Method for producing superabsorbents

Abstract

The present invention relates to a process for the continuous production of superabsorbents, in which superabsorbent particles are coated by spray application of a surface postcrosslinker solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1, and the thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2 having two horizontal shafts, in which particulate solids, aqueous solutions and / or dispersions are metered into the contact dryer 2 by at least one first nozzle in the region of the first horizontal shaft and by at least one second nozzle in the region of the second horizontal shaft, respectively.

Description

[Technical Field]

[0001] The present invention relates to a process for the continuous production of superabsorbents, in which superabsorbent particles are coated by spray application of a surface postcrosslinking agent solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1, and the thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2 having two horizontal shafts, and a particulate solid, an aqueous solution and / or a dispersion is metered into the contact dryer 2 by at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft, respectively. [Background technology]

[0002] Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene articles, but are also used as moisture-retaining agents in commercial horticulture. Superabsorbents are also called water-absorbing polymers.

[0003] The production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] Performance characteristics, such as gel bed permeability (GBP) and 2 To improve absorbency at a pressure of 49.2 g / cm (AUL 0.7 psi), it is common to surface postcrosslink superabsorbent particles. This increases the level of crosslinking at the particle surface, resulting in a 49.2 g / cm 2The absorbency under a pressure of (AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially separated. This surface post-crosslinking can be carried out in an aqueous gel phase. However, it is preferred to surface-coat dried, ground, and sieved polymer particles (base polymer) with a surface post-crosslinking agent and then thermally surface-post-crosslink them. Suitable crosslinking agents for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles. Summary of the Invention [Problem to be solved by the invention]

[0005] It was an object of the present invention to provide an improved process for coating surface postcrosslinked superabsorbent particles, in particular to provide a uniform coating, avoiding agglomerates. [Means for solving the problem]

[0006] This object was achieved by a process for continuously producing superabsorbent particles by spray-applying a surface postcrosslinking agent solution to coat the superabsorbent particles, thermally surface postcrosslinking the coated superabsorbent particles in a contact dryer 1, and cooling the thermally surface postcrosslinked superabsorbent particles in a contact dryer 2, wherein the contact dryer 2 has two horizontal shafts with mixing devices, the speed of which corresponds to a Froude number of 0.005 to 0.25, and the particulate solid, the aqueous solution and / or the dispersion are metered in by at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft of the contact dryer 2, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0007] In the context of the present invention, the horizontal shaft on the right side in the product flow direction is the first shaft and the horizontal shaft on the left side in the product flow direction is the second shaft.

[0008] Examples of contact dryers suitable for the continuous process of the present invention include paddle dryers and disk dryers. In contact dryers, the material to be dried is moved and layered along a heated surface by a dynamic tool. Contact dryers can also be used for cooling.

[0009] The speed of the mixing tool preferably corresponds to a Froude number of 0.01 to 0.21, more preferably 0.02 to 0.18, most preferably 0.05 to 0.15.

[0010] For a mixer where the mixing device is mounted horizontally, the Froude number is defined as:

number

[0011] The particulate solid, aqueous solution and / or dispersion may be metered into contact dryer 2 to a level at least 10 mm below the product bed surface, more preferably at least 50 mm below, and most preferably at least 100 mm below the product bed surface.

[0012] The particulate solids may be metered into the contact dryer 2 in the form of a dispersion in a gas stream. The aqueous solution and / or dispersion may be metered into the contact dryer 2 by means of a two-phase nozzle or a hydraulic nozzle.

[0013] Two-phase nozzles allow atomization into fine droplets or a spray mist. The atomization shapes used are round or elliptical solid or hollow cones. Two-phase nozzles can be configured with either external mixing or internal mixing. In the case of an external mixing two-phase nozzle, the liquid and atomizer gas exit the nozzle head through separate orifices. They are mixed in the spray jet only after exiting the atomizing nozzle. This allows for independent control of a wide range of droplet size distribution and throughput. The spray cone of the atomizing nozzle can be adjusted by adjusting the air cap. In the case of an internal mixing two-phase nozzle, the liquid and atomizer gas are mixed within the atomizing nozzle, and the two-phase mixture exits the nozzle head through the same bore or multiple parallel bores. In the case of an internal mixing two-phase nozzle, the volume ratio and pressure conditions are more closely related than in the case of an external mixing atomizing nozzle. Therefore, small changes in throughput result in changes in droplet size distribution. Adjustments to the desired throughput are made through the selected cross-section of the nozzle bore.

[0014] Useful atomizer gases include compressed air, gas, or steam at 0.5 bar or above. Droplet size can be adjusted independently by the ratio of liquid to atomizer gas, and also by the gas and liquid pressures.

[0015] The particulate solids, aqueous solutions and / or dispersions are respectively metered below the product bed surface by two opposing nozzles of contact dryer 2. The opposing sides are fixed via the positions and conveying directions of the two horizontal shafts. The particulate solids, aqueous solutions and / or dispersions must be metered via at least one nozzle on the right side of contact dryer 2 and via at least one nozzle on the left side of contact dryer 2. Contact dryer 2 may also have two or more nozzle pairs of this type. It is advantageous if the two nozzles of a nozzle pair are approximately opposite each other.

[0016] In contact dryer 2, the angle between the horizontal shaft and the nozzle is preferably about 90°. The particulate solid, aqueous solution and / or dispersion may be fed vertically from above. Feeding from below or obliquely from the side is equally possible, the angle to the vertical being preferably 0° to 90°, more preferably 0° to 70°, and most preferably 0° to 50°. Arranging the feed obliquely allows for the use of shorter feed paths, thereby reducing mechanical stresses during operation of contact dryer 2.

[0017] The present invention is based on the finding that particulate solids, aqueous solutions and dispersions can be difficult to mix uniformly in the cooler (contact dryer 2) simply because the mixing capacity of the contact dryer is insufficient. Therefore, it is necessary to meter particulate solids, aqueous solutions or dispersions on both sides of the contact dryer 2.

[0018] The temperature of the superabsorbent particles when the surface postcrosslinking agent solution is spray coated is preferably 30 to 80°C, more preferably 35 to 75°C, and most preferably 40 to 70°C.

[0019] The surface postcrosslinker solution preferably contains 0.001 to 2% by weight, more preferably 0.01 to 1% by weight, and most preferably 0.03 to 0.7% by weight of the surface postcrosslinker, in each case based on the superabsorbent particles. The surface postcrosslinker solution further contains preferably 0.5 to 5% by weight, more preferably 1.0 to 4% by weight, and most preferably 1.5 to 3% by weight of water, in each case based on the superabsorbent particles.

[0020] The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220° C., more preferably 120 to 210° C., and most preferably 130 to 200° C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, and most preferably 20 to 40 minutes.

[0021] Contact dryer 1 and the connections to contact dryer 2 may be slightly heated and / or insulated.

[0022] The amount of particulate solid used is preferably 0.001% to 2.0% by weight, more preferably 0.01% to 1.0% by weight, and most preferably 0.1% to 0.5% by weight, in each case based on the superabsorbent particles. The average particle size of the particulate solid is preferably 0.1 to 100 μm, more preferably 0.5 to 50 μm, and most preferably 1 to 25 μm. The average particle size is the volume-average particle size and can be determined by light scattering. Suitable particulate solids are aluminum trihydroxide, silicon dioxide, and / or aluminum oxide.

[0023] The temperature of the superabsorbent particles when coated with the particulate solid is preferably less than 180°C, more preferably less than 160°C, and most preferably less than 140°C.

[0024] The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80° C., more preferably 35 to 70° C., and most preferably 40 to 60° C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, and most preferably 20 to 40 minutes.

[0025] The mixing device of the contact dryer 2 preferably has a diameter of 0.2 to 2 m, more preferably 0.4 to 1.2 m, and most preferably 0.6 to 1.2 m. The speed of the mixing device is preferably less than 25 revolutions per minute, more preferably less than 20 revolutions per minute, and most preferably less than 15 revolutions per minute.

[0026] The manufacture of superabsorbents is described in detail below.

[0027] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0028] The ethylenically unsaturated monomers having acid groups are preferably water-soluble, i.e., their solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water, and most preferably at least 35 g / 100 g water.

[0029] Suitable monomers are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particularly preferred is acrylic acid.

[0030] Ethylenically unsaturated monomers having acid groups are typically partially neutralized. Neutralization is carried out at the monomer stage. Neutralization is typically carried out by mixing a neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol %, more preferably 50 to 80 mol %, and most preferably 60 to 75 mol %. Conventional neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, alkali metal bicarbonates, and mixtures thereof, can be used. Instead of alkali metal salts, ammonium salts can also be used. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate, or sodium bicarbonate, or mixtures thereof, especially sodium hydroxide, are very particularly preferred.

[0031] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0032] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups include, for example, a free radical polymerizable ethylenically unsaturated group in the polymer chain and a functional group that can form a covalent bond with the acid group of the monomer. In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0033] Suitable crosslinking agents include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, diacrylates and triacrylates, as described in DE 03 / 104301 A1, WO 03 / 104301 A1, and DE 10331450 A1; mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, as described in DE 10331456 A1, DE 10355401 A1, or crosslinker mixtures, as described, for example, in DE 19543368 A1, DE 19646484 A1, WO 90 / 15830 A1, and WO 02 / 032962 A2.

[0034] The amount of crosslinker, calculated in each case based on the total amount of monomer used, is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.6% by weight. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency exceeds the maximum value under pressure (AUL 0.3 psi).

[0035] The initiator used may be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0036] The water content of the monomer solution is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. If the water content is high, the energy consumption in the subsequent drying process will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0037] The temperature of the monomer solution is preferably 10 to 90°C, particularly preferably 20 to 70°C, very particularly preferably 30 to 50°C.

[0038] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen prior to polymerization. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight prior to polymerization.

[0039] Suitable reactors for polymerization are, for example, kneader reactors or belt reactors. In kneaders, the polymer gel formed during the polymerization of aqueous monomer solutions or suspensions is continuously comminuted, for example, by counter-rotating stirrer shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 3825366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be comminuted, for example, in an extruder or kneader.

[0040] To improve drying performance, the ground polymer gel obtained by the kneader may be further extruded.

[0041] The polymer gel is then typically dried by an air circulating belt dryer until the residual moisture content is preferably 0.5% to 10% by weight, more preferably 1% to 7% by weight, and most preferably 2% to 5% by weight, as measured by EDANA recommended test method No. WSP230.2-05 "Mass Loss on Heating." If the residual moisture content is too high, the glass transition temperature T of the dried polymer gel may increase. g If the residual moisture content is too low, the dried polymer gel will be too brittle, and the subsequent grinding step will produce an unnecessarily large amount of overly small superabsorbent particles ("fines"). The solids content of the polymer gel before drying is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then ground, and optionally coarsely pulverized.

[0042] The dried polymer gel is then typically ground and classified, and the equipment used for grinding may typically be a single or multi-roll mill, preferably a two or three roll mill, a pin mill, a hammer mill or a vibratory mill.

[0043] The average particle size of the superabsorbent particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most preferably 300 to 500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," in which the mass proportions of the selected fractions are plotted in cumulative form, and the average particle size is determined graphically. The average particle size in this specification refers to the mesh size resulting from a cumulative weight of 50%.

[0044] The properties of superabsorbent particles can be further improved by thermal surface postcrosslinking.Suitable surface postcrosslinking agents are compounds that contain a group capable of forming a covalent bond with at least two carboxylate groups of superabsorbent particles.Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides as described in EP 0083022A2, EP 0543303A1 and EP 0937736A2, difunctional or polyfunctional alcohols as described in DE 3314019A1, DE 3523617A1 and EP 0450922A2, or β-hydroxyalkylamides as described in DE 10204938A1 and U.S. Pat. No. 6,239,230.

[0045] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0046] Polyvalent cations that can be used in the process of the present invention include, for example, divalent cations such as zinc, magnesium, calcium, and strontium cations; trivalent cations such as aluminum, iron, chromium, rare earth, and manganese cations; and tetravalent cations such as titanium and zirconium cations. Possible counterions include chloride, bromide, hydroxide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0047] The amount of polyvalent cation used is, for example, 0.001% to 1.5% by weight, preferably 0.005% to 1% by weight, and more preferably 0.02% to 0.8% by weight, in each case based on the polymer.

[0048] The surface postcrosslinking is carried out in such a way that a solution of the surface postcrosslinker is sprayed onto the dried superabsorbent particles. After spray application, the superabsorbent particles coated with the surface postcrosslinker are subjected to thermal surface postcrosslinking.

[0049] The spray application of the solution of surface postcrosslinker is preferably carried out using a mixer with moving mixing implements, such as a screw mixer, a disk mixer, and a paddle mixer.Horizontal mixers, such as a paddle mixer, are particularly preferred, and vertical mixers are particularly preferred.Horizontal mixers and vertical mixers are distinguished by the way the mixing shaft is mounted, that is, horizontal mixers have horizontally mounted mixing shafts, and vertical mixers have vertically mounted mixing shafts.Suitable mixers are, for example, the horizontal Pflugschar® plowshare mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands).However, it is also possible to spray the solution of surface postcrosslinker in a fluidized bed.

[0050] The surface postcrosslinking agent is typically used in the form of an aqueous solution. The penetration depth of the surface postcrosslinking agent into the superabsorbent particles can be adjusted by the content of the nonaqueous solvent and the total amount of the solvent.

[0051] Thermal surface postcrosslinking is carried out in contact dryer, more preferably paddle dryer, most preferably disk dryer.Suitable dryer is for example Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® disk dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA) and Nara paddle dryer (NARA Machinery Europe; Frechen; Germany).

[0052] The surface postcrosslinked superabsorbent particles can then be classified again to remove overly small and / or overly large superabsorbent particles and reuse them in the process.

[0053] The properties of the surface postcrosslinked superabsorbent particles can be further improved by coating or rewetting.

[0054] Remoistening is preferably carried out at 30 to 80°C, more preferably 35 to 70°C, and most preferably 40 to 60°C. At excessively low temperatures, the superabsorbent particles tend to form agglomerates, while at higher temperatures, water evaporates to a significant extent. The amount of water used for remoistening is preferably 1 to 10% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 5% by weight. Remoistening increases the mechanical stability of the superabsorbent particles and reduces their tendency to become electrostatically charged. Remoistening is advantageously carried out in a cooler after thermal surface postcrosslinking.

[0055] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert materials such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust adsorption include, for example, polyols. Suitable coatings for combating the undesirable tendency of superabsorbent particles to caking include, for example, fumed silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20. [Example]

[0056] Example 1 (present invention): A monomer solution was prepared by successively mixing deionized water, 48% by weight sodium hydroxide solution, and acrylic acid to a degree of neutralization corresponding to 72.0 mol %. The water content of the monomer solution was 57.0% by weight.

[0057] The crosslinking agent used was triethoxylated glyceryl triacrylate (purity approximately 85% by weight). The amount used was 0.99 kg per ton of monomer solution. In addition, polyethylene glycol with an average molar mass of 4000 g / mol was added to the monomer solution. The amount used was 2.64 kg per ton of monomer solution.

[0058] Free radical polymerization was initiated using 0.99 kg of a 0.25 wt % aqueous hydrogen peroxide solution, 4.38 kg of a 15 wt % aqueous sodium peroxodisulfate solution, and 0.88 kg of a 1 wt % aqueous ascorbic acid solution per ton of monomer solution.

[0059] The monomer solution was added to 6.3 ml 3 The monomer solution was introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution was approximately 20 t / h. The feed temperature of the reaction solution was 23.5°C.

[0060] Between the addition of the crosslinker and the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution was inerted with nitrogen. Ascorbic acid was metered directly into the reactor. In addition, a 20 wt. % aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1'-diphosphonic acid (etidronic acid) was metered into the reactor. The amount of solution used was 6.17 kg per ton of monomer solution.

[0061] After about 50% of the residence time, an additional 1000 kg / h of superabsorbent particles having a particle size of less than 150 μm, obtained in a manufacturing process by grinding and classification, were metered into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0062] The resulting polymer gel was applied to the conveyor belt of an air circulation belt dryer by vibrating the conveyor belt. The air circulation belt dryer had a length of 48 m. The conveyor belt of the air circulation belt dryer had an effective width of 4.4 m. On the air circulation belt dryer, the aqueous polymer gel was exposed to a continuous flow of air / gas mixture (about 175°C) and dried. The residence time in the air circulation belt dryer was 37 minutes.

[0063] The dried polymer gel was crushed in a three-roll mill and sieved to particle sizes of 150-710 μm. Superabsorbent particles with particle sizes less than 150 μm were separated. Superabsorbent particles with particle sizes greater than 710 μm were recycled for crushing. Superabsorbent particles with particle sizes in the range of 150-710 μm were thermally surface postcrosslinked.

[0064] Superabsorbent particles were coated with the surface postcrosslinker solution in Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then thermally surface postcrosslinked in a NARA paddle dryer (contact dryer 1, GMF Gouda, Waddinxveen, Netherlands) at 120°C for 45 minutes.

[0065] The following amounts were metered into the Schugi Flexomix®: 7.5t / hour of superabsorbent particles 361.5 kg / hour of surface post-crosslinker solution

[0066] The surface postcrosslinker solution contained 0.87 wt% ethylene glycol diglycidyl ether, 32.78 wt% propane-1,2-diol, 1.10 wt% aluminum sulfate, and 65.25 wt% water.

[0067] The surface-postcrosslinked superabsorbent particles were transferred by star feeder to a NARA paddle cooler (contact dryer 2, GMF Gouda, Waddinxveen, the Netherlands) and cooled to approximately 60°C. Simultaneously, the surface-postcrosslinked superabsorbent particles were coated with 576.75 kg / h of an aqueous solution. The aqueous solution contained 0.65% by weight of aluminum sulfate and 0.065% by weight of sorbitan monolaurate (Span® 20). Two opposing two-phase nozzles were used to meter the aqueous solution below the surface of the product bed. The angle to the vertical was 48°. The distances from the end walls of the two-phase nozzles were 1135 mm and 1280 mm. The paddles had a diameter of approximately 0.9 m. The paddles rotated at approximately 10 revolutions per minute. The residence time was approximately 20 minutes.

[0068] The resulting superabsorbent particles contained very few agglomerates.

[0069] Example 2 (not according to the invention) The procedure of Example 1 was repeated. The aqueous solution was metered using only the first two-phase nozzle. The distance from the end wall of the two-phase nozzle was 1280 mm.

[0070] The resulting superabsorbent particles contained numerous agglomerates.

[0071] Example 3 (not according to the invention) The procedure of Example 1 was repeated. The aqueous solution was metered using two successive two-phase nozzles. The distances of the two-phase nozzles from the end wall were 1280 mm and 2295 mm.

[0072] The resulting superabsorbent particles contained agglomerates.

Claims

1. 1. A method for continuously producing superabsorbents by coating superabsorbent particles by spray application of a surface postcrosslinking agent solution, thermally surface postcrosslinking the coated superabsorbent particles in a contact dryer 1, and cooling the thermally surface postcrosslinked superabsorbent particles in a contact dryer 2, wherein the contact dryer 2 has two horizontal shafts with mixing devices, the speed of which corresponds to a Froude number of 0.005 to 0.25, and the particulate solid, aqueous solution and / or dispersion are metered into the contact dryer 2 by at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft, respectively.

2. 10. The method of claim 1, wherein the speed of the mixing implement corresponds to a Froude number of 0.05 to 0.

15.

3. 3. The method of claim 1 or 2, wherein the particulate solid, the aqueous solution, and / or the dispersion are metered below the product bed surface in the contact dryer (2).

4. The method according to any one of claims 1 to 3, wherein the residence time of the superabsorbent particles in the contact dryer 1 is between 10 and 60 minutes.

5. 5. The method according to claim 1, wherein the amount of particulate solid, aqueous solution and / or dispersion used is in each case from 0.001% to 2.0% by weight, based on the superabsorbent particles.

6. The method according to any one of claims 1 to 5, wherein the mixing device has a diameter of 0.2 to 2 m.

7. The method according to any one of claims 1 to 6, wherein the speed of the mixing device is less than 25 revolutions per minute.

8. A method according to any one of claims 1 to 7, wherein the particulate solid used is silicon dioxide, aluminium oxide and / or aluminium trihydroxide.

9. The method according to any one of claims 1 to 8, wherein the aqueous solution used is an aqueous solution of polyethylene glycol, an aluminum salt and / or a surfactant.

10. The method according to any one of the preceding claims, wherein the superabsorbent particles are cooled in the contact dryer (2) to a temperature of from 30 to 80°C.

11. 11. The method according to any one of claims 1 to 10, wherein partially neutralized cross-linked polyacrylic acid is the superabsorbent used.

12. The method according to any one of claims 1 to 11, wherein the surface postcrosslinker is capable of forming a covalent bond with the superabsorbent.

13. The method according to any one of claims 1 to 12, wherein the temperature of the superabsorbent particles during spray application of the surface postcrosslinker solution is 30 to 80°C.

14. A method according to any one of the preceding claims, wherein the superabsorbent particles are heated in the contact dryer 1 to a temperature of from 110 to 220°C.

15. The method according to any one of the preceding claims, wherein the residence time of the superabsorbent particles in the contact dryer 1 is between 10 and 60 minutes.

Images