Method for preparing alpha calcium sulfate hemihydrate

Abstract

According to a first aspect, the invention relates to a method for the preferably continuous production of alpha calcium sulfate hemihydrate, the method comprising: - preparing an aqueous suspension of gypsum, comprising gypsum in particulate form, water, and at least one superplasticizer polymer selected from phosphated polyethers, sulfonated melamine formaldehyde (SMF) and sodium polystyrene sulfonate; - calcining the aqueous suspension of gypsum at a temperature and pressure that are suitable for obtaining a suspension of alpha calcium sulfate hemihydrate; - optionally filtering the suspension of alpha calcium sulfate hemihydrate in order to separately obtain a suspension of concentrated alpha hemihydrate and an aqueous filtrate. According to a second aspect, the invention also relates to a method for preparing a gypsum-based product, comprising preparing a suspension of plaster comprising the alpha calcium sulphate hemihydrate obtained by the method according to the invention. The invention also relates to a plaster-based product obtained by means of a method according to the invention. The invention also relates to a gypsum-based product obtained by means of a method according to the invention, such as plasterboard.

Description

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 specifically, the present invention relates to a process for preparing alpha-type calcium sulfate hemihydrate, as well as a plasterboard obtained by such a process. TECHNOLOGICAL BACKGROUND

[0002] Plaster is a versatile material widely used in construction 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 calcium sulfate dihydrate is partially dehydrated to produce a powder consisting mainly of calcium sulfate hemihydrate (CaSO4.2H2O). 1 / 2 H2O). In the presence of water, 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 none other than calcium sulfate dihydrate (gypsum). Thus, gypsum and plaster can be used and recycled indefinitely.

[0003] Rehydrating gypsum plaster during the manufacture of shaped gypsum-based products (such as plasterboard or plaster tiles) requires adding large quantities of water to the plaster to obtain a suspension with sufficient fluidity for homogeneous mixing and good spreading or shaping. However, such large quantities of water also result in longer drying times and higher 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 plaster depend heavily on the calcination process used. In particular, depending on the firing temperature, the resulting calcium sulfate hemihydrate crystals can have very different structures or sizes. Two main crystalline types of hemihydrate are distinguished: - the P (beta) hemihydrate of calcium sulfate, whose crystals are porous, cracked, and irregularly shaped; and - the o (alpha) hemihydrate of calcium sulfate, consisting of denser crystals and of regular prismatic shape.

[0005] Beta hemihydrate is the most common type of gypsum hemihydrate and can be obtained using simple, dry-fired processes under atmospheric conditions. Alpha hemihydrate may be preferred for certain applications because it produces gypsum-based products with superior 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, wet-fired processes under high pressure.

[0006] In addition to these complex processes, further difficulties arise in the case of continuous alpha hemihydrate production compared to batch production. Firstly, the gypsum particles introduced into the calcination reactor (or pressurized tank) have a residence time distribution (rather than a single residence time in a batch process) that must be controlled. Secondly, 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 and to minimize the amount of water and energy used to heat the gypsum suspension.Finally, the production process involves dissolution and recrystallization phenomena during the transformation of gypsum into plaster, which have an impact on the specific surface area and shape of the alpha hemihydrate crystals obtained and which. These processes can be disrupted 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 gypsum particle size 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, as well as to reduce the quantities of water and energy required. Therefore, there is a continued need to efficiently produce alpha hemihydrate for the production of plaster and / or gypsum-based products of the desired quality and conformity, while minimizing water and energy consumption.

[0009] It is to the applicant's credit that she 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 selected from phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium polystyrene sulfonate; - the calcination of the aqueous gypsum suspension at a suitable temperature and pressure to obtain a suspension of alpha hemihydrate of calcium sulfate; - 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 excellent fluidity both within and at the calcination chamber outlet at the required temperatures, and while ensuring the formation and quality of alpha hemihydrate crystals, including in terms of size and dimension ratio. Furthermore, the process according to the invention is particularly well-suited to the continuous production of alpha plaster, that is, with a constant supply of raw materials and a continuous flow of product.The invention therefore has the advantage of providing an improved process for the large-scale manufacture of alpha plaster and gypsum-based products of desired quality and 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 to include the boundaries.

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

[0019] The gypsum used can 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. Particulate gypsum may advantageously have a Dw value greater than or equal to 2 pm, a D value 90less 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 application and / or to decrease the viscosity of these materials (thinning agent). This type of polymer also 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, and others.

[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 motif" means a polymer or motif comprising a phosphate or phosphonate group, preferably a motif or polymer comprising a linear side chain including a phosphate or phosphonate group. This type of phosphated polymer forms polyanions in water which enable the anionic repulsive function of the fluidizer. Preferably, in the process according to the invention, the phosphate polyether is a phosphate polyarylether, more particularly a polyarylether comprising a phosphate 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, W02012 / 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 containing a sodium sulfonate group. Such polymers are described, for example, in document W02023 / 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. + ) 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 mass of 4,000 to 2,000,000 Da, preferably from 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 in a 30% by mass solution in water, from the company Sigma Aldrich.

[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 calcium sulfate alpha hemihydrate according to the invention comprises a step of calcining the aqueous gypsum suspension, in particular in a calcination reactor, at a temperature and pressure enabling the production of a calcium sulfate alpha hemihydrate suspension. More particularly, the calcination step of the aqueous gypsum suspension is carried out in an autoclave reactor. An "autoclave reactor" is understood to be a confined environment under pressure and at a high temperature, in particular at a pressure greater than 0.15 MPa, for example from 0.15 MPa to 0.85 MPa, and a temperature of at least 90°C, for example from 100°C to 170°C.

[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 0.15 MPa to 0.85 MPa, more preferably 0.25 MPa to 0.50 MPa.

[0029] Preferably, during the calcination step, the average residence time of the suspension is at least 15 minutes, preferably from 20 minutes to 1 hour, and more preferably from 30 minutes to 40 minutes. The "average residence time" (denoted T) of the suspension is defined as the average duration for which the suspension particles (gypsum particles, possibly partially dehydrated into plaster) remain under the reactor conditions. The average residence time can be expressed, in particular, by the formula T = V / Q, where V represents the effective volume of the reactor (in m³). 3 ) and Q represents the volumetric flow rate of the suspension passing through the reactor (in m³ 3 / s).

[0030] Preferably, the step of preparing the aqueous gypsum suspension includes the addition of a crystallizing agent.

[0031] A crystallizing agent is a substance that facilitates the transformation of gypsum into its hemihydrate, particularly its alpha form. This type of agent can accelerate and / or influence crystal growth and control the shape of calcium sulfate hemihydrate crystals during their formation (during the conversion of gypsum to 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. 2+ 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.

[0032] Preferably, the crystallizing agent is a dicarboxylic acid, especially aliphatic, more preferably succinic acid.

[0033] 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.

[0034] Advantageously, the process according to the invention may further include an optional step of filtering the calcium sulfate alpha hemihydrate suspension to separately obtain a concentrated alpha hemihydrate suspension and an aqueous filtrate. Preferably, the concentrated alpha hemihydrate suspension obtained comprises 4 to 8% water by mass. The filtration step allows for the separation of at least a portion 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 (some of which also remains in the concentrated alpha hemihydrate suspension), and possibly a residual amount of crystallizing agent (when added during the preparation of the aqueous gypsum suspension). Therefore, recovering the aqueous filtrate saves water and reduces the amount of superplasticizer polymer and crystallizing agent required. crystallization used. Also, 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.

[0035] 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.

[0036] 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.

[0037] In aqueous gypsum suspension, the water / gypsum mass ratio can be at most 1. Preferably, in aqueous gypsum suspension, the water / gypsum mass ratio is 0.3 to 2, preferably 0.4 to 0.9, more preferably 0.5 to 0.75.

[0038] The aqueous gypsum suspension may contain a superplasticizing polymer content of not more than 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.

[0039] Advantageously, in the process according to the invention, the suspension of alpha hemihydrate of calcium sulfate, optionally concentrated, comprises alpha hemihydrate particles having a length / width aspect ratio of 0.8:1 to 1.2:1.

[0040] 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 fluidity of the suspension throughout the process. The process according to the invention advantageously allows obtaining a good aspect ratio despite the use of superplasticizing polymers during the calcination step. The aspect ratio can also be improved with the aid of the previously mentioned crystallizing agent.

[0041] 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 1 using a Malvern Kinexus rheometer equipped with a Vayne geometry. Viscosity is measured 10 seconds after suspension in the rheometer.

[0042] Advantageously, the process according to the invention further includes, 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 specifically, 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 instance, at the outlet of the gypsum aqueous suspension preparation mixer or within the mixer itself. Preferably, the viscosity measurement is performed 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.

[0043] 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.

[0044] 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).

[0045] More preferably, the process according to this embodiment may further include a step of online measurement of the viscosity of the aqueous suspension of gypsum, 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.

[0046] 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 could, 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.

[0047] 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. Furthermore, 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.

[0048] 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.

[0049] The invention also relates to a plaster-based product, for example a dry composition, 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, molding objects, etc.). The plaster-based product according to the invention may optionally to include superplasticizing polymer residues resulting from the process according to the invention. However, a large portion of the superplasticizing polymer can be removed during the filtration step, along with the aqueous filtrate. Preferably, in the plaster-based product, the mass content of the superplasticizing polymer is less than 0.75%, for example, from 0.05% to 0.5%. It is understood that this refers here to the superplasticizing polymer selected from among phosphate polyethers, sulfonated melamine formaldehyde, and sodium polystyrene sulfonate.

[0050] The invention also relates to a gypsum-based product obtained using a production process according to the invention. More particularly, the invention relates to a gypsum-based product obtained by mixing a plaster-based product, for example a dry composition, with water, comprising the alpha hemihydrate of calcium sulfate obtained using the process according to the invention. Examples of gypsum-based products include plaster tiles and plasterboard. Indeed, 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. The gypsum-based product according to the invention may optionally include superplasticizing polymer residues resulting from the process according to the invention.However, a large portion of the superplasticizer polymer can be removed during the filtration step, along with the aqueous filtrate. Preferably, in the gypsum-based product, the mass content of the superplasticizer polymer is less than 0.75%, for example, from 0.05% to 0.5%. It is understood that this refers to the superplasticizer polymer selected from among phosphate polyethers, sulfonated melamine formaldehyde, and sodium polystyrene sulfonate.

[0051] The invention also relates to the use of a superplasticizer selected from among phosphate polyethers, sulfonated melanin formaldehyde and sodium polystyrene sulfonate, for the calcination, preferably continuous, of an aqueous suspension of gypsum into alpha hemihydrate of calcium sulfate.

[0052] According to a preferred embodiment, 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 gypsum suspension by mixing particulate gypsum, water and at least one superplasticizing polymer chosen from the phosphate polyethers, sulfonated melamine formaldehyde (SMF) and sodium polystyrene 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 polystyrene sulfonate, to form an aqueous gypsum suspension; - 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.

[0053] In this way, the measured viscosity allows for the control and regulation of the superplasticizing polymer concentration and the overall production process, resulting in improved quality and consistency of the continuously produced products. 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 the concentrated suspension. The properties and / or quality of the calcium sulfate alpha hemihydrate are therefore controlled and optimized throughout the continuous production process.

[0054] The invention is illustrated in more detail by the non-limiting examples presented below. Examples

[0055] The invention is illustrated in more detail by the following non-limiting examples. 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.

[0056] Various suspensions "S", numbered 1 to 8 and listed in Table 1 below, are tested. They are prepared from aqueous gypsum suspensions with a water / gypsum ratio of 0.7, 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 is expressed as a mass percentage of the 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 (the best fluidizing efficiency obtained among the concentrations of 0.25%, 0.5%, and 0.75% for each tested superplasticizer polymer).

[0057] 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. 1 The rheometer has a heated measuring cell that maintains the temperature during 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 VMI TURBOTEST at a speed of 3300 rpm for 1 minute to ensure all suspensions are conditioned with the same mixing energy.

[0058] The fluidizing efficiency EF (at 50 s' 1and 85°C) is also calculated for each suspension according to the following formula: EF = ((V o -V s ) / V o )xlOO, where V s is the viscosity of the suspension with the superplasticizer (viscosity at 50 s' 1 and 85°C) and V o is the viscosity of the suspension without superplasticizer (suspension viscosity no. 1, always at 50 s -1 and 85°C).

[0059] The results are listed in Table 1 below. [Table 1]

[0060] The results show that most of these superplasticizers proved very effective at reducing suspension viscosity, except for lignosulfonate (suspension 8). Without wanting to be bound by any particular 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%).

[0061] 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.

[0062] Gypsum and water are loaded into a 2-liter autoclave reactor with a loading ratio of approximately 40% liquid and 60% solid, 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 is raised to 140°C for the time required to convert the gypsum into plaster (approximately 20 minutes at a pressure of (0.25 to 0.50 MPa). 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, then filtered through a Büchner funnel (under reduced pressure), and the resulting powder is washed with isopropanol.

[0063] The size of the alpha hemihydrate particles obtained was measured using a HORIBA PARTICA LA-950-v2 analyzer. The results are given in Table 2 below. [Table 2]

[0064] The results show that sample S2 has the largest particles.

[0065] The resulting plaster particles were then observed by scanning electron microscopy (SEM) using an ITACHI TM3030 instrument. The samples were first dispersed in an isopropanol solution, deposited onto an observation stage previously coated with carbon tape, and gold-plated using a JEOL Smart Coater sample preparation device. Comparative results are shown in Figure 1, where samples SI 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 lead to a plaster with poor rheology, with smaller particles, more fragmented and / or in the form of needles.

[0066] 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, then succinic acid is added (always at a concentration of 0.86 g / L). The plasticizer (superplasticizer) is then added, again according to the The concentrations mentioned previously (Table 1) are used. The reactor is then sealed, and the temperature inside 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 Büchner funnel (under reduced pressure), and the resulting powder is washed with isopropanol.

[0067] The plaster crystals obtained were evaluated as before (scanning electron microscopy and particle distribution). The results are equivalent to those previously described.

[0068] 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 obtaining of alpha hemihydrate particles exhibiting good size and shape properties.

[0069] Figure 2 is a diagram illustrating a continuous production process of alpha hemihydrate of calcium sulfate according to the invention.

[0070] 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).

[0071] The gypsum suspension 110 is then calcined in a calcination vessel A to provide a calcium sulfate alpha hemihydrate suspension 120 (wet calcination under temperature and pressure conditions that allow the transformation of gypsum into alpha hemihydrate). To obtain crystals with an aspect ratio close to 1, the gypsum suspension 110 further includes 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 with a length-to-width aspect ratio close to 1:1, ensuring good flowability of the alpha hemihydrate suspension throughout the process 100.

[0072] The alpha 120 hemihydrate suspension is then passed over a liquid / solid separator "B" (e.g., a belt filter or a centrifugal decanter) to The aqueous filtrate 122 is separated from the concentrated suspension 130 (obtaining an alpha hemihydrate cake). This removes excess aqueous solution. The aqueous filtrate 122 contains water, residual superplasticizer polymer (some of which is also found in the concentrated suspension 130), and residual crystallizing agent (some of which is also found in the concentrated suspension 130). The efficiency of filtration step "B" can also be affected by the shape of the alpha plaster crystals, and therefore by the succinic acid concentration.

[0073] To improve the efficiency of process 100 and reduce water demand (and therefore thermal energy and costs), the filtrate 122 containing residual superplasticizer polymer and crystallizing agent is recycled and reinjected to prepare a new, continuously fed gypsum suspension fraction. This allows for a reduction in the additional quantities of water, superplasticizer polymer, and crystallizing agent required during the preparation of the new gypsum suspension fraction. Furthermore, the still-warm filtrate saves energy on heating the gypsum suspension.

[0074] 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 allows the superplasticizing polymer content to be deduced and the quantity of superplasticizing polymer added to the suspension 110 to be regulated if necessary, 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.

[0075] 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 onto the surface of the alpha gypsum crystals). This adsorbed portion also improves the fluidity of the gypsum suspension when water is subsequently added for the production of gypsum-based products (such as plasterboard).

[0076] To regulate the concentration of crystallizing agent (succinic acid), the pH of filtrate 122 can also be measured online.

[0077] The concentrated suspension of alpha hemihydrate 130 may contain 4 to 8% by mass of water, preferably around 6%. It may undergo various further processing. For example, the 130 suspension may undergo drying and / or grinding (e.g., in a screw mill to reduce particle size) to obtain an alpha 140 plaster-based product of the desired quality (e.g., plaster or a formulation containing plaster 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 plaster 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 suspension of gypsum (110) at a suitable temperature and pressure to obtain a suspension of alpha hemihydrate of calcium sulfate (120); - optionally, the filtration of the calcium sulfate alpha hemihydrate suspension (120) to obtain separately 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 suspension of gypsum (110) is from 0.3 to 2, preferably from 0.4 to 0.9, more preferably from 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 process 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 polyethers phosphates, sulfonated formaldehyde melamine (SMF) and sodium polystyrene 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: - the preparation of 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 onto 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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