WO2004049885A2

WO2004049885A2 - Fine particle separation from pelletized-granular media

Info

Publication number: WO2004049885A2
Application number: PCT/US2003/038526
Authority: WO
Inventors: Charles Gordon Noll
Original assignee: Avogadro Industrial Products Corporation
Priority date: 2002-12-04
Filing date: 2003-12-03
Publication date: 2004-06-17
Also published as: WO2004049885A3; AU2003298870A8; AU2003298870A1

Abstract

A cascading air wash particle system comprising a pick-up wand (2), a cleaner (23) with a series of linear funnels (5), and static eliminators (6), wherein the cleaner operates from a single air-conveying source to separate fine particulate matter from bulk solids.

Description

FINE PARTICLE SEPARATION FROM PELLETIZED-GRANULAR MEDIA

FIELD OF THE INVENTION

[0001] This invention relates to cascading air wash particle systems. In particular, this invention relates to separation of fine particulate matter from bulk solids, using static elimination and an airflow management system.

BACKGROUND OF THE INVENTION

[0002] The bulk solids industry and those using their products are generally beset by unwanted dust or very small particulates that either intermix with or adhere to larger particles or particulate material (Lowe and Nelson, US 5,259,510). i the first instance, the small, intermixed particulates are not adhesively attached to the larger particulate material and therefore have a tendency to become airborne when the larger particulate material is transferred or transported. This may create an undesirable condition in that it may be unhealthy to breathe the airborne particles and there is a tendency for such mixed suspensions to become explosive (N. Kalkert and H.G. Schecker, Influence of Particle Size Distribution on the Minimum Ignition Energy of Explosive Dusts, Chemie Ingenieur Technik. vol. 52, no. 6, 515-7, (1980)). This concern is especially prevalent in the grain handling or milling industries and also in mining, quarrying and processing of minerals. In the latter instance, instead of intermixing, the small particulates often adhere to the larger particulate materials by static charge, moisture or physical impingement. This typically occurs during the processing of the particulate material wherein small particulates are generated by the friction between the particles themselves or with equipment surfaces. Such an occurrence often renders the larger particulate material less suitable for its intended use. For example, in the clay industry, particulate material is prepared for a variety of uses, including for oil and grease absorption or for use as animal litter. If, during the processing of these materials, the fine particulates adhere to the larger particles, the ability of the larger particulates to absorb oil and grease or to effectively neutralize odor (in the case of animal litter) is diminished. The present invention achieves physical separation of fine particulate from particulate granules or pellets, whether they are intermixed or adhering. Applications extend through industries as diverse as agriculture, pharmaceutical, chemical/petrochemical, plastics and rubber, and recycling.

[0003] The separation of fine particles from bulk solids (granular or pelletized material) has been a subject of inventors since before the establishment of patent offices around the world. The present work is focused on the separation of fine particles from dry, insulating bulk solids. The scope is broadly bounded by technology such as air classification systems, gas phase elutriators, drop out chambers, cyclonic collectors, fluidized beds, and cascading air wash systems. The handling of the insulating materials results in triboelectric charging of the fines and bulk solids, so many of the technologies have static charge eliminators either in their design or added by necessity after placed in operation. The present invention falls into the class of cascading air wash systems with integral static elimination. The static elimination is intended to improve dust separation and subsequent handling of the bulk solids.

[0004] Disposal of solid waste is an increasing problem that has reached crisis levels in some parts of the country. An obvious solution is to recycle materials that normally are land filled. Further, other industries, e.g., wire & cable and automotive, have a number of materials that would have value if they were reclaimable. It is very difficult to cost- effectively recycle commingled plastics at the post-consumer level, because failure to separate the plastics completely may substantially reduce the properties of one or more of the plastic materials. In order to achieve some reduction on the burden to land fills, trade associations and government promote in-plant recycling through regrind of manufacturing waste. [0005] The intent of this inventor is to focus on the cleaning of in-plant regrind and polymer resin-pellets that have gained fine particulate fractions through manufacture and handling. The invention is of additional value for those post-consumer recycling streams where separation has already occurred and fine-particulate separation is needed at the polymer resin-pellet shipment level. The approach is also broadly applicable to bulk solids applications in general industry.

[0006] Synthetic resin pellets carry fine particles that are generated during a step of producing the pellets or transportation of them. The presence of such fine particles causes a variety of troubles in a molding process with the use of the synthetic resin pellets, decreasing commercial value of the synthetic resin pellets. Accordingly, the fine particles have to be removed as much as possible.

[0007] The synthetic resin pellets typically have a short size of 1-3 mm and a long size of 1-8 mm. The fine particles are those passing a screen having 16 mesh size (JIS standard), and their shape is not limited in particular. The fine particles are usually formed of the same resin as the synthetic resin pellets and of a foreign matter. The synthetic resin is not limited to a particular polymer: polycarbonate, polyesters, polyolefins, and polyamides are typical resins.

[0008] Injection molding and extrusion devices typically load granular plastic material from a source, such as a material hopper into an injection machine where the material is melted and forced under pressure into a mold to form a finished product. The finished product is then separated from the scrap plastic portions. The scrap plastic portions are then reground (regrind material) and reused by mixing the regrind material with additional source material. The reground material is sometimes called a secondary source of polymer resin. Regrinding the scrap plastic creates fine particulate matter (fines), dust, and other contaminants. The regrind material is typically recycled by adding it to the source plastic material for reuse in the injection process. Since the regrind material contains fines, dust, and other contaminants, only a percentage of the source material may be composed of regrind. Although this percentage is completely dependent on the raw material being utilized and the requirements of the particular part being formed, for purposes of illustration, the percentage of required material maybe for example, 20-40% of the source material. The reason for such a limitation is that a higher percentage of regrind material produces unacceptably high defect levels, due to either the loss of physical properties of the material or an increase in level of visual defects.

[0009] In the production of small parts, the percentage of scrap plastic created may be as great as 70-90% since the amount of plastic used to create the part maybe small relative to the amount of the runner material (channels) and feed lines (Stiglianese, US 5,735,403). Hence, a proportionately large amount of regrind material is available. In typical injection molding and extrusion devices, such regrind material cannot be fully utilized since the amount of fines found in the regrind limits the injection mixture to approximately 20-40% of the source material. The mechanism of many of the defects is the inconsistency of particle size in the regrind material. Larger particles of material may take a long time to melt completely. During this time, the fine dust particles may have become burned and degraded. Increasing the ratio of regrind to source material may reduce the yield of the final product. Unused regrind material may be stored for subsequent use or may be discarded. Clearly, such alternatives are inefficient and costly. If the regrind contained fewer fines, the percentage of regrind added to the source material could be increased, thus utilizing a greater portion of the regrind material.

[0010] In some applications, such as the use of polycarbonate resins to make information recording substrate or optical equipment, the resins must be free of foreign matter and fines - the molding process for optical equipment itself is generally carried out in a clean room. Fine particle content in the resins is also restricted. Accordingly, a permissible amount of 16-mesh-pass fine particles (JIS standard) carried with the polycarbonate resin pellets that are used in the optical equipment manufacture is considered to be 40 ppm or less (Kazamoto and Oishi, US 5,494,171).

[0011] Typical particle separators available for injection-molding devices operate in batch mode. In batch mode, a quantity of source material is processed to separate the fines and dust from larger particles. The separated material is then loaded into a source hopper or other container for transport into the injection-molding device. A typical molder uses several resins to make a variety of products in a number of molding machines. Preparation of resins suggests the importance of ease of cleaning, portability, and Gaylord/FIBC size batch mode systems that can clean secondary materials for mixing or compounding. [0012] A variety of devices for separating small particulates from larger particles have been developed. These devices are generally known as dedusters. Interests here are distinguished from those systems intended to separate or sort plastic materials by particle size and density, as well as with selective electrostatic charges. Dedusters are also distinct from magnetic separators that are intended for tramp metal separation and liquid-based flotation systems. Dry dedusters do not include wetting and drying of the material and are therefore much less expensive than flotation. Also, there is no wastewater to be concerned with, and no hazardous by-products in the wastewater.

[0013] Dry dusters using static-charge eliminators can employ humidification, radioactive, and electrical methods. The central interest here is in those methods and apparatuses that use electrical static eliminators.

[0014] Typically, a deduster of importance to this work is intended to separate electrically-insulating particulates from electrically-insulating particles. The present deduster has unique advantages over those of the prior art. These prior-art devices can be classified by their means of handling the air and particle flow: cascading particle devices, emulators (fast fluidized beds), and centripetal systems.

[0015] Ruepp et al, (US 2,679,316) describes a drier/separator using inclined, vibrating and rotating agitators in the countercurrent flow channel. Such mechanical systems illustrate the need for mechanical vibration but pose maintenance concerns.

[0016] Kazamoto and Oishi (US 5,494,171) found that systems where the particles are caused to fall while successively colliding against wall surfaces of a tower which is bent in a zigzag configuration and exposed to a rising air flow yield 70-80% efficiency when measured from 16-mesh-pass particles. Kazamoto and Oishi mention a method in which fine particles are moved through a wire net by colliding synthetic resin pellets against the wire net through a high-speed airflow (15 m/s or more). Such method, however, causes trouble when the wire net is broken; metal wire becomes incorporated into the synthetic resin pellets being cleaned. When perforated plates are used instead of the wire net to avoid this trouble, problems such as crushing of the synthetic resin pellets tend to occur. [0017] There is also a method in which fine particles are removed through a rotary drum covered with a wire net or a vibratory screen, hi this method, high removal percentage of 9-mesh-pass and 16-mesh-on particulates can be gained. Kazamoto and Oishi performed measurements on such a system and found 80-90% removal. However, removal percentage of the 16-mesh-pass particulate is insufficient - 60-75%.

[0018] hi their patent, Kazamoto and Oishi bring the particles and particulate into contact with an ionized gas through nozzles such as Model NI-01 B nozzles manufactured by Kasuga Denki K.K. The ionized gas eliminates the electrostatic adhesion between the synthetic resin pellets and the fine particles. The resulting fine particles may thereafter be readily separated from the pellets and the freed fine particles may be withdrawn in a gas stream. Removal of particulate is claimed to reach levels less than 40 ppm. Apparatus for carrying out the method is also described, as is a system for stocking synthetic resin pellets using the described apparatus.

[0019] The amount of the ionized gas is preferably 240-300 L/min per kg/min of the synthetic resin pellets treated. The amount of non-ionized gas is preferably 6000-7000 L/min per kg/min of pellets. A residence time of the synthetic resin pellets is preferably 2-5 s. Kazamoto and Oishi recognize the need for physical agitation of the pellets and the need for their physical separation. They fail to notice the effect of field redirection to grounded surfaces and direct corona particle interaction. Also, the incorporation of compressed gas is a contaminant and yields inefficient delivery of ions.

[0020] Lyras (US 5,269,424) describes a mobile separation system for polymer abrasive blasting material. It includes an "airwash separating apparatus" with Simco Air Curtain Transvectors at the air inlets. Compressed air or internal vacuum draws charge carriers into the pellet environment. The injection of ions through a channel is also very inefficient. Air curtain Transvectors use compressed air and are expensive to purchase and operate. They provide little to improve charge delivery to the particulate.

[0021] Paulson (US 5,035,331) discloses a "kinetic gravity" deduster employing gravity to feed the dust and impurity laden particulate material through "linear kinetic energy" cells. The linear kinetic energy cells generate a steady magnetic field to neutralize the electrostatic charges causing the dust to adhere to the particulate material. Paulson's specification does not provide a substantiated method of static elimination by a magnetic field. Paulson continued to claim he doesn't know why it works, as he did in an earlier patent for Allied Industries. Air injection through the inclined perforated plates is an interesting feature of the design.

[0022] Erickson (US 2,579,228) describes the control of particle separation in a vertical flow column. The control is achieved at the exit of the pipe. Static elimination is handled by humidification.

[0023] Matheson (US 2,683,685) found that size fractions are obtained at 30 times the free-falling velocity and at 100 times the particle concentration by maintaining a turbulent fluid bed of solid at the bottom of the elutriation column. The solids to be separated are introduced into the system preferably in a freely flowing fluidized condition, as by way of a standpipe. The bottoms fraction may contain anywhere from the major portion of the original feed stock to a minor fraction thereof. A tall column (3-6 m or more) is used to produce the size separation. Particle can be removed near the top of the column or introduced to another bed of larger diameter for further fractionation.

[0024] Iannazzi (US 4,127,476) describes a fluidizing process for the segregation and separation of mixed office-paper- waste. In this process an upwardly directed airflow is used to suspend heavy, medium, and coarse fractions of shredded office waste paper. Air jets are used to separate the light and medium fractions for further processing.

[0025] Eissenberg and Liu (US 4,212,651) used changes in the diameter of a fluidized bed to achieve particle separation. There was also a residual fine fraction that was carried on to a dust collector. Their focus was on magnetic separation.

[0026] Frei (US 4,895,642) reports a separation system where shredded waste is tribocharged and introduced to an elutriation column. As the material rises it is exposed to a charged belt (variable voltage), captured and removed. There can be several levels of charging and removal along the column. Heavy material drops out and lightest can move on to a collector. [0027] Rodrigo and Good (US 5,289,921) incorporate static eliminators in the feed and drop-out channels of an air elutriation column. The ionizers are directly in contact with the pellets and fines and operated in a dual-phase, direct-coupled mode. A unique feature of the ionizers is a floating "reference electrode" which is claimed to balance the ionization - the emitters see a floating counter electrode to capture the current from the corona sources. A fabric filter dust collector is placed on the outlet stream and provision made to regulate pressure in the column. Material is introduced by compressed air.

[0028] Leitman, Pickering, and Rychlicki (US 5,397,066) describe a fluidized bed system wherein the flow rate of the gaseous stream is controlled to provide a relatively low density fraction of the plastic mixture exiting the upper end of the column, and a relatively high density fraction of the plastic mixture exiting at the lower end of the column. In one embodiment, electrostatic charges are induced on the particles (primarily by tribocharging) of the plastic mixture prior to introducing the plastic of similar size particles into the column and the column is charged to attract thereto the plastic particles having the highest electrostatic charge. The gas flow is cycled through successive beds in one embodiment of the invention. The intent is to separate or sort plastic materials by particle size and density, as well as with selective electrostatic charges.

[0029] The present invention falls into the class of cascading particle systems. It contains integral static elimination and a unique airflow arrangement to improve static elimination and throughput of solids. The resulting design yields a portable, compact assembly for flexible plant use.

SUMMARY OF THE INVENTION

[0030] The present invention provides a deduster for the separation of fine particulate matter from granules, pellets, and small parts that might be pneumatically conveyed, hi one aspect, the invention features a pick-up wand that contains excess air ports to assure dilute phase conveying of the parts to the cleaner. The cleaner operates from a single air-conveying source, be it electrical or compressed air operated, to transport the bulk solids to the cleaner and to aspirate fine particles from the bulk solids. A linear funnel arrangement is also provided to direct nearly free fall, and air assisted direction of particles through the cleaner, the directed airflow and free fall to inclined surfaces enhances particle- collision intensity and flow direction for separation of fines from bulk solids. Additionally, a static eliminator system is used to directly expose the falling bulk solids to the ionizers without air assist or ducting arrangements. The ionizers are so positioned to optimize neutralizing charge transfer to the bulk solids, while being free of collisional impacts that might otherwise damage the corona emitters.

[0031] Another aspect of the invention also features, an angular arrangement of linear funnel surfaces to separate aspiration air from air guided to lower levels of the cleaner. Aspirated air will be free from heavier particles and will contain the fines. Thus, a filter arrangement where primary containment of aspirated particles is used for cleanup and recycling and a secondary filter for protection of the motor from particles that might accidentally by-pass the filter. A variable speed fan for aspiration of fine particles without carryover of the bulk solids to the dust collection system is also used. Yet another feature of the invention is a compact cleaner for bulk solids, wherein a single air source is used to convey and clean the granular material. The bulk solids can be pushed through the system using compressed air at the inlet or pulled through the system by electrically or compressed- air driven vacuum equipment at the exhaust. The deduster system is portable on an adjustable and easily relocatable stand and also features a fine particle cleaner that is modular in construction for ease of cleanup and resin changeover.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view of a preferred embodiment of the invention. Figure 2 is a front view of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0032] The elements of the invention are summarized in Figure 1- Side View, and Figure 2-Front View. A single vacuum conveyor (1) is used to load particles to the cleaner and withdraw separated fines. The vacuum conveyor can be operated by an electric motor or compressed air. The pick-up wand (2) for granules includes excess-air ports (3) to assure that granules are picked up at the nozzle (4) in the dilute phase and carried as such to the cleaner. This is necessary to assure continuous cleaning and adequate air for fines separation. Whereas prior art systems employ cross flow air systems and often secondary dust extractors, the present invention uses a single gas stream. This arrangement prevents contaminants from entering through the sides of the cleaner and resin pellets from bouncing out. The airflow arrangement of the present system propels the pellets entering the system to assure larger impacts for dislodgement of particles during cleaning.

[0033] Once the pellets have been conveyed to the cleaner, they fall through a series of linear funnels (5) near which fine particle separation occurs. The air flow and vertical fall regions increase the intensity of pellet collisions below each funnel element and allow exposure of the pellets to the static eliminators (6) from both sides. The static eliminators are of the dual-phase design to aid in charge transfer to the resin pellets. Unlike prior art the resins are directly exposed to the ionizers - there are no ducts or air flow assists. Also, charged pellets are in free space for neutralization, so that their electric fields are not directed towards underlying metal surfaces (field suppressed) and can attract charge from the ionizers.

[0034] The surfaces of the funnels are inclined to capture heavier particles and carry some gas flow to lower levels of the cleaner. Some air and fine particles are aspirated through slots (7) at each level of the cleaner to the dust collection system. It was found that three levels of cleaning is adequate to achieve static elimination and fine particle aspiration. Claims should also cover one to about five levels. At higher levels, there is no improvement in static elimination and in return air flow.

[0035] Bulk solids are withdrawn from the bottom of the cleaner through a gravity- assisted flapper valve (8) or other conventional discharge device consistent with the operating pressure and mechanics of the system. Air will be inspired during this emptying process to aid in fines removal.

[0036] Air from the conveying system is uniquely used in the aspiration process. The aspirated air is drawn through disposable filter bags (9) and a final particle filter (10). The disposable filter bags allow capture of the fines for easy cleanup or recycling. Alternatively, the filter (10) and collection chamber (11) can be used to collect the fines. The vacuum motor (1) and filter (10) can be easily removed for cleanup.

[0037] The use of a single air system yields a compact design. Conventional systems, such as those marketed by Pelletron, Kice, and Kongskilde, have separate cleaning and filtration systems. By integrating these systems the cleaner can be mounted on a portable stand (12) with facility in height adjustment (13) for various containers to receive cleaned resins. The cleaner can then be easily relocated on casters (14) to locations where it is most needed.

[0038] The cleaner is controlled by air flow and static elimination. First, the vacuum system (1) is controlled by changing the speed of the motor or by restricting gas flow through it by various known methods. Alternatively, compressed air driven vacuum devices may replace an electric vacuum motor. In this case air pressure adjustments are used to control gas flow through the cleaner.

[0039] The flow through the cleaner is adjusted to convey pellets and fines from the nozzle (4) to the cleaner. The gas flow rate is reduced to prevent pellets from being conveyed through the dust collector channels (15), as observed at site windows (16) or by other methods. The speed of gas flow will be determined by density and weight of pellets being cleaned. It should be noted that, typically, highest cleaning rates are achieved at highest flow rates without significant pellet entrainment in the dust collection channels (15). Further, when the density of pellets becomes lower, lower gas flows are needed, and this is consistent with lower vacuum and weight needed to operate the gravity assisted flapper valve (8) for emptying the cleaner. The latter valve (8) will also contain a counterweight adjustment or other mechanism for system setup.

[0040] The static eliminators use a dual-phase a-c power source, so that a-c voltages are applied out-of-phase to pairs of ionizers (6) placed about each linear funnel (5) of the cascade. The individual ionizer bars may be direct- or capacitvely-coupled to the high voltage sources and fitted with safety devices available to the static-elimination art.

[0041] The deduster is modular in construction for ease in cleaning during change of resins. The vacuum motor (1) and filters (9,10) are easily removed by lifting the motor from its cradle (17) from the shell of the cleaner (23). The dust collection module (11, 15) is lifted from the cleaner after the retaining ring (18) is removed. Similarly, removal of the retaining ring (19) allows removal of the transition (21) and flapper valve (8). The blocks (21), support the cascade module (22) containing the ionizers (6) and linear funnels (5). The remaining shell (23) is then clear of equipment and exposed for cleanup.

Claims

I Claim:

1. A deduster for the separation of fine particulate matter from bulk solids that may include granules and pellets, comprising:

A) a pick-up wand containing excess air ports to assure dilute phase conveying of the bulk solids to the cleaner;

B) an air-conveying source for transporting the bulk solids to the cleaner and aspirating fine particles from the bulk solids;

C) a linear funnel arrangement that allows the bulk solids to nearly free fall through the cleaner; and

D) an air assisting device that enhances the nearly fall of the bulk solids through the cleaner, the air assisted nearly free fall flow of the bulk solids through the cleaner enhancing particle-collision intensity in the bulk solids.

2. The deduster of claim 1, wherein the air-conveying source comprises a single air- conveying source.

3. The deduster of claim 1, wherein the air-conveying source comprises an electrical air- conveying source.

4. The deduster of claim 1, wherein the air-conveying source comprises a compressed air operated source.

5. A deduster comprising:

A) a pick-up wand operatively connected to a body structure, the pick-up wand having excess air ports;

B) a cleaner operatively connected to the body, the cleaner operating from a single air-conveying source;

C) an angular arrangement of linear funnels located within the body; and

D) a static eliminator operatively connected to the body.

6. The deduster of claim 5, further comprising an arrangement of filters within the body.

7. The deduster of claim 5, further comprising a variable speed fan operatively connected to the body.

8. The deduster of claim 5, further comprising a fine particle cleaner operatively connected to the body.

9. The deduster of claim 5, further comprising a bulk solids compact cleaner operatively connected to the body.

10. The deduster of claim 5, further comprising a portable stand, the body being located on the stand.