvem Kinexus rheometer equipped with a Vayne geometry. The viscosity is measured, in particular, 10 seconds after suspension in the rheometer.

[0041] Advantageously, the process according to the invention further comprises, prior to the calcination step, a measurement step, preferably inline, of the viscosity of the aqueous gypsum suspension. Viscosity measurement allows control of the mixture's fluidity and adjustment of the amount of superplasticizing polymer to be added to the gypsum suspension. More particularly, when the process is continuous, inline viscosity measurement allows direct regulation of the concentration of superplasticizing polymer added to the gypsum suspension, based on the viscosity measured inline. An inline viscometer can, for example, be installed on a section of the suspension transport line, for example, at the outlet of the gypsum aqueous suspension preparation mixer or inside the mixer. Preferably, the viscosity measurement is carried out using a viscometer located on a transport line between the mixer and the reactors.Examples of in-line viscometers suitable for use on a pipeline include the Coriolis Proline Promass I 300 flowmeter from Endress+Hauser (Belgium). For in-line viscometers suitable for use in mixers and reactors, the XL7 viscometer from Hydramotion is a good example.

[0042] According to an advantageous embodiment of the process according to the invention, the process is continuous and further comprises, after the filtration step, a recirculation step of the aqueous filtrate for the continuous preparation of the aqueous gypsum suspension. The aqueous filtrate can thus be conveyed to the inline mixer for preparing the aqueous gypsum suspension.

[0043] Preferably, in the process according to this embodiment, the step of preparing the aqueous gypsum suspension includes combining the aqueous filtrate with additional gypsum, additional water and an additional amount of at least one superplasticizing polymer selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate).

[0044] More preferably, the process according to this embodiment may further comprise an online measurement step of the viscosity of the aqueous gypsum suspension, for example between the mixer and the calcination tank, and the additional quantity of superplasticizing polymer is continuously regulated according to the online measured value of the viscosity of the aqueous gypsum suspension.

[0045] Furthermore, the process according to the invention may also include, preferably after the filtration step, a drying step for the potentially concentrated alpha hemihydrate suspension. This may, for example, involve drying by circulating hot air (possibly "flash" drying with a short residence time) in equipment such as a grinder, or by heating with a rotary kiln, possibly with a scraper and heated wall, etc. This drying step advantageously allows the production of calcium sulfate alpha hemihydrate in powder form.

[0046] The invention also relates to a process for preparing a gypsum-based product, comprising the preparation of a plaster suspension comprising the alpha hemihydrate of calcium sulfate obtained by the process according to the invention. The plaster suspension may, for example, be prepared from previously dried alpha hemihydrate of calcium sulfate to which a predetermined quantity of water is added, or it may be prepared from the suspension of alpha hemihydrate of calcium sulfate (before filtering and / or before complete drying) to which an additional quantity of water is optionally added. In addition, various additives may optionally be added to the prepared plaster suspension, such as foaming agents, setting-time accelerators, etc. The plaster suspension is preferably capable of hardening upon drying.

[0047] The process for preparing a gypsum-based product according to the invention may further include, after the step of preparing the plaster suspension, shaping the plaster suspension on a support of suitable shape to form a gypsum-based product. The shaping step is optionally followed by a resting and / or drying step.

[0048] The invention also relates to a plaster-based product comprising the alpha hemihydrate of calcium sulfate obtained using the process according to the invention. Examples of plaster-based products include plaster powder, plasters, and plaster-based formulations (for example, mixed with other materials for floor leveling, for molding objects, etc.).

[0049] The invention also relates to a gypsum-based product obtained using a production process according to the invention. Examples of gypsum-based products include plaster tiles and plasterboard. These products are essentially composed of calcium sulfate dihydrate (gypsum obtained by rehydrating plaster). Preferably, the gypsum-based product according to the invention is a plasterboard.

[0050] The invention also relates to a process for the continuous production of alpha hemihydrate of calcium sulfate, the process comprising the following steps: - the preparation of an aqueous suspension of gypsum by mixing particulate gypsum, water and at least one superplasticizing polymer chosen from among phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate); - calcination of the aqueous gypsum suspension to obtain a suspension of alpha hemihydrate of calcium sulfate; - the filtration of the calcium sulfate alpha hemihydrate suspension to separate at least one aqueous filtrate of the alpha hemihydrate suspension; - the combination of said at least one aqueous filtrate with additional gypsum, additional water and an additional quantity of at least one superplasticizing polymer selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate), to form an aqueous gypsum suspension; - the online measurement of the viscosity of the aqueous gypsum suspension; the process may further include regulating the additional quantity of said at least one superplasticizing polymer as a function of the online measured value of the viscosity of the aqueous gypsum suspension.

[0051] In this way, the measured viscosity makes it possible to control and regulate the concentration of the superplasticizing polymer and to regulate the entire production process for improved quality and consistency of the products obtained continuously. The quality and properties of the new alpha hemihydrate suspension produced by the process are thus maintained, regardless of the properties of the previous alpha hemihydrate suspension and variations in the superplasticizing polymer content found in the aqueous filtrate and in the concentrated suspension. The properties and / or quality of the calcium sulfate alpha hemihydrate are thus controlled and optimized during the continuous production process.

[0052] The invention is illustrated in more detail by the non-limiting examples presented below. Examples

[0053] The invention is illustrated in more detail by the non-limiting examples presented below. Various superplasticizing polymers are tested and added to aqueous gypsum suspensions prepared according to the procedure described below. The viscosity of the suspensions at 85°C and the impact of the superplasticizers on the formation of hemihydrate crystals are evaluated.

[0054] Various suspensions "S" numbered 1 to 8 and listed in Table 1 below are tested. They are prepared from aqueous gypsum suspensions having a water / gypsum ratio of 0.7 and to which 0.86 g of succinic acid per liter of suspension is added. Different superplasticizers are then added. to compositions 2 to 8 according to the proportions given in Table 1 below (the superplasticizer content being expressed as a mass percentage of active ingredient relative to the mass of solid gypsum). Suspension no. 1 corresponds to the reference suspension (without superplasticizer). It should be noted that, for each of suspensions 2 to 8, the active ingredient content of the superplasticizer was previously optimized based on the fluidizing efficiency obtained for each superplasticizer (best fluidizing efficiency obtained among the concentrations of 0.25%, 0.5%, and 0.75% for each tested superplasticizer polymer).

[0055] The suspensions are heated to 85°C before their viscosity is measured. The viscosity of the suspensions is measured using a MALVERN KINEXUS rheometer equipped with a Vane cylindrical geometry module (more suitable for suspensions that settle and at high temperatures), at a temperature of 85°C and a shear rate of 50 s⁻¹. The rheometer has a heated measuring cell that maintains the temperature during the measurement. For each suspension, the instrument takes a measurement every 10 seconds for 200 seconds. The viscosity value measured at 10 seconds serves as the reference value for the comparative study. Just before the viscosity measurement, the suspension is vigorously mixed using a TURBOTEST VMI at a speed of 3300 rpm for 1 minute, in order to condition all the suspensions in the same way with the same mixing energy.

[0056] The fluidizing efficiency EF (at 50 s 1 and 85°C) is also calculated for each suspension according to the following formula: EF = ((Vo-Vs) / Vo)xlOO, where Vs is the viscosity of the suspension with the superplasticizer (viscosity at 50 s 1 and 85°C) and Vo is the viscosity of the suspension without superplasticizer (viscosity of suspension no. 1, also at 50 s 1 and 85°C).

[0057] The results are listed in Table 1 below. [Tables 1] S Type of superplasticizer (trade name) Content (in %) Viscosity at 50 s and 85°C (in Pa.s) EF (in %) 1 - 0 0.195 0 2 Phosphated polyether (BASF Melflux plus 1087) 0.5 0.012 94.1 3 Formaldehyde-melanin sulfonated polycondensate (BASF MelmentF15G) 0.25 0.055 71.9 4 Sodium polystyrene sulfonate (Poly(sodium 4-styrenesulfonate) solution, 30wt% solution in water, Mw~200,000) 0.25 0.082 57.7 5 Naphthalene sulfonic polymer (BOZZETTO Flub 40F CA) 0.25 0.098 49.7 6 Polycarboxylate (ARKEMA Rhéosperse 3206) 0.75 0.069 64.9 7 Ether polycarboxylate (BASF Melvis C9100F) 0.75 0.049 74.8 8 Lignosulfonate (BORRESPERSE CAF) 0.25 0.190 2.1

[0058] The results show that most of these superplasticizers proved very effective in reducing the viscosity of the suspension, except in the case of lignosulfonate (suspension 8). Without wishing to be bound by any theory, it seems that the fluidizing efficiency of lignosulfonate (BORRESPERSE CAF) deteriorated in the presence of succinic acid (since in the absence of succinic acid, the fluidizing efficiency at 0.25% is 61.2%).

[0059] The calcination of suspensions 1 to 7 was then studied, first in a 2-liter reactor (batch calcination), then in a 120-liter autoclave. It should be noted that suspension 8 was not subjected to calcination due to the poor viscosity results observed.

[0060] Gypsum and water are loaded into a 2-liter autoclave reactor with a loading ratio of approximately 40% liquid and 60% solid, and then succinic acid is added (always at a concentration of 0.86 g / L). The plasticizer (superplasticizer) is then added according to the concentrations mentioned previously (Table 1). The reactor is then sealed, and the temperature inside the reactor is raised to 140°C for the time required to convert the gypsum into plaster (approximately 20 minutes). The temperature is regulated using a thermostatically controlled oil bath pumped into the reactor's double jacket. The suspension is then cooled to 90°C, filtered through a Buchner funnel (under reduced pressure), and the resulting powder is washed with isopropanol.

[0061] The size of the alpha hemihydrate particles obtained is measured using a HORIBA PARTICA LA-950-v2 analyzer. The results are given in Table 2 below. [Tables 2] Dio, pm D50, pm D90, pm SI 8,3 25,0 39,9 S2 7,9 32,2 52,5 S3 8,3 24,8 38,6 S4 5,9 25,9 41,8 S5 4,2 14,6 41,3 S6 4,2 26,7 47,5 S7 2,3 21,5 47,2

[0062] The results show that sample S2 has the largest particles.

[0063] The resulting plaster particles are then observed by scanning electron microscopy (SEM) using an ITACHI TM3030 instrument. The samples are first dispersed in an isopropanol solution, deposited on an observation stage previously coated with carbon tape, and gold-plated using a JEOL Smart Coater sample preparation device. Comparative results are shown in [Fig. 1], where samples S1 to S7 correspond to suspensions 1 to 7, respectively. The results show that the plasticizers have an impact on the crystallization of the plaster.Superplasticizers S2, S3, and S4 allow for good recrystallization into alpha hemihydrate crystals of satisfactory size and shape, with particles having a length-to-width ratio closer to 1 (few elongated or needle-like particles). In contrast, superplasticizers S5 to S7 result in plaster with poor rheology, containing smaller, more fragmented, and / or needle-like particles.

[0064] Calcination is also carried out in a 120-liter autoclave. Gypsum and water are loaded into the reactor with a loading ratio of approximately 40% water to 60% solids, and then succinic acid is added (always at a concentration of 0.86 g / L). The plasticizer (superplasticizer) is then added, again according to the concentrations mentioned previously (Table 1). The reactor is then sealed, and the temperature inside the reactor is raised to 140°C for approximately 15 to 60 minutes. The temperature is regulated using a thermostatically controlled oil bath pumped into the reactor's double jacket. The autoclave is then cooled to 90°C before collecting the suspension. The suspension is then filtered using a Buchner funnel (under reduced pressure), and the resulting powder is washed with isopropanol.

[0065] The plaster crystals obtained are evaluated as before (scanning electron microscopy and particle distribution). The results are equivalent to those previously described.

[0066] In conclusion, the process according to the invention, comprising the addition of superplasticizers selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and Sodium poly(styrene sulfonate) allows the production of alpha hemihydrate particles exhibiting good size and shape properties.

[0067] Fig. 2 is a diagram illustrating a continuous production process of alpha hemihydrate of calcium sulfate according to the invention.

[0068] The illustrated process 100 includes the preparation of an aqueous suspension of gypsum 110 by mixing particulate gypsum 101, water 102 and at least one superplasticizing polymer 103 selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate).

[0069] The gypsum suspension 110 is then calcined in a calcination tank A to provide a calcium sulfate alpha hemihydrate suspension 120 (wet calcination under temperature and pressure conditions enabling the transformation of gypsum into alpha hemihydrate). To obtain crystals with an aspect ratio close to 1, the gypsum suspension 110 further comprises a crystallizing agent 104 (in particular succinic acid at 0.86 g / L of suspension). After calcination (step "A"), the alpha hemihydrate suspension 120 comprises alpha hemihydrate particles having a length / width aspect ratio close to 1:1, ensuring good fluidity of the alpha hemihydrate suspension throughout the process 100.

[0070] The alpha hemihydrate suspension 120 is then passed through a liquid / solid separator "B" (for example, a belt filter or a centrifugal decanter) to separate an aqueous filtrate 122 from the concentrated suspension 130 (obtaining an alpha hemihydrate cake). In this way, the excess aqueous solution is removed. The aqueous filtrate 122 includes, in particular, water but also residual superplasticizing polymer (some of the superplasticizing polymer is also always found in the concentrated suspension 130). The aqueous filtrate also includes residual crystallizing agent (some of the crystallizing agent is also always found in the concentrated suspension 130). The efficiency of the filtration step "B" can also be affected by the shape of the alpha plaster crystals, and therefore by the succinic acid concentration.

[0071] To improve the efficiency of process 100 and reduce water demand (and therefore thermal energy and costs), the filtrate 122 containing the residual superplasticizer polymer and crystallizing agent is recycled and reinjected for the preparation of a new, continuously fed gypsum suspension fraction. Thus, during the preparation step of the new gypsum suspension fraction, the additional quantities of water, superplasticizer polymer, and crystallizing agent can be reduced. Furthermore, the still-warm filtrate saves energy from heating the gypsum suspension.

[0072] To ensure the correct concentration of superplasticizing polymer in the continuation of the continuous process, the viscosity of the suspension 110 is measured online using a viscometer V. The viscosity measurement makes it possible to deduce the superplasticizing polymer content and to regulate, if necessary, the quantity of superplasticizer added to the suspension 110, using a flow regulator R, based on the measurements transmitted by the viscometer V, for example in response to the comparison of the measured viscosity with a predefined setpoint value.

[0073] It should be noted that the superplasticizing polymer and succinic acid can be consumed during both calcination “A” and filtration “B” (a portion may remain adsorbed on the surface of the alpha gypsum crystals). This remaining adsorbed portion also improves the fluidity of the gypsum suspension upon the subsequent addition of water for the production of gypsum-based products (such as plasterboard).

[0074] To regulate the concentration of crystallizing agent (succinic acid), the pH of the filtrate 122 can also be measured online.

[0075] The concentrated suspension of alpha hemihydrate 130 may comprise 4 to 8% by mass of water, preferably around 6%. It may undergo various further treatments. For example, the suspension 130 may undergo drying and / or grinding (e.g., in a screw mill to reduce particle size) to obtain a product based on alpha gypsum 140 of the desired quality (e.g., plaster or a formulation comprising gypsum in powder form). The suspension may also be subjected, optionally after drying and / or grinding, to the addition of water and optional additives (such as accelerators to reduce setting time, foaming agents, or others) to produce a curable gypsum suspension.Additional water 105 and any additives are then added to the alpha hemihydrate suspension 130 in a mixer "D" to obtain a plaster suspension with the required fluidity and setting characteristics for molding the mixture. During this shaping step, the plaster is gradually hydrated into gypsum and hardens into the desired form (plasterboard or plaster tile). After pouring and shaping, setting and drying can be carried out in open air or with the aid of heating devices to obtain a gypsum-based product 150.

Claims

Demands

1. A process for the production, preferably continuous, of calcium sulfate alpha hemihydrate (120, 130), the process comprising: - the preparation of an aqueous gypsum suspension (110) comprising gypsum in particulate form (101), water (102) and at least one superplasticizing polymer (103) selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate); - the calcination of the aqueous gypsum suspension (110) at a suitable temperature and pressure to obtain a suspension of calcium sulfate alpha hemihydrate (120); - optionally, the filtration of the calcium sulfate alpha hemihydrate suspension (120) to separately obtain a concentrated alpha hemihydrate suspension (130) and an aqueous filtrate (122).

2. A process according to claim 1, wherein the calcination temperature is from 100°C to 170°C.

3. A process according to claim 1 or 2, wherein the calcination pressure is from 0.15 MPa to 0.85 MPa.

4. A process according to any one of the preceding claims, wherein the step of preparing the aqueous gypsum suspension (110) includes the addition of a crystallizing agent (104), preferably a dicarboxylic acid, more preferably succinic acid.

5. A process according to any one of the preceding claims, wherein the aqueous gypsum suspension (110) comprises from 30% to 60% by mass of water, preferably from 30% to 50% by mass of water.

6. A method according to any one of the preceding claims, wherein the water / gypsum mass ratio in the aqueous gypsum suspension (110) is 0.3 to 2, preferably 0.4 to 0.9, more preferably 0.5 to 0.

75.

7. A process according to any one of the preceding claims, wherein the content of superplasticizing polymer (103) in the aqueous gypsum suspension (110) is 0.1% to 1.5%, preferably 0.25% to 1%, by mass per liter of suspension.

8. A method according to any one of the preceding claims, wherein the superplasticizing polymer (103) is a phosphated polyether, preferably a polyarylether comprising a phosphated side chain.

9. A method according to any one of the preceding claims, wherein the calcium sulfate alpha hemihydrate suspension (120, 130) comprises alpha hemihydrate particles having a length / width ratio of 0.8:1 to 1.2:

1.

10. A method according to any one of the preceding claims, further comprising, prior to the calcination step, a measurement step, preferably online, of the viscosity of the aqueous gypsum suspension (110).

11. A process according to any one of the preceding claims, the process being continuous and further comprising, after the filtration step, a recirculation step of the aqueous filtrate (122) for the continuous preparation of the aqueous gypsum suspension (110).

12. A process according to claim 11, wherein the step of preparing the aqueous gypsum suspension (110) comprises combining the aqueous filtrate (122) with additional gypsum (101), additional water (102) and an additional amount of at least one superplasticizing polymer (103) selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium poly(styrene sulfonate).

13. A method according to claim 12, wherein the method further comprises an online measurement step of the viscosity of the aqueous gypsum suspension (110), and the additional quantity of superplasticizing polymer (103) is continuously regulated as a function of the online measured value of the viscosity of the aqueous gypsum suspension (110).

14. A process according to any one of the preceding claims, further comprising a drying step of the calcium sulfate alpha hemihydrate suspension (120) or of the concentrated calcium alpha hemihydrate suspension (130).

15. A process for preparing a gypsum-based product comprising: - preparing a plaster suspension comprising the alpha hemihydrate of calcium sulfate obtained by the process according to any one of claims 1 to 14; and - shaping the plaster suspension on a support of suitable shape to form a gypsum-based product.

16. Plaster-based product comprising calcium sulfate alpha hemihydrate (140) obtained using a process according to any one of claims 1 to 14.

17. Gypsum-based product (150) obtained using a process according to claim 15, the gypsum-based product preferably being a plasterboard.

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