ARRANGEMENT AND METHOD FOR DRYING AND RECRUITING AN AQUEOUS POLYMER GEL

Description

[0001] The invention relates to a belt dryer arrangement for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles according to the preamble of claim 1. The invention further relates to a method for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles according to the preamble of claim 11 and a manufacturing method according to claim 12.

[0002] Water-absorbing or superabsorbing polymers (SAP, or superabsorbers for short) are cross-linked hydrophilic polymers that can absorb many times their mass in the dry state (sometimes over a thousand times) in liquids, such as water or similar liquids.

[0003] The main application of superabsorbent polymers is in the hygiene sector, and they also play a significant role in the medical field in wound dressings and plasters. Other important applications for superabsorbent polymers are in agriculture and horticulture, where they are used to improve soil's ability to retain moisture.

[0004] The requirements for a superabsorbent depend on the specific application, which is why the properties of the superabsorbent (e.g., swelling capacity and swelling rate) must be adapted accordingly. It is important to consider whether the absorption of the fluid to be absorbed will occur at a higher temperature and / or under pressure, which is particularly relevant for the use of superabsorbents in incontinence products. The type and composition of the fluid to be absorbed are also of great importance, as the swelling capacity of a superabsorbent is strongly influenced by the salt content of the swelling agent.

[0005] Water-absorbing polymers are, in particular, polymers made from (co-)polymerized hydrophilic monomers, graft copolymers of one or more hydrophilic monomers on a suitable graft base, cross-linked cellulose or starch ethers, cross-linked carboxymethylcellulose, partially cross-linked polyalkylene oxide, or natural products that swell in aqueous liquids, such as garnet derivatives. Such water-absorbing polymers are used in the manufacture of diapers, tampons, and sanitary napkins, as well as water-retaining agents in agricultural horticulture.

[0006] A manufacturing process for water-absorbing polymer particles comprises the following steps: polymerizing an aqueous monomer solution or suspension to produce a polymer gel; drying the aqueous polymer gel in a belt dryer with a conveyor belt, picking up the aqueous polymer gel on the conveyor belt and conveying the polymer gel on the conveyor belt in one conveying direction; breaking and / or grinding the dried polymer gel into polymer particles.

[0007] One of the manufacturing processes mentioned above is described in US 4,138,539, in which a partially dried polymer gel is initially transported from a conveyor belt of a belt dryer and crushed into particles of a certain size using a dry cake crusher in order to dry them further.

[0008] A special type of belt dryer arrangement mentioned above is described in US 4,417,697, in which a so-called "cutter" is used to scrape the dried material from the conveyor belt in contact with the conveyor belt.

[0009] Another type of belt dryer arrangement is described in US 4,559,235 A for drying and shredding agricultural products, namely freshly cut green chopped alfalfa.

[0010] The production of water-absorbing polymers is described, for example, in the monograph "Modern Super Absorbent Polymer Technology", by FL Buchholz and AT Graham, Wiley-VCH, 1998, or in Ullmann's "Encyclopedia of Industrial Chemistry", 6th edition, volume 35, pages 73 to 103.

[0011] For drying, a belt dryer, particularly a recirculating air belt dryer, is designed with a dryer assembly that largely encompasses the conveyor belt and incorporates an air return system connected to the dryer assembly. Return air is drawn from the dryer assembly and fed back to it via the air return system. The air return system includes an air intake section located between an upstream inlet section on the dryer assembly and a downstream outlet section within the air return system.

[0012] A superabsorbent polymer in the state of an aqueous polymer gel is considered to be in a wet state and can therefore be more generally referred to as a wet product; that is, the aqueous polymer gel still contains a considerable amount of water before drying, particularly as described below. The aqueous polymer gel is obtained by polymerizing a monomer solution or suspension. The aqueous polymer gel, consisting of still aqueous polymer particles, is preferably introduced into the belt dryer as granules, e.g., with a solids content of 40–60%. In this state, the polymer gel is generally already cross-linked to a desired degree of cross-linking, in particular initially homogeneously cross-linked, in particular with a comparatively low degree of cross-linking, and in particular initially with very little surface cross-linking.

[0013] A superabsorbent polymer in the state of water-absorbing polymer particles is considered to be in a post-drying state; that is, it has a low residual water content in the polymer particles after drying the aqueous polymer gel, particularly as described below. Thus, the superabsorbent polymer is preferably present as a dried polymer gel, or more specifically, as dried polymer particles. In this state, the water-absorbing polymer particles can preferably be post-crosslinked, particularly surface-crosslinked, with the degree of surface crosslinking then preferably being higher than the comparatively low initially homogeneous degree of crosslinking mentioned above. Preferably, an aqueous polymer gel of the water-absorbing polymers is obtained after polymerization, which is then dried.The principles of drying the aqueous polymer gel to a water-absorbing polymer with water-absorbing, especially dried, polymer particles are also described in the monograph "Modern Super Absorbent Polymer Technology", by FL Buchholz and AT Graham, Wiley-VCH, 1998, on pages 87 to 93.

[0014] In the belt dryer, the aqueous polymer gel is dried to a partially dried polymer gel, thus existing as a dry cake. The dry cake preferably exists as a strand of partially dried polymer gel, i.e., as a partially dried polymer strand, on the belt of the dryer, extending through the dryer assembly.

[0015] The dry cake is located at the end of the belt dryer, i.e., upon exiting the dryer assembly, as a largely dried strand of dried polymer gel, roughly in the form of a plate or a board-like strand – that is, as a dried polymer strand. In the following, the term "dried polymer particles" will sometimes refer to the partially dried polymer gel and to the dried polymer gel of the dry cake; both cases are described using the terms "superabsorbent or water-absorbing polymer gel" or "dried polymer gel" as distinct from "aqueous polymer gel".

[0016] A feed module of the belt dryer serves to introduce the superabsorbent polymer in the form of aqueous polymer particles. In practice, the drying conditions are then selected to represent a compromise between utilizing the dryer's capacity and the processability of the water-absorbing polymer particles. Compared to other dryer designs, the belt dryer has the advantage that (apart from gravity) no significant mechanical stress affects the product, as the aqueous polymer gel or the water-absorbing polymer particles rest loosely on a conveyor belt. In principle, a belt dryer offers the possibility of designing one or more control zones using one or more drying zones.

[0017] A belt dryer, for example, comprises a product feed module for polymer input, several dryer modules to form one or more drying zones, and a discharge module for polymer discharge. The discharge module serves to discharge the superabsorbent polymer in the form of water-absorbing polymer particles; in particular, the conveyor belt terminates in the discharge module or has a reversing point there. The superabsorbent polymer can fall onto a crusher or similar shredder at the end of the belt dryer in the discharge module, or it can be fed into it. The dry cake can be fed to the shredder whole, as fragments of the dry cake, or as other coarse pieces. The dried, but not yet shredded, polymer particles then exist partly as fragments of dried polymer gel, e.g., relatively coarse pieces, and partly as unavoidable residue of dried polymer gel.

[0018] The process consistently yields comminuted, dried polymer particles from dried polymer gel through comminution in the grinder. In particular, the residue from the dried polymer gel and the remaining material resulting from the comminution process comprise fine powder particles, including fine and ultrafine particles. After comminution, the dried and comminuted polymer particles are preferably conveyed pneumatically and subjected to a grinding process; thus, they are further processed into ground, dried polymer particles.

[0019] WO2015 / 163512 A1 describes a manufacturing process for water-absorbing polymer particles in which a polymer gel is dried. After drying, the dried polymer gel undergoes a first milling step in a first milling unit, followed by a dwell time unit, and then a second milling step in a second milling unit. The dwell times in the first and second milling units are in the range of 3 minutes, and the dwell time in the dwell time unit is 30 minutes or more, always significantly longer than the dwell time in a single milling unit. The dwell time unit prevents clumped material from entering the subsequent conveying process. Milling is performed to achieve particle sizes of 150 µm.

[0020] In particular, ground, dried polymer particles can then be subjected to a sieving process. A good-sized fraction already has a preferred desired particle size and can be separated. An oversized or fine-sized fraction can, if necessary, be ground, sieved, or processed again and added to the good-sized fraction. The dried, ground, and sieved polymer particles of the good-sized fraction can undergo surface finishing. The dried, ground, sieved, and surface-finished polymer particles can then be subjected to protective sieving.

[0021] Continuously operating convection belt dryers can be used in the drying process; in the following, this refers to a belt dryer of the type mentioned above, in particular a recirculating air belt dryer. The aforementioned belt dryer is specifically designed for an aqueous polymer gel, especially for the formation of a limited-free-flowing, lumpy product. In a continuously operating belt dryer, the product layer of an aqueous polymer gel, laid down as an aerated bulk, is transported as a polymer strand through the drying chamber on a perforated conveyor belt, where it is first dried to a partially dried polymer gel and finally to a dried polymer gel cake; this is then further processed into the aforementioned dried polymer particles as water-absorbing polymer particles.

[0022] The drying gas flowing through the product layer of the dry cake, consisting of partially dried and then fully dried polymer gel, serves both to introduce heat into the aqueous polymer gel being dried or into the partially dried water-absorbing polymer particles, and to remove evaporating moisture. Air is preferably used as the drying gas. In a recirculating air belt dryer, the drying air flowing through the product layer is also recirculated.

[0023] Belt dryers with conveyor belts are to be distinguished from belt reactors. While a belt reactor is used to produce aqueous polymer gel from its raw materials, a belt dryer is used to produce water-absorbing polymer particles from an aqueous polymer gel, in particular to produce the aforementioned water-absorbing polymer particles preferably from an aqueous polymer gel that is initially homogeneously cross-linked with the desired degree of cross-linking, and optionally also surface-cross-linked.

[0024] WO2015 / 074966A1 describes a plate belt dryer for superabsorbent polymers (SAP, or superabsorbents for short) with a shredder. Such a shredder, in the form of a cross-wing shredder, is generally known and used for shredding SAP and is commercially available. An example can be seen, for instance, in the online presentation of the company Grenzebach. https: / / www.yumpu.com / de / document / view / 6323642 / thermische-verfahrenstechnik-grenzebach-maschinenbau-gmbh .

[0025] A cross-wing shredder of the applicant is described in principle in relation to SAP production in WO2013 / 072419. This cross-wing shredder comprises a shaft on which a plurality of ingots are mounted. In addition to the ingots arranged on the shaft, the cross-wing shredder includes a plurality of fixed ingots that engage in the spaces between the ingots arranged on the shaft. The poly(meth)acrylate lumps fed into the shredder fall onto the fixed ingots and remain there. The ingots, rotating with the shaft, break up the lumps.

[0026] In WO2014 / 044780A1, a crusher in combination with an adjustable guide in the form of a grid is generally disclosed, which additionally causes a certain crushing of SAP lumps.

[0027] A desirable outcome is improved comminution of the dried polymer strand, i.e., the dry cake or its coarse fragments, into dried polymer particles at the end of a conveyor belt in a belt dryer. This is primarily due to the fact that, firstly, the properties of the dried polymer gel can vary in terms of hardness and consistency, thus impacting the comminution capacity of a shredder. Secondly, for pneumatic conveying—especially for pneumatic conveying with an intermediate grinding and / or sieving step—it is advantageous to provide a largely uniform product stream of already well-comminuted polymer particles from the dried polymer gel, and in particular, to introduce them into the pneumatic conveying system with comparatively constant parameters.

[0028] One object of the invention is to provide an improved device and an improved method for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles. In particular, the dried polymer gel, as a dry cake, is to be comminuted into dried polymer particles in an improved manner after drying or after passing through a belt dryer. Specifically, the dried polymer particles obtained through the improved comminution process for the dried polymer gel of the polymer strand, i.e., the dry cake, are to be comminuted with a particle size distribution that is optimal / suitable for subsequent pneumatic conveying and / or grinding. Another object of the invention is to provide an improved manufacturing process for producing water-absorbing polymer particles.

[0029] Regarding the apparatus, the problem is solved by the invention with a belt dryer arrangement for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles according to claim 1.

[0030] The invention relates to the belt dryer arrangement for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles, and assumes that the belt dryer arrangement comprises: a belt dryer with a dryer assembly and a conveyor belt for receiving and drying the aqueous polymer gel into a dry cake on a receiving surface of the conveyor belt, a comminution arrangement which is arranged in the product flow direction after the dryer assembly for comminuting the dried polymer gel of the dry cake or, if applicable, breaking it up into dried comminutionary polymer particles.

[0031] It is intended that The comminution arrangement comprises a comminutor formed as a milling drum, which is arranged with an upper working line of a working edge of the milling drum on or above a receiving surface of a conveyor belt for milling the dry cake or, if applicable, the breakage, wherein the milling drum is designed to discharge comminuted dried polymer particles of the dry cake or breakage downwards.

[0032] According to the invention, the milling roller has a diameter such that an axis of the milling roller is located at or below the height of the receiving surface of the conveyor belt, and a radius of the milling roller is greater than or equal to the thickness of the dry cake or the break, and a lower working line of the milling roller lies below the receiving surface of the conveyor belt.

[0033] Regarding the method, the problem is solved by the invention using a method of claim 11.

[0034] The invention relates to a process in which an aqueous polymer gel is dried and the dried polymer gel is comminuted into dried polymer particles using a comminution arrangement. For this purpose, a belt dryer arrangement is provided, comprising: a belt dryer with a dryer assembly and a conveyor belt for receiving and drying the aqueous polymer gel into a dry cake on a receiving surface of the conveyor belt, a comminution arrangement which is arranged in the product flow direction after the dryer assembly for comminuting the dried polymer gel of the dry cake or, if applicable, breaking it up into dried comminutionary polymer particles.

[0035] It is intended that The comminution arrangement comprises a comminutor designed as a milling drum, wherein in the process: the dried polymer gel of the dry cake is comminuted into dried comminuted polymer particles, wherein with an upper working line of a working edge of the milling drum on or above a receiving surface of a conveyor belt the dry cake or, if applicable, fragment thereof is milled and the milling drum throws the comminuted dried polymer particles of the dry cake or fragment downwards.

[0036] According to the invention, the milling roller has a diameter such that an axis of the milling roller is located at or below the height of the receiving surface of the conveyor belt, and a radius of the milling roller is greater than or equal to the thickness of the dry cake or the break, and a lower working line of the milling roller lies below the receiving surface of the conveyor belt.

[0037] The upper working line of the milling drum has an upper distance to the receiving surface of the conveyor belt, and the milling drum has a rotatable shaft with functional elements, and the conveyor belt has a deflection roller, wherein the rotatable shaft is designed to rotate with a direction of rotation opposite to a direction of rotation of the deflection roller.

[0038] A working line of a milling drum's working edge is defined as the tangent at the upper apex of the milling drum's working edge. Similarly, a lower working line of the milling drum is defined as the tangent at the lower apex of the milling drum's working edge. The working edge is understood to be the outer edge of the milling drum that operates on the dry cake; specifically, the circular edge that encloses the functional elements and whose radius is determined by the outer end of functional elements of the milling drum.

[0039] According to the concept of the invention, in simplified terms, a milling drum is arranged at or above the level of a conveyor belt for milling the dry cake and dropping milled dried polymer particles of the dry cake downwards.

[0040] The invention is based on the consideration that improved comminution of dried polymer particles of the dried polymer strand, in particular of the dry cake or possibly fragments of the dry cake, on the conveyor belt at the end of a conveyor belt of a belt dryer with only one cross-wing shredder is problematic or hardly achievable.

[0041] Based on this consideration, the invention has recognized that the milling roller can be better adapted to a distribution of polymer particles in the dry cake and breakup of the dry cake immediately after the conveyor belt, and that, on the other hand, the product of well or finely crushed dried polymer particles can be adapted independently with regard to the needs of pneumatic conveying and / or grinding.

[0042] The invention also leads to a manufacturing process according to claim 12.

[0043] According to the invention, the manufacturing process for producing water-absorbing polymer particles comprises the following steps: Polymerization of a monomer solution or suspension to water-absorbing polymer particles of an aqueous polymer gel, wherein the solution contains a) at least one ethylene-unsaturated, acid-group-bearing monomer, which may be at least partially neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylene-unsaturated monomers copolymerizable with the monomers mentioned under a) and optionally one or more water-soluble polymers, conveying the aqueous polymer gel to the belt dryer arrangement according to the invention.

[0044] According to the invention, the following is further provided: Drying the aqueous polymer gel and comminuting the dried polymer gel into dried polymer particles using the belt dryer arrangement according to the invention, comprising: a belt dryer with a dryer assembly and a conveyor belt for receiving and drying the aqueous polymer gel into a dry cake or fraction on a receiving surface of the conveyor belt, a comminution arrangement which is arranged in the product flow direction downstream of the dryer assembly for comminuting the dried polymer gel of the dry cake or fraction into dried comminutionary polymer particles, wherein the comminution arrangement comprises a comminutor which is formed as a milling drum, and in the process: the dried polymer gel of the dry cake is comminutioned into dried comminutionary polymer particles,wherein with an upper working line of a working edge of the milling drum on or above a receiving surface of a conveyor belt the dry cake or the fraction is milled and the milling drum throws the crushed dried polymer particles of the dry cake or fraction downwards, wherein , According to the invention, the milling roller has a diameter such that an axis of the milling roller is mounted at or below the height of the receiving surface of the conveyor belt, and a radius of the milling roller is greater than or equal to the thickness of the dry cake or the breakage, and a lower working line of the milling roller lies below the receiving surface of the conveyor belt, wherein the upper working line of the milling roller has an upper distance to the receiving surface of the conveyor belt, and the milling roller has a rotatable shaft with functional elements, and the conveyor belt has a deflection roller, wherein the rotatable shaft is designed to rotate with a direction of rotation opposite to a direction of rotation of the deflection roller, and optionally grinding and / or classifying the dried and crushed polymer particles.

[0045] Further advantageous embodiments of the invention can be found in the dependent claims and specify in detail advantageous possibilities for realizing the explained concept within the scope of the task and with regard to further advantages.

[0046] Advantageously, the dryer assembly includes a conveyor belt for receiving the aqueous polymer gel and conveying it through the assembly in one direction, as well as polymer gel input and output points. In particular, the dryer assembly features an air duct connected to it for supplying and exhausting air to dry the aqueous polymer gel.

[0047] The comminution arrangement is advantageously arranged downstream of a polymer gel discharge for comminuting the dried polymer gel into dried polymer particles, wherein the dried polymer gel is fed to the comminution arrangement as a dry cake or fragments of the dry cake made from dried polymer gel and comminuted into dried polymer particles.

[0048] According to the invention, the milling drum has a rotatable shaft with functional elements, and the conveyor belt has a deflection roller, wherein the rotatable shaft is configured to rotate in a direction opposite to the direction of rotation of the deflection roller. These preferred counter-rotating directions of rotation facilitate the downward discharge of crushed, dried polymer particles of the dry cake or the broken material.

[0049] According to the invention, the radius of the milling drum is greater than or equal to the thickness of the dry cake or the crushed material, and a lower working line of the milling drum lies below a receiving surface of a conveyor belt. These features ensure that the dry cake or crushed material is guided almost completely directly onto the milling drum and fully captured by it. These features, individually or alone, significantly reduce the risk of the dry cake or crushed material being guided wholly or partially past the milling drum.

[0050] Preferably, the upper working line of the milling drum is located above the top edge of the dry cake or above the top edge of the breakage. This facilitates the downward discharge of crushed, dried polymer particles from the dry cake or breakage.

[0051] According to the invention, the upper working line of the milling roller has a distance above the receiving surface of the conveyor belt, and preferably this distance above the height of the receiving surface of the conveyor belt is at least 5 cm, preferably at least 8 cm, and preferably at least 10 cm. This enables the milling roller to work efficiently directly on the dry cake or curd. It ensures that the milling roller strikes the dry cake or curd more or less from above.

[0052] Preferably, a support for the dry cake or the crushed material is arranged between the conveyor belt and the milling drum, extending along the conveyor belt. This prevents pieces of the dry cake and crushed material from falling through between the milling drum and the end of the conveyor belt without being ground up. Furthermore, the support absorbs the impact forces of the milling drum, thus preventing the conveyor belt and / or a rear deflection roller from having to absorb such forces.

[0053] Preferably, the support is designed as a support table with a closed support surface, maintaining a rear gap between a rear edge of the support table and a front working edge of the milling drum. A support table offers particularly good support properties and, with its closed support surface, efficiently prevents pieces of the dry cake and the broken pieces from falling through uncrushed behind the end of the conveyor belt. The remaining gap between a rear edge of the support table and a front working edge of the milling drum can therefore be advantageously adjusted.

[0054] Advantageously, the support is designed as a support grid with laterally spaced support elements, in particular rods, bars, poles, or the like. A support grid is relatively easy to implement. Furthermore, a support grid can enhance the crushing action of the milling drum if the dry cake or the broken material breaks against the spaced support elements of the grid.

[0055] Particularly advantageous is a front part of the support designed as a support table with a closed support surface, and a rear part of the support designed as a support grid comprising laterally spaced support elements, in particular rods, bars, poles, or the like. This preferred embodiment combines the advantages of a previously described support table and a previously described support grid.

[0056] Advantageously, the functional elements of the milling drum can engage in the spaces between the spaced support elements of a support grid. This has the advantage that the functional elements of the milling drum strike the dry cake or aggregate, causing it to break against the spaced support elements of the support grid. The spacing of the laterally spaced support elements can therefore be advantageously adjusted.

[0057] A milling drum can, but does not necessarily have to, be the sole shredder in a shredding arrangement. In a further development, the shredding arrangement may include, in addition to the milling drum and an optional auger, another shredding and / or guiding element, in particular another shredder and / or a deflector.

[0058] For the purposes of this application, a further crusher shall generally be understood to be a device comprising at least one rotating roller (rotor); i.e., a rotating roller alone (milling cutter, cutting mill) or a rotating roller, in particular a spiked roller, in combination with a stationary part (crusher, in particular a cross-wing crusher), or also a rotating roller in combination with one or more rotating rollers (mill). This also includes a crusher in the form of a screw conveyor, which, although primarily serving a conveying purpose, can also perform a crushing function. A crusher thus comprises at least all types of milling cutters, crushers, mills, and screws, but not screens or similar conveying devices, since the latter do not have a rotating roller that actively serves to crush the dried polymer gel of the drycake into dried polymer particles.

[0059] Preferably, the comminution arrangement comprises a screw conveyor, wherein the milling roller comminution arrangement is arranged directly upstream of the screw conveyor and / or the screw conveyor acts as a further comminution.

[0060] Preferably, the conveying screw is arranged next to or below the milling drum and / or is arranged to receive freely falling dried polymer particles from the milling drum.

[0061] Furthermore, this advanced design takes advantage of the fact that the comminution arrangement is positioned—preferably immediately after the polymer discharge at the end of the conveyor belt and directly downstream of the pneumatic conveying system. This allows the comminution arrangement to be housed in a space-saving manner between the belt dryer and the pneumatic conveying system. Nevertheless, it can be operated independently of the belt dryer, and in particular, largely independent of the properties of the coarse fragments of dried polymer particles broken off from the dried dry cake.

[0062] Preferably, the milling drum is designed to comminute the dry cake into dried polymer particles with a predefined size distribution, enabling them to be conveyed directly by at least one conveying unit, particularly a mechanical or pneumatic conveying unit, especially a screw conveyor. Preferably, the comminution arrangement, particularly the milling drum, is designed to comminute the dry cake into dried polymer particles, wherein the comminuted dried polymer particles have a mean particle diameter between 0.5 mm and 10 mm, preferably between 1 mm and 9 mm, and particularly between 1 mm and 5 mm. The mean particle diameter after comminution is determined according to EDANA test method no. WSP 220.2-05 "Particle Size Distribution".

[0063] The comminution arrangement is advantageously designed for a residence time of less than 90 seconds for polymer particles within the comminution unit. However, the residence time of the polymer particles in the milling drum is significantly lower.

[0064] Preferably, the process and the belt dryer with the comminution arrangement and / or grinding unit are robust against temperature fluctuations. The comminution arrangement is specifically designed to comminute the polymer particles at a temperature between 40°C and 140°C, particularly between 60°C and 120°C, and preferably between 80°C and 120°C. This offers advantages with regard to the comminution and conveying of the polymer particles. This allows for advantageous control of the comminution effect and the size distribution of the comminuted polymer particles.

[0065] In a preferred further development, the milling drum is designed to rotate the shaft at a speed of more than 50 rpm and less than 250 rpm. This allows for advantageous control of the comminution effect and size distribution of the comminuted polymer particles.

[0066] Advantageously, a number of functional elements of the milling drum are formed as a number of bars, teeth, ingots, paddle surfaces, or toothed, serrated, or knurled ribs or edges, in particular tooth spirals or knurled screws. These functional elements have proven advantageous in the comminution or fragmentation of the dry cake.

[0067] Advantageously, the milling drum has a number of functional elements arranged along a spiral path, forming a working edge. In particular, the spiral has a thread with a pitch angle between 20° and 70°. This results in improved comminution and / or provides a uniform load distribution over time for the milling drum, especially for its drive, and / or for the aforementioned support.

[0068] Advantageously, the spiral can have a single thread or several threads running in the same or opposite directions, particularly separate or interlocking threads. This allows the product flow of comminuted, dried polymer particles to be directed transversely to the conveying direction, especially towards the center.

[0069] Advantageously, the comminution arrangement is positioned downstream of the product flow, upstream of pneumatic conveying and / or grinding.

[0070] Advantageously, a grinding unit is provided for grinding comminuted dried polymer particles from the comminution arrangement into dried, comminuted, and ground polymer particles, with a mean particle diameter of at least 200 µm, particularly preferably 250 to 500 µm, and most preferably 300 to 700 µm. The mean particle diameter after grinding and sieving is determined on the product stream according to EDANA test method no. WSP 220.2-05 "Particle Size Distribution," analogous to the mean particle diameter after comminution.

[0071] Exemplary embodiments of the invention are now described below with reference to the drawing. These drawings are not necessarily to scale; rather, for illustrative purposes, they are presented in a schematic and / or slightly distorted form. For further information on the teachings directly apparent from the drawings, reference is made to the relevant prior art. The general idea and concept of the invention is not limited to the exact form or detail of the preferred embodiments shown and described below, nor is it limited to an object that would be restricted compared to the object claimed in the claims. Within specified design ranges, any values ​​are to be disclosed as limit values ​​and can be used and claimed as desired.Further advantages, features and details of the invention will become apparent from the following description, the preferred embodiments and the drawing.

[0072] The drawing shows in detail: FIG. 1 A schematic representation of a manufacturing arrangement for producing water-absorbing polymer particles with a belt dryer and, in the product flow direction after the belt dryer, with a comminution arrangement for comminuting a dry cake of dried polymer gel into dried polymer particles, which is also arranged in the product flow direction upstream of a pneumatic conveying system; FIG. 2 A schematically represented preferred embodiment of the comminution arrangement of the Fig.1 with a milling drum and a screw conveyor; namely, for the comminution of a dry cake of dried polymer gel into dried polymer particles downstream of a belt dryer and upstream of subsequent pneumatic conveying; FIG. 3A an arrangement of a rotatable shaft of a milling drum with a number of functional elements relative to a dry cake, wherein the dry cake as a whole and substantially unbroken is supported by a horizontal support and wherein the milling drum is designed to mill off dried polymer particles directly from the dry cake; FIG.3. A modified arrangement of a rotatable shaft of a milling drum with a number of functional elements relative to a dry cake, wherein the dry cake as a whole and substantially unbroken or partially broken off is supported by an angled support such that the milling drum is designed to mill dried polymer particles directly from the dry cake or directly from a break in the dry cake, as shown here by way of example; FIG. 4. A detail of a rotatable shaft of a milling drum with the number of functional elements, here in the form of impact bars, designed to mill dried polymer particles from the dry cake or a break therein; FIG. 5. A support in the form of a closed planar support table extending from the conveyor belt, in which a gap is left between its edge and a working edge of the milling drum; FIG.5Legs Support in the form of a support grid, at least partially open, essentially on the side facing the milling drum, with bars or bars or similar fixed support elements spaced apart; functional elements of the milling drum engage in these spaces, so the support grid is arranged in overlap with the working edge of the milling drum; FIG. 6 in views A, B and C three different spirally extending basic courses of working edges of a milling drum; FIG. 7A, FIG. 7Legs cumulative application of mass fractions of the sieve fractions for the graphical determination of the mass mean particle diameter after comminution (. Fig.7A ) and grinding ( Fig.7B ), in particular to explain an analogous procedure for determining the mass means particle diameter after grinding using three examples.

[0073] For the sake of simplicity, the same reference symbols are used in the drawing for identical or similar parts or parts with the same or similar function.

[0074] A manufacturing process, for example, includes the following steps: Processing a monomer solution or suspension under polymerization to an aqueous polymer gel, drying the aqueous polymer gel in a belt dryer, wherein the belt dryer has a circulating conveyor belt and the aqueous polymer gel is conveyed on the conveyor belt.

[0075] It is preferred that the conveyor belt is formed as a plate conveyor belt, comprising a number of belt plates separated at a joint line of a joint construction and wherein each belt plate has a surface for receiving the aqueous polymer gel.

[0076] In particular, this manufacturing process may include: processing a monomer solution or suspension under polymerization to form a cross-linked aqueous polymer gel.

[0077] In the context of a particularly preferred further development, it was recognized that the concept of the invention or one of its further developments is of particular advantage for a special manufacturing process for superabsorbents, in particular for a special manufacturing process for a polymer gel for superabsorbents, which is described below with some further developments and is also partly explained in WO2011 / 104152 and WO2006 / 100300 A1.

[0078] In particular, this concerns a manufacturing process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension containing a) at least one ethylene-unsaturated, acid-group-bearing monomer, which may be at least partially neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylene-unsaturated monomers copolymerizable with the monomers mentioned under a) and optionally one or more water-soluble polymers.

[0079] The water-absorbing polymer particles are produced by polymerization of a monomer solution or suspension and are insoluble in water.

[0080] The aqueous polymer gel is then dried using a belt dryer until a desired, preferably low, water content is reached, in particular a residual moisture content of preferably 0.5 to 15 wt.%, more preferably 1 to 10 wt.%, and most preferably 2 to 8 wt.%, wherein the residual moisture content is determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel has a glass transition temperature (Tg) that is too low and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large quantities of polymer particles with an excessively small particle size ("fines") are produced in the subsequent comminution steps. The solids content of the gel before drying is preferably between 25 and 90 wt.%, more preferably between 35 and 70 wt.%, and most preferably between 40 and 60 wt.%.Alternatively, a fluidized bed dryer or a paddle dryer can be used for drying.

[0081] The dried polymer gel is then ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills can be used for grinding.

[0082] Fig.1 This shows a schematic representation of a manufacturing process for poly(meth)acrylates; i.e., generally SAP. The schematic representation in FIG.1 The process of manufacturing poly(meth)acrylates up to dried polymer particles can be derived from this.

[0083] The starting materials 31 for the production of the poly(meth)acrylates are, for example, fed into a mixing kneader, belt reactor, or other reactor 30. The mixing kneader comprises, for example, two axially parallel, rotating shafts, on the surfaces of which disc surfaces with kneading bars arranged around their circumference are mounted. In a polymerization reaction, poly(meth)acrylate is produced as the product, which leaves the reactor 30 in the form of lumps 33 with a gel-like consistency. The lumps 33 enter a gel bunker 32, from which the lumps are conveyed by a swiveling belt or similar conveying device 34 onto a Fig.2 The illustrated conveyor belt 36F of a belt dryer 36 is used to apply the lumps 33 as a superabsorbent polymer in the state of an aqueous polymer gel, consisting of still aqueous polymer gel particles. The belt dryer 36 removes liquid from the aqueous polymer gel particles on the conveyor belt 36F at a temperature of up to 200°C, so that they are conveyed through the belt dryer as partially dried, i.e., still moist, polymer gel particles. The dried poly(meth)acrylate lumps emerge from the belt dryer as dried polymer gel particles forming a board-like, solid dry cake 35. The dry cake 35 then enters a comminution assembly 38.

[0084] The dry cake 35 can enter the crushing assembly 38 unbroken at the end of the conveyor belt (as in Fig.3A (to be seen) or it may be broken or fractured as fracture 37 (as in Fig.3B (to be seen); then the fraction 37, i.e. coarse fragments or chunks, subsequently enters the comminution arrangement 38.

[0085] Provided that the dry cake 35 is intact without breaking, i.e. in one piece, already according to a first modified embodiment ( Fig.3A When the polymer particles reach a milling cutter of a comminution arrangement 38, they are milled directly from the dry cake 35. However, according to a second modified embodiment ( Fig.3B ) a break 37 of the dry cake 35 or other coarse lumps are milled by the milling machine into crushed polymer particles.

[0086] As part of a comminution arrangement according to the prior art, for example WO2013 / 072419 specifies only a shredder designed as a cross-wing shredder. A cross-wing shredder according to the prior art comprises, for example, a shaft on which a plurality of functional bars are mounted. The functional bars are welded to the shaft 10. In addition to the functional bars arranged on the shaft, a cross-wing shredder comprises a plurality of fixed bars that engage in the spaces between the functional bars 14 arranged on the shaft. The poly(meth)acrylate lumps, consisting of dried polymer particles of the dry cake, fed into the shredder typically fall from the conveyor belt onto the fixed bars and remain there. The functional bars, rotating with the shaft, break up the lumps.After passing through the cross-wing shredder, the coarsely ground, dried polymer particles are fed via a pneumatic conveying system to, for example, a mill or similar device. There, the poly(meth)acrylate particles are further ground until the product is in powder form.

[0087] A problem with this relatively simple and efficient state-of-the-art system, which uses only one cross-wing shredder, is that it may not achieve sufficiently fine comminution of the fragments. Depending on the product being dried (solids content of the monomer solution, degree of neutralization, degree of cross-linking, etc.), the drying conditions, and the throughput of the belt dryer, the dry cake 35 may vary considerably in hardness. Therefore, depending on the system's dimensions, a single cross-wing shredder may not be sufficient to produce finely ground fragments. These fragments could, for example, fall through an excessively large gap between the shredder bars or simply remain on the bar. Both scenarios are detrimental to the system's operation.In particular, if excessively large pieces enter the pneumatic conveying system, this could lead to transport problems. Specifically, such large pieces might be processed inefficiently by the mill located further down the conveying path, or might not be drawn in at all.

[0088] In the present case, the invention has recognized that a Fig.2 The milling roller 38.1 shown as a preferred embodiment has proven itself in several respects. The milling roller 38.1 operates directly on the dry cake 35 ( Fig.3A ) according to a first modification or directly at the break 37 of the dry cake 35 according to a second modification ( Fig.3B ) - i.e. the milling roller 38.1 mills directly on an unbroken dried polymer strand, i.e. a dry cake 35, or a broken polymer strand, i.e. a break 37 of the dry cake 35, which is taken over more or less horizontally from the conveyor belt 36F of the belt dryer 36.

[0089] It is therefore suitable for the comminution arrangement 38 of the FIG.1 a crusher is provided which is designed as a milling drum 38.1, i.e. the milling drum 38.1 mills the dry cake 35 or the fragment 37 of the dry cake, which is pushed directly more or less horizontally against the milling drum by the conveyor belt 36F of the belt dryer 36, into crushed dried polymer particles 39.

[0090] Optionally, the milling drum 38.1 is combined with a further shredding and / or guiding element 38.2 of the shredding device 38, which is located downstream of the milling drum 38.1. The further shredding and / or guiding element 38.2 of the shredding device 38 can be another shredder. The further shredding and / or guiding element 38.2 of the shredding device 38 can also be a deflecting device, such as a hopper or the like.

[0091] According to the concept of the invention, the milling roller 38.1 is arranged, in simplified terms, at or above the level of a receiving surface of a conveyor belt for milling the dry cake and dropping milled dried polymer particles of the dry cake downwards.

[0092] Specifically, it is planned that --as in Fig.2 As can be seen, an upper working line H of a working edge 38.1R of the milling roller 38.1 is arranged on or above a receiving surface F of a conveyor belt 36F for milling the dry cake 35 or the fracture 37 thereof, wherein the milling roller 38.1 is designed to discharge crushed dried polymer particles 39 of the dry cake 35 downwards.

[0093] The downward discharge of comminuted, dried polymer particles 39 from the dry cake 35 follows gravity and is preferably assisted by the direction of rotation of the milling drum 38.1. A working line H of the working edge 38.1R of the milling drum 38.1 is defined as the tangent at the upper vertex of the working edge 38.1R of the milling drum 38.1. Similarly, a lower working line h of the milling drum is defined as the tangent at the lower vertex of the working edge 38.1R of the milling drum 38.1. The working edge 38.1R is defined as the circle whose radius is determined by the end of the functional elements ZF2 of the milling drum; that is, primarily the outer edge of the milling drum 38.1 that operates on the dry cake 35 or its fragment 37.

[0094] This ensures that the crushed polymer particles from the milling drum are sufficiently finely ground to be conveyable by the pneumatic transport system and preferably suitable for grinding by a mill. Details of a comminution arrangement 38 of the Fig.1 will be in relation to the further FIG.2 bis FIG.6 explained.

[0095] Further referring to Fig.1 The shredding arrangement 38, comprising the milling drum 38.1 and the optional further shredding and / or guiding element 38.2, optionally also includes a conveyor screw 40, which is Fig. 2 shown and further explained.

[0096] The milling drum 38.1 of the comminution assembly 38 is arranged directly upstream of the conveyor screw 40 if an optional further comminution and / or guide element 38.2 is not provided. The conveyor screw 40 is in Fig.1 The milling drum 38.1 is arranged in the product flow direction immediately after the milling drum 38.1 or an optional further comminution and / or guide element 38.2. The milling drum 38.1 of the comminution arrangement 38 is thus arranged – in the case that an optional further comminution and / or guide element 38.2 is provided – immediately upstream of the further comminution and / or guide element 38.2 and indirectly upstream of the conveyor screw 40.

[0097] In this case, dried polymer particles (more precisely, poly(meth)acrylate particles), i.e., sufficiently sized for pneumatic conveying and comminuted polymer particles suitable for grinding, which are here designated together with reference numeral 39, are fed to the screw conveyor 40. The screw conveyor 40 preferably provides for homogenizing the main flow of a product.

[0098] The sufficiently comminuted and homogenized dried polymer particles of the main stream 41 are then fed to a pneumatic conveying system 42, and these sufficiently comminuted polymer particles in the homogenized product stream of the pneumatic conveying system are designated by reference numeral 43. These dried polymer particles 43 are then fed to a mill 44 for a grinding process and are ground there, and as ground, dried polymer particles 45 enter a screening device 46. After the screening device 46, the screened, ground, and dried polymer particles 47 with a desired particle size distribution of the product fraction leave the screening device, and these screened, ground, and dried polymer particles 47 are, if necessary,The material undergoes further treatment, such as surface post-crosslinking and subsequent drying or other thermal treatment, and is made available as a product after protective sieving. Any oversize fractions remaining after sieving 46 can be subjected to further grinding 44 in the mill until they exhibit the desired particle size distribution of the usable fraction.

[0099] The mean particle size of the superabsorbent polymer particles separated as ground polymer particles is preferably at least 200 µm, particularly preferably 250 to 600 µm, and most preferably 300 to 500 µm. The mean particle size of the polymer particles can be determined using the EDANA recommended test method No. WSP 220.2-05 "Particle Size Distribution", whereby the mass fractions of the sieve fractions are plotted cumulatively and the mean particle size is determined graphically. The mean particle size is the mesh size value obtained for a cumulative 50 wt%.

[0100] The proportion of polymer particles with a particle size greater than 150 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, and most preferably at least 98 wt.%.

[0101] Polymer particles with too small a particle size reduce permeability (corresponding to an SFC value as a measure of the permeability of liquid between polymer particles; measurement method e.g. EP 0 752 892 B1 pp. 33-36

[0224] -

[0251] ). Therefore, the proportion of very small polymer particles ("fines") should be low.

[0102] Polymer particles that are too small are therefore separated and recycled back into the manufacturing process. Recycling preferably takes place before, during, or immediately after polymerization, i.e., before the polymer gel is dried. The polymer particles that are too small can be moistened with water and / or aqueous surfactant before or during recycling.

[0103] It is also possible to separate excessively small polymer particles in later manufacturing process steps, for example after surface post-crosslinking or another coating step. In this case, the recycled excessively small polymer particles are surface post-crosslinked or otherwise coated, for example with pyrogenic silica.

[0104] If a kneading reactor is used for polymerization, the polymer particles that are too small are preferably added in the last third of the kneading reactor.

[0105] If the polymer particles, which are too small, are added very early, for example, already to the monomer solution, this reduces the centrifugal retention capacity (CRC) of the resulting water-absorbing polymer particles. This can be compensated for, for example, by adjusting the amount of crosslinker used (b). The polymer particles, which are too small, can also be added later, but then they might not be incorporated sufficiently.

[0106] Insufficiently incorporated, too small polymer particles detach from the dried polymer gel during milling, are therefore separated again during classification, and increase the amount of too small polymer particles to be returned.

[0107] The proportion of particles with a particle size of at most 850 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, and most preferably at least 98 wt.%. The proportion of particles with a particle size of at most 600 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, and most preferably at least 98 wt.%. Polymer particles with an excessively large particle size reduce the swelling rate. Therefore, the proportion of excessively large polymer particles should also be low. Excessively large polymer particles are therefore removed and recycled back into the milling process of the dried polymer gel.

[0108] According to the concept of the invention, the above-described Fig.1 the following based on Fig.2 Preferred comminution arrangements 38 are described which comminute the dried polymer strand of polymer gel, i.e., the dry cake 35 or fragments 37 of the dry cake 35, after drying or after a belt dryer 36, in an improved manner into dried polymer particles 39 – in any case with a mass medium particle diameters significantly above 0.5 mm. In particular, the dried polymer particles in the improved comminution process for the dried polymer gel of the dry cake 35 or fragments 37 of the dry cake 35 are to be comminuted to a particle size suitable or determined for subsequent pneumatic conveying and / or grinding.

[0109] This shows Fig.2 A comminution arrangement 38 with a first comminutor in the form of a milling drum 38.1, which is also referred to as a comminution milling unit ZF. The comminution arrangement 38 also includes a feed screw 40. The feed screw 40 is in Fig. 2 For simplicity, it is shown rotated by 90°. However, realistically, its axis is aligned parallel to the axis of the milling drum 38.1 (i.e., actually rotated by 90° compared to...). Fig. 2 The screw conveyor 40 is thus designed to transport the polymer particles 39 laterally, i.e., along the axis, and / or (depending on the design of the screw conveyor's spiral) to draw them together; that is, preferably to transport them towards the center of this axis. The screw conveyor 40 also advantageously serves to homogenize the already comminuted and dried polymer particles 39. Additionally, it can advantageously be designed for further comminution of the dried polymer particles 39. In this respect, the screw conveyor 40 can also serve as a further comminutor for already comminuted polymer particles and to generate a homogeneous stream 41 of them. These comminuted and sufficiently homogeneous dried polymer particles 41 then enter the pneumatic conveying system 42 as a product stream 43.

[0110] An advantage of this arrangement is that the milling drum 38.1 carries the dry cake 35, --that is, the unbroken dry cake 35 which enters the milling drum 38.1 directly, practically horizontally ( Fig.3A ) or the more or less horizontally directly into the milling drum 38.1 break 37 from the dry cake 35 ( Fig.3B )-- to crush into dried polymer particles 39 which have a predefined size distribution in order to be conveyed directly by the pneumatic conveying system 42.

[0111] The in Fig.2 The conveying screw 40 shown is therefore optional. It can also be used exclusively for transporting and homogenizing the polymer particles 39. Advantageously, however, it can also function as a second crusher for the polymer particles 39. The polymer particles 41 and also the homogenized polymer particles have a mean particle diameter between 0.5 mm and 10 mm, preferably between 1 mm and 9 mm, and particularly between 1 mm and 5 mm.

[0112] The entire comminution arrangement 38 preferably operates at a temperature above about 40°C up to a maximum of 140°C, preferably 60-120°C, particularly preferably 80-120°C - this ensures comparatively constant hardness levels of the dry cake 35 in the area of ​​the comminution arrangement 38 and prevents condensation of moisture and caking.

[0113] The relative arrangement of the milling roller 38.1 in relation to the dry cake 35 in the dryer assembly 36 ( Fig.3A ) or the fraction 37 of the same ( Fig.3B ) will now be discussed in more detail in Fig.3A or Fig.3B shown, with the conveyor belt 36F of the assembly 36 depicted with the dry cake 35. For example, a milling roller 38.1 is shown in Fig. 4 shown.

[0114] The dry cake 35 can be guided unbroken over a support, which is shown here in a variant as support table 36T, and processed by the milling drum according to Fig.3A However, it can also be done according to Fig.3B At the end of conveyor belt 36F, the dry cake 35 breaks. The resulting fragment 37 of dried polymer particles from the dry cake 35 then slides or is pushed over a support, shown here in one variant as a support table 36T, towards the milling drum 38.1, which is the primary crusher. It has been found to be advantageous to position the milling drum 38.1 directly at the dry cake for milling off dried polymer particles, or to guide the dry cake, or possibly its fragment 37, directly onto the milling drum 38.1.

[0115] The milling drum 38.1, designed as a comminution milling cutter ZF, has a rotatable shaft ZF1 with a number of functional elements ZF2, which are designed for milling dried polymer particles directly from the dry cake 35 or from the fragment or lump 37 thereof. The working edge 38.1R of the milling drum 38.1 is to be understood as the outer edge of the milling drum that operates on the dry cake; in particular, the circumferential circle whose radius is defined by the end of functional elements of the milling drum.

[0116] Further referring to Fig.3A und Fig.3B In the embodiment shown here, the milling roller 38.1 is arranged at a distance D of 0.5 to 5 cm from the end of the support.

[0117] A support for the dry cake 35 or the fragment 37 thereof is arranged between the conveyor belt 36F and the milling roller 38.1 in extension of the receiving surface F and is oriented practically parallel to the conveyor belt 36F or to the transport direction C or at an angle δ to the horizontal or to the transport direction C, i.e. preferably downwards but alternatively possibly also upwards.

[0118] In the embodiment of the Fig.3A The angle δ is nearly or in the range of 0°; i.e. the support in the form of a support table 36T is practically parallel to the conveyor belt 36F or to the transport direction C.

[0119] In the embodiment of the Fig.3B The angle δ to the horizontal or to the downward transport direction C is slightly greater than 0°, meaning the support table 36T is slightly inclined, specifically with an angle δ of 1° to 5° downwards. In a modified embodiment, the angle δ can be up to 10° or possibly up to 15°.

[0120] In a modified embodiment, not shown here, an angle δ' to the horizontal or to the transport direction C can alternatively also be directed upwards. For example, the support table 36T can be practically parallel to the conveyor belt 36F or, alternatively, slightly inclined upwards, so that the dry cake 35 or fragments 37 of the dry cake are tilted slightly upwards due to the upward inclination of the support table 36T. Such an upward angle δ' relative to the horizontal or to the transport direction C can, for example, be an angle δ' of 1° to 5°, or possibly up to 10°, or possibly up to 15°. In principle, angles δ, δ' downwards or upwards are theoretically conceivable up to 30°.

[0121] To achieve the most uniform and sufficient comminution of the dry cake 35 or the fraction 37 into dried polymer particles, the relative height of the milling drum 38.1 is adjusted with respect to the conveyor belt 36F. That is, the upper working line H of the milling drum 38.1 is located at least at or slightly above the level of the receiving surface of the conveyor belt 36F for milling the dry cake 35 or the fraction 37, as shown in Fig.3A und Fig.3B This is evident. The milling drum is thus already positioned downwards for the discharge of milled, dried polymer particles 39 from the dry cake. Furthermore, the milling drum 38.1 has a rotatable shaft ZF1 with functional elements ZF2, and the conveyor belt 36F has a deflection roller 36U, wherein the rotatable shaft ZF1 is designed to rotate with a direction of rotation R2 opposite the direction of rotation R1 of the deflection roller 36U. This also facilitates the downward discharge of milled, dried polymer particles 39 from the dry cake.

[0122] The relative orientation of milling drum 38.1 to the dry cake 35 and the break 37 is indicated by the working line H as described above. In this case, the working line H of the working edge 38.1R of the milling drum 38.1 is positioned at or above the receiving surface F of the conveyor belt 36F. This is evident from the leveling of the working line H relative to the receiving surface F of the conveyor belt 36F. Fig.3A und 3B In the case of an unbroken dry cake 35, the upper working line H of the milling roller 38.1 is particularly preferably located above an upper edge 35C of the dry cake or relative to the upper edge 35C of the dry cake 35 or the upper edge 37C of the break 37.

[0123] On the other hand, in accordance with the concept of the invention, it is ensured that the milling drum 38.1 has a sufficiently large diameter. This is achieved by ensuring that the axis 38.1A of the milling drum 38.1 is located at or below the height of the receiving surface F of the conveyor belt 36F. Specifically, it is ensured that the radius r of the milling drum 38.1 is greater than or equal to the thickness 35D, 37D of the dry cake 35 or the fraction 37) and that a lower working line of the milling drum 38.1 lies below a receiving surface F of a conveyor belt 36F.

[0124] Within the framework of this relative alignment of the milling drum 38.1 to the dry cake 35 or fraction 37 and with the selected parameters of the aforementioned rotational speed and temperature conditions at the shredder 38, a surprisingly uniform and well-shredded discharge of shredded polymer particles 39 is already achieved. This is further explained with regard to Fig.7A and Fig.7B received.

[0125] Fig.4 Figure 1 shows a particularly preferred embodiment of such a milling drum 38.1 in the form of a comminution milling unit ZF with the aforementioned functional elements ZF2 in the form of impact bars, which form a working edge 38.1R for milling the fraction 37 or dry cake 35. The working edge 38.1R is recognizably designed following the course of a spiral; here, a spiral with a thread-like profile, wherein the thread forms a pitch angle between 40° and 50°.

[0126] To achieve the predetermined particle size, the milling drum rotates at a speed of more than 50 revolutions per minute and less than 250 revolutions per minute, preferably in the range of 100 revolutions per minute, depending on the actual diameter of the working edge 38.1R of the milling drum. In general, it has proven advantageous for the milling drum to rotate at a speed that tends to be higher the smaller the diameter of the working edge 38.1R of the milling drum. This is particularly relevant at an ambient temperature of 40°–140°C, preferably 60–120°C, and most preferably 80–120°C in the area of ​​the comminution arrangement 38.

[0127] Fig.5A und 5B each shows in a side view "1" and in a top view "2" the relative arrangement of a relative to Fig.3A und Fig.3B The support is explained by way of example; namely, a support table 36T and a support grid 36G for the supporting guidance of the dry cake 35 or – in the event that it should break off – of the break 37 of the dry cake 35 directly to the milling roller 38.1. For this purpose, in Fig.5A und Fig.5B Each milling drum 38.1 with corresponding functional elements ZF2 on a shaft ZF1 and with the working edge 38.1 R formed thereby is shown.

[0128] According to the embodiment of the Fig.5A The support is designed as a support table 36T with a closed support surface, wherein a distance D is maintained between an edge of the support table 36T and a working edge 38.1R of the milling drum 38.1. The distance D can be in the range between 0.5 cm and 20 cm, preferably less than 10 cm, and particularly preferably less than 2 cm.

[0129] According to the embodiment of the Fig.5B If such a distance does not exist between the support and the milling drum 38.1, the support is at least partially formed in the form of a support grid 36G with laterally spaced rods or bars or similar support elements 36B, which engage in the spaces between functional elements ZF2 of the milling drum 38.1.

[0130] In both embodiments, either the distance D is kept as small as possible ( Fig.5A ) or avoided ( Fig.5B ), so that any smaller fragments of the dry cake 35 or the fracture 37 that may be present are prevented from passing through (bypassing) between the support (for example, a support table 36T or a support grid 36G) and the milling drum 38.1. The support, whether a support table 36T or a support grid 36G, also has the advantage that the impact pressure of the functional elements ZF2 of the milling drum 38.1 is absorbed by the support.

[0131] Fig.6 schematically shows possible basic contours of a work edge 38.1R (in the sense of an enveloping boundary line for the work edge); for example, in view A for a in Fig.4 or Fig.5A, Fig.5B Example of shaft ZF1 of the milling drum 38.1.

[0132] The working edge 38.1R follows a spiral-shaped basic course in view A, which rotates clockwise from left to right. View B of the Fig.6 Figure 38.1 R shows a similar spiral path of the working edge, with the spiral rotating counterclockwise from left to right in this case. Depending on the direction of rotation of the milling drum 38.1 R, this also results in a corresponding lateral conveyance of a product stream of milled polymer particles. View C of the Fig.6 Figure 1 shows another embodiment in which, depending on the direction of rotation of the milling drum 38.1R, a product flow of milled polymer particles is conveyed towards the center, since the spiral paths for a working edge 38.1R are arranged oppositely to the center M of the milling drum 38.1R on a left or right side. Other embodiments are conceivable, such as interlocking, parallel spiral working lines, etc.

[0133] Fig.7A Figure 1 shows a cumulative plot of the mass fractions of the sieve fractions for the graphical determination of the mean particle diameter after comminution. The result of the measurement is plotted in Table 1 below with the sieve sizes; the values ​​are in Fig.7A The mean particle diameter in product stream 41 after comminution is approximately 3.0 mm, which can be determined from the plotting and assignment of a mean particle diameter for particles at a 50 wt.% value in the Fig.7A The mean particle diameter after comminution is determined according to EDANA test method no. WSP 220.2-05 "Particle Size Distribution". For determining the particle size distribution on product stream 41 after comminution, sieves with mesh sizes of 0.6 / 1 / 2 / 3.15 / 4 / 5 / 6.3 / 8 / 10 / 14 / 20 mm are used (see fraction). The mean particle diameter is the value of the "mesh size" that is determined for the Fig.7A The indicated cumulative 50 wt.% results. Tab.1 Fraktionsgröße SAP Gem.Gew(%) Fraktion (mm) SAP Kum.Gew.(%) 0-600µm 3,62 0,6 3,6 5 600µm-1mm 6,55 1 10,2 1mm-2mm 21,38 2 31,6 2mm-3,15mm 21,52 3,15 53,1 3,15mm-4mm 7,91 4 61,0 4mm-5mm 8,40 5 69,4 5mm-6,3mm 6,49 6,3 75,9 6,3mm-8mm 7,10 8 83,0 8mm-10mm 4,95 10 87,9 10mm-14mm 5,39 14 93,3 14mm-20mm 1,92 20 95,2 >20mm 4,75 100,0 15

[0134] The mass particle diameter after grinding and sieving is determined on the product stream 47 analogously to the mass particle diameter after comminution and is in Fig.7B shown. For determining the particle size distribution of a distribution, sieves with mesh sizes of 45 / 150 / 212 / 300 / 425 / 500 / 600 / 710 / 850 µm are used (su fraction). A particle size distribution determination was carried out after milling and sieving of product stream 47 on products that were sieved between 150 and 850 µm, between 100 and 700 µm, and between 100 and 600 µm.

[0135] The respective measurement results are plotted in the following tables with the sieve sizes; the values ​​are in Fig.7B The average particle diameter after grinding and sieving is 570 µm for product sieving at 150 and 850 µm (Table 2A), 425 µm for product sieving at 100 and 700 µm (Table 2B), and 348 µm for product sieving at 100 and 600 µm (Table 2C). Tab.2A Produktabsiebung 150 und 850 µm: Fraktionsgröße SAP (%) grob Fraktion grobKum Gew.% 0-45µm 0,1 45µm 0,1 45µm-150µm 1,6 150µm 1,7 150µm-212µm 4,0 212µm 5,7 212µm-300µm 7,6 300µm 13,3 300µm-425µm 14,1 425µm 27,4 425µm-500µm 11,0 500µm 38,4 500µm-600µm 16,9 600µm 55,3 600µm-710µm 26,9 710µm 82,2 710µm-850µm 17,7 850µm 99,9 >850µm 0,1 100,0 Tab.2B Produktabsiebung 100 und 700 µm: Fraktionsgröße SAP (%) Fraktion mittelKum Gew.% 0-45µm 0,0 45µm 0,0 45µm-150µm 5,5 150µm 5,5 150µm-212µm 7,2 212µm 12,7 212µm-300µm 12,6 300µm 25,3 300µm-425µm 25,1 425µm 50,4 425µm-500µm 18,8 500µm 69,2 500µm-600µm 20,0 600µm 89,2 600µm-710µm 9,6 710µm 98,8 710µm-850µm 1,2 850µm 100,0 >850µm 0,0 100,0 Table 2C Product screening 100 and 600 µm: Fraction size SAP (%) Faction fine powder wt.% 0-45µm 0,0 45µm 0,0 45µm-150µm 2,8 150µm 2,8 150µm-212µm 11,4 212µm 14,2 212µm-300µm 21,5 300µm 35,7 300µm-425µm 38,0 425µm 73,7 425µm-500µm 16,2 500µm 89,9 500µm-600µm 8,6 600µm 98,5 600µm-710µm 1,4 710µm 99,9 710µm-850µm 0,1 850µm 100,0 >850µm 0,0 100,0 Reference symbol:

[0136] 31 Reactants 30 Reactor 33 Lumps 32 Yellow Bunker 34 Conveyor 35 Dry Cake 35C Top Edge 35D Dry Cake Thickness 35 36 Belt Dryer 36A Dryer Structure 36F Conveyor Belt 36U Deflection Roller 37 Dry Cake Break 35 37C Top Edge 37D Break Thickness 37F Receiving Surface 36T Support Table 36G Support Grid 36B Laterally spaced bars or ingots or the like Support elements of a support grid 36G 38 Crushing Assembly 38.1 Milling Roller 38.1R Working Edge 38.1A Axis 38.2 Further Crushing and / or Guide Element of the Crushing Assembly Working Line Working Line R1 Direction of Rotation of a Deflection Roller 36U R2 Direction of Rotation of a Rotating Shaft ZF1 R Radius of milling drum 38.1 D Distance of milling drum 38.1 from the end of the support 40 screw conveyor 41 polymer particles in the main flow 42 pneumatic conveying 43 product flow in pneumatic conveying 39, 45, 47 crushed (39) and ground (45) and sieved (47) polymer particles 44 grinding 46 sieving device ZF crushing mill ZF1 rotatable shaft of the crushing mill ZF2 functional elements of the crushing mill.

Claims

1. A belt dryer arrangement for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles, comprising: - a belt dryer (36) with a dryer structure (36A) and a conveyor belt (36F) for receiving and drying the aqueous polymer gel to form a drying cake (35) or brittle product (37) on a receiving surface (F) of the conveyor belt (36F), - a comminution arrangement (38) arranged downstream of the dryer structure in the product flow direction for comminuting the dried polymer gel of the drying cake (35) or of the brittle product (37) into dried comminuted polymer particles (39), wherein - the comminution arrangement (38) comprises a comminutor formed as a milling roller (38.1) having an upper working line (H) of a working edge (38.1R) above the receiving surface (F) of the conveyor belt (36F), the milling roller (38.1) being configured to discharge the comminuted dried polymer particles (39) of the drying cake (35) or the brittle product (37) downward, - the milling roller (38.1) has a diameter such that an axis (38.1A) of the milling roller (38.1) is arranged at the height of or below the height of the receiving surface (F) of the conveyor belt (36F), and a radius (r) of the milling roller (38.1) is greater than or equal to the thickness (35D, 37D) of the drying cake (35) or the brittle product (37), - a lower working line (h) of the milling roller (38.1) lies below the receiving surface (F) of the conveyor belt (36F), - the upper working line (H) of the milling roller (38.1) has an upper spacing from the receiving surface (F) of the conveyor belt (36F), and - the milling roller (38.1) comprises a rotatable shaft (ZF1) with functional elements (ZF2), and the conveyor belt (36F) comprises a deflection roller (36U), the rotatable shaft (ZF1) being configured to rotate with a direction of rotation (R2) opposite to the direction of rotation (R1) of the deflection roller (36U).

2. Belt dryer arrangement according to claim 1, characterized in that the upper spacing above the height of the receiving surface (F) of the conveyor belt is at least 5 cm.

3. Belt dryer arrangement according to claim 1 or 2, characterized in that a support for the drying cake (35) or the brittle product (37) is arranged between the conveyor belt (36F) and the milling roller (38.1), in an extension of the conveyor belt.

4. Belt dryer arrangement according to claim 3, characterized in that the support is formed as a support table (36T) having a closed support surface, wherein a rear spacing (D) is left between a rear edge of the support table (36T) and a front working edge (38.1R) of the milling roller (38.1).

5. Belt dryer arrangement according to claim 3, characterized in that - the support is formed as a support grid (36G) having laterally spaced support elements (36B), in particular rods, bars, shafts or the like, or - a front part of the support is formed as a support table with a closed support surface, and a rear part of the support is formed as a support grid having laterally spaced support elements, in particular rods, bars, shafts or the like.

6. Belt dryer arrangement according to claim 5, characterized in that functional elements (ZF2) of the milling roller (38.1) can engage into the gaps between spaced support elements (36B) of the support grid (36G).

7. Belt dryer arrangement according to any one of claims 1 to 6, characterized in that the comminution arrangement, in addition to the milling roller (38.1) and an optional screw conveyor (40), comprises a further comminuting and / or guiding element (38.2), in particular a further comminutor and / or a deflection means.

8. Belt dryer arrangement according to claim 7, characterized in that a number of the functional elements (ZF2) of the milling roller (38.1) is formed as a number of rods, teeth, bars, shovel surfaces or tooth-, serration-, or knurl-ridges or - edges, in particular spiral teeth or knurled screws.

9. Belt dryer arrangement according to claim 8, characterized in that the functional elements (ZF2) are arranged along a basic course of a spiral, the spiral in particular having a thread with a thread angle between 20° and 70°.

10. Belt dryer arrangement according to claim 9, characterized in that the spiral has a single thread or several co-directional or counter-directional, in particular separate or intermeshing, threads.

11. A method for drying an aqueous polymer gel and for comminuting the dried polymer gel into dried polymer particles with a belt dryer arrangement according to any one of claims 1 to 10, comprising: - a belt dryer (36) with a dryer structure (36A) and a conveyor belt (36F) for receiving and drying the aqueous polymer gel to form a drying cake (35) or brittle product (37) on a receiving surface (F) of the conveyor belt (36F), - a comminution arrangement (38) arranged downstream of the dryer structure in the product flow direction for comminuting the dried polymer gel of the drying cake (35) or brittle product (37) into dried comminuted polymer particles (39), wherein - the comminution arrangement (38) comprises a comminutor formed as a milling roller (38.1), and in the method: - the dried polymer gel of the drying cake (35) or brittle product (37) is comminuted into dried comminuted polymer particles (39), - with an upper working line (H) of a working edge (38.1R) above the receiving surface (F) of the conveyor belt (36F), the drying cake (35) or brittle product (37) is milled, and the milling roller (38.1) discharges the dried comminuted polymer particles (39) downward, - the milling roller (38.1) has a diameter such that an axis (38.1A) of the milling roller (38.1) is arranged at or below the height of the receiving surface (F) of the conveyor belt (36F), and a radius (r) of the milling roller (38.1) is greater than or equal to the thickness (35D, 37D) of the drying cake (35) or brittle product (37), - a lower working line (h) of the milling roller (38.1) lies below the receiving surface (F) of the conveyor belt (36F), - the upper working line (H) of the milling roller (38.1) has an upper spacing from the receiving surface (F) of the conveyor belt (36F), and - the milling roller (38.1) comprises a rotatable shaft (ZF1) with functional elements (ZF2), and the conveyor belt (36F) comprises a deflection roller (36U), the rotatable shaft (ZF1) being configured to rotate with a direction of rotation (R2) opposite to the direction of rotation (R1) of the deflection roller (36U).

12. A manufacturing process for producing water-absorbing polymer particles, comprising the steps of: - polymerizing a monomer solution or suspension to water-absorbing polymer particles of an aqueous polymer gel, the solution comprising a) at least one ethylenically unsaturated, acid-group-containing monomer which may be at least partially neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomers copolymerizable with those under a), and optionally one or more water-soluble polymers, - conveying the aqueous polymer gel to a belt dryer arrangement according to any one of claims 1 to 10, - drying the aqueous polymer gel and comminuting the dried polymer gel into dried polymer particles with the belt dryer arrangement, the belt dryer (36) and comminution arrangement (38) being as defined in claim 11.

13. Method according to claim 11 or 12, characterized in that the milling roller (38.1) of the comminution arrangement (38) comminutes the drying cake (35) or brittle product (37) into dried comminuted polymer particles having a mass-average particle diameter between 0.5 mm and 10 mm, preferably between 1 mm and 9 mm, and in particular between 1 mm and 5 mm.