Field of the invention

The present invention relates to a process to modify particulate by removing undesired fines from the bulk of the material as well as to an equipment for executing such handling. Such equipment and process can also be applied to treatment of superabsorbent polymer materials, both during production thereof or when recycling such materials,

Background

In the handling of particulate it is often desired to do so in the absence of fine particles, e.g. so as to maintain a safe working environment by avoiding dust explosions or to avoid other detrimental effects of the fines in filtering or other material performance related aspects or to avoid dust inhalation by operators.

In regards to the work environment, the amount of dust which is produced in the working environment is related in some respects with the manner in which the fine particles are received by the intended users. Some recipients of fine particles rely on silos or storage rooms which are capable of handling large volumes of material. With such storage facilities, bulk delivery is typically the most economical and often features road or rail- bom silo vehicles with pneumatic pumps that can pump the material from the silo vehicle directly to the storage facility and/or to other transport units, like bags, e.g. bulk storage bags, often referred to as “big bags”. In course of any such handling, fines may be created such as by abrasion in the transport systems.

In JP8504735, a system is described that aims at removing fines from particulate bulk material by fluidizing the powder by air and removing fines with the air.

Another approach for dealing with fine particles is disclosed in KR603264B, where fine particles are removed from a surface by a dual slit nozzle expelling pressurized air through a first slit to loosen particles from surface and sucking air with loosened particles through second slit, applicable e.g. in clean room environment.

In DE2846499C2, fine particles are removed from a stream of particles containing coarser granulate as well as fine dust particles moving generally in a common direction are directed in a specific way to fall into a moving air stream which is moving in another direction compared to the direction of the particles stream. Thus, the fine particles are removed from the coarser particles. Whilst the need to deal with fine particles, and in particular to remove such fines from the bulk of the other materials, is important in many applications fields, it is also highly important in the hygiene industry, including but not limited to absorbent articles such as diapers for babies or adults, training pants, pull-up diapers, diaper pants, sanitary napkins, panty liners, etc... .

In modem products, the liquid absorption is typically achieved by using so called and well known “superabsorbent polymers” (SAP), often, though not necessarily, in particulate form. Such SAP particles maybe exhibit various shapes, such as spherical, but typically exhibit an irregular shape. The SAP particles often exhibit a particle size of between about 50 pm and 2 mm. If these SAP particles are too small, they may create undesirable dust, which even may be unhealthy. However, also within these broad ranges specific particle size distributions (PSD) may be desirable for absorbency performance reasons. First, the particle size impacts on the speed of absorption. Second, fine particles may penetrate through a porous top-sheet and thereby create a slimy feel or “gel-on-skin”. Further, in particular in absorbent cores with high amounts of or even pure SAP powder, fine particles may hinder liquid distribution by blocking liquid transport capillaries.

Fine SAP particles are created in course of the manufacturing process wherein SAP particles are created from a polymerized gel by grinding, typically followed by surface cross-linking and optionally a further surface treatment by adding small particles, such as silica, to the surface for liquid handling and particle flow enhancement. SAP powders are handled in large bulk amounts, often through chutes and/or pneumatic transport, all of which may cause abrasion and creation of fines - within the present context referring to particles of less than about 150 pm, when determined in a sieve analysis (e.g. by ISO 17190-3:2001 or EDANA WSP 220.2). Typical current manufacturing processes employ various sieving steps throughout the manufacturing process to control the PSD and minimize fines. The smaller the particles to be separated out are, the less effective the sieving becomes and more effort is required, such as by mechanically supporting the sieving by movement of the equipment, by applying sieving aids such as tapping balls or air brushes.

In US20090321682A1 a sorting step is disclosed, wherein by means of an image analysis and air gun system defective SAP particles, in particular particles with a discolourization, out of single layer particle curtain falling from chute, on which the optical detection takes place. Thus there is still a need to improve the removal of fines in such a manufacturing process. Further, in the production of absorbent articles certain off specification products are taken out as scrap, and effort has been spent against recycling these and reclaiming as much of the materials of these scrap products, see e.g. WO2014132128A2 (Diaper Recycling Technology), creating among others a stream of SAP powder, preferably a high purity, of at least 99 w-%, more preferably more than about 99.9 w-%, even more preferably more than about 99.99 w-%. Such purities allow re-use of the SAP powder in the article manufacturing. However, even if the feedstock is essentially free of fines, which is often not the case, a certain amount of fines maybe created during the article manufacturing, and during the reclaiming process even if executed very gently. Thus there is a need to remove small amounts of fines from the reclaimed SAP powder before recycling into the manufacturing process.

Summary

The present invention is an apparatus for removing fine particles from a particle stream, comprising

- a drum mounted rotatably around a longitudinal, x-directional or length directional axis, comprising

- an inner surface and an inner diameter, and a length;

- a collection zone in the lower portion, along gravity, inside the drum;

- a departure zone in the upper portion, along gravity, inside the drum;

- and surface irregularities protruding from the inner surface, preferably baffles mounted on the inner surface;

- a particle stream inlet and a particle stream outlet, defining the process r x- direction of the apparatus.

The apparatus further comprises

- a drive adapted to allow rotation of the drum;

- a plurality of static suction plates, preferably at least 3, more preferably at least 5 and most preferably at least 7, positioned inside the drum and along the longitudinal axis, inclined at an angle to the axis, preferably equidistantly spread along the length of the drum, and such that the lowermost part of the suction plates being positioned towards the collection zone of the drum is positioned x-directionally further away from the particle stream inlet than the uppermost part of the neighbouring suction plate along the process direction, whereby each of the suction plates comprises an interior space by being hollow or comprising at least one cavity, wherein the lowermost part of the suction plate positioned towards the collection zone is executed as a suction slit in the suction plate, adapted to allow air and entrained particles to pass into the interior space;

- a static air duct, positioned inside the drum and parallel to the longitudinal axis; whereby an inner space of the air duct is connected to an air suction device, and whereby each of the suction plates is mounted to the air duct such that the interior space of aid suction plates is connected to the inner space of the air duct.

The apparatus for removing fine particles from a particle stream may further comprise one or more features selected from the group consisting of: the longitudinal axis is inclined versus the horizontal by less than 45° , or less than 30° or less than 10° or less than 5°; the suction plates are positioned at an angle of between 20° and 70° versus the longitudinal axis; the suction plates exhibit optionally a generally sectional elliptic shape.

In another aspect, the present invention is a process for removing fine particles from a particle stream, comprising the steps of

- providing an apparatus as described in the above;

- feeding a particle stream through the drum inlet into the collection zone of the drum;

- rotating the drum at a rotational speed;

- transferring the particles of the particle stream from the collection zone to the departure zone by entraining the particles by the inner surface of the drum, further supported by the centrifugal force and the surface irregularities of the drum;

- removing the particles from the inner surface of the drum in the departure zone, supported by gravity exceeding the centrifugal force;

- depositing the particles on the surface of the inclined suction plates;

- creating a curtain of falling particles at the lower end of suction plate and over the suction slit;

- creating an air flow through the suction slit through the suction plate space to the air duct by the air suction device;

- adapting the air flow through the suction slit such that fines, exhibiting a size of less than the cut-off point of the suction plate, are sucked through the suction slit and through the space to the air duct for further removal, whilst other particles of a size of more than the cut-off point of the suction plate are not sucked into the suction slit and accumulating in the collection zone, whereby the particle stream moves generally x-directionally along the apparatus by being moved from one suction plate to the next, thereby being depleted of the fines, and exiting the drum at the particle stream outlet.

The process for removing fine particles may further comprise one or more steps selected form the group consisting of: rotating the drum at a rotational speed thereby exhibiting a centrifugal force of less than gravity as determined on the inner surface of the drum.

Brief description of the Figures

Fig. 1 A to F depict schematically an equipment according to the present invention.

Same numerals refer to same or equivalent features, apostrophe(s) like xx’, xx”, xx’” denote multiples of a feature or element. All figures are schematic and not to scale.

Detailed description

The present invention is an apparatus and its operation for removing fine particles from a particle or powder mass. The particles may exhibit various shapes and often exhibit an irregular shape, such as may result from break up or particle size reduction processes such as grinding. The particles may comprise secondary smaller particles attached to the surface of the particles, that may detach therefrom and contribute to the fraction of fines.

The present invention is applicable to a broad range of PSD, such as having maximum particle sizes of about 5 mm, or of about 2 mm, or of about 1 mm, or of about 0.5 mm or even for certain applications even of about 0.1 mm. The powder may comprise extremely fine particles, even at nano-size or 0.001pm, or very fine particles of less than about 45 pm, or fine particles of less than about 150 pm. The amount of particles of less than about 150pm may be in the range of less than about 10 w-%, often less than about 5 w-%, or even less than about 3 w-%, wherein all percentages as referred to herein are based on the weight (w-%), unless expressly stated.

The present invention is now explained further by referring to Fig. 1A to F, however these explanations or figures should not be seen limiting the present invention.

Thus, in a first aspect, the present invention is an apparatus 900 for removing fine particles from a particle stream. The apparatus comprises a drum 910 rotatably mounted around a longitudinal axis 915. The axis may be inclined versus the horizontal by less than 45°, or less than 30°, or less than 10°, or less than 5°, so as to have a downward slope from inlet 911 to outlet 919, or may even be essentially horizontal, or even may exhibit a slight upward slope. Drum 910 further comprises an inner surface 912, an inner diameter 914, and a length 916. Further, an collection zone 922 is positioned in the lower portion inside the drum, and a drop-off or departure zone 928 in the upper portion inside the drum, wherein all terms in the present description relating to “up” or “down(ward)” refer to the direction of gravity 901.

The drum comprises surface irregularities 930 protruding from the inner surface 912, here shown exemplarily as baffles mounted on the inner surface. The drum further comprises a particle stream inlet 911 and a particle stream outlet 919, thereby defining the process or x- direction of the apparatus from inlet to outlet. The apparatus comprises a drive adapted to allow rotation of the drum (not shown) around the longitudinal axis 915.

To facilitate multiple particle removal steps, the apparatus further comprises a plurality of static suction plates 940, 940’, 940’” ... , , preferably at least 3, more preferably at least 5 and most preferably at least 7. The suction plates are positioned inside the drum and along the longitudinal axis, inclined at an angle to the axis, preferably between 20° and 70°. Preferably, the suction plates are equidistantly spread along the length of the drum, such that the lowermost part 942 of a suction plate 940’, which is positioned towards the collection zone 922 of the drum, is positioned x-directionally further away from the particle stream inlet 911 than the uppermost part 948 of the neighbouring suction plate 940”, along the process direction. Depending on the rotational speed as well as inclination of the drum impacting on the x-directional component of the particle movement, this overlap may be reduced, and even a certain x-directional spacing apart may then be acceptable.

The shape of the suction plates is not critical, and may be a polygon or curvilinear. In the exemplary execution as depicted in Fig. 1, each of the suction plates 940 exhibits the optional generally sectional elliptic shape, and comprises an interior space 945 by being hollow or comprising at least one cavity 946. The lowermost part of the suction plate positioned towards the collection zone is executed as a suction slit 943 in the suction plate, adapted to allow air and entrained particles to pass into the interior space.

Further, the apparatus comprises a static air duct, positioned inside the drum 910 and preferably parallel to the longitudinal axis 915. The interior space of the air duct is connected to an air suction device (not shown).

The suction plates 940 mounted to the air duct 915 such that the interior space of air suction plates 940 is connected to the inner space of the air duct.

The drum 910 may exhibit a length 916 of more than about 100 mm, or more than about 1000 mm to less than about 3000 mm, or less than about 2000 mm, and an inner diameter 914 of more than about 50 mm or more than about 100 mm, or more than about 400 mm, and less than about 1500 mm, or less than about 1000 mm, or less than about 800 mm. The protruding irregularities 930 may exhibit a height (directing into the drum) of more than about 0.05 mm, or more than about 0. 1 mm, or more than about 1 mm, or more than about 2 mm, and less than about 500 mm, or less than about 200 mm, or less than about 100 mm, or less than about 50 mm, or even less than about 10 mm, or even less than about 6 mm, and a circumferential width (e.g. when executed as bars) of more than about 0.1 mm, or more than about 1 mm, or more than about 4 mm, and less than about 200 mm, or less than about 100 mm, or less than about 30 mm. Preferably, the irregularities are evenly spaced circumferentially on the inner surface of the drum and may be unitary with the drum or connected thereto, e.g. by screws or welding (not shown). Preferably they extend over the full length of the drum, and are preferably, though not necessarily continuous along their length.

Preferably, for cleaning and maintenance reasons, all parts inside the drum are treated to minimize adherence of particles thereto, e.g. by being polished or more preferably by being coated with anti-sticking material, e.g. PTFE.

The dimensions of the slits of the suction plates are adapted to the size of the particles that should be removed through the slit, and can exhibit a width along the cross-machine direction 908 of the drum of from about 0.5 mm, or more than about 1 mm, or more than about 5 mm or more than about 10 mm, and less than about 200 mm, or less than about 100 mm, or less than about 50 mm. The slits may further exhibit a height projected to the height or z-direction of the drum of 0.05 mm, or more than about 0. 1 mm or more than about 0.5 mm, or more than about 1 mm, and less than about 20 mm, or less than about 10 mm, or less than about 5 mm.

The interior space 942 of the suction plates may be executed merely by a hollow space, e.g. when the suction plates are connected sheets, e.g. by welding. Preferably, the interior space is formed by channels 946, which allows an optimized air flow and avoids deposition of sucked fine particles in the interior space.

The selection of the materials for the suction plates is not critical, preferably being compatible with the treatment for minimizing the adherence of particles thereto. Suitably, the plates may be manufactured of thermoformable material allowing to use 3D printing technology.

In another aspect, the present invention is a process for removing fine particles from a particle stream, by employing an apparatus as described in the above.

The process comprises the following steps:

- Feeding a particle stream through the drum inlet 911 into the collection zone 922 of the drum 910.

- Rotating the drum 910 at a pre-set rotational speed, e.g. looking along the longitudinal axis against the processing direction, the drum would rotate counter-clockwise in Fig 1A. The rotational speed is set such that the centrifugal force exerted on the particles is less than the gravitational force, preferably more than 70%, or 80%, but less than 100%, most preferably around 90% of the gravitational force. The skilled person will readily realize that the setting of the rotational speed will also depend on the diameter of the drum.

- Transferring the particles of the particle stream from the collection zone 922 to the departure zone 928 by entraining the particles by the inner surface 912 of the drum 910, further supported by the centrifugal force and the surface irregularities 930 of the drum.

- Removing the particles from the inner surface 912 of the drum in the departure zone 928, supported by gravity exceeding the centrifugal force.

- Depositing the particles 962, 968 on the upwardly facing surface 941 of the inclined suction plates 940 (see Fig. IE), whereon they are moving downwardly by gravity.

- Creating a curtain of falling particles 962, 968 at the lower end of suction plate 940 over the suction slit 943.

- Creating an air flow through the suction slit 943 through the suction plate interior space 943 to the air duct 950 by the air suction device.

- Adapting the air flow through the suction slit 943 such that (as indicated by respective arrows in Fig. IE) fine particles 962, exhibiting a size of less than the cut-off point of the suction plate, are sucked through the suction slit 943 and through the interior space 943 to the air duct for further removal, whilst other particles 968 of a size of more than the cut-off point of the suction plate are not sucked into the suction slit and accumulating in the collection zone.

Through this operation, the particle stream moves generally x-directionally along the apparatus by being moved from one suction plate 940’ to the next 940”, etc. thereby being depleted of the fines 962, and exiting the drum at the particle stream outlet 919.

The present invention removes the fines from a stream of particles, that typically is more than about 10 kg/hr, often more than about 50 kg /hr, or more than about 100 kg/hr, or - in particular when employed in powder manufacturing units - more than about 300 kg/hr.

Preferably, the present invention has a cut-off point of separating out fines of less than about 150 pm. However, the skilled person will readily realize that the present invention can be applied to other cut-off points, such as having a cut-off point of 100 pm, or 50pm, but also 300pm or even 500pm.

Preferably, the present invention allows to eliminate at least 99w-%, preferably more than about 99.9w-%, more preferably more than about 99.99w-% and most preferably essentially all of the particles smaller than the size of the cut-off point.

When considering the present invention in the context of hygiene products, it may be applied in relation to post-industrial recycling, i.e. when such absorbent articles are manufactured on “converter”, where the various materials are brought together and combined to form the product that may then be further confectioned and packed. Such converters can run at very high production speed, and a linear speed of a continuous web or composite may exceed 500 m/minute, resulting in more than 1000 articles per minute. Products not meeting the specification are sorted out, often at a scrap level of more than about 1%, or even more than 2%. The present invention may then be applied in the context of an overall scrap recycling unit, as may be connected to a manufacturing line, or to a plurality of manufacturing lines directly or via an interim storage system.

In relation to post-consumer recycling, such used absorbent articles are collected and sent to a recycling process where they are sterilized and separated back into the raw material components, SAP, pulp and plastic.

Maximizing re-use of materials both from an environmental and financial point is preferred and improving SAP quality is therefore of importance with post-consumer recycling as well as post-industrial recycling.