System for handling powder materials

Abstract

A method for handling polytetrafluoroethylene (PTFE) powder, comprising receiving the PTFE powder in a hopper having a conical portion, reducing (1) the adhesion force between the inner surface of the conical portion of the hopper and the PTFE powder, (2) the adhesion force between the particles of the PTFE powder, or both adhesion forces, discharging the PTFE powder into a transfer channel from an outlet located near the base of the conical portion of the hopper, applying a pressure difference to the transfer channel to convey the PTFE powder in a dilute phase containing gas and PTFE powder along the transfer channel, and separating the PTFE powder from the gas at the outlet of the transfer channel, wherein the separated PTFE powder has a particle form sufficient for the dry production of film battery electrodes.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 340,118, filed May 10, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] (Field of the Invention) This disclosure relates to the handling of powder materials, and more specifically, to the handling of fine powders of polymeric materials.

Background Art

[0003] Polytetrafluoroethylene (PTFE) is widely used to fabricate electrode materials for batteries, fuel cells, and supercapacitors. PTFE can be used as a binder for battery materials or as an electrospun nanofiber separator. Conventionally, PTFE has been manufactured and transported as a powder material or in a liquid suspension. As the demand for batteries increases, the need for the transport, storage, and handling of PTFE powder is increasing on a larger scale. However, PTFE powder is susceptible to fibrillation, in which the PTFE particles in the powder undergo a morphological change from a particulate form to a fibrous form in response to an applied stress or force. For example, PTFE particles can fibrillate under handling and storage conditions that apply stress or frictional force to the particles.

Summary of the Invention

Means for Solving the Problems

[0004] Disclosed herein are systems and methods for handling fine PTFE powder on a large scale, including storing and transporting the powder from a source to an outlet while maintaining a substantially unfibrillated state of a sufficient amount of PTFE powder suitable for use in downstream applications, such as in the dry manufacturing process of battery electrodes.

[0005] In one example, disclosed herein is a method for handling polytetrafluoroethylene (PTFE) powder, the method comprising receiving the PTFE powder in a hopper having a conical portion, reducing (1) the adhesion force between the inner surface of the conical portion of the hopper and the PTFE powder, (2) the adhesion force between the particles of the PTFE powder, or both, discharging the PTFE powder into a transfer channel from an outlet located near the base of the conical portion of the hopper, applying a pressure difference to the transfer channel to convey the PTFE powder in a dilute phase containing gas and PTFE powder along the transfer channel, and separating the PTFE powder from the gas at the outlet of the transfer channel, wherein the separated PTFE powder has a particle form sufficient for dry manufacture of a film battery electrode.

[0006] Examples may include the following features. At least a portion of the separated PTFE powder may include PTFE aggregates that are not substantially fibrillated. The separated PTFE powder may include an amount of PTFE aggregates that are not substantially fibrillated sufficient to enable the manufacture of film battery electrodes. At least 40 weight % of the PTFE powder received in the hopper may be separable from gas for use in the manufacture of film battery electrodes. Receiving the PTFE powder in the hopper may include receiving at least 200 pounds of PTFE powder in the hopper. Receiving the PTFE powder in the hopper may include receiving a volume of PTFE powder that may be less than a threshold volume of the PTFE powder, where the threshold volume of the PTFE powder may be the volume of PTFE powder that agglomerates due to the force exerted by its own weight when received in the hopper. The method may further include applying suction to an inlet channel to convey an initial dilute phase that may include the PTFE powder into the hopper along the inlet channel. Receiving the PTFE powder in the hopper may include receiving the PTFE powder from an upstream hopper. The PTFE may be transferred from the upstream hopper to the hopper by gravity only. The hopper may include a first hopper and may include discharging the PTFE powder into a transfer channel from an outlet of a second hopper. Receiving the PTFE powder in the hopper may include receiving the PTFE powder by directly mechanically transferring the PTFE powder from a drum containing the PTFE powder. Reducing the adhesion between the inner surface of the hopper and the PTFE powder may include aerating the inner surface of the hopper. Aerating the inner surface of the hopper may include flowing an aeration gas between an outer wall of a conical portion of the hopper and a porous inner wall of the conical portion of the hopper. The porous inner wall of the conical portion may extend from an outlet of the conical portion along the inner wall of the conical portion to a position where the diameter of the conical portion may be at least 75% of the maximum diameter of the conical portion. The method may further include cooling the aeration gas before flowing the aeration gas. The aeration gas may include an inert gas. The aeration gas may be substantially free of water.

[0007] The method may further include cooling the wall of the conical portion of the hopper. The cooling may include cooling the wall of the conical portion of the hopper to a temperature below the beta transition temperature of the PTFE powder. The hopper may include a cylindrical portion connected to the conical portion, and may include cooling the walls of the cylindrical portion and the conical portion of the hopper. The conical portion of the hopper may include a cooling jacket disposed on the wall of the conical portion, and cooling the wall of the conical portion may include flowing a cooling fluid through the cooling jacket. The method may further include providing a layer of cooling gas in the hopper between the PTFE powder and the inlet of the hopper. The height of the hopper may be at least twice the size of the maximum diameter of the conical portion of the hopper. The inner surface of the hopper may include stainless steel. The inner surface of the hopper may include polished stainless steel, and the polishing may be in the direction of the flow of the PTFE powder. Reducing the adhesion between the inner surface of the hopper and the PTFE powder may include applying mechanical vibrations to the PTFE powder in the hopper. Reducing the adhesion between the inner surface of the hopper and the PTFE powder may include injecting gas into the PTFE powder in the hopper. Applying a pressure difference to the transfer channel may include applying suction to the pressure channel. Applying a pressure difference to the transfer channel may include applying a positive pressure to the pressure channel. Applying a pressure difference to the transfer channel may include applying a pressure difference to generate a pickup speed of at least 2,500 feet per minute. Applying a pressure difference to the transfer channel may include operating a variable frequency drive to apply a pressure difference to the transfer channel.

[0008] The method may further include operating a variable frequency drive to control the velocity of the lean phase within the transfer channel. The method may further include cooling the transfer channel. The transfer channel may include a jacket, and cooling the transfer channel may include flowing a fluid through the jacket of the transfer channel. The method may further include cooling the lean phase gas. The lean phase gas may include an inert gas. The inner surface of the transfer channel may include stainless steel. The inner surface of the transfer channel may be free of weld joints. Conveying the lean phase along the transfer channel may include conveying the lean phase around elbows designed to reduce consolidation and shear. The method may further include maintaining the lean phase gas at a temperature that may exceed the dew point of the transfer channel environment. The method may further include separating PTFE powder from the gas in a cyclone separator. The method may further include applying suction to the cyclone separator. The method may further include separating PTFE powder from the gas in a bag filter. Separating PTFE powder from the gas may include separating PTFE powder from the gas within a separator and flowing the PTFE powder through a discharge valve at the outlet of the separator. The discharge valve of the separator may include a flapper valve. The method may further include separating PTFE powder from the gas in a plurality of separators arranged in series or parallel along the transfer channel. Each of the plurality of separators has a discharge valve connecting to a common receiving vessel. The method may further include using a screen to sieve the separated PTFE powder to crush or remove lumps of material. The method may further include vibrating the screen. The screen may include a mesh having openings with a diameter of 2 mm.

[0009] In a second example, a PTFE powder product obtained by the method according to any one of the preceding claims, wherein the separated PTFE is a powder product, is disclosed herein.

[0010] In a third example, disclosed herein is a system for handling PTFE, the system comprising a hopper having a conical portion, the hopper containing PTFE powder including substantially non-fibrillated PTFE aggregates, the hopper including a mechanism for reducing (1) the adhesion force between the inner surface of the conical portion of the hopper and the PTFE powder, (2) the adhesion force between the particles of the PTFE powder, or both; a transfer channel, the hopper being communicatively coupled to the transfer channel via an outlet defined at the base of the conical portion, the transfer channel containing the PTFE powder; a flow control device configured to apply a pressure differential to the transfer channel, the pressure differential applied to the transfer channel causing conveyance along the transfer channel of the PTFE powder in a dilute phase including gas and the PTFE powder; and a separator disposed at an outlet of the transfer channel and configured to separate the PTFE powder from the gas, the separated PTFE powder having a particle form sufficient for dry manufacture of a film battery electrode.

[0011] The example may include one or more of the following features. The hopper may contain at least 200 pounds of PTFE powder. The mechanism for reducing adhesion may include a porous sheet. The porous sheet may extend along the inner wall of the conical portion from the outlet of the conical portion to a position along the inner wall of the conical portion where the diameter of the conical portion is at least 75% of the maximum diameter of the conical portion. The mechanism for reducing adhesion may include a bin actuator. The mechanism for reducing adhesion may include an air injector. The system may further include a cooling jacket disposed on the wall of the conical portion. The height of the hopper may be at least twice the size of the maximum diameter of the conical portion of the hopper. The inner surface of the hopper may include stainless steel. The inner surface of the hopper may include polished stainless steel, and the polishing may be in the direction of the flow of the PTFE powder. The flow control device may include a suction source. The flow control device may include a positive pressure flow device. The flow control device may include a variable frequency drive. The transfer channel may include a cooling jacket. The inner surface of the transfer channel may include stainless steel. The inner surface of the transfer channel may not include weld joints. The transfer channel may include an elbow designed to reduce consolidation and shear. The separator may include a cyclone separator. The separator may include a bag filter. The separator may include a flapper valve. The system may further include a sieve positioned to receive the PTFE powder separated from the separator. The system may further include a plurality of hoppers connected in series. The system may further include a plurality of hoppers connected in parallel, and each hopper may be communicably coupled to the transfer channel via its respective outlet.

[0012] Certain implementations of the subject matter described herein may be implemented to realize one or more of the following technical advantages.

[0013] The approaches described herein enable the handling (e.g., storage and conveyance) of large quantities (e.g., at least 200 pounds) of fine PTFE powder in bulk, and at least a portion of the input PTFE powder is recoverable in the output of the system in a substantially unfibrillated form. The unfibrillated PTFE powder recovered from the systems and methods described herein can be used for blending with other components in a dry battery manufacturing process.

[0014] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.

Brief Description of the Drawings

[0015]

Figure 1A

Figure 1B

Figure 2A

Figure 2B

Figure 2C

Figure 3

Figure 4A

Figure 4B

Figure 5

[0016] In the drawings, like reference numerals designate like elements.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The fine powder of PTFE contains individual PTFE aggregates, which themselves contain compressed subunits of PTFE particles. The fine PTFE powder is susceptible to the effects of fibrillation, which can be induced, for example, by shear forces applied to the powder during powder transport, conveyance, or storage. Fibrillation is an irreversible process in which the particles undergo polymer unwind and mechanical linkage, often leading to aggregation of the individual particles and the formation of lumps and agglomerated materials. Fibrillated PTFE is often not suitable for downstream processes. For example, fibrillated PTFE powder is generally not suitable for use in dry battery electrode manufacturing processes. An explanation of the fibrillation of PTFE powder can be found in Ebnesajjad et al, (2015) Fluoroplastics (Second Edition), 1:11, 234-277, the contents of which are hereby incorporated by reference in their entirety.

[0018] The present disclosure describes a method for storing and handling large quantities of fine PTFE powder, including active aeration of sections of a storage hopper to reduce adhesion forces, such as friction and / or shear, between the inner surfaces of the storage hopper and to reduce bulk agglomeration and / or compaction of the PTFE powder. A conveying method that reduces the shear imparted to the PTFE powder and maintains the powder temperature below standard ambient temperature facilitates maintaining flow characteristics and reducing bulk agglomeration in the output material. These approaches enable bulk handling of large quantities of PTFE powder while obtaining substantially non-fibrillated PTFE aggregates having sufficient quality (e.g., particle morphology) to enable dry manufacture of film battery electrodes as an output. A significant amount of the input PTFE powder can be recovered as substantially non-fibrillated PTFE suitable for use in battery manufacturing applications at the output. For example, at least 200 pounds of fine PTFE powder can be provided as a bulk input and stored and conveyed using these approaches, with at least 40 wt%, at least 50 wt%, or at least 60 wt% of the input PTFE powder being recovered as the output PTFE powder.

[0019] FIG. 1 is an exemplary PTFE powder handling system 100 that reduces shear and compaction of large quantities of bulk PTFE powder. The system 100 receives bulk PTFE powder in a free-flowing, non-compressed form from one or more sources. In one example, the bulk PTFE powder is received from a separate source such as a barrel 10 or tote. Alternatively or additionally, the bulk PTFE powder is received from a continuous source such as a conveyor 20 or an upstream manufacturing process.

[0020] The components of the powder handling system 100 are manufactured from a hard and durable material and are configured to have a low-roughness contact surface, such as a polished surface. As an example, the components of the system 100 are made of stainless steel or include a contact surface made of stainless steel. The contact surfaces of the components have a surface finish (e.g., surface roughness of 0.5 μm or less) of 2B or higher (e.g., 2G, 2R, 2J) according to the EN 10088-2 standard. The contact surfaces of the components can be polished in a direction aligned with the expected flow direction of the PTFE powder through the system.

[0021] The system 100 receives bulk PTFE powder into a storage unit 102 such as a hopper. In the example of FIG. 1, the powder handling system 100 includes one storage unit 102, but in some implementations, the powder handling system 100 includes two or more storage units 102 connected in series (e.g., the output of one storage unit is fed to the next storage unit in series) or connected in parallel (e.g., the output of each storage unit is fed to the same destination). The storage unit 102 defines an internal volume large enough to contain a large amount of PTFE powder. For example, the storage unit 102 is sized to receive more than 100 pounds of PTFE powder (e.g., more than 200 pounds, more than 500 pounds, or more than 1000 pounds). In some examples, the amount of PTFE powder that can be stored in the storage unit 102 is limited by the weight of the PTFE powder, e.g., the amount of PTFE powder is limited to an amount that does not undergo agglomeration, fibrillation, or both due to the force exerted by the weight of the PTFE powder itself.

[0022] The storage unit 102 receives PTFE powder through an inlet 104 that connects the internal volume of the storage unit 102 to the external environment. The inlet 104 can be reversibly sealed, for example, using a threaded screw connection, a hinge, a flange, or a clamp, such that the inlet 104 seals the internal volume against the flow of fluids, gases, or materials when sealed. The inlet 104 is sized and positioned to receive PTFE powder. The storage unit 102 is generally configured such that the PTFE powder received through the inlet 104 flows under gravity through the upper section 106 of the storage unit 102 to the lower section 108. The inlet 104 is sized and positioned to receive PTFE powder into the upper section 106 of the storage unit 102 through a mechanism including a conveyor, a bucket, manual loading, or automatic loading.

[0023] The storage unit 102 of the exemplary system 100 has a cylindrical upper section 106 and a conical lower section 108, both having a circular cross-section, although other cross-sections may be utilized. The height of the upper section is greater than the lateral dimension (e.g., diameter) of the conical lower section 108, for example, at least twice as large.

[0024] In some implementations, the storage unit 102 is cooled below ambient temperature to reduce the handling temperature of the received PTFE powder. For example, the storage unit 102 includes a gas or liquid-based temperature control system that functions to maintain the internal volume and the material stored therein at a temperature lower than ambient temperature (e.g., <25°C). Operation at a low temperature helps to reduce shear on the PTFE powder, thereby avoiding fibrillation.

[0025] An example of a storage unit 102 that includes a gas or liquid-based temperature control system is a hopper that includes a cooling jacket disposed around all or a portion of the outer wall of the storage unit 102 (e.g., the outer wall of the upper section 106, the outer wall of the lower section 108, or both), through which a cold liquid is circulated. The storage unit 102 is temperature controlled (e.g., may include a temperature control system) across the entire outer surface of the storage unit 102 or a portion of the storage unit 102, such as the upper section 106, the lower section 108, or both. In some examples, the storage unit 102 is temperature controlled to a temperature below the beta transition temperature of the PTFE powder (e.g., 20 °C or less, 19 °C or less, 15 °C or less, 12 °C or less, 10 °C or less, 5 °C or less) and above the dew point of the environment.

[0026] Generally, the storage and handling of PTFE powder at low temperatures reduces the occurrence of bulk agglomeration and fibrillation of the PTFE. The storage and handling of PTFE powder at temperatures below the beta transition temperature reduces the occurrence of bulk agglomeration.

[0027] Here, and throughout this specification, references to measurable values such as amounts, durations, etc., and listings of values include exact values, approximate values, and values within ± 10% of the value. For example, in this specification, a reference to a temperature of 15 °C includes exactly 15 °C, approximately 15 °C, and values within ± 10% of 15 °C.

[0028] Powder handling system 100 includes or is connected to a gas source 110 that provides a pressurized gas source to the components of powder handling system 100 including storage unit 102. Examples of gas source 110 include static sources such as cylinders or tanks, or continuous on-demand sources such as compressors. Gas source 110 may include a filter for supplying a substantially pure (e.g., 99.99% or greater purity) gas to powder handling system 100. In some implementations, gas source 110 supplies a gas with a purity of 99.999% to powder handling system 100. Gas source 110 supplies a dry gas having a dew point (e.g., the temperature at which air needs to be cooled at a constant pressure to achieve 100% relative humidity (RH)) of less than 15 °C (e.g., less than 10 °C, less than 8 °C) to powder handling system 100. By supplying a dry gas to powder handling system 100 and maintaining a temperature above the dew point, the occurrence of condensation during the handling of PTFE powder is reduced, which can be important for downstream processing such as in dry battery manufacturing processes.

[0029] The gas supplied by gas source 110 to storage unit 102 is non-reactive, e.g., inert, to reduce contamination of the PTFE powder by reactants such as oxidation reaction products. The gas supplied by gas source 110 is cooled to a temperature lower than the ambient temperature, e.g., <25 °C. In some implementations, gas source 110 supplies gas to the upper section 106 of storage unit 102. Supplying a cold, dry, inert gas to the upper section 106 of storage unit 102 moves atmospheric gas from the internal volume of storage unit 102 and reduces agglomeration due to the effects of thermal and chemically reactive effects.

[0030] In some implementations, the gas source 110 supplies pressurized gas to the lower section 108 of the storage unit 102. The gas received by the lower section 108 is applied to the internal volume of the storage unit 102 in a region referred to as the “active zone”. FIG. 1B is a cross-sectional view of the internal volume of the lower section 108 and a portion of the upper section 106. The active zone 112 is a region of the lower section 108 that includes a mechanism for reducing adhesion forces such as friction or shear between the inner surfaces 114 of the storage unit 102 that contact the PTFE powder, a mechanism for reducing inter-particle friction within the PTFE powder itself, or both. The active zone 112 fluidizes a portion of the fine PTFE powder that contacts or is near the inner surface 114 in order to initiate and maintain flow out of the storage unit 102. In some examples, gas is not supplied to the active zone 112 and a mechanism for reducing adhesion forces without using gas is employed.

[0031] The active zone 112 includes all or a portion of the total surface area of the lower section 108. In one example, the upper section 106 of the storage unit 102 shown in FIG. 1B has a maximum dimension, e.g., diameter D. The active zone 112 of the lower section 108 extends to a height corresponding to 3 / 4D from the lowermost point of the lower section 108. Generally, the active zone 112 can extend over the entire lower section 108 (e.g., to a height corresponding to 1D). 3 / 4D is generally sufficient to reduce the friction between the inner surface 114 and the PTFE powder and to maintain the flow of the contents of the storage unit 102 when the outlet 122 is open.

[0032] Referring now to FIGS. 2A-2C, an example of the active zone 112 including the active zones 112a, 112b, and 112c is shown. In FIG. 2A, the storage unit 102 includes an active zone 112a including a porous inner wall 116. The pores are sized to allow gas flow but restrict the flow of PTFE powder through the porous inner wall 116, thereby enabling aeration of the inner wall 116. In one example, the porous inner wall 116 is manufactured from a sintered metal material, for example, refer to Dynapore® porous metal laminate manufactured by Parker Hannifin (Cleveland, OH, USA). In another example, the porous inner wall 116 is composed of a filter material such as a cloth, for example, a PTFE-coated polyester woven fabric medium such as the BTS discharge bottom manufactured by Zeppelin Systems (Garching, DE). In another example, the porous inner wall 116 of the active zone 112a includes uniformly spaced aeration holes such as the SIPERM® aeration insert manufactured by Tridelta Siperm (Dortmund, DE). In another example, the active zone 112c includes a multilayer wire mesh having a finished contact surface that generates a smooth and uniformly distributed air flow, such as the TransFlow® Powder Fluidization Pads manufactured by Young Industries (Muncy, PA, USA).

[0033] In FIG. 2B, the active zone 112b is the region of the lower section 108 including an array of gas ports 118. The gas source 110 supplies pressurized gas to discrete gas ports 118 that provide local injection of gas at discrete points along the inner surface 114 of the lower section 108, thereby aerating the inner surface 114. Local injection of gas can be provided using, for example, Airsweep® technology or Solimar® technology. The aerated active zones, such as the active zone 112a or the active zone 112b, promote fluidization over a substantially uniform distribution across the inner surface 114 of the lower section 108.

[0034] In Figure 2C, the active zone 112c includes a vibrating cone 120 inverted in the lower section 108. The vibrating cone 120 is powered to generate vibrations along the surface of the vibrating cone 120. The vibrations transfer energy to the surrounding PTFE powder according to the frequency and energy of the generated vibrations, and fluidize the surrounding PTFE powder by reducing the surface friction and inter-particle friction between the vibrating cone 120 and the PTFE powder. See, for example, the bin activator manufactured by Vibra Screw (Totowa, NJ, USA). In the example of Figure 2C, the maximum dimension of the active zone 112c is 3 / 4D or more. The vibrating active zone 112c induces more shear and consolidation than the active zones 112 that utilize gas flow to fluidize PTFE powder, such as the exemplary active zones 112a and 112b. The vibrating active zone 112c is an example of an active zone that does not rely on gas supply.

[0035] In some examples, when the system 100 includes multiple storage units 102, only some of the storage units 102 are provided with the active zone 112. For example, the first storage unit may be configured such that the PTFE powder flows from the first storage unit to the next storage unit only by gravity. The next storage unit is provided with the active zone 112.

[0036] Referring again to FIG. 1, the storage unit 102 is connected to the outlet 122 at the base of the lower section 108. The outlet 122 operates to gate-control the flow of PTFE powder from the storage unit 102. When the outlet 122 is in a flow-permissive state (e.g., at least partially open), the PTFE is discharged from the storage unit 102. When the outlet 122 enters a flow-restrictive state (e.g., closed state), the outlet 122 stops the discharge of PTFE powder from the storage unit 102. In some implementations, the system controller interlocks the operation of the active zone 112 and the outlet 122 such that when the outlet 122 is in a flow-permissive state, the active zone 112 fluidizes the PTFE powder within the lower section 108. When the outlet 122 enters a flow-restrictive state, the active zone 112 stops fluidizing the PTFE powder within the lower section 108. Such operation reduces the shear applied to the PTFE powder during storage and increases the flow rate from the storage unit 102 during discharge. In some examples, the outlet 122 is configured to allow an average PTFE powder flow rate of 100 pounds per hour. In other examples, the outlet 122 is configured to allow 1000 pounds per hour or more.

[0037] Referring to FIG. 3, an example of an outlet that can be used for the outlet 122 is shown. The pickup wand 302 entrains the PTFE powder in the carrier gas by flowing the carrier gas along a central channel. A negative pressure is applied to the central channel such that as the flowing carrier gas entrains the PTFE powder and thereby directs the PTFE powder into a dilute phase, the dilute-phase PTFE powder is directed into the central channel and downstream components. In some examples, the pickup wand 302 withdraws the PTFE powder from the storage unit 102 within the upper section 106. The pickup wand 302 is utilized when the storage unit 102, e.g., the hopper, is configured to withdraw material from above, e.g., from the upper section 106.

[0038] The baffle outlet 304 receives the PTFE powder such that the powder assumes the angle of repose of the material. This increases the overall material surface area for mixing the PTFE powder and flowing a carrier gas therethrough to induce a dilute phase of the powder.

[0039] The sliding gate outlet 306 restricts the supply rate of the PTFE powder by a slidable gate 308. The gate 308 is continuously slidable between an open state and a closed state for adjusting the supply rate, and a flap connected to the gate 308 directs the flowing PTFE powder toward the outlet.

[0040] The example of FIG. 3 is non-limiting. The illustrated example may be used alone, or in combination with the described examples, or in combination with other examples that enable the PTFE powder to flow from the storage unit 102 under relatively low shear and pressure differentials.

[0041] Referring again to FIG. 1, the outlet 122 is in fluid connection with the transfer channel 124. The PTFE powder discharged from the outlet 122 enters the transfer channel 124 and is conveyed downstream away from the storage unit 102 in a dilute phase. In some implementations, the transfer channel 124 is angled downward such that the PTFE is conveyed at least in part through the transfer channel 124 by gravity. Additionally or alternatively, a portion of the transfer channel 124 is substantially planar and includes no height variation through the planar portion. In some implementations, the transfer channel 124 is connected to a pressure differential generating system. The pressure differential can be a positive or negative pressure differential (e.g., suction). Due to the pressure differential, a carrier gas flows through the transfer channel 124, whereby the PTFE powder enters the dilute phase while being conveyed through the transfer channel 124.

[0042] In the dilute phase, the PTFE powder particles are uniformly suspended in the carrier gas. In one example, the pressure generating system is a positive pressure system (e.g., a blower). Additionally or alternatively, the pressure generating system is a negative pressure system (e.g., a vacuum source 126). A variable frequency drive can be used to control the gas flow rate (and thus the pressure difference), and consequently, the velocity of the dilute phase of the PTFE powder conveyed through the transfer channel 124. Generally, a lower carrier gas velocity reduces the fibrillation of the PTFE powder during conveyance.

[0043] Negative pressure conveyance reduces the leakage of PTFE powder into the processing environment of the powder handling system 100 because the leak points in the transfer channel 124 draw environmental gas into the system. In one example, the vacuum source 126 generates sufficient negative pressure to achieve a pickup velocity (e.g., the minimum velocity required for particle entrainment) of at least 2,500 feet per minute (fpm) for a 2-inch inner dimension (e.g., ID) of the transfer channel 124. The pickup velocity can be adjusted based on at least the PTFE grade (e.g., density, morphology), pipe diameter, gas pressure / density, and solid loading (e.g., the mass of PTFE powder per mass of carrier gas).

[0044] The carrier gas is cooled to a temperature below, for example, the beta transition temperature of the PTFE powder (e.g., below 19 °C). The carrier gas is a dry gas substantially free of water and contaminants. In some examples, the carrier gas is an inert gas.

[0045] The transfer channel 124 provides a flow path for the dilute-phase PTFE powder from the storage unit 102 to the channel outlet 128. The transfer channel 124 is constructed from one or more substantially straight pipe sections 130, one or more elbows 132, or both. The interior of the transfer channel 124, e.g., the surfaces that come into contact with the dilute phase of the PTFE powder during handling, has a smooth finish to reduce friction and the resulting agglomeration of fine powder. For example, the connections of the transfer channel 124 are welded and ground to a smooth finish (e.g., such that there are substantially no weld points or seams on the inner surface of the transfer channel) to facilitate the reduction of friction during handling. The number of elbows 132 within the transfer channel 124 is minimized to reduce the overall shear and friction of the flowing material within the transfer channel 124. The interior of the transfer channel 124 is a non-reactive material such as stainless steel.

[0046] In some implementations, the transfer channel 124 is cooled, for example, using a countercurrent cooling pipe or a cooling jacket, to maintain the bulk temperature of the PTFE powder at a temperature lower than the ambient temperature. In some examples, the transfer channel 124 is temperature-controlled to a temperature below the beta transition temperature of the PTFE powder (e.g., below 19 °C) and above the dew point of the environment. Operation at a low temperature helps to reduce the shear on the PTFE powder, thereby avoiding fibrillation. A temperature significantly lower than the beta transition temperature can be considered to account for the heat that may potentially be generated due to the friction of the gas and the friction of the PTFE particles.

[0047] Generally, the straight pipe section 130 has a length of at least 10 pipe diameters between the pickup and the elbow 132 to establish a smooth flow. For example, a pipe section 130 with an inner diameter of 2 inches has a length of at least 20 inches. In some implementations, the pipe section 130 has a length of 10 to 20 pipe diameters.

[0048] The elbow 132 is manufactured to reduce friction and shear forces on the PTFE powder in the dilute phase. In some implementations, the elbow 132 has a high radius of curvature. For example, the radius of curvature is greater than the pipe diameter. For example, R c ≧ 1.5D, for example, a long radius elbow (LR elbow) (for example, Rc ≧ 5D, Rc ≧ 10D). In some implementations, the elbow 132 is manufactured to reduce friction between the inner surface of the elbow and the PTFE powder, such as a Gamma Bend manufactured by Coperion (Stuttgart, DE) or a Pellbow Bend from Pelletron Corp. (Lancaster, PA).

[0049] In some examples, the transfer channel 124 includes a switch, manifold, or valve (e.g., a diverter valve) 134 for controlling the flow of dilute phase PTFE powder through the transfer channel 124.

[0050] In the exemplary system of FIG. 1A, a vacuum source 126 is connected to the transfer channel 124 that generates a negative pressure within the internal volume of the transfer channel 124. The outlet 122 enters a flow-permissive state and the PTFE is discharged from the storage unit 102. Due to the negative pressure in the internal volume of the transfer channel 124, the PTFE powder enters the dilute phase while being conveyed through the transfer channel 124.

[0051] The dilute-phase PTFE powder is transported along the transfer channel 124 to the separator 136 together with the carrier gas. The separator 136 functions to separate the carrier gas from the dilute-phase PTFE powder, whereby the PTFE powder enters the dense phase. FIGS. 4A and 4B show exemplary separators that can be used for the separator 136. FIG. 4A is a depiction of a cyclone separator 400 that receives the dilute-phase PTFE powder through the inlet 402 and cyclones the dilute phase within the body. The PTFE powder undergoes a centripetal motion as the carrier gas follows the circulation path shown as an exemplary dashed line. The PTFE particles are transported to the inner surface of the body 404, where they fall under gravity to the outlet 406. The cyclone separator is composed of stainless steel with a smooth surface finish, for example, a surface finish of 4B or better. The carrier gas is exhausted from the exhaust port 408 that is substantially free of PTFE powder. The cyclone separator has a medium to high collection efficiency and generally low capital and maintenance costs. In some implementations, the cyclone separator applies a certain amount of shear to the PTFE powder that can induce fibrillation.

[0052] A valve at the outlet 406 of the cyclone separator prevents the flow of gas into the body of the cyclone separator and helps ensure that the carrier gas escapes through the exhaust port 408. In some implementations, the outlet 406 includes a flapper valve having an extended spool component. The flapper valve opens when there is sufficient material inside the spool component, reducing the flow of the carrier gas through the spool component and the flapper valve. The flapper valve may be provided with a level detector to maintain a desired level before discharging the material. Implementations that utilize the extended spool component may include a flow promotion device, such as a vibrator, that operates when the valve is open to facilitate the flow of the PTFE powder from the outlet 406.

[0053] Figure 4B shows a bag filter system 410 that separates PTFE powder from the carrier gas using a bag filter 412. The dilute-phase PTFE enters the bag filter system 410 and is separated from the carrier gas. The bag filter system 410 is sized such that the empty tower filtration rate (also known as the air-to-cloth ratio) is maintained below 3 feet per minute. The material of the bag filter 412 is selected to be compatible with the carrier gas, PTFE powder, and the temperature of the process. A cloth filter, for example, a non-pleated filter, reduces the compression that the PTFE powder undergoes during filtration and reduces powder agglomeration. The bag filter system generally has a higher collection efficiency than a cyclone separator, but is costly to install and operate. In some implementations, the bag filter system applies a compression to the PTFE powder that can induce fibrillation or agglomeration.

[0054] The outlet valve of the bag filter system 410 prevents the flow of gas into the bag filter system 410. In some implementations, the outlet of the bag filter system 410 includes a flapper valve with an extended spool component. The flapper valve opens when there is sufficient material inside the spool component, reducing the flow of the carrier gas through the spool component and the flapper valve. The flapper valve may include a level detector to maintain a desired level before discharging the material. Implementations that utilize an extended spool component may include a flow promotion device, such as a vibrator, that operates when the valve is open to facilitate the flow of PTFE powder from the outlet.

[0055] The bag filter system can be cleaned using a reverse jet pulse system or mechanical oscillation to separate the PTFE powder. When reverse jet pulsing is used, a cooling compressed gas that does not contain moisture and contaminants is used as the jet gas.

[0056] In some implementations, multiple separators are used in series or in parallel, such as multiple separators of the same type, or both one or more cyclone separators and one or more bag filter systems.

[0057] Referring again to FIG. 1, the channel outlet 128 is in fluid connection with the separator 136 and regulates the flow of PTFE powder from the transfer channel 124 and the powder handling system 100. In some implementations, the channel outlet 128 includes a mechanical sieve, such as a mesh filter, for screening or pulverizing the agglomerated PTFE powder. For example, the channel outlet 128 includes a No. 4 mesh (e.g., 4.75 mm openings) for screening and / or breaking up the agglomerates. In some implementations, the sieve is vibrated. The screened agglomerates are discarded.

[0058] Referring to FIG. 5, in an exemplary method for handling PTFE powder, the PTFE powder is received in a hopper (50). The hopper has a conical lower section and a cylindrical upper section. For example, the hopper can receive less than a threshold amount of PTFE powder that, when at least 200 pounds of PTFE powder is received in the hopper, would cause its own agglomeration due to the force exerted by its own weight. The PTFE powder can be received in the hopper from an upstream hopper or drum.

[0059] The adhesion force between the inner surface of the conical portion of the hopper and the PTFE powder, or the inter-particle adhesion force of the PTFE powder, is reduced (52). The adhesion force can be reduced, for example, by aerating the inner surface of the hopper using a substantially water-free cooled insert aeration gas. The adhesion force can be reduced by applying mechanical vibration to the PTFE powder within the hopper. The adhesion force can be reduced by injecting gas into the PTFE powder within the hopper.

[0060] The walls of the conical portion of the hopper are cooled to a temperature lower than the beta transition temperature of the PTFE powder and above the dew point of the environment (54). For example, the walls can be cooled using a cooling jacket.

[0061] The PTFE powder is discharged into the transfer channel from an outlet located near the base of the conical portion of the hopper (56).

[0062] A pressure difference, such as suction or positive pressure, is applied to the transfer channel to convey a dilute-phase PTFE powder containing gas and PTFE powder along the transfer channel (58). For example, the pressure difference is applied using a variable frequency drive to control the velocity of the dilute phase within the transfer channel, for example, to generate a pickup velocity of at least 2,500 feet per minute.

[0063] The transfer channel is cooled, for example, using a cooling jacket (60). The dilute-phase gas is cooled to a temperature below the beta transition temperature of the PTFE powder and above the dew point of the environment (62).

[0064] At the outlet of the transfer channel, the PTFE powder is separated from the gas within a separator, such as a cyclone separator or a bag filter (64). The separated PTFE powder flows through a discharge valve, such as a flapper valve, at the outlet of the separator (66) and is screened to break up or remove agglomerates of the PTFE powder (68).

[0065] The separated PTFE powder has a particle morphology (e.g., size, shape, or both) sufficient for use in blending with other electrode film components for the dry manufacture of film battery electrodes. For example, at least a portion of the separated PTFE powder includes substantially non-fibrillated PTFE aggregates, e.g., an amount of substantially non-fibrillated PTFE aggregates sufficient to enable the manufacture of film battery electrodes.

[0066] This specification includes many details, but these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. The specific features described herein in the context of separate implementations can also be combined. Conversely, the various features described in the context of a single implementation can also be implemented separately, or in any suitable sub-combination, in multiple implementations.

[0067] Multiple implementation forms have been described. However, it will be understood that various modifications may be made without departing from the spirit and scope of the present invention. Accordingly, other implementation forms are within the scope of the following claims.

Claims

1. A method for handling polytetrafluoroethylene (PTFE) powder, wherein the method is: The hopper having a conical section receives the PTFE powder, (1) to reduce the adhesive force between the inner surface of the conical portion of the hopper and the PTFE powder, (2) to reduce the adhesive force between the particles of the PTFE powder, or both. The PTFE powder is discharged into the transfer channel from an outlet located near the base of the conical portion of the hopper. A pressure difference is applied to the transfer channel to transport the gas and the PTFE powder in the dilute phase containing the PTFE powder along the transfer channel. The transfer channel outlet is used to separate the PTFE powder from the gas, The separated PTFE powder has a particle form sufficient for the dry manufacturing of film battery electrodes, and is used in this method.

2. The method according to claim 1, wherein at least a portion of the separated PTFE powder comprises substantially non-fibrillated PTFE aggregates.

3. The method according to claim 1, wherein the separated PTFE powder comprises a substantially unfibrillated PTFE aggregate in an amount sufficient to enable the manufacture of the film battery electrode.

4. The method according to claim 1, wherein at least 40% by weight of the PTFE powder received in the hopper is separated from the gas for use in manufacturing the film battery electrodes.

5. The method according to claim 1, wherein receiving the PTFE powder into the hopper includes receiving at least 200 pounds of PTFE powder into the hopper.

6. The method according to claim 5, wherein receiving the PTFE powder into the hopper includes receiving a volume of PTFE powder less than a threshold volume of PTFE powder, the threshold volume of PTFE powder being the volume of PTFE powder that, when received into the hopper, undergoes agglomeration due to the force exerted by its own weight.

7. The method according to claim 1, comprising applying suction to the inlet channel to transport the initial dilute phase containing the PTFE powder along the inlet channel into the hopper.

8. The method according to claim 1, wherein the hopper includes a first hopper and includes discharging PTFE powder into the transfer channel from the outlet of the second hopper.

9. The method according to claim 1, wherein reducing the adhesion between the inner surface of the hopper and the PTFE powder includes aerating the inner surface of the hopper.

10. The method according to claim 9, wherein aeration of the inner surface of the hopper includes flowing aeration gas between the outer wall of the conical portion of the hopper and the porous inner wall of the conical portion of the hopper.

11. The method according to claim 10, wherein the porous inner wall of the cone extends from the outlet of the cone to a position along the inner wall of the cone where the diameter of the cone is at least 75% of the maximum diameter of the cone.

12. The method according to claim 10, further comprising cooling the aeration gas before flowing the aeration gas.

13. The method according to claim 10, wherein the aeration gas includes an inert gas.

14. The method according to claim 10, wherein the aeration gas is substantially free of water.

15. The method according to claim 1, further comprising cooling the wall of the conical portion of the hopper.

16. The method according to claim 15, comprising cooling the wall of the conical portion of the hopper to a temperature below the beta transition temperature of the PTFE powder.

17. The method according to claim 15, wherein the hopper comprises a cylindrical portion connected to the conical portion, and includes cooling the wall of the cylindrical portion and the wall of the conical portion of the hopper.

18. The method according to claim 15, wherein the conical portion of the hopper is provided with a cooling jacket disposed on the wall of the conical portion, and cooling the wall of the conical portion includes flowing a cooling fluid through the cooling jacket.

19. The method according to claim 1, comprising providing a layer of cooling gas in the hopper between the PTFE powder and the inlet of the hopper.

20. The method according to claim 1, wherein the height of the hopper is at least twice the maximum diameter of the conical portion of the hopper.

21. The method according to claim 1, wherein the inner surface of the hopper comprises polished stainless steel, and the polishing is in the direction of the flow of the PTFE powder.

22. The method according to claim 1, wherein reducing the adhesion between the inner surface of the hopper and the PTFE powder includes applying mechanical vibration to the PTFE powder in the hopper.

23. The method according to claim 1, wherein reducing the adhesion between the inner surface of the hopper and the PTFE powder includes injecting gas into the PTFE powder in the hopper.

24. The method according to claim 1, wherein applying a pressure difference to the transfer channel includes applying suction to the pressure channel.

25. The method according to claim 1, wherein applying a pressure difference to the transfer channel includes applying a positive pressure to the pressure channel.

26. The method according to claim 1, wherein applying a pressure difference to the transfer channel includes applying a pressure difference to generate a pickup speed of at least 2,500 feet / minute.

27. The method according to claim 1, wherein applying a pressure difference to the transfer channel includes operating a variable frequency drive device to apply the pressure difference to the transfer channel.

28. The method according to claim 27, comprising operating the variable frequency drive device to control the velocity of the dilute phase in the transfer channel.

29. The method according to claim 1, comprising cooling the transfer channel.

30. The method according to claim 29, wherein the transfer channel comprises a jacket, and cooling the transfer channel includes flowing a fluid through the jacket of the transfer channel.

31. The method according to claim 1, comprising cooling the gas in the dilute phase.

32. The method according to claim 1, wherein the gas in the dilute phase includes an inert gas.

33. The method according to claim 1, wherein the inner surface of the transfer channel is made of stainless steel.

34. The method according to claim 1, wherein the inner surface of the transfer channel does not include any welding points.

35. The method according to claim 1, wherein transporting the dilute phase along the transport channel includes transporting the dilute phase around an elbow designed to reduce consolidation and shear.

36. The method according to claim 1, comprising maintaining the gas of the dilute phase at a temperature above the dew point of the environment of the transfer channel.

37. The method according to claim 1, comprising separating the PTFE powder from the gas in a cyclone separator.

38. The method according to claim 37, comprising applying suction to the cyclone separator.

39. Separating the PTFE powder from the gas is The separation of the PTFE powder from the gas in the separator, The method according to claim 1, comprising flowing the PTFE powder through a discharge valve at the outlet of the separator.

40. The method according to claim 1, comprising separating the PTFE powder from the gas in a plurality of separators arranged in series or in parallel along the transfer channel.

41. The method according to claim 40, wherein each of the plurality of separators has a discharge valve connected to a common receiving container.

42. The method according to claim 1, comprising sieving the separated PTFE powder using a sieve to crush or remove material clumps.

43. The method according to claim 42, comprising vibrating the sieve.

44. The method according to claim 42 or 43, wherein the sieve includes a mesh having an opening with a diameter of 2 mm.

Images